ULTRAVIOLET DISINFECTION GUIDANCE MANUAL 

United States 

Environmental 

Protection Agency

Office of Water

(4601) 

EPA 815-D-03-007 

June 2003 

Draft 
Note on the Ultraviolet Disinfection Guidance Manual, June 2003 Draft

Purpose: The purpose of this guidance manual, when finalized, is solely to provide technical

information on the application of ultraviolet light for the disinfection of drinking water by public

water systems. EPA is developing this manual to support two upcoming drinking water

regulations: the Long Term 2 Enhanced Surface Water Treatment Rule, which would require

certain systems to provide additional treatment for Cryptosporidium, and the Stage 2

Disinfection Byproducts Rule, which would place more stringent limits on certain disinfection

byproducts. Chapter 1 of this manual contains additional information about these regulations.

This guidance is not a substitute for applicable legal requirements, nor is it a regulation itself.

Thus, it does not impose legally-binding requirements on any party, including EPA, states, or the

regulated community. Interested parties are free to raise questions and objections to the

guidance and the appropriateness of using it in a particular situation. Although this manual

covers many aspects of implementing a UV system, it is not comprehensive in terms of all types

of UV systems, design alternatives, and validation protocols that may provide satisfactory

performance. The mention of trade names or commercial products does not constitute

endorsement or recommendation for use.

Authorship: This manual was developed under the direction of EPA’s Office of Water, and was

prepared by Malcolm Pirnie, Inc., Carollo Engineers, P.C., The Cadmus Group, Inc., Dr. Karl G.

Linden, and Dr. James P. Malley, Jr. Questions concerning this document should be addressed

to:

Dan Schmelling

U.S. Environmental Protection Agency

Mail Code 4607M

1200 Pennsylvania Avenue NW

Washington, DC 20460-0001

Tel: (202) 564-5281

Fax: (202) 564-3767

Email: schmelling.dan@epa.gov

Request for comments: EPA is releasing this manual in draft form in order to solicit public

review and comment. The Agency would appreciate comments on the content and organization

of technical information presented in this manual. A list of topics for comment pertaining to

specific chapters and appendices is provided later in this manual. Please submit any comments

no later than 90 days after publication of the Long Term 2 Enhanced Surface Water Treatment

Rule proposal in the Federal Register. Detailed procedures for submitting comments are stated

below.
Acknowledgements: The following provided valuable technical information to assist in the 

development of this manual: 

American Water Works Association 

Dave Battigelli (Clancy Environmental Consultants) 

William Bellamy (CH2M Hill) 

Jim Bolton (Executive Director, International Ultraviolet Association) 

Calgon Carbon Corporation 

Tom Hargy (Clancy Environmental Consultants) 

Oluf Hoyer (DVGW)

Richard Hubel (American Water) 

Chris McMeen (then with Washington Department of Health) 

Alexander Mofidi (Metropolitan Water District of Southern California) 

 Ondeo Degremont 

Richard Sakaji (California Department of Health Services) 

Severn Trent Services 

Regina Sommer (University of Vienna) 

Paul Swaim (CH2M Hill) 

 Trojan Technologies 

 Wedeco-Ideal Horizons

John Young (American Water) 

Procedures for submitting comments: Comments on this draft guidance manual should be 

submitted to EPA’s Water Docket. You may submit comments electronically, by mail, or 

through hand delivery/courier. 

• To submit comments using EPA’s electronic public docket, go directly to EPA Dockets at 

http://www.epa.gov/edocket, and follow the online instructions for submitting comments. 

Once in the system, select “search,” and then key in Docket ID No. OW-2002-0039. 

• To submit comments by e-mail, send comments to OW_Docket@epa.gov, Attention Docket 

ID No. OW-2002-0039. If you send an e-mail comment directly to the Docket without going 

through EPA’s electronic public docket, EPA’s e-mail system automatically captures your email address, which is included as part of the comment that is placed in the official public 

docket. 

• To submit comments on a disk or CD ROM, mail it to the address identified below. These 

electronic submissions will be accepted in WordPerfect or ASCII file format. Avoid the use 

of special characters and any form of encryption. 

• To submit comments by mail, send three copies of your comments and any enclosures to: 

Water Docket, Environmental Protection Agency, Mail Code 4101T, 1200 Pennsylvania 

Ave., NW, Washington, DC, 20460, Attention Docket ID No. OW-2002-0039. 

• To submit comments by hand delivery or courier, deliver your comments to: Water Docket, 

EPA Docket Center, Environmental Protection Agency, Room B102, 1301 Constitution 

Ave., NW, Washington, DC, Attention Docket ID No. OW-2002-0039. 
Please identify the appropriate docket identification number in the subject line on the first page 

of your comment. If you submit an electronic comment, please include your name, mailing 

address, and an e-mail address or other contact information in the body of your comment. Also 

include this contact information on the outside of any disk or CD ROM you submit, and in any 

cover letter accompanying the disk or CD ROM. 

For public commenters, please note that EPA’s policy is that public comments, whether 

submitted electronically or in paper, will be made available for public viewing in EPA’s 

electronic public docket as EPA receives them and without change, unless the comment contains 

copyrighted material, confidential business information, or other information whose disclosure is 

restricted by statute. 
Table of Contents 

Table of Contents............................................................................................................................. i 

List of Figures................................................................................................................................ vi 

List of Tables ................................................................................................................................ vii 

Glossary ....................................................................................................................................... viii 

Acronyms and Abbreviations ...................................................................................................... xiii 

1.0 Introduction.................................................................................................................... 1-1 

1.1 Guidance Manual Objectives............................................................................... 1-1 

1.2 Organization......................................................................................................... 1-2 

1.3 Regulations Summary.......................................................................................... 1-3 

1.3.1 Long Term 2 Enhanced Surface Water Treatment Rule.......................... 1-4 

1.3.1.1 Filtered Systems..................................................................... 1-5

1.3.1.2 Unfiltered Systems................................................................. 1-6 

1.3.1.3 UV Requirements For Filtered And Unfiltered Systems....... 1-7 

1.3.2 Stage 2 DBPR .......................................................................................... 1-9 

1.4 Alternative Approaches for Disinfecting with UV Light .................................... 1-9 

2.0 Overview of UV Disinfection......................................................................................... 2-1 

2.1 History of UV Light for Drinking Water Disinfection ........................................ 2-1 

2.2 Fundamental Aspects of UV Light ...................................................................... 2-2 

2.2.1 Nature of UV Light.................................................................................. 2-2 

2.2.2 Propagation of UV Light ......................................................................... 2-3 

2.3 Microbial Response to UV Light......................................................................... 2-6 

2.3.1 Mechanisms of Microbial Inactivation by UV Light............................... 2-6 

2.3.2 Microbial Repair...................................................................................... 2-7 

2.3.3 UV Dose and Dose Distribution ............................................................ 2-10 

2.3.4 Microbial Response (UV Dose-Response)............................................ 2-11 

2.3.5 Microbial Spectral Response ................................................................. 2-11 

2.4 UV Reactors....................................................................................................... 2-12 

2.4.1 Reactor Configuration............................................................................ 2-13 

2.4.2 UV Lamps.............................................................................................. 2-14 

2.4.3 Lamp Power Supply and Ballasts.......................................................... 2-17 

2.4.4 Lamp Sleeves......................................................................................... 2-17 

2.4.5 Cleaning Systems................................................................................... 2-18 

2.4.6 UV Intensity Sensors............................................................................. 2-19 

2.4.7 UV Transmittance Monitors.................................................................. 2-20 

2.4.8 Temperature Sensors.............................................................................. 2-21 

2.4.9 Monitoring UV Disinfection Performance ............................................ 2-21 

UV Disinfection Guidance Manual i June 2003

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Table of Contents (Continued)

UV Disinfection Guidance Manual ii June 2003

2.5 Water Quality Impacts and Byproduct Formation............................................. 2-22 

2.5.1 Water Quality Impacts........................................................................... 2-22 

2.5.2 Byproducts from UV Disinfection......................................................... 2-25 

3.0 Planning and Design Aspects for UV Installations..................................................... 3-1 

3.1 UV Installations Planning.................................................................................... 3-4 

3.1.1 Defining UV Disinfection Goals ............................................................. 3-4 

3.1.2 Identifying Potential Locations For UV Installations.............................. 3-5 

3.1.2.1 Combined Filter Effluent Installation .................................... 3-6 

3.1.2.2 Individual Filter Effluent Piping Installation......................... 3-6 

3.1.2.3 UV Disinfection Downstream of The Clearwell ................... 3-8 

3.1.3 Defining Design Parameters.................................................................... 3-9 

3.1.3.1 Assessing Water Quality...................................................... 3-10 

3.1.3.2 Determining Design Flow Rate ........................................... 3-17 

3.1.3.3 Assessing Electrical Power.................................................. 3-17 

3.1.4 Evaluating Potential UV Reactors......................................................... 3-20 

3.1.4.1 UV Reactors......................................................................... 3-20 

3.1.4.2 UV Reactor Control Strategies ............................................ 3-22 

3.1.4.3 Equipment Validation Issues............................................... 3-22 

3.1.5 Evaluating Operational Strategies.......................................................... 3-24 

3.1.6 Evaluating Hydraulics and Process Footprint........................................ 3-25 

3.1.6.1 Hydraulic Considerations..................................................... 3-25 

3.1.6.2 Process Footprint ................................................................. 3-27 

3.1.7 Preparing Preliminary Costs and Selecting the UV Installation Option 3-28 

3.2 Equipment Procurement Options....................................................................... 3-29 

3.3 UV Installation Design Elements....................................................................... 3-30 

3.3.1 UV Installation Hydraulics.................................................................... 3-30 

3.3.1.1 Inlet and Outlet Piping Configuration.................................. 3-31 

3.3.1.2 Flow Distribution, Control, and Measurement .................... 3-31 

3.3.1.3 Level Control ....................................................................... 3-36 

3.3.1.4 Air Relief and Pressure Control Valves............................... 3-36 

3.3.1.5 Flow Control and Isolation Valves...................................... 3-37 

3.3.1.6 Intermediate Booster Pumps................................................ 3-37 

3.3.2 Operational Strategy Determination ...................................................... 3-38 

3.3.3 Instrumentation and Control .................................................................. 3-38 

3.3.3.1 UV Reactor Start-Up............................................................ 3-39 

3.3.3.2 UV Reactor Automation ...................................................... 3-39 

3.3.3.3 UV Intensity and Calculated Dose (If Applicable).............. 3-39 

3.3.3.4 UV Transmittance................................................................ 3-40 

3.3.3.5 Flow Measurement............................................................... 3-40 

3.3.3.6 Lamp Age............................................................................. 3-40 

3.3.3.7 Lamp and Reactor Status..................................................... 3-41 

3.3.3.8 Alarms and Control Systems Interlocks .............................. 3-41 

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Table of Contents (Continued)

UV Disinfection Guidance Manual iii June 2003

3.3.4 Electrical Power Configuration.............................................................. 3-42 

3.3.4.1 Power Requirements............................................................ 3-43 

3.3.4.2 Backup Power Supply.......................................................... 3-43 

3.3.4.3 Ground Fault Interrupt and Electrical Lockout.................... 3-44 

3.3.5 UV Installation Layouts......................................................................... 3-45 

3.3.5.1 Site Layout........................................................................... 3-45 

3.3.5.2 UV Installation Layout ........................................................ 3-45 

3.3.6 Elements Of UV Reactor Specifications................................................ 3-47 

3.3.6.1 Information Provided by Manufacturer in UV 

Reactor Bid .......................................................................... 3-49 

3.3.7 Final UV Installation Design ................................................................. 3-51 

3.3.7.1 Design Drawings.................................................................. 3-51 

3.3.7.2 Specifications....................................................................... 3-52 

3.4 Reporting to the State......................................................................................... 3-52 

3.4.1 Planning ................................................................................................. 3-52 

3.4.2 Equipment Procurement......................................................................... 3-53 

3.4.3 Drawings and Specifications.................................................................. 3-53 

3.4.4 Validation Report/Start-up Confirmation .............................................. 3-53 

4.0 Overview of Validation Testing .................................................................................... 4-1 

4.1 LT2ESWTR UV Disinfection Requirements ...................................................... 4-1 

4.2 Overview of Validation Process .......................................................................... 4-2 

4.2.1 Relating the Experimental RED to Log Inactivation Credit.................... 4-4 

4.2.1.1 Tier 1 and Tier 2 Approaches for Establishing Inactivation 

 Credit...................................................................................... 4-5 

4.2.2 Location and Application of Validation Testing ..................................... 4-5 

4.2.3 Third-Party Oversight.............................................................................. 4-6 

4.3 Considerations for Validation Testing................................................................. 4-6 

4.3.1 Inlet and Outlet Hydraulics...................................................................... 4-7 

4.3.2 UV Equipment ......................................................................................... 4-7 

4.3.2.1 UV Reactor Documentation................................................... 4-7 

4.3.2.2 Control Strategies................................................................... 4-7 

4.3.2.3 UV Intensity Sensor............................................................... 4-8 

4.3.2.4 Lamp Aging ........................................................................... 4-8 

4.3.3 Additives Used in Validation Testing...................................................... 4-8 

4.3.3.1 Challenge Microorganism...................................................... 4-8 

4.3.3.2 UV-Absorbing Material......................................................... 4-9 

4.4 Validation Testing ................................................................................................ 4-9 

4.4.1 Microorganism Preparation ..................................................................... 4-9 

4.4.2 Collimated Beam Testing ........................................................................ 4-9 

4.4.3 Biodosimetry of Full-Scale Reactors..................................................... 4-10 

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Table of Contents (Continued)

UV Disinfection Guidance Manual iv June 2003

4.5 Data Analysis..................................................................................................... 4-11 

4.5.1 Developing Challenge Microorganisms Dose-Response Curve............ 4-12 

4.5.1.1 Calculate Dose-Response Data From Collimated Beam 

 Testing.................................................................................. 4-12 

4.5.1.2 Fitting Dose-Response Data to a Curve .............................. 4-13 

4.5.2 Determining Log Inactivation from Biodosimetry Testing ................... 4-13 

4.5.3 Determining the RED ............................................................................ 4-14 

4.5.3.1 Calculating the RED Values................................................ 4-14 

4.5.3.2 Selecting the Appropriate RED for Log Inactivation 

Credit Determination ........................................................... 4-14 

4.5.3.3 Interpolating RED as a Function of Test Conditions........... 4-15 

4.5.4 Determining Inactivation Credit ............................................................ 4-15 

4.6 Tier 1 Criteria..................................................................................................... 4-17 

4.6.1 UV Intensity Sensors............................................................................. 4-17 

4.6.2 UV Lamp Output ................................................................................... 4-19 

4.6.3 Flow Measurements............................................................................... 4-19 

4.6.4 Collimated Beam Apparatus.................................................................. 4-19 

4.6.5 Challenge Microorganism Dose-Response............................................ 4-19 

4.6.6 Medium Pressure Lamps........................................................................ 4-20 

4.6.7 Biodosimetry Sampling ......................................................................... 4-21 

5.0 Start-Up and Operation of UV Installations............................................................... 5-1 

5.1 Start-up of UV Installation................................................................................... 5-3 

5.1.1 Final Inspection........................................................................................ 5-3 

5.1.2 Functional Testing ................................................................................... 5-3 

5.1.2.1 Verification of Mechanical Operation ................................... 5-4 

5.1.2.2 Verification of Monitoring Equipment .................................. 5-4 

5.1.2.3 Verification of Instrumentation and Control Systems........... 5-5 

5.1.2.4 Verification of Flow Distribution and Headloss.................... 5-6 

5.1.3 Performance Testing ................................................................................ 5-7 

5.1.4 Operations and Maintenance Manual ...................................................... 5-9 

5.2 Operation of UV Installations............................................................................ 5-10 

5.2.1 Operational Requirements ..................................................................... 5-11 

5.2.2 Recommended Operational Tasks ......................................................... 5-11 

5.2.3 Start-up and Shutdown of UV Reactors................................................. 5-12 

5.2.3.1 Routine Start-up................................................................... 5-12 

5.2.3.2 Routine Shutdown................................................................ 5-13 

5.2.3.3 Winterization........................................................................ 5-13 

5.3 Maintenance of UV Reactors............................................................................. 5-13 

5.3.1 Summary of Recommended Maintenance Tasks................................... 5-14 

5.3.2 General Guidelines for UV Reactor Maintenance ................................. 5-15 

5.3.2.1 UV Lamp Characteristics..................................................... 5-15 

5.3.2.2 UV Intensity Sensors ........................................................... 5-16 

5.3.2.3 Lamp Sleeves....................................................................... 5-18 

5.3.2.4 Fouling ................................................................................. 5-18 

Proposal Draft 
Table of Contents (Continued)

UV Disinfection Guidance Manual v June 2003

5.3.2.5 On-line UVT Monitor Calibration........................................ 5-20 

5.3.2.6 Flowmeter Calibration .......................................................... 5-20 

5.3.2.7 UV Reactor Temperature...................................................... 5-20 

5.3.2.8 Electrical Concerns............................................................... 5-21 

5.3.3 Spare Parts ............................................................................................. 5-22 

5.4 Monitoring, Recording, and Reporting of UV Installation Operation............... 5-24 

5.4.1 Monitoring and Recording Frequency for Compliance Parameters...... 5-24 

5.4.2 Monitoring and Recording for Other Operational Parameters .............. 5-25 

5.4.3 Reporting to the State............................................................................. 5-26 

5.5 Determination of Validated Operational Parameters......................................... 5-27 

5.6 Operational Challenges...................................................................................... 5-32 

5.6.1 Low UV Intensity or Low Calculated UV Dose.................................... 5-32 

5.6.2 Low UV Transmittance.......................................................................... 5-34 

5.6.3 Rapid Flow Increase or High Flow........................................................ 5-36 

5.6.4 Unreliable UV Intensity Sensor Readings............................................. 5-36 

5.6.5 Power Quality Problems........................................................................ 5-37 

5.7 Staffing Issues.................................................................................................... 5-37 

5.7.1 Staffing Levels....................................................................................... 5-38 

5.7.2 Training.................................................................................................. 5-38 

5.7.3 Safety Issues........................................................................................... 5-39 

6.0 References....................................................................................................................... 6-1 

 

Appendices 

Appendix A Fundamentals of UV Disinfection .......................................................... A-1 

Appendix B Derivation of UV Dose-Response Requirements ...................................B-1 

Appendix C Validation of UV Reactors ......................................................................C-1 

Appendix D Microbiological Methods ....................................................................... D-1 

Appendix E Measuring Challenge Microorganism UV Dose-Response ....................E-1 

Appendix F Background to the UV Reactor Validation Protocol ..............................F-1 

Appendix G Issues for Unfiltered Systems ................................................................ G-1 

Appendix H Issues for Ground Water Systems .......................................................... H-1 

Appendix I Issues for Small Systems .........................................................................I-1 

Appendix J Pilot-Scale and Demonstration-Scale Testing .........................................J-1 

Appendix K Preliminary Engineering Report ............................................................ K-1 

Appendix L Regulatory Timeline ...............................................................................L-1 

Appendix M Compliance Forms .................................................................................M-1 

Appendix N UV Lamp Breakage Issues ..................................................................... N-1 

Appendix O Case Studies [This appendix will be included in the final 

draft when more information being available.] ...................................... O-1 

Appendix P Validation Protocol Calculator Tool .......................................................P-1 

Proposal Draft 
List of Figures 

Figure 2.1 UV Light in the Electromagnetic Spectrum......................................................... 2-3 

Figure 2.2 Refraction of Light............................................................................................... 2-4 

Figure 2.3 Reflection of Light............................................................................................... 2-4 

Figure 2.4 Scattering of Light ............................................................................................... 2-5 

Figure 2.5 Structure of DNA and Nucleotide Sequences Within DNA................................ 2-6 

Figure 2.6 UV Absorbance of Nucleotides and Nucleic Acid at pH 7 ................................ 2-7 

Figure 2.7 Hypothetical Dose Distributions for Two Reactors with Differing 

 Hydraulics............................................................................................................ 2-9 

Figure 2.8 Shapes of UV Dose-Response Curves............................................................... 2-11 

Figure 2.9 Comparison of Microbial UV Action and DNA UV Absorbance..................... 2-12 

Figure 2.10 UV Disinfection System Schematic................................................................... 2-13 

Figure 2.11 Example of Closed and Open Channel Reactors............................................... 2-14 

Figure 2.12 UV Output of LP and MP Mercury Vapor Lamps ............................................ 2-16 

Figure 2.13 UV Lamp Output and its Relation to the UV Absorbance of DNA .................. 2-17 

Figure 2.14 UV Transmittance of Quartz that is 1 mm Thick at a Zero Degree 

 Incidence Angle ................................................................................................. 2-18 

Figure 2.15 Mechanical Wiper System and Physical-Chemical Wiper System ................... 2-19 

Figure 2.16 UV Intensity Sensor Viewing Lamps within a UV Reactor.............................. 2-20 

Figure 2.17 UV Transmittance Monitor Design ................................................................... 2-21 

Figure 3.1 Flowchart for Planning, Design, and Construction of UV Facilities................... 3-3 

Figure 3.2 Schematic for UV Installation (Upstream of Clearwell) ..................................... 3-6 

Figure 3.3 Schematic of Individual Filter Effluent Piping Installation in Filter Gallery ...... 3-7 

Figure 3.4 UV Disinfection Downstream of High Service Pumps ....................................... 3-8 

Figure 3.5 Example CF Diagram for Three Filtered Waters............................................... 3-12 

Figure 3.6 Example Flow and UV Absorbance (at 254) Data ............................................ 3-13 

Figure 3.7 Example Effect of Pre-ozonation on UV Absorbance if Ozone is 

Quenched Prior to UV Disinfection................................................................... 3-16 

Figure 3.8 Open-Channel Flow Distribution Options......................................................... 3-33 

Figure 3.9 Flow Measurement and Control Options........................................................... 3-36 

Figure 4.1 Steps of a Validation Process............................................................................... 4-3 

Figure 4.2 Collimated Beam Test Apparatus...................................................................... 4-10 

Figure 4.3 Biodosimetry Test Components......................................................................... 4-11 

Figure 4.4 Examples of UV Intensity Sensor Spectral Response Ranges........................... 4-18 

Figure 4.5 Dose-Response with a Shoulder ........................................................................ 4-20 

Figure 4.6 Criteria for the Minimum UVT of MP UV Systems Under Tier 1.................... 4-21 

Figure 5.1 Start-Up and Operation Flowchart....................................................................... 5-2 

Figure 5.2 Example 2-Interpoloation of Validation Data to Determine UV Intensity 

Setpoints............................................................................................................. 5-29 

Figure 5.3 Interpolation of Validation Data to Determine UV Intensity Setpoints at 

Different Flows and Cryptosporidium Inactivation........................................... 5-30 

Figure 5.4 Low UV Intensity of Low Calculated UV Dose Decision Chart....................... 5-33 

Figure 5.5 High UV Absorbance Decision Chart................................................................ 5-35 

UV Disinfection Guidance Manual vi June 2003

Proposal Draft 
List of Tables 

Table 1.1 Summary of Microbial and Disinfection Byproduct Rules ................................. 1-4 

Table 1.2 Bin Requirements for Filtered Systems............................................................... 1-6 

Table 1.3 Bin Requirements for Unfiltered Systems........................................................... 1-7 

Table 1.4 UV Dose Requirements Used During Validation Testing................................... 1-7 

Table 2.1 Mercury Vapor Lamp Characteristics................................................................ 2-15 

Table 2.2 Mercury Vapor Lamp Comparison.................................................................... 2-15 

Table 2.3 Water Quality Data and Fouling Observed for UV Disinfection Pilot and 

Demonstration Studies....................................................................................... 2-24 

Table 3.1 Potential Method to Determine Design Flow .................................................... 3-17 

Table 3.2 Start and Restart Times for LPHO and MP Lamps........................................... 3-18 

Table 3.3 UV Reactor Control Strategies .......................................................................... 3-22 

Table 3.4 Summary of Recommended Hydraulic Configurations for 

Validation and Installation................................................................................. 3-23 

Table 3.5 Potential Operational Strategies......................................................................... 3-25 

Table 3.6 Potential UV Reactor Procurement Options...................................................... 3-30 

Table 3.7 Comparison of Techniques for UV Installation Flow Measurement................. 3-35 

Table 3.8 Typical Alarm Conditions for UV Systems....................................................... 3-42 

Table 3.9 Recommended Content for UV Reactor Specifications .................................... 3-48 

Table 3.10 Recommended Information to be Provided by UV Manufacturer/Vendor ....... 3-50 

Table 4.1 Tier 1 RED Targets for UV System with LP of LPHO Lamps......................... 4-16 

Table 4.2 Tier 1 RED Targets for UV System with MP Lamps........................................ 4-16 

Table 5.1 Example Monitoring During a Four Week Performance Test............................. 5-9 

Table 5.2 Recommended Operational Tasks for the UV Reactor...................................... 5-11 

Table 5.3 Recommended Maintenance Tasks.................................................................... 5-14 

Table 5.4 Design and Guaranteed Lives of Major UV Components................................. 5-23 

Table 5.5 Off-Specification Operations for Each Control Strategy................................... 5-24 

Table 5.6 Monitoring Parameters and Recording Frequency ............................................ 5-25 

Table 5.7 Recommended Monitoring Parameters and Recording Frequency ................... 5-26 

Table 5.8 UV Reactor Control Strategies .......................................................................... 5-27 

Table 5.9 Example Validation Data for Variable Setpoint Operation ............................... 5-28 

Table 5.10 UV Intensity Setpoint for Different Flow Ranges............................................. 5-28 

Table 5.11 Example Validation Data for Variable Setpoint Operation ............................... 5-29 

Table 5.12 UV Intensity Setpoint for Different Flow Ranges............................................. 5-30 

Table 5.13 Dose Setpoints for Various Log Inactivation of Cryptosporidium.................... 5-31 

Table 5.14 Factors Impacting Staffing Needs...................................................................... 5-38 

UV Disinfection Guidance Manual vii June 2003

Proposal Draft 
Glossary 

The following definitions were derived from existing UV literature, standard physics

textbooks, and/or industry standards and conventions. Some concepts have more than one 

acceptable term or definition, but for consistency within the document, only one term is used.

Absorption – the transformation of UV light to other forms of energy as it passes through a 

substance. 

Action Spectrum – the relative efficiency of UV energy at different wavelengths in inactivating 

microorganisms. Each microorganism has a unique action spectrum.

Ballast – provides the proper voltage and current required to initiate and maintain the gas 

discharge within the UV lamp. 

Bioassay – a procedure used to determine the response of a specific microorganism after 

exposure to UV light, usually in UV reactors. Bioassay is a term typically utilized in toxicology, 

describing the testing of the bio-toxicity of a contaminant. Bioassay has been used in the UV 

disinfection literature in the same context as “biodosimetry” (see biodosimetry). 

Biodosimeter – the challenge microorganism used to measure UV inactivation and ultimately 

calculate the reduction equivalent dose (RED; see UV dose) in a UV reactor. 

Biodosimetry – a procedure used to determine the reduction equivalent dose (RED) of a UV 

reactor. Biodosimetry involves measuring the inactivation of a challenge microorganism after 

exposure to UV light in a UV reactor and comparing the results to the known UV dose-response 

curve of the challenge microorganism (determined using collimated beam testing) to determine 

the RED (see UV Dose). 

Challenge Organism – a microorganism used in UV reactor biodosimetry testing. 

Collimated Beam Test – a carefully controlled bench-scale test that is used to determine the UV 

dose-response of a microorganism. Both time and UV light intensity are accurately measured, 

resulting in a specific calculation of delivered UV dose for the microorganism being tested. 

Collimated beam tests are described in detail in Appendix C. 

Dark Repair – an enzyme-mediated microbial process that removes and regenerates a damaged 

section of deoxyribonucleic acid (DNA), using an existing complimentary strand of DNA. Dark 

repair refers to all microbial repair processes not requiring reactivating light. 

Delivered UV Dose – see UV Dose

Dose Control Strategy – the technique used by a UV system to control the delivered dose that 

typically involves adjusting the lamp power or turning "on" or "off" banks of UV lamps to 

respond to changes in UV absorbance, lamp intensity, and flow. Typically, the dose control 

strategy is different for LP/LPHO and MP systems. 

UV Disinfection Guidance Manual viii June 2003

Proposal Draft 
 Glossary (Continued) 

UV Disinfection Guidance Manual ix June 2003

Dose Distribution – see UV Dose, Delivered UV Dose Distribution. 

Emission Spectrum – the relative light power emitted by a lamp as a function of wavelength. 

Fluence – see UV Dose

Fluence Rate – see UV Intensity

Gas Discharge – a mixture of non-excited atoms, excited atoms, cations, and free electrons 

formed when a sufficiently high voltage is applied across a volume of gas. Most commercial UV 

lamps use mercury gas discharges to generate UV light. 

Germicidal Effectiveness – the relative inactivation efficiency of each UV wavelength in a 

polychromatic emission spectrum. This value is usually approximated by the relative absorbance 

of DNA at each wavelength, although individual microorganisms may respond differently. By 

convention, germicidal effectiveness of the 254 nm emission line by LP UV lamps is considered 

to be unity. The germicidal effectiveness is typically used to weight a polychromatic, MP UV 

lamp output to reflect the germicidal energy of that specific source. 

Germicidal Range – the range of UV wavelengths responsible for microbial inactivation in 

water (200 to 300 nm). 

Lamp Envelope – the exterior surface of the UV lamp, which is typically made of quartz. 

Lamp Sleeve – the quartz tube that surrounds and protects the UV lamp. The exterior is in 

direct contact with the water being treated. There is typically an air gap (approximately 1 cm) 

between the lamp envelope and the quartz sleeve.

Light Pipe – a quartz cylinder that transmits light from the interior of the UV reactor to the 

photodetector of a UV intensity sensor. 

Lignin Sulfonate – a commercially available reagent grade chemical that can simulate the UV 

absorbance spectrum of natural waters and be used to adjust UV transmittance during validation 

testing. 

Low Pressure (LP) Lamp – a mercury vapor lamp that operates at an internal pressure of 0.001 

to 0.01 torr (2 x 10-5 to 2 x 10-4 psi) and electrical input of 0.5 watts per centimeter. This results 

in essentially monochromatic light output at 254 nanometers. 

Low Pressure High Output (LPHO) Lamp – a low pressure mercury vapor lamp that operates 

under increased electrical input (1.5 to 10 W/cm), resulting in a higher UV intensity than LP 

lamps. It also has essentially monochromatic light output at 254 nanometers. 

Medium Pressure (MP) Lamp – a mercury vapor lamp that operates at an internal pressure of 

100 to 10,000 torr (2 to 200 psi) and electrical input of 50 to 150 W/cm. This results in a 

polychromatic (or broad spectrum) output of UV and visible light at multiple wavelengths, 

including the germicidal range. 

Proposal Draft 
 Glossary (Continued) 

UV Disinfection Guidance Manual x June 2003

Monochromatic – light output at only one wavelength. For example, because low pressure and 

low pressure high output lamps only significantly emit light at 254 nanometers, they are 

considered monochromatic UV light sources. 

Monitoring Window – a quartz disc that transmits light from the interior of the UV reactor to 

the photodetector of a UV intensity sensor. 

Offline Chemical Clean (OCC) – a process to clean lamp sleeves where the UV reactor is taken 

off-line and a cleaning solution (typically an acid) is manually pumped into the reactor. After 

the foulant has dissolved, the reactor is drained, rinsed, and returned to service. Also called 

flush-and-rinse systems. 

Online Mechanical Clean (OMC) – a process to clean lamp sleeves where an automatic 

mechanical wiper (e.g., O-ring, brush) wipes the surface of the lamp sleeve at a prescribed 

frequency. 

Petri Factor – a ratio used in collimated beam testing that is equal to the average intensity 

measured across the surface of a suspension in a petri dish divided by the intensity at the center 

of a petri dish. The petri factor is used to help calculate delivered UV dose as described in 

Appendix C. 

Photodetector – a device that produces an electrical current proportional to the UV light 

intensity at the detector's surface. 

Photoreactivation – a microbial repair process where enzymes activated by light in the near UV 

and visible range (310 to 490 nm) split pyrimidine dimers, thereby repairing UV induced 

damage. Photoreactivation requires the presence of light. 

Polychromatic – light energy output at several wavelengths such as with MP lamps. 

Quartz Sleeve – see lamp sleeve

Radiometer – an instrument used to measure UV irradiance 

Reduction Equivalent Dose (RED) – see UV Dose, RED. 

Reflection – the change in direction of light propagation when deflected by an interface or 

surface. 

Refraction – the change in direction of light propagation as it passes through one medium to 

another. 

Scattering – the change in direction of light propagation caused by interaction with a particle. 

Spectral UV Absorbance – the determination of UV Absorbance (A) over a range of 

wavelengths (e.g. 200 to 400 nm) 

Proposal Draft 
 Glossary (Continued) 

UV Disinfection Guidance Manual xi June 2003

State – the agency of the state, tribal, or federal government that has jurisdiction over public 

water systems. 

UV absorbance (A) – a measure of the amount of UV light that is absorbed by a substance (e.g., 

water, microbial DNA, lamp envelope, quartz sleeve) at a specific wavelength (e.g., 254 nm). 

This measurement accounts for absorption and scattering in the medium (e.g., water). Typically 

the absorbance is measured on a per centimeter (cm) basis in a 1 cm quartz cuvette. Standard 

Method 5910B details this measurement method. However, for UV disinfection applications, the 

sample should not be filtered or adjusted for pH as described in Standard Methods. 

UV Dose – the energy per unit area incident on a surface, typically in units of mJ/cm2

 or J/m2

(older literature also used the units mW-s/cm2

 where 1 mW-s/cm2 = 1 mJ/cm2). The UV dose 

received by a waterborne microorganism in a reactor vessel accounts for the effects on UV 

intensity of the absorbance of the water, absorbance of the quartz sleeves, reflection and 

refraction of light from the water surface and reactor walls, and the germicidal effectiveness of 

the UV wavelengths. This guidance also uses the following terms related to UV dose: 

• Delivered UV dose distribution – the probability distribution of delivered UV doses 

that microorganisms receive in a flow-through UV reactor; typically shown as a 

histogram. An example is shown in Figure 2-7. 

• Reduction Equivalent Dose (RED) – a calculated dose for a flow through UV 

reactor that is based on biodosimetry (i.e., measuring the level of inactivation of a 

challenge microorganism with a known UV dose-response). The RED is set equal to 

the UV dose in a collimated beam test that achieves the same level of inactivation of

the challenge microorganism as measured for the flow-through UV reactor during 

biodosimetry testing. 

UV Dose-Response – the relationship indicating the level of inactivation of a microbe as a 

function of UV dose. Inactivation is often plotted as log10(N0/N) where N0 is the number of 

microbes present prior to UV light exposure and N is the number of microbes present after UV 

light exposure. Examples are shown in Figure 2-8. 

UV Installation – all of the components of the UV disinfection process, including (but not 

limited to) UV reactors, control systems, piping, valves, and building or enclosure. 

UV Intensity – the power per unit area passing through an area perpendicular to the direction of 

propagation. UV intensity is used in this guidance manual to describe the magnitude of UV light 

in a UV reactor and in bench-scale UV experiments. 

UV Intensity Sensor – a photosensitive detector used to measure the UV intensity at a point 

within the UV reactor. 

UV Irradiance – the power per unit area incident to the direction of light propagation at all 

angles, including normal. 

UV Light – electromagnetic radiation with wavelengths from 200 to 400 nm. 

Proposal Draft 
 Glossary (Continued) 

UV Disinfection Guidance Manual xii June 2003

UV Reactor – the vessel or chamber where exposure to UV light takes place, consisting of UV 

lamps, quartz sleeves, UV intensity sensors, quartz sleeve cleaning systems, and baffles or other 

hydraulic controls. The UV reactor also includes additional hardware for controlling UV dose; 

typically comprised of (but not limited to): UV intensity sensors, UV transmittance monitors, 

ballasts, and control panels. 

UV Reactor Validation – a process by which a UV reactor’s disinfection performance is 

determined relative to operating parameters that can be monitored. Reactors are validated to 

indicate that they achieve a certain delivered UV dose for a range of flow, UV intensity, and 

water quality conditions (e.g., UV transmittance). Appendix C details the protocol for validating 

UV reactors.

UV Transmittance (UVT) – a measure of the fraction of incident light transmitted through the 

water column. The UV transmittance is the ratio of the light entering the water to that exiting the 

water. The UVT is usually reported for a pathlength of 1 cm. In an alternate pathlength is used, 

it should be specified. UVT is often represented as a percentage and is related to the UV 

absorbance by the following equation: %UVT = 100 x 10-A. As the UV absorbance increases, 

the UV transmittance decreases. 

Proposal Draft 
List of Acronyms and Abbreviations

A254 ultraviolet absorbance at 254 nanometers 

AC alternating current 

ACGIH American Conference of Governmental Industrial Hygienists 

ACS Automatic cleaning system

ANSI American National Standards Institute 

AOC assimilable organic carbon 

APHA American Public Health Association

ATCC American Type Culture Collection 

atm atmospheres 

AWWA American Water Works Association 

AwwaRF American Water Works Association Research Foundation 

BDL below detectable limits 

BDOC biodegradable dissolved organic carbon 

°C degrees Centigrade 

CCPP calcium carbonate precipitation potential 

CF cumulative frequency 

CFD computational fluid dynamics 

CFR Code of Federal Regulations 

cfu colony forming unit 

CIP clean-in-place 

cm centimeter 

CPEL ceiling level permissible exposure limit 

CSI Construction Specifications Institute 

CT residual disinfectant concentration (mg/L) x time (min) 

CWS community water system

DBP disinfection byproduct 

DBPR disinfection byproduct rule 

DC direct current 

D/DBP disinfectants/disinfection by-product 

DNA deoxyribonucleic acid 

DOC dissolved organic carbon 

DVGW Deutsche Vereinigung des Gas- und Wasserfaches (German Association 

for Gas and Water) 

e exponent of the base of the natural logarithm

EPA United States Environmental Protection Agency 

°F degrees Fahrenheit 

ft feet

g gram

GAC granular activated carbon 

UV Disinfection Guidance Manual xiii June 2003

Proposal Draft 
List of Acronyms and Abbreviations (Continued)

UV Disinfection Guidance Manual xiv June 2003

gal gallon 

GFI ground fault interrupt 

gpm gallons per minute 

GWR ground water rule 

GWUDI ground water under the direct influence [of surface water] 

h hour 

HAA haloacetic acid 

HDPE high-density polyethylene 

HGL hydraulic grade line 

hp horsepower 

HPC heterotrophic plate count 

HSP high service pump

Hz hertz 

I UV intensity 

IDLH Immediately Dangerous to Life or Health 

IDSE initial distribution system evaluation 

IESWTR Interim Enhanced Surface Water Treatment Rule 

IT UV intensity x time 

J joule 

kW kilowatt 

kW-hr kilowatt-hour 

ln natural logarithm

LP low pressure 

LPHO low pressure high output 

LRAA locational running annual average 

LSI Langlier Saturation Index 

LT1ESWTR Long Term 1 Enhanced Surface Water Treatment Rule 

LT2ESWTR Long Term 2 Enhanced Surface Water Treatment Rule 

λ wavelength 

m meter 

mA milliamp

MCL maximum contaminant level 

mg milligram

mgd million gallons per day 

min minutes

mJ millijoule 

mL milliliter 

mm millimeter 

MP medium pressure 

MS2 male specific-2 bacteriophage 

μg microgram

μm micrometer, micron 

Proposal Draft 
List of Acronyms and Abbreviations (Continued)

UV Disinfection Guidance Manual xv June 2003

NEL National Electric Code 

nm nanometer 

NIOSH National Institute for Occupational Safety and Health 

NIST National Institute of Standards and Technology

NOM natural organic matter 

NSF National Science Foundation 

NTNCWS non-transient non-community water system

NTU nephelometric turbidity units 

NWRI National Water Research Institute 

O&M operation and maintenance 

OCC offline chemical clean 

OMC online mechanical clean 

ÖNORM Österreichisches Normungsinstitut (Austrian Standards Institute)

OSHA Occupational Safety and Health Administration 

PAC powdered activated carbon 

PEL permissible exposure limit 

% percent 

PER preliminary engineering report 

pfu plaque forming unit 

pH negative logarithm of the effective hydrogen ion concentration 

PHA process hazard analysis 

PLC programmable logic controller 

POE point of entry 

psi pounds per square inch 

psig pounds per square inch gauge 

PVC polyvinyl chloride 

QA/QC quality assurance/quality control 

r radial distance from center 

r

2 correlation coefficient 

RAA running annual average 

RCRA Resource Conservation and Recovery Act 

RED reduction equivalent dose 

RMS root-mean-square 

RNA ribonucleic acid 

rpm revolutions per minute 

RPZ reduced pressure zone 

s second 

SARA Superfund Amendments and Reauthorization Act 

SCADA supervisory control and data acquisition 

SDWA Safe Drinking Water Act 

SMCL secondary maximum contaminant level 

SMP standard monitoring program

Proposal Draft 
List of Acronyms and Abbreviations (Continued)

UV Disinfection Guidance Manual xvi June 2003

SOP standard operating procedure 

SSS system-specific study 

SUVA specific ultraviolet absorbance 

SWTR Surface Water Treatment Rule 

T10 time at which ten percent of water has passed through the reactor 

TCLP toxic characteristic leaching procedure 

TCR total coliform rule 

TDH total dynamic head 

TDS total dissolved solids 

THM trihalo methane 

TLV threshold limit values 

TNTC too numerous to count 

TOC total organic carbon 

TOX total organic halides

TSA tryptic soy agar 

TSB tryptic soy broth 

TSS total suspended solids 

TTHM total trihalomethane 

UPS uninterruptible power supply 

UV ultraviolet 

UVT ultraviolet transmittance

VFD variable frequency drive 

W watt 

WTP water treatment plant 

Proposal Draft 
Requested Feedback on the 

UV Disinfection Guidance Manual 

Chapter or Appendix Title 

Specific Issues for Comment 

Glossary

1. Are there additional terms that should be defined?

2. Is each definition accurate and clearly presented?

1. Introduction 

1. Does this chapter provide the appropriate amount of information on the relevant 

regulations? 

2. Overview Of UV Disinfection 

1. Is the level of detail appropriate? 

2. Is there additional information that should be provided?

3. Planning And Design Aspects For UV Installations 

1. Is the overall UV installation design flowchart realistic? Is the chapter organization 

reader-friendly?

2. Is the issue of off-specification operation and its implications on the UV installation 

design clearly described? 

3. Are the recommendations on developing design criteria helpful? Are there other 

approaches that should be discussed? 

4. Is the power quality information clear? Is more information needed?

5. Are there additional planning or design issues that should be discussed?

4. Overview of Validation 

1. Are the elements of validation clearly presented?

2. Is there other information from the detailed validation protocol (Appendix C) that 

should be described here?

5. Start-Up And Operation Of UV Installations 

1. Are there other elements of the UV installation start-up that should be discussed?

2. Are the organization of the chapter and presentation of information appropriate?

3. Are the operational requirements examples clearly described?

4. Are there other operation and maintenance issues that should be discussed?

5. Are the operational challenges described realistic, and are the solutions helpful? 

6. References 

1. Are there any references that were overlooked that should be added to help clarify 

any points made in the UVDGM?

UV Disinfection Guidance Manual xvii June 2003

Proposal Draft 
Requested Feedback on the UV Disinfection Guidance Manual

UV Disinfection Guidance Manual xviii June 2003

A. Fundamentals of UV Disinfection 

1. Is the level of detail appropriate? 

2. Is there additional information that should be provided?

B. Derivation of UV Dose-Response Requirements 

1. Are there published or unpublished data available that are not included in this 

analysis? 

C. UV Validation Protocol Testing 

1. Is the description of the testing methods clear?

2. Are the distinctions between Tier 1 and 2 clearly described?

3. To provide a better assessment of the RED bias, please provide dose distributions for 

UV reactors you have modeled at UVTs of 95, 90, 85, and 80%. 

4. Are the Tier 1 criteria acceptable? If not, please provide data and rational to support 

alternative criteria. 

5. During validation, the uncertainty of some measurements will not be random. In 

particular, errors associated with measurements made by the radiometer will likely be 

a systematic error (i.e., the radiometer will always read high or read low for the 

duration of the validation testing). Other such errors could occur with the intensity 

sensors or the reference sensor used to calibrate the duty sensors. Currently, the 

UVGM combines these sources of uncertainty with other random sources of 

uncertainty to define an expanded uncertainty. Because these sources of error are not 

random during a given validation, should the following approach be used: 

If the error of a measurement during validation is constant and 

systematic, should the uncertainty of the measurement be used to 

define a bias error that is applied to the validation results? 

Under the current approach, this would apply to the uncertainty of the 

radiometer and move it from the expanded uncertainty to its own bias 

error. For example, if the uncertainty of the radiometer is 8 %, a 

safety factor of 1.08 is added to the RED bias, polychromatic bias, and 

expanded uncertainty. This will increase RED targets for Tier 1 and 2. 

6. The expanded uncertainty is calculated for an 80 percent confidence interval to ensure

at least nine out of ten cases of UV system operation meet target dose values. Should 

the expanded uncertainty calculation be based on a 90 or 95 percent confidence 

interval to ensure a higher percentage of UV systems meet requirements? 

D. Validation Microbial Methods 

1. The bounds provided for the MS2 and B. subtilis data come from an analysis of data 

published in the literature. Should these bounds be used? If not, please provide data to 

support using alternative bounds? Should any of the literature data used to develop 

these bounds not be included? If yes, please provide a rational for not including that 

data. 

2. Are the methods for analyzing the collimated beam data and subsequent UV doseresponse curve clearly stated and appropriate? Are there other options that should be 

considered? 

Proposal Draft 
Requested Feedback on the UV Disinfection Guidance Manual

UV Disinfection Guidance Manual xix June 2003

E. Collimated Beam Apparatus – Measuring Challenge Microbe UV DoseResponse 

1. Is the collimated beam testing description clear? 

F. Validation Background 

1. The following is an alternate approach for monitoring dose delivery that is not 

included in the manual because it has not been applied or referenced. 

Calculate the percent UV output from the lamp to the water based on the sensor 

readings using the formula: 

100 '( ) = ∗

S UVT

S PL

where 

PL = UV output from the lamp to the water (%) 

S = Sensor reading 

S’(UVT) = Sensor reading expected with a new lamp operating with unfouled 

sleeves at a given UVT 

UVT = UVT of the water at 254 nm

The calculated lamp power and measured UVT should be above setpoint values 

established during validation. The relation S’(UVT) is measured during validation as 

opposed to being calculated. 

This approach has the following potential benefits: 

• No requirement on sensor position 

• Could measure S’(UVT) with NOM and compare with LSA or coffee as 

an experimental approach for reducing the Polychromatic Bias to one. 

Should this approach be discussed in the manual? 

G. Issues for Unfiltered Systems 

1. Is the level of detail appropriate? 

2. Is there additional water quality related design or operational concerns for unfiltered 

systems that should be addressed? 

H. Issues for Ground Water Systems 

1. Are there design or operational issues with UV disinfection of groundwater that are 

not addressed?

I. Issues for Small Systems 

1. Is the level of detail appropriate? 

2. Are the design concerns facing small systems adequately addressed?

3. Design information is presented in Chapter 3, and this appendix only includes areas 

where small system design differs from the design issues discussed in Chapter 3. Is 

this approach effective?

Proposal Draft 
Requested Feedback on the UV Disinfection Guidance Manual

UV Disinfection Guidance Manual xx June 2003

J. Pilot-Scale and Demonstration-Scale Testing 

1. Is the level of detail appropriate? 

2. Were all of the recommended testing methods clearly explained?

3. Are there any other example testing protocols that should be included in this 

appendix?

K. Preliminary Engineering Report 

1. Are there any elements of this report that would benefit from more detail? 

2. Is there any information missing from this report that you would like to see included in a 

standard template (i.e., in this example Design Engineering Report)?

L. Regulatory Timeline 

1. Is this appendix helpful for UV installation planning?

2. Are the time allocations for the tasks listed in the timeline appropriate? 

M. Compliance Forms 

1. Are the example compliance forms well organized and easy to complete? 

2. Are there other forms that would be helpful to the utility or the State?

N. UV Lamp Breakage Issues 

1. Considering available information, are the major issues surrounding lamp breakage 

adequately presented in this appendix? Are there additional issues or sources of 

information to be discussed? 

2. Are there additional methods for the prevention or mitigation of on-line lamp breaks 

that should be presented?

O. Case Studies 

There are no questions related to this appendix because it is not included in this draft. 

P. Validation Protocol Calculator Tool 

There are no questions related to this appendix. 

Proposal Draft 
1

 Throughou t this document, the terms “State” or “States” are used to refer to all types of primacy agenc ies,

including U.S. Territories, Indian Tribes, and EP A Regions.

UV Disinfection Guidance Manual 1-1 June 2003

Propo sal Draft

1. Introduction

There is growing interest among public water systems in using ultraviolet (UV) light to

disinfect drinking water, based on its ability to inactivate certain microorganisms without

forming harmful disinfection byproducts (DBPs). Some pathogens, such as Cryptosporidium,

are resistant to commonly used disinfectants, whereas UV light has proven effective against these

microorganisms. 

The United States Environmental Protection Agency (EPA) is developing the Long Term

2 Enhanced Surface Water Treatment Rule (LT2ESWTR) to further control microbial

contamination of drinking water. The rule requires additional treatment for some systems based

on their source water Cryptosporidium concentrations. UV disinfection is one of the options

utilities have to comply with the treatment requirements. 

UV light has been widely used to disinfect effluent from wastewater treatment facilities,

particularly those that reuse effluent for irrigation. Until recently, the use of UV treatment for

drinking water applications was primarily limited to small ground water systems, due to the

belief that it was not effective for inactivating protozoa and was not cost-effective for large

systems. In 1998, however, research demonstrated that UV light could effectively inactivate

Cryptosporidium at low dosages (Buhkari et al. 1998), prompting more research to better

understand its potential for widespread application.

UV disinfection design, operation, and maintenance needs differ from those of traditional

chemical disinfectants used in drinking water applications. EPA is therefore developing this

guidance manual to familiarize States1

 and utilities with these important issues as well as

regulatory requirements. Areas of particular design and operational importance include hydraulic

control, reliability, redundancy, lamp cleaning and replacement, and lamp breakage. Regulatory

requirements are addressed through UV reactor validation, monitoring, and reporting.

1.1 Guidance Manual Objectives

This manual provides guidance to utilities, States, manufacturers, and other interested

parties on the disinfection of drinking water with UV light, including the regulatory requirements

associated with UV disinfection. The LT2ESWTR requirements do not cover all aspects of the

disinfection process. In the areas not directly addressed by the rule, the manual provides
1. Introduction 

UV Disinfection Guidance Manual 1-2 June 2003

Propo sal Draft

recommendations to assist utilities and regulatory agencies in assessing the disinfection

capability and performance of UV installations. The manual’s objectives are as follows:

• Provide public water systems and designers with technical information and guidance

on the selection, design, and operation of UV installations and the UV-related

requirements for compliance with the LT2ESWTR.

• Provide States with guidance and the necessary tools to assess UV installations at the

design, start-up, and routine operation phases.

• Provide manufacturers with testing and performance standards for UV components

and systems for treating drinking water.

1.2 Organization

This manual consists of six chapters and appendices:

• Chapter 1 – Introduction. The remainder of this chapter summarizes the LT2ESWTR

and Stage 2 DBPR and discusses regulatory requirements for disinfection of drinking

water with UV light.

• Chapter 2 – Overview of UV Disinfection. This chapter describes the principles of

disinfection with UV light including inactivation mechanisms, dose-response

relationships, water quality impacts, and UV reactors.

• Chapter 3 – Planning and Design Aspects for UV Installations. This chapter

discusses the key design features for UV disinfection facilities and presents some

common approaches to facility design. Key design features include treatment goals,

existing infrastructure, water quality, hydraulics, and operation and control strategies.

• Chapter 4 – Overview of UV Reactor Validation. This chapter describes the

LT2ESWTR requirements for validating UV reactors and provides an overview of

validation protocol presented in Appendix C.

• Chapter 5 – Start-up and Operation of UV Installations. This chapter discusses

start-up and operation issues of UV disinfection facilities as well as required

monitoring for regulatory compliance.

• Chapter 6 – References. This chapter lists the full references from Chapters 1-5.
1. Introduction 

UV Disinfection Guidance Manual 1-3 June 2003

Propo sal Draft

• The appendices and their titles follow:

Appendix A. Fundamentals of UV Disinfection

Appendix B. Derivation of UV Dose-Response Requirements

Appendix C. Validation of UV Reactors

Appendix D. Microbiological Methods

Appendix E. Collimated Beam Apparatus: Measuring Challenge Microorganism

UV Dose-Response

Appendix F. Background to the UV Reactor Validation Protocol

Appendix G. Issues for Unfiltered Systems

Appendix H. Issues for Ground Water Systems

Appendix I. Issues for Small Systems 

Appendix J. Pilot-Scale and Demonstration Scale Testing

Appendix K. Preliminary Engineering Report

Appendix L. Regulatory Time Line

Appendix M. Compliance Forms

Appendix N. UV Lamp Breakage Issues

Appendix O. Case Studies [This appendix will be included in the final draft at

which time EPA anticipates more information being available.]

Appendix P. Validation Protocol Calculator Tool

1.3 Regulations Summary

This section summarizes the drinking water regulations for microbial and DBP control. 

The Stage 2 Disinfectants and Disinfection Byproduct Rule (DBPR) aims to reduce peak DBP

concentrations in the distribution system by modifying the Stage 1 DBPR monitoring

requirements and procedures for compliance determination. The LT2ESWTR and Stage 2 DBPR

are to be promulgated together to address the risk-risk trade off between microbial disinfection

and the byproducts formed by commonly used disinfectants. Consequently, when a utility

assesses its disinfection strategy, not only the disinfection of target pathogens is important, but

also the DBP formation from each disinfectant. Table 1.1 summarizes the microbial treatment

requirements and DBP maximum contaminant levels (MCLs) from the Surface Water Treatment

Rule (SWTR), Interim Enhanced Surface Water Treatment Rule (IESWTR), Long Term 1

Enhanced Surface Water Treatment Rule (LT1ESWTR), LT2ESWTR, Stage 1 DBPR, and Stage

2 DBPR. 
1. Introduction 

UV Disinfection Guidance Manual 1-4 June 2003

Propo sal Draft

Table 1.1 Summary of Microbial and Disinfection Byproduct Rules

Surface W ater Treatmen t Rules - Minim um Trea tment Req uirements

Regulation Giardia Virus Cryptosporidium

SWTR 3 log removal and

inactivation

4 log removal and

inactivation Not addressed

IESWTR and

LT1ESWTR No change from SWTR 2 log removal

LT2ESWTR No change from SWTR

0-2.5 log additional

treatment1

2-3 log treatment2

Disinfection Byproduct Rules - MCLs Based on Run ning Annual Averages (RAAs)

Regulation

Trihalomethanes

(TTHM) (μg/L)

Haloacetic Acids

(HAA5) (μg/L)

Bromate

(μg/L)

Chlorite

(μg/L)

Stage 1 DBPR 80 as RAA 60 as RAA 10 1000

Stage 2A DBPR3

120 as LRAA 100 as LRAA No change from Stage 1

Stage 2B DBPR480 as LRAA 60 as LRAA No change from Stage 1

1Requirement for filtered systems is in addition to removal achieved by conventional treatment complying with the IESWTR and LT1ESWTR. Specific requirements for each plant depend on source water monitoring results (40

CFR 141.720).

2Unfiltered systems must provide 2-3 log inactivation; specific requirements for each plant depend on source water monitoring results (40 CFR 141.721(b)).

3Stage 2A bases compliance on a locational running annual average (LRAA) at the Stage 1 monitoring locations. Stage 1 RAAs must still be met during this time. Stage 2A begins [3 years after rule promulgation] for all

systems.

4Stage 2B bases compliance on an LRAAs at revised monitoring locations identified during the Initial Distribution System Evaluation. Stage 2B begins [6 years after rule promulgation] for large systems and [7.5-8.5 years after

rule promulgation] for small systems dependent on their LT2ESWTR requirements.

1.3.1 Long Term 2 Enhanced Surface Water Treatment Rule

The LT2ESWTR applies to all public water systems that use surface water or ground

water under the direct influence of surface water (GWUDI), except those that purchase all their

surface and GWUDI water. It builds on the SWTR, IESWTR, and the LT1ESWTR by

improving control of microbial pathogens, specifically the contaminant Cryptosporidium. Unlike

the previous rules, the LT2ESWTR bases treatment requirements on a system's source water

Cryptosporidium concentration and type of treatment provided. This section describes the rule

requirements for filtered and unfiltered systems.
1. Introduction 

UV Disinfection Guidance Manual 1-5 June 2003

Propo sal Draft

1.3.1.1 Filtered Systems

The LT2ESWTR requires systems that use a surface water or GWUDI source (referred to

collectively in this manual as surface water systems) to conduct source water monitoring to

determine average Cryptosporidium concentrations, unless they have historical Cryptosporidium

data equivalent to what is required under the LT2ESWTR (40 CFR 141.701(a)). Based on its

average source water Cryptosporidium concentration, filtered systems will be classified in one of

four possible bins. A system's bin assignment determines the extent of any additional

Cryptosporidium treatment requirements. The rule requires systems to comply with additional

treatment requirements by using one or more management or treatment techniques from a

toolbox of options (40 CFR 141.720(b)). The process is described in more detail below; the full

monitoring requirements are described in the Source Water Monitoring Guidance Manual for

Public Water Systems for the Long Term 2 Enhanced Surface Water Treatment Rule (USEPA

2003).

Bin Classification

Table 1.2 presents the bin classifications and their corresponding additional treatment

requirements for all filtered systems (40 CFR 141.709 and 40 CFR 141.720). Systems with

average Cryptosporidium concentrations of less than 0.075 oocysts per liter are placed in Bin 1,

for which no additional treatment is required. For concentrations of 0.075 or more, additional

treatment is required on top of that required by existing rules. The additional treatment required

for each bin, specified in terms of log removal, depends on the type of treatment already in place

by the system.
1. Introduction 

UV Disinfection Guidance Manual 1-6 June 2003

Propo sal Draft

Table 1.2 Bin Requirements for Filtered Systems1

If your 

Cryptosporidium

concentration

(oocy sts/L) is...

Your bin

classification

is...

And if you use the follo wing filtration tre atment in full

comp liance w ith existing regulations , then you r additional

treatm ent req uirem ents ar e...

Conventional

Filtration

Treatment

(includes

softening)

Direct

Filtration

Slow Sand or

Diatomaceous

Earth

Filtration

Alternative

Filtration

Technologies

< 0.075 1 No additional

treatment

No

additional

treatment

No additional

treatment

No additional

treatment

> 0.075 an d < 1.0 2 1 log

treatment2

1.5 log

treatment2

1 log

treatment2

As determined

by the State 2,4

> 1.0 and < 3.0 3 2 log

treatment3

2.5 log

treatment3

2 log

treatment3

As determined

by the State 3,5

> 3.0 4 2.5 log

treatment3

3 log

treatment3

2.5 log

treatment3

As determined

by the State 3,6

1

 (40 CFR 141.709 and 40 CFR 141.720)

2

 Systems may use any technology or combination of technologies from the microbial toolbox.

3

 Systems must achieve at least 1 log of the required treatment using ozone, chlorine dioxide, UV disinfection, membranes, bag/cartridge filters, or bank filtration.

4

 Total Cryptosporidium treatment must be at least 4.0 log.

5

 Total Cryptosporidium treatment must be at least 5.0 log.

6

 Total Cryptosporidium treatment must be at least 5.5 log.

1.3.1.2 Unfiltered Systems

All existing requirements for unfiltered systems under the SWTR (40 CFR 141.71 and

141.72(a)) remain in effect. This includes disinfection to achieve at least 3 log inactivation of

Giardia and 4 log inactivation of viruses and to maintain a disinfectant residual in the

distribution system (e.g., free chlorine or chloramines). The IESWTR and LT1ESWTR did not

change the disinfection requirements for unfiltered systems. The LT2ESWTR requires 2 log or 3

log inactivation of Cryptosporidium, depending on the source water concentration of

Cryptosporidium (40 CFR 141.721(b)).

The arithmetic mean concentration of all Cryptosporidium samples taken is used to

determine the amount of treatment required, as shown in Table 1.3 (40 CFR 141.721(a)). If the

mean concentration is less than or equal to 0.01 oocysts/L, the system must provide 2 log

inactivation of Cryptosporidium (40 CFR 141.721(b)). If the mean concentration of

Cryptosporidium exceeds 0.01 oocysts/L, the system must provide at least 3 log inactivation of

Cryptosporidium (40 CFR 141.721(b)).
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Table 1.3 Bin Requirements for Unfiltered Systems

Bin

Number

Average Cryptosporidium Concentration

(oocysts/liter)

Additional Cryptosporidium inactivation

requirements

1 < 0.01 2 log1

2 > 0.01 3 log1

1

 Overall disinfection requirements must be met with a minimum of two disinfectants (40 CFR 141.721(d)).

1.3.1.3 UV Disinfection Requirements for Filtered and Unfiltered Systems

To receive disinfection credit for a UV reactor, the LT2ESWTR requires utilities to

demonstrate through validation testing that the reactor can deliver the required UV dose (40 CFR

141, Subpart W, Appendix D). EPA developed dose requirements for Cryptosporidium,

Giardia, and virus as presented in Table 1.4 and described in Appendix B of this guidance

manual. These dose requirements account for uncertainty associated with the dose-response of

the microorganisms in controlled experimental conditions. In practical application, other sources

of uncertainty are introduced due to the hydraulic effects of the UV installation, UV reactor

equipment, and monitoring approach (e.g., UV intensity sensors). Therefore, the validation

protocol (described in Chapter 4 and Appendix C of this guidance manual) applies a safety factor

to the Table 1.4 dose requirements to account for these areas of uncertainty and variability.

Table 1.4 UV Dose Requirements Used During Validation Testing1

Log Inactivation

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Cryptosporidium 1.6 2.5 3.9 5.8 8.5 12 - -

Giardia 1.5 2.1 3.0 5.2 7.7 11 - -

Virus 39 58 79 100 121 143 163 186

1

 40 CFR 141.729(d)

The LT2ESWTR (40 CFR 141, Subpart W, Appendix D) specifies the following with

respect to reactor validation:

C Validation testing must determine a range of operating conditions that can be

monitored by the system and under which the reactor delivers the required UV dose. 

C Operating conditions must include flowrate, UV intensity, and lamp status, at a

minimum. 
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C Validated conditions determined by testing must account for UV absorbance of the

water, lamp fouling and aging, measurement uncertainty of on-line UV intensity

sensors, UV dose distributions arising from the velocity profiles through the reactor,

failure of UV lamps or other critical installation components, and inlet and outlet

piping or channel configurations of the UV reactor. 

Using the above requirements as a basis, Appendix C provides guidance for several

possible approaches to reactor validation. States may approve modifications to these approaches

or alternative approaches at their discretion.

Monitoring Requirements (40 CFR 141.729(d))

The LT2ESWTR requires utilities to monitor their reactors to demonstrate that they are

operating within the range of conditions that were validated for the required UV dose. At a

minimum, utilities must monitor each reactor for flowrate, lamp outage, UV intensity as

measured by a UV intensity sensor, and any other parameters required by the State. UV

absorbance should also be measured where it used in a dose control strategy. Systems must

check the calibration of UV intensity sensors and must recalibrate sensors in accordance with a

protocol approved by the State. The LT2ESWTR does not specify monitoring frequency (section

5.4 of this guidance describes the monitoring requirements with recommended frequencies). 

Reporting Requirements (40 CFR 141.730)

The LT2ESWTR requires utilities to report the following items:

• Initial reporting - Validation test results demonstrating operating conditions that

achieve the UV dose required for the inactivation credit desired for compliance with

the LT2ESWTR.

• Routine reporting - Volume of water entering the distribution system that was not

treated by the UV reactors operating under validated conditions on a monthly basis. 

For the purposes of this guidance manual, when a UV reactor is operating outside of its

validated limits, it is considered “off-specification.” 

Additional Requirement for Unfiltered Systems (40 CFR 141.721(c)(2))

For unfiltered systems using UV disinfection to meet the LT2ESWTR requirements, the

required Cryptosporidium log inactivation by UV disinfection must be achieved in at least 95

percent of the water delivered to the public during each calendar month.
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1.3.2 Stage 2 DBPR

The requirements of the Stage 2 DBPR will apply to all community water systems

(CWSs) and nontransient noncommunity water systems (NTNCWSs)—both ground and surface

water systems—that add a disinfectant other than UV light, or that deliver water that has been

treated with a disinfectant other than UV light.

Initial Distribution System Evaluations

The Stage 2 DBPR is designed to reduce DBP occurrence peaks in the distribution system

by changing compliance monitoring requirements. Compliance monitoring will be preceded by

an initial distribution system evaluation (IDSE) to identify compliance monitoring locations that

represent high TTHM and HAA5 levels. The IDSE consists of either a standard monitoring

program (SMP) or a system-specific study (SSS). NTNCWSs serving fewer than 10,000 people

are not required to perform an IDSE, and other systems may receive waivers from the IDSE

requirement.

Compliance Determination and Schedule

The Stage 2 DBPR changes the way sampling results are averaged to determine

compliance. The determination for the Stage 2 DBPR is based on a LRAA (i.e., compliance

must be met at each monitoring location) instead of the system-wide RAA used under the Stage

1 DBPR.

The Stage 2 DBPR will be implemented in two phases, Stage 2A and Stage 2B. Under

Stage 2A, all systems must comply with TTHM/HAA5 MCLs of 120/100 μg/L measured as

LRAAs at each Stage 1 DBPR monitoring site, while continuing to comply with the Stage 1

DBPR MCLs of 80/60 μg/L measured as RAAs. Under Stage 2B, systems must comply with

TTHM/HAA5 MCLs of 80/60 μg/L at locations identified under the IDSE.

Significant Excursion Evaluations

Because Stage 2 DBPR MCL compliance is based on an annual average of DBP

measurements, a system could from time to time have DBP levels significantly higher than the

MCL (referred to as a significant excursion) while still being in compliance. This is because the

high concentration could be averaged with lower concentrations at a given location. If a

significant excursion occurs, a system must conduct a significant excursion evaluation and

discuss the evaluation with the State no later than the next sanitary survey.

1.4 Alternative Approaches for Disinfecting with UV Light

This manual provides technical information about using UV disinfection for drinking

water treatment. Although it covers many aspects of implementing a UV installation, from

design and validation to operation, it is not comprehensive in terms of all types of UV

installations, design alternatives, and validation protocols that may provide satisfactory
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performance. For example, pulsed UV and eximer lamps are two types of UV technologies not

included in this manual, but they may provide effective disinfection. Currently, a significant

level of research is being conducted surrounding UV disinfection and its applications in various

industries. As more information becomes available, other UV equipment or methods of

operation, design, and validation will evolve. States may recognize alternatives in UV installation

design, operation, and validation that are not described in this manual.
2. Overview of UV Disinfection 

Chapter 2 provides an overview of UV disinfection. The material ranges from an 

explanation of the process in terms of basic chemical and physical principles to a description of 

the components of a UV installation and performance monitoring. Appendix A, Fundamentals 

of UV Disinfection, serves as a companion to this chapter by providing more detailed 

information on each of the topics discussed. The corresponding appendix sections are noted 

throughout the text. The organization of this chapter is presented below, including the key 

question each section addresses. 

• What are the fundamental characteristics of UV light, and what 

happens to UV light as it propagates through water? .................................. Section 2.2 

• How does UV light inactivate microorganisms? .......................................Section 2.3.1 

• Can microorganisms undergo repair and become infectious 

after inactivation by UV light? ..................................................................Section 2.3.2 

• How are UV dose and microbial response 

determined? .............................................................................. Sections 2.3.3 and 2.3.4 

• How does UV dose vary in a UV reactor? .................................................Section 2.3.3 

• What affects a microorganism's response to 

UV light? .................................................................................. Sections 2.3.4 and 2.3.5 

• What do UV reactors look like and how do the key 

components function? .................................................................................. Section 2.4 

• What are the differences between low pressure and medium

pressure lamps?...........................................................................................Section 2.4.2 

• How do the utility and the State know the UV reactor is 

delivering the required UV dose? ...............................................................Section 2.4.9 

• How does water quality affect UV reactor performance? ..........................Section 2.5.1 

• Do any disinfection byproducts form as a result of UV 

disinfection?................................................................................................Section 2.5.2 

2.1 History of UV Light for Drinking Water Disinfection 

UV disinfection is an established technology supported by decades of fundamental and 

applied research and practice in North America and Europe. Downes and Blunt (1887) 

discovered the germicidal properties of sunlight. The development of mercury lamps as artificial 

UV light sources in 1901 and the use of quartz as a UV transmitting material in 1906 was soon 

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followed by the first drinking water disinfection application in Marseilles, France in 1910. In 

1929, Gates identified a link between UV disinfection and absorption of UV light by nucleic 

acid. The development of the fluorescent lamp in the 1930s led to the production of germicidal 

tubular lamps. Considerable research on the mechanisms of UV disinfection and the inactivation 

of microorganisms occurred during the 1950s (Dulbecco 1950; Kelner 1950; Powell 1959; 

Brandt and Giese 1956). 

While there was substantial research on UV disinfection during the first half of the 20th

century, the low cost of chlorine and operational problems with early UV disinfection systems 

limited the growth of UV disinfection as a drinking water treatment technology. The first 

reliable applications of UV light for disinfecting municipal drinking water occurred in 

Switzerland and Austria in 1955 (Kruithof and van der Leer 1990). By 1985, the number of 

installations in these countries had risen to approximately 500 and 600, respectively. With the 

discovery of chlorinated disinfection byproducts (DBPs), UV disinfection became popular in 

Norway and the Netherlands with the first installations occurring in 1975 and 1980, respectively. 

As of 1996, there were over 2000 UV disinfection systems treating drinking water in 

Europe (USEPA 1996), primarily treating flows less than 1 million gallons per day (MGD). A 

survey conducted in 2000 found that UV disinfection is currently being used to treat larger flows, 

including two installations treating a combined flow of 76 MGD in Helsinki, Finland (Toivanen 

2000), and that the number of installations is increasing (USEPA 2000). Several large 

installations across the United States are currently under design. Because of the susceptibility of

Cryptosporidium to UV disinfection and the emphasis in recent regulations on controlling 

Cryptosporidium, the number of utilities using UV disinfection is expected to increase

significantly over the next decade. 

2.2 Fundamental Aspects of UV Light 

The use of UV light to disinfect drinking water involves (1) the generation of UV light 

with the desired germicidal properties and (2) the delivery (or transmission) of that light to 

pathogens. This section provides a basic description of how UV light is generated and the 

environmental conditions that affect its delivery to pathogens. 

2.2.1 Nature of UV Light 

UV light is the region of the electromagnetic spectrum that lies between x-rays and 

visible light (Figure 2.1). The UV spectrum is divided into four regions as shown in Figure 2.1: 

vacuum UV (100 to 200 nm), UV-C (200 to 280 nm), UV-B (280 to 315 nm), and UV-A (315 to 

400 nm) (Meulemans 1986). UV disinfection occurs due to the germicidal action of UV-B and 

UV-C with microorganisms. The germicidal action of UV-A is small relative to UV-B and 

UV-C and therefore needs very long exposure times to be effective as a disinfectant. Light in the 

vacuum UV range is very effective in disinfecting microorganisms (Munakata et al. 1991). 

However, it is impractical for water disinfection applications because it rapidly attenuates over 

very short distances in water. For the purposes of this manual, the practical germicidal

wavelength for UV light ranges between 200 and 300 nm. 

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Figure 2.1 UV Light in the Electromagnetic Spectrum 

254 nm

100 nm 400 nm

100 nm 200 nm 300 nm 400 nm

280 nm 315 nm

X-ray

Gamma

Rays

UV Visible Infrared

Vacuum UV UV-C UV-B UV-A

Typically, UV light is generated by applying a voltage across a gas mixture, resulting in a 

discharge of photons. The specific wavelengths of light emitted from photon discharge depend 

on the elemental composition of the gas and the power level of the lamp (section A.1.1). Nearly 

all UV lamps designed for water treatment use a gas mixture containing mercury vapor. 

Mercury is an advantageous gas for UV disinfection applications because it emits light in the 

germicidal wavelength range, as discussed in section 2.3.5. The light output depends on the 

concentration of mercury atoms, which is directly related to the mercury vapor pressure. 

Mercury at low vapor pressure (near vacuum; 0.001 to 0.01 torr, 2 x 10-5 to 2 x 10-3 psi) and 

moderate temperature (40 oC) produces essentially monochromatic UV light at 253.7 nm. At 

higher vapor pressures (100 to 10,000 torr, 2 to 200 psi) and higher operating temperatures (600 

to 900 oC), the frequency of collisions between mercury atoms increases, producing UV light 

over a broad spectrum (polychromatic) with an overall higher intensity. Mercury vapor pressure 

between 0.01 and 100 torr does not efficiently produce UV light. 

2.2.2 Propagation of UV Light 

As UV light propagates from its source, it interacts with the materials it encounters 

through absorption, reflection, refraction, and scattering. In disinfection applications, these 

phenomena result from interactions between the emitted UV light and UV reactor components 

(i.e., lamp envelopes, lamp sleeves, and reactor walls) and also the water being treated. When 

assessing water quality, UV absorbance or UV transmittance is the parameter that incorporates 

the impact of absorption and scattering. This section briefly describes both the phenomena that 

influence light propagation and measurement techniques to quantify UV light propagation. More 

detailed information is provided in sections A.1.2.1 through A.1.2.5. 

Absorption is the transformation of light to other forms of energy as it passes through a 

substance. UV absorption of a substance will vary with the wavelength of the light. The 

components of the reactor and the water passing through the reactor all absorb UV light to 

varying degrees, depending on their material composition. When UV light is absorbed, it is no 

longer available to disinfect microorganisms. 

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Unlike absorption, the phenomena of refraction, reflection, and scattering change the 

direction of UV light, but the UV light is still available to disinfect microorganisms. 

Refraction (Figure 2.2) is the change in the direction of light propagation as it passes 

from one medium to another. In UV reactors, refraction occurs when light passes from the UV 

lamp through an air gap, through the lamp sleeve, and through the water. These changes alter the 

angle that UV light strikes target pathogens. 

Figure 2.2 Refraction of Light 

Incident Light

from UV Lamp

Air

Gap

Quartz

Sleeve

Water

Refracted Light

θA θQ

θW

θA > θW > θQ Incident Light

from UV Lamp

Air

Gap

Quartz

Sleeve

Water

Refracted Light

θA θQ

θW

θA > θW > θQ

Reflection is the change in direction of light propagation when it is deflected by a surface 

(Figure 2.3). Reflection may be classified as specular or diffuse. Specular reflection occurs 

from smooth polished surfaces and follows the Law of Reflection (the angle of incidence is equal 

to the angle of reflection). Diffuse reflection occurs from rough surfaces and scatters light in all 

directions with little dependence on the incident angle. In UV reactors, reflection will take place 

at interfaces that do not transmit UV light (e.g., the reactor wall) and also at UV transmitting 

interfaces (e.g., the inside of a lamp sleeve). The type of reflection observed and intensity of 

light reflected from a surface depends on the material of the surface. 

Figure 2.3 Reflection of Light 

Specular Reflection

Incident Light Reflected Light

Diffuse Reflection

Incident Light

θ1 θ1

Reflected Light

Specular Reflection

Incident Light Reflected Light

Diffuse Reflection

Incident Light

θ1 θ1

Reflected Light

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Scattering of light is the change in direction of light propagation caused by interaction 

with a particle (Figure 2.4). Particles can cause scattering in all directions, including towards the 

incident light source (back-scattering). Scattering of light caused by particles smaller than the 

wavelength of the light is called Rayleigh scattering (section A.1.2.4). Particles larger than the 

wavelength of light scatter more light in the forward direction but also cause some

backscattering. Rayleigh scattering depends inversely on wavelength to the fourth power (1/λ4

) 

and thus is more prominent at shorter wavelengths. Scattering by particles larger that the 

wavelength of the light is relatively independent of wavelength. 

Figure 2.4 Scattering of Light 

Incident

Light

Target

Pathogens

Forward

Scattered

Light

90° Scattered Light

Back

Scattered

Light

Incident

Light

Target

Pathogens

Forward

Scattered

Light

90° Scattered Light

Back

Scattered

Light

UV absorbance (A254) is a commonly used water quality parameter that characterizes the 

decrease in the amount of incident light as it passes through a water sample over a specified 

distance or pathlength. Various procedures call for filtering the sample through a 0.45 μm 

membrane before measuring the absorbance. If the measurement is made according to a 

modified version of Standard Method 5910B (APHA et al. 1998), the water sample is not pH 

adjusted or filtered. Since most particles in drinking water are strong absorbers of UV light, it is 

recommended that absorbance measurements be made without filtering the sample. Therefore, 

the modified measurement accounts for scattering and some absorption from particles in the 

water sample that may interfere with UV disinfection. Although Standard Methods identifies 

this measurement as UV absorption, this manual will refer to it as absorbance since the latter 

term is widely used in the water treatment industry. 

The term UV transmittance (UVT) has also been used extensively in the literature when 

describing the behavior of UV light. UVT is the percentage of light passing through a water 

sample over a specified distance and is related to UV absorbance by Equation 2.1: 

254 % 100 10 A UVT − = ∗ Equation 2.1

where 

UVT = UV transmittance at specified wavelength (e.g., 254 nm) and pathlength (e.g., 1 

cm) 

A254 = UV absorbance at specified wavelength, based on 1 cm pathlength (unitless; UV 

absorption as measured by Standard Method 5910B) 

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2.3 Microbial Response to UV Light 

The mechanism of disinfection by UV light differs considerably from chemical 

disinfectants such as chlorine and ozone. Chemical disinfectants inactivate microorganisms by 

destroying or damaging cellular structures, interfering with metabolism, and hindering 

biosynthesis and growth (Snowball and Hornsey 1988). UV light inactivates microorganisms by 

damaging their nucleic acid, thereby preventing the microorganism from replicating. A 

microorganism that cannot replicate cannot infect a host. 

When studying UV disinfection effectiveness, it is important to use microbial assays that 

measure infectivity, not viability. Until recently, viability assays such as excystation and vital 

dyes were used to determine inactivation. However, these assays do not evaluate changes in the 

ability of a microorganism to reproduce and infest a host. The importance of using assays that 

measure inactivation is highlighted by the history of UV disinfection for Cryptosporidium. It 

was believed that UV disinfection was not effective for Cryptosporidium inactivation because 

results of early Cryptosporidium inactivation studies were based on viability assays. The ability 

of UV light to inactivate Cryptosporidium at low doses was revealed when infectivity was 

assessed by inoculating mice with UV treated water, which showed greater than 4-log 

inactivation of Cryptosporidium at doses less than 20 mJ/cm2

 (Bukhari et al. 1999). 

This section discusses the damage that causes microbial inactivation, the ability of 

microorganisms to repair the damage, methods for determining microbial inactivation, and how 

wavelength of UV light affects inactivation. 

2.3.1 Mechanisms of Microbial Inactivation by UV Light 

UV light inactivates microorganisms by damaging deoxyribonucleic acid (DNA) or 

ribonucleic acid (RNA), thereby interfering with replication of the microorganism (section 

A.2.2). In normal DNA replication, the double helix strand separates allowing the single strands 

to serve as a template for reconstructing the opposite strand of nucleotides: adenine bonds to 

thymine and guanine bonds to cytosine (Figure 2.5). 

Figure 2.5 Structure of DNA and Nucleotide Sequences Within DNA 

DNA STRUCTURE

SugarPhosphate

Backbone

DNA SEQUENCE

A = Adenine

C = Cytosine

T = Thymine

G = Guanine

— A — T —G — C — G —A — T — C —

— T — A— C — G — C — T — A — G —

Hydrogen Bonded

Nitrogenous

Base Pairs (A, T, G, C)

Purines Pyrimidines

| | | | | | | |

DNA STRUCTURE

SugarPhosphate

Backbone

DNA SEQUENCE

A = Adenine

C = Cytosine

T = Thymine

G = Guanine

— A — T —G — C — G —A — T — C —

— T — A— C — G — C — T — A — G —

Hydrogen Bonded

Nitrogenous

Base Pairs (A, T, G, C)

Purines Pyrimidines

| | | | | | | |

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Light that is absorbed by a system can induce a chemical reaction. As shown in 

Figure 2.6, each of the nucleotides absorbs UV light from 200 to 300 nm (section A.2.2). The 

UV absorption of DNA results from the combination of nucleotides and has a peak near 260 nm

and a local minimum near 230 nm. DNA absorbs light in the wavelength range emitted by UV 

lamps, enabling photobiological effects that lead to nucleic acid damage. 

Figure 2.6 UV Absorbance of Nucleotides (left) and Nucleic Acid (right) at pH 7 

(adapted from Jagger 1967) 

200 260 300

Wavelength (nm)

U

V absorbance

(Relative Scale)

Adenine

Guanine

Thymine

Cytosine

220 240 280

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

DNA

200 260 300

Wavelength (nm)

220 240 280

U

V absorbance

(Relative Scale)

200 260 300

Wavelength (nm)

U

V absorbance

(Relative Scale)

Adenine

Guanine

Thymine

Cytosine

220 240 280

0.0

0.2

0.4

0.6

0.8

1.0

200 260 300

Wavelength (nm)

U

V absorbance

(Relative Scale)

Adenine

Guanine

Thymine

Cytosine

220 240 280

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

DNA

200 260 300

Wavelength (nm)

220 240 280

U

V absorbance

(Relative Scale)

0.0

0.2

0.4

0.6

0.8

1.0

DNA

200 260 300

Wavelength (nm)

220 240 280

U

V absorbance

(Relative Scale)

Damage to nucleic acid does not prevent the cell from undergoing metabolism and other 

cell functions. Although the microbial cell is alive after exposure to UV light, it cannot 

reproduce, and therefore it is incapable of infecting a host. To kill the microbial cell, the UV 

dose would need to be increased by orders of magnitude as compared to the UV dose needed to 

prevent replication. 

Variations in DNA content cause microorganisms to absorb UV light differently, thereby 

contributing to the differences in microorganism susceptibility to UV disinfection. There can be 

significant disparity in the susceptibility of different strains of bacteria and viruses to UV 

disinfection (section A.2.7). Among the pathogens of interest in drinking water, viruses are most 

resistant to UV disinfection followed by bacteria and Cryptosporidium oocysts and Giardia

cysts. Appendix B provides statistical evaluations for dose-response data of Giardia cysts, 

Cryptosporidium oocysts, and viruses, and Chapter 1 contains the regulatory requirements for 

inactivating these pathogens. 

2.3.2 Microbial Repair 

Because microorganisms that have been exposed to UV light still retain metabolic 

functions, some are able to repair the damage done by UV light to a limited degree as described 

in section A.2.3. In some cases, the microorganism regains infectivity. These microorganisms 

have evolved enzyme-mediated mechanisms for reversing UV damage. Repair of UV lightinduced DNA damage includes photoreactivation and dark repair (Knudson 1985). In 

photoreactivation (or photorepair), enzymes energized by exposure to light between 310 and 490 

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nm (near and in the visible range) repair damaged sections of DNA. Photoreactivation needs the 

presence of reactivating light. Dark repair is defined as when a repair process does not need 

reactivating light. The term is somewhat misleading because dark repair can occur in the 

presence of light, and therefore does not need dark conditions. Excision repair, a form of dark 

repair, is an enzyme-mediated process where the damaged section of DNA is removed and 

regenerated using the existing complimentary strand of DNA. 

Knudson (1985) found that bacteria are able to repair in light and dark conditions, 

suggesting that bacteria may have the enzymes necessary for photorepair and dark repair. Viral 

DNA lacks the necessary enzymes for repair, but can repair using the enzymes of a host cell 

(Rauth 1965). Linden et al. (2002a) did not observe photoreactivation or dark repair of Giardia

at UV doses typical for UV disinfection applications (16 and 40 mJ/cm2

). However, unpublished 

data from the same study show Giardia reactivation in light and dark conditions at very low UV 

doses (0.5 mJ/cm2

; Linden 2002). Shin et al. (2001) reported Cryptosporidium does not regain 

infectivity after inactivation by UV light. One study has shown that Cryptosporidium contains 

the capability to undergo some DNA repair (Oguma et al. 2001). However, even though the 

DNA is repaired, infectivity is not restored. 

Knudson (1985) demonstrated that photorepair can be overcome by increasing the 

damage to the DNA through higher UV doses. However, it is unknown if higher UV doses can 

reduce dark repair because it is more difficult to study experimentally. Research is continuing to 

evaluate this phenomenon. At the doses typically used in UV disinfection, microbial repair can 

be controlled and accounted for as discussed in section 3.1.1. 

2.3.3 UV Dose and Dose Distribution 

UV dose is a measurement of the energy per unit area that is incident on a surface. UV 

dose is the product of the average intensity acting on a microorganism from all directions and the 

exposure time. Units commonly used for UV dose are J/m2

, mJ/cm2, and mWs/cm2

(10 J/m2

 = 1 mJ/cm2 = 1 mWs/cm

2

) with mJ/cm2 being the most common units in North America 

and J/m2

 being the most common in Europe. 

In a batch system such as a bench scale collimated beam test (described in Appendix E), 

the average intensity is determined mathematically. For collimated beam tests using a lowpressure lamp, the UV intensity measured by a radiometer, the UV absorbance of the water, the 

thickness of the water layer, the distribution of light across the water surface, and the reflection 

and refraction of light from the water surface all are considered in calculating the average 

intensity. The UV dose can be determined in a batch system by multiplying the calculated 

average intensity by the specific exposure time. 

When using polychromatic light sources (e.g., medium-pressure lamps), UV dose 

calculations in batch, bench scale experiments also incorporate the same parameters as a lowpressure lamp collimated beam test. In addition, the intensity at each wavelength in the 

germicidal range and the germicidal effectiveness at the associated UV wavelengths are also 

considered because microorganisms absorb different amounts of UV light at different 

wavelengths. The UV dose-response measured with polychromatic lamps will match the UV 

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dose-response of monochromatic lamps when the UV dose delivered by the polychromatic 

source is properly calculated (Cabaj et al. 2001; section A.2.4.1). 

Dose delivery in a continuous-flow UV reactor is subject to hydrodynamic irregularities 

and a variable UV intensity distribution and is a function of the UV absorbance of the water, the 

flowrate through the reactor, the UV output from the lamps, and the hydraulic characteristics 

within the reactor. As such, it is difficult to calculate directly UV dose within a UV reactor. If

the reactor has plug flow with complete mixing perpendicular to that flow, all microorganisms 

leaving the reactor receive the same dose, and the reactor would be termed an “ideal” reactor. 

However, these ideal conditions do not generally do not exist in continuous-flow UV reactors. 

As such, microorganisms passing through a UV reactor are exposed to different doses. The 

difference in UV doses experienced by microorganisms in a flowing reactor is best characterized 

by a dose distribution. 

A dose distribution is the probability distribution of UV doses that microorganisms 

receive in a flow-through UV reactor; typically shown as a histogram (Figure 2.7). Some

microorganisms travel close to the UV lamps and experience a higher dose while others that 

travel close to the reactor walls may experience a lower dose. Some microorganisms move 

through the reactor quickly while others travel a more circuitous path. A narrow dose 

distribution (Figure 2.7a) indicates more ideal hydrodynamic conditions. A wider distribution 

(Figure 2.7b) indicates less efficient reactor performance and results in a greater degree of 

“overdosing” to ensure that the minimum desired dose is achieved for the microorganisms at the 

lower end of the dose distribution. 

Figure 2.7 Hypothetical Dose Distributions for Two 

Reactors with Differing Hydraulics 

0.1

0.2

0.3

0.4

0.5

0.6

UV Dose (mJ/cm2)

O

c

c

urre

n

ce Probabilit

y

a. Narrow Dose

Distribution

(Better

Hydraulic

Conditions)

a. Narrow Dose

Distribution

(Better

Hydraulic

Conditions)

0 15 30 45 75 90

0.0 O

c

c

urre

n

ce Proba

bilit

y

b. Wide Dose

Distribution

(Worse

Hydraulic

Conditions)

b. Wide Dose

Distribution

(Worse

Hydraulic

Conditions)

0.1

0.2

0.3

0.4

0.5

0.6

0.0

UV Dose (mJ/cm2)

0 15 30 45 75 90

0.1

0.2

0.3

0.4

0.5

0.6

UV Dose (mJ/cm2)

O

c

c

urre

n

ce Probabilit

y

a. Narrow Dose

Distribution

(Better

Hydraulic

Conditions)

a. Narrow Dose

Distribution

(Better

Hydraulic

Conditions)

0 15 30 45 75 90

0.0

0.1

0.2

0.3

0.4

0.5

0.6

UV Dose (mJ/cm2)

O

c

c

urre

n

ce Probabilit

y

a. Narrow Dose

Distribution

(Better

Hydraulic

Conditions)

a. Narrow Dose

Distribution

(Better

Hydraulic

Conditions)

0 15 30 45 75 90

0.0 O

c

c

urre

n

ce Proba

bilit

y

b. Wide Dose

Distribution

(Worse

Hydraulic

Conditions)

b. Wide Dose

Distribution

(Worse

Hydraulic

Conditions)

0.1

0.2

0.3

0.4

0.5

0.6

0.0

UV Dose (mJ/cm2)

0 15 30 45 75 90

O

c

c

urre

n

ce Proba

bilit

y

b. Wide Dose

Distribution

(Worse

Hydraulic

Conditions)

b. Wide Dose

Distribution

(Worse

Hydraulic

Conditions)

0.1

0.2

0.3

0.4

0.5

0.6

0.0

UV Dose (mJ/cm2)

0 15 30 45 75 90

There are currently no methods to measure directly the dose distribution in a continuous 

flow UV reactor, but mathematical models can help to characterize dose distribution. Therefore, 

the UV dose in a UV reactor is estimated as the reduction equivalent dose (RED). The RED is a 

calculated dose for a flow through UV reactor that is based on biodosimetry (i.e., measuring the 

level of inactivation of a challenge microorganism with a known UV dose-response). The RED 

is set equal to the UV dose in a collimated beam test that achieves the same level of inactivation 

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2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-10 June 2003

of the challenge microorganism as measured for the flow-through UV reactor during 

biodosimetry testing. Methods for collimated beam testing and biodosimetry are in Appendix E 

section 4.2, respectively. 

2.3.4 Microbial Response (UV Dose-Response) 

The response of microorganisms to UV light is calculated by determining the 

concentration of infectious microorganisms before and after exposure to a measured UV dose 

and applying Equation 2.2. 

N

N Log Inactivation 0

10 = log Equation 2.2

Where 

N0 = Concentration of infectious microorganisms before exposure to UV light 

N = Concentration of infectious microorganisms after exposure to UV light 

UV dose-response relationships can be expressed as either the proportion of 

microorganisms inactivated (log inactivation, results in a dose-response curve with a positive 

slope) or the proportion of microorganisms remaining (log survival, results in a dose-response 

curve with a negative slope) as a function of UV dose. The proportion of microorganisms 

remaining and the log inactivation are typically shown on a logarithmic (base 10) scale, while the 

UV dose is typically shown on a linear scale. This manual will present microbial response as log 

inactivation since the terminology is widely accepted in the industry. Therefore, all doseresponse curves presented will have a positive slope. 

Although several approaches may be used to measure microbial dose-response, the 

bench-scale collimated beam test has evolved as the customary method because it has carefully 

controlled conditions, allowing for accurate and repeatable determination of UV dose. Accurate 

determination of UV dose is beneficial for developing meaningful relationships between UV 

dose and microbial response. 

Figure 2.8 presents examples of UV dose-response curves. In general, the UV doseresponse of disperse microorganisms follows first order inactivation (Figure 2.8, E. coli curve; 

section A.2.5.1). However, some microorganisms are slower to respond, producing a shoulder at 

low UV doses followed by near-linear inactivation (Figure 2.8, B. subtilis curve; section 

A.2.5.2). UV dose-response is generally independent of how the germicidal UV light is 

produced (i.e., low-pressure or medium-pressure UV light), UV absorbance, temperature, and 

pH. 

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2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-11 June 2003

Figure 2.8 Shapes of UV Dose-Response Curves 

(adapted from Chang et al. 1985) 

0

1

2

3

4

5

6

0 20 40 60 80 100

UV Dose (mJ/cm2)

Log I

n

a

ctiv

ation

E. coli

B. subtilis spores

Total coliform-wastewater

Rotavirus

0

1

2

3

4

5

6

0 20 40 60 80 100

UV Dose (mJ/cm2)

Log I

n

a

ctiv

ation

E. coli

B. subtilis spores

Total coliform-wastewater

Rotavirus

E. coli

B. subtilis spores

Total coliform-wastewater

Rotavirus

UV dose-response is affected by particle-association and clumping of microorganisms. 

Solids present in wastewater samples can cause a tailing or flattening of the dose-response curve 

at higher inactivation levels (Figure 2.8, total coliform curve; section A.2.5.3) because clumping 

or particle association shields a fraction of the microorganisms from UV light. In these 

wastewater experiments, the microorganisms are present in the treated water at very high 

concentrations so that any particle association with turbidity reflects the impact of upstream

treatment processes. 

Research by Linden et al. (2002b) indicated that the UV dose-response of 

microorganisms added to filtered drinking waters is not altered by variation in turbidity that

meets regulatory requirements (40 CFR 141.73). For unfiltered waters, Passantino and Malley 

(2001) found that source water turbidity up to 10 NTU does not impact the UV dose-response of 

separately added (seeded) microorganisms. In these experiments, however, microorganisms 

were added to waters containing various levels of treated or natural turbidity. Therefore, it was 

not possible to examine microorganisms associated directly with particles in their natural or 

treated states. Consequently, these drinking water studies can only suggest the impact of 

turbidity on dose-response as it relates to the impact of UV light scattering by particles, rather 

than particle-association or clumping of microorganisms. 

2.3.5 Microbial Spectral Response 

The action spectrum (also called UV action) of a microorganism is a measure of 

inactivation effectiveness as a function of wavelength. Figure 2.9 illustrates the UV action for 

three microbial species and also the UV absorbance of DNA as a function of wavelength. 

Because of the similarity between UV action and DNA absorbance, and because DNA 

absorbance is easier to measure than UV action, the DNA absorbance spectrum of a 

microorganism is often used as a surrogate for its UV action spectrum. The scale of the y-axis 

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2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-12 June 2003

represents the ratio of inactivation effectiveness at a given wavelength to the inactivation 

effectiveness at 254 nm. For most microorganisms, the UV action peaks at or near 260 nm, has a 

local minimum near 230 nm, and drops to zero near 300 nm. Although the sensitivity of the 

organism often increases below 230 nm, the strong absorption of UV light by components in 

natural water at these wavelengths offsets the increased organism sensitivity in this region. 

Nevertheless, an operating definition of the effective germicidal range for UV light in water 

includes wavelengths from 200 to 300 nm. 

Figure 2.9 Comparison of Microbial UV Action and DNA UV Absorbance

(adapted from Rauth 1965 and Linden et al. 2001) 

0.0

0.5

1.0

1.5

2.0

200 300

Wavelength (nm)

U

V A

ctio

n

or D

N

A

A

bsorbance

R

elative t

o 254 

n

m

DNA

Cryptosporidium

MS2

Herpes simplex virus

220 240 260 280

0.0

0.5

1.0

1.5

2.0

200 300

Wavelength (nm)

U

V A

ctio

n

or D

N

A

A

bsorbance

R

elative t

o 254 

n

m

DNA

Cryptosporidium

MS2

Herpes simplex virus

220 240 260 280

2.4 UV Reactors 

The goal in designing UV reactors for drinking water disinfection is to deliver efficiently 

the necessary dose to inactivate pathogenic microorganisms. An example UV reactor is shown 

in Figure 2.10. Commercial UV reactors consist of open or closed-channel vessels containing 

UV lamps, lamp sleeves, UV intensity sensors, lamp sleeve wipers, and temperature sensors. 

UV lamps are housed within the lamp sleeves, which protect and insulate the lamps. Some

reactors include automatic cleaning mechanisms to keep the lamp sleeves free of deposits that 

may form due to contact with the water. UV intensity sensors, flow meters, and in some cases, 

UVT monitors are used to monitor dose delivery by the reactor. This section briefly describes 

UV reactor components. A more detailed discussion of these components is provided in section 

A.3. 

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2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-13 June 2003

Figure 2.10 UV Disinfection System Schematic 

(courtesy of Severn Trent Services) 

Influent

Pipe

UV

Transmittance

Monitor

Control

Panel

UV Lamp Housed in

Quartz Sleeve

UV Intensity

Sensor

UV Intensity

Sensor

Reactor

Casing

Temperature

Sensor

Electrical

Connection

to Lamp

Effluent

Pipe

Quartz Sleeve

Wiper

Wiper

Motor

Influent

Pipe

UV

Transmittance

Monitor

Control

Panel

UV Lamp Housed in

Quartz Sleeve

UV Intensity

Sensor

UV Intensity

Sensor

Reactor

Casing

Temperature

Sensor

Electrical

Connection

to Lamp

Effluent

Pipe

Quartz Sleeve

Wiper

Wiper

Motor

2.4.1 Reactor Configuration 

UV reactors are typically classified as either open or closed channel. Water flows under 

pressure (i.e., no free surface) in closed channel reactors (Figure 2.11a). Drinking water UV 

applications have used only closed reactors to-date. Open channel reactors (Figure 2.11b) are 

open basins with channels containing racks of UV lamps. Open channel reactors are most 

commonly used in wastewater applications. 

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2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-14 June 2003

Figure 2.11 Example of Closed (a) and Open (b) Channel Reactors 

(courtesy of Trojan Technologies) 

a. Closed-Channel Reactor b. Open-Channel Reactor

Reactors are designed to optimize dose delivery, and the reactor hydrodynamics play an 

important role in design. Lamp placement, inlet and outlet conditions, and baffles all affect 

mixing within a reactor. Improvements to the hydraulic behavior of a reactor are often obtained 

at the expense of headloss. Individual reactor designs employ various methods to optimize dose 

delivery (e.g., higher lamp output versus lower lamp output and improved hydrodynamics 

through increased headloss). 

2.4.2 UV Lamps 

UV light can be produced by the following variety of lamps: 

• Low-pressure (LP) mercury vapor lamps 

• Low-pressure high-output (LPHO) mercury vapor lamps 

• Medium-pressure (MP) mercury vapor lamps 

• Electrode-less mercury vapor lamps 

• Metal halide lamps 

• Xenon lamps (pulsed UV) 

• Eximer lamps 

• UV lasers 

Full-scale drinking water applications generally use LP, LPHO, or MP lamps. As such, 

the subsequent discussions in this manual are limited to these UV lamp technologies. Table 2.1 

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2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-15 June 2003

lists characteristics associated with these lamps, and Table 2.2 lists operational advantages and 

disadvantages of the lamp types. 

Table 2.1 Mercury Vapor Lamp Characteristics 

Parameter Low-pressure

Low-pressure

high-output Medium-pressure 

Germicidal UV light Monochromatic at 

254 nm 

Monochromatic 

at 254 nm 

Polychromatic, including 

germicidal range 

(200 to 300 nm) 

Mercury Vapor Pressure (torr) Optimal at 0.007 0.76 300 – 30,000

Operating Temperature (°C) Optimal at 40 130 – 200 600 – 900 

Electrical Input (W/cm) 0.5 1.5 – 10 50 – 250 

Germicidal UV Output (W/cm) 0.2 0.5 – 3.5 5 – 30 

Electrical to Germicidal UV 

Conversion Efficiency (%) 35 – 38 30 – 40 10 – 20 

Arc length (cm) 10 – 150 10 – 150 5 – 120 

Relative Number of Lamps 

Needed for a Given Dose 

High Intermediate Low

Lifetime (hrs) 8,000 – 10,000 8,000 – 12,000 4,000 – 8,000

Table 2.2 Mercury Vapor Lamp Comparison

Low-pressure Medium-pressure

Comparative

Advantages

• Higher germicidal efficiency; nearly all 

output at 254 nm 

• Smaller power draw per lamp (less 

reduction in dose if lamp fails) 

• Longer lamp life 

• Higher power output 

• Fewer lamps for a given application 

• Smaller reactors 

• Smaller footprint 

Comparative

Disadvantages 

• More lamps needed for a given 

application 

• Larger footprint 

• Higher operating temperature can 

accelerate fouling (section 2.5.1) 

• Shorter lamp life 

• Lower electrical to germicidal UV 

conversion efficiency 

The light emitted by LP and LPHO lamps is essentially monochromatic at 253.7 nm

(Figure 2.12a) and is near the maximum of the microbial action spectrum. MP lamps emit at a 

wide range of wavelengths across the action spectra (Figure 2.12b). Therefore, LPHO lamps 

convert power to germicidal light more efficiently. In either lamp type, power not converted to 

light is primarily lost as heat. 

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2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-16 June 2003

Figure 2.12 UV Output of LP (a) and MP (b) Mercury Vapor Lamps 

(Sharpless and Linden 2001) 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

200 250 300 350 400

Wavelength (nm)

R

elativ

e Lamp O

utput La

m

p O

utput R

elativ

e t

o

M

a

xim

u

m O

utput i

n R

ange

b. Medium Pressure Lamp

0.0

0.2

0.4

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1.2

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Wavelength (nm)

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elativ

e Lamp O

utput

La

m

p O

utput R

elativ

e t

o

M

a

xim

u

m O

utput i

n R

ange

a. Low Pressure Lamp

0.0

0.2

0.4

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1.2

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Wavelength (nm)

R

elativ

e Lamp O

utput La

m

p O

utput R

elativ

e t

o

M

a

xim

u

m O

utput i

n R

ange

b. Medium Pressure Lamp

0.0

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Wavelength (nm)

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elativ

e Lamp O

utput La

m

p O

utput R

elativ

e t

o

M

a

xim

u

m O

utput i

n R

ange

b. Medium Pressure Lamp

0.0

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elativ

e Lamp O

utput

La

m

p O

utput R

elativ

e t

o

M

a

xim

u

m O

utput i

n R

ange

a. Low Pressure Lamp

0.0

0.2

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1.2

200 250 300 350 400

Wavelength (nm)

R

elativ

e Lamp O

utput

La

m

p O

utput R

elativ

e t

o

M

a

xim

u

m O

utput i

n R

ange

a. Low Pressure Lamp

Figure 2.13 shows the output of LP and MP lamps superimposed with the DNA 

absorption spectrum. In Figure 2.13, the DNA absorbance is plotted relative to the maximum 

absorbance in the range (260 nm). The lamp outputs are also presented on a relative scale. 

However, in absolute terms, there is a significant difference in the intensity and power of LP and 

MP lamps (see Table 2.1 for more information on lamp operating characteristics). 

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2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-17 June 2003

Figure 2.13 UV Lamp Output and its Relation to the UV Absorbance of DNA 

(courtesy of Bolton Photosciences, Inc.) 

200 250 300

Wavelength (nm)

La

m

p O

utput

R

elativ

e t

o

M

a

xim

u

m O

utput i

n R

ange

DNA

Absorbance

MP

Output Æ

LP OutputÆ

D

NA Ab

sorb

a

n

c

e R

elati

v

e to

M

a

ximum A

b

sorb

a

n

c

e in R

a

n

g

e

0.00

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

200 250 300

Wavelength (nm)

La

m

p O

utput

R

elativ

e t

o

M

a

xim

u

m O

utput i

n R

ange

DNA

Absorbance

MP

Output Æ

LP OutputÆ

D

NA Ab

sorb

a

n

c

e R

elati

v

e to

M

a

ximum A

b

sorb

a

n

c

e in R

a

n

g

e

0.00

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

UV lamps may be oriented parallel, perpendicular, or diagonal to flow or ground. 

Orienting MP lamps horizontally relative to the ground prevents differential heating of the lamps 

and reduces the potential for lamp breakage. Lamp breakage is discussed further in Appendix N. 

UV lamps degrade as they age resulting in a reduction in output (section A.3.1.6). MP 

lamps may have a shift in spectral output as well. Lamp degradation will impact dose delivery 

over time. 

2.4.3 Lamp Power Supply And Ballasts 

Ballasts supply the UV lamps with the appropriate power to energize and operate the UV 

lamps. Ballasts use inductance (coil or transformer), capacitance, and a starting circuit. Power 

supplies and ballasts are available in many different configurations and are tailored to a unique 

lamp type and application. UV reactors may use electronic ballasts, magnetic ballasts, or 

transformers. The various ballast types and their differences are detailed in section A.3.2. 

2.4.4 Lamp Sleeves 

UV lamps are housed within lamp sleeves to help keep the lamp at optimal operating 

temperature and to protect the lamp from breaking. Lamp sleeves are tubes of quartz (or vitreous 

silica). The sleeve length is sufficient to include the lamp and associated electrical connections. 

The sleeve diameter is typically 2.5 cm for LP lamps and 5 to 10 cm for MP lamps. The distance 

between the exterior of the lamp and interior of the lamp sleeve is approximately 1 cm. Sleeve 

walls are typically 2 to 3 mm thick and absorb some UV light (Figure 2.14). UV lamps are 

usually centered radially within lamp sleeves using spacers. 

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2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-18 June 2003

Figure 2.14 UV Transmittance of Quartz that is 1 mm Thick at a Zero Degree 

Incidence Angle (GE Quartz 2001) 

50

60

70

80

90

100

200 220 240 260 280 300 320 340 360 380 400

Wavelength (nm)

U

V Transmittance (

%)

Lamp sleeves can fracture and foul, and their transmittance will decrease as they age. 

Fractures can occur from internal stress and external mechanical forces such as wiper jams, 

water hammer, resonant vibration, and impact by objects. Microscopic fractures may also occur 

if lamp sleeves are not handled properly when removed for manual cleaning. If the sleeve 

fractures while in service, water can enter the sleeve, making the lamp vulnerable to breakage as 

a result of temperature differences between the lamp and the water. Lamp breakage is 

undesirable due to potential for mercury release. Appendix N discusses the potential effects of 

lamp breakage and possible response plans. 

Fouling on the internal lamp sleeve surface arises from the deposition of material from

components within the lamp or sleeve due to temperature and exposure to UV light. The UV 

reactor manufacturer can control internal lamp sleeve fouling through appropriate material 

selection. Fouling on external surfaces is caused by the reaction of compounds in the water with 

the lamp sleeve surface. Compounds that contribute to fouling are discussed in section 2.5.1. 

External fouling must be removed by cleaning. In addition, exposure of quartz contaminated 

with metal cations can cause solarization as lamp sleeves age. Both fouling and solarization can 

decrease the UV transmittance of the sleeve. 

2.4.5 Cleaning Systems 

UV reactor manufacturers have developed different approaches for cleaning lamp

sleeves, depending on the application. These approaches include both off-line chemical cleaning 

(OCC) and on-line mechanical cleaning (OMC) methods. 

In OCC systems, the reactor is shut down, drained, and flushed with a cleaning solution. 

Solutions used to clean lamp sleeves include citric acid, phosphoric acid, or a food grade 

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UV Disinfection Guidance Manual 2-19 June 2003

proprietary solution provided by the UV reactor manufacturer. The reactor is rinsed and returned

to operation after sufficient time to dissolve the substances fouling the sleeves is allowed. LPHO 

systems typically use OCC systems. 

OMC systems are built-in UV reactor components that consist of wipers that are driven 

by either screws attached to electric motors or pneumatic pistons. There are two types of wipers 

used in OMC systems: mechanical wipers and physical-chemical wipers. Mechanical wipers 

may consist of stainless steel brush collars or Teflon® rings that move along the lamp sleeve 

(Figure 2.15a). Physical-chemical wipers have a collar filled with cleaning solution that moves 

along the lamp sleeve (Figure 2.15b). The wiper physically removes fouling on the lamp sleeve 

surface while the cleaning solution within the collar dissolves fouling materials. The use of 

mechanical and physical-chemical wipers does not necessitate that the UV reactor be drained. 

Therefore, the reactor can remain on-line while the lamp sleeves are cleaned. MP systems 

typically use OMC systems because the higher lamp temperatures can accelerate fouling under 

certain water qualities. 

Figure 2.15 (a) Mechanical Wiper System (courtesy of Calgon Carbon 

Corporation), (b) Physical-Chemical Wiper System 

(courtesy of Trojan Technologies) 

a b

2.4.6 UV Intensity Sensors 

UV intensity sensors are photosensitive detectors that measure the UV intensity at a point 

within the UV reactor (Figure 2.16). Sensors are used to indicate dose delivery by providing 

information related to UV intensity at different points in the reactor. The measurement responds 

to changes in lamp output due to lamp power setting, lamp aging, lamp sleeve aging, and lamp

sleeve fouling. Depending on sensor position, UV intensity sensors may also respond to changes 

in UV absorbance of the water being treated (section A.3.8.2). UV intensity sensors are 

composed of optical components, a photodetector, an amplifier, a housing, and an electrical 

connector. The optical components may include monitoring windows, light pipes, diffusers, 

apertures, and filters. Monitoring windows and light pipes are designed to deliver light to the 

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UV Disinfection Guidance Manual 2-20 June 2003

photodetector. Diffusers and apertures are designed to reduce the amount of UV light reaching 

the photodetector, thereby reducing sensor degradation that is caused by UV energy. Optical 

filters are used to modify the spectral response such that the sensor only responds to germicidal 

wavelengths (i.e., 200 to 300 nm). At the time of publication, sensors are specific to each 

manufacturer and are subject to validation as described in sections 4.3.2.3 and C.4.7. 

Figure 2.16 UV Intensity Sensor Viewing Lamps within a UV Reactor 

(courtesy of Severn Trent Services) 

UV intensity sensors can be classified as wet or dry. Dry sensors monitor UV light 

through a monitoring window, whereas wet UV intensity sensors are in direct contact with the 

water flowing through the reactor. Monitoring windows and the wetted ends of wet sensors can 

foul over time and need cleaning similar to lamp sleeves. 

2.4.7 UV Transmittance Monitors 

As stated previously, UVT is an important parameter in determining the efficiency of UV 

disinfection. Therefore, monitoring UV transmittance (or UV absorbance to calculate UVT) is 

critical to ensure the success of a UV disinfection application. UVT can be determined either 

through grab samples with a laboratory instrument or on-line. Several commercial UV reactors 

use the measurement of UVT to help monitor and control the calculated UV dose in the reactor. 

In general, commercial on-line UVT monitors calculate UVT by measuring the UV 

intensity at various distances from a lamp. One such monitor is schematically displayed in 

Figure 2.17. In this monitor, a stream of water passes through a cavity containing a LP lamp

with three UV intensity sensors located at various distances from the lamp. The difference in 

sensor readings is used to calculate UVT. 

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UV Disinfection Guidance Manual 2-21 June 2003

Figure 2.17 UV Transmittance Monitor Design 

 (courtesy of Severn Trent Services) 

UV Lamp UV Intensity

Sensor

Inlet Outlet

A

B

C

UV Lamp UV Intensity

Sensor

Inlet Outlet

A

B

C

UV Lamp UV Intensity

Sensor

Inlet Outlet

A

B

C

2.4.8 Temperature Sensors 

Energy input per unit volume is relatively high for a UV reactor. The water flowing 

through a reactor efficiently absorbs the wasted heat and maintains operating temperatures within 

a desirable range. Nevertheless, temperatures can become elevated under the following 

circumstances: 

• Water level in the reactor drops and lamps are exposed to air. 

• Water stops flowing in the reactor. 

UV reactors are equipped with temperature sensors that monitor the water temperature 

within the reactor. If the temperature is above the recommended operating temperature range, 

the reactor will shut off to minimize the potential for the lamps overheating. 

 

2.4.9 Monitoring UV Disinfection Performance 

The performance of an operating UV disinfection system must be monitored to 

demonstrate that adequate disinfection is being achieved (40 CFR 141, Subpart W, Appendix D). 

Because the concentration of pathogenic organisms cannot be measured continuously in the UVtreated water and the dose distribution cannot be measured directly in real time, various 

strategies have been developed to monitor dose delivery. Any dose monitoring method must be 

evaluated during reactor validation (as described in section 4.3.2.2), and the outputs measured 

during validation will be part of the monitoring requirements described in section 5.4.1 (40 CFR 

141.729(d)). 

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UV Disinfection Guidance Manual 2-22 June 2003

Currently, there are three fundamental approaches to monitor UV disinfection 

performance in a UV reactor: 

1. UV Intensity Setpoint Approach. In this approach, measurements made by the UV 

intensity sensor are used to control the UV reactor. The UV intensity sensor is 

located in a position that allows it to properly respond to both changes in UV 

intensity output of the lamps and also UVT of the water. The UV intensity sensor 

output and the flowrate are used to monitor dose delivery. The setpoint value for UV 

intensity over a range of flowrates is determined during validation. 

2. UV Intensity and UVT Setpoint Approach. This approach is similar to the UV 

intensity sensor setpoint approach, except that the UV sensor is placed close to the 

lamp such that it only responds to changes in UV lamp output. UVT is monitored 

separately. For a specific flowrate, the UV intensity and UVT measurements are used 

to monitor dose delivery. The setpoints for UV intensity and UVT over a range of 

flowrates are determined during validation. 

3. Calculated UV Dose Approach. In this approach, the UV intensity sensor is placed

close to the lamp, which is similar to the UV intensity and UVT setpoint approach. 

Flowrate, UVT, and UV intensity are all monitored, and the outputs are used to 

calculate UV dose via a validated computational algorithm developed by the UV 

reactor manufacturer. 

The strategy for dose monitoring depends on the manufacturer and may be proprietary. 

Dose monitoring recommendations are discussed in section 5.4.2. 

2.5 Water Quality Impacts and Byproduct Formation 

Constituents in the water subjected to treatment affect the performance of UV 

disinfection. In addition, all disinfectants can form byproducts, and the goal of the overall 

disinfection process is to maximize disinfection while minimizing byproduct formation. This 

section discusses water quality characteristics impacting UV disinfection performance and 

finishes with a discussion of byproducts formed during the UV disinfection process. 

2.5.1 Water Quality Impacts 

UVT, particle content, and constituents that foul lamp sleeves and other wetted 

components are the most significant water quality factors impacting UV disinfection 

effectiveness. In spite of these effects, the impact of water quality on dose delivery can be 

adequately addressed in virtually all drinking water applications if carefully considered during 

the design of the UV disinfection system, as discussed in section 3.1.3.1. 

The most important water quality parameter affecting reactor performance is UVT. As 

UVT decreases, the intensity throughout the reactor decreases for a given lamp configuration. 

This results in a reduction in UV dose delivered to the microorganism and the measured UV 

Proposal Draft 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-23 June 2003

intensity for a given lamp output. Section 3.1.3.1 discusses how to incorporate the impact of 

UVT into UV disinfection system design. 

Several chemicals used in water treatment processes can decrease the UVT of water (e.g., 

Fe+3 and ozone). However, some oxidants (including ozone) can increase the UVT (APHA et al. 

1998) by degrading natural organic matter. Water treatment processes upstream of the UV 

reactors can be operated to control UVT, thereby optimizing the design and costs of the UV 

reactor (section A.4.1.3 and section 3.1.3.1). 

Particle content can also impact UV disinfection performance. Particles may scatter light 

and reduce the UV intensity delivered to the microorganisms. Particles may also shield 

microorganisms from UV light, effectively reducing disinfection performance. 

Compounds in the water can cause fouling in a UV reactor on the external surfaces of the 

lamp sleeves and other wetted components (e.g., monitoring windows of UV intensity sensors). 

Fouling on the lamp sleeves reduces the transmittance of UV light through the sleeve into the 

water, thereby reducing power efficiency. Fouling on the monitoring windows impacts UV 

intensity and dose monitoring. Hardness, alkalinity, temperature, iron concentration, and pH all 

influence the rate of fouling and, subsequently, the frequency of sleeve cleaning. The following 

compounds can cause fouling: 

• Compounds whose solubility decreases as temperature increases will precipitate (e.g., 

CaCO3, CaSO4, MgCO3, MgSO4, FePO4, FeCO3, Al2(SO4)3). These compounds will 

foul MP lamps faster than LP lamps due to differences in operating temperature. 

• Compounds with low solubility will precipitate (e.g., Fe(OH)3, Al(OH)3). 

• Particles will deposit on the lamp sleeve surface due to gravity settling and 

turbulence-induced collisions (Lin et al. 1999a). 

Fouling rate kinetics have been reported as first order over time following a short 

induction period (Lin et al. 1999b). Depending on the water quality and UV lamp type, 

significant fouling may occur in hours or take up to several months. Although there is currently 

not sufficient information to predict fouling based on water quality, a facility can use the 

Langelier Saturation Index (LSI) or the Calcium Carbonate Precipitation Potential (CCPP) as a 

tool to determine if precipitation is likely to occur (section A.4.1.4). Data have been generated 

from pilot-scale testing on waters of low to moderate hardness and iron content (Mackey et al. 

2001 and Mackey et al. 2003). At total and calcium hardness levels less than 140 mg/L and iron 

less than 0.1 mg/L, standard cleaning protocols and wiper frequencies (one sweep every 15 

minutes to an hour) were sufficient to overcome the impact of sleeve fouling at all sites tested. 

At sites with high hardness or iron in the feed water, it may be advantageous to evaluate fouling 

rates as described in section J.5.1 on a site-specific or worst case basis via pilot-scale or 

demonstration-scale testing to identify how best to keep the lamp sleeves clean. 

Table 2.3 is a summary of water quality data and the fouling observed for various pilot 

and full-scale UV reactors. All of the MP systems shown had mechanical cleaning (except at 

Boxalls Lane), and the LPHO systems used manual chemical cleaning. The fouling observed at 

individual sources is reported as shown in the following list: 

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2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-24 June 2003

• Not Significant – no significant drop in UV intensity (based on UV intensity sensor 

readings) 

• Moderate – slight decrease in UV intensity and slight scale observed on sleeves 

• Significant – large decrease in UV intensity and significant deposits observed on 

sleeves 

Table 2.3 Water Quality Data and Fouling Observed for UV Disinfection 

Pilot and Demonstration Studies

Name of Plant Boxalls 

Lane1

Atlanta2 

Ulrich 

Water 

Treatment 

Plant2

Central 

Utah2

Neenah 

Water 

Utility2

Cudahy

Water 

Utility2

Location Hampshire, 

UK 

Atlanta, 

GA Austin, TX Orem, UT Neenah, 

WI 

Cudahy, 

WI 

Lamp Type MP MP/LPHO MP/LPHO MP/LPHO MP/LPHO/

LP 

MP/LPHO/

LP 

A254 (cm-1) NA 0.01-0.04 0.03-0.08 0.01-0.04 0.03-0.10 0.00-0.03

LSI NA NA NA 0.5 0.7 -0.1

Iron (mg/L) NA <0.04 0.01 <0.02 0.02 0.01

Manganese (mg/L) NA <0.015 <0.001 <5.03 0.003 0.012

Calcium Hardness 

(mg/L as CaCO3) NA NA 40 162 54 80

Hardness 

(mg/L as CaCO3) 325-370 21.5 101 180 87 138

Alkalinity 

(mg/L as CaCO3) 260-280 13.7 60 159 52 125

pH 7.1-7.2 6.6 9.6 7.8 9 7.7

Fouling Observed not 

significant 

not 

significant4

moderate5

not 

significant moderate6 not significant 

not 

significant 

1

 Bourgine et al. 1995 2

 Mackey et al. 2001 3

 Detection Limit 4

 Cleaning wipers on (MP system) 5

 Cleaning wipers off (MP system) 6

 After 8 months of operation (LPHO system)

NA = Not available 

None of the systems studied and listed in Table 2.3 exhibited "significant" fouling, and in 

all cases, the observed fouling was controllable by regular system maintenance and cleaning. 

