______________________________________________________________________________________ 

ULTRAVIOLET DISINFECTION GUIDANCE MANUAL 

FOR THE FINAL LONG TERM 2 ENHANCED 

SURFACE WATER TREATMENT RULE 

Office of Water (4601) 

EPA 815-R-06-007 

November 2006 
U.S. Environmental Protection Agency 

Office of Water (4601) 

1200 Pennsylvania Avenue NW 

Washington, DC 20460 

EPA 815-R-06-007 

http://www.epa.gov/safewater/disinfection/lt2/compliance.html

November 2006 
UV Disinfection Guidance Manual 3 November 2006 

For the Final LT2ESWTR 

Purpose:

The purpose of this guidance manual is solely to provide technical information on the application 

of ultraviolet light for the disinfection of drinking water by public water systems. 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 disinfection 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 

Acknowledgements: 

This document was prepared by the United States Environmental Protection Agency, Office of 

Water. The Work Assignment Manager was Daniel Schmelling, and the Contract Project Officer 

was Jane Holtorf. Technical consultants played a significant role in the development of this 

document. The work was conducted jointly by Malcolm Pirnie, Inc and Carollo Engineers, P.C. 

under contract with The Cadmus Group, Inc. The primary contributors to the document included: 

• Daniel Schmelling (USEPA) 

• Christine Cotton and Douglas Owen (Malcolm Pirnie) 

• Erin Mackey and Harold Wright (Carollo Engineers) 

• Karl Linden (Duke University) 

• James Malley, Jr. (University of New Hampshire) 

Other contributors included Laurel Passantino, Amy Samuelson, Matthew Yonkin, and James 

Collins (Malcolm Pirnie); Dennis Greene, Trinia Dzurny and Stuart Hurley (Carollo Engineers); 

Laura Dufresne (The Cadmus Group, Inc.); and Maureen Donnelly and James Melstad 

(previously of The Cadmus Group, Inc.). 

The following provided valuable technical information to assist in the development of this 

manual: 
UV Disinfection Guidance Manual 4 November 2006 

For the Final LT2ESWTR 

American Water Works Association 

 Jim Bolton (Executive Director, International Ultraviolet Association) 

 Calgon Carbon Corporation 

 David Cornwell (Environmental Engineering and Technology) 

 Steven Deem (Washington Department of Health) 

 Joel DuCoste (North Carolina State University) 

 Tom Hargy (Clancy Environmental Consultants) 

 Oluf Hoyer (DVGW) 

 Richard Hubel (West Yost & Associates) 

 Chris McMeen (then with Washington Department of Health) 

 Alexander Mofidi (Metropolitan Water District of Southern California) 

 Mike Montysko (State of New York) 

 Issam Najm (Water Quality and Treatment Solutions, Inc.) 

 Ondeo Degremont 

 Jeffrey Rosen (Clancy Environmental Consultants, Inc.) 

 Richard Sakaji (California Department of Health Services) 

 Severn Trent Services 

 Regina Sommer (University of Vienna) 

 Paul Swaim (CH2M Hill) 

 Trojan Technologies, Inc. 

 Matthew Valade (Hazen and Sawyer) 

 WEDECO UV Technologies, Inc. 
UV Disinfection Guidance Manual i November 2006 

For the Final LT2ESWTR 

Table of Contents

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

List of Figures.............................................................................................................................. vii

List of Tables ................................................................................................................................ ix 

List of Examples............................................................................................................................ x 

List of Checklists........................................................................................................................... x 

Glossary ........................................................................................................................................ xi

List of Units, Abbreviations, and Acronyms ........................................................................... xxi

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

 1.1 Guidance Manual Objectives........................................................................................ 1-2 

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

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

 1.3.1 Filtered PWSs ................................................................................................. 1-4 

 1.3.2 Unfiltered PWSs ............................................................................................. 1-6 

 1.3.3 PWSs with Uncovered Finished Water Storage Facilities.............................. 1-6 

 1.4 UV Disinfection Requirements for Filtered and Unfiltered PWSs............................... 1-6 

1.4.1 UV Dose and Validation Testing Requirements............................................. 1-6 

1.4.2 UV Disinfection Monitoring Requirements [40 CFR 141.720(d)(3)(i)] ........ 1-8 

1.4.3 UV Disinfection Reporting Requirements [40 CFR 141.721(f)(15]) ............. 1-8 

1.4.4 Off-specification Operational Requirement for Filtered and Unfiltered Systems 

 [40 CFR 141.720(d)(3)(ii)] ............................................................................. 1-8 

1.5 Regulations Timeline.................................................................................................... 1-9

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

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

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

 2.2 UV Light Generation and Transmission....................................................................... 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 ............................................................................................. 2-8 

2.3.2.1 Photorepair ...................................................................................................... 2-8 

2.3.2.2 Dark Repair ..................................................................................................... 2-9 

2.3.3 UV Intensity, UV Dose, and UV Dose Distribution....................................... 2-9 

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

2.3.5 Microbial Spectral Response ........................................................................ 2-13 

 2.4 UV Disinfection Equipment ....................................................................................... 2-14 

2.4.1 UV Reactor Configuration ............................................................................ 2-15 

2.4.2 UV Lamps ..................................................................................................... 2-16 

2.4.2.1 Lamp Start-up ............................................................................................... 2-19 

2.4.2.2 Lamp Output ................................................................................................. 2-19 

2.4.2.3 Lamp Sensitivity to Power Quality............................................................... 2-21 

2.4.2.4 Lamp Aging .................................................................................................. 2-22 

2.4.3 Ballasts .......................................................................................................... 2-24 

2.4.4 Lamp Sleeves ................................................................................................ 2-25 
Table of Contents (Continued) 

UV Disinfection Guidance Manual ii November 2006 

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2.4.5 Cleaning Systems.......................................................................................... 2-27 

2.4.6 UV Sensors ................................................................................................... 2-28 

2.4.7 UVT Analyzers ............................................................................................. 2-29 

2.4.8 Temperature Sensors..................................................................................... 2-30 

2.4.9 UV Reactor Dose-Monitoring Strategy ........................................................ 2-31 

2.5 Water Quality Effects and Byproduct Formation....................................................... 2-31 

2.5.1 Effect of Water Quality on UV Reactor Performance .................................. 2-32 

2.5.1.1 UVT .............................................................................................................. 2-32 

2.5.1.2 Particle Content............................................................................................. 2-32 

2.5.1.3 Upstream Water Treatment Processes .......................................................... 2-32 

2.5.1.4 Fouling Potential ........................................................................................... 2-34 

2.5.1.5 Algal Occurrence and Growth ...................................................................... 2-36 

2.5.2 Chlorine Reduction through UV Reactors .................................................... 2-36 

2.5.3 Byproducts from UV Disinfection................................................................ 2-37 

 2.5.3.1 Trihalomethanes, Haloacetic Acids, and Total Organic Halides .................. 2-37 

 2.5.3.2 Biodegradable Compounds ........................................................................... 2-37 

 2.5.3.3 Nitrite ............................................................................................................ 2-37 

3. Planning Analyses for UV Facilities..................................................................................... 3-1 

 3.1 UV Disinfection Goals.................................................................................................. 3-1 

 3.2 Evaluating Integration of UV Disinfection into the Treatment Process....................... 3-3 

3.2.1 UV Disinfection Effects on Treatment ........................................................... 3-3 

3.2.2 Upstream Treatment Process Effect on UV Disinfection ............................... 3-4 

3.3 Identifying Potential Locations for UV Facilities......................................................... 3-5 

3.3.1 Installation Locations for Filtered Systems .................................................... 3-5 

3.3.1.1 Combined Filter Effluent Installation (Upstream of the Clearwell) ............... 3-5 

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

3.3.1.3 UV Disinfection Downstream of the Clearwell.............................................. 3-8 

3.3.2 Unfiltered System Installation Locations........................................................ 3-9 

3.3.3 Groundwater System Installation Locations ................................................. 3-10 

3.3.4 Uncovered Reservoir Installation Locations................................................. 3-10 

3.4 Defining Key Design Parameters ............................................................................... 3-10 

3.4.1 Off-specification Requirements .................................................................... 3-10 

3.4.2 Target Pathogen Inactivation and Required UV Dose.................................. 3-12 

3.4.3 Design Flow Rate.......................................................................................... 3-12 

3.4.4 Water Quality................................................................................................ 3-12 

 3.4.4.1 UVT and UVT Scans .................................................................................... 3-13 

 3.4.4.2 Water Quality Parameters That Affect Fouling ............................................ 3-18 

 3.4.4.3 Additional Water Quality Considerations for Unfiltered Supplies and 

 Treatment of Uncovered Reservoir Water .................................................... 3-19 

3.4.5 Fouling/Aging Factor.................................................................................... 3-21 

3.4.5.1 Testing to Determine the Fouling Factor ...................................................... 3-22 

3.4.5.2 Testing to Determine the Aging Factor......................................................... 3-23 

3.4.6 Power Quality Evaluations............................................................................ 3-25 

3.5 Evaluating UV Reactors, Dose Monitoring Strategy, and Operational Approach ..... 3-26 

 3.5.1 Characteristics of LPHO and MP Reactors................................................... 3-26 

3.5.2 Dose-monitoring Strategy and Operational Approach ............................................... 3-27 

3.5.2.1 UV Intensity Setpoint Approach................................................................... 3-28 
Table of Contents (Continued) 

UV Disinfection Guidance Manual iii November 2006 

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3.5.2.2 Calculated Dose Approach............................................................................ 3-29 

3.5.2.3 Advantages and Disadvantages..................................................................... 3-30 

3.6 Assessing UV Equipment Validation Issues .............................................................. 3-31 

3.6.1 Off-site Versus On-site Validation................................................................ 3-31 

3.6.2 Validation and Installation Hydraulics Recommendations........................... 3-32 

3.6.3 Selection of Validation and Hydraulic Approach ......................................... 3-33 

3.7 Assessing Head Loss Constraints ............................................................................... 3-35 

3.7.1 Eliminating Existing Hydraulic Inefficiencies.............................................. 3-35 

3.7.2 Modifying Clearwell Operation .................................................................... 3-35 

3.7.3 Modifying Filter Operation........................................................................... 3-36 

3.7.4 Installing Intermediate Booster Pumps ......................................................... 3-36 

3.7.5 Modifying Operation of HSPs ...................................................................... 3-36 

3.8 Estimating UV Facility Footprint ............................................................................... 3-36 

3.8.1 UV Equipment Constraints ........................................................................... 3-36 

3.8.2 Develop UV Facility Layout......................................................................... 3-37 

3.9 Preparing Preliminary Costs and Selecting the UV Facility Option .......................... 3-38 

3.10 Reporting to the State ................................................................................................. 3-38 

4. Design Considerations for UV Facilities.............................................................................. 4-1 

4.1 UV Facility Hydraulics................................................................................................. 4-2 

4.1.1 Inlet and Outlet Piping Configuration............................................................. 4-3 

4.1.2 Flow Distribution, Control, and Measurement ............................................... 4-3 

 4.1.2.1 Flow Distribution and Control ........................................................................ 4-3 

 4.1.2.2 Flow Rate Measurement ................................................................................. 4-3 

4.1.3 Water Level Control........................................................................................ 4-5 

4.1.4 Air Relief and Pressure Control Valves.......................................................... 4-6 

4.1.5 Flow Control and Isolation Valves ................................................................. 4-6 

4.1.6 Installation of Intermediate Booster Pumps.................................................... 4-6 

4.1.7 Evaluating Existing Pumps and Potential Water Hammer Issues................... 4-7 

4.1.8 Groundwater System Hydraulic Issues ........................................................... 4-7 

4.1.9 Uncovered Finished Water Reservoir Hydraulic Issues ................................. 4-8 

4.2 Operating Approach Selection...................................................................................... 4-8 

4.3 Instrumentation and Control ......................................................................................... 4-9 

4.3.1 UV Reactor Start-up and Sequencing ............................................................. 4-9 

4.3.1.1 UV Reactor Start-up...................................................................................... 4-10 

4.3.1.2 UV Reactor Sequencing................................................................................ 4-10 

4.3.1.3 Groundwater Pump Cycling Effects on UV Reactor Start-up ...................... 4-11 

4.3.2 UV Equipment Automation .......................................................................... 4-11 

4.3.3 Alarms and Control Systems Interlocks........................................................ 4-12 

4.3.4 UV Reactor Control Signals ......................................................................... 4-12 

4.3.4.1 UV Intensity.................................................................................................. 4-13 

4.3.4.2 UV Transmittance ......................................................................................... 4-13 

4.3.4.3 Flow Rate Measurement ............................................................................... 4-14 

4.3.4.4 Calculated and Validated UV Dose (If Applicable) ..................................... 4-14 

4.3.4.5 Operational Setpoints.................................................................................... 4-14 

4.3.4.6 Lamp Age...................................................................................................... 4-14 

4.3.4.7 Lamp Power, Lamp Status, and Reactor Status ............................................ 4-14 

4.3.4.8 UV Reactor Sleeve Cleaning ........................................................................ 4-14 
Table of Contents (Continued) 

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4.3.4.9 Alarms........................................................................................................... 4-15 

4.4 Electrical Power Configuration and Back-up Power.................................................. 4-15 

4.4.1 Considerations for Electrical Power.............................................................. 4-15 

4.4.2 Back-up Power Supply and Power Conditioning ......................................... 4-15 

4.4.2.1 Back-up Power Supply.................................................................................. 4-16 

4.4.2.2 Power Conditioning Equipment.................................................................... 4-16 

4.4.3 Ground Fault Interrupt and Electrical Lockout............................................. 4-17 

4.5 UV Facility Layout ..................................................................................................... 4-17

4.5.1 Additional Considerations for Unfiltered and Uncovered Reservoir UV 

 Facility Layouts ............................................................................................ 4-18 

4.5.2 Additional Considerations for Groundwater UV Facility Layouts............... 4-19 

4.6 Elements of UV Equipment Specifications ................................................................ 4-19 

4.6.1 UV Equipment Specification Components ................................................... 4-19 

4.6.2 Information Provided by Manufacturer in UV Reactor Bid ......................... 4-22 

4.7 Final UV Facility Design............................................................................................ 4-23 

4.8 Reporting to the State during Design.......................................................................... 4-24 

5. Validation of UV Reactors .................................................................................................... 5-1 

5.1 Minimum Requirements for Validation Testing........................................................... 5-2 

5.2 Overview of the Recommended Validation Protocol................................................... 5-3 

5.2.1 Key Steps in Recommended Validation Protocol........................................... 5-3 

5.2.2 Alternative Validation Protocols..................................................................... 5-6 

5.2.3 Third Party Oversight...................................................................................... 5-6 

5.2.4 Emerging Methods.......................................................................................... 5-7 

5.3 Selecting the Challenge Microorganism....................................................................... 5-7 

5.4 Equipment Needs for Full-scale Reactor Testing......................................................... 5-9 

5.4.1 Water Source................................................................................................... 5-9 

5.4.2 UV Absorbing Chemical............................................................................... 5-10 

5.4.3 Mixing........................................................................................................... 5-10 

5.4.4 Sampling Ports .............................................................................................. 5-11 

5.4.5 Configuration of Inlet and Outlet Piping ...................................................... 5-12 

5.4.6 Accounting for Non-uniform Lamp Aging................................................... 5-12 

5.4.7 Lamp Positioning to Address Lamp Variability ........................................... 5-14 

5.4.8 UV Sensors ................................................................................................... 5-14 

5.4.9 UV Sensor Port Windows ............................................................................. 5-15 

5.5 Accuracy of Measurement Equipment ....................................................................... 5-15 

5.5.1 Flow Meters .................................................................................................. 5-18 

5.5.2 UV Spectrophotometers................................................................................ 5-18 

5.5.3 Power Measurements .................................................................................... 5-19 

5.5.4 UV Sensors ................................................................................................... 5-19 

5.6 Identifying Test Conditions ........................................................................................ 5-20 

5.6.1 Test Conditions for the UV Intensity Setpoint Approach............................. 5-22 

5.6.2 Test Conditions for the Calculated Dose Approach...................................... 5-23 

5.6.3 Test Conditions for Confirming an Existing Validation Equation................ 5-26 

5.6.4 Quality-control Samples ............................................................................... 5-26 

5.7 Guidelines for Conducting Experimental Tests.......................................................... 5-27 

5.7.1 Preparing the Challenge Microorganism ...................................................... 5-27 
Table of Contents (Continued) 

UV Disinfection Guidance Manual v November 2006 

For the Final LT2ESWTR 

5.7.2 Full-scale UV Reactor Testing...................................................................... 5-28 

5.7.3 Collimated Beam Testing.............................................................................. 5-30 

5.8 Analyzing Experimental Data..................................................................................... 5-30 

5.8.1 Calculating the Reduction Equivalent Dose (RED)...................................... 5-30 

5.8.2 Selecting the Minimum RED for the UV Intensity Setpoint Approach ....... 5-34 

5.8.3 Developing the Dose-monitoring Equation for the Calculated Dose 

 Approach....................................................................................................... 5-34 

5.9 Deriving the Validation Factor (VF) .......................................................................... 5-36 

5.9.1 RED Bias Factor ........................................................................................... 5-37 

5.9.2 Uncertainty in Validation (UVal) ................................................................... 5-38 

5.9.2.1 Calculating USP for the UV Intensity Setpoint Approach............................. 5-41 

5.9.2.2 Calculating UIN for the Calculated Dose Approach...................................... 5-41 

5.10 Determining the Validated Dose and Validated Operating Conditions...................... 5-42 

5.10.1 Determining the Validated Dose and Operating Conditions for the UV 

 Intensity Setpoint Approach.......................................................................... 5-42 

5.10.2 Determining the Validated Dose and Operating Conditions for the Calculated 

 Dose Approach.............................................................................................. 5-43 

5.11 Documentation............................................................................................................ 5-44 

5.11.1 UV Reactor Documentation.......................................................................... 5-44 

5.11.2 Validation Test Plan...................................................................................... 5-46 

5.11.3 Validation Report.......................................................................................... 5-48 

5.12 Guidelines for Reviewing Validation Reports............................................................ 5-50 

5.13 Evaluating the Need for “Re-validation”.................................................................... 5-54 

6. Start-up and Operation of UV Facilities............................................................................ 6-1 

6.1 UV Facility Start-up...................................................................................................... 6-1 

6.1.1 O&M Manual.................................................................................................. 6-1 

6.1.2 State Coordination during Start-up ................................................................. 6-3 

6.1.3 Functional Testing........................................................................................... 6-3 

6.1.3.1 Verification of UV Equipment Components................................................... 6-4 

6.1.3.2 Verification of Instrumentation and Control Systems .................................... 6-5 

6.1.3.3 Verification of Flow Distribution and Head Loss........................................... 6-6 

6.1.4 Determining Validated Operational Conditions and Setting Operational 

 Controls........................................................................................................... 6-7 

6.1.5 Performance Testing ..................................................................................... 6-10 

6.1.6 Final Inspection............................................................................................. 6-12 

6.2 Operation of UV Facilities.......................................................................................... 6-12 

6.2.1 Operational Requirements............................................................................. 6-13 

6.2.2 Recommended Operational Tasks................................................................. 6-13 

6.2.3 Start-up and Shut-down of UV Reactors ...................................................... 6-14 

6.2.3.1 Routine Start-up ............................................................................................ 6-14 

6.2.3.2 Start-up Following Maintenance................................................................... 6-14 

6.2.3.3 Routine Shut-down ....................................................................................... 6-15 

6.2.3.4 Shut-down Prior to Maintenance .................................................................. 6-15 

6.2.3.5 Winterization................................................................................................. 6-15 

6.3 Maintenance of UV Reactors...................................................................................... 6-16 

6.3.1 Summary of Recommended Maintenance Tasks.......................................... 6-16 

6.3.2 General Guidelines for UV Reactor Maintenance ........................................ 6-18 
Table of Contents (Continued) 

UV Disinfection Guidance Manual vi November 2006 

For the Final LT2ESWTR 

6.3.2.1 Fouling .......................................................................................................... 6-18 

6.3.2.2 Lamp Output Variability.............................................................................. 6-19 

6.3.2.3 Reference UV Sensors .................................................................................. 6-20 

6.3.2.4 Electrical Concerns ....................................................................................... 6-20 

6.3.2.5 UV Reactor Temperature and Water Level .................................................. 6-21 

6.3.2.6 UV Lamp Replacement................................................................................. 6-21 

6.3.2.7 Lamp Sleeves ................................................................................................ 6-21 

6.3.2.8 On-line UVT Analyzer.................................................................................. 6-22 

6.3.3 Spare Parts..................................................................................................... 6-22 

6.4 Monitoring and Recording of UV Facility Operation ................................................ 6-24 

6.4.1 Monitoring and Recording for Compliance Parameters ............................... 6-24 

6.4.1.1 Monitoring of Duty UV Sensor Calibration.................................................. 6-24 

6.4.1.2 Monitoring of UVT Analyzer Calibration .................................................... 6-29 

6.4.1.3 Off-specification Events................................................................................ 6-31 

6.4.1.4 Monitoring and Recording Frequency of Required Parameters ................... 6-32 

6.4.2 Monitoring and Recording for Operational Parameters Not Related to 

 Compliance ................................................................................................... 6-36 

6.5 UV Facility Reporting to the State ............................................................................. 6-37 

6.5.1 Required Reporting ....................................................................................... 6-38 

6.5.2 Example Reporting Forms and Calculation Worksheets .............................. 6-39 

6.6 Operational Challenges............................................................................................... 6-48 

6.6.1 Low UV Intensity or Validated Dose Below the Setpoint............................ 6-48 

6.6.2 Low UV Transmittance................................................................................. 6-48 

6.6.3 Failure to Meet UV Sensor Calibration Criterion......................................... 6-50 

6.7 Staffing, Training, and Safety Issues.......................................................................... 6-50 

6.7.1 Staffing Levels .............................................................................................. 6-53 

6.7.2 Training......................................................................................................... 6-53 

6.7.3 Safety Issues.................................................................................................. 6-53 

7. Bibliography ......................................................................................................................... 7-1 

Appendices 

Appendix A. Preparing and Assaying Challenge Microorganisms ..................................... A-1 

Appendix B. UV Reactor Testing Examples........................................................................B-1 

Appendix C. Collimated Beam Testing to Develop a UV Dose-response Curve ................C-1 

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

Appendix E. UV Lamp Break Issues ...................................................................................E-1 

Appendix F. Case Studies ....................................................................................................F-1 

Appendix G. Reduction Equivalent Dose Bias Tables........................................................ G-1 
UV Disinfection Guidance Manual vii November 2006 

For the Final LT2ESWTR 

List of Figures 

Figure 1.1. LT2ESWTR Compliance Timeline for Initial Source Water Monitoring and 

Treatment Installation ................................................................................................................ 1-10 

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

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 ...................................... 2-7 

Figure 2.6. UV Absorbance of Nucleotides (left) and Nucleic Acid (right) at pH 7................... 2-7 

Figure 2.7. Hypothetical Dose Distributions for Two Reactors with Differing Hydraulics...... 2-10 

Figure 2.8. Shapes of UV Dose-Response Curves .................................................................... 2-12 

Figure 2.9. Comparison of Microbial UV Action and DNA UV Absorbance .......................... 2-14 

Figure 2.10. Example of UV Disinfection Equipment .............................................................. 2-15 

Figure 2.11. Examples of UV Reactors: (a) Closed-channel and (b) Open-channel................. 2-15 

Figure 2.12. Construction of a UV Lamp .................................................................................. 2-19 

Figure 2.13. UV Output of LP (a) and MP (b) Mercury Vapor Lamps..................................... 2-20 

Figure 2.14. UV Lamp Output and its Relationship to the UV Absorbance of DNA ............... 2-21 

Figure 2.15. Reduction in UV Output of (a) LPHO and (b) MP Lamps Over Time................. 2-23 

Figure 2.16. Lamp Aging for an MP Lamp ............................................................................... 2-23 

Figure 2.17. Aged UV Lamp (right) Compared to a New UV Lamp (left)............................... 2-24 

Figure 2.18. UVT of Quartz that is 1 mm Thick at a Zero-degree Incidence Angle................. 2-26 

Figure 2.19. Examples of (a) Mechanical Wiper System and (b) Mechanical-chemical Wiper 

 System................................................................................................................... 2-28 

Figure 2.20. Example of a Dry UV Sensor Mounted on a UV Reactor .................................... 2-29 

Figure 2.21. Example UVT Analyzer Design............................................................................ 2-30 

Figure 2.22. Example Effect of Ozonation on UV Absorbance if Ozone is Quenched Prior to UV 

 Disinfection........................................................................................................... 2-33 

Figure 2.23. Example Effect of UV Disinfection on Free Chlorine Residual Loss................... 2-36 

Figure 3.1. Example Flowchart for Planning UV Facilities ........................................................ 3-2 

Figure 3.2. Schematic for UV Facility Upstream of the Clearwell ............................................. 3-5 

Figure 3.3. Schematic of Individual Filter Effluent Piping Installation in Filter Gallery............ 3-6 

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

Figure 3.5. Example Cumulative Frequency Diagram for Three Filtered Waters..................... 3-16 

Figure 3.6. Example Flow Rate and UVT (at 254 nm) Data ..................................................... 3-17 

Figure 3.7. Schematic of Hydraulic Option #1 (90o-Bend, T-Bend, S-Bend Inlet Piping 

 Scenarios) ................................................................................................................ 3-33 

Figure 3.8. UV Reactor Validation Options and How They Affect Installation Hydraulics..... 3-34 

Figure 4.1. Flowchart for Planning, Design, and Construction of UV Facilities ........................ 4-2 

Figure 4.2. Open-channel Flow Distribution Options ................................................................. 4-4 

Figure 5.1. Overview of Recommended Validation Protocol...................................................... 5-4 

Figure 5.2. Block Diagram of a Typical Biodosimetry Test Stand for Full-scale Reactor 

 Testing...................................................................................................................... 5-10 
List of Figures (Continued) 

UV Disinfection Guidance Manual viii November 2006 

For the Final LT2ESWTR 

Figure 5.3. Hypothetical Examples of the Spectral Response of a Germicidal and a 

 Non-germicidal UV Sensor...................................................................................... 5-18 

Figure 5.4. UVAL Decision Tree for the UV Intensity Setpoint Approach................................. 5-39 

Figure 5.5. UVAL Decision Tree for the Calculated Dose Approach.......................................... 5-40 

Figure 5.6 UVT of Standard and “Ozone-Free” Quartz Assuming Air-Quartz and Quartz-Water 

 Interfaces................................................................................................................... 5-55 

Figure 6.1. Start-up and Operation Flowchart ............................................................................. 6-2 

Figure 6.2. Example Summary Monthly Report........................................................................ 6-41 

Figure 6.3. Example Daily Operating Log for Calculated Dose Approach............................... 6-42 

Figure 6.4. Example Daily Operating Log for UV Intensity Setpoint Approach...................... 6-43 

Figure 6.5. Example Off-specification Calculation Worksheet................................................. 6-44 

Figure 6.6. Example Monthly UV Sensor Calibration Check Log............................................ 6-45 

Figure 6.7. Example UV Sensor CF Calculation Worksheet..................................................... 6-46 

Figure 6.8. Example Monthly UVT Analyzer Calibration Check Log...................................... 6-47 

Figure 6.9. Low UV Intensity or Low Validated Dose Decision Chart .................................... 6-49 

Figure 6.10. Low UV Transmittance Decision Chart................................................................ 6-51 

Figure 6.11. Monitoring of UV Sensor Calibration Flowchart.................................................. 6-52 
UV Disinfection Guidance Manual ix November 2006 

For the Final LT2ESWTR 

List of Tables 

Table 1.1. Summary of Microbial and DBP Rules ...................................................................... 1-4 

Table 1.2. Bin Requirements for Filtered PWSs ......................................................................... 1-5 

Table 1.3. Requirements for Unfiltered PWSs ............................................................................ 1-6 

Table 1.4. UV Dose Requirements – millijoules per centimeter squared (mJ/cm2) ................... 1-7 

Table 2.1. Typical Mercury Vapor Lamp Characteristics ......................................................... 2-17 

Table 2.2. Mercury Vapor Lamp Operational Advantages........................................................ 2-17 

Table 2.3. Typical Start-up and Restart Times for LPHO and MP Lamps................................ 2-22 

Table 2.4. Comparison of Magnetic and Electronic Ballasts..................................................... 2-25 

Table 2.5. UV Absorbance Characteristics of Common Water Treatment Chemicals ............. 2-34 

Table 3.1. Potential Method to Determine Design Flow ........................................................... 3-12 

Table 3.2. Summary of UVT Data Collection and Analysis ..................................................... 3-14 

Table 3.3. Summary of Fouling Data Collection and Analysis ................................................. 3-19 

Table 3.4. Summary of Particle and Algal Data Collection and Analysis................................. 3-20 

Table 3.5. Power Quality Triggers for UV Reactors ................................................................. 3-25 

Table 3.6. Dose-monitoring Approaches – Key Characteristics................................................ 3-28 

Table 3.7. Advantages and Disadvantages of Single-setpoint and Variable-setpoint Operations 

 for the UV Intensity Setpoint Approach .................................................................. 3-29 

Table 3.8. Advantages and Disadvantages of Off-site and On-site Validation ......................... 3-31 

Table 4.1. Comparison of Techniques for UV Facility Flow Rate Measurement for Combined 

 Filter Effluent and Post-clearwell UV Facilities........................................................ 4-5 

Table 4.2. Typical Alarm Conditions for UV Reactors............................................................. 4-13 

Table 4.3. Possible Content for UV Equipment Specifications................................................. 4-20 

Table 4.4. Suggested Information to Be Provided by UV Manufacturer .................................. 4-22 

Table 5.1. Summary of LT2ESWTR Validation Requirements.................................................. 5-2 

Table 5.2. UV Sensitivity of Challenge Microorganisms............................................................ 5-8 

Table 5.3. Hypothetical Example of the Spectral Response of a Germicidal UV Sensor ......... 5-16 

Table 5.4. Hypothetical Example of the Spectral Response of a Non-germicidal UV Sensor.. 5-17 

Table 5.5. Factors to be Considered in Validation Test Design ................................................ 5-21 

Table 5.6. Minimum Test Conditions for the UV Intensity Setpoint Approach........................ 5-23 

Table 5.7. Minimum Test Conditions for the Calculated Dose Approach ................................ 5-25 

Table 6.1. Functional Testing of Cleaning Systems .................................................................... 6-5 

Table 6.2. Example Monitoring During a Four Week Performance Test.................................. 6-12 

Table 6.3. Recommended Operational Tasks for the UV Reactor ............................................ 6-13 

Table 6.4. Recommended Maintenance Tasks .......................................................................... 6-16 

Table 6.5. Typical Design and Guaranteed Lives of Major UV Components (Based on 

 Manufacturers’ Input) .............................................................................................. 6-23 

Table 6.6. Off-specification Examples for Each Monitoring Approach.................................... 6-32 

Table 6.7. Recommended Recording Frequency for Required Monitoring Parameters............ 6-33 

Table 6.8. Recommended Monitoring Parameters and Recording Frequency .......................... 6-37 

Table 6.9. Factors Impacting Staffing Needs............................................................................. 6-53 
UV Disinfection Guidance Manual x November 2006 

For the Final LT2ESWTR 

List of Examples 

Example 5.1. Determining Flow Conditions for Validation Testing......................................... 5-24 

Example 5.2. Calculating RED Using Validation Test Data..................................................... 5-31 

Example 5.3. Validation Testing Using Two Challenge Microorganisms ................................ 5-33 

Example 5.4. Determining the RED Bias factor........................................................................ 5-38 

Example 6.1. Setting Operational Controls for the UV Intensity Setpoint Approach – Single 

 Setpoint Operation ................................................................................................. 6-8 

Example 6.2. Setting Operational Controls for the Calculated Dose Approach.......................... 6-9 

Example 6.3. Duty UV Sensors are Verified using Reference Sensor ...................................... 6-26 

Example 6.4. Duty UV Sensors that Do Not Meet Calibration Criteria.................................... 6-28 

Example 6.5. UVT Analyzer Calibration Check ....................................................................... 6-31 

Example 6.6. Routine and Off-specification Recording............................................................ 6-34 

Example 6.7. Off-specification Computation ............................................................................ 6-39 

List of Checklists 

Checklist 5.1. UV Reactor Documentation................................................................................ 5-45 

Checklist 5.2. Key Elements of the Validation Test Plan.......................................................... 5-47 

Checklist 5.3. Key Elements of the Validation Report.............................................................. 5-49 

Checklist 5.4. Review for Quality Assurance/Quality Control.................................................. 5-51 

Checklist 5.5. Review for Key Validation Report Elements ..................................................... 5-52 
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. 

A254 (UV Absorbance at 254 nm)– a measure of the amount of UV light that is absorbed 

by a substance at 254 nm.

Action Spectra Correction Factor (CFas) – a correction factor to account for greater 

proportional inactivation of a challenge microorganism compared to the target pathogen 

that results from differences in action spectra. 

Action Spectrum – the relative efficiency of UV energy frequencies at inactivating 

microorganisms. Each microorganism has a unique action spectrum. 

Bacteriophage – a virus that infects bacterial cells and can be used a microbial surrogate 

during validation testing. 

Ballast – an electrical device that provide the proper voltage and current required to 

initiate and maintain the gas discharge within the UV lamp. 

Beer’s Law –an empirical equation describing the absorption of light as a function of the 

transmitting medium’s properties; also know as the Beer-Lambert law. 

Bioassay – in the context of this document, an empirical assessment of the inactivation 

response of a specific microorganism to a controlled dose of UV light, usually in UV 

reactors. Bioassay has been used in the UV disinfection literature in the same context as 

“biodosimetry” (see Biodosimetry). 

 

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 via 

bench-scale collimated beam testing). 

Calculated Dose Approach – See Dose-monitoring Strategy. 

Challenge Microorganism – a non-pathogenic microorganism used in validation testing 

of UV reactors. 

UV Disinfection Guidance Manual xi November 2006 

For the Final LT2ESWTR 
 Glossary (Continued) 

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

dose-response of a challenge microorganism. Both time and UV light intensity are 

directly measured; the UV dose is calculated using the intensity of the incident UV light, 

UV absorbance of the water, and exposure time. 

 

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. 

Design Flow Rate – the maximum flow that can be treated at the UV facility. See 

Section 3.4 for potential methods for determining design flow rate. 

Design UVT – The minimum UVT that will typically occur at the design flow of the UV 

facility. The design UVT and design flow are typically used by the UV manufacturer to 

determine the appropriate UV equipment for a target pathogen inactivation. The design 

UVT may not necessarily be the minimum operating UVT (see Minimum Operating 

UVT). 

Diffuse Reflection – that portion of light reflected by a rough surface that radiates in all 

directions. 

Dose Distribution – see UV Dose. 

Dose-monitoring Strategy – the method by which a UV reactor maintains the required 

dose at or near some specified value by monitoring UV dose delivery. Such strategies 

must include, at a minimum, flow rate and UV intensity (measured via duty UV 

sensor[s]) and lamp status. They sometimes include UVT and lamp power. Two 

common Dose-monitoring Strategies that are discussed in this manual are the UV 

Intensity Setpoint Approach and the Calculated Dose Approach. 

• The UV Intensity Setpoint Approach relies on one or more “setpoints” for UV 

intensity that are established during validation testing to determine UV dose. 

During operations, the UV intensity as measured by the UV sensors must meet or 

exceed the setpoint(s) to ensure delivery of the required dose. Reactors must also 

be operated within validated operation conditions for flow rates and lamp status 

[40 CFR 141.720(d)(2)]. In the UV Intensity Setpoint Approach, UVT does not 

need to be monitored separately. Instead, the intensity readings by the sensors 

account for changes in UVT. The operating strategy can be with either a single 

setpoint (one UV intensity setpoint is used for all validated flow rates) or a 

variable setpoint (the UV intensity setpoint is determined using a lookup table or 

equation for a range of flow rates). 

• The Calculated Dose Approach uses a dose-monitoring equation to estimate the 

UV dose based on operating conditions (typically flow rate, UV intensity, and 

UVT). The dose-monitoring equation may be developed by the UV manufacturers 

UV Disinfection Guidance Manual xii November 2006 

For the Final LT2ESWTR 
 Glossary (Continued) 

using numerical methods; however, EPA recommends that systems use an 

empirical dose-monitoring equation developed through validation testing. During 

reactor operations, the UV reactor control system inputs the measured parameters 

into the dose-monitoring equation to produce a calculated dose. The system 

operator divides the calculated dose by the Validation Factor (see Chapter 5 for 

more details on the Validation Factor) and compares the resulting value to the 

required dose for the target pathogen and log inactivation level. 

Dose-pacing Strategy – the method by which a UV reactor maintains the required dose 

at or near some specified value that typically involves adjusting the lamp power or 

turning "on" or "off" banks of UV lamps or whole UV reactors to respond to changes in 

UVT, lamp intensity, or flow rate. A programmable logic controller (PLC) makes 

adjustments using an equation(s) developed during the UV reactor validation process. 

Duty UV Sensor (or Duty Sensor) – the duty (on-line) UV sensor installed in the UV 

reactor that monitors UV intensity during UV equipment operations. 

Emission Spectrum – the relative power emitted by a lamp at different wavelengths. 

End-of-Lamp Life – The duration of lamp operations after which the lamp should be 

replaced 

First-order Inactivation – in the context of this document, inactivation of a 

microorganism that is directly proportional to the UV dose. 

Fluence – see the definition for UV Dose. 

Fluence Rate – see the definition for UV Intensity.

Fouling/Aging Factor – a site-specific factor (the product of a fouling factor and aging 

factor) that is used to account for the decline in UV transmittance through the lamp 

sleeve due to fouling (e.g., by water quality parameters) and aging of the lamp and lamp 

sleeve. The lamp fouling portion of the factor is the estimated fraction of UV light 

passing through a fouled sleeve as compared to a new sleeve. The lamp aging portion of 

the factor is the fraction of UV light emitted from aged sleeves and lamps compared to 

new sleeves and lamps. It can be estimated by the lamp and sleeve aging characteristics 

obtained from the UV manufacturer. 

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

of DNA at each wavelength. 

UV Disinfection Guidance Manual xiii November 2006 

For the Final LT2ESWTR 
 Glossary (Continued) 

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

in water (200 to 300 nm). 

Germicidal Sensor – A UV sensor with a spectral response that peaks between 250 and 

280 nm and has less than 10 percent of its total measurement due to light above 300 nm 

when mounted on the UV reactor and viewing the UV lamps through the water that will 

be treated at the water treatment plant.

Inactivation – in the context of UV disinfection, a process by which a microorganism is 

rendered unable to reproduce, thereby rendering it unable to infect a host. 

Lamp Burn-in – During the first few hours of mercury-vapor lamp operation, output will 

diminish rapidly, then stabilize as the impurities within the lamp are burned off. This 

initial “burn-in” period is typically assumed to be complete at 100 hours. 

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

quartz. 

Lamp Sleeve – the quartz tube or thimble 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. 

Lamp Status – see UV Lamp Status 

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 liquid lignin mixture (typically procured 

from paper mills) used to adjust the UV transmittance of natural waters during validation 

testing. 

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

of 0.13 to 1.3 Pa (2 x 10-5 -4 to 2 x 10 psi) and electrical input of 0.5 watts per centimeter 

(W/cm). This results in essentially monochromatic light output at 254 nm. 

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 low-pressure lamps. It also has essentially monochromatic light output at 

254 nm. 

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

pressure of 1.3 and 13,000 Pa (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 wavelengths in the germicidal range. 

UV Disinfection Guidance Manual xiv November 2006 

For the Final LT2ESWTR 
 Glossary (Continued) 

Microbial Repair – enzyme-mediated microbial process where damaged strands of 

deoxyribonucleic acid (DNA) are repaired. Energy for this process can be derived by 

light energy (photorepair) or chemical energy (dark repair). 

Minimum Operating UVT: The lowest UVT expected to occur during lifetime of the 

UV facility. Understanding the minimum UVT is critical because the UV reactor should 

be designed and validated for the range of UVT and flow rate combinations expected at 

the WTP to avoid off-specification operation. 

Monochromatic – light output at only one wavelength, such as UV light generated by 

low-pressure and low-pressure high-output lamps. 

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

reactor to the photodetector of a UV sensor. 

MS-2 Bacteriophage – a non-pathogenic bacteriophage commonly used as a challenge 

organism in UV reactor validation testing. 

Non-germicidal Sensor – A UV sensor with a spectral response that is not restricted to 

the germicidal range (see “Germicidal Sensor” for more details). 

Off-line Chemical Clean (OCC) – a process to clean lamp sleeves where the UV reactor 

is taken off-line and a cleaning solution (typically a weak acid) is sprayed into the reactor 

through a service port. After the foulants have dissolved, the reactor is drained, rinsed, 

and returned to service. Also called “flush-and-rinse” systems. 

Off-specification – A UV facility that is operating outside of the validated operating 

conditions (e.g., at a flow rate higher than the validated range or a UVT below the 

validated range). 

On-line Mechanical Clean (OMC) – a process to clean lamp sleeves where an 

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

prescribed frequency. 

On-line Mechanical-Chemical Clean (OMCC) – a process to clean lamp sleeves where 

an automatic mechanical wiper (e.g., O-ring) with a chemical solution located within the 

cleaning mechanism wipes the surface of the lamp sleeve at a prescribed frequency. 