Lastly, algae may grow upstream or downstream of UV reactors. Visible light emitted

from the lamps is transmitted through water at further distances than germicidal wavelengths. 

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2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-25 June 2003

Depending on the concentration of nutrients in the water and the amount of visible light 

transmitted beyond the reactor, algae growth may need to be controlled through periodic 

maintenance. 

2.5.2 Byproducts from UV Disinfection 

UV DBPs arise either directly through photochemical reactions or indirectly through 

reactions with products of photochemical reactions (section A.4.2). Photochemical reactions will 

only take place if a chemical species absorbs UV light, and the resulting excited state reacts to 

form a new species. The resulting concentration of new species will depend on the concentration 

of the reactants and the UV dose. 

In drinking water, research has focused on the impact of UV light on the formation of 

halogenated DBPs after subsequent chlorination and the transformation of organic material to 

more degradable components. For ground water and filtered drinking water, UV disinfection at 

typical doses has been shown not to impact the formation of trihalomethanes or haloacetic acids, 

two categories of DBPs currently regulated by the United States Environmental Protection 

Agency (EPA) (Malley et al. 1995; Kashinkunti et al. 2003). 

Several studies have shown low-level formation of non-regulated DBPs (e.g., aldehydes) 

as a result of applying UV light to wastewater and raw drinking water sources. However, a study 

performed with filtered drinking water indicates no significant change in aldehydes, carboxylic 

acids, or total organic halides (TOX) (Kashinkunti et al. 2003). The difference in results can be 

attributed to the difference in water quality, most notably the higher concentration of organic 

material in raw waters and wastewaters. 

Finally, the conversion of nitrate to nitrite is possible with MP lamps that emit at low 

wavelengths (von Sonntag and Schuchman 1992). However, due to the low conversion rate 

(about 1 percent; Sharpless and Linden 2001), this is of minimal concern in drinking water 

applications.

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3. Planning and Design Aspects 

for UV Installations 

This chapter discusses the key planning and design features for UV installations. The 

planning section helps identify the parameters and constraints to be considered prior to design of 

the UV installation, and the design section presents factors that should be considered during 

detailed design. 

The focus of Chapter 3 is UV disinfection implementation issues, not the determination

of whether UV disinfection is the most appropriate technology. Throughout Chapter 3, it is 

assumed, unless otherwise stated, that the water to be disinfected is filtered water meeting 

applicable regulatory requirements. Appendices G, H, and I provide additional information on 

unfiltered, ground water, and small systems, respectively. The planning and detailed design for 

any UV installation is site-specific. Given the wide range of treatment scenarios that are 

possible, a document of this nature cannot address or anticipate all possible treatment conditions. 

The information presented here should be used within the context of sound engineering judgment 

as it can be applied on a case-by-case basis. In addition, this Guidance Manual was written with 

the understanding that UV technology will continue to expand and evolve.

The organization of this chapter is presented below by the question that each section 

addresses. 

• What are the goals of the UV installation? .................................................Section 3.1.1 

• What are the potential installation locations? .............................................Section 3.1.2 

• What design parameters need to be defined?..............................................Section 3.1.3 

• How does the UV reactor selection affect design? .....................................Section 3.1.4 

• What are the options for validation?........................................................Section 3.1.4.3 

• How should potential installation locations be evaluated? .........................Section 3.1.6 

• What are the existing hydraulic conditions and UV installation 

hydraulic needs?.......................................................................................Section 3.1.6.1 

• What should be considered when estimating the process 

footprint of the UV installation? ..............................................................Section 3.1.6.2 

• How can the installation options be evaluated?..........................................Section 3.1.7 

• What are the options for UV reactor procurement?...................................... Section 3.2 

• What are the options for addressing hydraulic constraints and 

what are the critical hydraulic system components?...................................Section 3.3.1 

UV Disinfection Guidance Manual 3-1 June 2003

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3. Planning and Design Aspects for UV Installations 

UV Disinfection Guidance Manual 3-2 June 2003

• How does the control strategy influence the design of the 

process instrumentation and control for the UV installation? ....................Section 3.3.2 

• What are the elements in the process instrumentation and 

control system? ...........................................................................................Section 3.3.3 

• What are the necessary electric power arrangements? ...............................Section 3.3.4 

• What elements need to be considered for the UV installation 

layout?.........................................................................................................Section 3.3.5 

• What information should the equipment specification include?.................Section 3.3.6 

• What are the necessary drawings and specifications for the UV 

installation? .................................................................................................Section 3.3.7 

• What should be reported to the State and when?.......................................... Section 3.4 

The process of planning and designing a UV installation is presented as a flowchart in 

Figure 3.1. In the United States to date, the majority of the utilities undertaking the construction 

of UV installations have pre-purchased the UV reactors prior to design. Therefore, the design 

flowchart is based on the pre-purchase of the UV reactors and the use of a traditional design-bidbuild approach for the project. Chapter 3 is generally organized to follow the flowchart. UV 

installations can be successfully constructed using any of the equipment procurement and 

contractor selection approaches currently used within the industry. It is the utility’s and 

engineer’s responsibility to select the most appropriate project approach. Whatever approach is 

utilized, the planning and design components discussed in Chapter 3 should be addressed even 

though the actual order of completion may vary. 

Proposal Draft 
3. Planning and Design Aspects for UV Installations 

Figure 3.1 Flowchart for Planning, Design, and Construction of UV Installation

Define the goals ofthe UV installationand identify thetargetmicroorganism(s)Section 3.1.1

Identify thepotential UVfacilitySection 3.1.2

Determine designparametersy Water qualityy Flowratey Power qualitySection 3.1.1

Evaluate potentialUV equipmentSection 3.1.4

Evaluate equipmentvalidationoptions**Section 3.1.4.2

Evaluateoperational andcontrol strategiesSection 3.1.5

Identify facility evaluatiy Hy

and 

y Fooy ExiSection 

Compaand sellocationSection EvaluaprocureSection Design Section Determcontrol Section Design controlSection Design systemsSection 

Develop specifications andprocure equipmentSection 3.3.6Complete facility layoutSection 3.3.5

Finalize hydraulic designSection 3.3.7Finalize design layoutSection 3.3.7Finalize P&IC andelectrical designSection 3.3.7

Develop and bid UVfacility drawings andspecificationsSection 3.3.7

Report to PrimacyAgencySection 3.4

CONSTRUCTION

PLANNING

DESIGN

3.1.4.3

2

Define the goals ofthe UV installationand identify thetargetmicroorganism(s)Section 3.1.1

Identify thepotential UVfacilitySection 3.1.2

Determine designparametersy Water qualityy Flowratey Power qualitySection 3.1.1

Evaluate potentialUV equipmentSection 3.1.4

Evaluate equipmentvalidationoptions**Section 3.1.4.2

Evaluateoperational andcontrol strategiesSection 3.1.5

Identify facility evaluatiy Hy

and 

y Fooy ExiSection 

Compaand sellocationSection EvaluaprocureSection Design Section Determcontrol Section Design controlSection Design systemsSection 

Develop specifications andprocure equipmentSection 3.3.6Complete facility layoutSection 3.3.5

Finalize hydraulic designSection 3.3.7Finalize design layoutSection 3.3.7Finalize P&IC andelectrical designSection 3.3.7

Develop and bid UVfacility drawings andspecificationsSection 3.3.7

Report to PrimacyAgencySection 3.4

CONSTRUCTION

PLANNING

DESIGN

3.1.4.3

2

1 Flowchart is based on pre-purchase of UV reactors and the traditional design-bid-build approach 2 The timing of UV reactor validation testing depends on whether it has been validated off-site or if on-site validation is necessary.

UV Disinfection Guidance Manual 3-3 

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3. Planning and Design Aspects for UV Installations 

UV Disinfection Guidance Manual 3-4 June 2003

3.1 UV Installations Planning 

The planning process for a UV installation is similar to the process that would be 

employed for any retrofit, upgrade, or new construction project at a water treatment plant (WTP). 

In the planning phase, it is important to identify alternatives and define criteria needed to select 

the appropriate application and to facilitate detailed design. For a UV installation, this includes 

the following steps: 

• Defining disinfection goals 

• Identifying potential locations for UV disinfection 

• Defining design parameters 

• Evaluating potential UV reactors 

• Evaluating control strategies 

• Evaluating hydraulic factors and process footprint 

• Preparing preliminary costs and selecting an installation option 

This section provides planning guidance for each of these steps with a focus on specific 

elements that should be considered for UV disinfection. 

3.1.1 Defining UV Disinfection Goals 

A comprehensive disinfection strategy provides multiple barriers to reduce microbial risk 

while minimizing disinfection byproduct (DBP) formation. UV disinfection is a tool that can 

contribute to a comprehensive disinfection strategy by providing a cost-effective method of 

inactivating target pathogens that are more resistant to more traditional disinfection methods. 

The specific objectives of a given UV installation should be clearly defined during the planning 

stages. This can ensure that the design meets the utility’s and the State’s expectations based on 

the regulatory requirements, target microorganism(s), and the overall disinfection strategy. 

Chapter 1 presents the regulatory requirements that must be met for the overall water treatment 

process and specific requirements for UV disinfection. 

The UV doses necessary for Cryptosporidium and Giardia inactivation are lower than

that those needed to inactivate viruses. Accordingly, the capital costs for inactivating 

Cryptosporidium and Giardia should be lower. One study estimated capital costs for 

Cryptosporidium and Giardia inactivation by UV disinfection to be approximately 50 percent 

lower than the costs associated with the UV inactivation of viruses (Cotton et al. 2002). 

Therefore, the target microorganism and inactivation level should be determined early in the 

planning process. 

Repair of UV light-induced damage is discussed in section 2.3.2. As discussed 

previously, repair has not been observed in Cryptosporidium and viruses, and Giardia only 

Proposal Draft
3. Planning and Design Aspects for UV Installations 

UV Disinfection Guidance Manual 3-5 June 2003

exhibited repair when exposed to very low UV doses (0.5 mJ/cm2). Therefore, repair of UVinduced damage of Cryptosporidium, Giardia, and viruses do not need to be considered in the 

UV installation design. However, bacteria have been shown to repair of UV damage. The 

residual disinfectant concentration (either chlorine or chloramines) in the distribution system will 

most likely prevent repair of UV damage in bacteria. Therefore, microbial repair of bacteria also 

does not affect UV installation design. 

To a degree, UV disinfection can replace chemicals used to disinfect chlorine-resistant

pathogens (e.g., Cryptosporidium and Giardia), thereby reducing DBP formation. However, UV 

disinfection is not as efficient at inactivating viruses as more traditional, chlorine-based

disinfection processes. Because of its effectiveness at treating viruses and the need to maintain a 

disinfectant residual in the distribution system, some chlorine-based disinfectant (chlorine or 

chloramines) will be needed even if UV disinfection is implemented. Also, chemicals that serve 

as disinfectants may be added in the treatment process to oxidize other constituents present in the 

water (e.g., iron, manganese, or taste and odor causing compounds). Utilities that currently add a 

chemical disinfectant prior to the location of a future UV installation and plan to curtail the use 

of such chemicals, following implementation of UV disinfection should assess the effect that a 

reduction in pre-oxidant use may have on water quality at the point of UV application. 

Therefore, a utility considering a change in disinfection strategy should evaluate all water quality 

goals to ensure they are met and must prepare a disinfection benchmark as discussed in 

Chapter 1. 

3.1.2 Identifying Potential Locations for UV Installations 

It is strongly recommended that the UV disinfection process be placed after filtration. 

Although UV disinfection can potentially be applied anywhere along the treatment train from the 

raw water intake to after high-service pumping, there are significant drawbacks to placing the 

UV installation upstream of filtration in conventional WTPs. Prior to filtration, UV absorbance 

at 254 nm (A254) is higher (UV transmittance (UVT) is lower) due to higher concentrations of

natural organic matter, turbidity, and particles. Coagulation can enmesh microorganisms in flocs 

and may block the UV light from reaching the microorganisms, which affects the UV doseresponse of the microorganism. In addition, Long Term 2 Enhanced Surface Water Treatment 

Rule (LT2ESWTR) UV dose requirements apply only to post-filter and unfiltered supplies that 

meet the criteria for filtration avoidance (40 CFR 141.729 (d)). Therefore, this section focuses 

on the post-filtration use of UV disinfection. 

This section presents the general post-filtration locations that may be considered for the 

UV installation. For a location to be feasible, the UV installation hydraulic needs should be met 

(section 3.1.6.1) and the equipment must physically fit in the proposed location. Hydraulic 

profiles and preliminary drawings should be developed for each location under consideration to 

address these controlling criteria. Also, LT2ESWTR requires that all UV reactors be validated 

(40 CFR 141.729 (d)), and the validation protocol (Chapter 4) recommends specific piping 

configurations for both validation testing and UV installation. These recommendations for inlet 

and outlet conditions can affect the feasibility of the potential locations. Detail on the 

recommended inlet and outlet hydraulics for both validation and installation is given in 

section 3.1.4.3. 

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3. Planning and Design Aspects for UV Installations 

UV Disinfection Guidance Manual 3-6 June 2003

3.1.2.1 Combined Filter Effluent Installation (Upstream of Clearwell) 

A combined filter effluent installation is defined here as the application of UV 

disinfection to the filter effluent after it has been combined (as opposed to individual filters) and 

prior to the clearwell as shown in Figure 3.2. This installation is typically in a separate building. 

Of the three options described, the combined filter installation is generally preferred when 

conditions permit. 

Figure 3.2 Schematic for UV Installation Upstream of Clearwell 

Source

Water Rapid

Mix

Flocculation

Sedimentation

Basin

Filters UV

Disinfection Clearwell

To

Distribution

System

Source

Water Rapid

Mix

Flocculation

Sedimentation

Basin

Filters UV

Disinfection Clearwell

To

Distribution

System

There are several advantages to this type of design and installation: 

• The UV reactor operation is largely independent of the operation of individual filters, 

which provides flexibility for design and operation. 

• If the entire UV installation failed, a WTP could still provide disinfection by adding a 

chemical disinfectant to the clearwell. (Note that backup chemical disinfection may 

not provide Cryptosporidium inactivation.) 

• Surge and pressure issues that are concerns with UV reactors installed immediately 

downstream or upstream of high service pumps (HSPs) are usually not an issue for 

this installation location. 

• Because the UV installation will typically be constructed in a new building for this 

installation location, there may be greater flexibility in maintaining the recommended 

inlet and outlet hydraulic conditions for the UV reactors (section 3.1.6.1). 

The primary disadvantages of this type of installation are that an additional building may 

be necessary and that piping and fittings may result in higher headloss than alternative 

configurations. 

3.1.2.2 Individual Filter Effluent Piping Installation 

Individual filter effluent piping installations are defined here as installations with UV 

reactors dedicated to each individual filter effluent pipe. The installation is typically within the 

existing filter gallery. Figure 3.3 schematically represents this type of installation. The main 

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3. Planning and Design Aspects for UV Installations 

UV Disinfection Guidance Manual 3-7 June 2003

advantage of this installation is that a new building would not be necessary, which may lower 

construction costs. 

Figure 3.3 Schematic of Individual Filter 

Effluent Piping Installation in Filter Gallery

Source

Water Rapid

Mix

Flocculation

Sedimentation 

Basin

Filters UV

Disinfection

Clearwell

To

Distribution

System

HSPs Source

Water Rapid

Mix

Flocculation

Sedimentation 

Basin

Filters UV

Disinfection

Clearwell

To

Distribution

System

HSPs Source

Water Rapid

Mix

Flocculation

Sedimentation 

Basin

Filters UV

Disinfection

Clearwell

To

Distribution

System

HSPs

However, there are several disadvantages to this installation location. Many filter 

galleries do not have sufficient space within existing effluent piping to accommodate a UV 

reactor. The existing piping may also put constraints on how the UV reactor is validated because 

of the unique inlet and outlet conditions that may be present (section 3.1.6.1). In addition to 

accommodating the UV reactors, there needs to be sufficient space in the filter gallery or a 

nearby area for the control panels and electrical equipment. Access to existing equipment may 

be impaired by the UV reactor, and access to UV reactor components for maintenance may be 

more restricted than for a combined filter effluent installation. Also, the environmental 

conditions (e.g., moisture) in the filter gallery may not be appropriate for the installation of the 

UV reactors, associated control panels, and electrical equipment without improvements to the 

heating, ventilating, and air conditioning system for the area. 

The in-line installation may also complicate treatment plant operations and limit 

operational flexibility as described below. 

• In general, this option results in an increased number of UV reactors compared to a 

combined filter installation because the number of filters dictates the number of UV 

reactors. This may increase operation and maintenance costs in comparison to the 

combined filter effluent installation where the number of UV reactors is determined 

by the design flow, water quality constraints, UV reactor capacity, and redundancy 

needs. 

• The increased headloss of the UV reactors may affect the operation of the filters and 

the clearwell. 

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3. Planning and Design Aspects for UV Installations 

UV Disinfection Guidance Manual 3-8 June 2003

• With one UV reactor for each filter, the operation of each filter will be dependent on 

the reliable operation of each UV reactor and vice versa. 

• The UV reactor operation during a filter backwash can complicate UV reactor 

operations. The lamp cooling need to addressed if it remains energized during a 

backwash because the lamps should not be energized in stagnant water or air. If a 

UV reactor is off during a backwash, the flow during the UV reactor warm-up 

(section 3.1.3.3) is off-specification, which may cause problems with exceeding offspecification requirements and recommendations (section 3.1.3). 

3.1.2.3 UV Disinfection Downstream of the Clearwell 

It may be possible for a WTP to build the UV installation after the clearwell either

upstream or downstream of the high service pumps (Figure 3.4). In many WTPs, the HSPs take 

water directly from the clearwell, limiting space and the availability of suitable piping for 

installation of the UV installation upstream of the HSPs. Installation downstream of the HSPs 

may provide greater space and flexibility in locating the UV reactors. Either configuration may 

be advantageous if there is insufficient space or head to allow installation of the UV reactors 

between the filters and the clearwell; however, there are significant disadvantages to these 

options. 

Figure 3.4 UV Disinfection Downstream of High Service Pumps 

Source

Water Rapid

Mix

Flocculation

Sedimentation

Basin Filters UV

Disinfection

Clearwell

To

Distribution

System

So HSPs urce

Water Rapid

Mix

Flocculation

Sedimentation

Basin Filters UV

Disinfection

Clearwell

To

Distribution

System

So HSPs urce

Water Rapid

Mix

Flocculation

Sedimentation

Basin Filters UV

Disinfection

Clearwell

To

Distribution

System

HSPs

UV installations located downstream of the clearwell will experience greater fluctuations 

in flowrate since actual flowrates are more closely matched to system demand changes. This 

may increase the UV reactor size or more UV reactors to accommodate the flow fluctuations. 

In post-HSP installations, the water will be at distribution system pressure. The UV 

reactor housing may need to be reinforced because of these high pressures, which would increase 

the cost of the UV reactors. In addition, these locations are more prone to water hammer 

because of their proximity to the HSPs and subsequent high pressures, which could lead to sleeve 

damage. If a lamp sleeve is damaged, the enclosed lamp may break, releasing mercury into the 

water. Hydropneumatic tanks or pressure relief valves may be necessary in this installation 

location to avoid water hammer. This issue is discussed in more detail in section 3.1.6.1 and 

section N.2.1.3. 

Proposal Draft
3. Planning and Design Aspects for UV Installations 

UV Disinfection Guidance Manual 3-9 June 2003

A UV reactor located after the HSPs will reduce the discharge pressure to the distribution 

system

In summary, UV installations downstream of the clearwell are not recommended because 

of the i

3.1.3 efining Design Parameters

Water quality, lamp fouling/aging factor, flowrate, and power quality affect the sizing of 

the UV

V reactors are required to be validated by LT2ESWTR to demonstrate the UV 

installa blishes

ng

is

o the extent practical, UV reactors should be designed with process monitoring and 

control ion

d

limit 

The UV reactors are off-specification when any of the following conditions occur: 

The flow, UV intensity, or lamp status is outside of the validated range. 

The UVT or UV intensity is outside of the validated range (if the UV intensity and 

• The calculated dose is outside of the validated range at a given flow (if the calculated 

• All UV lamps in all UV reactors are off because of a power interruption or power 

, and a UV installation located between the clearwell and HSPs will reduce the suction 

head available for the pumps. As a result, discharge pressures and storage utilization could be

impacted at these two locations. 

ncreased potential for adverse pressure conditions within the UV reactor and the increased 

reliability and size considerations. In general, these installations should only be considered if the 

combined filter effluent and in-line filter effluent locations are not feasible.

D

reactors and associated support facilities. These design parameters need to be 

determined to ensure compliance with LT2ESWTR requirements. 

U

tion achieves the required UV dose (40 CFR 141.729(d)). Validation testing esta

the conditions under which the UV reactors must be operated to ensure the required dose 

delivery (40 CFR 141.729(d)). Off-specification is defined as a UV reactor that is operati

outside of its validated limits. (For example, the UV reactor is operating with a flowrate that

higher than the UV reactor was validated.) 

T

components (e.g., alarms, shut-off valves) to prevent water from entering the distribut

system when a UV reactor is operating outside of validated conditions. Unfiltered systems that 

use UV disinfection to meet the Cryptosporidium treatment requirement of the LT2ESWTR must 

demonstrate that at least 95 percent of the water delivered to the public during each month is 

treated by UV reactors operating within validated limits (40 CFR 141.721(c)(2)). Or in other

words, the UV reactor cannot be off-specification for more than 5 percent of the water delivere

to the public. The LT2ESWTR does not state an off-specification requirement for filtered 

systems; however, States may establish requirements for their filtered systems, including a

for off-specification operation. 

• 

• 

UVT setpoint approach is used (section 3.1.5)). 

dose approach is used (section 3.1.5)). 

quality problem, and water is flowing through the reactors. 

Proposal Draft
3. Planning and Design Aspects for UV Installations 

UV Disinfection Guidance Manual 3-10 June 2003

It is important to determine the appropriate design values for water quality, lamp

nts and 

ctors

3.1.3.1 Assessing Water Quality 

As highlighted in Chapter 2, the following water quality parameters are the primary 

parame

Parameters that affect UV dose delivery 

0 - 300 nm (germicidal range) 

• Parameters that typically determine sleeve and UV intensity sensor fouling 

emperature

It should be reiterated that this manual is focused on post-filtration applications; 

therefo s) and

y

Water quality data should be collected from locations that are representative of the 

potenti and

on

e

The four main considerations for assessing water quality are A254, fouling potential, lamp 

fouling

fouling/aging factor, flowrate, and power quality because of these LT2ESWTR requireme

recommendations. If the design parameters are not chosen conservatively enough, the UV 

reactors may be operating off-specification and be out of compliance. However, overly 

conservative design values may result in unnecessarily large UV reactors or more UV rea

than necessary. 

ters that affect UV installation planning and design: 

• 

- UV absorbance/transmittance from 20

- Upstream chemical additives 

- Calcium

- Alkalinity

- Hardness 

- Iron 

- pH 

- Lamp t

re, it is assumed that turbidity is low (1 nephelometric turbidity units (NTU) or les

results in insignificant particle effects on UV dose delivery (Linden et al. 2002b). It is also 

assumed that the water meets applicable maximum contaminant levels (MCLs) and secondar

MCLs. 

al UV installation location. The duration of sampling, number of samples collected,

data analyses used to evaluate water quality for UV disinfection are similar to the approaches 

taken for other water treatment technologies. The data collection frequency should be a based

flow variability, the consistency of the source and treated water qualities, and the potential for 

obtaining cost and energy savings by refining the design criteria. The extent of water quality 

data collected should be left to the discretion of the utility and the designer based on experienc

and professional judgment. States may desire to provide input on data collection needs. 

/ aging factor and upstream chemical impacts. Each of these is discussed in the following 

sections. 

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UV Absorbance

As discussed in Chapter 2, the A254

1

 of the water directly influences UV dose delivery. 

The A254 254 254

254

 data should be evaluated to select a design A value. The design A along with the 

specified UV dose and flowrate will be used by the UV manufacturer to determine the 

appropriate UV reactor. In addition, UV manufacturers may use the A range at the WTP to 

determine the turndown (i.e., power modulation) needs of the UV reactors. 

Overly conservative design A254 values (i.e., low UVT) can result in over-design and 

increased capital costs. Conversely, inappropriately low design A254 values can result in UV 

reactor operation outside the validated operating range and potential non-compliance. As with 

most designs, the larger the data set, the more refined the final design can be. A utility with very 

stable A254 might only need one or two months of data, while a utility that experiences seasonal 

changes would benefit from more frequent data collection during seasonal events and over a 

longer recording period. 

The A254 sampling plan should include collection of A254

2

 measurements in grab samples 

or continuously with an on-line A254 monitor. If A254 peaks occur regularly during the Spring 

and Fall, increased sampling frequency during these periods will better capture the magnitude 

and duration of the peaks. If different sources or combination of sources (i.e., blending) are used 

during the year, the A254 of the potential source water blends should be characterized to properly 

identify the appropriate water quality conditions. In addition, the maximum A254 may not 

correspond to the period of maximum water production. The relationship between seasonal 

production rates and A254 data should be considered when developing design criteria. 

A cumulative frequency (CF) diagram of the A254 data may assist the utility in 

determining its design A254 value. Figure 3.5 presents a CF diagram for three filtered waters; the 

CF percentile (x-axis) shows the percentage of the dataset that is lower than a given value of A254

over the data collection period. For example, if the 90th percentile A254 is 0.043 cm-1, then 90 

percent were lower and only 10 percent of the measurements were higher than 0.043 cm-1 over 

the period of record. 

 

1

 A254 in this section implies A254 measurement specifically at 254 nm unless otherwise noted 

2

 A254 measurements for developing the design basis for UV disinfection systems should be performed on unfiltered 

samples, not with the 0.45 μm pre-filtered samples typically used to characterize NOM. However, if only 

measurements that have been filtered are available, they still provide valuable information. It should be noted that 

pre-filtered measurements are typically biased low (in terms of absorbance), but this bias is generally minimal. 

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Figure 3.5 Example CF Diagram for Three Filtered Waters 

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Cumulative Frequency Percentile

UV Absorbance at 254 nm (cm-1)

Filtered Water 1 Data Collected from May 1991 to July 2001

Filtered Water 2 Data Collected from January 1991 to March 1998

Filtered Water 3 Data Collected from February 1993 to July 2001

In Figure 3.5, the A254 data for Filtered Waters 1, 2, and 3 display different 

characteristics. The A254 values for Filtered Water 1 are relatively constant between the 5th and 

85th percentiles, indicating consistent water quality approximately 80 percent of the time. Values 

above the 85th percentile increase to a plateau, and then increase again above the 95th percentile. 

A254 data for Filtered Waters 2 and 3 exhibit greater variability by the gradually increasing slope 

between the 5th and 95th percentile. Selecting an appropriate design A254 value for these waters 

depends on an assessment of this variability as compared to the percentage of time that offspecification water could be delivered. 

For example, a CF percentile of 95 percent would most likely meet the off-specification 

criteria for unfiltered systems. However, a 95 percent CF percentile may be overly conservative, 

depending on the flow observed at the planned UV installation. Therefore, plotting the A254 with 

the WTP flow can indicate if high A254 and high flow co-occur, which would be the worse case 

water quality condition. Figure 3.6 presents Filtered Water 3’s flow and A254 variation and 

illustrates a seasonal variation in A254. For this example WTP, the high A254 typically occurs 

during the high flow period; therefore, a more conservative design A254 of 0.077 cm

-1

(84% UVT), which is a CF percentile of 95 percent, may be warranted for Filtered Water 3. 

However, a less conservative design A254 would be appropriate if the high A254 occurred during 

flows less than the design flow (e.g., average flow) because the UV reactors should have enough 

turndown (e.g., power modulation) to accommodate high A254 at lower flows than the design 

flow. 

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Figure 3.6 Example Flow and UV Absorbance (at 254 nm) Data

20

60

100

140

180

220

Jan-97

Apr-97

Jul-97

Oct-97

Jan-98

Apr-98

Jul-98

Oct-98

Jan-99

Apr-99

Jul-99

Oct-99

Jan-00

Apr-00

Jul-00

Oct-00

Jan-01

Apr-01

Jul-01

Plant Flo

w (mgd) 

0.00

0.04

0.08

0.12

0.16

0.20

UV Absorb

a

nce at 254 n

m (cm-1)

Flow

UV Abs at 254 nm

Filtered Water 3

WTP Capacity: 220 mgd 

The design A254 (e.g., a CF percentile) also should be a function of the utility’s preferred 

level of conservatism and the site-specific A254and flow data. The UV reactor sizing and cost are 

not directly proportional to A254 but will increase for increased A254 design values. However, by 

evaluating the CF plot and collaborating with the UV manufacturer to assess the cost 

implications of using a lower A254 value, the utility and designer can select the most appropriate 

design A254 for the water quality and disinfection objectives of the project. 

Typically, the UV manufacturers work in terms of UVT; therefore, the design A254 is 

typically converted to a design UVT3

. Because UV manufacturers use UVT in their design and 

control of the UV reactors, the remainder of this chapter will use UVT as opposed to A254.

The spectral absorbance of the water over a range of wavelengths (200 - 400 nm) should 

also be collected, especially if medium pressure (MP) reactors are being considered. MP lamps 

emit light at a range of wavelengths across the 200 nm to 300 nm range. The UV absorbance of 

water varies with wavelength, typically decreasing with increasing wavelength. As such, the 

attenuation of UV light in a UV reactor, the corresponding disinfection performance, and the UV 

intensity sensor response depend on the absorbance at each of the emitted wavelengths. Sitespecific spectral absorbance can be used to model MP reactors and may be incorporated into UV 

dose monitoring and control systems by some UV manufacturers. Spectral absorbance may 

 

3 254 (%) 100 10 A UVT − = ∗

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exhibit seasonal variation; therefore, spectral absorbance should be collected at different times 

during the year to assess this variation. Also, the spectral absorbance may be used to determine 

the appropriate UV-absorbing chemical for validation of the UV reactors that will be installed, 

which is discussed in section 4.3.3.2. 

Fouling Potential

The rate of fouling and the corresponding frequency of sleeve cleaning depend on 

hardness, alkalinity, lamp temperature, pH, and certain inorganic constituents (e.g., iron and 

calcium). Fouling is typically caused by precipitation of compounds with low solubility or 

compounds where the solubility decreases as temperature increases (e.g., CaCO3). If significant 

seasonal shifts in any of the parameters are expected, these trends should be captured in the 

monitoring period. Again, a CF diagram may assist in the selection of the appropriate design 

criteria. 

While the specific rate of fouling and optimal cleaning protocol for any given application 

cannot currently be predicted, a proper cleaning protocol and sleeve-fouling factor can be 

adequately estimated for most water sources without pilot- or demonstration-scale testing and 

then adjusted during normal operation. Extensive data have been generated from pilot-scale 

testing on waters of low to moderate hardness and iron content (Mackey et al. 2001 and Mackey 

and Cushing 2003). At total and calcium hardness levels below 140 mg/L and low iron (less 

than 0.1 mg/L), standard cleaning protocols and wiper frequencies (one sweep every 15 minutes 

to an hour) were more than adequate to address the impact of sleeve fouling at the sites tested. 

At sites with hardness or iron that exceed these levels, it may be advantageous to evaluate 

fouling rates on a site-specific or worst case basis via pilot or demonstration testing (described in 

Appendix J) or during UV reactor start-up (section 5.1) to identify how best to address fouling. 

Although fouling is not expected to be a significant problem for most utilities, the listed 

water quality parameters (page 3-10) should be monitored prior to designing the UV installation 

unless adequate water quality data are available. It is important to provide these data to the UV 

manufacturer to assist them in a qualitative assessment of the fouling potential for their UV 

reactors and to assist the designer in determining what cleaning system should be specified. In 

addition, the lamp fouling/aging factor will depend on the initial assessment of potential fouling, 

which is discussed in the next section. 

Lamp Fouling/Aging Factor

Sleeve fouling, lamp aging, and UV intensity sensor window fouling (if applicable) affect 

long-term UV reactor performance. Accumulation of foulants on the lamp sleeve surface can 

reduce transmittance of the lamp energy to the water. The rate of fouling depends on the factors 

discussed in the previous section. In addition, lamp output decreases over time due to its 

physical aging. The rate at which lamp output will decrease is a function of the lamp physical 

characteristics, lamp hours in operation, number of on/off cycles, and power applied per lamp

length. In MP reactors, UV lamp aging can also result in a change in the spectral output over 

time. Lamp aging is discussed in detail in section A.3.1.6. 

A reduction in lamp output results in a reduction in UV dose. The effects of these 

parameters are typically incorporated into the UV reactor design by specifying a lamp

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fouling/aging factor, which includes the effects of both sleeve fouling and lamp aging. The lamp

fouling/aging factor will be site-specific and based on the assessment of fouling described 

previously and lamp aging information. The lamp aging characteristics can be obtained from the 

UV manufacturer and should be certified by an independent third party. The lamp fouling/aging 

factor is used by the manufacturer to assist in the selection of the appropriate UV reactor. For 

example, if a 0.5 lamp fouling/aging factor is specified, the UV manufacturer will choose a UV 

reactor the appropriate lamps (or number of lamps) where the specified UV dose can be achieved 

at half of the initial UV lamp output (after burn-in) with all the lamps energized at full power. 

The lamp fouling/aging factor typically ranges from 0.5 (NWRI 2000) to 0.9. 

The lamp fouling/aging factor is typically specified with a corresponding guaranteed UV 

lamp life (e.g., 5000 hours). These items are typically specified together to ensure that the UV 

lamp replacement frequency does not occur more frequently than specified by the guaranteed 

lamp life given the specified lamp fouling/aging factor. The lamp fouling/aging factor can be 

estimated based on the designer’s experience and UV manufacturer input. In addition, pilot and 

demonstration tests can be completed to estimate the lamp fouling/aging factor as described in 

Appendix J. 

Selection of a lamp fouling/aging factor and a guaranteed lamp life value is a trade-off 

between maintenance costs (the frequency of lamp replacement or chemical cleans necessary) 

and capital costs (the size of the UV reactors). A lower lamp fouling/aging factor means the 

utility will have less frequent lamp replacements because the UV reactors are designed with 

higher powered lamps or more lamps to achieve the necessary UV output at the guaranteed lamp

life. However, designing a UV reactor with higher powered lamps or more lamps will increase 

the size of the needed UV reactor. Thus, the use of a fouling/aging factor that is too conservative 

could result in the over-design of the UV reactors. Conversely, the use of a lamp fouling/aging 

factor that is not conservative enough may result in the underestimated reduction in the output of 

the lamp due to fouling/aging and potentially result in off-specification operation or more 

frequent lamp replacement. 

Impacts of Upstream Treatment Processes

Unit processes upstream of UV reactors can have a significant impact on the UV reactor 

performance. The three potential ways that upstream processes may affect UV performance are 

(1) to increase UVT by increasing organics removal or oxidizing organics, (2) to decrease UVT 

because certain chemicals will absorb UV light, and (3) to affect the lamp sleeve fouling rate. 

It is possible to increase filtered water UVT by increasing the coagulant dose; however, 

the results will be site-specific. In one study, the UVT was increased from 80% to 89% by 

increasing the alum dose from 15 to 45 mg/L (Cushing et al 2001). However, the UVT increase 

from an increased alum dose should be considered against the increased alum chemical costs and 

sludge production. UVT increases would also probably be observed if other iron coagulant and 

poly-aluminum chloride coagulant doses were increased. 

Properly implemented, ozone disinfection prior to UV disinfection has the potential to 

increase the UVT from oxidation of organic matter. Conversely, ozone disinfection can decrease 

UVT if a residual ozone concentration is present in the UV reactors. If the ozone residual is 

adequately quenched, a net increase in the UVT will be observed (Malley 2002); an example of 

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this increase for an unfiltered water is shown in Figure 3.7. If a UVT increase is desired, then a 

combination of coagulant increase and ozone disinfection will likely give the greatest UVT 

increase (Cushing et al 2001). 

Figure 3.7 Example Effect of Pre-ozonation on UV Absorbance 

if Ozone is Quenched Prior to UV Disinfection

W a v e leng th ( n m )

2 0 0 2 2 0 240 2 6 0 280 3 0 0 3 2 0

UV Absorbance (cm-1) 0 .0

0 .1

0 .2

0 .3

0 .4

0 .5

P r e o z o nated U V In fluen t

( no d e te c table oz o n e r e s idual)

U V In fluen t - N o o z o ne 

Most common water treatment chemicals themselves will not significantly impact UVT. 

The following common water treatment chemicals do not significantly affect UVT at typical 

concentrations present in filtered water: Alum, aluminum, ammonia, ammonium, zinc, 

phosphate, calcium, hydroxide, and ferrous iron (Fe+3) (Cushing et al 2001). 

However, hypochlorite (ClO-

), ferric iron (Fe+2), permanganate, and ozone were the only 

commonly used chemicals examined that might reduce UVT (Cushing et al 2001) as described 

below. 

 

• Residual ClO- has only a slight effect on UVT. For example, a ClO- residual of 3.5 

mg/L will cause the UVT to decrease from 91% to 90% (Cushing et al 2001). 

However, in most cases, a hypochlorite residual that high will not be flowing through 

the UV reactor. 

• It is unlikely that ferric iron will be present in filtered waters because ferric iron is 

only present when there is low dissolved oxygen. 

• Permanganate is a strong absorber of UV light; however, it is typically added in the 

raw water to oxidize taste and odor or iron and manganese. Therefore, when applied 

to raw water, there should not be a significant permanganate concentration in the 

filtered water. 

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• Ozone residual can be quenched, and then the UVT will not be decreased. Care 

should be taken when choosing the quenching agent because one popular choice, 

thiosulfate (often used in the form of calcium thiosulfate), is a strong absorber of UV 

light (section A.4.1.3, Table A.5) and will decrease the UVT. Sodium bisulfite, an 

alternative to calcium thiosulfate, will not significantly impact UVT. 

The possible UVT variation from upstream processes should be assessed by collecting 

UVT data during various operating conditions (e.g., a range of alum doses) that are typically 

observed. Potential treatment process upsets should also be considered in the water quality 

analysis to determine the extent to which they impact the design UVT and cleaning regime. 

Some unit processes that use metal-based coagulants may affect the rate of fouling; these 

effects will be site-specific. Mackey et al. (2001) found that iron levels less that 0.1 mg/L could 

be adequately cleaned by standard protocols as described previously. In addition, lime softening 

has been shown to reduce fouling potential (Mackey et al. 2001). Overall, the effect of upstream

coagulant addition and residual metals should be considered in the fouling data monitoring 

described previously. 

3.1.3.2 Determining Design Flowrate 

Flowrate is a fundamental design parameter that, in combination with water quality, UV

dose, and lamp fouling/aging factor determines the necessary size and number of UV reactors. 

The design criteria should identify the average, maximum, and minimum flowrates that the UV 

reactors will experience. Potential methods for determining the design flow for the three 

described retrofit locations are shown in Table 3.1. In addition, potential future changes in plant 

capacity should be considered when determining the UV installation design flow. 

Table 3.1 Potential Method to Determine Design Flow

Retrofit Location Design Flow Basis 

Combined filter retrofit Combined rated capacity of all duty filters1

Individual filter retrofit Rated design flow for individual filter 

Post-HSP Rated capacity of the HSP station 1

Flow does not include redundant filters 

3.1.3.3 Assessing Electrical Power 

The sensitivity of UV reactors to power fluctuations make electrical power supply a 

critical component of the UV installation planning and design. In addition, the electrical system

design needs to ensure that the UV installation will meet the requirements or recommendation of

operating within validated conditions (i.e., maximum allowed off-specification). Also, it is 

impossible to meet inactivation goals if the power quality causes the reactor to go down (i.e., no 

disinfection) for longer than the need to obtain the desired treatment level. For example, if a 2-

log Cryptosporidium inactivation is desired, the UV reactors cannot be down while more than 1 

percent of the flow passes through them. 

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UV lamps can potentially their lose arc if a voltage fluctuation, power quality anomaly, 

or a power interruption occurs. For example, voltage sags that vary from 10 to 15 percent from

normal operating conditions for as low as 2 to 5 cycles (0.03 to 0.08 seconds) may cause UV 

lamps to lose their arc. 

Low pressure (LP) lamps generally can return to full operating status within 15 seconds 

after power is restored. However, low-pressure high output (LPHO) and MP reactors that are 

more typically used in drinking water applications exhibit significant restart times if power is 

interrupted. The start-up and restart behavior for LPHO and MP lamps is summarized in Table 

3.2. 

Table 3.2 Start and Restart Times for LPHO and MP Lamps 1

Lamp Type Cold Start

2 Warm Start3

LPHO 

2 min warm-up 

+ 

4-5 min to full power

total time: 6 – 7 minutes 

2 min warm-up 

+ 

2-5 min to full power

total time: 4 – 7 minutes 

MP 

No warm-up or cool down

+ 

5 min to full power

4

total time: 5 minutes

5 min cool down 

+ 

5 min to full power

4

total time: 10 minutes 1 Information shown in table is compiled from Calgon Carbon, Severn Trent, Trojan, and Wedeco. 2

 A cold start occurs when UV lamps are started when they have not been operating for a significant 

period of time. 3

 A warm start occurs when UV lamps are started after they have just lost their arc (e.g., due to voltage 

sag). 4

 60 percent intensity is obtained after 3 minutes. 

The effects of temperature can increase or decrease the times listed in Table 3.2 and 

should be discussed with the UV manufacturer. Individual manufacturers report that colder 

water temperatures (below 50 degrees Fahrenheit, 10 degrees Centigrade) can result in slower 

startups for LPHO lamps than listed in Table 3.2. Conversely, MP manufacturers report shorter 

re-start times with colder temperatures because the cold water accelerates the condensation of 

mercury (i.e., cool down), which is necessary for re-striking the arc. 

To minimize the potential for off-specification operation, utilities should evaluate the 

reliability and quality of their power supply. Local power suppliers can often provide power 

quality and reliability data and should be the first source of information on power quality. For 

those locations where power quality is unknown, a power quality assessment is recommended. 

An assessment may be as simple as reviewing operating records of power quality incidents (if 

available) and power interruptions or Supervisory Control and Data Acquisition (SCADA) 

information for the existing plant. More advanced assessments may include the installation of 

power quality monitors or the retention of an outside consultant to conduct a detailed power 

quality assessment. Generally, personnel with a working knowledge of electrical supply and 

installation will be able to review power supply data and determine if power quality problems 

exist. If a problem is identified, however, tracing it back to its source and determining an 

appropriate remedy is often best left to an expert that specializes in this area. 

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The most common sources of power quality problems are as follows: 

• Faulty wiring and grounding 

• Off-site accidents (e.g., transformer damaged by a car accident) 

• Weather-related damage 

• Animal-related damage 

• Facility and equipment modifications 

• Power transfer to emergency generator or alternate feeders 

In specific locations that are subject to frequent power fluctuations or outages, the 

following options should be considered to minimize off-specification operation and ensure 

regulatory compliance: 

1. Installation of a backup generator 

2. Connection to a second, independent power source 

3. Installation of power conditioning equipment or a battery-supported uninterruptible 

power supply (UPS) 

These options will have different response and backup periods associated with them. For 

example, a backup generator cost-effectively provides backup power if an extended power 

interruption occurs; however, it will not ensure a continuous power supply to avoid UV reactor 

shutdown due to voltage sags. Connection to a second, independent power source may have the 

same issues as the backup generator, depending on the power quality associated with the second 

power source. 

Power conditioning equipment will provide high quality power even if voltage sags or 

other power quality problems occur. However, power-conditioning equipment does not provide 

backup power for extended power outages. A battery-supported UPS provides continuous, high 

quality power (i.e., prevent voltage sags) and a specific amount of backup power for a longer 

outage. UPS systems can provide as much battery backup as specified; however, typically UPS 

systems for this purpose range between 2 and 15 minutes of battery backup. 

The most suitable option will depend on the power quality of the utility, requirements

limiting off-specification operation, and preferences of the utility and State. For example, an 

unfiltered system with poor power quality that experiences multiple voltage sags everyday and 

periodic interruptions lasting over 3 minutes may consider installing a UPS system with 5 

minutes of backup batteries to ensure the 95 percent requirement of operating within validated 

ranges is met (40 CFR 141.721(c)(2)). However, a filtered system that experiences two or three 

voltage sags a month and no long-term power interruptions may not need to provide any 

additional power or power conditioning equipment. 

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Any equipment needed to address power quality problems affects both the cost and the 

feasibility of implementing UV disinfection. For example, the UV reactor cost and installation 

footprint has been estimated to increase by approximately 25 percent if a UPS system with 

5 minutes of backup capacity is installed (Cotton et al. 2002). Other power conditioning options 

without backup power are less expensive and have lower footprint needs. 

It is important that a utility have a complete WTP-wide assessment of its power quality 

when considering UV disinfection. Any actions involving the electrical system may also affect 

the WTP power quality and equipment performance. For example, the impact of the WTP’s 

maintenance program for backup generators (e.g., routine startup and exercise) should be 

considered during the planning and design of the UV reactors to ensure that the program 

supports compliance goals and does not cause excessive UV reactor shutdown times. Other 

items that may affect power quality include future integration or upgrade of equipment 

(particularly equipment with a large power demand or variable frequency operation), testing of 

backup power supplies, deterioration of existing facility wiring (resulting in poorly grounded 

circuits), overload of electrical circuits, and any other activity that may affect the electrical 

supply or distribution within the facility. 

3.1.4 Evaluating Potential UV Reactors 

It is important to evaluate the available UV reactors in the planning process because each 

manufacturer’s UV reactors are unique and proprietary. Process footprints and related 

installation needs (e.g., UV reactor to control panel distances) are different, depending on the UV 

manufacturer. This section provides a brief overview of different UV reactors, their impact on 

space requirements, and UV reactor validation issues. More detailed UV reactor information is 

presented in section 2.4. In addition, UV manufacturers should be contacted directly to gain a 

better understanding of the UV reactors available and what UV reactors are applicable to the 

utility’s installation locations given the design criteria developed in section 3.1.3. 

3.1.4.1 UV Reactors 

There are different types of UV reactors for disinfecting drinking water with unique 

characteristics, such as lamps, lamp configuration in the reactor, cleaning systems, ballasts, and 

control systems (section 2.4.). This section briefly highlights the differences in UV reactors that 

affect design of the UV installation.

UV reactors can generally be characterized based on lamp type with LPHO and MP 

lamps being the most applicable to WTPs. One of the fundamental differences between LPHO 

and MP reactors is the lamp intensity output, which influences the UV reactor configuration and 

size, lamp life, number of lamps, electrical needs, and ballasts. Each has its inherent advantages 

and disadvantages. While a competitive procurement can be made among these two reactor 

types when the construction contract is bid, the overall layout and supporting facilities will be 

different for each. 

The UV reactor footprint depends on the UV reactor configuration and UV lamp type. 

There are several different UV reactor configurations. Typically, LPHO reactors are in-line (i.e., 

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configured like a pipe). However, MP reactors can be in-line, S-shaped, or U-shaped, depending 

on the UV manufacturer and the site constraints of the specific installation location. Typically, 

LPHO reactors have a larger footprint than MP reactors because more UV lamps are needed to 

deliver the same UV dose. MP reactor footprints will also vary, depending on lamp orientation 

(e.g., parallel versus perpendicular to flow). When evaluating locations for installation, the 

largest UV reactor footprint of those being considered should be used to estimate the UV 

installation footprint. 

Lamp life also varies between LPHO and MP reactors. Most manufacturers provide 

warrantees of 8,000 to 12,000 hours for LPHO lamps. Guaranteed life for MP lamps range from

4,000 to 8,000 hours. Although the lamp life for LPHO is greater than that for MP reactors, due 

to the need for a greater number of lamps, the actual number of lamps that are replaced during a 

given period may be less for a MP reactor. It is important to consider the labor associated with 

lamp replacement, as well as the actual unit cost of the replacement lamps, when estimating the 

operating and maintenance costs of the two technologies. In addition, while LPHO reactors 

typically have more lamps, the actual power input is less than that for similarly sized MP 

reactors because MP lamps are less efficient in converting the power input to germicidal 

wavelengths for disinfection. This may result in a higher input power and an increase in the 

overall power consumption for MP reactors compared to LPHO reactors.

The lamp sleeve cleaning systems can also be different between LPHO and MP reactors. 

LPHO reactors may have off-line chemical cleaning (OCC) systems instead of on-line 

mechanical cleaning (OMC) because of the larger number of lamps. With OCC systems, the UV 

reactors must be taken off line to be cleaned. OMC and OCC systems are described in 

section 2.4.5. This may result in higher maintenance costs for LPHO reactors, depending on the 

extent to which cleaning is necessary.

Finally, the type of ballast used will affect the UV installation layout. Ballasts regulate 

the power supply at the appropriate level needed for energizing and driving the UV lamps. UV 

reactors may use electronic ballasts, electromagnetic ballasts, or transformers. Transformers are 

typically more stable than electronic or electromagnetic ballasts and allow a greater separation 

distance between the UV reactor and control panel. However, most transformers allow only step

adjustment of lamp intensity. Compared to transformers, ballasts have the capability to provide 

almost continuous intensity adjustment but may increase lamp aging and spectral shift and have 

lower allowable separation distances between the UV reactor and control panel. It is important 

to discuss the implications of these various components with the UV manufacturers to determine 

their effect on the UV installation layout and design. Specific items that should be discussed 

include ballast cooling needs, allowable separation distances, and intensity adjustment 

capabilities. 

The differences described above imply that UV reactor evaluation should not be based 

solely on capital costs. Operation and maintenance costs, including energy usage and labor, will 

be important in an overall life cycle cost comparison. This is discussed in greater detail in 

section 3.1.7. 

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3.1.4.2 UV Reactor Control Strategies 

There are currently three different control strategies for UV reactors, which affect how 

UV reactors are validated and operated. The three general control strategies relate to three 

methods for monitoring dose-delivery and are summarized in Table 3.3. The first strategy 

utilizes one or more UV intensity sensors located at a distance from the lamps that yields an 

intensity signal that is proportional to UV dose (UV intensity setpoint approach), and the 

intensity sensor measurement and flowrate are used to monitor dose delivery. The second and 

third methods utilize UV intensity sensors that are positioned close to the lamps (so that there is 

minimal absorbance by the water) and separate monitors for UVT. The second approach 

incorporates a validated setpoint value for UVT, in addition to setpoints for UV intensity and 

flowrate, to ensure a given dose (UV intensity and UVT setpoint approach). In the third 

approach, the UV dose is calculated based on these measurements of flowrate, UV intensity, and 

UVT via a validated computational algorithm developed by the manufacturer (calculated dose 

approach). 

Table 3.3 UV Reactor Control Strategies

Control Strategy Dose Delivery Monitoring and Control Basis 

UV Intensity Setpoint UV intensity sensor measurement 

UVT and UV Intensity Setpoint UV intensity sensor and UVT measurement 

Calculated Dose The calculated UV dose1

1 The UV reactor calculates a UV dose, using the UV intensity sensor measurement, the 

UVT of the water, and the flowrate.

In the planning phase, these control strategies need to be evaluated by the designer and 

utility to determine if a particular control strategy is preferable based on the ease of integration 

into their existing operation and control system. The impacts of the control strategy on the 

instrumentation and controls are discussed in section 3.3.2, and the specific validation 

recommendations for each control strategy are presented in section C.4.9. 

3.1.4.3 Equipment Validation Issues 

The LT2ESWTR requires that UV reactors be validated (40 CFR 141.729(d)). A utility’s 

approach to UV reactor validation will affect the UV installation design. The issues to consider 

are the hydraulic parameters for validation and whether equipment will be validated on-site or 

off-site.

This section describes how these issues affect the design and installation footprint 

estimation. Chapter 4 provides an overview of validation, and Appendix C details UV reactor 

validation guidelines in detail. 

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Validation Hydraulics

The inlet and outlet hydraulics of the UV reactor can significantly affect dose delivery; 

therefore, the following validation and corresponding installation strategies are recommended in 

the validation protocol (section C.3.1.5) and are presented in Table 3.4. 

Table 3.4 Summary of Recommended Hydraulic 

Configurations for Validation and Installation 

Option Validation UV Installation

1 

The inlet and outlet configuration is the 

same as the installation for 10 diameters 

upstream and 5 diameters downstream of 

the UV reactor. 

Inlet and outlet configuration is the same as 

when the UV reactor was validated for 10

diameters upstream and 5 diameters 

downstream of the UV reactor. 

2 

The UV reactor is validated with a 90-

degree bend directly upstream of the UV 

reactor. The UV reactor is defined to 

include a specific amount of straight pipe

upstream or downstream of the UV reactor 

as specified by the UV manufacturer. 

The UV reactor should be installed with a 

minimum of 5 pipe diameters of straight piping 

between the UV reactor and any upstream 

hydraulic configuration.1

 

3 

The velocity at the validation facility is 

measured at evenly spaced points through 

a given cross section of the flow upstream 

and downstream of the UV reactor. 

The velocity at the installation is measured at 

evenly spaced points through a given cross 

section of the flow upstream and downstream 

and is within 20 percent of the theoretical

velocity determined during validation. 1

 This approach is not acceptable if the upstream fitting is an expansion or if the upstream valve will be used for flow

control. A valve that will be exclusively used for open/close service (e.g., isolation) is acceptable. 

Option 1 is most applicable when unique piping configurations are needed or if the inlet 

and outlet conditions validated in Option 1 cannot be achieved because of site constraints. For 

example, Option 1 may be the only validation option for an individual filter effluent location, 

which probably does not have 5 diameters of straight pipe before the UV reactors (Option 2) 

because of existing site constraints. 

The validation and installation of a particular UV reactor should meet one of these 

options. Option 2 provides more general applicability for validation and installation of UV 

reactors. For example, the inlet and outlet piping configuration for installations in a new 

building could be designed based on how the procured UV reactor was validated. Option 3 also 

provides flexibility but may have the practical limitation of measuring the velocity through a 

cross section at the installation. 

Off-site Versus On-site Validation 

Manufacturers will likely validate UV reactors over a wide range of flowrates and water 

quality (e.g., UVT) conditions at off-site testing facilities. The inlet and outlet hydraulic 

conditions during validation will probably be selected so the UV reactors can be installed in most 

WTPs. Off-site validation has several advantages, including simplicity, cost, and the ability to 

design around a UV reactor with known performance characteristics and inlet and outlet 

hydraulics. However, the LT2ESWTR requires that the site-specific installation and operating 

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conditions must fall within the range of conditions used when the installed UV reactor was 

validated off-site (40 CFR 141.729(d)). If the validation conditions do not encompass the 

utility’s design criteria or inlet and outlet piping configurations, the utility may request that the 

UV manufacturer re-validate the unit off-site under specific testing conditions that closely match 

those of the proposed installation. Alternatively, on-site validation can be performed. 

The advantage of on-site validation are that the UV reactors can be validated under the 

exact piping hydraulic conditions at which it will operate, and the UVT will more accurately 

represent the UV installation even if a UV-absorbing chemical is added. In addition, the 

equipment necessary for on-site validation will also provide the flexibility for future testing to 

optimize the UV reactor performance under specific hydraulic and water quality conditions even 

if they are not completed for the initial validation. However, a disadvantage of on-site validation 

is that the UV installation is designed and constructed without prior validation of the 

performance of the UV reactors. This may lead to the UV installation failing to meet 

performance requirements, and it may be difficult to increase UV disinfection efficiency after the 

UV reactors are already installed. In addition, on-site validation is limited to the highest UVT 

available at the time of testing. Consequently, UV reactor performance characteristics cannot be 

determined at higher UVT, and the UV reactors may need to be operated at conditions other than 

optimal, resulting in higher power use and faster lamp and ballast replacement frequencies. 

Other disadvantages include the logistics and cost of the testing. For example, one unit must be 

isolated from the system to allow validation testing to occur, and a permit may be needed to 

discharge the non-pathogenic challenge microorganism. 

If on-site validation is desired, then the UV installation design should be adapted to 

enable testing. The UV reactor design would need to incorporate feed and sample ports, static 

mixers, space for tanks near the UV installation (for the addition of the challenge microorganism

and UV absorbing chemical), and adequate facilities for laboratory testing, and discharge of the 

treated water. 

3.1.5 Evaluating Operational Strategies 

The operational strategy is defined in this manual as the method in which the utility 

chooses to operate the UV reactors given the UV reactor’s control strategy and validation data. 

It is important for the utility to understand the control strategies unique to various UV reactors 

(section 3.1.4.2) and select equipment consistent with their operating philosophy and energy 

efficiency objectives. The control strategy is defined as the method that the UV reactor uses to 

monitor and control the UV lamp power based on flow and UVT to deliver the specified UV 

dose. For each UV reactor, the operating conditions must be defined based on validation testing 

results (40 CFR 141, Subpart W, Appendix D), and the validation data will vary with different 

control strategies. The validation data can be utilized in different ways that facilitate a simple or 

complex operating strategy; three potential approaches are described in Table 3.5. Detailed 

examples of how to determine the operational parameters for these operational strategies are 

described in section 5.5. 

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Table 3.5 Potential Operational Strategies 

Operational 

Strategy

Description Advantages Disadvantages

Single 

Operation 

Setpoint 

One setpoint is used for 

all flows and UVT values

that were validated 

Simplest operational 

strategy 

Not as energy efficient 

because the UV reactor is

over-dosing at low flows 

Variable 

Setpoint 

Operation 

A setpoint would be used 

for a given flowrate and 

UVT range using a 

lookup table 

Increased energy efficiency 

over the single setpoint 

approach 

More complex operation 

compared to single setpoint

approach and may 

necessitate more advanced 

controls for the UV reactor 

Setpoint 

Interpolation 

The setpoints are 

calculated as a function 

of flowrate, typically 

automatically using the 

UV reactor controls1

The most energy efficient 

operation and may reduce 

operational hours needed if

operated automatically 

Potentially more validation 

data is needed (which may 

increase validation costs)

and necessitates advanced 

reactor controls 

1

 Only an option for UV intensity setpoint and calculated dose setpoint approach because the UV intensity 

and UVT setpoint approach is controlled as function of flowrate and UVT (as opposed to only flowrate) 

3.1.6 Evaluating Hydraulics and Process Footprint 

The potential locations for UV disinfection identified in section 3.1.2 can be evaluated 

based on an understanding of the candidate UV reactors, the hydraulics, and the estimated 

process footprint. This section discusses the principle criteria that affect the feasibility of a UV 

installation location – (1) hydraulic needs and limitations and (2) space availability and site 

constraints. 

3.1.6.1 Hydraulic Considerations

When selecting the appropriate location for UV reactors, the hydraulic needs should be

addressed. Headloss through a UV installation is dependent on the specific UV reactor and 

flowrate and generally varies from 0.5 to 3 feet. Characteristic headloss data should be obtained 

from the UV manufacturer(s) for all candidate UV reactors. In addition to the headloss 

associated with the UV reactor itself, the headloss associated with piping, valves, flow meters, 

and flow distribution devices should be considered when assessing the feasibility and location of 

the installation. The overall headloss of a UV installation is typically between 1 and 8 feet. 

If the headloss through the UV installation is greater than the available head, 

modifications to the plant design and/or operation may be necessary. Some potential 

modifications, alone or in combination, that may be considered to address hydraulic limitations 

are listed below followed by details about each: 

• Eliminating existing hydraulic inefficiencies within the facility to improve head 

conditions (e.g., replace undersized or deteriorated piping and valves) 

• Modifying the operation of the clearwell to accommodate the UV installation 

• Modifying the operation of the filters to accommodate the UV installation

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• Installing booster pumps 

• Modifying the UV reactor design (through the UV manufacturer) to reduce headloss. 

If the UV reactor design is modified, it must be validated in its modified condition to 

ensure it meets performance requirements 

Eliminating Existing Hydraulic Inefficiencies

Replacing undersized piping and valves with larger diameter piping and valves may 

increase the available head for the proposed UV installation. Older piping can also produce 

excessive headloss if the inner pipe surface is pitted or scaled or if the original pipe material has 

a high coefficient of friction. Slip-lining the interior of existing pipe with a lower coefficient of 

friction pipe material (e.g., high density polyethylene) is one method of reducing friction losses. 

Re-lining the existing pipe interior with a smooth coating will also reduce headloss. 

Modifying Clearwell Operation

A utility may increase head available to a UV installation by lowering the surface water 

level of the clearwell. However, this strategy decreases the storage volume available to meet 

peak demands. In addition, a lower clearwell level will reduce the contact time available in the 

clearwell for chemical disinfectants and may affect pump discharge head. It is important to 

evaluate any potential reduction in disinfection credit if contact time in the clearwell is used for 

calculating CT. The UV installation, though, may reduce the Giardia CT requirements 

sufficiently to offset the reduction in contact time . 

Modifying Filter Operation

A treatment facility may alter the operation of its filters to increase the head available for 

the UV installation. However, this may reduce filter run times, unit filter run volumes, and result 

in more frequent backwashing. If conditions upstream of the filters are such that additional 

freeboard and hydraulic head are available, a second option is to increase the water surface 

elevation over the filters to help minimize the reduction in head available for filtration. 

Installing Booster Pumps

When modifications to the existing facility or operations will not provide adequate head 

for the UV reactors, booster pumps can be installed. Booster pumping provides additional 

flexibility in the location of the UV reactors. The installation of booster pumps will increase

facility operation and maintenance cost and space requirements. The reliability of the pumps 

should also be considered in the evaluation because the pumps become a critical operating 

component. Additional detail on booster pump design is provided in section 3.3.1.6. 

Modifying UV reactors

Modifying a UV reactor to reduce headloss (e.g., removing baffles) can affect 

disinfection performance and should only be considered in careful collaboration with the UV 

manufacturer. Any resulting gains in system head must be weighed against diminished 

disinfection efficiency, which could result in more UV reactors being needed to accommodate 

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the flow and provide the necessary UV dose. Any modified UV reactors will also need to be 

validated in its modified condition. 

Other Options to Address Hydraulic Constraints

If none of the above options are feasible, the utility could consider installing the UV 

reactors upstream or downstream of the HSPs. If a location adjacent to the HSPs is selected, the 

potential for damage from pressure surges is increased and a surge analysis should be completed. 

Most lamp sleeves are designed to withstand continuous positive pressures of at least 120 pounds 

per square inch gauge (psig) (Roberts 2000; Aquafine 2001; Dinkloh 2001). However, lamp

sleeves are vulnerable to negative gauge pressure transients associated with water hammer. The 

tolerance level of the sleeve depends on the quality of the quartz and the thickness and length of 

the sleeve. However, pressures of negative 1.5 psig have been shown to negatively affect sleeve 

integrity (Dinkloh 2001). Hydropneumatic tanks, surge relief valves, air release valves, or air 

vacuum valves on pumps or at different locations along the pipeline can be used to help control 

surge conditions. 

3.1.6.2 Process Footprint 

The process footprint should be estimated in the planning phase to determine potential 

UV installation locations. One critical component needed to estimate the UV installation 

footprint is the number, capacity, and configuration of the UV reactors. The number of UV 

reactors depends on the redundancy chosen. UV reactor redundancy should be determined early 

in the planning process and should use sound engineering approaches similar to those used for 

other major equipment (e.g., capacity to provide full treatment with the largest unit out-ofservice). The specific level of redundancy should be determined by the utility based on 

operating history and process requirements and should take into account the site constraints. For 

example, one UV reactor dedicated to each filter may have different redundancy needs than UV 

installations treating combined filter effluent. Any excess capacity that may be available within 

the UV reactors (e.g., incorporation of additional lamps or change in lamp power) should also be

considered. 

The number of UV reactors necessary is also affected by the acceptable power turndown 

of the UV reactors and the LT2ESWTR requirement that the UV reactors must operate within 

their validated flow range (40 CFR 141.729(d)). Some UV reactors will operate at low power 

efficiency at reduced flowrates, and more UV reactors with a lower capacity may increase 

energy efficiency, depending on water quality and flowrates. For the potential combinations of

number and capacity of UV reactors, the available turndown should be determined for each 

configuration with respect to the anticipated flow range and power modulation capabilities of the 

UV reactors. 

The overall UV reactor and piping configuration will be affected by site constraints. For 

example, a vertical orientation of the UV reactors may be necessary to reduce building footprint 

because of little land availability. Ultimately, the selected configuration should balance the 

capital cost of the equipment, which may be lower for designs incorporating high capacity 

reactors, with the improved operating efficiency and flexibility that may be achieved using a 

larger number of lower capacity reactors. 

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The following items should be considered when estimating the UV installation footprint 

in the planning phase: 

• The number, capacity, and configuration of UV reactors (including redundancy and 

connection piping) 

• The configuration of the connection piping and the inlet/outlet piping necessary 

before and after each UV reactor, based on validated hydraulic conditions and UV 

manufacturer recommendations 

• Booster pumps (if necessary) 

• The space needed for electrical equipment including control panels, transformers, 

ballasts, backup generator(s), and possible UPS systems 

• The maximum allowable separation distance between the UV reactors and electrical 

controls since distance limitations may apply 

• Access for maintenance and replacement, room for storage of spare parts, and 

chemicals (if needed), and lifting capability for heavy equipment 

• Provisions for on-site validation (if applicable) 

Once the UV installation footprint is estimated, the feasible site locations may be 

determined based on the available land and buildings to accommodate the installation footprint. 

UV installation layout is discussed in more detail in section 3.3.5. 

3.1.7 Preparing Preliminary Costs and Selecting the UV Installation Option 

The amount of analysis necessary to determine the appropriate application point for a UV 

installation is site-specific. Some options will clearly be infeasible while others may necessitate 

a more detailed comparison of the installation options. Once feasible alternatives are identified, 

the development of life cycle costs can be useful in selecting among alternatives. 

Preliminary life-cycle cost estimates should include both capital cost and operation and 

maintenance cost elements. Capital cost elements includes the cost of the UV reactors, pumping 

(if necessary), electrical and instrumentation provisions, and site work; contractor overhead and 

profit; piloting and validation costs; engineering, legal, and administrative costs. Depending on 

the detail of the cost estimates being developed, the existing infrastructure may need to be 

evaluated to develop the cost estimate. These issues are discussed in detail in section 3.3.5. 

The average conditions for flowrate and UVT are typically the most representative for 

determining annual operating costs, as opposed to the maximum design flowrate and minimum 

UVT. Nevertheless, the specific operating limitations of the equipment and the electrical cost 

rate structure for the installation should be considered. If a utility’s electricity charge includes 

both a usage and a demand component, the demand charge may need to be estimated based on 

the worst-case operating conditions to accurately represent the cost to the utility. Similarly, it 

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may be important to correlate the anticipated energy demand for the UV reactors to the actual 

rate structure for the facility if power costs vary based on the time of day and the flowrate and 

UVT fluctuate significantly. 

Selection of the best option may not be based solely on capital and operation and 

maintenance costs. The final selection criteria should also consider the following factors: 

• Cost-effectiveness and ability to meet the utility disinfection and design objectives. 

• Ease of installation (where applicable).

• Operational flexibility and reliability.

• Specific maintenance needs. 

• Flexibility for future treatment expansion (if applicable) 

3.2 Equipment Procurement Options

The same equipment procurement options that are used to acquire traditional equipment 

(e.g., pumps) within the water industry can also be used for UV reactors. Owner pre-purchase; 

base bid, under which the design is based on a single UV manufacturer but is open to alternatives 

at the discretion of the owner; and contractor selection, in which operating or performance 

criteria are established and final equipment selection is left to the discretion of the contractor are 

the most common methods of procurement for traditional design-bid-build projects. Because the 

use of UV reactors in drinking water treatment plant applications has been limited in the United 

States, many of the projects to date have pre-purchased the UV reactors. Pre-purchase allows the 

designer to work more closely with the UV manufacturer during design, reducing the potential 

for errors that could occur with an evolving technology. However, pre-purchasing may 

necessitate that a more detailed assessment be completed during the planning stage of the project 

to ensure that the appropriate equipment is selected and that a second set of contract documents 

be prepared. Further, this may result in the owner assuming increased responsibility for 

equipment delivery and performance when compared to base bid or contractor selection. If 

owner pre-purchase is selected, these factors need to be carefully considered and addressed by 

the designer during development of the equipment procurement document. 

The advantages and disadvantages of the procurement methods with respect to designing 

and constructing a UV installation are described in Table 3.6. It should be noted that funding 

sources or municipalities might have specific bidding and procurement requirements. These 

requirements are site-specific and should be reviewed prior to establishing a project approach to 

ensure all requirements are met. 

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Table 3.6 Potential UV Reactor Procurement Options 

Procurement 

Method Advantages Disadvantages

Owner

Pre-Purchase 

• Single design around selected equipment. 

• Actual UV reactor pricing is better defined 

earlier in project. 

• Owner receives equipment warranty 

directly from UV manufacturer. 

• May result in shorter project schedule if 

equipment fabrication time occurs during

design and bidding phases of the UV 

installation. 

• Option may necessitate the preparation

of two sets of documents; an equipment

procurement document and contract 

documents for the construction of the 

overall UV installation. 

• Option may not be possible under some

procurement codes.

• Except where procurement is assigned, 

installation contractor is not single point 

of responsibility for equipment. 

• Equipment disputes need to be dealt 

with by owner. 

Base Bid • Single design around selected UV 

reactors. 

• Contractor handles all pricing and 

coordination with UV manufacturer. 

• Low incentive for contractor to bid 

alternates to selected UV manufacturer. 

• It is difficult to prevent supplier 

“packaging” of UV reactors. 

• UV reactor disputes are problematic

because contractor was directed to use 

equipment. 

Contractor 

Selection 

• Fits most procurement codes.

• UV reactor disputes are the responsibility 

of the contractor. 

• Contractor is likely to select UV reactors

with lowest capital cost rather than 

lowest life-cycle cost.

• Multiple UV installation designs may be 

necessary, increasing engineering effort

and cost. 

As discussed previously, Chapter 3 is organized in the same manner as the flow chart 

shown in Figure 3.1, utilizing equipment pre-purchase and a design-bid-build approach for 

project implementation. It should be noted that successful implementation of UV installations 

can be accomplished using any of the equipment procurement and contractor selection 

approaches currently available. 

3.3 UV Installation Design Elements 

Additional design concepts are expanded and refined in this section, particularly with 

regard to hydraulic issues, control strategy, instrumentation, and electrical power. The section 

concludes with considerations for the layout of UV installations. 

3.3.1 UV Installation Hydraulics 

Following the selection of an installation option during the planning phase, a more 

detailed evaluation of system hydraulics should be conducted, including flow control, 

distribution, and measurement. It is important that design of the inlet and outlet conditions be 

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UV Disinfection Guidance Manual 3-31 June 2003

effectively validated and will provide hydraulic conditions that result in dose delivery equal to or

better than that provided during validation testing. 

3.3.1.1 Inlet and Outlet Piping Configuration 

Optimal hydraulic conditions vary based on the UV reactor design and lamp 

configuration, but turbulent flow with a reasonably uniform velocity profile is generally 

preferred. Turbulent flow conditions can be achieved at very low flowrates when compared to 

the actual capacity of a given pipe cross section. 

The recommended inlet and outlet conditions for validation and the installation are 

summarized in section 3.1.4.3 and described in detail in section C.3.1.5. These 

recommendations should be considered when designing the inlet and outlet conditions for the 

UV reactors. The designer should contact the UV manufacturer to determine how the procured 

UV reactors were validated and what the inlet and outlet piping constraints are for the UV 

installation. If on-site validation is planned, the inlet and outlet hydraulics should be designed as 

recommended by the UV manufacturer and as the site-specific constraints permit. 

 

3.3.1.2 Flow Distribution, Control, and Measurement 

Regulations specify flowrate, UV intensity, and lamp status as the minimum operating 

conditions a utility must routinely monitor (40 CFR 141, Subpart W, Appendix D). Accordingly, 

proper flow distribution and measurement are essential for compliance monitoring of the UV 

reactors. Confirmation of compliance will be dependent on understanding the flow through each

UV reactor, regardless of the dose monitoring or control strategy used by the utility. Moreover, 

UV reactors are validated within specific flow ranges and have associated operating 

characteristics that demonstrate dose delivery as a function of flow. Therefore, the flowrate 

through the UV reactor must be known to ensure that proper dose delivery is achieved. 

This section discusses different methods for ensuring proper flow distribution and 

measurement through UV reactors. In some instances, flow can be determined through flow 

splitting and proper hydraulic design without an individual flow measurement device for each 

UV reactor. Nevertheless, the need for individual flow measurement for each UV reactor is at 

the discretion of the State. Utilities implementing UV disinfection are encouraged to discuss 

flow measurement requirements with their State during the planning and preliminary design 

phases. 

Flow Distribution and Control

Two approaches for flow measurement and control have generally been used. The first 

involves the installation of a dedicated flow meter and flow control valve for each UV reactor. 

The second involves the use of passive flow distribution, with confirmation of equal flow split by 

monitoring pressure differential across identical pipe segments (or the UV reactor) or with flow 

meters. For identical reactors, the differential pressure across each parallel UV reactor train 

should be the same if equal flow distribution is occurring and valves are in the same operating 

position. The use of dedicated flow meters and modulating downstream valves to control flow 

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through the UV reactors provides the greatest hydraulic control in applications with widely 

varying flowrates. 

Assuming multiple, parallel UV reactors of the same capacity, the UV reactors should be 

sized and configured to provide approximately equal headloss through each treatment train (i.e., 

portion of distribution and recombination channel or manifold, lateral piping, and UV reactor 

with associated valves and flow measurement). This is particularly important if passive flow 

distribution is used. Because flowrates may deviate from equal distribution, the maximum

design flowrate for each reactor should account for any potential distribution imbalance. 

Equation 3.1 can be used to determine the appropriate upper design flowrate for each UV 

reactor: 

N

Q E Q total

reactor

∗ (1+ ) = Equation 3.1

where 

Qreactor = UV reactor design flow 

Qtotal = Plant maximum design flow 

E = Calculated maximum flow distribution error (percent as a decimal) 

N = Number of on-line UV reactors 

The maximum flow distribution error (E) should be determined through hydraulic 

calculations or hydraulic modeling of the UV installation. For example, ideally two identical, 

parallel reactors would have a 50/50 flow split. If the actual flow split between the reactors is 

calculated or modeled to be 60/40 percent, then a 20 percent (E=0.20) maximum flow 

distribution error (E=(60-50)/50=0.2) would be used in the above equation to estimate the proper 

design flow for the reactor. 

The reactor flow should be estimated over the range of anticipated operating reactors 

(i.e., number of operating reactors). In general, with passive distribution, as the number of UV 

reactors increases and flowrate decreases, the potential for flow distribution imbalance is 

magnified. Effective passive distribution relies on the headloss through each treatment lateral 

being significantly greater than the headloss through the common influent manifold or chamber. 

Under the conditions of reduced flow and an increased number of operating reactors, the relative 

amount of headloss through each lateral becomes less significant when compared to the headloss 

through the manifold, resulting in less controlled distribution.

For utilities that use distribution and recombination channels (as opposed to influent and 

effluent manifolds), designers typically have two basic choices to achieve passive flow 

distribution (Figure 3.8): a series of individual weirs set at the same elevation or a series of 

orifices submerged into the individual UV reactor laterals. 

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Figure 3.8 Open-Channel Flow Distribution Options

A. Flow Splitting Weirs

Plan

Weir

Plan

Section

Section

B. Submerged Orifices

Flow

Flow

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Flow Measurement

Depending on the design and control strategy of the UV reactor, a number of options are 

available for flow measurement. As discussed previously, flow measurement devices installed 

specifically for the UV reactors may not be needed in all applications. It should be noted that 

some level of inaccuracy or drift is likely to occur with all flow meters. This potential error 

should be accounted for during design and validation. 

If a single UV reactor is installed, the plant’s raw water metering station can be used to 

determine a reasonably accurate flow through the reactor. The use of raw water flow metering 

data, however, may not account for backwash and residuals flow losses, which would create flow 

measurement inaccuracies for UV reactors installed downstream of the filters or clearwell. For 

applications where the UV reactor is dedicated to a rate-of-flow control filter, flow information 

from the filters may be used to determine the flowrate through the UV reactors. 

If equal flow distribution between multiple UV reactors can be achieved passively under 

all hydraulic conditions, a single, common flow meter (new or existing) may be used to measure 

flow. The total flow can then be divided by the number of operating UV reactors to determine 

the flow through each UV reactor. If this approach is selected, some method of confirming the 

equal flow split should also be incorporated (e.g., differential pressure measurement). 

A single flow meter for the entire UV installation or individual meters (with or without 

rate-of-flow control) should be considered to provide increased flow measurement accuracy. 

Magnetic flow meters or other meter types, such as doppler, that do not protrude into the flow 

path have the least effect on the velocity profile, which minimizes the potential effect on reactor 

inlet or outlet hydraulics. The desired means of flow measurement for the UV reactors should be 

selected based on the level of flow measurement accuracy needed to accomplish the operating 

and control strategy for the installation and satisfy validation criteria, as well as an understanding 

of the variability in the plant flowrate. Several options are listed in Table 3.7 and are illustrated 

in Figure 3.9. 

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Table 3.7 Comparison of Techniques for UV Installation Flow Measurement 

Flow Measurement Method Description Flow Control Method Advantages DisadvantA. Raw or Filtered Water 

Flow Measurement 

(Figure 3.8A) 

Use plant flowinformation upstream of UV reactors. 

Passive flow control such as a weir or orifice. 

• New flow meters not needed• Minimizes UV installation 

capital cost

• Simplifies control strategy

• Introdu

measurUV rea

• Relies o

distribureactor

• Relies o• May ne

reactordose de

B. Single Flow Meter for Flow

Measurement to Entire UV Installation 

Measure total UV reactor flow. 

(Figure 3.8B) 

Passive flow control such as a weir or orifice. 

• Measures flows accurately • Only one new flow meter 

needed 

• Relies o

distribureactor

• Relies o• May ne

reactordose de

C. Individual UV Reactor 

Flow Measurement 

(Figure 3.8C without flow control valve) 

Measure flow for each UV reactor. 

Passive flow control such as a weir or orifice. 

• Measures UV reactor flows 

accurately 

• Does not have one meter as 

a single point of failure 

• Equal flow distribution is not 

necessary for dose control

• Relies o

distribu

• Increas• Increas

comple

• Increas

footprinnecess

• Increas

D. Individual UV Reactor 

Flow Measurement and Control 

(Figure 3.8C) 

Measure and control flow for each UV reactor. 

Individual flow control (valve) for each UV reactor. 

• Does not introduce potential 

errors in measured flow 

• Does not rely on adequate 

flow distribution 

• Does not rely on a single 

meter 

• Increas• Increas

comple

• Increas

footprinUV reavalves

• Increas

UV Disinfection Guidance Manual 3-35 

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Figure 3.9 Flow Measurement and Control Options 

A. Raw Water Meter for Flow Measurement

M

C. Individual UV Reactor Flow Measurement and Control

M

M

B. Single UV Disinfection Facility Meter for Flow Measurement

M

M

M

M

Raw Water 

Flowmeter 

Raw Water 

Flowmeter 

UV Facility 

Flowmeter 

Raw Water 

Flowmeter 

Individual 

Flowmeters 

3.3.1.3 Level Control

The UV reactors must be flowing full at all times during operation. Therefore, the 

reactors should be placed below the hydraulic grade line elevation. There are two basic options 

commonly used to maintain the level in the reactors. One option is with a fixed downstream

weir; in many WTPs, a fixed weir is already located in a clearwell and can be used for this 

purpose. If not, another option is to install a weir immediately downstream of the UV reactor or 

at another location that ensures full pipe flow through the UV reactors. A final option is to use 

flow control valves to monitor and maintain the downstream hydraulic grade line. 

3.3.1.4 Air Relief and Pressure Control Valves

UV reactors should be kept free of air to prevent lamp overheating. The formation of 

negative pressures or surge effects within the UV reactors should also be prevented to avoid 

damage to the lamp sleeve and UV lamps. The use of air release valves, air/vacuum valves, or 

combination air valves may be appropriate to prevent air pockets and negative pressure 

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conditions. The locations of the valves will be dictated by the specific configuration of the 

installation and should be determined during design. 

3.3.1.5 Flow Control and Isolation Valves 

Each UV reactor should have the capability of being isolated and taken out of service. 

This will necessitate a valve, gate, or other isolation device upstream and downstream of the UV 

reactor. Valves are generally preferred, since they provide a tighter seal. Utilities that use 

passive flow distribution will rely on the valves primarily for isolation and sequencing of UV 

reactor operation (as opposed to flow control). Valves downstream of the UV reactor should be 

equipped with an actuator to automatically open or close on a critical alarm occurrence and to 

enable start-up sequencing. 

If the isolation valves are used for flow control, either the upstream or the downstream 

valve can be used. However, it is generally recommended that the valve downstream of the UV 

reactor be used to minimize disturbance of the flow entering the UV reactor, particularly if the 

separation distance between the upstream valve and the UV reactor is relatively small. The flow 

characteristic curve of the valve and the operating speed of the actuator should be matched to the 

flow control needs of the UV reactors. During design, the valve configuration should be 

discussed with the UV manufacturer to ensure that UV reactor performance will not be adversely 

affected by the location or operation of the valves. It is important to coordinate the location of 

the valves with the validation conditions for the reactor, as discussed in section C.3.1.1. 

Valve seats and other in-pipe seals and fittings within the straight pipe lengths adjacent to 

the UV reactors should be constructed of materials that are resistant to UV light to avoid 

degradation. If in-place rehabilitation of existing piping is used to improve system hydraulics, 

the materials used to slip-line or reline the piping adjacent to the proposed UV reactors should 

also be resistant to degradation from exposure to UV light. Organic materials and plastics that 

have not incorporated UV-resistant additives are typically most susceptible to UV degradation. 

3.3.1.6 Intermediate Booster Pumps 

A detailed evaluation and design of a booster pumping system is recommended if it is 

determined during the planning phase that head constraints necessitate the installation of a 

pumping system. Pumps common in water treatment plants (i.e., vertical turbine, end-suction 

centrifugal, and split-case centrifugal pumps) tend to have higher discharge pressures than 

intermediate pumping applications and are generally not appropriate for this application. Mixed 

flow or axial flow pumps with high-flow and low-head operating characteristics are typically 

more appropriate. However, additional headloss may need to be added to the system, based on 

the capabilities of the pump. Smaller diameter piping, backpressure valves, or control valves can 

be used to increase the system head to more closely match the pump discharge curve. 

Pumps may be installed before or after the UV reactors, allowing more flexibility in the 

UV installation’s design elevations and the location of the UV reactors. Regardless of pump

location, some form of wetwell should be provided upstream of the pump station. Existing 

clearwells, recombination channels, or dedicated pump wetwells may be used. Direct connection 

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clearwells, recombination channels, or dedicated pump wetwells may be used. Direct connection 

to filter effluent piping may adversely affect upstream process performance and should be 

avoided. Booster pump operation may be controlled by the water level within the upstream

wetwell. The use of variable frequency drives (VFDs) to moderate flow peaks is recommended. 

This is especially important if the pump station is upstream of the UV reactors. By minimizing 

hydraulic peaks, the UV reactors can be sized to more closely match the flow through the WTP. 

If pumps are located adjacent to the UV reactors, the impact of surge conditions should 

be evaluated. Of particular concern is the potential for surge if the pumps are operating and 

power is lost. Pump start-up procedures should be carefully selected with possible inclusion of 

pump control valves. Control of individual UV reactor isolation valves should be coordinated 

with pump starts and stops and with pump control valves where appropriate. Likewise, the 

warm-up time associated with the start-up of the UV reactors must be taken into account with the 

sequencing of the pump operation. 

3.3.2 Operational Strategy Determination 

Once the UV reactors are procured, a utility should determine the preferred operational 

strategy given the UV reactor’s control strategy and available validation data. The different 

operational strategies are described in section 3.1.5, and an example of how to interpret the 

validation data to develop an operational strategy is described in section 5.5. 

The power needs for UV reactors can be moderately high, and an inefficient UV 

installation can result in unnecessarily high operating costs. When considering what operational 

strategy to use for a particular installation, the operational complexity should be compared to the 

potential for energy savings. It should be noted, however, that intensity adjustment does not 

correlate directly to the amount of energy that is saved. Lamp output efficiency may decrease as 

the lamp intensity is reduced, resulting in a reduced energy savings. Lamp output efficiency as a 

factor of intensity should be discussed with the UV manufacturer and considered when 

determining the potential cost savings associated with dose pacing. An operational strategy 

consistent with the procured UV reactor should be selected to facilitate the instrumentation and 

control design. 

3.3.3 Instrumentation and Control 

After the hydraulic needs of the UV reactors have been addressed and a dose control 

strategy has been selected, the instrumentation and controls necessary to satisfy both can be 

identified. The level of instrumentation and control that is needed will depend on the flow 

control, flow distribution, and flow measurement approach that is selected, as well as the dose 

control strategy that is employed. Passive flow distribution with an intensity setpoint dose 

control strategy is a relatively simple operation and demands limited instrumentation and control. 

Operating flexibility and the ability to optimize UV reactor energy efficiency, however, are 

reduced. The use of dedicated flow meters and flow control valves, in combination with on-line 

transmittance monitors and dose pacing, demands a higher level of instrumentation and control. 

However, this approach provides significant operating flexibility and the ability to optimize 
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The specific instrumentation and control elements included with the UV reactors may not 

be known until a final UV reactor selection is made. Most of the equipment manufacturers, 

however, share common instrumentation and control attributes and alarm conditions in the 

designs of their UV reactors. To enable a procurement document to be prepared, a control 

strategy should be established. To the extent practicable, the designer should identify the 

elements of the control system that are preprogrammed into the UV reactor control panel and 

those that will be addressed through the installation of supplemental controls and equipment. At 

a minimum, the LT2ESWTR requires that UV lamp intensity, flowrate, and lamp status be 

monitored (40 CFR 141.729(d)). The final instrumentation and control design can be modified 

as needed after equipment is selected. 

3.3.3.1 UV Reactor Start-up

Regardless of the UV reactors that are selected, the start-up cycle will likely be the same. 

For a UV reactor that is starting cold (i.e., previously off as opposed to shutdown for a very short 

period due to power interruption), a typical control sequence will open the isolation valves, ignite 

the lamps, and bring the lamps to full power. During the typical control sequence, the water 

being treated will be off-specification until the lamps reach full operating power, which can take

up to 10 minutes. However, the amount of off-specification water can be reduced by providing a 

low flow of cooling water that can be discharged to waste. Alternatively, if a LP or LPHO 

reactor is procured, the downstream valve may remain closed as the UV lamps are warming up. 

However, the designer should consult the LP or LPHO manufacturer to ensure this strategy is 

feasible. It is recommended that the utility discuss these practices with the State to confirm their 

acceptance. 

3.3.3.2 UV Reactor Automation

Depending on the size and complexity of the UV reactor, its operation can range from 

manual to fully automatic. Manual operation includes manual initiation of lamp start-up and 

shut down, and appropriate valve actuation. Different levels and types of automation can be 

added to the manual sequence. A first level of automation includes the sequencing of lamp startup and valve actuation to bring individual UV reactors on-line after manual initiation. Further 

levels of automation could include starting up UV reactors, activating rows of lamps, or making 

lamp intensity adjustments based on lamp condition, water quality, and/or flowrate. 

Automatic UV reactor shutdown under critical alarm conditions (e.g., high temperature, 

lamp or sleeve failure, loss of flow) is important for all operating approaches, including manual 

operation. The shutdown cycle will be site-specific. However, to the extent practicable, the 

downstream flow control or isolation valve should be closed whenever the UV reactor is shut

down to minimize the distribution of water that has not been disinfected by the UV installation. 

3.3.3.3 UV Intensity and Calculated Dose (If Applicable)

Signals from UV intensity sensors should be displayed locally or on the UV reactor 

control panel. Because the output from the UV intensity sensor is integral to the determination 

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of adequate dose delivery, the UV intensity sensor output should be monitored continuously. If 

the calculated dose control strategy is used, the calculated dose should be displayed locally and 

be monitored continuously. 

3.3.3.4 UV Transmittance

An on-line UVT monitor or bench-top spectrophotometer may be used to monitor UVT, 

depending on the control strategy (section 3.1.4.2). An on-line UVT monitor is typically used 

for the UV intensity and UVT setpoint approach and the calculated UV dose setpoint approach. 

However, for utilities that have water with a stable UVT, periodic grab samples may be 

adequate. Results from a bench-top spectrophotometer can be manually input into a SCADA 

system or other control system. Output from an on-line UVT monitor can be input directly into a 

control loop for most UV reactors, a SCADA system, or both. 

If the UV intensity setpoint approach is used, UVT does not need to be monitored 

because the UVT is accounted for in the UV intensity measurement. However, it may be 

advantageous to monitor UVT with an on-line UVT monitor or bench-top unit to assist with 

troubleshooting UV reactor performance issues. 

The specific size and operating characteristics of the UVT monitor will vary dependent 

on the UV manufacturer. If an on-line UVT monitor is included in the design, it is important to 

provide adequate space and access to an electrical supply for installation of the monitor and to 

include appropriate sample taps and drains in the design for the withdrawal and discharge of 

sample water. The sample line should be equipped with a valve to isolate the unit. If insufficient 

pressure is available in the system, then a sample pump should be installed. 

3.3.3.5 Flow Measurement

Flowrate is one of the operating conditions a utility must routinely monitor (40 CFR 141, 

Subpart W, Appendix D). To maintain regulatory compliance, the flowrate through a UV reactor 

must be known to ensure that flow is within the validated range. Section 3.3.1.2 discusses flow 

measurement and control options. If flow meters are installed, the flow signal should be 

displayed locally or be input directly into a control loop for the UV reactor, a SCADA system, or

both. 

3.3.3.6 Lamp Age

Each lamp or an integral bank of lamps should be monitored for operating time. Lamp

replacement should be based on the dose delivery and the age of the lamp. Initially, the number 

of lamp hours used to trigger lamp replacement can be estimated based on UV manufacturer 

recommendation and validation data. Later, the actual frequency of replacement should be 

correlated to the operating performance of the UV installation. Frequent restarting of the lamps 

may reduce their useful life. 

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3.3.3.7 Lamp and Reactor Status

Lamp status is one of the operating conditions a utility must routinely monitor (40 CFR 

141, Subpart W, Appendix D). In addition to the status of individual lamps, whether the reactor 

is on-line or off-line should also be monitored and indicated locally and remotely. Power and 

valve status are two methods that utilities can consider to perform such monitoring. 

3.3.3.8 Alarms and Control Systems Interlocks

Many UV reactor signals and alarms are specific to the UV installation and the level of 

automation employed. Alarms may be designated as minor, major, or critical, depending on the 

severity of the condition being indicated. The same alarm condition may represent a different 

level of severity dependent on the conditions under which the UV reactor was validated, the type 

of UV reactor, the control strategy, and the disinfection objectives of the utility. For example, if 

a UV reactor was validated with one lamp out of service, a single lamp failure alarm may be a 

minor alarm. Had the reactor been validated with all lamps in operation, then a single lamp

failure may be a major alarm. At a minimum, alarm conditions should be displayed locally. The 

use of an audible alarm may be beneficial. If UV reactors will frequently be unstaffed, 

provisions should also be included in the design to allow remote monitoring. 

A minor alarm generally indicates that a UV reactor needs maintenance but that the UV 

reactor is not operating out of compliance. For example, a minor alarm would occur when the 

end-of-lamp-life is reached, indicating the possible need for lamp replacement. A major alarm

indicates that the UV reactor needs immediate maintenance (e.g., the UV intensity sensor value 

has dropped below the validated setpoint) and that the unit may be operating off-specification. 

Based on the utility’s disinfection objectives, this condition may also be handled as a critical 

alarm. A critical alarm typically shuts the unit down until the cause of the alarm condition is 

remedied. An example of a more typical critical alarm is the UV reactor temperature exceeding 

a pre-determined maximum value, resulting in automatic shutdown to prevent overheating and 

equipment damage. 

The designer should work with the UV manufacturer to determine what elements of the 

control system are integral to the UV reactor and what will be addressed through the installation 

of supplemental controls and equipment. For installations with multiple UV reactors, a common, 

master control panel may be necessary to enable sequencing of the UV reactors and to allow the 

UV reactor operations to be optimized. Table 3.8 summarizes typical UV reactor monitoring and 

alarms; additional detail is provided in section 5.4. Many of the alarms shown will be integral to 

the UV reactor control panel. 

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Table 3.8 Typical Alarm Conditions for UV Reactors

Alarm/Sensor Purpose/Descriptions

Lamp Age • Minor alarm occurs when run-time for lamp indicates end of 

defined operational lamp life. 

Calibrate UV Intensity Sensor • Minor alarm occurs when UV intensity sensor needs 

calibration based on operating time. 

Differential Pressure Out of 

Range 

(When Differential Pressure is 

Used for Flow Split 

Confirmation)

• Necessary only if a single master flow meter is used.

• Minor alarm occurs if pressure drop across parallel, identical 

UV reactors indicates unequal flow split. 

• Major alarm occurs if differential pressure across a given UV 

reactor indicates flow outside of the validated range.

Low UV Dose • Major alarm occurs when dose condition falls below required 

dose. 

• Triggered by signals gathered by control system and 

compared to validated UV reactor dose requirements.

Low UV Intensity • Major alarm occurs when intensity falls below design 

conditions. 

Low UV Transmittance • Major alarm occurs when UVT falls below design conditions. 

High/Low Flow • Major alarm occurs when flowrate falls outside of validated 

range. 

• Based on measurement from dedicated flow meters or 

calculated based on total flowrate divided by number of units 

operating. 

Lamp/Ballast Failure • Major alarm occurs when a single lamp/ballast failure is 

identified. 

• Critical alarm occurs when multiple lamp/ballast failures are 

identified. 

Low Liquid Level • Critical alarm occurs when liquid level within the UV reactor 

drops and potential for overheating increases. 

High Temperature • Critical alarm occurs when the temperature within the UV 

reactor or ballast exceeds a setpoint. 

Mechanical Wiper Function 

Failure 

• Needed only if a mechanical wiper system is used. 

• Critical alarm occurs if wiper function fails. 

Note: Alarm conditions and relative severity shown above may vary dependent on specific conditions under which 

the UV reactor is validated, the type of UV reactor, the control strategy, and the disinfection objectives of the 

utility.

3.3.4 Electrical Power Configuration 

The electrical power configuration that is used should take into account the findings of 

the power quality assessment conducted during the planning phase described in section 3.1.3.3, 

the power requirements of the selected equipment, and the disinfection objectives and control 

strategy of the utility. Issues that should be addressed include harmonic distortion and offspecification operation due to power quality problems (fluctuation in line voltage). 

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3.3.4.1 Power Requirements

The proper supply voltage and total load requirements must be coordinated with the UV 

manufacturer, considering the available power supply. In addition, the power needs for each of 

the UV reactor components may differ. For example, the UV reactors may require a 3-phase, 

480-volt service while the on-line UVT monitor may need a single phase, 110-volt service. The 

method of handling the power feed must be carefully coordinated to ensure all electrical 

equipment and services are included and to clearly establish the responsible party for each 

element of the electrical supply (e.g., primary service, transformer, secondary service). 

Excluding high service pumping, the electrical load from UV reactors will typically be one of the 

larger loads at the WTP. 

Due to the varying nature of UV reactor loads, current and voltage harmonic distortion 

can be induced. Such disturbances can result in electrical system problems, including 

overheating of some power supply components and effects on other critical systems such as 

VFDs, program logic controllers (PLCs), and computers. Proper selection of the UV reactors, 

including a thorough analysis of the potential for the equipment to induce harmonic distortion, 

should minimize the potential for harmonic distortion. Another method for controlling 

harmonics is to use a transformer with Delta Wye connections to isolate the UV reactors from

the remainder of the WTP power system. The delta-connected primary feed could be designed 

and sized to trap and moderate any induced harmonics. The Wye-connected secondary should 

be solidly grounded so that the ballasts are powered from a grounded source in accordance with 

electrical code requirements. If a separate transformer for the UV reactors is impractical, 

harmonic filters could be added to the UV reactor power supply to control distortion. Regardless 

of the method used to address harmonic issues, electrical acceptance testing during start-up 

should include a harmonic analysis to verify that the UV reactor harmonics are not affecting 

other electrical components at the WTP. 

3.3.4.2 Backup Power Supply

The continuous operation of the UV reactors is highly dependent on its power supply. 

This dependence, when combined with the sensitivity of the UV reactors to power fluctuations, 

increases the importance of a high quality, dependable power supply. The utility should work 

with the State to establish specific power reliability objectives for the UV installation, as power 

reliability may directly affect the utility’s ability to meet the State’s allowable off-specification 

requirements. As discussed in section 3.1.3.3, minor power transients can lead to lamp outages. 

If the power reliability objectives, and, consequently, the disinfection objectives, cannot be met 

solely by relying on the commercial power supply, then the use of a backup power supply (i.e., 

backup generator, separate commercial service, and/or battery-supported UPS) may be 

necessary. If an existing backup power supply is in place, the load capacity of this supply should 

be assessed to determine if it is able to accept the additional load associated with the UV

reactors. Additionally, the time needed to transfer from the primary power supply to a backup 

power supply and the potential effect of the transfer time on compliance with the State’s 

allowable off-specification operation should be determined. 

An alternate backup power supply may be needed if a backup power supply is not inplace or the available load capacity is insufficient to handle the new load associated with the UV 

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reactor equipment. The type of backup power supply that is needed will depend on the 

frequency and duration of the power interruptions and the potential for those interruptions to 

result in off-specification operation of the UV reactors. If power quality issues are infrequent 

and of short duration (on the order of seconds or minutes), it is possible that a backup power 

supply may be unnecessary, or a simple backup power supply may be sufficient. If the 

frequency of power outages increases or the duration of the outages increases, the need for a 

more extensive backup power supply becomes more significant. 

If a backup power supply is necessary, but continuous power is not needed, the use of a 

traditional diesel or natural gas-fired backup generator set, a standby UPS, or a rotary UPS may 

be adequate. Should a continuous power supply be needed to meet reliability objectives, the use 

of a line-interactive UPS will be necessary. The line-interactive UPS provides a continuous 

power supply, but is generally less efficient, has a lower starting current, and costs more than a 

similarly sized standby UPS. Typically, a line-interactive UPS would be installed in conjunction 

with a backup generator to provide a cost-effective backup power supply for longer duration 

power interruptions or for frequent, shorter duration power interruptions. Although unlikely to 

be a requirement for compliance monitoring, it may be beneficial to include a data logger that 

records instances of UPS operation as part of the UPS system design. 

The elements that should be considered when assessing the need for a backup power 

supply for a UV reactor are somewhat unique when compared to those associated with more 

typical WTP equipment. However, once it is determined that a backup power supply is 

necessary, the design for a UV reactor is very similar to that for any other equipment or 

treatment process. Factors that should be considered during design include isolation, in-rush 

current, purchase and installation cost, maintenance requirements, voltage regulation, electrical 

surge protection, attenuation of harmonic current, run-time, transformer continuity, and the 

ability to operate with other power supply equipment. In most circumstances, a UPS should not 

be used without a backup generator because of the battery reserve necessary to power a UV 

installation for longer durations. To minimize capital cost, the battery reserve time should be 

sufficient to allow the power supply to switch to the backup generator. 

3.3.4.3 Ground Fault Interrupt and Electrical Lockout

Ground fault interrupt (GFI) is an important safety feature for any electrical system in 

contact with water, including UV reactors. All UV reactor suppliers should provide GFI circuits 

for their lamps, which should be included in the specifications that are developed for equipment 

procurement. For a GFI to function properly, the transformer in the UV reactor ballast must not 

be isolated from the ground. If the UV reactor ballast isolates the output from the ground, 

ground faults will not be properly detected, and safety can be compromised. 

Provisions enabling the UV reactors to be isolated and locked out for maintenance, both 

hydraulically and electrically, should be included in the design. Control of all lockout systems 

should remain local; however, when appropriate, the status of local lockouts could be monitored 

remotely. In all cases, the design must comply with electrical code and policy requirements for 

equipment lockout. 

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3.3.5 UV Installation Layouts 

Once the previous design elements (i.e., section 3.3.1 through section 3.3.4) have been 

evaluated, an installation layout can be prepared as part of the equipment procurement document. 

The layout should take into account the findings of all previous work. Because the design 

process is iterative and many elements of the layout are dependent on the specific UV reactors 

that are used and the validation scenario that is proposed, the layout may change after the UV 

reactors are selected and any additional space constraints are identified. 

3.3.5.1 Site Layout

Site layout for a UV installation is generally similar to the layout of any treatment 

process. When locating the UV installation, access for construction, operation, and maintenance 

should be addressed. The availability of adequate existing infrastructure (e.g., power, drains, 

lifting devices) is also important. In general, when compared to other treatment processes at a 

WTP, the UV installation has a relatively small footprint. 

3.3.5.2 UV Installation Layout 

In large part, the piping layout will be dictated by the validated hydraulic conditions 

because the inlet and outlet conditions for the installed UV reactors should be equal to or better 

than the hydraulic conditions used during validation. Additional details on the relationship 

between the validated inlet and outlet configuration and the actual installed configuration are 

given in section C.3.1.5. Nevertheless, the designer can prepare a reasonable UV installation 

layout based on the type of technology (i.e., LPHO versus MP), the number of UV reactors that 

is needed, and the manner in which flow is controlled and measured. This layout can then be 

used in the selection and procurement of the UV reactors. 

Most UV reactors available for drinking water applications are of a closed-chamber type. 

Filtered water is conveyed via pipes or covered channels to a series of UV reactors for primary 

disinfection. As such, laying out UV installation typically involves designing the method by 

which water is divided between UV reactors (channel or piping), and routing the sections of pipe 

between inlet and discharge headers in which the UV reactors are inserted via flanged 

connections (although other types of connections may be used). The number and configuration 

of the UV reactors will vary depending on lamp type/reactor design, reactor size, flow range to 

be treated, control strategy, and degree of redundancy. 

Although most components of UV reactors are fairly compact, it is important not to 

underestimate the necessary space for the building that will house the UV installation. In 

addition to those items identified in section 3.1.6.2, the following factors should be considered in 

the layout for the UV installation: 

• The length of straight-run piping before and after each flow meter to achieve the 

proper hydraulic conditions for accurate and repeatable flow measurement (if 

applicable) 

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• Field instrumentation 

• Isolation valves and flow control devices 

• Control and power panels, and code-required clear space 

• Potential space for power monitors and UPS systems 

• Drain provisions for the process area and to permit UV reactor draining 

• Provisions for future expansion of UV disinfection capacity 

Components of the UV reactors are typically located inside a building for protection from

the weather and to provide a clean, convenient area for maintenance. The UV reactors 

themselves, associated electrical components and controls, and electrical support equipment such 

as a UPS should be enclosed. There are installations, however, where UV reactors and control 

panels are uncovered. Prior to implementing an uncovered installation, it is recommended that 

the State and UV manufacturer be consulted. Any exposed equipment and control panels should 

be rated for the anticipated environment and appropriate site security should be in-place to 

restrict public access. 

The power and control panels associated with UV reactors should be located so that there 

is adequate space for panel doors to be opened without interference, and to allow unhindered 

access to the UV reactors with panel doors open. In selecting the location of the power and 

control panels, UV manufacturer cable length limitations should not be exceeded. The 

maximum allowable cable length is UV manufacturer-specific and may be less than 30 feet. If 

harmonic feedback is a concern, extra room should be provided for power conditioning 

equipment. 

When allotting space for maintenance activities, adequate space to remove the lamps and 

the lamp wiper assembly should be provided. In some cases, access may be needed on both 

sides of the UV reactor. In addition, provisions should be included to collect and convey water 

that is discharged during maintenance activities. 

Certain UV reactors need maintenance involving an OCC procedure in which a UV 

reactor is taken off-line, isolated, drained, filled with a cleaning solution, cleaned, flushed, and 

returned to service. The OCC equipment is typically self-contained and the cleaning chemical is 

recirculated. Where applicable, sufficient space around the UV reactors should be maintained to 

provide access for the OCC procedure. In addition, the OCC solution often has specific handling 

requirements. Appropriate drains, storage, and health and safety equipment (e.g., emergency 

eyewash station) should be provided as recommended by the chemical manufacturer. 

Sample taps are recommended upstream and downstream of each UV reactor within the 

lateral pipe. The sample taps may be used for the collection of water quality samples or may be 

used during validation testing if on-site validation is necessary. If on-site validation will be

conducted, the number and location of sample and feed ports should be coordinated with the UV 

manufacturer or third party validation service to comply with the recommendations of the 

selected validation protocol. Additional detail on the locations of sample taps and other 

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validation-related appurtenances, as well as the methods used to validate a reactor are provided 

in section C.3.1.4. 

Drain valves or plugs should be located on each lateral between the two isolation valves. 

In many cases, the UV manufacturer may have already incorporated a drain into the UV reactor 

design. Drain valves should also be provided at one or more low points in the UV installation to 

enable the UV reactor to be fully drained for future maintenance activities.

3.3.6 Elements of UV Reactor Specifications 

Table 3.9 summarizes the elements that should be considered in developing equipment 

specifications for the UV reactors. The information included in Table 3.9 is not exhaustive and 

should be modified to meet the specific needs of the utility. 

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Table 3.9 Recommended Content for UV Reactor Specifications 

Specification Item Purpose/Description 

Flowrate Maximum, minimum, and average flowrates should be clearly identified. 

The minimum and maximum flowrates must be within the range of

validation flowrates. The minimum flowrate is important to avoid 

overheating with MP reactors. 

UV Dose The required reduction equivalent dose as well as the validation 

technique that will be used to measure the dose should be established. 

Additional detail is provided in Chapter 4. 

Water Quality and 

Environment 

The following water quality criteria should be included: 

- Influent temperature - Total Hardness 

- Turbidity - pH 

- UV Transmittance at 254 nm - Iron 

- Spectral absorbance 200-300 

nm (MP reactors only) 

For some parameters, a design range may be most appropriate. 

UV Intensity Sensors It is recommended that at least one UV intensity sensor be specified per 

UV reactor. The number of reference sensors that should be determined 

based on the time and labor associated with checking and maintaining 

the duty sensors. 

Redundancy If a combined filter effluent UV reactors are used, it is recommended that 

at least one completely redundant UV reactor be specified as a standby. 

For other configurations, the designer should determine the appropriate 

redundancy based on the State’s requirements and the utility’s 

disinfection objectives. 

Hydraulics The following hydraulic information should be specified: 

- Maximum system pressure at the UV reactor 

- Maximum allowable headloss through the UV reactor 

- Special surge conditions that may be experienced 

- Hydraulic constraints based on site-specific conditions and 

validated conditions (e.g., upstream and downstream straight pipe 

lengths) 

Size/Location 

Constraints 

Any size constraints or restrictions on the location of the UV reactor or 

control panels (e.g., space constraints with in-line installation). 

Validation The specifications should establish the validation protocol that will be 

followed, provide the conditions under which the validation will be 

conducted (e.g., water quality, flow range, hydraulic conditions, UVT), 

and require the submittal of a validation report (40 CFR 141.730). 

Control Strategy and 

Operating Sequence 

The specification should provide a narrative description of the operating 

sequence and control strategy for the UV reactors. 

Lamp Sleeves At a minimum, the following items should be specified:

- Lamp sleeves should be annealed to remove internal stress. 

- UV manufacturer should perform QA / QC checks of a fraction of 

each lot using a polarized light or other approved method. 

- UV manufacturer should submit documentation on the integrity of 

their sleeve, monitoring practices, and rationale for using a given 

internal QA / QC frequency. 

- UV manufacturer should submit calculations showing the maximum

allowable pressure for the lamp sleeves and the maximum bending

stress experienced by the lamp sleeves under the maximum 

specified flow conditions. 
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Table 3.9 Recommended Content for UV Reactor Specifications (continued) 

Specification Item Purpose/Description 

Safeguards 

At a minimum, the following UV reactor alarms should be specified: 

- Lamp or ballast failure 

- Low UV intensity or low UV dose (dependent on control strategy 

used) 

- High temperature 

- Low or high flow 

- Wiper failure (as applicable) 

- Other alarms discussed in section 3.3.3.8, as appropriate 

Control Systems 

At a minimum the following signals and indications should be specified: 

- UV reactor status 

- UV intensity 

- Individual lamp status 

- Lamp cleaning cycle and history 

- Accumulated runtime for individual lamps 

- Influent flowrate 

At a minimum the following UV reactor controls (as applicable) should be 

specified: 

- UV dose setpoints, lamp intensity setpoints, or UVT setpoints 

(dependent on control strategy used) 

- UV reactor on/off control 

- UV reactor manual/auto control 

- UV reactor local/remote control 

- Manual lamp power level control 

- Manual lamp cleaning cycle control 

- Automatic lamp cleaning cycle setpoint control 

Performance 

Guarantee 

The performance guarantee should specify that the equipment provided 

under the UV reactor specification should meet the performance 

requirements stated in the specification for an identified period. The 

following specific performance criteria may be included: 

- Allowable headloss at each of the design flowrates. 

- Estimated power consumption under the design operating 

conditions. 

- Disinfection capacity of each reactor under the design water quality 

conditions. 

Warranties

A physical equipment guarantee and UV lamp guarantee should be

specified. The specific requirements of these guarantees will be at the 

discretion of the utility and engineer. 

3.3.6.1 Information Provided by Manufacturer in UV Reactor Bid 

It is important that UV manufacturers provide adequate information when bidding to 

enable the designer to conduct a proper, timely review of the proposed equipment. Suggested 

information to be obtained from the UV manufacturer is presented in Table 3.10. 

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Table 3.10 Recommended Information to be Provided by UV 

Manufacturer/Vendor 

Item Purpose

Design 

Parameters 

Demonstration of an understanding of the design parameters for the UV reactors. 

All UV reactor design parameters from the contract documents should be repeated 

in the proposed UV reactor submittal information. 

Summary of

Design 

A summary of the equipment proposed (number of UV reactors, lamp type) and 

specify equipment redundancies.

Reactor 

Technical 

Specifications 

Ability of proposed UV reactors to meet technical specifications and an explanation 

of any exceptions taken.

UV 

Manufacturer’s 

Experience 

Information on project experience, including previous installations and references. 

UV Intensity

Sensor 

Information on the UV intensity sensor(s) including acceptance angle, external 

dimensions, working range in mW/cm2

, spectral response, measurement 

uncertainty, environmental requirements, linearity and temperature stability. Data 

and calculations should be provided showing how the total measurement 

uncertainty of the sensor is derived from the individual sensor properties. 

(See sections 4.3.2.3 and C.4.7 ) 

Validation 

Data

UV reactor validation data as described in Appendix C of these Guidelines. If onsite validation is proposed, validation data for the UV reactors from other, similar 

installations should be included to provide a baseline comparison to the proposed

operating conditions. 

Upstream and 

Downstream

Hydraulic 

Requirements 

A statement of the length of straight pipe and hydraulic conditions necessary 

upstream and downstream from the UV reactor to ensure the desired flow profile is

maintained and the design conditions are met.

Power 

Requirements 

The power needs of each UV reactor and which elements, including electrical cable 

and wiring, are included as part of their equipment.

Cleaning 

Strategy

The strategy that will be employed for cleaning the UV lamps in the UV reactor. 

Control 

Strategy

The proposed UV reactor control strategy, including manual and automatic control 

schemes and a listing of inputs, outputs, and the types of signals that are available 

for remote monitoring and control. 

Reactor Data The materials of construction, dimensions of the UV reactors and ancillary 

equipment, a listing of spare parts, and a sample operations and maintenance 

manual. 

Safeguards The safeguards built into the UV reactor and accompanying equipment, such as

high temperature protection, wiper failure alarms, and lamp failure alarms.

Warranties A statement of the proposed UV reactor guarantees, including the physical 

equipment, the UV lamp, and the system performance guarantee. Any exceptions 

should be indicated and explained.

Warranties

The UV reactor specification should include suitable written guarantees regarding 

physical equipment, UV lamps, and performance. 

It is recommended that the UV lamp guarantee specify that each lamp is warranted to

provide the lamp output necessary to meet the required reduction equivalent dose (RED) under 

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the design conditions for a minimum number of operating hours, which will vary depending on 

lamp type. To limit the UV manufacturer’s liability and to potentially reduce the contingency 

costs included in their equipment bid prices, the guarantee could be prorated after a specified 

number of operating hours. It is important that the appropriate lamp fouling/aging factor be 

included in the design conditions as discussed in section 3.1.3.1. If these specifications are not 

met, significant operation and maintenance costs may occur because lamps may need to be 

replaced frequently for the UV reactors to operate within the validated range. The combination 

of lamp fouling/aging factor and the guaranteed lamp life will make the UV manufacturer 

responsible if the UV lamps do not meet these specifications. The guaranteed lamp life will 

depend on the available technology at the time of the UV installation design and will likely 

change as lamp technology improves. 

3.3.7 Final UV Installation Design 

After the equipment procurement document is developed and competitively bid, and all 

bids have been carefully reviewed, the UV reactors can be selected. Once the UV reactors are 

selected, the designer can work with the selected UV manufacturer to develop the final 

disinfection installation design based on the specific needs and design of the selected equipment. 

The hydraulic design, instrumentation and control design, electrical design, and installation 

layout should be modified to address the specific needs of the selected equipment and to ensure 

that the control strategy can be implemented within the constraints established during the 

validation testing. 

Particular emphasis should be given to the integration of the overall control strategy with 

the alarms, signals, and interlocks that are integral to the UV reactor design. For designs with 

multiple UV reactors, a master control panel may be necessary to enable the sequenced operation

of the individual UV reactors and to optimize the efficiency of the UV installation. It is critical 

that the final design be coordinated with the validation testing to ensure that validation criteria 

are sufficient to implement the proposed control strategy and to ensure that the UV reactors will 

meet the utility’s disinfection objectives under the anticipated operating conditions. 

3.3.7.1 Design Drawings 

The drawings may include the following content: 

• Existing conditions 

• Site work 

• Structural work 

• Architectural work 

• Mechanical work (heating, ventilation, and air conditioning) 

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• Electrical work 

• Instrumentation work 

3.3.7.2 Specifications 

The content of the specifications will vary dependent on the complexity and size of the 

UV installation and the selected method of project delivery. However, it is likely that portions of 

nearly all of the 16-Division Construction Specifications Institute (CSI) MasterFormat may be 

necessary. For those UV installations that pre-purchase the UV reactors, the equipment 

procurement document should be included as an appendix to the specifications to facilitate 

contractor review and installation of the equipment. 

3.4 Reporting To The State 

Interaction with the State throughout the planning and design phases, as well as during 

development of the reactor validation protocol, is recommended to ensure that the objectives of 

both the utility and the State are met. 

Given the relatively limited past use of UV disinfection in drinking water treatment and 

the unique technical characteristics of this technology, State agencies may not have developed 

approval requirements specifically for UV disinfection. This section provides guidance on the 

information that may be included in submittals to the State. Utilities are urged to consult with 

their State early in their UV disinfection planning process to understand what approvals and 

documentation will be required for the use of UV disinfection.

3.4.1 Planning 

The State may require that a pre-design report be submitted that summarizes the decision 

logic used to identify, evaluate, and select UV disinfection. Appendix K is an example predesign report, including installation alternatives and analysis. The following items may be 

addressed in the pre-design report: 

• Disinfection objectives (target organism and inactivation) 

• Overall disinfection strategy 

• Summary of reasons for incorporating UV disinfection 

• Description of the overall process train 

• Description of the proposed UV reactors 

• Water quality data 

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• UV reactor reliability targets (i.e., off-specification limits) 

• UV reactor validation 

3.4.2 Equipment Procurement 

If the utility pre-purchases the equipment, a separate procurement document would be 

prepared. The equipment procurement document should be consistent with the pre-design report 

and should include technical specifications and a preliminary layout of the UV installation. 

Details on the recommended content of the specifications are given in section 3.3.7. While the 

State may not require submittal of the equipment procurement document prior to equipment 

purchase, it is recommended that acceptance of a pre-design report be received from the State 

prior to proceeding with equipment purchase. The State should also be notified of any deviations 

from the pre-design report. 

3.4.3 Drawings and Specifications 

The UV installation drawings and specifications should be submitted to the State for 

approval. Under the equipment pre-purchase option, the drawings and specifications should 

address the installation of the UV reactors and related equipment as well as other necessary 

facility modifications. The specific items that would be included in this submittal are discussed 

in section 3.3.7. If an alternative approach is used (e.g., design-build or design-build-operate) 

the level of detail included in the design documents will differ. 

3.4.4 Validation Report/Start-up Confirmation

States may request that a validation report or other preliminary testing results be 

submitted. As discussed in section 3.1.4.3, validation may occur off-site or on-site. If the UV 

reactors are validated at an off-site location, the validation report should be available from the 

UV manufacturer and should be a required submittal from the UV manufacturer as part of either 

the equipment procurement documents or the UV installation specifications. If on-site validation 

is used, a validation protocol should be developed and accepted by the State prior to 

implementation. Following completion of the on-site validation, a validation report should be 

prepared and submitted to the State. Recommended validation protocols are provided in 

Appendix C. 

In addition, some States may request that the utility provide as-built documentation (i.e., 

start-up confirmation) certifying construction was completed in accordance with the approved 

drawings and specifications. Start-up confirmation may be most important where alternative 

project delivery approaches are used and the State does not have the benefit of reviewing 100 

percent design drawings and specifications prior to construction.

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The Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) requires the 

use of validated UV reactors for receiving Cryptosporidium, Giardia, or virus inactivation credit 

(40 CFR 141.729(d)). The purpose of validating a UV reactor is to provide confidence that the 

UV reactor can provide the level of inactivation required for a given application. The rule 

specifies only basic components of a validation process (presented in section 4.1). Using those 

requirements as a framework, this guidance manual describes recommended procedures and data 

analysis for one possible approach to validating a UV reactor. Other approaches or 

modifications to this approach may be used at the discretion of the State. 