Operating Strategy – the strategy used by the PWS to operate the UV equipment with 

the UV Intensity Setpoint Approach. Typically, single setpoint or variable setpoint 

operation is used. 

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. 

UV Disinfection Guidance Manual xv November 2006 

For the Final LT2ESWTR 
 Glossary (Continued) 

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

intensity at the detector's surface. 

Photorepair– a microbial repair process where enzymes are activated by light in the near 

UV and visible range, thereby repairing UV induced damage. Photoreactivation requires 

the presence of light. 

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

Polychromatic Bias – a potential bias in validation test data resulting from 

polychromatic differences between validation and operation of a UV reactor at a water 

system. Polychromatic bias can occur in MP reactors when non-germicidal sensors are 

used. 

Quartz Sleeve – see lamp sleeve. 

Radiometer – an instrument used to measure UV irradiance. 

Rayleigh Scattering – light scattering by particles smaller than the wavelength of the 

light. 

Reduction Equivalent Dose (RED) – see UV Dose. 

Reduction Equivalent Dose (RED) Bias – a correction that accounts for the difference 

between the UV dose measured with a surrogate microorganism and the UV dose that 

would be delivered to a target pathogen due to differences in the microorganisms’ 

inactivation kinetics. 

Reference UV Sensor (or Reference Sensor) – a calibrated, off-line UV sensor used to 

monitor duty UV sensor calibration and to determine UV sensor uncertainty. 

Required Dose – the UV dose required for a certain level of log inactivation. Required 

doses are set forth by the LT2ESWTR. 

Sensor Correction Factor – a correction factor that may need to be temporarily applied 

during operations when duty sensor(s) fail a calibration check and can not be 

immediately replaced. The sensor correction factor allows the UV facility to remain in 

operation while the problem is resolved. 

Setpoint (also called “operational setpoint”) – a specific value for a critical parameter, 

such as UV intensity, that is related to UV dose. Setpoints are established during 

validation testing. During operations, the PWS compares the measured parameter to the 

setpoint to confirm performance. 

Solarization – a change in the structure of a material due to exposure to UV light that 

increases light scattering and attenuation. 

UV Disinfection Guidance Manual xvi November 2006 

For the Final LT2ESWTR 
 Glossary (Continued) 

Spectral Response – A measure of the output of the UV sensor as a function of 

wavelength. 

State – the agency of the state or Tribal government that has jurisdiction over public 

water systems. During any period when a state or Tribal government does not have 

primary enforcement responsibility pursuant to section 1413 of the Act, the term “state” 

means the Regional Administrator, U.S. Environmental Protection Agency. 

Subpart H Systems – public water systems using surface water or ground water under 

the direct influence of surface water as a source that are subject to the requirements of 

subpart H of 40 CFR Part 141. 

Target Log Inactivation - For the target pathogen, the specific log inactivation the PWS 

wants to achieve using UV disinfection. The target log inactivation is driven by 

requirements of the SWTR, LT1ESWTR, IESWTR, and LT2ESWTR. 

Target Pathogen (also called “target microorganism”) – For the purposes of this 

manual, the target pathogen is defined as the microorganism for which a PWS wants to 

obtain inactivation credit using UV disinfection. 

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). 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 Absorbance at 254 nm (A254) – a measure of the amount of UV light that is 

absorbed by a substance at 254 nm. 

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

inactivating microorganisms. Each microorganism has a unique action spectrum.

UV Dose – the UV energy per unit area incident on a surface, typically reported in units 

of mJ/cm2 or J/m2. 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 transmitted. This 

guidance manual also uses the following terms related to UV dose: 

• 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 2-8. 

UV Disinfection Guidance Manual xvii November 2006 

For the Final LT2ESWTR 
 Glossary (Continued) 

• Reduction Equivalent Dose (RED) – The UV dose derived by entering the log 

inactivation measured during full-scale reactor testing into the UV dose-response 

curve that was derived through collimated beam testing. RED values are always 

specific to the challenge microorganism used during experimental testing and the 

validation test conditions for full-scale reactor testing. 

• Required Dose (D ) – The UV dose in units of mJ/cm2

req needed to achieve the 

target log inactivation for the target pathogen. The required dose is specified in 

the LT2ESWTR and presented in Table 1.4 of this guidance manual. 

• Validated Dose (Dval) – The UV dose in units of mJ/cm2 delivered by the UV 

reactor as determined through validation testing. The validated dose is compared 

to the Required Dose (Dreq) to determine log inactivation credit. 

• Calculated Dose - the RED calculated using the dose-monitoring equation that 

was developed through validation testing. 

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

microorganism as a function of UV dose. 

UV Equipment – the UV reactor and related components of the UV disinfection process, 

including (but not limited to) UV reactor appurtenances, ballasts, and control panels.

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

limited to) UV reactors, control systems, piping, valves, and building (if applicable). 

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

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

UV light measured by UV sensors in a reactor and with a radiometer in bench-scale UV 

experiments. 

UV Intensity Setpoint Approach – See Dose-Monitoring Strategy. 

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

all angles, including normal. 

UV Lamp Status – a parameter that is monitored during validation testing and long-term 

operation of UV reactors that indicates whether a particular UV lamp is on or off. 

UV Light – light emitted with wavelengths from 200 to 400 nm. 

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

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

other hydraulic controls. The UV reactor also includes additional hardware for 

monitoring UV dose delivery; typically comprised of (but not limited to): UV sensors and 

UVT monitors. 

UV Disinfection Guidance Manual xviii November 2006 

For the Final LT2ESWTR 
 Glossary (Continued) 

UV Reactor Validation – Experimental testing to determine the operating conditions 

under which a UV reactor delivers the dose required for inactivation credit of 

Cryptosporidium, Giardia lamblia, and viruses. 

UV Sensitivity – the resistance of a microorganism to inactivation by UV light, 

expressed as mJ/cm2

 per log inactivation. 

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

the UV reactor that converts the signal to units of milliamps (mA). 

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

through a material (e.g., water sample or quartz). The UVT is usually reported for a 

wavelength of 254 nm and a pathlength of 1-cm. If an alternate pathlength is used, it 

should be specified or converted to units of cm-1. UVT is often represented as a 

percentage and is related to the UV absorbance (A254) by the following equation (for a 1-

cm path length): % UVT = 100 x 10-A. 

Validated Dose – see UV Dose. 

Validation Factor – an uncertainty term that accounts for the bias and uncertainty 

associated with validation testing. 

Validated Operating Conditions – the operating conditions under which the UV reactor 

is confirmed as delivering the dose required for LT2ESWTR inactivation credit . These 

operating conditions must include flow rate, UV intensity as measured by a UV sensor, 

and UV lamp status. Also commonly referred to as the “validated range” or the 

“validated limits.” 

Validation Uncertainty – an uncertainty term that accounts for error in measurements 

made during validation testing to develop the UV intensity setpoint(s) (for the UV 

Intensity Setpoint Approach) or dose-monitoring equation (for the Calculated Dose 

Approach). 

Visible Light – Wavelengths of light in the visible range (380 – 720 nm). 

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 Glossary (Continued) 

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UV Disinfection Guidance Manual xx November 2006 

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List of Units, Abbreviations, and Acronyms 

λ wavelength 

μg microgram 

μg/L microgram per liter 

μm micrometer, micron 

A254 ultraviolet light absorbance at 254 nanometers 

ACGIH American Conference of Governmental Industrial Hygienists 

AIAA American Institute of Aeronautics and Astronautics 

ANSI American National Standards Institute 

AOC assimilable organic carbon 

APHA American Public Health Association 

ATCC American Type Culture Collection 

AWA Australian Water Association 

AWWA American Water Works Association 

BB

Poly polychromatic bias 

°C degree Centigrade 

CCPP calcium carbonate precipitation potential 

CCWA Clayton County Water Authority 

CEC Clancy Environmental Consultants 

CF correction factor 

CFas action spectra correction factor 

CFD computational fluid dynamics 

CFR Code of Federal Regulations 

cfu colony forming unit 

cfu/mL colony forming units per milliliter 

cm centimeter 

cPEL ceiling permissible exposure limit 

CT contact time 

DBP disinfection byproduct 

DBPR Stage 2 Disinfectants and Disinfection Byproducts Rule 

DNA deoxyribonucleic acid 

DOC dissolved organic carbon 

DReq required UV dose 

DVGW Deutsche Vereinigung des Gas- und Wasserfaches 

ENR BCI Engineering News Record Building Cost Index 

EOLL end-of-lamp-life 

EPA U.S. Environmental Protection Agency 

EPRI Electric Power Research Institute 

°F degree Fahrenheit 

g gram 

g/L gram per liter 

g/mL gram per milliliter 

GAC granular activated carbon 

GFI ground fault interrupter 

UV Disinfection Guidance Manual xxi November 2006 

For the Final LT2ESWTR 
List of Units, Abbreviations, and Acronyms (Continued) 

gpm gallon per minute 

gpm/sf gallon per minute per square foot 

GWUDI ground water under the direct influence of surface water 

HAA haloacetic acid 

HAA5 five haloacetic acids (monochloroacetic, dichloroacetic, trichloroacetic, 

monobromoacetic, and dibromoacetic acids) 

HazMat hazardous materials 

hr hour 

HSP high-service pump 

HVAC heating, ventilating, and air conditioning system 

Hz Hertz 

I&C instrumentation and control 

IDLH Immediately Dangerous to Life or Health 

IEEE Institute of Electrical and Electronic Engineers 

IESWTR Interim Enhanced Surface Water Treatment Rule 

IFE individual filter effluents 

J joule 

J/m2 joule per meter squared 

kVA kilovolt ampere 

kW kilowatt 

L liter 

LED light emitting diode 

LID light intensity distribution 

log I log Inactivation 

LP low pressure 

LPHO low pressure high output 

LRAA locational running annual average 

LSA lignin sulfonic acid 

LT1ESWTR Long Term 1 Enhanced Surface Water Treatment Rule 

LT2ESWTR Long Term 2 Enhanced Surface Water Treatment Rule 

M molar 

-1

 cm

-1 M molar absorption coefficient 

2 m/s meter per second squared 

mA milliampere 

mA/mW milliampere per milliwatt 

MCL maximum contaminant level 

mg milligram 

mg/cm milligram per centimeter 

mg/L milligram per liter 

mgd million gallon per day 

mg-Hg/m3 milligrams mercury per meter cubed 

min minute 

mJ millijoule 

mJ/cm2 millijoule per centimeter squared 

mL milliliter 

mm millimeter 

UV Disinfection Guidance Manual xxii November 2006 

For the Final LT2ESWTR 
List of Units, Abbreviations, and Acronyms (Continued) 

m-mhos/cm millimhos per centimeter 

MP medium pressure 

MS2 male-specific-2 bacteriophage 

mW milliwatt 

mW/cm milliwatt per centimeter 

mW/cm2 milliwatt per centimeter squared 

mWs/cm2 milliwatt second per centimeter squared 

NEC National Electric Code 

NIOSH National Institute for Occupational Safety and Health 

NIST National Institute of Standards and Technology 

nm nanometer 

NOM natural organic matter 

NPL National Physical Laboratory 

NSF National Science Foundation 

NTU nephelometric turbidity unit 

NWRI National Water Research Institute 

NYSERDA New York State Energy Research and Development Authority 

O&M operation and maintenance 

OCC off-line chemical cleaning 

OMC on-line mechanical cleaning 

OMCC on-line mechanical-chemical cleaning 

ÖNORM Österreichisches Normungsinstitut 

oocysts/L oocysts per liter 

ORP oxidation-reduction potential 

OSHA Occupational Safety and Health Administration 

Pa pascal 

PAC powder activated carbon 

PEL permissible exposure limit 

pfu plaque forming unit 

pfu/mL plaque forming units per milliliter 

PLC programmable logic controller 

psi pounds per square inch 

psig pounds-force per square inch gauge 

PTB Physikalisch Technische Bundesanstalt 

PWS public water system 

PWSID public water system identification 

QA/QC quality assurance/quality control 

RAA running annual average 

RCRA Resource Conservation and Recovery Act 

RED reduction equivalent dose 

RNA ribonucleic acid 

s second 

SCADA Supervisory Control and Data Acquisition 

SDWA Safe Drinking Water Act 

SMCL secondary maximum contaminant level 

SUVA specific ultraviolet absorbance 

UV Disinfection Guidance Manual xxiii November 2006 

For the Final LT2ESWTR 
List of Units, Abbreviations, and Acronyms (Continued) 

SWTR Surface Water Treatment Rule 

TCU total color unit 

THM trihalomethane 

TLV threshold limit value 

TNTC too numerous to count 

TOC total organic carbon 

TSA tryptic soy agar 

TSB tryptic soy broth 

TTHM total trihalomethane 

UPS uninterruptible power supply 

UV ultraviolet 

UVal Uncertainty in Validation 

UV-A ultraviolet range from 315 to 400 nm 

UV-B ultraviolet range from 280 to 315 nm 

UV-C ultraviolet range from 200 to 280 nm 

UDR Uncertainty of the Dose-response Fit 

UIN Uncertainty in Interpolation 

US Uncertainty in UV Sensor Measurements 

USP Uncertainty in the Setpoint Value 

UVT ultraviolet transmittance 

VF validation factor 

VFD variable frequency drive 

W watt 

W/cm watt per centimeter 

W/cm2 watt per centimeter squared 

W/m2 watt per meter squared 

W/nm watt per nanometer 

WEF Water Environment Federation 

WTP water treatment plant 

UV Disinfection Guidance Manual xxiv November 2006 

For the Final LT2ESWTR 
1. Introduction 

 Interest in using ultraviolet (UV) light to disinfect drinking water is growing among 

public water systems (PWSs)1 due to its ability to inactivate pathogenic microorganisms without 

forming regulated disinfection byproducts (DBPs). UV light has proven effective against some 

pathogens, such as Cryptosporidium, that are resistant to commonly used disinfectants like 

chlorine. 

 The United States Environmental Protection Agency (EPA) developed the Long Term 2 

Enhanced Surface Water Treatment Rule (LT2ESWTR) to further reduce microbial 

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

their source water Cryptosporidium concentrations and current treatment practices. UV 

disinfection is one option PWSs have to comply with the additional treatment requirements. 

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

traditional chemical disinfectants used in drinking water applications. EPA has developed this 

guidance manual to familiarize states2 and PWSs with these distinctions, as well as associated 

regulatory requirements in the LT2ESWTR. Particularly important design and operation 

considerations include monitoring, reliability, redundancy, lamp cleaning and replacement, and 

lamp breakage. Regulatory requirements include UV dose, UV reactor validation, monitoring, 

reporting, and off-specification compliance. 

EPA developed the requirements for UV disinfection in the LT2ESWTR and the 

guidance in this manual solely for PWSs using UV light to meet drinking water disinfection 

standards established under the Safe Drinking Water Act (SDWA). EPA has not addressed and 

did not consider the extension of these requirements and guidance to other applications, 

including point-of-entry or point-of-use devices for residential water treatment that are not 

operated by PWSs to meet SDWA disinfection standards. 

Chapter 1 covers: 

1.1 Guidance Manual Objectives 

1.2 Organization 

1.3 Regulations Summary 

1.4 UV Disinfection Requirements for Filtered and Unfiltered PWSs 

1.5 Regulations Timeline 

1.6 Alternative Approaches for Disinfecting with UV Light 

 

UV Disinfection Guidance Manual 1-1 November 2006

For the Final LT2ESWTR 

 

1

 Throughout this document, the terms “PWS” and “water system” are used interchangeably. 2 Throughout this document, the terms “state” and “states” are used to refer to all regulatory agencies, including 

both state and federal, with primary enforcement authority for PWSs.
1. Introduction 

UV Disinfection Guidance Manual 1-2 November 2006 

For the Final LT2ESWTR 

1.1 Guidance Manual Objectives 

 

 This manual’s objectives are as follows: 

• Provide PWSs and designers with technical information and guidance on selecting, 

designing, and operating UV installations and complying with the UV disinfectionrelated requirements in the LT2ESWTR. 

 

• Provide states with guidance and the necessary tools to assess UV installations during 

the design, start-up, and routine operation phases. 

• Provide manufacturers with testing and performance standards for UV reactors and 

components intended for treating drinking water. 

1.2 Organization

 

 This manual consists of seven chapters and seven appendices: 

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

treatment and UV disinfection requirements of the LT2ESWTR. 

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

UV disinfection, dose-response relationships, water quality impacts, and UV reactors. 

• Chapter 3 – Planning Analyses for UV Facilities. This chapter discusses planning 

for UV disinfection facilities, including disinfection goals, potential locations, basic 

design parameters, UV reactor evaluation, operational strategies, facility hydraulics, 

pilot- and demonstration-scale testing, and preliminary costs. 

• Chapter 4 – Design Considerations for UV Facilities. This chapter discusses the key 

design features for UV disinfection facilities and presents some common approaches 

to facility design. Key design features include hydraulics, operational optimization, 

instrumentation and controls, electrical power considerations, facility layout, and 

specifications. 

• Chapter 5 –Validation of UV Reactors. This chapter summarizes the LT2ESWTR 

requirements for validation testing and presents EPA’s recommended validation 

protocol. 

• Chapter 6 – Start-up and Operation of UV Facilities. This chapter discusses startup and operation issues for UV disinfection facilities, recommended maintenance 

tasks, and monitoring requirements and recommendations. 

• Chapter 7 – Bibliography. This chapter lists the references used in Chapters 1 

through 6 and Appendices A through G. 
1. Introduction 

UV Disinfection Guidance Manual 1-3 November 2006 

For the Final LT2ESWTR 

• Seven appendices provide supplemental information to Chapters 1 – 6. 

 Appendix A. Preparing and Assaying Challenge Microorganisms 

Appendix B. UV Reactor Testing Examples 

 Appendix C. Collimated Beam Testing to Develop a UV Dose-response Curve 

 Appendix D. Background to the UV Reactor Validation Protocol 

 Appendix E. UV Lamp Break Issues 

 Appendix F. Case Studies 

 Appendix G. Reduction Equivalent Dose Bias Tables 

1.3 Regulations Summary 

 

 This section summarizes general microbial treatment and specific UV disinfection 

requirements in the LT2ESWTR. The rule applies to all PWSs that use surface water or 

groundwater under the direct influence of surface water (GWUDI). It builds on existing 

regulations—the Surface Water Treatment Rule (SWTR), Interim Enhanced Surface Water 

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

(LT1ESWTR)—to improve control of Cryptosporidium and other microbial pathogens. 

 EPA has developed a Stage 2 Disinfectants and Disinfection Byproducts Rule (DBPR) 

with the LT2ESWTR to address the risk-risk trade off between microbial disinfection and the 

DBPs formed by commonly used disinfectants. The Stage 2 DBPR aims to reduce peak DBP 

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

requirements and procedures for compliance determination. Consequently, when a PWS assesses 

its disinfection strategy, it must consider both the disinfectant effectiveness against the target 

pathogen and the DBPs formed as a result of the disinfectant. 

 Table 1.1 highlights microbial treatment requirements and DBP maximum contaminant 

levels (MCLs) from the SWTR, IESWTR, LT1ESWTR, LT2ESWTR, Stage 1 DBPR, and Stage 

2 DBPR. See the original regulations or the Code of Federal Regulations (CFR) for complete 

requirements. Details on the Stage 2 DBPR can be found in 40 CFR 141.600 – 141.629. 
1. Introduction 

UV Disinfection Guidance Manual 1-4 November 2006 

For the Final LT2ESWTR 

 

Table 1.1. Summary of Microbial and DBP Rules 

Surface Water Treatment Rules – Minimum Treatment Requirements1

Regulation Giardia Virus Cryptosporidium

SWTR 3-log removal 

and/or inactivation 

4-log removal 

and/or inactivation Not addressed 

IESWTR and LT1ESWTR No change from SWTR 2-log removal 

0- to 2.5-log additional treatment 

for filtered systems2

LT2ESWTR No change from SWTR 

2- or 3-log inactivation for 

unfiltered systems 2

DBP Rules – MCLs Based on Running Annual Averages (RAAs) or Locational RAAs (LRAAs) 

Regulation

Total 

Trihalomethanes 

(TTHM) 

(μg/L)3

Five Haloacetic 

Acids (HAA5) 

(μg/L)3

Bromate 

(μg/L)3 Chlorite (μg/L)3

Stage 1 DBPR 80 as RAA 60 as RAA 10 1000

Stage 2 DBPR4 80 as LRAA 60 as LRAA No change from Stage 1 

1

 The term “log” means the order of magnitude reduction in concentration; e.g., 2-log removal equals a 99% 

reduction, 3-log removal equals a 99.9% reduction, and 4-log removal equals a 99.99-percent reduction. 2

 Specific requirements for each plant depend on source water monitoring results and current treatment practices 

(40 CFR 141.710 – 141.712). 3

 micrograms/liter (μg/L) 4

 Monitoring locations for LRAAs are identified from the Initial Distribution System Evaluation. 

 The following sections describe LT2ESWTR requirements for filtered and unfiltered 

PWSs. 

1.3.1 Filtered PWSs

 The LT2ESWTR requires filtered PWSs to conduct source water monitoring3

 to 

determine average Cryptosporidium concentrations. Based on the monitoring results, filtered 

PWSs will be classified in one of four possible treatment bins. A PWS’s bin classification 

determines the extent of any additional Cryptosporidium treatment requirements. The rule 

requires filtered PWSs to comply with additional treatment requirements by using one or more 

management or treatment techniques from a “microbial toolbox” of options (40 CFR 141.711). 

UV is one option in the microbial toolbox; see the LT2ESWTR for additional options (40 CFR 

141.715). 

 

3

 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 2006). 
1. Introduction 

UV Disinfection Guidance Manual 1-5 November 2006 

For the Final LT2ESWTR 

 Filtered PWSs are exempt from Cryptosporidium monitoring if the PWS provides, or will 

provide, a total of at least 5.5-log Cryptosporidium treatment—the maximum treatment required 

by the LT2ESWTR for filtered PWSs4

—by the treatment compliance date, which varies, 

depending on population (see Section 1.5 for compliance dates) [40 CFR 141.701(d)]. Installing 

a UV disinfection system that is validated for the appropriate inactivation credit in addition to 

filtration treatment can achieve this objective. 

Treatment Bin Classification

 

 Table 1.2 presents the bin classifications and their corresponding additional treatment 

requirements for filtered PWSs (40 CFR 141.711). PWSs with average Cryptosporidium

concentrations of less than 0.075 oocysts per liter (oocysts/L) are placed in Bin 1 where no 

additional treatment is required. For concentrations of 0.075 oocysts/L or more, treatment 

beyond that required by existing rules is necessary. The additional treatment required for each 

bin, specified in terms of log removal, depends on the type of treatment the PWS already uses. 

Table 1.2. Bin Requirements for Filtered PWSs 1

 

And if the following filtration treatment is operating in full 

compliance with existing regulations, then the additional treatment 

requirements are2

...

Cryptosporidium

Concentration 

(oocysts/L) 

Bin 

Classification

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 and < 1.0 2 1 log 

treatment3

1.5 log 

treatment3

1 log 

treatment3

As determined 

by the state3,5

≥ 1.0 and < 3.0 3 2 log 

treatment4

2.5 log 

treatment4

2 log 

treatment4

As determined 

by the state4,6

≥ 3.0 4 2.5 log 

treatment4

3 log 

treatment4

2.5 log 

treatment4

As determined 

by the state4,7

1

 From 40 CFR 141.711 

2

 Additional treatment requirements reflect a Cryptosporidium removal credit of 3 log for a conventional, slow sand, 

or diatomaceous earth filtration, and a 2.5-log credit for direct filtration plants. 3

 PWSs may use any technology or combination of technologies from the microbial toolbox. 4

 PWSs must achieve at least 1 log of the required treatment using ozone, chlorine dioxide, UV light, membranes, 

bag/cartridge filters, or bank filtration. 5

 Total Cryptosporidium treatment must be at least 4.0 log. 6

 Total Cryptosporidium treatment must be at least 5.0 log. 

7

 Total Cryptosporidium treatment must be at least 5.5 log.

 

 

4

 Treatment requirements for filtered PWSs [40 CFR 141.711] are based on a determination that conventional, slow 

sand, and diatomaceous earth filtration plants in compliance with the IESWTR and LT1ESWTR achieve an 

average of 3-log removal of Cryptosporidium. EPA has determined that direct filtration plants achieve an average 

of 2.5-log removal of Cryptosporidium (their removal is less than in conventional filtration because they lack a 

sedimentation process). 
1. Introduction 

UV Disinfection Guidance Manual 1-6 November 2006 

For the Final LT2ESWTR 

1.3.2 Unfiltered PWSs

 

 All existing requirements for unfiltered PWSs remain in effect, including disinfection to 

achieve at least 3-log inactivation of Giardia and 4-log inactivation of viruses. The LT2ESWTR 

requires 2- or 3-log inactivation of Cryptosporidium, depending on the source water 

concentration of Cryptosporidium, as shown in Table 1.3 [40 CFR 141.712)]. 

Table 1.3. Requirements for Unfiltered PWSs 

Average Cryptosporidium Concentration

(oocysts/L)

Additional Cryptosporidium Inactivation 

Requirements

≤ 0.01 2 log1 

> 0.01 3 log1 

1

 Overall disinfection requirements must be met with a minimum of two disinfectants [40 CFR 

141.712(d)].

 Unfiltered PWSs are exempt from Cryptosporidium monitoring if the PWS provides, or 

will provide, a total of at least 3-log Cryptosporidium inactivation—the maximum treatment 

required by the LT2ESWTR for unfiltered systems [40 CFR 141.701(d)]—by the treatment 

compliance date. (See Figure 1.1.) Installing a UV disinfection system that is validated for the 

appropriate inactivation credit can achieve this objective. 

1.3.3 PWSs with Uncovered Finished Water Storage Facilities 

 

 The LT2ESWTR requires PWSs with uncovered finished water storage facilities to either 

cover the storage facility or treat the discharge of the storage facility that is distributed to 

consumers to achieve inactivation and/or removal of 4-log virus, 3-log Giardia, and 2-log 

Cryptosporidium. UV disinfection is a treatment option that can help water systems meet these 

requirements. 

1.4 UV Disinfection Requirements for Filtered and Unfiltered PWSs 

 

 The LT2ESWTR has several requirements related to the use of UV disinfection. They 

address the UV doses for different levels of inactivation credit, performance validation testing of 

UV reactors, monitoring, reporting, and off-specification operation. 

1.4.1 UV Dose and Validation Testing Requirements 

 

 EPA developed UV dose requirements for PWSs to receive credit for inactivation of 

Cryptosporidium, Giardia, and viruses (Table 1.4). The UV dose values in Table 1.4 are 

applicable only to post-filter applications of UV disinfection in filtered systems and to unfiltered 

systems. 
1. Introduction 

UV Disinfection Guidance Manual 1-7 November 2006 

For the Final LT2ESWTR 

 Unlike chemical disinfectants, UV leaves no residual that can be monitored to determine 

UV dose and inactivation credit. The UV dose depends on the UV intensity (measured by UV 

sensors), the flow rate, and the UV transmittance (UVT).5

 A relationship between the required 

UV dose and these parameters must be established and then monitored at the water treatment 

plant to ensure sufficient disinfection of microbial pathogens. 

Table 1.4. UV Dose Requirements – 

millijoules per centimeter squared (mJ/cm2) 1

Target Log Inactivation 

Pathogens 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 15 22 

Giardia 1.5 2.1 3.0 5.2 7.7 11 15 22 

Virus 39 58 79 100 121 143 163 186 

1 40 CFR 141.720(d)(1)

 The UV dose requirements in Table 1.4 account for uncertainty in the UV dose-response 

relationships of the target pathogens but do not address other significant sources of uncertainty in 

full-scale UV disinfection applications. These other sources of uncertainty are due to the 

hydraulic effects of the UV installation, the UV reactor equipment (e.g., UV sensors), and the 

monitoring approach. 

 Due to these factors, the LT2ESWTR requires PWSs to use UV reactors that have 

undergone validation testing. This validation testing must determine the operating conditions 

under which the reactor delivers the required UV dose for treatment credit [40 CFR 

141.720(d)(2)]. These operating conditions must include flow rate, UV intensity as measured by 

a UV sensor, and UV lamp status. Further, validation testing must meet the following 

requirements: 

• Validated operating conditions must account for UV absorbance of the water, lamp 

fouling and aging, measurement uncertainty of online sensors, UV dose distributions 

arising from the velocity profiles through the reactor, failure of UV lamps or other 

critical system components, and inlet and outlet piping or channel configurations of 

the UV reactor [40 CFR 141.720(d)(2)(i)]. 

• Validation testing must involve full-scale testing of a reactor that conforms uniformly 

to the UV reactors used by the PWS, and it also must demonstrate inactivation of a 

test microorganism whose dose-response characteristics have been quantified with a 

low-pressure mercury vapor lamp [40 CFR 141.720(d)(2)(ii)]. 

Using the above requirements as a basis, Chapter 5 presents EPA’s recommended 

validation protocol. Water systems are not required to follow this protocol but may follow 

alternatives that achieve compliance with the regulatory requirements as long as they are 

acceptable to the state. Also, states may have additional requirements than are provided in the 

federal rule. 

 

5

 UV intensity measurements may account for UVT depending on sensor locations. 
1. Introduction 

UV Disinfection Guidance Manual 1-8 November 2006 

For the Final LT2ESWTR 

1.4.2 UV Disinfection Monitoring Requirements [40 CFR 141.720(d)(3)(i)] 

 The LT2ESWTR requires PWSs to monitor their UV reactors to demonstrate that they 

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

minimum, PWSs must monitor each reactor for flow rate, lamp status, UV intensity as measured 

by a UV sensor, and any other parameters required by the state. UV absorbance should also be 

measured when it is used in a dose-monitoring strategy. PWSs must verify the calibration of UV 

sensors and recalibrate sensors in accordance with a protocol the state approves. Section 6.4.1.2 

of this guidance describes recommended frequencies for checking sensors. 

1.4.3 UV Disinfection Reporting Requirements [40 CFR 141.721(f)(15]) 

 

 The LT2ESWTR requires PWSs to report the following items: 

• Initial reporting – Validation test results demonstrating operating conditions that 

achieve the UV dose required for compliance with the LT2ESWTR. 

• Routine reporting – Percentage of water entering the distribution system that was 

not treated by the UV reactors operating within validated conditions on a monthly 

basis. 

1.4.4 Off-specification Operational Requirement for Filtered and Unfiltered 

Systems [40 CFR 141.720(d)(3)(ii)] 

 To receive disinfection credit for UV, both filtered and unfiltered PWSs must treat at 

least 95 percent of the water delivered to the public during each month by UV reactors operating 

within validated conditions for the required UV dose. EPA views this 95-percent limit as a 

feasible minimum level of performance for PWSs to achieve, while ensuring the desired level of 

health protection is provided. For purposes of design and operation, PWSs should strive to 

deliver the required UV dose at all times during treatment. 

In this manual, operating outside the validated limits is defined as off-specification. Offspecification compliance is based on the volume of water treated. Guidance for calculating offspecification is provided in Chapter 6. 
1. Introduction 

UV Disinfection Guidance Manual 1-9 November 2006 

For the Final LT2ESWTR 

1.5 Regulations Timeline 

 

Figure 1.1 provides a timeline for LT2ESWTR initial source water monitoring and 

treatment installation. Compliance dates vary among the following PWS sizes: 

• Systems serving 100,000 or more people 

• Systems serving 50,000 to 99,999 people 

• Systems serving 10,000 to 49,999 people 

• Systems serving fewer than 10,000 people 

 

 Treatment installation dates pertain only to PWSs that are required to provide additional 

treatment for Cryptosporidium. Further, the actual duration of the treatment installation phase 

will be contingent on a number of PWS-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, 4, and 5). 

1.6 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, it is not 

comprehensive in terms of all types of UV installations, design alternatives, and validation 

protocols that may provide satisfactory 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, substantial research is being conducted on UV disinfection and its applications 

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

operation, design, and validation will evolve. Water systems are not limited by the information 

provided in this guidance manual but must meet the requirements of the LT2 and other drinking 

water rules, as well as any state-specific requirements. States may approve alternatives in UV 

installation design, operation, and validation that are not described in this manual. 
1. Introduction 

UV Disinfection Guidance Manual 1-10 November 2006 

Figure 1.1. LT2ESWTR Compliance Timeline for Initial Source Water Monitoring 

and Treatment Installation 

100,000 People or More50,000 to 99,999 People10,000 to 49,999 People

Fewer than 10,000 People

2016 2015 2014 2013 2012 2011 2010 2009 2008 2007 2006 System Size

Cryptosporidium

Monitoring

Oct. 1, 2006

Treatment

Installation

Possible 

Extension1

Cryptosporidium

Monitoring

Treatment

Installation

Possible 

Extension1

April 1, 2007

April 1, 2012

Oct. 1, 2012

Cryptosporidium

Monitoring

April 1, 2008

Treatment

Installation

Possible 

Extension1

Oct. 1, 2013

E. coli

Monit.2

Oct. 1, 2008

Monit.

Extension4

Crypto.

Monit.3

Treatment

Installation

Possible 

Extension1

Oct. 1, 2014 April 1, 2010

Regulatory Compliance Date

1 Two-year extension may be granted at the discretion of the state for systems requiring capital improvements. 2 E. coli monitoring applies only to filtered systems or unfiltered systems that are required to install filtration. 3 Cryptosporidium monitoring for small systems is necessary only if E. coli monitoring indicates an annual mean concentration greater than 50 E. coli per 100 mL. 4 Systems serving fewer than 10,000 people may monitor Cryptosporidium either by collecting two samples per month for one year or

one sample per month for two years.

100,000 People or More50,000 to 99,999 People10,000 to 49,999 People

Fewer than 10,000 People

2016 2015 2014 2013 2012 2011 2010 2009 2008 2007 2006 System Size

Cryptosporidium

Monitoring

Oct. 1, 2006

Treatment

Installation

Possible 

Extension1

Cryptosporidium

Monitoring

Treatment

Installation

Possible 

Extension1

April 1, 2007

April 1, 2012

Oct. 1, 2012

Cryptosporidium

Monitoring

April 1, 2008

Treatment

Installation

Possible 

Extension1

Oct. 1, 2013

E. coli

Monit.2

Oct. 1, 2008

Monit.

Extension4

Crypto.

Monit.3

Treatment

Installation

Possible 

Extension1

Oct. 1, 2014 April 1, 2010

Regulatory Compliance Date

1 Two-year extension may be granted at the discretion of the state for systems requiring capital improvements. 2 E. coli monitoring applies only to filtered systems or unfiltered systems that are required to install filtration. 3 Cryptosporidium monitoring for small systems is necessary only if E. coli monitoring indicates an annual mean concentration greater than 50 E. coli per 100 mL. 4 Systems serving fewer than 10,000 people may monitor Cryptosporidium either by collecting two samples per month for one year or

one sample per month for two years.

For the Final LT2ESWTR 
UV Disinfection Guidance Manual 2-1 November 2006 

2. Overview of UV Disinfection 

 Chapter 2 provides an overview of UV disinfection. This overview includes discussion of 

basic chemical and physical principles, the components of UV equipment, and performance 

monitoring for UV facilities. The overview material in Chapter 2 is intended to present generally 

accepted facts and research results related to UV disinfection. The material is not intended to 

provide guidance or recommendations for designing, validating, or installing UV disinfection 

facilities. Some guidance is included in this chapter to enhance the information presented, but 

any guidance that appears in this section is also documented in the appropriate subsequent 

chapters in this manual. 

Chapter 2 covers: 

2.1 History of UV Light for Drinking Water Disinfection 

2.2 UV Light Generation and Transmission 

2.3 Microbial Response to UV Light 

2.4 UV Disinfection Equipment 

2.5 Water Quality Effects and Byproduct Formation 

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 (1877) 

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 were soon 

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 

(Gates 1929). 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, Brandt 

and Giese 1956, Powell 1959). 

 Although substantial research on UV disinfection occurred during the first half of the 20th

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

limited its growth 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 such installations in these countries 

had risen to approximately 500 and 600, respectively. After chlorinated disinfection byproducts 

(DBPs) were discovered, UV disinfection became popular in Norway and the Netherlands with 

the first installations occurring in 1975 and 1980, respectively. 

As of the year 2000, more than 400 UV disinfection facilities worldwide were treating 

drinking water; these UV facilities typically treat flows of less than 1 million gallons per day 

(mgd) (USEPA 2000). Since 2000, several large UV installations across the United States have 

been constructed or are currently under design. The largest of these facilities includes a 180-mgd 

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-2 November 2006 

facility in operation in Seattle, Washington, and a 2,200-mgd facility under design for the New 

York City Department of Environmental Protection (Schulz 2004). Because of the susceptibility 

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

Cryptosporidium, the number of public water systems (PWSs) using UV disinfection is expected 

to increase significantly over the next decade. 

2.2 UV Light Generation and Transmission 

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

desired germicidal properties and (2) delivering (or transmitting) that light to pathogens. This 

section summarizes 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: vacuum UV [100 to 200 

nanometers (nm)]; UV-C (200 to 280 nm); UV-B (280 to 315 nm); and UV-A (315 to 400 nm) 

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

UV-C light on microorganisms. The germicidal action of UV-A light is small relative to UV-B 

light and UV-C light; therefore, very long exposure times are necessary for UV-A light to be 

effective as a disinfectant. Although light in the vacuum UV range can disinfect microorganisms 

(Munakata et al. 1991), vacuum UV light is impractical for water disinfection applications 

because it rapidly dissipates in water over very short distances. For the purposes of this manual, 

the practical germicidal wavelength for UV light is defined as the range between 200 and 300 

nm. The germicidal range is discussed further in Section 2.3.1. 

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

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-3 November 2006 

For the Final LT2ESWTR 

 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. Nearly all UV lamps 

currently designed for water treatment use a gas mixture containing mercury vapor. Mercury gas 

is advantageous for UV disinfection applications because it emits light in the germicidal 

wavelength range. Other gases such as xenon also emit light in the germicidal range. 

 The light output from mercury-based UV lamps depends on the concentration of mercury 

atoms, which is directly related to the mercury vapor pressure. In low-pressure (LP) UV lamps, 

mercury at low vapor pressure [near vacuum; 2 x 10-5 to 2 x 10-3 pounds per square inch (psi)] 

and moderate temperature [40 degrees centigrade (oC)] produces essentially monochromatic (one 

wavelength) UV light at 253.7 nm. In medium-pressure (MP) UV lamps, a higher vapor pressure 

[2 – 200 psi] and higher operating temperature (600 – 900 oC) is used to increase the frequency 

of collisions between mercury atoms, which produces UV light over a broad spectrum 

(polychromatic) with an overall higher intensity. The characteristics of LP and MP lamps are 

discussed in Section 2.4.2 and summarized in Table 2.1. 

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 

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

assessing water quality, UV absorbance or UV transmittance (UVT) is the parameter that 

incorporates the effect of absorption and scattering. This section briefly describes both the 

phenomena that influence light propagation and the measurement techniques used to quantify 

UV light propagation. 

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

substance. UV absorbance of a substance varies with the wavelength (λ) of the light. The 

components of a UV 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. 

 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 

through the interface between one medium and another. In UV reactors, refraction occurs when 

light passes from the UV lamp into an air gap, from the air gap into the lamp sleeve, and from 

the lamp sleeve into the water. Refraction changes the angle that UV light strikes target 

pathogens, but how this ultimately affects the UV disinfection process is unknown. 

 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 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-4 November 2006 

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 and intensity of light reflected 

from a surface depends on the material of the surface. 

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

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

 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 toward the 

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

wavelength of the light is called Rayleigh scattering. Rayleigh scattering depends inversely on 

wavelength to the fourth power (1/λ4

) and thus is more prominent at shorter wavelengths. 

Particles larger than the wavelength of light scatter more light in the forward direction but also 

cause some backscattering that is relatively independent of wavelength. 

 UV absorbance (A) quantifies the decrease in the amount of incident light as it passes 

through a water sample over a specified distance or pathlength. UV absorbance at 254 nm (A254) 

is a water quality parameter commonly used to characterize the DBP formation potential of the 

water (e.g., specific UV absorbance calculations). In UV disinfection applications, A254 is used to 

measure the amount of UV light passing through the water and reaching the target organisms. 

A254 is measured using a spectrophotometer with 254 nm incident light and is typically reported 

on a per centimeter (cm-1) basis. 

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-5 November 2006 

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

 Standard Method 5910B (APHA et al. 1998) calls for filtering the sample through a 

0.45-μm membrane and adjusting the pH before measuring the absorbance. For UV disinfection 

applications, however, A254 measurements should reflect the water to be treated. Therefore, water 

samples should be analyzed without filtering or adjusting the pH. More information on collecting 

A254 data is provided in Section 3.4.4.1. Although Standard Methods defines this measurement as 

UV absorption, this manual refers to it as UV absorbance because the latter term is widely used 

in the water treatment industry. 