The validation protocol provided in this manual has two tiers, specifying two different 

methods for addressing uncertainty with a safety factor to determine the log inactivation credit. 

These tiers differ in level of complexity. Tier 1 is simplified while Tier 2 is more complex, 

potentially allowing for a less conservative safety factor based on detailed knowledge and testing 

of equipment performance. Appendix C provides all the necessary procedures and descriptions 

to complete a validation for both Tier 1 and Tier 2 methods. This chapter provides a brief 

overview of the validation process, describing all the basic steps of the testing procedures and 

interpretation of results, with references to Appendix C for more detailed descriptions. For those 

conducting a validation test of a given reactor, it is important to understand the background and 

detailed procedures described in Appendix C. 

4.1 LT2ESWTR UV Disinfection Requirements 

This section reviews the LT2ESWTR requirements related to UV reactor validation 

specified under 40 CFR 141.729(d) and 40 CFR141, Subpart W, Appendix D. 

Validation testing must determine a set of operating conditions that can be monitored by 

a utility to ensure that the UV dose required for a given pathogen inactivation credit is delivered; 

and the utility must then monitor to demonstrate it is operating within the range of conditions 

under which the reactor was validated. 

Validation operating conditions must include, at a minimum, the following: 

• UV intensity (as measured by a UV intensity sensor) 

• Flowrate 

• Lamp status 

Many design and equipment factors affect the UV dose delivered by the reactor. The 

validated operating conditions must account for the following factors: 

• Lamp aging

• Lamp sleeve fouling 

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• UV transmittance (UVT) of the water

• Inlet and outlet piping or channel configurations of the UV reactor 

• Dose distributions arising from the velocity profiles through the reactor 

• Failure of UV lamps or other critical system components 

• Measurement uncertainty of on-line sensors 

Unless the State approves an alternative approach, validation testing must involve the 

following: 

• Full-scale testing of a UV reactor that conforms uniformly to the reactors used by the 

utility. 

• Inactivation of a test microorganism whose dose-response characteristics have been 

quantified with a low-pressure (LP) mercury vapor lamp. 

4.2 Overview of Validation Process 

The validation process determines the log inactivation achieved for a specific pathogen 

and relates it to the operating conditions at the time of the testing (e.g., UV transmittance at 

254 nm, or UVT, flowrate). Figure 4.1 shows the key steps of a validation process, with the 

differentiation of Tier 1 and Tier 2 approaches. 

The experimental portion of the validation process is referred to as "biodosimetry." It 

consists of a UV reactor test that measures log inactivation of a surrogate (challenge) 

microorganism under various flowrate, UVT, and lamp power combinations. Log inactivation is 

then benchmarked to the corresponding operational conditions and UV intensity sensor values. 

Since the true UV doses delivered to the challenge microorganisms cannot be measured directly 

by the UV reactor, a separate test must be conducted to relate the inactivation measured in the 

field to a UV dose value. Current practice in the UV industry uses a collimated beam to generate 

a UV dose-response curve for a given challenge microorganism (log inactivation versus UV 

dose). The log inactivation from the biodosimetry test is then related to a UV dose from the UV 

dose-response curve. This dose is termed the reduction equivalent dose (RED). 

Hydraulic effects, UV reactor equipment, and error in on-line UV intensity sensors all 

create uncertainty in translating an RED measured during a validation test to a given level of 

pathogen inactivation during routine operation. To account for this uncertainty, a safety factor 

should be applied to the required UV dose values for pathogen inactivation credit. The required 

UV dose value multiplied by the appropriate safety factor is the RED that should be 

demonstrated during a validation test for a given level of pathogen inactivation credit. Tier 1 and 

Tier 2 approaches provide methods for incorporating the safety factor to determine the log 

inactivation credit. 

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Figure 4.1 Steps of a Validation Process 

Collimated Beam Test

Log Inactivation and corresponding UV intensity

sensor values

Biodosimetry Test

Challenge Microorganism

Influent 

Sample

Effluent 

Sample

1 1

Log Inactivation 

UV Dose

Generate UV-dose Response for 

Challenge Microorganism

Determine UV Dose for 

Log Inactivation Measured

Reduction Equivalent Dose (RED)

Log Inactivation 

(fro

m biodosimetry)

UV Dose (from collimated beam)

2

Determine Log Inactivation Credit

Tier I (Preset Safety Factors)

- Experimental plan and results should meet

specified criteria

- Uses Tier I RED Target Tables (Table 4.1 and 

4.2)

Tier 2 (Derive Safety Factors)

-Calculate uncertainties associated

with lamps, sensors, microbial

measurements, and interpolation of

data.

-Calculate bias associated with RED

measurements of challenge

microorganism vs. pathogen.

-Calculate bias of MP lamp

measurements (if applicable).

Calculate safety factor

from the above three results.

Calculate safety factor

from the above three results.

3

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4.2.1 Relating the Experimental RED to Log Inactivation Credit 

Chapter 1 presents the UV dose needed to achieve various inactivation credits for 

Cryptosporidium, Giardia, and viruses. These dose requirements were derived from batch 

(collimated beam) dose-response data and account for the uncertainty and statistical variability in 

the dose-response of the pathogen. 

There is significant, additional uncertainty associated with applying these batch data to

full-scale, continuous flow testing results. This additional uncertainty associated with UV 

reactor validation and on-line dose monitoring should also be considered when determining the 

log inactivation credit from UV reactor validation. To account for this uncertainty, the RED 

measured during validation should be greater than the dose requirement multiplied by a safety 

factor. The safety factor incorporates random uncertainty and corrections for expected variation, 

and is defined according to Equation 4.1: 

SF B B ( e = RED × Poly × 1+ ) Equation 4.1

where 

BRED = RED bias 

BPoly = Polychromatic bias

e = Expanded uncertainty expressed as a fraction 

The RED bias is a correction that accounts for the difference between the expected dose 

delivered to the target pathogen and the actual dose measured using a challenge microorganism

during biodosimetry. That is, the RED measured for two microorganisms is not identical if the 

dose-response behavior of the two microorganisms is different. The magnitude of the difference 

will depend on the dose distribution of the UV reactor and the unique inactivation kinetics of the 

challenge microorganism and target pathogen. If the challenge microorganism is more resistant 

to UV light than the target pathogen, the RED measured during validation will be greater than 

the expected dose delivered to the pathogen. If the challenge microorganism is as sensitive or 

more sensitive to UV light than the target pathogen, the RED bias has a value of one. Appendix 

F describes this concept in more detail. 

The polychromatic bias is a correction for the spectral differences in the lamp output, 

lamp sleeve UV transmittance, water UVT, and action spectrum between validation and 

operation of a UV reactor. This bias only applies to polychromatic lamps. 

The expanded uncertainty, e, accounts for the uncertainty in the measurements taken 

during validation and associated with the equipment (e.g., UV intensity sensors) used to monitor 

dose delivery. 

Appendix F discusses in greater detail the basis for the uncertainty and bias terms of the 

safety factor. Later sections of this chapter and Appendix C describe the application of the 

safety factor. 

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4.2.1.1 Tier 1 and Tier 2 Approaches for Establishing Inactivation Credit 

As stated previously, the Tier 1 and Tier 2 approaches differ in the complexity of the 

method used to determine the log inactivation credit based on the RED measured during 

biodosimetry. The Tier 1 approach provides RED target values to be met during validation that 

correspond to the log inactivation credit (presented in Tables 4.1 and 4.2). These RED values 

incorporate pre-determined safety factors based on characteristics of the UV reactor and 

validation testing (section 4.6 provides further details). In the Tier 2 approach, the user 

calculates the safety factor using detailed knowledge of the equipment and testing conditions and

then applies it to the required dose. This allows the user to optimize their experimental methods 

which may reduce the safety factor. 

4.2.2 Location and Application of Validation Testing 

Validation testing may be conducted either on-site, being the location where the UV 

reactor will be installed and operating, or off-site. Off-site validation may be conducted at either 

a manufacturer’s facility or at a centralized facility dedicated to validating a variety of UV 

equipment. 

Reactors may be validated for a specific WTP or may validate under a wide array of 

conditions for a variety of treatment applications. In addition to a range of operating conditions 

(e.g., flowrate, UVT), the reactors may also be validated for a wide range of target doses, thereby 

allowing reactor operation to be tailored to achieve different levels of pathogen inactivation 

credit at different WTPs. The test conditions and target doses can allow interpolation of the 

validation data to conditions of flowrate, UVT, and lamp output specific for application to 

various WTP applications. Section C.4.9.3 describes interpolation of validation results as a 

function of those variables. 

Utilities installing a pre-validated UV reactor should ensure that validation conditions are 

appropriate for their plant operations and the quality of testing is acceptable to their State. At a 

minimum, the following hydraulic and operating test conditions impact the application of prevalidated UV reactors: 

• UV reactor inlet and outlet configurations 

• Flowrate 

• UVT 

Validating on-site at the WTP is not trivial and should be regarded as a relatively 

complex experimental procedure. Utilities conducting on-site validation should consider the 

following issues (section C.3.1 provides further details): 

• Obtaining water with a sufficiently high UVT to allow validation over the entire UVT 

range expected at the WTP 

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• Adequate facilities to culture the challenge microorganism to the necessary levels to 

demonstrate the desired inactivation

• Adequate facilities and chemicals to adjust UVT to the range expected during fullscale operation 

• Providing sufficient mixing of additives prior to entering the UV reactor and mixing 

of the challenge microorganisms after the reactor 

• Obtaining permits for the disposal of water used for validation 

• Verification of the behavior of UV intensity sensors used during validation (sections 

C.3.2 and C.4.7) 

• Testing with inlet and outlet conditions representative of those conditions used at the 

WTP (issue for off-site validation)

UV reactors previously validated under existing protocols may receive inactivation credit 

if the validation used the appropriate challenge microorganism(s) and test conditions met the 

needs of the operating conditions at the WTP. Both the Austrian Standard ONORM M 5873-1 

and German Guideline DVGW W294 require an RED of 40 mJ/cm2

, using a microorganism

more representative of Cryptosporidium (B. subtilis) than that used to develop Tier 1 criteria 

(MS2 phage). Based on criteria in this document, UV reactors validated with those protocols 

should be granted 3 log Cryptosporidium and Giardia inactivation credit. Validation by 

NWRI/AwwaRF Guidelines and NSF Standard 55 should be evaluated on a case-by-case basis 

as indicated in Appendix C. 

4.2.3 Third-Party Oversight 

Third-party oversight is recommended to ensure that validation testing and data analyses 

are conducted in a technically-sound manner and without bias. The validation testing should be 

overseen by a registered professional engineer, independent of the UV manufacturer, with 

experience in testing and evaluating UV reactors. Furthermore, expert opinion should be sought 

from additional parties in areas of UV validation where the engineer has limited experience. 

These areas can include, but are not limited to, lamp physics, optics, hydraulics, microbiology, 

and electronics. 

4.3 Considerations for Validation Testing 

This section highlights the key factors that should be considered in the early planning 

stages of UV reactor validation. 

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4.3.1 Inlet and Outlet Hydraulics 

The inlet and outlet configurations of the validation location should produce conditions 

that result in equal or worse dose delivery than those that will be obtained at the WTP. Sections 

3.1.4.3 and C.3.1.5 provide recommended approaches to ensure such hydraulic conditions. 

Computation fluid dynamics (CFD)-based dose modeling can also be used in conjunction 

with any approach to conservatively address reactor hydraulics during testing. However, due to 

uncertainty in the CFD predictions, the predicted dose delivery during validation should be at 

least 20 percent greater than the dose delivery predictions at the WTP. 

4.3.2 UV Equipment 

This section discusses the following UV equipment related issues: documentation, 

monitoring control strategies, UV intensity sensors, and lamp aging effects. 

4.3.2.1 UV Reactor Documentation 

In the weeks prior to testing, the UV manufacturer should provide documentation 

identifying and describing the UV reactor to the testing organization (or to third-party oversight

if the manufacturer is conducting the testing with their facilities). This documentation should 

include all reactor and component information relating to dose delivery and monitoring, such as 

technical descriptions of all internal components, lamp and sleeve specifications, UV intensity 

sensor and sensor port information. See section C.2.2 for a complete list and discussion of the 

documentation requirements. 

4.3.2.2 Control Strategies 

The UV reactor’s control strategy for monitoring dose delivery affects the selection of

test conditions (i.e., flowrate, UV intensity, and UVT). At present, three strategies are 

commonly used for monitoring UV dose delivery. Sections C.4.9.4.1 to C.4.9.4.3 describe these 

strategies in detail and recommend validation conditions for each. (Sections 3.1.5 and 5.5 also 

describe these strategies with relation to design and operation, respectively). 

• UV intensity setpoint – relies on UV intensity measurements (i.e., UV intensity 

sensors) and flowrate to confirm dose delivery. The system is in compliance 

when the measured intensity value is greater than the setpoint at that flowrate. 

• UV intensity/UVT setpoint – relies on the UVT as well as the UV intensity and 

flowrate to determine dose delivery. The system is in compliance when both the 

UV intensity and UVT are greater than the preset setpoint values. 

• Calculated dose – relies on calculated dose delivery from UV intensity, UVT (in 

some cases), lamp power and flowrate using an algorithm provided by the UV 

reactor manufacturer. Typically, this method is tested over a range of 

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combinations of flow, UVT, and lamp power to determine the UV dose and 

validate the algorithm.

4.3.2.3 UV Intensity Sensor 

Monitoring of the UV dose is achieved through the use of on-line UV intensity sensors. 

The properties of both on-line and reference sensors should be measured by an independent 

laboratory that is equipped to confirm sensor calibration and measure the sensor’s angular and 

spectral response, linearity over the working range, and temperature response. The Tier 1 

approach specifies criteria for sensor placement in the UV reactor, sensor spectral response, and 

measurement uncertainty. 

4.3.2.4 Lamp Aging 

Prior to the initiation of validation testing, all lamps should undergo 100 hours of burn-in. 

This practice improves the stability of lamp output. Additional testing may also be performed, if

requested, in order to assess the effects of lamp age on dose delivery. With time, mediumpressure (MP) UV lamps can undergo non-uniform aging that causes spectral shifts in output. 

These changes can have an impact on the dose delivery registered by the monitoring systems. 

Manufacturers should test dose delivery of new and aged lamps to determine if the aged lamps 

reduce disinfection performance. If so, validation should be conducted using both new and aged 

lamps. (Section C.4.8 describes a procedure for testing new versus aged lamps.) 

4.3.3 Additives Used in Validation Testing 

4.3.3.1 Challenge Microorganism 

UV reactor validations should be performed with a microorganism with the following 

characteristics: inactivation kinetics closely resembling those of the target pathogen and the 

ability to be cultured in a reproducible manner to high concentrations. Currently, research has 

not identified such a microorganism that is ideal for Cryptosporidium. Challenge 

microorganisms typically used include MS2 phage and Bacillus subtilis, both of which are 

significantly more resistant to UV than Cryptosporidium. 

The RED bias, an important component of the safety factor, is due to the differences in 

inactivation kinetics between the challenge microorganism and the target pathogen. Under the 

Tier 1, the RED bias is based on MS2 phage as the challenge organism. If a challenge 

microorganism is identified in the future that exhibits a dose-response similar to the target 

pathogen (e.g., Cryptosporidium), the RED bias could be decreased. 

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4.3.3.2 UV-Absorbing Compound 

During validation, the UVT can be lowered through the addition of a UV-absorbing 

compound to simulate the range of UVT that may be encountered for a given UV application. 

For the validation of MP UV systems, the absorbing compound should have a UV absorbance 

spectrum similar to the water being treated in the full-scale application. However, obtaining an 

exact replica is usually not possible. Coffee and lignin sulfonate are commonly used UV 

absorbing compounds; however, sodium thiosulfate and fluorescein have also been used with 

some success. 

The polychromatic bias, a component of the safety factor for only MP reactors, is 

determined as a function of the UV-absorbing compound. The Tier 1 approach specifies criteria 

for minimum UVT for MP reactors using UV-absorbing compounds and applies a correction 

factor based on validation testing performed to-date with various UV absorbing compounds. 

4.4 Validation Testing 

Validation provides an assessment of UV reactor dose delivery and monitoring under 

specific conditions of flowrate, UVT, and lamp output. This section briefly discusses the steps 

involved in conducting a validation test and provides references to more detailed procedures in 

Appendices C and D. 

4.4.1 Microorganism Preparation 

Challenge microorganisms should be prepared in accordance with peer-reviewed 

methods. All information regarding the source of the host, media descriptions, and preparation 

steps should be documented. It is expected that the microorganism stock will be prepared by 

laboratory personnel familiar with methodologies designed to prevent microbial stock 

contamination. The use of these same techniques in the field during validation is critical and any 

personnel participating in the validation should be familiar with them to avoid sample 

contamination. 

Preparation methods for the two most common challenge microorganisms, MS2 phage 

and B. subtilis spores, are provided in Appendix D. Note, the same batch of challenge 

organisms should be used for both collimated beam and biodosimetry testing, as described 

below. 

4.4.2 Collimated Beam Testing 

The collimated beam data are used to develop the dose-response curve for the challenge 

microorganism. A collimated beam apparatus typically consists of an enclosed low-pressure UV 

lamp and a tube with a non-reflective inner surface (see Figure 4.2). A sample of the challenge 

microorganism (preferably taken from the influent to the biodosimetry test stand) is placed in a 

petri dish and exposed to the UV light for a predetermined amount of time. The UV dose is 

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calculated using the intensity of the incident UV light, UV absorbance of the water, and exposure 

time. Appendix E provides a complete description of collimated beam testing. 

At least two water quality conditions should be tested−one with the highest UVT (no 

absorbing chemical added) and a second with the lowest UVT used in the biodosimetry test. UV 

doses should be selected to target microorganism inactivations of approximately 0.5, 1.0, 2.0, 

3.0, 4.0, and 5.0 log. 

Figure 4.2 Collimated Beam Test Apparatus 

Low-Pressure

Mercury Arc Lamp Lamp Enclosure

Petri Dish Containing

Microbial Suspension

Magnetic Stirrer

UV Light @ 254 nm

Collimating Tube

4.4.3 Biodosimetry of Full-Scale Reactors 

The biodosimetry test is used to determine the inactivation of the challenge 

microorganism by the UV reactor under continuous-flow test conditions. Figure 4.3 provides a 

schematic of the components used in a typical biodosimetry test. Section C.3.1 describes the key 

features. 

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UV Disinfection Guidance Manual 4-11 June 2003 

Figure 4.3 Biodosimetry Test Components 

he following facilities are typically required in biodosimetry testing: 

Injection of the challenge microorganism and UV absorbing compound 

• ixing of the added compounds upstream and downstream of the reactor before 

• low measurement 

• ressure measurement upstream and downstream of the reactor 

• ample collection before and after the UV reactor 

Proper facilities should be provided, along with appropriate permits, to discharge the 

treated the

ent and

p

detailed description of sampling requirements is provided in Appendix C. 

4.5 Data Analysis 

esults from the collimated beam testing, biodosimetry testing, and uncertainties 

associa ved by

. Developing a UV dose-response curve for the challenge microorganism from the 

Challenge

Microbe

Backflow

Prevention

Static

Mixer

Static

Mixer

Flow

meter

UV

Reactor

Valve

To

Waste

Effluent

Sample

Port

Influent

Sample

Port

Valve

Water

Supply Inlet

Piping

UV

Absorber

Outlet

Piping

Pressure

Gage

Pressure

Gage

Influent

Quenching

Agent

T

• 

M

sampling 

F

P

S

water. The testing should be conducted after steady-state conditions are achieved for

desired matrix of experimental conditions evaluating variations in challenge organism

concentration, flowrate UVT, and lamp power/output. Samples collected from the influ

effluent sample ports are used to determine the inactivation achieved for the specific reactor 

condition being tested. Operational parameters, such as UV intensity, flowrate, UVT, and lam

power, are measured during the test.

A

R

ted with equipment and data are used to determine the log inactivation credit achie

the UV reactor. Data analysis consists of four steps: 

1

collimated beam test 

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UV Disinfection Guidance Manual 4-12 June 2003 

2. Calculating log inactivation from the biodosimetry test 

. Determining the RED(s) from the results of steps 1 and 2 

4. Applying safety factors to determine log inactivation credit (Tier 1 or Tier 2 

he following sections describe these steps. References to the appropriate sections in 

Append

4.5.1 eveloping Challenge Microorganism Dose-Response Curve 

ose-response curves should initially be generated separately for each collimated beam

test con

nt

er

he following sub-sections describe how to calculate the log inactivation from collimated 

beam t

.5.1.1 Calculate Dose-Response Data From Collimated Beam Testing 

he log inactivation for each dose delivered by the collimated beam should be calculated 

using E

3

approach) 

T

ix C are provided for further details and examples. 

D

D

dition (a minimum of two conditions—lowest and highest UVT—is recommended). The

curves should predict similar dose-response relationships, as indicated by statistical analyses. If 

statistically similar, the data can be combined and one curve generated for the entire dataset. If

the curves are statistically different, the cause of the difference should be determined, and the 

test should either be redone or the different dose-response curves should be used for the differe

test conditions. Differences in UV dose-response could occur if the dose-response were 

determined with different batches of the challenge microorganism or if coagulation or oth

water quality interferences impacted the dose-response. 

T

est data and generate a dose-response curve. Section C.4.9.7.2 discusses the statistical 

analysis for comparing curves and combining data. 

4

T

quation 4.2:

⎟

⎠

⎞ ⎜

⎝

⎛ = N

N Log Inactivation 0 log Equation 4.2 

here

= Challenge microorganism concentration in an aliquot of sample 

o dose aliquots

w

N 

N0 = Average concentration of the challenge microorganism in the zer

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UV Disinfection Guidance Manual 4-13 June 2003 

4.5.1.2 Fitting Dose-Response Data to a Curve 

The following steps describe how to develop a dose-response curve: 

1) Plot log inactivation achieved as a function of UV dose in the collimated 

beam test 

2) Use regression analysis to derive an equation that best fits the data 

• For first-order kinetics, a linear equation should fit best: 

Dose = A× Log Inactivation + B

• For dose-responses showing tailing effects, a quadratic equation should fit 

best: 

( )

2 Dose = C × Log Inactivation + D × Log Inactivation

• For dose-response showing shoulder effects, other polynomial equations 

should be used. 

3) Evaluate fit of equation 

• Equation coefficients should be significant at a 95 percent confidence 

level (section C.4.9.7.1 provides an example that uses p-statistics to 

evaluate the coefficients). 

• Confidence intervals for the fit should be determined at an 80 percent 

confidence level. (The Tier 1 approach specifies criteria the confidence 

intervals must meet and the Tier 2 approach includes an uncertainty term

for the confidence intervals in the safety factor calculation.) 

• The differences between the predicted dose and measured dose at a given

log inactivation should be randomly distributed around zero and not 

dependent on dose. In other words, the data points should be randomly 

distributed above and below the curve (section C.4.9.7.1 provides an 

example of this evaluation). 

4.5.2 Determining Log Inactivation from Biodosimetry Testing 

At each test condition—flowrate, UVT, and lamp output—the arithmetic mean and 

standard deviation of the log influent and effluent challenge microorganism concentrations 

should be calculated. From the mean concentrations, log inactivation should calculated using the 

following equation:

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UV Disinfection Guidance Manual 4-14 June 2003 

( ) ( NI NE Log Inactivation = log − log ) Equation 4.3

where 

log(NI) = Mean challenge microorganism log concentration of the reactor influent samples 

log(NE) = Mean challenge microorganism log concentration of the reactor effluent samples 

The standard deviation is used in the safety factor calculation for Tier 2, while Tier 1 specifies a 

limit for the standard deviation. 

4.5.3 Determining the RED 

This section describes how to calculate RED values for all test conditions and select the 

appropriate RED for subsequent log inactivation credit determination. 

 

4.5.3.1 Calculating the RED Values 

The RED is calculated by inputting the biodosimetry log inactivation values for each test 

condition into the equation describing the UV dose-response curve of the challenge 

microorganism. 

Example. For 0.5 MGD flow, 80 percent UVT, and lamp output of 70 percent, the 

inactivation calculated from Equation 4.3 was 4.0 log. The UV dose-response equation 

was best fit with the equation:

Dose = 15.5× Log Inactivation − 6.0

Inputting 4.0 log into the above equation results in an RED of 56 mJ/cm2

. This 

calculation should be repeated for each test condition (i.e., flowrate, UVT, and lamp

output combination). 

4.5.3.2 Selecting the Appropriate RED for Log Inactivation Credit 

Determination

Since the biodosimetry test is conducted at various flowrates, UVT, and lamp output 

combinations, the validation results will have more than one RED value for each setpoint. 

Choosing the appropriate RED to determine log inactivation credit depends first on the 

monitoring approach used to indicate dose delivery. The following three approaches are 

considered in this text: 

• UV intensity setpoint approach - the UV reactor should be rated at the lowest 

inactivation observed for each set point condition tested. 

• UV intensity and UVT setpoint approach - the UV reactor should be rated at the 

inactivation observed with UV reactor operation under setpoint conditions. 

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UV Disinfection Guidance Manual 4-15 June 2003 

• Calculated dose approach - the UV reactor should be rated at the lowest inactivation 

observed for each calculated dose setpoint evaluated. 

Section C.4.9.4 recommends validation conditions for each of the above approaches. 

Section C.5 provides examples of interpreting validation results for the different approaches. 

4.5.3.3 Interpolating RED as a Function of Test Conditions 

The RED measured during validation testing can be interpolated as a function of inverse 

flowrate, UVT, or UV intensity by fitting an equation to the data being interpolated (e.g., RED as 

a function of inverse flowrate). The equation should not be used for extrapolation (i.e., 

projecting RED outside the range of tested conditions). The following provides guidelines for 

interpolation:

• The equation should pass through the origin (0,0) if the RED is interpolated as a 

function of measured intensity or inverse flowrate 

• The equation coefficients should be significant at a 95 percent confidence level 

• The differences between the values measured and predicted by the equation should be 

randomly distributed around zero 

• An 80 percent confidence interval should be used to determine the uncertainty of the 

equation used to interpolate the RED values. For Tier 1, the uncertainty of the 

interpolation should be 10 percent or less at an 80 percent confidence level. For Tier 

2 it should be included as an uncertainty term in the safety factor calculation as 

described in section C.4.10.2.3. 

4.5.4 Determining Inactivation Credit 

As discussed in the introduction to this chapter, there are two approaches described for 

determining log inactivation. 

• Tier 1 - pre-determined safety factor. 

• Tier 2 - calculated safety factor from the following dose delivery monitoring and 

validation bias and uncertainties:

- RED bias 

- Polychromatic bias (for MP reactors)

- Measured RED 

- Interpolation of RED as a function of flowrate, UVT, or UV intensity 

- Sensors used during validation (UV intensity, UVT) 

- On-line and reference sensors used at WTP (UV intensity, UVT) 

- Lamp output quantification 

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UV Disinfection Guidance Manual 4-16 June 2003 

The remainder of this chapter describes how to determine the log inactivation credit 

achieved using the Tier 1 approach and the criteria that should be met in order to use this 

approach. Appendix C contains a detailed description of the basis the Tier 2 approach. 

Tier 1 Log Inactivation Credit

Tables 4.1 and 4.2 present the RED target values for UV reactors using LP/LPHO and 

MP lamps, respectively. The values in these tables are derived by multiplying the required dose 

values by the Tier 1 safety factors (see Appendix C for details). The values in Table 4.2 (MP) 

are higher than in Table 4.1 (LP/LPHO) because they include the polychromatic bias, which is 

not a factor in monochromatic (LP/LPHO) reactors. 

For a given pathogen and level of log inactivation credit, the RED measured during 

validation should be greater than or equal to the corresponding RED target listed in the table. 

Note, validation testing with multiple setpoints may result in different log inactivation credits for 

the different setpoints. 

Example. Using an LP reactor and meeting the Tier 1 validation criteria (see section 

4.6), the lowest RED measured for the challenge microorganism during validation was 

29 mJ/cm2

. Consequently, the log inactivation credits achieved are 2.5 for 

Cryptosporidium and 2.5 for Giarida. No inactivation credit is achieved for viruses. 

Table 4.1 Tier 1 RED Targets for UV Reactors with LP or LPHO Lamps 

RED Target (mJ/cm2 Log )

Inactivation 

Credit 

Cryptosporidium Giardia Virus

0.5 6.8 6.6 55

1.0 11 9.7 81

1.5 15 13 110

2.0 21 20 139

2.5 28 26 169

3.0 36 34 199

3.5 - - 227

4.0 - - 259

Table 4.2 Tier 1 RED Targets for UV Reactors with MP Lamps 

RED Target (mJ/cm2 Log )

Inactivation 

Credit 

Cryptosporidium Giardia Virus

0.5 7.7 7.5 63

1.0 12 11 94

1.5 17 15 128

2.0 24 23 161

2.5 32 30 195

3.0 42 40 231

3.5 - - 263

4.0 - - 300

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UV Disinfection Guidance Manual 4-17 June 2003 

4.6 Tier 1 Criteria 

The safety factors derived for the Tier 1 approach are based on assumed uncertainties and 

corrections for given experimental methods. For these assumptions to be practical, and thus the 

use of Tier 1 numbers appropriate, the validation conditions should meet the criteria specified in 

this section. Note, the equipment criteria should be provided by the UV manufacturer and 

reviewed by a third-party for verification. 

4.6.1 UV Intensity Sensors 

• UV reactors with MP lamps should be equipped with one sensor per lamp. UV 

reactors with LP or LPHO lamps should be equipped with at least one sensor per 

bank of lamps. 

• UV intensity sensors should view a point along the length of the lamp that is between 

the electrode (lamp end) and within 25 percent of the arc length away from the 

electrode. 

• UV intensity sensors should have a spectral response that peaks between 250 and 280 

nm. When mounted on the UV reactor and viewing the lamps through water, the 

measurement of UV light greater than 300 nm made by the sensor should be less than 

10 percent of the total measurement made by the sensor. Conformance to these 

criteria can be demonstrated using UV intensity field modeling. Figure 4.4 presents 

examples of two sensors where both have the appropriate peaks, but one has too 

much UV light in the >300 nm range. 

• The UV intensity sensors used during validation and the duty and reference sensors 

used during operation of the UV reactor at the WTP should provide National Institute 

of Standards and Technology (NIST)-traceable measurements with an uncertainty of 

± 15 percent or less at an 80 percent confidence level. 

• During operation of the UV reactor at the WTP, measurements made by the duty UV 

intensity sensor should be checked using a reference UV intensity sensor. If the duty 

sensor reads higher than the reference sensor (i.e., overestimating dose delivery), or 

substantially lower, it should be recalibrated. For a recommended control standard, 

the duty sensor should not read less than the reference by the following amount: 

( ) 2

1 2

Duty

2

Ref

Ref

Duty 1 100

I

I

× ≤ σ + σ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − Equation 4.4

where 

 IRef = Intensity measured by the reference sensor 

 IDuty = Intensity measured by the duty sensor 

σRef = Measurement uncertainty of the reference sensor (%) 

σDuty = Measurement uncertainty of the duty sensor (%) 

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UV Disinfection Guidance Manual 4-18 June 2003 

• If the dose-monitoring strategy uses an on-line UVT monitor, the UV absorbance at 

254 nm (A254) calculated from the measured UVT should have an uncertainty of ±10 

percent or less at an 80 percent confidence level.

Figure 4.4 Examples of UV Intensity Sensor Spectral Response Ranges 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

200 250 300 350 400

Wavelength (nm)

S

p

e

ctral R

e

spons

e

R

elative t

o 254 n

m

Unfiltered 

Sensor

Filtered Sensor

250 280

Filtered Sensor. Detected UV light with a 0 cm

sensor-to-lamp water layer. Detected UV > 300 nm

is 0.7% of total UV light detected. 

Unfiltered Sensor. Detected UV light with a 0 cm

sensor-to-lamp water layer. Detected UV > 300 nm

is 41% of total UV light detected. 

0

5

10

15

20

25

200 250 300 350 400

Wavelength (nm)

D

etect

e

d Irra

diance

0

10

20

30

40

50

200 250 300 350 400

Wavelength (nm)

D

etect

e

d Irra

diance

Filtered Sensor. Detected UV light with a 20 cm

sensor-to-lamp water layer. Detected UV > 300 nm

is 5% of total UV light detected. 

Unfiltered Sensor. Detected UV light with a 20 cm

sensor-to-lamp water layer. Detected UV > 300 nm is

85% of total UV light detected. 

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

200 250 300 350 400

Wavelength (nm)

Detected Irradiance

0.0000

0.0100

0.0200

0.0300

0.0400

200 250 300 350 400

Wavelength (nm)

Detected Irra

diance

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UV Disinfection Guidance Manual 4-19 June 2003 

4.6.2 UV Lamp Output 

• The standard deviation of the UV output of LP or LPHO lamps should be 15 percent 

or less of the mean output. The standard deviation should be determined using either 

life test or field test data on aged lamps. 

4.6.3 Flow Measurements 

• The flow measurements made during validation and during operation of the UV 

reactor at the WTP should have an uncertainty of ± 5 percent or less at an 80 percent 

confidence level. 

4.6.4 Collimated Beam Apparatus 

• The calculated dose delivered by the collimated beam apparatus should have a 

measurement uncertainty of ± 15 percent or less at an 80 percent confidence level. 

4.6.5 Challenge Microorganism Dose-Response 

• Over the range of doses within one log unit of the log inactivation demonstrated 

during validation, the UV sensitivity of the challenge microorganism should be less 

than or equal to 25 mJ/cm2

 per log inactivation (the dose-response of a resistant strain 

of MS2). For example, if the challenge microorganism log inactivation measured by 

the UV reactor ranges between 1.5 and 3.5 log, the dose-response of the challenge 

microorganism should be less than or equal to 25 mJ/cm2

 per log inactivation 

between 0.5 and 4.5 log inactivation. 

• If the dose-response of the challenge microorganism has a shoulder, that shoulder 

should not occur over a dose range greater than 50 percent of the RED demonstrated 

during validation. The shoulder is defined by extrapolating the exponential reduction 

region of the dose-response curve to the dose-axis (see Figure 4.5). 

• If the dose-response demonstrates tailing, the tailing should not occur until one log 

reduction greater than the log reduction demonstrated during validation. 

• A plot of dose versus log inactivation for the collimated beam test should have an 80 

percent confidence interval of 10 percent or less at the log inactivation demonstrated 

by the UV reactor. 

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UV Disinfection Guidance Manual 4-20 June 2003 

Figure 4.5 Dose-Response With a Shoulder 

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 25 50 75

UV Dose (mJ/cm2

)

Log I

n

a

ctiv

atio

n

100

This range should be less than 1/2

the RED demonstrated during

validation. In this case, the RED

should be >20 mJ/cm2

.

4.6.6 Medium Pressure Lamps 

• During validation, the UVT of the water at 254 nm should be greater than the values 

specified in Figure 4.6 for a given sensor-to-lamp water layer and UV-absorbing 

chemical. The sensor-to-lamp water layer is defined as the distance traveled through 

water by UV light passing from the lamp to the sensor. The values in Figure 4.6 were 

taken from Figure C.7 of Appendix C for a polychromatic bias of 1.2. 

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UV Disinfection Guidance Manual 4-21 June 2003 

Figure 4.6 Criteria for the Minimum UVT of MP Reactors under Tier 1 

75

80

85

90

95

100

0 5 10 15 20 25 30

Sensor-to-lamp wate r layer (cm)

Mini

m

u

m

U

V

T

at 2

5

4 nm (%)

Coffee

Lignin Sulphonate

NOM

4.6.7 Biodosimetry Sampling 

• Five influent and five effluent samples should be collected for each test condition and

evaluated as described in section C.4.9.5. 

• The standard deviation of the challenge microorganism concentration measured with 

the influent and effluent samples should be less than or equal to 0.20 log units. 

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5. Start-Up and Operation of UV Installations 

This chapter describes the start-up activities and routine operational issues associated 

with a UV disinfection facility. The start-up discussion focuses on the functional and 

performance testing that should be conducted during the start-up process. The remainder of the 

chapter describes the requirements and recommendations for operation, maintenance, 

monitoring, and reporting for UV installations. The organization of this chapter is presented 

below by the key question each section addresses.

• What is included in final UV installation inspection? ................................Section 5.1.1 

• What testing should be completed during start-up?.................. Sections 5.1.2 and 5.1.3 

• What items should be included in the operations and 

maintenance manual?..................................................................................Section 5.1.4 

• What are the operational requirements and recommended 

tasks? ........................................................................................ Sections 5.2.1 and 5.2.2 

• What are the routine start-up and shutdown procedures? ..........................Section 5.2.3 

• What maintenance tasks are recommended? .............................................Section 5.3.1 

• What spare parts are recommended to be kept on hand?............................Section 5.3.3 

• What monitoring is required for regulatory compliance?...........................Section 5.4.1 

• What additional monitoring is recommended? ..........................................Section 5.4.2 

• What should be reported to the State? .......................................................Section 5.4.3 

• How do you determine the operational requirements from

validation testing? ........................................................................................ Section 5.5 

• What should be done if there is: .................................................................. Section 5.6 

- Low UV intensity? 

- High UV absorbance? 

- Rapid flow increase/high flow?

- Unreliable UV intensity sensor readings?

- Power loss? 

• What staffing issues are associated with operation, 

maintenance, and monitoring of UV installations? ...................................... Section 5.7 

Given the wide range of UV installations and UV reactors available, this document 

cannot address or anticipate all scenarios. The guidelines provided in this manual are a 

compilation of industry experience and manufacturers’ recommendations. Therefore, they may 

UV Disinfection Guidance Manual 5-1 June 2003

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UV Disinfection Guidance Manual 5-2 June 2003

differ from those provided by specific manufacturers for their equipment. In these situations, the 

manufacturer’s standards should be followed. 

The general process to be followed for the start-up and routine operation of a UV 

installation is shown in Figure 5.1. A detailed description of each activity is given in the 

remainder of this chapter. 

Figure 5.1 Start-up and Operation Flowchart 

Routine Operations

Sections 5.2.1 to 5.2.4

Report to State

Section 5.4.3

Completion of

Construction and

Inspection

Final Inspection

Section 5.1.1

Maintenance

Section 5.3

Monitoring and

Recording

Sections 5.4.1 to

5.4.2

Determination of

Operational

Requirements

Section 5.5

Appendix C

Staffing Issues

Section 5.7

Operational

Challenges

Section 5.6

Routine Operation

Start-Up Activities

Functional Testing

Section 5.1.2

Performance Testing

Section 5.1.3

Operations and Maintenance

Development

Section 5.1.4

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5. Start-up and Operation of UV Installations 

UV Disinfection Guidance Manual 5-3 June 2003

5.1 Start-up of UV Installation 

For the purposes of this manual, the start-up of the UV installation is considered as the 

transition from the construction phase to the operation phase. Start-up activities include final 

inspection of the UV reactors and ancillary equipment, functional testing, performance testing, 

operations and maintenance (O&M) manual development. Functional testing confirms the 

mechanical, instrumentation and controls, and hydraulic conditions of the UV installation to 

ensure they meet the requirements of the contract documents. It also verifies that the operational 

conditions are consistent with the validated conditions. Performance testing verifies that the UV 

reactors are operating in accordance with the contract documents. In addition, an O&M manual 

should be developed during UV installation start-up. 

A start-up plan should be developed in collaboration with the UV installation designer, 

plant operations staff, and the UV manufacturer. The designer will be most familiar with the 

layout of the reactors, piping, and how to integrate the UV installation with the other treatment 

processes. The operations staff will be able to identify potential impacts on routine plant 

operations. The manufacturer will be most familiar with operation of the UV reactors. The startup plan should include a pre-start checklist, a procedure for checking equipment installation and 

calibration (functional testing), a procedure for verifying system operation, and a procedure for 

checking alarm settings and system controls (performance testing). 

5.1.1 Final Inspection 

As the first step in the start-up process, a detailed inspection of the UV installation should 

be completed. The inspection should include a visual assessment to ensure that all components 

meet the technical specifications and that the UV installation was completed in accordance with 

the construction documents. The configuration of the piping and UV reactors should meet the 

constraints established during validation testing (see section 4.3.1). If on-site validation will be 

performed, the availability of the necessary features (e.g., feed and sample ports, mixing 

systems, drains) should be confirmed. In addition, leak testing should be performed, and then all 

UV installation components and associated valves and piping should be thoroughly cleaned and 

disinfected (State requirements may apply). 

5.1.2 Functional Testing 

Functional testing consists of a series of short duration tests that assess the ability of each 

component of the system to function in accordance with the specifications detailed in the 

contract documents. Some of the evaluations are conducted by monitoring performance during 

normal operations. However, the majority of functional testing is completed through simulations 

of specific operating conditions and monitoring the UV reactor operation and response. 

Functional testing entails flooding and energizing the UV reactors to confirm the operation of the 

following items: 

• UV lamps and UV intensity sensors

• Operating sequence and control logic for the reactor 

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UV Disinfection Guidance Manual 5-4 June 2003

• Ancillary equipment, including UV transmittance (UVT) monitors, flowmeters, and 

control valves 

• Electrical system components, including ballasts, uninterruptible or standby power 

supplies, and the ballast cooling system

It is strongly recommended that the UV manufacturer inspect the UV installation prior to 

energizing the UV reactors and be present when the UV reactors are first energized. 

Manufacturers may require the presence of one of their representatives during these activities as 

a condition of their equipment warranty. 

5.1.2.1 Verification of Mechanical Operation 

UV reactors may incorporate mechanical elements such as valves and on-line mechanical 

cleaning (OMC). During functional testing, the satisfactory operation of these mechanical 

components should be confirmed. The procedures used to confirm valve operation for a UV 

installation are not different from those for other applications that use valves for isolation or flow 

control and, therefore, are not described here. The OMC system, if provided, should be checked 

for proper operation. Specifically, the following items should be verified: 

• Smooth movement of the wiper with no jamming or binding of the wiper on the 

sleeve 

• Extension of wiper stroke to the full length of the sleeve with no impact at the end of 

travel that could damage or break the sleeve 

• Proper operation of the wiper drive mechanism and motor with no slipping or binding 

5.1.2.2 Verification of Monitoring Equipment

The monitoring equipment is important for UV reactor operation, and its proper operation 

should be verified during functional testing. 

Flowmeter

Accurate measurement of the flow is essential to ensure that the UV reactors are 

operating within the validated conditions. Not all utilities will install dedicated flowmeters. For 

those facilities that rely on flow measurement using an existing, common flowmeter (e.g., raw 

water flowmeter), the functionality of the flowmeter should be verified in conjunction with its 

intended use with the UV installation. Specifically, the accuracy and operating range of the 

flowmeter should be verified and the availability of the necessary output signals from the meter 

should be confirmed. If pressure gauges are used to monitor the flow split between UV reactors, 

the calibration and installation of the pressure gauges should be verified as well. 

The uncertainty associated with the existing flowmeter should be determined to ensure 

that the appropriate validation constraints were used. It is recommended that the original 

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UV Disinfection Guidance Manual 5-5 June 2003

certification of calibration be reviewed in conjunction with the equipment specifications to 

establish the measurement uncertainty for the existing flowmeter. There are three methods to 

verify the flowmeter operation: flow verificators, a time-discharge test, and a clamp-on 

flowmeter. The flow verificators assess the physical condition of the installed equipment relative 

to its condition at the time of factory calibration to confirm that the original uncertainty can be 

maintained. For example, verification of a magnetic flowmeter would consist of an insulation 

test of the entire flowmeter system and cable; testing of the sensor magnetic properties; testing of 

signal converter gain, linearity and zero point; testing of digital output; and testing of analog 

output. A time-discharge test compares the flowrate measured by the flowmeter against the 

value calculated by measuring the volume of water discharged over a predetermined amount of 

time (using a bucket, clearwell, or tank of known volume). A temporary or clamp-on flowmeter 

can be used to assess the accuracy of the existing flowmeter. It is important to consider the 

uncertainty of the reference flowmeter when using this approach. 

If a new flowmeter is used to measure the flow through the reactor, the flowmeter 

manufacturer should provide a certification of calibration at the time of equipment delivery. It is 

also recommended that the manufacturer inspect the UV installation and confirm that it was 

completed in accordance with their recommendations to ensure the certified accuracy of the 

flowmeter is achieved. The flowmeter measurement uncertainty should be equal to or better than

that used during validation. 

On-line UVT Monitor

An on-line UVT monitor may be included as part of the UV reactor, especially if a UV 

intensity and UVT setpoint or calculated dose control strategy (section 3.1.4.2) is used. The online UVT monitor should be calibrated and its operation verified. Calibration can be completed 

using a buffer solution of known UVT and may be operation may be verified by collecting and 

analyzing grab samples, using a bench top spectrophotometer. 

5.1.2.3 Verification of Instrumentation and Control Systems 

The amount of testing needed for the instrumentation and control systems is proportional 

to the complexity of the control strategy that is used. Testing should include verification of 

monitoring equipment (including calibration of all instruments), tuning of control loops, 

checking operation functions, and verifying all final control actions. As described below, the 

UV reactors should be run through a series of simulations that represent the possible operating 

scenarios in order to confirm that the appropriate UV reactor response occurs. Typically, the 

packaged UV reactor control panel contains all of the components to control and operate the UV 

reactor. The panel should provide the operating status, diagnostic information, and operator 

interface capability. It should also include lamp status indicators and programmable logic 

controllers (PLC) and may include ballasts, and lamp starters. The PLCs are typically used to 

control the operation of a UV reactor based on certain input signals. A manufacturer 

representative should be present during the simulations to assist in troubleshooting and 

addressing any issues that may result from the packaged UV reactor controls. 

Simulations should be used to confirm the operation of the UV reactors and the operation 

of all ancillary equipment and instrumentation, including valves, flowmeters, and UVT monitors. 

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As applicable, specific operating conditions that should be simulated include the following 

conditions: 

• Cold start of the UV reactors 

• Cool down and restart of the UV reactors 

• Sequencing of the UV reactors in multiple reactor installations

• Adjustment of lamp intensity in response to varying water quality or flowrate 

• Shutdown of the UV reactors 

• Operation of the UV reactors during line power failure (when backup or 

uninterruptible power supplies (UPS) are available) 

• Manual override, safety interlocks, and report generation 

During these simulations, the utility should record the amount of off-specification time 

and discharge volume (i.e., operation outside of validated conditions) associated with each 

action. This is necessary to assess the potential effect of the conditions associated with these 

actions on the utility’s ability to meet its disinfection goals and comply with the State-established 

limitations for off-specification operation. In addition to simulating possible operating 

conditions, each of the alarm conditions and monitoring functions incorporated in the design 

should be verified. Possible monitoring functions and alarm conditions are discussed in section 

3.3.3.8 and may include the following conditions: 

• Low UV dose and UV intensity 

• Low UVT 

• Low and high flowrate 

• Lamp age 

• Lamp or ballast failure 

• Low liquid level in the UV reactor 

• High temperature 

• OMC system failure 

5.1.2.4 Verification of Flow Distribution and Headloss 

If each reactor is not equipped with a dedicated flowmeter, then it will be necessary to

verify the flow split between reactors over the entire operating flow range. This flow split and 

the total plant flow should be used to estimate the flow through each UV reactor and confirm

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operation is within the validated conditions. Clamp-on type flowmeters or differential pressure 

readings across each parallel reactor are alternatives for field verification of the flow split. 

The allowable difference in flow among reactors (flow split differential) is established 

during validation and should be accounted for in the validation protocol safety factor (section 

4.2.1 and section F.5). If the actual flow split differential is greater than assumed in validation, 

then steps should be taken to improve the flow split. The Tier 1 recommendations for validation 

of UV reactors (section 4.6) necessitates a flow split differential of 10 percent or less. If this is 

not observed during functional testing, then a Tier 2 analysis for validation safety factor needs to 

be completed for the UV reactor. Appendix C provides details about the Tier 1 and 2 analysis 

and Appendix F provides details about the development of the safety factor. 

The headloss should be measured for each reactor and compared to the headloss specified 

in the contract documents (if applicable). Pressure transducers or pressure gauges can be used to 

measure the headloss. 

5.1.3 Performance Testing

Performance testing is intended to assess the operating performance of the UV reactor as 

a whole, as well as the individual performance of its components. While functional testing is 

primarily completed through simulations of specific operating conditions, performance testing is 

generally accomplished through extensive monitoring of reactor performance during the early 

stages of continuous operation. It is important to note that performance testing is not intended to 

validate disinfection performance, which is completed during validation testing (as described in 

Chapter 4). However, performance testing can be used to confirm that the actual operating 

conditions are within the constraints established during validation testing. Performance testing 

focuses on the accuracy, reliability, and repeatability of UV reactor operation, whereas validation 

is used to measure the effectiveness of the UV reactor at delivering the UV doses required for 

target pathogen inactivation credit. 

When UV lamps are first energized, they go through a stabilizing period called “burn-in.” 

For some UV lamp designs, the initial lamp output may significantly exceed the design value. 

During burn-in, the lamp output may rapidly decrease to a value more consistent with the design. 

Following burn-in, lamp output becomes relatively stable until the end of lamp life is 

approached. Typically, new UV lamps will not have undergone burn-in prior to installation. 

Because performance testing should compare actual operating conditions to validated conditions, 

it is important that the lamps be in the same condition as they were during validation testing. 

Therefore, UV lamps should be burned-in prior to performance testing, which typically takes 

100 hours of continuous operation. The actual required burn-in time should be discussed with 

the manufacturer and confirmed through documented operating experience at other UV 

installations. 

The duration of performance testing and the extent of monitoring will be project-specific 

and should be established by the utility and designer based on the objectives of the performance 

testing. Performance testing may range in duration from as little as 48 hours of uninterrupted 

operation to greater than four months of demonstrative operation. Similarly, the scope of the 

testing may range from an increased monitoring frequency to confirm performance to an 

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extensive testing protocol to fully optimize reactor performance and establish long-term

operating procedures. During performance testing, treated water may be sent to the distribution 

system if upstream treatment has not changed and meets existing regulations. However this 

should be confirmed with the State. 

Performance testing may include the following items: 

• Operation of each UV reactor in automatic mode and demonstration that actual 

operating conditions are within the constraints established during validation testing 

• Demonstration of UV reactor start-up and switchover sequences that result from

water quality and/or flowrate changes 

• Observation of operation, including periods of off-specification operation, due to 

power quality problems, and other alarm conditions 

• Measurement of electrical service voltage, current, and power consumption with 

different flow and water quality combinations to optimize energy use within the 

constraints established during validation 

• Assessment of the effectiveness of the cleaning system by inspecting sleeve clarity 

and condition at regular intervals throughout the test period 

• Confirmation that the programmed cleaning frequency correlates with the actual 

frequency of cleaning 

• Verification of UV intensity sensor operation 

• Confirmation of duty sensor accuracy using reference sensors (see section 5.3.2.2) 

• Observation of ballast temperature and cooling system performance 

• Verification of the accuracy and repeatability of the on-line UVT monitor through the 

collection of grab samples and analysis using a bench-top spectrophotometer (if 

applicable) 

• Confirmation of backup generator and/or UPS power transfer to the UV reactor. This 

may necessitate simulation of line power failure to trigger the backup power supply. 

It is recommended that the backup power supply be tested for a minimum of two 

separate one-hour periods. 

The performance testing should be tailored to the specific UV installation. An example 

monitoring program for a 4-week performance test is shown in Table 5.1. 

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Table 5.1 Example Monitoring During a Four Week Performance Test

Frequency Task Notes

Confirm the operating 

setpoint(s) 

Monitor reactor operation to confirm compliance with the 

setpoint(s) established during validation.

Continuous 

Develop energy 

efficient operation 

Monitor the power consumption. Test the automatic 

operation and power consumption under the flow and 

water quality variations to determine if energy efficiency 

improvements can be made within the validation 

constraints. 

Check the on-line UVT 

monitor calibration 

Check the on-line UVT monitor against a bench-top

spectrophotometer to determine if the on-line unit is in 

calibration. 

Weekly

Check UV intensity

sensor calibration 

Check the duty sensor against a reference sensor, using 

the recommended protocol (section 5.3.2.2) to 

determine whether the duty sensor is in calibration. 

Switch to standby 

reactor 

Monitor the time it takes to switch to a standby reactor to 

determine if there will be off-specification operation 

during switchover. Twice during 

testing period

Switch to standby 

power or UPS 

Monitor the time it takes to switch to the standby power 

supply to determine if there will be off-specification 

operation because of power transfer. 

After 4 weeks, 

100 OMC 

cycles or one 

Off-line 

chemical clean 

(OCC) 

Inspect lamp sleeves

for fouling 

Remove a sleeve from the reactor and inspect as 

recommended in section 5.3.2.3. 

Any off-specification time and flow should be recorded during all performance tests, and 

these results should be evaluated to ensure that off-specification requirements are met. During 

performance testing, any component that is not operating properly should be corrected and 

retested to ensure satisfactory operation. This may necessitate manufacturer involvement, 

especially if specifications in the contract documents were not met. Following performance 

testing, ongoing monitoring and recording of reactor operation should continue at a reduced 

frequency as discussed in section 5.4 and as required by the State. 

5.1.4 Operations and Maintenance Manual

The O&M manual should be site-specific and based on as-built drawings, manufacturer’s 

shop drawings, operating procedures, recommended maintenance tasks, and results from the 

performance testing. If possible, the O&M manual should be developed prior to routine 

operations. At a minimum, O&M manuals should include the following items: 

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• Federal and State regulatory requirements and guidelines 

• Overall treatment objectives 

• Role of the UV installation in the overall disinfection strategy 

• Relationship to adjoining unit processes 

• UV reactor design criteria 

• UV reactor validation criteria 

• General description of UV installation 

• Controls and monitoring 

• Standard operating procedures 

• Start-up procedures 

• Shutdown procedures (manual and automatic) 

• Safety issues

• Emergency procedures and contingency plan 

• Alarm response plans 

• Preventative maintenance needs and procedures 

• Equipment calibration needs and procedures 

• Troubleshooting guide 

• Equipment component summary 

• Spare parts inventory 

• Contact information for equipment manufacturers and technical services 

5.2 Operation of UV Installations 

The operation of UV installations will vary based on the UV manufacturer, the UV 

reactor configuration, and the dose control strategy. This section discusses the required and 

recommended operational and routine start-up and shutdown procedures that are common to all 

UV reactors. The operational tasks presented in this section are general in nature, and the 

specific operational procedures for the installed UV reactors should be developed with assistance 

from the manufacturer and UV installation designer. Examples of how to determine the 

operational requirements are presented in section 5.5. 

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5.2.1 Operational Requirements 

To receive inactivation credit, the UV reactors are required to operate within the 

validated limits (40 CFR 141, Subpart W, Appendix D). When a UV reactor is operating outside 

of these limits, the UV reactor is operating off-specification as described previously. Unfiltered 

systems that use UV disinfection to meet the Cryptosporidium treatment requirement of the 

Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) must demonstrate that at 

least 95 percent of the water delivered to the public during each month is treated by UV reactors 

operating within validated limits (40 CFR 141.721(c)(2)). In other words, the UV reactors 

cannot operate off-specification for more than 5 percent of the water delivered to the public. 

The LT2ESWTR does not establish an off-specification requirement for filtered systems; 

however, States may adopt a 5 percent off-specification or more stringent requirement. 

Although the specific criteria limiting off-specification water are defined by the State, the United 

States Environmental Protection Agency (EPA) recommends that the UV reactors be operated to 

minimize off-specification water. The UV reactors must operate under the validated conditions 

that are determined based on validation testing (section 5.5) (40 CFR 141, Subpart W, Appendix 

D). The specific monitoring requirements associated with off-specification are described in 

section 5.4. 

5.2.2 Recommended Operational Tasks 

UV reactors typically use automatic control systems and do not need significant 

operational attention. This section outlines the general operational tasks that are recommended 

(Table 5.2). Site-specific operational tasks should be determined by the manufacturer, UV 

installation designer, and facility operators, and should be described in the O&M manual (section 

5.1.4). Recommended maintenance tasks are discussed in section 5.3.1. 

Table 5.2 Recommended Operational Tasks for the UV Reactor 

Frequency Recommended Tasks 

Daily • Perform overall visual inspection of the all UV reactors. 

• Ensure system control is on automatic mode (if applicable). 

• Check control panel display for status of system components and alarm status and 

history. 

• Ensure all on-line analyzers, flowmeters, and data recording equipment are operating 

normally. 

• Review 24-hour monitoring data to ensure that the reactor has been operating within 

validated limits during that period. 

Weekly • Initiate manual operation of wipers (if provided) to ensure proper operation. 

Monthly • Check lamp run time values. Consider changing lamps if operating hours exceed

design life or UV intensity is low. 

Semiannually 

• Check ballast cooling fans for unusual noise. 

• Check operation of automatic and manual valves. 

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5.2.3 Start-up and Shutdown of UV Reactors

UV reactors may be turned on and off regularly in response to varying flowrate and water 

quality. This section describes the routine start-up procedures, shutdown procedures, and 

winterization of the UV reactors. The routine start-up and shutdown procedures shown are not 

all inclusive. Utilities should modify these procedures based on the specific manufacturer’s

recommendations and operating requirements for their system. 

5.2.3.1 Routine Start-up 

The following start-up procedure serves as an example procedure. The UV reactors 

should be operating within validated conditions once the start-up sequence is complete.

1. Follow site-specific procedures for removal of lockouts and tag-outs of the power 

supply and control panel. 

2. Ensure all lamp and ground connections are properly made. Verify that all incoming 

power conductors, including ground conductors are properly terminated. 

3. Ensure that the lamp ends and all other reactor ports are covered and/or sealed to 

eliminate the potential for operator exposure to UV light. 

4. Ensure the breakers are turned on, and all electrical cabinets and equipment are clear 

and closed. 

5. Initiate the UV reactors’ start-up sequence. 

6. Initiate water flow (if it is not automatically done in UV reactor controls) to the 

reactor and gradually increase the flow until the minimum flow required for lamp

cooling is reached. The water exiting the reactor is not disinfected and is considered 

off-specification. 

7. Verify that all air is purged from reactors (i.e., reactor completely full). Check the 

top of the reactor for heat buildup, which indicates an air pocket. 

8. Check the UV reactor control panel to ensure that all of the lamps are on and all of 

the monitoring parameters are being displayed. 

9. Check and resolve any system alarms being displayed. 

10. Ensure all of the on-line analyzers (UV intensity sensors and UVT monitors, if 

applicable) and flowmeters are operating as intended. 

11. After lamp warm-up period, increase flow to the minimum validated flow (if flow is 

not automatically adjusted with UV reactor control sequence). 

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12. Verify correct flow split between parallel UV reactors using flowmeters and/or 

differential pressure gauges. 

13. Verify that the UV reactor is operating within validated limits. 

5.2.3.2 Routine Shutdown 

UV reactors will need to be shut down periodically for maintenance or to accommodate

water quality or flow changes. The main steps involved in shutting reactors down are as follows: 

1. Throttle the effluent valve (if not part of the control sequence) to reduce flow through 

the reactor to the minimum required for cooling. If complete closure of the effluent 

valve can be accomplished without overheating the lamps, it is recommended. 

2. De-energize the reactors.

3. Close effluent valve if not completed in Step 1. The water exiting the de-energizing 

reactor is considered off-specification. 

4. If maintenance is being performed, the following steps should be followed. If the UV 

reactor is to be placed on standby, the following steps are not necessary. 

5. Follow lock out and tag-out procedures for the facility. 

6. Drain the reactor if necessary for the specific maintenance task. 

7. Inspect and repair or replace any necessary equipment. 

After an extended shutdown period (greater than 30 days), the operator should perform a 

cleaning and then inspect the lamp sleeves for fouling. Additional cleaning may be necessary 

prior to start-up. 

5.2.3.3 Winterization 

In most drinking water applications, the UV reactors will probably be located within a 

building. However, in some instances, the reactors may be located in unheated concrete vaults. 

When it is necessary to shut down a UV reactor for an extended period of time and freeze 

damage is possible, the UV reactors should be winterized in accordance with the manufacturer’s 

recommendations. 

5.3 Maintenance of UV Reactors 

There are no specific regulatory requirements for maintenance of a UV reactor. 

However, the UV reactors need to be maintained to ensure that disinfection requirements are 

met. Poor maintenance may cause the UV reactors to be operating off-specification. As part of 

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the maintenance tasks, UV reactor components will need to be replaced; therefore, an inventory 

of spare parts is necessary. These tasks are described in this section. 

5.3.1 Summary of Recommended Maintenance Tasks 

Table 5.3 summarizes the recommended maintenance tasks and refers to the general 

guidelines for those tasks that are discussed in section 5.3.2. Before any maintenance is 

performed, the main electrical supply to the UV reactors should be disconnected, lockout and 

tag-out protocol should be followed, and the operator should wait at least 5 minutes (or as 

recommended by the manufacturer) for the lamps to cool down and energy to dissipate. 

Table 5.3. Recommended Maintenance Tasks 

Frequency

Task 

General Guideline 

Section Reference 

Action

Weekly Check on-line UVT 

monitor calibration 

section 5.3.2.5 

Calibrate UVT monitor when manufacturer’s guaranteed 

measurement uncertainty is exceeded. 

Monthly Check reactor

housing, sleeves, and 

wiper seals for leaks 

Replace housing, sleeve, or wiper seals if damaged or

leaking. 

Monthly UV intensity sensor 

calibration check 

protocol 

section 5.3.2.2 

Check the sensor calibration at the lamp power utilized during 

routine operating conditions (e.g., the majority of operation). 

A sensor is out of calibration when it fails the criteria shown in 

section 5.3.2.2 

When UV 

intensity 

sensor fails

calibration 

check 

Replace duty sensor

with calibrated backup 

sensor

section 5.3.2.2 

• Check the reference sensor with second reference sensor

or two other duty sensors to ensure the first reference 

sensor is calibrated. 

• If reference sensor is properly calibrated, replace the duty 

sensor with calibrated sensor, and send the duty sensor 

that failed calibration to the manufacturer. 

• Check the replaced sensor one hour later. 

Monthly 

(OCC) 

Semi-annually

(OMC) 

Check cleaning 

efficiency

section 5.3.2.4 

• Record UV intensity sensor reading.

• Extract one sleeve per reactor (or bank of lamps for low 

pressure high output (LPHO) reactors) for inspection. 

• Check remaining sleeves if fouling is observed on the first 

sleeve. 

• Manually clean sleeve(s) if fouling is seen on the sleeves. 

• Record UV intensity sensor reading and compare to original

reading after cleaning. 

• Replace sleeve if UV intensity is not restored to validated 

level. 

Semi-annually

(OMC) 

Check cleaning fluid 

reservoir (if provided) 

section 5.3.2.4

Replenish solution if the reservoir level is low. Drain and 

replace solution if the solution is discolored. 

Annually Calibrate reference

sensor

section 5.3.2.2

Send the reference sensor to the manufacturer for calibration. 

Annually Test-trip GFI

section 5.3.2.8

Maintain ground fault interrupt (GFI) breakers in accordance 

with the manufacturer’s recommendations. 

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Table 5.3. Recommended Maintenance Tasks (continued) 

Frequency

Task 

General Guideline 

Section Reference 

Action

Manufacturer’s 

recommended 

frequency 

Check flowmeter 

calibration 

section 5.3.2.6 

Calibrate flowmeter when manufacturer’s guaranteed 

measurement uncertainty is exceeded. 

Lamp/ 

manufacturer 

specific 

Replace lamp

section 5.3.2.1 

Replace lamps when any one of the following conditions 

occur: 

• Initiation of low UV intensity alarm (UV intensity equal to or 

less than set point value) after verifying that this condition is 

caused by low lamp output. 

• Initiation of lamp failure alarm. 

When lamps are 

replaced

Properly dispose of 

lamps 

section 5.3.2.1

Send spent lamps to a mercury recycling facility or back to the 

manufacturer. 

Sleeve/ 

Manufacturer 

specific 

Replace sleeve 

section 5.3.2.3

Replace sleeve every 3 to 5 years or when damage, cracks, 

or excessive fouling significantly decreases UV intensity of an 

otherwise acceptable lamp to the minimum validated intensity 

level. The replacement frequency should be adjusted based

on operational experience.

Pressure gauge 

manufacturer 

specific 

Check operation of 

the pressure gauges 

that are used to 

confirm flow split (if 

applicable)

section 5.3.2.6 

Replace the pressure gauge if deemed faulty by 

manufacturer’s evaluation procedure. 

Manufacturer 

specific 

Clean UVT monitor Clean according to manufacturer’s recommended procedure. 

Manufacturer 

specific 

Inspect OMC drive 

mechanism 

Inspect and maintain OMC drive routinely as recommended 

by the manufacturer. 

Manufacturer 

specific 

Inspect ballast 

cooling fan 

Check the ballast cooling fans for dust buildup and damage. 

Replace if necessary. 

5.3.2 General Guidelines for UV Reactor Maintenance 

This section describes general guidelines for UV reactor components that relate to 

maintenance tasks. Specific operations, maintenance, and monitoring tasks are described 

individually in later sections. These latter sections also refer back to this section as a reminder of 

the general recommendations. 

5.3.2.1 UV Lamp Characteristics 

UV lamp output decreases over time, and UV lamps will need to be replaced periodically 

to maintain sufficient UV intensity (i.e., the validated UV intensity setpoint). Replacement 

lamps should be identical to those used during reactor validation with respect to arc length, lamp

envelope material and dimensions, amount of mercury, and spectral output. If the lamps 

supplied are not equal to the lamps used during validation, the UV reactor is not operating as 

validated and is considered off-specification. 

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If the mercury content or power rating changes, the different lamp should be assessed by 

comparing UV intensity sensor readings, after burn-in, to the lamps that were validated to 

determine if the new lamps are equal to the validated lamps. If the sensor reading is equal to or 

greater than that of the validated lamps after burn-in, the different lamps are acceptable and 

comparable to the validated lamps. However, if a utility replaces the lamps with higher power 

lamps to receive higher log inactivation credit, validation testing should be performed to confirm

performance. Lamp manufacturers should also provide documentation of lamp output decay 

characteristics, guaranteed life, and lamp burn-in period. This information will help the utility 

determine the lamp replacement frequency. It should be noted that different lamps might have 

different aging characteristics, which may affect operations and maintenance costs. 

The frequency of UV lamp replacement can be based on a utility-determined schedule, 

lamp operating hours, or the UV intensity reduction as measured by the UV intensity sensor 

(after sleeve and sensor window cleaning); lamp replacement recommendations are discussed in 

section 5.3.2.1. During replacement, the lamps and sleeves should be handled in accordance 

with manufacturer recommendations, using clean cotton, powder-free latex, or vinyl gloves 

because fingerprints can cause damage to the lamps or sleeves during operation. 

Lamp manufacturers are required to determine whether their products exhibit the toxicity 

characteristic for mercury and whether their lamp is regulated as a universal hazardous waste 

under Subtitle C of Resource Conservation and Recovery Act (RCRA) [40 CFR Part 260, 261, 

264 and 273]. Currently, most UV lamps exceed these toxicity characteristics and require 

regulated disposal. As such, these lamps should be sent to a mercury recycling facility where the 

mercury is recovered and lamp components are recycled. Some UV reactors and lamp 

manufacturers will accept spent or broken lamps for recycling or proper disposal (Dinkloh 2001; 

Lienberger 2002; Gump 2002). Utilities should contact their lamp manufacturer to determine if 

they accept spent lamps or should contact their State for a list of local mercury recycling 

facilities. 

5.3.2.2 UV Intensity Sensors 

Well performing UV intensity sensors are necessary to assess whether the validated UV

intensity is being achieved. Sensor calibration, rotation, and placement affect operation. This 

section describes these effects and provides recommendations to minimize them. 

There are two types of sensors used for UV reactor operation: duty and reference sensors. 

Duty sensors are on-line sensors and continuously monitor UV intensity, while the reference 

sensors are off-line sensors used to assess the duty sensor performance. Therefore, the reference 

sensor specifications should exactly match those of the duty sensors, so that a valid comparison 

can be completed. Both duty and reference sensors are described in this section. 

Duty UV Intensity Sensor Calibration

Prior to installation, manufacturers calibrate the UV intensity sensors. However, over 

time the sensor may drift out of calibration. Because these sensors are vital to assessing the UV 

disinfection performance, the calibration of each sensor should be checked at least monthly 

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against the reference sensor. To assess the calibration, the following sensor calibration check 

protocol should be followed: 

1. Measure the UV intensity with the duty sensor, and record the measurement result. 

2. Replace the duty sensor with the reference sensor in the same location (i.e., port) as 

the duty sensor used in Step 1. 

3. Measure the UV intensity with the reference sensor and record the measurement 

result. 

4. Determine if Equation 5.1 holds true for the two UV intensity sensor readings: 

( ) 2

1 2

Duty

2

Ref

Ref

Duty 1 *100

I

I

≤ σ + σ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − Equation 5.1

where 

IRef = Intensity measured with the reference sensor (mW/cm

2

) 

IDuty = Intensity measured with the on-line sensor (mW/cm

2

) 

σ Duty = Measurement uncertainty of the on-line UV intensity sensor (%) as

provided by the UV manufacturer in the validation report 

σ Reference = Measurement uncertainty of the reference UV intensity sensor (%) 

as provided by the UV manufacturer in the validation report 

5. Replace the duty sensor with another calibrated duty sensor if the relationship 

Equation 5.1 does not hold true. 

The calibration of the UV intensity sensor is sensitive to the power level of the UV lamps 

(Swaim et al. 2002). To most effectively compare the duty sensor to the reference sensor, the 

power level should be set at the level typically used during routine operation (e.g., the majority 

of operation). 

UV Intensity Sensor Rotation

Some UV intensity sensors are sensitive to their rotational alignment within the sensor

port and will have different readings at different rotations. This may be due to the UV intensity 

sensor configuration (e.g., acceptance angle). Section A.3.5 discusses UV intensity sensors 

configurations in more detail. The sensors should be rotated until the lowest UV intensity 

reading is obtained for routine monitoring purposes. Alternatively, UV reactors may be designed 

so the UV intensity sensors are keyed in the same rotational position at all times. This may not 

be an issue for all UV intensity sensors.

Measuring Lamp Output Variability

UV lamp output differs for each lamp, depending on lamp age and lot. As discussed in 

section 2.4.6, a sensor measures the UV intensity at its location in the UV reactor and cannot 

assess lamp output variability unless there is one sensor per lamp. Many low pressure (LP) or 

LPHO reactors have one sensor to monitor a bank of lamps, and some MP reactors use one UV 

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intensity sensor to monitor more than one lamp in the reactor. The effect of variable lamp output 

is accounted for in the validation protocol safety factor as discussed in section F.3. For routine 

operation, the oldest lamp should be placed in the position closest to the UV intensity sensor if 

one sensor monitors multiple lamps. 

Reference UV Intensity Sensor 

The reference sensor should be calibrated at least once per year at a qualified facility 

(e.g., manufacturer) to ensure that it is calibrated properly for the regular duty sensor calibration 

checks. The reference sensor should not be exposed to UV light for longer than it takes to 

perform the reference sensor measurement. When not in use, the reference sensor should be 

stored under conditions that will maintain its integrity and accuracy as recommended by the 

manufacturer. If the reference sensor is found to be out of calibration, the calibration interval 

should be shortened. One indicator that the reference sensor itself may be out of calibration is if 

it shows that all on-line sensors are out of calibration. Some utilities may choose to have 

multiple reference sensors to help determine if one reference sensor is out of calibration, as a 

replacement reference sensor, or to allow multiple duty sensors to be checked simultaneously. 

5.3.2.3 Lamp Sleeves 

Lamp sleeves degrade over time due to solarization (section 2.4.4) and internal sleeve 

fouling, resulting in cloudiness and a loss of UV transmittance. Abrasion of the sleeve surface 

during handling or mechanical cleaning may also be a contributing factor to the loss of UV 

transmittance. Sleeve transmittance loss is reflected in the UV intensity sensor reading and, 

therefore, does not need to be monitored. However, a low UV intensity sensor reading may be 

from sleeve transmittance loss and should be considered when troubleshooting the cause of this 

problem (as discussed in section 5.6.1). Sleeves will need to be replaced in the case of UV 

transmittance loss or other damage. 

Sleeves should be replaced every 3 to 5 years or when damage, cracks or excessive

fouling diminishes UV intensity to the minimum validated intensity level, whichever occurs first. 

This replacement frequency should be increased or decreased based on operational experience. 

Replacement sleeves should be identical to the sleeves used during validation, meet the design 

and UV manufacturer’s material and construction specifications, and be certified as described in 

section F.6.3. The sleeves should be handled in accordance with manufacturer 

recommendations, using clean cotton, powder-free latex, or vinyl gloves because fingerprints can 

cause damage to the sleeves during operation. When the sleeves are replaced, the manufacturer’s 

procedure should be closely followed because the lamp sleeve can crack and break from overtightening of the compression nuts that hold it in place. 

5.3.2.4 Fouling 

As discussed in Chapters 2 and 3, the lamp sleeves and UV intensity sensors/windows 

may foul over time, depending on the water quality, lamp type, and cleaning regime. This 

section describes possible cleaning techniques and provides some specific recommendations for 

addressing fouling issues. 

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Sleeve and UV Intensity Sensor Surface/Window Fouling

There are two types of sleeve cleaning techniques as discussed in section 2.4.5. The first 

type is an OMC system, which typically utilizes an automatic mechanical wiper (e.g., O-ring, 

brush) to wipe the surface of the sleeve at a prescribed frequency. Some OMC systems have Orings with cleaning fluid enclosed in them to enhance cleaning. The second type is an OCC, 

which is also a called flush and rinse system. OCC systems are off-line, manual systems that 

pump cleaning solution (typically an acid) into the reactor and circulate the solution for a period 

of time. Helsinki Water uses an OCC system; a description of their cleaning regime is discussed 

in Appendix O. Also, OCC systems clean the sensor wetted surface/window; however, OMC 

systems may not, depending on the UV reactor. 

The frequency of cleaning is site-specific. An appropriate sleeve cleaning frequency 

(manual or automatic) can be determined based on the rate of fouling during the start-up period, 

which can be assessed by monitoring the UV intensity sensor measurement. For routine 

operation, the cleaning frequency should be increased or decreased based on the amount of 

fouling left on the sleeves after the cleaning cycle and the loss of UV intensity prior to cleaning. 

Sleeves should initially be inspected for fouling every six months if OMC is employed 

and every month if OCC is used. This frequency should be adjusted after 2 years of operating 

data are available. A decrease in UV intensity may indicate sleeve fouling, and sleeves should 

be inspected if fouling is the suspected cause of the UV intensity drop. In addition, the sensor 

windows (if applicable) should be inspected for fouling and supplemental cleaning should be 

conducted if necessary, according to the manufacturers recommendation. 

For sleeve inspection, one sleeve per reactor (or bank of lamps for LP or LPHO reactors) 

should be inspected. The sleeves should be handled in the same manner as described for UV 

lamps. If damage or fouling is observed, the remaining sleeves should be inspected. External 

fouling can be difficult to identify. Sleeve discoloration is more easily seen by laying the sleeve 

on a clean, white, lint-free cloth along side of a new sleeve. If streaks are observed, this may 

indicate that the OMC wiper material may be worn or damaged or not aligned properly; 

therefore, the wiper should also be inspected. If fouling is observed, the cleaning frequency 

should be increased, and supplemental manual cleaning should be conducted as necessary. 

If manual cleaning (i.e., beyond routine OCC or OMC cleaning) of lamp sleeves is 

necessary, this should be done according to manufacturer recommendations and procedures. 

Abrasive cleaners or pads that might scratch the lamp sleeve should not be used. In addition, the 

inside of the sleeve should be dry prior to re-installation because water or cleaning solutions 

could cause a coating to form during operation. One method of drying the sleeve is to use 

isopropyl alcohol and a lint-free cloth; however, there should not be any alcohol left inside the 

sleeve after this procedure. As noted earlier, when the sleeves are re-installed after inspection, 

the manufacturer’s procedure should be closely followed to avoid over-tightening of the 

compression nuts. 

If OMC cleaning is used, the OMC wipers should be checked for deformation or 

degradation at the same time the sleeves are checked. If the OMC cleaning uses a cleaning 

solution, the cleaning solution reservoir should be checked every six months to determine 

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whether more solution should be added. In addition the solution should be replaced if it is 

discolored or if the OMC system is not effectively cleaning the sleeve. 

Fouling During Periods of Standby

When the UV reactors are out-of-service and full of water, the sleeves may become

fouled (Toivanen 2000). The rate of fouling is site-specific and depends on the influent water 

quality. UV reactors equipped with OMC should continue to clean the sleeves even though the 

UV reactor is off-line. This should prevent fouling of the sleeves. For UV reactors that do not 

include OMC, the utility should consider draining the UV reactor if it is off-line for more than 

one week. However, this period could be shorter or longer, depending on the water quality. 

After an extended shutdown period of greater than 30 days, the operator should perform a 

cleaning (OCC or OMC) and then inspect the lamp sleeves for fouling. Additional cleaning may 

be necessary prior to start-up after extended periods of standby. 

5.3.2.5 On-line UVT Monitor Calibration 

On-line UVT measurements should be compared to those obtained using a bench-top 

spectrophotometer every week. The grab samples that are used to check calibration should be 

collected from a location close to the on-line UVT monitor sampling point. The frequency may 

be decreased or increased based on the performance demonstrated over a one-year period. For 

example, the frequency could be reduced to once per month if the UVT monitor was consistently 

within the calibration specification for over a month during the first year of monitoring. 

5.3.2.6 Flowmeter Calibration 

The flowmeter calibration should be checked at the frequency recommended by the 

manufacturer. Techniques for verifying calibration are discussed in section 5.1.2.2. 

Some UV installations will not have dedicated flowmeters and may use a combination of

an upstream flowmeter and differential pressure gauges to verify flow split as described in 

section 3.3.1.2. If differential pressure is used to verify the flow split, the calibration of the main 

flowmeter should be checked at the manufacturer’s recommended frequency and the accuracy of 

the pressure gauges should be periodically verified using a reference gauge or redundant gauge 

to confirm measurement consistency between the gauges. 

5.3.2.7 UV Reactor Temperature 

UV lamps operate at high temperatures (as discussed in section 2.4.2) and need water 

flow to maintain them at their optimal temperature and to prevent overheating. Another concern 

related to overheating is the formation of air pockets in the UV reactor. Air pockets can cause 

the UV reactor temperature to increase and may alter the flow pattern in the UV reactor. UV 

lamps can break if their threshold temperature is exceeded, which is discussed in more detail in 

section N.2.1.2. 

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The water temperature should be monitored. If the water temperature exceeds 

manufacturer recommendations, the UV reactor should be shut down. Water level monitoring or 

reactor temperature monitoring are typically included in the packaged control systems for the 

UV reactor. The water level monitoring should detect any air pockets in the UV reactor. During 

start-up and whenever necessary, air should be bled from the UV reactors. The UV reactor 

surface can become hot during operation if air pockets or stagnant water are present in the UV 

reactor. As a result, nothing unrelated to reactor equipment should be in external contact with 

the reactor while in service. 

5.3.2.8 Electrical Concerns 

UV reactors operate at high voltages. Before any maintenance on the UV reactor is 

performed, the main electrical supply to the UV reactors should be disconnected and the operator 

should wait at least 5 minutes for the lamps to cool down and energy to dissipate. Lockout, tagout procedures and all applicable codes should be followed. The UV reactors should not be 

operated if any of the control panel doors are open, and water should not be sprayed around the 

electrical equipment. 

Typically, power to the UV reactors are provided via a distribution transformer, a circuit 

breaker, a disconnect switch at the UV reactor, and related wires and conduits. If maintenance is 

necessary on the control panel, the main electrical supply should be disconnected. The power to 

the lamps is typically delivered through individual GFI circuit breakers and ballasts. 

Maintenance of the GFI breakers is important because they are safety devices that protect the 

operators when they are working around the powered equipment. The GFI breakers should be 

test-tripped at least once per year and should be maintained in accordance with the 

manufacturer’s recommendations. Ballast output should be monitored through the UV reactor's 

control panel. Irregularities or instabilities in ballast output may indicate a problem with the 

electrical feed or the ballast itself. 

The ballasts, typically connected between the GFI breakers and the lamps, are electrical 

components that regulate the line power to match the input requirement of the lamps. Three 

types of ballasts are typically used with UV reactors for converting power: electronic ballasts, 

electromagnetic ballasts, and transformers. Electromagnetic ballasts and transformers are very 

similar in that both contain a specially wound coil of wire that is used to control the current to 

the lamp. Typically inductors or capacitors are used to allow step adjustment of the lamp output. 

Electronic ballasts, sometimes referred to as solid-state ballasts, contain semiconductors and 

other electronic components that allow the ballast to behave like a switching power supply. 

Electronic ballast technology allows nearly continuous adjustment of lamp output. 

Power regulation, particularly with electromagnetic ballasts and transformers, will result 

in significant heat build-up within the ballast enclosure. If the excess heat is not dissipated, it 

can damage the ballast electronics and cause failure. A cooling system is normally provided 

with LPHO and medium pressure (MP) reactors to maintain the ballast temperature below the 

maximum specified limit. LP reactors typically do not need ballast cooling. The ballast cooling 

system should be inspected and maintained as recommended by the manufacturer. 

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Power use depends on the specific UV reactor and how it adjusts to changes in water 

quality and flow. Power use should be monitored as operational adjustments are made for 

changes in flow, UV intensity, UVT, lamp aging and output, and other factors. This information 

can be used to determine the most energy efficient operating strategies. For example, some UV 

reactors can both increase lamp output and energize additional lamps to respond to a low UV 

intensity reading. The power use under these two strategies can be compared to determine which 

is more energy efficient.

5.3.3 Spare Parts 

The actual life of a component is a function of many variables, including operating 

conditions, maintenance practices, the quality of the materials of construction, and fabrication 

practices. As a consequence, it is impossible to predict the actual life of a component. To 

overcome the operational impacts of this uncertainty, an adequate inventory of critical spare 

parts should be maintained to ensure reliable and consistent performance of the UV reactors and 

minimize the delivery of off-specification water. 

All UV components have a design life and a guaranteed life. The design life represents 

the expected duration of operation. The guaranteed life incorporates the risk, assumed by the 

manufacturer, to account for the uncertainties associated with the quality of materials used, 

production, and operating conditions. Generally, guarantees are conditional in nature and are 

valid under certain operating conditions. For example, guaranteed lamp life is normally linked to 

the lamp power setting or the number of on/off cycles per 24-hour period. If equipment failure 

occurs during the warranty period and if all of the warranty conditions are satisfied, the 

manufacturer will typically replace the component and charge the owner a prorated fee for the 

use of the replaced component. 

Table 5.4 provides typical design and guaranteed lives for major UV reactor components. 

These represent current industry trends and are likely to change as more operation and 

maintenance information becomes available and technological advances occur. Manufacturers 

should be contacted directly for details specific to their equipment. 

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Table 5.4. Design and Guaranteed Lives of Major UV Components 

(Based on Manufacturers’ Input) 

Component Design Life1 Guaranteed Life 2

Low pressure lamps (LP and LPHO) 12,000 hours 8,000 - 12,000 hours 

MP lamps 10,000 hours 4,000 - 8,000 hours 

Sleeve 8 to 10 years 1 to 3 years 

UV Intensity Sensor 3 to 10 years 1 year 

UVT monitor 3 to 5 years 1 year 

Cleaning systems 3 to 5 years 1 to 3 years 

Ballasts 10 to 15 years 1 to 3 years 1

 Expected duration of operation 2

 Accounts for variability of material quality, production, and operating conditions. 

The following is a suggested minimum inventory of spare parts, expressed as a 

percentage of the installed number. A full list of spare parts will vary depending on the specific 

equipment installed and should be coordinated with the UV manufacturer. The number of spare 

parts needed depends on the guaranteed life of the specific equipment. For example, a higher 

percentage of MP lamps may be necessary compared to LP lamps because the guaranteed lamp

life is less for MP lamps, and therefore they need to be replaced more frequently. 

• UV lamps- 10 percent with a minimum of two lamps 

• Sleeves- 5 percent with a minimum of one sleeve 

• O-ring Seals- 5 percent with a minimum of two seals 

• OMC wipers- 5 percent with a minimum of two wipers 

• OMC wiper drive mechanisms- 2 percent with a minimum of one drive 

• Ballasts- 5 percent with a minimum of one unit 

• Ballast cooling fan- 1 unit 

• Duty UV intensity sensor- minimum of 2 units (adjust number based on operating 

experience) 

• Reference UV intensity sensor- 2 units 

• On-line UVT monitor- 1 unit (if used for control strategy) 

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5.4 Monitoring, Recording, and Reporting of UV Installation Operation 

Operation of the UV reactors should be monitored to ensure the reactors are operating 

within validated limits, to diagnose operating problems, to determine when maintenance is 

necessary, and to maintain safe operation. This section discusses the required and recommended 

monitoring, recording, and reporting activities for UV installations. 

5.4.1 Monitoring and Recording Frequency for Compliance Parameters 

Utilities must monitor each reactor to determine whether it is operating within validated 

conditions. They also must determine the percentage of flow that was treated within validated 

limits (40 CFR 141, Subpart W, Appendix D). The flow is off-specification when a reactor is 

operating outside of validated limits. The monitoring parameters depend on the control strategy 

used and the validation results. Table 5.5 presents the monitoring parameters for each control 

strategy, the criteria for when off-specification occurs, and examples of off-specification 

operating conditions. 

Table 5.5 Off-Specification Operations for Each Control Strategy

Control 

Strategy

Parameters 

Monitored Off-Specification Examples

UV intensity 

setpoint 

UV intensity, 

flowrate, lamp 

status 

Anytime these values are 

outside of the validated limits 

for these parameters 

1) UV intensity below setpoint

2) Flowrate outside validated limits 

3) UV lamp failure 

4) UV intensity sensor failure 

UVT and UV 

intensity 

setpoints 

UV intensity, 

flowrate, UVT,

lamp status 

Anytime these values are 

outside of the validated limits 

for these parameters 

1) UV intensity below setpoint

2) Flowrate outside validated limits 

3) UV lamp failure 

4) UV intensity sensor failure 

5) UVT below setpoint 

Calculated

dose 

Calculated

dose, flowrate, 

UVT, lamp 

status

Anytime the calculated dose 

is below the validated 

setpoint (if validation certifies 

that the calculated dose can 

be used to control the UV 

reactor – see section F.2)1

1) Calculated dose below setpoint 

2) Flowrate outside validated limits 

3) UV lamp failure 

4) UV intensity sensor failure 

5) UVT below setpoint 

1 If validation deems that the calculated dose control is not acceptable, the UV reactor should use the 

UVT and UV intensity setpoint control strategy. 

It is recommended that the required monitoring parameters be continuously monitored for 

each UV reactor and that these values be recorded at least once every four hours. These fourhour records should be used to determine the percentage of flow that is off-specification. Very 

small systems (e.g., systems serving less than 500 people) that are unable to record reactor status 

every 4 hours (e.g., manual recording is practiced) can consider a reduced recording frequency; 

however, the frequency should not be less than once per day and should be approved by the 

State. The monitoring guidelines are summarized in Table 5.6. 

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Table 5.6 Monitoring Parameters and Recording Frequency

Parameter 

General Guideline 

Section Reference 

 (if applicable)

Recommended 

Recording 

Frequency

Notes 

UV intensity Every 4 hours The UV intensity must be above the validated 

setpoint 

UVT1 Every 4 hours The UVT must be above the validated setpoint. If 

not required to be monitored, this information will 

assist in determining if low UV intensity readings 

are because of low UVT 

Calculated dose1 Every 4 hours The calculated dose must be above the validated 

setpoint 

Lamp status Every 4 hours The lamps should be energized if water is flowing 

through the UV reactor 

Calibration of UV 

intensity sensors 

section 5.3.2.2

Monthly The UV intensity sensor calibration must be 

checked, using sensor calibration check protocol 

1 Only required if necessary for the control strategy (Table 5.11) 

5.4.2 Monitoring and Recording for Other Operational Parameters 

In order to minimize operational problems, facilitate regulatory compliance, and evaluate 

UV reactor performance, it is recommended that additional parameters, beyond those needed for 

regulatory compliance, be monitored. Table 5.7 presents these additional parameters 

recommended for monitoring and the recommended recording frequency. These recommended 

parameters and their monitoring frequency should be adjusted based on site-specific operating 

experience. For example, if sleeve fouling is a maintenance issue and supplemental reactor 

cleaning is frequent (e.g., monthly), then the fouling parameters should be monitored on a daily 

basis as opposed to weekly as shown in the table below. 

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Table 5.7 Recommended Monitoring Parameters and Recording Frequency

Parameter 

General Guideline 

Section Reference 

 (if applicable)

Monitoring 

Frequency

Recording 

Frequency

Notes 

Power draw 

section 5.3.2.8 

Continuous Every 4 hours This information can be used to 

determine the most energy 

efficient operation strategies 

Water Temperature 

section 5.3.2.7 

Continuous Daily Monitor to ensure the high 

temperature limit is not exceeded 

(usually part of packaged UV 

control system) 

UV lamp on/off cycles

section 5.3.2.1 

Continuous Weekly

(Total cycles in 

a week) 

Monitor to assess status of the 

UV lamps since the of on/off 

cycles can help assess lamp

aging 

Turbidity Daily Weekly Monitor if chemicals (e.g., lime) 

are added post-filtration or prior to 

UV disinfection (monitoring may 

not be necessary for many UV 

reactors) 

pH, iron, calcium, 

alkalinity, hardness 

section 5.3.2.4

Weekly (reduce if 

fouling is not 

prevalent) 

Weekly Monitor to help assess fouling 

issues if necessary 

UVT monitor calibration 

section 5.3.2.5

Weekly (reduce if 

appropriate based 

on operational

experience) 

Weekly Information can assist in planning

scheduled maintenance and O&M 

budget 

Age of the following

equipment: 

• Lamp 

• Ballast 

• Sleeve 

• UV intensity sensor 

Monthly Monthly Information can assist in planning

scheduled maintenance and O&M 

budget 

Calibration of flowmeter 

section 5.3.2.6

Monthly Monthly Information can assist in planning

scheduled maintenance and O&M 

budget 

All data related to UV reactor operation should be gathered, compiled, and stored for 

easy retrieval. The recorded data should be stored for at least two years. Appendix M provides 

example logs for many of the parameters listed in Table 5.13. 

5.4.3 Reporting to the State 

Monthly reports must be prepared and submitted to the State. The report must include 

the percentage of off-specification flow, which should be based on at least 4-hour records for 

each reactor. The State may have additional reporting requirements. In addition, the percentage 

of the UV intensity sensors that were checked for calibration must be reported monthly; all 

sensors should be checked every month. An example monthly monitoring form is shown in the 

Appendix M. The State should be contacted to determine the specific content of the monthly 

reports and to coordinate with other reporting requirements. 

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5.5 Determination of Validated Operational Parameters 

For each UV reactor, the operating conditions associated with a given level of 

inactivation credit must be defined based on validation testing results (40 CFR 141, Subpart W,

Appendix D). The validation testing and resultant data that are used to determine these operating 

conditions will vary with different control strategies. A detailed discussion of the three common 

control strategies is presented in section 3.3.2. A brief description of each of the control 

strategies is shown in Table 5.8. 

Table 5.8 UV Reactor Control Strategies 

Control Strategy Dose Delivery Monitoring and Control Based On 

UV Intensity Setpoint UV intensity sensor measurement 

UV Intensity and UVT setpoints UV intensity sensor and UVT measurement 

Calculated Dose The calculated UV dose1

1 The UV reactor calculates a UV dose using the UV intensity sensor measurement, 

the UVT of the water, and the flowrate.

This section provides example operational requirements based on the validation examples 

described in section C.5 of the validation protocol. Each example describes how the operating 

requirements are determined based on the control and operation strategy used and the validation 

results. 

Example 1. UV Intensity Setpoint Control - Single Operational Setpoint for all Conditions 

(Section C.5.1) 

The simplest operational strategy uses one single UV intensity setpoint for all flows. In 

this example, a LPHO reactor that uses the UV intensity setpoint control strategy was validated 

at flows between 100 and 500 gallons per minute (gpm) and a UVT range of 84 to 98 percent. 

This reactor passed the criteria for 2-log inactivation of Cryptosporidium with an intensity sensor 

setpoint of 5 mW/cm2

. The validation testing verified that the UV intensity setpoint control 

strategy is appropriate for this reactor 

Based on this validation, this reactor must operate at a minimum UV intensity sensor 

setpoint of 5.0 mW/cm2

 and a flow range between 100 and 500 gpm to claim 2 log 

Cryptosporidium credit. The UV intensity setpoint approach accounts for the UVT in the UV 

intensity measurement. Therefore, the intensity setpoint of 5.0 mW/cm2

 can be used for any 

UVT. Although this is a simple and straightforward operating strategy, single setpoint operation 

will not be as energy efficient as using a variable setpoint approach, which is described in 

Example 2. 

Example 2. UV Intensity Setpoint Control - Variable Setpoint Operation for Different 

Flow Conditions (Section C.5.2) 

The variable UV intensity setpoint approach has a different UV intensity setpoint at 

different flowrates. This operation promotes more energy efficient operation compared to the 

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single setpoint approach because the UV intensity setpoint can be decreased at lower flows. For 

this example, a LPHO reactor that uses the UV intensity setpoint control strategy was validated 

under the conditions shown in Table 5.9 and passed the criteria for 3-log inactivation of 

Cryptosporidium at each condition. 

Table 5.9 Example Validation Data for 

Variable Setpoint Operation 

Flow (mgd) UVT (%) UV Intensity (mW/cm2

) 

0.90 70 6.1

1.2 75 7.5

1.7 83 10

2.4 92 14

The UV intensity measurements recorded during validation verified that the UV intensity 

setpoint approach is appropriate for this reactor. Because of the data collected, this UV reactor

can be operated at a different setpoint for each flow range. These intensity setpoints could be 

used in three ways. 

1. A single setpoint as described in Example 1. For example, a setpoint of 14 mW/cm2

could be used at between 0.90 and 2.4 mgd with any UVT. 

2. Each intensity setpoint could be used over a given flow range as shown in Table 5.10. 

The higher UV intensity measurement from each flow range should be used as the 

UV intensity setpoint to be conservative. 

Table 5.10 UV Intensity Setpoint for Different 

Flow Ranges 

Minimum Flow

(mgd) 

Maximum Flow

(mgd) 

UV Intensity

(mW/cm2

) 

0.90 1.2 7.5

1.2 1.7 10

1.7 2.4 14

3. The intensity setpoints could be interpolated as a function of flowrate. Figure 5.2 

presents an equation based on interpolation. For example, for a flowrate of 2 mgd, 

interpolation indicates that a setpoint of 12 mW/cm2 is needed to achieve 3-log 

inactivation.

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Figure 5.2 Example 2 – Interpolation of 

Validation Data to Determine UV Intensity Setpoints 

y = 5.1979x + 1.3063

0

5

10

15

0 1 2 3

Flowrate (mgd)

Intensity setpoint

(mW/c

m

2

)

yy = = 5. 5.20x 1979x + 1. +3 1.3063

0

5

10

15

0 1 2 3

Flowrate (mgd)

Intensity setpoint

(mW/c

m

2

)

y = 5.20x + 1.31

Example 3. UV Intensity Setpoint Control - Variable Setpoint Operation for Different 

Flow Conditions and Inactivation Goals (Section C.5.3) 

For this example, a UV manufacturer has completed a matrix of tests at different 

flowrates, UVT, and lamp power to develop a relationship between UV intensity readings, log 

inactivation credit, and flow. Table 5.11 shows the results of the validation tests. 

Table 5.11 Example Validation Data for Variable Setpoint Operation 

Flow

(mgd) 

UV Intensity

(mW/cm2

) 

Cryptosporidium

Log Credit 

5 5.1 3.0

5 3.3 2.5

5 1.8 1.0

10 9.1 3.0

10 5.6 2.5

10 2.6 1.0

20 15 3.0

20 11 2.0

20 5.6 1.0

The UV intensity measurements recorded during validation verified that the UV intensity 

setpoint approach is valid for this reactor. In contrast to Example 2, this reactor was validated 

for three different levels of Cryptosporidium inactivation credit. For a utility that only is 

required to achieve a 2.0-log inactivation, using this reactor would reduce energy costs compared 

to a reactor that had only been validated for 3.0-log Cryptosporidium inactivation. 

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These intensity setpoints could be used in three ways to achieve 2.0-log Cryptosporidium

inactivation credit with this reactor, 

1. A single setpoint as described in Example 1. For example, a setpoint of 11 mW/cm2

could be used at or between 5 and 20 mgd with any UVT. 

2. Each intensity setpoint could be used over a given flow range as shown in Table 5.12. 

The higher UV intensity measurement from each flow range should be used as the 

UV intensity setpoint to be conservative. 

Table 5.12 UV Intensity Setpoint for Different 

Flow Ranges 

Minimum Flow

(mgd) 

Maximum Flow

(mgd) 

UV Intensity

(mW/cm2) 

5 10 5.6

10 20 11

3. The intensity setpoints could be interpolated as a function of flowrate. Figure 5.3 

presents an equation based on interpolation for three different levels of 

Cryptosporidium inactivation. For example, for a flowrate of 12 mgd, interpolation 

indicates that a setpoint of 3.8 mW/cm2 is needed to achieve 2-log inactivation. 

Figure 5.3 Example 3 – Interpolation of Validation Data to Determine UV Intensity

Setpoints at Different Flows and Cryptosporidium Inactivation 

y = 0.001x2 + 0.517x + 0.872

y = 0.005x2 + 0.176x + 0.968

y = 0.003x2 + 0.346x + 0.920

0

2

4

6

8

10

12

14

0 5 10 15 20 25

Flowrate (gpm)

Intensity set

p

oint

(m

W/

c

m2

)

2.0 log Crypto

2.5 log Crypto

3.0 log Crypto

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Example 4. UV Intensity and UVT Setpoint Control Strategy - Single Operational Setpoint 

for all Conditions (Section C.5.4) 

This example uses single operational setpoint as the operating strategy, which is the same

as example 1. However, this example uses both a UV intensity and a UVT setpoint to control 

the reactor operation. In this example, a MP reactor that uses the UV intensity and UVT setpoint 

control strategy was validated at flows between 0.1 and 0.5 mgd and a UVT range of 75 to 98 

percent. This reactor passed the criteria for 3-log Cryptosporidium inactivation credit with a UV 

intensity sensor setpoint of 41 mW/cm2

 and a UVT setpoint of 85 percent. 

Therefore, to claim 3-log Cryptosporidium, this reactor must operate under the following 

conditions: 

• Maintain minimum UV intensity sensor setpoint of 41.0 mW/cm

2

. 

• Operate within a flow range of 0.1 mgd and 0.5 mgd. 

• Operate within a UVT range of 85 to 98 percent. 

Example 5. Calculated Dose Setpoint Control - Variable Setpoint Operation for Different 

Flow Conditions and Inactivation Goals (Section C.5.5) 

The calculated dose control strategy uses UVT, UV intensity, and flow measurements to 

estimate a UV dose. For this example, a UV manufacturer has completed a matrix of tests at 

different flowrates, UVT, and lamp power to develop a relationship between calculated dose, log 

inactivation, and flow. A MP reactor that uses the calculated dose control strategy was validated 

at flows between 10 to 40 mgd and a UVT range of 75 to 98 percent. Table 5.13 shows the 

results of the validation tests. 

Table 5.13 Dose Setpoints for Various Log Inactivation of Cryptosporidium

Cryptosporidium

Log Inactivation 

Calculated Dose 

Setpoint 

(mJ/cm2

) 

UVT Range 

(%)

1.0 14 75 - 98 

1.5 18 75 - 98 

2.0 23 75 - 98 

2.5 28 75 - 98 

3.0 30 79 - 98 

The validation tests as described in section C.5.5 verified that the calculated dose 

approach is valid for this reactor and that the calculated dose setpoints could be used for the 

ranges of flows tested (10 – 40 mgd). In addition, this reactor can be utilized by utilities that 

need different levels of Cryptosporidium inactivation credit. For a utility that only is required to 

achieve a 2.0 log inactivation credit, using this reactor would reduce energy costs compared to a 

reactor that had only been validated for 3-log Cryptosporidium inactivation credit. Therefore, 

this reactor could be operated at any flow between 10 and 40 mgd, the UVT range specified in 

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Table 5.8, and at the specified calculated dose in Table 5.13 to achieve a specific level of 

Cryptosporidium inactivation credit. For example, a reactor must operate at a minimum

calculated dose of 28 mJ/cm2

 and a flow range between 10 and 40 mgd and UVT between 75 and 

98 percent to achieve 2.5-log Cryptosporidium inactivation credit. 

5.6 Operational Challenges 

An excursion from validated limits can be caused by low UV intensity, low UVT, high or 

low flowrate, poor UV intensity sensor performance, power quality problems, or a combination 

of these conditions. These conditions will need to be resolved quickly to ensure regulatory 

compliance because they can result in prolonged off-specification operation. This section 

discusses some of the potential operational challenges and suggested corrective measures. 

5.6.1 Low UV Intensity or Low Calculated UV Dose 

Although the UV intensity and calculated dose control strategies are different, approaches 

for addressing either a low UV intensity or low calculated dose are typically the same. This is 

because the UV intensity setpoint control strategy uses UV intensity as an indicator for UV dose; 

therefore, the causes of a low UV intensity in a UV intensity control strategy and a low 

calculated dose in a calculated dose control strategy are similar. 

The output of the UV lamps, UV transmittance of the sleeves, status of the UV intensity 

sensor, and fouling of both lamp sleeves and sensor windows affect UV intensity sensor 

readings. In the UV intensity setpoint control strategy, UV intensity sensors are placed far 

enough from the UV lamp to be affected by UVT. In the UV intensity and UVT setpoint or 

calculated dose setpoint control strategy, the UV intensity sensors are close to the lamps and 

should not be affected by UVT changes. 

If one or more UV intensity sensors reads below the required setpoint, the cause could be 

low UV lamp output. If the UV lamp life is greater than the design life, the lamp should be 

replaced. If the UV intensity is still low, sensor accuracy should be determined by replacing the 

duty sensor with the reference sensor. If the duty and reference sensor agree within the required 

uncertainty (from validation), the cause of the low intensity reading may be due to UV intensity 

sensor surface or sensor window fouling or sleeve UV transmittance loss. Potential corrective 

measures include cleaning of fouled surfaces and replacement of defective sleeves. 

Figure 5.4 presents a decision tree for evaluating low UV intensity problems. If the 

above strategies cannot be implemented or are not successful in reducing the low UV intensity, 

the UV manufacturer or UV installation designer should be contacted to investigate the problem

further. The utility should activate any backup disinfection or consider shutting down the water 

treatment plant (WTP) until the UV intensity is within the validated limits. Anytime that the UV 

intensity is lower than the validated limit, it should be recorded as off-specification even if this 

does not occur at precisely the time (e.g., 4-hour interval) when the 4-hour recording is 

completed. 

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5. Start-up and Operation of UV Installations 

UV Disinfection Guidance Manual 5-33 June 2003

Figure 5.4 Low UV Intensity or Low Calculated UV Dose Decision Chart

UV Intensity or calculated

dose is below validated

limits

Is the UVT low or

below validated limits?

Replace duty sensor(s)

with reference sensor(s)

Is UV intensity or

calculated dose still low?

Evaluate and repair

faulty sensor

Is UV intensity or

calculated dose still low?

Replace lamp

Is UV intensity or

calculated dose still low? Replace sleeve

Check other lamps and/or

sleeves in other reactors

to see if they need to

be replaced

No

No

See evaluation of low UVT

(Figure 5.4) Yes

Is the UV lamp age beyond

the design life?

Clean sleeve and or

sensor surface/ window

Inspect other reactors and

sensors for fouling

Take out quartz sleeve

and/or sensor window and

inspect for fouling

Are the sleeves or sensor

surface /windows fouled?

Is the quartz sleeves age

beyond the design life?

Contact UV manufacturer

and/or UV facility designer

to evaluate problem further

and activate backup

disinfection or consider

WTP shutdown

Yes

Yes

No

Yes

Yes

No

Yes

Yes

No

Adjust UV system operation

to compensate for low UV

intensity if not done

automatically by the control

system.

Is the UV intensity or

calculated dose still low?

Yes

No Continue operation

No

Is the UV intensity or

calculated dose still low?

Yes

No Continue operation

No

Continue operation

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5. Start-up and Operation of UV Installations 

UV Disinfection Guidance Manual 5-34 June 2003

5.6.2 Low UV Transmittance 

This evaluation of low UVT assumes either that the low intensity evaluation has been 

completed and the cause of the low UV intensity was low UVT or that the operational staff has 

observed low UVT. Some UV reactors may increase lamp output or number of lamps in service 

to accommodate a decrease in UVT. If the system does not sufficiently compensate, or if the UV 

reactor cannot adjust lamp output, the UV intensity may go below the validated limits. The steps 

for evaluating low UVT are described below. 

The first step is to evaluate the UVT monitor function. If UVT is monitored using an online instrument, the utility should verify the low reading with a bench-top spectrophotometer. If 

the second measurement differs significantly from the on-line instrument response, appropriate 

repair and calibration of the on-line instrument is necessary. 

If UVT is determined using grab samples, a duplicate sample should be obtained and 

analyzed. If the UVT of the duplicate sample remains low, the spectrophotometer response 

should be checked using a phthalate standard (EPA ICR UV254 method or Standard Method 

5910). If the spectrophotometer response is determined to be inaccurate, the spectrophotometer 

monitor should be calibrated or repaired. 

If the low UVT is determined to be real and not due to a faulty instruments, it should be 

compared to the validated UVT set point. If UVT is below the validated UVT set point, the 

following operational changes should be considered: 

• Vary source water blending ratio (if available) to increase UVT. 

• Evaluate whether the coagulation process has been optimized for natural organic 

matter (NOM) removal and whether the coagulant dose should be increased. Poor 

coagulation caused by coagulant under-dosing can lead to increased NOM 

concentration and an associated decrease in UVT. 

• Increase oxidant dose prior to the UV installation if possible. However, this strategy 

may increase disinfection byproduct (DBP) formation, which must also be evaluated 

if this option is used. 

• Investigate potential upstream chemical interferences that may be from a process 

failure or upset. For example, if the ozone quenching system failed, the UVT would 

decrease. 

If the above strategies cannot be implemented or are not successful in reducing the low 

UVT, the UV manufacturer or UV installation designer should be contacted to investigate the 

problem further. The utility may consider shutting down the WTP or activating any backup 

disinfection capacity until the UVT is within the validated limits. A decision tree that

summarizes the approach for troubleshooting low UVT is shown on Figure 5.5. Anytime the 

UVT is lower than the validated limit, it should be recorded as off-specification even if it does 

not occur at precisely the time (e.g., 4-hour interval) that recording is completed. 

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5. Start-up and Operation of UV Installations 

UV Disinfection Guidance Manual 5-35 June 2003

Figure 5.5 High UV Absorbance Decision Chart 

Resample and

Reanalyze

Low UVT

Is on-line UVT monitor used?

Check grab

UVT sample with

bench-top

spectrophotometer

No Yes

Is repeat UVT similar

and low?

Evaluate sampling

and analytical

procedures

No

Check

spectrophotometer

with phthalate

standard

Yes

Is spectrophotometer

response acceptable?

Is grab UVT sample low

after repeat sampling?

Recalibrate or repair

bench-top

spectrophotometer

No

Is the UV system operating

off-specification because of

low UVT?

Recalibrate or repair

on-line UVT monitor No

Yes Yes

Continue operation No

Can UVT be increased through

WTP operation changes?

Yes

Is UVT below the

validation limit?

Is UV intensity

or calculate dose below

validation limits?

Continue operation

Yes

No

No

Contact manufacturer

or UV system designer

to investigate this

issue further. Consider

WTP shutdown

Yes

See low UV intensity

and calculated dose

decision tree

(Figure 5.3)

Yes

No

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5. Start-up and Operation of UV Installations 

UV Disinfection Guidance Manual 5-36 June 2003

5.6.3 Rapid Flow Increase or High Flow

It may be possible to compensate for increased flow (depending on validation data) by 

completing one or more of the following actions: 

• Increasing the output of the UV lamps 

• Using additional lamps or banks of lamps 

• Using additional UV reactors 

The success of these strategies depends on the magnitude of the flowrate increase and the 

type and configuration of the UV reactors. These changes should occur automatically for 

reactors that are controlled using PLCs. 

If the measured flowrate is higher than the validated limits and cannot be reduced, the 

flowmeter and/or differential pressure meter (if used) should be evaluated to determine if it is 

functioning properly. Instrument error can be assessed by comparing signals from individual 

flowmeters or differential pressure devices to anticipated values based on facility flowrate and 

historic operating data. Alternatively, a calibrated clamp-on type flowmeter may be used to 

verify flowrates. If the flowmeter is not operating properly, it should be repaired or replaced. If 

flow monitoring devices appear to be functioning properly, valve position or blockage may be 

the cause of unequal flow distribution and should be evaluated. 

If the flow is below the validated limits, one UV reactor should be taken off-line, which 

will transfer that flow to the other energized reactors. This change in operation should result in 

the UV reactors being within the validated flow range. Anytime the flow is lower or higher than 

the validated limit, it should be recorded as off-specification even if it does not occur at precisely 

the time (e.g., 4-hour interval) that the recording is completed. 

5.6.4 Unreliable UV Intensity Sensor Readings 

Consistent UV intensity sensor readings are important to ensure that the UV reactors are 

operating within the validated limits. Unreliable UV intensity sensor readings can be described 

by one or more of the following behaviors: 

• Calibration checks outside of uncertainty specified in the validation testing 

• Random fluctuations of greater than 25 percent 

• Biased readings (UV intensity sensor reading is offset from the reference sensor 

readings by a certain value) 

Unreliable UV intensity sensor readings can be due to UV intensity sensor malfunction, 

condensation in the sensor or between the sensor and sensor window, lamp malfunction, poor 

grounding, degradation of sensor electronics, or electronic short circuits. 

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5. Start-up and Operation of UV Installations 

UV Disinfection Guidance Manual 5-37 June 2003

The UV intensity sensor and lamp electrical cables should be secured, and a reference or 

standby sensor should be compared to the duty sensor reading. If the duty sensor is found to be 

defective or out of calibration, it should be sent to the manufacturer for repair, and the standby 

sensor used in its place. 

5.6.5 Power Quality Problems 

UV lamps can potentially lose their arc if a voltage sag, power quality anomaly, or a 

power interruption occurs. Voltage sags as little as 10 to 15 percent from the nominal voltage for 

as few as 2 to 5 cycles can cause a UV lamp to lose its arc. LP lamps generally can return to full 

operating status within 15 seconds after power is restored. LPHO and MP lamps will need to be 

re-struck, which generally requires between 4 and 10 minutes to get to full lamp power, to 

restart. LPHO and MP lamps are affected differently from power losses as discussed in more 

detail in section 3.1.3.3. 

The corrective actions for short-term power failures (e.g., voltage sag) are different for 

LPHO and MP reactors. LPHO lamps need to warm-up before the arc can be struck, and MP 

lamps need to be cooled before the arc can be struck. Standby MP reactors (i.e., not in operation 

when voltage sag occurred) should be energized instead of “warm” reactors because they will 

take less time to restore operation to within validated limits because the UV lamps do not have to 

cool down before re-striking. However, installations using LPHO reactors should energize their 

“warm” reactors (i.e., the reactors on-line when the voltage sag occurred) instead of standby 

LPHO reactors because the UV lamp warm-up time will be less compared to a cold LPHO 

reactor. 

For long-term power failure (e.g., > 5 minutes) without a UPS system, the UV reactors 

should be powered by the backup generator until power is restored. When power is restored, the 

shift from the backup generator will likely cause the UV lamps to lose their arc again. 

Given the restrictions on operation outside of validated limits (section 1.3.1.3), the utility 

should stop water flow through the UV reactors when the lamps are not operating. Also, utilities 

should consider installing a UPS if power quality problems are frequent because a standby 

generator alone may not adequately alleviate frequent, off-specification flows due to power 

quality problems. A UPS system delivers consistent, continuous power even when power 

problems occur. 

5.7 Staffing Issues 

In order to provide consistent and reliable operation of UV reactors, the utility needs to 

have appropriate staffing, training, and safety measures in place. This section discusses these 

issues. 

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5. Start-up and Operation of UV Installations 

UV Disinfection Guidance Manual 5-38 June 2003

5.7.1 Staffing Levels 

During start-up operation, a UV reactor will need more operator attention to assist with

functional and performance testing and to establish site-specific O&M procedures (described in 

section 5.1.4). However, a typical UV installation needs little operator attention during normal 

operation, depending on the level of automation. Generally, UV installations use PLCs to 

monitor operating parameters, control the UV reactor, and generate alarms. Increased 

automation (e.g., remote monitoring capability) may be incorporated to further reduce operator 

requirements. Table 5.14 describes how various site-specific factors affect staffing needs for a 

UV installation.

Table 5.14 Factors Impacting Staffing Needs 

Factor Impact on Staffing 

Type of UV reactor LP and LPHO reactors may need more maintenance compared

to a MP reactor because they have more lamps and usually 

employ OCC cleaning. However, MP lamps will probably need 

to be replaced more often than LP lamps. 

Instrumentation and control strategy More automated control strategies will result in lower staffing 

levels due to enhanced remote operation and monitoring 

capability. 

Water quality Sleeve fouling and cleaning frequency is affected by water 

quality and the design of the UV reactor. These in turn impact 

the staffing needs for manual cleaning for OCC systems and for 

maintaining the OMC system. 

5.7.2 Training 

Training is necessary for all personnel who are associated with the UV installation, 

including operators, maintenance workers, instrumentation technicians, electricians, laboratory 

staff, custodial staff, engineers, and administrators. The training program should incorporate any 

State requirements and should emphasize both normal and emergency operating procedures, 

safety issues, process control and alarm conditions, validated operation, and response to 

deviations. 

The UV manufacturer and UV installation designer should provide training on the UV 

reactors, UV installation design, and operation and maintenance activities. It is recommended 

that training include both classroom instruction and field training. In addition, actively involving 

the operating staff during start-up will provide another opportunity to reinforce classroom

instructions. Continued training should be provided when new employees are hired or when a 

process or control alteration is made. 

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5. Start-up and Operation of UV Installations 

UV Disinfection Guidance Manual 5-39 June 2003

5.7.3 Safety Issues 

The Office of Safety and Health Administration (OSHA) issues regulations and guidance 

to support operator safety in the workplace. There may also be specific safety requirements 

imposed by the State. In addition to the standards and procedures established for WTP 

operations, the following safety issues pertain specifically to UV reactors: 

• UV light exposure 

• Electrical safety 

• Burns from hot lamps or equipment 

• Abrasions or cuts from broken lamps 

• Potential exposure to mercury from broken lamps - Over-exposure to UV light can 

cause eye injury and skin damage. 

Threshold Limit Values (TLVs) are issued biannually by the American Conference of 

Governmental Industrial Hygienists (ACGIH). The TLVs for UV radiation apply to 

occupational exposure to UV incident on the skin or eye. The recommended TLVs depend on 

the lamp wavelengths emitted and the irradiance (mW/cm2

); the utility can determine the 

appropriate TLV for their UV reactors, using the TLVs for Chemical Substances and Physical 

Agents and Biological Exposure Indices (ACGIH 2002). These values are not enforceable 

standards but should be considered when establishing operational procedures. To limit or 

prevent operator exposure to the UV light, UV reactors should have interlocks that deactivate the 

lamps when reactors are accessed. Viewing ports, if provided, should be fitted with UV filtering 

windows, or operators should wear a UV resistant face shield when looking at lamps or the 

reaction chamber. In addition, warning signs should be placed to minimize the danger of 

exposure. 

To reduce the risk of electrical shock, the main electrical supply to the UV reactors 

should be disconnected and the operator should wait at least 5 minutes for the lamps to cool 

down and energy to dissipate before any maintenance is performed. All safety and operation 

precautions required by the National Electric Code (NEC), OSHA, local electric codes, and the 

UV manufacturer should be followed and include the following precautions: 

• Proper grounding 

• Lockout, tag-out procedures 

• Use of proper electrical insulators 

• Installation of safety cut-off switches 

The ballasts and the reactor chamber can also become hot during operation. The 

temperature of these components should be assessed prior to contact. 

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5. Start-up and Operation of UV Installations 

UV Disinfection Guidance Manual 5-40 June 2003

Broken lamps pose two potential safety hazards. The lamps and sleeves are constructed 

of quartz tubing, which can fracture and cause serious cuts or injury. In addition, broken lamps 

may release mercury. Operators should be trained in proper mercury cleanup and disposal 

procedures to prevent mercury inhalation or absorption through the skin. Appendix N discusses 

potential mercury cleanup procedures. 

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6. References 

APHA-AWWA-WEF. 1998. Standard methods for the examination of water and wastewater, 

20th ed. Washington D.C. 

Aquafine Corporation. 2001. CSL series: Installation, maintenance and operation manual. Part 

no.115-1. 

Bourgine, F.P., J.I. Chapman, H. Kerai, J.I. Duval, J.G. Green and D. Hamilton. 1995. The 

degradation of atrazine and other pesticides by photolysis. The chartered institution of

water and environmental management 9: 417-422. 

Brandt, C.L., and A.C. Giese. 1956. Photoreversal of nuclear and cytoplasmic effects of short 

ultraviolet radiation on paramecium caudatum. J. Gen. Physiol. 39, no. 5: 735-751. 

Bukhari, Z., T.M Hargy, J.R. Bolton, B. Dussert, and J.L. Clancy. 1999. Medium-pressure UV 

for oocyst inactivation. Journal AWWA 91: 86-94. 

Cabaj, A., R. Sommer, W. Pribil, and T. Haider. 2001. What means “dose” in UV-disinfection 

with medium pressure lamps. Ozone Science & Engineering 23: 239-244. 

Carollo Engineers. 2001. Weber basin water treatment plant no. 3 expansion. Layton, Utah. 

Chang, J.C.H., S.F. Osoff, D.C. Lobe, M.H. Dorfman, C.M. Dumais, R.G. Qualls, and J.D. 

Johnson. 1985. UV inactivation of pathogenic and indicator microorganisms. Applied 

and Environmental Microbiology 49, no.6: 1361-1365. 

Cushing, R.S., E.D. Mackey, J.R. Bolton, and M.I. Stefan. 2001. Impact of common water 

treatment chemicals on UV disinfection. Proceedings of the AWWA Annual Conference 

and Exposition, June 17-21, Washington, D.C. 