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 material 

(e.g., a water sample or quartz) over a specified distance. The UVT can be calculated using 

Beer’s law (Equation 2.1): 

0

% *100

I

I UVT = Equation 2.1 

 where 

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

(e.g., 1 cm) 

I = Intensity of light transmitted through the sample [milliwatt per centimeter 

squared (mW/cm2

)] 

I0 = Intensity of light incident on the sample (mW/cm2

) 

UVT can also be calculated by relating it to UV absorbance using Equation 2.2: 

 Equation 2.2 A UVT − % ∗= 10100

 where 

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

(e.g., 1 cm) 

A = UV absorbance at a specified wavelength and pathlength (unitless) 

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-6 November 2006 

For the Final LT2ESWTR 

 UVT is typically reported at 254 nm because UV manufacturers and PWSs widely use 

A254. This manual assumes UVT is at 254 nm unless specifically stated otherwise. 

2.3 Microbial Response to UV Light 

 The mechanism of disinfection by UV light differs considerably from the mechanisms of 

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 them from replicating. A 

microorganism that cannot replicate cannot infect a host. 

 It is important that the assays used to quantify microorganism inactivation measure the 

ability of the microorganism to reproduce (Jagger 1967). For bacteria, assays measure the ability 

of the microorganism to divide and form colonies. For viruses, assays measure the ability of the 

microorganism to form plaques in host cells. For protozoan cysts, the assays measure the ability 

of the microorganism to infect a host or tissue culture. Assays that do not measure a response to 

reproduction may result in misleading information on the inactivation of microorganisms using 

UV light. 

 This section describes how UV light causes microbial inactivation, discusses how 

microorganisms can repair the damage, and introduces the concept of UV dose-response. 

2.3.1 Mechanisms of Microbial Inactivation by UV Light 

 Nucleic acid is the molecule responsible for defining the metabolic functions and 

reproduction of all forms of life. The two most common forms of nucleic acid are 

deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA and RNA consist of single- or 

double-stranded polymers comprising building blocks called nucleotides (Figure 2.5). In DNA, 

the nucleotides are classified as either purines (adenine and guanine) or pyrimidines (thymine 

and cytosine). In RNA, the purines are the same as in DNA, but the pyrimidines are uracil and 

cytosine. 

 As shown in Figure 2.6, the nucleotides absorb UV light at wavelengths from 200 to 300 

nm. The UV absorption of DNA and RNA reflects their nucleotide composition and tends to 

have a peak near 260 nm and a local minimum near 230 nm. 

 All purines and pyrimidines strongly absorb UV light, but the rate of UV-induced 

damage is greater with pyrimidines (Jagger 1967). Absorbed UV light induces six types of 

damage in the pyrimidines of nucleic acid (Setlow 1967, Snowball and Hornsey 1988, Pfeifer 

1997). The damage varies depending on UV dose. The following three types of damage 

contribute to microorganism inactivation: 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-7 November 2006 

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

| | | | | | | |

Figure 2.6. UV Absorbance of Nucleotides (left) and 

Nucleic Acid (right) at pH 7 

 

200 260 300

Wavelength (nm)

UV 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

UV absorbance(Relative Scale)

200 260 300

Wavelength (nm)

UV 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)

UV 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

UV absorbance(Relative Scale)

0.0

0.2

0.4

0.6

0.8

1.0

DNA

200 260 300

Wavelength (nm)

220 240 280

UV absorbance(Relative Scale)

Source: Adapted from Jagger (1967)

• Pyrimidine dimers form when covalent bonds are present between adjacent 

pyrimidines on the same DNA or RNA strand, and they are the most common 

damage resulting from UV disinfection. 

• Pyrimidine (6-4) pyrimidone photoproducts are similar to pyrimidine dimers and 

form on the same sites. 

• 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. 

 The other three types of damage do not significantly contribute to UV disinfection: 

pyrimidine hydrates occur much less frequently than dimers, and single- and double-strand 

breaks and DNA-DNA cross-links occur only at doses that are several orders of magnitude 

higher than the doses typically used for UV disinfection (Jagger 1967). 

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2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-8 November 2006 

For the Final LT2ESWTR 

 Pyrimidine dimers are the most common form of nucleic acid damage, being 1000 times 

more likely to occur than strand breaks, DNA-DNA cross-links, and protein-DNA cross-links. 

Of the three possible pyrimidine dimers that can form within DNA (thymine-thymine, cytosinecytosine, and thymine-cytosine), thymine-thymine dimers are the most common. For RNA, 

because thymine is not present, uracil-uracil and cytosine-cytosine dimers are formed. 

Microorganisms with DNA rich in thymine tend to be more sensitive to UV disinfection (Adler 

1966). 

 Pyrimidine dimer damage and other forms of nucleic acid damage prevent the replication 

of the microorganism. The damage, however, does not prevent the metabolic functions in the 

microorganism such as respiration. UV doses capable of causing oxidative damage that prevent 

cell metabolism and kill the microorganism (similar to the damage caused by chemical 

disinfectants) are several orders of magnitude greater than doses required to damage the nucleic 

acid and prevent replication. 

2.3.2 Microbial Repair 

 Many microorganisms have enzyme systems that repair damage caused by UV light. 

Repair mechanisms are classified as either photorepair or dark repair (Knudson 1985). Microbial 

repair can increase the UV dose needed to achieve a given degree of inactivation of a pathogen, 

but the process does not prevent inactivation. 

 Even though microbial repair can occur, neither photorepair nor dark repair is anticipated 

to affect the performance of drinking water UV disinfection, as described below: 

• Photorepair of UV irradiated bacteria can be prevented by keeping the UV disinfected 

water in the dark for at least two hours before exposure to room light or sunlight. 

Treated water typically remains in the dark in the piping, reservoirs, and distribution 

system after UV disinfection. Most facilities also use chemical disinfection to provide 

further inactivation of bacteria and virus and protection of the distribution system. 

Both of these common practices make photorepair unlikely to be an issue for PWSs. 

• Dark repair is also not a concern for PWSs because the required UV doses shown in 

Table 1.4 are derived from data that are assumed to account for dark repair. 

2.3.2.1 Photorepair 

 In photorepair (or photoreactivation), enzymes energized by exposure to light between 

310 and 490 nm (near and in the visible range) break the covalent bonds that form the pyrimidine 

dimers. Photorepair requires reactivating light and repairs only pyrimidine dimers (Jagger 1967). 

Knudson (1985) found that bacteria have the enzymes necessary for photorepair. Unlike 

bacteria, viruses lack the necessary enzymes for repair but can repair using the enzymes of a host 

cell (Rauth 1965). Linden et al. (2002a) did not observe photorepair 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 conditions at very low UV doses (0.5 mJ/cm2

, 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-9 November 2006 

For the Final LT2ESWTR 

 

Linden 2002). Shin et al. (2001) reported that Cryptosporidium does not regain infectivity after 

inactivation by UV light. One study showed that Cryptosporidium can undergo some DNA 

photorepair (Oguma et al. 2001). Even though the DNA is repaired, however, infectivity is not 

restored. 

2.3.2.2 Dark Repair 

 

 Dark repair is defined as any repair process that does not require the presence of light. 

The term is somewhat misleading because dark repair can also occur in the presence of light. 

Excision repair, a form of dark repair, is an enzyme-mediated process in which the damaged 

section of DNA is removed and regenerated using the existing complementary strand of DNA. 

As such, excision repair can occur only with double stranded DNA and RNA. The extent of dark 

repair varies with the microorganism. With bacteria and protozoa, dark repair enzymes start to 

act immediately following exposure to UV light; therefore, reported dose-response data are 

assumed to account for dark repair. 

 Knudson (1985) found that bacteria can undergo dark repair, but some lack the enzymes 

needed for dark repair (Knudson 1985). Viruses also lack the necessary enzymes for repair but 

can repair using the enzymes of a host cell (Rauth 1965). Oguma et al. (2001) used an assay that 

measures the number of dimers formed in nucleic acid to show that dark repair occurs in 

Cryptosporidium, even though the mircroorganism did not regain infectivity. Linden et al. 

(2002a) did not observe dark repair of Giardia at UV doses typical for UV disinfection 

applications (16 and 40 mJ/cm2

). Shin et al. (2001) reported Cryptosporidium does not regain 

infectivity after inactivation by UV light. 

2.3.3 UV Intensity, UV Dose, and UV Dose Distribution 

 

UV intensity is a fundamental property of UV light and has the units of watts per meter 

squared (W/m2

) (Halliday and Resnick 1978). UV intensity has a formal definition that is derived 

from Maxwell’s equations, which are fundamental equations that define the wavelike properties 

of light. The total UV intensity at a point in space is the sum of the intensity of UV light from all 

directions. 

 UV dose is the integral of UV intensity during the exposure period (i.e., the area under an 

intensity versus time curve). If the UV intensity is constant over the exposure time, UV dose is 

defined as the product of the intensity and the exposure time. Units commonly used for UV dose 

are joule per meter squared (J/m2

), mJ/cm2, and milliwatt seconds per centimeter squared 

(mWs/cm2

), with mJ/cm2 being the most common units in North America and J/m2 being the 

most common in Europe.5

 

 In a completely mixed batch system, the UV dose that the microorganisms receive is 

equal to the volume-averaged UV intensity within the system. An example of a completely 

mixed batch system is the collimated beam study in which a petri dish containing the stirred 

5

 10 J/m2 = 1 mJ/cm2 = 1 mWs/cm2
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-10 November 2006 

microbial solution is irradiated by a collimated UV light beam (see Appendix C for details). In 

this case, the average UV intensity is calculated from the measured UV intensity incident on the 

surface of the microbial suspension, the suspension depth, and the UV absorbance of the water 

(see Appendix C for details). When using polychromatic light sources (e.g., MP lamps), UV dose 

calculations in batch system also incorporate the intensity at each wavelength in the germicidal 

range and the germicidal effectiveness at the associated UV wavelengths. 

 Dose delivery in a continuous flow UV reactor is considerably more complex than in a 

completely mixed batch reactor. 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. The result is that each microorganism leaving the reactor receives a different UV 

dose. Accordingly, UV dose delivered to the microorganisms passing through the reactor is best 

described using a dose distribution (Cabaj et al. 1996) as opposed to a single dose value. A dose 

distribution can be defined as a histogram of dose delivery (see Figure 2.7). Alternatively, the 

dose distribution can be defined as a probability distribution that a microorganism leaving a UV 

reactor will receive a given dose. 

Figure 2.7. Hypothetical Dose Distributions for Two Reactors 

with Differing Hydraulics 

 

 The width of the dose distribution is indicative of the dose delivery efficiency of the 

reactor. A narrow dose distribution (Figure 2.7a) indicates a more efficient reactor, and a wider 

dose distribution (Figure 2.7b) indicates a less efficient reactor. In particular, the average log 

inactivation a reactor achieves with a given microorganism is strongly affected by 

microorganisms that receive the lowest UV doses. 

 The dose distribution a UV reactor delivers can be estimated using mathematical models 

based on computational fluid dynamics (CFD) and the light intensity distribution (LID). CFD is 

used to predict the trajectories of microorganisms as they travel through the UV reactor. LID is 

used to predict the intensity at each point within the UV reactor. UV dose to each microorganism 

is calculated by integrating the UV intensity over the microorganism’s trajectory through the 

reactor. Biodosimetry (discussed below) is often used to verify these modeling results. 

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-11 November 2006 

 Currently, dose delivery is measured using a technique termed biodosimetry. With 

biodosimetry, the log inactivation of a surrogate microorganism is measured through the UV 

reactor and related to a dose value termed the reduction equivalent dose (RED) using the UV 

dose-response curve of the surrogate microorganism. Methods for conducting biodosimetry are 

presented in Chapter 5. Although alternatives to biodosimetry are being developed (e.g., the use 

of actinometric microspheres) for measuring the dose distribution of a reactor, such methods 

have not yet been proven for measuring dose delivery in UV reactors. 

2.3.4 Microbial Response (UV Dose-Response) 

 

 Microbial response is a measure of the sensitivity of the microorganism to UV light and 

is unique to each microorganism. UV dose-response is determined by irradiating water samples 

containing the microorganism with various UV doses using a collimated beam apparatus (as 

described in Appendix C of this manual) and measuring the concentration of infectious 

microorganisms before and after exposure. The microbial response is calculated using Equation 

2.3. 

N

N Log onInactivati 0

10 = log Equation 2.3 

 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 or the proportion of microorganisms remaining as a function of UV 

dose. Microbial inactivation has a dose-response curve with a positive slope, while microbial 

survival has a dose-response curve with a negative slope. This manual presents microbial 

response as log inactivation because the terminology is widely accepted in the industry. 

Therefore, all dose-response curves presented (log inactivation as a function of dose) have a 

positive slope with log inactivation on a logarithmic (base 10) scale and UV dose on a linear 

scale. 

 Figure 2.8 presents examples of UV dose-response curves. The shape of the UV doseresponse curve typically has three regions. At low UV doses, the UV dose-response shows a 

shoulder region where little if any inactivation occurs (e.g., Bacillus subtilis curve, Figure 2.8). 

The shoulder region has been attributed to dark repair (Morton and Haynes 1969) and 

photorepair (Hoyer 1998). Above some threshold dose level, the dose-response shows first-order 

inactivation where inactivation increases linearly with increased dose. In many cases, the doseresponse shows first-order inactivation without a shoulder (e.g., E. coli curve, Figure 2.8). At 

higher UV doses, the dose-response shows tailing, a region where the slope of the dose-response 

decreases with increased dose (e.g., rotavirus and total coliform curves, Figure 2.8). Tailing has 

been attributed to the presence of UV-resistant sub-populations of the microorganism and the 

presence of particulate-associated and clumped microorganisms (Parker and Darby 1995). The 

shape of the dose-response curve can affect validation results, and information on how to 

account for tailing and shoulders in validation testing is included in Section C.6. 

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-12 November 2006 

Figure 2.8. Shapes of UV Dose-Response Curves 

0

1

2

3

4

5

6

0 20 40 60 80 100

UV Dose (mJ/cm2)

Log Inactivation

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 Inactivation

E. coli

B. subtilis spores

Total coliform-wastewater

Rotavirus

Source: Adapted from Chang et al. (1985) 

 Microbial response to UV light can vary significantly among microorganisms. The UV 

sensitivity of viruses and bacteriophage can vary by more than two orders of magnitude (Rauth 

1965). With bacteria, spore-forming and gram-positive bacteria are more resistant to UV light 

than gram-negative bacteria (Jagger 1967). Among the pathogens of interest in drinking water, 

viruses are most resistant to UV disinfection followed by bacteria, Cryptosporidium oocysts, and 

Giardia cysts. 

 UV dose-response is generally independent of the following factors: 

• UV intensity: In general, UV dose-response follows the Law of Reflectivity over an 

intensity range of 1 – 200 mW/cm2

, where the same level of inactivation is achieved 

with a given UV dose regardless of whether that dose was obtained with a high UV 

intensity and low exposure time or vice versa (Oliver and Cosgrove 1975, Rice and 

Ewell 2001). Non-reciprocity has been observed at low intensities where repair may 

compete with inactivation (Sommer et al. 1998, Setlow 1967). 

• UV absorbance: UV absorbance of the suspension is considered when calculating 

UV dose. Increasing intensity or exposure time, however, may be necessary to 

achieve a constant UV dose as the absorbance of a suspension changes. 

• Temperature: Temperature effects on dose-response are minimal and depend on the 

microorganism. For male-specific-2 (MS2) bacteriophage, inactivation is not 

temperature-dependent (Malley 2000). Severin et al. (1983) studied three 

microorganisms to determine the dose required to achieve 2-log inactivation as a 

function of temperature. For E. coli and Candia parapsilosis, the dose requires 

decreases by less than 10 percent as the temperature increases from 5 to 35 °C, and 

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-13 November 2006 

For the Final LT2ESWTR 

for f2 bacteriophage, the dose requires decreases by less than 20 percent over the 

same temperature interval (Severin et al. 1983). 

• pH: Dose-response is independent of the suspension pH from pH 6 to pH 9 (Malley 

2000). 

 

 Particle association and clumping of microorganisms affects UV dose-response. Small 

floc particles can enmesh and protect MS2 bacteriophage, and potentially other viruses, from 

exposure to UV light (Templeton et al. 2003). Similarly, the inactivation rate of particleassociated coliforms is slower than that of non-particle-associated coliforms (Örmeci and Linden 

2003). The shielding effect of clumping or particle association can cause a tailing or flattening of 

the dose-response curve at higher inactivation levels (Figure 2.8, total coliform curve). 

 Several studies have examined the effect of particles on UV disinfection performance. 

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 for filtered effluents. For unfiltered waters, source water turbidity up to 10 

nephelometric turbidity units (NTU) did not affect the UV dose-response of separately added 

(seeded) microorganisms (Passantino et al. 2004, Oppenheimer et al. 2002). The effect of 

particle enmeshment on the UV dose-response of seeded microorganisms in water has been 

studied by adding clays or natural particles. When coagulating suspensions containing kaolinite 

or montmorillonite clay using alum or ferric chloride, no difference was observed in the log 

inactivation of the seeded microorganisms (Templeton et al. 2004, Mamane-Gravetz and Linden 

2004). When humic acid particles and a coagulant were added to the suspensions, however, 

significantly less inactivation was achieved (Templeton et al. 2004). Further research is needed 

to understand fully the effect of coagulation and particles on microbial inactivation by UV light. 

2.3.5 Microbial Spectral Response 

 

 Microbial response varies as a function of wavelength of the UV light. 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 spectrum for three microbial 

species and the UV absorbance of DNA as a function of wavelength. Because of the similarity 

between the UV action and DNA absorbance spectra 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. In Figure 2.9, the scale of the y-axis represents the ratio of 

inactivation effectiveness at a given wavelength to the inactivation effectiveness at 254 nm. 

 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-14 November 2006 

Figure 2.9. Comparison of Microbial UV Action and DNA UV Absorbance 

 

0

0.5

1

1.5

2

2.5

3

3.5

4

200 210 220 230 240 250 260 270 280 290 300

Wavelength (nm)

UV Action or DNA Absorbance 

Relative to 254 nm

MS2 - Linden et al. 2001

Cryptosporidium - Linden et al. 2001

Adenovirus - Malley et al. 2004

Herpes Simplex - Linden et al. 2001

DNA - Rauth 1965

Source: Adapted from Rauth (1965), Linden et al. (2001), and Malley et al. (2004)

 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, which means that UV light at 260 nm is the most 

effective at inactivating microorganisms. Because no efficient way to produce UV light at 260 

nm is available and mercury produces UV light very efficiently at 254 nm, however, the latter 

has become the standard. Although the action spectrum of various microorganisms is similar at 

wavelengths above 240 nm, significant differences occur at wavelengths below 240 nm (Rauth 

1965). 

2.4 UV Disinfection Equipment 

 

 The goal in designing UV reactors for drinking water disinfection is to efficiently deliver 

the dose necessary to inactivate pathogenic microorganisms. An example of UV equipment is 

shown in Figure 2.10. Commercial UV reactors consist of open or closed-channel vessels, 

containing UV lamps, lamp sleeves, UV sensors, and temperature sensors. UV lamps typically 

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. UV sensors, flow 

meters, and, in some cases, UVT analyzers, are used to monitor dose delivery by the reactor. 

This section briefly describes the components of the UV equipment and its monitoring systems. 

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-15 November 2006 

Figure 2.10. Example of UV Disinfection Equipment 

Influent

Pipe

UV

Transmittance

Analyzer

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

Analyzer

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

Source: Courtesy of and adapted from Severn Trent Services 

Note: Not to scale 

2.4.1 UV Reactor Configuration 

 UV reactors are typically classified as either closed or open 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 and are most commonly used in 

wastewater applications. 

Figure 2.11. Examples of UV Reactors: (a) Closed-channel 

and (b) Open-channel 

a. b. 

Source: (a) Courtesy of Calgon Carbon Corporation and (b) Courtesy of WEDECO UV Systems

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-16 November 2006 

For the Final LT2ESWTR 

 UV equipment manufacturers design their UV reactors to provide efficient and costeffective dose delivery. Lamp placement, baffles, and inlet and outlet conditions all affect mixing 

within a reactor and dose delivery. Individual reactor designs use various methods to optimize 

dose delivery (e.g., higher lamp output versus lower lamp output and improved hydrodynamics 

through increased head loss). 

 The lamp configuration in a reactor is designed to optimize dose delivery. In a reactor 

with a square cross-section, lamps are typically 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. However, 

UV lamps may be oriented parallel, perpendicular, or diagonal to the flow direction. Depending 

on the reactor installation, lamps may consequently be oriented horizontally, vertically, or 

diagonally relative to the ground surface. Orienting MP lamps parallel to the ground prevents 

overheating at the top of the lamps and reduces the potential for lamp breakage due to 

temperature differentials. 

 The thickness of the water layer between lamps and between the lamps and the reactor 

wall influences dose delivery. If the water layer is too thin, the reactor wall and adjacent lamps 

will absorb UV light. If the water layer is too thick, 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 hydraulic mixing within the reactor. 

 The flow through UV reactors is turbulent. Residence times are on the order of tenths of a 

second for MP lamps and seconds for LP lamps. In theory, optimal dose delivery is obtained 

with plug flow hydraulics through a UV reactor. In practice, however, UV reactors do not have 

such ideal hydrodynamics. For example, turbulence and eddies form in the wake behind lamp 

sleeves oriented perpendicularly to flow. Some manufacturers insert baffles to improve 

hydrodynamics in the reactor. Improvements to the hydraulic behavior of a reactor are often 

obtained at the expense of head loss. 

 Inlet and outlet conditions can significantly affect reactor hydrodynamics and UV dose 

delivery. For example, changes in flow direction of 90 degrees at inlets and outlets promote 

short-circuiting, eddies, and dead zones within the reactor. Straight inlet configurations with 

gradual changes in cross-sectional area will help create flow conditions for optimal dose 

delivery. 

2.4.2 UV Lamps 

 

 UV light can be produced by the following variety of lamps: 

• LP mercury vapor lamps 

• Low-pressure high-output (LPHO) mercury vapor lamps 

• MP mercury vapor lamps 

• Electrode-less mercury vapor lamps 

• Metal halide lamps 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-17 November 2006 

For the Final LT2ESWTR 

• Xenon lamps (pulsed UV) 

• Eximer lamps 

• UV lasers 

• Light emitting diodes (LEDs) 

Full-scale drinking water applications generally use LP, LPHO, or MP mercury vapor 

lamps. Therefore, this manual limits discussion to these UV lamp technologies. Table 2.1 lists 

characteristics of these lamps, and Table 2.2 lists operational advantages of the lamp types. 

 

Table 2.1. Typical 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 – 300 nm) 

Mercury Vapor Pressure (Pa) Approximately 0.93 

(1.35x10-4 psi) 

0.18 – 1.6 

(2.6x10-5 – 2.3x10-4 psi) 

40,000 – 4,000,000 

(5.80 – 580 psi) 

Operating Temperature (°C) Approximately 40 60 – 100 600 – 900 

Electrical Input [watts per 

centimeter (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 – 35 10 – 20 

Arc Length (cm) 10 – 150 10 – 150 5 – 120 

Relative Number of Lamps 

Needed for a Given Dose High Intermediate Low 

Lifetime [hour (hr)] 8,000 – 10,000 8,000 – 12,000 4,000 – 8,000 

Note: Information in this table was compiled from UV manufacturer data.

Table 2.2. Mercury Vapor Lamp Operational Advantages 

Low-pressure and Low-pressure High-output Medium-pressure 

• 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 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-18 November 2006 

For the Final LT2ESWTR 

LP, LPHO, and MP lamps consist of the following elements, arranged as shown in 

Figure 2.12: 

 

• Lamp Envelope: The envelope of the lamp is designed to transmit germicidal UV 

light, act as an electrical insulator, and not react with the lamp’s fill gases. A noncrystalline form of quartz, vitreous silica, is often used for the lamp envelope because 

of its high UVT and its resistance to high temperatures. The UVT of the envelope 

affects the spectral output of lamps, especially with MP lamps at lower wavelengths. 

Because of this, lamp envelopes can be made from doped quartz (quartz that is altered 

to absorb specific wavelengths) to prevent undesirable non-germicidal photochemical 

reactions. Envelopes are approximately 1 – 2 millimeters (mm) thick, and the 

diameter is selected to optimize the UV output and lamp life. 

• Electrodes: Electrode design and operation are critical for reliable long-term 

operation of lamps. Electrodes promote heat transfer so that lamps can operate at an 

appropriate temperature. The electrodes in LP and LPHO lamps are made of a coil of 

tungsten wire embedded with oxides of calcium, barium, or strontium. In MP lamps, 

electrodes consist of a tungsten rod wrapped in a coil of tungsten wire. 

• Mercury Fill: The mercury fill present in UV lamps can be in the solid, liquid, or 

vapor phase. Amalgams (alloys of mercury and other metals such as indium or 

gallium in the solid phase) are typically used in LPHO lamps, while LP and MP 

lamps contain liquid elemental mercury. As the lamps heat, the vapor pressure of 

mercury increases. LP and LPHO lamps operate at lower temperatures and have 

lower mercury vapor pressures than MP lamps. In MP lamps, the concentration of 

mercury in the vapor phase is controlled by the amount of mercury in the lamp. In 

LPHO lamps, an excess of mercury is placed in the lamp, and the amount of mercury 

entering the vapor phase is limited by 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. 

• Inert Gas Fill: In addition to mercury, lamps are filled with an inert gas (typically 

argon). The inert gas aids in starting the gas discharge and reduces deterioration of 

the electrode. The vapor pressure of the inert gas is typically 0.02 – 1 psi. 

 In addition to amalgam LPHO lamps, another method is used to increase the output from 

LP lamps. In this application, a standard LP lamp with reinforced filaments is used, allowing for 

an increase in current through the lamp. The higher current increases the output from the lamp. 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-19 November 2006 

Figure 2.12. 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

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

2.4.2.1 Lamp Start-up 

 As lamps start up, the following series of events occurs to generate an arc (i.e., produce 

UV light). First, the electrode emits electrons that collide with the inert gas atoms, causing the 

inert gas to ionize. This creates a plasma that allows current to flow, which heats the gas. The 

mercury in operating lamps vaporizes in the presence of the hot inert gas, and collisions between 

the vapor-phase mercury and high-energy electrons in the plasma cause the mercury atoms to 

reach one of many excited states. As the mercury returns from a given excited state to ground 

state, energy is released (according to the difference in the state energies) in the wavelength 

range of the UV spectrum. 

2.4.2.2 Lamp Output 

The light that LP and LPHO lamps emit is essentially monochromatic at 253.7 nm 

(Figure 2.13a) in the ultraviolet range and is near the maximum of the microbial action spectrum. 

These lamps also emit small amounts of light at 185, 313, 365, 405, 436, and 546 nm due to 

higher energy electron transition in the mercury. Lamp output at 185 nm promotes ozone 

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-20 November 2006 

For the Final LT2ESWTR 

use ozone is corrosive, toxic, and absorbs UV light, LP and LPHO lamps used in 

water dis

 

Figure 2.13. UV Output of LP (a) and MP (b) Mercury Vapor Lamps 

formation. Beca

infection applications are manufactured to reduce the output at 185 nm. 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

200 250 3

0.0

0.2

0.4

0.6

0.8

1.0

1.2

200 250 300 350 400

Wavelength (nm)

Relative Lamp Output

0

Waveleng

0 350 400

th (nm)

Relative Lamp Output Lamp Output Relative to Maximum Output in Range

b. Medium-Pressure Lamp

Lamp Output Relative to

Maximum Output in Range

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)

Relative Lamp Output

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 350 400

th (nm)

Relative Lamp Output

200 250 30

Waveleng

Lamp Output Relative to

Maximum Output in Range

b. Medium-Pressure Lamp

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 350 400

th (nm)

Relative Lamp Output

200 250 30

Waveleng

Lamp Output Relative to

Maximum Output in Range

b. Medium-Pressure Lamp

Lamp Output Relative to

Maximum Output in Range

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)

Relative Lamp Output Lamp Output Relative to Maximum Output in Range

a. Low-Pressure Lamp

Source: Sharpless and Linden (2001)

 2.13b). The 

ombination of free electrons and mercury in the lamp creates a broad continuum of UV energy 

All UV lamps also emit light in the visible range. Visible light can promote algal growth 

on 

 in 

d on a relative scale. In absolute terms, 

owever, the intensity and power of LP and MP lamps differ significantly (see Table 2.1 for 

more information on lamp operating characteristics). 

 MP lamps emit a wide range of UV wavelengths from 200 to 400 nm (Figure

c

below 245 nm. Electron transitions in the mercury cause the peaks in the spectrum. 

as discussed in Section 2.5.1.5. 

 Figure 2.14 shows the output of LP and MP lamps superimposed on the DNA absorpti

spectrum. In Figure 2.14, the DNA absorbance is plotted relative to the maximum absorbance

the range (260 nm), and the lamp outputs are presente

h
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-21 November 2006 

Figure 2.14. UV Lamp Output and its Relationship to 

the UV Absorbance of DNA 

200 250 300

Wavelength (nm)

Lamp Output Relative to

Maximum Output in Range 

for Each Data Set

DNA

Absorbance

MP 

Output

LP Output

DNA Absorbance Relative to

Maximum Absorbance in Range

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)

Lamp Output Relative to

Maximum Output in Range 

for Each Data Set

DNA

Absorbance

MP 

Output

LP Output

DNA Absorbance Relative to

Maximum Absorbance in Range

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)

Lamp Output Relative to

Maximum Output in Range 

for Each Data Set

DNA

Absorbance

MP 

Output

LP Output

DNA Absorbance Relative to

Maximum Absorbance in Range

0.00

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Source: Courtesy of Bolton Photosciences, Inc.

2.4.2.3 Lamp Sensitivity to Power Quality 

A UV lamp can lose its arc if a voltage fluctuation, power quality anomaly, or power 

interruption occurs. For example, voltage sags that vary more than 10 – 30 percent from the 

nominal voltage for as few as 0.5 – 3 cycles (0.01 – 0.05 seconds) may cause a UV lamp to lose 

its arc. 

 

The most common sources of power quality problems that may cause UV lamps to lose 

their arcs 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 

• Starting or stopping equipment with large electrical needs on the same circuit at the 

water plant 

• Power transfer to emergency generator or alternate feeders 

 LP lamps generally can return to full operating status within 15 seconds after power is 

restored. LPHO and MP reactors that are more typically used in drinking water applications, 

however, exhibit significant restart times if power is interrupted. The start-up time for lamps 

should be considered in the design of UV disinfection systems as start-up time can contribute to 

off-specification operations (see Section 3.4.1). The start-up and restart behaviors for LPHO and 

MP lamps are summarized in Table 2.3. 

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-22 November 2006 

For the Final LT2ESWTR 

Table 2.3. Typical Start-up and Restart Times for LPHO and MP Lamps 1

Lamp Type Cold Start2 3 Warm Start

LPHO 

total time es (min) tota in 

plus r) 

: 4 – 7 minut

(0 – 2 min warm-up 

plus 4 – 5 min to full power) 

l time: 2 – 7 m

(0 – 2 min warm-up 

 2 – 5 min to full powe

MP (No n 

p plus 4

) 

total time: 1 – 5 min 

 warm-up or cool dow

lus 1 – 5 min to full power4

) 

total time: 4 – 10 min 

(2 – 5 min cool down 

 2 – 5 min to full power

1 Information shown i arbo an, and 

nt 

2

The effects of temperature can increase or decrease the times listed in Table 2.3 and 

ater 

.4.2.4 Lamp Aging 

V lamps degrade as they age, resulting in a reduction in output that causes a drop in UV 

dose de

amp degradation occurs with both LP and MP lamps and is a function of the number of 

lamp ho

e 

 reliminary findings from ongoing research into lamp aging at water and wastewater UV 

ging 

d 

n table is compiled from Calgon C n Corporation, Severn Trent, Troj

WEDECO. Contact the manufacturer to determine the start-up and restart times for specific equipme

models. 

 A cold start occurs when UV lamps have not been operating for a significant period of time. 

3

 A warm start occurs when UV lamps have just lost their arcs (e.g., due to voltage sag). 

4

 60 percent intensity is reached after 3 min. 

Source: Cotton et al. (2005) 

should be discussed with the UV manufacturer. Individual manufacturers report that colder w

temperatures (below 10 oC) can result in slower start-ups for LPHO lamps than those listed in 

Table 2.3. Conversely, MP manufacturers report shorter restart 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. 

2

U

livery over time. Lamp aging can be accounted for with the fouling/aging factor 

(described in Section 3.4.5) in the design of the UV facility. 

L

urs in operation, number of on/off cycles, power applied per unit (lamp) length, water 

temperature, and heat transfer from lamps. The rate of decrease in lamp output often slows as th

lamp ages (Figure 2.15). The reduction in output occurs at all wavelengths across the germicidal 

range as shown in Figure 2.16, which is an example of MP lamp output reduction after 8,220 

hours of operation. 

P

facilities shows that LPHO and MP lamp aging is non-uniform with respect to axial and 

horizontal output and varies greatly from lamp to lamp (Mackey et al. 2005). The lamp a

study by Mackey et al. is still ongoing, and any future findings from this or other studies shoul

be evaluated and considered once results are available. 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-23 November 2006 

Figure 2.15. Reduction in UV Output of (a) LPHO and (b) MP Lamps Over Time 

a. Low-Pressure High Output Mercury Lamps b. Medium-Pressure Mercury Lamps

70%

80%

90%

100%

110%

0 2000 4000 6000 8000 10000 12000

Time (Hrs)

UV Output (%)

70%

80%

90%

100%

110%

0 2000 4000 6000 8000 10000 12000

Time (Hrs)

UV Output (%)

a. Low-Pressure High Output Mercury Lamps b. Medium-Pressure Mercury Lamps

70%

80%

90%

100%

110%

0 2000 4000 6000 8000 10000 12000

Time (Hrs)

UV Output (%)

70%

80%

90%

100%

110%

0 2000 4000 6000 8000 10000 12000

Time (Hrs)

UV Output (%)

Source: (a) Adapted from WEDECO, (b) adapted from Linden et al. (2004)

Figure 2.16. Lamp Aging for an MP Lamp 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

200 225 250 275 300 325 350

Wavelength (nm)

UV Irradiance (mW/cm2)

0 hr

8,220 hr

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

200 225 250 275 300 325 350

Wavelength (nm)

UV Irradiance (mW/cm2)

0 hr

8,220 hr

Source: Adapted from Linden et al. (2004)

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-24 November 2006 

 

 Any deposits on the inner or outer surfaces of the lamp envelope and metallic impurities 

within the envelope can absorb UV light and cause premature lamp aging. In LP and LPHO 

lamps using UV-transmitting glass, mercury may combine with sodium in the glass to create a 

UV-absorbing coating. Electrode sputtering during start-up can also coat the inside surface of the 

lamp envelope with tungsten as the lamp ages. The tungsten coating is black, non-uniform, 

concentrated within a few inches of the electrode, and can absorb UV light (Figure 2.17). If the 

lamps are not sufficiently cooled during operation, electrode material in MP lamps may 

evaporate and condense on the inside of the envelope. 

Figure 2.17. Aged UV Lamp (right) Compared 

to a New UV Lamp (left) 

Source: Mackey et al. (2004) 

 UV lamp manufacturers can reduce electrode sputtering by designing lamps that pre-heat 

the electrode before applying the start voltage, are driven by a sinusoidal current waveform, or 

have a higher argon (inert gas) content. Electrode sputtering can be reduced by minimizing the 

number of lamp starts during operation. 

2.4.3 Ballasts 

 Ballasts are used to regulate the incoming power supply at the level needed to energize 

and operate the UV lamps. Power supplies and ballasts are available in many different 

configurations and are tailored to a unique lamp type and application. UV reactors typically use 

magnetic ballasts or electronic ballasts. 

 Electronic and magnetic ballasts each have specific advantages and disadvantages. UV 

reactor manufacturers consider these advantages and disadvantages when determining what 

technology to incorporate into their equipment designs. Electronic and inductor-based magnetic 

ballasts can provide almost continuous adjustment of lamp intensity. Most transformer-based 

magnetic ballasts, however, allow only step adjustment of lamp intensity. Transformer-based 

magnetic ballasts are typically more electrically efficient than inductor-based ballasts but are less 

efficient than electronic ballasts. However, higher efficiency and additional features can increase 

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-25 November 2006 

For the Final LT2ESWTR 

the electronic ballast cost. UV lamps that are powered by magnetic ballasts tend to have more 

lamp end-darkening (i.e., electrode sputtering) and have shorter lives compared to lamps 

powered by electronic ballasts due to the higher frequencies used by electronic ballasts. 

Electronic ballasts are generally more susceptible to power quality problems (Section 2.4.2.3) 

compared to magnetic ballasts; however, the power quality tolerances of both ballast types 

depend on the electrical design. A comparison of magnetic and electronic ballast technologies is 

shown in Table 2.4. 

Table 2.4. Comparison of Magnetic and Electronic Ballasts 

 

Magnetic Ballast Electronic Ballast 

• Less expensive 

• Continuous power adjustment occurs with 

inductor-based magnetic ballast (but not with 

transformer-based magnetic ballast) 

• More resistant to power surges 

• Proven technology (in use for nearly 70 years) 

• Greater separation distance allowed between the 

UV reactor and control panel 

• Continuous power adjustment and ability to 

adjust to lower power levels (e.g., 30 %) 

• More power efficient 

• Lighter weight and smaller size 

• Allows for longer lamp operating life and less 

lamp end-darkening 

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 (vitreous 

silica) that are open at one or both ends. The sleeve length is sufficient to include the lamp and 

associated electrical connections. The sleeve diameter is typically 2.5 – 5.0 cm for LP and LPHO 

lamps and 3.5 – 10.0 cm for MP lamps. The distance between the exterior of the lamp and 

interior of the lamp sleeve is approximately 1 cm. The positioning of the UV lamp along the 

length of the sleeve can vary, depending on reactor configuration. Lamp sleeves absorb some UV 

light (Figure 2.18), which may influence dose delivery by the reactor. 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-26 November 2006 

Figure 2.18. UVT of Quartz that is 1 mm Thick 

at a Zero-degree Incidence Angle 

 

50

60

70

80

90

100

200 220 240 260 280 300 320 340 360 380 400

Wavelength (nm)

UV Transmittance (%)

Source: GE Quartz (2004a)

 The lamp sleeve assemblies are sealed to prevent water condensation within the sleeve 

and contain 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, UV light exposure can cause component 

deterioration and off-gassing of any impurities present in the quartz sleeve. Off-gassed materials 

can form UV-absorbing deposits on the inner surfaces of the lamp sleeve. Off-gassing and ozone 

formation are of greater concern 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. 

 Lamp sleeves are vulnerable to fractures. Fractures can occur from internal stress and 

external mechanical forces such as wiper jams, water hammer, resonant vibration, and impact by 

objects. Fractures may also occur if lamp sleeves are not handled properly when removed for 

manual cleaning. 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, pressures of negative 1.5 psig have been shown to adversely affect sleeve integrity 

(Dinkloh 2001). Section 4.1.4 discusses design considerations to reduce the potential for 

pressure-related incidents. If a lamp sleeve fractures while in service, water can enter the sleeve. 

The temperature difference between the hot lamp and cooler water may cause the lamp to break. 

Lamp breaks are undesirable due to the potential for mercury release. Appendix E discusses the 

lamp sleeve and lamp breaks. The tolerance level of the sleeve depends on the quality of the 

quartz and the sleeve’s thickness and length. 

 Lamp sleeves can also foul, decreasing the UVT of the lamp sleeve. 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. For example, some 

UV reactors using LP or LPHO lamps have sleeves made of Teflon® or Teflon-coated quartz. 

Teflon sleeves have a lower UVT, however, and their transmittance reduces faster than quartz 

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-27 November 2006 

For the Final LT2ESWTR 

sleeves without Teflon. Deposition of compounds in the water on the lamp sleeve surface cause 

fouling on external surfaces. A combination of thermal effects and photochemical processes 

causes the external fouling (Derrick and Blatchley 2005). Some compounds that may contribute 

to fouling are discussed in Section 2.5.1. External fouling can be removed by cleaning. 

 Solarization can also decrease the UVT of the sleeve. Solarization is photo-thermal 

damage to the quartz that increases light scattering and attenuation (Polymicro Technologies 

2004). Quartz solarizes if exposed to prolonged high energy radiation such as UV light. 

Resistance to this type of solarization increases as the purity of the quartz increases. Solarization 

on quartz can be reversed by heating the quartz to about 500 °C (GE Quartz 2004b). 

2.4.5 Cleaning Systems 

 

 UV reactor manufacturers have developed different approaches for cleaning lamp 

sleeves, depending on the application. These approaches include off-line chemical cleaning 

(OCC), on-line mechanical cleaning (OMC), and on-line mechanical-chemical cleaning (OMCC) 

methods. 

 For 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 solution the UV 

reactor manufacturer provides that is consistent with National Sanitation Foundation 

International/American National Standards Institute (NSF/ANSI) 60 Standard (Drinking Water 

Treatment Chemicals – Health Effects). The reactor is filled with the cleaning solution for a time 

sufficient to dissolve the substances fouling the sleeves (approximately 15 minutes), rinsed, and 

returned to operation. The entire cleaning cycle typically lasts approximately 3 hours. 

Alternatively, instead of rinsing the UV reactor with a cleaning solution, the sleeves can be 

removed and manually cleaned. Some LPHO UV equipment uses OCC systems. The frequency 

of OCC can range from monthly to yearly and depends on the site-specific water quality and 

degree and frequency of fouling. 