Dinkloh, L. 2001. Interview by Ben Hauck. Telephone conversation. Tucson, AZ., August 10. 

Downes, A. and T.P. Blunt. 1877. Researches on the effect of light upon bacteria and other 

organisms. Proceedings Royal Society London. Volume 26. 

Dulbecco, R. 1950. Experiments on photoreactivation of bacteriophages inactivated with 

ultraviolet radiation. Journal of Bacteriology 59: 329-347. 

Gates, F. L. 1929. A Study of the Bactericidal Action of Ultraviolet Light. J. Gen. Physiol. 13: 

231-260. 

GE Quartz. 2001. Available from Internet location http://www.gequartz.com/en/lamptube.htm. 

Internet. 

Gump, D. 2002. Severn Trent. Personal communication with Jennifer Hafer, Malcolm Pirnie, 

Inc., regarding UV reactors. Tucson AZ, November 6. 

UV Disinfection Guidance Manual 6-1 June 2003

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Chapter 6. References 

UV Disinfection Guidance Manual 6-2 June 2003

Jagger, J. 1967. Introduction to research in ultraviolet photobiology. Englewood Cliffs, N.J: 

Prentice-Hall, Inc.

Kashinkunti, R., K.G. Linden, G. Shin, D.H. Metz, M.D. Sobsey, M. Moran, and A. Samuelson. 

2003. Achieving multi-barrier inactivation in Cincinnati: UV, byproducts, and 

biostability. JAWWA. Submitted January 9, 2003. 

Kelner, A. 1950. Light-induced recovery of microorganisms from ultraviolet radiation injury, 

with special reference to Escherichia coli. Bulletin of the New York Academy of Medicine

26: 189-199. 

Knudson, G.B. 1985. Photoreactivation of UV-irradiated Legionella Pneumoplila and other 

Legionella species. Applied and Environmental Microbiology 49, no. 4: 975-980. 

Kruithof, J.C., and R.C. van der Leer. 1990. Practical experiences with UV-disinfection in the 

Netherlands. Proceedings of the AWWA seminar on emerging technologies in practice, 

AWWA Annual Conference, June 17-21, Cincinnati, OH. 

Lienberger, J. 2002. Trojan Technologies. Personal communication with Jennifer Hafer, Malcolm Pirnie, 

Inc, regarding UV reactors. November 5.

Lin, L.S., C.T. Johnston, and E.R. Blatchley III. 1999a. Inorganic fouling at quartz:water 

interfaces in ultraviolet photoreactors I: chemical characterization. Water Research 33, 

no. 15: 3321-3329. 

Lin, L.S., C.T. Johnston, and E.R. Blatchley. 1999. Inorganic fouling at quartz: water interfaces 

in ultraviolet photoreactors – II Temporal And Spatial Distribution. Water Research 33, 

no. 15: 3330-3338. 

Linden, K.G., G.A. Shin, and M.D. Sobsey. 2001. Comparative effectiveness of UV 

wavelengths for the inactivation of Cryptosporidium parvum oocysts in water. Water 

Science & Technology 43, no. 12: 171-174. 

Linden, K.G., G.A. Shin, G. Faubert, W. Cairns, and M.D. Sobsey. 2002a. UV disinfection of 

Giardia lamblia cysts in water. Environmental Science and Technology 36, no. 11: 2519-

2522. 

Linden, K.G., L. Batch, and C. Schulz. 2002b. UV Disinfection of filtered water supplies: water 

quality impacts on MS2 dose-response curves. Proceedings of the AWWA Annual 

Conference, June 16-20, New Orleans, LA. 

Linden, K.G. 2002. Personal email communication by Laurel Passantino. April 24-May 6. 

Mackey, E.D., R.S. Cushing, and H.B. Wright. 2001. Effect of water quality on UV disinfection 

of drinking water. Proceedings of the First International Ultraviolet Association 

Congress, June 14-16, Washington, D.C. 

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Chapter 6. References 

UV Disinfection Guidance Manual 6-3 June 2003

Mackey, E.D., R.S. Cushing, M.L. Janex, N. Picard, J.M. Laine, and J.P. Malley. 2003. 

Bridging Pilot-Scale Testing to Full-Scale Design of UV Disinfection Systems, Draft 

Report. Final report to be published in 2004. Denver, CO: AWWA Research 

Foundation. 

Malley, J.P, J.P. Shaw and J.R. Ropp. 1995. Evaluation of by-products produced by treatment of 

groundwaters with ultraviolet irradiation. Denver, CO.: AWWA Research Foundation. 

Malley, J.P. 2002. Historical perspective of UV use. Presented at the AWWA Water Quality 

Technology Conference, November 10-14, Seattle, W.A. 

Meulemans, C.C.E. 1986. The basic principles of UV-sterilization of water. In: Ozone + 

Ultraviolet Water Treatment, Aquatec Amsterdam, Paris: International Ozone 

Association, B.1.1-B.1.13. 

Munakata, N., M. Daito, and K. Hieda. 1991. Inactivation spectra of Bacillus Subtilis spores in 

extended ultraviolet wavelengths (50-300nm) obtained with Synchrotron radiation.

Photochemistry and Photobiology 54, no 5:761-768. 

NWRI. 2000. Ultraviolet Disinfection: Guidelines for Drinking Water and Water Reuse. 

National Water Research Institute, Fountain Valley, CA. 

Oguma K., H. Katayama, H. Mitani, S. Morita, T. Hirata, and S. Ohgaki. 2001. Determination of 

pyrimidine dimers in Escherichia coli and Cryptosporidium parvum during UV light 

inactivation, photoreactivation and dark repair. Applied and Environmental 

Microbiology 67, no 10: 4630 – 4637. 

Passantino, L.B. and J.P. Malley. 2001. Impacts of turbidity and algal content of unfiltered 

drinking water supplies on the ultraviolet disinfection process. Proceedings of the 2001 

AWWA Annual Conference, Washington D.C. 

Powell, W. F. 1959. Radiosensitivity as an index of Herpes Simplex virus development. Virology

9:1-19. 

Rauth, A.M. 1965. The physical state of viral nucleic acid and the sensitivity of viruses to 

ultraviolet light. Biophysical Journal 5: 257-273. 

Roberts, Andrew. 2000. Letter to Dan Schmelling of the USEPA, April 10. 

Sharpless C.M., and K.G. Linden. 2001. UV photolysis of nitrate: effects of natural organic 

matter and dissolved inorganic carbon, and implications for UV water disinfection. 

Environmental Science and Technology 35, no 14: 2949-2955. 

Shin G.A., K.G. Linden, M.J. Arrowood, G. Faubert, and M.D. Sobsey. 2001. DNA repair of 

UV-irradiated Cryptosporidium parvum oocysts and Giardia lamblia cysts. Proceedings 

of the First International Ultraviolet Association Congress, June 14-16, Washington, D.C. 

Proposal Draft 
Chapter 6. References 

UV Disinfection Guidance Manual 6-4 June 2003

Snowball, M.R., and I.S. Hornsey. 1988. Purification of water supplies using ultraviolet light. 

Developments in food microbiology, edited by R.K. Robinson, 171-191. NY: Elsevier 

Applied Science. 

Swaim, P.D., M. A. Morine, R.G. Brauer, M.A. Neher, J.L Gebhart, W.D. Bellamy. 2002. 

Operating data from Henderson’s full-scale UV disinfection facility. Presented at the 

water quality technology conference, November 2002. 

Toivanen, E. 2000. Experiences with UV disinfection at Helsinki water. IUVA News 2, no. 6: 4-8 

U.S. Environmental Protection Agency. 2000. Technical memorandum: Worldwide ultraviolet 

disinfection installations for drinking water. Office of Groundwater and Drinking Water, 

Washington, DC. 

U.S. Environmental Protection Agency. 2000. Technological costs for control of microbial 

contaminants and disinfection by-products. Office of Groundwater and Drinking Water, 

Washington, DC. 

U.S. Environmental Protection Agency. 2002. Microbial Toolbox Guidance Manual. Office of 

Groundwater and Drinking Water, Washington, DC. 

U.S. Environmental Protection Agency. 2003. Public water system guidance manual for source 

water monitoring under the LT2ESWTR. Office of Groundwater and Drinking Water, 

Washington, DC. 

von Sonntag, C., and H.P. Schuchmann. 1992. UV disinfection of drinking water and by-product 

formation—some basic considerations. Journal of Water Supply Research and 

Technology 41, no 2: 67-74 

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Appendix A. Fundamentals of UV Disinfection 

This appendix supplements Chapter 2, Overview of UV Disinfection, with an additional 

level of detail. The purpose of this appendix is to provide technical information regarding the 

physical mechanisms of UV light generation, biological reactions causing disinfection, and UV 

reactor equipment. The organization of this appendix is presented below, including the key 

questions addressed by each section. 

• How is UV light generated?.......................................................................Section A.1.1 

• What happens to UV light as it propagates through water?.......................Section A.1.2 

• How does UV light inactivate microorganisms? .......................................Section A.2.2 

• Can microorganisms undergo repair and become

infectious after inactivation by UV light?..................................................Section A.2.3 

• How is UV dose determined in a bench-scale (batch) 

system?....................................................................................................Section A.2.4.1 

• How does UV dose vary in a UV reactor?..............................................Section A.2.4.2 

• How do microbial dose-response curves differ?........................................Section A.2.5 

• What factors influence microbial dose-response? .....................................Section A.2.6 

• Do all microorganisms have the same sensitivity to UV 

light? ..........................................................................................................Section A.2.7 

• What are the components of a UV installation? ...........................................Section A.3 

• How do low pressure, low-pressure high-output, and 

medium pressure lamps differ?................................................. Section A.3.1.2-A.3.1.4 

• What happens to UV lamps as they age?................................................Section A.3.1.6 

• How are UV lamps powered? ....................................................................Section A.3.2 

• What is the function of the lamp sleeve? ...................................................Section A.3.3 

• How are lamp sleeves cleaned and why is it necessary 

to clean them? ............................................................................................Section A.3.4 

• How is UV light monitored in a reactor?...................................................Section A.3.5 

• How are the components of a UV reactor arranged?.................................Section A.3.8 

UV Disinfection Guidance Manual A-1 June 2003

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Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-2 June 2003

• How do the utility and the State know the UV reactor is 

delivering the required UV dose? ..............................................................Section A.3.9 

• What are the impacts of water quality on UV 

disinfection?...............................................................................................Section A.4.1 

• Do any disinfection byproducts form as a result of UV 

disinfection?...............................................................................................Section A.4.2 

A.1 UV Light Generation and Propagation Through Liquid Media 

Using UV light to disinfect drinking water involves generating UV light with the desired 

germicidal properties and subsequently delivering that light to the target pathogens. This section 

describes fundamental concepts related to the generation and transmission of UV light. 

A.1.1 UV Light Generation 

Atoms and ions emit light when they change from a higher to a lower energy state. An 

atom and most ions consist of electrons orbiting a nucleus of protons and neutrons. The 

electrons in each orbital occupy a unique energy state, where the electrons closest to the nucleus 

have a lower energy and the electrons further away have a higher energy. When an electron 

makes a transition from a higher energy state to a lower energy state, a discrete amount of energy 

is released as photons of light at a particular wavelength (λ) according to Equation A.1. 

λ

hc E2 − E1 = Equation A.1

where 

E1 = Lower energy state (J) 

E2 = Higher energy state (J) 

h = Planck's Constant (6.626 x 10-34

 J•s) 

c = Speed of light (2.997 x 108

 m/s)

λ = Wavelength (m) 

Energy levels of a given atom or ion are unique and depend on the number of electrons, 

protons, and neutrons within that atom or ion and their interaction with external force fields. As 

such, each element emits a unique spectrum of light. If the difference between energy levels is 

appropriate, the light emitted is in the UV range. 

A transition from a lower to a higher energy state requires an energy input. This energy 

may be derived from the collision of the atom with a photon of light of wavelength λ or by 

collision with other atoms, ions, or electrons. Energy transferred to the atom may result in an 

increase in the atom’s kinetic energy, the transfer of an electron to a higher energy level, or the 

removal of an electron from the atom. Removal of an electron from the atom is termed 

ionization and results in a positively charged cation and a negatively charged free electron. The 

energy required to remove an electron from an atom is termed the ionization energy. 

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Recombination of a free electron and a cation may result in the emission of light. Since 

the free electron and cation may have a range of kinetic energies, the wavelength of emitted light 

will vary. The wavelength range will be bound by the ionization energy of the atom, and there 

will be a peak within the rage that depends on the temperature of the electrons and cations. The 

following sections discuss the relationship between atomic energy states and the generation of 

UV light through gas and mercury discharges. 

A.1.1.1 Gas Discharges 

A gas discharge is a mixture of non-excited atoms, excited atoms, cations, and free 

electrons formed when a sufficiently high voltage is applied across a volume of gas. The 

wavelength of light emitted from the gas discharge depends on the elemental composition of the 

gas discharge and the excitation, ionization, and kinetic energy of those elements. 

The formation of the gas discharge within a UV lamp involves several stages. When a 

voltage is first applied, free electrons and ions present in the gas are accelerated by the electric 

field formed between two electrodes. Initially, the concentration of free electrons and ions arises 

from natural radioactivity and is very low. With sufficient voltage, the electrons are accelerated 

to high kinetic energies. Collisions of the free electrons with atoms result in a transfer of energy 

to the atoms. If the energy transferred is sufficient, the atoms are ionized. This ionization 

provides a rapid increase in the number of free electrons and cations, a corresponding increase in 

lamp current, and a drop in the voltage across the lamp. 

Cations colliding with an electrode cause electrons to be emitted. If sufficient electrons 

are emitted, a self-sustaining discharge termed a glow discharge occurs. Initially, only a small 

fraction of each electrode emits electrons. With an increase in current, this area increases until 

the entire electrode is in use. To increase the current beyond that point, the voltage is increased 

to provide more kinetic energy to the cations. High energy cations that collide with the electrode 

increase the electrode’s temperature. At sufficiently high temperatures, the electrode begins to 

thermally emit electrons, and a further increase in current reduces the voltage requirement. At 

this point, the electrode discharge is termed an arc discharge.

The start voltage, which is the voltage required to start the gas discharge, is typically 

higher than the ionization potential of the gas unless a means is used to introduce electrons. 

Preheating the electrode or producing a strong local field using a third electrode located close to 

one of the electrodes can be used to introduce electrons and aid in starting the gas discharge. 

A gas discharge has a negative impedance that is intrinsically unstable unless a ballast is 

placed in series to provide a positive impedance to the power supply. With a direct current (DC) 

supply powering the gas discharge, the ballast is a resistor. With an alternating current (AC) 

supply, the ballast is either an inductor, capacitor, or some combination of those components. 

Inductors and capacitors are preferred over resistors because they do not consume power. More 

detail on ballasts is presented in section A.3.2. 

The frequency of the AC supply impacts the performance of the gas discharge. If the 

frequency of the AC supply is low (<< 1 kHz), electron-cation recombination extinguishes the 

discharge every half cycle of the lamp voltage. Re-ignition during the next half cycle is 

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UV Disinfection Guidance Manual A-4 June 2003

facilitated by electron emission from the still warm electrodes. If the frequency of the AC 

supply is greater than 1 kHz, the free electrons and cations do not have sufficient time to 

recombine and the discharge does not extinguish. 

A.1.1.2 Mercury Discharges 

Mercury in a gas discharge is used to generate the UV light produced in most commercial 

UV lamps. Mercury is an advantageous element for UV disinfection due to the following 

factors: 

• Electron transitions within mercury provide electromagnetic energy in the germicidal 

wavelength range. 

• Mercury at low vapor pressure and near room temperature produces light at 

wavelength 253.7 nm from electrical energy with high efficiency. This wavelength is 

near optimal for UV disinfection (section A.2.2). 

• Mercury at high vapor pressures produces high intensity polychromatic UV light with 

reasonably high efficiency. 

• Mercury has a low ionization energy; therefore, free electrons and cations required 

for the formation of a gas discharge are easily created using a relatively low start 

voltage. 

• Mercury reacts minimally with the lamp envelope and electrode materials. 

The wavelength and magnitude of light output from a mercury discharge depend on the 

concentration of mercury atoms, which is directly related to the mercury pressure. At low 

pressures of 0.001 to 0.01 torr (2 x 10-5 to 2 x 10-4 psi), the concentration of mercury is low, and 

the distance electrons travel between collisions is relatively long. Electrons achieve higher 

kinetic energies with the longer travel distance. Collisions between those free electrons and 

mercury atoms excite mercury to the first energy state above the lowest or ground state. 

Transition of electrons back to ground state results in the emission of electromagnetic energy at 

253.7 and 185 nm. UV lamps with this type of mercury discharge are commonly referred to as 

low pressure (LP) lamps. 

At higher mercury pressures (100 to 10,000 torr; 2 to 20 psi), a much greater collision 

frequency occurs between free electrons and mercury. This increases the energy state of the 

mercury atoms and cations to near that of the electrons and increases the temperature of the gas 

discharge to near 6,000 °C. When the atoms return to lower energy states, electromagnetic 

energy at several wavelengths in the UV light and visible light regions is produced. 

Recombination of free electrons and mercury cations produces a small continuum of UV light 

between 200 and 245 nm. UV lamps with this type of discharge are called medium pressure 

(MP) lamps. 

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UV Disinfection Guidance Manual A-5 June 2003

A.1.2 UV Light Propagation

This section details the effects that the UV reactor and the water being treated have on the 

propagation of UV light. As UV light propagates, it interacts with the materials it encounters 

through absorption, reflection, refraction, and scattering. 

A.1.2.1 Absorption 

Absorption is the transformation of light to other forms of energy as it passes through a 

substance. UV absorbance is the water quality parameter that measures the extent to which the 

intensity of UV light is reduced as it passes through water. The impact of absorption on the 

intensity light as it travels through a substance is calculated as follows: 

icid a d A ed

e

I

I − ε − − −α = = = ∑ = 10 254 10 10 10

1

2 Equation A.2

where 

I1 = Light intensity incident on a cell (mW/cm

2

) 

I2 = Light intensity passing through a distance, d, in the cell containing a solution with 

various absorbing components (mW/cm2

) 

d = Distance traveled by light through the cell (cm) 

εi = Molar absorption coefficient of component i (L/mol/cm)

ci = Concentration of component i (mol/L) 

a10 = Decadic (base 10) absorption coefficient, (cm-1) 

A254 = Decadic (base 10) absorbance (unitless) 

αe = Naperian (base e) absorption coefficient (cm-1) 

When UV light is absorbed, it is no longer available to disinfect microorganisms. 

A.1.2.2 Refraction 

Refraction (Figure A.1) is the change in the direction of light propagation as it passes 

from one medium to another. 

Figure A.1 Refraction of Light 

Incident Light

Medium 1 Medium 2

Refracted Light

θ1

θ2

Incident Light

Medium 1 Medium 2

Refracted Light

θ1

θ2

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UV Disinfection Guidance Manual A-6 June 2003

Refraction is governed by Snell’s Law, which is shown in Equation A.3: 

1 1 2 2 n sinθ = n sinθ Equation A.3

where 

n1 = Index of refraction of the first media 

n2 = Index of refraction of the second media 

θ1 = Incident angle on the interface 

θ2 = Exit angle from the interface 

In UV reactors, refraction occurs when light passes from the lamp through an air gap, 

through the lamp sleeve, and into the water. Although refracted light is still available for 

disinfection, refraction changes the angle that the light strikes target pathogens. 

A.1.2.3 Reflection 

Reflection is the change in the direction of light propagation when it is deflected by the 

interface between two media (Figure A.2). Reflection may be classified as specular or diffuse. 

Specular reflection occurs at smooth polished surfaces where the roughness of the surface is 

smaller than the wavelength of light. Reflection from specular surfaces follows the Law of 

Reflection, which states that the angle of incidence is equal to the angle of reflection. Diffuse 

reflection occurs at rough surfaces. Light is scattered in all directions with little dependence on 

the incident angle. The intensity of diffuse reflected light is proportional to the cosine of the 

reflectance angle. Reflected light is still available for disinfection. 

Figure A.2 Specular and Diffuse Reflection of Light 

Specular Reflection

Incident Light Reflected Light

Diffuse Reflection

Incident Light

θ1 θ1

Reflected Light

Specular Reflection

Incident Light Reflected Light

Diffuse Reflection

Incident Light

θ1 θ1

Reflected Light

In a UV reactor, reflection will take place at UV-transmitting interfaces like an air-quartz 

interface and at also interfaces that do not transmit UV light like the reactor wall. The intensity 

of reflected light from a UV-transmitting interface is governed by Fresnel’s Law, which is shown 

in Equation A.4. 

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Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-7 June 2003

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

− + ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

− =

2

1 2 2 1

2 1 1 2

2

1 1 2 2

1 1 2 2

cos cos

cos cos

cos cos

cos cos

2

1

θ θ

θ θ

θ θ

θ θ

n n

n n

n n

n n R Equation A.4

where 

R = the ratio of reflected intensity to incident intensity 

n1 = Index of refraction of the first media 

n2 = Index of refraction of the second media 

θ1 = Incident angle onto the interface 

θ2 = Reflected angle from the interface 

The intensity of reflected light from a non-transmitting interface depends on the material, 

incident angle, wavelength of light, and nature of the surface. Currently, most UV reactors are 

constructed of stainless steel, which reflects 24 percent of UV light at 254 nm at a zero degree 

incident angle (Jagger 1967). This indicates that 76 percent of the light energy reaching the 

reactor wall is lost. In the future, UV reactors may be developed using materials that reflect 

more light, which may improve efficiency. 

A.1.2.4 Scattering 

Scattering of light is the change in direction of light propagation caused by interaction 

with a particle (Figure A.3). Scattered light is still capable of disinfecting microorganisms. 

Figure A.3 Scattering of Light 

Incident

Light

Forward

Scattered

Light

90° Scattered Light

Back

Scattered

Light

Incident

Light

Forward

Scattered

Light

90° Scattered Light

Back

Scattered

Light

Rayleigh scattering is the scattering of light by particles that are smaller than the 

wavelength of the light. With Rayleigh scattering, light is scattered uniformly in all directions at 

an intensity inversely proportional to the wavelength of light to the fourth power (1/λ4

). As such, 

scattering is more evident at shorter wavelengths. For example, the intensity of scattered light at 

200 nm is five times greater than at 300 nm because 1/(2004

) is over five times greater than 

1/(3004

). Particles in water that cause Rayleigh scattering of UV light at 254 nm include small 

viruses and large molecules (25 to 300 nm). With larger particles, the scattering observed is nonuniform, and more light is scattered in the forward direction. The larger particles also cause 

backscattering, which is nearly independent of the wavelength of light. 

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UV Disinfection Guidance Manual A-8 June 2003

A.1.2.5 UV Absorbance and UV Transmittance 

UV absorbance (A254) is a commonly used water quality parameter that characterizes the 

decrease in the amount of incident light as it passes through a water sample over a specified 

distance or pathlength. If the measurement is made according to a modified version of Standard 

Method 5910B (APHA et al. 1998) where the water sample is not filtered or pH adjusted, the 

modified measurement accounts for scattering and some absorption from particles in the water 

sample that may interfere with UV disinfection. Although the Standard Method identifies this 

measurement as UV absorption, this manual will refer to it as UV absorbance since the latter 

term is widely used in the water treatment industry. 

The term UV transmittance (UVT) has also been used extensively in the literature when

describing the behavior of UV light. UVT is the percentage of light passing through a water 

sample over a specified distance and is related to UV absorbance by Equation A.5: 

254 % 100 10 A UVT − = ∗ Equation A.5

where 

UVT = UV Transmittance at specified wavelength (e.g., 254 nm) and pathlength 

(e.g., 1 cm) 

A254 = UV Absorbance at specified wavelength, based on 1 cm pathlength (unitless; UV 

absorption as measured by Standard Method 5910B) 

Since UV light scattered from particles is capable of disinfecting microorganisms, it 

should be considered when assessing UVT. Much of the scattered light is in the forward 

direction and is a significant portion of the transmitted UV light. Typically, conventional 

spectrophotometers use narrow beams of light and small detectors that will not measure the 

forward scattered light and therefore underestimate the effective UVT of the water sample 

(Jagger et al.1975; Linden and Darby 1998). However, spectrophotometers can be equipped 

with integrating spheres (Linden and Darby 1998) or detectors capable of measuring forward 

scattered light (Jagger et al. 1975) in order to provide a proper assessment of the UVT of water 

samples with significant scattering. 

A.1.2.6 Estimating UV Light Intensity Within a UV Reactor 

The distribution of light intensity about a UV lamp is influenced by the shape of the lamp

and the absorption, refraction, scattering, and reflection of light. Complex models factoring all 

of these effects can be used to determine the intensity profile about a lamp, and simplified 

models can be used to approximate those profiles. These models are useful tools for 

understanding the impact of UV absorbance, UV reactor properties, and UV reactor dimensions 

on UV dose delivery and measurements of UV intensity. 

If the distance from the lamp is greater than the radius of the arc discharge, the lamp can 

be treated as a line source to estimate the intensity. For LP lamps, since the arc discharge fills 

the entire lamp, the radius is the same as the lamp radius. For MP lamps, the arc discharge is 

much smaller than the radius of the lamp. There are two approaches commonly used for 

modeling a line source: the radial model and the point source summation model. If the distance 

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UV Disinfection Guidance Manual A-9 June 2003

from the lamp is smaller than the radius of the gas discharge, more complex modeling tools must 

be used. 

The radial model provides a two-dimensional representation of a three-dimensional 

intensity profile. The model assumes light is emitted perpendicular from the line source in the 

radial direction as per Equation A.6: 

I r

P

r

e

L re ( ) = −

2π

α Equation A.6

where 

PL = UV power emitted per unit arc length of the line source (mW/cm) 

r = Radial distance from the line source (cm)

αe = Naperian (base e) absorption coefficient of the media (cm-1) 

I(r) = UV intensity (mW/cm

2

) at a distance r from the line source 

The Point Source Summation model (Jacob and Dranoff 1970) treats the lamp as a 

series of point sources radiating uniformly in all directions. The UV intensity at a point within 

the reactor is the sum contribution from each of these points as per Equation A.7. 

I r( )z

P

e p

i

e i , = −

∑ 4 2 π ρ

α ρ Equation A.7

where 

Pp = UV power emitted by each point source (mW)

i = Number of point sources used to simulate the lamp 

ρi = Distance from the ith point source (cm)

αe = Naperian (base e) absorption coefficient of the media (cm-1) 

r = Radial distance from the lamp (cm)

z = Axial distance along the lamp (cm)

I(r,z) = UV intensity (mW/cm

2

) at a coordinate position (r,z) 

The radial and axial distance from the lamp are shown in Figure A.4. 

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UV Disinfection Guidance Manual A-10 June 2003

Figure A.4 Radial and Axial Distance from a UV Lamp 

UV Lamp

z

(axial distance)

r

(radial distance)

ρ

(r, z)

UV Lamp

z

(axial distance)

r

(radial distance)

ρ

(r, z)

Note: The point where z = 0 is arbitrary. It can be at the lamp

ends or anywhere along the lamp length. 

For a 25 cm long UV lamp housed in a lamp sleeve (radius = 4 cm) and immersed in 

water, Figure A.5 presents the intensity profile predicted using Point Source Summation as a 

function of radial and axial distance and the water UV absorbance. For a given radial distance, 

the model predicts a greater UV intensity at an axial position corresponding to the center of the 

lamp than at an axial distance corresponding to the lamp ends. The model also demonstrates that 

UV intensity will decrease with increased distance from the lamp even in water that does not 

absorb UV light (i.e., A254 = 0) due to the divergence of UV light from the source. Last, the 

model predictions show that the water UV absorbance has a profound impact on the UV intensity 

profile about a UV source. 

Figure A.5 UV Intensity Profile of a 25 cm Medium-Pressure Mercury Arc Lamp 

as a Function of (a) Radial and (b) Axial Position for Waters with 

Different UV Absorbance 

Y-axis =

UV Intensity

10 cm Radial Distance

From Lamp

mW/cm2

Lamp is 25 cm long,

centered at x = 0

0

50

100

150

-30 -20 -10 0 10 20 30

Axial Distance along Lamp (cm) b.

0

100

200

300

400

500

0 5 10 15 20

Radial Distance from Lamp Center (cm)

UV Intensity (m

W/cm2)

A254 = 0 cm-1, UVT = 100 %

A254 = 0.046 cm-1, UVT = 90 %

A254 = 0.097 cm-1, UVT = 80 %

a.

Y-axis =

UV Intensity

10 cm Radial Distance

From Lamp

mW/cm2

Lamp is 25 cm long,

centered at x = 0

0

50

100

150

-30 -20 -10 0 10 20 30

Axial Distance along Lamp (cm) b.

Y-axis =

UV Intensity

10 cm Radial Distance

From Lamp

mW/cm2

Lamp is 25 cm long,

centered at x = 0

0

50

100

150

-30 -20 -10 0 10 20 30

Axial Distance along Lamp (cm) b.

0

100

200

300

400

500

0 5 10 15 20

Radial Distance from Lamp Center (cm)

UV Intensity (m

W/cm2)

A254 = 0 cm-1, UVT = 100 %

A254 = 0.046 cm-1, UVT = 90 %

A254 = 0.097 cm-1, UVT = 80 %

a.

0

100

200

300

400

500

0 5 10 15 20

Radial Distance from Lamp Center (cm)

UV Intensity (m

W/cm2)

A254 = 0 cm-1, UVT = 100 %

A254 = 0.046 cm-1, UVT = 90 %

A254 = 0.097 cm-1, UVT = 80 %

a.

More advanced models of the intensity profile about a lamp account for the impacts of 

refraction and reflection from reactor components as the light propagates from the discharge 

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UV Disinfection Guidance Manual A-11 June 2003

(Bolton 2000), the three-dimensional nature of the gas discharge (Irazoqui et al. 1973), and the 

direction of light emission (Phillips 1983). 

A.2 Microbial Response to UV Light 

Disinfection by UV light differs from chemical disinfectants such as chlorine and ozone. 

Chemical disinfectants inactivate microorganisms by destroying or damaging cellular structures, 

thereby interfering with metabolism, biosynthesis, and growth (Snowball and Hornsey 1988). In 

UV disinfection, microorganisms are inactivated by inducing damage to their nucleic acid such 

that they can no longer reproduce. This section discusses nucleic acid structure, the damage that 

causes microbial inactivation, the ability of microorganisms to repair the damage, methods for 

determining microbial inactivation, and factors that affect inactivation. 

A.2.1 DNA/RNA Structure

Nucleic acid is a fundamental building block of life and is responsible for reproduction 

and defining the nature of life. The nucleic acid is either deoxyribonucleic acid (DNA) or 

ribonucleic acid (RNA). The nucleic acid within the nucleus of most cells, including bacteria 

and protozoa, is composed of double stranded DNA. DNA contains the information necessary 

for the synthesis of ribosomal, transfer, and messenger RNA, which are responsible for 

synthesizing enzymes that drive metabolic processes within the cell. The genetic material of

viruses may either be DNA or RNA and can be single or double stranded. 

DNA and RNA are long polymers comprised of combinations of four nucleotides. In 

DNA, the nucleotides are purines (adenine and guanine) and pyrimidines (thymine and cytosine). 

In RNA, the nucleotides are the same except that uracil replaces thymine. Each nucleotide can 

be broken down into two parts – a sugar-phosphate backbone and a nitrogenous base (Figure 

A.6). If the nucleic acid is double-stranded, nucleotides on one strand will compliment those on 

the other strand. Adenine pairs with thymine in DNA (or uracil in RNA) while guanine pairs 

with cytosine. Hydrogen bonds form between each pair (Figure A.6). 

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UV Disinfection Guidance Manual A-12 June 2003

Figure A.6 Structure of DNA and Nucleotide Sequences Within DNA 

DNA STRUCTURE

SugarPhosphate

Backbone

DNA SEQUENCE

A = Adenine

C = Cytosine

T = Thymine

G = Guanine

— A — T — G — C — G — A — T — C —

— T — A— C — G — C —T — A — G —

Hydrogen Bonded

Nitrogenous

Base Pairs (A, T, G, C)

Purines Pyrimidines

| | | | | | | |

DNA STRUCTURE

SugarPhosphate

Backbone

DNA SEQUENCE

A = Adenine

C = Cytosine

T = Thymine

G = Guanine

— A — T — G — C — G — A — T — C —

— T — A— C — G — C —T — A — G —

Hydrogen Bonded

Nitrogenous

Base Pairs (A, T, G, C)

Purines Pyrimidines

| | | | | | | |

A.2.2 Mechanism of Inactivation by UV Light 

UV light inactivates microorganisms by damaging DNA or RNA, thereby interfering 

with replication of the microorganism. Only light that is absorbed by a system can induce a 

chemical reaction (First Law of Photochemistry). As shown in Figure A.7, nucleotides absorb 

UV light in from 200 to 300 nm, which enables the photobiological effects that lead to nucleic 

acid damage. The UV absorption of nucleic acid is a combination of the absorbance of the 

nucleotides and has an absorption peak near 260 nm and a local minimum near 230 nm. 

Figure A.7 UV Absorbance of Nucleotides and Nucleic Acid at pH 7 

(adapted from Jagger 1967) 

200 260 300

Wavelength (nm)

U

V absorbance

(Relativ

e Scal

e)

Adenine

Guanine

Thymine

Cytosine

220 240 280

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

DNA

200 260 300

Wavelength (nm)

220 240 280

U

V a

bsorban

c

e

(Rel

ative S

cale)

200 260 300

Wavelength (nm)

U

V absorbance

(Relativ

e Scal

e)

Adenine

Guanine

Thymine

Cytosine

220 240 280

0.0

0.2

0.4

0.6

0.8

1.0

200 260 300

Wavelength (nm)

U

V absorbance

(Relativ

e Scal

e)

Adenine

Guanine

Thymine

Cytosine

220 240 280

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

DNA

200 260 300

Wavelength (nm)

220 240 280

U

V a

bsorban

c

e

(Rel

ative S

cale)

0.0

0.2

0.4

0.6

0.8

1.0

DNA

200 260 300

Wavelength (nm)

220 240 280

U

V a

bsorban

c

e

(Rel

ative S

cale)

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UV Disinfection Guidance Manual A-13 June 2003

While both purines and pyrimidines strongly absorb UV light, the rate of UV-induced 

damage is greater with pyrimidines (Jagger 1967). Absorbed UV light induces six types of 

damage within the pyrimidines of nucleic acid (Setlow 1967; Snowball and Hornsey 1988; 

Pfeifer 1997), with varying levels of effectiveness dependent on UV dose: 

• Single and double strand breaks are only significant with UV doses several orders 

of magnitude higher than those practical for UV disinfection. 

• DNA-DNA cross-links are covalent bonds between two different strands of DNA, 

and they are also only significant with UV doses orders of magnitude higher than 

those practical for UV disinfection. 

• Protein-DNA cross-links are covalent bonds between a protein and a DNA strand, 

and they may be important for the disinfection of certain microorganisms such as 

Micrococcus radiodurans. 

• Pyrimidine hydrates do not contribute to UV disinfection. 

• Pyrimidine (6–4) pyrimidine photoproducts are a major class of UV damage. 

• Pyrimidine dimers are covalent bonds between two pyrimidines on the same DNA 

strand, and they are the most common damage resulting from UV disinfection. 

While it is possible for thymine-thymine, cyctosine-cytosine, and thymine-cytosine 

dimers to form within DNA, thymine-thymine dimers are the most common. However, since 

thymine is not present in RNA, uracil-uracil and cytosine-cytosine dimers are formed. 

Microorganisms with DNA rich in the thymine tend to be more sensitive to UV disinfection 

(Adler 1966). 

Dimers cause faults in the transcription of information from DNA to RNA, which in turn 

results in disruption of cell metabolism. However, damage to nucleic acid does not prevent the 

cell from undergoing metabolism and other cell functions. As discussed in the next section, 

enzyme mechanisms within the cell are capable of repairing some of the damage to the nucleic 

acid. To directly damage the internal structure of the cell, UV doses much higher than those 

required for inactivation are necessary (Brandt and Giese 1956). 

A.2.3 Repair Mechanisms 

Because microorganisms that have been exposed to UV light still retain metabolic 

functions, some are able to repair the damage done by UV light and regain infectivity. Repair of

UV light-induced DNA damage includes photoreactivation and dark repair (Knudson 1985). At 

the doses typically used in UV disinfection, microbial repair can be controlled and accounted for 

as discussed in section 3.1.1. 

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UV Disinfection Guidance Manual A-14 June 2003

A.2.3.1 Photoreactivation 

Photreactivation or photorepair is the cleaving of pyrimidine dimers by the enzyme DNA 

photolyase (Setlow 1967). The repair mechanism is termed photorepair because exposure of the 

enzyme to light between 310 and 490 nm is needed to activate the enzyme and provide it with 

the energy necessary to split the paired dimers. 

Figure A.8 shows the difference in UV dose necessary to achieve a certain log 

inactivation with and without considering photoreactivation for two organisms. Photorepair 

varies with different microorganism types, different species, and different strains of a given 

species. The extent of photorepair depends on many factors, including type of microorganism, 

degree of inactivation, time between exposure to UV light and photoreactivating light, and the 

nutrient state of the microorganism. 

Figure A.8 Repair of L. Pneumophila and E. Coli (adapted from Knudson 1985) 

0

1

2

3

4

5

6

7

0 10 20 30 40

UV Dose (mJ/cm2)

Log Ina

ctiv

ation

L. pneumophila - No Photorepair

L. pneumophila - Photorepair

E. coli - Photorepair

E. coli - No Photorepair

0

1

2

3

4

5

6

7

0 10 20 30 40

UV Dose (mJ/cm2)

Log Ina

ctiv

ation

L. pneumophila - No Photorepair

L. pneumophila - Photorepair

E. coli - Photorepair

E. coli - No Photorepair

Photoreactivation increased the UV dose necessary to achieve 3-log inactivation of seven 

Legionella species between 1.1 and 6.3 fold (Knudson 1985). Photoreactivation also increased 

the dose necessary for 4-log inactivation of twelve species of bacteria by 1.2 to 3.5 fold (Hoyer 

1998). However, Shin et al. (2001) did not observe photorepair with Cryptosporidium parvum. 

Although viral DNA does not have the necessary enzymes for repair, the photorepair of 

viral DNA can occur using the enzymes of their host cells. Lytle (1971) reported that the 

photorepair of Herpes simplex virus by mammalian cells varies significantly, depending on the 

host cell. RNA viruses lack the ability to photorepair in a host cell (Rauth 1965). 

Kelner (1950) reported that the ratio of UV dose necessary to achieve a certain log 

inactivation with and without considering photorepair is independent of the degree of 

inactivation. However, more recent research by Lindenauer and Darby (1994) reported that the 

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-15 June 2003

effect of photorepair of coliform bacteria in wastewater becomes less pronounced as UV dose 

increases. Knudsen (1985) also found a slight reduction in the ability of Legionella in 

wastewater to repair after higher inactivation levels. 

The time between UV light exposure and exposure to photoreactivating light has a 

signific of

a

he rate of photorepair is constant with time until it reaches saturation, where saturation 

is defin

E. 

was

sm. 

he nutrient state of the microorganism also impacts the ability to photorepair. Giese et 

al. (195

.2.3.2 Dark Repair 

ark repair is when repair processes do not need reactivating light. The term is 

somew e does

ir,

. Repair endonnuclease enzyme recognizes the DNA damage and cleaves the DNA 

2. xonuclease enzyme excises the damaged section. 

3. NA polymerase rebuilds the removed section using the complementary strand as a 

4. olynucleotide ligase rejoins the severed strand.

One study (Knudsen 1985) examined two different strains of E. coli: one that has the 

enzyme cAthe

ant effect on the ability to photoreactivate. Dulbecco (1950) reported that the ability

T2 phage to repair using E. coli as a host organism decreases as the time between exposure to 

UV light and photoreactivating light increases. Kelner (1950) reported that E. coli at 37 °C in

nutrient broth lost the ability to photorepair after 140 minutes in the dark after exposure to UV 

light (the same time the survivors took to attempt cell division). In the same study, cells kept at

colder temperatures maintained their ability to photorepair for several hours longer. 

T

ed as the maximum amount of repair possible by the microorganism given its repair 

ability and the extent of damage. Kashimada et al. (1996) reported photorepair saturation of

coli occurs after 2 hours of exposure under fluorescent lighting. With exposure to sunlight, 

however, they reported photorepair saturation after 15 minutes followed by inactivation that

attributed to the UV component of sunlight. The rate of repair increases with temperature 

(Kelner 1950) but is nearly independent of the reactivating light intensity (Setlow 1967), 

suggesting photorepair is rate limited by the enzyme concentration within the microorgani

T

4) reported that a starved strain of paramecium, Colpidium colpoda, needed more 

reactivating light to reach saturation than organisms with sufficient nutrients. 

A

D

hat misleading because dark repair can occur in the presence of light and therefor

not need dark conditions. The forms of dark repair include excision repair, recombinational 

repair, and inducible error prone repair. Excision repair, the most common form of dark repa

is an enzyme-mediated process involving four steps: 

1

strand. 

E

D

template. 

P

s necessary for repair (B/R strain) and one that lacks the necessary repair enzymes (re

uvr-

 strain). The difference in UV dose needed for 1-log inactivation of the strain capable of 

repair was over two orders of magnitude higher than the dose needed for 1-log inactivation of

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-16 June 2003

repair deficient strain, indicating that dark repair impacts the UV dose-response of 

microorganisms. 

Based on the difference in UV sensitivity of repair proficient and deficient bacteria, 

Jagger er,

zymes 

A.2.4 V Dose and Dose Distribution

V dose is a measurement of the amount of the energy per unit area that is incident on a 

surface

.2.4.1 Calculation of UV Dose in Bench-Scale, Batch Systems 

he most carefully controlled method of determining UV dose is in a batch system with a 

bench-s

is

he general definition of UV dose is the product of UV intensity multiplied by the 

exposu

 Equation A.8 

here

= UV intensity (mW/cm2

) 

If intensity is not constant with respect to time, the integral of the intensity output over 

the exp

UV Dose I dt

0

 Equation A.9 

where 

s are defined as in Equation A.8.

(1967) discovered that roughly 99 percent of repair that occurs is dark repair. Howev

the rate at which dark repair occurs is unknown. It is possible that microorganisms have dark 

repaired prior to the microbial assay, and dark repair is not detected. Therefore, the effects of 

dark repair can be difficult to measure. Unlike bacteria, viruses do not have the enzymes 

necessary for dark repair. However, virus can repair in the host cell using the host cells' en

(Rauth 1965). 

U

U

. UV dose is the product of the average intensity acting on a microorganism from all 

directions and the exposure time. Units commonly used for UV dose are J/m2

, mJ/cm2, and 

mWs/cm2

 (10 J/m2 = 1 mJ/cm

2

 = 1 mWs/cm

2

) with mJ/cm2 being the most common units in

North America and J/m2

 being the most common in Europe. This section discusses how UV 

dose is calculated in bench-scale, batch systems and also how the UV dose distribution is 

determined in continuous flow pilot- or full-scale UV reactors. 

A

T

cale collimated beam apparatus. Appendix E presents procedures for collimated beam

testing. The factors impacting UV dose calculation in collimated beam tests are described in th

section. 

T

re time. 

UV Dose = I ⋅t

w

I 

t = Exposure time (s) 

osure time should be used in place of intensity as in Equation A.9. t

∫ = ⋅

variable

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-17 June 2003

Due to several conditions present in collimated beam testing, the intensity measured by 

the radiometer does not accurately represent the intensity of light that reaches the target

organisms. To get an accurate calculation of the UV dose delivered to the microorganisms, the 

following factors are considered as shown in Equation A.10 (Bolton and Linden 2003): 

absorbance/transmittance of the water, thickness of the water layer, distribution of light across 

the surface of the suspension, and reflection and refraction of light from the water. 

t

a d

I P R

UV Dose I t

a d

f

avg ⋅ −

− − = ⋅ =

−

ln10

(1 )(1 10 )

10

0

254

10

Equation A.10

where 

Iavg = Average intensity within the suspension (mW/cm

2

) 

t = Exposure time (s) 

I0 = Intensity measured at the suspension's surface (mW/cm2

) 

R = Fraction of light reflected at the suspension’s surface (from Fresnel’s Law) 

a10 = Decadic (base 10) absorption coefficient of the suspension, A254 (cm-1) 

d = Thickness of water layer (cm)

Pf = Petri factor, ratio of measured intensity at the center of the exposure dish to 

average intensity across the surface area of the exposure dish (unitless)

Because microorganisms respond differently to different wavelengths of light, if a 

polychromatic light source (e.g., MP lamp) is used, it is also critical to incorporate the light 

intensity and the inactivation effectiveness of each wavelength in the germicidal range when 

determining UV dose. For microorganisms that exhibit inactivation kinetics that are independent 

of wavelength, the equivalent dose at 254 nm from a polychromatic source is calculated as 

follows (Meulemans 1986): 

D = ∑I G ⋅t =

300

200

254 ( ) ( )

λ

λ λ Equation A.11

where 

D254 = UV dose equivalent at 254 nm

λ = Wavelength of light (nm)

I(λ) = Intensity at wavelength λ over 1 nm increments 

G(λ) = Relative action spectrum of the microorganism defined as kλ/k254 

kλ = First order inactivation constant at wavelength λ

k254 = First order inactivation constant at 254 nm wavelength 

t = Exposure time (s) 

However, if the microorganism does not exhibit the same inactivation kinetics at each

wavelength, the dose-response curve may be characterized by a shoulder (section A.2.5.2), and 

the dose equivalent at 254 nm is calculated using Equation A.12 (Cabaj et al. 2001): 

∑=

− − = − − = − −

300

0 200

1 (1 ) 1 (1 ) 254 254 254

λ

λDλ dλ k D d k

e e

N

N Equation A.12

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-18 June 2003

where 

Dλ = Dose delivered at wavelength λ

kλ = First order inactivation constant at wavelength λ

dλ = Intercept of the exponential region with the y-axis at wavelength λ

A.2.4.2 Dose Distribution in Continuous Flow UV Reactors 

Determining the UV dose in a continuous flow pilot- or full-scale UV reactor is 

complicated by hydrodynamics (particle paths) and variations in UV intensity throughout the 

reactor. 

In an ideal reactor that has plug-flow conditions and complete mixing perpendicular to 

the flow, all microorganisms entering the reactor will receive the same UV dose, which is 

calculated according to Equation A.13. 

Q

V UV Dose I t I = avg r = avg Equation A.13

where 

Iavg = Volume-averaged UV intensity within the reactor (mW/cm2

) 

tr = Theoretical residence time of the reactor (s) 

V = Volume of water within the reactor (gal) 

Q = Flowrate through the reactor (gal/s) 

Equation A.13 calculates the maximum UV dose possible in an ideal reactor. However in 

practice, microorganisms take different paths through a reactor and thus do not all receive the 

maximum dose. Instead, the UV dose delivered to the organisms is best described using a dose 

distribution (Figure A.9). A dose distribution is a curve or histogram that indicates the 

probability of a microorganism receiving a certain dose as it travels through the UV reactor. 

Figure A.9. Hypothetical Dose Distribution Delivered by a UV Reactor 

0

0.05

0.10

0.15

0.25

UV Dose (mJ/cm2)

O

c

c

urrence Pro

b

a

bility

0.20

0 10 20 30 40 50 60 70 80 90 100

0

0.05

0.10

0.15

0.25

UV Dose (mJ/cm2)

O

c

c

urrence Pro

b

a

bility

0.20

0 10 20 30 40 50 60 70 80 90 100

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-19 June 2003

The width of the dose distribution is an indication of the hydraulic conditions in the 

reactor luge and

he dose distribution of a UV reactor cannot be measured in a practical manner with 

current a

activation achieved by a reactor with a modeled dose distribution can be calculated by 

summi

. The more narrow the distribution, the better the hydraulic conditions approximate p

flow with complete mixing. However, a narrow dose distribution does not always imply 

efficient dose delivery. An annular reactor with a thin water layer between the lamp sleev

the reactor wall will deliver a narrow dose distribution. However, if the reactor wall absorbs UV

light, energy losses at the wall will be excessive and the reactor will not efficiently utilize the UV 

output of the lamp. The most cost effective design of a UV reactor will have a dose distribution 

that reflects a compromise between inefficiency due to energy losses at the reactor wall and by 

adjacent lamps as well as inefficiency due to hydrodynamics. 

T

technology. However, by predicting particle trajectories through the intensity field of

UV reactor using computational fluid dynamics (CFD), dose distributions can be calculated 

(Wright and Lawryshyn 2000). 

In

ng the inactivation achieved by each dose in the dose distribution according to 

Equation A.14. 

( ) ( ) N

N p DfD i

i

i

0

= ∑ Equation A.14 

here

= Probability of dose Di occurring 

microorganism inactivation as a function of 

sing the inactivation kinetics of the microorganism, the inactivation is related to a single 

dose va

w

pi(Di) 

f(Di) = Mathematical function describing

dose 

U

lue termed the reduction equivalent dose (RED) by Equation A.15. 

( ) ( ) ( )

0

p D f D f RED

N

N

i

= ∑ i i = Equation A.15 

here

= Concentration of organisms after exposure to UV light 

t

i) = 

ibing inactivation as a function of dose 

valent dose

w

N 

N0 = Concentration of organisms before exposure to UV ligh

p(D Probability of Di occurring 

f(Di) = Mathematical function descr

Di = UV Dose 

RED = Reduction equi

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-20 June 2003

If microorganism inactivation can be described using first order kinetics (section 

A.2.5.1), inactivation is related to RED by Equation A.16. 

( ) N

N p D e e i

i

kDi k RED

0

= ∑ = − − Equation A.16

where 

variables are defined as in Equation A.15 and 

k = First order inactivation coefficient 

By re-arranging Equation A.16, the reduction equivalent dose is calculated according to 

Equation A.17. 

RED ( ) k p D e i

i

kDi = ⎡

⎣

⎢ ⎤

⎦ ∑ ⎥ − 1

ln Equation A.17

where 

variables are defined as in Equation A.15 and A.16 

Because UV reactors do not exhibit ideal dose delivery, the RED of a reactor delivering a 

dose distribution depends on the UV sensitivity of the microorganisms used to calculate RED. 

The RED determined when using a challenge microorganism that is more resistant to UV 

disinfection will be higher compared to when using a less resistant microorganism. In contrast, 

the RED of an ideal reactor has the same value for all microorganisms. Also, the RED of a 

reactor delivering a dose distribution will vary in a non-linear fashion with the lamp power and a 

flowrate while the ideal reactor model predicts a proportional relationship. Lastly, the 

dependence of RED on UV absorbance of the water will be more pronounced with a reactor 

delivering a dose distribution than an ideal reactor. The RED will decrease with increased UV 

absorbance at a greater rate with the reactor with a dose distribution than is predicted by ideal 

models. 

The inactivation of a microorganism and the associated RED are measured using 

biodosimetry (described in section 4.2). 

A.2.5 Dose-Response Relationships

UV dose-response relationships can be expressed as either the proportion of 

microorganisms inactivated (log inactivation, results in dose-response curves with positive 

slope) or the proportion of microorganisms remaining (log survival; results in dose-response 

curves with negative slope) as a function of UV dose. The proportion of microorganisms 

remaining and the log inactivation are typically shown on a logarithmic (base 10) scale, while the 

UV dose is typically shown on a linear scale. This manual will present microbial response as log 

inactivation since the terminology is widely accepted in the industry. Therefore, all doseresponse curves presented will have a positive slope. The log inactivation of the microorganisms 

is determined by measuring the concentration of replicating microorganisms after exposure to a 

measured UV dose and is calculated according to Equation A.18. 

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-21 June 2003

N

N Log Inactivation 0

10 = log Equation A.18

where 

N0 = Concentration of viable microorganisms before exposure to UV light 

N = Concentration of viable microorganisms after exposure to UV light 

Many UV dose-response curves for disperse microorganisms follow first order 

inactivation, but in some cases, the dose-response curves take other shapes such as shoulders or 

tailing. A shoulder is characterized by a period of very little inactivation at lower doses followed 

by linear or exponential inactivation. A dose-response curve that exhibits tailing is characterized 

by a decrease in the inactivation rate after a certain degree of inactivation has been observed. 

Figure A.10 shows various shapes of dose-response curves. Note that the terms "shoulder" and 

"tailing" refer to the shape the curves take when the y-axis of the dose-response curve is 

presented as log survival with negative slopes, which is the inverse of log inactivation. 

Figure A.10. UV Dose-Response Curves (adapted from Chang et al. 1985) 

0

1

2

3

4

5

6

0 20 40 60 80 100

UV Dose (mJ/cm2)

Log I

n

a

ctiv

ation

E. coli

B. subtilis spores

Total coliform-wastewater

Rotavirus

0

1

2

3

4

5

6

0 20 40 60 80 100

UV Dose (mJ/cm2)

Log I

n

a

ctiv

ation

E. coli

B. subtilis spores

Total coliform-wastewater

Rotavirus

E. coli

B. subtilis spores

Total coliform-wastewater

Rotavirus

A.2.5.1 First Order Response 

The E. coli data shown in Figure A.10 exhibit first order dose-response behavior. The 

equation for first order inactivation is shown in Equation A.19: 

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-22 June 2003

10 10

0

D

D

kD

e

N

N − − = = Equation A.19

where 

N0 = Concentration of viable microorganisms before UV exposure 

N = Concentration of viable microorganisms after UV exposure 

k = First order inactivation coefficient of the microorganisms (cm2

/mJ) 

D = UV dose delivered to the microorganisms (mJ/cm2

) 

D10 = UV dose needed to inactivate microorganisms by one log (i.e., 90 percent 

inactivation) (mJ/cm2

) 

In first-order response, only one photon of light is needed to inactivate a microorganism. 

A.2.5.2 Shoulders 

The B. subtilis data shown in Figure A.10 exhibit a shoulder followed by first order doseresponse behavior. The shoulder is attributed to a delayed response of a microorganism when 

exposed to UV light. Unlike first order inactivation, more than one photon of light is needed to 

inactivate a microorganism. Although the number of photons can not be measured directly, it 

can be related to first order response through curve fits of empirical equations. Equation A.20 

(Cabaj et al. 2001) is one of the many equations derived from empirical curve fits that can be 

used to model inactivation curves with shoulders. 

( N

N e kD d

0

= − 1 1− − ) Equation A.20

where 

N0 = Concentration of viable microorganisms before UV exposure 

N = Concentration of viable microorganisms after UV exposure 

k = First order inactivation coefficient of the microorganisms (cm2

/mJ) 

D = UV dose delivered to the microorganisms (mJ/cm2

) 

d = Intercept of the exponential region of the dose-response with the y-axis 

Morton and Haynes (1969) reported a decrease in the shoulder with nutrient-depleted E. 

coli and proposed that the shoulder was associated with dark repair. Photoreactivation 

significantly increased the shoulder observed with E. coli (Hoyer 1998) and Legionella

(Knudson 1985). 

Note that the equation presented is only one of the many equations derived from

empirical curve fits. There are many methods to model UV dose-response data not presented 

here that may better describe specific UV dose-response data (Severin et al. 1984). 

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-23 June 2003

A.2.5.3 Tailing 

If the irradiated microorganisms are a mixture of disperse microorganisms and clumped 

or particle-associated microorganisms, the UV dose-response will demonstrate tailing, or a 

flattening of the curve at higher UV doses (Parker and Darby 1995). With wastewaters, tailing 

begins after 2 to 3 log of disperse microorganism inactivation, with diminishing inactivation 

occurring beyond that level despite increasing UV dose (Figure A.10, Total coliforms). Doseresponse with tailing can be modeled using Equation A.21. 

k D

p

kD p N N e N e − − = 0 + Equation A.21

where 

N = Concentration of viable microorganisms after UV exposure 

N0 = Concentration of disperse microorganisms before UV exposure 

k = First order inactivation coefficient of the microorganisms (cm2

/mJ) 

D = UV dose delivered to the microorganisms (mJ/cm2

)

Np = Concentration of particles containing the microorganisms 

kp = Pseudo first order inactivation constant of particle-associated microorganisms 

(cm2

/mJ)

A.2.6 Factors Impacting Microbial Response 

Several factors impact the response of microorganisms to UV light. This section 

discusses these factors, including UV intensity, UV absorbance, temperature, pH, particles, and 

UV wavelength. 

A.2.6.1 UV Intensity 

Oliver and Cosgrove (1975) reported that UV dose-response of microorganisms follows 

the Law of Reciprocity over an intensity range of 1 to 200 mW/cm2

. For example, the 

inactivation effectiveness observed with UV intensity of 1 mW/cm2

 and an exposure time of 200 

seconds is equivalent to the inactivation observed with an exposure time of 1 second and UV 

intensity of 200 mW/cm2

 as well as all intensity-time combinations between 1 and 200. 

Exceptions to this reciprocal relationship between time and intensity occur at very low 

and high intensities (Setlow 1967). With low UV intensities and long exposure times, repair 

may compete with inactivation. Sommer et al. (1998) found less inactivation of E. coli at a given 

dose with low intensities ranging from 0.002 to 0.2 mW/cm2

. However, inactivation of B. 

subtilis spores, MS2 bacteriophage (MS2), φx174 phage, and B40-8 phage at a given dose was 

the same regardless of UV intensity. At UV intensities on the order of 1010 to 1011 mW/cm2

(several orders of magnitude higher than the intensity from lamps used for UV disinfection), 

Gurzadyan et al. (1981) reported an increase in single strand breaks and a reduction in 

dimerization in the nucleic acid of φx174 phage. 

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-24 June 2003

A.2.6.2 UV Absorbance 

Because the calculation of dose delivered to a microbial suspension (Equation A.10, 

section A.2.4.1) accounts for UV absorbance in bench-scale batch experiments, measured UV 

dose-response curves like those presented in Figure A.10 are independent of the suspension's UV 

absorbance. However, as the UV absorbance of the suspension increases, the UV intensity 

incident on the sample may need to increase in order to deliver a given dose. In bench-scale, 

batch experiments, there are several ways to keep the UV dose constant such as increasing 

exposure time or decreasing the depth of the solution, thereby decreasing the pathlength. 

A.2.6.3 Temperature 

Temperature affects the configuration of nucleic acid and the activity of repair enzymes; 

however, existing research shows temperature effects on UV dose-response are minimal and 

depend on the microorganism. Severin et al. (1983) found the UV dose needed for a given log 

reduction of E. coli, Candida parapsilosis, and f2 phage increases slightly as temperature 

decreased (Table A.1). Malley (2000) reported the dose-response of MS2 is independent of 

temperature from 1 to 23°C (Figure A.11). 

Table A.1 Impact of Temperature on UV Disinfection 

UV dose (mJ/cm2) needed to achieve 2 log inactivation at a temperature of 

Microorganism 

5 o

C 20 oC 35 oC 

E. coli 11.8 11.2 10.7

C. parapsilosis 30.9 28.8 28.0

f2 phage 72.5 65.6 60.7 

Figure A.11 Impact of Temperature on MS2 UV Dose-Response (Malley 2000) 

Delivered UV Dose (mJ/cm2)

10 20 30 40 50 60 70 80 90 100

Log Inactivation of MS2 0

1

2

3

4

5

pH 7 and 10 oC

pH 7 and 23 oC

pH 7 and 1 oC

Delivered UV Dose (mJ/cm2)

10 20 30 40 50 60 70 80 90 100

Log Inactivation of MS2 0

1

2

3

4

5

pH 7 and 10 oC

pH 7 and 23 oC

pH 7 and 1 oC

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-25 June 2003

A.2.6.4 pH 

UV dose-response is usually independent of the pH of the water. The UV absorbance of 

nucleic acid and repair enzyme activity vary with pH (Jagger 1967). However, the pH within a 

cell is buffered to a relatively constant value, independent of water pH. For example, Malley 

(2000) reported the dose-response of MS2 is independent of the suspension pH from pH 6 to 9 

(Figure A.12). 

Figure A.12 Impact of pH on MS2 UV Dose-Response (Malley 2000) 

Delivered UV Dose (mJ/cm2)

10 20 30 40 50 60 70 80 90 100

Log Inactivation of MS2

0

1

2

3

4

5

pH 9 and 1oC 

pH 7 and 1oC 

pH 6 and 1oC 

Delivered UV Dose (mJ/cm2)

10 20 30 40 50 60 70 80 90 100

Log Inactivation of MS2

0

1

2

3

4

5

pH 9 and 1oC 

pH 7 and 1oC 

pH 6 and 1oC 

A.2.6.5 Particle Association 

To date, research examining the effects of naturally occurring particles and 

microorganisms is limited to wastewater studies. Due to the limited concentration of

microorganisms in drinking water sources, methods of directly examining the impact of particles 

do not currently exist. However, the phenomena observed in wastewater studies may also apply 

to particle association occurring in drinking water. The effects of individual particles (such as 

those that cause turbidity) on UV disinfection are discussed in section A.4.1.2. 

Results from research with wastewaters have indicated that clumping or particle 

association will shield microorganisms from UV light. The UVT at 260 nm through 10 microns 

of cell tissue is roughly 10 percent (Jagger 1967), suggesting that clumps of organisms would 

offer protection. The water content of cells and intracellular material and the porous nature of

flocculated particles will influence the penetration of light into waterborne particles. Qualls et al.

(1983) reported that filtration of secondary effluent through an 8 micron filter removes the 

particles responsible for the tailing in the dose-response of coliforms. With 8 micron filtration, 

coliform inactivation at 12 mJ/cm2

 increased from 3 log to over 4.5 log inactivation. Loge et al. 

(1999) reported the UV absorbance of wastewater solids varied from 0.33 to 56.9 μm-1 (3,300 to 

569,000 cm-1) with the high absorbance associated with activated sludge plant using iron to 

remove phosphorus. Petri et al. (2000) reported coagulation of MS2 by iron in ground water 

increased the UV dose needed to inactivate MS2 by a factor of 2.5 to 3.5. 

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-26 June 2003

A.2.6.6 Wavelength 

Microbial dose-response varies with the wavelength of UV light. The action spectrum 

(also called UV action) of a microorganism is a measure of inactivation as a function of the 

wavelength for a given UV dose. The dependence of UV action on wavelength is similar to the 

dependence of the UV absorbance of DNA on wavelength (Figure A.13). UV action peaks near 

260 nm, has a local minimum near 230 nm, and drops to zero near 300 nm. While it is generally 

believed that microorganisms are most sensitive near 260 nm, there are exceptions. For 

example, the UV sensitivities of tobacco mosaic virus (Hollaender and Duggar 1936), reovirus 

(Rauth 1965), and Herpes simplex virus (Powell 1959) are greater below 230 nm. Although the 

UV action increases below 230 nm for most microorganisms, the UV absorbance of natural 

waters at these wavelengths make this region impractical for UV disinfection of microorganisms 

in water. Because of the similarity between UV action and DNA absorbance, and because DNA 

absorbance is easier to measure than UV action, the DNA absorbance spectrum of a 

microorganism is often used as a surrogate for its UV action spectrum. 

Figure A.13 Comparison of Microbial Action to DNA UV Absorbance 

(adapted from Rauth 1965 and Linden et al. 2001) 

0.0

0.5

1.0

1.5

2.0

200 300

Wavelength (nm)

U

V A

ctio

n or D

N

A

A

bsorbance

R

elative t

o 254 n

m

DNA

Cryptosporidium

MS2

Herpes simplex virus

220 240 260 280

0.0

0.5

1.0

1.5

2.0

200 300

Wavelength (nm)

U

V A

ctio

n or D

N

A

A

bsorbance

R

elative t

o 254 n

m

DNA

Cryptosporidium

MS2

Herpes simplex virus

220 240 260 280

A plot of the first order inactivation coefficient as a function of wavelength can be used 

to show the action spectrum if the dose-response follows first order inactivation. Plots of two 

kinetic terms as a function of wavelength are necessary to plot the action spectrum if the doseresponse has a shoulder (Cabaj et al. 2001) as discussed in section A.2.5.2. Plots of UV action 

spectra are often presented relative to some wavelength, typically 254 nm. 

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-27 June 2003

A.2.7 UV Dose-Response of Differing Microorganisms 

The UV dose-responses of microorganisms have been tabulated in a number of review 

articles and are summarized in this section. 

Data presented in Tables A.2 and A.3 show that the UV sensitivity of microorganisms 

varies with different species. Of the pathogens of interest in drinking water, viruses are most 

resistant to UV disinfection followed by bacteria, and Cryptosporidium oocysts and Giardia

cysts. The most UV resistant viruses of concern in drinking water are adenovirus Type 40 and 

41. Appendix B provides dose-response data for Giardia cysts, Cryptosporidium oocysts, and 

viruses, and Chapter 1 (Table 1.4) contains the regulatory UV dose requirements for inactivating 

these pathogens. 

Table A.2 provides average dose reported without photoreactivation for incremental log 

inactivation of various pathogenic bacteria, virus, and protozoa of concern in drinking water. 

Table A.3 provides similar information for various non-pathogenic indicator bacteria, spore 

forming bacteria, and bacteriophage. All data in Tables A.2 and A.3 are for microorganisms 

suspended in water and irradiated using a collimated beam apparatus with UV light at 254 nm. 

Spore-forming and gram-positive bacteria are more resistant to UV light than gram

negative bacteria (Jagger 1967). With microorganisms larger than 1 micron, the absorption of 

UV light by the cytoplasm can be significant, depending on the wavelength, and therefore can 

affect UV sensitivity. 

Rauth (1965) found that the UV sensitivity of virus and bacteriophage varies over two 

orders of magnitude from the most sensitive to the most resistant. The same study showed 

viruses with single-stranded nucleic acid are ten times more sensitive than viruses with doublestranded nucleic acid. 

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-28 June 2003

Table A.2 UV Sensitivity of Pathogenic Microorganisms in Water1 

UV Dose (mJ/cm2) inactivation 

Microorgan indicated ism Type

1-log 2-log 3-log 4-log

Reference 

Aeromonas hydrophila Bacteria 1.1 2.6 3.9 5 Wilson et al. 1992 

Campylobacter jejuni Bacteria 1.6 3.4 4 4.6 Wilson et al. 1992 

Escherichia coli O157:H7 Bacteria 1.5 2.8 4.1 5.6 Wilson et al. 1992 

Legionella pneumophila Bacteria 3.1 5 6.9 9.4 Wilson et al. 1992 

Salmonella anatum Bacteria 7.5 12 15 Tosa and Hirata 1998 

Salmonella enteritidis Bacteria 5 7 9 10 Tosa and Hirata 1998 

Salmonella typhi Bacteria 1.8 4.8 6.4 8.2 Wilson et al. 1992 

Salmonella typhimurium Bacteria 2 3.5 5 9 Tosa and Hirata 1998 

Shigella dysenteriae Bacteria 0.5 1.2 2 3 Wilson et al. 1992 

Shigella sonnei Bacteria 3.2 4.9 6.5 8.2 Chang et al. 1985 

Staphylococcus aureus Bacteria 3.9 5.4 6.5 10.4 Chang et al. 1985 

Vibrio cholerae Bacteria 0.8 1.4 2.2 2.9 Wilson et al. 1992 

Yersinia enterocolitica Bacteria 1.7 2.8 3.7 4.6 Wilson et al. 1992 

Adenovirus Type 40 2 Virus 30 59 90 120 Meng and Gerba 1996 

Adenovirus Type 41 2 Virus 22 50 80 Meng and Gerba 1996 

Coxsackievirus B5 Virus 6.9 14 21 Battigelli et al. 1993 

Hepatitis A HM175 Virus 5.1 14 22 30 Wilson et al. 1992 

Hepatitis A Virus 5.5 9.8 15 21 Wiedenmann et al. 1993 

Hepatitis A HM175 Virus 4.1 8.2 12 16 Battigelli et al. 1993 

Poliovirus Type 1 Virus 4.0 8.7 14 21 Meng and Gerba 1996 

Poliovirus Type 1 Virus 6 14 23 30 Harris et al. 1987 

Poliovirus Type 1 Virus 5.6 11 16 22 Chang et al. 1985 

Poliovirus Type 1 Virus 5.7 11 18 13 Wilson et al. 1992 

Rotavirus SA11 Virus 7.6 15 23 Battigelli et al. 1993 

Rotavirus SA11 Virus 7.1 15 25 Chang et al. 1985 

Rotavirus SA11 Virus 9.1 19 26 36 Wilson et al. 1992 

Cryptosporidium parvum 2 Protozoa < 2 < 3 < 5 Shin et al. 2001 

Cryptosporidium parvum 2 Protozoa < 3 < 6 Clancy et al. 2000 

Giardia lamblia 2 Protozoa <1 <2 Linden et al. 2002a 

Giardia lamblia 2 Protozoa <1 < 3 < 6 Mofidi et al. 2002 1

 Adapted from Wright and Sakamoto 1999

2 Additional data for adenovirus, Cryptosporidium, and Giardia are in Appendix B.

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Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-29 June 2003

Table A.3 UV Sensitivity of Non-Pathogenic Bacteria, Bacteriophage, and 

Spore-Forming Bacteria in Water1 

UV Dose (mJ/cm2) 

inactivation indicated Reference 

Microorganism Type

1-log 2-log 3-log 4-log

Escherichia coli Bacteria 2.5 3 3.5 5 Harris et al. 1987 

Escherichia coli Bacteria 3 4.8 6.7 8.4 Chang et al. 1985 

Escherichia coli Bacteria 4.0 5.3 6.4 7.3 Sommer et al. 1998

Escherichia coli Bacteria 4.4 6.2 7.3 8.1 Wilson et al. 1992

Streptococcus faecalis Bacteria 6.6 8.8 9.9 11 Chang et al. 1985 

Streptococcus faecalis Bacteria 5.5 6.5 8 9 Harris et al. 1987 

MS-2 Phage 4 16 38 68 Wiedenmann et al. 1993 

MS-2 Phage 16 34 52 71 Wilson et al. 1992

MS-2 Phage 12 30 Tree et al. 1997 

MS-2 Phage 21 36 Sommer et al. 1998 

MS-2 Phage 17 34 Rauth 1965

MS-2 Phage 14 29 45 62 Meng and Gerba 1996

MS-2 Phage 19 40 61 Oppenheimer et al. 1993 

φX174 Phage 2.2 5.3 7.3 10 Sommer et al. 1998 

φX174 Phage 2.1 4.2 6.4 8.5 Battigelli et al. 1993

φX174 Phage 4 8 12 Oppenheimer et al. 1993 

PRD-1 Phage 9.9 17 24 30 Meng and Gerba 1996 

B-40 Phage 12 18 23 28 Sommer et al. 1998

Bacillus subtilis spores Spores 36 49 61 78 Chang et al. 1985 

Bacillus subtilis spores Spores 29 40 51 Sommer et al. 1998 1 Adapted from Wright and Sakamoto 1999. 

A.3 UV Reactors 

This section discusses UV reactor components, UV reactor configurations, and how 

reactor performance is monitored. The following UV reactor components are discussed: 

• Mercury lamps 

• Ballasts and power supplies 

• Lamp sleeves 

• Cleaning systems 

• UV intensity sensors 

• UV transmittance monitors 

• Temperature sensors 

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Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-30 June 2003

A.3.1 Mercury Lamps 

This section describes mercury lamps, including how they are constructed, their 

components, efficiency, spectral output, and aging. A majority of the material in this section was 

derived from Sources and Applications of Ultraviolet Radiation by Roger Phillips (1983). 

Section 2.4.2 (Table 2.1) compares the operating characteristics of LP, LPHO, and MP mercury 

lamps. 

A.3.1.1 Lamp Construction 

Mercury vapor discharge lamps consist of a UV-transmitting envelope made from a tube 

of quartz sealed at both ends (Figure A.14). An electrode is located at each end of the envelope 

connected to the outside through a seal. The envelope is filled with mercury and an inert gas. 

Figure A.14 Construction of a UV Lamp 

LOW-PRESSURE MERCURY LAMP - HOT CATHODE TYPE

Tungsten Coil Electrode

Electrical

Connection

Mercury & Inert Gas Fill

End Seal

Envelope

LOW-PRESSURE HIGH-OUTPUT MERCURY LAMP - AMALGAM TYPE

MEDIUM-PRESSURE MERCURY LAMP

Tungsten Coil Electrode

Electrical

Connection

Inert Gas Fill Seal

Mercury Amalgam

Envelope

Electrode - Tungsten Coils on a Tungsten Rod

Electrical

Connection

Mercury & Inert Gas Fill

Seal

Envelope Molybdenum Foil

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Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-31 June 2003

Lamp Envelope

The envelope of the lamp should transmit germicidal UV light, act as an electrical 

insulator, and not react with the lamp’s fill gases. A non-crystalline form of quartz, vitreous 

silica, is often used for the lamp envelope because of its high UVT and its resistance to high 

temperatures. However, some LP lamps use UV-transmitting glass instead of quartz. Envelopes 

are approximately 1 to 2 mm thick, and the diameter is selected to optimize the UV output and 

lamp life. 

As quartz is exposed to high temperatures, it begins to crystallize. Crystallization 

reduces the UVT of the quartz and changes its coefficient of expansion, which causes internal 

stresses. Envelopes for MP lamps must be able to withstand thermal shocks associated with 600 

to 900 °C operating temperatures without the quartz transforming to its crystalline form. LP 

lamps have lower operating temperatures where crystallization is not a concern, which is why 

some LP lamps use UV-transmitting glass rather than quartz for the lamp envelope. 

Envelopes of MP lamps may be covered with a reflective coating at the ends. This is to 

keep the ends warm and prevent the condensation of mercury behind the electrodes. 

The UV transmittance of the envelope affects the spectral output of MP mercury lamps, 

especially at lower wavelengths. Lamp envelopes can be made from doped quartz, or quartz that 

is altered to absorb specific wavelengths, in order to prevent undesirable non-germicidal 

photochemical reactions. If the lamp envelope is not made from doped quartz, the lamp sleeves 

can also be used to restrict the wavelengths emitted (described in section A.3.3). 

Electrodes

With a LP mercury lamp, electrode design depends on whether the lamp operates with a 

glow or arc discharge. With a glow discharge, free electrons are formed from the bombardment 

of the electrode by cations. The electrode used is typically a cylinder of iron or nickel. Lamps 

of this type of electrode operate near 150 °C and are termed cold-cathode lamps. With an arc 

discharge, free electrons are emitted thermally from a hot electrode operating near 900 °C and 

are referred to as hot-cathode lamps. The electrode is made of a coil of tungsten wire embedded 

with oxides of calcium, barium, or strontium. The high melting point of tungsten prevents 

evaporation of electrode materials that could coat the inside of the lamp and reduce output of UV 

light. The oxides embedded within the tungsten coil reduce the temperature needed for the 

emission of electrons. LP and LPHO lamps used in UV disinfection are usually hot-cathode 

lamps. 

In order to reduce the start voltage of a hot-cathode lamp, each electrode may have two 

electrical connections to pass current through the electrode. Resistive heating of the electrode 

raises the electrode’s temperature, thereby facilitating rapid transition from a glow discharge to 

an arc discharge at a lower voltage. Rapid transition from a glow to an arc discharge reduces 

electrode sputtering and improves lamp life. The process of transitioning from glow to arc 

discharges and how it produces UV light is described in section A.1.1.1. 

Electrode design and operation is critical for reliable long term operation. In order for 

lamps to operate at an optimal temperature, electrode design should promote heat transfer. The 

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UV Disinfection Guidance Manual A-32 June 2003

electrodes of a MP mercury lamp consist of a tungsten rod wrapped in a coil of tungsten wire. 

To improve thermal emission of electrons, thorium or alkaline-earth oxides are embedded within 

the coils, and the tungsten rod may contain thorium oxide. The electrode must warm-up within 

seconds to allow transition from a glow to an arc discharge and minimize sputtering of tungsten 

onto the envelope. The electrode operating temperature must be high enough to promote thermal 

emission of electrons and low enough to prevent the evaporation of tungsten. 

The electrode of a MP lamp is connected to the external electrical supply via a thin 

molybdenum foil sealed in the quartz at the lamp ends. The molybdenum foil is ductile and 

therefore does not crack the quartz when the lamp expands and contracts due to changes in 

operating temperature. If the temperature of the seal increases beyond 350 °C, the molybdenum

will oxidize and the seal will fail. Because the seal is located behind the electrode, its 

temperature is lower than the temperature of the arc. 

Mercury Fill

The mercury fill present in UV lamps can be in the solid, liquid, or vapor phase. At 

typical LP and LPHO lamp operating temperatures, mercury remains predominantly in the liquid 

or solid amalgam phase with a small proportion in the vapor phase (which is responsible for 

producing UV light). An amalgam is an alloy of elemental mercury with another metal 

(typically indium or gallium in lamp applications) that can be either solid or liquid at room

temperature, depending on the relative proportions of the two metals. Amalgams are typically 

used in LP and LPHO lamps, while MP lamps contain liquid elemental mercury. 

Vapor pressure (the pressure of mercury in the vapor phase) depends on the temperature. 

LP lamps operate with an envelope temperature near 40 °C, resulting in a mercury vapor 

pressure near 0.007 torr (1.4 x 10-4 psi), which is optimal for the production of UV light at 254 

nm. MP lamps operate at a much higher envelope temperature (600 to 900 °C), resulting in a 

mercury vapor pressure ranging from 100 to 10,000 torr (2 to 200 psi). In MP lamps, the 

concentration of mercury in the vapor phase is controlled by the amount of mercury in the lamp, 

as opposed to LP and LPHO lamps where an excess of mercury is placed in the lamp and only a 

portion of the elemental mercury enters the vapor phase. 

With a conventional LP lamp, increasing the operating current will not produce a higher 

UV output. Instead the operating temperature will increase causing an increase in vapor pressure 

and the UV light output of the lamp will decrease. LPHO lamps hold the mercury vapor pressure

constant at the optimal value, allowing the UV light output to increase as current increases until a 

saturation value is reached. Methods of controlling the vapor pressure within the lamp include 

using either a mercury amalgam attached to the lamp envelope, a cold spot on the lamp wall, or a 

mercury condensation chamber located behind each electrode. With each method, the 

temperature of the mercury within the lamp, and hence the vapor pressure, is controlled, 

allowing efficient production of UV light at higher currents. 

Inert Gas Fill

In addition to mercury, lamps are filled with an inert gas (typically argon) at 1 to 50 torr 

(0.02 to 1 psi). The inert gas aids in starting the gas discharge and reduces deterioration of the 

electrode. When the lamp is started at room temperature, the concentration of mercury atoms is 

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Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-33 June 2003

low and there are few collisions between free electrons and mercury. However, there are a 

significant number of collisions between free electrons and argon atoms. These collisions excite 

the argon atoms to a metastable higher energy state that does not return quickly to a ground state. 

Collisions between the excited metastable argon and mercury or free electrons ionizes the 

mercury and argon, respectively. The free electrons released by ionization reduce the start 

voltage and aid in the formation of the gas discharge. However, if lamps are manufactured with 

a non-ideal argon pressure (>50 torr; 1 psi), the collisions between electrons and argon cause 

energy losses, and therefore the electrons never achieve sufficient kinetic energy to excite the 

mercury atoms. 

A.3.1.2 Low-Pressure Lamp Efficiency 

LP lamps are designed and manufactured to efficiently convert electrical energy to 

germicidal UV light. An optimal LP lamp design typically includes the following components: 

• 3.6 cm lamp envelope diameter 

• 0.007 torr mercury fill (1.3 x 10-4 psi) 

• 3 torr argon fill (0.06 psi) 

• 400 mA operating current 

• 40 °C operating temperature 

• 0.5 W/cm power input per arc length 

Under such conditions, the power input efficiency is as follows: 

• 60 percent converted to UV light at 185 and 254 nm

• 3 percent converted to other wavelengths 

• 15 percent to electrode losses 

• 22 percent to thermal losses from the arc 

A.3.1.3 Low-Pressure High Output Lamp Efficiency 

Theoretically, LPHO lamps have the same efficiency as LP lamps because they operate at 

similar vapor pressures. However in practice, LPHO lamp efficiency can be significantly lower, 

depending on lamp construction, ballast operation, power settings, and lamp cooling. The 

energy input to a LPHO lamp can be converted to energy in the following forms: 

• 30 to 45 percent converted to UV light at and 254 nm

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Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-34 June 2003

• 5 to 25 percent converted to light at other wavelengths, primarily 185, 313, 365, and 

436 nm

• 50 to 65 percent to thermal losses from the arc 

A.3.1.4 Medium-Pressure Lamp Efficiency 

For the purposes of UV disinfection, the efficiency of a MP lamp can be defined as the 

ratio of its germicidal output to its electrical input. Equation A.22 defines germicidal efficiency 

as a function of power input, lamp output, and the action spectra of a given microorganism. 

E

nm

nm

E

G

P

P G

P

P ∑= = =

300

200

( ) ( )

λ

λ λ

η Equation A.22 

where 

η = Germicidal efficiency of the lamp

PG = Germicidal lamp output (W) 

PE = Electrical power input (W) 

λ = Wavelength (nm)

P(λ) = Lamp output measured over 1 nm increments at wavelength λ (W) 

G(λ) = Action spectra of the microorganism at wavelength λ (unitless) 

Figure A.15 presents the output versus electrical input between 248 and 320 nm for three 

MP lamps containing different mercury doses. For the lamps considered, lamp efficiency varied 

slightly with input power to the lamp but did not vary with mercury dose. Lamp efficiency on 

average was 10 percent. Because lamp data used to generate Figure A.15 were based only on 

lamp output from 248 to 320 nm, the lamp efficiency may be underestimated. 

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Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-35 June 2003

Figure A.15 Germicidal Output from 248 to 320 nm of Three MP 

Lamps Calculated for MS2 as a Function of Electrical Power Input 

(adapted from Phillips 1983) 

0 

5 

10 

15 

0 50 100 

Electrical Power Consumed (W/cm)

Germicidal Output (W/cm)

150 

4.8 mg/cm Hg dose

8.0 mg/cm Hg dose 

10.1 mg/cm Hg dose 

A.3.1.5 Spectral Output of Lamps 

LP lamps emit primarily resonant light at 253.7 nm (Figure A.16a) that is formed from 

electron transitions from the first excited states to the ground state of mercury. They also emit 

light at 185 nm with intensity varying from 12 to 34 percent of the UV intensity at 253.7 nm

depending on the operating current, wall temperature, and inert gas fill. UV light at 185 nm will 

react with oxygen and promote the formation of ozone within the lamp sleeve. Ozone is a 

corrosive and toxic compound that absorbs UV light. As such, LP lamps for UV disinfection 

applications are manufactured to reduce or eliminate the emission of UV light at 185 nm. Other 

wavelengths of light including 313, 365, 405, 436, and 546 nm also are emitted from LP lamps at 

low intensities due to higher energy electron transitions within the mercury. 

The spectral output of LPHO lamps is similar to LP lamps. Although all of the 

wavelengths emitted are identical, the intensity of light from LPHO lamps is higher. 

The spectral output of MP mercury lamps involves peaks overlying a continuum (Figure 

A.16b). The combination of free electrons and mercury cations within the arc creates a broad 

continuum of UV energy lines between 200 and 245 nm. This continuum does not occur with 

LP lamps, where non-radiating recombination occurs at the envelope walls. Electron transitions 

within the mercury produce numerous narrow peaks of electromagnetic energy in the visible and

ultraviolet range. These transitions result in a broadening of the emitted light and a shift in its 

peak, usually to longer wavelengths. For example, the peak from 260 to 270 nm arises primarily 

due to the 254 nm electron transition. 

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Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-36 June 2003

Figure A.16. UV Output of LP (a) and MP (b) 

Mercury Lamps (Sharpless and Linden 2001) 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

200 250 300 350 400

Wavelength (nm)

R

elativ

e Lamp O

utput La

m

p O

utput R

elativ

e t

o

M

a

xim

u

m O

utput i

n R

ange

b. Medium Pressure Lamp

UV

Continuum

0.0

0.2

0.4

0.6

0.8

1.0

1.2

200 250 300 350 400

Wavelength (nm)

R

elativ

e Lamp O

utput

La

m

p O

utput R

elativ

e t

o

M

a

xim

u

m O

utput i

n R

ange

a. Low Pressure Lamp

0.0

0.2

0.4

0.6

0.8

1.0

1.2

200 250 300 350 400

Wavelength (nm)

R

elativ

e Lamp O

utput La

m

p O

utput R

elativ

e t

o

M

a

xim

u

m O

utput i

n R

ange

b. Medium Pressure Lamp

UV

Continuum

0.0

0.2

0.4

0.6

0.8

1.0

1.2

200 250 300 350 400

Wavelength (nm)

R

elativ

e Lamp O

utput La

m

p O

utput R

elativ

e t

o

M

a

xim

u

m O

utput i

n R

ange

b. Medium Pressure Lamp

UV

Continuum

0.0

0.2

0.4

0.6

0.8

1.0

1.2

200 250 300 350 400

Wavelength (nm)

R

elativ

e Lamp O

utput

La

m

p O

utput R

elativ

e t

o

M

a

xim

u

m O

utput i

n R

ange

a. Low Pressure Lamp

0.0

0.2

0.4

0.6

0.8

1.0

1.2

200 250 300 350 400

Wavelength (nm)

R

elativ

e Lamp O

utput

La

m

p O

utput R

elativ

e t

o

M

a

xim

u

m O

utput i

n R

ange

a. Low Pressure Lamp

A.3.1.6 Lamp Aging 

Over time, UV lamps can degrade, resulting in a reduction in output where lower 

germicidal wavelengths degrade faster than higher wavelengths. Lamp output will decrease over 

time as a function of lamp hours in operation, number of on/off cycles and power applied per 

unit (lamp) length. The rate of decrease in lamp output slows as the lamp ages (Figure A.17). 

Proposal Draft 
Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-37 June 2003

Figure A.17. Reduction in UV Output of LP and MP Lamps Over Time 

(adapted from Schenck 1981) 

a. Low-Pressure Mercury Lamps

0

20

40

60

80

100

120

0 5000 10000

Time (Hrs)

UV Output (

%)

b. Medium-Pressure Mercury Lamps

0

20

40

60

80

100

120

0 500 1000

Time (Hrs)

UV Output (%)

UV A

UV B

UV C

a. Low-Pressure Mercury Lamps

0

20

40

60

80

100

120

0 5000 10000

Time (Hrs)

UV Output (

%)

b. Medium-Pressure Mercury Lamps

0

20

40

60

80

100

120

0 500 1000

Time (Hrs)

UV Output (%)

UV A

UV B

UV C

Lamp aging can be affected by the following factors: 

• Ballast operations, including power setting, frequency, and harmonic distortion of the 

voltage and current driving the lamp

• Water temperature and heat transfer from lamps 

• Vibration of the lamp sleeves caused by water flowing through the reactor 

• The frequency of on-off cycles 

With LP and MP lamps, sputtering of the electrode during the glow phase of start-up can 

coat the inside surface of the lamp envelope with tungsten. The tungsten coating is black in 

color, non-uniform, concentrated within a few inches of the electrode, and can absorb UV light 

(Figure A.18). Sputtering from the electrode can be reduced by the following conditions: 

• Pre-heating the electrode before applying the start voltage 

• Driving the lamp with a sinusoidal current waveform

• Using a lamp with a higher argon content 

• Minimizing the number of lamp starts during operation 

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Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-38 June 2003

Figure A.18. Aged UV Lamp (right) in Comparison to a New UV Lamp (left) 

(Mackey et al. 2003) 

If a MP lamp is not sufficiently cooled during operation, tungsten and oxides between the 

tungsten coils may evaporate and coat the inside of the envelope. LP lamps using 

UV-transmitting glass may have mercury combine with sodium in the glass to create a UV 

absorbing coating. Any deposits on the inner or outer surfaces of the lamp envelope and by 

metallic impurities within the envelope will absorb UV light. The absorption of UV light can 

raise the temperature, which may lead to localized overheating of the lamp envelope. If the lamp

envelope is quartz, the increase in temperature can lead to devitrification (crystallization), 

contributing to an additional decrease in UVT. 

With MP lamps, reaction of the electrode with any water molecules that have entered the 

lamp envelope as a result of lamp seal failure will form an oxide and hydrogen and also increase 

the start voltage. The molybdenum seal of a MP lamp will oxidize and fail if the seal 

temperature exceeds 350 °C. High operating temperatures of a MP lamp can also lead to bubbles 

and distortion of the lamp envelope materials and devitrification (crystallization), which leads to 

a decrease in UVT. The coefficient of expansion of crystalline quartz is higher than that of noncrystalline quartz, and rapid changes in temperature will also stress the envelope, which may lead 

to lamp breakage. 

A.3.2 Lamp Power Supply and Ballasts 

UV lamps are typically operated with an AC supply. Unlike an incandescent lamp, a 

mercury vapor lamp cannot be connected directly to the electrical service because it has a nonlinear voltage to ampere characteristic (Persson and Kuusisto 1998). In order for the mercury 

vapor lamp to function properly, a ballast must be inserted into the circuit to limit the current 

flow through the lamp. When placed in series with the lamp, the ballast provides an impedance 

to the power supply with a positive voltage-current characteristic. The power supply and ballast 

are designed to provide the following features: 

• Reliable and rapid starting of the gas discharge 

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UV Disinfection Guidance Manual A-39 June 2003

• Re-ignition of the gas discharge every half cycle of the power supply

ductors, or combinations of these can be used as ballasts;

owever, resistors are not used because they consume power and therefore reduce electrical 

efficien ic) or

.

.3.2.1 Magnetic Ballasts 

two types of magnetic ballasts: capacitive (those with capacitors) and inductive 

(those with inductors). Each ballast type is designed to control the current to the lamp. 

e

capacitance used and does not vary significantly with the applied voltage or the lamp properties. 

An adv

asts,

the applied voltage, and the lamp properties. As electrical current flows through the inductor, it 

generat f

er

tion

netic ballasts are currently the most common type of ballast used for medium

pressur ps due to their durability and proven operating stability in the higher power 

applica ent to 

agnetic 

• An appropriate current waveform

• A high power factor 

• Stable light output 

Resistors, capacitors, in

h

cy. Lamp ballasts are often termed either magnetic (also known as electromagnet

electronic. Magnetic ballasts can be inductive or capacitive and operate at the line frequency

Electronic ballasts operate at frequencies higher than that of the line voltage and involve solid 

state devices or a mixture of solid state devices, inductors, and capacitors. 

A

There are

With a capacitive ballast, the current through the lamp is primarily a function of th

antage of the capacitive ballast is that the power delivered to the lamp and the lamp

output are independent of line voltage. A disadvantage is that electrode sputtering can increase, 

which accelerates electrode aging. Capacitive ballasts are more efficient than inductive ball

but less efficient than electronic ballasts. Because of the stored energy in the capacitor and the 

coil, capacitive ballasts are less prone to failure as a result of small fluctuations in power quality.

With the inductive ballast, the current through the lamp is a function of the inductance, 

es a magnetic field. The magnetic field opposes the electrical current, and the strength o

the field is proportional to the current passing through the inductor. Therefore, as the current 

increases, so does the resistance to the current. This interaction limits the total current flow to 

the lamps to a specific amperage. The highest power achieved with the inductive ballast is low

than with the capacitive ballast. However, electrode sputtering is less than with capacitive 

ballasts, leading to extended electrode life. With capacitive ballasts, the UV lamp output varies 

with the line voltage. Inductive ballasts provide more stable current output and better resolu

and control than capacitive ballasts, but are generally less efficient, larger, heavier, and more 

expensive. 

Mag

e lam

tions. Medium pressure reactors typically incorporate some form of power adjustm

optimize energy efficiency and control dose delivery. Because of the manner in which m

ballasts operate, power can only be adjusted by incorporating capacitors or inductors into the 

circuit. Adjustment occurs in a series of steps, and the number of steps is limited by the number 

of capacitors or inductors that are included in the ballast. 

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Appendix A. Fundamentals of UV Disinfection 

UV Disinfection Guidance Manual A-40 June 2003

A.3.2.2 Electronic Ballasts 

Electronic ballasts, sometimes referred to as solid state ballasts, contain semiconductors 

and other electronic components such as low-pass filters, rectifiers, buffer capacitors, and high 

frequen

lamp

allasts and are therefore less

proven. Although they have limited operational experience, electronic ballasts offer increased 

efficien

d

.3.2.3 Comparison of Ballast Types 

advantages and disadvantages.

Manufacturers consider these advantages and disadvantages when determining the technology to 

incorpo

r

cy oscillators that allow the ballast to behave like a switching power supply. A chopped

electrical current with up to 50,000 pulses per second of electricity is supplied to the lamp, 

whereas a magnetic ballast typically produces only 100 to 120 pulses per second. With an 

electronic ballast, the frequency of electrical pulses supplied to the lamp is longer when the

is cold. As the lamp approaches its optimum operating temperature, the electronic ballast 

provides shorter and less frequent pulses of current to the lamp. 

Electronic ballasts are a newer technology than magnetic b

cy, smaller size and weight, and the opportunity for nearly continuous power adjustment

over a wide range of settings. Reliability has improved significantly since electronic ballasts 

were initially developed. Currently, manufacturers of low pressure reactors and smaller medium

pressure reactors often use electronic ballasts in their design. Because of the reduction in store

energy, electronic ballasts are more susceptible to failure due to power inconsistencies; however, 

by incorporating a buffer capacitor, minor power disturbances can be smoothed out, reducing the

occurrence of lamp failure. 

A

Electronic and magnetic ballasts each have specific

rate into their equipment designs. The final selection takes into account the relative

importance of each of the advantages and disadvantages for a given application. A single 

manufacturer may have equipment designs based on both ballast types. For example, one UV

manufacturer uses electronic ballasts for its smaller units and magnetic ballasts for its large

units. A summary of some of the advantages and disadvantages of each ballast technology is 

shown in Table A.4. 

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UV Disinfection Guidance Manual A-41 June 2003

Table A.4. Comparison of Magnetic and Electronic Ballasts. 

Magnetic Ballast Electronic Ballast

Comparative 

Advantages 

• Less potential for power 

interference due to stored energy 

• More resistant to power surges 

• More resistant to high 

temperatures. 

• Less prone to interference with 

electronic devices 

• Less prone to sputtering (inductive 

less than capacitive) 

• Proven technology (in use for 

nearly 70-years) 

• Less expensive 

• More efficient

• Lighter weight 

• Smaller size

• Less potential for heat generation 

• Less potential for noise 

• Continuous power adjustment 

• Longer lamp operating life 

Comparative 

Disadvantages 

• Less efficient (capacitive more 

efficient than inductive) 

• Heavier weight 

• Larger size 

• More potential for heat generation 

• More potential for noise. 

• Step-function power adjustment 

(number of steps proportional to 

number of inductors/capacitors) 

• Shorter lamp operating life 

• More potential for power 

interference due to stored energy 

(can be minimized by incorporating 

a capacitor) 

• Less resistant to power surges 

• Less resistant to high temperatures 

• More prone to interference with 

electronic devices 

• More potential for sputtering 

• Newer technology (limited operating 

experience, especially in larger 

sizes) 

• More expensive 

A.3.2.4 Lamp Startup 

The voltage applied to the lamps must be sufficiently high to start and operate the lamps. 

Step-up transformers are needed to increase the voltage above the mains to start cold-cathode 

lamps. Hot-cathode lamps are classified as either instant or switch start. Instant-start lamps have 

a single connection with each electrode. Starting instant-start lamps needs the application of a 

high voltage. As the electrodes warm-up, the needed voltage drops. Switch-start lamps have 

two electrical connections with each electrode, and the electrodes are preheated for 1 to 2 

seconds before the start voltage is applied. This reduces the start voltage and lengthens the lamp

life. Because of their relatively high impedance, MP lamps typically need a higher voltage than 

LP lamps for starting and stable operation. Operating voltage ranges from 5 to 30 volts/cm,

depending on arc length, mercury dose, lamp diameter, and electrode losses. With the exception 

of short lamps, step-up transformers are needed to operate MP lamps and high voltage pulses are 

used to start them. 

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A.3.2.5 Voltage Frequency Converters 

With LP and LPHO lamps, frequency converters may be used to increase the voltage 

frequency from that of the mains (typically 60 Hz) to 20 to 100 kHz. Typically, the efficiency of 

UV light output from electrical power increases by as much as 10 percent as the frequency 

increases above 500 Hz. Furthermore, the higher frequency reduces electrode deterioration, 

makes the lamps easier to start, and extends the lamp life. These benefits, however, can be offset 

by power losses associated with the frequency converter. 

A.3.3 Lamp Sleeves 

In UV reactors, lamps are housed within lamp sleeves. Sleeve length is sufficient to 

include the lamp and associated electrical connections. Sleeve diameter is typically 1 inch (2.5 

cm) for LP mercury lamps and 2 to 4 inches (5 to 10 cm) for MP lamps. Sleeve walls are 

typically 2 to 3 mm thick and absorb some UV light. Sleeves made of doped quartz are used to 

prevent the transmission of low-wavelength UV light, thereby reducing undesirable 

photochemical reactions. 

Lamp sleeves have several functions other than housing the lamps. They maintain the

lamp temperature at an optimal value and control heat transfer from the lamps. Heat transfer 

from the MP lamp prevents failure of the molybdenum seal, distortion of the lamp envelope, and 

evaporation of the tungsten electrode. Also, lamp sleeves isolate the lamp and its electrical 

connections from the water. Lastly, they protect the lamp from mechanical forces such as water 

hammer and protect the lamp from thermal shock arising from differences in water and lamp

envelope temperature. 

Typically, LP lamps are centered using Teflon® rings, and MP lamps are centered using 

ceramic or metal disks. The positioning of the lamp along the length of the sleeve can influence 

dose delivery by the reactor. 

Sealing the lamp sleeve assembly prevents water condensation within the sleeve and 

contains any ozone formed between the lamp envelope and lamp sleeve. Components within the 

sleeve should withstand exposure to UV light, ozone, and high temperatures. If the components 

are not made of the appropriate material, exposure can cause component deterioration and offgassing of any impurities present in the quartz from manufacturing. Off-gassed materials can 

form UV-absorbing deposits on the inner surfaces of the lamp sleeve. Off-gassing and ozone 

formation will be a greater issue with MP lamps because they operate at a higher temperature 

and emit low-wavelength ozone-forming UV light. Off-gassing can be minimized through 

proper manufacturing of the lamp sleeves. 

The UVT of a lamp sleeve influences the intensity of UV light transmitted from the lamp

into the water. The UVT is a function of the reflectance and absorbance of UV light by the 

sleeve, as per Equation A.23. 

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UV Disinfection Guidance Manual A-43 June 2003

( ( ) ) ( ) [1 ( )][1 ( )] L

s AS SW UVT R R e α λ λ λ λ − = − − Equation A.23

where 

UVTS(λ) = Sleeve UVT at wavelength λ

RAS(λ) = Reflectance of UV light at the air-sleeve interface at wavelength λ

RSW(λ) = Reflectance of UV light at the sleeve-water interface at wavelength λ

α(λ) = Sleeve absorption (Base e) at wavelength λ

L = Pathlength of light through the sleeve 

Because the refractive indices of the lamp sleeve and water are similar, the reflectance of 

UV light at the sleeve-water interface (RSW) is often considered negligible in this equation. The 

absorption coefficient of the sleeve varies strongly with wavelength and the material of the 

sleeve. For a zero degree incidence angle, Figure A.19 presents the UVT over the germicidal 

range of two types of quartz: standard and wavelength-selective. Quartz can be manufactured to 

select for a variety of wavelengths depending on the desired application. For UV disinfection 

applications, wavelength-selective quartz is primarily used to prevent the transmission of low 

wavelength (<200 nm) UV light into the water. 

Figure A.19. UV Transmittance of Two Types of Quartz Commonly Used to Make 

Lamp Sleeves (GE Quartz 2001) 

Standard Quartz

Wavelength-Selective

Quartz

100

80

60

40

20

0

U

V Tr

a

n

s

mittanc

e (

%)

200 220 240 260 280 300 320 

Wavelength (nm)

Standard Quartz

Wavelength-Selective

Quartz

100

80

60

40

20

0

U

V Tr

a

n

s

mittanc

e (

%)

200 220 240 260 280 300 320 

Wavelength (nm)

In order to reduce fouling on the sleeve surfaces, some UV reactors using LP lamps have 

sleeves made of Teflon® or Teflon®-coated quartz. However, Teflon® sleeves have a lower UV 

transmittance, and their transmittance degrades faster than conventional quartz. 

Failure mechanisms for sleeves include fractures and fouling. Fractures arise from

internal stresses created during the production of the quartz and external mechanical forces. 

Annealing the quartz at high temperatures during production removes internal stresses. Visual 

inspection using polarized light can also reveal whether or not sleeves are stress free. Fractures 

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UV Disinfection Guidance Manual A-44 June 2003

may arise from mechanical forces such as wiper jams, water hammer, resonant vibration, and 

impact by foreign objects. Fouling may occur on both internal and external surfaces and is 

discussed in more detail in section A.4.1.4. Exposure of quartz contaminated with metal cations 

from the manufacturing process can cause solarization and an increase in UV absorption. 

A.3.4 Cleaning Systems 

Due to fouling on the lamp sleeves, cleaning the external surface of sleeves is important 

to maintain dose delivery. UV reactor manufacturers have developed different approaches for 

cleaning lamp sleeves, depending on the application. Both manual and automatic cleaning 

regimes are used. A reactor must be shut down and drained prior to manual cleaning. The 

sleeves are removed once the reactor is drained and wiped with a cloth and cleaning solution. 

Manual cleaning is primarily used for smaller systems with relatively few sleeves and lower 

fouling potential. 

Automatic cleaning approaches are typically used for larger systems. They may be 

classified as off-line chemical cleaning (OCC) or on-line mechanical cleaning (OMC). OCC 

systems, also referred to as flush and rinse systems, involve a sequence of events controlled by 

the UV reactor. In OCC systems, the reactor is shut down, drained, and flushed with a cleaning 

solution. Solutions used to clean sleeves include citric acid, phosphoric acid, or a food grade 

proprietary solution provided by the UV reactor manufacturer. The reactor is rinsed and returned

to operation after sufficient time to dissolve the substances fouling the sleeves is allowed. OCC 

cleaning approaches are typically used by reactors with LPHO lamps. 

In OMC systems, the UV reactor remains on-line while the lamp sleeves are cleaned. 

OMC systems have mechanical or physical-chemical wipers that are built-in to the UV reactor. 

The wipers are either driven by screws attached to electric motors or pneumatic pistons. 

Mechanical wipers may consist of steel brush collars or Teflon® rings that move along the lamp

sleeve. Physical-chemical wipers have a collar filled with cleaning solution that move along the 

lamp sleeve. The wiper physically removes fouling on the lamp sleeve surface while the 

cleaning solution within the collar dissolves fouling materials. UV reactors with MP lamps 

typically use wipers because the higher lamp temperatures accelerate fouling under certain water 

qualities. 

The time between sleeve cleaning will depend on the rate of fouling. Sleeve cleaning can 

be initiated manually, at regular intervals, or triggered by a calculated UV dose or measured UV 

intensity, depending on the reactor control logic. In physical-chemical wipers, solution 

replacement varies with the rate of fouling and is on the order of months. Replacing the cleaning 

solution is necessary because reaction with the foulant and dilution with water reduces the ability 

of the cleaning solution to dissolve the foulant. 

A.3.5 UV Intensity Sensors 

UV intensity sensors are photosensitive detectors that are used to indicate dose delivery 

by providing information related to UV intensity at different points in the UV reactor. UV 

intensity sensors include the following components arranged as shown in Figure A.20. 

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UV Disinfection Guidance Manual A-45 June 2003

• Monitoring windows and light pipes deliver light to the photodetector. Monitoring 

windows are typically quartz discs and light pipes are cylindrical probes made of 

quartz (quartz silica probe). 

• Diffusers and apertures reduce the UV light incident on the photodetector, thereby 

reducing UV intensity sensor degradation. Diffusers also modify the UV intensity 

sensor’s angular response. 

• Filters limit the light delivered to the diode, often restricting it to germicidal 

wavelengths. 

• Photodetectors are solid-state devices that produce a current proportional to the 

irradiance on the detector’s active surface. The responsivity of typical photodetector 

to UV light is on the order of 0.1 to 0.4 milliamps/mW (mA/mW). 

• Amplifiers convert the output of the photodetector from a low-level current to a 

standardized output proportional to the irradiance (e.g., converts intensity to a 4 to 20

mA output for use in process control interfaces). 

• The housing of the UV intensity sensor protects the components from the external 

environment. The housing should be electrically grounded to shield the photodetector 

and amplifier, thereby reducing electrical noise and bias. 

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UV Disinfection Guidance Manual A-46 June 2003

Figure A.20. Interior UV Intensity Sensor Schematics 

(courtesy of (a) Severn Trent Services and (b) Wedeco-Ideal Horizons) 

Quartz

Window

Filter

Photodetector

Housing

Signal

Output

UV Light

UVC Filter

Signal Amplifier

Printed Circuit Boards

Quartz Silica Probe

Digital

Signal

Output

Photodetector

UVC Light

Housing

a. b.

Quartz

Window

Filter

Photodetector

Housing

Signal

Output

UV Light

Quartz

Window

Filter

Photodetector

Housing

Signal

Output

UV Light

UVC Filter

Signal Amplifier

Printed Circuit Boards

Quartz Silica Probe

Digital

Signal

Output

Photodetector

UVC Light

Housing

UVC Filter

Signal Amplifier

Printed Circuit Boards

Quartz Silica Probe

Digital

Signal

Output

Photodetector

UVC Light

Housing

a. b.

Note that the sensor shown in Figure A.20b is cylindrical in shape. All dimensions are 

standardized as detailed in the German standards for UV disinfection. 

A.3.5.1 UV Intensity Sensor Properties 

UV intensity sensor properties that impact the measurement of UV intensity and dose 

delivery monitoring include angular response, acceptance angle, spectral response, working 

range, detection limit and resolution, linearity, temperature response, long term drift, calibration 

factor, and measurement uncertainty. An ideal UV intensity sensor will have a linear response 

over the working range, provide a response unaffected by ambient temperature, be stable over 

time, have zero measurement noise and bias, respond only to germicidal UV light, and have zero 

measurement uncertainty. 

Angular response is a plot of the sensor measurement as a function of the incident angle 

of UV light at the sensor’s window. Angular response is affected by the UV intensity sensor’s 

aperture size, the size of the photodetector’s active surface, the distance between the aperture and 

the active surface, and the impact of any diffusers and reflecting surfaces within the UV intensity 

sensor. An ideal sensor has a cosine response (Equation A.24) because a cosine response results 

in an accurate measure of the light incident on the surface of the photodetector. 

I I ( ) m i = cos θ Equation A.24

where 

Im = Intensity measured by photodetector 

Ii = Intensity incident on photodetector’s surface 

θ = Incident angle at the photodetector surface 

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In practice, sensors deviate from cosine response; some potential responses are shown in 

Figure A.21. 

Figure A.21. Angular Response of Two UV Intensity Sensors 

Relative to Ideal Cosine Response 

0.0

0.2

0.4

0.6

0.8

1.0

-90 -60 -30 0 30 60 90

Incident Angle (degrees)

A

n

g

ular R

esp

o

nse

(R

elativ

e Scal

e)

Cosine

Sensor 1

Sensor 2

0.0

0.2

0.4

0.6

0.8

1.0

-90 -60 -30 0 30 60 90

Incident Angle (degrees)

A

n

g

ular R

esp

o

nse

(R

elativ

e Scal

e)

Cosine

Sensor 1

Sensor 2

The opening or acceptance angle of the UV intensity sensor is the angle over which the 

sensor detects UV light. The opening angle is typically measured by either the threshold 

detection of UV light or detection at some percentage of the maximum detection value (e.g., 50 

percent). The acceptance angle is a characteristic of the sensor but does not affect sensor 

performance. 

The spectral response is a measure of the output of the UV intensity sensor as a function 

of wavelength. The sensor spectral response depends on the response of the photodetector and 

filters and the UVT of the monitoring windows, light pipes, and filters. Some sensors use filters 

to limit the spectral response to the wavelengths within the germicidal range (200 to 300 nm) 

because it can be advantageous for sensors to only respond to UV light that causes damage to 

microorganisms. 

The working range of the UV intensity sensor is the range that the sensor is able to 

measure. The low end of the working range is defined by the detection limit of the 

measurement. The high end of the measurement range is limited by the saturation of the 

photodetector and the amplifier. Saturation is the point at which the sensor can no longer 

respond to an increase in intensity. 

The detection limit of the UV intensity sensor is the lowest UV intensity that can be

detected and quantified at a known confidence level. The detection limit is calculated as a 

confidence of repeated measurements of low intensity UV light, usually at a specific percentage 

confidence interval. The measurement resolution is the smallest difference in UV intensity that

can be differentiated at a given confidence limit. The detection limit and the resolution depend 

on the measurement noise and on any digitalization of the analog output from the UV intensity 

sensor by the system's electronics. The measurement noise is the root-mean-square (RMS) of the 

random variation in the sensor measurement over time. The measurement bias is the timeaveraged sensor measurement obtained with no incident light. The measurement bias and noise 

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UV Disinfection Guidance Manual A-48 June 2003

of a photodetector are increased by electromagnetic fields within the UV reactor if the sensor is 

not properly shielded and grounded. 

An ideal UV intensity sensor responds proportionally to the intensity incident on the 

sensor (Figure A.22). The linearity of the UV intensity sensor is a measure of the deviation of 

the sensor response from that proportional relationship. Linearity is affected by bias and 

saturation. The linearity is reported as the ratio of the measured response to the known incident 

intensity, usually at a specific percentage confidence interval. 

Figure A.22 Example of Sensor Linearity

0

1

2

3

4

5

6

7

0 1 2 3 4 5 6 7

Incident Intensity (mW/cm2)

Sensor Reading (

m

W/cm2

)

Ideal

Response

Region of

Saturation

Bias

Region of

Linear Response

0

1

2

3

4

5

6

7

0 1 2 3 4 5 6 7

Incident Intensity (mW/cm2)

Sensor Reading (

m

W/cm2

)

Ideal

Response

Region of

Saturation

Bias

Region of

Linear Response

UV intensity sensor measurement is also affected by ambient temperature. The changes 

in sensor response arise from thermal expansion of the optical components, the photodetector, 

and the amplifier. UV intensity sensor electronics can compensate to reduce the effects of 

temperature.

The long-term drift of the UV intensity sensor is the change in response as a function of 

time. Exposure to UV light damages optical and electronic components within the UV intensity 

sensor. The damage caused by UV light is typically greater at higher UV intensities and lower 

wavelengths. Degradation of the filter can increase the filter’s bandwidth (the wavelength range 

passed by the filter), thereby increasing the UV intensity sensor measurement even though the 

UV lamp output has not increased. Degradation of the monitoring windows and light pipes may 

cause a decrease in UVT due to solarization. Off-gassing from damaged components can coat 

optical components, reducing the measured intensity. 

The calibration factor of the UV intensity sensor is a value used to convert the standard 

electrical output of the UV intensity sensor (mA or volts) to UV intensity (mW/cm2

 or W/m2). 

The calibration factor is the ratio of the known intensity of the UV light to the electrical output of 

the sensor. Sensors used in UV reactors equipped with LP or LPHO lamps are calibrated with 

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UV Disinfection Guidance Manual A-49 June 2003

UV light at 254 nm. Sensors used in UV reactors equipped with MP lamps can either be 

calibrated with light only at 254 nm or can be calibrated with polychromatic UV light from

lamp. 

 a MP 

he uncertainty of a UV intensity sensor represents the difference in intensity between 

that me

A.3.6 UV Transmittance Monitors 

As stated previously, UVT is an important parameter in determining the efficiency of UV 

disinfec

general, commercial on-line UVT monitors calculate transmittance by measuring UV 

intensi

P

ce

Figure A.23. UV Transm

T

asured by the sensor and an accepted reference sensor. This uncertainty incorporates the

uncertainty that arises due to variability in calibration, linearity, spectral response, angular 

response, temperature response, and long-term drift. 

tion. Therefore, monitoring UVT (or UV absorbance, A254, to calculate UVT) is critical

to the success of a UV disinfection application. UVT can be determined either by grab samples 

with a laboratory instrument or by an on-line instrument. Several commercial UV reactors use 

the measurement of UVT to help monitor and control the calculated UV dose in the reactor. 

In

ty at various distances from a lamp. One such monitor is schematically displayed in 

Figure A.23. In this monitor, a stream of water passes through a cavity containing a short L

lamp with three UV intensity sensors located at various distances from the lamp. The differen

in sensor readings is used to calculate UVT. 

ittance Monitor Design 

(courtesy of Severn Trent Services) 

UV Lamp UV Intensity

Sensor

Inlet Outlet

A

B

C

UV Lamp UV Intensity

Sensor

Inlet Outlet

A

B

C

UV Lamp UV Intensity

Sensor

Inlet Outlet

A

B

C

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A.3.7 Temperature Sensors 

Energy input per unit volume is relatively high for a UV reactor. The water flowing 

through a reactor efficiently absorbs the waste heat and maintains operating temperatures within 

a desirable range. Nevertheless, temperatures can become elevated under the following 

circumstances: 

• Water level in the reactor drops and lamps are exposed to air. 

• Water stops flowing in the reactor. 

Most temperature sensors are located at the top of the UV reactor. The temperature 

sensor can either measure the water temperature or the reactor shell temperature. In either case, 

if the temperature exceeds a setpoint value, it will register a high-temperature alarm. The 

temperature alarms can be integrated into a supervisory control and data acquisition (SCADA) 

system such that the alarm results in an operations change to reduce the potential for lamp

breakage. For instance, the reactor can be shut down or valves can open or close to change the 

flow of water to the reactor. 

A.3.8 Reactor Configuration 

This section describes the configuration of UV lamps and UV intensity sensors as well as 

the hydraulic considerations of the overall reactor design. 

A.3.8.1 Lamp Placement 

The lamp configuration in a reactor is designed to optimize dose delivery. UV lamps 

may be oriented parallel, perpendicular, or diagonal to flow. Depending on the installation of the 

reactor, this can result in lamps oriented horizontally, vertically, or diagonally relative to the 

ground. Orienting MP lamps horizontal relative to the ground prevents overheating at the top of 

the lamps and reduces the potential for lamp breakage due to temperature differentials.

In a reactor with a square-cross section, typically lamps are placed with lamp arrays 

perpendicular to flow. This pattern may be staggered to improve disinfection efficiency. With a 

circular cross-section, lamps typically are evenly spaced on one or more concentric circles 

parallel to flow. The water layer between lamps and between the lamps and the reactor wall 

influences dose delivery. If the water layer is too small, the reactor wall and adjacent lamps will 

absorb UV light. If the water layer is too large, water will pass through regions of lower UV 

intensity and experience a lower UV dose. The optimal spacing between lamps depends on the 

UVT of the water, the output of the lamp, and the degree of hydraulic mixing within the reactor. 

A.3.8.2 UV Intensity Sensor Placement 

UV intensity sensors may be located to view either one or more lamps. The measurement 

of UV intensity from a given lamp depends on the following conditions: 

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UV Disinfection Guidance Manual A-51 June 2003

• Output of the lamp

• UVT of the water 

• Distance from the lamp to the UV intensity sensor

• Incident angle of the light on the UV intensity sensor 

As such, a given measurement by a UV intensity sensor viewing more than one lamp can 

have many interpretations, and such measurements should be properly understood to avoid 

misinterpretation. Also, UV intensity sensors may be located to view the output from the center 

or ends of the lamp. The optimal sensor placement will give a representative or conservative 

measure of the lamp output, given that lamp aging and sleeve fouling is non-uniform along the 

length of the lamp. 

The number of UV intensity sensors used in a reactor can vary from one per lamp to one 

per reactor. The appropriate number of sensors will depend on the type of lamp used, the 

variance in lamp-to-lamp output (especially after the lamps have aged), and the impact of that 

variance on dose delivery and dose monitoring. The implications of the number of sensors used 

per reactor are discussed in the background to the validation protocol (section F.3.5) 

UV intensity sensors may view the lamps either from a UV intensity sensor port located 

on the reactor wall or through a lamp sleeve located within the reactor. UV intensity sensors are 

classified as wet or dry. A dry UV intensity sensor views the UV light through a monitoring 

window as shown in Figure A.20b. A wet UV intensity sensor is in direct contact with the water 

flowing through the reactor and is shown in Figure A.20a. While checking the on-line UV 

intensity sensor with a reference UV intensity sensor is easier with a separate monitoring 

window, condensation on the window can interfere with the measurement of UV intensity. 

A.3.8.3 Hydraulic Considerations 

The flow through UV reactors is turbulent with residence times on the order of tenths of a 

second for MP lamps or seconds for LP lamps. In theory, optimal dose delivery by a UV reactor

is obtained with plug flow hydrodynamics and complete mixing perpendicular to the flow. In 

practice, however, UV reactors do not have these ideal hydrodynamics. 

Lamp placement, inlet and outlet conditions, baffles, and mixers all affect hydrodynamics 

within a reactor. Turbulence and eddies form in the wake behind lamp sleeves oriented 

perpendicular to flow. Staggered lamp arrays promote mixing within the reactor, thereby 

minimizing short-circuiting of flow.

Inlet and outlet conditions can have a significant impact on reactor hydrodynamics. 

Ninety-degree inlet and outlets promote short-circuiting, eddies, and dead zones within the 

reactor. Straight inlet conditions with gradual changes in cross sectional area can be used to 

develop flow for optimal dose delivery. 

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UV Disinfection Guidance Manual A-52 June 2003

Some manufacturers insert baffles to improve hydrodynamics in the reactor. Perforated 

plates can be used to even the flow throughout the reactor’s cross-section. Plates with a single 

opening are used to direct flow towards high intensity regions within the reactor. Mixers used 

within reactors are designed to promote either turbulent or vortex mixing. 

Improvements to the hydrodynamics through the reactor are often obtained at the expense 

of headloss. Perforated baffle plates and turbulent mixers can increase dose delivery but will 

significantly increase headloss. However, inlet and outlet conditions surrounding the reactor can 

be changed to reduce headloss without changing the disinfection effectiveness within the reactor. 

Also, using vortex mixers allows the spacing between lamps to increase, thereby reducing 

headloss through the reactor. 

A.3.9 Monitoring UV Disinfection Performance 

Some method of monitoring the performance of an operating UV installation is required 

to demonstrate to the utility and primacy agency that adequate disinfection is being achieved (40 

CFR 141.729(d)). Because the concentration of pathogenic organisms cannot be measured 

continuously in the UV-treated water and the dose cannot be measured directly in real time, 

various strategies have been developed to demonstrate adequate dose delivery. Any dose 

monitoring method selected must be evaluated during reactor validation (described in Appendix 

C) and the outputs measured during validation will be part of the monitoring setpoints.