 OMC and OMCC systems use wipers that are attached to electric motors or pneumatic 

piston drives. In OMC systems, mechanical wipers may consist of stainless steel brush collars or 

Teflon rings that move along the lamp sleeve (Figure 2.19a). In OMCC systems, a collar filled 

with cleaning solution moves along the lamp sleeve (Figure 2.19b). The wiper physically 

removes fouling on the lamp sleeve surface while the cleaning solution within the collar 

dissolves fouling materials. 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-28 November 2006 

Figure 2.19. Examples of (a) Mechanical Wiper System and 

(b) Mechanical-chemical Wiper System 

 

a b

Source: (a) Courtesy of Infilco Degremont, (b) Courtesy of Trojan Technologies

 Draining the reactor is unnecessary when mechanical and mechanical-chemical wipers 

are used. Therefore, the reactor can remain on-line while the lamp sleeves are cleaned. MP 

equipment typically uses OMC or OMCC systems because the higher lamp temperatures can 

accelerate fouling under certain water qualities. The cleaning frequency for these OMC and 

OMCC systems ranges from 1 – 12 cycles per hour (Mackey et al. 2004). 

2.4.6 UV Sensors 

 UV sensors measure the UV intensity at a point within the UV reactor (Figure 2.20) and 

are used with measurements of flow rate and, potentially, UVT to indicate UV dose delivery. 

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 sensors may also 

respond to changes in UVT of the water being treated. UV sensors comprise optical components, 

a photodetector, an amplifier, its housing, and an electrical connector. The optical components 

may include monitoring windows, light pipes, diffusers, apertures, and filters. Monitoring 

windows and light pipes deliver light to the photodetector. Diffusers and apertures reduce the 

amount of UV light reaching the photodetector, thereby reducing the sensor degradation that UV 

light causes. Optical filters modify the spectral response such that the sensor responds only to 

germicidal wavelengths (i.e., 200 – 300 nm). Verification of sensor performance is described in 

Chapter 5. 

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-29 November 2006 

Figure 2.20. Example of a Dry UV Sensor 

Mounted on a UV Reactor 

Source: Courtesy of WEDECO

 UV sensors can be classified as dry or wet. Dry sensors monitor UV light through a 

monitoring window, whereas wet UV sensors directly contact the water flowing through the 

reactor. Monitoring windows and the wetted ends of wet sensors can foul over time and may 

require cleaning similar to lamp sleeves. 

2.4.7 UVT Analyzers 

 As stated previously, UVT is an important parameter in determining UV dose delivery. 

UVT analyzers are essential if UVT is part of the dose-monitoring strategy (see Section 2.4.9 for 

a discussion of dose monitoring approaches). If UVT is not part of the dose-monitoring strategy, 

analyzers may be provided for the purpose of monitoring water quality and helping to diagnose 

operational problems. Several commercial UV reactors use the measurement of UVT to calculate 

UV dose in the reactor and, if necessary, change lamp output or the number of energized lamps 

to maintain appropriate UV dose delivery. 

 Two types of commercial on-line UVT analyzers are available. One analyzer calculates 

UVT by measuring the UV intensity at various distances from a lamp. This type of analyzer is 

schematically displayed in Figure 2.21. In this analyzer, which is external to the UV reactor, a 

stream of water passes through a cavity containing an LP lamp with three UV sensors located at 

various distances from the lamp. The difference in sensor readings is used to calculate UVT. 

For the Final LT2ESWTR 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-30 November 2006 

Figure 2.21. Example UVT Analyzer Design 

 

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

Source: Courtesy of Severn Trent Services 

 The other type of on-line UVT analyzer is a flow-through spectrophotometer that uses a 

monochromatic UV light source at 253.7 nm. The instrument measures the A254 and calculates 

and displays UVT. 

2.4.8 Temperature Sensors 

 The energy input to UV reactors that is not converted to light (approximately 60 – 90 

percent, depending on lamp and ballast assembly) is wasted as heat. As it passes through a 

reactor, water can absorb the heat, keeping the reactor from overheating. Nevertheless, 

temperatures can increase when either of the following events occurs: 

• Water level in the reactor drops and lamps are exposed to air. 

• Water stops flowing in the reactor. 

 UV reactors can be equipped with temperature sensors that monitor the water temperature 

within the reactor. If the temperature is above the recommended operating range, the reactor will 

shut off to minimize the potential for the lamps to overheat. Because of the high operating 

temperature of MP lamps, dissipating heat can be more difficult than in reactors that use LP or 

LPHO lamps. As such, UV reactors with MP lamps typically have temperature sensors; however, 

reactors with LP or LPHO lamps may not because of the lower lamp operating temperature. 

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2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-31 November 2006 

For the Final LT2ESWTR 

 

2.4.9 UV Reactor Dose-Monitoring Strategy 

The dose-monitoring strategy establishes the operating parameters used to confirm UV 

dose delivery. This guidance manual focuses on UV reactors that use one of these two strategies, 

described below. Other existing dose-monitoring strategies or new strategies developed after this 

manual is published, however, may also be suitable for reactor operations provided they meet 

minimum regulatory requirements.6

 

1. UV Intensity Setpoint Approach. This approach relies on one or more “setpoints” 

for UV intensity that are established during validation testing to determine UV dose. 

During operations, the UV intensity as measured by the UV sensors must meet or 

exceed the setpoint(s) to ensure delivery of the required dose. Reactors must also be 

operated within validated operation conditions for flow rates and lamp status [40 CFR 

141.720(d)(2)]. In the UV Intensity Setpoint Approach, UVT does not need to be 

monitored separately. Instead, the intensity readings by the sensors account for 

changes in UVT. The operating strategy can be with either a single setpoint (one UV 

intensity setpoint is used for all validated flow rates) or a variable setpoint (the UV 

intensity setpoint is determined using a lookup table or equation for a range of flow 

rates). 

2. Calculated Dose Approach. This approach uses a dose monitoring equation to 

estimate the UV dose based on the flow rate, UV intensity, and UVT, as measured 

during reactor operations. The dose monitoring equation may be developed by the 

UV manufacturers using numerical methods; however, EPA recommends that water 

systems use an empirical dose monitoring equation developed through validation 

testing. During reactor operations, the UV reactor control system inputs the measured 

parameters into the dose monitoring equation to produce a calculated dose. The water 

system operator divides the calculated dose by the Validation Factor (see Chapter 5 

for more details on the Validation Factor) and compares the resulting value to the 

required dose for the target pathogen and log inactivation level. 

 The dose-monitoring strategies are described in more detail in Section 3.5.2. Any dose 

monitoring strategy must be evaluated during reactor validation (as described in Section 5.1), 

and the outputs measured during validation will be part of the monitoring requirements described 

in Section 6.4.1 [40 CFR 141.720(d)]. 

2.5 Water Quality Effects and Byproduct Formation 

 Constituents in the water to be treated can affect the performance of UV disinfection. 

Additionally, all disinfectants can form byproducts, and the goal of the overall disinfection 

process is to maximize disinfection while controlling byproduct formation. This section 

6 At a minimum, water systems must monitor flow rate, lamp status, and UV intensity plus any other parameters 

required by the State to show that a reactor is operating within validated conditions [40 CFR 141.720(d)(3)(i)]. 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-32 November 2006 

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discusses water quality characteristics affecting UV disinfection performance and the byproducts 

that may form during the UV disinfection process. 

2.5.1 Effect of Water Quality on UV Reactor Performance 

 

 UVT, particle content, upstream water treatment processes, constituents that foul reactor 

components, and algae affect the performance of UV reactors. These effects can be adequately 

addressed through proper design of the UV disinfection equipment. The design guidelines are 

discussed in Section 3.4. 

2.5.1.1 UVT 

 UVT has a strong effect on the dose delivery of a UV reactor. As UVT decreases, the 

intensity throughout the reactor decreases, which reduces the dose the reactor delivers. UV 

reactors are typically sized to deliver the required UV dose under specified UVT conditions for 

the application. Section 3.4.4.1 discusses approaches for selecting the UVT for UV facility 

design. 

 UV absorbers in typical source waters include soluble and particulate forms of 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). UV absorbance will vary 

over time due to changing concentrations of these compounds and seasonal effects—rainfall, 

lake stratification and destratification (turnover), and changes in biological activity of 

microorganisms within the water source. 

2.5.1.2 Particle Content 

 As described in Section 2.3.4, particle content can also affect UV disinfection 

performance. 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 

turnover, and spring runoff are some events that increase the concentration of particles. 

2.5.1.3 Upstream Water Treatment Processes 

 

 Unit processes and chemical addition upstream of UV reactors can significantly affect 

UV reactor performance because they can change the particle content and UVT of the water. 

Additionally, when UV disinfection is used in combination with another disinfectant, synergistic 

disinfection potentially may occur (i.e., the combination of disinfectants may be more effective 

than either disinfectant acting alone). 

 Water treatment processes upstream of the UV reactors can be operated to maximize 

UVT, thereby optimizing the design and costs of the UV reactor (Section 3.2.2). For example, 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-33 November 2006 

coagulation, flocculation, and sedimentation remove soluble and particulate material, and 

filtration removes particles. Activated carbon absorption also reduces soluble organics. 

 Adding oxidants (such as chlorine and ozone) can increase the UVT (APHA et al. 1998) 

by degrading natural organic matter, reducing soluble material, and precipitating metals. An 

example of the effect ozone has on decreasing UV absorbance is shown in Figure 2.22. Ozone is 

also a strong absorber of UV light, however, and will decrease the UVT if an ozone residual is 

present in significant concentrations in the water passing through a UV reactor. Quenching 

agents that do not absorb UV light (such as sodium bisulfite) can be used to destroy the ozone 

residual upstream of the UV reactors. Thiosulfate is not recommended as a quenching agent 

because it absorbs UV light and can decrease the UVT. 

Figure 2.22. Example Effect of Ozonation on UV Absorbance 

if Ozone is Quenched Prior to UV Disinfection 

W a ve le n g th (n m )

200 220 240 260 280 300 320

UV Absorbance (cm-1) 0.0

0.1

0.2

0.3

0.4

0.5

P re o zo n a te d U V In flu e n t

(n o d e te cta b le o zo n e re sid u a l)

U V In flu e n t - N o o zo n e 

Source: Malley (2002)

 In addition to ozone, other chemicals used in water treatment such as ferric iron and 

permanganate also absorb UV light and can decrease UVT. Table 2.5 lists the UV absorption 

coefficients at 254 nm of several common water treatment chemicals along with their “impact 

threshold concentration,” which is the concentration that will decrease the UVT at 254 nm from 

91 to 90 percent (Bolton et al. 2001). Note that these data are only for 254 nm, and the effect 

these chemicals have on UVT may be significantly different at other wavelengths generated by 

MP or other polychromatic lamps. The following chemicals were also evaluated in the same 

study (Bolton et al. 2001) and were found to have no significant absorbance: ammonia (NH3), 

ammonium ion (NH4

+

), calcium ion (Ca2+), hydroxide ion (OH-), magnesium ion (Mg2+), 

manganese ion (Mn2+), phosphate species, and sulfate ion (SO4

2-). 

 UV disinfection is often used in combination with other disinfectants, and the interaction 

of the disinfectants can affect the overall inactivation achieved. Research shows that applying 

ozone prior to UV disinfection is beneficial: the ozone increases the UVT, while the UV 

disinfection provides Cryptosporidium inactivation (Malley et al. 2003, Crozes et al. 2003). 

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2. Overview of UV Disinfection 

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Whether the effects of multiple disinfectants are synergistic (i.e., more inactivation observed 

when processes are used in combination than is expected from the sum of 

Table 2.5. UV Absorbance Characteristics of Common 

 Water Treatment Chemicals 

Compound1

Molar Absorption 

Coefficient 

(M-1 cm-1) 

Impact Threshold 

Concentration2

(mg/L) 

Ozone (O3) (aqueous) 3,250 0.071 

Ferric iron (Fe3+) 4,716 0.057 

Permanganate (MnO4

-

) 657 0.91 

Thiosulfate (S2O3

2-) 201 2.7 

Hypochlorite (ClO-) 29.5 8.4 

Hydrogen peroxide (H2O2) 18.7 8.7 

Ferrous iron (Fe2+) 28 9.6 

Sulfite (SO3

2-) 16.5 23 

Zinc (Zn2+) 1.7 187 

1 The following chemicals were also evaluated in the same study (Bolton et al. 2001) 

and were found to have no significant absorbance: ammonia (NH3), ammonium ion 

(NH4

+

), calcium ion (Ca2+), hydroxide ion (OH-), magnesium ion (Mg2+), manganese 

ion (Mn2+), phosphate species, and sulfate ion (SO4

2-) 2 Concentration in mg/L resulting in UVT decrease from 91 % to 90 % (A254 increase 

from 0.041 cm-1 to 0.046 cm-1) 

Source: Adapted from Bolton et al. (2001) 

the disinfectants acting alone) is currently under debate. Two studies reported synergistic effects 

when using UV disinfection and free chlorine, monochloramine, or chlorine dioxide (Ballester et 

al. 2003, Lotierzo et al. 2003), while others did not observe synergism (Coronell et al. 2003, 

Oppenheimer et al. 2003). The importance of the sequence of the disinfectants is also a subject of 

debate. Ballester et al. (2003) obtained improved disinfection with UV disinfection followed by 

monochloramine addition than with chloramination followed by UV disinfection, while the 

sequence of disinfectants did not affect the disinfection effectiveness in the study by Lotierzo et 

al. (2003). 

2.5.1.4 Fouling Potential 

 Compounds in the water can foul the external surfaces of the lamp sleeves and other 

wetted components (e.g., monitoring windows of UV sensors) of UV reactors. Fouling on the 

lamp sleeves reduces the transmittance of UV light through the sleeve into the water, thereby 

reducing the output from the UV lamp into the water. Also, fouling on the monitoring windows 

affects measured UV intensity and dose monitoring. Sleeve fouling can be accounted for with the 

fouling/aging factor (described in Section 3.4.5) in the design of the UV facility. 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-35 November 2006 

For the Final LT2ESWTR 

 Hardness (as CaCO3), alkalinity, temperature, ion concentration, oxidation reduction 

potential (ORP), and pH all influence the rate of fouling and, subsequently, the necessary 

frequency of sleeve cleaning. Fouling can occur for the following reasons: 

• Compounds for which the solubility decreases as temperature increases may 

precipitate [e.g., CaCO3, CaSO4, MgCO3, MgSO4, FePO4, FeCO3, Al2(SO4)3]. These 

compounds will foul MP lamps faster than LP or LPHO lamps because MP lamps 

operate at higher temperatures. 

• Photochemical reactions that are independent of sleeve temperature may cause sleeve 

fouling (Derrick 2005). 

• Compounds with low solubility may precipitate [e.g., Fe(OH)3, Al(OH)3]. 

• Particles may deposit on the lamp sleeve surface due to gravity settling and 

turbulence-induced collisions (Lin et al. 1999a). 

• Organic fouling can occur when a reactor is left off and full of water for an extended 

period of time (Toivanen 2000). 

• Inorganic constituents can oxidize and precipitate (Wait et al. 2005). 

 Fouling rate kinetics has been reported as independent of 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. 

Pilot studies lasting 5 – 12 months using UV reactors with LP, LPHO and MP lamps 

found that standard cleaning protocols and wiper frequencies (1 – 12 cleaning cycles per hour) 

were sufficient to overcome the effect of sleeve fouling with water that had total and calcium 

hardness levels less than 140 milligrams per liter (mg/L) and iron less than 0.1 mg/L (Mackey et 

al. 2001, Mackey et al. 2004). 

Inorganic fouling is a complex process, however, and is not related only to hardness and 

iron concentrations. The solubility of inorganic constituents depends on whether they are in an 

oxidized or reduced state, which can be affected by both the ORP and pH of the water (Wait et 

al. 2005). ORP is a measurement of the water’s ability to oxidize or reduce constituents in the 

water. Both pH and ORP are needed to predict the oxidation state of an inorganic constituent. 

Studies have found that fouling rates increase as ORP increases (Collins and Malley 2005, Wait 

et al. 2005, Derrick 2005). In some waters with high ORP, however, fouling rates can be 

minimized if the iron and manganese are removed through oxidation, precipitation, and filtration 

(Wait et al. 2005, Derrick 2005, Jeffcoat 2005). Although ORP can provide valuable 

information, measuring it can be challenging and may not be possible in all instances. 

Ultimately, the fouling potential is difficult to predict, but standard cleaning equipment 

can remove fouling and may need to be included. Also, pilot-scale or demonstration-scale testing 

can determine the fouling tendencies and cleaning regime if the PWS is concerned about fouling. 
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-36 November 2006 

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.5.1.5 Algal Occurrence and Growth 

The presence of algae in the water being treated may reduce UVT and interfere with the 

s 

.5.2 Chlorine Reduction through UV Reactors 

is applied to water with a free or total chlorine residual, some 

reducti the 

ter 

Figure 2.23. Example Effect of UV Disinfection on 

2

UV disinfection process. Algae also may grow upstream or downstream of UV reactors, which 

has been observed in MP pilot studies (Mackey et al. 2004). Visible light emitted from the lamp

is transmitted through water farther than germicidal wavelengths. Algal growth depends on the 

concentration of nutrients in the water, hydraulics (i.e., dead spaces), and the amount of visible 

light transmitted beyond the reactor. 

2

 

When UV disinfection 

on of the residual may occur. The reduction in free chlorine residual is proportional to 

delivered dose and independent of flow rate (Brodkorb and Richards 2004). The reduction in 

total chlorine residual is also proportional to the delivered dose (Wilczak and Lai, 2006). The 

reduction in chlorine residual further depends on the chlorine species, UV light source, and wa

quality characteristics (Örmeci et al. 2005, Venkatesan et al. 2003). An example of the effect of 

UV light on the free chlorine residual is shown in Figure 2.23. In other evaluations, a loss of 

about 0.3 mg/L of the free chlorine residual was observed in a WTP at a dose between 80 and

120 mJ/cm2 (Kubik 2005), and a loss of 0.2 mg/L of the total chlorine residual was observed in

bench-scale testing at doses up to 40 mJ/cm2

 (Wilczak and Lai 2006). 

Free Chlorine Residual Loss 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150 200

Calculated MS-2 Reduction Equivalent Dose 

(mJ/cm2

)

Free Chlorine Loss (mg/L) 

1.72 mgd

3.33 mgd

4.83 mgd

5.68 mgd

7.00 mgd

7.93 mgd

Source: Brodkorb and Richards (2004)
2. Overview of UV Disinfection 

UV Disinfection Guidance Manual 2-37 November 2006 

For the Final LT2ESWTR 

Studies indicate that UV disinfection at UV doses up to 200 mJ/cm2

 do not change the 

hen a 

studies have shown low-level formation of non-regulated DBPs (e.g., aldehydes) 

as a res

The conversion of nitrate to nitrite is possible with MP lamps that emit at wavelengths 

uld 

d. 

2.5.3 Byproducts from UV Disinfection 

pH, turbidity, dissolved organic carbon level, UVT, color, nitrate, nitrite, bromide, iron, or 

manganese of the water being treated (Malley et al. 1996). Byproducts from UV disinfection, 

however, can arise either directly through photochemical reactions or indirectly through 

reactions with products of photochemical reactions. Photochemical reactions occur only w

chemical species absorbs UV light and the resulting excited state reacts to form a new species. 

The resulting concentration of new species depends on the concentration of the reactants and the 

UV dose. In drinking water, research on potential byproducts of UV disinfection has focused on 

the effect of UV light on the formation of halogenated DBPs after subsequent chlorination, the 

transformation of organic material to more degradable components, and on the potential 

formation of other DBPs (e.g., biodegradable compounds, nitrite, mutagenicity, and other

byproducts). 

2.5.3.1 Trihalomethanes, Haloacetic Acids, and Total Organic Halides 

Trihalomethanes (THMs) and haloacetic acids (HAAs) are two categories of halogenated 

DBPs that EPA currently regulates. UV light at doses less than 400 mJ/cm2

 has not been found to 

significantly affect the formation of THMs or HAAs upon subsequent chlorination (Malley et al. 

1996, Kashinkunti et al. 2003, Zheng et al. 1999, Liu et al. 2002, Venkatesan et al. 2003). 

2.5.3.2 Biodegradable Compounds 

 

Several 

ult of applying UV light at doses greater than 400 mJ/cm2

 to wastewater and raw drinking 

water sources (Liu et al. 2002, Venkatesan et al. 2003). At the doses typical for UV disinfection 

in drinking water (< 140 mJ/cm2

), however, no significant change was observed (Kashinkunti et 

al. 2003). UV disinfection has not been found to significantly increase the assimilable organic 

carbon (AOC) of drinking water at UV doses ranging from 18 – 250 mJ/cm2

 (Kruithof and van

der Leer 1990, Akhlaq et al. 1990, Malley et al. 1996). 

2.5.3.3 Nitrite 

below 225 nm [von Sonntag and Schuchmann (1992), Mack and Bolton (1999), IJpelaar et al. 

(2003), Peldszus et al. (2004)]. Sharpless and Linden (2001) reported a conversion rate from 

nitrate to nitrite of approximately 1 percent. Therefore, the nitrate-to-nitrite conversion is 

unlikely to be a significant issue for PWSs under current regulations. The nitrate levels wo

have to be higher than the nitrate MCL of 10 mg/L for the nitrite MCL of 1 mg/L to be exceede
2. Overview of UV Disinfection 

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This page intentionally left blank. 
3. Planning Analyses for UV Facilities 

This chapter provides information on the elements that should be addressed during the 

UV disinfection planning or preliminary design phase. 

Chapter 3 covers: 

3.1 UV Disinfection Goals 

3.2 Evaluating Integration of UV Disinfection into the Treatment Process 

3.3 Identifying Potential Locations for UV Facilities 

3.4 Defining Key Design Parameters 

3.5 Evaluating UV Reactors, Dose Monitoring Strategy, and Operational 

Approach 

3.6 Assessing UV Equipment Validation Issues 

3.7 Assessing Head Loss Constraints 

3.8 Estimating UV Facility Footprint 

3.9 Preparing Preliminary Costs and Selecting the UV Facility Option 

3.10 Reporting to the State 

The planning for any UV facility is site-specific. Given the wide range of possible 

treatment scenarios, a guidance document such as this one cannot address or anticipate all 

possible treatment conditions. The information presented here should be used within the context 

of sound engineering judgment and applied appropriately on a case-by-case basis. Appendix F 

presents case studies that illustrate how various public water systems (PWSs) have implemented 

UV disinfection in their water systems. Additionally, this manual was written with the 

understanding that UV technology will continue to expand and evolve, so the information 

presented is current only as of the publication date. Furthermore, unless otherwise stated, 

throughout Chapter 3 the water to be disinfected is assumed to be from surface water systems

[(i.e., filtered water, an unfiltered source water, or groundwater under the direct influence 

(GWUDI)], meeting applicable regulatory requirements that pre-date the Long Term 2 Enhanced 

Surface Water Treatment Rule (LT2ESWTR). 

The process of planning and designing a UV facility is presented in Figure 3.1. Once the 

design parameters are defined and the implementation issues are identified, they are incorporated

into the detailed design phase, which is discussed in Chapter 4. 

3.1 UV Disinfection Goals 

 

The first step in planning a UV disinfection facility is to define the goals for the facility 

as part of a comprehensive disinfection strategy for the entire treatment process. Additionally, 

the target pathogen(s), target log-inactivation, and corresponding required UV dose should be 

identified. 

UV Disinfection Guidance Manual 3-1 November 2006

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3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-2 November 2006 

For the Final LT2ESWTR 

Figure 3.1. Example Flowchart for Planning UV Facilities

Identify 

potential 

UV facility 

location(s) 

Section 3.3 

Evaluate potential UV 

equipment and dosemonitoring strategy 

Section 3.5

Evaluate UV reactor 

validation issues 

Section 3.6 

Design UV facility 

Chapter 4 

Estimate UV facility 

footprint for all 

potential locations and 

UV reactor 

configurations 

Section 3.8 

Define the key design 

parameters 

• Water quality 

• Fouling/aging factor 

• Flow rate 

• Power quality 

Section 3.4 

Define the goals of 

the UV facility and 

identify the target 

microorganism(s) 

and loginactivation 

Section 3.1 

Report to 

Primacy 

Agency 

Section 3.10

Determine head loss 

constraints 

Section 3.7

Evaluate 

integration of UV 

disinfection into 

the treatment 

process 

Section 3.2 

Calculate costs for 

installation options, 

compare options, and 

select facility location

Section 3.9 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-3 November 2006 

For the Final LT2ESWTR 

 

• Comprehensive Disinfection Strategy: A comprehensive disinfection strategy 

provides multiple barriers to reduce microbial risk, while minimizing disinfectant 

byproduct (DBP) formation. UV disinfection is a tool that can contribute to a 

comprehensive disinfection strategy by providing a cost-effective method of 

inactivating pathogens that are more resistant to traditional disinfection methods. 

Also, UV disinfection can replace chemicals for primary disinfection of chlorineresistant pathogens (e.g., Cryptosporidium and Giardia), thereby reducing DBP 

formation. Note that PWSs that plan to significantly change their disinfection process, 

including adding UV disinfection, must prepare a disinfection benchmark1

 (40 CFR 

141.708) and consult with the state before making any changes. Further, PWSs must 

continue to provide 2-log Cryptosporidium removal by meeting filtered water 

turbidity requirements (40 CFR 141.173 for PWSs serving at least 10,000 people and 

40 CFR 141.551 for PWSs serving fewer than 10,000 people) unless they meet the 

filtration avoidance criteria. 

• Target Pathogen and Log Inactivation: The required UV doses for 

Cryptosporidium and Giardia inactivation are lower than those needed to inactivate 

viruses. (See Table 1.4.) Accordingly, the capital and operational costs for 

inactivating Cryptosporidium and Giardia should be lower than for viruses. One 

study estimated capital costs for Cryptosporidium and Giardia inactivation by UV 

disinfection on a log removal basis to be about half the cost associated with the UV 

inactivation of viruses (Cotton et al. 2002). Additionally, most viruses can be easily 

inactivated with chlorine so UV disinfection for virus inactivation may not be 

necessary. The target log inactivation also should be considered because higher target 

inactivation requires higher UV doses that will affect the design and cost of the UV 

facility. Therefore, the target microorganism(s) and their log-inactivation level should 

be determined early in the planning process. 

3.2 Evaluating Integration of UV Disinfection into the Treatment Process 

When installed, UV disinfection will typically be one of several treatment processes to 

help meet water quality goals. Accordingly, UV disinfection should be evaluated in the context 

of the complete treatment process, and the impacts on UV disinfection on other treatment 

processes should be considered. These issues are summarized in this section. 

3.2.1 UV Disinfection Effects on Treatment 

Typically, UV disinfection cannot entirely replace chemical disinfectants used in the 

treatment process. Some of the reasons are listed below. 

• Surface water systems must maintain a disinfectant residual in the distribution system 

(40 CFR 141.72). 

1

 More information on completing a disinfection benchmark can be found in Disinfection Profiling and 

Benchmarking Guidance Manual (EPA 1999). 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-4 November 2006 

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• UV disinfection is not as efficient in inactivating viruses as more traditional, chlorinebased disinfection processes. 

• Chemical disinfectants may also be needed to oxidize other constituents present in the 

water (e.g., iron, manganese, or taste- and odor-causing compounds). 

• Some water systems apply chlorine to reduce algal growth in sedimentation basins. 

Consequently, some level of chlorine-based disinfectant (chlorine or chloramines) usually 

will be needed even when UV disinfection is implemented. Therefore, any reduction in chlorinebased disinfectants should be evaluated in the context of other water quality and treatment goals. 

When UV disinfection is applied to water having a chlorine residual, some chlorine 

residual reduction may occur, depending on the UV dose, chlorine species, UV light source, and 

water quality characteristics (Brodkorb and Richards 2004, Örmeci et al. 2005, Venkatesan et al. 

2003). Brodkorb and Richards (2004) reported chlorine residual reduction between 0.1 and 

0.7 milligrams per liter (mg/L) at a wide range of UV doses (described in Section 2.5.2). 

Significant chlorine reduction could occur inadvertently if the UV equipment cannot provide 

enough power modulation capacity and actually operates at much higher doses than designed. 

Two options are available to avoid chlorine reduction by UV disinfection: 

1. Consider moving the chlorine addition point to after the UV facility if possible, 

especially when targeting viruses (because their required UV doses are higher). 

2. Procure the UV equipment that has adequate power modulation to prevent overdosing 

and subsequent chlorine reduction. 

In addition, UV disinfection of water having a chlorine residual, which results in a higher 

oxidation-reduction potential (ORP), could result in sleeve fouling (Section 2.5.1.4) if iron or 

manganese are present even at low levels and a proper cleaning system is not in place (Malley et 

al. 2001). Several studies have shown that fouling occurs at iron levels below the secondary 

maximum contaminant level (SMCL) when the water has a high oxidation-reduction potential 

(ORP) (Collins and Malley 2005, Derrick 2005, Wait et al. 2005). Again, moving the point of 

chlorination to after the UV facility can possibly reduce sleeve fouling (Section 3.4.4.2). 

Alternatively, oxidation and removal of iron and manganese (e.g., by adding potassium 

permanganate upstream of the sedimentation basin) reduces the fouling potential. 

3.2.2 Upstream Treatment Process Effect on UV Disinfection 

 

Water treatment processes upstream of the UV reactors can be operated to maximize the 

ultraviolet transmittance (UVT), thereby optimizing the design and costs of the UV equipment 

(Section 3.4.4.1). For example, coagulation, flocculation, and sedimentation remove soluble and 

particulate material, and optimizing coagulation for organics removal will increase the UVT, 

which could reduce the UV facility costs. Also, upstream chemicals may affect UV disinfection 

performance as described in Sections 2.5.1.3 and 3.4.4.1. 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-5 November 2006 

3.3 Identifying Potential Locations for UV Facilities 

 

The UV dose tables (see Table 1.4) in the LT2ESWTR apply to post-filter applications of 

UV disinfection in filtration plants and to unfiltered systems that meet filtration avoidance 

criteria. In general, installing UV disinfection prior to filtration in conventional water treatment 

plants (WTPs) is not recommended because of the potential particle interference in raw and 

settled waters. As such, only post-filter locations are discussed for filtered systems in this 

section. 

After the potential locations are identified, design criteria, hydraulics, validation issues, 

and footprint estimations should be evaluated at each location to identify which location is most 

feasible for the UV facility. These evaluations are described in subsequent sections. 

3.3.1 Installation Locations for Filtered Systems 

 

 In conventional WTPs, the three most common installation locations are downstream of 

the combined filter effluent (upstream of the clearwell), on the individual filter effluent piping 

(upstream of the clearwell), and downstream of the clearwell. 

3.3.1.1 Combined Filter Effluent Installation (Upstream of the Clearwell) 

A combined filter effluent installation is defined as the application of UV disinfection to 

the filtered effluent after the effluent from individual filters has been combined (as opposed to 

applying UV disinfection to the individual filter effluents) and ahead of the clearwell, as shown 

in Figure 3.2. For retrofits on existing WTPs, these installations are usually housed in a separate 

building. 

Figure 3.2. Schematic for UV Facility Upstream of the 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

This type of design and installation has several advantages: 

• The UV reactor operation is largely independent of the operation of individual filters, 

which provides flexibility for design and operation. 

 

For the Final LT2ESWTR 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-6 November 2006 

• If the entire UV facility failed, a WTP can continue to disinfect by adding a chemical 

disinfectant to the clearwell. (Note that backup chemical disinfection will likely not 

provide Cryptosporidium inactivation.) 

• Surge and pressure fluctuations typically are not a concern for this installation 

location unless membrane filtration, pressure filters, or intermediate booster pumps 

are used. 

• Because this type of UV facility is typically constructed in a new building, there may 

be greater flexibility to maintain the recommended inlet and outlet hydraulic 

conditions for the UV reactors (Section 3.6.2). 

The primary disadvantages of this type of installation are: 

• An additional building and space may be necessary. 

• The piping and fittings may result in greater head loss than alternative configurations, 

which may result in the need for intermediate booster pumps. 

3.3.1.2 Individual Filter Effluent Piping Installation 

 

Individual filter effluent piping installations are defined as UV reactors installed on each 

filter effluent pipe (Figure 3.3). This type of installation is typically located within an existing 

filter gallery. 

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

For the Final LT2ESWTR 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-7 November 2006 

For the Final LT2ESWTR 

The primary advantages of this type of installation are: 

• A new building is not necessary, which will decrease construction costs. 

• The hydraulic effect of the UV facility is less because the only additional head loss is 

from the UV reactors (most necessary valves and appurtenances are already present in 

the filter gallery). 

• If the UV reactors fail, a WTP can continue to disinfect by adding a chemical 

disinfectant to the clearwell. (Note that backup chemical disinfection likely will not 

provide Cryptosporidium inactivation.) 

This installation location, however, has several disadvantages: 

 

• Many filter galleries have insufficient space within existing effluent piping to 

accommodate the UV reactors. 

• Sufficient space is needed in the filter gallery or nearby for the control panels and 

electrical equipment. 

• Access to existing equipment may be impeded by the UV reactor, and access to UV 

reactor components for maintenance may be more restricted than for a combined filter 

effluent installation. 

• 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. This situation would necessitate improvements to the heating, ventilating, 

and air conditioning (HVAC) system. 

• The existing piping may constrain how the UV reactor is validated because of the 

unique inlet and outlet conditions that may be present (Section 3.6.2). 

• Surge and pressure fluctuations would need to be investigated if UV reactors are 

installed directly downstream of pressure filters or membrane filtration because water 

hammer can damage lamp sleeves. 

Additionally, the individual filter effluent installation may also complicate treatment 

plant operations and limit operational flexibility, as described below: 

• In general, this option increases the number of UV reactors required compared to a 

combined filter installation because the number of filters dictates the number of UV 

reactors. More reactors may increase operation and maintenance costs. 

• The head loss of the UV reactors may affect the operation of the filters and the 

clearwell. 

• The operations of the UV reactor and the filter are closely related. If one reactor or 

one filter is off-line, the other process may not be operable. 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-8 November 2006 

• When a UV reactor goes off-line, the corresponding filter also should be taken offline to minimize off-specification operation. 

• The filter backwash cycle can complicate UV reactor operation. 

– Lamps that remain energized during a backwash may require cooling water 

because some lamps should not be energized in stagnant water. The designer 

should consult the UV manufacturer to determine whether the UV reactor 

requires cooling water during start-up. 

 

– If a UV reactor is off-line during a backwash, the UV reactor may be operating 

outside of its validated limits (i.e., off-specification—discussed in Section 3.4.1) 

if water is being treated during lamp warm-up. If the piping configuration permits, 

energizing the UV reactors during the filter-to-waste period and having the filterto-waste water pass through the reactors during the warm-up period would cool 

the lamps and reduce the volume of the off-specification water. 

 

3.3.1.3 UV Disinfection Downstream of the Clearwell 

 

 A WTP may be able to locate the UV facility downstream of the clearwell, either 

upstream or downstream of the high-service pumps (HSPs), as shown in Figure 3.4. In many 

WTPs, the HSPs pump water directly from the clearwell, which limits space and the availability 

of suitable piping for installing the UV facility upstream of the HSPs. Installation downstream of 

the HSPs may provide greater space and flexibility in locating the UV facility. 

Figure 3.4. UV Disinfection Downstream of High Service Pumps 

Source 

Water Rapid 

Mix

Flocculation 

Sedimentation 

Basin

Filters UV 

Disinfection

Clearwell

To 

Distribution 

System

Source HSPs

Water Rapid 

Mix

Flocculation 

Sedimentation 

Basin

Filters UV 

Disinfection

Clearwell

To 

Distribution 

System

Source HSPs

Water Rapid 

Mix

Flocculation 

Sedimentation 

Basin

Filters UV 

Disinfection

Clearwell

To 

Distribution 

System

HSPs

 The primary advantage of this type of installation is that UV reactor installation is 

possible even if the space or available head is insufficient to allow installation of the UV 

equipment between the filters and the clearwell. However, these options have significant 

disadvantages: 

• UV facilities located downstream of the clearwell may experience greater fluctuations 

in flow rate because the flow rate is more closely related to demand changes. 

For the Final LT2ESWTR 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-9 November 2006 

For the Final LT2ESWTR 

Accommodating flow rate fluctuations may necessitate increasing the UV reactor size 

or number of UV reactors. 

• Post-clearwell installation locations are more prone to water hammer because of their 

proximity to the HSPs and subsequent high pressures, and water hammer could 

damage lamp sleeves and the lamps. Hydropneumatic tanks or pressure-relief valves 

may be needed to avoid water hammer. 

• In the event of a lamp break, post-clearwell installations may have less ability to 

contain mercury and quartz resulting from the break in a low-velocity collection area 

(depending on the distribution system configuration). 

• In post-HSP installations, the water is at distribution system pressure. The UV reactor 

housing may need reinforcement to accommodate high pressure, which would 

increase the cost of the UV reactors. 

• A UV facility located after the HSPs will reduce the discharge pressure to the 

distribution system, and a UV facility 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 affected at these two locations unless the HSPs are 

upgraded to account for the UV facility hydraulic needs. 

• When UV disinfection is applied to water with a free or total chlorine residual, some 

reduction of the residual may occur, which may necessitate increasing the chlorine 

dose in the clearwell or moving the chlorination point to downstream of the UV 

facility. 

3.3.2 Unfiltered System Installation Locations 

In an unfiltered system, UV facilities can be located either before or after a storage 

reservoir. If the storage is covered, UV disinfection facilities can be installed in either location. If 

the storage reservoir is uncovered, however, the PWS is subject to the uncovered reservoir 

requirements of the LT2ESWTR and as such should install UV disinfection on the discharge side 

of the reservoir to provide the necessary treatment. Most unfiltered systems flow to the 

distribution system by gravity; however, water hammer may still be a concern if the facility is 

located near HSPs (if applicable). This installation location is similar to installations downstream 

of the clearwell, and as such, the items described in Section 3.3.1.3 also apply to this location. 

More debris may be present in the influent to UV reactors in unfiltered applications than 

in post-filter applications. Debris entering the UV reactor with sufficient momentum can cause 

the lamp and sleeve to break. The mass and size of an object that might cause damage are 

installation-specific and depend on UV reactor configuration (e.g., horizontal versus vertical 

reactor orientation) and water velocity through the reactor. Methods of addressing debris are 

described in Section 4.5.1, and additional information on lamp breakage is presented in 

Appendix E. 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-10 November 2006 

For the Final LT2ESWTR 

3.3.3 Groundwater System Installation Locations 

 For groundwater applications of UV disinfection, UV facilities may be installed either at 

each well in a production system or at a centralized facility. If installed at or near well pumps, 

the hydraulic and water hammer considerations described in Section 3.3.1.3 will also apply. An 

engineering cost analysis can be conducted to compare centralized versus wellhead UV 

disinfection treatment, as well as any other treatment needs, such as removing iron, manganese, 

or sulfides. 

3.3.4 Uncovered Reservoir Installation Locations 

The LT2ESWTR requires PWSs with uncovered finished water storage facilities to either 

cover the storage facility or treat the discharge of the storage facility that is distributed to 

consumers to achieve inactivation and/or removal of 4-log virus, 3-log Giardia, and 2-log 

Cryptosporidium [40 CFR 141.714(c)]. When applying UV disinfection to uncovered reservoirs, 

the UV facility should be on the outlet of the uncovered reservoir. In some cases, the inlet and 

outlet to the uncovered reservoir is the same pipe, and the UV facility should be designed so it 

operates when the water flows from the uncovered reservoir to the customer. Water from most 

uncovered reservoirs flows by gravity to the distribution system; however, water hammer may 

still be a concern if the UV reactors are located close to HSPs. As such, the items described in 

Section 3.3.1.3 also apply to this location. 

3.4 Defining Key Design Parameters 

Off-specification requirements (see Section 3.4.1 below), target pathogen inactivation, 

flow, water quality, the fouling/aging factor, and power quality affect the sizing of the UV 

reactors and associated support facilities. Specifically, UV manufacturers use the design flow, 

design UVT, the range of UVT expected, and the fouling/aging factor to determine the 

appropriate number of UV reactors to achieve the required UV dose. 

Pilot- and demonstration-scale testing for UV disinfection systems can be helpful in 

determining key design parameters but typically are unnecessary. For example, pilot- or 

demonstration-scale testing may be warranted when bench-scale analysis cannot determine the 

design criteria (e.g., prediction of fouling/aging factor in waters with high inorganic 

constituents). This section also describes some pilot- or demonstration-scale testing that can be 

used to determine key design criteria if deemed necessary by the PWS or design engineer. 

3.4.1 Off-specification Requirements 

 

The LT2ESWTR requires validation of UV reactors to demonstrate that they achieve the 

required UV dose [40 CFR 141.720(d)]. Validation testing establishes the conditions under 

which the UV reactors must be operated to ensure the required UV dose delivery [40 CFR 

141.720(d)]. 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-11 November 2006 

For the Final LT2ESWTR 

 

Receiving log inactivation credit to meet the treatment requirement of the LT2ESWTR 

requires 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 141.720(d)(3)]. In other words, the 

UV reactors cannot be operated outside of their validated limits for more than 5 percent of the 

volume of water that is treated each month. Operating outside of the validated limits is defined in 

this manual as off-specification operation. 