Currently, there are three fundamental approaches to monitor UV disinfection 

performance in a UV reactor: 

1. UV Intensity Setpoint Approach. In this approach, measurements made by the UV 

intensity sensor are used to control the UV reactor. The UV intensity sensor is 

located in a position that allows it to properly respond to both changes in UV 

intensity output of the lamps and also UVT of the water. The UV intensity sensor 

output and the flowrate are used to monitor dose delivery. The setpoint value for UV 

intensity over a range of flowrates is determined during validation (see Chapter 4). 

2. UV Intensity and UVT Setpoint Approach. This approach is similar to the UV 

intensity sensor setpoint approach, except that the UV sensor is placed close to the 

lamp such that it only responds to changes in UV lamp output. UVT is monitored 

separately. For a specific flowrate, the UV intensity and UVT measurements are used 

to monitor dose delivery. The setpoints for UV intensity and UVT over a range of 

flowrates are determined during validation (see Chapter 4). 

3. Calculated UV Dose Approach. In this approach, the UV intensity sensor is placed

close to the lamp, which is similar to the UV intensity and UVT setpoint approach. 

Flowrate, UVT, and UV intensity are all monitored, and the outputs are used to 

calculate UV dose via a validated computational algorithm developed by the UV 

reactor manufacturer. 

The strategy for dose monitoring depends on the manufacturer and is typically 

proprietary. Dose monitoring recommendations are discussed in section 5.4. 

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UV Disinfection Guidance Manual A-53 June 2003

A.4 Water Quality Impacts and Byproduct Formation 

Constituents in the water affect the performance of UV reactors. In addition, most 

disinfectants form byproducts, and the goal of the overall disinfection process is to maximize 

disinfection while minimizing byproduct formation. 

A.4.1 Water Quality Impacts 

UV absorbance, particle content, and constituents that foul lamp sleeves and other wetted 

components are the most significant water quality factors impacting UV disinfection 

effectiveness. In spite of these effects, the impact of water quality on dose delivery can be 

adequately addressed in virtually all drinking water applications if carefully considered during 

the design of the UV reactors. 

A.4.1.1 UV Absorbance 

The most important water quality parameter affecting reactor performance is UV 

absorbance. As UV absorbance increases, the intensity throughout the reactor decreases for a 

given lamp output. This results in a reduction in UV dose delivered to the microorganism and 

measured UV intensity. Section 3.1.3.1 discusses how to incorporate the impact of UV 

absorbance into UV reactor design. 

UV absorbers in typical source waters include humic and fulvic acids, other aromatic 

organics (e.g., phenols), metals (e.g., iron), and anions (e.g., nitrates and sulfites) (Yip and 

Konasewich 1972; DeMers and Renner 1992). Both soluble and particulate forms of these 

compounds will absorb UV light. UV absorbance will vary over time due to changing

concentrations of these compounds. Temporal variability in UV absorbance is greater in rivers 

and small lakes than in large lakes and reservoirs. UV absorbance will vary seasonally due to 

rainfall, lake stratification and destratification (turnover), and changes in biological activity of 

organisms within the water source. 

Water treatment processes can reduce the UV absorbance of water. Coagulation, 

flocculation, and sedimentation remove soluble and particulate material, and filtration removes 

particles. Oxidants such as chlorine and ozone reduce soluble material, precipitate metals, and 

reduce UV absorbance. Activated carbon absorption also reduces soluble organics. Because 

these treatment processes reduce UV absorbance, the lowest UV absorbance occurs at the end of 

the treatment train, and therefore UV disinfection is most effective when applied after filtration. 

Chemicals used in the water treatment process can also increase the UV absorbance of the water,

and their impacts are discussed in section A.4.1.3. 

A.4.1.2 Particles 

Particle content can also impact UV disinfection performance. Particles may absorb and 

scatter light, thereby reducing the UV intensity delivered to the organisms. Particle-associated 

microorganisms also may be shielded from UV light, effectively reducing disinfection 

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performance as discussed in section A.2.6.5 and causing a tailing or flattening of the doseresponse curve when higher inactivation levels are desired. Particles in source waters are diverse 

in composition and size and include large molecules, microorganisms, clay particles, algae, and 

flocs. Sources of particles include wastewater discharges, erosion, runoff, microbial growth, and 

animal waste. The particle concentration will vary over time both seasonally and over the short 

term. Storm events, lake turn over, and spring runoff are some events that increase the 

concentration of particles. 

Recent research by Linden et al. (2002b) indicated that the UV dose-response of 

microorganisms added to filtered drinking waters is not altered by variation in turbidity of

filtered water that met regulatory requirements (40 CFR 141.73). For unfiltered raw waters, 

Passantino and Malley (2001) found that source water turbidity up to 10 NTU does not impact 

the UV dose-response of separately added (seeded) organisms. In these experiments, however, 

organisms were added to waters containing various levels of treated or natural turbidity. 

Therefore, it was not possible to examine microorganisms associated directly with particles in 

their natural or treated states. Consequently, these drinking water studies can only suggest the 

impact of turbidity on dose-response as it relates to the impact of UV light scattering by particles 

rather than particle-association or clumping of microorganisms. 

Water treatment unit processes such as coagulation, flocculation, sedimentation, and 

filtration are designed to remove particles from water. Organisms within coagulated and 

flocculated particles will be more difficult to inactivate; however, they will typically be removed 

during filtration. 

A.4.1.3 Water Treatment Chemicals 

Water treatment chemicals affect the UVT of the water, the formation of conglomerate 

particles, and the fouling potential of the water. 

Water treatment processes upstream of the UV reactors can be operated to control and 

increase UVT, thereby optimizing the design and costs of the UV reactor. Chemicals such as 

chlorine, ozone, and hydrogen peroxide oxidize UV-absorbing compounds but may also absorb 

UV light with ozone showing the most pronounced effect on UV absorbance. Oxidant residuals 

can be quenched with chemicals such as sodium thiosulfate or sodium bisulfite. However, the 

use of these chemicals can also increase the UV absorbance of water. 

Table A.5 lists the UV absorption coefficients of common water treatment chemicals and 

their "impact threshold concentration", defined as the concentration that will decrease the UVT 

from 91 to 90 percent. Of these chemicals, ozone and ferric iron have the greatest potential of

impacting the UV absorbance of water. 

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Table A.5 UV Absorbance Characteristics of Common Water Treatment 

Chemicals (Adapted from Bolton et al. 2001)

Compound 

Molar Absorbtion 

Coefficient 

(M-1 cm-1) 

Mass-based 

Absorbance

(L/mg cm-1) 

Impact 

Threshold 

Concentration 1

(mg/L) 

Ozone (O3) (aqueous) 3,250 0.0677 0.071 

Ferric iron (Fe3+) 4,716 0.0844 0.057

Permanganate (MnO4

-

) 657 0.0055 0.91

Thiosulfate ion (S2O3

2-) 201 0.00178 2.7

Hypochlorite ion (ClO-) 29.5 0.000573 8.4

Hydrogen peroxide (H2O2) 18.7 0.00055 8.7

Ferrous iron (Fe2+) 28 0.0005 9.6

Sulfite ion (SO3

2-) 16.5 0.000206 23

Zinc ion (Zn2+) 1.7 0.000026 187

Ammonia (NH3) NSA NSA N/A

Ammonium ion (NH4

+

) NSA NSA N/A

Calcium ion (Ca2+) NSA NSA N/A

Hydroxide ion (OH-) NSA NSA N/A

Magnesium ion (Mg2+) NSA NSA N/A

Manganese ion (Mn2+) NSA NSA N/A

Phosphate species NSA NSA N/A 

Sulfate ion (SO4

2-) NSA NSA N/A

NSA No significant absorbance 

N/A Not applicable 1

 Concentration in mg/L resulting in UVT decrease from 91 percent to 90 percent 

(A254 increase from 0.041 cm-1 to 0.046 cm-1) 

A.4.1.4 Fouling Potential 

Wetted components within a UV reactor can become fouled over time. Fouling on the 

external surfaces of the lamp sleeve reduces the transmittance of UV light from the lamps into 

the water, thereby reducing dose delivery. Fouling on UV intensity sensor windows reduces the 

intensity of UV light measured by the sensors, resulting in under prediction of dose delivery. 

Fouling on the inside surfaces of the reactor reduces reflection of UV light from those surfaces, 

which reduces the amount of UV light available for disinfection. 

Fouling on the wetted surfaces of a UV reactor has been attributed to the following 

events: 

• Compounds whose solubility decreases as temperature increases will precipitate (e.g., 

CaCO3, CaSO4, MgCO3, MgSO4, FePO4, FeCO3, Al2(SO4)3). These compounds will 

foul MP lamps faster than LP lamps due to differences in operating temperature. 

• Compounds with low solubility will precipitate (e.g., Fe(OH)3, Al(OH)3). 

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• Particles will deposit on the lamp sleeve surface due to gravity settling and 

turbulence-induced collisions (Lin et al. 1999a). 

Precipitation will depend on the water temperature, pH, alkalinity, ion concentration, 

total hardness, and the particle concentration. Residual concentrations of coagulants like ferric 

sulfate can also affect fouling. The fouling will vary spatially along and around the lamp sleeve, 

and will depend on the operating temperature of the lamp. Precipitation of compounds whose 

solubility decreases with increasing temperature is more notable with lamps operating at higher 

temperatures (e.g., MP lamps; Sheriff and Gehr 2001). Organic fouling can occur when a reactor 

is left off and full of water for an extended period of time (Toivanen 2000). 

Fouling rates on lamp sleeves are reported to follow first order kinetics after an initial

induction period (Lin et al. 1999b). Currently, there is not sufficient information to predict 

quantitatively the fouling based on water quality. The potential for fouling and the frequency of 

sleeve cleaning will be site and equipment specific. The fouling observed during several pilotand full-scale UV facilities is shown in section 2.5.1 (Table 2.3). 

The Langelier Saturation Index (LSI) or the calcium carbonate precipitation potential 

(CCPP) can be used to help indicate fouling potential. The LSI is defined as the difference 

between the pH of the water and the pH at which calcium and carbonate are at equilibrium with 

solid CaCO3. The CCPP is the amount of calcium carbonate that will precipitate when 

equilibrium conditions in the water have been reached. Both the LSI and CCPP are functions of 

temperature, pH, calcium hardness, total dissolved solids (TDS), and alkalinity. For UV 

disinfection, the temperature of the lamp sleeve surface should be used to calculate the LSI and 

CCPP. The LSI and CCPP will depend on upstream processes, such as pH adjustment and lime

softening, and may vary daily or seasonally. 

A.4.1.5 Algae Growth 

Visible light emitted from UV lamps may promote algae growth in UV reactors and the

surrounding piping. Depending on the species, algae growth could cause taste and odor 

problems in the finished water. Algae growth is a greater issue with MP lamps than LP lamps 

because MP lamps produce more light in the visible range. Algae growth also depends on water 

temperature, pH, and nutrient concentration (Sterner and Grover 1998). 

A.4.2 Disinfection Byproducts 

UV disinfection byproducts (DBPs) arise either directly through photochemical reactions 

or indirectly through reactions with products of photochemical reactions. Photochemical 

reactions will only take place if a chemical species absorbs UV light, and the resulting excited 

state reacts to form a new species. The resulting concentration of new species will depend on the 

concentration of the reactants and the UV dose. 

When UV light is absorbed by an atom, electrons within the atom are excited to higher 

energy states. An excited atom may return to its original ground state releasing the absorbed 

energy as light, or it may interact with other atoms forming or breaking bonds. The formation or 

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UV Disinfection Guidance Manual A-57 June 2003

breaking of bonds between atoms results in the formation of a new chemical species. Chemical 

reactions promoted by light are termed photochemical reactions. In some cases, the products of 

photochemical reactions are radical species. Radical species may react with other chemicals to 

form new chemical species (i.e., UV DBPs). 

In drinking water, research has focused on the impact of UV light on the formation of 

halogenated DBPs following subsequent chlorination and the transformation of organic material 

to more degradable components. For ground water and filtered drinking water, UV disinfection 

at typical doses is not shown to impact the formation of trihalomethanes (THM) or haloacetic 

acids (HAA), two categories of DBPs currently regulated by EPA (Malley et al. 1995; 

Kashinkunti et al. 2003). Several studies have shown low-level formation of degradable, nonregulated DBPs (e.g., aldehydes) as a result of applying UV light to wastewater and raw drinking 

water sources. However, a study performed with filtered drinking water indicates no significant 

change in aldehydes, carboxylic acids, or total organic halides (TOX) (Kashinkunti et al. 2003). 

The difference in results can be attributed to the difference in water quality, most notably the 

higher concentration of organic material in raw waters and wastewaters. 

Akhlaq et al. (1990) reported that UV doses of 250 mJ/cm2

 from an LP lamp do not break 

down alginic acid, a model compound for polysaccharides in drinking water. They concluded 

that UV disinfection does not increase the assimilable organic carbon (AOC) of drinking water. 

With UV doses ranging from 18 to 161 mJ/cm2

, Kruithof et al. (1989) reported no increase in 

AOC or mutagenicity of a granular activated carbon (GAC) filtrate. 

Malley et al. (1995) evaluated the impact of UV doses of 60, 130, and 200 mJ/cm2

 on 

DBP formation in ground waters and treated surface waters. They reported no change in pH, 

turbidity, dissolved organic carbon, A254, color, nitrate, nitrite, bromide, iron, or manganese. 

Formaldehyde increased from 1.2 to 12.1 μg/L with one highly colored ground water. 

Formaldehyde increased from less than 2 μg/L up to 14 μg/L with untreated surface waters but 

only 2 to 3 μg/L with treated surface waters. A small but insignificant increase in AOC was 

observed with all waters. 

Zheng et al. (1999) observed an 8 to 17 percent decrease in THM and a 9 to 19 percent 

increase in HAA when MP UV light was applied at a dose of 2000 mJ/cm2

 after chlorination. 

However, at a lower dose of 100 mJ/cm2

, they observed a 1 to 7 percent decrease in THM and no 

change in HAA. 

A low conversion of nitrate to nitrite by UV light has been observed (approximately 1 

percent; Sharpless and Linden 2001). Von Sonntag and Schuchmann (1992) also reported 0.001 

and 0.072 mg/L nitrite formed from 50 mg/L nitrate exposed to 25 mJ/cm2

 from LP and MP 

lamps, respectively. Conversion is lower with LP lamps than MP lamps because the UV 

absorbance of nitrate is higher below 240 nm than it is at 254 nm. 

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Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements 

In support of the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR), 

the U.S. Environmental Protection Agency (EPA) developed UV dose requirements for 

Cryptosporidium, Giardia, and virus inactivation. The requirements represent the UV dose 

necessary to achieve a given inactivation level, similar to the concentration * time (CT) 

requirements for chemical disinfectants. 

The UV dose requirements were developed to account for uncertainty associated with the 

dose-response of microorganisms (Cryptosporidium, Giardia, and virus) in controlled 

experimental conditions. In practical application, other sources of variability and uncertainty are 

introduced due to the hydraulic effects of the UV installation, UV reactor, and UV intensity 

sensors. The validation protocol, as described in Chapter 4 and Appendix C, addresses these and 

other areas of variability and uncertainty by applying safety factors to the UV dose requirements 

derived in this appendix. Therefore, the dose requirements presented in this appendix are not the 

actual dose levels at which utilities will be required to validate and operate UV reactors for a 

given log inactivation. 

This appendix explains the derivation of the UV dose requirements through a three-step 

process of data collection, qualitative review to establish working data sets, and mathematical 

analyses. 

B.1 Data Collection 

EPA collected UV dose-response research data for adenovirus, Giardia lamblia, Giardia 

muris, and Cryptosporidium parvum. Adenovirus was evaluated because, of the data available, it 

is considered the most resistant to inactivation by UV light of the pathogenic waterborne viruses. 

In compiling data, EPA reviewed published and unpublished studies conducted over the past 50 

years as provided in published literature, electronic databases, research reports, and conference 

proceedings. The experimental conditions varied among batch and continuous flow UV 

apparatuses, types of UV lamps, and water quality conditions. Table B.1 summarizes these 

studies.

UV Disinfection Guidance Manual B-1 June 2003

Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

Table B.1 Summary of Data Collected (cont) 

General Microbial Information Experiment

Reference 

Peer 

Reviewed 

Literature 

(Y/N) Organism Species Host Strain Type Assay Used

Experiment 

Type 

Lamp 

Type 

Gerba 2000 No Adenovirus N/A Human N/A 2 Cell Culture

(PLC/PRF/5) 

Batch LP 

Gilead and Ginsberg 1966 

Yes Adenovirus N/A Human N/A 12 Cell Culture 

(KB cells) 

Batch LP 

Hara et al. 1990 

Yes Adenovirus N/A Human N/A 19 Cell Culture

(Vero) 

Batch LP 

Malley 2000b No Adenovirus N/A Human N/A 41 Cell Culture 

(Hep-2) and 

RT-PCR 

Batch MP M

Meng and Gerba 1996 

Yes Adenovirus N/A Not

given 

N/A 40

41 

Cell Culture 

(PLC/PRF/5) 

Batch LP 

Shin et al. 2001a 

No Adenovirus N/A Human N/A 5 Cell Culture

(A549) 

Batch LP 

Thompson et al. 2002 

Yes Adenovirus N/A Human N/A 2

15 

Cell Culture 

(A-549) 

Batch LP 

Thurston et al. 2002 

No Adenovirus N/A Human N/A 40 Cell Culture

(PLC/PRF/5) 

Batch LP 

UV Disinfection Guidance Manual B-2 Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

Table B.1 Summary of Data Collected (cont) 

General Microbial Information Experiment

Reference 

Peer 

Reviewed 

Literature 

(Y/N) Organism Species Host Strain Type Assay Used

Experiment 

Type 

Lamp 

Type 

Craik et al. 2000 

Yes Giardia muris Bovine N/A N/A Mouse

Infectivity

(C3H/HeN) 

Batch MP M

Danielson et al. 2001 

No Giardia muris Bovine N/A N/A Mouse

Infectivity

Batch LP/

LPHO 

Hayes et al. 2001 

No Giardia muris Hamster N/A N/A Mouse

Infectivity

Batch LP 

Oppenheimer et al. 2002 

No Giardia muris Mouse N/A N/A Mouse

Infectivity

Batch LP/MP 

Campbell and Wallis 2002 

Yes Giardia lamblia Human N/A N/A Gerbil

Infectivity

Batch LP 

Linden et al. 2002 

No Giardia lamblia Bovine N/A N/A Gerbil

Infectivity

Batch LP 

Malley 2000a No Giardia lamblia Bovine N/A N/A Gerbil

Infectivity

Batch P-UV 

Mofidi et al. 2002 

Yes Giardia lamblia Bovine N/A N/A Mouse

Infectivity

Gerbil 

Infectivity

Batch LP 

UV Disinfection Guidance Manual B-3 Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

Table B.1 Summary of Data Collected (cont) 

General Microbial Information Experiment

Reference 

Peer 

Reviewed 

Literature 

(Y/N) Organism Species Host Strain Type Assay Used

Experiment 

Type 

Lamp 

Type 

Bukhari et al. 1999 

Yes Crypto. parvum Bovine Iowa N/A Mouse

Infectivity

(CD-1) 

Continuous 

Flow

MP 

Clancy et al. 2000 

Yes Crypto. parvum Bovine Iowa N/A Mouse

Infectivity

(CD-1) 

Batch MP/

LP 

M

Clancy Env 2000 

No Crypto. parvum Bovine Iowa N/A Mouse

Infectivity

(CD-1) 

Batch LP/MP 

M

Clancy et al. 2002 

Yes Crypto. parvum Bovine TAMU

Moredum

Iowa

Maine 

Glasgow

N/A Mouse

Infectivity

Batch LP 

Craik et al. 2001 

Yes Crypto. parvum Bovine Iowa N/A Mouse

Infectivity

(CD-1) 

Batch LP/

MP 

Hargy et al. 2000 

Yes Crypto. parvum Bovine Iowa N/A Mouse

Infectivity

(CD-1) 

Continuous 

Flow

MP 

Landis et al. 2000 

No Crypto. parvum Bovine Iowa N/A Cell Culture

(HCT-8) 

Batch LP 

Mackey et al. 2000 

No Crypto. parvum Bovine Iowa N/A Mouse

Infectivity

(CD-1) 

Continuous 

Flow

LPHO 

UV Disinfection Guidance Manual B-4 Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

Table B.1 Summary of Data Collected (cont) 

General Microbial Information Experiment

Reference 

Peer 

Reviewed 

Literature 

(Y/N) Organism Species Host Strain Type Assay Used

Experiment 

Type 

Lamp 

Type 

Mofidi et al. 1999 

No Crypto. parvum Bovine Iowa N/A Cell Culture

(HCT-8) and 

RT-PCR 

Batch MP/

P-UV 

Oppenheimer et al. 2002 

No Crypto. parvum Bovine Iowa N/A Mouse

Infectivity

(CD-1) 

Batch LP/MP 

Shin et al. 2001b 

Yes Crypto. parvum Bovine Iowa N/A Cell Culture

(MDCK) 

Batch LP 

Kashinkunti et al. 2002 

No Crypto. parvum Bovine Iowa N/A Cell Culture

(MDCK) 

Batch LP 

Sommer et al. 2001 

No Crypto. parvum Bovine Iowa N/A Cell Culture

(HCT-8) 

Batch LP 

N/A – Not applicable; LP – low pressure lamp; LPHO – low pressure-high output lamp; MP – medium pressure 1 Water Quality Definitions: 

Lab - Tap water treated in the lab by de-ionization and buffering (in some cases). Reclaimed Wastewater - Tertiary treated wastewater. Low Turbidity Wastewater - Tertiary treated wastewater with turbidity less than 1 NTU. Unfiltered - Water that meets EPA's filtration avoidance criteria. Untreated Surface Water - Water from an untreated surface water (e.g., lake, river). WTP Filtered Water – Filtered water from a water treatment plant. Lab Filtered Surface Water - Filtered water from a water treatment plant that is filtered subsequently in the lab 

UV Disinfection Guidance Manual B-5 Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

UV Disinfection Guidance Manual B-6 June 2003

B.2 Data Review—Criteria for Inclusion in Statistical Analysis 

EPA evaluated the data presented in Table B.1 to determine the data sets to be used in

analyzing dose-response for each target microorganism. To be included in the statistical 

analysis, the experimental design had to be sufficiently documented with respect to 

experimental conditions, methodology, and calculation of results to allow an accurate 

assessment of UV dose-response. For instance, studies were not included if the report did 

not provide sufficient information to determine the UV dose measurement method or whether 

the reported UV dose accounted for appropriate parameters (e.g., UV absorbance). The 

statistical dose-response analysis combines data across different experimental designs and 

conditions; therefore, it is important to ensure the differences between studies do not affect 

the UV dose-response relationship. 

B.2.1 Appropriate Experimental Design and Conditions 

Research studies with the following criteria were selected for the statistical analyses: 

• Batch experimental design 

• Low pressure (LP) lamps as the UV light source 

• Filtered water, high quality unfiltered water, laboratory water, or low turbidity 

reclaimed wastewater 

• UV dose of the target microorganism inactivation directly measured and not derived 

from the inactivation response of another microorganism

Data from continuous flow studies were not included in the analyses because flowthrough UV reactors apply a distribution of UV doses as opposed to a single dose. Moreover, 

UV dose in a reactor is difficult to calculate precisely due to the variability in hydraulic detention 

time and UV intensity distributions in reactors. 

Studies were not included if the researchers utilized a UV light source that did not have a 

widely accepted dose measurement methodology, such as pulsed UV lamps. Medium pressure 

(MP) lamps pose a challenge of dose measurement due to the polychromatic nature of the MP 

UV light and the absence of a standard method for calculating dose from MP lamps. The results 

of a t-test indicated the LP and MP UV dose-response data, as reported, were statistically 

different1

; therefore, only LP lamp data were used in the statistical analyses. 

Given the potential interference of particles in water and the fact that utilities installing

UV disinfection would need to meet finished water turbidity levels, EPA restricted media to 

water with turbidity values less than or equal to 1 nephelometric turbidity unit (NTU). 

 

1

 The t-test was calculated with Cryptosporidium data at low doses. The Giardia data and higher dose 

Cryptosporidium data had too many data reported as “greater than a value” (referred to as censored data) and thus,

could not be used in a t-test. The adenovirus data had too few MP data to conduct a t-test. 

Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

UV Disinfection Guidance Manual B-7 June 2003

Studies utilizing non-standard microbial assay methods (i.e., not generally accepted in 

standard microbiological methods references) or studies not providing an evaluation of pathogen 

infectivity were not included. 

Note that many research studies evaluated multiple experimental conditions, but only the 

subset of data meeting the criteria specified for this statistical analysis were used. 

B.2.2 Research Studies and Data Included in Statistical Analysis 

UV dose-response data sets for adenovirus, Cryptosporidium parvum, and Giardia 

lamblia and Giarida muris that met the criteria specified previously are presented in this section. 

B.2.2.1 Viruses 

For adenovirus, 4 of the 9 studies met the criteria discussed for inclusion in the statistical 

analysis. Figure B.1 shows the data of the selected studies.

Figure B.1 Observed Adenovirus Data from Selected Research Studies

0

1

2

3

4

5

6

7

0 50 100 150 200 250 300

UV Dose (mJ/cm2

)

Log Inactivation

Gerba 2000 (Type 2) Meng and Gerba, 1996 (Type 40 and 41) Shin et al., 2001a (Type 5)

Thompson et al., 2002 (Type 2) Thompson et al., 2002 (Type 15) Thompson et al., 2002 (Type Not Avail.)

Thurston et al., 2002 (Type 40)

Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

UV Disinfection Guidance Manual B-8 June 2003

B.2.2.2 Protozoa 

 For Cryptosporidium parvum, 9 of the 13 studies met the criteria for inclusion in the 

statistical analysis. For Giardia (including both lamblia and muris), 6 of the 8 studies were 

included. Figures B.2 and B.3 show the Cryptosporidium parvum and Giardia data of the 

selected studies, respectively. The data are both censored and uncensored and noted as such on 

each graph. Censored data are those with log inactivation of “greater than” a particular value 

rather than an absolute value (termed uncensored). 

Figure B.2 Cryptosporidium Data from Selected Research Studies 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0 5 10 15 20 25 30 35 40 45 50

UV Dose (mJ/cm2

)

Log Inactivation

Clancy et al., 2000 Clancy et al., 2000 Clancy et al., 2002 Clancy et al., 2002

Craik et al., 2001 Craik et al., 2001 Landis et al., 2000 Landis et al., 2000

Clancy Envionmental, 2002 Clancy Envionmental, 2002 Oppenheimer et al., 2002 Oppenheimer et al., 2002

Shin et al., 2001b Shin et al., 2001b Kashinkunti et al., 2002 Kashinkunti et al., 2002

Sommer et al., 2001 Open symbols report Log Inactivation Closed symbols report > Log

Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

UV Disinfection Guidance Manual B-9 June 2003

Figure B.3 Giardia Data from Selected Research Studies

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0 5 10 15 20 25 30 35 40 45 50

UV Dose (mJ/cm2

)

Log Inactivation

Mofidi et al., 2002 - G. lamblia Mofidi et al., 2002 - G. lamblia

Linden et al., 2002 - G. lamblia Linden et al., 2002 - G. lamblia

Hayes et al., 2001 - G. muris Hayes et al., 2001 - G. muris

Mofidi et al., 2002 - G. muris Mofidi et al., 2002 - G. muris

Campbell and Wallis, 2002 - G. lamblia Oppenheimer et al., 2002 - G. muris

 Open symbols report Log Inactivation

 Closed symbols report > Log Inactivation

B.3 Statistical Analysis

To determine the relationships between UV dose and log inactivation of

Cryptosporidium, Giardia, and virus, a mathematical model with hierarchical Bayesian 

parameter estimation techniques was used. This model performs a meta-analysis that 

summarizes and integrates the findings of multiple research studies. It can be considered as a 

compromise of two extreme methods of combining data from different sources. One extreme 

method treats the data from different sources as identical replications and computes a regression 

as if the data were from a single source. The second extreme method treats each individual study 

as totally unrelated to other studies. In this second method, the separately estimated regression 

coefficients are pooled only to reflect the possible range. The Bayesian meta-analysis treats the 

studies as exchangeable, but not identical or completely unrelated (Hedges 1997). Regression

coefficients for each study are estimated using the same calculations and allowed to differ 

between studies. A Bayesian hierarchical modeling approach represents a more general and 

reasonable approach for combining information (Gelman et al. 1995; Condon 2001). 

Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

UV Disinfection Guidance Manual B-10 June 2003

B.3.1 Model Description 

The model used to relate UV dose to Cryptosporidium, Giardia, and virus log 

inactivation is described by Equation B.1. Qian et al. (2003) provides a complete description of 

the model and further statistical analyses. 

 

( ) ( ) ijk ij Cijk Y N I 1 ~ μ ,τ Equation B.1

( )

( )

( )

~ ( ) 0,0.0001

~ 0.001,0.001

~ ,

exp( )log 1

1,2

2

10

N

gamma

N

X

i

ij i ij

β

τ

β β τ

μ = β +

where 

Yijk = Log inactivation of the kth observation exposed to the jth UV dose level in the ith

study 

N(μij,τ1)= Normal distribution with mean μ and precision τ

I(Cijk) = Censor operator with Cijk as the estimated lower bound of the log inactivation 

value for the kth observation exposing to the jth UV dose level in the ith study 

Xij = j

th dose level of study i, 

βi = Regression coefficient for study i 

β = Integrated regression coefficient, combining information from all studies 

When an observation is known to be greater than a value (right-censored), the reported 

value is used as a lower bound value (Cijk.). The prior distributions on precision (inverse of 

variance), τ1,2, are modeled using gamma(0.001, 0.001), which is considered “non-informative” 

(the log variance is almost uniform). The prior distribution on β is N(0, 0.0001), a practically 

flat distribution. 

One of the benefits of using a Bayesian modeling approach is it allows known 

information that can better explain the data relationships to be incorporated into the model. In 

this model, two known pieces of information were incorporated: (1) as UV dose increases the 

number of microorganisms inactivated increases—incorporated by taking the exponential of βi in 

the second line of Equation B.1, which restricts the slope of the regression between log 

inactivation and UV dose to a positive value; and (2) when the UV dose is zero, no 

microorganism inactivation due to UV light occurs—incorporated by setting the intercept of the 

regression line to zero (the second line in Equation B.1 has no intercept term). 

A Markov Chain Monte Carlo simulation method is used for estimating the model 

parameters. To impute the censored data, an iterative procedure is used. At a given iteration, a 

random sample of log inactivation is taken from a normal distribution with the mean and 

variance calculated by the current estimates of βi and τ1:2. If the generated value is less than the 

reported value (the lower bound), it is not used and a new value is generated until one that is 

larger than the reported value is found. The model is then refitted with new estimates of βi and 

τ1,2. This process is repeated many times (200,000 in this case). Mathematical theories indicate 

that the effect of a set of random initial values for all model coefficients and the censored values 

Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

UV Disinfection Guidance Manual B-11 June 2003

will gradually disappear, and the samples will converge to their respective posterior marginal

distributions after a number of iterations. In this case, the first 140,000 iterations were discarded 

and 1,000 samples for each of the unknown quantities (i.e., coefficients, predictions, and 

censored values) were taken from the remaining 60,000 iterations. The computation is 

implemented under WinBUGS (Spiegelhalter et al. 1996). 

The Bayesian hierarchical model of Equation B.1 estimates the integrated model 

coefficients using the coefficient estimates from each study. As the model indicated, β is 

assumed to be the mean of the parent distribution of βi. This integration accounts for the 

uncertainty of each study and “weights” each study accordingly. 

B.3.2 Cryptosporidium and Giardia Modeled Results 

The modeled results for Cryptosporidium and Giardia are shown graphically in Figures 

B.4 and B.5, respectively. The graphs show the estimated regression for each study. The model 

incorporates the coefficients from each study and calculates the predicted median and 80 percent 

credible intervals, shown by the black solid line (median) and dark dotted lines (credible 

intervals). 

Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

UV Disinfection Guidance Manual B-12 

Figure B.4 Cryptosporidium Modeled Data and Predictive Credible Intervals 

UV Dose

Log Inactivation

o Uncensored datax Censored data+ Predicted Censored

Predicted Median80% Credible IntervalClancy et al. 2000Clancy et al. 2002Craik et al. 2001Landis et al. 2000Oppenheimer et al. 2002Sommer et al. 2001Kashinkunti et al. 2002Shin et al. 2001bClancy Environmental 2002

05 10 15

0 1 2 3 4 6 8 11 30 45 75 120 195 300

+ +

+

+

+

+

+

+ +++ +

+

+

+

+

++

++

+

+

oo

ooooo

o

ooooooo oooo o

oo

oooooo

o

oo oo

o

oo

o

oo

oo o

oo

o

o

oo

o

oo

o

o

x

x x

xx

x x x

x xxx

x

x

xxx

xx

x x 

x

xx

xx

x

xx

3 log inactivat

Lower bound credible interval

Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

UV Disinfection Guidance Manual B-13 Proposal Draft 

Figure B.5 Giardia Modeled Data and Predictive Credible Intervals 

UV Dose

Log Inactivation

o Uncensored datax Censored data+ Predicted Censored

Predicted Median80% Credible IntervalCampbell and Wallis 2002Mofidi et al. 2001Hayes et al. 2001Shin et al. 2000Oppenheimer et al. 2002

010 2030 4050 60

0 1 2 3 4 5 7912 30 45 60 90 13

+

+

+

+

++ +++

+

+ +

+ + ++

+

+

+

++

++

o o

o o

o oo o

o o o o o o o o o oo oo o o o o o o o o o o o o o o x x x x xx xxx x x x x x xx xx x x x x 

3 log inactivation

Lower bound credible interval
Appendix B. Derivation of UV Dose-Response Requirements

UV Disinfection Guidance Manual B-14 June 2003

B.3.3 Virus Modeled Results 

The model for the virus data is slightly different from the Cryptosporidium and Giardia

model described in Equation B.1. First, there were no censored data points; as a result, the term

I(Cijk) is not included. Second, based on the data, a log transformation on the UV dose is not 

necessary, i.e., the mean is modeled by βi Xij. Figure B.6 displays the modeled results for 

adenovirus. 

Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

UV Disinfection Guidance Manual B-15 Proposal Draft 

Figure B.6 Virus Modeled Data and Predictive Credible Intervals 

UV Dose

Log Inactivation

o Data

Predicted Median80% Credible IntervalGerba, 2000Meng et al., 1996Shin 2001aThompson et al., 2000Thompson et al., 2002 (Type 15)Thompson et al., 2002 (Type 2)Thurston, 2002 (Type 40)

0 2 468

0 50 100 150 200 25

o oooo

oo

oo

o

oo oo

ooo

o

oo

oo

oo oo

o

o

o

o

oooo

o

o

o

oo o

ooo

o

o

o

o

ooooo

o

oooo o

o

oo

oooo

o

o

oo

ooo

o

o

ooo o

o

ooo

oo oo

o

o

o

o

oo o

4 log inactivation

Lower bound credible interval
Appendix B. Derivation of UV Dose-Response Requirements

UV Disinfection Guidance Manual B-16 June 2003

B.3.4 Calculating UV Dose Requirements from Modeled Results

Table B.2 presents the UV dose requirements for Cryptosporidium, Giardia, and viruses. 

Each of the graphs presented in Figures B.4 through B.6 show the 80 percent credible interval. 

The UV dose requirements for given log inactivation levels were calculated from the fitted 

model’s lower bound of the credible interval (as called out in Figures B.4-B.6). Using the lower 

bound means that at a given UV dose, the corresponding log inactivation is expected to be 

achieved 90 percent of the time. 

Table B.2 UV Dose Requirements for Inactivation of Cryptosporidium, Giardia

and Viruses During Validation Testing 

Log Inactivation

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Cryptosporidium 1.6 2.5 3.9 5.8 8.5 12 - -

Giardia 1.5 2.1 3.0 5.2 7.7 11 - -

Virus 39 58 79 100 121 143 163 186

B.4 References

Bukhari, Z., T.M. Hargy, J.R. Bolton, B. Dussert, and J.L. Clancy. 1999. Medium-pressure UV 

for oocyst inactivation. JAWWA. 91(3):86-94. 

Campbell, A.T. and P. Wallis. 2002. The effect of UV irradiation on human derived Giardia 

lamblia cysts. Water Research. 36(4):963-969. 

Clancy Environmental Consultants. 2000. UV Disinfection of Cryptosporidium and MS2 

Coliphage in Catskill-Delaware Water. Report to Hazen and Sawyer. 

Clancy, J.L., Z. Bukhari, T.M. Hargy, J.R. Bolton, B.W. Dussert, and M.M. Marshall. 2000. 

Using UV to inactivate Cryptosporidium. Journal AWWA. 92(9):97-104. 

Clancy, J. L., T. M. Hargy, D.A Battigelli, M. M. Marshall, D. Korich, and W.L. Nicholson. 

2002. Susceptibility of Multiple Strains of C. parvum oocysts to UV Light. Final report to 

AWWA Research Foundation And American Water Works Association. 

Condon, P. 2001. Bayesian Statistical Modeling. Wiley, London. 

Craik, S.A., G.R. Finch, J.R. Bolton, and M. Belosevic. 2000. Inactivation of Giardia muris

cysts using medium-pressure ultraviolet radiation in filtered drinking water. Water 

Research. 34(18):4325-4332. 

Craik, S.A., D. Weldon, G.R. Finch, J.R. Bolton, and M Belosevic. 2001. Inactivation of 

Cryptosporidium parvum oocysts using medium- and low-pressure ultraviolet radiation. 

Water Research. 35(6):1387-1398. 

Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

UV Disinfection Guidance Manual B-17 June 2003

Danielson, R.E., J. Phillips, and F. Soroushian. 2001. Inactivation of Giardia muris, Bacillus 

subtilis spores and MS2 coliphage by ultraviolet light. Proceedings IUVA Congress, 

Washington D.C. 

Gelman, A., J.B. Carlin, H.S. Stern, and D.B. Rubin. 1995. Bayesian Data Analysis. Chapman & 

Hall. London. 

Gerba, C.P. et al. 2000. Treatability Screening for CCL Microbial Contaminants - Interim

DRAFT Report to US EPA Office of Water. 

Gilead, Z. and H.S. Ginsberg. 1966. Comparison of the rates of ultraviolet inactivation of the 

capacity of Type 12 Adenovirus to infect cells and to induce T antigen formation. J. 

Bacteriol. 92(6):1853-1854. 

Hara, J. et al, 1990, Survival and Disinfection of adenovirus type 19 and enterovirus 70 in 

ophthalmic practice. Jpn J ophthamol. 34: 421-427. 

Hargy, T.M., J.L. Clancy, Z. Bukhari, and M.M. Marshall. 2000. Shedding UV Light on the 

Cryptosporidium Threat. Journal of Environmental Health, 63: 19-22. 

Hayes, S.L., C.D. Barnes, E.W. Rice, M.W. Ware, and F.W. Schaefer III. 2001. Low Pressure 

Ultraviolet Studies for Inactivation for Giardia muris cysts, Proceedings IUVA Congress, 

Washington D.C. 

Hedges, L. 1997. Bayesian meta-analysis. In Statistical Analysis of Medical Data, eds B. Everitt 

and I. Dunn. Arnold. London. pp. 251-275. 

Kashinkunti, R.D., D.H. Metz, K.G. Linden, M.D. Sobsey, M. Moran, and A. Samuelson. 2002. 

Microbial Inactivation Strategies for the Future: UV, Chlorine, and DBPs. Proceedings

WQTC 2002, Seattle, November 10-14, 2002. 

Landis, H.E., Thompson, J.E., Robinson, J.P., Blatchley, E.R. 2000. Inactivation responses of 

Cryptosporidium parvum to UV radiation and gamma radiation. Proceedings WQTC 

2000, Salt Lake City, WA. November 5-9, 2000. 

Linden, K.G., G-A. Shin, G. Faubert, W. Cairns, and M.D. Sobsey. 2002. UV disinfection of 

Giardia lamblia in water. Environmental Science and Technology. 36(11):2519-2522. 

Mackey, E.D., J.P. Malley, T.M. Hargy, and R.S. Cushing. 2000. MS-2 Bioassays and 

Cryptosporidium Challenges: Comparing and Contrasting UV Reactor Validation 

Techniques. Proceedings AWWA Water Quality Technical Conference, Salt Lake City, 

UT. 

Malley, J.P. 2000a. Reduction of Microbial and Chemical Pollution of Coastal Waters Using 

Pulsed-UV Disinfection, Interim Report, NOAA. 

Malley, J.P. 2000b. AWWARF Project 2593: “Inactivation of Pathogens by Innovative UV 

Technologies” Second Quarterly Report. 

Proposal Draft 
Appendix B. Derivation of UV Dose-Response Requirements

UV Disinfection Guidance Manual B-18 June 2003

Meng, Q.S. and C.P. Gerba. 1996. Comparative inactivation of enteric adenovirus, poliovirus 

and coliphages by ultraviolet irradiation. Water Research. 30(11):2665-2668. 

Mofidi, A.A., H. Baribeau and J.F. Green. 1999. Inactivation of Cryptosporidium parvum with 

polychromatic systems. Proceedings WQTC, Tampa, Florida, October 31- November 4, 

1999. 

Mofidi, A.A., E.A. Meyer, P.M. Wallis, C.I. Chou, Meyer, B.P., S. Ramalingam, and B.M. 

Coffey. 2002. The effect of UV light on the inactivation of Giardia lamblia and Giardia 

muris cysts as determined by animal infectivity assay. Water Research. 36:2098-2108. 

Oppenheimer, J., T. Gillogly, G. Stolarik, and R. Ward. 2002. Comparing the Efficiency of 

Low and Medium Pressure UV Light for Inactivating Giardia muris and 

Cryptosporidium parvum. Proceedings AWWA Annual Conference, New Orleans, LA. 

Qian, S. S., M. Donnelly, D. C. Schmelling, M. Messner, K. G. Linden, and C. Cotton. 2003. 

Ultraviolet light inactivation of Cryptosporidium and Giardia in drinking water: A 

Bayesian meta-analysis. Manuscript submitted to Water Research. 

Shin, G-A, K. G. Linden, and M.D. Sobsey. 2001a. UV Disinfection of Adenovirus. WERF 

Progress Report 6, 98-HHE-2. 

Shin, G-A, K. G. Linden, M. Arrowood, and M.D. Sobsey. 2001b. Low pressure UV inactivation 

and subsequent DNA repair potential of Cryptosporidium parvum oocysts. Appl. Environ. 

Micro. 67(7):3029-3032. 

Sommer, R., S. Appelt, W. Pribil, and T. Slifko. 2001. UV inactivation of C. parvum evaluated 

in cell culture. Unpublished data, personal communication. 

Spiegelhalter, D.J., A. Thomas, A., and W.R. Best. 2000. WinBUGS Version 1.3: User Manual. 

Medical Research Council, Biostatistics Unit, Institute of Public Health, Cambridge, UK. 

Thompson, S.S., J.L. Jackson, M. Suva-Castillo, W.A. Yanko, Z. El Jack, J. Kuo, C.L. Chen, 

F.P. Williams, and D.P. Schnurr. 2002. Detection of Infectious Human Adenoviruses in 

Tertiary Treated and UV Disinfected Wastewater. Accepted for publication, Water 

Environment Research. 

Thurston-Enriquez, J., C. Hass, J. Jacangelo, K. Riley, and C.P. Gerba. 2002. Inactivation of 

Feline Calicivirus and Adenovirus Type 40 by Ultraviolet Irradiation. Proceedings, WEF 

Disinfection 2002, St. Petersburg FL, February 17-20, 2002. 

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UV Disinfection Guidance Manual C-1 June 2003

Appendix C. Validation of UV Reactors 

To receive credit for Cryptosporidium, Giardia, or virus inactivation using UV light, the 

Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) requires systems to 

demonstrate that the UV reactor can deliver the required dose through validation testing (40 CFR 

141.729(d)). Furthermore, validation testing must determine a set of operating conditions that 

can be monitored by the control system to ensure that the UV dose required for a given pathogen 

inactivation credit is delivered during operation. At a minimum, these operating conditions must 

include flowrate, UV intensity measured by a UV intensity sensor, and lamp status. The 

validated operating conditions must account for the following factors (40 CFR 141, Subpart W,

Appendix D): 

• Lamp aging

• Lamp sleeve fouling 

• UV transmittance of the water 

• Inlet and outlet piping or channel configurations of the UV reactor 

• Dose distributions arising from the velocity profiles through the reactor 

• Failure of UV lamps or other critical system components 

• Measurement uncertainty of on-line sensors 

Unless the State approves an alternative approach, validation testing must involve the 

following components: 

• Full-scale testing of a UV reactor, which conforms uniformly to the reactors used by 

the system

• Inactivation of a test microorganism whose dose-response characteristics have been 

quantified with a low-pressure (LP) mercury vapor lamp

This appendix presents one approach for validating UV reactors. Other approaches or

modifications to this approach may be used at the discretion of the State. This appendix begins 

with an overview of the approach for conducting validation testing and interpreting validation 

results. This is followed by a description of the materials, equipment, and personnel used to 

conduct validation testing and a description of the steps involved in validating UV reactors. The 

appendix ends with descriptive examples showing how validation test results can be related to 

inactivation credit. 

Appendix F provides more detailed background information on validation testing and 

includes several examples. 

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UV Disinfection Guidance Manual C-2 June 2003

C.1 Overview

UV reactor validation should provide confidence that the UV reactor is appropriately 

sized for a given disinfection application and should allow a water treatment plant (WTP) to 

receive inactivation credit based on on-line measurements of flow, UV intensity, lamp status, 

and, in some cases, UV transmittance (UVT) of the water at 254 nm. To ensure a UV reactor is 

appropriately-sized for a given WTP, validation testing should provide data on dose delivery and 

monitoring under design conditions of flow, UVT, and lamp output. This should be done either 

by validating UV reactor performance under those conditions or by validating UV reactor 

performance over a range of conditions that can be interpolated to obtain performance under 

design conditions. To allow a WTP to obtain inactivation credit with UV disinfection, validation 

testing should provide data relating on-line measurements of flow, UV intensity, lamp status, and 

UVT to UV dose levels required to achieve target pathogen inactivation credit. This should be 

done over the range of those on-line measurements expected with operation of the UV reactor at 

the WTP. 

UV manufacturers typically produce UV reactors as part of a product line where each 

reactor is manufactured to the same specifications. If a representative UV reactor from that 

product line undergoes validation testing, the test results can be applied to all other UV reactors 

within that product line if those reactors are manufactured to the same specifications as the 

validated reactor. If the design specifications of the product line that impact dose delivery and 

monitoring change, this new UV reactor design must be re-validated. 

C.1.1 Test Protocol 

The validation protocol in this guidance document builds on well-established protocols 

used in Europe and North America. A UV manufacturer typically delivers a UV reactor to a test 

facility. Test personnel inspect the UV reactor and document features of the design that impact 

dose delivery and monitoring (e.g., reactor dimensions and sensor properties). The UV reactor is 

installed within a biodosimetry test stand with inlet and outlet piping that should result in equal 

or worse dose delivery than with the reactor installed at the WTP. The UV reactor is operated 

under various test conditions of flow, UVT, and lamp power. The test condition of UVT is 

typically obtained using a UV-absorbing compound injected into the flow upstream of the UV 

reactor. A challenge microorganism is injected into the flow upstream of the UV reactor. The 

concentration of viable challenge microorganisms is measured in samples collected at the 

reactor’s inlet and outlet. The results are used to calculate the log inactivation of the challenge 

microorganism achieved by the UV reactor. The UV dose-response of the challenge 

microorganism present in the inlet sample is measured using a bench-scale device termed a 

collimated beam apparatus. The UV dose-response curve is used to relate the log inactivation 

observed through the reactor to a UV dose value termed the Reduction Equivalent Dose (RED). 

A safety factor is applied to the results to account for any bias and random uncertainty associated 

with the validation of the UV reactor and the on-line monitoring approach used to indicate dose 

delivery both during validation and during operation at the WTP. Last, a validation report is 

prepared that describes the UV reactor tested, the test protocol, the test results, and the 

inactivation credits that can be assigned to the UV reactor under given conditions of flow, UVT, 

and lamp output. Figure C.1 presents the organization of this validation protocol and the 

sections within this appendix that address each of these issues. 

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UV Disinfection Guidance Manual C-3 June 2003

Figure C.1 Elements of UV Reactor Validation 

UV System

Section C.2

Test Facility

Section C.3

Provisions

for Testing

Section C.2.1

UV System

Documentation

Section C.2.2

Biodosimetry

Test Stand

Section C.3.1

Collimated Beam

Appendix E

Microbiology

Methods

Appendix D

UV Intensity

Sensor Test Stand

Section C.3.2

Third Party

Oversight

Section C.3.3

Testing

Section C.4

Evaluate Influent and

Effluent Mixing

Section C.4.5 – C.4.6

UV System Inspection

and Installation

Sections C.4.2 – C.4.4

Evaluate UV Sensors

Intensity

Section C.4.7

Evaluated Impact of

Lamp Aging

Section C.4.8

Biodosimetry

Section C.4.9

Determining Inactivation

Credit

Section C.4.10

Reporting

Section C.4.11

Develop Approved

Test Plan

Sections C.4.1

Examples

Section C.5

UV System

Section C.2

Test Facility

Section C.3

Provisions

for Testing

Section C.2.1

UV System

Documentation

Section C.2.2

Biodosimetry

Test Stand

Section C.3.1

Collimated Beam

Appendix E

Microbiology

Methods

Appendix D

UV Intensity

Sensor Test Stand

Section C.3.2

Third Party

Oversight

Section C.3.3

Testing

Section C.4

Evaluate Influent and

Effluent Mixing

Section C.4.5 – C.4.6

UV System Inspection

and Installation

Sections C.4.2 – C.4.4

Evaluate UV Sensors

Intensity

Section C.4.7

Evaluated Impact of

Lamp Aging

Section C.4.8

Biodosimetry

Section C.4.9

Determining Inactivation

Credit

Section C.4.10

Reporting

Section C.4.11

Develop Approved

Test Plan

Sections C.4.1

Examples

Section C.5

C.1.2 Relating RED to Target Pathogen Inactivation Credit 

Chapter 1 (Table 1.4) presents the UV dose needed to achieve various inactivation credits 

for Cryptosporidium, Giardia, and viruses. The dose values provided in Chapter 1 were 

obtained by analyzing UV dose-response data measured using a bench-scale collimated beam

device. To account for variability in the dose-response of the pathogen, an 80 percent predictive 

credible interval was used to determine dose values needed to achieve a given log inactivation of 

the pathogen. The derivation of the UV dose requirements is presented in Appendix B. This 

assessment, however, does not account for the measurement uncertainty associated with UV 

reactor validation and on-line dose monitoring. To account for this uncertainty, the RED 

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UV Disinfection Guidance Manual C-4 June 2003

measured during reactor validation should be equal to or greater than a target RED defined using 

the following equation: 

( ) T RED Poly DP RED = B × B × 1+ e × Equation C.1

where 

REDT = Target RED that should be demonstrated during validation 

BRED = RED bias 

BPoly = Polychromatic bias

e = Expanded uncertainty expressed as a fraction 

DP = UV dose in Chapter 1 (Table 1.4) required for a given level of target pathogen 

inactivation credit. 

The RED bias term accounts for the difference between the dose delivered to the target 

pathogen and the dose measured using a challenge microorganism. If the challenge 

microorganism is more resistant to UV light than the target pathogen, the RED measured during 

validation will be greater than the dose delivered to the pathogen. The magnitude of the 

difference will depend on the dose distribution of the UV reactor and the inactivation kinetics of

the challenge microorganism and the target pathogen. If the challenge microorganism is as 

sensitive or more sensitive to UV light than the target pathogen, the RED bias has a value of 

1.00. A recommended approach for obtaining the value of the RED bias is given in section 

C.4.10.2. 

The polychromatic bias term accounts for spectral differences in the lamp output, lamp

sleeve UV transmittance, UVT, and action spectrum of the challenge microorganism between 

validation and operation of a UV reactor equipped with medium-pressure (MP) lamps. These 

differences can cause the dose delivered at the WTP to differ from the dose measured during 

validation. Depending on the spectral response and positioning of the UV intensity sensor and 

the dose distribution of the UV reactor, the dose delivered at the WTP can be less than dose 

measured during validation and indicated by the monitoring system. The polychromatic bias 

term accounts for this issue. The polychromatic bias only applies to UV reactors that use 

polychromatic UV lamps. With UV reactors using LP or low-pressure high-output (LPHO) 

lamps, the polychromatic bias equals 1.00. A recommended approach for obtaining the value of 

the polychromatic bias is given in section C.4.10.2. 

The expanded uncertainty, e, accounts for the uncertainty in the measurements taken 

during validation and used with dose delivery monitoring. In this protocol, the numeric value of 

the expanded uncertainty is estimated using an 80 percent confidence level by summing the 

individual measurement uncertainties associated with on-line sensors used in the field and during 

validation, influent and effluent challenge microorganism concentrations, challenge 

microorganism UV dose-response, and quantification of the UV output from the lamps. This 

approach is described in section C.4.10.2. 

Two approaches, termed Tier 1 and Tier 2, are presented in section C.4.10 for applying 

the RED bias, polychromatic bias, and the expanded uncertainty to define the target RED values. 

The Tier 1 approach, described in section C.4.10.1, is a standardized approach that uses 

prescribed values for the RED bias, the polychromatic bias, and the expanded uncertainty to 

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UV Disinfection Guidance Manual C-5 June 2003

define RED targets to be demonstrated during validation. To use the Tier 1 approach, the dose 

monitoring and validation should meet defined criteria on reactor design, challenge 

microorganism UV dose-response, UV absorber used during validation, sensor properties, 

monitoring approach, and microbiology. 

The Tier 2 approach, described in section C.4.10.2, allows the user to calculate the values 

of the RED bias, the polychromatic bias, and the expanded uncertainty, and to use those values 

to define the RED target to be demonstrated during validation. The approach does not prescribe 

criteria for reactor design, challenge microorganism dose-response, the UV absorber used during 

validation, sensor properties, monitoring approach, or microbiology. 

C.1.3 Other Validation Protocols 

Validation of UV reactors used in drinking water applications has been practiced in North 

America and Europe using well-established protocols that include the following, shown in 

chronological order of development: 

• National Sanitation Foundation/American National Standards Institute (NSF/ANSI) 

Standard 55 

• Austrian Standards Institute (ÖNORM ; Österreichisches Normungsinstitut) M 5873-

1 

• German Association for Gas and Water (DVGW; Deutsche Vereinigung des Gas- und 

Wasserfaches) W294 

• National Water Research Institute/American Water Works Association Research 

Foundation (NWRI/AwwaRF) UV Guidelines 

UV validation conducted as per DVGW and ÖNORM demonstrates that a UV reactor

will deliver a RED of 40 mJ/cm2

 measured using Bacillus subtilis spores. Validation as per these 

protocols should meet criteria for the UV reactor and its validation. UV validation conducted as 

per NWRI/AwwaRF Guidelines and NSF Standard 55 both use MS2 bacteriophage (MS2) as a 

challenge microorganism. NSF standard 55 specifies a target RED of 40 mJ/cm2

 while 

NWRI/AwwaRF Guidelines does not specify a target RED. Validation testing as per

NWRI/AwwaRF Guidelines and NSF Standard 55 should be assessed for consistency with the 

guidance for test conditions provided in section C.4.9. Results should be interpreted as per the 

guidance provided in sections C.4.9 and C.4.10. 

C.1.4 Planning UV Validation 

In general, validation testing will be conducted either for a UV manufacturer who wishes 

to validate a given UV reactor for the drinking water market or for a utility that wishes to 

validate a UV reactor for a specific application. Regardless of the end user, parties conducting 

validation testing should develop a test plan that addresses the following questions: 

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• Where will validation take place? 

• What test conditions of flow, UVT, and lamp output should be tested?

• What UV absorbers and challenge microorganisms should be used?

• What inlet and outlet conditions will be used during validation?

• Who will provide the challenge stock solutions and assay water samples?

• How will UV intensity sensor properties during validation be verified? 

• Who will conduct collimated beam testing?

• What is the expected safety factor and is it acceptable? 

• Who will provide third party oversight? 

• What State review and approval is needed for the test protocol? 

When planning how validation testing will be done, utilities and manufacturers should 

determine if they want to evaluate validation results under Tier 1 or Tier 2. They should assess if 

the planned validation will meet the Tier 1 criteria and develop preliminary estimates of the 

safety factor that would be applied under Tier 2. They should explore opportunities to optimize 

validation testing by identifying approaches that minimize the values of the RED bias, 

polychromatic bias, and expanded uncertainty terms used to determine the safety factor. To 

provide flexibility in using Tier 1 and 2, one approach would be to ensure validation meets Tier 1 

criteria and then to optimize for Tier 2. 

C.1.4.1 UV Validation for Manufacturers

UV manufacturers will conduct validation either for a specific WTP or to allow broad 

application of their UV reactor to many WTPs. If validation is being done to allow broad 

application of the UV reactor, the test conditions of flowrate, UVT, and lamp output will likely 

span a larger range than the test conditions that would be used when validating for a specific 

WTP. The UV manufacturer may also validate the UV reactor for a range of dose targets that 

allow the UV reactor to achieve credit for a range of pathogen log inactivation values. The 

number of test conditions and dose targets chosen should be sufficient to allow interpolation of 

validation data to conditions of flowrate, UVT, and lamp output specific for a given WTP 

application.

For broad application of validation results, inlet and outlet conditions should be chosen to 

provide a conservative yet practical representation of inlet/outlet piping used at WTPs. For 

example, if the UV reactor is typically applied in a filter gallery, it may make sense to test with a 

90 degree bend immediately upstream of the reactor to represent a “worst case” scenario. On the 

other hand, if a UV reactor is typically installed with 5 or 10 pipe diameters of straight pipe 

upstream of the reactor inlet, it may make sense to test with a 90-degree bend immediately 

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UV Disinfection Guidance Manual C-7 June 2003

upstream of a 5 pipe diameters of straight pipe. UV manufacturers can use computational fluid 

dynamics (CFD) as a tool to understand the impact of inlet and outlet conditions on the dose 

delivery of their UV reactors in order to best identify the inlet and outlet conditions most 

representative of a wide range of applications. 

In order to facilitate regulatory approval in the States, validation testing should be 

conducted using recognized and accepted protocols. Alternatively, the UV manufacturer should 

solicit feedback and approval for the validation test plan from the State(s) before testing. 

C.1.4.2 UV Validation for Utilities 

Utilities have the option of validating UV reactors either at a UV test facility or on-site at 

their WTP. Utilities considering on-site validation should address recommendations on water 

quality, disposal, and test train requirements provided in section C.3.1. Potential issues include 

obtaining water with a sufficiently high UVT that allows validation over the entire UVT range 

expected at the WTP, providing sufficient mixing of additives prior to entering the UV reactor

and mixing of the challenge microorganisms after the reactor, and obtaining permits for the 

disposal of the water used for validation. Utilities considering off-site validation at a test facility 

should ensure that the inlet and outlet conditions used during validation are representative of 

those conditions used at the WTP. Recommendations for inlet and outlet conditions to be used 

during UV validation are provided in section C.3.1.5. 

If on-site validation is considered, the utility should identify who will provide 

microbiological support for validation testing. The utility could use either their own 

microbiological lab or a third party lab. Regardless of the approach, the microbiology lab should 

have demonstrated experience working with the challenge microorganism and be able to provide 

timely analysis of water samples collected during validation testing. Appendix D provides detail 

on the microbiological lab qualifications and includes growth and assay methods for two 

commonly used challenge microorganisms. 

With on-site validation, the utility should also identify how it will verify the performance 

of UV intensity sensors used during validation. Because utility staff typically do not have 

experience in optoelectronic instrumentation, they should use a third party laboratory to 

benchmark sensor performance. Sections C.3.2 and C.4.7 describe the laboratory needs and the 

measurements used to benchmark sensor performance. 

C.2 UV Reactor 

This section describes the hardware and documentation that the UV manufacturer should 

provide to the validation facility. 

C.2.1 Provisions for Testing 

The UV manufacturer should provide for validation a UV reactor with the following 

characteristics: 

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UV Disinfection Guidance Manual C-8 June 2003

• A UV reactor that matches the technical description in the documentation provided as

per section C.2.2. 

• UV lamps that have undergone appropriate burn-in. The recommended burn-in 

period is 100 hours. 

• Lamps aged to give end-of-lamp-life conditions if the reactor is to be tested with aged 

lamps. 

• Provisions to reduce lamp output as per section C.4.9.4. 

• Provisions to measure the UV output of each lamp and the electrical power delivered 

to the lamps as per section C.4.9.2. 

• On-line and reference UV intensity sensors that meet the technical description 

provided in the documentation. 

• A safety cut-off switch to prevent overheating if LPHO or MP lamps are used. 

C.2.2 UV Reactor Documentation 

Prior to validation testing, the UV manufacturer should provide to the party conducting 

the tests documentation identifying and describing the UV reactor. Documentation should 

include all reactor and component information that impacts dose delivery and monitoring 

including the following: 

• Technical descriptions of the reactor and all internal components, including lamps, 

sleeves, UV intensity sensors, baffles, and cleaning mechanisms. The technical 

description should include dimensions and placement of all wetted components. 

• Technical descriptions of the inlet and outlet piping to the reactor undergoing 

validation, including the length and cross-sectional dimensions of any pipes, 

channels, and bends, and dimensions of any hydraulic structures affecting flow. If 

reactors are validated in series, technical descriptions of the piping between reactors 

should be provided. 

• Lamp specification stating the lamp manufacturer and product number, electrical 

power rating, length from electrode to electrode, spectral output of new and aged 

lamps, mercury content, and envelope diameter. The spectral output should be 

specified for 5 nm intervals or less over a wavelength range that includes the response 

range of the UV intensity sensors and the germicidal range. 

• Sleeve specifications indicating sleeve dimensions, material, and UV transmittance 

from 200 to 400 nm. 

• Technical description of the placement of the lamp within the sleeve. 

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UV Disinfection Guidance Manual C-9 June 2003

• Specifications for the reference and on-line UV intensity sensors indicating 

manufacturer and product number, external dimensions, and measurement properties. 

Measurement properties include spectral and angular response, working range and 

linearity, calibration factor, temperature stability, long-term stability, and 

measurement uncertainty. Data and calculations should be provided showing how the 

total measurement uncertainty of the sensor is derived from the individual sensor 

properties. Table C.1 gives an example of the calculation of sensor measurement 

uncertainty from the uncertainty that arises due to each sensor property. 

Table C.1 Example of a UV Intensity Sensor Uncertainty Datasheet 

Property Uncertainty (%)

Calibration 8

Linearity 5

Temperature response 3 

Angular response 5 

Spectral response 1 

Long term drift 10 

Total Uncertainty1 15 1 Total uncertainty is calculated as the square root of the 

sum of the squared individual uncertainties. In this example,

total uncertainty is (82

+52+32+52+12+102)

1/2 = 15%. 

• Specifications for the UV intensity sensor port indicating all dimensions and 

tolerances that impact the positioning of the sensor relative to the lamps. 

• If the sensor port contains a monitoring window separate from the sensor, 

specifications giving the window material, thickness, and UV transmittance from 200 

to 400 nm should be provided. 

• Technical description of the algorithm used by the reactor to monitor dose delivery, 

including the use of sensors, signal processing, and calculations. 

Documentation should also be provided on the proper installation and operation of the 

reactor to ensure proper and safe validation testing, including: 

• Flowrate, headloss, and pressure rating of the reactor 

• Assembly and installation instructions 

• Electrical requirements including required line frequency, voltage, amps, and power 

• Operation and maintenance manuals that include cleaning procedures, required spare 

parts, and safety requirements. Safety requirements should include information on 

electrical lockouts, eye and skin protection from UV light, safe handling of lamps, 

and mercury cleanup recommendations in the event of a lamp breakage 

Lastly, the UV manufacturer should consider providing the following information 

relevant to the test procedure: 

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UV Disinfection Guidance Manual C-10 June 2003

• Specifications for the challenge microorganism to be used during validation that 

includes protocols required for growth and enumeration, expected UV dose-response, 

and suitability for use in validation testing as discussed in section F.1.4. 

• Specifications for the UV absorber to be used during validation. 

• A description of the test conditions of flowrate, UVT, and lamp output used to 

validate the reactor, and the expected measurements of UV intensity and challenge 

microorganism RED. 

C.3 Test Equipment, Facilities, and Personnel

This section describes the test equipment, laboratory facilities, and personnel that are 

typically used during validation testing, including the following components: 

• Biodosimetry test stand for measuring challenge microorganism inactivation by the 

UV reactor 

• UV intensity sensor test stand for measuring sensor properties 

• Third party oversight 

Appendix D provides information on the microbiological laboratory with specific 

information on the growth and assay of MS2 bacteriophage and B. subtilis spores. Appendix E 

provides information on collimated beam apparatus used to measure the UV dose-response of the 

challenge microorganism. 

C.3.1 Biodosimetry Test Stand 

The biodosimetry test stand is used to measure the inactivation of a challenge 

microorganism by the UV reactor operating under controlled conditions of flowrate, UVT, and 

lamp output. 

Figure C.2 presents a block diagram of such a test stand with the following features: 

• Water supply with rate-of-flow control and backflow prevention 

• Dosing pumps and ports for injecting the challenge microorganism, the UV-absorbing 

compound, and, if required, a disinfectant residual-quenching agent 

• Influent-mixing device (static mixer or length of pipe) upstream of the reactor to 

ensure the challenge microorganism and UV-absorbing compound are well-mixed 

prior to entering the reactor 

• Influent sampling port after the influent-mixing device and before the reactor 

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UV Disinfection Guidance Manual C-11 June 2003

• Inlet and outlet piping to the reactor that results in a dose delivery equal to or less 

than the dose delivery expected with the installation of the reactor at a WTP 

• UV reactor under test 

• Ports to allow head-loss measurements across the UV reactor 

• Effluent-mixing device (static mixer or length of pipe) downstream of the reactor to 

ensure that the challenge microorganisms that survive inactivation by the reactor are 

well-mixed prior to sampling 

• Effluent sampling port after the effluent-mixing device 

• Water disposal facilities

Figure C.2 Block Diagram of the Biodosimetry Test Stand

Challenge

Microbe

Backflow

Prevention

Static

Mixer

Static

Mixer

Flow

meter

UV

Reactor

Valve

To

Waste

Effluent

Sample

Port

Influent

Sample

Port

Valve

Water

Supply Inlet

Piping

UV

Absorber

Outlet

Piping

Pressure

Gage

Pressure

Gage

Influent

Quenching

Agent

C.3.1.1 Water Supply 

Validation testing should prove that the monitoring of dose delivery by the UV reactor is 

valid over the full range of UVT values expected with the application of the UV reactor at the 

WTP. Typically, the UVT of the water supply used for validation is high and UV absorbing 

chemicals are added upstream of the reactor to simulate different, lower UVTs over the test

range. For validation results to be generally applied to all WTPs, the water supply should have a 

UVT at 254 nm greater than 97 percent (UV absorption coefficient less than 0.013 cm-1 with a 

10 nm path length). 

Whether coagulants are naturally present (e.g., reduced iron in ground water) or added as 

part of water treatment, they can affect the challenge microorganism concentration, the turbidity, 

and the UVT of water samples collected during reactor validation (Petri et al. 2000). 

Coagulation of the challenge microorganism can lead to reduced counts and poor sample-tosample repeatability. To avoid these effects, the water supply should not contain coagulants that 

interfere with the validation results. Alternatively, chelating agents or coffee can be used as an 

additive to counter these effects (Petri et al. 2000). 

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UV Disinfection Guidance Manual C-12 June 2003

The water passing through the reactor should not contain disinfectant residuals that 

inactivate the challenge microorganism during testing. If the water does contain a disinfectant 

residual, a quenching agent should be injected into the water upstream of the microorganism

injection port. The quenching agent should have a minimal impact on the UVT. 

The water supply (volume and flowrate) should be sufficient to allow testing over the 

rated flow range of the UV reactor. A flow-control device (e.g., variable speed pump or valve) 

can be used to vary the flow over that range. A flowmeter with a known measurement 

uncertainty should monitor the flowrate through the UV reactor. 

Backflow prevention should be used with a potable water supply. Backflow prevention 

can be obtained using reduced pressure zone (RPZ) backflow preventers, air gaps, or check 

valves. 

C.3.1.2 Dosing of Additives 

Challenge microorganisms, UV-absorbing compounds, and possibly disinfectant 

quenching agents may be injected into the flow upstream of the UV reactor during validation. If 

pumps are used to inject the additives, they should provide a pulseless flowrate or have a cycle 

time an order of magnitude less than the residence time of the reactor. The flowrate generated by 

the pump should be stable over the time required to take samples as per section C.4.9.5. An 

injection port using standardized injector technologies can be used to disperse the additives into 

the flow. 

C.3.1.3 Mixing of Reactor Influent and Effluent 

Additives passed through the reactor should be well-mixed through the cross-section of 

the pipe prior to the reactor influent sampling port. The challenge microorganisms surviving UV 

disinfection should be well-mixed through the pipe cross-section prior to the reactor effluent 

sampling port. Mixing can be achieved either using static mixers or by relying on the turbulent 

mixing present in the lengths of pipe upstream of the sampling ports. If the water passed through 

the UV reactor is obtained from a large tank, the additives can be premixed in the tank to obtain 

a uniform concentration for testing. 

C.3.1.4 Sample Taps 

The sample taps should be located to provide representative samples of undisinfected 

water entering the reactor and the disinfected water leaving the UV reactor. If the influent 

sample tap is located too close to the reactor influent, the samples collected may be exposed to 

UV light, resulting in underestimation of the influent concentration of the challenge 

microorganism. If the effluent sample tap is located too close to the reactor effluent, the effluent 

samples will be collected before full exposure to UV light and the effluent concentration of the 

challenge microorganism will be overestimated. The UVT of the water can be used to calculate 

how far UV light from the reactor penetrates the water upstream and downstream of the reactor. 

The sampling points should be located far enough from the UV reactor that the germicidal UV 

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UV Disinfection Guidance Manual C-13 June 2003

intensity at the point of sampling is less than 0.1 percent of the germicidal intensity within the 

UV reactor.

Sample taps may sample from a single point within the flow or from multiple points at 

the same time. Samples taken from multiple points within the flow should have the same

concentration of additives and microorganisms within the measurement error. 

Sampling taps should remain open over the duration of the test. Sample collection should 

meet standards of good practice as defined by Standard Methods Section 9060 (APHA et al. 

1995). Samples should be collected in bottles that have been cleaned and sterilized. Samples 

collected should be immediately stored on ice within a cooler in the dark until needed for 

analysis. 

C.3.1.5 UV Reactor Inlet and Outlet Conditions 

As stated previously, the inlet and outlet structures to the UV reactor during validation 

should result in equal or worse dose delivery than with the reactor installed at the WTP. EPA 

recommends using any one or combination of the following approaches: 

• Inlet and outlet conditions used at the WTP match those used during validation for at 

least 10 pipe diameters upstream and 5 pipe diameters downstream of the reactor. 

• UV reactor is validated either with a 90-degree bend immediately upstream of the 

reactor inlet or a with 90-degree bend followed by a length of straight pipe 

immediately upstream of the reactor inlet. The reactor is installed at the WTP with a 

length of straight pipe immediately upstream of the reactor equal to 5 pipe diameters 

plus any length used after the 90-degree bend during validation. To avoid jetting 

effects, piping upstream of the straight pipe length should not have expansions for at 

least 10 pipe diameters and any valves located in that length of pipe should always be 

fully open during operation of the reactor. With this approach, it is assumed that the 

90-degree bend immediately upstream of the reactor inlet provides worse hydraulics 

than the installation. This approach assumes that the reactor design has not been 

optimized for the 90-degree bend inlet. 

• Velocity of the water measured at evenly-spaced points through a given cross section 

of the flow upstream and downstream of the reactor is within 20 percent of the 

theoretical velocity with both the validation test stand and the installation. The 

theoretical velocity is defined as the flowrate divided by the cross-sectional area. 

CFD-based dose modeling can be used, in tandem with one of the above-mentioned 

approaches, to show that dose delivery with the installation is better than dose delivery during 

validation for given conditions of flowrate, UVT, and lamp output. To account for uncertainty in 

CFD predictions of dose delivery (Petri and Olson 2001, Wright and Hargreaves 2002), CFD 

predictions of dose delivery during validation should be at least 20 percent greater than 

predictions of dose delivery at the WTP. 

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UV Disinfection Guidance Manual C-14 June 2003

C.3.1.6 Quality Assurance and Quality Control 

Flowmeters, injection pumps, pressure gauges, and other measuring devices used should 

bear evidence of being in calibration. Accuracy of instrumentation should be checked by 

comparison with standard measurements. The documentation describing the test facility should 

be provided and verified including the following items: 

• A description of the validation test stand, including all piping, valves, flowmeters, 

mixers, pumps, sampling locations, and measurement instrumentation 

• The measurement uncertainty and the last calibration date of all measurement 

instrumentation 

• Comparisons of on-line instrumentation with standard measurements 

C.3.2 UV Intensity Sensor Test Stand 

The properties of the on-line and reference UV intensity sensors should be measured by 

an independent laboratory that is equipped to confirm sensor calibration and measure the 

sensor’s angular and spectral response, linearity over the working range, and temperature 

response. Measurements should be National Institute of Standards and Technology (NIST) 

traceable or equivalent with quantified measurement uncertainties. Personnel who test UV 

intensity sensors should be qualified to undertake optical testing, understand the test protocols 

for the sensors as provided by the manufacturer, and be aware of all safety requirements 

associated with UV-irradiation devices. 

C.3.3 Third-Party Oversight 

Validation of UV reactors and their components should be conducted at facilities and by 

personnel that are acceptable to the State. At a minimum, personnel independent of the 

manufacturer of the UV reactor should oversee validation testing. A registered professional 

engineer with knowledge and experience in testing and evaluating UV reactors should witness 

the validation testing to verify that the documented validation protocol was followed and the 

reported data and results are accurate. The engineer should be responsible for supervising the 

preparation of the engineering report on validation testing and should review and approve that 

report prior to its release. The engineer should not have a personal stake in the outcome of the 

validation testing or any conflict of interest with respect to the ultimate use of the UV reactor 

being tested. Where necessary, the engineer should use other third parties to provide expert 

opinion on various aspects of UV validation testing. 

C.4 Testing 

This section describes the recommended steps for validating the UV reactor provided by 

the UV manufacturer. At the discretion of the State, variations or alternatives to the procedures 

or steps may be accepted.

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UV Disinfection Guidance Manual C-15 June 2003

C.4.1 Develop Approved Test Plan 

The first step in validating a UV reactor should be the development and review of a test 

plan. The test plan should be developed with input and approval from the utility, manufacturer, 

third party oversight, and the State. The test plan should resolve the questions identified in 

section C.1.4. 

C.4.2 UV Reactor Inspection 

Prior to installing the UV reactor in the biodosimetry test stand, the UV reactor should be 

inspected to confirm that it matches the descriptions and dimensions provided in the 

manufacturer's documentation as described in section C.2.2. 

C.4.3 UV Reactor Installation 

The UV reactor and its inlet and outlet piping should be installed at the test facility in 

accordance with the manufacturer's installation and assembly instructions. If reactors are 

installed in series, the piping between the reactors should conform to specifications provided by 

the UV reactor manufacturer. The piping should be inspected to ensure compliance with the 

manufacturer's documentation. 

C.4.4 Headloss and Integrity Evaluation 

The physical integrity of the UV reactor and the test train should be checked before 

conducting further testing. Personnel who operate the UV reactor during all tests should be 

familiar with its operation and maintenance manual and with any safety requirements. 

Procedure 

1. Pass water through the reactor at the minimum and maximum flowrates. 

2. Measure and record the headloss across the reactor at each flowrate. 

3. On completion of the test, visually inspect the sleeves, UV intensity sensors, and/or 

monitoring windows for mechanical integrity. 

4. If the headloss across the reactor exceeds specifications provided by the manufacturer, or

if component integrity has been compromised, investigate the cause and resolve the issue

before further testing.

C.4.5 Evaluation of the Mixing of Additives 

The mixing of the UV-absorbing chemical and the challenge microorganism prior to 

entering the UV reactor should be confirmed. Mixing can be confirmed by comparing the UV 

absorbance of the water at 254 nm (A254) of samples collected at the influent and effluent 

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UV Disinfection Guidance Manual C-16 June 2003

sampling ports using the following procedure. This test should not be necessary if a static mixer 

is used between the injection port and the reactor entrance and the flowrate through the static 

mixer meets manufacturer specifications.

Procedure 

1. Prepare a stock solution of the UV-absorbing compound. 

2. Pass water through the reactor at the minimum flowrate. 

3. Inject sufficient UV-absorbing compound into the flow of water passing through the 

reactor to give a UVT less than the minimum that will be used during challenge 

testing. 

4. Collect water samples from the influent and effluent sampling ports at 1-minute 

intervals and measure the UVT. The sample volume should be less than 5 mL and 

collected over a time not exceeding 2 seconds. 

5. Calculate the A254 from the measured UVT. Mixing of the injected compounds 

should be sufficient if the average A254 of the influent samples and the average A254

of the effluent samples agree within 2 percent and the standard deviation of each is 

less than 5 percent. If these conditions are not met, the mixing between the injection 

port and the influent sampling port should be increased and retested. 

C.4.6 Evaluation of the Mixing of Surviving Microorganisms 

Mixing of the surviving challenge microorganisms leaving the UV reactor should be 

confirmed. Mixing can be confirmed by comparing the challenge microorganism concentration 

of samples collected at the effluent sampling port and a sampling port downstream of the effluent 

sampling port using the following procedure. This test should not be necessary if a static mixer 

is located between the reactor exit and the effluent sampling port and the flowrate through the 

static mixer meets manufacturer specifications.

Procedure 

1. Prepare a stock solution of the challenge microorganism and a stock solution of the 

UV- absorbing compound. 

2. Pass water through the reactor at the minimum flowrate that will be used during 

challenge testing. 

3. Operate the UV reactor with the lamps power set at 100 percent. 

4. Inject the challenge microorganism into the water flowing through the reactor. 

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UV Disinfection Guidance Manual C-17 June 2003

5. Collect at least three UV-disinfected samples spaced 1 minute apart from the effluent 

sampling point and from a location at least 5 pipe diameters downstream of the 

effluent sampling point. 

6. Measure the concentration of the challenge microorganism in each sample in 

triplicate. 

7. If the concentration in the effluent samples is below the detection limit, repeat steps 2

to 6 with the UV absorber injected into the flow to reduce the dose delivery by the 

reactor. 

8. Repeat steps 3 to 7, passing the water through the reactor at the minimum flowrate 

that will be used during the challenge test. 

9. The mixing should be sufficient if there is no statistical difference at a 95 percent 

confidence level between the geometric means of the samples collected from the two 

effluent sample points. If statistical differences are observed, the mixing between the 

reactor and the effluent sampling port should be increased and the test repeated. 

C.4.7 UV Intensity Sensor Evaluation 

The measurement uncertainty of the UV intensity sensors used on the UV reactors should 

be confirmed. This may be achieved either by comparing the UV intensity sensor measurements 

made on the reactor to a reference measurement, or by measuring the properties of the sensors 

using a UV intensity sensor test stand. The following sections discuss each of these approaches. 

C.4.7.1 Assessing Uncertainty Using Reference Sensors 

If the measurement uncertainty of the reference intensity sensor is known, the following 

procedure can be used to check the uncertainty of the UV intensity sensors used during 

validation. 

Procedure 

1. Pass water through the reactor without the addition of UV-absorbing chemicals. 

2. Using at least three recently calibrated reference sensors, install each sensor on the UV

reactor at each port and record the measured UV intensity. Repeat using each duty 

sensor. If the sensors can be rotated, then measure the minimum and maximum sensor 

readings with rotation. 

3. Record the water temperature as an indicator of the operating temperature of the sensors. 

4. Repeat the test with the UVT decreased to the minimum value expected during testing.

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UV Disinfection Guidance Manual C-18 June 2003

5. For a given lamp output and UVT, the difference between the reference sensor 

measurements should follow Equation C.2: 

( ) 2

1 2

Ref 2

2

Ref 1

Ref 2

Ref 1 1 *100

I

I − ≤ σ + σ Equation C.2

 where 

I = Intensity measured by a reference sensor designated by the subscript 

σ = Measurement uncertainty of reference sensor designated by the subscript 

(%) 

6. For a given lamp output and UVT, the difference between the reference and duty sensor

measurements should follow Equation C.3: 

( ) 2

1 2

Duty

2

Ref

Ref

Duty 1 *100

I

I − ≤ σ + σ Equation C.3

where 

IRef = Intensity measured by the reference sensor 

IDuty = Intensity measured by the duty sensor 

σRef = Measurement uncertainty of the reference sensor (%) 

σDuty = Measurement uncertainty of the duty sensor (%) 

7. UV intensity sensors that do not meet these criteria should be replaced. Alternatively, the 

UV manufacturer can re-evaluate their stated measurement uncertainty and use a higher 

value. 

C.4.7.2 Assessing Uncertainty Using a Sensor Test Stand 

The measurement uncertainty of the UV intensity sensors can be assessed by a laboratory 

capable of confirming sensor calibration and properties with a known measurement uncertainty. 

The laboratory should measure linearity, spectral and angular response, and temperature 

response. Results should be used to calculate the measurement uncertainty. Sensors that do not 

meet manufacturer specifications should be replaced. Alternatively, the UV manufacturer can 

re-evaluate their stated measurement uncertainty and use a higher value. 

C.4.8 Evaluation of Lamp and Sleeve Aging on Dose Monitoring

With operation over time, UV lamps and sleeves can experience non-uniform aging along 

their length and around their circumference. Lamps can also experience spectral shifts in output 

and sleeves can experience spectral shifts in UV transmittance. If these effects have a significant 

impact on how the dose delivery indicated by the monitoring system compares to the delivered 

dose, validation should be conducted using both new and aged lamps and sleeves. The following 

procedure compares dose delivery monitoring with new and aged lamps to identify if validation 

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Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-19 June 2003

should be conducted with both new and aged lamps and sleeves. Alternatively, data on the UV 

output of new and aged lamps and the UV transmittance of new and aged sleeves can be 

compared and used to demonstrate if validation should be conducted with new and aged lamps 

and sleeves. In both approaches, an aged lamp or sleeve is one that has reached the end of its 

useful service life. 

Procedure 

1. Prepare a stock solution of the challenge microorganism. 

2. Fit the UV reactor with aged lamps and sleeves.

3. Pass water through the reactor at a constant UVT and at the maximum flowrate that will 

be used during challenge testing. 

4. Operate the UV reactor at peak lamp power. 

5. Inject the challenge microorganism into the flow passing through the reactor. 

6. Collect at least three microbiological samples spaced one minute apart from the influent 

and effluent sampling ports. 

7. Record the UV intensity sensor measurements. 

8. Fit the UV reactor with new lamps that have undergone 100-hour burn-in and new 

sleeves. 

9. Lower the lamp power to give a UV intensity sensor reading equivalent to the reading 

obtained in step 7. 

10. Repeat steps 5 and 6. 

11. If the mean log inactivation achieved with new lamps differs from the mean log 

inactivation achieved with aged lamps, lamp aging impacts the relationship between dose 

delivery and UV intensity sensor reading, and validation with aged lamps and sleeves 

should be considered.

C.4.9 Dose Delivery Validation 

Dose delivery validation via biodosimetry provides an assessment of dose delivery and 

monitoring by the UV reactor under specific conditions of flowrate, UVT, and lamp output. 

C.4.9.1 Preparation of Challenge Microorganism Stock Solution 

The challenge microorganism is used to measure the dose delivery of the UV reactor 

during validation. Because MS2 and B. subtilis spores are typically used, methods for their 

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UV Disinfection Guidance Manual C-20 June 2003

preparation and assay are provided in this manual in Appendix D. Other peer-reviewed methods 

may be used. A rationale for selecting challenge microorganisms other than MS2 and B. subtilis

spores is provided in section F.1. 

The challenge microorganism stock solution should be prepared in accordance with peerreviewed methods. The source of the challenge microorganism, the source of the host (if used), 

a description of all media used, the steps involved in propagating the challenge microorganism, 

and the steps involved in purifying the challenge microorganism to create a mono-disperse stock 

solution should be documented. The volume of stock solution needed should be estimated prior 

to testing based on the test plan and the expected stock concentration. 

C.4.9.2 Reactor Preparation 

If the number of sensors is less than the number of lamps, the UV intensity sensors 

should be directly monitoring the lamps with the highest output and those lamps should be the 

closest lamps to the sensor. The lamps with the highest output can be identified by taking 

measurements using either a dedicated test stand or the UV reactor. One approach for using the 

UV reactor is described below. This preparation should not be necessary if the UV reactor has 

one UV intensity sensor per lamp.

 

Procedure 

1. Install a lamp within a lamp sleeve located near one of the reactor's UV intensity 

sensors. 

2. Pass water through the reactor at a constant flowrate and UVT. 

3. With only the lamp under evaluation on, record the measured UV intensity. 

4. Repeat the test for each lamp and rank the results. 

5. Install the lamps in the UV reactor so that the lamps with the highest output are 

closest to the UV intensity sensors monitoring those lamps. 

C.4.9.3 Flowrates 

At a minimum, the reactor should be validated at the minimum and maximum flowrates 

as defined by the UV manufacturer. Other flowrates within that range can be tested. For 

interpolation of validation results as a function of flowrate, a recommended approach for 

selecting intermediate flowrates is to approximate a geometric series using Equation C.4: 

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UV Disinfection Guidance Manual C-21 June 2003

n Qn QMax− = 1 β Equation C.4

where 

Qn = nth flowrate to be tested 

Qmax = Maximum flowrate to be tested 

β = Rate term with a recommended value between 1.5 and 2 

n = Number of flowrates to be tested 

The value of β should not exceed 2 and should be sufficient to obtain at least three measured data 

points for interpolation. 

Example. Interpolation will be used to predict RED as a function of flowrate for a UV 

reactor rated over a flow range of 2 to 20 mgd. If a rate term of 2 was used with Equation C.4, 

the UV reactor would be validated at flowrates of 20, 10, 5, 2.5, and 2 mgd. If a rate term of 1.5 

was used with Equation C.4, the UV reactor would be validated at flowrates of 20, 13, 8.9, 5.9, 

4.0, 2.6, and 2 mgd. 

C.4.9.4 Lamp Power and UV Transmittance 

At a given flowrate, the UV reactor should be validated under conditions of UVT and 

lamp output that demonstrate the UV reactor is sized to deliver a given dose and the UV

reactor’s dose monitoring system provides a valid measure of that dose. Typically, the UVT of 

the source water used during validation is high and UV absorbing chemicals are added to that 

water to achieve the UVT used during validation testing. Different levels of lamp output can be 

obtained using one or more of the following approaches: 

• Using new and aged lamps 

• Using different lamp types with the same spectral output (e.g., using LP and LPHO 

lamps) 

• Changing the ballasts’ power settings

• Using specially modified ballasts capable of operating at different power levels 

• Changing the supply voltage to the lamp ballasts 

If lamp aging affects the relationship between the inactivation achieved by the UV 

reactor and the measurements made by the on-line UV intensity sensor, aged lamps should be 

used when validation testing involves reduced lamp output. 

The conditions of lamp power and UVT used during validation should depend on the 

monitoring approach of the UV reactor. The next three sections describe recommended 

approaches for defining these test conditions for UV reactors that use the following monitoring 

approaches: 

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UV Disinfection Guidance Manual C-22 June 2003

• UV intensity setpoint approach 

• UV intensity and UVT setpoint approach 

• Calculated dose approach 

Section F.2 provides background on the development of these approaches. 

UV Intensity Setpoint Approach

With the UV intensity setpoint approach, measurements of UV intensity and flowrate are 

used directly to indicate dose delivery. Dose delivery at or above a given level is indicated when 

the measured intensity reads above an alarm setpoint value defined as a function of flowrate. 

With the UV intensity setpoint approach, the UV intensity sensor is positioned within the 

UV reactor to respond to the impacts of both lamp output and UVT. As such, dose delivery can 

be monitored without the need to measure the UVT. 

Strategies for implementing this approach include: 

1. Using a single UV intensity setpoint value from minimum to maximum flow to verify 

dose delivery at some minimum level. 

Example. A UV intensity setpoint of 10 mW/cm2 is used to verify a minimum

MS2 RED of 40 mJ/cm2

 from 1 to 5 mgd. 

2. Several UV intensity setpoint values are used, each one applying over a specific flow 

range. 

Example. UV intensity setpoints of 10 and 20 mW/cm2 are used to verify a 

minimum MS2 RED of 40 mJ/cm2

 from 1 to 2.5 mgd and from 2.5 to 5 mgd, 

respectively.

3. UV intensity setpoint values are interpolated as a function of flowrate. 

Example. UV intensity setpoints defined by the following equation are used to 

indicate an MS2 dose of 39 mJ/cm2

 from 1 to 2.4 mgd: 

Intensity setpoint (mW/cm2

) = 15.6 x flow rate (mgd) + 3.9 

4. UV intensity setpoints are defined as a function of flowrate for multiple levels of dose 

delivery. 

Example. A UV intensity setpoint of 10 mJ/cm2 is used to verify a minimum

RED of 40 mJ/cm2

 from 1 to 5 mgd. A UV intensity setpoint value of 7 mW/cm2

is used to verify a minimum RED of 25 mW/cm2

 from 1 to 5 mgd. 

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UV Disinfection Guidance Manual C-23 June 2003

With UV reactors using this monitoring approach, validation testing provides data on the 

relationship between dose delivery and measured intensity at a given flowrate. Dose delivery at 

a given flowrate and UV intensity is measured under two conditions of lamp power and UVT, 

described as follows: 

1. Lamps at peak power and the UVT decreased to give a UV intensity sensor reading at 

a setpoint value. 

2. High UVT and the lamp power lowered to give a UV intensity sensor reading at a 

setpoint value. 

The RED assigned to the reactor is the lower value observed between the two test 

conditions. 

If the lamp power cannot be sufficiently lowered to obtain a UV intensity sensor reading 

at the setpoint value, an alternative to the second test condition is to test with the lowest possible 

lamp power setting and the UVT decreased until an intensity reading at the setpoint is obtained. 

This alternative second test condition is acceptable if the following conditions are met: 

• The adjusted lamp power results in a lamp output equal to or lower than the lamp

output used for sizing the UV reactor for a WTP. The lamp output used for sizing the 

UV reactor is the product of the lamp-aging factor and the fouling factor.

• The RED measured with the second condition is equal to or greater than the RED 

measured with the first test condition or the UVT with the second test condition is 

less than the UVT expected at the WTP. 

There are several approaches for defining the UV intensity setpoint values evaluated 

during validation testing: 

1. If a UV reactor is being validated for an application with specific design conditions of 

flowrate, lamp output, and UVT, the intensity setpoint at design flow is equal to or 

greater than the intensity reading obtained with the reactor operating under these 

design conditions. 

2. A UV reactor manufacturer can usually provide model estimates of dose delivery as a 

function of flowrate and UV intensity. The model estimates would be used to define 

the intensity setpoint values associated with a target dose delivery. Since model 

estimates may not be accurate, trial and error testing may be used to establish the 

optimal intensity setpoint necessary for a target level of dose delivery. Alternatively, 

testing can be used to define the relation between dose delivery and measured 

intensity, and interpolation can be used to define the optimal setpoint associated with

a target dose delivery. 

During the validation of a UV reactor using the intensity setpoint monitoring approach, 

the UVT used will likely be less than the design UVT and the lamp output will be less than the 

design lamp output. While it may appear that these test conditions are more stringent than the 

design conditions, it should be recognized that design conditions do not represent the worst-case 

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conditions that can occur at a WTP. For example, lamps can age below their expected end-oflife output, lamp sleeves can foul internally, wiper mechanisms can fail, and dose-pacing

strategies can reduce lamp output. These factors in combination can result in a UV output well 

below the design output. If the design UVT is selected at a 95 percent confidence level, then a 

UVT below the design value is expected 5 percent of the time. Because intensity setpoints 

should provide a valid measure of dose delivery, regardless of the combination of lamp output 

and UVT values, a UV reactor using intensity setpoint monitoring should be validated over the 

full range of conditions giving rise to the setpoint, even if they exceed design conditions. 

Example. A UV reactor that uses the intensity setpoint approach for monitoring is sized 

using a design UVT of 90 percent, a lamp aging/fouling factor of 70 percent, and a flow of 5 

mgd. With lamp power and UVT adjusted to 70 and 90 percent, respectively, the UV intensity 

sensor reads 14 mW/cm2

. The UV reactor is tested at a flow of 5 mgd using the following 

conditions of lamp output and UVT that give rise to a UV intensity of 14 mW/cm2

: 

• 100 percent lamp power, 87 percent UVT 

• 27 percent lamp power, 98 percent UVT 

By testing the reactor using these conditions, dose delivery associated with a setpoint of

14 mW/cm2

 is validated. 

UV Intensity and UVT Setpoint Approach

With the UV intensity/UVT setpoint approach, measurements of UV intensity, UVT, and 

flowrate are used to indicate dose delivery. Dose delivery at or above a given level is indicated 

when both the measured UV intensity and UVT read above their respective alarm setpoint 

values. Strategies for implementing this approach include: 

1. Using a single UV intensity setpoint value and UVT setpoint value from minimum to 

maximum flowrate to indicate dose delivery at some level. 

Example. A minimum MS2 RED of 40 mJ/cm2 from 1 to 5 mgd is verified when 

the measured UV intensity is equal to or greater than 10 mW/cm2

 and the 

measured UVT is equal to or greater than 85 percent. 

2. Several sets of UV intensity and UVT setpoint values are used, each set applying over 

a specific flow range. 

Example. For an MS2 RED of 40 mJ/cm2, a UV intensity setpoint value of 10 

mW/cm2

 and a UVT setpoint value of 80 percent are used from 1 to 2.5 mgd. A 

UV intensity setpoint value of 20 mW/cm2

 and a UVT setpoint value of 85 

percent are used from 2.5 to 5 mgd. 

3. Sets of UV intensity and UVT setpoint values are interpolated as a function of 

flowrate. 

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4. Sets of UV intensity and UVT setpoint values are defined as a function of flowrate 

for multiple levels of dose delivery. 

With UV reactors using this monitoring approach, validation testing provides data on the 

dose delivery with the reactor operating at the setpoint values and proof that the sensor is 

appropriately positioned for this monitoring approach. As such, each set of UV intensity and 

UVT setpoints should be tested using two conditions as follows: 

1. UVT decreased to give a reading at the UVT setpoint followed by a decrease in lamp

power to give a UV intensity sensor reading at the UV intensity setpoint. 

2. Lamp power at 100 percent and UVT decreased to give a UV intensity sensor reading 

at the intensity setpoint. 

The first condition provides data on dose delivery with the reactor operating with UV 

intensity and UVT at the setpoint values. The second condition provides data on the positioning 

of the UV intensity sensor. If the RED measured with the second test condition is greater than 

the RED measured with the first, the UV intensity sensor is not appropriately positioned for this 

monitoring strategy and this monitoring strategy cannot be used (see section F.2 for a rational for 

this criteria).

There are several approaches for defining the UV intensity and UVT setpoints used 

during validation testing. 

1. At design flow, the UVT setpoint is the design UVT. The intensity setpoint is the UV

intensity measured with the lamp output and UVT adjusted to their design values. 

2. At other flowrates, model estimates of dose as a function of UVT and lamp output 

can be used to identify the setpoint values that will be assessed during validation 

testing. Trial and error testing or interpolation of test results can be used to refine and

optimize those values for a given target dose delivery. 

Example. A UV reactor that uses the UV intensity and UVT setpoint approach for 

monitoring is sized for a WTP using a design UVT of 90 percent, a design lamp fouling/aging 

factor of 70 percent, and a design flowrate of 5 mgd. Operating under those conditions, the 

intensity sensor measures 14 mW/cm2

. The UV reactor is validated under two test conditions at 

a flowrate of 5 mgd: 

• 70 percent lamp power and 90 percent UVT resulting in a UV intensity reading of 14 

mW/cm2

• 100 percent lamp power and 75 percent UVT resulting in a UV intensity reading of 

14 mW/cm2

The first condition provides data on the dose delivery of the reactor operating at the 

setpoint. The second condition provides data to assess the positioning of the UV intensity 

sensor. 

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Calculated Dose Approach

With the calculated dose approach, dose delivery is calculated from measurements of UV 

intensity, UVT, and flowrate using an algorithm developed by the UV reactor manufacturer. For 

UV reactors that use this approach, the UV reactor should be tested over a range of combinations 

of flowrate, UVT, and lamp power that result in a given calculated dose. At a given flowrate, 

that range should include the following combinations: 

• Maximum power and decreased UVT 

• Maximum UVT and decreased lamp power 

• One or two intermediate combinations of UVT and lamp power 

If the algorithm for calculating dose accounts for lamps operating at different power 

levels or specific lamps operating either on or off, test conditions should include combinations of 

these conditions. 

Example. A UV reactor that uses a calculated dose for compliance will be used at a 

WTP with a design UVT of 90 percent, a design lamp fouling/aging factor of 70 percent, and a 

design flowrate of 5 mgd. The target RED is 40 mJ/cm2

. At 5 mgd, test conditions that result in 

a calculated dose of 40 mJ/cm2

 by the monitoring system are as follows: 

• 100 percent lamp power, 80 percent UVT 

• 58 percent lamp power, 90 percent UVT 

• 34 percent lamp power, 98 percent UVT 

C.4.9.5 Measuring Challenge Microorganism Inactivation by the UV 

Reactor 

The reactor should be operated at each of the test conditions of flowrate, UVT, and lamp 

power in accordance with sections C.4.9.3 and C.4.9.4. Prior to sampling, steady-state 

conditions should be confirmed by monitoring the UV intensity sensor measurements and the 

UVT. The challenge microorganism should be injected into the flow upstream of the reactor and 

well-mixed prior to its entering the UV reactor. At least three influent and effluent samples 

should be collected for each test condition. The time interval between sample collections should 

be greater than or equal to the residence time between the inlet and outlet sampling ports. Water 

samples should be collected by personnel who are familiar with good sampling practices as 

specified in Standard Methods (APHA et al. 1995) and the guidance for collecting UV-irradiated 

samples. Sample volumes should be sufficient for assessing the challenge microorganism

concentrations in the influent and effluent. 

Before and after the samples are collected, the flowrate through the reactor, all UV 

intensity sensor measurements, on-line UVT measurements, and any calculated dose values 

should be measured and recorded. With the validation of LP or LPHO UV reactors, the UVT 

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should be measured and recorded with each influent sample. With MP reactors, the UVT from

200 to 400 nm should be measured and recorded. The electrical power delivered to the lamps by 

each ballast should also be measured and recorded. The challenge test should be repeated if the 

flowrate, UV intensity, lamp power, or UVT changes by more than the error of the measurement 

over the course of sampling. 

The challenge microorganism concentration in the samples should be measured within 24 

hours of collection using a peer-reviewed method. Suggested methods for measuring MS2 and 

B. subtilis spore concentrations in water samples are provided in Appendix D. Reported 

challenge microorganism concentrations should include dilutions, volumes used, and the number 

of plaques or colonies counted on each plate. 

C.4.9.6 Quality Assurance and Quality Control Samples 

During testing of the UV reactor, samples should be collected to ensure quality assurance 

and control (QA/QC) including: 

• Trip controls - sample bottles of challenge microorganism stock solution of known 

concentration that travel with the stock solution from the microbiological laboratory 

to the location of reactor testing and back to the laboratory. The concentration of the 

challenge microorganism in the trip controls measured at the beginning and end 

should be the same at a 90 percent confidence level. 

• Reactor blanks - influent water samples taken without any addition of challenge 

microorganism to the flow passing through the reactor. The concentration of the 

challenge microorganism measured with the blank should not interfere with the 

determination of RED delivered by the reactor. 

• Reactor controls - influent and effluent water samples taken with the UV lamps (in 

the reactor) turned off. The challenge microorganism concentrations in both samples 

should be the same at a 90 percent confidence level. 

• Method blanks - sample bottle of sterilized reagent grade water that undergoes the 

challenge microorganism assay procedure. The concentration of challenge 

microorganism with the method blank should be non-detectable. 

C.4.9.7 Challenge Microorganism Dose-Response 

The UV dose-response of the challenge microorganism within samples collected from the 

reactor influent should be measured with the collimated beam apparatus as described in 

Appendix E. At least two dose-response curves should be generated. One sample should have 

UVT unadjusted by UV-absorbing additives and one sample should have UVT adjusted to give 

the minimum UVT used in section C.4.9.4. A one-liter influent sample should be sufficient for 

measuring the challenge microorganism UV dose-response. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-28 June 2003

The collimated beam tests should be conducted within 24 hours of sample collection. 

Based on the expected dose-response of the challenge microorganism, UV doses should be 

applied to achieve log reductions of approximately 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0. For each log 

reduction, at least three aliquots of the influent sample should be irradiated. Three aliquots 

should also be collected as zero dose samples. Aliquots should be packed on ice and stored in 

the dark until they are assayed. Aliquots should be assayed within 24 hours of irradiation. 

The log inactivation for each applied dose delivered by the collimated beam should be 

calculated using Equation C.5: 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

= N

N

Log Inactivation 0 log Equation C.5

where

N0 = Average concentration of the challenge microorganism in the zero dose aliquots 

N = Challenge microorganism concentration in an aliquot of sample 

Fitting Dose-Response Data

The dose-response of the challenge microorganism should be plotted as UV dose versus 

log inactivation. An equation that best expresses the UV dose as a function of log (N0/N) should 

be obtained using regression analysis. A linear equation should best-fit first-order kinetics. A 

quadratic equation should provide a better fit with tailing, and other equations should be used if 

inactivation kinetics involves shoulders (DVGW 1997, ONORM 2001). Equation coefficients 

obtained from the regression analysis should be significant at a 95 percent confidence level. The 

differences between the values measured and predicted by the equation should be randomly 

distributed around zero and not show a dependence on dose. Confidence intervals for the fit 

should be determined at an 80 percent confidence level. The equation should be used for 

interpolating dose-response data but should not be used for extrapolation outside of the measured 

UV dose range. 

Example. The dose-response of MS2, presented in the following table, was measured 

using a collimated beam apparatus. 

UV Dose

(mJ/cm2

) 

Log 

Inactivation 

Log 

Inactivation 

0 0.016 -0.119

10 0.805 1.06

30 1.87 2.16

60 3.40 3.62

100 4.71 4.83

Regression analysis was used to fit the equations to the MS2 dose-response data: 

Dose = A× Log Inactivation + B

and 

( )

2 Dose = C × Log Inactivation + D × Log Inactivation

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-29 June 2003

The following table lists the coefficients derived from the regression analysis, the pstatistics for those coefficients, and the R-squared value for the fit. 

Equation Coefficient Value p-statistic

A 20.5 3.13 × 10-7

Dose = A× Log Inactivation + B

R-squared = 0.967 B -6.01 0.15

C 8.90 1.22 × 10-4 ( )

2 Dose = C × Log Inactivation + D × Log Inactivation

R-squared = 0.995 D 2.47 4.39 × 10-5

In evaluating the two equations, a first check was done to determine if the equation 

coefficients were significant at a 95 percent confidence level. While the R-squared value for the 

first equation was high, the p-statistic for coefficient B was greater than 0.05, indicating that it 

was not significant at a 95 percent confidence level. Thus, the first equation was not a good fit to 

the dose-response data. On the other hand, the p-statistics for coefficients C and D with the 

second equation were both less than 0.05, indicating that they were significant at a 95 percent 

confidence level. Thus, Equation 2 was a valid fit to the dose-response data. 

A second check of the two equations was to determine if the difference between the 

measured and predicted dose was randomly distributed as a function of the log inactivation. 

Figure C.3 presents the dose-response data and the fits to the data with confidence levels. As 

shown, the first equation under-predicts UV dose at low and high levels of inactivation and overpredicts dose at mid levels of dose. On the other hand, the second equation does not show a bias 

in the prediction of dose as a function of log inactivation. This second check further 

demonstrates that the second equation was a valid fit to the dose-response data while the first 

equation was not valid. 

To illustrate the importance of using an appropriate equation to fit the dose-response data, 

the following table compares the dose predicted using the two equations for 2-log inactivation. 

UV Dose for 2 log inactivation 

(mJ/cm2

) 

Equation 

Mean Lower Bound

Dose = A× Log Inactivation + B 35 30

( )

2 Dose = C × Log Inactivation + D × Log Inactivation 28 26

As shown, the first equation over-predicts the mean dose needed for 2-log inactivation by 

27 percent, as compared to the second equation. Large errors can occur predicting the UV dose 

associated with a given log inactivation if the equation used to fit the data is not appropriate. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-30 June 2003

Figure C.3 UV Dose Plotted as a Function of MS2 

Log Inactivation and Fitted Using Two Equations 

Combining Dose-Response Data

During validation, the UV dose-response of the challenge microorganism is used to relate 

the inactivation measured through the reactor under each test condition to an RED value. 

Typically, it is assumed that the dose-response measured with a subset of the test conditions 

assessed during validation can be used to calculate the RED for all test conditions. This 

assumption is valid if the dose-response of the challenge microorganism does not vary from test 

condition to test condition. To prove this assumption, the regression coefficients generated for 

each set of dose-response data should be equal at a 95 percent confidence level (Draper and 

Smith 1981). If the coefficients are the same, the equation fitting the combined dataset should be 

used for determining the RED. If the coefficients are different, the cause of the difference should 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-31 June 2003

be determined. Difference in UV dose-response could occur if the dose-response was 

determined with different batches of the challenge microorganism or if water quality 

interferences are impacting the dose-response (e.g., MS2 coagulation). The following example 

presents an approach that can be used to determine if two sets of UV dose-response data can be 

combined. 

Example. The following table gives the dose-response data for MS2 measured on two 

influent samples during validation testing. 

UV Dose Log Inactivation (N0/N) 

(mJ/cm2) Influent Sample 1 Influent Sample 2 

0 0.02 0.09 0.24 -0.10

10 0.33 0.709 0.54 0.40

20 1.1 1.4 1.0 1.4

40 1.8 2.4 2.3 2.3

60 2.7 3.2 3.2 3.3

80 3.5 4.4 3.4 3.9

100 3.9 4.4 4.3 4.8

Each dataset can be described using the following equation: 

2

0 0

N

N B Log NN Dose A Log ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ⎟

⎠

⎞ ⎜

⎝

⎛ ⎟ + ×

⎠

⎞ ⎜

⎝

⎛ = ×

To determine if the two datasets could be combined, a general equation is defined for 

both datasets as: 

2

0 0

2

0 0

N

N D d Log NN C d Log NN B Log NN Dose A Log ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ⎟

⎠

⎞ ⎜

⎝

⎛ ⎟ + × ×

⎠

⎞ ⎜

⎝

⎛ + × × ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ⎟

⎠

⎞ ⎜

⎝

⎛ ⎟ + ×

⎠

⎞ ⎜

⎝

⎛ = ×

The term d is set to zero with the first dataset and set to one with the second dataset. 

Multiple regression analysis using the full dataset is used to determine the values of coefficients 

A, B, C and D with the following results: 

Coefficient Value p-statistic

A 17.5 0.000

B 1.03 0.202

C -2.43 0.553

D 0.435 0.689

As shown by the p-statistic, the term A was statistically significant at the 95 percent 

confidence level (p ≤ 0.05) and the terms B, C, and D were not (p ≥ 0.05). The regression 

analysis was repeated in a step-wise fashion, removing the term with the highest p-value from

the equation. With the second regression, the terms A and B were statistically significant and the 

term C was not. With the third regression, the terms A and B were both statistically significant. 

Because neither terms C nor D were significant, it can be concluded that the regression

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-32 June 2003

coefficients generated by the fits to each dose-response are equal at a 95 percent confidence 

level. Thus, the two datasets can be combined. 

C.4.9.8 Reactor Log Inactivation and RED 

For each condition of flowrate, UVT, and lamp output as defined in sections C.4.9.3 and 

C.4.9.4, the arithmetic mean and standard deviation of the log of the influent and effluent 

challenge microorganism concentrations should be calculated. For each test condition, the log 

inactivation should be calculated using equation C.6: 

( ) ( I NE Log Inactivation = log N − log ) Equation C.6

where 

log(NI) = Mean challenge microorganism log concentration of the influent samples 

log(NE) = Mean challenge microorganism log concentration of the effluent samples 

The uncertainty of the log inactivation should be calculated using Equation C.7: 

( ) ( )

%

Log Inactivation

n

t

n

t

E

E E

I

I I

100

2

1

2 2

×

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

=

σ σ

Uin Equation C.7

where 

Uin = Percent uncertainty of the log inactivation through the UV reactor

tI = t-statistic of the influent samples at an 80 percent confidence level 

σI = Standard deviation of the challenge microorganism log concentration of the 

influent samples 

nI = Number of influent samples 

tE = t-statistic of the effluent samples at an 80 percent confidence level 

σE = Standard deviation of the challenge microorganism log concentration of the 

effluent samples 

nE = Number of effluent samples 

The RED should be calculated from the log inactivation using the equation describing the 

UV dose-response curve of the challenge microorganism. The percent measurement uncertainty 

of the RED can be calculated using Equation C.8: 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-33 June 2003

( ) 2

1 2

D

2

DR

2 URED = Uin + U + U Equation C.8 

where 

URED = Percent uncertainty of the measured RED 

UDR = Percent uncertainty of the regression equation fitting the challenge 

microorganism’s UV dose-response data at an 80 percent confidence level (see 

section C.4.8.7, Figure C.3) 

UD = Percent uncertainty of the collimated beam dose calculation that is not captured in 

the variability of the measured dose-response data (see Appendix E). This 

typically includes the uncertainty of the radiometer and the Petri factor 

Example. A UV reactor was validated using MS2. The UV dose-response measured 

using a collimated beam apparatus is given in Figure C.3. The dose-response was fitted using 

the following equation:

( )

2 Dose = 8.90× Log Inactivation + 2.47 × Log Inactivation

The uncertainty of the radiometer used with the collimated beam apparatus was 8 percent. 

The Petri factor was measured with an uncertainty of 2 percent. Thus the uncertainty of the 

collimated beam dose calculation, UD is calculated as follows: 

U ( ) 8 2 2 8.2%

1 2 2

D = + =

The following table presents the microbiology results obtained with the influent and 

effluent samples collected with one of the test conditions assessed during validation. 

The mean and standard deviation of the influent and effluent log concentrations of the 

MS2 are 6.32 ± 0.075 and 4.26 ± 0.13, respectively. The log inactivation through the reactor is 

calculated as follows: 

Log Inactivation = 6.32 − 4.26 = 2.06

Table C.2 Estimated Log Inactivation and Corresponding RED Values Using 

Bioassay Results 

Plate Counts - Dilution = 104 Plate Counts - Dilution = 105 Before UV Concentration 

1 2 3 1 2 3 PFU/mL log

Sample 1 148 180 TNTC 15 18 20 1.77×106 6.24 

Sample 2 173 TNTC TNTC 11 32 22 2.17×106 6.33 

Sample 3 TNTC 192 150 37 15 22 2.47×106 6.39 

Plate Counts - Dilution = 102 Plate Counts - Dilution = 103 After UV Concentration 

1 2 3 1 2 3 PFU/mL Log

Sample 1 166 181 TNTC 17 18 42 2.57×104 4.40 

Sample 2 133 TNTC 101 13 28 10 1.70×104 4.23 

Sample 3 165 141 123 17 14 12 1.43×104 4.15 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-34 June 2003

The t-statistic for 3 samples and an 80 percent confidence level is 1.88. The percent 

uncertainty of the log inactivation is calculated as follows: 

( ) ( )

. % .

. . . .

Uin 100 7 91

2 06

3

1 88 0 13

3

1 88 0 075 2

1

2 2

× =

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ×

+

×

=

The RED associated with a log inactivation of 2.06 is calculated as follows: 

( )2 2 Dose = 8.90 × 2.06 + 2.47 × 2.06 = 28.8mJ/cm

The percent uncertainty of the regression equation, UDR, at a log inactivation of 2.06 is 6 percent. 

The percent uncertainty of the RED is calculated as follows: 

U ( ) 7.9 6 8.2 2 12.9%

1 2 2 2

RED = + + =

 

C.4.9.9 Interpretation of Results

Interpretation of the results should depend on the monitoring approach used to guarantee 

dose delivery: 

• With the UV intensity setpoint approach, the UV reactor should be rated at the lowest 

inactivation observed for each setpoint condition evaluated. 

• With the UV intensity and UVT setpoint approach, the UV reactor should be rated at 

the inactivation observed with UV reactor operation under setpoint conditions. 

• With the calculated dose approach, the UV reactor should be rated at the lowest 

inactivation observed for each calculated dose setpoint evaluated. 

C.4.9.10 Interpolation of Results

The RED measured by validation testing can be interpolated as a function of flowrate, 

UVT, and UV intensity by fitting an equation to the data being interpolated. If the RED is 

interpolated as a function of the measured intensity or the inverse flowrate, the equation used 

should pass through the origin (0,0). The equation coefficients should be significant at a 95 

percent confidence level. The differences between the values measured and predicted by the 

equation should be randomly distributed around zero. The equation should be used for 

interpolating between measured data but should not be used for extrapolation. 

The uncertainty of the equation used to interpolate the RED should be assessed by 

determining the 80 percent confidence level. If significant, the uncertainty should be included as 

an uncertainty term in the determination of the expanded uncertainty, as described in section 

C.4.10.2.3. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-35 June 2003

C.4.10 Determining Inactivation Credit 

This guidance presents two approaches, termed Tier 1 and Tier 2, which can be used to 

relate the RED demonstrated during reactor validation to target pathogen inactivation. Other 

approaches or modifications to this approach may be used at the discretion of the State. 

With both approaches, the RED demonstrated during validation should be equal to or 

greater than a target RED that is related to the dose tables in Chapter 1 using a safety factor. 

With Tier 1, fixed safety factors have been defined and applied to the dose tables in Chapter 1 to 

define target RED values. The Tier 1 safety factors are based on specific Tier 1 criteria for the 

UV reactor and its validation protocol. The Tier 1 approach can be used with a given UV reactor 

provided it meets all the Tier 1 criteria. With Tier 2, the safety factors are calculated based on 

the validation results for, and certain properties of, the UV reactor that are calculated from the 

validation results and certain properties of the UV reactor undergoing validation. 

C.4.10.1 Tier 1 Approach 

For a UV reactor using LP or LPHO lamps, Table C.3 presents the Tier 1 RED values 

that should be demonstrated during validation to achieve the specified log-inactivation credits for 

Cryptosporidium, Giardia, and virus. Table C.4 presents the Tier 1 RED values for MP reactors. 

The Tier 1 RED values are applicable with all UV reactors that meet the Tier 1 criteria provided 

in this section. 

Example. To receive 2.5 log Cryptosporidium inactivation credit, a LP reactor under 

Tier 1 should demonstrate an RED of 28 mJ/cm2

. 

Table C.3 Tier 1 RED Targets for UV Reactors with LP or LPHO Lamps 

RED Target (mJ/cm2 Log )

Inactivation 

Credit 

Cryptosporidium Giardia Virus

0.5 6.8 6.6 55

1.0 11 9.7 81

1.5 15 13 110

2.0 21 20 139

2.5 28 26 169

3.0 36 34 199

3.5 - - 227

4.0 - - 259

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-36 June 2003

Table C.4 Tier 1 RED Targets for UV Reactors with MP Lamps 

RED Target (mJ/cm2 Log )

Inactivation 

Credit 

Cryptosporidium Giardia Virus

0.5 7.7 7.5 63

1.0 12 11 94

1.5 17 15 128

2.0 24 23 161

2.5 32 30 195

3.0 42 40 231

3.5 - - 263

4.0 - - 300

Tier 1 criteria for the UV reactor are as follows: 

• UV reactors equipped with MP lamps should be equipped with one sensor per lamp. 

UV reactors equipped with LP or LPHO lamps should be equipped with at least one 

sensor per bank of lamps. 

• The standard deviation of the UV output of LP or LPHO lamps should be 15 percent 

or less of the mean output. The standard deviation should be determined using either 

life test or field data on aged lamps. 

• UV intensity sensors should view a point along the length of the lamp that is within 

25 percent of the arc length away from the electrode. 

• UV intensity sensors should have a spectral response that peaks between 250 and 280 

nm. When mounted on the UV reactor and viewing the lamps through water, the 

measurement of UV light greater than 300 nm made by the sensor should be less than 

10 percent of the total measurement made by the sensor. Conformance to these 

criteria can be demonstrated using UV intensity field modeling. Figure C.4 presents 

an example of how two sensors would conform to this criterion. 

• The UV intensity sensors used during validation and the duty and reference sensors 

used during operation of the UV reactor at the WTP should provide NIST traceable 

measurements with a measurement uncertainty of ± 15 percent or less at an 80 

percent confidence level.

• During operation of the UV reactor at the WTP, measurements made by the duty UV 

intensity sensor should be checked using a reference UV intensity sensor. The 

difference between the measurement made by the duty and reference sensors should 

meet the following criteria: 

 ( ) 2

1 2

Duty

2

Ref

Ref

Duty 1 100

I

I

× ≤ σ + σ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − Equation C.9 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-37 June 2003

Figure C.4 Comparison of the Spectral Response of Two UV Intensity Sensors 

Estimated Using UV Intensity Field Modeling 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

200 250 300 350 400

Wavelength (nm)

S

pectr

al R

esp

o

n

se 

R

elative t

o 254 n

m

Unfiltered 

Sensor

Filtered Sensor

250 280

Filtered Sensor. Detected UV light with a 0 cm 

sensor-to-lamp water layer. Detected UV >

300 nm is 0.7% of total UV light detected. 

Unfiltered Sensor. Detected UV light with a 0 

cm sensor-to-lamp water layer. Detected UV >

300 nm is 41% of total UV light detected.

0

5

10

15

20

25

200 250 300 350 400

Wavelength (nm)

D

etect

e

d Irra

diance

0

10

20

30

40

50

200 250 300 350 400

Wavelength (nm)

D

etect

e

d Irra

diance

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

200 250 300 350 400

Wavelength (nm)

D

etect

e

d Irra

diance

0.0000

0.0100

0.0200

0.0300

0.0400

200 250 300 350 400

Wavelength (nm)

D

etect

e

d Irra

diance

Unfiltered Sensor. Detected UV light with a 20 

cm sensor-to-lamp water layer. Detected UV >

300 nm is 85% of total UV light detected.

Filtered Sensor. Detected UV light with a 20 

cm sensor-to-lamp water layer. Detected UV >

300 nm is 5% of total UV light detected. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-38 June 2003

• If the dose monitoring strategy uses an on-line UVT monitor, the A254 calculated from 

the measured UVT should have a measurement uncertainty of ± 10 percent or less at 

an 80 percent confidence level. 

Tier 1 criteria for the flow measurements are as follows: 

• The flow measurements during validation and during operation of the UV reactor at 

the WTP should have a measurement uncertainty of ± 5 percent or less at an 80 

percent confidence level.

Tier 1 criteria for the collimated beam apparatus are as follows: 

• The calculated dose delivered by the collimated beam apparatus should have a 

measurement uncertainty of ± 15 percent or less at an 80 percent confidence level. 

Tier 1 criteria for the challenge microorganism dose-response are as follows: 

• Over the range of doses within one log of the log reduction demonstrated during 

validation, the UV sensitivity of the challenge microorganism should be less than or 

equal to 25 mJ/cm2

 per log inactivation (the dose-response of a resistant strain of 

MS2). For example, if you measure log inactivation values between 1.5 and 3.5 log, 

the test organism you use should have a dose-response less than or equal to 25 

mJ/cm2

 per log inactivation between 0.5 and 4.5 log inactivation. 

• If the dose-response of the challenge microorganism has a shoulder, that shoulder 

should not occur over a dose range greater than 50 percent of the RED demonstrated 

during validation. The shoulder is defined by extrapolating the exponential reduction 

region of the dose-response curve to the dose-axis. 

• If the dose-response demonstrates tailing, the tailing should not occur until one log 

reduction greater than the highest log reduction demonstrated during validation. 

Tier 1 criteria for the UVT used for validating UV reactors using medium-pressure lamps 

are as follows: 

• The UVT at 254 nm of the water during validation should be greater than the values 

specified in Figure C.5 for a given sensor-to-lamp water layer and UV-absorbing 

chemical (the polychromatic bias should be 1.0). The sensor-to-lamp water layer is 

defined as the distance traveled through water by UV light passing from the lamp to 

the sensor. The values in Figure C.5 were taken from Figure C.7 for a polychromatic 

bias of 1.2. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-39 June 2003

Figure C.5 Criteria for the Minimum UVT of MP UV Reactors under Tier 1 

Tier 1 criteria for the challenge microorganism dose-response data are as follows: 

• A plot of dose versus log inactivation should have an 80 percent confidence level of 

10 percent or less at the log inactivation demonstrated by the UV reactor.

Tier 1 criteria for the challenge microorganism measurements through the reactor are as

follows: 

• Five influent and five effluent samples should be collected per test condition 

evaluated as per section C.4.9.5. 

• The standard deviation of the challenge microorganism concentration measured with 

the influent and the effluent samples should be less than or equal to 0.20 log. 

Tier 1 criteria for the interpolation of challenge microbe results are as follows: 

• The uncertainty of the interpolation should be 10 percent or less at an 80 percent 

confidence level. 

C.4.10.2 Tier 2 Approach 

The safety factor used to relate the RED demonstrated during validation to the dose 

required to inactivate the target pathogen should be defined using Equation C.10: 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-40 June 2003

SF B B ( e = RED × Poly × 1+ ) Equation C.10

where 

SF = Safety Factor 

BRED = RED bias 

BPoly = Polychromatic bias

e = Expanded uncertainty as a fraction 

The following sections describe an approach for defining each of these terms. 

Determining the RED Bias

If a single challenge microorganism is used to demonstrate dose delivery during 

validation, the RED bias should be determined using Figure C.6 and the following procedure. 

(Section F.1 provides the background on the development of Figure C.6 and the procedure for 

determining the RED bias.) 

Procedure 

1. Calculate the UV sensitivity of the target pathogen as the dose requirement specified 

in Chapter 1 divided by the corresponding log inactivation credit. 

2. Calculate the UV sensitivity of the challenge microorganism as the calculated RED 

divided by the log inactivation. 

3. If the target pathogen is more resistant to UV light than the challenge microorganism, 

the RED bias equals 1.0. Otherwise, calculate the RED bias using Equation C.11: 

P

C

RED

RED

RED Bias = Equation C.11

where 

REDC = RED of the challenge microorganism obtained from Figure C.6 

REDP = RED of the target pathogen obtained from Figure C.6 

Example. An MS2 inactivation of 2 log corresponding to an RED of 36 mJ/cm2 is 

measured during validation. A 2-log Cryptosporidium credit is required. The UV dose required 

to achieve that level of inactivation from Chapter 1 is 5.8 mJ/cm2

. Thus, the UV sensitivity of

MS2 and Cryptosporidium is defined as 36/2.0 = 18 and 5.8/2.0 = 2.9 mJ/cm2

 per log 

inactivation, respectively. Because MS2 is more resistant than Cryptosporidium, the RED bias is 

greater than one. In Figure C.6, REDs of 19 and 8.2 correspond to UV sensitivities of 18 and 2.9 

mJ/cm2

 per log, respectively. Thus, using Equation C.11, the RED bias is 19/8.2 = 2.3. 

Example. An MS2 inactivation of 4 log and a corresponding RED of 80 mJ/cm2 is 

measured during validation. A 2.0-log adenovirus credit requiring a dose of 100 mJ/cm2

 is 

required. Thus, the UV sensitivity of the challenge microorganism and pathogen are 20 and 50 

mJ/cm2

 per log inactivation, respectively. Because the UV sensitivity of adenovirus is greater 

than that of the challenge microorganism, the RED bias equals 1.0. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-41 June 2003

Figure C.6 RED versus Microorganism UV Sensitivity 

for Use in Determining the RED Bias 

0

5

10

15

20

25

30

0 10 20 30 40 5

UV Sensitivity (mJ/cm2

 per log inactivation)

R

eduction Equi

v

alent

D

o

s

e

(mJ/cm2

)

0

If two challenge microorganisms with different UV sensitivities are used during 

validation to demonstrate dose delivery, the RED delivered to the target pathogen can be 

determined by interpolation using the following procedure. (Section F.1.3 provides the 

background on the use of two challenge microorganisms to demonstrate RED delivered to a 

target pathogen.) 

Procedure

1. For a given test condition of flowrate, UVT, and lamp output, calculate the UV 

sensitivity of the challenge microorganisms as their respective measured REDs divided 

by their corresponding log inactivations. 

2. Determine the UV sensitivity of the target pathogen as the dose listed in Chapter 1 

divided by the log inactivation. 

3. Calculate the RED delivered to the target pathogen using the following equation:

( ) ( )

( ) C2 C1

P C1

P C1 C2 C1 D10 D10

D10 D10 RED RED RED RED −

− = + − Equation C.12 

where 

REDP = Estimate of the target pathogen’s RED 

REDC1 = The RED measured with the first challenge microorganism

REDC2 = The RED measured with the second challenge microorganism 

D10P = UV sensitivity of the target pathogen (mJ/cm2

 per log inactivation)

D10C1 = UV sensitivity of the first challenge microorganism (mJ/cm2

 per log 

inactivation) 

D10C2 = UV sensitivity of the second challenge microorganism (mJ/cm2

 per log 

inactivation) 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-42 June 2003

4. Calculate the percent uncertainty of the estimated RED of the target pathogen using 

Equation C.13: 

( ) { } ( ) ( )

P

2

1

2

C2 RED,C1

2

C1 RED,C1

2

C2 C1

2 p C1

C1 RED,C1

REDp RED

RED U RED U

D10 D10

D10 D10

RED U

U

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

+ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

− +

=

Equation C.13

where 

UREDp = Percent uncertainty of the RED estimated for the pathogen 

URED,C1= Percent uncertainty of the RED measured with the first challenge 

microorganism (see Equation C.8) 

As an alternative two-microorganism approach, the log inactivation measured with the 

challenge microorganisms can be interpolated as a function of the microorganisms’ first-order 

inactivation coefficients.

Example. A UV reactor was validated using MS2 and φX174 at 1 and 2 mgd. The UV 

sensitivities of the MS2 and φX174 were 18 and 2 mJ/cm2

 per log inactivation, respectively. The 

following table gives the RED and percent uncertainties measured with MS2 and φX174. At the 

lower flowrate of 1 mgd, the φX174 was inactivated to below the detection limit and the 

measured RED was estimated as greater than 10 mJ/cm2

. The table also gives the RED delivered 

to Cryptosporidium estimated using Equation C.12 and the percent uncertainty of that RED 

estimated using Equation C.13. These estimations assumed a UV sensitivity of Cryptosporidium

of 4.0 mJ/cm2

 per log inactivation based on the dose in Chapter 1 for a 3.0-log inactivation 

credit. 

 

MS2 φX174 Cryptosporidium Flow

(mgd) RED 

(mJ/cm2

) 

Uncertainty

(%)

RED 

(mJ/cm2) 

Uncertainty

(%)

RED 

(mJ/cm2) 

Uncertainty

(%)

1 40 6 > 10 0 14 3.2

2 20 11 9 4 10 2.9

Determining the Polychromatic Bias

For a UV reactor using a germicidal UV intensity sensor (the spectral response meets 

Tier 1 criteria), the polychromatic bias can be assigned a value of one if the UV intensity sensor 

is located where dose delivery is proportional to measured UV intensity or closer to the lamps 

than that location. This can be shown experimentally by demonstrating under fixed conditions of 

flow and measured UV intensity that the RED obtained with peak UVT and lowered lamp power 

is greater than or equal to the RED measured with peak lamp power and lowered RED. 

If data are not available showing the UV intensity sensor location meets the above 

criteria, the polychromatic bias should be determined by calculating, at a given flowrate, UVT, 

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Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-43 June 2003

and measured UV intensity, the ratio of RED during validation to the RED at the WTP. This 

calculation should be done conservatively by assuming ideal dose delivery where dose is the 

product of the average intensity within the reactor and the theoretical mean residence time. The 

calculation should include the following factors: 

• The spectral UV transmittance of the water during validation and at the WTP. 

• The spectral lamp output during validation and expected at the WTP with aged lamps. 

• The spectral sleeve UV transmittance during validation and expected at the WTP with 

aged and fouled sleeves.

• The spectral response of the sensor used during validation and at the WTP. 

• The action spectra of the challenge microorganism used during validation and the 

action spectra of the target pathogen taken from the literature. 

If the above ratio is less than one, the polychromatic bias should be assigned a value of 

one. 

Figures C.7 to C.9 present the polychromatic bias for reactors with UV intensity sensor 

spectral response curves shown in Figure C.10. Each figure presents, for a given sensor spectral 

response, the polychromatic bias as a function of the UVT, the sensor to sleeve water layer, and 

the UV absorbing chemical used during validation (coffee, lignin sulphonate, and natural organic 

matter (NOM)). The spectral UV absorption coefficient of the UV absorbers and the WTP water 

used to define the polychromatic bias values is provided in Figures C.11 and C.12. Figures C.7 

to C.9 can be used to determine the polychromatic bias if the spectral response of the UV 

intensity sensor used in the figure is representative of the spectral response the UV reactor’s 

intensity sensor. Alternatively, the polychromatic bias can be calculated using a model that

meets the above-mentioned criteria. 

The polychromatic bias shown in Figures C.7 to C.9 was determined for an annular 

reactor with a reactor radius of 18.8 cm and a sleeve radius of 3.81 cm. The UV intensity field 

was calculated using a radial intensity model. Section F.4.2 presents details on the models used 

to develop Figures C.7 to C.9. 

The polychromatic bias values in Figures C.7 to C.9 only account for differences between the 

spectral UV absorbance during validation and the spectral UV absorbance at the WTP. They do 

not account for the impact of spectral shifts in the optical properties of the UV reactor (e.g., lamp

output, sleeve UVT). If spectral shifts in UV reactor properties occur with operation of the UV 

reactor at the WTP, the polychromatic bias should be multiplied by terms that account for those 

shifts. Section F.4.3 describes spectral shifts and provides estimates of the polychromatic biases 

that can occur with those shifts.

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Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-44 June 2003

Figure C.7 Polychromatic Bias as a Function of Water UVT and Sensor-to-Lamp 

Water Layer for UV Reactors using Sensors with Germicidal Response (response 

A in Figure C.10) Validated using Coffee, Lignin Sulphonate, or NOM 

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Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-45 June 2003

Figure C.8 Polychromatic Bias as a Function of Water UVT and Sensor-To-Lamp 

Water Layer for UV Reactors Using Sensors with SiC Response (Response B In 

Figure C.10) Validated Using Coffee, Lignin Sulphonate, or NOM 

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Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-46 June 2003

Figure C.9 Polychromatic Bias as a Function of Water UVT and Sensor-To-Lamp 

Water Layer for UV Reactors Using Sensors with Germicidal Response 

(Response C In Figure C.10) Validated Using Coffee, Lignin Sulphonate, or NOM 

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Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-47 June 2003

Figure C.10 Spectral Response of Sensors Used in Defining Figures C.7 to C.9 

Figure C.11 UV Absorption Coefficient of Coffee, Lignin Sulphonate, and the 

Target Water used to Define Figures C.7 to C.9 

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Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-48 June 2003

Figure C.12 Spectral UV Absorbance of NOM and the 

Target Water Used to Define Figures C.7 to C.9 

0.00

0.05

0.10

0.15

0.20

200 250 300 350 400

Wavelength (nm)

UVa (cm-1)

Target

Water

NOM

Example. A UV reactor equipped with “Sensor B” located 10 cm from the lamp sleeve 

(10 cm water layer) is validated using coffee as a UV absorbing chemical. The UV reactor is 

validated at three intensity setpoints, each tested at lowered UVT values of 95 percent, 90 

percent, and 85 percent. The polychromatic bias values taken from Figure C.8 are 1.11, 1.29, 

and 1.56, for UVT values of 95 percent, 90 percent and 85 percent, respectively. 

Example. A UV reactor equipped with “Sensor A” located 15 cm from the sleeve (20 

cm water layer) is being considered at a WTP with a design UVT of 80 percent. From Figure 

C.7, the polychromatic bias with coffee, lignin sulphonate, and NOM are 1.7, 1.3, and 1.2, 

respectively. Comparing these values, a strong incentive exists to select the UV absorber that 

minimizes the polychromatic bias. 

Determining the Random Uncertainty

The random uncertainty associated with monitoring and validation should be calculated 

at an 80 percent confidence level using the uncertainty of the terms listed in Table C.5. The 

expanded uncertainty should be calculated as the square root of the sum of squares of 

uncertainties of each term. 

If one challenge microorganism is used during validation, the uncertainty of the RED is 

calculated using Equation C.8. If two challenge microorganisms are used, the uncertainty of the 

RED is calculated using Equation C.13. The uncertainty of the interpolation is obtained from the 

confidence bands of the equation used for the interpolation (see section C.4.9.10). The 

uncertainty of the UV intensity sensors used during validation and used at the WTP should be 

obtained from manufacturer data with supporting documentation as per Table C.1 in section 

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Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-49 June 2003

Table C.5 Factors Impacting Expanded Uncertainty of 

Dose Delivery Monitoring and Validation 

Uncertainty

Measured RED 

Any interpolation of RED as a function of flowrate, UVT, or UV intensity 

Sensors used during validation (UV intensity, UVT) 

On-line and reference sensors used at the WTP (UV intensity, UVT)

Lamp output quantification

C.2.2. If the dose monitoring approach uses a UVT monitor, include the measurement 

uncertainty of the UVT monitor obtained from data provided by the manufacturer. 

The uncertainty of lamp output quantification is zero if each lamp is monitored by an 

individual UV intensity sensor. Otherwise, the uncertainty can be calculated using Equation 

C.14: 

1 2

1.28

n n

Uncertainty

σ = Equation C.14

where 

σ = Standard deviation of lamp-to-lamp output expressed as a percentage of the mean 

n1 = Number of banks of lamps in series in the reactor 

n2 = Number of sensors monitoring each bank 

The variability of UV output from lamp-to-lamp can be obtained from either life test or 

field data on aged lamps. 

Example. A UV reactor consists of two banks of four lamps. Each bank is equipped 

with two UV intensity sensors. Dose delivery is monitored using the UV intensity setpoint 

approach. The manufacturer provides data showing the standard deviation of lamp-to-lamp

output is 12 percent of the mean output at the end of lamp life. Thus, the lamp output 

quantification uncertainty is 1.28×12/(20.5

×20.5) = 7.7 percent. When operating at a WTP, the online UV intensity sensors have a measurement uncertainty of 20 percent. The on-line sensors 

will be checked using a reference sensor with an uncertainty of 5 percent. During validation, the 

flowmeter and UV intensity sensors had an uncertainty of 0.5 percent and 5 percent. The 

collimated beam dose calculation has an uncertainty of 8 percent. The regression fit to the doseresponse of the phage has an uncertainty of 10 percent. The UV reactor is tested at peak flowrate 

with the results shown in Table C.6. The uncertainty of the measured log inactivation is 

determined as 4.4 percent. As summarized in Table C.7, a total uncertainty of 26 percent is 

calculated as the square root of the sum of the squares of the individual uncertainties.

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-50 June 2003

Table C.6 Sample Calculation of the Log Inactivation Uncertainty

Influent Effluent

N Log N N Log N 

3.60×105 5.56 154 2.19

4.90×105 5.69 206 2.31

4.10×105 5.61 263 2.42

Mean 5.62 Mean 2.31 

St Dev. 0.067 St Dev. 0.116 

T-statistic 2.92 T-statistic 2.92 

Uncertainty 0.0729 Uncertainty 0.126 

Inactivation 

Log Inactivation 3.31 

Uncertainty 0.145 

Uncertainty (%) 4.40 

Table C.7 Sample Calculation of the Expanded Uncertainty

Uncertainty Uncertainty

(%)

Uncertainty

Squared 

Log Inactivation by reactor 4.4 19 

Collimated beam dose calculation 8 64 

Regression fit to UV Dose-Response Data 10 100 

Validation UV intensity sensor 5 25 

WTP on-line UV intensity sensor 20 400 

WTP reference UV intensity sensor 5 25 

Quantification of lamp-to-lamp variability 7.7 59 

Expanded Uncertainty 26 692 

Determining the Safety Factor 

The safety factor relating the RED measured during validation to the pathogen 

inactivation requirements should be calculated as the product of the RED bias, the polychromatic 

bias, and the expanded uncertainty as per Equation C.10. 

Example. MS2 inactivation of 2.0 log corresponding to an RED of 40 mJ/cm2 is 

measured during validation with a LP reactor. The expanded uncertainty of 35 percent is 

calculated. Because LP lamps are used, the polychromatic bias is 1.00. An RED bias of 2.0 is 

determined using the observed UV sensitivity of MS2 and the UV sensitivity associated with a 

3.0-log Cryptosporidium inactivation credit. A safety factor of (1+0.35) × 2.0 × 1.0 = 2.7 is 

calculated. Hence, the Cryptosporidium RED demonstrated by validation is 40 / 2.7 = 

15 mJ/cm2

. Because the demonstrated Cryptosporidium RED is greater than the 3.0-log 

requirement of 12 mJ/cm2

, the UV reactor is validated for a 3.0-log Cryptosporidium inactivation 

credit. 

Example. The UV reactor in the above example is instead equipped with MP lamps 

monitored with UV intensity sensors matching the spectral response of Sensor A. The UV 

intensity sensors view the lamp through a 15 cm water layer. The UV reactor is validated using 

lignin sulphonate at a maximum UVT of 80 percent. Using Figure C.7, the polychromatic bias 

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Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-51 June 2003

of 1.3 is determined. Thus, the safety factor is (1+0.35) × 2.0 × 1.3 = 3.5 and the 

Cryptosporidium RED demonstrated by validation is 40/3.5 = 11.4 mJ/cm2. In this case, the 

demonstrated RED is less than the required RED of 12 mJ/cm2

 for 3.0-log Cryptosporidium

inactivation credit, and the UV reactor should not be considered validated for 3.0-log 

inactivation of Cryptosporidium. However, for a 2.5-log Cryptosporidium inactivation requiring 

a dose of 8.5 mJ/cm2

, the RED bias is 2.1, resulting in a safety factor of 3.7 and a demonstrated 

Cryptosporidium RED of 40 / 3.7 = 10.8 mJ/cm2. Because the demonstrated RED of 10.8 

mJ/cm2

 is greater than the target RED of 8.5 mJ/cm2, the UV reactor can be considered validated 

for a 2.5 log Cryptosporidium inactivation credit.

C.4.11Validation Test Report 

The engineer responsible for third-party oversight should collect all documentation and 

test results and prepare summary and detailed reports. 

C.4.11.1 Summary Report

The summary report should describe the UV reactor validated under this protocol in 

general terms including the following components: 

• Inlet and outlet conditions

• Number of UV lamps and their location within the reactor 

• Lamp characteristics including type, electrical power consumption, and spectral 

output 

• Monitoring and controls approach used for dose compliance 

• Number of UV intensity sensors and their locations 

• UVT monitor, if used 

• Safety features used to ensure water disinfection 

The summary report should provide the challenge microorganism UV dose-response, 

including the regression fit and the confidence intervals. The report should tabulate each reactor 

test condition evaluated, including the flowrate, UV intensity setpoint, UVT setpoint (if used), 

calculated dose (if used), log inactivation achieved, and calculated RED. The number of samples

evaluated, the standard deviation of the influent and effluent samples, and the uncertainty of the 

inactivation through the reactor should also be tabulated. 

If interpolation of bioassay results is part of dose monitoring, tables or charts should 

present the results of the interpolation.

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Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-52 June 2003

If the reactor is evaluated under Tier 1, documentation should be provided supporting that 

the validation met Tier 1 criteria. The report should state the pathogen credits that the UV 

reactor can achieve based on the Tier 1 designation. 

If the UV reactor is evaluated under Tier 2, documentation should be provided describing 

the Tier 2 analysis including the determination of the RED bias, polychromatic bias, and 

expanded uncertainty. For the expanded uncertainty, each term used in the calculation should be 

provided. The report should state the pathogen credits that the UV reactor can achieve based on 

the Tier 2 results. 

Based on the values used to determine the safety factor applied to the validation data 

(Tier 1 or 2), the summary report should specify all criteria for the measurement uncertainty of

the UV intensity sensors, and UVT monitors used at the WTP. 

C.4.11.2 Detailed Report

The detailed report should provide a comprehensive description of the test methodology 

that includes the following components: 

• Identity and qualifications of personnel involved in the validation test 

• UV reactor specifications

• UV intensity sensor specifications and calibration documentation 

• Physical test set-up 

• Summary of QA/QC procedures 

• Materials and methods employed during the test 

• Complete test results, including raw data and analyses performed 

C.5 UV Reactor Validation Examples 

This section provides examples of UV reactor validation for the following reactors and

monitoring approach combinations: 

• LP reactor using a single intensity setpoint (section C.5.1) 

• LP reactor using multiple setpoints as a function of flowrate (section C.5.2) 

• LP reactor using multiple setpoints as a function of flowrate and dose (section C.5.3) 

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Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-53 June 2003

• MP reactor using a single intensity setpoint and UVT setpoint (section C.5.4) 

• MP reactor using the calculated dose method for monitoring (section C.5.5) 

C.5.1 LP Reactor Using a Single Intensity Setpoint

A UV reactor consists of two banks in series of nine LPHO lamps oriented perpendicular 

to flow. Dose delivery is monitored using the UV intensity setpoint approach. Each bank is 

equipped with one UV intensity sensor. 

The UV reactor is considered for use at a WTP. The application requires a 2.5 log 

inactivation credit of Cryptosporidium. The design flowrate and UVT at the WTP are 500 gpm

and 90 percent, respectively. The UV manufacturer states the lamp fouling/aging factor for the 

reactor is 70 percent. During operation at a WTP, the on-line and reference UV intensity sensors 

are expected to have a measurement uncertainty of 15 and 5 percent, respectively. The reactor 

will operate at the WTP using a single intensity setpoint to indicate dose delivery over a flow 

range of 100 to 500 gpm. 

The reactor is validated using coffee as the UV absorber and MS2 as the challenge 

microorganism. Figure C.13 gives the dose-response of the MS2 measured during validation 

with a collimated beam. The dose-response is fitted using the following equation: 

4.3

N

N Dose 20.3 log 0 ⎟ +

⎠

⎞ ⎜

⎝

⎛ = ×

Figure C.13 also provides 80 percent confidence levels for the fit and the percent 

uncertainty of the UV dose calculated from those confidence levels. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-54 June 2003

Figure C.13 Dose-Response of the MS2 Challenge 

Microorganism Used in Example C.5.1 

A reference sensor is used to monitor UV intensity during validation testing. The 

intensity setpoint to be validated is determined by operating the reactor under design conditions 

of 70 percent lamp output and 90 percent water UVT. Under these conditions, the UV intensity 

sensor reads 5.0 mW/cm2

. 

Table C.8 gives the validation test conditions and results. The reactor is tested at 

flowrates of 100 and 500 gpm with the intensity sensor reading 5.0 mW/cm2

. At each flowrate, 

the reactor is tested under conditions of low UVT - high lamp output and high UVT - low lamp

output. Each test condition is evaluated using five influent and five effluent samples. 

Table C.8 Validation Test Conditions and Results for Example C.5.1 

Test Conditions Test Results

Flow

(gpm) 

UVT 

(%)

Lamp 

(%)

UV 

Intensity

(mW/cm2

) 

Influent 

(log) 

Effluent

(log) 

Inactivation 

(log) 

RED 

(mJ/cm2

) 

100 98 44 4.98 4.97 ± 0.08 < 0 > 4.97 > 105 

100 84 100 4.90 5.02 ± 0.10 < 0 > 5.02 > 106 

500 98 44 4.98 5.03 ± 0.06 4.02 ± 0.08 1.00 24.6

500 84 100 4.92 5.02 ± 0.10 3.52 ± 0.18 1.49 34.6

Note. Influent and effluent data presented as mean ± standard deviation.

Based on the results, the reactor is rated at an MS2 RED of 24.6 mJ/cm2

 for a flow range 

of 100 to 500 gpm and a sensor setpoint of 5 mW/cm2

. 

A Tier 2 analysis was used to assess if the reactor achieved 2.5 log Cryptosporidium. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-55 June 2003

RED Bias. Since a 2.5 log inactivation of Cryptosporidium requires a dose of 

8.5 mJ/cm2

, the UV sensitivity of Cryptosporidium is defined as 8.5/2.5 = 3.4 mJ/cm2 per log 

inactivation. Since 1.00-log MS2 inactivation occurred with a dose of 24.6 mJ/cm2

, the UV 

sensitivity of MS2 is defined as 24.6 mJ/cm2

 per log inactivation. In Figure C.6, an RED of 9.2 

and 21 mJ/cm2

 occurs with a UV sensitivity of 3.4 and 24.6 mJ/cm2 per log inactivation, 

respectively. Accordingly, the RED bias is 21/9.2 = 2.28. 

Polychromatic Bias. The polychromatic bias equals 1.0 because the UV reactor uses 

LPHO lamps. 

Expanded uncertainty. A t-statistic of 1.53 is associated with 5 samples and an 80 

percent confidence level. Using the standard deviations for the influent and effluent counts in 

Table C.8, the uncertainty of the log inactivation through the reactor is calculated as follows 

using Equation C.7: 

( ) ( )

*100% 6.8%

1.00

5

0.08 1.53

5

0.06 1.53

Error

2

1

2 2

=

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ×

+

×

=

The uncertainty of the collimated beam dose calculation was determined to be 

8.9 percent. At a UV dose of 24.6 mJ/cm2

, the uncertainty in the dose calculation based on the 

confidence bands in Figure C.13 is 9.6 percent. 

The uncertainties of the sensors used during validation and at the WTP are as follows: 

• Validation UV intensity sensor 5 percent 

• WTP on-line UV intensity sensor 15 percent 

• WTP reference UV intensity sensor 5 percent 

The total uncertainty of the sensors is calculated according to the following equation:

Error ( ) 5 15 5 2 16.6%

1 2 2 2 = + + =

The UV vendor states the standard deviation of the UV output from lamp to lamp is 25 

percent. Given two banks of lamps and one UV intensity sensor per bank, the uncertainty of the 

lamp output is calculated as follows using Equation C.14: 

Error . % . 22 6

1 2

1 28 25 = × =

Including each of these random uncertainty terms, the expanded uncertainty is calculated 

as follows: 

Error ( ) 6.8 8.9 9.6 16.6 22.6 2 31.7%

1 2 2 2 2 2 = + + + + =

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-56 June 2003

Safety factor. Using Equation C.10, the safety factor is calculated as follows: 

SF = ( ) 1+ 0.317 × 2.28×1.00 = 3.00

Based on this safety factor value and the Cryptosporidium dose target for 2.5-log 

inactivation credit, the MS2 RED demonstrated during validation should be as follows: 

2 MS2 RED = 3.00 × 8.5 = 25.5mJ/cm

Because the demonstrated RED of 24.6 mJ/cm2

 is less than this value, the reactor cannot 

get 2.5-log Cryptosporidium inactivation credit operating at a sensor setpoint of 15 mW/cm2

. 

However, with a 2.0-log Cryptosporidium credit target, the RED bias would be 2.6, resulting in a 

safety factor of 3.42 and an MS2 RED target of 19.8 mJ/cm2

. Because the demonstrated MS2 

RED is greater than this value, the reactor can get 2.0 log Cryptosporidium credit operating at a 

setpoint of 15 mW/cm2

 over a flow range of 100 to 500 gpm. 

The reactor does not meet Tier 1 criteria because the standard deviation of the UV output 

from lamp-to-lamp is greater than 15 percent. If the reactor did meet all Tier 1 criteria, the 

reactor would receive credit for 2.0-log Cryptosporidium based on a comparison of the 

demonstrated MS2 RED of 24.6 mJ/cm2

 with the dose criteria in Table C.3. 

C.5.2 LP Reactor with a Intensity Setpoint Interpolation as a Function of Flow 

A UV reactor consists of four banks of six LPHO lamps oriented perpendicular to the 

flow. Dose delivery is monitored using the UV intensity setpoint approach. Each bank is 

equipped with two UV intensity sensors. 

The UV reactor is rated by the manufacturer for flows ranging from 0.9 to 2.4 mgd. The 

manufacturer states that sensor setpoints of 6.0, 7.5, 10, and 14 mW/cm2

 should indicate a 3.0-

log Cryptosporidium inactivation credit at flows of 0.9, 1.2, 1.7 and 2.4 mgd, respectively. 

During operation at a WTP, the on-line and reference UV intensity sensors will have a 

measurement uncertainty of 15 and 5 percent respectively. 

 

The reactor is validated using lignin sulphonate as the UV absorber and MS2 as the 

challenge microorganism. Figure C.14 gives the dose-response of the MS2 measured during 

validation with a collimated beam. The dose-response is fitted using the following equation:

0.144

N

N

Dose 15.6 log 0 ⎟ −

⎠

⎞ ⎜

⎝

⎛ = ×

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-57 June 2003

Figure C.14 Dose-Response of the MS2 Challenge 

Microorganism Used in Example C.5.2 

Confidence intervals are fitted to the data at an 80 percent level. 

Table C.9 gives the validation test conditions and results. The reactor is tested at four 

flowrates, 0.9, 1.2, 1.7 and 2.4 mgd, with the lamp power and UVT adjusted to give a UV 

intensity sensor reading at the setpoint values. At each flowrate, the reactor is tested under 

conditions of reduced UVT - maximum lamp output and maximum UVT - reduced lamp output. 

A reference sensor with an uncertainty of 5 percent is used during validation to measure UV 

intensity. Each test condition is evaluated using five influent and five effluent samples. 

Table C.9 Validation Test Conditions and Results for Example C.5.2 

Test Conditions Test Results

Inactivation Flow

(mgd) 

UVT 

(%)

Lamp 

(%)

UV 

Intensity

(mW/cm2

) 

Influent 

(Logs) 

Effluent

(Logs) Log Uncertainty

(%)

RED 

(mJ/cm2

) 

0.90 98 37 6.15 5.99 ± 0.096 2.95 ± 0.080 3.04 3.8 47.5

0.90 70 100 6.06 5.94 ± 0.127 3.21 ± 0.087 2.73 5.4 42.5

1.2 98 45 7.48 6.09 ± 0.100 2.98 ± 0.108 3.11 4.5 48.5

1.2 75 100 7.46 6.04 ± 0.070 3.34 ± 0.088 2.70 4.0 42.1

1.7 98 61 10.1 6.03 ± 0.150 3.27 ± 0.112 2.76 6.5 43.1

1.7 83 100 10.1 5.98 ± 0.116 3.45 ± 0.120 2.52 6.2 39.4

2.4 98 83 13.8 6.03 ± 0.102 3.37 ± 0.090 2.67 4.9 41.6

2.4 92 100 13.7 6.02 ± 0.136 3.37 ± 0.062 2.66 5.4 41.4

Table C.10 presents the MS2 RED and reactor setpoint assigned to each flowrate based 

on the validation results. A Tier 2 analysis was used to determine the Cryptosporidium

inactivation credit that can be assigned to the reactor given the validation test results. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-58 June 2003

Table C.10 Summary of Validation Results for Example C.5.2 

Flow

(mgd) 

UV Intensity

Setpoint 

(mW/cm2

) 

MS2 RED 

(mJ/cm2

) 

0.90 6.06 42.5

1.2 7.46 42.1

1.7 10.1 39.4

2.4 13.7 41.4

RED Bias. Since a 3.0-log inactivation credit for Cryptosporidium requires a dose of 12 

mJ/cm2

, the UV sensitivity of Cryptosporidium is defined as 12/3.0 = 4.0 mJ/cm2 per log 

inactivation. The UV sensitivity of MS2 is 16 mJ/cm2

 per log inactivation 42.5/2.73 = 

16 mJ/cm2

. In Figure C.6, an RED of 10 and 18 mJ/cm2 is associated with a UV sensitivity of

4.0 and 16 mJ/cm2

 per log inactivation. Accordingly, the RED bias is 18/9.8 = 1.84. 

Polychromatic Bias. The polychromatic bias equals 1.0 because the UV reactor uses 

LPHO lamps. 

Expanded uncertainty. The uncertainty of the log inactivation through the reactor, 

calculated using Equation C.7, is tabulated in Table C.9. A mean value of 5.1 percent is used as 

the uncertainty of the log inactivation in this analysis. The uncertainty of the collimated beam

dose calculation was determined as 8.9 percent. For an RED near 40 mJ/cm2

, the uncertainty in 

the RED arising from the scatter in the dose-response in Figure C.14 is 4 percent. 

The uncertainties of the sensors used during validation and at the WTP are as follows: 

• Validation UV intensity sensor 5 percent 

• WTP On-line UV intensity sensor 10 percent 

• WTP Reference UV intensity sensor 5 percent 

The total uncertainty of the sensors is calculated as follows: 

Error ( ) 5 10 5 2 12.2%

1 2 2 2 = + + =

The UV vendor states the standard deviation of the UV output from lamp to lamp is 15 

percent. Given four banks of lamps and two sensors per bank, the uncertainty associated with 

the number of sensors is calculated as follows using Equation C.14: 

6.8%

2 4

1.28 15 Error = × =

Including each of these random uncertainty terms, the expanded uncertainty is calculated 

as follows: 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-59 June 2003

Error ( ) 5.1 8.9 4 12.2 6.8 2 17.8%

1 2 2 2 2 2 = + + + + =

Safety Factor. Using Equation C.10, the safety factor is calculated as follows: 

SF = ( ) 1+ 0.178 ×1.84×1.00 = 2.17

Cryptosporidium Credit. Using this safety factor, the target RED that should be 

demonstrated during validation is 12 × 2.17 = 26 mJ/cm2

. Because the demonstrated RED of 

39.4 mJ/cm2

 is greater than this number, the UV reactor operating at the validated intensity 

setpoints can get credit for 3.0-log Cryptosporidium inactivation. 

The validation results can be used to define three strategies for operating the UV reactor 

at a WTP: 

1. The UV reactor can operate using one intensity setpoint over the full range of 

flowrates. In this case, a setpoint of 13.7 mW/cm2

 can be used to indicate a 3.0-log 

Cryptosporidium inactivation at all flows of 2.4 mgd or less. 

2. The UV reactor can operate using multiple intensity setpoints where each setpoint

functions over a given range of flows. In this case, a setpoint of 13.7 mW/cm2

 would 

be used at all flows from 1.7 to 2.4 mgd, a setpoint of 10.1 mW/cm2

 would be used at 

all flows from1.2 to 1.7 mgd, and a setpoint of 7.46 mW/cm2

 would be used at all 

flows from 0.90 to 1.2 mgd. 

3. The UV reactor can be operated using intensity setpoints interpolated as a function of 

flowrate using the validation data. In this case, using the plot of sensor setpoint 

versus flowrate in Figure C.15, a setpoint value of 11.7 mW/cm2

 can be used at a 

flow of 2 mgd to indicate an MS2 RED of 39.4 mJ/cm2

 and hence a 3.0-log 

Cryptosporidium inactivation credit.

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-60 June 2003

Figure C.15 Interpolation of Intensity Setpoint Values 

Indicating an MS2 Dose of 39.3 mJ/cm2

0

20

40

60

80

100

60 70 80 90 100

Water UV Transmittance (%)

La

mnp O

utput (

%)

6.1 mW/cm2

7.5 mW/cm2

10.1 mW/cm2

13.8 mW/cm2

The UV reactor and validation test conditions met all prerequisites to be considered under 

Tier 1. The Tier 1 requirement for 3.0-log inactivation of Cryptosporidium by a LPHO reactor is 

36 mJ/cm2

. Since the RED demonstrated during validation is greater than this amount, the 

reactor can receive 3.0-log Cryptosporidium inactivation credit under Tier 1. 

Validation data obtained in this example can be related to design criteria by plotting 

combinations of lamp output and water UVT that result in a given measured UV intensity 

setpoint value. For example, Figure C.16 plots combinations of lamp output and water UVT that 

result in the intensity setpoint values validated in Table C.9. Any combination of UVT and lamp

output along that curve can be used as design criteria for each setpoint value shown. For 

example, a setpoint of 10.1 mW/cm2

 indicates 3.0-log Cryptosporidium inactivation at a flow of

1.7 mgd. A setpoint of 10.1 mW/cm2

 occurs with a combination of 70 percent lamp output and 

93 percent UVT. Thus, the reactor could be used in a design application where the design flow, 

UVT, and lamp fouling/aging factor are 1.7 mgd, 93 percent, and 70 percent, respectively. The 

setpoint of 10.1 mW/cm2

 is also obtained with a combination of 80 percent lamp output and 89 

percent UVT. Thus, the reactor could also be used in a design application where the design flow, 

UVT, and lamp fouling/aging factor is 1.7 mgd, 89 percent, and 80 percent, respectively. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-61 June 2003

Figure C.16 Combinations of Lamp Output and Water UV Transmittance that 

Result in Given Sensor Setpoint Values 

C.5.3 LP Reactor with Intensity Setpoint Interpolation as a Function of Flow and 

Target Inactivation 

A UV reactor consists of twelve rows of twelve LPHO lamps oriented perpendicular to 

flow. Dose delivery is monitored using the UV intensity setpoint approach. Each row is 

equipped with one UV intensity sensor. 

The UV reactor is rated by the UV vendor for flows ranging from 5 to 20 mgd. During 

operation at a WTP, the on-line and reference UV intensity sensors will have a measurement 

uncertainty of 15 and 5 percent, respectively. 

The UV manufacturer wants to validate the UV reactor using test conditions that allow

interpolation of intensity setpoints as a function of flowrate and measured RED. Table C.11 

gives the validation test conditions and results. To allow interpolation of sensor setpoints as a 

function of flowrate, the reactor is tested at a three flowrates of 5, 10, and 20 mgd. To allow 

interpolation of sensor setpoints as a function of dose delivery, the reactor is tested at each 

flowrate at setpoint values that the manufacturer states will result in MS2 RED values of 10, 20, 

and 30 mJ/cm2

. At each setpoint evaluated, the reactor is tested under conditions of reduced 

UVT - maximum lamp output and maximum UVT - reduced lamp output. Each test condition is 

evaluated using five influent and five effluent samples. A reference sensor with an uncertainty 

of 5 percent is used during validation to measure the UV intensity. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-62 June 2003

Table C.11 Validation Test Conditions and Results for Example C.5.3 

Test Conditions Test Results

Flow Inactivation 

(mgd) 

UVT 

(%)

Lamp 

(%)

UV 

Intensity

(mW/cm2

) 

Influent 

(logs) 

Effluent

(logs) log Uncertainty

(%)

RED 

(mJ/cm2

) 

5 98 31 5.20 6.00 ± 0.074 1.86 ± 0.098 4.14 2.8 42.8 

5 66 100 5.10 5.98 ± 0.136 2.68 ± 0.090 3.30 4.7 34.4 

5 97.5 20 3.28 6.02 ± 0.088 3.23 ± 0.039 2.80 3.3 29.4 

5 57 100 3.27 6.02 ± 0.129 3.87 ± 0.060 2.15 6.3 23.0 

5 80 20 1.81 5.97 ± 0.075 4.55 ± 0.176 1.42 12.8 15.8 

5 47 100 1.84 5.96 ± 0.118 4.84 ± 0.110 1.12 13.7 12.8 

10 98 55 9.20 6.09 ± 0.141 2.31 ± 0.114 3.79 4.6 39.3 

10 80 100 9.10 5.96 ± 0.076 2.66 ± 0.121 3.30 4.1 34.4 

10 98 33 5.50 6.00 ± 0.068 3.74 ± 0.070 2.26 4.1 24.1 

10 68 100 5.60 6.00 ± 0.130 4.04 ± 0.086 1.97 7.6 21.2 

10 90.5 20 2.62 5.96 ± 0.066 4.91 ± 0.072 1.05 8.8 12.1 

10 53 100 2.63 5.99 ± 0.135 4.92 ± 0.076 1.07 13.7 12.3 

20 98 91 15.1 5.97 ± 0.080 2.93 ± 0.104 3.04 4.1 31.8 

20 95 100 15.2 5.97 ± 0.117 2.97 ± 0.125 3.00 5.4 31.5 

20 98 67 11.2 6.03 ± 0.117 3.62 ± 0.121 2.41 6.7 25.6 

20 86 100 11.2 5.91 ± 0.079 3.82 ± 0.038 2.09 4.0 22.4 

20 98 33 5.50 6.00 ± 0.167 4.91 ± 0.104 1.09 17.2 12.5 

20 68 100 5.60 5.97 ± 0.032 4.89 ± 0.110 1.08 10.2 12.4 

During validation, lignin sulphonate and MS2 are used as the UV absorber and challenge 

microorganism, respectively. Figure C.17 gives the dose-response of the MS2 measured during 

validation with a collimated beam apparatus. The dose-response is fitted using the following 

equation: 

1.70

N

N

Dose 9.91 log 0 ⎟ +

⎠

⎞ ⎜

⎝

⎛ = ×

Confidence intervals are fitted to the data at an 80 percent level. 

Table C.12 presents the MS2 RED assigned to each reactor setpoint based on the 

validation results. For a given flowrate, Figure C.18 presents the measured RED interpolated as 

a function of measured UV intensity. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-63 June 2003

Figure C.17 Dose-Response of the MS2 Challenge 

Microorganism Used in Example C.5.3 

Table C.12 Summary of Validation Results for Example C.5.3 

Flow

(mgd) 

UV Intensity

(mW/cm2

) 

MS2 RED 

(mJ/cm2) 

5 5.10 34.4

5 3.27 23.0

5 1.84 12.8

10 9.10 34.4

10 5.60 21.2

10 2.63 12.3

20 15.2 31.5

20 11.2 22.4

20 5.60 12.4

A Tier 2 analysis was used to determine the Cryptosporidium inactivation credit that can 

be assigned to the UV reactor given the validation test results. Because the validation results 

will be interpolated as a function of dose delivery, the Tier 2 safety factors are determined as a 

function of measured RED. For 1.5, 2.0, 2.5, and 3.0 log Cryptosporidium inactivation, Table 

C.13 presents the RED bias as a function of the measured RED. RED bias values were 

determined using the approach cited in section C.4.10.2. The polychromatic bias equals 1.0 

because the UV reactor uses LPHO lamps. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-64 June 2003

Table C.13 RED Bias as a Function of the Target Pathogen 

Target Inactivation and the Demonstrated RED in Example C.5.3 

Demonstrated RED Bias for a Cryptosporidium log Inactivation of 

RED 

(mJ/cm2

) 

Challenge 

Microorganism 

UV Sensitivity

(mJ/cm2

 per log) 

1.5 log 2.0 log 2.5 log 3.0 log 

12 11.6 1.98 1.89 1.76 1.65

16 11.3 1.96 1.87 1.74 1.63

20 11.1 1.95 1.86 1.73 1.62

24 11.0 1.94 1.85 1.72 1.61

28 10.8 1.93 1.84 1.71 1.61

32 10.8 1.93 1.84 1.71 1.60

36 10.7 1.92 1.83 1.70 1.60

Figure C.18 Measured RED as a Function of Sensor 

Setpoint Values for Given Flowrates 

y = 6.64 + 0.89 y = 3.45 + 2.83

y = 1.97 + 1.09

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

0.0 5.0 10.0 15.0 20.0

Setpoint (mW/cm2

)

RED (

m

J/c

m2

)

20 mgd

10 mgd

5 mgd

Table C.14 presents the random uncertainty terms and the expanded uncertainty as a 

function of the demonstrated RED. Using data from Table C.11, Figure C.19 presents the 

uncertainty of the log inactivation as a function of the demonstrated RED. An empirical fit to 

this data was used to obtain the uncertainty of the log inactivation as a function of demonstrated 

RED in Table C.14. The uncertainty of the RED due to the dose-response data was obtained 

from Figure C.17. The uncertainty of the collimated beam dose calculation was 8.9 percent. The 

uncertainties of the sensors used during validation and at the WTP are as follows: 

• Validation UV intensity sensor 5 percent 

• WTP On-line UV intensity sensor 15 percent 

• WTP Reference UV intensity sensor 5 percent 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual CThe total uncertainty of the sensors is calculated as follows: 

Error ( ) 5 15 5 2 16.6%

1 2 2 2 = + + =

The UV vendor states the standard deviation of the UV output from lamp to lamp is 25 

percent. Given four rows of lamps and two sensors per row, the uncertainty associated with the 

number of sensors is calculated as follows: 

9.2%

1 12

1.28 25 Error = × =

Table C.14 Random Uncertainty Terms as a Function 

of the Demonstrated RED for Example C.5.3 

Uncertainty (%)

Demonstrated

RED (mJ/cm2

) 

Challenge 

Microorganism 

Log 

Inactivation 

Challenge 

Microorganism 

Dose-response

Collimated 

Beam Dose 

Calculation

Intensity

and 

Flow

Sensors 

Number 

of 

Sensors 

Total 

Expanded 

Uncertainty

12 12.4 11.7 8.9 16.6 9.2 27.0

14 10.5 9.7 8.9 16.6 9.2 25.3

16 9.1 8.2 8.9 16.6 9.2 24.3

18 8.0 7.1 8.9 16.6 9.2 23.5

20 7.1 6.3 8.9 16.6 9.2 23.0

22 6.4 5.6 8.9 16.6 9.2 22.6

24 5.9 5.1 8.9 16.6 9.2 22.3

Figure C.19 Uncertainty of the Measured Log Inactivation 

as a Function of Demonstrated RED 

y = 198.01x-1.10

0

5

10

15

20

0.0 10.0 20.0 30.0 40.0 50.0

Demonstrated RED (mJ/cm2

)

Log I

n

a

ctiv

ation 

U

ncertai

nty

65 June 2003

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-66 June 2003

Safety factors calculated from the RED bias, polychromatic bias, and expanded 

uncertainty are tabulated in Table C.15. The safety factors were multiplied by the dose 

requirements for Cryptosporidium to obtain target RED values which are tabulated in 

Table C.16. For a given demonstrated RED, the UV reactor can achieve a given level of 

Cryptosporidium credit if the demonstrated RED is greater than the target RED. Interpolation of 

these data can be used to identify the RED required to obtain a given level of Cryptosporidium

inactivation. Using this approach, an RED of 14.0, 19.1, and 25 mJ/cm2

 is required to show 2.0, 

2.5, and 3.0-log Cryptosporidium inactivation, respectively.

For each flowrate validated, interpolation of the data in Figure C.18 will provide the UV 

intensity setpoints that will indicate an RED of 14.0, 19.1, and 25 mJ/cm2

. For a given RED, 

Figure C.20 presents those intensity setpoints as a function of flowrate. Interpolation of the data 

in Figure C.20 can be used to identify the intensity setpoint required at a given flowrate. For 

example, at a flowrate of 15 mgd, intensity setpoints of 4.7, 6.7, and 9.0 mW/cm2

 can be used to 

indicate Cryptosporidium log inactivation of 2.0, 2.5, and 3.0. 

Intensity setpoints obtained from Figure C.20 for a given design flow can be related to 

design values of water UVT and lamp output using an approach similar to that used in 

section C.5.2 (see Figure C.16). 

Table C.15 Safety Factors Applicable to Validation Results 

Demonstrated Safety Factors Needed Given a Cryptosporidium Inactivation of 

RED (mJ/cm2

) 1.5 log 2.0 log 2.5 log 3.0 log 

12 2.54 2.42 2.25 2.11

14 2.48 2.37 2.20 2.06

16 2.44 2.33 2.17 2.03

18 2.42 2.30 2.14 2.01

20 2.40 2.28 2.13 1.99

22 2.38 2.27 2.11 1.98

24 2.37 2.26 2.10 1.97

Table C.16 Comparison of Demonstrated RED and RED Required for 

Various Log Inactivation of Cryptosporidium for Example C.5.3 

Demonstrated RED Needed to Achieve a Cryptosporidium Inactivation of 

RED (mJ/cm2

) 1.5 log 2.0 log 2.5 log 3.0 log 

12 9.9 14.0 19.1 25

14 9.7 13.7 18.7 25

16 9.5 13.5 18.4 24

18 9.4 13.3 18.2 24

20 9.4 13.2 18.1 24

22 9.3 13.2 17.9 24

24 9.2 13.1 17.9 24

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-67 June 2003

Figure C.20 Intensity Setpoint Values Indicating Various Log 

Inactivation of Cryptosporidium for example C.5.2 

y = 0.001x2 + 0.517x + 0.872

y = 0.005x2 + 0.176x + 0.968

y = 0.003x2 + 0.346x + 0.920

0

2

4

6

8

10

12

14

0 5 10 15 20 25

Flowrate (gpm)

Intensity setpoi

nt (

m

W/cm2

)

2.0 log Crypto

2.5 log Crypto

3.0 log Crypto

C.5.4 MP Reactor Using a Single UV Intensity Setpoint and UV Transmittance 

Setpoint 

A UV reactor consists of two MP lamps oriented parallel to the flow. Each lamp is 

monitored by a UV intensity sensor. Lamps are spaced 40 cm apart and 20 cm from the wall. 

The lamp sleeve radius is 5 cm. The sensor is located on the wall, 20 cm away from the lamp. 

The sensor’s spectral response matches that of “Sensor A” in Figure C.10. Dose delivery is 

indicated using the UV intensity and UVT setpoint approach. 

The reactor is rated for a flow from 0.1 to 0.5 mgd. The reactor will be used with a 

design UVT of 85 percent. The manufacturer states that the lamp output at the end-of-lamp life 

will be 78 percent of the 100 hr burn-in value. The fouling factor for the reactor is 90 percent. 

Accordingly, the lamp output factor for the reactor is 0.78 × 0.90 = 0.70. A UV intensity 

setpoint of 2.8 mW/cm2

 is obtained by measuring the UV intensity with the water UVT set to 85 

percent and the lamp output lowered to 70 percent. 

During operation at a WTP, the on-line and reference UV intensity sensors will have a 

measurement uncertainty of 10 and 5 percent, respectively. 

The UV reactor is validated using lignin sulphonate as a UV absorber and MS2 as a 

challenge microorganism. The measured dose-response of the challenge microorganism is 

provided in Figure C.14. Table C.17 gives the validation test conditions and results. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-68 June 2003

Table C.17 Validation Test Conditions and Results for 

Example C.5.4 with the Sensor Located 20 cm from the Lamp 

Test Conditions Test results

Flow Inactivation 

(mgd) 

UVT 

(%)

Lamp 

(%)

UV 

Intensity

(mW/cm2

) 

Influent 

(Logs) 

Effluent

(logs) Log Uncertainty

(%)

RED 

(mJ/cm2

) 

0.1 85 70 2.9 6.00 ± 0.07 0.00 

>6.0

0 - > 82.6

0.5 85 70 2.9 6.06 ± 0.08 3.04 ± 0.16 3.03 5.6 47.1 

0.5 82.7 100 2.8 5.99 ± 0.11 2.13 ± 0.14 3.85 4.3 60.0 

0.5 93.5 20 2.9 6.12 ± 0.11 4.52 ± 0.07 1.60 7.6 24.8 

The first two test conditions evaluate dose delivery at minimum and maximum flow with 

the reactor operating at the intensity and UVT setpoint values. Based on these results, the UV 

reactor is rated at an MS2 RED of 47.1 mJ/cm2

 when operating at the setpoint conditions. 

The last two test conditions evaluate the sensor position and the validity of using the UV 

intensity and UVT setpoint approach for indicating dose delivery. As indicated, the UV reactor 

delivers an MS2 RED of 60.0 mJ/cm2

 when operating with peak lamp output and the UVT 

lowered to 82.7 percent to give a measured intensity at the setpoint. The UV reactor delivers a 

dose of 24.8 mJ/cm2

 when operating at high UVT and lowered lamp output to give a measured 

intensity at the setpoint value. In other words, an intensity setpoint of 2.9 mW/cm2

 and a UVT 

setpoint of 85 percent does not ensure the reactor delivers an MS2 RED of 47.1 mJ/cm2

. 

The manufacturer has three options for resolving this problem:

• Relocate the sensor closer to the lamp. 

• Switch from the dose monitoring method to the calculated dose approach. 

• Switch from the dose monitoring method to the intensity setpoint approach either 

with the sensor in its current location or with the sensor in a more optimized location.

In this example, the manufacturer chooses to relocate the sensor to 8 cm from the lamp

and revalidates the UV reactor. Table C.18 gives the test conditions and results. In this case, a 

UV intensity of 41.0 mW/cm2

 is measured with the UV reactor operating with a UVT of 85 

percent and a lamp output of 70 percent. This value is greater than the UV intensity measured 

with the sensor on the wall because the sensor is located closer to the lamp. Based on the results, 

the UV reactor is rated at an MS2 RED of 48.5 mJ/cm2

 when operating at setpoint conditions of 

85 percent UVT and a 41.0 mW/cm2

 UV intensity value. With the measured intensity at the 

intensity setpoint value, the UV reactor delivers an RED greater than 48.5 mJ/cm2

 when

operating with a UVT greater than the UVT setpoint value and an RED less than 48.5 mJ/cm2

when operating with a UVT less than the setpoint value. Thus, the intensity sensor is properly 

located for using the intensity and UVT setpoint approach for indicating dose delivery and the 

setpoints will ensure the dose delivery meets an RED of 48.5 mJ/cm2

. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-69 June 2003

Table C.18 Validation Test Conditions and Results for 

Example C.5.4 with the Sensor Located 12 cm from the Lamp 

Test Conditions Test Results

Inactivation Flow

(mgd) 

UVT 

(%)

Lamp 

(%)

UV 

Intensity

(mW/cm2

) 

Influent 

(Logs) 

Effluent

(Logs) Log Uncertainty

(%)

RED 

(mJ/cm2

) 

0.1 85 70 41.0 6.00 ± 0.07 0.00 > 6.00 - > 82.6 

0.5 85 70 41.0 6.01 ± 0.10 2.90 ± 0.10 3.12 4.3 48.5

0.5 75 100 41.4 6.03 ± 0.07 3.26 ± 0.18 2.77 6.7 43.0

0.5 98 46 41.1 5.96 ± 0.12 1.59 ± 0.12 4.37 3.7 68.1

A Tier 2 analysis is used to determine the Cryptosporidium inactivation credit that can be 

assigned to the UV reactor given the validation test results. 

RED Bias. Since a 3.0 log inactivation of Cryptosporidium requires a dose of 

12 mJ/cm2

, the UV sensitivity of Cryptosporidium is defined as 12/3.0 = 4 mJ/cm2 per log 

inactivation. The UV sensitivity of MS2 is 48.5/3.12 = 15.5 mJ/cm2

. In Figure C.6, a RED of 

9.78 and 18.0 mJ/cm2

 is associated with UV sensitivities of 4 and 15.5 mJ/cm2 per log 

inactivation, respectively. Accordingly, the RED bias is 18.0/9.78 = 1.84. 

Polychromatic Bias. The sensor-to-lamp water layer is 3 cm. For a sensor with the 

response of “Sensor A” in Figure C.10, a polychromatic bias 1.00 is obtained from Figure C.7 

for a UVT of 85 percent. 

Expanded uncertainty. The uncertainty of the log inactivation through the UV reactor 

calculated using Equation C.7 is 4.3 percent. The uncertainty of the collimated beam dose 

calculation was 8.9 percent. The uncertainty in the RED arising from the scatter in the doseresponse obtained from Figure C.14 is 3.9 percent at an RED of 48.5 mJ/cm2

. The uncertainties 

of the sensors used during validation and at the WTP are as follows: 

• Validation UV intensity sensor 5 percent 

• WTP On-line UV intensity sensor 10 percent 

• WTP Reference UV intensity sensor 5 percent 

The total uncertainty of the sensors is calculated as follows: 

Error ( ) 5 10 5 2 12.2%

1 2 2 2 = + + =

Because each lamp is monitored, the uncertainty knowing the output of the lamps is 0 

percent. Including each of these random uncertainty terms, the expanded uncertainty is 

calculated as follows: 

Error ( ) 4.3 8.9 3.9 12.2 0 2 16.2%

1 2 2 2 2 2 = + + + + =

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-70 June 2003

Safety factor. Using Equation C.10, the safety factor is calculated as follows: 

SF = ( ) 1+ 0.162 ×1.84 ×1.00 = 2.14

Cryptosporidium Credit. Using this safety factor, the target RED that should be 

demonstrated during validation is 12 × 2.14 = 26 mJ/cm2

. Because the demonstrated RED is 

greater than this number, the UV reactor operating at or above the validated intensity and UVT 

setpoints can get credit for 3.0-log Cryptosporidium inactivation. 

C.5.5 MP Reactor Using Calculated Dose Monitoring

A UV reactor consists of twelve MP lamps oriented perpendicular to the flow. Each 

lamp is monitored by a UV intensity sensor whose spectral response matches that of “Sensor A” 

in Figure C.10. The UV intensity sensors view the UV lamps through a 15 cm water layer. 

During operation at a WTP, the on-line and reference UV intensity sensors will have a 

measurement uncertainty of 10 and 5 percent respectively. During validation, a reference sensor 

is used to measure UV intensity. 

The UV reactor is validated at flows ranging from 10 to 40 mgd using lignin sulphonate 

as a UV absorber and MS2 as a challenge microorganism. Figure C.3 gives the dose-response of 

the challenge microorganism. The lamp’s power supplies vary lamp power from 30 to 100 

percent. The UV reactor is validated at flowrate, UVT, and lamp power combinations that give a 

calculated UV doses of 30, 20 and 10 mJ/cm2

. Table C.19 gives the validation test conditions 

and results. 

Table C.19 Validation Test Conditions and Results for Example C.5.5 

Test Conditions Test Results

Inactivation 

Flow

(mgd) 

UVT 

(%)

Lamp 

(%)

Calculated

RED 

(mJ/cm2

) 

Intensity

(mW/m2

) Log Uncertainty

(%)

Measured 

RED 

(mJ/cm2

) 

40 98 40.5 30.1 22.7 2.38 3.0 35.2

40 90 68 30.2 11.8 2.43 3.2 36.3

40 82.8 100 30.2 5.8 2.38 4.2 35.1

20 90 33.8 30.0 5.9 2.57 3.0 39.1

20 85 44.5 30.1 3.6 2.36 3.6 34.7

20 75 70 29.7 1.2 2.31 3.5 33.8

10 79 30 30.3 1.0 2.22 4.8 32.0

10 75 35.5 30.1 0.6 2.76 2.9 43.3

40 96.5 30 20.1 13.5 1.42 4.0 17.7

40 85 59.5 20.1 4.9 1.34 6.2 16.3

40 73.5 100 19.9 1.3 1.31 7.3 15.9

20 85 30 20.3 2.5 1.75 4.0 23.1

20 75 47 19.9 0.8 1.72 2.9 22.7

40 84.5 30 9.9 2.3 0.96 6.5 10.8

40 75 47 10.0 0.8 1.24 8.1 14.9

40 80 38 10.1 1.4 0.93 7.8 10.4

20 70 30 10.4 0.2 0.86 10.4 9.5

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-71 June 2003

Figure C.21 provides a plot of the measured RED as a function of the calculated RED. 

As shown, there is a range of measured RED values associated with a given calculated RED. 

The UV reactor is rated at the lower end of that range for a given calculated dose. A power 

function (y=AxB

) is used to define the relationship between the calculated RED and the lower 

bound of the measured RED. 

Figure C.21 Relationship Between Measured and Calculated 

Dose for the MP Reactor in Example C.5.5 

A Tier 2 analysis was used to determine the calculated RED values required for 3.0-log 

credit for Cryptosporidium. For various log inactivation credit values for Cryptosporidium, 

Table C.20 gives the RED bias as a function of the MS2 RED predicted from the calculated dose 

using the power function in Figure 21. 

Table C.20 RED Bias as a Function of the Target Pathogen 

Target Inactivation and the Calculated Dose in Example C.5.5

RED Bias for Cryptosporidium

log inactivations of Calculated 

Dose 

(mJ/cm2

) 

MS2 RED 

(mJ/cm2

) 

MS2 Log 

Inactivation 

MS2 Sensitivity

(mJ/cm2

 per log 

inactivation) 3.0 

log 

2.5 

log 

2.0 

log 

1.5 

log 

1.0 

log 

10 4.8 0.43 11.2 1.61 1.73 1.86 1.95 1.99

12 6.6 0.58 11.3 1.62 1.74 1.87 1.97 2.00

14 8.6 0.75 11.5 1.63 1.76 1.89 1.98 2.01

16 10.8 0.92 11.7 1.64 1.77 1.90 1.99 2.03

18 13.3 1.11 11.9 1.65 1.78 1.92 2.01 2.04

20 15.9 1.30 12.2 1.67 1.80 1.93 2.03 2.06

22 18.7 1.50 12.5 1.68 1.82 1.95 2.05 2.08

24 21.8 1.69 12.9 1.70 1.84 1.97 2.07 2.11

26 25.0 1.88 13.3 1.72 1.86 2.00 2.10 2.13

28 28.4 2.07 13.7 1.74 1.88 2.02 2.12 2.16

30 32.0 2.25 14.2 1.77 1.91 2.05 2.15 2.19

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-72 June 2003

Table C.21 gives the random uncertainty terms and the resulting total expanded 

uncertainty as a function of the MS2 RED and calculated dose. Using data from Table C.19, 

Figure C.22 presents the uncertainty of the log inactivation as a function of measured MS2. An 

empirical fit to this data was used to predict the uncertainty of the log inactivation as a function 

of MS2 RED in Table C.21. The uncertainty of the RED due to the dose-response data was 

obtained from Figure C.3. The uncertainty of the collimated beam dose calculation was 8.0 

percent. The uncertainties of the sensors used during validation and at the WTP are as follows: 

• Validation UV intensity sensor 5 percent 

• WTP On-line UV intensity sensor 10 percent 

• WTP Reference UV intensity sensor 5 percent 

The total uncertainty of the sensors is calculated as follows: 

Error ( ) 5 10 5 2 12.2%

1 2 2 2 = + + =

Because each lamp is monitored by a UV intensity sensor, the uncertainty associated with 

quantifying lamp output is zero. 

Table C.21 Random Uncertainty Terms as a Function 

of the Calculated Dose for Example C.5.5 

Uncertainty (%) Calculated 

Dose 

(mJ/cm2

) 

MS2 RED 

(mJ/cm2

) 

Challenge 

Microorganism 

Log Inactivation

Challenge 

Microorganism 

Dose-response

Collimated 

Beam Dose 

Calc 

Intensity

Sensors 

Total 

Expanded 

Uncertainty

10 4.8 13.9 1.9 8.0 12.2 20.3

12 6.6 11.1 1.3 8.0 12.2 18.4

14 8.6 9.2 0.9 8.0 12.2 17.3

16 10.8 7.8 0.7 8.0 12.2 16.6

18 13.3 6.8 0.5 8.0 12.2 16.1

20 15.9 5.9 0.4 8.0 12.2 15.8

22 18.7 5.3 0.3 8.0 12.2 15.6

24 21.8 4.8 0.3 8.0 12.2 15.4

26 25.0 4.3 0.2 8.0 12.2 15.3

28 28.4 3.9 0.2 8.0 12.2 15.1

30 32.0 3.6 0.2 8.0 12.2 15.1

Table C.22 gives the polychromatic bias as a function of the UVT. The polychromatic 

bias values were taken from Figure C.7 for sensor “A” located with a 15 cm water layer and 

lignin sulphonate as the UV-absorbing chemical. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-73 June 2003

Figure C.22 Uncertainty of the Measured Log Inactivation 

as a Function of Demonstrated RED 

Table C.22 Polychromatic Bias for Example C.5.5 

UVT 

(%)

Polychromatic 

Bias 

98 1.03

95 1.06

90 1.10

85 1.20

80 1.30

75 1.55

Safety factors calculated from the RED bias, polychromatic bias, and expanded 

uncertainty are tabulated in Table C.23. The safety factors were multiplied by the 3.0-log dose 

requirement for Cryptosporidium of 12 mJ/cm2

 to obtain target RED values which are tabulated 

in Table C.24. For a given calculated dose, the UV reactor can achieve receive 3.0-log 

Cryptosporidium credit if the measured MS2 RED associated with that calculated dose is greater 

than the target RED. 

Table C.25 presents the calculated dose needed to achieve a given level of 

Cryptosporidium inactivation and the lower limit of UVT over which the calculated dose applies. 

The values in Table C.22 for 3.0-log inactivation credit were obtained from Table C.21. The 

values for other log inactivation credit levels were obtained by repeating the analysis in Tables 

C.23 and C.24. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-74 June 2003

Table C.23 Safety Factors for 3-Log Cryptosporidium Inactivation as a Function 

of the Calculated Dose for Example C.5.5 

Calculated Safety Factors for 3.0-log Cryptosporidium Inactivation for UVT of 

Dose 

(mJ/cm2

) 

MS2 

RED 

(mJ/cm2

) 98% 95% 90% 85% 80% 75%

10 4.8 1.99 2.05 2.13 2.32 2.51 3.00

12 6.6 1.97 2.03 2.10 2.30 2.49 2.97

14 8.6 1.97 2.02 2.10 2.29 2.48 2.96

16 10.8 1.97 2.03 2.10 2.29 2.48 2.96

18 13.3 1.98 2.03 2.11 2.30 2.49 2.97

20 15.9 1.99 2.05 2.12 2.32 2.51 2.99

22 18.7 2.00 2.06 2.14 2.34 2.53 3.02

24 21.8 2.02 2.08 2.16 2.36 2.55 3.04

26 25.0 2.04 2.10 2.18 2.38 2.58 3.08

28 28.4 2.07 2.13 2.21 2.41 2.61 3.11

30 32.0 2.09 2.16 2.24 2.44 2.64 3.15

Table C.24 Comparison of the Target MS2 RED Needed to Demonstrate 3.0-Log 

Cryptosporidium Inactivation Credit to the Calculated Dose and Measured 

MS2 RED 

Target MS2 RED (mJ/cm2 Calculated ) 

Dose 

(mJ/cm2

) 

MS2 

RED 

(mJ/cm2

) 98% 95% 90% 85% 80% 75%

10 4.8 23.9 24.6 25.5 27.8 30.1 35.9

12 6.6 23.7 24.3 25.3 27.6 29.9 35.6

14 8.6 23.6 24.3 25.2 27.5 29.8 35.5

16 10.8 23.6 24.3 25.2 27.5 29.8 35.5

18 13.3 23.7 24.4 25.3 27.6 29.9 35.7

20 15.9 23.9 24.6 25.5 27.8 30.1 35.9

22 18.7 24.1 24.8 25.7 28.0 30.4 36.2

24 21.8 24.3 25.0 25.9 28.3 30.6 36.5

26 25.0 24.5 25.2 26.2 28.6 31.0 36.9

28 28.4 24.8 25.5 26.5 28.9 31.3 37.3

30 32.0 25.1 25.9 26.8 29.3 31.7 37.8

Table C.25 Dose and UVT Alarm Setpoints for 

Various Log Inactivation Credit Levels of Cryptosporidium

3.0 log 2.5 log 2.0 log 1.5 log 1.0 log 

Dose 

(mJ/cm2

) 

UVT 

(%)

Dose 

(mJ/cm2) 

UVT 

(%)

Dose 

(mJ/cm2) 

UVT 

(%)

Dose 

(mJ/cm2) 

UVT 

(%)

Dose 

(mJ/cm2) 

UVT 

(%)

25 98.0 22 94.7 19 88.8 15 93.0 12 89.6

26 95.6 23 88.5 20 83.2 16 84.7 13 80.9

27 90.4 24 83.9 21 79.4 17 79.5 14 76.3

28 85.9 25 80.5 22 76.8 18 76.3 15 77.5

29 82.2 26 78.1 23 75.1 - - - -

30 79.2 27 76.3 - - - - - -

- - 28 75.0 - - - - - -

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-75 June 2003

The data in Table C.25 represents calculated dose and UVT alarm setpoints that can be 

used to ensure the reactor delivers a given log inactivation of Cryptosporidium. Alternatively, as 

shown in Table C.26, a single calculated dose alarm setpoint can be defined over the validated 

range of UVT. 

Table C.26 Dose Setpoints for Various Log Inactivation Credit Levels of 

Cryptosporidium 

Cryptosporidium

Log Inactivation 

Credit 

Calculated Dose 

Setpoint 

(mJ/cm2

) 

UVT Range 

(%)

1.0 14 75 - 98 

1.5 18 75 - 98 

2.0 23 75 - 98 

2.5 28 75 - 98 

3.0 30 79 - 98 

C.6 References

APHA/AWWA/WEF. 1995. Standard methods for the Examination of Water and Wastewater, 

19th Edition. American Public Health Association, American Water Works Association, 

and Water Environment Federation, Washington, DC. 

Chang, J.C.H., S.F. Osoff, D.C. Lobe, M.H. Dorfman, C.M. Dumais, R.G. Qualls and J.D. 

Johnson. 1985. UV inactivation of pathogenic and indicator microorganisms. Applied 

and Environmental Microbiology 49, no. 6:1361-1365. 

Draper, N. and Smith, H. 1981. Applied regression analysis, Second Edition. New York: Wiley. 

DVGW. 1997. UV Disinfection Devices for Drinking Water Supply––Requirements and 

Testing. German Gas and Water Management Union (DVGW), Bonn, Germany. 

NWRI/AwwaRF. 2000. Ultraviolet Disinfection Guidelines for Drinking Water and Water 

Reuse. National Water Research Institute and the AwwaRF 

ÖNORM. 2001. ÖNORM M 5873-1 Plants for the Disinfection of water Using Ultraviolet 

Radiation: Requirements and Testing Low Pressure Mercury Lamp Plants. 

Österreichisches Normungsinstitut, Vienna, Austria. 

Petri, B.M., G. Fang, J.P. Malley, D.C. Moran, and H. Wright. 2000. Ground water UV 

Disinfection: Challenges and Solutions. Proceedings of the American Water Works 

Association Water Quality Technology Conference. Salt Lake City, UT, November 5-9, 

2000. 

Proposal Draft
Appendix C. Validation of UV Reactors 

UV Disinfection Guidance Manual C-76 June 2003

Petri, B. and D. Olson. 2001. Bioassay-validated numerical models for UV reactor design and 

scale-up IUVA First International Congress on UV Technologies, Washington, DC, June 

14-16, 2001. 

Wright, N.G. and D.M. Hargreaves. 2001. The use of CFD in the evaluation of UV treatment 

systems Journal of Hydroinformatics 3:59-70. 

Proposal Draft
Appendix D. Microbiological Methods 

D.1 General Recommendations 

The challenge microorganism used to validate UV reactors should be cultured and 

analyzed by a laboratory staffed by professional microbiologists and equipped to perform 

microbiological examinations as per Standard Methods for the Examination of Water and 

Wastewater (APHA et al. 1998, sections 1020-1050). Protocols for culturing the challenge 

microorganism and measuring its concentration should be based on published and peer-reviewed 

methods and should be clearly defined and followed. Measurement of the concentration of the 

challenge microorganism before and after exposure to UV light should be initiated within 

24 hours of exposure. If the challenge microorganism has the ability to photorepair, exposure of 

samples to visible light should be kept to a minimum.

Because MS2 bacteriophage (MS2) and B. subtilis spores are commonly being used as 

challenge microorganisms for UV reactor validation, the following sections describe procedures 

that can be used for preparing stock solutions of MS2 and B. subtilis spores and assaying the 

concentration of those microorganisms in water samples. Procedures for preparing stock 

solutions can be scaled to provide the volumes needed for UV reactor validation. Alternative 

procedures and challenge microorganisms can be used if they are acceptable to the State. 

Section F.1 provides a rational for selecting challenge microorganisms. 

D.2 MS2 Bacteriophage Stock Preparation 

MS2 (ATCC 15597-B1) can be propagated using a variety of host bacteria including 

Escherichia coli C3000 (ATCC 15597), E. coli F-amp (ATCC 700891), and others (Meng and 

Gerba 1996, Oppenheimer et al. 1993, NWRI/AwwaRF 2000). The following propagation 

method was adapted from NWRI/AwwaRF (2000): 

1. Inoculate sterile tryptic soy broth (TSB) (DIFCO, Detroit, Michigan) with host 

bacterium transferred from a single colony grown on a nutrient agar plate. Incubate 

the culture with constant stirring at 35 to 37°C for 18 to 24 hours. 

2. Transfer 0.5 mL of the host bacterium culture to 50 mL of fresh TSB and incubate at 

35 to 37°C for 4 to 6 hours with continuous shaking at 100 Hz to obtain a culture in 

its log growth phase (approx. 3x108

 cfu/mL) (cfu = colony forming units). 

3. Dilute stock MS2 using Tris-buffered saline (pH 7.3) to a concentration of 

approximately 108

 pfu/mL (pfu = plaque forming units). 

4. Add 1mL of diluted MS2 stock solution to the 50 mL volume of E. coli in TSB and 

incubate overnight at 35 to 37°C. 

5. Centrifuge the MS2/E. coli culture at 8000 × g (g = 9.82 m/s

2

) for 10 minutes at 4°C 

to remove cellular debris. 

UV Disinfection Guidance Manual D-1 June 2003

Proposal Draft
Appendix D. Microbiological Methods 

UV Disinfection Guidance Manual D-2 June 2003

6. Filter the supernatant by passing it through a 0.45 μm low protein-binding filter. 

7. Assay the concentration of MS2 in the stock solution as per section D.3. 

8. Collect and refrigerate the filtrate at 4°C and use within a one-month period. 

Propagation should result in a highly concentrated stock solution of essentially monodispersed phage whose UV dose-response follows first order kinetics with minimal tailing. 

Figure D.1 presents the dose-response of MS2 as reported in the literature. Over the range of 

REDs demonstrated during validation testing, the mean dose-response of the MS2 stock solution 

should lie within the 90 percent prediction interval of the mean response in Figure D.1. Over a 

dose range of 0 to 120 mJ/cm2

, the predictions intervals may be defined using the following 

equations: 

Lower Bound Inactivation = − × × Dose + × × Dose −4 2 −2 : log 1.4 10 7.6 10

Upper Bound Inactivation = − × × Dose + × × Dose −5 2 −2 : log 9.6 10 4.5 10

Figure D.1 UV Dose-Response of MS2 

0

1

2

3

4

5

6

7

8

0 20 40 60 80 100 120

UV Dose (mJ/cm

2

)

L

o

g I

nactivatio

n

Meng and Gerba, 1996

Oppenheimer et al, 1993

Sommer et al, 1998

Tree et al, 1997

Havelaar et al, 1990

Nieuwstad et al, 1994

Mean

Mean Response 90% Prediction Interval

Proposal Draft
Appendix D. Microbiological Methods 

UV Disinfection Guidance Manual D-3 June 2003

D.3 MS2 Assay 

The concentration of MS2 (ATCC 15597-B1) in water samples can be assayed using the 

agar overlay technique with E coli (ATCC 15597) as a host bacterium (Adams 1959, Yahya et 

al. 1992, Oppenheimer et al. 1993, Meng and Gerba 1996). The following procedure can be 

used: 

1. Inoculate TSB (Difco, Detroit, MI) with the host bacterium and incubate at 35 to 

37°C for 18 to 24 hours to obtain an approximate concentration of 108

 CFU/mL. 

2. Transfer 1 mL of the culture to 50 mL of fresh TSB and incubate at 35 to 37°C for 4

to 6 hours with continuous shaking at 100 Hz to obtain a culture in its log growth 

phase. 

3. Obtain serial dilutions of the MS2 sample using a 0.001 M phosphate-saline buffer or 

TSB. 

4. Combine and gently stir 1 mL of host cell solution, 0.1 mL of diluted MS2 sample, 

and 2 to 3 mL of molten tryptic soy agar (TSA) (0.7 percent agar, 45 - 48°C) (Difco, 

Detroit, MI).

5. Pour the mixture onto solidified TSA (1.5 percent agar) contained within Petri dishes. 

The time between the mixing the MS2 sample with the E. coli host and the plating of 

the top agar layer should not exceed 10 minutes. After plating, the agar should 

harden in 10 minutes. 

6. After the top agar layer hardens, cover, invert the Petri dishes, and incubate 16 to 

24 hours at 35 to 37°C. 

7. Count the plaques with the aid of a colony counter. Plaques are identified as clear 

circular zones 1 to 10 mm in diameter in the lawn of host bacteria. 

8. Record the number of plaques per dish, and the MS2 sample volume and dilution. If 

it is not possible to distinguish individual plaques because of confluent growth, record

the plate counts as “TNTC” (too numerous to count). 

9. Calculate the MS2 concentration in the water samples: 

∑

∑=

i

i

V

n

Concentration Equation D.1

where 

ni = The number of counts on each plate 

Vi = The volume of undiluted sample used with each plate 

Example. A water sample containing MS2 was diluted 10, 100 and 1,000-fold using a 

0.1 mL aliquot dilution of the sample for each. Each dilution was assayed in triplicate. Plaque 

Proposal Draft
Appendix D. Microbiological Methods 

UV Disinfection Guidance Manual D-4 June 2003

forming units observed on the plates were 2, 5 and 6 for the 1,000-fold diluted sample and 32, 

40, and 47 for the 100-fold diluted sample. With the 10-fold dilution, plate counts were too 

numerous to count. The concentration in the original sample is calculated as follows: 

( ) pfu mL

mL mL

pfu Concentration 40,000 /

0.1*3/1000 0.1 3/100

2 5 6 32 40 47 = + ×

+ + + + + =

D.4 Bacillus Subtilis Spore Preparation 

Bacillus subtilis spores (ATCC 6633) can be propagated using Schaeffer’s media 

(Munakata and Rupert 1972, Sommer et al. 1995, DVGW 1997). The following propagation 

method was adopted from DVGW (1997): 

1. Prepare Columbia agar (Oxoid CM 331) as a 1 L solution of 23.0 g special peptone 

(Oxoid L 72), 1.0 g starch, 5.0 g NaCl, and 10.0 g agar (Oxoid L 11) in distilled 

water. Adjust pH to 7.0 and autoclave 15 minutes at 121oC. 

2. Prepare the sporulation media as a 1 L solution of 280 mg MgSO4·H2O, 1.11 g KCl, 

3.1 mg FeSO4·7H2O, and 8.9 g nutrient broth (Oxoid CM 67) in distilled water. 

Adjust the pH to 7.0 and autoclave it for 15 minutes at 121oC. 

3. Inoculate Columbia agar plates (Oxoid CM 331) with three smears of B. subtilis and 

incubate 24 hours at 37oC. 

4. Inoculate 300 mL of sporulation media with three colonies collected from the agar 

plates. 

5. Incubate the sporulation media 72 hours at 37oC on a shaker operating at 2 Hz. 

6. Sonicate the resulting culture for 10 minutes at 50 kHz and 10oC. 

7. Harvest the spores by centrifuging 80 mL aliquots at 5000 g for 10 minutes and 10oC. 

8. Wash the spores three times by resuspending in 20 mL of distilled water and 

centrifuging at 5000 × g for 10 minutes at 10oC. 

9. Resuspend the washed spores in 100 mL of 0.001 M phosphate-saline buffer. 

10. Inactivate the vegetative B. subtilis by heat treatment at 80oC for 10 minutes. 

11. Sonicate the resulting culture for 10 minutes at 50 kHz and 10oC. 

12. Collect the resulting stock solution and assay the B. subtilis spore concentration as per 

section D.5. 

13. Refrigerate the filtrate at 4oC and use within a one-month period. Sonicate for 

10 minutes at 50 kHz and 10oC before use. 

Proposal Draft
Appendix D. Microbiological Methods 

UV Disinfection Guidance Manual D-5 June 2003

Propagation should result in a highly concentrated stock solution of mono-dispersed 

B. subtilis spores with a UV dose-response that follows the dose-response reported in the 

literature and presented in Figure D.2. Over the range of reduction equivalent doses (REDs) 

demonstrated during validation testing, the mean dose-response of the B. subtilis stock solution

should lie within the 90 percent prediction interval of the mean response provided in Figure D.2. 

Over a dose range of 0 to 70 mJ/cm2

, the predictions intervals may be defined using the 

following equations: 

Lower Bound Inactivation = − × × Dose + × × Dose − × × Dose −5 3 −3 2 −2 : log 2.0 10 2.7 10 5.3 10

Upper Bound Inactivation = × × Dose + × × Dose −4 2 −2 : log 5.7 10 4.3 10

Figure D.2 UV Dose-Response of Bacillus Subtilis Spores 

-1

0

1

2

3

4

5

6

7

0 50 100 150

UV Dose (mJ/cm2

)

Log I

n

a

ctiv

ation

DVGW, 1997

Hoyer, 2002

Chang et al, 1985

Sommer et al, 1996

Sommer et al, 1995

Sommer et al, 1998

Sommer and Cabaj, 1993

Sommer and Cabaj, 1993

Mean

90% Prediction Interval of the Mean Response

Proposal Draft
Appendix D. Microbiological Methods 

UV Disinfection Guidance Manual D-6 June 2003

D.5 Bacillus Subtilis Spore Assay 

The concentration of B. subtilis spores (ATCC 6633) in water samples can be assayed by 

the plate method using plate count agar. The following procedure was adopted from Deutscher 

Verein des Gas- und Wasserfaches (DVGW) (1997): 

1. Prepare plate count agar (Oxoid CM 325) as a 1 L solution of 5.0 g casein peptone 

(Oxoid L 42), 2.5 g yeast extract (Oxoid L 21), 1.0 g glucose, and 9.0 g agar (Oxoid L 

11) in distilled water. Adjust pH to 6.8 ± 0.2 and autoclave for 15 minutes at 121o C. 

2. Obtain serial dilutions of the B. subtilis spore sample using 0.001 M phosphate-saline 

buffer. 

3. Vacuum filter 100 mL of diluted sample through a 47 mm x 0.45 μm membrane filter 

(Gelman Sciences, Ann Arbor, MI). 

4. Place filter onto a Petri dish containing hardened agar and cover plates. 

5. Incubate plates 24 ± 2 hours at 37 ± 1oC. 

6. Count the number of colonies formed with the aid of a colony counter. 

7. Record the number of colonies per dish, and the B. subtilis spore sample volume and 

dilution. If it is not possible to distinguish individual colonies because of confluent 

growth, record the plate counts as “TNTC”. 

8. Calculate the B. subtilis spore concentration in the original samples as cfu/mL using 

Equation D.1. 

D.6 References

Adams, M.H.1959. Bacteriophage. New York: Interscience publication.

APHA/AWWA/WEF. 1998. Standard methods for the examination of water and wastewater, 

20th edition. Washington DC: American Public Health Association, American Water 

Works Association, and Water Environment Federation. 

DVGW. 1997. UV disinfection devices for drinking water supply––Requirements and testing. 

Bonn, Germany: German Gas and Water Management Union (DVGW). 

Havelaar, A.H., C.C.E. Meulemans, W.M. Pot-Hogeboom, and J. Koster. 1990. Inactivation of 

bacteriophage MS2 in wastewater effluent with monochromatic and polychromatic 

ultraviolet light. Water Research 24, no. 2:1387-1393. 

Hoyer, O. 2002. Personal communication. 

Proposal Draft
Appendix D. Microbiological Methods 

UV Disinfection Guidance Manual D-7 June 2003

Meng, Q.S. and C.P. Gerba. 1996. Comparative inactivation of enteric adenovirus, poliovirus 

and coliphages by ultraviolet irradiation. Wat. Res. 30:2665-2668. 

Munakata, N. and C.S. Rupert. 1972. Genetically controlled removal of “spore photoproduct” 

from deoxyribonucleic acid of ultraviolet-irradiated bacillus subtilis spores. J. Bacteriol. 

111:192-198. 

Nieuwstad, T.J. and A.H. Havelaar. 1994. The kinetics of batch ultraviolet inactivation of 

bacteriophage MS2 and microbiological calibration of an ultraviolet pilot plant. J. 

Environ. Sci. Health A29, no.5:1993-2007. 

NWRI/AwwaRF. 2000. Ultraviolet disinfection guidelines for drinking water and water reuse. 

National Water Research Institute and AwwaRF

Oppenheimer, J.A., J.E. Hoagland, J.-M. Laine, J.G. Jacangelo, and A. Bhamrah. 1993. 

Microbial inactivation and characterization of toxicity and by-products occurring in 

reclaimed wastewater disinfected with UV radiation. Water Environment Federation 

(WEF) Specialty Conference: Planning, design & operation of effluent disinfection 

systems. Whippany, New Jersey. 

Sommer, R. and A. Cabaj. 1993. Evaluation of the efficiency of a UV plant for drinking water 

disinfection. Water Science & Technology 27, no. 7-8:357-362. 

Sommer, R., A. Cabaj, D. Schoenen, J. Gebel, A. Kolch, A.H. Havelaar, and F.M. Schets. 1995. 

Comparison of three laboratory devices for UV-inactivation of microorganisms. Wat. Sci. 

Tech. 31:147-156. 

Sommer, R., A. Cabaj, and T. Haider. 1996. Microbiocidal effect of reflected UV radiation in 

devices for water disinfection. Water Science & Technology 34, no.5-6:173-177. 

Sommer, R., T. Haider, A. Cabaj, W. Pribil, and M. Lhotsky. 1998. Time dose reciprocity in UV 

disinfection of water. IAWQ, Vancouver. 

Tree, J.A., M.R. Adams, and D.N. Lees. 1997. Virus inactivation during disinfection of 

wastewater by chlorination and UV irradiation and the efficacy of f+ bacteriophage as a 

'viral indicator'. Water Science & Technology 35:227-232. 

Yahya, M.T., T.M. Straub, and C.P.Gerba. 1992. Inactivation of coliphage MS-2 and poliovirus 

by copper, silver, and chlorine. Canadian Journal of Microbiology. 38, no.6:430-435. 

Proposal Draft
Appendix E. Collimated Beam Apparatus: Measuring 

Challenge Microorganism UV Dose-Response 

The challenge microorganism's UV dose-response should be measured using a benchscale device referred to here as a “collimated beam apparatus” (Figure E.1). The apparatus 

delivers UV light to a microbial suspension usually contained within a completely mixed batch 

reactor. The UV light enters the suspension with a near zero degree angle of incidence and is 

relatively homogenous across the surface area. UV dose delivered to the suspension is 

calculated using measurements of incident UV intensity, exposure time, suspension depth, and 

the absorption coefficient of the suspension. By measuring microbial inactivation in the 

suspension as a function of UV dose, the microorganism's dose-response is determined. 

Figure E.1 Collimated Beam Apparatus 

Low-Pressure

Mercury Arc Lamp Lamp Enclosure

Petri Dish Containing

Microbial Suspension

Magnetic Stirrer

UV Light @ 254 nm

Collimating Tube

UV Disinfection Guidance Manual E-1 June 2003

Proposal Draft 
Appendix E. Collimated Beam Apparatus: Measuring Challenge Microorganism UV Dose-Response 

 

UV Disinfection Guidance Manual E-2 June 2003

This appendix describes the following components of collimated beam testing: 

• Collimated beam apparatus design and operation (section E.1)

• Procedure for irradiating samples using apparatus (section E.2) 

• Calculation of UV dose delivered by the apparatus (section E.3) 

• Quality Assurance / Quality Control (QA/QC) procedures (section E.4) 

• Reporting results (section E.5) 

E.1 Apparatus Design and Operation 

Because UV dose requirements are based on the pathogen inactivation achieved using

253.7 nm light, the collimated beam apparatus must use a lamp that emits germicidal UV light 

only at 254 nm (e.g., a low-pressure lamp). To prevent ozone formation, lamps that emit 185 nm 

light should not be used. The output from the lamp measured using a radiometer or equivalent 

should not vary more than 5 percent over the exposure time. A stable lamp output can be 

obtained by driving the lamp with a constant power source and maintaining the lamp at a 

constant operating temperature. A voltage regulator may be used to obtain a stable power supply 

to the lamps if the line voltage is not sufficiently stable. A stable temperature can be obtained by 

controlling the airflow around the lamp. 

The UV lamp should be located far enough above the surface of the microbial suspension 

that uniform irradiance is obtained across the sample's surface and UV light enters the 

suspension with a near zero degree angle of incidence (Blatchley 1997). A recommended 

minimum distance from the lamp to the suspension is six times the longest distance across the 

suspension’s surface. In order to vary the UV intensity incident on the suspension, the distance 

between the suspension and the lamp can be adjusted. 

The uniformity of the intensity field across the sample’s surface should be assessed by 

measuring the “Petri Factor,” the ratio of the average irradiance across the suspension surface to 

the irradiance measured at the center (Bolton and Linden 2003). The average irradiance is 

determined by averaging radiometer measurements taken at each point in a 5 mm spaced grid 

across an area defined by the suspension's surface. If the radiometer's sensing window is wider 

Proposal Draft 
Appendix E. Collimated Beam Apparatus: Measuring Challenge Microorganism UV Dose-Response 

UV Disinfection Guidance Manual E-3 June 2003

than 5 mm, it should be reduced using a cover slip with a small hole. In general, the collimated 

beam apparatus should have a Petri Factor greater than 0.9. 

The lamp and the light path from the lamp to the suspension should be enclosed to protect 

the user from exposure to UV light. A box-like enclosure made of aluminum is often used. A 

length of pipe is often used to enclose the light path from the lamp to the microbial suspension. 

The inside surface of the pipe should have a low UV reflectance and incorporate apertures to 

improve UV light collimation (Blatchley 1997). A shutter mechanism is sometimes used to 

control the exposure of the suspension to UV light. The exposure times should be measured with 

an uncertainty of 5 percent or less. Exposure times less than 20 seconds are not recommended. 

The microbial suspension should be irradiated in an open cylindrical container with a 

constant cross-sectional area (e.g., Petri dish). The diameter of the container should be smaller 

than the diameter of the light beam incident on the container. Sample depth should be 0.5 to 

2 cm. The material of the container should not adsorb the challenge microorganism enough to 

impact its measured dose-response. 

Sample volumes irradiated in the container should be sufficient for measuring the 

challenge microorganism's concentration after irradiation. The microbial suspension should be 

mixed using a stir bar and a magnetic stirrer at a rate that does not induce vortices. The volume 

and diameter of the stir bar should be small relative to the volume and depth of the sample 

volume. 

The irradiance at the center of the suspension's surface before and after exposure to UV 

light should be measured using a radiometer calibrated at 254 nm. The radiometer calibration 

should be National Institute of Standards and Technology (NIST) traceable or equivalent with a 

known measurement uncertainty. During measurement, the radiometer's calibration plane should 

match the height of the suspension's surface and be perpendicular to the incident UV light. The 

calibration plane of the radiometer should be specified in the radiometer’s calibration certificate. 

The depth of the suspension, including the stir bar volume, should be measured with an 

uncertainty of 10 percent or less. The UV absorption coefficient of the microbial suspension at 

254 nm should be measured using a spectrophotometer with a measurement uncertainty of 

10 percent or less. If scattering of light by the microorganisms and other particulate matter 

within the suspension is significant, the UV absorption coefficient should be measured using a 

spectrophotometer equipped with an integrating sphere (Linden and Darby 1998). While 1 cm

cuvettes are typically used for measuring UV absorption coefficients, cuvettes with longer 

pathlengths are recommended to reduce the measurement uncertainty with low UV absorbance 

samples. 

E.2 Procedure

Personnel who perform bench-scale UV irradiation should be experienced with the use 

and safety requirements of the equipment. Safety goggles and latex gloves should be worn. Skin 

should be shielded from exposure to UV light. The following procedure is recommended for 

irradiating a water sample using the collimated beam apparatus: 

1. Define the target UV dose. 

Proposal Draft 
Appendix E. Collimated Beam Apparatus: Measuring Challenge Microorganism UV Dose-Response 

UV Disinfection Guidance Manual E-4 June 2003

2. Measure the UV absorption coefficient of the water sample. 

3. Place a known volume from the water sample into a container and add a stir bar. 

4. Measure the water depth in the container. 

5. Measure the UV irradiance delivered by the collimated beam. 

6. Calculate the exposure time to deliver the target dose. 

7. Block the light from the collimating tube using a shutter or equivalent. 

8. Center the container containing the water sample under the collimating tube. 

9. Unblock the light from the collimating tube and start the timer. 

10. When the target exposure time has elapsed, block the light from the collimating tube. 

11. Remove the container and collect the sample for measurement of the challenge 

microorganism concentration. If the sample is not assayed immediately, store in the 

dark at 4oC. 

12. Re-measure the UV irradiance and calculate the average of the two measurements. 

13. Using Equation E.1, calculate the applied dose using the measured irradiance, UV 

absorption coefficient, sample depth, and exposure time. 

14. Repeat the procedure for various target dose values. The UV dose-response curve is 

a plot of the microorganism concentration as a function of the applied dose. 

E.3 Dose Calculation 

Dose delivered to the sample is calculated using Equation E.1: 

t

d L ad

L D E P R

ad

s f ln(10)

(1 10 )

( ) (1 ) −+ = − Equation E.1 

where 

D = UV dose in mJ/cm2

Es = UV irradiance at the center of the suspension’s surface in mW/cm2

Pf = Petri Factor

R = Reflectance at the air-water interface at 254 nm

L = Distance from lamp centerline to suspension surface in cm

d = Depth of the suspension in cm

a = UV absorption coefficient (Base 10) of the suspension at 254 nm in cm-1

t = Exposure time in seconds 

Proposal Draft 
Appendix E. Collimated Beam Apparatus: Measuring Challenge Microorganism UV Dose-Response 

UV Disinfection Guidance Manual E-5 June 2003

The term L/(d+L) accounts for the divergence of the UV light from the collimated beam 

as it passes through the suspension. The reflectance at the air-water interface estimated using 

Fresnel’s Law is 0.025 given an index of refraction of 1.000 and 1.372 for air and water, 

respectively.

Alternatively, given a target dose, the exposure time may be calculated by re-arranging 

Equation E.1 to form Equation E.2: 

(1 )(1 10 )

ln(10)( )

ad EsPf L R

ad d L

t D − −

+ = Equation E.2

where 

variables are defined as in Equation E.1 

The measurement uncertainty of the dose delivered by the collimated beam should be 

assessed at an 80 percent confidence interval with consideration of each term in Equation E.1. 

The measurement uncertainty of each term in Equation E.1 can be determined from the 

measurement uncertainty stated for the instrumentation used to measure those quantities and the 

standard deviation of repeated measurements made with that instrumentation (Taylor 1997). If 

the uncertainty of the measurement of the suspension depth and the UV absorption coefficient is 

less than 10 percent at a 80 percent confidence level and the product ad is less than 0.1, the 

uncertainty of the term (1-10-ad)/ad can be assumed as 4 percent. This assumption allows the use 

of the sum of variances approach to calculate the uncertainty of the dose delivered by the 

collimated beam. 

Example. A pipette with an accuracy of 0.2 mL is used to place a 25 mL microbial 

sample in a Petri dish. The incident irradiance of 1.00 mW/cm2

 is measured using a radiometer. 

The uncertainty of the radiometer measurement indicated by the calibration certificate is 

7 percent. The suspension is irradiated for 60 seconds. The irradiation time is monitored using a 

stopwatch with an uncertainty of ±1 second. The Petri dish radius, measured using a ruler with 

1 mm graduations, is 2.5 ± 0.1 cm. The stir bar volume is estimated as 1 ± 0.1 mL. The UV 

absorption coefficient of the microbial sample at 254 nm is 0.050 ± 0.005 cm-1. The Petri factor 

of 0.90 ± 0.02 is calculated for the collimated beam apparatus. The distance from the lamp to the 

surface of the suspension is determined using a ruler as 25 ± 1 cm. 

The depth in the Petri dish is calculated as the sum of the suspension and stir bar volumes 

divided by the area of the Petri dish. 

( ) ( )

( ) l

Volume

Area

25 0.2 cm 1 0.1 cm

p 2.5 0.1cm

1.32 0.07cm

3 3

= = 2

± + ±

±

= ±

The UV dose is calculated as: 

Proposal Draft 
Appendix E. Collimated Beam Apparatus: Measuring Challenge Microorganism UV Dose-Response 

UV Disinfection Guidance Manual E-6 June 2003

( )( )( )

[ ( )

( ) ]

( )( ) ( )

( )( ) ( ) D ( ) 1.00 mW/ cm 0.90 1 0.025

 1

1.32cm

25cm

1 10

0.050cm 1.32 cm ln 10

60s 46 mJ/cm

2 0.050 cm 1.32 cm

1

2

1

= −

+

−

=

−

−

−

Because the uncertainty of the sample depth (± 0.07 cm) and the measured UV 

absorption coefficient (± 0.005 cm-1) is less than or equal to 10 percent of the sample depth and 

the product of the sample depth and UV absorption coefficient is less than 0.1, the uncertainty of 

the term (1-10-ad)/ad is assumed as 4 percent. The uncertainties of the terms in the dose 

calculation are as follows: 

• Incidence irradiance 7 percent 

• Petri factor 2 percent 

• L/(d+L) 0.3 percent 

• Time 2 percent 

• (1-10-ad)/ad 4 percent 

The uncertainty of the dose calculation is calculated using the sum of variances approach 

as: 

Uncertainty ( ) 7 2 0.3 2 4 8.5% 1/ 2 2 2 2 2 2 = + + + + =

E.4 Quality Assurance and Quality Control 

QA/QC measures include: 

• Designing the collimated beam apparatus with a Petri factor greater than 0.9 

• Selecting instrumentation and methods that minimize the measurement uncertainty of

dose delivery by the collimated beam apparatus 

• Calibrating all radiometers at regular intervals as recommended by the manufacturer 

• Using a reference radiometer or equivalent method to regularly check the 

measurement accuracy of the radiometer used to measure incident irradiance 

• Ensuring irradiance measurements before and after exposure to UV light do not differ 

by more than 5 percent 

• Ensuring replicate UV inactivation curves do not differ significantly 

Proposal Draft 
Appendix E. Collimated Beam Apparatus: Measuring Challenge Microorganism UV Dose-Response 

UV Disinfection Guidance Manual E-7 June 2003

• Ensuring the UV dose-response of the challenge microorganism lies within expected 

bounds as defined by published dose-response data 

E.5 Reporting 

The following information should be documented and included with the validation test 

report: 

• Lamp type 

• Distance from the light source to the sample surface 

• Radiometer make and model 

• Measurement uncertainty of the radiometer and date of last calibration 

• Comparison of working and reference radiometers 

• Volume and depth of the microbial suspension 

• UV absorption coefficient of the microbial suspension measured at 254 nm 

• Irradiance measurement before and after each irradiation 

• Petri factor calculations and results 

• Method of dose determination 

• UV dose calculations 

• Uncertainty assessment 

E.6 References

Blatchley, E.R. 1997. Numerical modeling of UV intensity: Application to collimated-beam

reactors and continuous-flow systems. Water Research 31:2205-2218. 

Bolton. J. and K. Linden. 2003. Standardization of methods for fluence (UV Dose) 

determination in bench-scale UV experiments. J. Environ. Eng. 129, no.3:209-216. 

Linden, K.G. and J.L. Darby. 1998. UV Disinfection of Marginal Effluents: determining UV 

Absorbance and Subsequent Estimation of UV Intensity. Water Environment Research

70(2).

Taylor, J.R. 1997. An introduction to error analysis: the study of uncertainties in physical 

measurements. Sausalito, CA: University Science Books.

Proposal Draft 
UV Disinfection Guidance Manual F-1 June 2003

Proposal Draft

Appendix F. Background to the UV Reactor

Validation Protocol

This appendix provides background material for the validation protocol given in

Appendix C. The background material is organized into the following six sections.

• Dose delivery by UV reactors. Section F.1 describes how the RED of a challenge

microorganism measured during UV reactor validation is related to the capacity of the

UV reactor to inactivate a target pathogen. This section describes why correction

factors should be applied to the reduction equivalent dose (RED) of the challenge

microorganism to account for systematic errors that arise if the challenge

microorganism is more resistant to UV light as compared to the target pathogen. The

section concludes by describing approaches for selecting one or more challenge

microorganisms to minimize those errors.

• Dose monitoring. Section F.2 describes three approaches whereby measurements of

flowrate, UV intensity, and water UV transmittance (UVT) are used by UV reactors

to indicate dose delivery. This section discusses the importance of UV intensity

sensor placement within a UV reactor and provides a rationale for defining test

conditions to validate UV reactors using a given dose monitoring approach.

• UV intensity sensors. Section F.3 describes the properties of UV intensity sensors,

how those properties impact the sensor’s measurement uncertainty, and how that

measurement uncertainty is used to define rejection criteria for using reference

sensors to check the accuracy of duty sensors. The section also discusses how nonuniform lamp aging and fouling and the variability in lamp output affects the use of

UV intensity sensors.

• Polychromatic considerations. Section F.4 describes systematic errors that can occur

with the validation of UV reactors equipped with medium-pressure UV lamps. This

section provides approaches for assessing those errors and for defining correction

factors that should be applied to validation data.

• Uncertainty of monitoring and dose factors. Section F.5 provides a rationale for

defining a safety factor that accounts for the random uncertainty associated with UV

reactor validation and monitoring.

• Re-validation. Section F.6 discusses how some changes to a UV reactor design made

by a manufacturer would trigger a need to re-validate the UV reactor.

F.1 Dose Delivery by UV Reactors

Dose delivered to an individual microorganism passing through a UV reactor is defined

as the integral of UV intensity over time:
Appendix F. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual F-2 June 2003

∫ =

rt

D Idt

0

 Equation F.1 

where 

D = Dose delivered to the microorganism by the UV reactor

I = UV intensity incident on the microorganism as it travels through the UV reactor 

t = time 

tr = Residence time of the microorganism within the UV reactor 

Because each microorganism passing through the UV reactor follows a unique trajectory, 

each microorganism is exposed to a unique dose. For example, microorganisms passing through 

the UV reactor close to the lamps are exposed to higher UV intensities as compared to 

microorganisms traveling near the reactor walls or between lamps. Microorganisms caught in 

eddies or dead zones spend more time within the UV reactor as compared to microorganisms that 

pass through the reactor quickly due to hydraulic short-circuiting. Because each microorganism

is exposed to a different UV dose, dose delivery by the UV reactor is best described using a dose 

distribution, as opposed to a single dose value. A dose distribution describes the probability that 

a microorganism passing through a UV reactor will receive a given dose. Figure F.1 presents an 

example of a dose distribution for a UV reactor.

Model-based and experimental approaches have been identified to determine the dose 

distribution of a UV reactor. Model-based approaches use computational fluid dynamics (CFD) 

to predict microorganism trajectories through a UV reactor and hence the dose delivered to each 

microorganism. Experimental approaches use microspheres that undergo a chemical reaction 

when exposed to UV light. The microspheres are injected upstream of the UV reactor and are 

collected downstream. The extent of the UV-induced chemical reaction within each sphere is 

measured and used to calculate the dose delivered to that sphere as it traveled through the 

reactor. While promising, both model and experimental-based approaches are subjects of current 

research. Further effort is necessary to prove these approaches as practical and accurate 

predictors of UV reactor performance. 

Dose delivery by UV reactors is currently measured using biodosimetry (Qualls and 

Johnson 1983). With biodosimetry, inactivation of a challenge microorganism passed through 

the UV reactor is measured and related to a single dose value based on the known UV 

dose-response of that microorganism. This dose value is termed the RED. 

Proposal Draft
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-3 June 2003

Proposal Draft

Figure F.1 Dose Distribution Delivered by a UV Reactor

1

1

(Adapted from Chiu et al. 1999)

F.1.1 Relationship Between RED and the Dose Distribution

The RED of a given microorganism depends on the dose distribution delivered by the

reactor and the UV inactivation kinetics (dose-response) of the challenge microorganism (Cabaj

et al. 1996). A general equation describing this dependence is Equation F.2:

( ) ( )

=

=

j

i 1

i Di

f RED p f Equation F.2

where

RED = RED measured using biodosimetry

f = Mathematical function describing the inactivation kinetics of the microorganism

j = Number of dose values in the dose distribution

Di = i

th

dose in the dose distribution

pi = Probability of occurrence of dose Di

For example, if the microorganism has first order inactivation kinetics, the function f is

shown in Equation F.3:

N f(D) N exp( kD.) = = 0 − Equation F.3

where

N = Microorganism concentration after exposure to dose D

N0 = Microorganism concentration with zero UV dose

D. = Applied UV dose

k = Microorganism’s first order inactivation coefficient
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-4 June 2003

Proposal Draft

Substituting Equation F.3 into F.2 gives the following equation for the RED of a

microorganism with first-order inactivation kinetics:

( )

















= − −

=

j

i

i i p kD

k

RED

1

ln exp

1

Equation F.4

In equation F.4, the RED depends on the dose distribution of the UV reactor and the first

order inactivation coefficient of the microorganism.

Figure F.2 presents the dependence of the RED on the first order inactivation coefficient

of the challenge microorganism for the dose distribution shown in Figure F.1. The relation was

calculated using Equation F.4. As shown, the RED of a microorganism with a small first-order

inactivation coefficient is greater than the RED of a microorganism with a large first-order

inactivation coefficient. Because the RED depends on the microorganism’s UV inactivation

kinetics, the RED of the challenge microorganism is an exact measure of the RED delivered to a

pathogen of interest only when the challenge microorganism has the same inactivation kinetics

as the pathogen (Wright and Lawryshyn 2000).

Example 1. A UV reactor delivers a dose distribution represented by Figure F.1. The

UV reactor is evaluated using biodosimetry. The challenge microorganisms are MS2

bacteriophage (MS2) with a first order coefficient of 0.13 cm

2

/mJ and X174 phage with a first

order coefficient of 1.2 cm

2

/mJ. As shown in Figure F.2, MS2 would have experienced 1.1 log

inactivation, corresponding to an RED of 19 mJ/cm

2

. φX174 would have experienced 3.6 log

inactivation, corresponding to an RED of 7.3 mJ/cm

2

. If the pathogen of interest has the same

inactivation kinetics as X174, the RED of MS2 would be 2.5 times greater than the RED

delivered to the pathogen, while the RED of X174 would be an exact measure of the RED

delivered to the pathogen.
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-5 June 2003

Proposal Draft

Figure F.2 Microorganism Log Inactivation and RED as a Function of the

Microorganism’s First Order Inactivation Coefficient for the UV Reactor

Represented in Figure F.1

F.1.2 Using RED to Demonstrate Target Pathogen Inactivation

If the UV dose-response of the challenge microorganism differs from that of the target

pathogen and the dose distribution of the UV reactor is not known, the results of biodosimetry

can only be used to estimate the target pathogen inactivation within a range bounded by the

inactivation expected assuming ideal and worst-case hydraulics. Figure F.3 provides a

comparison of the dose distribution of reactors with ideal and worst-case hydraulics to a dose

distribution that might be seen with a real reactor.

Figure F.3 Comparison of the Dose Distributions

of Ideal, Realistic, and Worst-Case UV Reactors

A reactor with ideal hydraulics delivers the same dose to all the microorganisms passing

through the reactor. Its dose distribution is represented by a single dose. Examples of a UV

reactor with ideal hydraulics include the stirred suspension irradiated during the measurement of

UV dose-response by a collimated beam device and a plug flowrate reactor with complete lateral

mixing. In both cases, the UV dose delivered is the product of the average UV intensity within

0 50 100

UV Dose (mJ/cm

2

)

Pro

b

a

bility

Ideal

0 50 100

UV Dose (mJ/cm

2

)

Pro

b

a

bility

Reality

0 80

UV Dose (mJ/cm

2

)

Pro

b

a

bility

Worst Case

0 Infinity
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-6 June 2003

Proposal Draft

the reactor and the residence time. With an ideal reactor, Equation F.5 shows the net microbial

inactivation achieved by the reactor:

p f(D ) f(D) N

N

j

i 1

i i

0

= =

=

Equation F.5

Accordingly, with an ideal reactor, the RED measured with a challenge microorganism is

a measure of the RED delivered to all microorganisms that pass through the reactor. If both the

challenge microorganism and the pathogen have first order inactivation kinetics, the log

inactivation of the pathogen is calculated using Equation F.6:

( ( )) p P

p

D

RED

k RED

N

N

0 10

log = − log exp − = 















Equation F.6

where

log (N/N0)p = Log inactivation of the pathogen

kp = First order inactivation coefficient of the pathogen

RED = RED observed with the pathogen

D10

p

= UV sensitivity of the pathogen expressed as mJ/cm

2

per log

The UV sensitivity of the pathogen is related to the first order inactivation coefficient

using Equation F.7:

( )

k k

D

ln 10 2.30

10 = = Equation F.7

A UV reactor with worst-case hydraulics delivers a UV dose of zero to all

microorganisms passing through the reactor. However, in the case of a reactor with a

measurable RED, worst-case hydraulics is defined as infinite dose delivered to one fraction of

the flowrate and zero dose delivered to the other fraction. Under these conditions, the net

microbial inactivation achieved by the reactor is calculated according to Equation F.8:

( ) ( ) ( ) 1 2 1

j

i 1

i i

0

p f D p f 0 p f p N

N = = + ∞ =

=

Equation F.8

As shown, the net inactivation achieved by the worst-case UV reactor with a measurable

RED is constant and independent of the inactivation kinetics of the microorganism. With a

worst-case UV reactor, the measured inactivation is a measure of the inactivation that would

occur with all microorganisms regardless of their UV sensitivity. In other words, the log

inactivation of the pathogen is calculated according to Equation F.9:
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-7 June 2003

Proposal Draft

p c

N

N

N

N

















= 















0 0

log log Equation F.9

where

log (N/N0)c = log inactivation of the challenge microorganism

Using the above definitions of an ideal and a worst-case reactor, the log inactivation of a

pathogen estimated from biodosmetry results will have a value between log(N/No)c and RED/Dp.

If the inactivation of the pathogen must be known with absolute confidence, the lower bound of

that range should be used. If the challenge microorganism is more resistant to UV light than the

pathogen, the lower bound is log(N/No)c. If the challenge microorganism is less resistant to UV

light than the pathogen, the lower bound is RED/Dp.

Example 2. A UV reactor is challenged using MS2 with a UV sensitivity of 18 mJ/cm

2

per log inactivation. Four log inactivation of the MS2 is observed corresponding to an MS2

RED of 4 × 18 = 72 mJ/cm

2

. The MS2 results are used to estimate the log inactivation of two

pathogens, one with a UV sensitivity of 10 mJ/cm

2

per log inactivation and the other with a UV

sensitivity of 25 mJ/cm

2

per log inactivation. The log inactivation of the first pathogen is

estimated between 4.0 and 72/10 = 7.2 log and the log inactivation of the second pathogen is

estimated between 72/25 = 2.9 and 4.0 log. The biodosimetry results can be used to state with

absolute confidence that the inactivation of the first pathogen was at least 4.0 log and the

inactivation of the second pathogen was at least 2.9 log.

Example 3. A UV reactor is designed for 3.0 log Cryptosporidium inactivation. MS2 is

used to measure the performance of the UV reactor. Because MS2 is more resistant to UV light

than Cryptosporidium, 3.0-log MS2 inactivation must be measured to state with absolute

confidence that the reactor achieves 3.0-log Cryptosporidium inactivation.

Example 4. A UV reactor is designed for two log adenovirus inactivation. Two-log

adenovirus inactivation occurs using a UV dose of 100 mJ/cm

2

. The UV reactor is validated

using MS2. Because adenovirus is more resistant to UV light than MS2, a RED of 100 mJ/cm

2

must be measured with MS2 to state with absolute confidence that the UV reactor achieves 2 log

adenovirus inactivation.

Because UV manufacturers strive to optimize the hydraulic design of their UV reactors,

using the worst-case dose distribution represented in Figure F.3 to define the lower bound of

pathogen inactivation is overly conservative. An alternative approach is to use the dose

distribution of a commercial UV reactor that is representative of worst-case reactor hydraulics.

However, defining a worst-case commercial UV reactor is difficult because little data are

available in the peer-reviewed UV disinfection literature on dose distributions. Chiu et al. (1999)

used measured velocity fields and a random walk model to predict the dose distribution delivered

by a wastewater reactor equipped with low-pressure (LP) lamps oriented perpendicular to

flowrate. The dose distribution was bimodal due to a short-circuiting path along the reactor

walls. Wright and Lawryshyn (2000) compared the dose distribution of four reactor designs

using CFD-based dose modeling including the reactor modeled by Chiu et al. Based on this

comparison, the dose distribution developed by Chiu et al. is believed to represent a worst-case

commercial UV reactor.
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-8 June 2003

Proposal Draft

Figure F.1 presents a dose distribution adapted from Chiu et al.’s data. For that dose

distribution, Figure F.2 presents log inactivation and RED as a function of the microorganism’s

UV sensitivity expressed as a first-order inactivation coefficient. Figure F.4 presents the same

relationship, but with UV sensitivity expressed as dose per log inactivation. Using these figures,

the RED delivered to a pathogen by a given UV reactor can be estimated from the measured

RED of the challenge microorganism using Equation F.10:

*

*

c

p

p c

RED

RED

RED = RED × Equation F.10

where

REDP = RED of the pathogen estimated for the UV reactor of interest

REDc = RED of the challenge microorganism measured during biodosimetry

REDp = RED of the pathogen estimated from Figure F.2 or F.4

REDc = RED of the challenge microorganism estimated from Figures F.2 or F.4

The RED determined using Equation F.10 represents the RED that would be delivered if

the reactor under consideration had a dose distribution representative of a worst-case commercial

reactor.

Figure F.4 Microorganism Inactivation and RED as a Function of Microorganism

UV Sensitivity for the UV Reactor Represented in Figure F.1

Example 5. A UV reactor is challenged using MS2 with a UV sensitivity of 18 mJ/cm

2

per log inactivation. Four log inactivation of the MS2 is observed corresponding to an MS2

RED of 4 × 18 = 72 mJ/cm

2

. The MS2 results are used to estimate the log inactivation of two

pathogens, one with a UV sensitivity of 10 /cm

2

per log inactivation and the other with a UV

sensitivity of 25 mJ/cm

2

per log inactivation. In Figure F.4, the RED delivered to the

microorganisms with a UV sensitivity of 10, 18, and 25 mJ/cm

2

per log inactivation is 15, 19,
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-9 June 2003

Proposal Draft

and 21 mJ/cm

2

, respectively. Assuming the UV reactor’s performance is bounded by a worst

case represented by Figure F.4, the RED delivered to the first pathogen is estimated between 72

mJ/cm

2

and (72 × 15)/19 = 57 mJ/cm

2

and the RED delivered to the second pathogen is

estimated between 72 and (72 × 21)/19 = 80 mJ/cm

2

. Inactivation of the first pathogen is

estimated between 5.7 (57/10) and 7.2 (72/10) log and inactivation of the second pathogen is

estimated between 2.9 (72/25) and 3.2 (80/25) log inactivation. This range of inactivation

estimated using the worst-case represented in Figure F.4 is notably less than the range estimated

in Example 3 using the worst-case represented in Figure F.3.

For regulatory purposes, the lower bound of the range of inactivation and RED estimated

for the pathogen should be used when relating challenge microorganism inactivation to target

pathogen inactivation. If the challenge microorganism is more sensitive to UV light than the

pathogen or if both have the same sensitivity, the RED delivered to the pathogen should be

estimated using the RED of the challenge microorganism. If the challenge microorganism is

more resistant to UV light than the pathogen, the RED delivered to the pathogen should be

estimated using Equation F.10.

Example 6. A UV reactor is designed for three log Cryptosporidium inactivation. The

dose needed for 3 log Cryptosporidium taken from Chapter 1 (Table 1.4) is 12 mJ/cm

2

.

Accordingly, the UV sensitivity of Cryptosporidium is defined as 12/3 = 4 mJ/cm

2

per log

inactivation. MS2 with a UV sensitivity of 18 mJ/cm

2

per log inactivation is used to measure the

performance of the UV reactor. Because MS2 is more resistant to UV light than

Cryptosporidium, Equation F.10 is used to relate the RED measured using MS2 to the dose

delivered to Cryptosporidium. From Figure F.4, the RED delivered to the microorganisms with

a UV sensitivity of 3.9 and 18 mJ/cm

2

per log inactivation is 9.8 and 19, respectively. Thus an

MS2 RED of 12×19/9.8 = 23 mJ/cm

2

should be demonstrated to show 3 log Cryptosporidium

inactivation.

Example 7. A UV reactor is designed for one-log adenovirus inactivation. The dose

needed for 1-log adenovirus taken from Chapter 1 (Table 1.4) is 58 mJ/cm

2

. MS2 with a UV

sensitivity of 18 mJ/cm

2

is used to measure the performance of the UV reactor. Because MS2 is

less resistant to UV light than adenovirus, an MS2 RED of 58 mJ/cm

2

should be demonstrated to

show 1-log adenovirus inactivation.

The RED of microorganisms with shoulders and tailing within the dose-response curve

depends on the overlap of the dose distribution with those regions (Cabaj et al.1996, Wright and

Lawryshyn 2000). To use Figure F.2 to define safety factors, the inactivation of the challenge

microorganism should demonstrate an exponential inactivation as a function of dose over the

range of doses in the dose distribution. This creates a dilemma if the dose distribution is not

known. To avoid this issue, the dose-response of an appropriate challenge microorganism

should not demonstrate a shoulder at a dose beyond 50 percent of the demonstrated RED and

should not demonstrate tailing until one log inactivation beyond the demonstrated inactivation.