Determining the appropriate design criteria related to flow, water quality (UVT and 

fouling), the fouling/aging factor, and power quality is important to comply with LT2ESWTR 

off-specification requirements. These design criteria also define the conditions under which the 

UV reactors must be validated and then operated. If the design parameters are not sufficiently 

conservative, the UV reactors may often operate off-specification and be out of compliance. 

The UV reactors are off-specification when any of the following conditions occur: 

 

• The flow rate is higher than the validated range. 

• The UVT is lower than the validated range [if the Calculated Dose Approach is used 

(see Section 3.5.2)]. 

• The UV intensity is below the validated setpoint [if the UV Intensity Setpoint 

Approach is used (see Section 3.5.2)]. 

• The validated dose 2 is less than the required UV dose at a given flow rate [if the 

Calculated Dose Approach is used (see Section 3.5.2)]. 

• One or more lamps are not energized unless the UV reactor was validated with these 

lamps off. 

• All UV lamps are off because of a power interruption or power quality problem, and 

water is flowing through the reactors. 

• One or more UV sensors are not within calibration criteria, and the remedial actions 

are not taken. (See Section 6.4.1.1). 

• A UVT analyzer is needed for the dose-monitoring strategy; the UVT analyzer is out 

of calibration; and a corrective action was not taken. (See Section 6.4.1.2.) 

• The UV equipment includes installed or replaced components (or both) that are not

equal to or better than the components used during validation testing unless the UV 

equipment was re-validated. (See Section 5.13.) 

2

 For the purposes of this manual, the “Validated Dose” is the UV dose in units of mJ/cm2 delivered by the UV 

reactor as determined through validation testing. The validated dose is compared to the required dose to determine 

log inactivation credit. For the Calculated Dose Approach, the validated dose equals the calculated dose from the 

dose-monitoring equation, divided by the Validation Factor. The Validation Factor accounts for key uncertainties 

and biases resulting from validation testing. 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-12 November 2006 

For the Final LT2ESWTR 

3.4.2 Target Pathogen Inactivation and Required UV Dose 

 

As described in Section 3.1, the UV facility design criteria should include the target 

pathogen, log inactivation level, and corresponding required UV dose. The required UV dose 

(DReq) for the various pathogens and inactivation are shown in Table 1.4; however, the PWS may 

consider increasing the required dose beyond those listed in Table 1.4 by 10 to 20 percent to 

provide flexibility and conservatism. Similar approaches are commonly used by many PWSs 

with chlorine disinfection where they provide higher chlorine residuals and contact times (CT) 

than required. 

3.4.3 Design Flow Rate 

 

The UV facility design criteria should identify the average, maximum, and minimum 

flow rates that the UV reactors will experience. Methods for determining the design flow rate for 

the installation locations described previously are listed in Table 3.1. 

Table 3.1. Potential Method to Determine Design Flow 

Installation Location Design Flow Basis 

Combined Filter Effluent Combined rated capacity of all duty filters1

Individual Filter Effluent Rated design flow for individual filter 

Downstream of the Clearwell Rated capacity of the HSP station 

Unfiltered Application Rated capacity of the treatment facility 

Groundwater Application Rated capacity of the well pump or well field 

Uncovered Reservoir Application Maximum reservoir outflow 

1 Does not include redundant filters 

3.4.4 Water Quality 

 

As highlighted in Chapter 2, the following water quality parameters and issues affect UV 

dose delivery and should be considered in UV facility planning: 

• UVT at 254 nanometers (nm) 

• UV transmittance scan from 200 – 300 nm (i.e., germicidal range) 

• Sleeve and UV sensor window fouling, including 

– Calcium 

– Alkalinity 

– Hardness 

– Iron 

– Manganese 

– pH 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-13 November 2006 

– Lamp temperature 

– ORP 

• Particle content and algae (unfiltered and uncovered reservoir applications) 

Water quality data should be collected from locations that are representative of the 

potential UV facility location(s). The duration of sampling, numbers of samples collected, and 

data analyses used to evaluate water quality for UV disinfection are similar to the approaches 

used for other water treatment technologies. The data collection should capture typical water 

quality and any water quality variation due to storm events, reservoir turnover, seasonal changes, 

source water blends, and variations in upstream treatment. The data collection frequency should 

be based on flow rate 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 to be collected and the data analysis should be left to the discretion of the 

PWS and the design engineer based on experience and professional judgment. 

Water quality information should be communicated to the UV manufacturers, so they can 

determine the applicable UV reactors for the target pathogen inactivation. This section provides 

more details on the data collection and analysis recommendations. 

3.4.4.1 UVT and UVT Scans 

 

 The most important water quality characteristic affecting UV facility design is UVT3,4

because the UVT of the water directly influences UV dose delivery, as discussed in Chapter 2. 

Overly conservative design UVT values (i.e., low UVT) can result in over-design and increased 

capital costs. Conversely, inappropriately high design UVT values can result in frequent UV 

reactor off-specification operation, which could violate LT2ESWTR requirements. 

Quantifying both a design UVT and the full range of UVT expected during operation is 

essential. Understanding the full range of UVT is critical because the UV reactor should be 

validated for the range of UVT and flow combinations expected at the WTP to avoid offspecification operation. Specifying a matrix of flow and UVT conditions for the UV reactors to 

meet the required UV dose may be appropriate. Also, the UV manufacturers may use the UVT 

range at the WTP to help determine the turndown (i.e., power modulation) needs of the UV 

reactors. 

This section discusses the issues with using existing UVT data and describes the data 

collection, UVT measurement, and data analysis that can be used to determine design UVT and 

UVT range. Table 3.2 summarizes the recommendations for collecting and analyzing UVT data. 

For the Final LT2ESWTR 

 

3

 UVT in this section implies UVT measurement specifically at 254 nm and 1 cm pathlength unless otherwise noted. 

4

⎟

⎠

⎞ ⎜

⎝

⎛ −= 100

(%) log 254

UVT A
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-14 November 2006 

For the Final LT2ESWTR 

Table 3.2. Summary of UVT Data Collection and Analysis1

Issue Recommendation 

Water Quality Events to 

Capture in Data Collection 

• Typical/average water quality conditions 

• Rainfall effects on source water 

• Reservoir turnover 

• Seasonal variations 

• Possible water quality blends if multiple source waters are used 

• Variation in upstream water treatment 

Water Quality Sampling 

Locations Locations that are representative of potential UV facility location(s) 

Sample Type for Various 

Installation Options2

• Composite samples from operating filters or grab samples from the 

combined filtered water header should be collected for combined filter 

effluent installations 

• Grab samples from representative filter(s) for individual filter piping 

effluent installations 

• Grab samples from any locations downstream of clearwell under 

consideration 

Collection Frequency and 

Period 

• Weekly for 1 – 2 months if water quality is stable 

• Weekly3 for 6 – 12 months (or more) if water quality changes seasonally 

Existing Data for Potential 

Use 

A254 is often collected in filtered waters to determine the specific UV 

absorbance (SUVA), and these measurements could be used in the data 

analysis. However, ultraviolet light absorbance at 254 nm (A254) is typically 

filtered for the SUVA calculation, which would bias the A254 low (high UVT). 

Therefore, such data should only be used with this understanding. 

Recommended Data 

Analysis 

• Cumulative frequency analysis 

• UVT occurrence with flows 

Recommended Data to 

Provide to UV 

Manufacturer 

• Matrix of flows with corresponding UVTs 

• Target pathogen(s) and log inactivation 

• Design UVT4 (corresponding to design flow) 

• Range of operating UVTs 

1 Existing A254 or UVT data may be available, which would reduce the sampling and analysis needed. 

2 The potential installation locations are described in detail in Section 3.3.1. 

3

 More frequent samples may be needed to capture a water quality event (e.g., storm events). 

4

 The design UVT is the UVT that will typically occur at the location of the facility.

Availability of Existing UVT Measurements 

UVT data collection may not be necessary if sufficient filtered water UVT data are 

available to perform the recommended data analysis described subsequently. Additionally, 

filtered water A254 is often collected to determine the SUVA, and these measurements could be 

used in the data analysis. However, the water sample is typically passed through a 0.45- 

micrometer (μm) filter for the A254 measurement needed for the SUVA calculation, which may 

bias the A254 low (high UVT). If the only available A254 measurements are on water that has been 

passed through a 0.45- μm) filter, they can still provide input to the planning process, but 

additional UVT data collection may be necessary to understand the magnitude of the bias. 
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UV Disinfection Guidance Manual 3-15 November 2006 

For the Final LT2ESWTR 

Data Collection 

 UVT measurements should be collected from locations that are representative of the 

potential facility location(s). UVT data can be collected using grab or composite samples, and 

the type of sample collected depends on the potential UV facility locations under consideration. 

For example, composite samples from operating filters or a grab sample from a combined filter 

effluent header should be collected for combined filter effluent UV facilities. For individual filter 

effluent pipe installations, grab samples from representative filters at the beginning and the end 

of filter runs are recommended. Grab samples from any location(s) downstream of the clearwell 

under consideration should be collected. 

As with most engineering designs, the larger the data set, the more refined the design 

UVT can be. If UVT data are not available, weekly UVT measurement is recommended, but the 

duration of the sampling period depends on the source water quality. For example, a PWS with 

very stable UVT measurements may need only one or two months of data. A PWS that 

experiences seasonal changes, however, would benefit from more frequent data collection during 

seasonal events and over a longer period (6 to 12 months or more). If seasonal UVT decreases 

occur regularly, increased sampling frequency (e.g., daily) during these periods will better 

capture the magnitude and duration of the decreases. The possible effect of upstream processes 

on UVT should be assessed by collecting UVT data during the various operating conditions (e.g., 

a range of alum doses). If different sources or combinations of sources are used during the year, 

the UVT of the potential source water blends should be characterized to properly identify the 

representative water quality conditions. 

UVT Measurement 

UVT can be measured with a bench-top spectrophotometer or can be continuously 

measured by an on-line UVT analyzer. During planning, UVT is typically measured using a 

spectrophotometer and is typically reported as a percent. The wavelength of the 

spectrophotometer should be set to 254 nm, and the pathlength of the quartz cuvette used to 

measure UVT is usually 1 centimeter (cm). If the UVT is high, however, longer pathlengths can 

be used to improve measurement resolution. When longer pathlengths are used, the A254 

measured on the spectrophotometer should be normalized by the specific pathlength to calculate 

the A254 on a per cm basis, and then the UVT should be calculated based on the A254 with the 

converted 1-cm pathlength. Because particles can affect the absorbance of UV light, samples for 

UVT should not be passed through a 0.45-μm filter before analysis. The sample pH also should 

not be adjusted. 

Data Analysis 

A cumulative frequency diagram of the UVT data can help the PWS determine its design 

UVT value and will also illustrate the UVT range. Cumulative frequency diagrams can be 

prepared by ranking UVT results from lowest to highest and then calculating the percentile for 

each value. Figure 3.5 presents an example cumulative frequency diagram for three filtered 

waters; the cumulative frequency percentile (x-axis) shows the percentage of the dataset that is 

less than a given value of UVT over the data collection period. For example, if the 90th percentile 

UVT is 91 percent, then 90 percent of the measurements are greater than 91 percent, and 10 

percent of the UVT measurements are less than 91 percent. 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-16 November 2006 

In Figure 3.5, the UVT data for Filtered Waters 1, 2, and 3 display different 

characteristics. Filtered Water 1 has a relatively stable UVT, while Filtered Waters 2 and 3 have 

gradually increasing cumulative frequency slopes that indicate greater variability. Selection of an 

appropriate UVT design value for these waters should consider the variability in UVT and flow 

values and the maximum allowable volume of off-specification finished water at different UVT 

design levels. The water supply’s preferred level of conservatism should also be taken into 

account in this comparison. 

Figure 3.5. Example Cumulative Frequency Diagram for Three Filtered Waters 

75%

80%

85%

90%

95%

100%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Cumulative Frequency Percentile

UV Transmittance (%)

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

Additionally, the minimum operating UVT may not correspond to the period with the 

highest flow rates. The relationship between seasonal flow rates and UVT data should be 

considered when selecting a design UVT value and the matrix of UVT and flow conditions to be 

defined for the UV manufacturer. Figure 3.6 presents flow rate and UVT variations and seasonal 

patterns for Filtered Water 3. For this example WTP, the low UVT typically occurs in September 

and October and not during the high flow rate period in the summer. In this example, the 

following conditions for UVT and flow could be communicated to the UV manufacturers, so 

they can determine the applicable UV reactors for the required UV dose: 

A 90th-percentile design UVT value of 86 percent at the design 220-million gallons per day 

(mgd) capacity 

Minimum UVT of 83 percent coupled with a flow of 140 mgd 

For the Final LT2ESWTR 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-17 November 2006 

Figure 3.6. Example Flow Rate and UVT (at 254 nm) Data 

20

40

60

80

100

120

140

160

180

200

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 Flow (mgd) 

79

82

85

88

91

94

97

100

103

106

109

UV Transmittance (%)

Flow

UVT

Filtered Water 3

WTP Capacity: 220 mgd 

Upstream Treatment Chemicals Effect on UVT 

 

As described in Section 2.5.1.3 and Bolton et al. (2001), the following chemicals alone 

will not significantly affect UVT under typical filtered water conditions: alum, aluminum, 

ammonia, ammonium, zinc, phosphate, calcium, hydroxide, ferrous iron (Fe+2), hypochlorite 

(ClO-

), ferric iron (Fe+3), and permanganate. However, ozone residual affects UVT, as described 

below. If other chemicals of concern are present, the effect of water treatment chemicals on UV 

absorbance can be assessed by preparing solutions of various concentrations and measuring their 

UV absorbance using a standard spectrophotometer. 

If ozone is added before UV disinfection, the UVT of the water can be increased 

measurably, thereby improving the efficiency of UV disinfection. Ozone also absorbs UV light, 

however, so if residual ozone enters the UV reactor, the resulting decrease in UVT can be 

significant and should be considered when determining the design UVT. To address this issue, 

PWSs can monitor the ozone residual and add an ozone-reducing chemical prior to the UV 

reactor to maintain the ozone residual below a specified setpoint value. Several chemicals can 

quench ozone, but some (such as sodium thiosulfate) also have a high UV absorbance value and 

can decrease UVT. Such chemicals should not be used prior to UV disinfection unless their 

application causes no residual concentration. Sodium bisulfite is an alternative to sodium 

thiosulfate that does not significantly affect UVT. 

For the Final LT2ESWTR 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-18 November 2006 

For the Final LT2ESWTR 

UVT Scans 

If MP lamps are being considered, measuring the UVT at the wavelengths in the 

germicidal range (in addition to 254 nm) may also be important. A UVT scan is used to 

determine the UVT of the water over 200 – 300 nm (i.e., germicidal range). In a UVT scan, the 

absorbance at each wavelength is measured and converted to UVT using Equation 2.2 (%UVT 

= 100 x 10-A). The UV absorbance of water typically decreases with increasing wavelength over 

the germicidal range. Thus, the UV light attenuation in a UV reactor and the corresponding 

disinfection performance depend on the absorbance at each emitted wavelength. Some UV 

manufacturers use site-specific UVT scans in their UV dose monitoring and control systems. 

UVT scans can also vary seasonally; therefore, UVT scans could be measured at different times 

during the year to account for this variation. Also, the UVT scans can be used to determine the 

appropriate UV-absorbing chemical for validating the UV reactors that will be installed. 

3.4.4.2 Water Quality Parameters That Affect Fouling 

Water quality can affect the amount and type of lamp sleeve fouling that occurs in UV 

reactors. The factors that affect fouling pertain to all UV equipment. 

Fouling is typically caused by precipitation of compounds on the lamp sleeve, as 

described in Section 2.5.1.4. The rate of fouling and the consequent frequency of sleeve cleaning 

depend on ORP, hardness, alkalinity, lamp temperature, pH, and the presence of certain 

inorganic constituents (e.g., iron and calcium). If significant seasonal shifts in any of the 

parameters or coagulant doses are expected, the duration of the monitoring period should be 

sufficiently long to capture the variations. 

Although fouling should not be a significant problem for most PWSs, the water quality 

parameters listed below should be monitored before the UV facility is designed, unless adequate 

water quality data are available. A summary of the data collection and analysis related to fouling 

parameters is provided in Table 3.3. Providing these data to UV manufacturers is recommended 

to help them qualitatively assess the fouling potential for their UV reactors and to assist 

designers in determining whether a particular cleaning system should be specified. These data 

will also help determine the fouling/aging factor, which is discussed in Section 3.4.5. (Note that 

ORP can be challenging to measure, so the data collected may have limited value.) 

• Calcium 

• Alkalinity 

• Hardness 

• Iron 

• Manganese 

• pH 

• ORP 
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Table 3.3. Summary of Fouling Data Collection and Analysis 

Issue Fouling Parameters1

Collection Location Locations that are representative of potential UV 

facility location(s) 

Collection Frequency2 and Period

• Monthly for 1 – 2 months if water quality is stable 

• Monthly for 6 – 12 months (or more) if water quality 

changes seasonally 

Recommended Data Analysis Based on design engineer’s and PWS’ best 

professional judgment 

Recommended Data to Provide to 

UV Manufacturer Median and maximum values 

1 Fouling parameters include calcium, alkalinity, hardness, iron, manganese, pH, and ORP. 2 More frequent samples may be necessary to capture a water quality event (e.g., storm events).

Pilot tests of waters with total hardness levels less than 140 mg/L and iron less than 0.1 

mg/L found that standard cleaning protocols and wiper frequencies (one sweep every 15 – 60 

minutes) addressed the effect of sleeve fouling at the sites tested (Mackey et al. 2001, Mackey et 

al. 2004). Recent research has shown, however, that the addition of a chemical oxidant directly 

upstream of UV reactors (i.e., downstream of filters) will increase the ORP and potential for 

fouling (Derrick 2005, Wait et al. 2005). Therefore, moving the chemical oxidation point from 

immediately upstream of the UV reactors to downstream of the UV reactors should be 

considered to reduce the potential for fouling. It should be noted that if oxidation and filtration 

occur prior to UV disinfection, the iron and manganese are typically oxidized and then filtered 

out prior to the UV reactor, and fouling will be minimal (Derrick 2005, Wait et al. 2005, Jeffcoat 

2005). 

If the ORP, pH, and inorganic constituent concentrations are low, fouling is not likely to 

be an issue, and a cleaning system may not be necessary. However, a cleaning system should be 

considered if iron and manganese are present. Also, if the chemical oxidation point cannot be 

moved from immediately upstream of the UV equipment and iron and manganese are present, 

pilot testing (Section 3.4.5.1) may be necessary to determine the fouling rate and effectiveness of 

sleeve cleaning. 

3.4.4.3 Additional Water Quality Considerations for Unfiltered Supplies and 

Treatment of Uncovered Reservoir Water 

 

Water supplies are susceptible to variable water quality, turbidity spikes, reservoir 

turnover, and seasonal algal blooms. Typically, water treatment processes at filtered WTPs 

dampen the effects of such variations on UV disinfection. Unfiltered supplies, however, 

generally do not have upstream treatment that mitigates these variations. Specifically, the 

presence of particles and algae may affect UV dose delivery, and water quality and UVT may 
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fluctuate more in unfiltered supplies and thus should be a consideration in the water quality data 

analysis. 

Uncovered reservoirs have similar water quality issues as unfiltered supplies. In most 

cases, however, the problems are less severe because the water has been treated before it enters 

the uncovered reservoir and the operation of uncovered reservoirs is more controlled (e.g., 

smaller volumes, storm water control, concrete lining, and bird control). One exception is that 

algal blooms may be more prevalent in uncovered reservoirs than in unfiltered supplies if 

phosphate-based corrosion inhibitors are added at the WTP. Phosphates can promote algal 

growth. 

Issues that should be considered in the water quality data analysis for unfiltered supplies 

and uncovered reservoirs are described in this section and summarized in Table 3.4. 

Table 3.4. Summary of Particle and Algal Data Collection and Analysis 

Issue Particles and Algae 

Collection Location Locations that are representative of potential UV facility 

location(s) 

Collection Frequency1 and Period 

• Monthly for 1 – 2 months for an Unfiltered PWS 

• Bi-weekly for the summer months2 for Uncovered 

Reservoirs 

Recommended Data Analysis Based on design engineer’s and PWS’ best professional 

judgment 

Recommended Data to Provide to 

UV Manufacturer Median and maximum values 

1 More frequent samples may be needed to capture a water quality event (e.g., storm events).

2 Algal blooms often occur in summer months in uncovered reservoir supplies.

Water Quality Fluctuations from Reservoir Turnover 

Reservoir turnover in unfiltered supplies and uncovered reservoirs may cause water 

quality changes that affect UV disinfection. The UVT and parameters that affect fouling should 

be monitored over a complete reservoir cycle to account for these issues in the design criteria. 

For example, reservoir turnover can cause increased iron levels, which is a factor that should be 

considered when assessing fouling potential. If the potential for increased iron levels is not 

assessed, the appropriate sleeve cleaning technology may not be installed, and UV dose delivery 

may be affected. 

Particle Content and UVT Variability 

 

For unfiltered systems, the Surface Water Treatment Rule (SWTR) allows turbidity up to 

5 nephelometric turbidity units (NTU) immediately prior to the first point of disinfection 

application (40 CFR 141.71). Storm-related turbidity spikes are more prevalent in unfiltered 

supplies than in filtered supplies because no upstream treatment is available to remove the 

particles. Particles in water absorb and scatter UV light to varying degrees based on their size 

and composition. Particles affect the disinfection process in two ways: 
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1. Particles can decrease the UVT of water and thereby affect UV dose delivery. 

2. Microorganisms can associate with particles and be shielded from UV light, thereby 

changing the characteristics of the UV dose-response curve that is obtained using 

collimated beam studies. 

Several studies have found that the effects of turbidity up to 10 NTU on UV disinfection 

can be accounted for in the UVT measurements (Passantino et al. 2004, Christensen and Linden 

2002). However, the most commonly used spectrophotometer (bench-top direct reading) may 

underestimate the UVT of water with turbidity greater than 3 NTU (Christensen and Linden 

2002). To reduce this underestimation, all unfiltered systems and uncovered reservoir 

applications should use a bench-top UV spectrophotometer with an integrating sphere to provide 

more accurate UVT measurements for planning purposes. 

For unfiltered waters susceptible to turbidity fluctuations, the UVT sampling should 

occur during these events and be accounted for in the design UVT and UVT range. If the design 

UVT is appropriate, the UV reactor will be able to respond to changes in UVT that arise due to 

particles. 

 

As described previously, particle content and UVT variability will probably be less 

prevalent in uncovered reservoirs compared to unfiltered supplies. The UVT sampling, however, 

should be conducted during a period sufficient to include seasonal events (e.g., rainstorms and 

runoff) that will affect the design UVT and the UVT range. 

Algae 

 Previous research with male-specific-2 bacteriophage (MS2) has shown that algal counts 

up to 70,000 cells/mL do not affect disinfection performance (Wobma et al. 2004). Whether 

algal counts greater than 70,000 cells/mL affect the UV disinfection process is unknown. 

Therefore, for both unfiltered supplies and uncovered reservoirs, UVT sampling should be 

conducted during algal blooms to enable their effects on UVT to be assessed. At high algal 

concentrations, bench-, pilot-, or demonstration-scale testing may be warranted to determine if 

UV disinfection is significantly affected. 

3.4.5 Fouling/Aging Factor 

 

Sleeve fouling, sleeve aging, lamp aging, and UV sensor window fouling (if applicable) 

affect long-term UV reactor performance, as described in Sections 2.4.2 and 2.4.4. The 

fouling/aging factor accounts for these issues. 

An acceptable fouling/aging factor and guaranteed lamp life should be determined based 

on experience and professional judgment. Alternatively, pilot- or demonstration-scale testing can 

be used to estimate the fouling factor and aging factor if deemed necessary by the PWS, as 

described in Sections 3.4.5.1 and 3.4.5.2, respectively. 

The lamp-fouling portion of the factor (i.e., fouling factor) is the estimated fraction of 

UV light passing through a fouled sleeve as compared to a new sleeve. A lamp sleeve can 
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UV Disinfection Guidance Manual 3-22 November 2006 

become fouled when inorganics (e.g. iron) precipitate onto a lamp sleeve and reduce the UV 

transmittance of the sleeve. Water quality parameters that affect fouling are described in Section 

3.4.4.2. 

The lamp aging portion of the factor (i.e., aging factor) is the fraction of UV light emitted 

from aged sleeves and lamps compared to new sleeves and lamps and can be estimated by the 

lamp and sleeve aging characteristics obtained from the UV manufacturer. The lamp aging factor 

is important because as UV lamps age, the output of the lamps decrease. 

The fouling/aging factor is calculated by multiplying the fouling factor by the aging 

factor and typically ranges from 0.4 (NWRI 2003) to 0.9. The fouling/aging factor is typically 

used in validation testing to ensure the UV equipment can meet the required dose in a fouled 

and/or aged condition. (See Equation 3.1.) 

CleanwithDoseUV Lamps ∗ Fouling Factor ∗ Aging Factor ≥ Required DoseUV Equation 3.1 

When purchasing a pre-validated reactor, the PWS should determine if validation testing 

was conducted under conditions of reduced lamp output (e.g., 70 percent) that is equal to or less 

than reduced lamp output expected for fouled/aged conditions at its water treatment plant (e.g., 

0.75, or 75 percent). If the site-specific fouling/aging factor is lower (e.g., 0.5, or 50 percent) 

than considered during validation testing, adjustments in validation test results or additional 

testing should be considered. 

Selection of a fouling/aging factor coupled with a guaranteed lamp life is a trade-off 

between maintenance costs (the frequency of lamp replacement or chemical cleanings necessary) 

and capital costs (the size of the UV reactors). Both a fouling/aging factor and a guaranteed lamp 

life should be selected because doing so will guarantee that the fouling/aging factor will not be 

exceeded within the guaranteed lamp life. Lamps for a UV reactor with a lower fouling/aging 

factor will require less frequent replacement because the UV reactors are designed with more or 

higher powered lamps to achieve the necessary UV output at the guaranteed lamp life. This 

strategy, however, may necessitate an increase in the size of the UV reactor and facility. 

Conversely, the use of an insufficiently conservative factor may underestimate the reduction in 

the lamp output and potentially result in off-specification operation or more frequent lamp 

replacement. 

3.4.5.1 Testing to Determine the Fouling Factor 

The specific fouling rate and optimal cleaning protocol for any given application cannot 

be predicted with existing empirically-proven, mathematical equations. A proper cleaning 

protocol and sleeve-fouling factor, however, can be adequately estimated for most water sources 

without pilot- or demonstration-scale testing and then adjusted during normal operation. 

Alternatively, fouling rates can be evaluated on a site-specific basis through pilot- or 

demonstration-scale testing or during UV reactor start-up. Testing could consist of the following 

test elements: 

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• Test setup: The UV sensors, lamp and sleeve type, power system, and cleaning 

system tested in a pilot- or demonstration-scale system should be identical to the fullscale reactor. Differences in lamp and lamp sleeve geometry can lead to erroneous 

conclusions based on pilot data alone. 

• Flow and UV equipment conditions: Water should flow through the reactor at the 

minimum flow rate, and the lamps should be operated at maximum power. 

• Establishment of cleaning settings: UV equipment with on-line chemical cleaning 

(OCC) systems should be operated for a prescribed length of time (e.g., 2 weeks) 

without a chemical cleaning to evaluate fouling. With water systems using on-line 

mechanical cleaning (OMC) and on-line mechanical-chemical cleaning (OMCC), the 

cleaning systems should be operated at the manufacturer’s recommended frequency 

to assess fouling. One sleeve should be unwiped, however, for the entire testing 

period to serve as a control to verify that fouling is occurring. 

• Assessment of fouling factor: Fouling is assessed by placing a new lamp inside a 

fouled sleeve, igniting it, and measuring the UV intensity. The UV intensity should be 

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 fouling factor. 

• Evaluation of sleeve cleaning efficiency: A sleeve cleaning assessment can also be 

performed to determine if more frequent cleaning could reduce the fouling factor. 

• Sensor window fouling (if applicable): To assess fouling on the UV sensor 

windows, the windows should be cleaned with phosphoric or citric acid at varying 

time intervals, and the change in UV sensor readings recorded. The fouling rate of the 

lamp sleeves is likely to be greater than the fouling rate of the sensor windows 

because the sleeves are hotter than the windows, and higher temperatures accelerate 

fouling. 

• Quality assurance: The fouled sleeve should be manually cleaned, which should 

restore the sleeve UV intensity value to very near that of a new, clean sleeve after the 

fouling factor has been determined. If not, the inside of the sleeve should be manually 

cleaned and the UV intensity measured again. If the UV intensity is still low, the 

sleeve material has likely degraded, and the test should be performed with a new 

sleeve to ensure that the test results indicate fouling only and not sleeve degradation. 

 The fouling factor data can be analyzed to determine the water system’s preferred fouling 

factor under the observed sleeve cleaning efficiencies. 

3.4.5.2 Testing to Determine the Aging Factor 

 

The aging factor is the fraction of UV light emitted from aged sleeves and lamps 

compared to the fraction emitted from new sleeves and lamps. The lamp aging factor is typically 

between 0.5 and 0.8. In most cases, the aging factor can be determined from manufacturer data 
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with existing empirically proven, mathematical equations. The PWS, however, may desire 

testing to better understand lamp aging characteristics. Lamp aging tests assess the reduction and 

variance in lamp germicidal output over time under defined worst-case operating conditions. 

Factors to consider in designing the test(s) include lamp batch, lamp assembly, electrical 

characteristics of the ballasts, heat transfer from the lamps to the water, and lamp operation. 

Because lamps are manufactured in batches, lamps from several different lots should be 

evaluated to ensure that collected data are representative. 

Lamp age can be tested with either a pilot- or demonstration-scale UV reactor or a test 

stand designed to simulate the UV lamp aging in full-scale operation. For either setup, lamps 

should be operated in an environment that reflects conditions expected when the UV equipment 

is installed at a WTP (e.g., use lamp sleeves, ballasts, and cleaning systems that will be used in 

the final application). 

During testing, the following activities should be considered: 

• Monitor the UV intensity, UVT, electrical power delivered to the ballast, electrical 

power delivered to the lamp, and water temperature over the lamp life. 

 

• Visually inspect the lamp sleeves at regular intervals to document any degradation of 

the lamp assembly, including electrodes and seals, and any darkening of the lamp 

envelope. 

 

• Document any fouling on the internal surfaces of the lamp sleeves. 

 

• Using either a radiometer equipped with a germicidal filter or a reference UV sensor, 

measure the germicidal output of the lamp under fixed conditions of ballast operation 

(e.g., power setting); heat transfer (e.g., lamp sleeves); and environment (water 

temperature and transmission). The following procedure should be used: 

– Take one measurement with lamps that have been aged 100 hours (“new”). 

 

– Measure the output from various positions along the lamp based on visual 

inspection (i.e., the pattern of darkening on the lamp). 

– Measure lamp output as a function of lamp power setting if lamp power is 

variable. 

– Assess the output from lamps of different lots. 

The lamp output measured under fixed operating conditions can be plotted over time and 

fit to estimate the mean expected performance for various lamp ages. To determine the aging 

factor, measure the output of a new lamp and the output at the end-of-lamp life. The aging factor 

is the ratio of the output at the guaranteed lamp life to new lamp output and is expressed as a 

fraction. 
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Although it does not impact reactor design, studies have shown that non-uniform lamp 

aging can occur. Non-uniform lamp aging should be considered during validation testing. (See 

Section 5.4.6) 

3.4.6 Power Quality Evaluations 

 

UV lamps can turn off if a voltage fluctuation, power quality anomaly, or a power 

interruption occurs. Power quality tolerances depend on the UV equipment design and vary 

significantly among UV manufacturers (Table 3.5). The UV manufacturer should be contacted to 

determine the power quality tolerance and the length of time for the equipment to reach full 

power after a power quality event. (See Section 2.4.2.3.) 

Table 3.5. Power Quality Triggers for UV Reactors1

Power Quality Event 

LPHO 

Manufacturer #1

LPHO 

Manufacturer #2

MP 

Manufacturer #1 

MP 

Manufacturer #2

Voltage2 Voltage ± 20% ± 10% ± 30% ± 20% 

Sag/Swell 

Tolerance Duration3 2 seconds (s) > 0.03 s > 0.02 s 2 s 

Power 

Interruption 

Tolerances4

Duration3 > 0.05 s > 0.03 s > 0.009 s > 0.05 s 

1 Information shown in the table is compiled from Calgon Carbon Corporation, Trojan Technologies, and 

WEDECO. 

2

 Percent of line voltage. For example, a 10-percent voltage loss is when the voltage is at 90% of the line 

voltage. 3

 1 cycle is 0.017 s. 

4

 Power interruption assumes total voltage loss. 

Source: Cotton et al. (2005)

Studies have shown that the typical industrial power user experiences an average of eight 

power quality events per month (Grebe et al. 1996). Accordingly, power quality problems alone 

likely will not cause UV reactors to exceed the maximum off-specification requirements even 

though UV reactors are sensitive to power quality (Cotton et al. 2005). Therefore, a power 

quality assessment is probably necessary only when the installation site is (1) known to have 

power quality problems (e.g., 30 power interruptions and/or brownouts per month); or (2) located 

in a remote area and the power quality is unknown. 

If power quality may be a problem at the intended installation location, a power quality 

assessment can be performed to quantify and understand the potential for off-specification 

operation, which consists of the following five steps: 

1. Estimate the power quality at the potential location(s) of the UV facility. Local power 

suppliers often can provide data on power quality and reliability and should be the 

first source of information. Other sources of information are operating records of 

power quality incidents (if available), power interruptions, or Supervisory Control 

and Data Acquisition (SCADA) information for the existing plant. 
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2. Understand the power quality tolerance of the UV equipment under consideration by 

contacting the UV manufacturer or consulting published data. 

3. Contact the UV manufacturer to determine how long it will take their equipment to be 

functioning at full power after a power quality event. 

4. Estimate the off-specification time for the potential UV equipment-based information 

gathered in Steps 1 through 3. Examples of how to estimate off-specification based on 

this information are presented in Cotton et al. (2005). 

5. Determine if backup power or power conditioning equipment is needed to reduce offspecification time or to improve UV equipment reliability. 

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. More 

advanced assessments can include the installation of power quality monitors or the retention of 

an outside consultant to conduct a detailed power quality assessment. 

3.5 Evaluating UV Reactors, Dose Monitoring Strategy, and Operational 

Approach

 

Selecting the appropriate UV reactor depends on the installation locations under 

consideration and the design parameters discussed in Section 3.4. The UV reactor manufacturer 

is a valuable resource for such evaluations and can determine what UV reactors are most 

appropriate for the installation locations under consideration. Evaluating the available UV 

reactors in the planning process is important because each manufacturer’s UV reactors are 

unique and proprietary, and installation needs (e.g., power requirements) differ. UV reactors can 

generally be characterized based on lamp type with low-pressure high-output (LPHO) lamps and 

medium-pressure (MP) lamps applicable to most WTPs. This section discusses the general 

characteristics of LPHO and MP reactors and describes the various control strategies. UV 

manufacturers should be contacted directly to gain a better understanding of the available and 

appropriate UV reactors. 

3.5.1 Characteristics of LPHO and MP Reactors 

 

The fundamental difference between LPHO and MP reactors is the lamp intensity output 

(which influences the UV reactor configuration and size), lamp life and replacement, power use, 

power modulation capabilities, and sleeve cleaning. 

• UV reactor configuration and size: Several UV reactor configurations are available. 

Reactors can be in-line (i.e., shaped like a pipe), 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 required UV dose. MP reactor footprints will 

also vary, depending on lamp orientation (e.g., parallel versus perpendicular to flow). 
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• Lamp life and replacement: Lamp life also varies between LPHO and MP reactors. 

Most manufacturers provide warranties of 8,000 – 12,000 hours for LPHO lamps and 

4,000 – 8,000 hours for MP lamps. Although the lamp life for LPHO reactors is 

greater than that for MP reactors, more lamps are needed for an LPHO reactor. The 

actual number of lamps replaced during a given period, therefore, may be less for MP 

reactors. 

• Power use: Even though LPHO reactors typically have more lamps, they require less 

power input than similarly sized MP reactors because LPHO lamps are more efficient 

in converting the power to germicidal UV light for disinfection. This decreased 

energy efficiency results in higher power needs and increases in overall power 

consumption for MP reactors compared to LPHO reactors. 

• Power modulation capabilities: The ability of the UV equipment to adjust lamp 

power or number of UV lamps energized will affect the energy use. Unlike the other 

issues described, power modulation capabilities depend on the UV equipment design 

and not the lamp type. 

• Sleeve Cleaning: The lamp sleeve cleaning systems for LPHO and MP reactors can 

also differ. LPHO reactors typically have OCC systems, and MP reactors typically 

have OMC systems. Although OCC systems tend to be more labor intensive than 

OMC systems, OMC systems typically have more parts to replace. The extent of 

fouling will determine the amount of maintenance (labor and parts) that is needed on 

a routine basis and will affect the overall maintenance costs. 

As described, the PWS should evaluate the differences between LPHO and MP reactors 

and determine any preferences based on the different characteristics and site-specific constraints. 

If one technology is precluded, it should not be evaluated further in the planning analyses. 

3.5.2 Dose-monitoring Strategy and Operational Approach 

The dose-monitoring strategy establishes the operating parameters used to confirm UV 

dose delivery. It affects how a reactor is validated, how instrumentation and controls are 

designed, and how the reactor is operated. In the planning phase, the water system should 

evaluate the various dose-monitoring strategies to determine whether a particular approach is 

preferable based on the ease of integration into their existing operation and control system. If a 

particular dose-monitoring strategy is preferred, the water system should select a UV equipment 

that has been validated for that strategy. The effect of the dose-monitoring strategy on the 

instrumentation and controls design is described in Section 4.3. 

UV manufacturers commonly design their reactors to operate using either: 

• The UV Intensity Setpoint Approach or 

• The Calculated Dose Approach 
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This guidance manual focuses on the design, validation, and operation of UV reactors 

that use one of these two approaches. Another existing dose-monitoring strategy or a new 

strategy developed after this manual is published, however, may also be suitable for reactor 

operations as long as they meet minimum regulatory requirements.5 Alternative strategies should 

be considered on a case-by-case basis. 

Table 3.6 summarizes key characteristics of the two dose-monitoring approaches 

discussed in this manual. The next two sections provide an overview of how the approaches 

operate. Advantages and disadvantages of each are discussed in Section 3.5.2.3, and Section 6.4 

provides additional guidance on monitoring frequency and reporting requirements for these 

control strategies. 

Table 3.6. Dose-monitoring Approaches – Key Characteristics 

Dose-monitoring 

Strategy 

Parameter Used as the 

Operational Setpoint 

Parameters Monitored During 

Operations to Confirm Dose Delivery 

UV Intensity Setpoint 

Approach UV Intensity 

Flow rate 

Lamp status 

UV intensity 

Calculated Dose 

Approach Calculated or Validated dose 1

Flow rate 

Lamp status 

UV intensity 

UVT 

1

 As noted in Section 3.4.1, the calculated dose is estimated using a dose-monitoring equation. For the Calculated 

Dose Approach, the validated dose is equal to the calculated dose divided by a Validation Factor, which 

accounts for biases and experimental uncertainty.

3.5.2.1 UV Intensity Setpoint Approach 

As indicated by its name, the UV Intensity Setpoint Approach relies upon one or more 

“setpoints” for UV intensity that are established during validation testing. During operations, the 

UV intensity, as measured by UV sensors, must meet or exceed the setpoint(s) to ensure delivery 

of the validated dose. Importantly, reactors must also be operated within the validated range of 

flow rates and lamp statuses (i.e., the “validated operating conditions”) [40 CFR 141.720(d)(2)]. 

 One key characteristic of the UV Intensity Setpoint Approach is that water systems need 

not monitor UVT during operations to confirm dose delivery. Instead, the approach relies on UV 

intensity readings by UV sensors to account for changes in UVT. In order for UV sensors to 

efficiently monitor dose delivery, they should be as close as possible to the “ideal” location. This 

means that they should be positioned so that the UV intensity is proportional to the UV dose, 

irrespective of changes in UVT and lamp output. If the sensor is too close to the lamp, changes in 

lamp output will disproportionately impact the measured UV intensity. If the sensor is too far 

from the lamp, changes in UVT of the water will disproportionately impact the measured UV 

 

5

 Systems must monitor flow rate, lamp status, and UV intensity, plus any other parameters required by the state at 

a minimum to show that a reactor is operating within validated conditions [40 CFR 141.720(d)(3)(i)].
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intensity. Water systems can check if sensors are in the ideal location by reviewing validation 

test data. (See Chapter 5.) 

 The recommended validation protocol in Chapter 5 will produce conservatively high UV 

intensity setpoint(s) under many water quality and lamp output conditions if the sensor is not in 

the ideal location, resulting in overdosing during operations. In some cases, UV manufacturers 

have developed modifications to the UV Intensity Setpoint Approach to account for non-ideal 

sensor placement. 

 Water systems can use one of the following operating strategies for the UV Intensity 

Setpoint Approach: single-setpoint operation or variable-setpoint operations. Table 3.7 describes 

these operating strategies and summarizes the advantages and disadvantages of each. 