In the case of a challenge microorganism with a shoulder and tailing in the dose-response, the

UV sensitivity will be defined as the sensitivity over the region of exponential inactivation that

occurs between the shoulder and the onset of tailing. The shoulder of the dose-response is

defined by the intersect of the exponential region with the dose axis (see Figure F.5).
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-10 June 2003

Proposal Draft

Example 8. Figure F.5 presents the measured UV dose-response of B. subtilis spores.

Because the measured dose-response has a shoulder of 16.5 mJ/cm

2

, the B. subtilis spores should

only be used to demonstrate RED values greater than or equal to 2 × 16.5 = 33 mJ/cm

2

.

Figure F.5. UV Dose-Response of B. subtilis Spores

(Adapted from Sommer et al. 1998)

The RED safety factor provides an incentive to select a challenge microorganism whose

UV sensitivity matches that of the target pathogen and a disincentive for overrating UV reactor

performance by using challenge microorganisms whose UV sensitivity is much greater than the

target pathogen.

F.1.3 Biodosimetry Using Two Challenge Microorganisms

In order to provide a better estimate of the target pathogen’s log inactivation and RED,

two microorganisms with different UV sensitivities can be used to validate UV reactors. The

target pathogen’s log inactivation should be estimated by interpolating the log inactivation of the

two microorganisms as a function of the UV sensitivity defined on a linear scale as a first-order

inactivation coefficient. Alternatively, the target pathogen’s RED should be estimated by

interpolating the RED of the two microorganisms as a function of the UV sensitivity defined on a

linear scale as dose per log inactivation. If interpolation does not meet these provisions, the

inactivation of the pathogen will be overestimated.

Example 9. A UV reactor with a dose distribution represented in Figure F.4 is tested

using MS2 and φX174. The MS2 and φX174 have a UV sensitivity of 18 and 2 mJ/cm

2

per log

inactivation. Using biodosimetry, 1.1 and 3.6 log inactivation of MS2 and φX174 are measured.

These log inactivations correspond to RED values of 20 and 7.2 mJ/cm

2

, respectively. The RED

measured with MS2 and φX174 is fit as a function of UV sensitivity resulting in the following

equation:
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-11 June 2003

Proposal Draft

RED = 0.731×UV Sensitivity + 5.83

This equation predicts that the RED delivered to Cryptosporidium, defined with a UV

sensitivity of 3.9 mJ/cm

2

per log inactivation, is 8.7 mJ/cm

2

.

If the inactivation of the more UV-sensitive of the two challenge microorganisms is

greater than the detection limit of the assay, interpolation should be based on the level indicated

by the limitation. Because the inactivation of the UV-sensitive microorganism is

underestimated, the interpolation will be conservative and two-microorganism validation may

not offer an advantage over single microorganism validation.

Example 10. A UV reactor is evaluated using MS2 and φX174 phage. MS2 and φX174

are injected into the flowrate upstream of the reactor. Influent and effluent samples are collected

and assayed. The assay has a detection limit of 1 pfu/mL. The concentrations of MS2 and

φX174 in the influent is determined as 1,000,000 and 10,000 pfu/mL, respectively. The

concentrations of MS2 and φX174 in the effluent samples are 10,000 and 0 pfu/mL, respectively.

The results indicate that the concentration of φX174 is below the detection limit of the assay.

Accordingly, the log inactivation of MS2 and φX174 is 2 log and > 4 log, respectively. If the

UV sensitivity of MS2 and φX174 are determined to be 20 and 2 mJ/cm

2

per log, the MS2 RED

is 40 mJ/cm2 and the φX174 RED is > 8 mJ/cm

2

. The following equation fits the measured RED

as a function of UV sensitivity:

RED =1.77 ×UV Sensitivity + 4.44

This equation predicts that the RED delivered to Cryptosporidium defined with a UV

sensitivity of 3.9 mJ/cm

2

per log is 11.3 mJ/cm

2

. This compares to an RED of 20 mJ/cm

2

that

would have been predicted by Equation F.10 using the MS2 data alone. In this case, twomicroorganism biodosimetry estimated lower dose delivery to Cryptosporidium than single

microorganism biodosimetry.

In the past, it has been assumed that the RED measured with a UV-resistant challenge

microorganism can be used to demonstrate compliance with a dose target while the log

inactivation demonstrated with a UV-sensitive challenge microorganism can be used to

demonstrate compliance to a log inactivation target. This approach is not recommended. It is

not possible to demonstrate compliance to a 3-log Cryptosporidium inactivation by using UVresistant MS2 to show an RED of 11.7 mJ/cm

2

and using UV-sensitive φX174 to show

3-log inactivation.

Example 11. In Example 9, even though the RED measured with MS2 was 18 mJ/cm

2

and the log inactivation measured with φX174 was 3.6 log, Figure F.4 shows that

Cryptosporidium, defined with a UV sensitivity of 3.9 mJ/cm

2

per log, experienced a log

inactivation of 2.5 corresponding to an RED of 9.8 mJ/cm

2

.
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-12 June 2003

Proposal Draft

F.1.4 Challenge Microorganism Selection

Ideally, UV reactor performance should be validated with a microorganism whose UV

sensitivity matches that of the target pathogen. In this guidance document, the UV sensitivity of

the target microorganisms is given by the dose requirements given in Chapter 1 for

Cryptosporidium, Giardia, and virus. Challenge microorganisms currently used to validate UV

reactors do not have a UV-sensitivity that matches the UV-sensitivity of the target pathogens as

defined in Chapter 1. The UV-resistance of MS2 and B. subtilis spores is notably greater than

that of Cryptosporidium and Giardia, and notably less than that of adenovirus. Furthermore,

demonstrating 3 or 4-log virus inactivation with these challenge microorganisms necessitates

demonstrating REDs greater than 150 mJ/cm

2

. These REDs correspond to greater than 6-log

inactivation of MS2 and B. subtilis spores. Currently, culturing titers of challenge

microorganisms needed to demonstrate greater than 6-log inactivation are not practical.

A challenge microorganism should have reproducible UV inactivation kinetics over the

dose range of interest. The challenge microorganism should be easily prepared in high titers,

easily enumerated by an assay based on microorganism replication, non-pathogenic to humans,

and not harmful to the environment. If the challenge microorganism is a phage, the host bacteria

used to assay the phage concentration should not be pathogenic to humans. MS-2 phage,

non-pathogenic Escherichia coli, B. subtilis spores, and Saccharomyces cerevisae have been

used to bioassay UV reactors designed to treat drinking water. Table F.1 summarizes the UV

sensitivity of commonly-used and candidate bioassay microorganisms.

Table F.1 UV Sensitivity of Bioassay Microorganisms and Candidates

Dose (mJ/cm

2

) Reported to Achieve Microorganism 1 log 2 log 3 log 4 log

Reference

MS-2 phage 16 34 52 71 Wilson et al. 1992

E. Coli 3.0 4.8 6.7 8.4 Chang et al. 1985

B. subtilis spores 28 39 50 62 Sommer et al. 1998

φx174 phage 2.2 5.3 7.3 11 Sommer et al. 1998

B40-8 phage 12 18 23 28 Sommer et al. 1998

PRD-1 phage 9.9 17 24 30 Meng and Gerba 1996

F.2 Dose Monitoring

There are three approaches currently used to monitor dose delivery. In this guidance

document, the terms used are as follows:

• UV intensity setpoint approach

• UV intensity and UVT setpoint approach

• Calculated dose approach
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-13 June 2003

Proposal Draft

With the UV intensity setpoint approach, dose delivery is indicated by measured flowrate

and UV intensity. The UV reactor complies with a required dose delivery when the measured

UV intensity is above an alarm setpoint value defined as a function of flowrate through the

reactor. With this approach, the UV intensity sensor should be positioned far enough from the

lamp that it provides measurable responses to changing water UV absorbance (and

corresponding UVT) as well as lamp output. With the UV intensity and UVT setpoint approach,

dose delivery is indicated by measured flowrate, UV intensity, and UVT. The UV reactor

complies with a required dose delivery when the measured UV intensity and UVT are above

alarm setpoint values, both defined as a function of flowrate through the reactor. With this

approach, the UV intensity sensor should be positioned relatively close to the lamp so that it

responds primarily to changing lamp output. With the calculated dose approach, dose delivery is

indicated by a dose value calculated from measured flowrate, UV intensity, and UVT. The UV

reactor complies with a required dose delivery when the calculated dose is above an alarm

setpoint value. With this approach, there are no requirements for sensor positioning.

To illustrate the UV intensity setpoint approach and the UV intensity and UVT setpoint

approach, Figures F.6, F.7, and F.8 present the relationship between UV dose and measured UV

intensity for an annular reactor containing a single LP lamp. UV intensity was calculated using a

radial UV intensity model and UV dose was calculated assuming ideal hydraulics (Haas and

Sakellaropoulos 1979). UV intensity and dose were calculated for a fixed flowrate of 400 gpm,

water UVT ranging from 60 to 98 percent, and lamp output ranging from 20 to 100 percent. In

each figure, data are presented as plots of dose versus UV intensity sensor reading for values of

UVT specified in the legend. For each of those plots, each point at a given UVT represents, in

order of increasing dose, operation at 20, 40, 60, 80, and 100 percent lamp power. The

differences between these figures are due to sensor placement.

Figure F.6 Relationship between UV Dose and Intensity for a UV Intensity

Sensor Located to Give Dose Proportional to Measured Irradiance

0

10

20

30

40

50

60

70

0 20 40 60

Sensor (mW/cm2)

D

o

s

e (m

J/c

m2

)

60%

70%

80%

85%

90%

94%

98%

S

UVT

400 GPM 254 nm
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-14 June 2003

Proposal Draft

Figure F.7 Relationship between UV Dose and Intensity

for a UV Intensity Sensor Located Close to the Lamp

Figure F.8 Relationship between UV Dose and Intensity

for a UV Intensity Sensor Located Far from the Lamp

F.2.1 UV Intensity Setpoint Approach

Figure F.6 presents the relationship obtained when the UV intensity sensor is located at a

distance from the lamps where UV dose is proportional to measured UV intensity regardless of

the UVT and lamp output. With an ideal reactor, this sensor location occurs where the measured

intensity equals the average intensity within the reactor. Because of the proportional relationship

between dose delivery and measured intensity, a given intensity can be related to a specific level

of dose delivery.

Example 12. The UV reactor characterized in Figure F.6 is used in a disinfection

application needing a UV dose of 20 mJ/cm

2

. At a flowrate of 400 gpm, a UV intensity value S

0

10

20

30

40

50

60

70

0 5 10 15 20

Sensor (mW/cm

2

)

D

o

s

e (m

J/c

m

2

)

60%

70%

80%

85%

90%

94%

98%

S

UVT

254 nm 400 GPM

S'

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120 140 160

Sensor (mW/cm2)

D

o

s

e (m

J/c

m2

)

60%

70%

80%

85%

90%

94%

98%

S

UVT

400 GPM 254 nm

S'
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-15 June 2003

Proposal Draft

of 18 mW/cm

2

is used as an alarm setpoint to indicate the UV reactor delivers a dose of

20 mJ/cm

2

. This alarm setpoint value will indicate a dose of 20 mJ/cm

2

regardless of the UVT of

the water and the output of the lamps.

Figure F.7 presents the relationship between dose delivery and measured UV intensity

when the UV intensity sensor is placed closer to the lamp than the sensor in Figure F.6. Because

the sensor views the lamp through a relatively thin water layer, the sensor response to changing

UVT is small compared to that in Figure F.6. Accordingly, the relationship between dose

delivery and measured intensity for different values of UVT cannot be described by a single

proportional relationship. Unlike Figure F.6, a given UV intensity is not related to a specific

level of dose delivery but is related to a range of delivered doses. Accordingly, the measured

UV intensity should only be used to indicate dose delivery at the lower end of that range, which

occurs under conditions of maximum lamp power and reduced UVT.

Example 13. The UV reactor characterized in Figure F.7 is used in an application

needing a UV dose of 20 mJ/cm

2

. The UV manufacturer states that a UV intensity value S of

80 mW/cm

2

will indicate a dose of 20 mJ/cm

2

under design conditions of 85 percent UVT and 60

percent lamp output. However, as shown in Figure F.7, an intensity of 80 mW/cm

2

corresponds

to a dose ranging from 5 to 37 mJ/cm

2

. The lower end of this range occurs with lamp powers

higher that 60 percent and water UVT lower than 85 percent. For a UV intensity alarm setpoint

to ensure a dose of 20 mJ/cm

2

under all possible conditions of the water UVT and lamp output, a

setpoint value S’ of 134 mW/cm

2

should be chosen.

Figure F.8 presents the relationship between dose delivery and measured UV intensity

when the UV intensity sensor is located further from the lamps than the sensor in Figure F.6.

Because the sensor views the lamp through a relatively thick water layer, the sensor response to

changing water transmittance is large compared to that in Figure F.6. Like Figure F.7, the

relationship between dose delivery and measured intensity for different values of UVT cannot be

described by a single proportional relationship. As such, a given intensity value is related to a

range of dose values as opposed to a single value. Again, the measured UV intensity should only

be used to indicate dose delivery at the lower end of that range. However, unlike Figure F.7, the

lower end of the range occurs under conditions of reduced lamp power and maximum UVT.

Example 14. The UV reactor characterized in Figure F.8 is used in an application

needing a UV dose of 20 mJ/cm

2

. The UV reactor uses the UV intensity setpoint approach to

monitor dose delivery. A UV intensity alarm setpoint value S of 4 mW/cm

2

is proposed based on

the UV intensity measured under design conditions of 85 percent UVT and 60 percent lamp

output. However, an intensity of 4 mW/cm

2

indicates a dose ranging from 9 to 26 mJ/cm

2

. To

indicate a dose of 20 mJ/cm

2

using the UV intensity setpoint approach, a setpoint value S’ of 8

mW/cm

2

should be chosen.

The location of the UV intensity sensor within a UV reactor is selected by the

manufacturer of the UV reactor. If the UV reactor uses the UV intensity setpoint approach for

dose monitoring, the UV manufacturer should optimize the UV intensity sensor’s location to

give a proportional relationship between dose delivery and measured UV intensity similar to the

example given in Figure F.6. If the UV manufacturer does not optimize the UV intensity

sensor’s location, a given UV intensity will correspond to a range of UV doses values as opposed

to a single value. While this does not prevent the UV reactor from using the UV intensity
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-16 June 2003

Proposal Draft

setpoint approach, the monitoring approach will not be as efficient as with an optimally located

sensor because the UV reactor will be overdosing at many combinations of UVT and lamp

power that given rise to operation at the setpoint.

F.2.2 UV Intensity and UVT Setpoint Approach

If the UV intensity sensor is not at a location optimal for the UV intensity setpoint

approach, measurements of UVT can be used to provide more efficient dose monitoring. UVT

alarm setpoints combined with UV intensity alarm setpoints can be used to indicate dose delivery

providing the UV intensity sensor is placed relatively close to the lamp. With the sensor located

relatively close to the lamp, dose delivery at a given intensity and flowrate decreases with

decreasing UVT (Figure F.7). Accordingly, a UVT alarm setpoint combined with a UV intensity

alarm setpoint provides a meaningful indicator of dose delivery.

Example 15. The UV reactor characterized in Figure F.7 is used in an application

needing a UV dose of 20 mJ/cm

2

. If the UV reactor used the UV intensity setpoint approach to

monitor dose delivery, an alarm setpoint S’ of 134 mW/cm

2

would be used to indicate a dose

delivery of 20 mJ/cm

2

. This approach is not efficient because a UV intensity of 134 mW/cm

2

is

associated with a UV dose ranging from 20 to 60 mJ/cm

2

. An alternative approach for dose

monitoring is to use the UV intensity and UVT setpoint approach. Under this approach, a UV

intensity alarm setpoint S of 80 mW/cm

2

combined with a UVT alarm setpoint of 85 percent will

indicate a dose delivery of 20 mJ/cm

2

. However, the approach is still inefficient because UV

dose may range from 20 to 38 mJ/cm

2

with operation of the reactor at the setpoint conditions.

If the UV intensity sensor is located at the optimal position for the UV intensity setpoint

approach (Figure F.6), the UVT reading does not provide any additional information on dose

delivery that is not provided by the measured UV intensity. However, the measured UVT could

be used to indicate whether a UV intensity alarm condition arises from low UVT.

If the UV intensity sensor is located too far from the lamp, dose delivery at a given UV

intensity and flowrate increases with decreasing UVT (Figure F.8). As such, the UVT reading

cannot be used as an alarm setpoint to indicate dose delivery.

Example 16. The UV reactor characterized in Figure F.8 uses the UV intensity and UVT

setpoint approach to show the UV reactor delivers a dose of 20 mJ/cm

2

. The intensity alarm

setpoint is set to 5 mW/cm

2

and the UVT alarm setpoint is set to 90 percent. If the reactor was

operating with a measured UVT and UV intensity of 85 percent and 5 mW/cm

2

, the delivered

dose would be 28 mJ/cm

2

. If the reactor was operating with a UVT and UV intensity of 98

percent and 5 mW/cm

2

, respectively, the delivered dose would be 12 mJ/cm

2

. Thus the two

alarm setpoint values are not ensuring the UV reactor complies with a dose of 20 mJ/cm

2

. To

remedy this problem, the UV manufacturer should either uses the UV intensity setpoint

approach, move the UV intensity sensor closer to the lamps, or use the calculated dose approach

to monitor dose delivery.
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-17 June 2003

Proposal Draft

F.2.3 Calculated Dose Approach

Measurements of flowrate, UV intensity, and UVT can be incorporated into theoretical,

empirical, or semi-empirical calculations of dose delivery. For example, the relationships

represented in Figures F.6 to F.8 could be defined experimentally and used in an empirical

manner to calculate dose. Relationships could also be defined using advanced modeling

approaches and used to relate measured intensity to dose delivery for a given flowrate and UVT.

In theory, the dose calculation does not necessitate that the sensor be placed at any one location

within the reactor. However, if the sensor placed at a location that gives dose delivery

proportional to the sensor reading, the dose calculation does not require UVT as an input

parameter.

F.2.4 Validating Dose Monitoring

The test conditions used to validate a UV reactor should depend on the approach used to

monitor dose delivery.

If the UV reactor uses the UV intensity setpoint approach, the UV reactor is validated by

measuring the dose delivery with the UV intensity adjusted to the UV intensity alarm setpoint

value. The combination of lamp power and UVT used to achieve operation at the alarm setpoint

should be selected to capture the lower end of the dose range associated with the setpoint. If the

UV intensity sensor is located closer to the lamp than the optimal location, the UV reactor should

be validated at peak lamp power and lowered UVT. If the UV intensity sensor is located further

from the lamp than the optimal location, the UV reactor should be validated at peak UVT and

lowered lamp power. If the positioning of the UV intensity sensor relative to the optimal

location is not known prior to validation testing, the UV reactor should be validated using both

test conditions. If the dose values measured with both test conditions are the same, the UV

intensity sensor is at the optimal location.

If the UV reactor uses the UV intensity and UVT setpoint approach, the UV reactor is

validated by measuring dose delivery with the UV intensity and UVT adjusted to the alarm

setpoint values. Validation should also confirm that the UV intensity sensor is located close

enough to the lamp that UVT alarm setpoint values provide a meaningful indicator of dose

delivery. This is accomplished by showing that dose delivery decreases with decreasing UVT

while the UV intensity is held constant at the intensity alarm setpoint value. If dose delivery

increases with decreased UVT, the UV intensity sensor is located too far from the lamp and this

monitoring approach will not work.

If the reactor uses dose calculations, validation testing confirms that dose delivery is

greater than or equal to the calculated dose. Validation testing is conducted at various

combinations of flowrate, lamp output, and UVT that result in performance at a target dose. This

proves the dose calculation is robust over the range of those variables expected with operation of

the reactor at a water treatment plant (WTP).
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-18 June 2003

Proposal Draft

F.3 UV Intensity Sensors

UV reactors should be equipped with at least one on-line UV intensity sensor that

measures the UV intensity at some point within the UV reactor. Measurements made by the

on-line UV sensors are used to indicate dose delivery by the UV reactor. Reference sensors are

used to check that the measurements made by the on-line sensors are valid.

F.3.1 UV Sensor Properties

The UV sensor may or may not measure the UV light through a monitoring window that

is separate from the sensor body. The monitoring windows should have a high UVT over the

spectral response range of the UV sensors.

The UV intensity sensor should detect germicidal UV radiation and produce a

standardized output signal (e.g., 4 to 20 mA) proportional to the UV irradiance incident on the

sensor. UV intensity sensors should be calibrated to an absolute irradiance standard and have a

suitable measurement range, angular response, spectral response, linearity, and stability for

monitoring and controlling UV dose delivery by the UV reactor. An ideal UV intensity sensor

has a linear response to incident UV irradiance that is independent of water temperature and does

not degrade with time. Furthermore, the ideal sensor has a fixed angular response and a

wavelength response that mimics the germicidal response of microorganisms.

UV intensity sensors provided by the manufacturer should be individually calibrated.

UV intensity sensors used to monitor LP lamps are often calibrated using the substitution method

(Larason et al. 1998). With this approach, the intensity of a collimated beam of UV light at 254

nm is measured using the UV sensor and compared to that made using a standard measurement,

such as a National Institute of Standards and Technology (NIST) traceable sensor or chemical

actinometer. The ratio of the standard measurement to the sensor output is the calibration factor.

With sensors designed to measure the output of medium-pressure (MP) lamps, the sensor can be

either calibrated at 254 nm, calibrated as a function of wavelength, or calibrated using

polychromatic light from a MP lamp with a known spectral output. Regardless of the approach

used, the calibration should be traceable to some absolute measurement standard and have a

quantified measurement uncertainty.

Sensor linearity is determined by comparing the sensor output as a function of incident

irradiance to standard measurements of that irradiance. Sensor temperature response is

determined by measuring the dependence of sensor output on the sensor’s operating temperature

with the sensor measuring a constant irradiance. Both linearity and temperature response should

be determined over the range of irradiance and temperature expected with the operation of the

UV reactor at the WTP. Angular response of a sensor is determined by measuring the

dependence of the sensor output on the incident angle of collimated UV light of fixed intensity.

The spectral response of a sensor is determined by measuring the dependence of the

sensor output on the wavelength of monochromatic light of known irradiance incident on the

sensor. Spectral response is typically presented as a plot of the ratio of sensor output to incident

irradiance as a function of the wavelength of light. Because it may be affected by infrared
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-19 June 2003

Proposal Draft

transmission of glass filters and fluorescence of diffusers that are part of the sensor (Larason and

Cromer 2001), UV intensity sensor spectral response should be evaluated from 200 to 1000 nm.

The long-term stability of a UV sensor is best-determined using field data but may be

estimated using accelerated life cycle testing. The measurement accuracy of UV sensors can

change over time with operation and environmental exposure. Temperature cycling, exposure to

UV light, mechanical vibration, and other factors will impact the linear, spectral, angular, and

temperature response of a sensor.

The UV sensor manufacturer should conduct regular testing on manufactured UV sensors

to develop a database on sensor properties. While some sensor properties may be measured with

each sensor, other properties, such as long-term stability, can only be measured on a

representative lot size. The sensor manufacturer should have available for inspection the

following information:

• Description of the properties measured

• Description of the measurement system used to measure each property

• Description of the measurement standards used

• Documented uncertainty of each measurement

• Description of QA/QC procedures used to ensure the measurements are traceable

• Data collected over time that demonstrates that the properties of the manufactured

sensors meet specifications

F.3.2 UV Intensity Sensor Measurement Uncertainty

The measurement uncertainty of a UV intensity sensor quantifies how the measurement

of UV intensity made by the sensor when mounted on the UV reactor compares with the true

value. For the purposes of this guidance document, UV intensity sensor uncertainty should be

determined at a 90 percent confidence level by summing the uncertainty that arises from the

calibration, linearity, angular and spectral response, temperature response, and long-term

stability.

The uncertainty of sensor calibration depends on the uncertainty of the standards and

instrumentation used to calibrate the sensor, such as voltmeters and amplifiers. Uncertainty

arises from linearity and temperature response because sensor calibration factors, determined at a

given temperature and UV irradiance, are used over a range of temperatures and irradiances with

operation of the sensor with the UV reactor. Uncertainty arises with sensor degradation because

calibration factors are determined on new sensors.

Uncertainty arises with angular response because sensors, calibrated using collimated UV

light, are used in UV reactors to measure UV light impacting from different directions.

Uncertainty arises with spectral response because sensors, calibrated at a fixed wavelength, are
Appendix F. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual F-20 June 2003

used in UV reactors equipped with MP lamps. Variability in spectral and angular response from 

sensor to sensor will result in a measurement uncertainty not accounted for in calibration. The 

impact of spectral and angular response variability on sensor measurement uncertainty can be 

determined either by calculation or by measurement. In the first approach, the sensor spectral 

and angular response measured on a representative lot size is used as an input to a model that 

predicts sensor readings in a UV reactor. The variability in the sensor readings predicted by the 

model is used to define an uncertainty term that is included in the calculation of sensor 

uncertainty. In the second approach, the variability in measurements made by a representative 

number of sensors mounted on the UV reactor is used to define the uncertainty. 

Example 17. A UV sensor manufacturer calibrates each manufactured UV intensity 

sensor at 20°C with an uncertainty of 5 percent. UV intensity sensor linearity, temperature 

response, angular response, and spectral response is evaluated on every tenth sensor 

manufactured. Linearity ranges from 1 to 3 percent over the measurement range of the sensor. 

Temperature response ranges from 0.1 to 0.2 percent per C°, or an uncertainty of 4 percent from

0 to 40°C. Models predict that the variability in angular and spectral response from sensor to 

sensor will cause uncertainties of 8 and 4 percent, respectively. An evaluation of sensors 

returned from the field indicates that the long-term drift over a one-year period is 10 percent. 

The measurement uncertainty of the sensors is calculated as the square root of the sum of the 

squares of the individual uncertainties as per: 

M 5 3 4 8 4 10 15 percent 2 2 2 2 2 2 easurement uncertainty = + + + + + =

F.3.3 On-line and Reference UV Intensity Sensors 

Degradation in UV intensity sensor performance can lead to significant under- or overestimations of dose delivery by the UV reactor’s on-line monitoring system. To prevent 

underdosing, the measurement uncertainty of the UV intensity sensors should be incorporated as 

a safety factor into the sizing and operation of a UV installation and the performance of the online sensor should be regularly checked by use of a reference sensor. Measurements made by the 

on-line and reference sensor should meet the following equation: 

[ ] 2 1/ 2

Duty

2

Ref

Ref

Duty 1 100 σ σ

I

I

× ≤ + ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − Equation F.11

where 

IRef = Intensity measured with the reference sensor (W/m2

) 

IDuty = Intensity measured with the duty sensor (W/m2

) 

σRef = Measurement Uncertainty of the reference UV intensity sensor (%) 

σDuty = Measurement Uncertainty of the duty UV intensity sensor (%)

If this condition is not met, the cause for the discrepancy should be determined and 

resolved. Typically, the discrepancy indicates degradation of the on-line sensor that necessitates 

recalibration or replacement. 

Proposal Draft
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-21 June 2003

Proposal Draft

Example 18. A UV reactor uses on-line sensors with an uncertainty of 15 percent. A

reference sensor with an uncertainty of 5 percent is used to check the on-line sensors when the

UV reactor is operating at the WTP. Measurements made by the on-line sensors are considered

out of spec when:

1 100 [15 5 ] 16%

I

I 1/2 2 2

Ref

Duty

× ≤ + = 















−

F.3.4 Positioning of UV Intensity Sensors

While the UV output along the length and around the circumference of a new UV lamp

will be relatively uniform, this may not be true with aged or fouled lamps. Sputtering of

electrode material leads to deposits on the inside of the lamp sleeve within 2 or 3 inches from the

electrode. Discoloration of the lamp sleeve with lamp aging varies along the length of the lamp.

Sleeve fouling varies spatially both along the length and circumference of the lamp sleeve (Lin et

al. 1999).

If lamps experience non-uniform aging along their length, the UV intensity sensor should

be located to monitor the section along the lamp that experienced the greatest decrease in UV

output with aging. The sensor should not be located to monitor the section that experiences the

least decrease in UV output.

F.3.5 Number of UV Intensity Sensors

Variability in UV output from lamp to lamp impacts both dose delivery and monitoring.

A lamp with a lower output will deliver lower doses to microorganisms passing in its vicinity,

thereby shifting the dose distribution to lower values and reducing the net performance of the

reactor. The shift in the dose distribution will be more pronounced with a reactor with fewer

lamps. Because the dose distribution is affected, the impact on net performance will be greater

with a more UV-sensitive microorganism. If the number of UV intensity sensors is less than the

number of lamps and the sensors monitor those lamps with the highest output, the monitoring

system will overestimate dose delivery by the UV reactor.

The monitoring strategy used to ensure that UV dose delivery meets regulatory targets

should account for the variability of UV output from lamp-to-lamp. If each lamp in the reactor is

monitored by a UV intensity sensor, dose delivery compliance should be based on the lowest

lamp output, unless an accepted and validated dose calculation methodology can account for

lamp-to-lamp variability. If the number of sensors used is less than the number of lamps, either

the lamp with the lowest output should be monitored and used for dose compliance, or the

setpoint used for dose delivery compliance should include a safety factor to account for lamp-tolamp variability.

Example 19. A UV reactor installed at a WTP is equipped with four lamps and two UV

intensity sensors. Because of variability in lamp output, the UV intensity 5 cm from each lamp

is 15, 10, 8, and 20 mW/cm

2

, respectively. If one sensor monitors the first lamp and the second
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-22 June 2003

Proposal Draft

monitors the forth lamp, the monitoring system will over-estimate the dose delivery by the UV

reactor because microorganisms passing by the second and third lamps will receive lower doses

than the microorganisms passing by the first and fourth lamps.

During UV reactor validation, variability in UV output from lamp to lamp should not

cause the UV reactor to be overrated. If the number of sensors is less than the number of lamps,

the sensors should be monitoring the lamps with the lowest output. If UV intensity sensors

record different values during validation, intensity setpoints and calculations should be based on

the lowest values recorded.

Example 20. A UV reactor undergoing validation is equipped with four lamps and two

sensors. Dose delivery is monitored using the UV intensity setpoint approach. Because of

variability in lamp output, the UV intensity 5 cm from each lamp is 10, 15, 8, and 12 mW/cm

2

,

respectively. To ensure validation results are meaningful, the sensors should be monitoring the

first and third lamps.

F.4 Polychromatic Considerations

With UV reactors equipped with LP or low pressure high output (LPHO) lamps, dose

delivery and monitoring occurs at a single wavelength of 254 nm. With UV reactors equipped

with MP lamps, dose delivery and monitoring involves a response to multiple wavelengths.

Dose delivery is an integrated response to UV light from 200 to 320 nm. The output from the

UV intensity sensor is an integrated response to UV light over wavelengths spanning the sensor’s

spectral response. UV absorbance monitors typically measure UV absorbance at a single

wavelength of 254 nm. If the spectral properties of the UV reactor that influence dose delivery

and monitoring during operation of the UV installation at a WTP are the same as the spectral

properties during validation, then the same dose delivery and monitoring characterized during

validation will occur at the WTP. However, if the spectral properties are different, dose delivery

and monitoring at the WTP will differ from dose delivery and monitoring measured during

validation. The following spectral properties may differ:

• Action spectra of the challenge microorganism used during validation and the target

pathogen

• Spectral UV absorbance of the water during validation and at the WTP

• UV output of the lamps during validation and at the WTP

• UVT of the lamp sleeves during validation and at the WTP

Safety factors should be applied to the validation data for polychromatic UV reactors if

spectral differences will lead to under dosing at the WTP. This section describes approaches for

assessing the impact of differences in spectral properties and deriving those safety factors.
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-23 June 2003

Proposal Draft

F.4.1 Action Spectra

The dependence of microorganism inactivation kinetics on UV wavelength may be

described using an action spectrum - the UV inactivation sensitivity as a function of wavelength

(Figure F.9). Ideally, the action spectrum of the challenge microorganism used to validate a

polychromatic UV reactor would either match that of the target microorganism or provide a

conservative estimate of inactivation.

Figure F.9 Action Spectra for Various Microorganisms

1

1

(Adapted from Rauth 1965)

The impact of various action spectra on UV dose delivery may be estimated by

calculating the germicidal lamp output using Equation F.12:

( ) ( )

=

= ∆

320

λ 200

PG P λ G λ λ Equation F.12

where

PG = Germicidal output of the MP lamp (W/cm)

 = Wavelength (nm)

P() = Lamp output (W/nm) measured over 1 nm increments at wavelength 

G() = Relative UV sensitivity of the microorganism at wavelength 

∆λ = 1 nm increment

Using the action spectra published for fourteen microorganisms (Rauth 1965, Cabaj et al.

2002, Linden 2001), Table F.2 presents the germicidal lamp output calculated for a MP lamp and

the ratio of that output to that of Cryptosporidium. A ratio greater than one indicates that the

action spectra of the microorganism favors greater inactivation than the action spectra of

Cryptosporidium. If a challenge microorganism with a ratio greater than one is used to validate a

0.0

0.5

1.0

1.5

2.0

2.5

225 235 245 255 265 275 285 295 305

Wavelength (nm)

A

ctio

n relativ

e to

2

5

4

n

m

Herpes simplex

MS2, R-17,fr, 7-S

oX-174

T2
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-24 June 2003

Proposal Draft

MP reactor for Cryptosporidium inactivation, the ratio should be used as a correction factor to

account for the greater inactivation of the challenge that arises from the differences in action

spectra. In the case of MS2 and B. subtilis, the ratio is close to one and the correction is small.

However, based on the data in Table F.2, a correction factor of 1.16 would be needed with UV

reactors equipped with MP lamps if φX174 was used to show Cryptosporidium inactivation.

Table F.2 Germicidal Output Delivered to 14 Microorganisms by a MP Lamp

Microorganism

Type / Nucleic acid

(SS = Single Strand,

DS = Double Strand)

Germicidal

Output (W/cm)

Germicidal Output

Relative to

Cryptosporidium

Cryptosporidium oocysts DS DNA 5.64 1.00

MS-2, R-17, fr, 7-S Phage / SS RNA 5.78 1.04

B. subtilis spores DS DNA 5.58 0.99

X174 Phage / DS DNA 6.53 1.16

Reovirus-3 Animal virus / DS RNA 7.46 1.32

Polyoma Animal virus / DS DNA 6.74 1.18

T2 Phage / DS DNA 6.05 1.07

VSV Animal virus / RNA 5.53 0.99

Vaccinia Animal virus / DS DNA 5.46 0.98

EMC Animal virus / SS RNA 5.98 1.07

Herpes simplex Human virus / DS DNA 7.00 1.26

The germicidal output of the MP lamp calculated using the action spectra of B. subtilis

spores and MS2 is equal to or less than that of most of the 14 microorganisms listed in Table F.2.

It is thus reasonable to assume that these microorganisms are acceptable as challenge

microorganisms for many pathogens whose action spectrum is not known, like adenovirus and

Giardia. However, if an alternative challenge microorganism is to be used, its action spectra

should be assessed for suitability.

As an alternate approach to measuring the action spectrum and using Equation F.12, the

correction factor can also be estimated by comparing the dose-response of the challenge

microorganism to that of MS2 measured with a LP and MP lamp. The correction factor would

be defined as:

MP MS2

LP

LP Challenge

MP

k

k

k

k

SafetyFactor 1.04

































= Equation F.13

where

kMP = Slope of the dose-response measure with the MP collimated beam (cm

2

/mJ)

kLP = Slope of the dose-response measure with the LP collimated beam (cm

2

/mJ)

1.04 = Germicidal output of MS2 relative to Cryptosporidium, from Table F.2
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-25 June 2003

Proposal Draft

The correction factor that accounts for differences in the action spectra is not the same

correction factor that accounts for differences in the UV sensitivity described in section F.1.2.

The correction factor described in section F.1.2 applies to all UV reactors regardless of lamp

type. The correction factor described in this section is applicable to MP reactors. It should be

used in addition to the correction factor described in section F.1.2.

F.4.2 Water Absorption

During UV reactor validation, a UV-absorbing chemical is added to the water passing

through the reactor in order to simulate high UV absorbance events that could occur at the WTP.

UV-absorbing chemicals that have been used to validate UV reactors include sodium thiosulfate,

fluorescein, coffee, tea, and parahydroxybenzoic acid. Ideally, the spectral absorption of the

water used to validate UV reactors equipped with MP lamps should match the spectral

absorption of the water at the WTP over the wavelength range associated with dose delivery and

monitoring (Figure F.10).

Figure F.11 compares the UV absorbance spectra of coffee and lignin sulphonate to that

of two drinking water sources (Water A and Water B). For a given UVT at 254 nm, the UV

absorption at wavelengths above and below 254 nm is greater with coffee, tea, and lignin

sulphonate than with the drinking water sources. If those chemicals are used during validation of

a MP reactor, the RED and UV intensity measured at a given flowrate, lamp output, and water

UVT will be lower during validation than at the WTP.

Figure F.10 Spectral UV Absorption of Water at Various WTPs
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-26 June 2003

Proposal Draft

Figure F.11 Comparison of the UV Absorbance Spectrum of Additives used

during UV Reactor Validation to the UV Absorbance of Two Finished Waters

The impact of the difference in the UV absorbance spectra on the measured intensity will

depend on sensor placement relative to the lamps. If the sensor is located close to the lamps, the

sensor reading during validation will be only slightly lower than the reading at the WTP.

Accordingly, for a given sensor reading, flowrate, and water UVT, the RED delivered at the

WTP will be greater than the RED measured during validation. However, if the sensor is placed

far enough from the lamp, the UV intensity measured during validation will be much lower than

the reading at the WTP. As such, for a given sensor reading, flowrate, and water UVT, the RED

delivered at the WTP will be less than the RED measured during validation. If the UV intensity

sensor’s spectral response mimics the microorganism’s action spectra and the sensor is located at

a position where the dose delivery is proportional to the sensor reading, the RED delivered at the

WTP will equal the RED measured during validation, even with the differences in the UV

absorbance spectra shown in Figure F.11 (Wright et al. 2002). However, this relationship will

not hold true if the sensor’s spectral response deviates sufficiently from the microorganism’s

action spectra.

Modeling approaches can be used to predict and compare the RED and UV intensity

sensor readings obtained during validation to those expected at a WTP. The modeling approach

can be used to define correction factors applicable to validation results to ensure dose monitoring

provides valid measurements at the WTP. UV intensity readings should be predicted using

polychromatic intensity models that factor in the spectral and angular response of the sensor.

While RED predictions could be obtained using CFD-based dose modeling approaches, ideal

dose delivery models should be used to provide conservative correction factors. The ideal dose

delivery model is conservative because the sensor location within a reactor where the dose

delivery is proportional to sensor reading is predicted to occur closer to the lamp with the ideal

model than with a CFD-based dose delivery model (Wright et al. 2002). As such, the transition

to a correction factor greater than one occurs with closer sensor-to-lamp distance with the ideal

dose delivery model than with the CFD-based dose delivery model.
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-27 June 2003

Proposal Draft

Table F.3 provides predictions of dose delivery and sensor measurements for an ideal

annular reactor. The reactor consists of a cylinder with an 18.81-cm radius and a length greater

than the arc length of the lamp. The reactor is equipped with a single MP lamp oriented along

the central axis of the cylinder (i.e., at a radius of 0 cm). The lamp is housed in a lamp sleeve

with a radius of 3.81 cm. The spectral output of the lamp is given in Figure F.12. The spectral

UV absorbances used in the model are provided in Figure F.11. UV intensity was modeled using

a polychromatic radial intensity model and the dose was calculated as the product of the average

germicidal intensity and the hydraulic residence time as per the following equation:

( ) ( ) ( )

( ( ) )

( ) =

















−

− −

=

320

200

exp 1

λ α λ

α λ

λ λ λ

e

e wl

arc q Q

r

D P L G T Equation F.14

where

D = Dose delivered by the reactor (mJ/cm

2

)

Larc = Arc length of the lamp (cm)

Tq(λ) = Lamp sleeve UVT

αe(λ) = Naperian UV absorbance

rwl = Reactor water layer, defined as the radial distance from the sleeve to the reactor

wall (cm)

Q = Flowrate through the reactor (cm

3

/s)

UV intensity sensor measurements were modeled at different lamp-to-sensor distances

for sensors with the spectral response shown in Figure F.13 as per Equation F.15:

( ) ( ) ( ) ( ( )( ))

=

− −

=

400

200 2

exp

λ π

λ λ λ α λ

r

P S T r r

I

q e S

Equation F.15

where

I = Intensity measured by the sensor

S(λ) = Sensor spectral response normalized to unity at 254 nm

r = Distance from the sensor to the lamp (cm)

rS = Lamp sleeve outer radius (cm)
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-28 June 2003

Proposal Draft

Table F.3 Dose and UV Intensity Sensor

Measurements Modeled for a MP Annular Reactor

Performance Parameters Water A Water B Coffee

Lignin

Sulphonate

MS2 RED (mJ/cm

2

) 72 67 60 61

Sensor Water Layer (cm)

1 Measured UV Intensity (254 nm equivalent mW/cm2

)

2.0 269 256 238 245

5.0 136 122 101 110

10 59.7 48.7 31.6 40.4

15 31.9 23.6 11.7 18.2

SiC

20 19.2 13.0 4.77 9.34

2.0 112 107 103 104

5.0 48.0 44.8 10.9 11.8

10 15.3 13.7 3.46 4.06

15 5.02 4.97 1.18 1.51

Filtered SiC

20 2.42 1.95 0.410 0.593

1 Water layer is defined as the distance between the lamp sleeve and the UV intensity sensor.

Figure F.12 UV Output of a MP Lamp

Table F.3 presents the MS2 RED and sensor measurements predicted for the annular

reactor operating at a flowrate of 200 gpm, a water UVT of 85 percent at 254 nm, and 100

percent lamp power. As expected, the dose delivered with coffee and lignin sulphonate for a

given flowrate, water UVT, and lamp power was less than the dose delivered with both WTP

waters.

For a given sensor reading, flowrate, and UVT, Table F.4 presents the ratio of the dose

measured during validation to the dose delivered at the WTP calculated using the data from

Table F.3. A ratio greater than one indicates that the dose measured during validation will be

greater than the dose delivered at the WTP. As expected, the ratio is less than one with the UV
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-29 June 2003

Proposal Draft

intensity sensor located close to the lamps and greater than one with the UV intensity sensor

located far from the lamps. For a given sensor position, the ratio with lignin sulphonate is closer

to one than the ratio with coffee indicated that lignin sulphonate better matches the UV

absorption spectra of WTP waters. The ratio is also closer to one with a germicidal sensor

spectral response compared to the non-germicidal response. This indicates that validation results

with a germicidal sensor are more representative of performance at a WTP than validation results

with a non-germicidal sensor.

Table F.4 Impact of Water UV Absorbance on the UV Intensity

Sensor Value Associated with a Given UV Dose Delivery

Ratio of Dose Delivered During Validation to Dose Delivered at

the WTP for a Given Sensor Reading

UV Sensor

Water Layer

(cm) Coffee to

Water A

Coffee to

Water B

Lignin

Sulphonate to

Water A

Lignin

Sulphonate to

Water B

2 0.93 0.96 0.93 0.95

5 1.12 1.09 1.04 1.01

10 1.56 1.37 1.25 1.10

15 2.25 1.80 1.48 1.19

SiC

20 3.34 2.44 1.74 1.27

2 0.89 0.93 0.91 0.94

5 0.98 0.99 0.98 0.99

10 1.16 1.12 1.09 1.06

15 1.39 1.29 1.21 1.12

Filtered SiC

20 1.70 1.48 1.35 1.18

For the germicidal sensor, Figure F.13 presents the ratio of the dose expected with coffee

to the dose expected with finished Water A as a function of sensor position and water UVT.

With the sensor located close to the lamp, the ratio is less than one over a wide range of water

UVT values. However, the ratio increases above one with increased sensor-to-lamp water layer

and, for the most part, increases with decreased UVT.
Appendix F. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual F-30 June 2003

Figure F.13 Comparison of Dose Expected with Coffee as a UV Absorber to Dose 

Expected with WTP Water for a MP Reactor Equipped with a Germicidal Sensor 

For a given UV reactor equipped with MP lamps, the impact of differences in the spectral 

UV absorbance between validation and operation at a WTP should be evaluated and used to 

establish correction factors. The correction factor is calculated for a given flowrate, sensor 

reading, and UVT, as the ratio of the dose expected during validation to the dose expected at the 

WTP. If the ratio is less than one, no correction factor is needed. 

F.4.3 Spectral Shifts 

Spectral shifts in the UV output of MP lamps may occur as MP lamps age. Spectral 

shifts in the UVT of light through lamp sleeves may occur as sleeves age and undergo internal 

and external fouling. Spectral shifts in the UVT of sensor windows may occur with window 

fouling. Spectral shifts associated with the lamp-sleeve assembly will impact both dose delivery 

and monitoring, while spectral shifts associated with window fouling will impact monitoring 

only.

Figure F.14 presents reported data on the spectral shift in MP lamp output and lamp

sleeve UVT experienced with aging. Figure F.15 presents data comparing the UVT of clean and 

fouled lamp sleeves. In both cases, aging and fouling have reduced the output of lowwavelength UV light from the lamp/sleeve assembly more than the output of higher wavelength 

UV light. The impact of lamp and sleeve aging and sleeve fouling can be assessed by validation 

testing. Alternatively, the impact can be modeled and used to define a correction factor 

applicable to validation results generated using new lamps. 

Proposal Draft
Appendix F. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual F-31 June 2003

Figure F.14 Spectral Shifts in the MP Lamp Output and 

Lamp Sleeve UVT Reported with Aging1

 

1

 Adapted from Phillips 1983 and Kawar et al. 1998. 

Figure F.15 Comparison of the UVT 

of New and Fouled Lamp Sleeves 

For a measured flowrate, water UVT, and UV intensity, Figures F.16, F.17, and F.18 

provide the ratio of the dose delivered with new lamps and sleeves to the dose delivered with 

aged lamps, aged sleeves, and fouled sleeves, respectively. In each figure, the dose ratio is 

presented as a function of water UVT and sensor-to-lamp water layer for two different sensors. 

One sensor had a SiC spectral response while the other had a germicidal response. Dose and UV 

Proposal Draft
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-32 June 2003

Proposal Draft

response. Dose and UV intensity values were predicted using Equations F.14 and F.15 applied

to the annular reactor described in section F.4.2.

Figure F.16 Comparison of Dose Delivered by a MP Reactor

with New and Aged Type 214 Lamp Sleeves
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-33 June 2003

Proposal Draft

Figure F.17 Comparison of Dose Delivered by a

MP Reactor with New and Aged Lamps
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-34 June 2003

Proposal Draft

Figure F.18 Comparison of Dose Delivered by a MP Reactor

with New Type 214 Lamp Sleeves to Fouled Sleeves

In each figure (Figures F.16 to F.18), the dose ratio increases with decreased water UVT

and increased sensor-to-lamp distance. The ratio is closer to one with germicidal sensors

compared with sensors with a SiC spectral response.

For a given UV reactor equipped with MP lamps, the impact of spectral shifts in lamp

output and sleeve UVT should be evaluated and used to establish correction factors. The

correction factor is calculated, for a given flowrate, sensor reading, and UVT, as the ratio of the

dose expected with and without the spectral shift expected with operation of the UV reactor at

the WTP. If the ratio is less than one, no correction factor is needed.
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-35 June 2003

Proposal Draft

Spectral shifts associated with lamp and sleeve aging can be avoided by regular

replacement of those components. Spectral shifts arising from fouling on external surfaces of

lamp sleeves and sensor windows can be minimized with good cleaning practices. However,

fouling can also occur on internal surfaces of lamp sleeves and sensor windows.

F.5 Uncertainty of Dose Monitoring and Safety Factors

UV installations should be sized and operated in a manner that accounts for the

measurement uncertainty associated with dose delivery monitoring. The objective of dose

delivery monitoring is to indicate the level of inactivation of the target pathogen. Safety factors

applied to UV installations that account for measurement uncertainty should be chosen to ensure

that UV reactors meet inactivation targets at a 90-percent confidence level. A 90 percent

confidence level is consistent with the confidence level used to define dose values for

Cryptosporidium, Giardia, and virus in Chapter 1.

F.5.1 Analytical Foundation for Defining Uncertainty

This section derives a measurement equation for UV dose monitoring. This equation is

used in this guidance document as the analytical foundation for defining the uncertainty of dose

monitoring.

Consider a UV installation operating at a WTP. Assuming first order kinetics, the log

inactivation of a target pathogen achieved by the UV reactor at some point in time can be

expressed using Equation F.16:

log N

RED

D

p

p

p

=

10

Equation F.16

where

log Np = Log inactivation of the pathogen

REDp = RED of the pathogen (mJ/cm

2

)

D10p = UV sensitivity of the pathogen (mJ/cm

2

per log inactivation)

If the UV reactor delivers a dose distribution, the log inactivation

of the pathogen is related to the inactivation of a challenge microorganism using Equation F.17:

p

c

p RED D

RED

N B

10

log = Equation F.17

where

REDc = RED of the challenge microorganism (mJ/cm

2

)

BRED = Ratio of the RED of the pathogen to that of the challenge microorganism

Assuming the challenge microorganism RED is proportional to the measured UV

intensity, log inactivation of the pathogen can be expressed according to Equation F.18:
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-36 June 2003

Proposal Draft

p

p RED D

I

N B

10

log

α

= Equation F.18

where

I = UV intensity measured at the WTP (mW/cm

2

)

α = Constant relating challenge microorganism inactivation to measured intensity

(J/W)

The constant k is determined during validation as the ratio of the measured RED of the

challenge microorganism to the measured intensity. Assuming that inactivation is proportional

to flowrate, Equation F.19 can be used:

Q

Q

I

I

D

RED

N B

v

p v

cv

p RED

10

log = Equation F.19

where

REDcv = RED of the challenge microorganism measured during validation

Iv = UV intensity measured during validation

Qv = Flowrate measured during validation (mgd)

Q = Flowrate measured at the WTP (mgd)

If spectral properties such as lamp output, sleeve UVT, and water UV absorbance during

validation differ from those during operation of the UV installation at the WTP, Equation F.19 is

expressed as Equation F.21:

Poly

v

p v

cv

p REF B

Q

Q

I

I

D

RED

N B

10

log = Equation F.20

where

variables are defined as in Equation F.19

The term BPoly is the ratio of challenge microorganism RED expected at the WTP to the

challenge microorganism RED expected during validation for the same conditions of flowrate,

water UVT, and UV intensity.

Assuming the dose-response of the challenge microorganism follows first order kinetics,

the challenge microorganism RED during validation is calculated using the log inactivation of

the challenge microorganism measured through the reactor as per Equation F.21:

cv

ef

in

cv c N

N

RED D 











= log 10 Equation F.21

where

D10c = UV sensitivity of the challenge microorganism (mJ/cm

2

per log inactivation)

Nin = Challenge microorganism concentration measured at the reactor influent

Nef = Challenge microorganism concentration measured at the reactor effluent
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-37 June 2003

Proposal Draft

The UV sensitivity of the challenge microorganism can be calculated according to

Equation F.22 from the UV dose-response measured using the collimated beam apparatus:

i

D

D

CB

c

log

10 = Equation F.22

where

DCB = Dose delivered by the collimated beam apparatus

log i = Log inactivation of the challenge microorganism observed with dose DCB

The dose delivered by the collimated beam apparatus is defined by Equation E.1 (section

E.3). Substituting Equations F.21 and F.22, and E.1 into Equation F.20 gives the measurement

equation for dose monitoring using the UV intensity setpoint approach:

) ln(10)

1

log (1

(1 )(1 10 )

log log

10

al

L

D i

E P R

Q

Q

I

I

N

N

N B B

p

al

v s f

v

cv

ef

in

p RED Poly

+

− −

















=

−

Equation F.23

F.5.2 Calculating Total Uncertainty

Errors in dose monitoring can be classified as either biases or random uncertainties.

Biases are systematic errors that favor either an over or under estimation of dose delivery.

A bias error will occur with dose monitoring if the monitoring approach does not account for

differences in the RED measured with the challenge microorganism and the RED delivered to

the target pathogen. A bias error will also occur if the monitoring approach does not account for

differences between the spectral properties of the UV reactor that impact dose delivery and

monitoring during validation and those properties during operation of the UV reactor at the

WTP. A bias error will occur if the radiometer, UV intensity sensor, flowmeter, or UVT monitor

used during validation always reads either high or low. Bias errors should be accounted for

using correction factors. The approaches for defining correction factors to account for bias

errors represented by the terms BRED and BPoly in the measurement equation are provided in

Sections F.1 and F.4, respectively.

Random uncertainty is associated with every term in the measurement equation

(Equation F.23). If the measurement equation consists of linear relationships of independent

variables whose random uncertainty is normally distributed, standard approaches can be used to

calculate the uncertainty of the measured variable from the uncertainty of each term in the

measurement equation. For example, if the measurement equation is y = x1 + x2 or y = x1 - x2,

the uncertainty of y due to the uncertainty of x1 and x2 is calculated using Equation F.24:
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-38 June 2003

Proposal Draft

( )

2

1

2

2

2

1

s = s + s Equation F.24

where

s = Uncertainty of y in absolute units

s1 = Uncertainty of x1 in absolute units

s2 = Uncertainty of x2 in absolute units

On the other hand, if the measurement equation is y = x1 × x2 or y = x1 / x2, the

uncertainty of y due to the uncertainty of x1 and x2 is calculated using Equation F.25:

( )

2

1

2

2

2

1

s = s + s Equation F.25

where

s = Uncertainty of y in percent

s1 = Uncertainty of x1 in percent

s2 = Uncertainty of x2 in percent

If the measurement equation involves non-linear relations like y= x1 exp(x2), Monte Carlo

approaches should be used to define the uncertainty of y.

Determining the random uncertainty of a measured quantity requires making assumptions

about the statistical distribution of measurements. If the distribution is normal, the uncertainty is

calculated as the product of the sample standard deviation and the t-statistic. If the number of

samples is high, the t-statistic can be approximated by the z-statistic. If the standard deviation of

the population is known, the uncertainty is calculated as the product of the population standard

deviation and the z-statistic. T and z-statistics are often given in the appendices of statistics

texts. The NIST provides recommendations for specifying the uncertainty for quantities that are

not normally distributed.

Table F.5 defines an approach for estimating the uncertainties of each term in the

measurement Equation F.23. The total random uncertainty of dose monitoring can be estimated

by summing the uncertainties associated with each term in Equation F.23 using the above stated

rules. Assuming the terms BRED and BPoly are the only bias terms, a safety factor for dose

monitoring can be defined according to Equation F.26:

SF B B ( e) = RED × Poly× 1+ Equation F.26

where

e = Total random uncertainty associated with the measurement equation.
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-39 June 2003

Proposal Draft

Table F.5 Terms Used to Define the Uncertainty of Dose Monitoring

Term Assumption

BRED and Babs No term used if values are selected as safety factors as described in Sections F.1

and F.4. If terms are calculated, use uncertainty of model predictions to define

uncertainty of these terms.

I and Iv UV intensity measurement uncertainty is often defined by the UV intensity sensor

manufacturer. If a reference sensor is used to check the uncertainty of a duty

sensor, the uncertainty of the duty sensor should be defined as the rejection criteria

used to determine if the on-line sensor is out of tolerance. See Equation F.11.

Q and Qv Use measurement uncertainty defined by flowmeter manufacturer

D10p Accounted for in dose targets provided in Chapter 1

Log(Nin/Nef) Calculated as a confidence interval using standard deviation and Student’s t-statistic

associated with samples collected during validation. See Equation C.7

DCB Calculated as a confidence interval using the measurement uncertainties of the

terms in Equation C.2. See Appendix E and Equation C.8.

Log(i) Use confidence interval of challenge dose-response. See sections C.4.9.7 and

C.4.9.8

The safety factor defines the relationship between the dose targets provided in Chapter 1

and the RED that should be delivered by the UV reactor at the WTP.

F.6 Re-validation

If the design of a validated UV reactor changes, the UV reactor should be re-validated if

the design change significantly impacts dose delivery or monitoring. Dose delivery and sensor

modeling can be used to assess the impact of the design change and justify the need, or lack of

need, for re-validation. This section discusses UV reactor modifications and provides guidance

on the need for re-validation.

F.6.1 Lamp Assembly

Design changes to the lamp assembly include changes made by the lamp manufacturer to

the lamp, selection of a new lamp type by the UV manufacturer, and changes made by the UV

manufacturer to the components associated with the lamp assembly. The relationship between

dose delivery and monitoring may be impacted by any design change involving modifications to

the following components:

• Lamp arc length

• Any reflectors, connectors, and spacers used at the lamp ends

• Lamp envelope diameter

• Lamp envelope UVT from 185 nm to 400 nm
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-40 June 2003

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• Mercury content of the lamp

• Argon content of the lamp

The lamp’s arc length and the use of components at the ends of the lamps (like reflectors,

spacers, and connectors) impact the UV intensity field in the region near the lamp ends. Design

changes to these components could impact dose delivery, especially if the lamps are oriented

perpendicular to flowrate. Design changes could also impact UV intensity sensor measurements

if the lamp ends are within the viewing angle of the sensors. Dose delivery and UV intensity

sensor modeling can be used to assess the impacts on changing the lamp arc length or

components used at the lamp ends. If the impacts are considered significant, the reactor should

be re-validated.

With LP lamps, the UV-emitting plasma occupies the space within the lamp envelope.

With MP lamps, the plasma forms a narrow arc that occupies a portion of the space within the

lamp envelope. In the presence of electromagnetic fields, the plasma within a MP lamp can be

displaced off center within the lamp. The diameter of a plasma centered within the lamp

envelope should have a small impact on the UV intensity field and dose delivery (Bolton 2000).

However, displacement of the plasma off-center within the envelope could impact the intensity

field and dose delivery. The reactor should be re-validated if design changes to the lamp

diameter significantly impact the intensity field.

The UVT of the lamp envelope will impact the UV output of both LP and MP lamps.

With LP lamps, envelope material can be selected to allow or prevent LP lamps from emitting

UV light at 185 nm. While UV light at 185 nm has a negligible impact on dose delivery and UV

intensity sensor measurements because of the high UV absorbance of water at this wavelength,

185 nm light may promote the formation of ozone within the lamp sleeve. Ozone will absorb

UV light at 254 nm and lower the output from the lamp. Ozone could degrade components

within the lamp assembly leading to internal sleeve fouling. Typically, LP lamps are selected

with envelopes that prevent output at 185 nm.

With MP lamps, the envelope material has a significant impact on the intensity of UV

light emitted below 260 nm. Lamp envelope material can be selected to eliminate or maximize

UV output at lower wavelengths. Since envelope transmittance decreases with increased

temperature, the UVT of the envelope of a MP lamp should be assessed at the operating

temperature of the lamp. Dose delivery and UV intensity sensor modeling can be used to assess

the impacts of changing lamp material and justify the need for re-validation.

LP lamps typically operate near 40oC with a relatively low mercury vapor pressure that

promotes UV output at 254 nm. Because the amount of mercury added to the lamp is well in

excess of the amount that enters the vapor state during lamp operation, the UV output of a LP

lamp is independent of the mercury dose added to the lamp during lamp manufacture. On the

other hand, MP lamps operate at a high temperature, near 600oC, with all of the added mercury

in the vapor phase. As such, the mercury vapor pressure is dependent on the mercury dose and

the lamp operating temperature. The vapor pressure influences the fraction of mercury that is

ionized or excited to higher energy states, and hence the spectral output of the MP lamp. Table

F.6 presents the calculated impact of mercury dose on the germicidal output and measured

intensity from a MP lamp operating with an electrical input of 70 W/cm. The results suggest that
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-41 June 2003

Proposal Draft

a change in mercury dose has no impact on the relationship between dose delivery and

monitoring with germicidal sensors and a small impact on the relationship with SiC sensors.

Table F.6 Impact of the Mercury Dose on the Relationship

Between Germicidal Output and Measured Output of a MP Lamp

1

Mercury Dose UV Output (W/cm) Weighted by Ratios

(mg/cm) MS2 Action SiC Sensor Filtered SiC Sensor SiC:MS2 Filtered SiC:MS2

4.8 6.88 11.4 6.82 1.65 0.990

8 6.53 10.3 6.46 1.58 0.989

10.1 7.10 10.8 7.06 1.52 0.993

1

Adapted from lamp output data from 248 to 400 nm provided by Phillips (1983).

F.6.2 Ballasts

Modifications to lamp ballasts include changing the operating voltage, current,

frequency, and waveform. With LP lamps, modifications will impact the amount of UV

generated by the lamp, but will not impact the relationship between dose delivery and UV

intensity measurements. With MP lamps and some LPHO lamps, changes in lamp operating

temperature and mercury pressure caused by changes in ballast power will impact the spectral

distribution of emitted light. Table F.7 presents the impact of changing the input power from

48 to 92 W/cm on the germicidal output and measured intensity from a MP lamp dosed

with 4.8 mg/cm of mercury. The results suggest a change in lamp operating power has no

impact on the relationship between dose delivery and monitoring with germicidal sensors and a

small impact with SiC sensors.

Table F.7 Impact of Operating Power on the Relationship Between

Germicidal Output and Measured Output of a MP Lamp

1

Lamp Input UV Output (W/cm) Weighted by Ratios

Power (W/cm) MS2 Action SiC Sensor Filtered SiC Sensor SiC:MS2 Filtered SiC:MS2

48 4.13 7.01 4.08 1.70 0.99

70 6.86 11.3 6.78 1.66 0.99

92 9.29 15.2 9.14 1.65 0.98

1

Adapted from lamp output data from 248 to 400 nm provided by Phillips (1983) for a MP lamp dosed with 4.8

mg/cm Hg.

F.6.3 Lamp Sleeves

Design changes to the lamp sleeves include changing the sleeve diameter, thickness, and

material. Changing the sleeve diameter may impact the hydraulics through the reactor, the

measurement of UV intensity, and the optimal placement of UV intensity sensors relative to the

lamp. Changing the thickness and material of the lamp sleeve will impact the spectral UVT,

thereby impacting both dose delivery and UV intensity measurements.
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-42 June 2003

Proposal Draft

Dose delivery and UV intensity sensor modeling may be used to assess the impact of

lamp sleeve design changes. Figure F.19 provides the ratio of dose delivered with a standard

sleeve to dose delivered with an “ozone-free” sleeve for a given sensor reading as a function of

water UVT, sensor-to-lamp distance, and sensor spectral response. Dose and UV intensity

values were predicted using Equations F.14 and F.15 applied to the annular reactor described in

section F.4.2. Sleeve UVT is provided in Figure F.20. The results show that a design change

from a regular sleeve to an ozone-free sleeve described in Figure F.20 would have a small impact

on the relationship between dose delivery and UV intensity sensor readings with a SiC sensor

and a negligible impact with a germicidal sensor. Modeling can also be used to show that the

dose delivery at a given lamp output, water UVT, and flowrate would be approximately 10

percent greater with the standard sleeve than with the ozone-free sleeve. If models indicate the

sleeve design change causes a significant impact on dose delivery and monitoring, the UV

reactor should be re-validated.

Figure F.19 Ratios of Dose Delivered with Standard Sleeve to

Dose Delivered with “Ozone-Free” Sleeves by an Annular Reactor
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-43 June 2003

Proposal Draft

Figure F.20 UVT of Standard and “Ozone-Free” Quartz Assuming Air-Quartz and

Quartz-Water Interfaces

F.6.4 Reactor and Component Dimensions

Modifications to the wetted dimensions and positioning of the components within the

reactor will impact the reactor hydraulics and dose delivery. Modifications could also impact the

intensity field within the reactor and the measurement of UV intensity. Modifications include

changes to the dimensions of the reactor, inlet piping, exit piping, baffles, lamp sleeves, wipers,

and UV intensity sensors. The impact of such modifications on dose delivery and UV intensity

measurements can be insignificant or large. Addition of a baffle plate will likely have a large

impact on dose delivery and a small impact on measured UV intensity, while changing the

position of a UV intensity sensor will likely have a small impact on dose and a large impact on

measured UV intensity. Dose delivery and UV intensity modeling may be used to assess the

impacts of these modifications. If the impacts are significant, the reactor should be re-validated.

F.6.5 UV Intensity Sensors

Modifications to the UV intensity sensors include changes made by the sensor

manufacturer to the sensor, changes by the UV manufacturer to the sensor housing and

associated optical components, and changes by the UV manufacturer to the number and

positioning of the sensors within the reactor.

Changes to the semi-conductor and optical components within the UV intensity sensor

could impact the sensor’s spectral response, linearity, angular response, and temperature

stability. Changes to those properties could impact the sensor’s measurement uncertainty. If the

new measurement uncertainty is quantified, it should be used to define a new safety factor for the

UV reactor. If the angular response or spectral response of the sensor changes, measurements

supported by calculations should be used to evaluate the impact of the change on dose delivery

monitoring.
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-44 June 2003

Proposal Draft

Changes to the measuring window of the UV intensity sensor include dimensional and

material changes. Changes may impact the UVT of the window and the detection angle.

Measurements supported by calculations should be used to evaluate the impact of the change on

dose delivery monitoring.

Modifications to the positioning of the UV intensity sensor within the reactor could

disturb the flowrate and impact dose delivery. If the impact on dose delivery is negligible,

measurements supported by calculations may be used to compare measured UV intensity at the

two positions and modify the dose monitoring approach without the need for re-validation.

Addition of UV intensity sensors to the reactor could disturb the flowrate through the UV

reactor and impact dose delivery. If sensors are added, they should be positioned relative to the

lamps in a similar manner as the other sensors. For example, if one sensor is positioned to view

two lamps through a 5-cm water layer, then all added sensor should view two lamps through a 5

cm water layer.

F.7 References

Bolton, J.R. 2000. Calculation of ultraviolet fluence rate distributions in an annular reactor:

significance of refraction and reflection. Water Research 34:3315-3324.

Cabaj, A., R. Sommer, and D. Schoenen. 1996. Biodosimetry: model calculations for U.V. water

disinfection devices with regard to dose distributions. Water Research. 30, no.4:1003-

1009.

Cabaj, A., R. Sommer, W. Pribil, and T. Haider. 2002. The spectral UV sensitivity of

microorgnisms used in biodosimetry. Water Science and Technology: Water Supply,

2(3):175-181.

Chang, J.C.H., S.F. Osoff, D.C. Lobe, M.H. Dorfman, C.M. Dumais, R.G. Qualls, and J.D.

Johnson.1985. UV inactivation of pathogenic and indicator microorganisms. Applied

and Environmental Microbiology 49(6):1361-1365.

Chiu, K.-P., D.A. Lyn, P. Savoye, and E.R. Blatchley.1999. Effect of UV system modifications

on disinfection performance. Journal of Environmental Engineering 125(5):459-469.

Haas, C.N. and G.P. Sakellaropoulos.1979. Rational analysis of ultraviolet disinfection.

National Conference on Environmental Engineering, Proc. ASCE Specialty Conf., San

Francisco, CA, July 9-11.

Kawar, K., J. Jenkins, B. Srikanth, and A. Shurtleff. 1998. Ultraviolet light deterioration of the

light transmittance of quartz sleeves due to continuous exposure to UV radiation.

Ultrapure Water October, 67-71.

Larason, T.C., S.S. Bruce, and A.C. Parr. 1998. Spectroradiometric Detector Measurements.

NIST Special Publication 250-41. U.S. Government Printing Office, Washington.
Appendix F. Background to the UV Reactor Validation Protocol

UV Disinfection Guidance Manual F-45 June 2003

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Larason T.C. and C. L. Cromer. 2001. Source of error in UV radiation measurements. Journal of

Research of the National Institute of Standards and Technology 106, no.4:649-656.

Lin, L., C.T. Johnston, E.R. Blatchley.1999. Inorganic fouling at quartz: water interfaces in

ultraviolet photoreactors I: chemical characterization. Water Research 33:3321-3329.

Linden K.G., G. Shin, and M.D. Sobsey. 2001. Comparative effectiveness of UV wavelengths

for the inactivation of Cryptosporidium parvum oocysts in water. Water Science &

Technology 43(12):171-174.

Meng, Q.S. and C.P. Gerba.1996. Comparative inactivation of enteric adenovirus, poliovirus and

coliphages by ultraviolet irradiation. Wat. Res. 30, no.11:2665-2668.

Phillips, R.1983. Sources and Applications of Ultraviolet Radiation. New York: Academic Press,

Qualls, R.G. and J.D. Johnson. 1983. Bioassay and dose measurement in UV disinfection.

Applied and Environmental Microbiology 45, no.3:872-877.

Rauth, A.M. 1965. The physical state of viral nucleic acid and the sensitivity of viruses to

ultraviolet light. Biophysical Journal 5:257-273.

Sommer, R., T. Haider, A. Cabaj, W. Pribil, and M. Lhotsky. 1998. Time dose reciprocity in UV

disinfection of water. IAWQ Vancouver 1998 Poster.

Wilson, B.R., P.F. Roessler, E. Van Dellen, M. Abbaszadegan, and C.P. Gerba. 1992 Coliphage

MS-2 as a UV water disinfection efficacy test surrogate for bacterial and viral pathogens.

Proceedings of the Water Quality Technology Conference, Nov. 15-19, 1992, Toronto,

219-235.

Wright, H.B. and Y.A. Lawryshyn. 2000. An assessment of the bioassay concept for UV reactor

validation. Disinfection 2000: Disinfection of Wastes in the New Millenium, New

Orleans, Louisiana, March 15-18, 2000; Water Environment Federation, Alexandria,

Virginia

Wright, H.B., E. Mackey, and P. White. 2002. UV Disinfection Compliance Monitoring for

Drinking Water Applications. Proceedings of the Water Quality technology Conference

and Exhibition, Seattle, November 10 - 14, 2002.
Appendix G. Issues for Unfiltered Systems

Unfiltered systems are utilities that use surface water sources and meet the filtration 

avoidance criteria of the Surface Water Treatment Rule (SWTR) (40 CFR 141.71). The Long 

Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) requires unfiltered systems to 

meet overall disinfection requirements (i.e., Cryptosporidium, Giardia, and virus inactivation)

using a minimum of two disinfectants (40 CFR 141.721(d)). The information presented in this 

manual is focused on post-filtration applications of UV disinfection; however, the information is 

also relevant to UV disinfection of unfiltered supplies. In addition, the UV dose requirements 

presented in section 1.3.1.3 are applicable to both filtered water and water supplies that meet the 

regulatory requirements for filtration avoidance (40 CFR 141.729(d)). This appendix identifies 

issues that are specific to unfiltered applications of UV disinfection. The following issues are of 

particular interest to unfiltered supplies because they make applying UV disinfection different 

from post-filter locations: 

• Water quality (especially particle content) 

• Debris 

• Ozone residual (when ozone is applied prior to UV disinfection) 

• Off-specification requirements recommendations 

G.1 Water Quality

Differences in the quantity and nature of particles in unfiltered surface water supplies are 

the most pertinent distinction between post-filtration and unfiltered supply water qualities. 

Typically, the turbidity in unfiltered surface waters is less than 1 nephelometric turbidity units 

(NTU). However, the SWTR allows turbidity up to 5 NTU immediately prior to the first point of

disinfection application (40 CFR 141.71). Several studies have examined the effects of turbidity 

up to 10 NTU on UV disinfection, including changes in UV absorbance measurements made 

with a spectrophotometer and inactivation of microorganisms. 

Particles in water absorb and scatter UV light to varying degrees based on size and 

composition. Particles impact the disinfection process in two distinct manners: 

1. Particles can decrease the UV transmittance (UVT) of water and thereby impact UV 

dose delivery (section A.4.1.2). 

2. Particle association can shield microorganisms from UV light, thereby changing the 

characteristics of the UV dose-response curve (section A.2.6.5). 

Christensen and Linden (2001) concluded that the light scattering and changes in 

absorbance caused by turbidity up to 10 NTU can be accounted for when calculating UV dose in 

collimated beam testing provided that the ultraviolet absorbance at 254 nanometers (A254) of the 

sample is measured according to a modified version of Standard Method 5910B (i.e., without 

UV Disinfection Guidance Manual G-1 June 2003

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Appendix G. Issues for Unfiltered Systems 

UV Disinfection Guidance Manual G-2 June 2003

0.45 μm filtration). Direct reading spectrophotometers, the most common type of 

spectrophotometer, may overestimate the A254 of water with turbidity greater than 3 NTU, 

resulting in an overly conservative UV dose calculation (Christensen and Linden 2002). To 

reduce this overestimation, an integrating sphere can be installed in a direct-reading 

spectrophotometer that will provide accurate A254 measurements. Regardless of the type of 

spectrophotometer used, the effects of increased absorbance due to particles can be accounted for 

in the A254 measurement, which can then be used to determine the design UVT. If an appropriate 

design UVT is used, the UV reactor will be able to respond to changes in UVT that arise due to 

particles. 

Particles and microorganisms in a water sample are either dispersed or aggregated 

together. Studies have demonstrated that dispersed coliform bacteria in wastewater are easier to 

disinfect than aggregated bacteria (Parker and Darby 1995). To date, research examining the 

effects of particles in drinking water on UV disinfection has been performed with seeded 

organisms and particles. It is unknown at this time how well these studies represent naturally 

occurring microorganism and particle interactions. However, since the concentration of 

microorganisms in unfiltered sources is typically below detectable limits, methods to examine 

this phenomenon directly (without seeding) do not currently exist. Consequently, seeded 

drinking water studies can only suggest the impact of turbidity on dose-response as it relates to 

the impact of UV light scattering by particles rather than particle-association or clumping of 

microorganisms. 

Recent research has shown that particles present in supplies meeting regulatory 

requirements for unfiltered drinking water do not impact the UV inactivation of seeded

microorganisms. Passantino and Malley (2001) reported that for unfiltered surface waters, 

turbidity up to 7 NTU does not affect the inactivation of seeded male specific-2 bacteriophage 

(MS2) in bench-scale, batch, collimated beam testing. In this study, turbidity was increased by 

adding natural sediment to waters collected from unfiltered water supplies. Therefore, naturally 

occurring interactions between particles and microorganisms could not be evaluated. In another 

study, batch (bench-scale) and continuous-flow (pilot-scale) studies showed that turbidity 

ranging from 0.65 to 7 NTU does not affect the UV dose necessary per log inactivation of seeded 

MS2, Giardia muris, or Cryptosporidium parvum in unfiltered waters (Oppenheimer et al. 2002). 

Womba et al. (2002) evaluated the impact of turbidity on UV inactivation of MS2 at the benchand pilot-scale. They found that on the bench-scale, when the impact of turbidity was accounted 

for in the UV dose determination, the inactivation of MS2 was not affected by turbidity. 

However, in this study on the pilot-scale, because the lamp intensity and flowrate (and therefore 

residence time in the reactor) remained constant, the effects of turbidity were not accounted for 

in the reactor control strategy. Therefore, the reduction equivalent dose (RED) observed 

decreased as turbidity increased. 

Unfiltered supplies are also susceptible to algal blooms. Womba et al. (2002) monitored 

algae levels in an unfiltered supply reservoir for over one year and found that algal counts were 

typically below 30,000 cells/mL; however one algae event had a higher level of nearly 300,000 

cells/mL. Although not regulated, the presence of algae may interfere with the UV disinfection 

process. Womba et al. (2002) and Passantino and Malley (2001) examined the effects of algae 

on UV disinfection of MS2 at the bench-scale in batch, collimated beam testing. Both studies 

found that up to algal counts up to 70,000 cells/mL and 42,000 cells/mL, respectively, do not 

affect the inactivation of MS2. 

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Appendix G. Issues for Unfiltered Systems 

UV Disinfection Guidance Manual G-3 June 2003

G.2 Debris 

Relative to post-filter applications of UV disinfection, there may be greater opportunity 

for debris to be present in the influent to UV reactors in unfiltered applications. Debris entering 

the UV reactor with sufficient momentum could cause lamp sleeve and lamp breakage. The 

mass and size of an object that might cause damage is installation-specific and depends on UV 

reactor configuration (e.g., horizontal versus vertical reactor orientation) and water velocity 

through the reactor. As such, designs should incorporate features that prevent potentially 

damaging objects from entering the system; the optimal approach is site-specific. Such features 

could include screens, baffles, or low velocity collection areas. Another option is to install the 

UV reactors vertically with the inlet closest to the ground, following a low velocity zone. This 

arrangement will decrease the momentum of larger debris and reduce the risk of lamp breakage. 

The effects of lamp breakage and methods of minimizing it are discussed in Appendix N. 

G.3 Ozone Impacts on Absorbance 

Some utilities using an unfiltered source may consider applying ozone in addition to UV 

disinfection. There are a number of benefits associated with this process combination, including 

addressing multi-barrier disinfection requirements. Additionally, if ozone is added prior to UV 

disinfection, the A254 of the water can be reduced as shown in Figure G.1, thereby improving the 

efficiency of UV disinfection. 

Figure G.1 Impact Of Pre-Ozonation On A254 (Malley 2002).

W a v e le n g th ( n m )

200 220 240 260 280 3 0 0 320

UV Absorbance (cm-1) 0 .0

0 .1

0 .2

0 .3

0 .4

0 .5

P r eoz onate d U V In fluen t

( no de te c table oz one r e s idual)

U V In flu e n t - N o o z o n e

It should be noted, however, that ozone is a strong UV absorber with a molar absorbance 

value of 0.0677 L/mg/cm at 254 nm. Figure G.2 illustrates the impact of ozone concentration on 

UVT1

 for three baseline transmittance values. If ozone is applied prior to UV reactors and 

residual ozone enters the UV reactor, the increase in UV absorbance due to ozone residual can be 

 

= ∗ 1 UVT 254 (%) 100 10 A

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Appendix G. Issues for Unfiltered Systems 

UV Disinfection Guidance Manual G-4 June 2003

significant and should be considered when determining the design UVT. To address this issue, 

utilities can monitor the ozone residual and add an ozone-reducing chemical to maintain the 

ozone residual below a specified setpoint value (e.g., 0.1 mg/L). There are several chemicals 

that can be used to quench ozone; however, some chemicals (such as sodium thiosulfate) have a 

high molar absorbance value (as shown in Table A.5, section A.4.1.3), and thus have the 

potential to decrease the UVT. Such chemicals should not be used prior to UV disinfection. 

Sulfite has a lower molar absorbance value and is therefore an acceptable chemical to quench 

ozone residual. The impact of water treatment chemicals on UV absorbance can be assessed by 

preparing solutions of various concentration and measuring their UV absorbance using a 

standard spectrophotometer (Bolton et al. 2001). 

Figure G.2. Impact of Ozone Residual on UVT (adapted from Cushing et al. 2001) 

60%

70%

80%

90%

100%

0.00 0.25 0.50 0.75 1.00

Aqueous Ozone Concentration (mg/L)

UVT (254 nm; 10 mm pathlength)

Initial UVT = 80%

Initial UVT = 87%

Initial UVT = 95%

In at least some cases, the increase in UVT resulting from ozone addition will improve 

overall UV disinfection effectiveness provided that any remaining ozone residual is adequately 

controlled. Each utility should explore the sequence of disinfectants that best fits their sitespecific objectives and constraints. 

G.4 Off-specification Requirements 

Off-specification is when the UV reactor is operating outside of its validated limits. UV 

installations should be designed with process monitoring and control components (e.g., alarms, 

shut-off valves) to prevent water from entering the distribution system when a UV reactor is 

operating outside of validated conditions. Unfiltered systems that use UV disinfection to meet 

the Cryptosporidium treatment requirement of the LT2ESWTR must demonstrate that at least 95

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Appendix G. Issues for Unfiltered Systems 

UV Disinfection Guidance Manual G-5 June 2003

percent of the water delivered to the public during each month is treated by UV reactors 

operating within validated limits (i.e., operating conditions that have been validated to achieve 

the necessary log inactivation) (40 CFR 141.721(c)). Failure to demonstrate this will result in a 

treatment technique violation. 

The UV reactors are off-specification when any of the following conditions occur (40 

CFR 141.729(d)): 

• The flow, UV intensity, or lamp status is outside of the validated range. 

• The UVT or UV intensity is outside of the validated range (if the UV intensity and 

UVT setpoint approach is used (section 3.1.5)) 

• The calculated dose is outside of the validated range at a given flow (if the calculated 

dose approach is used (section 3.1.5)) 

• All UV lamps in all UV reactors are off because of a power interruption or power 

quality problem (as discussed in section 3.1.3.3), and water is flowing through the 

reactors. 

More information on off-specification is in section 3.1.3, and compliance information is 

in section 5.4.1. 

G.5 References 

Bolton, J.R., M.I. Stefan, R.S. Cushing, and E. Mackey. 2001. Importance of water 

absorbance/transmittance on the efficiency of ultraviolet disinfection reactors. 

Proceedings of the IUVA 1st International Congress, June 14-16, Washington, D.C. 

Christensen, J. and K. Linden. 2001. Ultraviolet disinfection of unfiltered drinking water: 

particle impacts. Proceedings of the IUVA 1st International Congress, June 14-16, 

Washington, D.C. 

Christensen, J. and K. Linden. 2002. New findings regarding the impacts of suspended particles 

on UV disinfection of drinking water. Proceedings of the AWWA Annual Conference, 

June 16-20, New Orleans, L.A. 

Cushing, R.S., E.D. Mackey, J.R. Bolton, and M.I. Stefan. 2001. Impact of common water 

treatment chemicals on UV disinfection. Proceedings of the AWWA Annual Conference 

and Exposition, June 17-21, Washington DC. 

Malley, J.P. 2002. Historical perspective of UV use. Presented at the AWWA Water Quality 

Technology Conference, November 10-14, Seattle, W.A. 

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Appendix G. Issues for Unfiltered Systems 

UV Disinfection Guidance Manual G-6 June 2003

Oppenheimer J., T. Gillogly, G. Stolarik, and R. Ward. 2002. Comparing the efficiency of low 

and medium pressure UV light for inactivating Giardia muris and Cryptosporidium 

parvum in waters with low and high levels of turbidity. Proceedings of the AWWA 

Annual Conference, June 16-20, New Orleans, L.A. 

Parker, J.A. and J.L. Darby. 1995. Particle-associated coliform in secondary effluents: shielding 

from ultraviolet light disinfection. Water Environment Research 67:1065-1075. 

Passantino, L. and J.P. Malley. 2001. Impacts of turbidity and algal content of unfiltered 

drinking water supplies on the ultraviolet disinfection process. Proceedings of the 

AWWA Annual Conference and Exposition, June 17-21, Washington D.C. 

Womba P., W. Bellamy, J. Malley, and C. Douglas. 2002. UV disinfection and disinfection 

byproduct characteristics of an unfiltered water supply, Project 2747. Denver, C.O.: 

AwwaRF periodic report. 

Proposal Draft 
Appendix H. Issues for Ground Water Systems 

The UV installation design, operation, and maintenance principles presented in Chapters 

3 and 5 of this manual are focused on the use of UV reactors to disinfect filtered surface water. 

Most of the information presented in those chapters is also applicable to ground water systems. 

Additional ground water-specific regulatory requirements and recommendations, site issues, 

hydraulic issues, and water quality issues that affect design and operation are discussed in this 

appendix. 

H.1 Ground Water Systems Background 

Regulations should be reviewed to determine the goals and requirements for disinfection. 

Existing treatment processes and distribution system parameters should also be analyzed before 

selecting a strategy for integrating UV disinfection into the system. 

H.1.1 Regulatory Background 

Currently, federal regulations do not require ground water systems to provide primary or 

secondary disinfection unless the water is a ground water source under the direct influence of 

surface water (GWUDI). However, some States require ground water systems to maintain a 

residual disinfectant in the distribution system. In addition, ground water systems are required to 

meet the requirements of the Total Coliform Rule (TCR) (54 FR 27544) and the Stage 1 

Disinfection and Disinfection Byproducts Rule (DBPR) (63 FR 69390) and are expected to be 

affected by the upcoming Stage 2 DBPR. 

The upcoming Ground Water Rule, as proposed, would require some ground water 

systems to provide 4-log removal or inactivation of viruses. Systems with significant 

deficiencies, as determined by States during sanitary surveys, and systems that detect fecal 

indicators in their source water will be affected. These systems will be required to correct any 

deficiencies, provide water from an alternative source, or install treatment that provides 4-log 

removal or inactivation of viruses. 

Ground water supplies that are designated as GWUDI, as defined in the Surface Water 

Treatment Rule (SWTR), 40 CFR Part 141.2, are classified as Subpart H Systems and must meet 

the same regulatory requirements as surface water systems. GWUDI systems often use many of 

the same treatment strategies as surface water systems, including filtration. The issues involved 

with implementing UV disinfection at filtered water utilities (including GWUDI) are discussed 

in detail in Chapters 1 through 5 of this manual. GWUDI systems are subject to the Stage 1 

DBPR and would be subject to the upcoming Stage 2 DBPR and the Long Term 2 Enhanced 

Surface Water Treatment Rule (LT2ESWTR). Both of these regulations are summarized in 

section 1.3. 

UV Disinfection Guidance Manual H-1 June 2003

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UV Disinfection Guidance Manual H-2 June 2003

H.1.2 Typical Ground Water System Design

Most ground water systems operate by cycling ground water pumps on and off in 

response to demand or storage capacity. Because significant treatment is not usually necessary 

beyond secondary disinfection, ground water systems typically do not have a single, centralized 

treatment system. Many ground water systems pump to storage (e.g., hydropneumatic tank, 

elevated storage tank), but some may discharge directly to the distribution system. A production 

well typically consists of a well pump, and may contain a chlorinator (for secondary disinfection) 

and corrosion control equipment (for Lead and Copper Rule compliance), air release valves, 

vacuum relief valves, and other ancillary equipment necessary for well operation (Figure H.1). 

Figure H.1. Typical Ground Water Well Site Layout 

UV reactors may be installed at each well in a production system. If multiple wells are

located in the same area, centralizing the flow through a common header minimizes the number 

of UV reactors needed and possibly reduces the project cost. In addition, treatment for other 

aesthetic issues (e.g., removal of iron and manganese or stripping of sulfides) may be more 

effectively accomplished with centralized treatment. An engineering cost analysis should be 

conducted to compare centralized treatment with the installation of individual reactors at each 

well. 

H.2 Water Quality Issues 

Although ground water typically exhibits small variations in water quality, specific 

parameters need to be analyzed when planning for a ground water system. As with surface water 

systems, UV absorbance at 254 nm (A254) and the corresponding UV transmittance (UVT) is the 

most important parameter when designing a UV installation because it affects the UV reactor 

size. In addition, many naturally occurring constituents present in ground water (e.g., calcium, 

iron, manganese, aluminum, chloride, carbonate, sulfide, and phosphate) are capable of fouling 

the lamp sleeves in UV reactors, and these constituents should be monitored. The potential for 

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Appendix H. Issues for Ground Water Systems 

UV Disinfection Guidance Manual H-3 June 2003

fouling is greater with medium pressure (MP) reactors than low pressure (LP) and low pressure 

high output (LPHO) reactors because MP lamps operate at higher temperatures (section 2.4.2). 

Mechanical wipers are often effective at removing fouling on the lamp sleeves. In situations 

where the ground water has detectable levels of iron and manganese, chlorination prior to the UV 

reactors may cause increased fouling or staining, necessitating chemical cleaning (Malley et al. 

2001). A complete discussion of the relevant water quality parameters and the determination of 

their design values is presented in section 3.1.3.1. 

With ground water systems, it is common for one or more wells to be taken out of service 

for an extended period due to fluctuations in water demand, ground water quality, operational 

problems, or other planned and unplanned circumstances. Toivanen (2000) reported that the 

lamp sleeves and internal surfaces of the UV reactors became fouled when the UV reactors were 

out-of-service and full of water. The amount of time it takes to foul the UV reactor while off-line 

is site-specific and depends on the water quality. At a minimum, it is recommended that the 

reactors be drained if the UV reactor is off-line for more than one week; however, the 

appropriate period for this could be shorter or longer depending on the water quality. If the UV 

reactor will be off-line for an extended period of time (longer than 30 days), it is recommended 

that the reactor be cleaned prior to re-starting the UV reactor. Routine shutdown and start-up 

procedures are discussed in section 5.2.3.

H.3 Off-Specification Issues 

UV reactors must be validated as discussed in Chapter 4 and operated within the 

conditions determined during validation. When a utility is operating outside of the validated 

limits, the utility is operating “off-specification." 

LT2ESWTR includes requirements limiting off-specification for compliance with the

LT2ESWTR for unfiltered supplies (40 CFR 141.721(c)(2)); however, the rule does not state an 

off-specification requirement for filtered systems or ground water systems. States may develop 

statewide or site-specific requirements off-specification requirements for ground water systems. 

There are two ways that a ground water system could be operating off-specification. 

First, off-specification can occur when the flow, UVT, or UV intensity is outside of the validated 

conditions. Second, UV lamps can lose arc if a voltage fluctuation, power quality anomaly, or a 

power interruption occurs. LP lamps generally can return to full operating status within 15 

seconds after power is restored. However, LPHO and MP UV lamps exhibit restart times 

between 4 and 10 minutes if power is interrupted (Cotton et al. 2002). During these restart times, 

the water being distributed is inadequately disinfected and is considered off-specification. 

H.3.1 Power Quality Assessment 

A power quality assessment at each well site should be performed to determine if power 

quality might cause off-specification operation. In addition, the reliability of commercial power 

at remote sites may be less than that of more populated areas. Backup power or an 

uninterruptible power supply (UPS) may be needed, depending on the findings of the power 

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Appendix H. Issues for Ground Water Systems 

UV Disinfection Guidance Manual H-4 June 2003

quality assessment. If backup power is already available for the well pumps, then the backup 

power supply should be assessed to determine if sufficient output is available for the UV reactor. 

However, UPS may also be needed if there are frequent power quality problems. For systems 

that have storage following UV disinfection, it may be possible to isolate the UV reactor and rely 

on stored water to meet demand during periods of power failure. Power quality assessments are 

discussed in more detail in section 3.1.3.3. 

H.3.2 Well Pump Cycling 

Well pumps may regularly cycle on and off in response to changes in distribution system 

pressure, causing the UV reactor to also be cycled. Frequent lamp cycling reduces lamp life. 

Manufacturers recommend that whenever possible lamps remain energized for a minimum of 6 

hours (Dinkloh 2001). In addition, the warm-up time when the UV reactor is coming on-line is 

considered off-specification until the UV intensity sensor reading reaches the validated value if

water is flowing to the distribution system. 

Depending on its current operation and direction from the State, the utility may need to 

consider changing the well pump cycling strategy or incorporate UV reactor controls to reduce 

off-specification time and to meet the needs of the distribution system. The utility should discuss

its proposed operating strategy with the UV manufacturer to ensure it is appropriate for the 

selected UV reactors. While there may be any number of operating strategies that a utility could 

use, two operational strategies that could be incorporated to sequence the well pumping with the 

operation of the UV reactor are presented below.

The first strategy is to incorporate a delay that prevents the well pump from starting until 

the UV reactor reaches its validated UV intensity sensor setpoint (i.e., no flow through the UV 

reactor). Under this control strategy there will be a period when the UV reactor will be “on” but 

no flow will be passing through it. This control strategy is only effective when LP or LPHO 

reactors are used because their lamps can operate for up to 1 hour under no-flow conditions 

(Dinkloh 2001) without overheating. However, MP UV reactors may heat the water above the 

safe operating temperature of 50 degrees Celsius in 1 to 15 minutes, causing the reactor’s 

internal safety devices to shut the reactor off (Miller 2001). As such, this control strategy may 

be infeasible for MP reactors unless they incorporate a low flow waste line that allows water to 

circulate through the reactor in order for MP lamps to reach the validated UV intensity sensor 

setpoint without overheating. The UV manufacturer should be contacted to confirm that this 

operational strategy is feasible with or without the waste line. 

The second strategy is to provide a system of automated valves that diverts the UV 

reactor discharge away from the distribution system until the reactor reaches its validated UV 

intensity sensor setpoint. Then the automated valves are repositioned to direct the water from the 

UV reactor to the distribution system. This strategy delivers the off-specification water to 

“waste” until the validated UV intensity sensor setpoint is reached. This ensures that sufficient 

cooling water will flow through MP reactors to prevent overheating and reactor shutdown. For 

this strategy, the utility needs to develop an approach for managing the water that is wasted 

during reactor warm-up. The water may be wasted to a sanitary sewer, storm sewer, on-site 

disposal or drainage system, or temporary storage tank. The utility should coordinate the 

discharge location with the State and other involved parties. 

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Appendix H. Issues for Ground Water Systems 

UV Disinfection Guidance Manual H-5 June 2003

Both operational strategies introduce a lag between the time when the pump is initiated

and the time when water is introduced into the distribution system. Because of this, existing 

controls may need to be adjusted to avoid insufficient system pressure or storage during periods 

of UV reactor warm-up. This will be particularly important for those ground water systems that 

have frequent on-off cycles or limited storage. 

H.4 Well Location Issues 

Ground water production wells are sited in a variety of locations, ranging from urban 

areas to remote installations. The well location will affect the design and operation of the UV 

installation, especially if there is limited space. 

H.4.1 Design Considerations

As discussed in section 3.3.5.2, the UV reactor should be installed within a building or

underground vault if possible to facilitate maintenance and protect sensitive equipment. The 

need for enclosure of the UV installation will ultimately be based on the manufacturer’s

recommendations, local regulatory and code requirements, environmental conditions, and sitespecific constraints. Site security should be appropriate to prevent tampering with the equipment 

and water supply and to protect people from injury (e.g., electrocution). 

Well sites, particularly in urban areas, may be spatially constrained by adjacent 

development. As a result, the amount of exposed pipe and available area for locating equipment 

may be limited. In these cases, it may be necessary to modify the pump discharge piping to 

accommodate a UV reactor. When constructing a UV installation in an extremely confined 

location, the designer must consider the area necessary for operation and maintenance and the 

area needed for installation (e.g., staging areas, personnel, and equipment access). In addition, 

the inlet and outlet piping should meet the criteria listed in section 3.3.1.1 as compared to the 

validated inlet and outlet piping. 

UV reactors are susceptible to damage by suspended sand particles or other debris that 

may be present in a ground water supply and pass through the well screens. Therefore, it is 

important to determine if sand, grit, or fines are present in a well supply and if it is necessary to 

install a sand/debris trap or removal equipment prior to UV disinfection. Particles flowing 

through the UV reactor may scratch the lamp sleeves, cause the sleeve wiping mechanisms to 

jam, or shield pathogens from UV light, thereby decreasing the UV disinfection effectiveness. In 

addition, larger particles could break the lamp sleeves and lamps (see Appendix N for lamp

breakage issues). 

H.4.2 Operational Issues 

Because most well sites are not continuously staffed, UV installations may need 

sufficient automation to allow remote monitoring and operation. Controls and alarms should be 

designed to ensure that real-time operational and monitoring data are communicated to the 

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Appendix H. Issues for Ground Water Systems 

UV Disinfection Guidance Manual H-6 June 2003

operators. These factors also emphasize the importance of a power quality assessment and the 

design of alarms, monitoring capabilities, and backup power facilities. 

Disposal of the chemicals used to clean the UV reactors may be an issue if an on-line 

chemical cleaning (OCC) system is used (section 2.4.5). If sewer connections or other standard 

means of disposal are not available, then chemical waste will need to be transported off-site for 

disposal or handled on-site. Utilities should consult with chemical suppliers and the State when 

developing disposal strategies. 

H.5 Hydraulic Issues 

The hydraulic issues associated with ground water systems include high operating 

pressures, piping configuration, air entrainment, and the potential of water hammer and surge 

events. 

Many well pumps discharge directly to the distribution system or to elevated or 

pressurized storage; therefore, the discharge will often be at system pressure. The UV reactor

design may need to be modified to accommodate these higher distribution system pressures. 

The actual inlet and outlet hydraulics of the UV reactor should be designed to match the 

validated hydraulics as discussed in section 3.3.1. Space is often limited with ground water 

installations so valves, flow meters, or other appurtenances may be directly upstream or 

downstream of the UV reactor. Consequently, these site constraints may need to be considered 

in determining how the UV reactor should be validated. Detailed discussions of UV installation 

layout and validation are given in section 3.3.5 and Chapter 4, respectively. 

UV reactors should be flooded at all times because air binding can interfere with the UV 

disinfection process or cause the lamps to overheat. UV reactors should be located downstream

of any existing or planned air removal equipment (if necessary). Otherwise, the UV installation 

design should include a means for automatically releasing air prior to the UV reactor. The UV 

reactor may have integral air release valves or valve ports, or air release valves can be installed 

in the inlet and outlet piping. 

Pressure surge events (water hammer) near the UV reactor may be more likely with 

ground water systems than surface water systems because of the UV reactor’s proximity to the 

well pumps. Surge events can cause positive or negative pressure transients in the well discharge 

piping. Negative pressures as small as -1.5 psi may cause the lamp sleeves to break (Dinkloh 

2001). A surge analysis is recommended to determine if surge protection is necessary. Many 

well sites and distribution systems are already equipped with surge control tanks to dampen 

surge effects. These tanks may provide sufficient protection for the UV reactors, depending on 

their location relative to the UV reactors. 

Other surge control devices, such as air/vacuum release valves, may rely on the 

introduction of air into the system to mitigate surge. As discussed previously, the presence of air 

can negatively affect the performance of the UV reactors. Air/vacuum valves should only be 

used if surge tanks are not an option and the design can eliminate the air prior to the UV reactor

(e.g., install the valve in a section of pipe at a higher elevation than the UV reactor). 

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Appendix H. Issues for Ground Water Systems 

UV Disinfection Guidance Manual H-7 June 2003

H.6 References

Cotton, C.A., R.S. Cushing, and D.M. Owen. 2002. The impact of the draft UV disinfection 

requirements on UV facility design and operation. Proceedings of the AWWA Annual 

Conference, June 16-20, New Orleans, LA. 

Dinkloh, L. 2001. Wedeco-Ideal Horizons. Telephone conversations and email correspondence 

by Ben Hauck, Malcolm Pirnie Inc., regarding UV reactors. April 25 and August 10. 

Malley, J.P., B.A. Petri, G.L. Hunter, D. Moran, M. Nadeau, and J. Leach. 2001. Full-scale 

implementation of UV in groundwater disinfection systems. Denver, CO: AwwaRF Final 

Report. 

Miller, A. 2001. Trojan Technologies. Telephone conversation by Ben Hauck, Malcolm Pirnie 

Inc., regarding UV reactors. August 10. 

Toivanen, E. 2000. Experiences with UV disinfection at Helsinki water. IUVA News 2, no.6: 4-8. 

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Appendix I. Issues for Small Systems 

The objectives of this appendix are to highlight the issues that small systems face when 

considering UV disinfection and to reference the more detailed discussion of these issues in this 

manual. 

To be classified as a public water system, a utility must provide water to a minimum of 

15 service connections or serve at least 25 people for at least 60 days per year (40 CFR 141.2). 

For the purpose of this appendix, the term small system includes those utilities serving fewer 

than 10,000 people or having a daily production rate of less than approximately 1.0 mgd. Most 

of the information regarding UV disinfection in Chapters 2, 3, 4, and 5 is valid for both small 

and large systems. 

I.1 Is UV Disinfection Applicable to Small Systems?

UV disinfection is applicable to small systems and may be attractive for the following

reasons: 

• It is a relatively low cost technology for the inactivation of Cryptosporidium (Cotton 

et al. 2001). 

• Chemical use is little to none. 

• Operation is relatively simple and maintenance is low. 

• Space needs are small. 

Two types of UV reactors can potentially be used by small systems, conventional and 

point-of-entry (POE) devices. Conventional UV reactors are manufactured for a wide range of 

flows (e.g., from 20 gallons per minute (gpm) up to 40 mgd) and are described in section 2.4. 

POE units are small UV reactors that are installed at the service connection of the customer. 

POE units contain the same components as conventional low-pressure (LP) installations but are 

more compact. They are primarily intended for use at individual properties and may be more 

suitable for utilities with a limited number of service connections. POE units are required to be 

owned, controlled, and maintained by the utility or by a person under contract with the utility to 

facilitate proper operation and maintenance and compliance with the treatment requirements 

(Safe Drinking Water Act (SDWA) Section 1412(b)(4)(E)). The use of POE units may result in 

higher total costs when compared to a centralized, conventional UV installation for all but the 

smallest water utilities. 

I.2 What Information is Necessary to Assess the Feasibility of UV 

Disinfection? 

Small systems generally need the same information to assess UV disinfection as larger 

systems. Chapter 3 describes the planning and design process for a UV installation in a 

UV Disinfection Guidance Manual I-1 June 2003

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Appendix I. Issues for Small Systems 

UV Disinfection Guidance Manual I-2 June 2003

conventional plant and discusses each of the elements that should be considered. In general, the 

utility should answer the following questions when assessing the suitability of UV disinfection: 

• What are the disinfection goals and can UV disinfection be used to meet these goals? 

(section 3.1.1) 

• What are the minimum, average, and maximum flowrates that the UV reactors will 

need to treat? (section 3.1.3.2) 

• What is the design UV absorbance at 254 nm (A254) and corresponding design UV 

transmittance (UVT)? What is the fouling potential of the water supply? What is the 

potential for process upset or variability in water quality? Do any of the existing 

processes have the potential to interfere with the performance of the UV reactors?

(section 3.1.3.1) 

• Can the UV reactors be incorporated into the existing hydraulic profile? If not, can 

the existing operations be modified to accommodate the UV reactors, or does 

intermediate pumping need to be installed? (section 3.1.6.1) 

• Can the UV installation be incorporated into the existing facility layout? Does a 

building need to be constructed to house the UV reactors? (section 3.1.6.2) 

• Is the quality and reliability of the electrical power supply adequate? Does a backup 

power supply or other supplemental electrical equipment need to be installed?

(sections 3.1.3.3 and 3.3.4) 

• How should the UV reactors be controlled? What level of automation and

operational complexity is appropriate? Does the potential for power savings justify 

using a more complex operating strategy? Is the existing operations staff sufficient?

(section 3.3.3) 

• Is the number of UV reactors installed appropriate to efficiently respond to the 

anticipated range of flowrates? Does the UV installation have the capability to be

expanded to meet future increases in demand? Is there sufficient redundancy to allow 

operating flexibility and to meet the disinfection goals under the operating scenarios?

(section 3.1.3.2) 

• How should the UV reactors be procured? (section 3.2) 

• Do the characteristics of the proposed UV application (e.g., flowrate, UVT, UV 

intensity) differ from those under which a selected UV reactor was validated? If so, 

should the selected equipment be validated on-site or off-site under characteristics 

that match those of the intended installation? (Chapter 4 and section 3.1.4.2) 

• What is the capital cost of the UV installation? What are the operating costs 

associated with a UV installation? (section 3.1.7) 

• What is the cost of the UV installation as compared to other disinfection alternatives? 

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Appendix I. Issues for Small Systems 

UV Disinfection Guidance Manual I-3 June 2003

• Is there a cost benefit to using POE units as opposed to a centralized UV installation? 

If so, how should the utility administer the POE units? Is some form of access 

agreement or water use ordinance necessary to allow administration of the POE units? 

I.3 Do the UV Reactors Need to be Housed in a Building? 

If possible, the UV reactors should be constructed within a building to facilitate 

maintenance and protect the UV reactors. Nonetheless, the need for enclosing the UV reactors 

will ultimately be based on manufacturers’ recommendations, State requirements, and 

environmental conditions. Although some current UV installations do not have a building (e.g., 

Hanovia facility in Australia), local building and electrical codes may necessitate a building or

other protection for the electrical equipment. Regardless of whether the UV reactors are 

enclosed, site security is important to prevent tampering with the equipment and water supply 

and to protect people from injury (e.g., electrocution). Section 3.3.4 discusses the electrical 

equipment issues that should be considered during the planning and design of a UV installation. 

I.4 Do the Components of a Small System Differ from Larger UV Reactors?

The main components of a UV reactor (including the necessary instrumentation and 

controls) do not differ between large and small systems. Components of the UV reactor may 

include the UV lamps, lamp sleeves, UV intensity sensors, ballasts, and cleaning mechanisms, 

which are described in section 2.4. 

Full-scale drinking water applications generally use LP, low-pressure high-output 

(LPHO), or medium-pressure (MP) lamps. Small systems may find reactors that use LP or 

LPHO lamps more economical because they convert power into germicidal wavelengths of UV 

light more efficiently than MP lamps. Additionally, LP lamps typically have a longer useful life 

than either MP or LPHO lamps. For small systems, UV reactors with LP lamps are likely to 

represent the most cost-effective disinfection solution. For systems that serve near 10,000 

people or treat near 1 mgd, more consideration should be given to LPHO or MP lamps. An 

additional discussion of the different lamp types is given in section 2.4.2. 

I.5 What are the Power Needs?

The power needs depend on the manufacturer. Common manufacturers’ power needs are 

as follows: 

• POE UV units – 120V/60Hz/1 phase 

• Conventional LP reactors – 120/208V/60Hz/3 phase 

• Conventional LPHO reactors – 480V/60Hz/3 phase 

• Conventional MP reactors – 480V/60Hz/3 phase 

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Appendix I. Issues for Small Systems 

UV Disinfection Guidance Manual I-4 June 2003

Backup power may be necessary, depending on the type of installation that is selected, 

the power quality at the installation site, and the regulatory requirements for the installation. 

However, backup power for small systems may not be necessary as some small systems can 

accommodate a shutdown for longer periods because there is sufficient storage to meet demand. 

Additional detail on the need for backup power and the factors that should be considered when 

assessing the power supply are discussed in section 3.1.3.3. 

I.6 Do Small UV Reactors Need to be Validated?

All UV reactors, including POE units, are required to be validated (40 CFR 141.729(d)). 

Small systems will probably purchase UV reactors that have been validated by the manufacturer. 

UV intensity sensor operating setpoints (and potentially UVT setpoints) are established at 

specific flowrates during validation testing. These are the setpoints that the systems are required 

to operate within to receive inactivation credit. For many small UV reactors and nearly all POE 

units, UV reactor control will be limited to “on” and “off” with UV reactor shutdown under 

specific critical alarm conditions. Chapter 4 discusses the UV reactor validation requirements, 

and Chapter 5 describes operating requirements. 

I.7 How are UV Reactors Monitored?

Monitoring UV reactors (conventional and POE) is required to ensure that the UV 

reactors are operating within the validated range (40 CFR 141.729(d)). Parameters that must be 

monitored include flowrate, UV intensity sensor readings, and UVT (if it is part of the control 

strategy) (40 CFR 141.729(d)). POE units should be equipped with mechanical warnings to 

ensure that customers are automatically notified of operational problems. Additional detail on 

monitoring requirements is provided in section 5.4.1. 

I.8 Can the UV Reactors be Operated Remotely?

UV reactors can be operated remotely if the monitoring components provide a 4-20 mA

analog output signal and are integrated into a control strategy. Even though UV reactors can be 

operated remotely, routine inspections and on-site maintenance will be necessary to confirm that 

the UV reactor is operating properly. Provisions for hydraulic and electrical lockout should be 

provided to enable local isolation and lockout for maintenance. Section 3.3.3 provides additional 

detail on control strategies for centralized UV installations, and section 5.3 discusses operations 

and maintenance needs. 

If the utility uses POE units, it may be beneficial to telemeter all alarm conditions to a

central location to facilitate administration and maintenance of the POE units. However, 

incorporating this remote capability will likely increase the cost of the UV installation. 

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UV Disinfection Guidance Manual I-5 June 2003

I.9 How Much Maintenance is Needed?

Maintenance is generally limited but will vary depending on the manufacturer and the 

specific application. Maintenance may include the following activities: 

• Periodic calibration verification of UV intensity sensors, UVT meters, or flowmeters 

• Periodic replacement of UV intensity sensors, UVT meters, or flowmeters (if 

applicable), depending on calibration or age of the equipment 

• Lamp sleeve and reactor cleaning 

• Replacement of UV lamps and other components 

• Maintenance of other operating components and the electrical systems 

Operators should be trained by the UV manufacturer on the proper operation and 

maintenance of the UV reactors. The utility should consider contracting trained service 

personnel to maintain the UV reactors if this is not possible. Additional detail on operations and 

maintenance is given in section 5.3. 

I.10 References

Cotton, C.A., D.M. Owen, G.C. Cline, and T.P. Brodeur. 2001. UV disinfection costs for 

inactivating Cryptosporidium. Journal of the American Water Works Association 93, no. 

2: 67-74. 

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Appendix J. Pilot-Scale and Demonstration-Scale Testing 

In some cases, pilot- or demonstration-scale testing may be warranted to aid in selection 

of design criteria. For example, long-term UV unit performance will be impacted by lamp aging 

and sleeve fouling. With increased use, lamp output decreases due to deposition of inorganic 

material on the outside and inside of the sleeve (i.e., “fouling”). Fouling reduces the 

transmittance of the lamp energy to the water. Over time, these phenomena will contribute to a 

reduction in UV dose. The effect of these parameters should be incorporated into the UV reactor 

design. A “lamp aging factor” and a “fouling factor” are usually specified by the design 

engineer. Pilot testing can provide useful information for the development of these factors. 

This appendix discusses when pilot or demonstration tests may be needed and the types 

of tests that may be performed on UV disinfection systems. The purpose(s) of pilot and 

demonstration testing is to establish or confirm system design factors, test system reliability, and 

evaluate operation and maintenance (O&M) needs. The tests described herein may be performed 

individually or in parallel. Validation of reactor microbial inactivation performance is addressed 

separately in Appendix C (Validation Protocol). 

J.1 When Is Pilot or Demonstration Testing Needed? 

Pilot and demonstration tests can be used to meet the following three goals:

1. Assess the impact of unusual water quality conditions (e.g., high calcium or iron 

concentrations). 

2. Improve estimation of safety factors for large water systems for which such an 

investment can yield a high return in reduced life cycle costs. 

3. Gain first-hand experience with operating and maintaining a UV installation. 

A UV disinfection system should be designed with some knowledge of the likely fouling 

potential of the water and lamp-aging characteristics to ensure the system operates as intended. 

If the design and the operation protocol do not properly account for the effects of lamp aging and 

sleeve fouling, the system may go into alarm frequently (indicating under dosing). 

While pilot or demonstration testing may be warranted in some cases, it is becoming less 

necessary as more performance and fouling information is developed. The need for pilot or 

demonstration testing should be carefully considered in light of the pre-existing data available on 

both system performance and water quality effects on sleeve fouling. Pilot or demonstration 

testing may be used to gain operational experience or primacy agency acceptance, as discussed 

in the following sections.

Microbiological challenge tests are not recommended during pilot studies because 

inactivation efficiency in a pilot system may not be indicative of full-scale performance. 

However, UV reactor validation bioassays could be conducted as part of a full-scale 

UV Disinfection Guidance Manual J-1 June 2003

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UV Disinfection Guidance Manual J-2 June 2003

demonstration test if on-site testing is planned. Appendix C presents a detailed discussion of UV 

reactor performance validation. 

Because UV disinfection is a relatively new drinking water treatment technology in the 

United States, State regulatory agency acceptance may depend in part on the confidence in the 

technology gained through pilot- and demonstration-scale studies. Identifying previous studies 

of similar scope that provide background and precedents may also be helpful in gaining 

acceptance of its planned use (see, for example, et al. Mackey 2001). 

If a utility chooses to or is required to conduct pilot or demonstration tests, the primacy 

agency should understand the objective(s) of the test(s) and the methodologies used. It is 

recommended that the primacy agency be contacted before testing and involved throughout the 

pilot and/or demonstration testing. Identifying and resolving State regulatory agency concerns 

when planning testing can help produce a more useful dataset. Additionally, it may be helpful to 

include the State in interim briefings on progress and results, and to give them a final report after 

completing the testing. 

J.1.1 Water Quality Impacts 

Extensive data have been generated from pilot-scale testing on waters of low to moderate 

hardness and iron content (Mackey et al. 2001, Mackey and Cushing 2003). At total hardness 

and calcium levels below 140 mg/L and low iron (less than 0.1 mg/L), standard cleaning 

protocols and wiper frequencies (one sweep every 15 minutes to an hour) were more than 

adequate to deal with the impact of sleeve fouling at all sites tested. At sites with hardness or 

iron in the feed water that exceed these levels, it may be advantageous to evaluate fouling rates 

on a site-specific or worst case basis via pilot or demonstration testing to identify how best to 

keep the sleeves clean. 

J.1.2 Lamp Fouling Factors for Large Systems 

In UV reactor design, a lamp aging factor of 0.7 is commonly used, as discussed in 

section 3.1.3.1 of the Manual. For larger systems, it may be economical to pilot or 

demonstration test lamp aging to provide data for selecting lamp aging and low-dose-alarm

design factors that will best balance operational costs (how many hours one wants to be able to 

operate a lamp before replacing it) with capital costs (the size of the system needed based on 

end-of-lamp-life). Lamp aging factors may also be obtained from a certified lamp age testing 

program performed by equipment or lamp manufacturers. A lower lamp aging factor means the 

utility will have less frequent lamp replacements, but may require a larger system to ensure 

compliance at all times. 

J.1.3 Gaining Operational Experience 

Due to the small number of U.S. drinking water UV installations, very few United States 

operators have experience with UV disinfection systems. It may be beneficial for a facility’s 

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Appendix J. Pilot-Scale and Demonstration-Scale Testing 

UV Disinfection Guidance Manual J-3 June 2003

staff to obtain operational experience with UV disinfection systems prior to selecting and 

implementing UV disinfection. If a utility staff becomes familiar with the operational aspects of

UV disinfection, that staff will be able to provide feedback input on the UV installation design. 

In addition, operational experience can help facility managers to determine the staffing/training 

needs and help maintenance staff understand and plan for the maintenance needs of the system

(e.g., time to change lamps and calibrate UV intensity sensors and perform manual cleaning). 

On-site testing is site-specific depends on the needs and preferences of the utility. 

Methods by which facility staff can gain operational experience (besides on-site testing) include: 

site visits and partnerships with systems already using UV disinfection; conversations and visits 

with manufacturers and attendance of seminars; and on-site training programs (a detailed 

discussion of training programs is provided in section 5.7.2). 

J.2 Pilot- Versus Demonstration-Scale Testing 

Table J.1 presents a comparison of the advantages and disadvantages associated with 

pilot-scale and demonstration-scale testing. Pilot-scale testing involves operating a smaller 

version of a full-sized UV disinfection reactor. It may or may not include all the components of 

the full-sized system. Demonstration-scale testing is essentially pilot testing of a full-scale UV 

disinfection reactor. 

Table J.1 Comparison of Pilot and Demonstration Testing 

Testing Method Advantages Disadvantages

Pilot-scale • Smaller footprint needed for UV 

reactors

• Less-expensive installation and 

operation 

• Operators gain O&M experience 

• High flexibility in placement of 

equipment 

• Lesser volumes of water to dispose 

• Design conditions for UV 

disinfection systems may not scaleup to full-scale systems 

• In rare cases it may be advisable to 

use pilot-scale treatment process 

equipment (filters, clarifiers, etc.) to 

simulate operational conditions 

(e.g., upstream ozone process) 

DemonstrationScale 

• Confidence in long-term operation 

of the UV unit due to the 

representative scale at which 

results are obtained 

• Scale-up factors need not be 

developed 

• Operators gain operations and 

maintenance experience on a fullscale system.

• Approval from the primacy agency 

may be required to conduct a 

demonstration study 

• Demonstration setups are not as 

flexible as pilot studies for 

operational experimentation

• Installation and operation more 

expensive than pilot scale

• Greater volumes of water to 

dispose. 

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Appendix J. Pilot-Scale and Demonstration-Scale Testing 

UV Disinfection Guidance Manual J-4 June 2003

J.3 Documenting the Test Reactor 

For a given test, should the properties of the components that may influence the final 

outcome of the test should be identified and recorded. That record may later be used to confirm

that key components of installed UV reactors match those of the systems tested. Table J.2 lists 

the components of a UV disinfection system that should be documented and compared between 

testing and the final design. 

Table J.2 Key Components Associated with UV Reactor Pilot-Scale 

and Demonstration Scale Testing 

Test Components to Document

Operational Experience Controls, alarms, cleaning mechanisms, 

operation, maintenance. 

Fouling Assessment Lamps, sleeves, ballasts, power settings, 

UV intensity sensor windows, flow velocity. 

Head loss Assessment

(demonstration-scale only) 

Reactor and wetted components, inlet/outlet 

conditions. 

Ballast Performance Lamps, sleeves, ballast, power settings, 

operation. 

Cleaning Mechanism Performance Lamps, sleeves, ballasts, power settings, 

ballast operation, UV intensity sensor 

windows (if wiper used), cleaning

mechanism, cleaning solutions, wiper 

maintenance and operation.

Lamp Aging/Failure Lamps, sleeves, ballasts, power settings, 

ballast operation, cleaning mechanism, 

cleaning solutions, wiper maintenance and 

operation. 

Sleeve Breakage Sleeves, cleaning mechanisms, flow 

velocity, water hammer. 

Controls/Alarms Lamps, sleeves, ballasts, UV intensity 

sensors, cleaning mechanisms, controls, 

operation. 

J.4 Testing Objectives 

Pilot/demonstration testing may be used to gain information on a specific UV reactor, a 

specific water treatment plant (WTP) site, or a combination of the two. Common test objectives 

include the following topics: 

• The long-term performance and failure modes of the lamps

• The efficacy of cleaning mechanisms for lamp sleeves and UV intensity sensor 

windows 

• The stability of UV intensity and UVT monitors 

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Appendix J. Pilot-Scale and Demonstration-Scale Testing 

UV Disinfection Guidance Manual J-5 June 2003

• The reliability of controls and alarm systems 

• The ease of lamp and UV intensity sensor replacement, the use of reference sensors, 

and the maintenance of cleaning devices and solutions 

• The rate of fouling on lamp sleeves and UV intensity sensor windows 

• The most appropriate cleaning method 

• The head loss across the reactor at various flow rates (demonstration-scale only) 

• The impact on other unit operations at the WTP 

The information obtained during pilot and demonstration testing should be applicable to 

the final UV disinfection system installed at the WTP. Accordingly, the equipment tested should 

be representative of the UV disinfection system that will be installed. Specific elements of a 

pilot/demo-scale system that should be identical include the UV intensity sensors, lamp and 

sleeve type, power system, cleaning system, cleaning frequency, and water quality. For 

example, lamp-aging data on a 3 kW 25 cm medium pressure (MP) lamp driven by an 

electromagnetic ballast cannot be used to predict the aging expected with a 10 kW 50 cm MP 

lamp driven by a transformer. 

J.5 Testing Protocols 

This section describes the major elements and benefits of a range of pilot and 

demonstration testing protocols to investigate sleeve fouling and cleaning, lamp aging, head loss 

and alarms and controls. 

J.5.1 Assessing Fouling 

A fouling assessment can be conducted to answer the following questions: 

• How fast do the lamps foul?

• How does water quality affect fouling?