Table 3.7. Advantages and Disadvantages of Single-setpoint and Variablesetpoint Operations for the UV Intensity Setpoint Approach 

Operating Strategy Description Advantages Disadvantages 

Single-setpoint 

One UV intensity 

setpoint is used for all 

flow rates that were 

validated 

Simplest to operate 

and control 

When flow rate is variable, 

not energy efficient under 

most conditions because 

reactor is overdosing at low 

flow rates 

Variable-setpoint 1

The UV intensity setpoint 

is determined using a 

lookup table or equation 

for a range of flow rates 

Lamp output can be 

reduced at low flow 

conditions to reduce 

energy costs 

More validation data are 

needed. More complex 

operation compared to 

single-setpoint approach. 

Necessitates more 

advanced UV reactor 

monitoring and control. 

1 For the purposes of this guidance manual, variable-setpoint operations refers to variations based on flow 

rate only, as this is the most common application. In theory, multiple setpoints could also be established for 

different lamp statuses and UVT ranges.

3.5.2.2 Calculated Dose Approach 

The Calculated Dose Approach uses a dose-monitoring equation to estimate the UV dose 

based the parameters measured during reactor operations. The most common operational 

parameters in dose-monitoring equation are: 

• Flow rate, 

• UV intensity, and 

• UVT 

Some manufacturers also consider lamp status as a variable in the dose-monitoring equation. 
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UV manufacturers may develop a theoretical dose-monitoring equation using numerical 

models (e.g., computational fluid dynamics [CFD]). Although the theoretical equations can be 

used as a starting point, EPA strongly recommends that water systems use an empirical dosemonitoring equation developed through validation testing. To generate the empirical dosemonitoring equation, validation tests are performed over a wide range of flow rates, UVT values, 

and lamp power combinations. Regression analysis is used to fit the observed validation data to 

an equation. Chapter 5 of this manual provides detailed guidance on how to derive an empirical 

dose-monitoring equation through validation testing. 

 During reactor operations, the UV reactor control system (i.e., the internal reactor 

electronics) typically inputs the measured parameters into the dose-monitoring equation to 

produce a calculated dose. The system operator divides the calculated dose by a Validation 

Factor that accounts for uncertainties and biases to determine the validated dose.6 The operator 

compares the validated dose to the required dose for the target pathogen and log inactivation 

level. 

3.5.2.3 Advantages and Disadvantages 

The principal operating advantage of the UV Intensity Setpoint Approach compared to 

the Calculated Dose Approach is that UVT monitoring is not needed to confirm dose delivery. 

Another important advantage is that the UV Intensity Setpoint Approach, single-setpoint 

operation is straightforward and simple to control with one operational setpoint and one 

maximum value for flow rate. For these reasons, EPA believes this option is good for small 

water systems. Other advantages are that the UV Intensity Setpoint requires fewer validation 

tests than the Calculated Dose Approach and data analyses are relatively straightforward. Data 

analyses to develop the dose-monitoring equation for the Calculated Dose Approach can be 

complex. 

Water systems may favor the Calculated Dose Approach over the UV Intensity Setpoint 

Approach because it offers significant flexibility to reduce operating costs by manipulating lamp 

power (e.g., turning off banks of lamps or powering down lamps when the UVT increases and/or 

the flow rate decreases). This process is also called “dose pacing.” Another potential advantage 

is that operations are more intuitive because the calculated dose, adjusted for uncertainties and 

biases, can be directly compared to the required dose for the target pathogen and log inactivation. 

Manufacturers may favor the Calculated Dose Approach because they have more 

flexibility in UV sensor positioning (i.e., because internal analyzers monitor UVT during 

operations instead of relying on sensors to respond to changes in UVT, positioning sensors as 

close as possible to the “ideal” location offers no advantages). As noted in Section 3.5.2.1, UV 

Intensity Setpoint Approach operations will be more efficient if the UV sensors are at or near the 

ideal location. 

6

 In some cases, the UV reactor control system will perform this step as well, outputting the validated dose 

automatically. 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-31 November 2006 

For the Final LT2ESWTR 

3.6 Assessing UV Equipment Validation Issues 

 

For disinfection credit, the LT2ESWTR requires UV reactors to be validated [40 CFR 

141.720(d)]. A water system’s approach to UV reactor validation and to UV facility design is 

interrelated. The issues to consider are whether equipment will be validated on-site or off-site 

and the hydraulic conditions of the UV reactor validation and installation. This section describes 

how these issues affect the design and the relationship between the validation and hydraulic 

installation approaches. Chapter 5 details the UV reactor validation guidelines. 

3.6.1 Off-site Versus On-site Validation 

UV reactors can be validated either off-site or on-site. With off-site validation, the UV 

reactors are validated before installation (i.e., pre-validated), typically at a third-party validation 

test center or a UV manufacturer facility. With on-site validation, the UV reactors are validated 

at the PWS after they have been installed. Many PWSs will use off-site validation to meet the 

LT2ESWTR requirements. In some cases, however, on-site validation may be appropriate (e.g., 

when the full UVT range was not tested in off-site validation). The advantages and 

disadvantages of off-site and on-site validation are presented in Table 3.8. 

Table 3.8. Advantages and Disadvantages of Off-site and On-site Validation 

 

Advantages Disadvantages 

Off-site 

• Broader ranges of flow and water quality are 

tested so a reactor can be validated for more 

than one application 

• Installation hydraulics are general, allowing 

for installation at most WTPs 

• Process is simpler for utilities because testing 

is conducted at a remote location 

• Cost is usually lower 

• Reactor performance is known before facility 

is designed and constructed 

• Re-validation or additional on-site validation 

testing may be necessary if site-specific 

hydraulics and water quality are not within 

the tested ranges 

• Water quality and hydraulics may not match 

the installation location, potentially resulting 

in less efficient operation 

On-site 

• Exact hydraulics of the installation are used 

• Water quality tested is specific to the 

installation 

• Having provisions for on-site testing (e.g., 

feed and sample ports and static mixers) 

enables flexibility for future testing to optimize 

performance 

• Facility may be designed and constructed 

before reactor performance is verified 

• Water quality is limited to the highest UVT at 

the facility during the testing period 

• Testing logistics can be complex, including 

isolation of the test reactor, assessment of 

additive mixing, and challenge 

microorganism stability 

• Cost may be higher 

• Disposal of test water may require special 

permits 

The PWS should determine whether off-site or on-site validation will be used to meet the 

LT2ESWTR requirements. If on-site validation is preferred, the UV facility design should be 

adapted to enable testing. The UV reactor design should incorporate feed and sample ports, static 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-32 November 2006 

For the Final LT2ESWTR 

mixers, space for tanks near the UV facility for adding the challenge microorganism and UV 

absorbing chemical, and a method to discharge the validation test water. If off-site validation is 

preferred, the UV facility need not incorporate provisions for on-site validation testing. 

If pre-validated reactors that were validated off-site are chosen, the PWS should confirm 

that the validation hydraulic recommendations in Section 3.6.2 can be met without additional onsite validation or PWS-specific off-site validation. 

3.6.2 Validation and Installation Hydraulics Recommendations 

 

 The inlet and outlet piping to the UV reactor in the UV facility should result in a UV dose 

delivery that is equal to or greater than the UV dose delivered when the UV reactor was 

validated. If off-site validation is used, the three preferred options for meeting this condition are 

presented below. 

1. Minimum five pipe diameters of straight pipe upstream of UV reactor: The 

length of straight pipe upstream of each UV reactor at the UV facility is the length of 

straight pipe used in the validation testing plus a minimum of five (5) pipe diameters. 

During validation testing, the inlet piping to the reactor consists of either a single 90-

degree bend, a “T” bend, or an “S” bend, followed by a length of straight pipe if 

necessary. See Figure 3.7 for validation and installation configuration options. 

2. Identical inlet and outlet conditions: Inlet and outlet conditions used during 

validation match those used at the WTP for at least ten (10) pipe diameters upstream 

and five (5) pipe diameters downstream of the UV reactor.

3. Velocity profile measurement: 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 WTP installation (NWRI 2003). The theoretical velocity is defined as 

the flow rate divided by the cross-sectional area.

 Jetting and swirling flow will impact the assumptions for Options 1 and 3. To avoid 

jetting flow, the inlet piping should have no expansions for at least ten (10) pipe diameters 

upstream of the reactor. Also, any valves located in that length of straight pipe should always be 

fully open during UV reactor operation. To avoid swirling flow, the validation piping should not 

include two out-of-plane 90°-bends in series. 

The most suitable validation option depends on the site-specific layout and piping 

constraints and on the validation data. Option 1 is more generally 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 2 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 2 may be the only validation option for an individual filter effluent location, 

which likely will not have 5 diameters of straight pipe before the UV reactors (Option 1) because 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-33 November 2006 

of existing site constraints. Option 3 also provides flexibility but may have the practical 

limitation of measuring the velocity through a cross-section at the installation. 

Figure 3.7. Schematic of Hydraulic Option #1 (90o-Bend, T-Bend, S-Bend 

Inlet Piping Scenarios) 

(x ≥ 0 where x is a multiple of the pipe diameter “D” and is the reactor.

Validation Options UV Facility Installation Options

(x ≥ 0 where x is a multiple of the pipe diameter “D” and is the reactor.

Validation Options UV Facility Installation Options

(x ≥ 0 where x is a multiple of the pipe diameter “D” and is the reactor.

Validation Options UV Facility Installation Options

If available, the validation report for pre-validated UV equipment under consideration 

should be reviewed to determine what the inlet/outlet conditions were during validation, which 

will help determine if Option 1 is feasible. The method for meeting these recommended 

inlet/outlet constraints should be determined in the planning stage and considered when 

developing the UV facility layout (Section 3.8.2). 

CFD modeling and CFD-based UV dose modeling of inlet and outlet conditions may be 

used to assess whether UV dose delivery at the WTP installation is better than UV dose delivery 

achieved during validation for given conditions of flow rate, UVT, and lamp output. The state 

should approve such models and their reliability should be properly evaluated before their results 

are accepted. Appendix D provides guidance on evaluating CFD models. 

3.6.3 Selection of Validation and Hydraulic Approach 

Whether or not the UV reactor was pre-validated off-site affects the inlet/outlet piping 

options for the UV facility. Completing on-site validation provides more inlet/outlet piping 

For the Final LT2ESWTR 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-34 November 2006 

flexibility, but on-site validation means additional design considerations and testing at the water 

treatment plant. If the selected UV equipment is not pre-validated, the PWS can choose either 

off-site or on-site validation based on their site-constraints and preferences. These options are 

described in Figure 3.8. 

Figure 3.8. UV Reactor Validation Options and How They Affect 

Installation Hydraulics 

All UV reactors must be 

validated [40 CFR 

141.720(d)] 

PWS purchases a 

pre-validated UV 

reactor that was 

validated off-site 

PWS purchases a 

UV reactor that has 

NOT been prevalidated 

A. PWS installs 

validated UV reactor 

in accordance to 

installation hydraulic 

Option 1 or 3 

(Section 3.6.2). Onsite validation is 

not necessary for 

this option. 

B. PWS installs a 

pre-validated UV 

reactor, develops a 

validation test plan, 

and conducts on-site 

validation because 

either: 

1. The UV facility 

has unique hydraulic 

conditions (i.e., 

installation 

hydraulic Options 1 

and 3 are not 

feasible)1

2. The PWS wants 

to refine the 

validated conditions 

to closely match 

their operating 

conditions 

Hydraulic Option 2 

is used with on-site 

validation by default

C. PWS develops 

detailed validation 

test plan for off-site 

validation and has 

flexibility of using 

any hydraulic 

installation option. 

D. PWS develops 

a validation test 

plan and conducts 

on-site validation 

and uses hydraulic 

Option 2 by 

default. 

1

 PWS could contract with an off-site validation center to perform validation testing with specific hydraulic 

conditions rather than perform on-site validation. 

For the Final LT2ESWTR 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-35 November 2006 

For the Final LT2ESWTR 

3.7 Assessing Head Loss Constraints 

 

When selecting a feasible location for UV reactors, the hydraulic requirements should be 

met. Head loss through a UV reactor is specific to the equipment and flow rate and generally 

varies from 0.5 – 3 feet (UV reactor only). Characteristic head loss data should be obtained from 

the UV manufacturer(s) for all candidate UV reactors. In addition to the head loss associated 

with the UV reactor itself, the head loss associated with piping, valves, flow meters, and flow 

distribution devices (e.g., baffles) should be considered when assessing the feasibility and 

location of the installation. When selecting a reactor that has been validated off-site (Options A 

of Figure 3.8), the UV reactor inlet/outlet piping used to estimate the head loss through the 

facility should be consistent with the validation recommendations described in Section 3.6.2. The 

head loss through the entire UV facility (i.e., piping, valves, joints, and UV reactors) can be 

between 1 and 8 feet. 

If the head loss through the UV facility is greater than the available head, the plant design 

or operation, or both, may require modification. Some potential modifications, alone or in 

combination, that may be considered to address hydraulic limitations are listed below, and details 

for each are provided in the sections that follow: 

• Eliminating existing hydraulic inefficiencies within the facility to improve head 

conditions (e.g., replacing undersized or deteriorated piping and valves) 

• Modifying the operation of the clearwell 

• Modifying the operation of the filters 

• Installing intermediate booster pumps 

• Modifying the operation of the HSPs 

3.7.1 Eliminating Existing Hydraulic Inefficiencies 

 

Replacing undersized piping and valves with larger diameter piping and valves may 

increase the available head for the proposed UV facility. Older piping can also produce excessive 

head loss if the inner pipe surface is pitted or scaled or if the pipe material has a high coefficient 

of friction. Slip-lining the interior of existing pipe with material having a lower coefficient of 

friction (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 head loss. 

3.7.2 Modifying Clearwell Operation 

 

A PWS may increase head available to a UV facility by lowering the surface water level 

of the clearwell. This strategy, however, decreases the storage volume available to meet peak 

demands, reduces the contact time available in the clearwell for chemical disinfectants, and may 

affect the pump discharge head and distribution system pressure. Evaluating any potential 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-36 November 2006 

For the Final LT2ESWTR 

reduction in disinfection credit is important if contact time in the clearwell is used for calculating 

chlorine disinfectant requirements (i.e., CT). The UV facility, however, may reduce the Giardia

CT requirements sufficiently to offset the reduction in CT. 

3.7.3 Modifying Filter Operation 

 

A treatment facility can alter the operation of its filters (e.g., increase the water elevation 

above the filters) to increase the head available for the UV facility. This approach, however, can 

reduce filter run times and reduce unit filter run volumes, resulting in a need for 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 above the 

filters to help minimize the reduction in head as the water is filtered. 

3.7.4 Installing Intermediate Booster Pumps 

 

When modifications to the existing facility or operations do not provide adequate head 

for the UV reactors, intermediate booster pumps can be installed. Booster pumping increases 

flexibility in locating the UV reactors. Installing booster pumps, however, increases facility 

operation and maintenance costs and space requirements. The reliability of the pumps should 

also be considered in the evaluation because they become a critical operating component. More 

information on intermediate booster pumps is presented in Section 4.1.6. 

3.7.5 Modifying Operation of HSPs 

 

When UV disinfection is installed close to the HSPs (e.g., after the clearwell in a 

filtration plant or after an unfiltered reservoir), one option to increase the head available for the 

UV facility is to modify the pumping operation of the HSPs. Modifications may not be practical, 

however, if they change the distribution system pressure. 

3.8 Estimating UV Facility Footprint 

 

The process footprint should be estimated in the planning phase to help determine 

feasible UV facility locations. The critical components for estimating the UV facility footprint 

are UV equipment constraints and UV facility layout. 

3.8.1 UV Equipment Constraints 

 

The UV equipment constraints that affect the footprint estimation are the number of UV 

reactors needed to meet the design criteria, the UV reactor orientation, and the control panel 

location constraints. 

• Number of UV reactors: The number of UV reactors depends on the redundancy 

chosen and the power modulation capabilities of the UV reactor. UV reactor 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-37 November 2006 

For the Final LT2ESWTR 

redundancy should be determined using sound engineering approaches similar to 

those used for other major equipment (e.g., capacity to provide full treatment with the 

largest UV reactor out-of-service). The ability of the UV equipment to modulate lamp 

power or change the number of lamps energized also should be considered, so that 

energy efficient operation is possible at the operating range of flows and UVTs 

expected for the UV reactors. The UV manufacturer should be contacted to determine 

a particular UV reactor’s power modulation capabilities. 

• UV reactor orientation: UV reactors can be oriented either parallel or perpendicular 

to the ground. Two advantages of vertical orientation (i.e., flow perpendicular to the 

ground) are that (1) the footprint will be smaller and (2) the potential for lamp breaks 

due to debris may be reduced (as described in Appendix E). 

• Control panel location constraints: Maximum allowable separation distance 

between the UV reactors and electrical controls should be considered in the UV 

facility layout and footprint estimation. This information is unique to each UV reactor 

and should be obtained from the UV manufacturer.

• Validation hydraulic restrictions: Section 3.6.2 describes how the validation piping 

configuration can dictate the possible UV facility piping configurations. 

3.8.2 Develop UV Facility Layout 

 

The UV facility layout is dictated by site constraints and the UV equipment constraints 

described in the previous section. The following items should be considered when developing the 

UV reactor and piping configuration and estimating the UV facility footprint in the planning 

phase: 

• Number, capacity, dimensions, and configuration of the UV reactors (including 

redundancy and connective piping) 

• Vertical or horizontal orientation of the UV reactor 

• Maximum allowable separation distance between the UV reactors and electrical 

controls if distance limitations apply 

• Adequate distance between adjacent reactors to afford access for maintenance tasks 

(e.g., lamp replacement) 

• Configuration of the connection piping and the inlet/outlet piping necessary before 

and after each UV reactor, based on validated hydraulic conditions (see Section 3.6.2) 

and UV manufacturer recommendations 

• Space and piping for booster pumps and wetwells (if necessary) 

• Space for electrical equipment, including control panels, transformers, ballasts, 

backup generators, and possible uninterruptible power supplies 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-38 November 2006 

For the Final LT2ESWTR 

• Room for storing spare parts and chemicals (if needed) 

• Lifting capability for heavy equipment 

• Provisions for on-site validation (if applicable) 

The dimensions of UV reactors and associated electrical equipment vary depending on 

the UV manufacturer. Installation footprint and layout should therefore be estimated for all UV 

manufacturers being considered. Once the UV facility footprint is estimated, feasible site 

locations can be determined based on the available land and buildings. 

3.9 Preparing Preliminary Costs and Selecting the UV Facility Option 

 

The amount of analysis necessary to determine the appropriate application point for a UV 

facility is site-specific. Some options clearly will be infeasible, while others may necessitate a 

more detailed comparison of the installation options. Once feasible alternatives are identified, 

development of life-cycle costs and consideration of the non-monetary factors (e.g., ease of UV 

facility operation) can be useful in selecting among alternatives. 

Preliminary life-cycle cost estimates should include capital costs and operation and 

maintenance (O&M) costs. Capital costs include the cost of the UV reactors; building (if 

necessary); piping; pumping (if necessary); electrical and instrumentation provisions; site work; 

contractor overhead and profit; pilot-testing (if necessary); validation costs; and engineering, 

legal, and administrative costs. The O&M costs should include the estimated labor, energy, and 

equipment replacement costs. The LPHO equipment and MP equipment have different O&M 

needs (Section 3.5.1) that should be considered in the O&M costs. 

Selection of the best option should be based on the disinfection and design objectives and 

consideration of the following and other PWS-specific criteria: 

• Cost-effectiveness and ability to meet the water system’s disinfection and design 

objectives 

• Ease of installation (where applicable)

• Operational flexibility and reliability

• Specific maintenance needs 

• Flexibility for future treatment expansion (if applicable) 

3.10 Reporting to the State 

 

Interaction with the state throughout the planning and design phases is recommended to 

ensure that the objectives of both the PWS and the state are met. This interaction may require 

several months and can have a significant effect on the implementation schedule, particularly 

when the state requires modifications. Given the relatively limited use of UV disinfection in the 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-39 November 2006 

For the Final LT2ESWTR 

United States to date, drinking water treatment, and the unique technical characteristics of this 

technology, state agencies may not have developed approval requirements specifically for UV 

disinfection. As such, PWSs are urged to consult with their state early in their UV disinfection 

planning process to understand the approvals and documentation that will be required for the use 

of UV disinfection. 

The state may require that a preliminary design report be submitted that summarizes the 

decision logic used to identify, evaluate, and select UV disinfection. The following items may be 

addressed in the preliminary 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 

• Design criteria 

• Validation Test Plan (if performing on-site or off-site validation- See Section 5.11 

for guidance on developing a Validation Test Plan) 
3. Planning Analyses for UV Facilities 

UV Disinfection Guidance Manual 3-40 November 2006 

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4. Design Considerations for UV Facilities 

This chapter presents the key factors that should be considered during the detailed design 

phase and is written under the premise that the necessary planning and evaluation work discussed 

in Chapter 3 has been completed. This chapter focuses primarily on the design of UV 

disinfection applications for filtered surface water. Most of the information presented, however, 

also applies to unfiltered systems, groundwater under the direct influence (GWUDI), and 

uncovered finished water reservoirs. Additional design issues specifically associated with 

unfiltered, GWUDI, and uncovered finished water reservoir systems are also described. 

4.1 UV Facility Hydraulics 

4.2 Operating Approach Selection 

4.3 Instrumentation and Control 

4.4 Electrical Power Consideration and Back-up Power 

4.5 UV Facility Layout 

4.6 Elements of UV Equipment Specifications 

4.7 Final UV Facility Design 

4.8 Reporting to the State during Design

Chapter 4 covers: 

 

In the United States, most public water systems (PWSs) purchase or select the UV 

equipment before the UV facility design is complete. Pre-purchase or pre-selection of the UV 

equipment enables the designer and the UV manufacturer to coordinate during the detailed, final 

design phase to consider manufacturer-specific design recommendations. Sometimes the 

equipment is pre-selected and the UV equipment manufacturer is included in the construction 

contract. Other procurement methods (e.g., base-bid and contractor selection of equipment) are 

also used, but these methods are less common. 

The process for designing a UV facility is presented as a flowchart in Figure 4.1. The 

illustrated process is based on pre-purchasing or pre-selecting the UV equipment using a 

traditional design-bid-build approach. Any of the equipment procurement and contractor 

selection approaches currently available within the industry, however, can be used to build UV 

facilities. The PWS and the engineer are responsible for selecting the most appropriate approach 

for their specific project. The order of the steps for other procurement approaches may differ 

from that shown in Figure 4.1, but the analyses completed are likely to be very similar. The steps 

described in this chapter follow the order presented in Figure 4.1. Some states may have design 

and plan review requirements that could impact the timing or sequence of steps shown in 

Figure 4.1. The appropriate state regulatory agency should be contacted early in the design 

process to discuss specific design requirements, plan review fees, and review scheduling. 

UV Disinfection Guidance Manual 4-1 November 2006

For the Final LT2ESWTR
4. Design Considerations for UV Facilities 

UV Disinfection Guidance Manual 4-2 November 2006 

Figure 4.1. Flowchart for Planning, Design, and Construction of UV Facilities1

CONSTRUCTION 

Design 

instrumentation 

and controls 

Section 4.3 

Design UV 

facility 

hydraulics 

Section 4.1 

Determine 

operating 

approach 

Section 4.2 

Report to Primacy Agency2

Section 3.10 (Planning) 

Section 4.8 (Design) 

Finalize UV facility 

design, drawings, 

and specifications 

and bid UV facility 

Section 4.7 

Planning analyses 

for UV facilities 

completed 

Chapter 3 

Design electric 

power systems 

Section 4.4 

Complete UV 

facility layout 

Section 4.5 

Develop 

specifications 

and procure 

equipment 

Section 4.6 

1 Flowchart is based on pre-purchase of UV reactors that have undergone validation testing and equipment 

installation using a traditional design-bid-build approach. 2

 Additional state coordination may be necessary. 

 

4.1 UV Facility Hydraulics 

 After the facility location and UV equipment are selected during the planning phase, a 

more detailed evaluation of system hydraulics for the UV facility layout developed in Section 3.8 

should be conducted. In most cases, the UV facility will be designed with multiple, parallel UV 

reactor trains of the same capacity. Each train consists of the lateral piping, UV reactor, valves, 

and flow meter (if applicable) and is joined to the other trains by the distribution and 

recombination channel or manifold. The hydraulic evaluation should include upstream and 

downstream processes for free water surfaces, the inlet/outlet piping configuration, flow control 

and distribution, flow rate measurement, level control, air and pressure controls, valving, and, 

where applicable, intermediate booster pumps. 

For the Final LT2ESWTR 
4. Design Considerations for UV Facilities 

UV Disinfection Guidance Manual 4-3 November 2006 

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4.1.1 Inlet and Outlet Piping Configuration 

 

The recommended inlet and outlet conditions for validation and for the UV facility are 

described in detail in Section 3.6.2. If validation is conducted at an off-site testing facility, the 

designer should refer to the validation report to determine the validated inlet and outlet 

conditions, and then use the recommendations in Section 3.6.2 to determine the recommended 

inlet and outlet piping for the UV facility. If on-site validation or custom off-site validation is 

planned, the inlet and outlet hydraulics should be designed according to manufacturer 

recommendations and to accommodate any site-specific constraints. In addition, to avoid jetting 

flow, the inlet piping should have no expansions for at least ten (10) pipe diameters upstream of 

the reactor. 

 

4.1.2 Flow Distribution, Control, and Measurement 

Regulations specify flow rate, UV intensity, and lamp status as the minimum operating 

conditions that a PWS must routinely monitor [40 CFR 141.720(d)(3)]. Accordingly, proper flow 

distribution and measurement are essential for compliance monitoring of the UV reactors. This 

section discusses various methods for designing proper flow distribution and measurement 

through the UV reactors. 

4.1.2.1 Flow Distribution and Control 

The lateral piping for each UV reactor train should be sized and configured to provide 

approximately equal head loss through each UV reactor train over the range of flow rates. 

Importantly, flow rate through each reactor must conform to the validated operating conditions, 

[40 CFR 141.729(d)] as described in the validation report. 

Two approaches for flow distribution and control are generally used. The first is active 

flow control and distribution, in which a dedicated flow meter and modulating control valve are 

installed for each UV reactor. Active flow control provides the greatest hydraulic control in 

applications with widely varying flow rates. The second method is passive flow distribution. For 

the passive approach, equal flow split is monitored with flow meters. 

For PWSs that use distribution and recombination channels (instead of influent and 

effluent manifolds), designers typically have two basic choices to achieve passive flow 

distribution (Figure 4.2): (1) a series of individual weirs set at the same elevation or (2) a series 

of orifices submerged in the individual UV reactor laterals. 

4.1.2.2 Flow Rate Measurement 

 

The method of flow rate measurement selected should be based on the variability in plant 

flow rate, the type of flow split used, and any state requirements. Selection of the flow rate 

measurement method should be at the discretion of the PWS and the design engineer based on 

experience and professional judgment. Generally, each UV reactor should have a dedicated flow 

meter (as described in Table 4.1) to confirm that the reactor is operating within the validated 
4. Design Considerations for UV Facilities 

UV Disinfection Guidance Manual 4-4 November 2006 

flow rate. The state, however, may approve other methods (e.g., one flow meter coupled with 

pressure differential measurements). 

Figure 4.2. Open-channel Flow Distribution Options 

For the Final LT2ESWTR 
4. Design Considerations for UV Facilities 

UV Disinfection Guidance Manual 4-5 November 2006 

For the Final LT2ESWTR 

Table 4.1. Comparison of Techniques for UV Facility Flow Rate Measurement for 

Combined Filter Effluent and Post-clearwell UV Facilities 1

Flow Rate 

Measurement 

Method

Flow 

Control 

Method Advantages Disadvantages

Individual UV 

Reactor Flow Rate 

Measurement 

Passive flow 

control such 

as a weir or 

an orifice 

• Measures individual UV reactor flow 

rates accurately 

• May have unequal flow distribution 

• Cannot control the UV reactor flow rate 

Individual UV 

Reactor Flow Rate 

Measurement and 

Control 

Individual flow 

control (valve) 

for each UV 

reactor 

• Measures individual UV reactor flow 

rates accurately 

• Does not rely on passive flow 

distribution 

• Increases capital cost 

• May increase facility footprint due to 

hydraulics of UV reactor, meter, and 

valves 

1

 For individual filter effluent installations, the flow rate from the filters can be used to determine the flow rate through the 

UV reactors. 

When selecting a flow meter, the flow meter’s effect on the inlet/outlet hydraulics of the 

UV reactor should be considered. Magnetic or other types of flow meters (such as Doppler) that 

do not protrude into the flow path exert the least effect on the velocity profile, which minimizes 

the potential effect on reactor inlet or outlet hydraulics. 

4.1.3 Water Level Control 

 

 The UV lamps in the UV reactor should be submerged at all times to prevent overheating 

and UV equipment damage. This is accomplished by installing the UV reactors at an elevation 

below the hydraulic grade line elevation. Two common methods for keeping the UV lamps 

submerged are to: 

1. Install a flow control structure (e.g., weir or orifice) immediately downstream of the 

UV reactor or at another location that ensures full pipe conditions through the UV 

reactors. 

2. Use flow control valves to monitor and maintain the hydraulic grade line. 

 Damage to UV lamps caused by operation in air is specific to each lamp type and size. 

Low-pressure (LP) lamps can typically operate in air for up to 24 hours with minimal damage. 

Low-pressure high-output (LPHO) lamps will begin experiencing damage as a result of 

dislodged amalgam or mercury adsorption to the inner surface of the lamps in 6 – 12 hours 

(Lawal 2006). Medium-pressure (MP) lamps can experience advanced aging or solarization in 

fewer than 6 hours and can break (see Appendix E). 
4. Design Considerations for UV Facilities 

UV Disinfection Guidance Manual 4-6 November 2006 

For the Final LT2ESWTR 

4.1.4 Air Relief and Pressure Control Valves 

UV reactors should be kept free of air to prevent lamp overheating. Negative gauge 

pressures or surge effects within the UV reactors should also be prevented to avoid damage to 

the lamps and lamp sleeves. Quartz sleeves are designed to accommodate continuous positive 

pressures of at least 120 pounds per square inch gauge (psig) but have been shown to break at 

negative pressures of 1.5 (Roberts 2000, Aquafine 2001, Dinkloh 2001). Negative pressures can 

result from line breaks or accidental dewatering of the reactor. The use of air release valves, 

air/vacuum valves, or combination air valves may be necessary to prevent air pockets and 

negative gauge pressure conditions. The UV manufacturer should be contacted to determine any 

equipment-specific air release and pressure control valve needs. The valve locations will be 

dictated by the specific configuration of the facility and should be determined during design. 

4.1.5 Flow Control and Isolation Valves 

Each UV reactor should be capable of being isolated and removed from service. Isolating 

or shutting down a UV reactor will require valves, gates, or similar devices upstream and 

downstream of the UV reactor. Valves are recommended because they provide a tighter seal. 

During design, the inlet and outlet valve configuration should be discussed with the UV 

manufacturer to ensure that UV reactor performance will not be adversely affected and that the 

required inlet conditions used during validation are met, as discussed in Section 3.6. 

If the isolation valves are also used for flow control, the flow control valve should be 

located downstream of the UV reactor to limit the disturbance of the flow entering the UV 

reactor. Valves downstream of the UV reactor can be equipped with an actuator to open or close 

automatically on a critical alarm occurrence and to enable start-up sequencing. 

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 and chemicals 

that may be used for reactor cleaning. Resistant materials will help avoid valve degradation. 

4.1.6 Installation of Intermediate Booster Pumps 

 

A detailed evaluation and design of a booster pumping system is recommended if head 

constraints indicate a pumping system is necessary. 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 needed for intermediate pumping applications and are generally 

not appropriate. Mixed- or axial-flow pumps with high-flow and low-head operating 

characteristics are usually better choices for intermediate pumping applications because typically 

only 1 – 8 feet of additional hydraulic head is needed to overcome the head loss through the UV 

facility. 

Pumps can be installed before or after the UV reactors, allowing more flexibility in the 

UV facility’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. 
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Booster pump operation may be controlled by the water level within the upstream 

wetwell. The use of variable frequency drives or a rate-of-flow controller with a modulating 

valve to dampen flow rate peaks is recommended, especially 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 rate through the water treatment plant (WTP). 

4.1.7 Evaluating Existing Pumps and Potential Water Hammer Issues 

In some WTPs, the most feasible location for installing the UV reactors may be 

immediately upstream or downstream of existing high-service pumps (HSPs) (Section 3.3.1.3). 

The HSP discharge curves should be analyzed to determine the effect of the increased head loss 

through the UV reactors and whether HSP modifications are necessary. 

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 defined, including procedures for 

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 should be taken into account with 

the sequencing of the pump operation. 

4.1.8 Groundwater System Hydraulic Issues 

Common hydraulic issues associated with groundwater systems include high operating 

pressures, air entrainment, and the potential for water hammer events. 

Lamp sleeves are designed to resist high external operating pressures. Before selecting 

equipment, however, the designer should determine the maximum expected operating pressure, 

which may occur during a failure event (e.g., downstream valve closes), and confirm that the 

proposed equipment can withstand that pressure. 

Pressure surge events (water hammer) near the UV reactor may be more likely with 

groundwater 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 and potentially break the sleeves and lamps. 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. 

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 facility design should include a means for automatically releasing 

air prior to the UV reactor. The UV reactor may have air release valves or valve ports, or air 

release valves can be installed in the inlet piping. 

 
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4.1.9 Uncovered Finished Water Reservoir Hydraulic Issues 

 

 

Many uncovered finished water reservoirs serve as distribution storage and are directly 

affected by the water system demand. Others may be used solely as an emergency supply or may 

function as both distribution storage and emergency supply. The specific hydraulic 

considerations that a PWS and designer should consider will vary depending on the function of 

the uncovered finished water reservoir. Regardless of reservoir function, however, specific 

hydraulic issues that should be considered when designing UV facilities at uncovered finished 

water reservoirs include widely varying flow rates, bi-directional flow (under certain piping 

configurations), and the effect a UV facility will have on system pressure. 

• Variable flow rate: The methodology described in Section 3.4 should be followed to 

determine the flow rate and UVT that are used to design the UV facility. Most UV 

facilities at uncovered finished water reservoirs should be designed to handle the peak 

instantaneous demand that must be met by the reservoir. The instrumentation and 

control (I&C) design must consider how the PWS will sequence the UV reactors with 

highly variable flow conditions, especially warm-up times for UV lamps (Section 

2.4.2.3). 

• Bi-directional flow: In some cases, the inlet and outlet to the uncovered finished 

water reservoir is the same pipe, and the UV facility should be designed so that 

disinfection continues when the water is flowing from the uncovered finished water 

reservoir. The PWS may also consider operating the UV reactors at a minimum level 

as the water flows into the reservoir so that the UV lamps are energized and ready for 

UV disinfection if the flow direction changes suddenly. The necessity for this latter 

approach depends on the number of directional changes per day in the context of 

meeting off-specification requirements. 

• UV facility effect on system pressure: As discussed in Section 3.7, head loss 

through a UV reactor generally varies from 0.5 to 3 feet, with the overall head loss 

through a UV facility typically about 1 to 8 feet. This head loss could affect the 

distribution system pressure. As discussed in Chapter 3, a hydraulic assessment 

should be completed to determine if head loss constraints occur for the UV facility or 

if booster pumping is needed. 

4.2 Operating Approach Selection 

 

The operating approach is the method of operating a UV reactor based on the dosemonitoring strategy (Section 3.5.2) and validation report data and should be determined 

before the I&C design is complete. The operating conditions for each UV reactor must be based 

on validation testing results [40 CFR 141.720(d)(3)]. 

As described in Section 3.5.2, this guidance manual focuses on two dose-monitoring 

strategies: UV Intensity Setpoint Approach and Calculated Dose Approach. The UV Intensity 

Setpoint Approach can be used with a single or variable setpoint operation; variable setpoint 
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operation allows for some energy savings. The Calculated Dose Approach typically uses a single 

setpoint (e.g., the required dose), and the UV equipment automatically compensates based on the 

UVT, UV sensor measurements, and flow rate, which increases energy savings. 

When considering the dose-monitoring strategy and operating approach, the operational 

complexity should be compared to the potential for energy savings. The UV manufacturer should 

be contacted to determine the potential energy savings with the available dose-monitoring 

strategies and operating approaches. For small water systems, the UV Intensity Setpoint 

Approach with a single setpoint may be the best option because the energy savings with a more 

complex operating approach may not be worth the additional operational needs. Detailed 

examples of how to determine the operational setpoints from validation reports for these 

operating strategies are described in Section 6.1.4. 

4.3 Instrumentation and Control 

 

The necessary level of I&C depends on the selected techniques for flow control and 

distribution, flow rate measurement, and the operating approach. For example, passive flow 

distribution with the UV Intensity Setpoint Approach that uses a single setpoint is simple and 

demands limited I&C but may result in reduced operating flexibility and energy efficiency. More 

complex control strategies, such as the use of dedicated flow meters and flow control valves with 

the Calculated Dose Approach, necessitate a higher level of I&C, but improve operating 

flexibility and enable optimization of disinfection performance. The control system complexity 

and operating flexibility should be balanced to meet the needs of the PWS. 

Most of the manufacturers’ equipment has similar I&C attributes and alarm conditions 

incorporated in the UV reactor designs. The designer should identify the 

• Elements that are preprogrammed in the UV reactor control panel 

• Necessary supplemental controls to coordinate the operations of the UV reactor trains 

• Actions necessary for each alarm condition. 

At a minimum, UV lamp intensity, flow rate, and lamp status must be monitored (40 CFR 

141.720(d)(3)). The final I&C design can be modified as needed after UV equipment is selected. 

The following sections describe the elements that should be considered in I&C design. 

4.3.1 UV Reactor Start-up and Sequencing 

 

This section describes the typical UV reactor start-up protocol, strategies for sequencing 

the start-up of multiple UV reactors, and considerations for groundwater UV facility start-up. 
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4.3.1.1 UV Reactor Start-up 

 

The UV reactor start-up sequence depends on whether the UV reactor requires cooling 

water while the UV lamps warm up. Some reactors require cooling water (Leinberger 2005, 

Larner 2005) and some do not (Larner 2005, Bircher 2005). Without water flow, some UV lamps 

may heat the water above the safe operating temperatures of 30 – 49 °C in 2.5 – 15 minutes, 

causing the reactor’s internal safety devices to shut the reactor off (Leinberger 2005, Bircher 

2005). LP and LPHO reactors typically do not and some MP reactors do not need cooling water 

as the UV lamps are warming up (Haubner 2005). UV lamp breaks (discussed in detail in 

Appendix E) can occur if the lamps become overheated because of no flow or minimal cooling 

water flow. The designer should consult the UV manufacturer to determine whether the UV 

reactor requires cooling water during start-up. 

The potential start-up sequences for UV reactors that do and do not need cooling water 

and are starting cold (i.e., previously off as opposed to shut down for a very short period) are 

summarized below: 

 

• UV reactors that do not require cooling water: The potential control sequence will 

ignite the lamps, get the UV reactor to its validated conditions, and open the isolation 

valves. With this strategy, the UV reactor will be “on” for some time when no water 

is flowing through it. Flow should be established in the UV reactor within an hour to 

prevent fouling of the quartz sleeves. 

• UV reactors that do require cooling water: The potential control sequence will 

open the isolation valves to allow the minimal cooling water flow, ignite the lamps, 

get the UV reactor to its validated conditions, and then fully open the isolation valves 

to allow the full flow through the UV reactor. The I&C should be designed to reduce 

the amount of off-specification water by providing the minimal flow necessary to 

keep the lamps cool during start-up. If the amount of off-specification water should 

be limited, methods are available to design the UV facility piping to minimize offspecification water (e.g., cooling water being diverted to waste). 

For facilities that do not operate continuously, the designer should discuss the specific 

operating schedule with the manufacturer to identify any special provisions that should be 

included in the design or the operating procedures (e.g., automatic cleaning before each start-up, 

draining for extended periods of downtime). 

4.3.1.2 UV Reactor Sequencing 

 

 UV facilities with multiple UV reactors should develop two types of UV reactor start-up 

sequences in I&C loop descriptions: 

• Routine operation: The UV reactor sequencing should be developed based on the 

validated conditions and the operational approach. 

• Start-up after a power quality event: The control system should monitor the power 

input to the UV reactors and the UV reactor status. LPHO and MP reactors have 
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different start-up characteristics after a power quality event (Section 2.4.2.3) and 

should have different start-up sequences to minimize warm-up and corresponding offspecification time. 

– LPHO reactors – UV reactors that were on-line (i.e., operating) before the power 

quality event and shut-down should be restarted first after normal power is 

restored. 

– MP reactors – UV reactors that were off-line before a power quality event that 

shuts down UV rectors should be started first when normal power is restored. 

4.3.1.3 Groundwater Pump Cycling Effects on UV Reactor Start-up 

 

Groundwater well cycling can adversely affect UV reactor performance, causing an 

increase in off-specification water. An analysis should be performed to estimate off-specification 

volume based on the current well cycling frequency. The well cycling approach may need to be 

changed if off-specification requirements cannot be met under current well cycling frequency. 

Two approaches that can minimize the effects of well cycling, depending on whether the UV 

reactors require cooling water, are discussed below. 

 

• UV reactors that do not require cooling water: A time delay can be incorporated in 

the I&C loops that prevents the well pump from starting until the UV reactor reaches 

its validated conditions. As described in Section 4.3.1.1, the UV reactor will be “on” 

for some time when no water is flowing through it. 

• UV reactors that do require cooling water: The I&C programming would supply 

the minimum water flow through the UV reactor until the reactor reaches validated 

conditions. Then, the groundwater flow can be increased to meet system demand. If 

desired, the cooling water can be discharged to waste if site conditions permit. 

4.3.2 UV Equipment Automation 

 

UV equipment operation can range from manual to fully automatic, depending on the 

reactor’s size and complexity. Manual operation includes manually initiating lamp start-up and 

shut-down, and activating the appropriate valves. Various levels and types of automation are 

typically part of the internal UV equipment controls and can be added to the manual sequence. A 

first level of automation includes the sequencing of lamp start-up and valve actuation to bring 

individual UV reactors on-line after manual initiation. Further levels of automation include 

starting UV reactors, activating rows of lamps, or making lamp intensity adjustments based on 

UV intensity, UVT, or flow rate. Automatic UV reactor shut-down under critical alarm 

conditions (e.g., high temperature, lamp or sleeve failure, loss of flow) is essential for all 

operating approaches, including manual operation. 
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4.3.3 Alarms and Control Systems Interlocks 

 

Many UV reactor signals and alarms are specific to the UV facility and the level of 

automation used. Alarms may be designated as minor, major, or critical, depending on the 

severity of the condition being indicated. 

• A minor alarm generally indicates that a UV reactor requires maintenance but that 

the UV reactor is operating in compliance. Minor alarms also can be set for 

conditions just short of failure conditions so that major alarm conditions are not 

reached. For example, a minor alarm would occur when the UVT is within 1 percent 

UVT of the minimum allowed UVT or when the end-of-lamp-life based on hours of 

operation is reached, indicating the possible need for lamp replacement. 

• A major alarm indicates that the UV reactor requires immediate maintenance (e.g., 

the UV sensor value has dropped below the validated setpoint) and that the unit may 

be operating off-specification. Based on the water system’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 critical alarm is the UV reactor’s temperature exceeding 

a pre-determined maximum value, resulting in automatic shut-down to prevent 

overheating and equipment damage. 

 The same alarm condition may represent a different level of severity depending on the 

validated conditions, the type of UV reactor, the operating approach, and the disinfection 

objectives of the PWS. For example, if a UV reactor was validated with one lamp out of service, 

a single lamp failure alarm may trigger a minor alarm. Had the reactor been validated with all 

lamps in operation, a single lamp failure may trigger a major alarm. Table 4.2 summarizes 

typical UV reactor monitoring and alarms that would likely be integral to the UV reactor control 

panel. 

 

4.3.4 UV Reactor Control Signals 

 

The designer should coordinate with the UV manufacturer to determine what elements of 

the control system are integral to the UV reactor and what elements should be addressed with 

supplemental controls and equipment (i.e., supervisory control and data acquisition or SCADA). For 

installations with multiple UV reactors, a common, master control panel may be necessary to 

optimize UV reactor operations. Typically, each UV reactor has a dedicated control panel, and 

the plant’s SCADA system receives control signals from each control panel to control the entire 

UV facility. The SCADA system also monitors and records the process parameters. 

Recommended monitoring and recording frequencies are provided in Chapter 6, and the designer 

should coordinate with the state to determine if expected frequencies differ. This section 

describes the control signals that could be transferred from each reactor’s control panel to the 

SCADA system. 
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4.3.4.1 UV Intensity 

 Signals from UV sensors should be displayed locally on the UV reactor control panel and 

in the plant’s SCADA system screen (if applicable). 

Table 4.2. Typical Alarm Conditions for UV Reactors 1

Sensor Alarm Type Purpose/Description 

Lamp Age Minor alarm Run-time for lamp indicates end of defined 

operational lamp life. 

Calibration Check of UV sensor Minor alarm UV sensor requires calibration check based 

on operating time. 

Low UV Validated Dose Major alarm Indicated validated UV dose (based on UV 

reactor parameters, i.e., flow rate, UV 

intensity, and UVT) falls below required UV 

dose. 

Low UV Intensity Major alarm Intensity falls below validated conditions. 

Low UV Transmittance Major alarm UVT falls below validated conditions. 

High Flow Rate (Not Integral to UV 

Reactor—Relies on Flow Meters) 

Major alarm2 Flow rate falls outside of validated range. 

Mechanical Wiper Function Failure (If 

Applicable) 

Major alarm Wipe function fails. 

Major alarm Single lamp/ballast failure identified.3 Lamp/Ballast Failure 

Critical alarm Multiple lamp/ballast failures identified. 

Low Liquid Level Critical alarm Liquid level within the UV reactor drops and 

potential for overheating increases. 

High Temperature Critical alarm Temperature within the UV reactor or ballast 

exceeds a setpoint. 

1 Alarm conditions and relative severity shown above may vary depending on the specific validated conditions, type 

of UV reactor, manufacturer, dose-monitoring strategy, and disinfection objectives of the PWS. 2

 Based on measurement from dedicated flow meters or calculated based on total flow rate divided by number of UV 

reactors operating. 3

 Coordinate with UV manufacturer to determine if a lamp/ballast failure could indicate a sleeve and lamp break, 

which should be classified as a critical alarm.

4.3.4.2 UV Transmittance 

 If the Calculated Dose Approach is used, the UVT must be known to ensure that it is 

within the validated range. An on-line UVT analyzer or a bench-top spectrophotometer may be 

used to monitor UVT. Output from an on-line UVT analyzer can be input directly into a control 

loop for most UV reactors, a SCADA system, or both. Results from a bench-top 

spectrophotometer can be manually input into a SCADA system or UV reactor control panel(s). 
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4.3.4.3 Flow Rate Measurement 

 

To maintain regulatory compliance, the flow rate through a UV reactor must be known to 

ensure that it is within the validated range [40 CFR 141.720 (d)(2)]. Section 4.1.2 discusses flow 

rate measurement and control options. The flow rate signal should be displayed locally or be 

input directly into a control loop for the UV reactor, a SCADA system, or both. 

4.3.4.4 Calculated and Validated UV Dose (If Applicable) 

 

If the Calculated Dose Approach is used, the calculated and validated doses should be 

displayed locally and transmitted to the SCADA system. The validated dose is equal to the 

calculated dose divided by the Validation Factor (See Section 5.10 for details). 

4.3.4.5 Operational Setpoints 

 

The operational setpoints should be displayed locally and remotely in the SCADA 

system. These setpoints will depend on the specific dose-monitoring strategy, operating approach 

(Section 4.2), and the validation data, and may include UV intensity, UVT, flow rate, calculated 

dose, and validated dose. 

4.3.4.6 Lamp Age 

 

The operating time of each lamp should be monitored, displayed locally, and transmitted 

to the SCADA system to facilitate O&M and lamp replacement, as discussed in Section 6.3.2.6. 

4.3.4.7 Lamp Power, Lamp Status, and Reactor Status 

 

Water systems must monitor lamp status to verify that UV reactors are operating within 

validated conditions [40 CFR 141.720(d)(3)]. Lamp status refers to whether the lamp is “on” or 

“off.” The operating power level should also be monitored and displayed at the control panel and 

remotely in the SCADA system. Each reactor’s on-line or off-line status should also be 

monitored and indicated locally and remotely, which can be accomplished by monitoring power 

and valve status. 

4.3.4.8 UV Reactor Sleeve Cleaning 

 

Sleeve cleaning information should be displayed locally and communicated between the 

local control panels and the SCADA system. This information should include the date and time 

of the last cleaning for off-line chemical cleaning (OCC) systems and the wiping frequency for 

on-line mechanical cleaning (OMC) or on-line mechanical-chemical cleaning (OMCC) systems. 
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4.3.4.9 Alarms 

 

At a minimum, alarm conditions should be displayed locally. The use of visual or audible 

alarms is also recommended. If the UV facility will frequently be unstaffed or a SCADA system 

is already in place, provisions should also be included in the design to allow remote monitoring 

and display through the SCADA system. 

4.4 Electrical Power Configuration and Back-up Power 

 

The electrical power configuration should take into account the power requirements of 

the selected equipment, the disinfection objectives, and power quality issues, if applicable. (See 

Section 3.4.6.) 

4.4.1 Considerations for Electrical Power 

 

The proper supply voltage and total load requirements should be coordinated with the UV 

manufacturer, considering the available power supply. In addition, the power needs for each UV 

reactor component may differ. For example, the UV reactor may require 3-phase, 480-volt 

service, while the on-line UVT analyzer may need single-phase, 110-volt service. Excluding 

high service pumping, the electrical load from UV reactors will typically be among 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 cause electrical system problems, including overheating 

of some power supply components and can affect other critical systems, such as variable 

frequency drives (VFDs), programmable logic controllers (PLCs), and computers. Selection of 

the UV reactors should be based on a thorough analysis of the potential for the equipment to 

induce harmonic distortion. Additionally, the UV facility design and UV equipment should meet 

the Institute of Electrical and Electronic Engineers (IEEE) 519 Standard that addresses 

harmonics. 

One method for controlling harmonics is to use a transformer with Delta Wye 

connections to isolate the UV reactor from the remainder of the WTP power system. The Deltaconnected primary feed can 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 can be added to the UV reactor power supply to 

control distortion. 

4.4.2 Back-up Power Supply and Power Conditioning 

 

The continuous operation of the UV reactor is highly dependent on the power supply and 

its quality (Section 3.4.6). If the power reliability requirements and, consequently, the 

disinfection objectives cannot be met by relying solely on the commercial power supply, the use 

of back-up power, power conditioning equipment, or both may be necessary. 
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4.4.2.1 Back-up Power Supply 

 

 A simple backup power supply (e.g., generator) may be sufficient if power quality issues 

are infrequent. If an existing backup power supply is in place, its load capacity should be 

assessed to determine whether it can accept the additional load associated with the UV facility. 

The time necessary for switching from the primary power supply to a backup power supply and 

how that time affects compliance with the allowable off-specification operation should be 

determined. 

4.4.2.2 Power Conditioning Equipment 

 

Power conditioning equipment may be necessary if the power quality analysis reveals 

frequent events (Section 3.4.6) that cause the UV facility not to meet disinfection objectives. A 

site-specific analysis should be completed to determine the most appropriate power conditioning 

approach (Cotton et al. 2005). Consideration should include off-specification compliance, quality 

of the power supply, the cost of power conditioning equipment, and site constraints (e.g., land 

availability). 

• Uninterruptible Power Supply (UPS) systems provide continuous power in the 

event of voltage sag or power interruption. The battery capacity is large enough to 

supply power to all connected equipment until a generator starts. UPS systems can 

either be on-line or off-line: 

On-line UPS: The unit and batteries are installed in series between the incoming 

power feed and all critical equipment. The incoming power feed charges the UPS 

batteries, and the batteries supply the electrical load. In this situation, the power feed 

is completely separated from the electrical load. This alternative is the most costly 

and has the largest footprint. 

Off-line UPS: The unit is installed in parallel with the connection from the incoming 

power feed to the critical equipment. During normal operations, the electrical load 

receives power directly from the power feed. When the off-line UPS senses a voltage 

fluctuation greater than or less than 10 percent of the nominal voltage, the load 

transfers to the UPS until the electrical feed stabilizes or the generator starts. Off-line 

UPS systems are less costly and have a smaller footprint than on-line UPS systems. 

• Active Series Compensators protect electrical equipment against momentary voltage 

sags and interruptions. These devices boost the voltage by injecting a voltage in series 

with the remaining voltage during a sag condition. The corrected response time is a 

fraction of a cycle, preventing the equipment from experiencing a voltage sag. Active 

series compensators are well suited for instantaneous sags and interruptions; however, 

they cannot correct sustained sags or interruptions. Active series compensators are the 

lowest cost and smallest power conditioning option. 

 
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4.4.3 Ground Fault Interrupt and Electrical Lockout 

 

Proper grounding and insulation of electrical components are critical for protecting 

operators from electrical shock and protecting the equipment. When combined with effective 

lockout/tagout procedures, the risk of electrical shock is further minimized. Ground fault 

interrupt (GFI) is another 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 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. 

4.5 UV Facility Layout 

 

Site layout for a UV facility is generally similar to the layout for any treatment process. 

Access for construction, operation, and maintenance should be considered. Typically, a 

preliminary layout is developed during project planning (Section 3.8.2). This preliminary layout 

may be modified to address space constraints or special installation conditions that result from 

the final equipment selection or based on more extensive site information gathered during 

detailed design. In addition to those items identified in Section 3.8.2, this section describes the 

items to be considered in the more detailed layout developed in the design phase. 

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 

should be enclosed. In some installations, UV reactors and control panels are uncovered. Before 

designing an uncovered facility, however, the state and UV manufacturer should be consulted. 

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 piping, valve, and flow meter design developed in the hydraulic evaluation 

(Section 4.1) should be considered in the UV facility configuration. For example, the length of 

straight-run piping before and after each flow meter to achieve the proper hydraulic conditions 

for accurate and repeatable flow rate measurement should be considered in the piping layout, 

depending on the flow control and measurement technique used (Section 4.1.2). 

 

The location of the power and control panels associated with UV reactors should allow 

adequate space for panel doors to be opened without interference, and to allow unhindered 

access to the UV reactors when the doors are 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 power 
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quality is a concern, room for power conditioning equipment should be provided. Such 

equipment may be located adjacent to the UV reactors or in a separate control room. 

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. If applicable, sufficient space should be maintained around the UV reactors to 

provide access for the OCC procedure. Also, 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 in the lateral pipe are recommended upstream and downstream of each UV 

reactor. The sample taps may be used for collecting water quality samples or 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 

oversight entity to comply with the recommendations of the selected validation protocol. 

Additional details on the locations of sample taps and other validation-related appurtenances are 

provided in Section 5.4. 

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 facility to 

enable the UV reactor and entire lateral to be fully drained for maintenance activities. These 

drains should be large enough to drain the reactor and adjacent piping in a reasonable amount of 

time. 

Additionally, the UVT analyzer installation (if necessary) should be considered in the 

layout. The specific size and operating characteristics of the UVT analyzer will vary depending 

on the UV manufacturer. If an on-line UVT analyzer is included in the design, adequate space 

and access to an electrical supply for monitor installation should be provided and appropriate 

sample taps and drains for withdrawing and discharging sample water should be included in the 

design. The sample line should be equipped with a valve to isolate the UVT analyzer. A sample 

pump (e.g., peristaltic) should be installed if insufficient pressure is available in the system. The 

UVT analyzer should be in a location that minimizes the likelihood of air bubbles (which can 

cause erroneous readings) passing through the monitor. 

4.5.1 Additional Considerations for Unfiltered and Uncovered Reservoir UV 

Facility Layouts 

 

Site issues that should be considered with unfiltered systems are generally consistent with 

those for filtered surface water systems. The most significant difference is the increased 

opportunity for debris to be present in the inflow to UV reactors in unfiltered applications. To 
4. Design Considerations for UV Facilities 

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address the increased potential for debris, UV facility designs for unfiltered applications should 

incorporate features that prevent potentially damaging objects from entering the UV reactor. 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 for 

minimizing it are discussed in Appendix E. 

4.5.2 Additional Considerations for Groundwater UV Facility Layouts 

 

Site issues that should be considered with groundwater systems are generally consistent 

with those for post-filtration surface water systems; the most significant difference is access of 

the site and potential sand particles affecting the UV reactor. Because well sites can be located in 

remote areas and may be more accessible to the general public or unauthorized individuals, the 

UV reactor should be installed within a building to protect sensitive equipment. The need to 

enclose the UV facility will ultimately be based on the manufacturer’s recommendations, local 

regulatory and code requirements, state requirements, environmental conditions, and site-specific 

constraints. Site security should be appropriate to prevent tampering with the equipment and 

water supply and to protect people from injury (e.g., electrocution). 

In addition, sand or debris flowing through the UV reactor may scratch the lamp sleeves 

or cause the sleeve wiping mechanisms to jam. Larger sand and debris could break the lamp 

sleeves and lamps. (See Appendix E for lamp breakage issues.) Intermittently used wells may 

accumulate sand or other particles; this initial concentration of particles should be discharged 

before operation and should bypass the UV reactor to avoid scratching the quartz sleeves. A 

sand/debris trap or other removal equipment prior to UV disinfection may be necessary if 

evidence suggests that the well pump will pull any sand or particles through the screen during 

normal well operation. 

4.6 Elements of UV Equipment Specifications 

 

When procuring the UV reactors, the UV facility layout and UV reactor specification are 

typically provided to the UV manufacturer. This section describes the potential elements 

included in a UV reactor specification and outlines the information that could be requested from 

the UV manufacturer. 

4.6.1 UV Equipment Specification Components 

Table 4.3 summarizes the factors that should be considered when developing equipment 

specifications for the UV equipment. The information included in Table 4.3 is not exhaustive and 

should be modified to meet the specific needs of the PWS and the requirements for the UV 

facility. 
4. Design Considerations for UV Facilities 

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Table 4.3. Possible Content for UV Equipment Specifications 

(Table Spans Pages 4-20 – 4-22)

Item Specification Content 

Flow rate Maximum, minimum, and average flow rates should be clearly 

identified. The maximum flow rate must be within the validated 

range documented in the validation report [40 CFR 141.720 

(d)(3)]. The minimum flow rate may be important to avoid 

overheating with MP reactors. One method for determining the 

maximum flow rate is described in Section 3.4.3. 

Target Pathogen(s) and Log 

Inactivation 

The log inactivation for the target pathogen(s) 

Required UV Dose The required UV dose for the target microorganism and log 

inactivation that must be verified by the validation process. 

Additional detail is provided in Chapter 5. 

Water Quality and Environment The following water quality criteria should be included: 

- Influent temperature - pH 

- Turbidity - Iron 

- UV transmittance at 254 nm - Calcium 

- UVT scan from 200 – 300 - Manganese 

nm (MP reactors only) - ORP 

- Total hardness 

For some parameters, a design range may be most appropriate. 

Operating Flow and UVT Matrix Appropriate matrix of paired flow and UVT values based on flow 

and UVT data (Section 3.4.4.1). 

Operating Pressure The expected operating pressures, including the maximum and 

minimum operating pressure to be withstood by the lamp sleeves 

and UV reactor housing. 

UV Sensors A germicidal spectral response should be specified (Section 5.4.8). 

A minimum of one UV sensor should be specified per UV reactor. 

The actual number should be identical to the UV reactor that was, 

or will be, validated. 

The uncertainty of the UV sensors used during validation should 

meet the criteria described in Section 5.5.4. 

The uncertainty of the duty UV sensors during operation should 

meet the criteria described in Section 6.4.1.1. 

Reference UV sensors should be calibrated against a traceable 

standard. For example, the following standards are currently being 

used by UV manufacturers: 

- National Physical Laboratory (NPL) 

- National Institute of Standards and Technology (NIST) 

- Deutsche Vereinigung des Gas- und Wasserfaches (DVGW) 

- Österreichisches Normungsinstitut (ONORM) 

Redundancy The reactor redundancy determined in Section 3.8.1. 

Hydraulics The following hydraulic information should be specified: 

- Maximum system pressure at the UV reactor 

- Maximum allowable head loss 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). 
4. Design Considerations for UV Facilities 

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Table 4.3. Possible Content for UV Equipment Specifications 

(Table Spans Pages 4-20 – 4-22)

Item Specification Content 

Size/Location Constraints Any size constraints or restrictions on the location of the UV 

reactor or control panels (e.g., space constraints with individual 

filter effluent installation). 

Validation The range of operating conditions (e.g., flow, UVT) that must be 

included in the validation testing, and submittal of a validation 

report (40 CFR 141.720) should be required. The validation testing 

should be completed in accordance to the procedures and data 

analysis described in detail in Chapter 5. 

Dose-Monitoring Strategy A description of the preferred dose-monitoring strategy for the UV 

reactors. 

Operating Approach A description of the intended operating approach for the UV 

reactors, as described in Section 4.2. 

Economic and Non-Economic 

Factors 

The necessary information to thoroughly evaluate the UV 

equipment based on the PWS’s specific goals. As appropriate, this 

information may include both economic (e.g., energy use, 

chemical use) and non-economic (e.g., future expansion, 

manufacturer experience) factors. 

Lamp Sleeves Lamp sleeves should be annealed to minimize internal stress. 

Safeguards At a minimum, the following UV reactor alarms should be 

specified: 

- Lamp or ballast failure 

- Low UV intensity or low validated UV dose (depending on 

dose-monitoring strategy used) 

- High temperature 

- Operating conditions outside of validated range 

- Wiper failure (as applicable) 

- Other alarms discussed in Section 4.3.3, as appropriate. 

Instrumentation and Control At a minimum the following signals and indications should be 

specified: 

- UV lamp status 

- UV reactor status 

- UV intensity 

- Lamp cleaning cycle and history 

- Accumulated run time for individual lamps or banks of lamps 

- Influent flow rate. 

At a minimum the following UV reactor controls (as applicable) 

should be specified: 

- UV dose setpoints, UV intensity setpoints, or UVT setpoints 

(depending on dose-monitoring strategy used) 

- UV lamps on/off 

- 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. 
4. Design Considerations for UV Facilities 

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Table 4.3. Possible Content for UV Equipment Specifications 

(Table Spans Pages 4-20 – 4-22)

Item Specification Content 

Performance Guarantee The equipment provided should meet the performance 

requirements stated in the specification for an identified period or 

during on-site performance testing (Section 6.1.5). The following 

specific performance criteria may be included: 

- Allowable head loss at each design flow rate 

- Estimated power consumption under the design operating 

conditions 

- Disinfection capacity of each reactor under the design water 

quality conditions 

- Sensitivity of equipment to variations in voltage or current 

- Reference UV sensor, duty UV sensor, and UVT analyzer (if 

provided) performance compared to specification 

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 PWS and engineer. Significant variation 

from common commercial standards should be discussed with the 

manufacturer. Lamps should be warranted to provide the lamp 

intensity under the design conditions for the fouling/aging factor 

and a minimum number of operating hours. To limit the UV 

manufacturer’s liability, the guarantee could be prorated after a 

specified number of operating hours. 

UVT Analyzer During operation, the difference between the UVT analyzer 

measurement and the UVT measured by a calibrated 

spectrophotometer should be less than or equal to 2 % UVT. 

4.6.2 Information Provided by Manufacturer in UV Reactor Bid 

 

The UV manufacturers should 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 4.4. 

Table 4.4. Suggested Information to Be Provided by UV Manufacturer 

Item Description of Information 

Design Parameters Demonstration of an understanding of the design parameters for the UV equipment. 

All UV equipment design parameters from the contract documents should be 

repeated in the proposed UV equipment submittal information. 

Summary of Design A summary of the equipment proposed (number of UV reactors, lamp type) and 

specified equipment redundancies.

Reactor Technical 

Specifications 

Ability of proposed UV reactors to meet technical specifications and an explanation 

of any exceptions taken.

UV Equipment 

Documentation and 

Specifications 

Documentation that identifies and describes the UV equipment components that 

were validated, as described in Section 5.11.1.1

UV Manufacturer’s 

Experience 

Information on project experience, including previous facilities and references. 

UV Lamps Detailed description of the lamp dimensions and electrical requirements. 
4. Design Considerations for UV Facilities 

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Table 4.4. Suggested Information to Be Provided by UV Manufacturer 

Item Description of Information 

UV Sensor Information on the UV sensor(s), including spectral response, acceptance angle, 

external dimensions, working range in mW/cm2

, measurement uncertainty, 

environmental requirements, linearity, and temperature stability. 

Data and calculations should be provided showing how the total measurement 

uncertainty of the UV sensor used during validation meets the criteria established 

in Section 5.5.4. 

Data that demonstrate duty UV sensors will meet the criteria described in Section 

6.4.1.1 will be met during operation. 

Lamp Sleeves Calculations showing the maximum allowable pressure for the lamp sleeves and the 

maximum bending stress the lamp sleeves may experience under the maximum 

specified flow rate conditions. 

UVT Analyzer 

(if applicable) 

Data that prove the UVT analyzer used during validation meets the criterion in 

Section 6.4.1.2 during operation. 

Validation Report UV reactor validation should be provided that includes the elements described in 

Section 5.11.3. If on-site validation is proposed, validation data for the UV reactors 

from off-site validation (if completed) should be included to provide a baseline 

comparison to the proposed 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. If pre-validated equipment is 

specified, a description of the hydraulic configuration used during validation testing 

should be provided.

Power 

Requirements 

The power needs of each UV reactor and which elements, including electrical cable 

and wiring, are included as part of the equipment.

Power Quality 

Tolerance 

The power quality tolerance of the UV equipment for voltage sags, surges, and 

interruptions. 

Cleaning Strategy The strategy that will be used for cleaning the UV lamps in the UV reactor. 

Dose-monitoring 

Strategy 

The proposed UV reactor dose-monitoring 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 list 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, UV lamps, lamp sleeves, fouling/aging factor, and the system 

performance guarantee. Any exceptions should be indicated and explained.

1 Key elements of this documentation are also listed in this table. 

4.7 Final UV Facility Design 

 

The UV reactors can be selected after all bids have been carefully reviewed. Once the UV 

reactors are selected, the designer can coordinate with the selected UV manufacturer to develop 

the final facility design based on the selected UV equipment. The hydraulic design, I&C design, 

electrical design, and facility layout should be modified based on the selected UV equipment. 
4. Design Considerations for UV Facilities 

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Particular emphasis should be given to the integration of the overall dose-monitoring 

strategy with the alarms, signals, and interlocks that are integral to the UV reactor design. That 

the final design be coordinated with the validation testing results is critical. The validation results 

must be sufficient to implement the proposed operations approach and should meet the water 

supply’s disinfection objectives under the specified operating conditions. 

4.8 Reporting to the State during Design 

 

Interaction with the state throughout the design phases is recommended and increases the 

likelihood that the objectives of both the PWS and the state are met. Currently many states have 

limited experience in the use of this technology; therefore, the appropriate level of state 

involvement during design should be greater than that for more traditional designs. Early 

agreement on the specific objectives and requirements of the project can significantly reduce the 

potential for conflict or costly design changes later in the project. The level of state involvement 

during design, as well as the specific submittal requirements, will vary by state and may vary by 

project. PWSs are urged to consult with their state early in their UV disinfection design process 

to understand what approvals and documentation will be required. 
5. Validation of UV Reactors 

 The purpose of validation testing is to determine the operating conditions under which a 

UV reactor delivers the validated dose.1 As noted elsewhere in this guidance document, the 

validated dose must be greater than or equal to the required dose (presented in Table 1.4) to 

receive log inactivation credit for a target pathogen. Validation testing also establishes the 

operational setpoints used during reactor operations to confirm delivery of the validated dose. 

 This chapter explains the key steps in EPA’s recommended validation protocol for UV 

reactors. It includes recommendations for selecting test conditions, quality assurance/quality 

control (QA/QC) steps, and data analysis procedures. It provides the rationale for the 

recommended steps and cites relevant research studies where appropriate. 

Chapter 5 covers: 

5.1 Minimum Requirements for Validation Testing 

5.2 Overview of the Recommended Validation Protocol 

5.3 Selecting the Challenge Microorganism

5.4 Equipment Needs for Full-scale Reactor Testing 

5.5 Accuracy of Measurement Equipment 

5.6 Identifying Test Conditions 

5.7 Guidelines for Conducting Experimental Tests 

5.8 Analyzing Experimental Data 

5.9 Deriving the Validation Factor (VF) 

5.10 Determining the Validated Dose and Validated Operating Conditions 

5.11 Documentation 

5.12 Guidelines for Reviewing Validation Reports 

5.13 Evaluating the Need for “Re-validation” 

Several appendices support this chapter: 

• Appendix A provides recommendations for preparing stock solutions of and assaying 

challenge microorganisms. 

• Appendix B presents validation testing examples for two hypothetical water systems. 

• Appendix C provides the recommended procedure for conducting collimated beam

tests, including test conditions, apparatus design, equipment accuracy, and QA/QC. 

Appendix C also provides guidelines for using collimated beam test data to develop a 

UV dose-response curve.

UV Disinfection Guidance Manual 5-1 November 2006

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1 For the purposes of this guidance manual, the validated dose is defined as the UV dose in units of millijoule per 

centimeter squared (mJ/cm2

) delivered by the UV reactor as determined through validation testing. The required 

dose is defined as the UV dose needed to achieve log inactivation credit. All UV dose terms are included in the 

glossary at the beginning of this manual.
5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-2 November 2006 

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• Appendix D contains the background theory used to support the recommended 

validation protocol.

The material in this chapter is intended to help water systems and states understand how 

the validation testing process works. It should be considered as a resource when reviewing 

validation reports or overseeing validation testing activities. Some of the terms and acronyms 

used in this chapter are unique to UV reactor validation. EPA has included an extensive glossary 

and acronyms list at the beginning of this guidance manual to help the reader keep track of new 

terms. 

5.1 Minimum Requirements for Validation Testing 

 Unlike chemical disinfection, UV light leaves no residual that can be monitored to 

determine the delivered dose. UV sensors can measure intensity of UV light, but they cannot 

measure the dose delivered to the microorganisms as they pass through the reactor at different 

trajectories. Therefore, to receive treatment credit for inactivating Cryptosporidium, Giardia, or 

viruses using UV light, the LT2ESWTR requires water systems to use UV reactors that have 

undergone validation testing. 

 Section 1.4 of this manual summarizes all LT2ESWTR requirements related to UV 

disinfection, including minimum dose, validation, monitoring, and reporting requirements. For 

easy reference Table 5.1 summarizes the regulatory requirements for validation. 

Table 5.1. Summary of LT2ESWTR Validation Requirements 

Requirement Conditions Citation 

Validated operating 

conditions must 

include 

• Flow rate 

• UV intensity as measured by a UV sensor 

• UV lamp status 

40 CFR 141.720 (d)(2) 

Validation testing 

must include 1

• Full-scale testing of a reactor that conforms uniformly 

to the UV reactors used by the water system 

• Inactivation of a test microorganism whose doseresponse characteristics have been quantified with a 

low-pressure mercury vapor lamp 

40 CFR 141.720 (d)(2)(ii)

Validation testing 

must account for 

• UV absorbance of the water 

• Lamp fouling and aging 

• Measurement uncertainty of on-line sensors 

• UV dose distributions arising from the velocity profiles 

through the reactor 

• Failure of UV lamps or other critical components 

• Inlet and outlet piping or channel configurations of the 

UV reactor 

40 CFR 141.720 (d)(2)(i) 

1

 The state may approve an alternative approach to validation testing. 
5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-3 November 2006 

The LT2ESWTR does not specifically address “re-validation” if the design of a validated 

UV reactor changes. If design modifications significantly impact UV dose delivery or 

monitoring, however, the UV reactor should be considered a different reactor (with 

unsubstantiated performance) than the one previously validated and as such, must be re-validated 

[40 CFR 141.720 (d)(2)]. Section 5.13 discusses some of the common types of UV reactor 

modifications and provides recommendations for which types of changes necessitate revalidation. 

5.2 Overview of the Recommended Validation Protocol 

 EPA’s recommended validation protocol uses biodosimetry. Under this approach, the log 

inactivation of a challenge microorganism is measured during full-scale reactor testing for 

specific operating conditions of flow rate, UV transmittance (UVT),2 and UV intensity. The 

dose-response equation for the challenge microorganism (relating UV dose to log inactivation) is 

determined using independent, bench-scale testing. Log-inactivation values from full-scale 

testing are input into the laboratory derived-UV dose-response relationship to estimate the 

Reduction Equivalent Dose (RED). The RED value is adjusted for uncertainties and biases to 

produce the validated dose of the reactor for the specific operating conditions tested. The 

validated dose is compared to the required dose for compliance purposes. 

 The protocol can be described in three main steps, as shown in Figure 5.1 and described 

in more detail in Section 5.2.1. Alternative approaches to validation are discussed in Section 

5.2.2. Sections 5.2.3 and 5.2.4 present recommendations for third-party oversight and emerging 

validation approaches, respectively. 

5.2.1 Key Steps in Recommended Validation Protocol 

 Consistent with other recommendations in this guidance manual, EPA developed the 

validation protocol working closely with industry representatives and experts in the field of UV 

disinfection. EPA believes that the approach produces reliable results and can be used to meet 

microbial treatment requirements of the LT2ESWTR and encourages water systems to use it 

where applicable. Water systems are not required, however, to follow the protocol as long as 

they meet the minimum regulatory requirements summarized in Section 5.1. EPA recommends 

that water systems contact their states early to discuss any additional state-specific requirements 

for reactor validation. 

For the Final LT2ESWTR

 

2

 In this Chapter, UVT implies UVT measurement specifically at 254 nanometers (nm) and 1 centimeter (cm) 

pathlength unless otherwise noted. UV absorbance at 254 nm (A254) can be related to UVT using the following 

equation: 

⎟

⎠

⎞ ⎜

⎝

⎛ −= 100

(%) log 254

UVT A
5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-4 November 2006 

Figure 5.1. Overview of Recommended Validation Protocol 

Step 3. Adjust for Uncertainty to Calculate 

the Validated Dose

Validated Dose = RED / VF

Where VF = Validation Factor that accounts for biases and 

experimental uncertainty. 

* Simple representations of testing equipment shown. For more details, see Figures C.1 and 5.2. 

Step 1. Experimental Testing Using a Challenge Microorganism*

1a: Bench Scale Testing 1b: Full-Scale Reactor Testing

UV Reactor

Measure effluent 

microorganism 

concentration, 

compare to influent to 

calculate the log 

inactivation

Measure UV 

intensity with 

a UV sensor. 

Inject challenge 

microorganism

Measure influent 

flow rate, UVT, and 

microorganism 

concentration

Collimated UV rays

Petri dish with challenge 

microorganism

Measure the log inactivation for different 

UV doses to develop a UV doseresponse curve:

Log inactivation

UV dose

(mJ/cm2)

Step 2. Determine the Reduction Equivalent Dose (RED)

Input the log inactivation from Step 1b into the dose-response curve from Step 

1a to estimate RED.

UV dose

(mJ/cm2)

RED

Log inactivation (from step 1b)

Dose-response curve 

( from step 1a)

Step 3. Adjust for Uncertainty to Calculate 

the Validated Dose

Validated Dose = RED / VF

Where VF = Validation Factor that accounts for biases and 

experimental uncertainty. 

* Simple representations of testing equipment shown. For more details, see Figures C.1 and 5.2. 

Step 1. Experimental Testing Using a Challenge Microorganism*

1a: Bench Scale Testing 1b: Full-Scale Reactor Testing

UV Reactor

Measure effluent 

microorganism 

concentration, 

compare to influent to 

calculate the log 

inactivation

Measure UV 

intensity with 

a UV sensor. 

Inject challenge 

microorganism

Measure influent 

flow rate, UVT, and 

microorganism 

concentration

Collimated UV rays

Petri dish with challenge 

microorganism

Measure the log inactivation for different 

UV doses to develop a UV doseresponse curve:

Log inactivation

UV dose

(mJ/cm2)

Step 2. Determine the Reduction Equivalent Dose (RED)

Input the log inactivation from Step 1b into the dose-response curve from Step 

1a to estimate RED.

UV dose

(mJ/cm2)

RED

Log inactivation (from step 1b)

Dose-response curve 

( from step 1a)

For the Final LT2ESWTR
5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-5 November 2006 

Step 1: Conduct Experimental Tests Using a Challenge Microorganism 

 Because handling of the target pathogen during validation testing is neither practical nor 

in the best interest of public health3

, a challenge microorganism whose sensitivity to UV light is 

similar to the target pathogen should be used in all experiments. Using a challenge 

microorganism instead of the target pathogen, however, introduces uncertainty into the testing 

results. This uncertainty is accounted for by applying a validation factor (see Step 3). 

 UV reactor validation includes two types of experimental tests, as described below. 

Importantly, the same stock solution of challenge microorganisms should be used for both tests 

because UV sensitivity for stock solutions can vary. 

1a. Bench-scale testing using a collimated beam apparatus. Collimated beam testing 

characterizes the UV dose-response relationship of the challenge microorganism. In 

these experiments, UV light is directed down a collimating tube to dose a sample of 

challenge microorganisms of a known concentration. After a specified exposure time, 

the sample is analyzed to determine the log inactivation (where log inactivation in 

this situation equals the log concentration prior to UV light exposure minus the log 

concentration after UV light exposure) as a function of UV dose. The UV dose 

delivered to the microorganisms is calculated based on the UV intensity, exposure 

time, and other experimental factors. Figure C.1 in Appendix C illustrates a typical 

collimated beam apparatus. 

 Collimated beam tests are performed at a range of doses to generate a UV doseresponse curve for the specific challenge microorganism. The functional forms of the 

equations for UV dose-response curves can vary depending on the results (guidance 

on developing UV dose-response curves is provided in Section C.3). A quadratic UV 

dose-response equation is provided below. 

( ) ( )

2

×= logADoseUV ×+ oninactivati logBoninactivati Equation 5.1

For this equation type, the coefficients “A” and “B” would be solved for using the 

collimated beam testing data. 

1b. Full-scale reactor testing. In these experiments, the challenge microorganisms are 

injected upstream of the UV reactor. Samples are analyzed to determine the log 

inactivation (where log inactivation in this situation equals log influent concentration 

minus log effluent concentration) at the test conditions of flow rate, UVT, lamp 

status, and UV intensity as measured by UV sensors. Full-scale reactor testing can be 

performed on-site at a water treatment plant or off-site at a validation test center (see 

Figure 5.2 in Section 5.4 for a diagram of a typical biodosimetry test stand used for 

off-site validation). 

 

 

3 Culturing pathogenic microorganisms introduces additional risks in terms of handling, disposal, and cross 

connections. Therefore, the industry regularly uses challenge microorganisms as surrogates for pathogenic target 

microorganisms.

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5. Validation of UV Reactors 

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Step 2: Estimate the Reduction Equivalent Dose 

 Step 2 combines results from the two experimental tests in Step 1. The log inactivation of 

the challenge microorganism measured during the full-scale testing is entered into the UV doseresponse equation (Equation 5.1 if the relationship is quadratic) to calculate the RED of the 

reactor. Another way to conceptualize this step is to consider the RED to be “back-calculated” 

using the field-measured log inactivation as the input variable. This approach is the opposite of 

most applications in which UV dose is the independent variable and log inactivation is the 

dependent variable. 

 RED values are always specific to the following: 

• The challenge microorganism used during experimental testing, 

• Validation test conditions during full-scale reactor tests (flow rate, UVT, lamp status, 

and UV intensity as measured by the UV sensor) 

Step 3: Adjust for Uncertainty to Calculate the Validated Dose 

In the last step shown in Figure 5.1, the RED is divided by a Validation Factor (VF) to 

produce the Validated Dose. The VF accounts for biases associated with using a challenge 

microorganism instead of the target pathogen and for experimental uncertainty (Section 5.9 

provides a detailed description of how the VF is derived). The Validated Dose is associated with 

the validation test conditions of flow rate, lamp status, UV intensity as measured by a UV sensor, 

and in some cases, UVT. As noted previously, the validated dose is compared to the required 

dose to determine the inactivation credit for the target pathogen. 

5.2.2 Alternative Validation Protocols 

 

 The Austrian Standards ÖNORM M 5873-1 and M 5873-2 (2001 and 2003, respectively)

and the German Guideline DVGW W294 (2006) define measured flow rate, UV intensity, and 

lamp status for a Bacillus subtilis RED of 40 mJ/cm2

. Based on the recommended validation 

protocol presented in this guidance manual, UV reactors certified by ÖNORM and DVGW for a 

B. subtilis RED of 40 mJ/cm2

 should be granted 3-log Cryptosporidium and 3-log Giardia 

inactivation credit. Validation by NWRI/AwwaRF Guidelines and NSF Standard 55 should be 

evaluated on a case-by-case basis (NWRI 2003, NSF 2004). 

5.2.3 Third Party Oversight 

EPA recommends that an independent third party provide oversight to ensure that 

validation testing and data analyses are conducted in a technically sound manner and without 

bias. A person independent of the UV reactor manufacturer should oversee validation testing. 

Individuals qualified for such oversight include engineers experienced in testing and evaluating 

UV reactors and scientists experienced in the microbial aspects of biodosimetry. Appropriate 

individuals should have no real or apparent conflicts of interest regarding the ultimate use of the 

UV reactor being tested. 
5. Validation of UV Reactors 

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At a minimum, independent oversight should include observing validation testing to 

verify that the individuals performing the validation follow the documented protocol and 

reviewing the report for accurate data and results. The independent third party should review the 

validation report before its release. When appropriate, the third party should rely on additional 

outside experts to review various aspects of UV validation testing, such as lamp physics, optics, 

hydraulics, microbiology, and electronics. 

5.2.4 Emerging Methods 

 In recent years, researchers have been working on alternative approaches to biodosimetry 

for UV reactor validation. Potential model-based approaches use computational fluid dynamics 

(CFD) to predict microorganism trajectories through a UV reactor, and hence the UV dose 

delivered to each microorganism. Section D.6 in Appendix D describes certain aspects of using 

CFD to predict UV dose delivery. A possible approach for verifying and validating hydraulic 

CFD models is outlined in the AIAA CFD guide (1998). Another emerging experimental 

approach uses microspheres that undergo a chemical reaction when exposed to UV light 

(Blatchley et al. 2005). The microspheres are injected upstream of the UV reactor and are 

collected downstream. The extent of the UV light-induced chemical reaction within each sphere 

is measured and used to calculate the UV dose delivered to that sphere as it traveled through the 

reactor. 