• What lamp fouling factor should be specified?

• What lamp cleaning interval is required?

• What sleeve replacement interval is required? 

• How do lamp/reactor configurations affect fouling?

• Is fouling of the UV intensity sensor window(s) significant and how should it be 

addressed? 

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Appendix J. Pilot-Scale and Demonstration-Scale Testing 

UV Disinfection Guidance Manual J-6 June 2003

Fouling may occur on the inner and outer surfaces of the lamp sleeves, the internal

surfaces of the reactor, and UV intensity sensor windows. Lamp sleeve fouling may have an 

impact

d to

on of

d for assessing sleeve fouling similar to that employed by Lin et al. (1999b), 

new lamp can be placed inside the fouled sleeve and ignited. Ultraviolet absorbance at 254 

nanom

chemical cleaning should restore the sleeve A254 to very near that of a new, clean 

leeve. If not, manually clean the inside of the sleeve and measure A254. If it is still low, the 

sleeve

s a

fouling, care should be taken to ensure that the results scalep to full-scale applications. Some differences in system geometry may lead to erroneous 

conclus

leeve

f

e UV intensity sensor windows, clean the sensor monitoring 

indows with phosphoric or citric acid at varying time intervals and record the change in sensor 

reading

.5.2 Evaluating Cleaning Systems 

ethods can be performed to answer the following 

uestions:

Does a particular cleaning protocol work for the UV reactor application? 

on dose delivery and cleaning requirements. Sensor window fouling may have an 

important impact on assessing dose delivery (e.g., the sensor will not be able to accurately 

measure lamp intensity). Fouling on the wetted surfaces of a UV reactor has been attribute

precipitation of compounds whose solubility decreases as temperature increases, precipitati

compounds with low solubility, and deposition of particles by gravity settling and turbulenceinduced impaction (Lin et al. 1999a). More detailed discussion on fouling is provided in sections

3.1.3.1 and A.4.1.4. 

In one metho

a

eters (A254) is measured by a calibrated radiometer and compared to a similar 

measurement made using a new, clean sleeve. The ratio of these two measurements (UV light

passing through the fouled sleeve to that passing through the new sleeve) is the lamp sleevefouling factor. 

Manual,

s

material has likely degraded. If A254 cannot be recovered, further testing may be used to 

identify a proper sleeve replacement interval. The lamp sleeve-fouling factor can be plotted a

function of time. Worst-case results can be analyzed to determine cleaning requirements and 

fouling factors for design purposes. 

When assessing lamp sleeve

u

ions based on pilot data alone. For instance, in parallel flow reactors, fouling has been 

found to be uneven along the length of the lamps (Lin et al. 1999a). If the lamp and lamp s

geometry (e.g., length or diameter) of the pilot unit is very different from the full-scale system,

the fouling that will occur in the full-scale plant may be markedly different from expectations 

based on pilot-scale data. The lamp lengths will be very different and end-effects may be more 

pronounced (i.e., the blackened lamp ends of an aged lamp will comprise a greater percentage o

the total length of the lamp). 

To assess fouling on th

w

s. It is expected that the rate of fouling on the lamps will be greater than the rate of

fouling on the sensor windows due to elevated lamp temperature. 

J

An evaluation of system cleaning m

q

• 

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Appendix J. Pilot-Scale and Demonstration-Scale Testing 

UV Disinfection Guidance Manual J-7 June 2003

• What is the long term effectiveness of the cleaning method?

• What cleaning frequency is required for each method considered?

nts that

accumu te on the lamp sleeves and UV intensity sensor windows. Lamp sleeve cleaning 

method ning),

ipers

g

.5.2.1 Assessing Lamp Sleeve Cleaning Protocols 

mp sleeves, and the 

sults b

lowest

r at the minimum flow rate and operate the lamps at 

maxim ith systems using mechanical or physicochemical wipers, an unwiped 

sleeve

se this 

ata to optimize the cleaning frequency. Sensor windows should be manually cleaned before 

measur

heduled sleeve cleaning cycle, 

remove the lamp sleeves and assess the sleeve A254 for low pressure (LP) lamps and absorbance 

from 20

hen manual, chemical cleaning is recommended, remove the sleeves 

and measure the sleeve A254 before and after cleaning the outer surfaces. If the new sleeve 

transm

ntify 

.5.2.2 Assessing UV Intensity Sensor Window Cleaning Protocols 

actor e cond

maxim

leaning

Various cleaning methods can be used to periodically remove the foula

la

s include off-line chemical cleaning (OCC) (off-line manual or mechanized clea

on-line mechanical cleaning (OMC) (e.g., brushes or rings), and on-line physicochemical w

(acid solution in a wiper collar). Sensor window cleaning methods also include manual cleanin

and mechanical wipers. 

J

A sleeve cleaning assessment should be performed on at least four la

re e used to identify a sleeve fouling design factor for sizing the UV reactor based on the 

individual sleeve-fouling factor observed. This will help ensure proper dose delivery for 

the entire life of the sleeve. One method for assessing lamp sleeve cleaning needs is detailed 

below: 

Pass water through the reacto

um power. W

can be used as a control to verify that fouling is occurring. The manufacturer’s 

recommendations regarding the maintenance of the cleaning device should be followed. 

Record the UV intensity sensor readings before and after the cleaning cycle and u

d

ements to ensure only lamp sleeve fouling is affecting the sensor values. If possible, 

check all UV intensity sensor readings with a reference sensor. 

At regular time intervals and immediately prior to the sc

0 - 400 nm for MP. The non-destructive method of Lin et al. (1999b) may be used as 

discussed in section J.5.1. 

After 6 months, or w

ittance is not restored by the cleaning, it is likely that the sleeve material has fouled 

internally or permanently degraded. Further monitoring and testing may be necessary to ide

the proper sleeve replacement interval. 

J

To assess fouling of the UV intensity sensor window, one alternative is to operate the 

re under th itions suggested in section J.5.2.1 (i.e., minimum water flow rate, 

um lamp power). After 6 months, or a time interval suggested by the manufacturer, a 

chemical cleaning of the monitoring sensor window could be performed. Alternatively, c

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Appendix J. Pilot-Scale and Demonstration-Scale Testing 

UV Disinfection Guidance Manual J-8 June 2003

could be performed when the sensor reading falls to a minimum value suggested by the 

manufacturer. The A254 of the window should be measured before and after cleaning. A sensor 

window cleaning frequency can then be estimated as discussed in section 3.1.3.1. 

J.5.3 Assessing Head Loss 

ld be performed for demonstration-scale (full-scale) systems 

verify that head loss constraints at the final install station will not be exceeded. Head loss data 

from pi

ion J.1: 

Head loss assessments shou

to

lot scale units should not be used to estimate head loss in a full-scale system. 

The head loss, ∆H, through a UV reactor may be calculated according to Equat

g

Kv

2

H Equation J.1 2 =

where 

= Head loss coefficient (unitless) for the UV reactor

= Water velocity (m/s) through the reactor 

, the UV unit can be installed with 

strumentation to measure pressure loss across the reactor (including baffles and specialized 

inlet/ou res due to 

um

and ma imum flow rates, these measured head loss values can be plotted as a function of the 

square

J.5.4 amp Aging and Failure

e conducted to answer the following questions: 

e?

?

ds of hours. The germicidal output of 

the lam will decline during this period (Phillips 1983). In MP systems, UV lamp aging can also 

result i

∆

K

v 

g = Gravitational constant (9.8 m/s

2

) 

To assess head loss through a UV reactor

in

tlet piping). Since the head loss coefficient will be higher at lower temperatu

decreased water viscosity, it may be desirable, if feasible, to measure head loss at the lowest 

water temperature expected at the UV reactor installation to assess the worst-case condition. 

If head loss is measured at various flow rates through the reactor, including the minim

x

of the calculated water velocity through the reactor to determine a head loss coefficient. 

L

A lamp aging evaluation can b

• What is the actual operating lamp life?

• How does lamp output degrade over tim

• What lamp aging factor should be specified

The service life of a UV lamp extends for thousan

p

n a change in the spectral output over time. With polychromatic (MP) UV lamps, lower 

wavelengths will likely decline at a faster rate than will higher wavelengths. The rate and 

manner in which a lamp ages is lamp- and operation-specific. A detailed discussion of lamp

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Appendix J. Pilot-Scale and Demonstration-Scale Testing 

UV Disinfection Guidance Manual J-9 June 2003

aging is presented in section A.3.1.6. Lamp output will decrease over time as a function of

lamp hours in operation, the number of on/off cycles, and the power applied per unit (lamp) 

length. 

La

the

mp aging tests should be designed to assess the reduction and variance in lamp

germicidal output over time under defined worst-case operating conditions. Lamp age testing 

may us igned

e

 batch, lamp assembly, electrical 

haracteristics of the ballasts, heat transfer from the lamps to the water, and operation of the 

lamps.

clude electrical power delivered to the ballast, 

electrical power delivered to the lamp, and water temperature. If UV intensity sensors alone 

monito

radation of

the lamp assembly, including electrodes and seals, and any darkening of the lamp

• any fouling on the internal surfaces of the lamp sleeves; 

ballast operation

(e.g., power setting), heat transfer (e.g., lamp sleeves), and environment (water 

measured using one of the following: a radiometer 

ter

− 

rkening on the lamp). 

y be 

• 

n be used to identify operational

sues and provide operational guidance. The output of the lamps measured under fixed 

operati and

e either a pilot/demonstration-scale UV reactor installed at a WTP or a test bed des

to emulate the reactor (i.e., identical power supply). It is strongly recommended that all tests b

done with the lamps housed in the sleeves and powered by the ballasts that will be used in the 

final application. It is best if the lamp sleeves are maintained free of external foulant during 

aging tests, in a manner similar to that of the final application. 

Factors to consider in designing the test(s) include lamp

c

Since lamps will be manufactured in batches, it is recommended that lamps from several 

different lots be evaluated. During demonstration and pilot scale testing, the lamps should be

operated in a manner and in an environment that reflects conditions expected when the UV 

disinfection system is installed at a WTP. 

Parameters to monitor over time in

r lamp output, it is recommended that the A254 of the water also be measured. 

During testing, it is recommended that the following analyses be considered: 

• Visually inspect the lamps at regular intervals to document any visible deg

envelope; 

Document

• Measure the germicidal output of the lamp under fixed conditions of

temperature and transmission). 

− One measurement should be made with the lamps aged 100 hours (“new”).

− The germicidal output may be

equipped with a germicidal filter; a reference UV intensity sensor or radiome

from 200 to 400 nm; or by bioassay. 

The output from various positions along the lamp may be measured based on 

visual inspection (i.e., the pattern of da

− If lamp power is variable, lamp output as a function of lamp power setting ma

measured. 

Assess the output from lamps of different lots. 

Pilot/demo-scale test data and visual inspections ca

is

ng conditions can be plotted over time and fit to provide mean expected performance

prediction intervals (e.g., 90th, 95th, and 99th percentiles) to estimate the range of performance in 

Proposal Draft 
Appendix J. Pilot-Scale and Demonstration-Scale Testing 

UV Disinfection Guidance Manual J-10 June 2003

lamp intensity at different lamp ages. In a robust system, all the lamps will age in a similar 

manner. If lamps age differently than expected, the results will affect dose delivery and UV 

intensity sensor measurements. This data can be used to assess a proper end-of-lamp-life. 

J.5.5 valuating Controls and Alarms

ls and alarms should be conducted to verify their 

erformance and to gain familiarity with alarm/control response procedures. A test plan is 

typicall

educing

ts, or

ified

.6 References

and E.R. Blatchley. 1999a. Inorganic fouling at Quartz: Water 

interfaces in ultraviolet photoreactors – I Chemical characterization. Wat. Res. 33, no 15: 

Lin, L.-S., C.T. Johnston, and E.R. Blatchley. 1999b. Inorganic fouling at Quartz: Water 

interfaces in ultraviolet photoreactors – II Temporal and spatial distribution. Wat. Res. 33, 

Mackey , and G.F. Crozes. 2001. Practical aspects of UV disinfection. 

Denver, CO: AWWA Research Foundation and AWWA. 

Mackey Bridging pilot-scale testing to 

full-scale design of UV disinfection systems. Denver, CO: AWWA Research Foundation 

Phillips rces and application of ultraviolet radiation. Academic Press: New York. 

E

An evaluation of the UV reactor contro

p

y organized prior to testing, describing the control function to be tested, the test 

procedure, and the expected response. Faults may be induced or simulated. Low-dose 

conditions may be simulated by reducing lamp power, increasing the A254 of the water, r

or increasing flow beyond the validation limits of the reactor, turning off lamps or ballas

disconnecting sensors. Valve failure, high temperature, and ground fault interrupts may be 

induced or simulated. Simulating faults may require disconnecting components of the UV 

disinfection system or using modified electronics. Accordingly, qualified personnel as ident

by the manufacturer of the UV disinfection system should undertake these simulations. All

operational functions can be verified, including startup and shutdown sequences and cleaning 

cycles. Dose pacing if used, may be verified by monitoring lamp power settings and dose 

compliance as flow rate, A254, and lamp output are varied.

J

Lin, L.-S., C.T. Johnston,

3321-3329. 

no 15: 3330-3338. 

, E.D., R.S. Cushing

, E.D. and R.S. Cushing. Publication anticipated in 2003.

and AWWA. 

, R. 1983. Sou

Proposal Draft 
Appendix K. Preliminary Engineering Report 

Critical design and implementation issues need to be resolved early in the planning phase 

of a UV disinfection facility. The purpose of a preliminary engineering report is 1) to provide 

conceptual level layouts and preliminary cost estimates for implementation of UV disinfection at 

the water treatment plant (WTP), and 2) to recommend an implementation plan for UV 

installation design and construction. Specific components of the preliminary engineering 

analysis are listed below: 

• Identification of UV reactor design criteria and implementation issues 

• Evaluation of UV reactor alternatives and potential locations for the proposed 

installations in the plant treatment train 

• Determination of the hydraulic characteristics of the UV reactor and incorporate it 

into the hydraulic model of the plant 

• Development of estimates for capital, operational, and life-cycle costs for each 

alternative 

• Comparison of feasible alternatives and development of implementation 

recommendations 

This appendix presents an example of a preliminary engineering report (PER) for 

retrofitting a UV disinfection facility into an existing WTP. The basic elements involved in the 

planning phase of the UV installation are discussed in this report. The format and content of a 

site-specific PER should be coordinated with the State. This example report is based largely on 

a predesign report prepared for North Shore Water Commission, Wisconsin (Carollo Engineers 

2001). 

Chapter 3 of the Guidance Manual presents a detailed discussion of UV installation 

planning and design principles. A flowchart depicting the planning and design process is 

included in Figure 3.1. Table K.1 presents a correlation between the flowchart elements 

discussed in Chapter 3 and respective sections in this appendix. 

Table K.1 Elements of the Planning and Design Process (Ref. Figure 3.1) 

Element Chapter 3 Section Appendix K 

Section(s)

Define UV disinfection goals 3.1.1 K.1, K.2 

Identify potential retrofit locations 3.1.2 K.5 

Determine design parameters 3.1.3 K.2.2 

Evaluate potential UV reactors 3.1.4 K.3 

Evaluate operational and control strategies 3.1.5 K.4 

Evaluate hydraulic profile and site layouts 3.1.6 K.5 

Compare retrofit options and costs; select retrofit locations 3.1.7 K.6, K.7, K.8 

UV Disinfection Guidance Manual K-1 June 2003 

Proposal Draft 
Appendix K. Preliminary Engineering Report 

UV Disinfection Guidance Manual K-2 June 2003 

K.1 Background of Example WTP 

A 20 million gallon per day (mgd) surface WTP is used as an example in this appendix. 

The water treatment processes employed are coagulation and sedimentation pretreatment, 

granular media (anthracite and sand) filtration followed by chlorine disinfection. Since the plant 

was put into service in the 1960s, water quality regulations have become more stringent. In 

addition, there are growing concerns over chlorine-resistant pathogens (e.g., Cryptosporidium) 

and production of chlorinated disinfection byproducts (e.g., trihalomethanes, haloacetic acids). 

In order to upgrade the facility to meet current and future regulations and health concerns, 

several research studies have been performed involving the use of ozone, membranes, and UV 

disinfection. From the results of those studies, it was concluded that the most feasible and cost 

effective solution to achieve disinfection of chlorine-resistant pathogens was to add UV 

disinfection to the current treatment train. 

The following are the general performance goals of the UV installation for the example 

WTP:

• Provide 2-log Cryptosporidium inactivation. 

• Provide an additional disinfection barrier for other chlorine-resistant pathogens. 

K.2 UV Disinfection Criteria 

This section includes general information regarding the optimal application point for UV 

disinfection at the WTP and design considerations for implementation. 

K.2.1 Application Point and UV Transmittance 

One of the important parameters controlling UV installation design is the UV 

transmittance (UVT) of the water to be treated (section 3.1.3.1). The lower the UVT, the greater 

the UV intensity is needed to provide a given UV dose at a given flowrate. UVT typically varies 

with source water, seasonally, and through the treatment processes. Consequently, a thorough 

UVT analysis was completed during development of design criteria. 

K.2.1.1 Point of Application for UV Disinfection 

In keeping with the content of this guidance manual, the UV disinfection alternatives 

assessed for the WTP were limited to applications after filtration. Based on statistical results of 

the filtered water ultraviolet absorbance at 254 nanometers (A254) data, the UV reactors are sized 

based on a 0.032/cm A254 (93 percent UVT; 10 mm path length; light at 254 nm), which is the 

99th percentile minimum of the available A254 data. 

Proposal Draft 

 
Appendix K. Preliminary Engineering Report 

UV Disinfection Guidance Manual K-3 June 2003 

K.2.1.2 Treatment Chemical Impact on Absorbance 

Some chemicals used in water treatment absorb UV light and hence, can influence the 

design absorbance value, as discussed in Chapter 3.1.2 of the manual. Ferric iron and 

permanganate are two of these, and are used at the example WTP. Ferric iron strongly absorbs 

UV light; however, post-filtration iron levels are generally low. Permanganate absorbs UV light, 

but at permanganate levels of less than 1 mg/L, which is typically the case post-filtration, the 

impact is not significant. Therefore, for the PER, chemical A254 is not considered to influence 

the UV design criteria. 

K.2.1.3 Power Quality Impact on Absorbance 

As stated in section 3.1.3.3, the sensitivity of UV reactors to power fluctuations make 

electrical power supply a critical component of the UV installation planning and design. 

Preliminary pilot testing of UV reactors over the course of a year at this site did not indicate any 

problems with existing water utility power quality for the UV reactor’s operational continuity. 

Therefore, for this PER, power quality is not considered to negatively impact the UV installation 

design. 

K.2.2 Inactivation Goals and UV Dose 

The goal of UV disinfection at the example WTP is to provide inactivation of chlorineresistant pathogens. By using UV disinfection, the general goal of improving public health 

protection will be met and compliance with the Long Term 2 Enhanced Surface Water Treatment 

Rule (LT2ESWTR) Cryptosporidium inactivation requirements may be achieved, if needed (40 

CFR 141.702). (Source water sampling for “Bin” determination has not yet been completed at 

this example WTP, so the level of additional credit needed in the future is unknown.) UV 

disinfection credit will also be available for Giardia and virus inactivation. This additional UV 

disinfection credit will most likely reduce chlorine disinfection requirements, and hence, reduce 

disinfection by-product formation. 

The desired UV dose (or validated reduction equivalent dose [RED]) depends on the 

disinfection strategy of the individual UV installation. The State, utility, and designer must 

decide the log inactivation requirements for a target pathogen. Once this information is known, 

the UV dose can be established (section 3.1.1). For this PER example, a UV dose of 40 mJ/cm2

is recommended to achieve 2-log Cryptosporidium inactivation and is used for UV disinfection 

pre-design purposes. 

A 12-month pilot study was conducted to assess the long-term disinfection efficiency and 

operation and maintenance (O&M) issues. The study results indicated that lamp fouling and 

power quality issues should not be a concern for the facility (Mackey et al. 2001). 

Proposal Draft 

 
Appendix K. Preliminary Engineering Report 

UV Disinfection Guidance Manual K-4 June 2003 

K.3 UV Installation Equipment 

General information on UV reactors and the types of reactor configurations used for 

water treatment is provided in this section. 

K.3.1 UV Lamp Types 

For the flowrates associated with the WTP applications in this example, the number of 

lamps needed for a low-pressure (LP) reactor would be excessive, so consideration is limited to 

low pressure high output (LPHO) and medium pressure (MP) lamps. The general relative 

characteristics of each of these lamp types are listed in Table K.2. The ratio of number of lamps 

needed to achieve equivalent RED for LPHO lamps as compared to MP lamps is on the order of 

6:1. 

Table K.2 Relative Characteristics of LPHO and MP Lamps 

LPHO MP

Lamp Power Output Low High 

Power Efficiency High Low 

Number of Lamps Needed High Low 

Operating Temperature (°C) 130 – 200 600 - 900 

Typical Lamp Life (hours) 8,000 - 12,000 3,000 - 8,000 

K.3.2 UV Reactor Configuration 

UV installations can be designed around open-channel or closed-vessel configurations. 

In keeping with the content of this guidance manual and the general trend of the drinking water 

industry, the conceptual designs developed herein are limited to MP and LPHO closed-vessel 

reactors. 

K.4 UV Reactor Description 

This section contains information on the UV reactor design criteria for disinfection at the 

WTP.

K.4.1 General UV Reactor Description 

Each UV reactor for the WTP should include appropriate control and electrical cabinets 

and an off-line chemical cleaning (OCC) system or an on-line mechanical cleaning (OMC) 

system for the lamps. The cleaning systems should allow for the removal of organic and 

inorganic foulants that have accumulated on the surfaces of the lamp sleeves. 

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Appendix K. Preliminary Engineering Report 

UV Disinfection Guidance Manual K-5 June 2003 

K.4.2 Process Control 

For disinfection of drinking water, the ability of the UV reactor to deliver the design 

RED of 40 mJ/cm2

 depends on the flowrate, feed water UVT, and UV intensity. UV intensity is 

subject to lamp aging, lamp sleeve cleanliness, and water quality (mainly water UVT). The UV 

reactor should be designed to deliver the appropriate dose of UV light to the process flow based 

on predetermined maximum flowrate and minimum water quality parameter setpoints with an 

appropriate factor of safety (see Chapter 3). 

UV intensity sensors in each UV reactor should provide continuous performance 

verification of the reactor during operation. In case of lamp failure, the UV reactor 

programmable logic controller (PLC) should be programmed to either replace one row of lamps 

with another row that was off, or turn the reactor off after replacing it with a stand-by reactor. 

(Note that Alternative 1, described in section K.5.1, involves placing a UV reactor on each filter 

effluent pipe, therefore stand-by reactors for individual filter installations are not provided). The 

failed lamp can then be replaced with minimal interruption of UV reactor operation. 

In case of incorrect operation of lamps or low level of UV intensity, the PLC should 

display a warning to indicate to the operator that cleaning of the reactor should be performed. 

The operator initiates the cleaning of any reactor through the local human machine interface 

(HMI). After selection, the UV reactor PLC turns on the stand-by reactor. Then, the PLC closes 

the inlet and outlet valves and isolates the reactor to be cleaned. 

K.4.3 Expected UV Reactor Maintenance 

Although maintenance methods are installation and site-specific, some general 

maintenance tasks have been developed and are briefly described in this section. As the UV 

reactor represents a critical disinfection process, preventative maintenance should be carried out 

on a routine basis to ensure that UV reactors reliably provide the specified dose (40 mJ/cm2

). 

Inadequate cleaning is a common cause of underdosing in UV reactors. The lamp sleeves should 

be cleaned regularly by OMC or periodic OCC, and manually cleaned periodically to supplement 

automatic cleaning. The cleaning frequency is dependent on the water quality. Chemical 

cleaning is most commonly done with dilute citric or phosphoric acid. 

The effective life of the UV lamps depends on the minimum UV dose. The UV lamps 

should be replaced either at the end of their expected lifetime or following failure. Generally, 

UV lamps are replaced when the intensity has dropped to 70 percent of the original new-lamp

intensity (following cleaning of the chamber). This typically occurs after about 8,000 to 12,000 

hours (approximately 300 to 500 days) of operation for LPHO lamps and about 3,000 to 8,000 

hours (approximately 100 to 300 days) for MP lamps. The front panel of the enclosures 

indicates the cumulative hours each lamp has operated. The lamp run time display will facilitate 

monitoring of lamp replacement needs. 

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K.4.4 Power Needs 

UV reactor power needs to vary depending on the type of equipment that is installed and 

UVT of the water being disinfected. The LPHO reactors used for this pre-design have power 

requirements of approximately 20 kW for treating 20 mgd. The MP UV reactors require about 

130 kW for treating 20 mgd. As indicated in Table K.3 through K.5, additional power would be 

necessary to allow for future expansion of the UV facilities. 

K.5 Site Plans and Facility Layouts 

The preferred process location for a UV installation at a WTP is downstream of the filters 

and upstream of the high-service pumps (section 3.1.2). At the example WTP, there are three 

viable alternatives for the UV installation downstream of the filters: 

• Alternative 1 – Filter Gallery 

• Alternative 2 – Existing Chemical Room

• Alternative 3 – New Building 

There are eight granular media filters at the WTP. Alternative 1 involves placing one UV 

reactor on the effluent pipe of each filter in the filter gallery between the filters and clearwell. 

Alternative 2 is to construct the UV installation in a chemical room in the WTP between the low 

service pumps and the reservoir. Alternative 3 involves constructing a new building between the 

low service pumps and the reservoir to house the UV reactors. Figure K.1 presents a portion of 

the plants hydraulic profile and indicates the vertical locations of the three viable alternatives. 

The construction requirements and preliminary drawings for each alternative are 

illustrated and described in the following section, along with preliminary design criteria. Costs 

for the three alternatives are also compared. The preliminary site-specific design criteria are 

provided for example purposes only. Application-specific design criteria should be provided by 

the UV manufacturer for each individual UV disinfection implementation project on a case-bycase basis. 

K.5.1 Alternative 1 - Filter Gallery

In Alternative 1, one 3 mgd UV reactor is installed on the discharge pipe of each of the 

eight filters, as shown in Figure K.2. The UV reactors would be installed below the hydraulic 

grade-line (HGL) of the existing clearwell to ensure constant submergence (section 3.1.6.1). 

Flow through the UV reactors is by gravity from the filters into the clearwell. During filter 

backwashing and filter-to-waste cycles, valves located at the influent of each UV reactor can be 

closed to keep the reactor flooded while it is taken off-line. 

Compared to the other alternatives, construction of Alternative 1 is the simplest. 

Construction would include lowering the level in the clearwell to below the filter discharge 

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UV Disinfection Guidance Manual K-7 June 2003 

pipes, then taking each filter off-line individually to install the new piping, valves, and UV 

reactor. This would preclude significant disruption of plant operation during construction. 

Figure K.1 Portion of the WTP Hydraulic Profile and Alternative Locations for UV 

Implementation (Carollo Engineers 2001) Low S

ervi

ce Pu

mp

Filters

Reservoir

Clearwell

Elev. 72.0’

Elev. 56.5’

El

ev. 69.0’

Alternative 2

Alternative 3

Altern

ative 1

 

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Figure K.2 Alternative 1 - Filter Gallery (Carollo Engineers 2001) 

Note: UV reactor is installed on discharge pipe of each filter. Valve on UV reactor influent is closed to maintain water 

in the UV reactor during filter backwashing. 

Some concerns associated with installing UV reactors in the filter gallery are space 

constraints, climate control, and impact on filtration. In the design, care would need to be taken 

to allow enough space for maintenance and construction of the UV installation. Although the 

UV installation in this example will fit into the existing filter gallery of the WTP, generally there 

is little room to work in these locations. In addition, depending on the location of the WTP, 

humidity and pipe sweating in this space might be a concern. However, there are protective 

climate controlled enclosures available for these conditions, though they add to cost and 

maintenance needs. A detailed hydraulic analysis of the filters would need to be completed prior 

to designing a UV installation on the filter effluent piping. There must be adequate head 

available from the filters to the clearwell to allow for the addition of a UV reactor that will not 

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adversely affect the filter performance. Furthermore, in the event of a UV reactor shutdown, the 

filter associated with that UV reactor shutdown would also need to be removed from service. 

The preliminary design criteria for Alternative 1 are provided in Table K.3. Due to the 

large size of the LPHO reactors, they will not fit into the filter gallery. Therefore, only the MP 

reactors are considered for Alternative 1. 

Table K.3 Preliminary Design Criteria – Alternative 1 – MP UV Reactors

Description Unit Criteria

Current Future

Treatment plant capacities 

Flowrate mgd 20 40

Water quality

UVT In a 10mm quartz cell @ 254 nm % UVT 93 93 

Ultraviolet reactors 

Type of reactors: medium-pressure 

Number of reactors No 8 16 

Number of banks per reactor No. 2 2 

Number of lamps per bank No. 4 4 

Total number of lamps per reactor No. 8 8 

Input power per lamp W 2000 2000 

Total operating electric load kW 128 256 

Total installed electric load kW 128 256 

Headloss through reactor (at current and future flowrates) Inches 12 36 

Approximate dimensions of each UV reactor 

Length Inches 22 50

Width Inches 36 36

Height Inches 26 26

Flanges diameter Inches 12 12 

As stated previously, the eight UV reactors listed in Table K.3 are designed for a 

maximum capacity of 3 mgd each. This design is based on the assumption that one UV reactor 

would be taken off-line periodically during a filter backwash cycle. The WTP must be able to 

treat 20 mgd with one filter out of service, so the remaining seven UV reactors would need to be 

able to disinfect the maximum plant flow. This arrangement also provides reactor redundancy. 

If one UV reactor were taken out of service, the associated filter would also be taken out of 

service. 

In this example, future plant expansions needed to be taken into account. For the present 

analysis, provisions are made so that future expansion of the UV installation to an ultimate flow 

of 40 mgd will be possible. If the filter capacities can be expanded to 40 mgd, the UV 

installation expansion will necessitate placing two UV reactors (16 total) in series along each 

filter effluent pipe. During the construction of the initial design for 20 mgd, adequate space and 

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mechanical layouts would be provided for the addition of a second UV reactor. For example, a 

section of pipe located at the outlet of the UV reactor for the 20 mgd design could be designed 

for easy removal and installation of a second UV reactor. 

K.5.2 Alternative 2 - Existing Chemical Room 

Alternative 2 consists of installing three 10 mgd reactors (2 operational + 1 stand-by) in 

an existing chemical room in the WTP, as shown in Figure K.3. This alternative necessitates 

placing the UV reactors above the existing HGL of the plant. The low-lift clearwell pumps 

provide the head through the UV reactors. (It is generally more advantageous to place the UV 

reactors below the HGL. However, due to the space constraints at the example WTP, and to 

provide an example of issues that may arise during design, this option is discussed.) 

Figure K.3 Alternative 2 - Existing Chemical Room with UV Reactors 

(Carollo Engineers 2001) 

In theory, an outlet weir structure would be a viable option to raise the HGL of the plant 

to ensure constant submergence of the UV reactors (section 3.1.6.1). In this case, there is not 

enough space for such a structure. To ensure constant submergence of the UV reactors, a 

vertical pipe at the outlet header would maintain the water level at an elevation above the top of 

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the UV reactors. Air vacuum valves would be installed on the inlet and the effluent vertical pipe 

to counteract siphon effects on the UV reactors. The discharge from the effluent header would 

then flow by gravity to the reservoirs. The hydraulic design of the inlet and outlet channels 

provides a continuous equal flow split between the reactors (section 3.3.1.2). 

The construction needs of Alternative 2 are more difficult than Alternative 1. A 36-inch 

finished water pipe from the low head pumps would need to be taken out-of-service long enough 

to cut the pipe and tie-in a new section with fittings and valves for connection of the new UV 

reactors. The existing equipment would need to be moved to alternate locations to accommodate 

the new large piping and UV reactors, and the floor would need to be cut to provide clearance 

for the piping to and from the lower floor. 

Other issues with Alternative 2 are that space constraints in the chemical room, possible 

structural upgrades of the building, and raising the HGL of the plant. To allow for adequate 

space for maintenance, piping, and instrumentation, the existing chemical equipment in the room

would need to be removed and reinstalled elsewhere in the plant. Since this is a second level 

room, a detailed structural analysis would need to be completed to ensure the floor is able to 

withstand the load of the UV installation. If structural upgrades are needed, they could prove to 

be expensive and difficult to design and construct. Installing the UV reactors in the second level 

room above the HGL would significantly increase the total dynamic head (TDH) placed on the 

low-lift clearwell pumps. Therefore, pump upgrades would be necessary to overcome the 

additional headloss of the UV installation. 

K.5.3 Alternative 3 - New Building 

Alternative 3 includes constructing a new building to house three 10 mgd UV reactors (2 

operational for 20 mgd + 1 standby) and related equipment. The building layout and UV design 

for Alternative 3 is presented in Figures K.4 and K.5. The 36-inch finished water line from the 

low-service clearwell pumps would be modified to provide flow to the UV reactors in the new 

building. The new facility would include a two-level structure to house mechanical and 

electrical equipment and large diameter piping to convey the filtered water through the UV 

reactors. The UV reactors would be installed in the basement of the new building below the 

HGL of the plant to ensure constant submergence. The hydraulic design of the inlet and outlet 

channels would provide an equal flow split between the reactors, and the discharge would flow 

under pressure to the reservoirs. 

The UV building design and mechanical piping shown in Figures K.4 and K.5 are for 

preliminary design and cost estimates only. If this option were selected, the building size and 

configuration for this alternative would need to be evaluated in more detail during the design 

phase and adjusted as necessary, depending on the final UV reactors selected to be used. 

Construction for Alternative 3 would be the most involved of the three options because it 

would include excavation and construction of a new building. Besides the building construction, 

the project would involve tying into the existing 36-inch finished water pipe in two locations 

below grade, and modifying site amenities such as pavement and landscaping. In addition, the 

new building would also need new power, control, and security systems as well as plumbing, 

HVAC, etc. 

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UV Disinfection Guidance Manual K-12 June 2003 

Figure K.4 Alternative 3 - New Building with UV Reactors – Plan View 

(Carollo Engineers 2001) 

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UV Disinfection Guidance Manual K-13 June 2003 

Figure K.5 Alternative 3 - New Building with UV Reactors – Section View 

(Carollo Engineers 2001) 

The preliminary design criteria for Alternatives 2 and 3 using the MP UV reactors are 

presented in Table K.4. 

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UV Disinfection Guidance Manual K-14 June 2003 

Table K.4 Preliminary Design Criteria – Alternatives 2 and 3 – MP UV Reactors 

Description Unit Criteria

Current Future

Treatment plant capacities 

Flowrate mgd 20 40

Water quality

UVT in a 10mm quartz cell at 254 nm % UVT 93 93 

Ultraviolet reactors 

Type of reactors: medium-pressure 

Number of reactors No. (Duty + Standby) 2+1 2 + 1 

Number of banks per reactor No. 2 2 

Number of lamps per bank No. 8 8 

Total number of lamps per reactor No. 16 16 

Input power per lamp W 4000 4500 

Total operating electric load kW 128 144 

Total installed electric load kW 192 216 

Headloss through reactor (at current & future flows) Inches 10 48 

Approximate dimensions of each UV reactor 

Length Inches 48 48

Width Inches 49 49

Height Inches 41 41

Flanges diameter Inches 30 30 

The three MP UV reactors selected have design capacities of 10 mgd each. Three 10 

mgd UV reactors for the 20 mgd design provide one stand-by reactor in the event of a 

malfunction, cleaning, or maintenance of one UV reactor. 

Note that for expansion of the UV installation using the MP reactors given in Table K.4, 

the size and number of UV reactors remains constant. In order to provide extra lamp intensity to 

meet dose requirements at the ultimate flow, 4000 W lamps would replaced with 4500 W lamps. 

 (Note that re-validation of the reactors with the 4500 W lamps would be required (40 

CFR141.729 (d)). 

The preliminary design criteria using the LPHO reactors for Alternatives 2 and 3 is 

provided in Table K.5. 

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Table K.5 Preliminary Design Criteria – Alternatives 2 and 3 – LPHO UV Reactors 

Criteria Description Unit Current Future

Treatment plant design capacities 

Plant flowrate mgd 20 40 

Water quality

UVT in a 10mm quartz cell at 254 nm % UVT 93 93 

Ultraviolet reactors 

Type of reactors: Low-Pressure High-Output 

Number of reactors No (Duty + Standby) 2+1 2+1 

Number of rows per reactor(1) No. 5 9

Number of rows with lamps installed No. 4 8 

Number of lamps per row No. 12 12 

Total number of lamps per reactor No. 48 96 

Input power per lamp W 200 200 

Total operating electric load kW 19.2 38.4 

Total installed electric load kW 28.8 57.6 

Headloss through reactor (at current & future flows) Inches 24 35 

Approximate dimensions of each UV reactor 

Length(2) Inches 110 144

Width Inches 51 51

Height Inches 100 100

Flanges diameter Inches 32 32 1 Preliminary design assumes one spare row in addition to current flow demand requirements for installation of lamps in 

the future. 2 Length varies depending on the number of rows installed.

The expansion from 20 mgd to 40 mgd using the LPHO UV reactors would be 

accomplished by adding additional rows of lamps to the reactor. The UV manufacturers would 

oversize the reactor and additional rows of lamps could be inserted as needed for increasing flow 

capacities. However, UV reactor validation would need to be done both with and without the 

additional rows for the maximum and ultimate flow conditions. 

Alternatively, the UV installation could be sized to allow additional UV reactors to be 

installed for expansion. Initially, three 10 mgd reactors would be installed for a capacity of 20 

mgd (2 operational and 1 standby). Space would be provided to install two additional reactors in 

the future for a capacity of 40 mgd (4 operational and 1 standby). 

These examples of UV installation expansion alternatives provide various options to the 

designer. To confidently design for future UV installation expansions, it will be critical to have 

an accurate flow projection and adequate space for the UV installation expansion. In addition to 

the UV reactors needed for an expansion, mechanical piping, controls, instrumentation, and 

wiring would need to be considered during the preliminary engineering phase. Furthermore, the 

designer should work closely with the UV manufacturer to decide on an expansion plan that has 

been proven to work effectively and efficiently for the specific UV installation design.

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UV Disinfection Guidance Manual K-16 June 2003 

K.6 Preliminary Capital and O&M Cost Estimates

The preliminary capital, operational, and maintenance costs for each alternative are 

summarized in this section. Estimated costs presented for Alternative 1 are based solely on the 

MP design. The LPHO UV reactors used for comparison here would not fit into the filter gallery 

and so was not considered for that alternative. 

K.6.1 Capital Cost Estimate Summary

The estimated capital improvement costs for each alternative are summarized in Table 

K.6. Total project cost includes UV reactors, construction cost, engineering services, and a 20 

percent estimating contingency. 

The cost estimates presented for Alternative 1 in Table K.6 are based on using eight MP 

UV reactors. The costs presented for Alternatives 2 and 3 were developed around using three 

MP and LPHO UV reactors. The equipment cost for installing the MP UV reactors for all three 

Alternatives is higher than installing LPHO UV reactors for Alternatives 2 and 3. However, due 

to the relatively simple construction details associated with Alternative 1, the total project cost is 

considerably lower than Alternatives 2 and 3, which require significant construction provisions. 

A comparison of these alternatives is provided in section K.7. 

Table K.6 Preliminary Capital Cost Estimates 

UV Reactor Cost Total Project 

Cost

Annualized 

Capital Cost1

Alternative 1- Filter Gallery (MP) $531,000 $1,900,000 $166,000 

Alternative 2- Chemical Room (MP) $556,000 $2,400,000 $209,000 

Alternative 2- Chemical Room (LPHO) $450,000 $2,300,000 $201,000 

Alternative 3- New Building (MP) $556,000 $2,800,000 $244,000 

Alternative 3- New Building (LPHO) $450,000 $2,700,000 $235,000 1

 Annualized costs calculated at 6 percent interest for 20 years. 

K.6.2 Preliminary Operating and Maintenance Costs 

Table K.7 presents a summary of the estimated O&M costs and total annualized costs for 

each alternative (four MP reactors in service for Alternative 1, two MP and LPHO UV reactors 

are in service for Alternatives 2 and 3). Detailed O&M costs for an average flow of 10 mgd are 

provided for each alternative and UV reactors in Tables K.8 and K.9. 

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Table K.7 Estimated UV Disinfection Costs 

Annul O&M 

Cost1

Annualized 

Capital Cost2

Total Annual 

Cost

Annual 

Difference3

Alternative 1 $72,000 $166,000 $238,000 $1,000 

Alternative 2 (MP) $111,000 $209,000 $320,000 $83,000 

Alternative 2 (LPHO) $36,000 $201,000 $237,000 -0- 

Alternative 3 (MP) 111,000 $244,000 $355,000 $118,000 

Alternative 3 (LPHO) $36,000 $235,000 $271,000 $34,000 

1. Costs for UV intensity sensor calibrations, lamp sleeve, ballast and sensor replacement are not included. 

2. Annualized costs calculated at 6 percent interest for 20 years. 

3. Relative to the least expensive alternative (Alternative 2-LPHO).

Table K.8 MP UV Installation O&M Cost Estimates 

for Alternatives 1, 2 and 3 

O&M Costs

Alt. 1 Alts. 2 & 3

Average Plant Flow 10 mgd 

1 - Power Consumption

Annual power consumption of lamps in kWh 530,155 883,5921

Price of electricity ($/kWh) 0.10 0.10 

Annual Expenses ($) 53,015 88,359 

2 - Consumables

Lamp replacement # operating 32 $/Lamp 500 (#1) 

600(#2&3) 

17,500 21,000

# replaced / yr 35 

Annual Expenses ($) 17,500 21,000 

3 - Labor

Lamps # replaced / yr 35 Time (hr) 8.8 

15 min / lamp 

Cleaning 1 clngs / yr / reactor 3 Time (hr) 9.0 

3 hrs / cleaning 

Total Time (hr) 17.8 $/hr 65 65

Annual Expenses ($) 1,153 1,153 

TOTAL COSTS 

1 - Power Consumption 53,015 88,359 

2 - Consumables 17,500 21,000 

3 - Labor 1,153 1,153 

4 - Chemicals 100 100 

Total Annual Costs 71,770 110,610 

COSTS PER MG TREATED 

Costs per MG Treated $/MG 20.00 30.00 1 Alternative 1 utilizes eight smaller UV reactors while Alternatives 2 and 3 utilize 2 large reactors. At an average flow of 

10 mgd, the large UV reactors operate at the lowest possible setting, which is higher than required at 10mgd. 

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UV Disinfection Guidance Manual K-18 June 2003 

Table K.9 LPHO UV Installation O&M Cost Estimates 

for Alternatives 2 and 3 

O&M Costs

Average Plant Flow 10 mgd 

1 - Power Consumption

Annual power consumption of lamps in kWh 168,303 

Price of electricity ($/kWh) 0.10 

Annual Expenses ($) 16,830 

2 - Consumables

Lamp replacement # operating 96 $/Lamp 150 12,600 

# replaced / yr 84 

Annual Expenses ($) 12,600 

3 - Labor

Lamps # replaced / yr 84 Time (hr) 21.0 

15 min / lamp 

Cleaning 1 clngs / yr / reactor 24 Time (hr) 72.0 

3 hrs / cleaning 

Total Time (hr) 93.0 $/hr 65

Annual Expenses ($) 6,045 

TOTAL COSTS

1 - Power Consumption 16,830 

2 - Consumables 12,600 

3 - Labor 6,045 

4 - Chemicals 600 

Total Annual Costs 36,070 

COSTS PER MG TREATED $/MG 10.00

Power consumption represents the majority of the operational cost. The power cost used 

for calculation of the annual O&M costs was $0.10/kWh. As expected, the MP reactors have 

considerably higher power costs associated with their operation than the LPHO reactors (Tables 

K.8 and K.9). 

Lamp replacement costs are also significant. Cost of lamp replacement is based on an 

estimated lamp life of 10,000 hours and $150/lamp equipment cost for the LPHO reactors. The 

MP estimated lamp life is 8,000 hours with a lamp replacement cost of $500/lamp for 

Alternative 1 and a lamp cost of $600/lamp for Alternatives 2 and 3. 

Estimated labor needs range from 18 hours for the MP UV installation to 93 hours per 

year for the LPHO UV installation. Labor estimates are based on lamp replacement at four 

lamps per hour and three hours per cleaning. 

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K.7 Summary of Alternatives and their Advantages and Disadvantages 

A comparison of the three alternatives is presented in this section. The alternative 

comparisons are based on cost, feasibility of construction, and ease of maintenance. The 

advantages and disadvantages of each alternative are summarized in Table K.10. 

Table K.10 Advantages and Disadvantages of Each Alternative 

Alternative 1 – Filter Gallery

• New building not necessary • Damp during periods of the year 

• Below existing hydraulic grade line (HGL) • Needs protective cabinet for each UV reactor 

• Relatively simple construction • Tight quarters for construction and maintenance

• Likely no plant down-time for construction • Less manufacturer flexibility

• Lowest capital cost • Must take a filter off-line for UV reactor 

maintenance 

• Does not accommodate expansion easily 

• No redundancy at maximum flow 

Alternative 2 – Chemical Building

• New building not necessary • UV reactors above the plant HGL 

• Main floor access • Difficult construction constraints 

• Uncertainty associated with structural upgrades 

• Very limited space for relocation of existing 

• Chemical equipment to other parts of the plant 

• Low lift pump upgrades necessary 

Alternative 3 – New Building

• Ample space for UV reactors and controls • Highest capital and total project cost 

• UV reactors placed below the plant HGL • Necessitates a new building 

• Room for future expansion • Longer construction schedule 

• Flexibility in UV installation design options 

• Custom designed space for UV reactors 

K.8 Conclusions and Recommendations 

Alternative 1, which involves the installation of the UV reactors in the filter gallery, is 

the least expensive option. However, there are concerns about the moisture associated with the 

location that may adversely affect the performance of the UV reactors and cause maintenance 

problems. Servicing and maintaining the UV reactors in the filter gallery and installing the 

necessary control panels might be problematic based on space constraints. In addition, to expand 

the capacity of the UV installation to accommodate the ultimate flow (40 mgd), the size of the 

pipes connecting the UV reactors to the filter effluent pipes would have to be increased and an 

additional UV reactor would have to be installed. Furthermore, Alternative 1 does not provide 

adequate redundancy at the ultimate flow. 

Alternative 2, retrofitting the existing chemical feed room has the advantage of not 

requiring a new building. However the disadvantages of this option include the space 

limitations, associated pump upgrade needs and the need to find a new location for the chemical 

feed equipment currently housed in that room. 

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Alternative 3, constructing a building addition to the WTP, is the most expensive 

alternative, but this option has some important advantages over Alternatives 1 and 2. The new 

building would offer flexibility in design options. The design would not be limited to using the 

MP or LPHO reactors; any UV reactors could be accommodated. There would be room for 

future expansion of the UV installation, if necessary, and ample space would be provided for 

mechanical and instrument layouts. The UV reactors would be installed below the existing 

hydraulic grade line of the plant to ensure submergence of the reactors. 

Although Alternative 3 has some distinct advantages over Alternatives 1 and 2, the 

capital cost is significantly higher, due to the cost of the new building and appurtenances. 

Alternative 1 is the most economical alternative, but does not accommodate expansion easily and 

provides no redundancy at maximum flow. The disadvantages of Alternative 2, including lack 

of space for the existing chemical equipment, make this alternative the least desirable. 

Given the above discussion, and based on both economical and non-economical criteria 

for comparison in this example, including anticipated future expansion needs, the ranking of the 

alternatives from most desirable to least desirable is as follows: 

• Most desirable – Alternative 3, New Building 

• Next best option – Alternative 1, Filter Gallery 

• Least desirable – Alternative 2, Existing Chemical Room

K.9 References

Carollo Engineers. 2001. Weber basin water treatment plant No. 3 expansion. Layton, Utah. 

Mackey, E.D., R.S. Cushing, and G.F. Crozes. 2001. Practical Aspects of UV Disinfection. 

Denver, Colo.: AWWA Research Foundation. 

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Appendix L. Regulatory Timeline 

The purpose of this appendix is to provide utilities with a timeline (Figure L.1) to assist 

in planning and implementation of tasks to achieve compliance with the Long-Term 2 Enhanced 

Surface Water Treatment Rule (LT2ESWTR). The timelines present the important tasks that 

utilities are likely to complete; however, the tasks and their duration will change based on utilityspecific priorities and constraints. 

Tasks have been divided into two general categories: regulatory and engineering. 

Compliance dates and resulting planning activities are based on utility size (i.e., systems serving 

fewer than 10,000 persons or systems serving 10,000 or more persons). 

L.1 Regulatory Tasks

Regulatory tasks and milestones include key dates in the regulatory schedule such as 

monitoring requirements and compliance dates. 

L.1.1 Cryptosporidium Monitoring 

One of the key provisions of the LT2ESWTR is the requirement to conduct monitoring to 

determine Cryptosporidium removal/inactivation requirements (40 CFR 141.702). Monitoring 

results will be used to determine a “bin classification,” which prescribes the Cryptosporidium

inactivation/removal required. More information regarding the monitoring requirements is 

available in the Source Water Monitoring Guidance Manual for Public Water Systems for the 

LT2ESWTR. 

L.1.2 Compliance Deadlines for Cryptosporidium Treatment 

For utilities required to provide additional treatment for Cryptosporidium, the compliance 

deadline is the date when a utility must have implemented the selected treatment techniques (40 

CFR 141.701). Table L.1 summarizes the Cryptosporidium treatment compliance deadlines for 

the LT2ESWTR. 

Table L.1 LT2ESWTR Compliance Schedule Summary1

System Size Compliance Deadline for Systems Making 

No Capital Improvements for Compliance2

Serving 10,000 or more people 6 years after LT2ESWTR promulgation

Serving fewer than 10,000 people 8 1⁄2 years after LT2ESWTR promulgation

1 (40 CFR 141.701) 

2 State may grant an additional two years for systems making capital improvements. 

UV Disinfection Guidance Manual L-1 June 2003

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Appendix L. Regulatory Timeline 

UV Disinfection Guidance Manual L-2 June 2003

L.2 Engineering Tasks and Milestones 

Engineering tasks and milestones include tasks that should be completed by a utility to 

develop and implement an LT2ESWTR compliance strategy.

L.2.1 Process Evaluation and Planning 

Compliance with the LT2ESWTR Cryptosporidium treatment requirements will 

necessitate varied levels of process evaluation and planning. After compliance strategy options 

have been reviewed (see section 3.1.5) and a decision has been made to implement UV 

disinfection, planning may include one or more of the following activities:

• Engaging the State during planning to ensure the installation of UV disinfection is 

approved 

• Conducting disinfection benchmarking and profiling if distribution system total 

trihalomethane (TTHM) and five haloacetic acids (HAA5) concentrations are at least 

80 percent of the Stage 1 DBPR maximum contaminant levels for TTHM and HAA5 

(40 CFR 141.711-713) 

• Developing a capital improvement program that includes the necessary modifications 

for LT2ESWTR compliance (i.e., UV disinfection) 

• Evaluating and implementing funding alternatives 

Utilities are encouraged to seek approval of their LT2ESWTR compliance plan from the 

State before implementation of a compliance strategy. This may take several months and can 

have a significant impact on the implementation schedule, particularly when the State requires 

modifications. Because UV disinfection is a relatively new technology, the State may take 

longer to approve UV disinfection or require more significant involvement in the compliance 

strategy development. 

L.2.2 UV Installation Design 

The duration of the facility design phase will be contingent on a number of 

utility-specific factors, including scope of design (i.e., new facility or retrofit), scale of design 

(size of facility), available in-house resources, procurement methods, and validation testing 

requirements (discussed in detail in chapters 3 and 4). The design will likely include one or 

more of the following tasks: 

• Evaluation of equipment and contractor procurement methods 

• UV reactor procurement 

• UV installation design 

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UV Disinfection Guidance Manual L-3 June 2003

• UV reactor validation strategy determination 

Many States require final approval of process improvements. As such, utilities should 

review the UV installation design and validation strategy with the State. If the State is not 

consulted during these phases, additional time may be necessary to receive final approval. 

L.2.3 Construction and Startup

The timeline in Figure L.1 reflects a construction period of two years for both large and 

small utilities. However, the actual duration of construction and startup can vary significantly, 

depending on the scope of the project, the significance of the changes to the existing treatment 

plant, and other utility specific factors. Utilities should consider these factors during planning 

phases and adjust accordingly to ensure regulatory milestones are achieved by the necessary 

dates. 

L.3 Example Timeline 

Figure L.1 presents example timelines that encompass the regulatory and engineering

tasks discussed in the previous sections. Utilities may have site-specific constraints that may 

shorten or extend the duration of the engineering tasks listed; however, regulatory milestones are 

not flexible. 

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Appendix L. Regulatory Timeline 

Figure L.1 Example LT2ESWTR Compliance Timeline 

2003 2004 2005 2006 2007 2008 2009 2010 2011 

Task Description

LT2ESWTR ProposalLT2ESWTR PromulgationSystems serving10,000 or more personsCryptosporidium monitoringInitial Bin ClassificationProcess Evaluation and PlanningFacility DesignConstruction and Startup*Compliance DeadlineSystems servingless than 10,000 personsCryptosporidium monitoringInitial bin classificationProcess Evaluation and PlanningFacility DesignConstruction and Startup*Compliance Deadline

* State may grant anadditional 2 years for systemmaking capital improvements

 Regulatory Tasks/Milestones Engineering Tasks/Milestones

UV Disinfection Guidance Manual L-4 Proposal Draft 
Appendix M. Compliance Forms 

This appendix is intended to supplement the monitoring information provided in section 

5.4 with examples of monthly compliance report and monitoring log forms that utilities might 

use for reporting to the State. (Note, these are only examples; the States may develop their own 

compliance forms and require additional monitoring.) The specific monitoring and reporting 

requirements for each utility should be confirmed with the State, and the forms should be 

modified accordingly. For those utilities with advanced control systems (e.g., Supervisory 

Control and Data Acquisition (SCADA)), it may be possible to automatically generate these 

reports and compliance forms. 

To receive disinfection credit, the Long Term 2 Enhanced Surface Water Treatment Rule 

(LT2ESWTR) requires validation testing of UV reactors to demonstrate a set of operating 

conditions where the UV reactor will deliver the required dose (40 CFR 141.729 (d)). These 

operating conditions must include flowrate, UV intensity, and UV lamp status, and the utility 

must monitor these parameters during routine operation to ensure dose delivery. (40 CFR 

141.729 (d)). States may specify additional monitoring or reporting requirements. The example 

forms presented in this appendix list both required and recommended monitoring parameters 

(required parameters are identified with the applicable rule citation). 

Table M.1 summarizes the recommended minimum level of monitoring and record 

keeping for utilities utilizing UV disinfection. For many of the UV reactor components, the 

required or recommended performance level is based on the measurement uncertainty of the 

specific equipment that was used when the UV reactor was validated. This uncertainty is used to 

determine the validation safety factor and recommended reduction equivalent dose (i.e., 

operational UV dose), as described in section 4.2. 

UV Disinfection Guidance Manual M-1 June 2003

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Appendix M. Compliance Forms

UV Disinfection Guidance Manual M-2 June 2003

Table M.1. Summary of Compliance Monitoring and Reporting Activities1, 2

Item Description

Measured 

Parameter 

Recommended 

Monitoring 

Frequency

Reporting 

Frequency

Offspecification/ 

Validated 

Parameters for 

UV Dose 

Monitor reactor to 

ensure operation 

within conditions 

validated for 

required UV dose 

(40 CFR 141,

Subpart W, 

Appendix D). 

Flowrate, UV 

intensity, lamp 

status, and other 

parameters (e.g., 

UVT) used to

monitor dose. 

Continuously. 

Record at least 

once every four 

hours (daily for 

very small 

systems).

Required monthly 

report of offspecification 

operation as a 

percent of distributed 

flow or operating time 

(40 CFR 141.730).3

 

Calibration of

UV Intensity 

Sensors 

Calibration checks 

compare the duty 

sensor to the 

reference sensor

and are 

recommended at 

the power setting 

utilized during 

normal operation. 

Percent difference 

between duty and 

reference sensors

relative to the level 

of uncertainty used 

in determining the 

RED. 

(see section C.4.7) 

Monthly. If a 

sensor fails for 

three consecutive 

months, then the 

sensor should be 

checked weekly 

and the 

manufacturer 

contacted. 

Requirements in a 

State approved 

protocol. 

Calibration of

UV 

Transmittance 

(UVT) Monitor 

It is recommended 

that grab samples 

be collected to 

confirm 

performance. 

Percent difference 

relative to the 

manufacturer’s 

guaranteed 

uncertainty. 

Weekly initially. 

Reduced 

frequency 

following oneyear of 

supporting data. 

NR. 

1

 Section 5.4.2 presents all recommended monitoring activities, including the compliance monitoring shown in this 

table. 2

 Unless noted in the table with an LT2ESWTR citation, the monitoring is recommended and not required. 3

 The reported off-specification value is the percentage of water entering the distribution that was not treated with

UV reactors operating within validated conditions. This is required by the LT2ESWTR (40 CFR 141.730).

NR – No requirement 

The LT2ESWTR requires utilities to submit monthly reports to the State (40 CFR 

141.730). At a minimum, the reports must detail operating performance during the reporting 

period and, specifically, the percent of total distributed volume treated during periods when the 

UV reactor(s) was off-specification. An example monthly monitoring form is shown in Table 

M.2. Tables M.3 through M.6 present a format that the utility can use to log operating data for 

development of the monthly reports. With minor modification, the example forms are applicable 

for any of the three control strategies discussed in section 4.3.2.2: UV intensity setpoint, UV 

intensity and UV transmittance (UVT) setpoint, and calculated dose. 

 

For those utilities utilizing multiple reactors, the operation of each reactor must be 

monitored, recorded, and reported. Requirements for compliance monitoring beyond those 

established by the LT2ESWTR and the specific content of the monthly report will be established 

by the State and coordinated with all other reporting requirements. Additional information on 

UV reactor monitoring and maintenance is provided in Chapter 5. 

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Appendix M. Compliance Forms

UV Disinfection Guidance Manual M-3 June 2003

Greater detail on each of the example forms is provided below:

• Form M.2 is an example of a summary report that would be completed by the utility 

and submitted to the State on a monthly basis. 

• Forms M.3A, M.3B, and M.3C are example reference forms for each of the three 

control strategies discussed in section 4.3.2.2. These forms would be completed by 

the utility based on validation results and then referenced throughout the operation of 

the UV installation to confirm compliance. 

• Form M.4 is an example operating log that would be completed on a daily basis. The 

form would be used to record the operating status of the UV installation and to 

estimate the volume of water that was discharged during off-specification operation. 

• Form M.5 is an example sensor calibration log. This log would be completed 

whenever sensor calibration checks are performed. The log would be used to record 

the results of the calibration testing as well to track any sensor recalibration or repair 

work that was completed. 

• Form M.6 is an example on-line UVT monitor calibration log. This log would only 

be completed by those utilities that have included on-line UVT monitors as part of 

their design. The log would be completed whenever UVT monitor calibration checks 

are performed. The log would be used to record the results of the calibration testing 

as well to track any recalibration or repair work that was completed. 

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Appendix M. Compliance Forms

Table M.2 Example Monthly Report to State

Reporting Period:

System/Treatment Plant:

PWSID:

Monthly Reactor Operating Report

Operating Data Off-Specification1

Unit No.

Operating Time 

(Hours)

Volume Treated

(Gallons)

No. of

Events Volume (Gallons) Time (Hours)

TOTALS:

Compliance Certification:By volume the total percent of off-specification operation during the reporting period = %Of the _____ sensors within those reactors that operated during this reporting period, _____ have been checked for calibration and were within the acceptaSignature of Principal ExecutiveOfficer or Authorized Agent: Date:

1 From Table M.4

UV Disinfection Guidance Manual M-4 Proposal Draft
Appendix M. Compliance Forms

Table M.3A UV Intensity Setpoint Control Strategy1

Validated Operating Requirements to Achieve Disinfection Credit for (Target Pathoge

(Reference Document - See Instructions Below)

Validated Operating Conditions

Flow Rate UV Intensity Log Inactivatio

1 This form is for use with a UV Intensity setpoint operating strategy. If a different operating strategy is employed by the utility, then one of the other sample 

should be used or a form summarizing the specific validation criteria necessary to confirm compliance using the selected operating strategy should be deve

2 For those utilities that do not employ variable power settings or that have a single target log inactivation, this column may not be necessary.

Instructions:

This form should be completed based on the validation testing results and used as a reference document.This form should be referenced to determine if the UV system is operating within its validated conditions and meeting the performance requirements for

inactivation credit for the target pathogen.

UV Disinfection Guidance Manual M-5 Proposal Draft
Appendix M. Compliance Forms

Table M.3B UV Intensity and UVT Setpoint Control Strategy1

Validated Operating Requirements to Achieve Disinfection Credit for (Target Pat

(Reference Document - See Instructions Below)

Validated Operating Conditions

Flow Rate UV Intensity UVT Log Inactiv

1 This form is for use with a UV Intensity/UVT setpoint operating strategy. If a different operating strategy is employed by the utility, then one of the other sa

reference forms should be used or a form summarizing the specific validation criteria necessary to confirm compliance using the selected operating strategshould be developed.

2 For those utilities that do not employ variable power settings or that have a single target log inactivation, this column may not be necessary.

Instructions:

This form should be completed based on the validation testing results and used as a reference document.This form should be referenced to determine if the UV system is operating within its validated conditions and meeting the performance requirements for

inactivation credit for the target pathogen.

UV Disinfection Guidance Manual M-6 Proposal Draft
Appendix M. Compliance Forms

Table M.3C Calculated Dose Control Strategy1 

Validated Operating Requirements to Achieve Disinfection Credit for (Targe

(Reference Document - See Instructions Below)

Validated Operating Conditions

Flow Rate UV Intensity UVT Calculated Dose Log In

1 This form is for use with a calculated dose operating strategy. If a different operating strategy is employed by the utility, then one of the other sample referenc

should be used or a form summarizing the specific validation criteria necessary to confirm compliance using the selected operating strategy should be de

2 For those utilities that do not employ variable power settings or that have a single target log inactivation, this column may not be necessary.

Instructions:

This form should be completed based on the validation testing results and used as a reference document.This form should be referenced to determine if the UV system is operating within its validated conditions and meeting the performance requirements for

inactivation credit for the target pathogen.

UV Disinfection Guidance Manual M-7 Proposal Draft
Appendix M. Compliance Forms

Table M.4 Daily UV Intensity Sensor and UVT Monitoring and Compliance Log

Date: _______________________________________________ Unit Did Not Operate During This Monitoring Interval:System/Treatment Plant: _______________________________PWSID: _____________________________________________ Operator Signature:Unit Number:

_______________________________

UV Transmittance Monitor: Grab Sample Collected: Y/N If yes, complete Calibration Log Sheet.

Calibration Check Performed: Y/N

UV Intensity Sensors: Calibration Check Performed: Y/N If yes, complete Calibration Log Sheet.

Cumulative UV Intensity Sensors

Flow Rate Volume

No. Time (gpd) (gals) Reading Reading Reading Reading Reading Reading Reading Reading

123456

Time Flow Rate

UVT

Acceptable

(Y/N)

UV Intensity

Acceptable

(Y/N) 2

Calculated 

Dose 2 Volume 3

Operating

Time

Off-Spec

(Y/N)

Volume OffSpec 4

Reading No.1Reading No.2Reading No.3Reading No.4Reading No.5Reading No.6

Daily Total:

A B

Percent Off-Specification by Volume: % >5% (Y/N) 5

(B/A x 100)

Notes:

1 As presented, this form is most applicable to a calculated dose control strategy, but can be modified to suit all control strategies.2 Only if applicable to selected control strategy.3 Volume treated since last reading.4 For systems that rely solely on manual readings, the volume off-specification shall be calculated as the total volume discharged since the last recording interval during which operation within

validated conditions was observed. If continuous monitoring is provided, then the volume off-specification shall be only that portion of the volume distributed during the recording interval thatwas outside of the validated conditions.

5 LT2ESWTR requires that off-specification be less than 5 percent by volume on a monthly basis for unfiltered systems (§141.721). For filtered systems, it is that off-specification operation 

be less than 5 percent by volume on a monthly basis. The specific requirements for allowable off-specification for filtered systems will be established by the state.

Instructions:

This form should be completed daily for each operating unit. If a unit did not operate during the 24-hour monitoring interval, please note as such in the box at the top of the log.This form is intended for periodic manual monitoring of system operating conditions at the recommended interval of once every four hours. If automated or more frequent monitoring is performed,

then this form should be modified by the utility.

UV Disinfection Guidance Manual M-8 Proposal Draft
Appendix M. Compliance Forms

Table M.5 Daily UV Intensity Sensor and UVT Monitoring and Compliance Log

where,

is measured inis measureme

Duty Sensor

Reading (A)

Reference Sensor

Reading (B)

Erro

[(A-B)/Bx

alibration.

ange of tolerance.

ented below.

Total Uncertainty Criteria = 

Ref

Duty 1 *1

I

I

⎟⎟⎠⎞ ⎜⎜⎝⎛ −

σ

I

Reporting Period:

System/Treatment Plant:

PWSID:

Duty UV Intensity Sensor Uncertainty (%): where,

Reference Intensity Sensor Uncertainty (%): is measured in

Total Uncertainty Criteria (%): is measureme

UVT Intensity Sensor Calibration Report (Make Additional Copies of Form as Necessary)

Reactor/Duty

Sensor No.

Reference Sensor 

ID Duty Sensor ID Date Time

Duty Sensor

Reading (A)

Reference Sensor

Reading (B)

Erro

[(A-B)/Bx

Certification:Of the ______ sensors within those reactors that operated during this reporting period, ______ have been checked for calibration.One or more of the sensors within those reactors that operated during this reporting period were outside the acceptable range of tolerance.

UV Intensity Sensor(s) sent to manufacturer to be recalibrated as documented below.

UV intensity sensors sent to manufacturer for calibration:

Sensor ID Unit No. Date Sent Date Received

Operator Signature: Date:

Total Uncertainty Criteria = 

Ref

Duty 1 *1

I

I

⎟⎟⎠⎞ ⎜⎜⎝⎛ −

σ

I

UV Disinfection Guidance Manual M-9 Proposal Draft
Appendix N. UV Lamp Breakage Issues 

Lamps used in UV reactors typically contain mercury or an amalgam composed of 

mercury and another element, such as indium or gallium. Other elements, such as xenon, 

cadmium, zinc, and magnesium, are also capable of generating UV light; however, the 

temperatures required to volatilize these elements are much higher than to volatilize mercury. In 

contrast, mercury has a sufficient vapor pressure at ambient temperatures to provide the optimum

pressure for efficient production of resonance radiation. Moreover, mercury has a low ionization 

energy to facilitate starting a lamp (Phillips 1983). In order to provide a cost-efficient lamp

while addressing perceived risks and disposal issues associated with mercury, lamp

manufacturers are continuing to develop ways to reduce the mercury content of lamps without 

impacting their efficiency (USEPA 1997b; Walitsky 2001). 

The mercury contained within a UV lamp is isolated from exposure to water by a lamp 

envelope and surrounding lamp sleeve. In order for mercury to be released into the water, both 

the lamp and lamp sleeve must break. For the purposes of this appendix, lamp breakage is 

defined as fracture of the lamp sleeve and the lamp envelope. This is further divided into off-line 

and on-line breaks. Off-line breaks occur during handling or maintenance functions when the 

lamps are not installed in the reactor. On-line lamp breaks occur while UV reactors are in 

operation. 

Due to the general public health concern with mercury, this appendix discusses the issues 

associated with UV lamps used for drinking water disinfection by addressing potential causes of 

lamp breakage, preventive measures, disposal issues, the fate of mercury after release, and 

regulatory issues. 

N.1 Off-Line Lamp Breaks 

Off-line breaks occur when a lamp breaks during shipping, handling, or storage. These 

releases do not pose a hazard to the water consumer but are a concern for operators or employees 

in the vicinity of the break. 

N.1.1 Potential Causes of Off-Line Lamp Breaks and Corresponding Prevention 

Measures 

Mercury is sealed in a UV lamp within the lamp envelope; therefore, there is no risk of 

mercury exposure from handling an unbroken UV lamp. The UV manufacturer should train 

operators in proper handling and maintenance of UV lamps to avoid mishandling and potential 

off-line breaks. In addition, proper storage procedures will also reduce the potential for lamp

breakage. Lamps should be stored horizontally in individual packaging. Lamps should not be 

stacked unpackaged on one another or vertically propped in corners (Dinkloh 2001a). 

UV Disinfection Guidance Manual N-1 June 2003

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Appendix N. UV Lamp Breakage Issues 

UV Disinfection Guidance Manual N-2 June 2003

N.1.2 Off-Line Mercury Release Cleanup Procedures 

Off-line lamp breaks resulting in a release of mercury can occur; therefore, Standard 

Operating Procedures (SOPs) should be developed that describe the procedures for containing 

and cleaning the off-line spills. The local poison control center, fire department, or public health 

board can assist in the development of SOPs. 

Small spills, defined as less than about 0.6 to 2.25 grams (USEPA 1992) or the amount in 

a broken thermometer (USEPA 1997a), can be contained and collected with commercially 

available mercury spill kits. Mercury and materials used during the cleanup procedure are 

regulated as hazardous wastes and should be disposed of properly as described in section N.3.3. 

The USEPA Office of Emergency and Remedial Response recommends that “[i]n the event of a 

large mercury spill (more than a broken thermometer’s worth), immediately evacuate everyone 

from the area, seal off the area as well as possible, and call your local authorities for assistance” 

(USEPA 1997a). Local authorities can help determine the appropriate response for various spill 

sizes to be included in SOPs. Given that the mercury content in a single UV lamp typically 

ranges from 0.005 to 0.4 grams (as discussed in section N.4.3), large mercury spill actions would 

not be warranted for a single lamp break or multiple lamp breaks that result in release of less 

than roughly two grams. 

Superfund Amendments and Reauthorization Act (SARA) Title III regulations address 

emergency release, inventory, and release reporting requirements for hazardous materials. The 

reportable quantity for mercury spills is one pound (454 grams) as mercury. Based on typical 

mercury levels in UV lamps (discussed in section N.4.3), this would necessitate the breakage of 

approximately 1,100 medium pressure (MP) lamps and up to 90,000 low pressure (LP) lamps; as 

such, spilling more than one pound of mercury is highly unlikely. 

N.2 On-Line Lamp Breaks 

A recent survey of domestic water and wastewater municipalities, UV lamp

manufacturers, and UV reactor manufacturers identified relatively few instances of on-line lamp

breaks and mercury release (Malley 2001). This section discusses potential causes of lamp

breakage and corresponding prevention measures, followed by a summary of documented 

incidents of on-line lamp breaks. 

N.2.1 Potential Causes of On-Line Lamp Breaks and Corresponding Prevention 

Measures 

Lamp breaks can potentially be caused by debris in the water, temperature variations,

exceeding positive or negative pressure limits (water hammer), electrical surges, or improper 

maintenance. Lamps may also break as a result of inherent mechanical or physical limitations of 

the lamp and improper material selection. 

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Appendix N. UV Lamp Breakage Issues 

UV Disinfection Guidance Manual N-3 June 2003

N.2.1.1 Debris 

Debris in the water can potentially break the lamp sleeves and lamps. Although the 

majority of UV reactors will be installed after the filters in the treatment train, it is possible that 

equipment failure upstream may release parts or fragments, such as nuts or bolts. In addition, if 

UV disinfection is applied prior to the filters the probability of having debris in the water might 

be higher compared to post-filter UV installation. Ground water systems have reported stones or 

gravel from wells entering UV reactors and breaking lamps (Malley 2001; Roberts 2000). 

Placement of screens, baffles, or low velocity collection areas upstream of UV reactors or 

vertical installation of UV reactors (when applicable) may reduce the risk of debris in the water 

from entering the reactor (Cairns 2000; Malley 2001, McClean 2001b). The extent of 

containment provided by these safety measures is unknown. Utilities and designers should 

determine the applicability of these isolation techniques on a site-specific basis. 

N.2.1.2 Loss of Water Flow and Temperature Considerations 

UV lamps are designed to operate within a specific temperature range to maximize the 

UV light output of the lamp. Without flowing water to cool the lamp, the lamp temperature can 

rise to dangerous levels and may break (Dinkloh 2001a; Malley 2001; Srikanth 2001a; Srikanth 

2001b). This overheating is more likely to occur with MP than LP lamps (due to lamp operating 

temperatures) and occurs much faster in air than stagnant water. Even if upper temperature 

levels are not exceeded, after restoration of water flow, the lower temperature water entering the 

reactor may cause the lamp sleeve and the lamp to break due to temperature differentials 

(Dinkloh 2001a; Malley 2001). In order to prevent lamp breaks, operating procedures should 

ensure that the following conditions are met: 

• Water is flowing through the UV reactor if the UV lamps are energized.

• The lamps are not energized while the reactor is not flowing full (i.e., no air in the 

reactor). 

Temperature sensors should be, and typically are, incorporated into the reactor design and 

will shut down the reactor before critical temperatures are exceeded (Cairns 2000; Dinkloh 

2001a; Malley 2001; Srikanth 2001b). Proper hydraulic design is also necessary to ensure that 

lamps are submerged at all times during reactor operation. Reactor designs should incorporate 

low flow alarms, air relief valves, or other devices to ensure that lamps are operating only when 

the reactor is completely flooded and water is flowing. These sensors should be linked to an 

alarm and automatic shutoff system (Cairns 2000; Dinkloh 2001a; Srikanth 2001b). Lamp

overheating and temperature differentials could break all the lamps within the affected reactor. 

N.2.1.3 Pressure-Related Issues 

Hydraulic pressures within the reactor that are not within UV installation operating limits 

may also break the lamp sleeve. Although breaking the lamp sleeve does not automatically 

break the lamp envelope, the lamp is more vulnerable when its lamp sleeve has been 

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Appendix N. UV Lamp Breakage Issues 

UV Disinfection Guidance Manual N-4 June 2003

compromised, potentially allowing the lamp envelope to come into direct contact with the 

surrounding water. 

Most lamp sleeves are designed to withstand continuous positive pressures of at least 120 

pounds per square inch gauge (psig) (Roberts 2000; Aquafine 2001; Dinkloh 2001c; Srikanth 

2001a; Srikanth 2001b). However, negative gauge pressures below -1.5 psig have been shown to 

adversely affect lamp sleeve integrity (Dinkloh 2001c). The tolerance level of the lamp sleeve 

depends on the quality of the quartz and the thickness and length of the lamp sleeve; therefore, 

pressure thresholds vary between lamp sleeves. Positive and negative pressures, such as those 

associated with water hammer, that exceed these levels may compromise the integrity of the 

lamp sleeve. Manufacturers should provide lamp sleeves with the appropriate material, 

thickness, geometry, and seals for the specified pressure and flow ranges of a given UV 

installation. Water hammer can affect all UV reactors and break all lamps; therefore, utilities 

should perform a surge analysis to determine if water hammer is a potential problem. 

N.2.1.4 Procedural Errors 

Operation and maintenance training can help prevent lamp breaks during on-line 

operations because a lamp damaged by off-line handling or improper maintenance operations 

may potentially break under on-line pressure or temperature stresses. For example, a common 

procedural error that can occur during lamp replacement is over-tightening compression nuts 

when securing the lamp sleeve (Aquafine 2001; Dinkloh 2001a; Srikanth 2001a; Srikanth 2001b; 

Swaim 2002). Over-tightening can cause a fracture of the lamp sleeve or a leak around the 

sleeve or compression nut cavity that may not become apparent until after start-up and operation 

of the UV reactor. 

N.2.1.5 UV Reactor Design 

The UV reactor manufacturer should design the UV reactor to reduce the possibility of 

lamp sleeve and lamp breaks. This subsection describes design problems that may cause lamp

sleeve and/or lamp breakage if not properly addressed. 

Electrical Considerations

If the UV installation electrical support system is improperly designed (e.g., inadequate 

circuit breakers and ground fault indicator circuits), electrical surges can cause short-circuiting 

and lamp socket damage (Srikanth 2001a; Srikanth 2001b). In addition, system electronics that 

can provide voltages that exceed lamp ratings (overdriving lamps) may also result in breaking 

the lamp (Malley 2001). 

Cleaning Mechanism Considerations

The cleaning mechanism may break the lamp sleeve and lamp envelope if it is not 

aligned properly. Although the cleaning mechanism closely surrounds the lamp sleeve for 

cleaning, manufacturers should ensure that the mechanism is flexible and able to adjust to minor 

misalignment of the lamp sleeves. 

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Appendix N. UV Lamp Breakage Issues 

UV Disinfection Guidance Manual N-5 June 2003

At high lamp temperatures, the cleaning mechanism in some UV reactors may fuse to the 

lamp sleeve when not in use. As a result, during the next cleaning event, the lamp sleeve may 

crack when the cleaning mechanism is activated or when the cleaning mechanism passes back 

over the residual left on the lamp sleeve (Dinkloh 2001a). Routine inspection according to 

manufacturers’ recommendations will help detect problems with the cleaning mechanism before 

damage occurs. In some UV reactors, wipers rest away from the lamp sleeve when not in use 

and an alarm sounds when the wiper stops along the lamp sleeve. 

Thermal Expansion and Contraction

Other potential causes of lamp breaks include improper matching of lamp materials with 

respect to thermal expansion characteristics. Compatible materials within the lamp should be 

used by the manufacturer to avoid stress and damage that can be caused by thermal expansion 

and contraction differences between materials under various operating, shipping, or handling 

conditions (Cairns 2000). In addition, improper seal design or lamp envelope swelling may 

cause water leaks around the seals that may result in electrical shorts and cracking of lamps 

(Cairns 2000). 

N.2.1.6 Summary of Potential Causes and Methods of Prevention of OnLine UV Lamp Breaks 

Table N.1 summarizes the potential causes of on-line lamp breaks and provides a brief 

description of the preventive measures that UV installation designers and operators can 

implement to reduce each risk. There are few documented cases where lamps have been broken 

during on-line operations, which are discussed in section N.2.2. 

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Appendix N. UV Lamp Breakage Issues 

UV Disinfection Guidance Manual N-6 June 2003

Table N.1 Summary of Potential Causes and 

Methods of Prevention of On-Line UV Lamp Breaks 

Potential 

Cause Description Preventive Measure

Debris • Physical impact of debris on 

lamp sleeves may cause lamp 

breaks. 

• Installation of screens, baffles, or low 

velocity collection areas upstream of UV 

reactors or vertical installation of UV 

reactors will help prevent debris from 

entering the reactor. 

Loss of Water 

Flow and 

Temperature

Considerations 

• Lamps may overheat and break. 

• The temperature differential 

between stagnant water or air 

and flowing water may cause 

lamp breaks.

• Reactors should always be completely 

flooded. Temperature and flow sensors 

that are linked to an alarm and automatic 

shutoff system can be used to indicate 

irregular temperature or flow conditions. 

PressureRelated 

Considerations 

• Excessive positive or negative 

pressures may exceed lamp

sleeve tolerances and break the 

lamp sleeve. 

• A surge analysis should be completed to 

determine the occurrence of water 

hammer. 

• Pressure relief valves or other measures

can be used to reduce pressure surges. 

• Applicable pressure ranges should be

specified for lamp sleeves. 

Procedural 

Errors 

• Improper handling or 

maintenance may compromise 

the integrity of the lamp sleeve 

and/or lamp. 

• Operators and maintenance staff should

be trained by the manufacturer. 

UV Reactor 

Design 

• Electrical surges can cause 

short-circuiting and lamp socket 

damage. 

• Applying power that exceeds 

design rating of lamps can cause 

lamps to burst from within. 

• Adequate circuit breakers/ground fault 

indicators should be specified to prevent 

damage to the reactor. 

• Replacement lamps should be electrically 

compatible with reactor design. 

• Misaligned or heat-fused 

cleaning mechanism may break 

or damage the lamp sleeve and 

lamp envelope. 

• Operators and maintenance staff should

perform routine inspection and 

maintenance according to manufacturers’ 

recommendations. 

• Thermally incompatible materials 

do not allow for expansion and 

contraction of lamp components 

under required temperature 

range. 

• Designers should specify temperature 

ranges likely to be encountered during 

shipping, storage, and operation of lamps 

to aid the manufacturer in the selection of 

thermally compatible materials. 

N.2.2 Frequency of On-Line Lamp Breaks 

There have been relatively few documented incidents of on-line lamp breaks. As part of 

a survey of domestic water and wastewater municipalities, UV lamp manufacturers, and UV 

reactor manufacturers, Malley (2001) identified nine cases of on-line lamp breaks. Both the 

lamp sleeve and lamp envelope were damaged in all nine cases, resulting in mercury release 

(Table N.2). No cases of on-line failures using LP or low pressure high output (LPHO) lamps 

were identified. However, LPHO lamps are relatively new to the UV disinfection market and all 

LPHO lamp installations have been operating for 5 years or fewer (Malley 2001). All nine cases 

involved MP lamps. Four of the nine lamp breaks were caused by impacts from stones on lamps 

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Appendix N. UV Lamp Breakage Issues 

UV Disinfection Guidance Manual N-7 June 2003

oriented perpendicular to flow. In one of the nine lamp breaks, the applied power exceeded 

design rating of lamp (30kW) causing the lamp to burst from within. Differential sleeve heating 

resulted in two of the nine documented lamp breaks. The lamps were mounted vertically in the 

UV reactor allowing heat to accumulate at the top of the lamp, eventually cracking the sleeve. In 

two of the nine instances, operating lamps reached extremely high temperatures (>600 °C) in air 

because the reactors lost water flow. When water flow resumed, the cooler water (20 °C) broke 

the lamps. Most of these documented cases of lamp failure were the result of design issues that 

have been addressed in modern reactor designs. As mentioned previously, temperature and flow 

alarms should shut the UV reactor down when the potential for overheating or differential 

heating exist. 

Another documented instance of MP lamp breakage occurred in a UV-peroxide reactor 

designed for well-head treatment of tetrachloroethene-contaminated ground water (Moss 2002a). 

The UV reactor was positioned between the ground water extraction pump and distribution 

system booster pumps. The 7-foot long MP lamp sleeve sagged and came into contact with the 

lamp envelope. The lamp envelope and lamp sleeve broke, releasing mercury to the water in the 

reactor. The lamp failure triggered an alarm, shutting down both the ground water extraction and 

distribution system booster pumps. Mercury liquid was found settled in the bottom of the 

reactor. Water sampling at a nearby fire hydrant detected mercury concentrations below the 

maximum contaminant level (MCL) of 2 micrograms per liter (μg/L) (Moss 2002a; Moss 

2002b). 

European drinking water utilities have an extensive history with UV technologies. 

Unfortunately, no written documentation of lamp failures was identified; however, two instances 

of lamp breakage during UV disinfection of drinking water were noted by European 

manufacturers (Roberts 2000; Table N.2). In one instance, a ground water well pump discharged 

gravel or stones into the reactor, resulting in a lamp break. A strainer was placed in-line prior to 

the reactor to prevent any future instances. The other documented case of a lamp breaking was 

due to operator error. A forklift was driven into an operating reactor and physically damaged the 

UV reactor. The event activated an alarm and pneumatic valve closure, which contained the 

contamination (Roberts 2000). In addition, there was an incident in which equipment debris (a 

bolt from the filter underdrain) impacted a lamp sleeve. Although the lamp sleeve was broken, 

the lamp envelope remained intact and mercury was not released because of the immediate UV 

installation shutdown and prompt operator response (McClean 2001a). Table N.2 summarizes 

the documented lamp breakages discussed in this section. 

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Table N.2 Mercury Release Incidents Involving UV Lamp Breaks

Identified 

Cause 

Number of

Incidents Description of Incident

Debris 5 (4)1 Stones entered the reactors and impacted and broke the lamps. 

(1)2

 Gravel entered reactor through the booster pump and impacted

and broke the lamp. 

Loss of Water 

Flow and 

Temperature

Considerations 

2 (2)1

 Lamps were left on and allowed to reach high temperatures

(600 oC) in empty non-operating reactors. Restoration of flow 

resulted in cooler water (20 o

C) breaking the lamps. 

Operator Error 1 (1)3 Forklift collided with on-line reactor resulting in lamp breakage.

Manufacturer 

Design 

4 (1)

1

 Applied power exceeded design rating of lamp (30kW) causing

the lamp to burst from within. 

(2)1

 Vertical orientation of lamps resulted in differential heating and 

eventual cracking of lamp sleeve as surrounding water cooled the 

submerged portion of lamp and the exposed portion of the lamp 

accumulated heat. 

(1)4

 High operating temperatures resulted in deformation of the lamp 

sleeve. The lamp sleeve sagged and on contact with the lamp 

envelope, both envelope and lamp sleeve broke. 1

 Survey of domestic water, wastewater, and hazardous waste treatment utilities (Malley 2001) 2 European drinking water facilities (Roberts 2000) 3

 European brewery (Roberts 2000) 4

 UV-peroxide ground water remediation reactor (Moss 2002a) 

N.2.3 On-Line Mercury Release Response Plan

On-line lamp breaks are rare occurrences that are preventable with appropriate design and 

operation of UV reactors. However, utilities may consider developing a mercury release 

response plan for an on-line UV lamp break. The plan may include the following components: 

• Site-specific containment measures 

• Mercury sampling and compliance monitoring guidelines 

• Clean-up procedures 

• Reporting requirements 

In the event of an on-line lamp failure alarm, the UV reactor should be immediately shut 

down and operators should attempt to determine the cause of the alarm. Unfortunately, lamp

failure alarms or sensors cannot typically determine the cause of the alarm, whether it is partial 

or complete breakage of the lamp sleeve or lamp envelope (Kolch 2001) or another problem

unrelated to the lamps. Thus, it is recommended that the reactor be taken off-line when 

investigating the cause of a lamp failure alarm (Kolch 2001). 

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In the event of an on-line lamp break and mercury release, operators should attempt to 

isolate the mercury in the reactor or downstream. Utilities may install spring-return actuated 

valves with a short closure time on the reactor inlet and outlet piping (McClean 2001b) to isolate 

the mercury. Given the short residence time of many MP reactors, the outlet-side valve may 

need to be located quite a distance downstream so that the valve has time to close and isolate the 

mercury upstream. UV installation designers should evaluate valve closure times with respect to 

creating water hammer. 

Condensed mercury may collect in areas of low water velocity such as the bottom of a 

shutdown reactor, sump areas, or a clearwell. In addition, a strainer positioned on the reactor 

outlet piping may prevent lamp fragments from entering the water supply system (McClean 

2001b; Srikanth 2001a; Srikanth 2001b). The headloss associated with such measures should be 

considered in the hydraulic profile. Designers may also consider installation of drains, vacuum

relief valves, and piping to allow disposal of potentially contaminated water in the reactor to a 

waste container or truck. 

The extent of containment provided by these safety measures is unknown. Utilities and 

designers should determine the applicability of these isolation techniques on a site-specific basis. 

Utilities should coordinate with their State primacy agency when developing the 

following action items: 

• Mercury sampling plan – Sampling procedures may outline sample locations, 

sampling frequencies, and analysis methods. Sample locations should be chosen with 

consideration of where mercury may settle and to assess the mercury concentrations

potentially reaching the consumer. Sampling frequencies should consider flowrate, 

detention time, and travel time to the first potential consumer. 

• Site-specific cleanup procedures – Site-specific cleanup procedures should be 

incorporated into a utility process hazard analysis (PHA). Issues to consider are 

detection and disposal of isolated or condensed mercury, potential disposal or 

treatment of contaminated water, and cleanup responsibilities (by utility staff or 

contracted hazardous materials team). 

• Reporting to State – Reporting may include a description of the release, estimated 

quantity of release, shutdown or containment procedures, cleanup or disposal 

methods, sampling procedures (including sampling locations, frequencies, and 

results). 

• Public notification requirements, if applicable – Revised public notification 

requirements (40 CFR 141.203) outline three tiers of public notification, depending 

on the severity of the violation or situation. Exceeding the mercury MCL of 2 μg/L is 

classified as a Tier 2 notice, where public notification is required within 30 days, 

unless extended to 90 days by the State primacy agency. Public notification 

requirements do not specifically address mercury releases due to UV lamp breakage 

where the MCL is not exceeded.

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N.3 Regulatory Review

This section presents a review of regulations that may apply to the use or breakage of UV 

lamps containing mercury in water treatment plants (WTPs). 

N.3.1 Safe Drinking Water Act 

The Safe Drinking Water Act (SDWA) established a primary MCL of 2 μg/L for 

inorganic mercury (40 CFR 141.62(b)). The required monitoring frequency depends on the 

water source and the frequency of detections. Utilities using ground water sources are required 

to sample once every 3 years. WTPs using surface water sources are required to sample 

annually. If mercury is detected above the MCL in any ground water or surface water utility, the 

utility must sample quarterly. 

These regulations are independent of the use of UV disinfection at a facility. As 

discussed in section N.2.3, utilities should consult with their primacy agencies when developing 

a sampling plan for responding to an on-line UV lamp break. 

N.3.2 Operator Health and Safety - Exposure Limits 

The Mercury Study Report to Congress (USEPA 1997c) provides detailed information on 

health effects associated with exposure to elemental mercury and mercury compounds. Mercury 

exposure to employees in WTPs falls under the regulatory authority of the Occupational Safety 

and Health Administration (OSHA). The exposure limits set by OSHA focus on exposure by 

inhalation. 

OSHA regulations have established permissible exposure limits (PELs) for mercury 

compounds and organo alkyls containing mercury. A PEL is a time weighted average 

concentration for an 8-hour workday during a 40-hour work week that is not to be exceeded. 

When a PEL is designated as a ceiling level (cPEL), the concentration cannot be exceeded during 

any part of the workday. PELs and cPELs are enforceable standards. The National Institute for 

Occupational Safety and Health (NIOSH) also publishes Immediately Dangerous to Life or 

Health (IDLH) concentrations for a variety of compounds. IDLH concentrations represent the 

maximum concentrations that one could escape within 30 minutes without symptoms of

impairment or irreversible health effects. These values are not enforceable, but can be used as 

guidance for safety procedures. Table N.3 outlines the PELs, cPELs, and IDLHs for mercury 

compounds and organo alkyls containing mercury. 

Table N.3 Health and Safety Standards for Mercury Compounds in Air 

Compound PEL (mg-Hg/m3) cPEL (mg-Hg/m3) IDLH (mg-Hg/m3) 

Mercury compounds NR 0.1 10 

Organo alkyls containing mercury 0.01 0.04 2

NR - not reported. 

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In the event of a spill, the volatilization and the resultant concentration of mercury in air 

depends on the vapor pressure (0.002 mm Hg; Table N.4), air currents, temperature, surface 

area/dispersion of mercury droplets, and time. Calculations using the ideal gas law (PV=nRT) 

indicate that these levels may be exceeded if cleanup of the mercury spills does not occur; 

however, prompt response and proper cleanup procedures should prevent exposure levels over 

these standards. 

N.3.3 UV Lamp Disposal Regulations 

Lamp manufacturers are required to determine whether their products exhibit the toxicity 

characteristic for mercury using a test called the Toxic Characteristic Leaching Procedure 

(TCLP, 40 CFR 261). If the TCLP level of a lamp is above the regulatory limit of 0.2 mg/L, the 

lamp is regulated as a universal hazardous waste (Universal Waste Rule, 40 CFR 273) under 

Subtitle C of the Resource Conservation and Recovery Act (RCRA). As such, these lamps 

should be sent to a mercury recycling facility where the mercury is recovered and lamp 

components are recycled. Although some mercury lamps do not exceed the TCLP regulatory 

level, utilities are encouraged to recycle these lamps to reduce mercury loading to the 

environment. Some UV reactor and lamp manufacturers will accept spent or broken lamps for 

recycling or proper disposal (Dinkloh 2001a; Lienberger 2002; Gump 2002). Alternatively, 

utilities should contact their primacy agency for a list of local recycling facilities. 

N.4 Additional Factors Affecting Risk 

This section provides further information that may be helpful in evaluating risk 

associated with on-line lamp breakage. The ultimate fate of mercury after a lamp is broken is 

currently unknown but is expected to depend on the following conditions: 

• Physical and chemical properties of mercury species in air and water 

• Mercury behavior in operating UV lamps 

• Quantity of mercury released (type, age, and number of broken lamps) 

• Potential mercury reactions in water treatment plants and the distribution system

N.4.1 Physical and Chemical Properties of Mercury

Mercury can exist in three oxidation states: elemental (Hg0

), mercurous (Hg+1), and 

mercuric (Hg+2). Mercury cycles between oxidation states as a function of the redox conditions 

of the surrounding environment and the availability of other reactive compounds. 

Elemental mercury is a liquid at ambient temperature and pressure; however, given its 

high vapor pressure (Table N.4), elemental mercury is easily vaporized at ambient temperatures. 

Other physical and chemical properties of elemental mercury that affect its fate and transport are 

outlined in Table N.4. 

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Table N.4 Physical and Chemical Properties of Elemental Mercury

(Merck & Co., Inc. 1983) 

Property Value

Melting point (

o

C) -38.87

Boiling point (oC) 356.72

Density (g/mL at 25 oC) 13.534

Solubility (g/L at 25 oC) 0.061

Vapor pressure (mm Hg at 25 oC) 0.002 1

 Further information regarding mercury solubility in water can

be found in Glew et al. 1971 

N.4.2 Mercury Behavior in UV Lamps 

It is important to characterize the quantity and form of mercury in an operating lamp

because they represent the starting point for mercury dispersion, speciation, and reaction 

chemistry in the water system following a lamp break. However, the quantity and form of 

mercury placed in UV lamps typically is considered proprietary information by manufacturers 

because these parameters affect the efficiency, operation, and life of the lamp. In general, the 

form of mercury contained in a UV lamp is elemental mercury (LP and MP) or a mercury 

amalgam (LPHO). An amalgam is an alloy of elemental mercury with another metal (typically 

indium in lamp applications) that can be either solid or liquid at room temperature, depending on 

the relative proportions of the two metals. In operating lamps, elemental mercury (from pure or 

amalgamated mercury) is vaporized in the presence of an inert gas. Vapor phase mercury is 

excited and then ionized by the energy transfer from the excited inert gas and the supply of 

electrons generated from the applied voltage (Phillips 1983). It is the transition of mercury 

electrons from excited state back to ground state that releases energy in the wavelength range of 

the UV spectrum.

The concentration of mercury in the vapor phase in LP and LPHO lamps is controlled 

predominantly by temperature. Manufacturers of these lamps use different methods to control or 

maintain the temperature of the liquid mercury or mercury amalgam to establish the desired 

vapor phase mercury concentrations. Methods of controlling the temperature of mercury and, 

consequently, the vapor pressure in LP and LPHO include using either a mercury amalgam

attached to the lamp envelope (LPHO only), a cold spot on the lamp wall, or a mercury 

condensation chamber located behind each electrode. At typical LP and LPHO lamp operating 

temperatures, mercury remains predominantly in the liquid or solid amalgam phase with a small 

proportion in the vapor phase. 

MP lamps are dosed with elemental mercury liquid. In operating MP lamps, mercury is 

primarily present in the vapor phase due to high operating temperatures (600 to 900 o

C; 

Table 2.1) that cause all liquid elemental mercury to volatilize (Phillips 1983). In order to 

control the concentration of vapor phase mercury, manufacturers strictly control the amount of 

mercury dosed or added to the lamp during production. This is different from the LP and LPHO 

lamps where an excess of mercury is placed in the lamp and only a portion of the elemental 

mercury enters the vapor phase. Once elemental mercury enters the vapor phase, mercury 

ionization in a MP lamp occurs the same way as in LP or LPHO lamps. 

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The relative proportion of mercury in the liquid/amalgam phase and the vapor phase 

becomes important when an operating lamp breaks in water. Vapor phase elemental or ionized 

mercury may be released as very fine particles. These particles may more readily dissolve in 

water as opposed to condensed liquid or amalgamated mercury that settles in low velocity areas. 

In addition to these functional mercury interactions, Altena (2001) reported reactions of 

vapor phase mercury with fluorescent lamp components, such as the glass bulb, glass stems, 

coatings, and the emission material (electrodes). This process results in the embedding of 

mercury in lamp components and the accumulation of mercury-containing deposits, such as 

mercury oxide, on the internal lamp envelope surface. Altena (2001) theorized that mercury 

reactions with UV lamp components would be comparable to fluorescent lamps. These deposits 

represent approximately 2 to 15 percent of the total mercury present in a lamp as calculated from

Altena (2001). After breakage, these deposits are available to dissolve in water; however, 

mercury oxide has a low solubility in water (Merck & Co. 1983). 

Figure N.1 outlines the expected forms of mercury in an operating lamp. Note that all 

liquid elemental mercury will volatilize in an operating MP lamp, leaving no mercury in the 

liquid phase. Also, amalgams are only used in LPHO lamps. 

Figure N.1 Mercury Speciation In Operating UV Lamps 

N.4.3 Quantity of Mercury in Lamps 

The amount of mercury in a UV reactor is a function of the type of lamp, the number of 

lamps in a reactor, and the number of reactors. Mercury content within lamps depends on type 

(LP, LPHO, or MP), length, and power rating. Although mercury content data are specific to 

manufacturer and lamp, lamps with higher pressures, power ratings, and lengths typically contain 

more mercury. Table N.5 summarizes the quantities of elemental mercury dosed into lamps 

during manufacturing according to a confidential manufacturer survey and published literature 

values. 

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Table N.5 Elemental Mercury Content in UV Lamps 

Lamp Type Mercury Content (mg per lamp) Electrical 

Power 

Rating (W) Phillips (1983) Clear et al. (1994) Manufacturer Survey

LP 15-70 “a single drop”

1 202 5-50 

120-260 NR 263, 364 LPHO 150 

400 NR 75.5 NR

MP 1000 NR 250 NR

1-25 kW 1.4 - 14.5 mg/cm5 NR 200-400,

0.3 - 7 mg/cm length, 

7.9 mg/cm length 1

 Phillips 1983 2

 75 W mercury vapor lamp 3

 175 W mercury vapor lamp 4

 250 W mercury vapor lamp 5

 mg per cm of lamp length, reported lamp lengths are 6-300 cm (Primarc Limited 2001) 

NR - Not Reported 

N.4.4 Quantification of Mercury in Example UV Installations 

This section illustrates example calculations of the amount of mercury contained in 

hypothetical UV installations. Two UV reactor manufacturers established design parameters for 

three treatment flowrates (0.18, 3.5, and 210 million gallons per day (mgd)) with a specified 

water quality and design dose (Table N.6). Design parameters included the number of lamps 

needed to obtain a dose of 40 mJ/cm2

 and the total number of reactors for each of the three 

design flows. Calculations assume 50, 150, and 400 mg of mercury per LP, LPHO, and MP 

lamp, respectively. Utilities should use site-specific UV installation information to determine 

quantities because mercury content varies with lamp type and manufacturer. 

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Table N.6 Mercury Quantity in Example UV Installations1,2

Design Flow

(mgd) 

Average 

Flow

(mgd) 

Lamp Type Average 

Number of

Reactors

Average Number 

of Lamps 

(per reactor) 

Total Hg in UV 

Installation3

(g) 

LP 1 2 0.1

LPHO 1 1 0.2

0.18 0.054

MP 1 1 0.4

3.5 1.4 LPHO 1 30 4.5

MP 1 4 1.6

210 120 LPHO 6 72 64.8

MP 6 7 16.8 1 UV Dose = 40 mJ/cm2

2 Water quality criteria: UVT = 89% (A254 = 0.05 cm-1), Turbidity = 0.1 NTU, Alkalinity = 60 mg/L as CaCO3, 

Hardness = 100 mg/L as CaCO3 3 Values given represent the amount of elemental mercury dosed in lamps during manufacturing. 

N.4.5 Fate of Mercury After Release 

The previous sections define the quantity and form of mercury in an operating lamp and 

thus define the starting point for the investigation of the fate of mercury in the water system. 

Unfortunately, little documentation exists on the fate of mercury in WTPs or distribution 

systems. The few case studies that do exist are mainly in the wastewater industry and focus 

primarily on removal of influent mercury by the following wastewater treatment processes: 

• Primary sedimentation (Lester 1983; Firk 1986; Balogh and Liang 1995; Goldstone et 

al. 1990; Oliver and Cosgrove 1974) 

• Activated sludge (Gilmour and Bloom 1995; Lester 1983; Chen et al. 1974; and Wu 

and Hilger 1985) 

• Conventional treatment process (Mugan 1996; Balogh and Liang 1995; Bodaly et al. 

1998) 

Much of the knowledge about mercury and its potential fate in water systems is derived 

from studies performed in natural environments. It is expected that this knowledge of mercury 

cycling within the natural environment can be applied to mercury dynamics within a WTP and 

distribution system where environmental conditions are largely controlled and remain fairly 

stable. However, WTPs employ a number of chemicals that are not typically found in natural 

environments. No studies were identified on influence and reaction of mercury with coagulants, 

polymers, corrosion inhibitors, ammonia, strong oxidants, and other disinfectants (e.g., chlorine 

and ozone). This subsection projects mercury reactions within a WTP and distribution system

based on documented mercury reactions in the natural environment. 

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N.4.5.1 Potential Mercury Reactions in Water Treatment Plants and 

Distribution System 

Liquid phase elemental mercury is considerably denser than water (specific gravity =

13.6; Table N.4) and does not readily dissolve in water. Therefore, liquid elemental mercury and 

mercury amalgams may settle in areas of low water velocity, thereby providing an option for 

early containment and removal. For example, in cases where mercury was released from other 

water treatment equipment (such as manometers, flow instrumentation, or pump seals), mercury 

was found to have condensed and settled in the clearwell. However, the amount of mercury 

recovered relative to the amount of mercury released is unknown (Cotton 2002). Kolch (2001) 

monitored the mercury concentrations in a 50-liter batch reactor following the destruction of one 

LPHO lamp (approximately 150 mg-Hg). Mercury concentrations reached approximately 2.5 

μg/L in water. Amalgamated mercury was found settled on the bottom at the reactor (Dinkloh 

2001b). However, it was not reported whether all the 150 mg of mercury present in the 

operating lamp was recovered with the amalgam or accounted for in the aqueous phase. 

Also, Kolch (2001) did not determine whether the source of aqueous phase mercury was 

dissolved mercury from the amalgam or vapor phase mercury present in the lamp prior to when it 

was broken. This issue may become important when considering the aqueous behavior of 

mercury following a MP lamp break. Mercury in a MP lamp is predominantly in the vapor phase 

during operation. It is unknown how the vapor phase mercury will react with the water. Vapor 

phase elemental and ionized mercury may become very fine particles when contacting the water 

as opposed to liquid or amalgamated mercury that settles in low velocity areas. 

Depending on the age of the lamp, mercury-containing deposits, such as mercury oxide, 

may accumulate on the inner surface of the lamp envelope in all lamp types. Dissolution of the 

deposits would result in additional ionized mercury entering the water. Prompt response to a 

lamp break would include removal of lamp fragments; therefore, the compounds on the lamp

fragments should not have the opportunity to enter the WTP. 

Once in the water, aqueous (dissolved) elemental and ionized mercury are expected to 

cycle between phases and oxidation states as determined by temperature, pH, organic carbon 

concentration, minerals and inorganic species, and dissolved oxygen level. The mercurous ion 

(Hg2

+2) is formed by the oxidation of elemental mercury or the reduction of the mercuric ion 

(Hg+2). The mercurous ion is capable of bonding with inorganic constituents; however, it does 

not bind with organic compounds. Hg2

+2 is rarely stable under typical environmental conditions 

and is readily reduced to Hg0

 or oxidized to Hg+2. 

Inorganic reactions involving the mercuric ion (Hg+2) include binding with inorganic 

ligands such as chloride, hydroxide, and sulfide. Considering sulfide is present in anoxic 

environments, mercury sulfide (HgS) is not expected to form in a water treatment environment. 

Reactions of Hg+2 with chloride and hydroxide resulting in mercuric chloride and mercuric 

hydroxide compounds depend on the pH and chloride concentrations (Beckvar et al. 1996). 

Additional discrepancies arise in the comparison of mercury reactions and fate in a 

drinking water environment versus the natural environment when organic carbon concentrations 

and existing microbial populations are considered. The mercuric ion (Hg+2) is the only oxidation 

state of mercury capable of association with organic compounds such as phenyl and methyl 

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groups. The resultant organic compounds, commonly known as methylated mercury, have 

different chemical, physical, and toxicological properties than inorganic mercury and offer more 

cause for concern due to toxicity and bioaccumulation properties (Beckvar et al. 1996). 

Methylation of mercury to form methyl and dimethyl mercury is primarily a biological process 

involving sulfate-reducing bacteria although it can also occur abiotically. The extent to which 

methylation occurs depends on the availability of Hg+2 and the presence of an appropriate 

microbial population. Methylation rates are higher under anoxic conditions, low pH, elevated 

temperatures, and high organic matter concentrations (USEPA 1997c). Therefore, even though 

Hg+2 may be present in the water column, all of the above factors oppose the occurrence of 

methylation in a drinking water environment. 

Another divergence of a water treatment environment from the natural environment is the 

presence of treatment chemicals such as coagulants, strong oxidants, polymers, corrosion 

inhibitors, and ammonia. Seigneur (1994) researched the reaction chemistry of mercury with 

inorganic species and strong oxidants such as chlorine and ozone in the aqueous and gas phases 

that are present in the atmosphere. Ionized mercury can form inorganic compounds with 

chloride and hydroxide ions. Depending on reactant concentrations, these compounds may be 

present in the aqueous phase and as solid precipitate. Also, based on reduction oxidation 

potentials, it is possible that the oxidation of elemental mercury would occur in the presence of 

chlorine and ozone, forming mercury ions and thereby increasing the solubility of elemental 

mercury. 

Further research and investigation is necessary to determine the mechanisms of any 

potential mercury reactions. Figure N.2 outlines this preliminary assessment of mercury 

speciation and reaction in a drinking water environment. 

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Figure N.2 Potential Reactions of Mercury in a Drinking Water Environment 

(Compounds released from a broken lamp are in boldface type.) 

Mercury (methylated and ionic) sorption to dissolved and particulate organic matter is

commonly found in natural environments (Beckvar et al. 1996; USEPA 1997c). This 

observation was also made in water and wastewater treatment plant studies, where mercury 

became incorporated into coagulant flocs and activated sludge waste, respectively (Logsdon 

1973; Gilmour and Bloom 1995; Lester 1983; Chen et al. 1974; Wu and Hilger 1985). 

If UV disinfection of raw water is used and a UV lamp breaks, the mercury could 

potentially be removed within the WTP. Logsdon (1973) investigated the efficiency of mercury 

removal in conventional WTPs. Bench-scale laboratory tests indicated that inorganic mercury 

was removed via coagulation, softening, adsorption on turbidity, powdered activated carbon 

(PAC) adsorption, and granular activated carbon (GAC) column adsorption. 

N.5 Summary and Conclusions 

The risk associated with a mercury release to the water system depends on many factors. 

More research is needed to close the knowledge gap that exists regarding the fate of mercury in a 

drinking water environment following a UV lamp break. The influence of treatment chemicals 

such as oxidants, disinfectants, and coagulants is largely unstudied. Likewise, dispersion and 

transport of mercury through a WTP and distribution system has yet to be evaluated. Although 

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these issues are, at present time, largely unknown, there are procedures and actions that can be 

taken to reduce or mitigate mercury release caused by UV lamp breakage. 

For the purposes of this appendix, lamp breakage was defined as fracture of the lamp

sleeve and the lamp envelope. This was further divided into off-line and on-line breaks. Off-line 

lamp breaks typically occur during storage or handling and cause small spills (< 2g). Small 

spills should be contained, cleaned up, and disposed of properly. 

On-line lamp breaks occur while the UV reactor is in operation. There have been 

reported incidents of on-line UV lamp breaks associated with impact from debris, loss of water 

flow, temperature differentials, faulty electrical equipment and design, and procedural errors. 

However, on-line lamp breaks are a rare occurrence and are largely preventable with appropriate 

design, operation, maintenance, and operator care. The following engineering and administrative 

methods have been proposed that may help prevent UV lamp breakage: 

• Screens, baffles, or low velocity collection areas prior to the reactor influent to 

prevent entrance of debris 

• Temperature and flow sensors and alarms to detect critical conditions and shut the 

UV reactors down 

• Surge analysis to determine if water hammer may be a potential problem 

• Comprehensive training and maintenance program 

In the event of a mercury release, the following engineering controls are additional 

precautions that may aid in the containment and collection of mercury: 

• Strainers and low velocity collection areas downstream of the reactor 

• Isolation valves activated by an alarm to attempt to isolate potentially contaminated 

water 

The extent of containment and prevention provided by these measures is unknown. 

Utilities and designers should consider the applicability of these isolation techniques on a sitespecific basis. Utilities should consult with their State primacy agency in the development of 

standard operating procedures, clean-up procedures, and reporting requirements in preparation 

for a potential UV lamp break and mercury release. It is recommended that a utility prepare a 

mercury release response plan to address these issues. This plan should address compliance with 

the SDWA, OSHA health and safety standards, and RCRA universal waste rules. Utilities are 

encouraged to recycle or return all mercury-containing lamps to mercury re-generating facilities 

or the lamp manufacturer. 

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N.6 References

Aquafine Corporation. 2001. CSL Series: Installation, Maintenance, and Operation Manual

Part No. 115-1. 

Altena, F.W., J.B.J. van Overveld, and H. Giller. 2001. Technological advances in disinfection 

lamps leading to more compact UV sources. Presented at the First International Congress 

on Ultraviolet Technologies, Washington, DC, June 14-16. 

Balogh, S. and L. Liang. 1995. Mercury pathways in municipal wastewater treatment plants. 

Water, Air, and Soil Pollution 80: 1181-1190. 

Beckvar, N., J. Field, S. Salazar, and R. Hoff. 1996. Contaminants in aquatic habitats at 

hazardous waste site: mercury. Hazardous Materials Response and Assessment Division 

of the National Oceanic and Atmospheric Administration Technical Memorandum NOS 

ORCA 100. Seattle, WA. 

Bodaly, R.A., J.W.M. Rudd, and R.J. Flett. 1998. Effect of urban sewage treatment on total and 

methyl mercury concentrations in effluents. Biogeochemistry 40: 279-291. 

Cairns, B. 2000. Comments on USEPA questions about the risk of using UV disinfection 

technologies in the potable water industry. Trojan Technologies Inc., London, Ontario, 

Canada, May 24. 

Chen, K.Y., C.S. Young and N. Rohatgi. 1974. Trace metals in wastewater effluents. J. Water 

Poll. Control Fed. 46: 2663. 

Clear, R., and S. Berman. 1994. Environmental and health aspects of lighting: mercury. 

Journal of the Illuminating Engineering Society 138-156. 

Cotton, C. 2002. Confidential communication with major water municipality. June. 

Dinkloh, L. 2001a. Wedeco Ideal Horizons. Personal communication with Jennifer Hafer, 

Malcolm Pirnie, regarding UV reactors. Tucson AZ, May 10. 

Dinkloh, L. 2001b. Wedeco-Ideal Horizons. Personal communication with Jennifer Hafer, 

Malcolm Pirnie, regarding UV reactors. Tucson AZ, August 10. 

Dinkloh, L. 2001c. Wedeco-Ideal Horizons. Email to Christine Cotton regarding UV negative 

pressure. Tucson AZ, August 15. 

Firk, W. 1986. Heavy metals in sewage and occurring sludges – the balance of three sewage 

treatment plants, edited by J.N. Lester, R. Perry and R.M Sterritt. Lisbon: International 

Conference Chemicals in the Environment. 

 

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Appendix N. UV Lamp Breakage Issues 

UV Disinfection Guidance Manual N-21 June 2003

Gilmour, C.C. and N.S. Bloom. 1995. A case study of mercury and methylmercury dynamics in 

a Hg-contaminated municipal wastewater treatment plant. Water Air and Soil Pollution 

80: 799-803. 

Glew, D.N. and D.A. Hames. 1971. Aqueous nonelectrolyte solution: mercury solubility in 

water. Canadian Journal of Chemistry 49: 3114-3118. 

Goldstone, M.E., C. Atkinson, P.W.W. Kirk, and J.N. Lester. 1990. The behavior of heavy 

metals during wastewater treatment III: mercury and arsenic. The Science of the Total 

Environment 95: 271-294. 

Gump, D. 2002. Severn Trent. Personal communication with Jennifer Hafer, Malcolm Pirnie, 

Inc., regarding UV reactors. Tucson AZ, November 6. 

Kolch, A. 2001. Assessing the risk of mercury release from ultraviolet discharge lamps in 

drinking water. Wedeco AG Water Technology. 

Lester, J.N. 1983. Significance and behaviour of heavy metals in waste water treatment 

processes. I. sewage treatment and effluent discharge. Science of the Total Environment 

30: 1-44. 

Lienberger, J. 2002. Trojan Technologies. Personal communication with Jennifer Hafer, 

Malcolm Pirnie, Inc, regarding UV reactors. November 5. 

Logsdon, G. and J. Symons. 1973. Mercury removal by conventional water-treatment 

techniques. Journal American Water Works Association 554-562. 

Malley, J. 2001. Addressing concerns over mercury releases from UV lamps. IUVA News 3, no 

3: 28-29. 

McClean, J. 2001a. Hanovia Corporation. Personal communication with Doug Owen and 

Christine Cotton, Malcolm Pirnie, Inc., regarding UV reactors. Slough, England, April 9. 

McClean, J. 2001b. Hanovia Corporation. Response to memo by Christine Cotton, Malcolm

Pirnie, Inc., regarding UV reactors. Tucson AZ, May 4. 

Merck & Co., Inc. 1983. The Merck Index 10th Edition, edited by M. Windholz. Rahway, NJ. 

Moss, L. 2002a. Personal communication with Jennifer Hafer, Malcolm Pirnie, Inc., regarding 

UV reactors. Tucson AZ, April 29. 

Moss, L. 2002b. Personal communication with Jennifer Hafer, Malcolm Pirnie, Inc., regarding 

UV reactors. Tucson AZ, November 18. 

Mugan, T.J. 1996. Quantification of total mercury discharges from publicly owned treatment 

works to Wisconsin surface waters. Water Environment Research 68: 2, 229-234. 

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Appendix N. UV Lamp Breakage Issues 

UV Disinfection Guidance Manual N-22 June 2003

Oliver, B.G., and E.G. Cosgrove. 1974. Efficiency of heavy-metal removal by a conventional 

activated-sludge treatment plant. Water Research 8, no 11: 869-874. 

Phillips, R. 1983. Sources and Applications of Ultraviolet Radiation. Academic Press Inc., 

New York, New York. 

Primarc Limited. 2001. September 5,. www.primarc.com. 

Roberts, A. 2000. Hanovia Corporation. Letter to Dan Schmelling, EPA. April 10. 

Seigneur, C., J. Wrobel, and E. Constantinou. 1994. A chemical kinetic mechanism for 

atmospheric inorganic mercury. Environmental Science and Technology 28: 1589-1597. 

Srikanth, B. 2001a. Aquafine Corporation. Letter to James Malley, UNH, regarding accidental 

UV lamp breakage and potential scenarios and prevention strategies. June 13. 

Srikanth, B. 2001b. Aquafine Corporation. Personal communication with Jennifer Hafer, 

Malcolm Pirnie, Inc., regarding UV reactors. Tucson, AZ, June 21. 

Swaim, P.D., M.A. Morine, R.G. Brauer, M.A. Neher, J.L. Gebhart, and W.D. Bellamy. 2002. 

Operating data from Henderson's full-scale UV disinfection facility. Proceedings from 

the Water Quality Technology Conference, November 10-14, Seattle, WA. 

U.S. Environmental Protection Agency. 1992. Characterization of Products Containing 

Mercury in Municipal Solid Waste in the United States, 1970-2000. Office of Solid 

Waste, Washington, D.C. 

U.S. Environmental Protection Agency. 1997a. Mercury - Emergency Spill & Release Facts. 

EPA 540-K-97-004, OSWER 9378.0-10FS, PB97-963405. Office of Emergency and 

Remedial Response, Washington, D.C. 

U.S. Environmental Protection Agency. 1997b. Mercury emissions from the disposal of 

fluorescent lamps. Office of Solid Waste, Washington, D.C. June 30. 

U.S. Environmental Protection Agency. 1997c. Mercury Study Report to Congress. EPA425/R-97-004: 2-14. Office of Air Quality Planning and Standards and Office of 

Research and Development, Washington, D.C. 

Walitsky, P. Philips Lighting. 2001. Personal communication with Jennifer Hafer, Malcolm

Pirnie, Inc., regarding UV reactors. Tucson AZ, May 14. 

Wu, J.S. and H. Hilger. 1985. Chemodynamic behavior of mercury in activated sludge process. 

Am. Inst. Chem. Eng. 81: 109. 

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Appendix O. Case Studies 

This appendix will be included in the final draft when more information is available. 

UV Disinfection Guidance Manual O-1 June 2003

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Appendix P. Validation Protocol Calculator Tool 

The validation protocol described in Chapter 4 and Appendix C of this guidance manual 

involves several calculations to determine the log inactivation credit achieved during validation 

of a UV reactor. For this protocol, a safety factor calculated from uncertainties and known 

variability associated with UV reactors, monitoring, and validation methods is used to relate the 

reduction equivalent dose (RED) demonstrated during validation to the UV dose required to 

achieve a specified log inactivation credit (Table 1.4). EPA developed a spreadsheet that enables 

a user to input information associated with the uncertainty of validation and monitoring and 

calculate the safety factor and resulting target RED. The calculator was used to develop Tier 1 

RED targets and can be used to apply the Tier 2 approach. (See section C.4.10.2 for a 

description of the Tier 2 approach, including the safety factor calculation.) 

 

The Microsoft Excel® spreadsheet contains the following five worksheets: 

• Instructions – provides step-by-step instructions for entering data into the “RED 

Bias”, “Polychromatic Bias”, and “Safety Factor” worksheets and executing macros 

to calculate safety factor and resulting target RED. 

• RED Bias – calculates RED bias from input of Chapter 1 UV dose requirements and 

inactivation and RED measured during validation. 

• Polychromatic Bias – calculates the polychromatic bias for medium-pressure UV 

systems using spectral data on the lamp output, sleeve UV transmittance, water UV 

transmittance, and sensor response. 

• Safety Factor – calculates safety factor from RED bias, polychromatic bias, and 

expanded uncertainty associated with reactor validation and monitoring. 

• Default Data – contains assumed data for calculating the polychromatic bias; 

alternatively, the user can provide validation testing data as specified in the 

instruction worksheet. 

UV Disinfection Guidance Manual P-1 June 2003

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