 Although model and experimental-based approaches clearly have potential for use in 

validating UV reactors, they are still subjects of current research. EPA anticipates that these 

methods will continue to develop and improve in the future.

5.3 Selecting the Challenge Microorganism 

 For the reasons stated in Section 5.2.1, the disinfection performance of the UV reactor is 

measured using a non-pathogenic “challenge” microorganism. Ideally, the challenge 

microorganism should have the same sensitivity to UV light (i.e., the same microbial doseresponse) as the target pathogen.4

 If medium-pressure (MP) lamps are used, the organism should 

display a similar action spectrum, which is the relative sensitivity of the organism at other 

wavelengths compared to its sensitivity at 254 nm. In addition, the challenge microorganism 

should be: 

• Easily cultured and enumerated, with repeatable results, 

• Culturable to high concentrations, and 

• Stable over long periods of time (to allow for shipment to and from the laboratory, 

on-site use, and enumeration without loss of viability or change in UV doseresponse). 

4

 In this guidance document, the UV sensitivity of the target microorganisms Cryptosporidium, Giardia, and 

viruses is defined by the required UV doses as presented in Table 1.4.
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UV Disinfection Guidance Manual 5-8 November 2006 

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• If the challenge microorganism is a phage, the host bacteria used to assay the phage 

concentration should not be pathogenic to humans. 

 Male-specific-2 bacteriophage (MS2) phage and B. subtilis spores historically have been 

used for validation testing to receive treatment credit for Cryptosporidium and Giardia. Because 

their UV resistance is notably greater than that of Cryptosporidium and Giardia, other, more 

sensitive microorganisms such as T1 and T7 phage are gaining favor (Mackey et al. 2006). 

 To demonstrate 3- or 4-log inactivation for viruses, validation testing would need to 

demonstrate greater than 6-log inactivation of MS2 phage and B. subtilis spores. Such a 

demonstration requires an extremely high concentration in the reactor influent to allow for 

enumeration of the organisms in the effluent samples. Because of the need for serial dilutions, 

these high concentrations are difficult to measure and can introduce error into the experiment. 

Research to find an alternative challenge microorganism for demonstrating virus inactivation is 

ongoing. 

 Other challenge microorganisms that have been used for validation testing include nonpathogenic Escherichia coli, Saccharomyces cerevisae, and Qβ phage. Table 5.2 summarizes the 

UV sensitivity of some commonly used and some candidate bioassay microorganisms. 

Table 5.2. UV Sensitivity of Challenge Microorganisms 

Reported Delivered UV Dose (mJ/cm2) 

to Achieve Indicated Log Inactivation 

Microorganism 1-log 2-log 3-log 4-log Reference 

Bacillus subtilis 28 39 50 62 Sommer et al. 1998

MS2 phage 16 34 52 71 Wilson et al. 1992 

Qß phage 10.9 22.5 34.6 47.6 Mackey et al. 2006 

PRD-1 phage 9.9 17 24 30 Meng and Gerba 1996 

B40-8 phage 12 18 23 28 Sommer et al. 1998 

φx174 phage 2.2 5.3 7.3 11 Sommer et al. 1998 

E. coli 3.0 4.8 6.7 8.4 Chang et al. 1985 

T7 3.6 7.5 11.8 16.6 Mackey et al. 2006 

T1 ~5 ~10 ~15 ~20 Wright 2006 

 

 Some microorganisms, such as B. subtilis, exhibit shoulders or tailing in their UV doseresponse, meaning that the shape of the UV dose-response curve is flat at either low or high 

doses. Shoulders and tailing limit the range of UV doses that can be used to validate the reactor. 

See Section C.6 in Appendix C for an example of shoulders and tailing and limitations of using 

challenge microorganisms exhibiting this response in developing the UV dose-response curve. 

 As noted in Section 5.2.1, the validation test results are adjusted by a VF to account for 

bias and experimental uncertainties. A portion of the VF accounts for the difference in microbial 

response between the challenge microorganism and target pathogen. Using a challenge organism 

with significantly higher UV resistance than the pathogen of interest (e.g., using MS2 to earn 

Cryptosporidium inactivation credit) may result in a high VF. To provide a better estimate of the 

UV dose that a UV reactor can deliver to a target pathogen, a challenge microorganism with 

similar UV sensitivity to the target pathogen can be used. Alternatively, two challenge 
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For the Final LT2ESWTR

microorganisms whose UV sensitivities bracket that of the target pathogen (i.e., one challenge 

microorganism is less resistant and the other is more resistant than the target pathogen) can be 

selected. One advantage to this second approach is that the factor used to account for the 

difference between the microbial response of the challenge microorganism and target pathogen 

can be set to 1.0 (see Section 5.9 for discussion of the RED bias factor). 

 If the UV reactor being validated uses MP lamps and a challenge microorganism other 

than MS2 Phage or B. subtilis, a correction factor should be applied to the test results to account 

for differences in action spectra between the challenge microorganism and the target pathogen. 

Section D.4.1 explains how the correction factor is derived and how it should be applied to 

validation testing results. 

5.4 Equipment Needs for Full-scale Reactor Testing 

 

 As noted in Section 3.6, full-scale reactor validation can occur on-site at the water 

treatment plant or off-site at a third-party validation test center or a UV manufacturer’s facility. 

If full-scale reactor testing is performed off-site, tests are often conducted using a Biodosimetry 

Test Stand as shown in Figure 5.2. Regardless of the testing location, testing equipment should 

include (1) injection pumps and ports to introduce the challenge microorganism, the UVabsorbing compound, and, if needed, a disinfectant residual quenching agent into the feed water, 

(2) rate-of-flow control and a flow meter either upstream or downstream of the reactor, and (3) a 

strategy to ensure that the water is well mixed before sampling (e.g., static mixers or appropriate 

number of pipe lengths with good mixing confirmed, see Section 5.4.3 for details). There should 

also be a state-approved plan for wastewater disposal with any associated required permits. 

 The next several sections provide detailed recommendations regarding water source, the 

UV-absorbing chemical to be used to simulate reduced UVT, mixing, sampling ports, 

configuration of inlet/outlet piping, accounting for non-uniform lamp aging, lamp positioning, 

UV sensors, and UV sensor port windows. 

5.4.1 Water Source 

When validation testing is conducted off-site, the source water for experiments is usually 

a potable water supply with a high UVT. To protect the potable water source, backflow 

prevention should be provided. 

The water passing through the reactor should not contain disinfectant residuals that 

inactivate the challenge microorganism during testing. If this is a concern, a quenching agent can 

be injected into the water upstream of the microorganism injection port. When validating UV 

reactors using MP lamps, the quenching agent should have a minimal impact on the spectral 

UVT from 200 to 400 nm. Some common quenching agents like sodium thiosulfate can have a 

significant impact on the UV absorbance spectra if added in high enough concentrations. Testers 

should use a quenching agent, such as sodium bisulfite, that does not influence UVT. 
5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-10 November 2006 

Figure 5.2. Block Diagram of a Typical Biodosimetry Test Stand for 

Full-scale Reactor Testing 

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

5.4.2 UV Absorbing Chemical 

Typically during validation, a UV-absorbing chemical is injected into the flow to produce 

UVT values that span the required range. Common UV-absorbing compounds include the 

following: 

• Coffee, 

• Lignin sulfonic acid (LSA), and 

• Humic acids, such as those derived from leonardite shales (Mackey et al. 2006, Bircher 

2004). 

5.4.3 Mixing 

Additives passing through the reactor (e.g., UV-absorbing compound, the challenge 

microorganism) should be well mixed through the cross-section of the influent pipe prior to the 

reactor influent sampling port. The challenge microorganisms surviving UV disinfection should 

be well mixed through the effluent pipe cross-section prior to the reactor effluent sampling port. 

Additives to the influent and effluent can be mixed by either using appropriately sized 

and designed static mixers or relying on the turbulent mixing in the lengths of pipe upstream of 

the sampling ports. If the water passing 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. If pumps are 

used to inject the additives, the mechanism should provide a pulse-free flow rate or have a cycle 

time (i.e., time between pulses) an order of magnitude less than the residence time of the reactor. 

The flow rate generated by the pump should be stable over the time required to take samples. 

Adequate mixing at the influent sampling port can be confirmed by comparing the UV 

absorbance at 254 nm (A254) of water samples collected from various locations across the pipe 

cross-section. Samples can be collected across a pipe section using a perforated stab tube. The 

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5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-11 November 2006 

standard deviation of the A254 values measured at different locations should be less than 5 percent 

of the mean A254 value. Another approach is to compare the A254 of water samples collected at 

the influent and effluent sampling ports. The average A254 values of the influent and effluent 

samples should agree within 5 percent and the standard deviation of each should be less than 5 

percent of their respective means. 

The mixing at the effluent sampling port can be confirmed by injecting a UV-absorbing 

chemical into the flow at a location immediately downstream of the UV reactor and comparing 

the A254 of water samples collected from various locations across the pipe cross-section. 

Alternatively, the A254 of water samples collected at the effluent sampling port and a second 

effluent sampling port located five pipe diameters or more downstream of the first can be 

compared. The water samples should meet the criteria given above for the influent samples. 

Mixing tests should be done at the minimum and maximum flow rates with the UVT 

adjusted to the minimum value that will be used during testing. If the water samples collected 

during the tests do not meet the above criteria for good mixing, the mixing should be increased 

and retested. 

5.4.4 Sampling Ports 

 The sampling points for microorganisms should be located far enough from the UV 

reactor that the germicidal UV intensity at the point of sampling is < 0.1 percent of the 

germicidal intensity within the UV reactor. If the outlet sample port is located downstream of a 

90o

 bend (or the inlet sample port is upstream of a 90o bend), incident light is not a concern. 

 To estimate intensity at a certain distance from the reactor, the following equation can be 

used: 

L re e

r

P rI α

π

− = 2 )( Equation 5.2 

 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 for water, (~0.015 cm-1) 

 I(r) = UV intensity (mW/cm2

) at a distance r from the line source 

For example, suppose the outlet sample port at a hypothetical UV test facility is located 10 feet 

downstream of the last UV lamp in a reactor. The lamp’s maximum power per unit arc length is 

100 watts per centimeter (W/cm). Using Equation 5.2, the intensity at a radial distance of 10 feet 

(305 cm) is calculated to be 5.4 x 10-4 mW/cm. Because this intensity is less than 0.1 percent of 

the intensity within the reactor, the sample port location is acceptable. 

Sample taps may draw water from a single point or simultaneously from multiple points 

across the pipe diameter. Samples taken from multiple points within the flow should have the 

same concentration of additives and microorganisms (within the measurement error of the 

analytical method). If samples from different points in the flow have different concentrations, the 

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5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-12 November 2006 

flow at the sampling point is either insufficiently mixed or not at steady state. Sampling from 

multiple points at the same time should not be used to compensate for poor mixing. 

5.4.5 Configuration of Inlet and Outlet Piping 

 

 Appendix D describes how flow conditions of the water can significantly impact dose 

delivery inside a UV reactor. Flow conditions are dependent on the velocity of the water and 

configuration of inlet and outlet piping. If the reactor is validated off-site, the inlet and outlet 

piping at the water treatment plant should result in a UV dose that is the same or greater than

the dose delivered at the validation test facility. Section 3.6 provides suggestions for inlet and 

outlet piping design for water treatment plants and validation testing facilities. 

Computational fluid dynamics (CFD) is a tool that can be used to assess whether the dose 

delivery at the treatment plant is the same or greater than the dose delivery at the validation 

testing facility. Section D.6 in Appendix D provides guidelines on using CFD for the purposes of 

modeling UV dose delivery. 

5.4.6 Accounting for Non-uniform Lamp Aging 

As will be discussed in Section 5.6, validation testing of full-scale reactors should 

account for decreased UV light transmittance caused by sleeve fouling, sleeve aging, lamp aging, 

and UV sensor window fouling. During design, the engineer and UV manufacturer will typically 

estimate a “fouling factor” and an “aging factor” for the reactor. The fouling factor is defined as 

the fraction of UV light passing through a fouled sleeve as compared to a new sleeve. The aging 

factor is the fraction of UV light emitted from aged lamps and sleeves at the end of the specified 

useful life compared to UV light emitted from new lamps and sleeves. The “fouling/aging 

factor” is equal to the fouling factor multiplied by the aging factor and typically ranges from 0.4 

to 0.9 (NWRI 2003). See Section 3.4.5 for a more detailed discussion on determining these 

factors. 

Traditionally, lamp power is turned down to simulate aging and fouling during validation 

testing. The magnitude of the power reduction (or power turn-down) is determined by calculating 

the relative sensor intensity, which is defined as follows: 

o Relativesensorintensity = SS Equation 5.3 

 Where 

 So = UV intensity measured at 100 percent lamp power 

 S = UV intensity measured at reduced lamp power 

For example, if the fouling/aging factor is 0.7, the lamp power would be reduced until the 

relative sensor intensity was 0.7, or 70 percent. 

Recent studies have shown, however, that UV lamps and sleeves can exhibit significant 

non-uniform aging along their length and around their circumference (e.g., Mackey et al. 2005 

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5. Validation of UV Reactors 

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and 2006). Turning down power during validation testing to simulate aging for lamps that 

experience non-uniform aging may result in under-dosing when the reactor is operated at the 

treatment plants, particularly at the end of useful lamp life. This guidance manual recommends 

that water systems use one of the following three alternatives to account for non-uniform lamp 

aging: 

1. Request data from the manufacturer to verify that the lamps age uniformly. The 

manufacturer can provide such verification by simulating lamp aging in a test bed, 

then measuring lamp output at different locations along the length of the lamp and 

around the circumference. Results from a recently-completed AwwaRF study showed 

that output at the lamp ends is usually less than in the middle of the lamp when 

significant non-uniform aging is observed (Mackey et al. 2006). If the manufacturer 

can show that the lamp aging factor either already accounts for non-uniform aging or 

that it is not an issue, power turn-down can be used to simulate lamp aging during 

validation tests. 

2. Use aged lamps (i.e., lamps that have been operated under similar conditions to the 

end of their specified lamp life) for validation testing. Power turn-down to simulate 

lamp aging during validation tests is not necessary in this approach (although power 

turn-down should still be considered to simulate lamp fouling). 

3. Conduct experimental testing to determine if lamp aging can be simulated by power 

turn-down: 

 

a. Prepare a stock solution of the challenge microorganism. 

b. Fit the UV reactor with aged lamps and sleeves. 

c. Pass water through the reactor at a constant UVT, flow rate, and lamp power that 

will be used during challenge testing. 

d. Inject the challenge microorganism into the flow passing through the reactor 

(ensure they are well-mixed prior to entering the reactor). 

e. Collect at least five (5) microbiological samples spaced one (1) minute apart from 

the influent and effluent sampling ports for analysis. 

f. Record the UV sensor measurements. 

g. Fit the UV reactor with new lamps that have undergone 100-hour burn-in and new 

sleeves. 

h. Operate the UV reactor at the flow rate and UVT used in Step c. Lower the lamp 

power to produce a UV sensor reading equivalent to the reading obtained in Step 

f. Repeat steps e and f. 

If the mean log inactivation achieved with aged lamps is similar to the log 

inactivation achieved with new lamps with reduced lamp power, power turn-down 
5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-14 November 2006 

For the Final LT2ESWTR

can be used to simulate lamp aging. If results show significant differences, 

Alternative 2 (aged lamps and sleeves) should be used during validation testing. 

5.4.7 Lamp Positioning to Address Lamp Variability 

Due to manufacturing tolerances and differences in operation and aging, UV output 

typically varies from lamp to lamp. If a UV reactor has fewer UV sensors than lamps and the 

lamps are randomly distributed in the reactor, the UV sensors may monitor the lamps with the 

lowest outputs during validation. If this were to occur, the validation data collected would 

typically lead to under-dosing at the treatment plant. 

To prevent underdosing, the lamps with the highest UV output should be placed closest 

to the UV sensors during validation testing. Other lamps should be randomly distributed in the 

lamp array throughout the reactor. This approach is unnecessary if the reactor uses one UV 

sensor per lamp. 

The lamps with the highest UV output can be identified by measuring UV output using 

either a dedicated test stand or the UV reactor. One approach for determining the UV lamp 

output using the UV reactor is described below. 

Procedure 

1. Install a lamp within a lamp sleeve located at the position nearest to one of the 

reactor’s UV sensors. 

2. Pass water through the reactor at a constant flow rate 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. For full-scale reactor testing, install the lamps with the highest output closest to the 

UV sensors. The rest of the lamps should be randomly distributed (with respect to 

lamp intensity). 

5.4.8 UV Sensors 

UV sensors are photosensitive detectors that measure UV intensity. UV sensors used in 

drinking water UV applications, particularly those with MP or other polychromatic lamps, 

should be germicidal. Germicidal sensors are defined as having the following properties: 

• A spectral response (i.e., UV intensity measured at various wavelengths) that peaks 

between 250 and 280 nanometers (nm). 

• Less than 10 percent of its total measurement is due to light above 300 nm when mounted 

on the UV reactor and viewing the UV lamps through the water that will be treated. 
5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-15 November 2006 

For the Final LT2ESWTR

 

Manufacturers should document the spectral response of the UV sensors. Tables 5.3 and 

5.4 provide two examples of spectral response for a hypothetical germicidal sensor and a 

hypothetical non-germicidal sensor, respectively. Figure 5.3 graphically depicts the spectral 

response for the data in Tables 5.3 and 5.4, respectively. 

Table 5.3 shows that nearly 100 percent of the area under the spectral response curve 

between 200 and 400 nm is at wavelengths below 300 nm - almost none of the sensor reading is 

due to wavelengths greater than 300 nm. Moreover, the peak spectral response is at 270 nm. 

Therefore, this UV sensor is classified as germicidal. Conversely, Table 5.4 reveals that only 74 

percent of the area under the curve is below 300 nm, which means that 26 percent of the area is 

measured at wavelengths greater than 300 nm. Because 26 percent is greater than the maximum 

allowable 10 percent, this UV sensor is classified as non-germicidal. 

EPA recognizes that, before the publication of this document, some UV reactors using 

MP lamps with non-germicidal sensors are in the final design phases, and some have been 

installed at water treatment plants. These water systems should apply a correction factor to 

validation test data to account for polychromatic bias. Section D.4 in Appendix D explains how 

polychromatic bias impacts sensor reading and provides guidelines for deriving the correction 

factor. As noted in Chapter 4, facilities installing new UV treatment systems should use reactors 

that are equipped with germicidal sensors. 

5.4.9 UV Sensor Port Windows 

UV sensors often view the lamps through a UV sensor port window. These windows are 

typically made of quartz and have a UVT greater than 90 percent. The UVT of the sensor port 

windows should be checked before and after validation testing. If the sensor port windows are 

fouled or contaminated, UV sensor readings will be low. If this were to occur during validation 

testing, it could lead to under-dosing at the water treatment plant whenever the sensor port 

windows are clean. A collimated beam apparatus and a radiometer can be used to measure the 

sensor port window UVT either before the reactor is shipped to the test site or during validation 

testing. 

5.5 Accuracy of Measurement Equipment 

During validation testing, all equipment should be carefully selected and calibrated to 

minimize uncertainty. All measurements of flow rate, electrical power consumption, and head 

loss5

 should be traceable to an independent standard. Moreover, because they are key parameters 

that affect UV dose delivery, measurements of UVT and UV intensity should be NIST6

-traceable 

or equivalent7

 with a known measurement uncertainty. 

5

 Although not part of UV validation, headloss as a function of flow rate is often measured during validation testing 

as it offers an opportunity to gather such design data on the system

6 National Institute of Science and Technology

7 For example, the German national testing and standards agency, Physikalisch Technische Bundesanstalt (PTB), or 

the United Kingdom’s National Weights and Measures Laboratory. 
5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-16 November 2006 

For the Final LT2ESWTR

Table 5.3. Hypothetical Example of the Spectral Response of a 

Germicidal UV Sensor 

 

Wavelength 

(nm) 

Spectral Response1

(mW/cm2

) 

Area 

Under the Curve 

Between Readings 

Cumulative 

Area Under the 

Curve 

Cumulative Area

as % of Total Area

200 0.11 – − −

210 0.21 1.600 2 1.6 3% 

220 0.30 2.550 3 4.15 8% 

225 0.35 1.625 5.775 11% 

230 0.40 1.875 7.65 14% 

235 0.48 2.200 9.85 18% 

240 0.58 2.650 12.5 23% 

245 0.72 3.250 15.75 29% 

250 0.88 4.000 19.75 36% 

255 1.03 4.775 24.525 45% 

260 1.15 5.450 29.975 55% 

265 1.23 5.950 35.925 66% 

270 1.30 6.325 42.25 77% 

275 1.21 6.275 48.525 89% 

280 0.30 3.775 52.3 95% 

285 0.19 1.225 53.525 98% 

290 0.08 0.675 54.2 99% 

295 0.03 0.275 54.475 99% 

300 0.02 0.125 54.6 100% 

310 0.01 0.150 54.75 100% 

320 0.00 0.050 54.8 100% 

330 0.00 0.000 54.8 100% 

340 0.00 0.000 54.8 100% 

350 0.00 0.000 54.8 100% 

360 0.00 0.000 54.8 100% 

370 0.00 0.000 54.8 100% 

380 0.00 0.000 54.8 100% 

390 0.00 0.000 54.8 100% 

400 0.00 0.000 54.8 100% 1

 UV intensity measured by the UV sensor. 

2

 (0.21 + 0.11) × (210 − 200) / 2 3

 (0.30 + 0.21) × (220 − 210) / 2
5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-17 November 2006 

For the Final LT2ESWTR

Table 5.4. Hypothetical Example of the Spectral Response of a 

Non-germicidal UV Sensor 

Wavelength 

(nm) 

Spectral 

Response 1

(mW/cm2

) 

Area 

Under the Curve 

Between Readings 

Cumulative Area 

Under the Curve 

Cumulative Area as % 

of Total Area 

200 0.12 – – –

210 0.30 2.100 2 2.1 2% 

220 0.47 3.850 3 5.95 6% 

225 0.55 2.550 8.5 8% 

230 0.63 2.950 11.45 11% 

235 0.71 3.350 14.8 14% 

240 0.79 3.750 18.55 17% 

245 0.87 4.150 22.7 21% 

250 0.94 4.525 27.225 25% 

255 1.01 4.875 32.1 30% 

260 1.07 5.200 37.3 35% 

265 1.12 5.475 42.775 40% 

270 1.14 5.650 48.425 45% 

275 1.15 5.725 54.15 50% 

280 1.12 5.675 59.825 56% 

285 1.07 5.475 65.3 61% 

290 1.00 5.175 70.475 66% 

295 0.91 4.775 75.25 70% 

300 0.82 4.325 79.575 74% 

310 0.63 7.250 86.825 81% 

320 0.48 5.550 92.375 86% 

330 0.38 4.300 96.675 90% 

340 0.31 3.450 100.125 93% 

350 0.23 2.700 102.825 96% 

360 0.15 1.900 104.725 98% 

370 0.10 1.250 105.975 99% 

380 0.06 0.800 106.775 99% 

390 0.03 0.450 107.225 100% 

400 0.00 0.150 107.375 100% 1 UV intensity measured by the UV sensor. 

2

 (0.31 + 0.12) × (210 − 200) / 2 3 (0.47 + 0.30) × (220 – 210 / 2
5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-18 November 2006 

Figure 5.3. Hypothetical Examples of the Spectral Response of a 

Germicidal and a Non-germicidal UV Sensor 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

200 250 300 350 400

Wavelength (nm)

Spectral Response (mW/cm2

)

Germicidal

Non-germicidal

Source: Data from Tables 5.3 (germicidal sensor) and 5.4 (non-germicidal sensor). 

The next three sections provide recommendations for verifying measurement uncertainty 

for flow meters, UV spectrophotometers, and power consumption. Section 5.5.4 provides the 

recommended approach for determining the measurement uncertainty of UV sensors, which the 

LT2ESWTR requires. Tests verifying equipment accuracy (particularly UV sensor checks as 

described in Section 5.5.4) should be documented in the Validation Report (See Section 5.11 for 

guidance). 

5.5.1 Flow Meters 

 

 During validation testing, the uncertainty of flow rate measurements should be less than 

or equal to 5 percent. The measurement uncertainty of the flow meter can be verified by 

comparing measured flow rate to a second, calibrated flow meter or a calibrated pitometer. 

5.5.2 UV Spectrophotometers 

Spectrophotometer measurements of A254 should be verified using NIST-traceable 

potassium dichromate UV absorbance standards and holmium oxide UV wavelength standards. 

Many UV spectrophotometers have their own internal QA/QC procedures to verify calibration. 

UV absorbance of solutions used to zero the spectrophotometer should be verified using reagentgrade organic-free water certified by the supplier to have zero UV absorbance.

For the Final LT2ESWTR
5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-19 November 2006 

The measurement uncertainty of the spectrophotometer should be 10 percent or less. A 

recommended approach for achieving this goal is as follows: 

1. Verify that the spectrophotometer reads the wavelength to within the accuracy of a 

holmium oxide standard (typically ± 0.11 nm at a 95-percent confidence level), 

2. Verify that the spectrophotometer reads A254 within the accuracy of a dichromate 

standard (e.g., 0.281 ± 0.004 cm-1 at 257 nm with a 20 mg/L standard), and 

3. Verify that the water used to zero the instrument has an A254 value that is within 

0.002 cm-1 of a certified zero absorbance solution. 

When the UVT is greater than 90 percent, it is recommended that a 4-cm or greater 

pathlength cuvette be used (as opposed to the standard 1-cm cuvette). This greatly improves the 

accuracy of the UVT measurement at values above 90 percent. Measurements made with a 4-cm 

cuvette can be converted to 1-cm UVT measurements using the following equation: 

 Equation 5.4 4/1 UVT1−cm = UVT4−cm

When validation testing is performed using unfiltered water, the UV spectrophotometer 

should be equipped with an integrating sphere, which will provide more accurate UV absorbance 

readings if there are particles in the water. 

 If the spectrophotometer provides biased readings, the measurements should be corrected 

to account for that bias, or another instrument with measurement uncertainty of 10 percent or less 

should be used. 

5.5.3 Power Measurements 

 Voltmeters, ammeters, and power meters used to measure (1) ballast and UV equipment 

input voltage, and (2) consumed current and power, should bear evidence of being in calibration 

(e.g., have a tag showing that it was calibrated). The accuracy of the measurements can be 

verified using a second instrument or a standard measurement. Power meters should provide a 

measure of true power as opposed to apparent power in units of kilovolt ampere (kVA). 

5.5.4 UV Sensors 

 During validation testing, duty UV sensor measurements should be within 10 percent of 

the average of two or more reference sensor measurements.8 Duty sensors that do not meet this 

criterion should be replaced or the measurement uncertainty should be incorporated into the VF 

(see Section 5.9). 

For the Final LT2ESWTR

 

8

 Note that this error range is smaller than recommended for operations (in Section 6.4.1.1, EPA recommends that 

sensor readings be within 20 percent of the average of two or more reference sensors). EPA believes that a 10-

percent error is easily attainable during validation testing and will help ensure good data quality for developing 

operational setpoints.
5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-20 November 2006 

The following procedure can be used to check the uncertainty of the duty and reference 

UV sensors used during validation (this calculation is also illustrated in the examples in 

Appendix B). 

1. Pass water through the reactor at the maximum UVT and the maximum lamp power 

setting to be used during validation testing. 

2. Using two recently calibrated reference UV sensors (which should agree within the 

calibration certificate-specified measurement uncertainty9

), install each sensor on the 

UV reactor at each port and record the measured UV intensity (Sref). Repeat using 

each duty UV sensor (Sduty). If the UV sensors can be rotated, measure the minimum 

and maximum sensor readings across the complete range of rotation. 

3. Repeat Steps 1 and 2 at either (a) maximum lamp power and UVT decreased to the 

minimum value expected during validation testing, or (b) maximum UVT and lamp 

power decreased to the minimum level expected to occur during validation testing. 

Duty UV sensors can be checked under both conditions, although this is not 

necessary. 

4. For a given lamp output and UVT value, the difference between the reference and 

duty UV sensor measurements should follow Equation 5.5: 

%101

avgRef,

duty ≤−

S

S

Equation 5.5

where: 

 Sduty = Intensity measured by a duty UV sensor 

 SAvg Ref = Average UV intensity measured by all the reference UV sensors in the 

same UV sensor port with the same UV lamp at the same UV lamp power. 

5. Duty and reference UV sensors that do not meet this criterion should be replaced. 

Alternatively, measurement uncertainty can be re-stated at the maximum uncertainty 

observed during validation testing and incorporated into the VF (see Section 5.9). 

Duty sensors should be checked prior to full-scale reactor testing to ensure that the data 

collected during testing will be useful. Sensors are also often spot-checked during and after fullscale testing to verify that they are still within the recommended uncertainty limit. 

5.6 Identifying Test Conditions 

Numerous combinations of experimental tests can be performed to validate a UV reactor. 

The number of tests could range from a few tests to a complex matrix spanning a range of UV 

 

9 If the reference sensors do not agree with the calibration certificate-specified measurement uncertainty, they should 

be sent back to the manufacturer.

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dose, flow rate, UVT, ballast power, and lamp status combinations. The test design (i.e., number 

of tests and test conditions) depends on several factors, as summarized in Table 5.5. 

Table 5.5. Factors to be Considered in Validation Test Design 

Factor Examples 

Section of Manual 

with Additional 

Guidance 

1. Purpose of validation testing Validation of new reactor by water 

system vs. validation to confirm an 

existing validation equation 

3.6 

5.6 

2. Dose-monitoring strategy of the UV 

reactor 

UV Intensity Setpoint Approach vs. the 

Calculated Dose Approach1

3.5.2 

3. Operational strategy (for the UV 

Intensity Setpoint Approach only) 

Single-setpoint operations vs. variablesetpoint operations 3.5.2 

4. Predicted lamp aging and fouling Aging factor of 0.8 vs. using aged lamps 

used during validation testing (where the 

aging factor would equal 1.0) 

3.4, 5.4.6 

5. Target pathogen and target log 

inactivation 

2.0 log inactivation of Cryptosporidium

vs. 3.0 log inactivation of viruses 

3.1 

6. Full operating range of flow rate and 

UVT 

Range of flow = 5 – 20 mgd, Range of 

UVT = 70 – 90 % 

3.4 

1

 As noted in Section 3.5.2, there are many dose-monitoring strategies for UV reactors. This guidance manual 

focuses on two common strategies, the UV Intensity Setpoint Approach and the Calculated Dose Approach. 

 Although all factors in Table 5.5 influence test design, the total number of experiments is 

highly dependent on the first three factors. For example, suppose a water system wants to 

validate a new UV reactor that uses the UV Intensity Setpoint Approach. The system decides to 

use single-setpoint operations, meaning that it will use one UV intensity setpoint for all operating 

conditions. Validation testing in this case would be fairly straightforward with a small number of 

tests. If another water system selects the same reactor but selects variable-setpoint operations to 

allow it to reduce lamp power at low flow rates, that system would conduct more validation tests 

to establish different setpoints for the different flow rate ranges. Another common scenario is 

when a manufacturer decides to validate a new UV reactor over a wide range of flow rates, UVT 

levels, and lamp status combinations to develop a dose-monitoring equation. This scenario likely 

would necessitate many tests. 

 As noted in Section 5.4.6, lamp fouling and aging are important factors that should be 

accounted for during validation testing. Power turn-down is typically used to simulate lamp 

aging and fouling at the end-of-lamp life. Instead of reducing power to simulate lamp aging, 

aged lamps can be used during validation testing (although power-turn down would still be 

needed to simulate lamp fouling). Section 3.4.5 provides information on how the fouling/aging 

factors are estimated, and Section 5.4.6 provides guidance on using new versus aged lamps 

during validation testing. 

 If a new, un-validated reactor is being tested for a specific water system, the last two 

items listed in Table 5.5 can help establish validation test conditions. The target pathogen and 

target log inactivation for the water system define the required dose that is the target for 
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validation testing. The full range of operating conditions for flow rate and UVT dictate the flow 

rate and UVT conditions used during validation testing. 

 Sections 5.6.1 and 5.6.2 discuss the validation test design for the UV Intensity Setpoint 

Approach and the Calculated Dose Approach, respectively. Test designs for other dosemonitoring strategies that use setpoints should be similar to recommendations in Section 5.6.1 

and should be developed using professional judgment. Section 5.6.3 provides considerations for 

water systems who are confirming an existing dose-monitoring equation (as developed for the 

Calculated Dose Approach). Section 5.6.4 lists the types of quality control samples that should 

be collected and analyzed during testing. Appendix C provides guidelines for identifying test 

conditions for collimated beam testing. 

 Experimental test conditions should be documented in a Validation Test Plan. 

Section 5.11.2 provides recommendations on what a Validation Test Plan should contain. EPA 

recommends including the Test Plan into the final Validation Report (see Section 5.11.3). 

5.6.1 Test Conditions for the UV Intensity Setpoint Approach 

For the UV Intensity Setpoint Approach, the purpose of validation testing is to determine 

the validated dose corresponding to the UV intensity setpoint for a reactor at a particular flow 

rate. Typically, the manufacturer determines the UV intensity setpoint for their reactor. If this is 

the case, water systems should work with the manufacturer to ensure that the setpoint is defined

conservatively low enough to account for combined conditions of minimum UVT and maximum 

fouling/aging (commonly represented by the fouling/aging factor). If the manufacturer does not 

establish the UV intensity setpoint for their reactor, the water system can select a setpoint using 

the following procedure: 

1. Record the UV intensity measurement at conditions of maximum UVT and 100 

percent power (So)

10. 

2. Reduce the lamp power until the measured UV intensity results in the following 

relative sensor intensity (S/So per Equation 5.3): 

a. If aged lamps are used during validation testing, the relative sensor intensity 

should be equal to the fouling factor. 

b. If new lamps are used during validation testing, the relative sensor intensity 

should be equal to the fouling/aging factor, which is the fouling factor multiplied 

by the aging factor. 

3. Reduce the UVT of the water to the minimum UVT (see Section 3.4 for guidance on 

determining the minimum UVT). 

10 The impacts of lamp power and UVT on UV sensor readings are not dependent on the specific rate of flow 

traveling through the reactor. Thus, any flow rate can be used for this procedure. 
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4. Record the UV Intensity at reduced power and reduced UVT conditions. This 

intensity is the UV intensity setpoint.

The UV Intensity Setpoint approach uses two validation test conditions, as specified in 

Table 5.6. The first involves reducing UVT until UV intensity measured by the UV sensor is 

equal to the UV intensity setpoint. The second involves testing at high UVT but reducing power 

until the UV intensity measured by the sensor is equal to the UV intensity setpoint. Additional 

test conditions should be evaluated if the water system will be using variable setpoint operations 

(i.e., each test condition in Table 5.6 should be repeated at different flow rates). 

Table 5.6. Minimum Test Conditions for the UV Intensity Setpoint Approach1

Test ID2 Flow Rate UVT Lamp Power 

1 Design (highest) Lowered to give the 

UV intensity setpoint 3

Maximum (100 %) 

2 Design (highest) Maximum Lowered to give the UV intensity 

setpoint 3

1

 Minimum test conditions shown are for single-setpoint operations. Additional tests should be conducted 

at different flow rates for variable setpoint operations. 2

 At least three replicate tests with the same stock solution of challenge microorganisms should be 

performed for each test condition. 3

 The UV intensity setpoint is typically established by the manufacturer. Alternatively, it can be 

established by the water system using the procedure in Section 5.6.1. 

 Water systems may decide to use two challenge microorganisms with different UV 

sensitivities for validation testing (see Section 5.3 for additional discussion). In many cases, 

challenge microorganisms can be tested at the same time if they have been proven not to 

interfere with each other. 

 The validation approach described herein produces a UV intensity setpoint and Validated 

Dose that are independent of UVT. Thus, UVT is not typically monitored during reactor 

operations. 

5.6.2 Test Conditions for the Calculated Dose Approach 

 For the Calculated Dose Approach, the purpose of validation testing is to develop a dosemonitoring equation relating RED11 to operating parameters such as flow rate, UVT, lamp power 

(quantified as relative sensor value), and in some cases lamp status. For each operating 

parameter used in the equation, at least three conditions should be evaluated during validation 

testing. Three data points are needed for interpolation of results because the relationship between 

RED and operating parameters such as flow rate and UVT is typically non-linear. 

 

11 As a reminder, RED is the reduction equivalent dose, which is determined by inputting the measured log 

inactivation (observed during full-scale reactor testing) into the UV dose-response curve (generated through 

collimated beam testing). 
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In many cases, three operating parameters (UVT, flow rate, and lamp power) are used in 

the dose-monitoring equation, resulting in a minimum of 27 test conditions (3 × 3 × 3). Fewer 

test conditions are needed when the dose monitoring equation is based on fewer than three 

parameters, such as when a minimum UVT is assumed for all operating conditions. More than 27 

test conditions may be needed when the water system plans to vary lamp status during operations 

(e.g., UVT, flow rate, and lamp power are used in the dose monitoring equation and individual 

banks of lamps will be turned off and on to conserve power). 

If validation tests are being conducted for a specific water system, the system’s operating 

range of flow rates, UVT, and the required UV dose for their target pathogen and log inactivation 

help establish test conditions. For flow rate, the water system’s maximum and minimum flow 

rates, as well as one or more intermediate flow rates, should be selected as test conditions. To 

select intermediate flow rates, EPA recommends using a geometric progression (because the 

relationship between UV dose and flow is non-linear) using the following equation: 

n

n QQ Max

− = 1 β Equation 5.6 

where: 

 Qn = nth flow rate to be tested 

 QMax = Maximum flow rate to be tested 

β = Constant with a recommended value between 1.5 and 2.0 to achieve good 

separation of flow measurements 

 n = Flow rate test # to be evaluated (must be ≥ 3, if interpolating results) 

The value of β should be sufficient to obtain at least three measured data points for 

developing the dose-monitoring equation. The value of n should be selected to span the range of 

flow rates. An example using Equation 5.6 is provided below. 

Example 5.1. Determining Flow Conditions for Validation Testing. A UV reactor using the 

Calculated Dose Approach and operating within the range of 5 – 20 mgd is to be validated. The 

test engineer selects a ß value of 1.6, resulting in the following test flow rates: 

 n Q (mgd)

1 20 

2 12.5 

3 7.8 

4 4.9 

 For UVT, test conditions should include the water system’s minimum UVT, maximum 

UVT, and at least one intermediate value. If the dose-monitoring equation will account for 

specific lamps operating either on or off or other power manipulations, validation test design 

should include these conditions. 

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 Table 5.7 summarizes the recommended minimum test conditions for the Calculated 

Dose Approach. Table B.9 in Appendix B presents an example test matrix for a hypothetical 

water system. 

Table 5.7 Minimum Test Conditions for the 

Calculated Dose Approach1

Test ID2 UVT Flow rate3 Flow Rate3 Lamp Power 4

1 Maximum Design Maximum 

2 Maximum Intermediate Maximum 

3 Maximum Minimum Maximum 

4 Maximum Design Minimum expected to occur during operations 

5 Maximum Intermediate Minimum expected to occur during operations 

6 Maximum Minimum Minimum expected to occur during operations 

7 Maximum Design Intermediate 

8 Maximum Intermediate Intermediate 

9 Maximum Minimum Intermediate 

10 Intermediate Design Maximum 

11 Intermediate Intermediate Maximum 

12 Intermediate Minimum Maximum 

13 Intermediate Design Minimum expected to occur during operations 

14 Intermediate Intermediate Minimum expected to occur during operations 

15 Intermediate Minimum Minimum expected to occur during operations 

16 Intermediate Design Intermediate 

17 Intermediate Intermediate Intermediate 

18 Intermediate Minimum Intermediate 

19 Minimum Design Maximum 

20 Minimum Intermediate Maximum 

21 Minimum Minimum Maximum 

22 Minimum Design Minimum expected to occur during operations 

23 Minimum Intermediate Minimum expected to occur during operations 

24 Minimum Minimum Minimum expected to occur during operations 

25 Minimum Design Intermediate 

26 Minimum Intermediate Intermediate 

27 Minimum Minimum Intermediate 

1

 Assuming validation on a non-validated UV reactor. Minimum test conditions shown are for all lamps turned on. 

Additional tests should be performed to evaluate other lamp on/off combinations or other power combinations. 2

 At least three replicate tests with the same stock solution of challenge microorganisms should be performed for each 

test condition. 

3

 See Section 3.4 for guidelines on identifying design flow and minimum and maximum UVT. 

4

 Minimum power should include reduction in lamp output caused by fouling and aging. 
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5.6.3 Test Conditions for Confirming an Existing Validation Equation 

 Water systems may decide to perform on-site validation testing to show that the hydraulic 

conditions at the water treatment plant result in a UV dose that is the same or greater than the 

UV dose delivered at the off-site validation test facility. Test conditions should generally span 

the range of operating conditions expected at the treatment plant (e.g., minimum and maximum 

UVT, minimum and maximum flow rate). See Section 3.6.2 for additional discussion on 

validation testing scenarios. 

 EPA cautions water systems on combining on-site validation testing data with off-site 

validation data to develop a new dose-monitoring equation. On-site and off-site testing is often 

done under different hydraulic conditions and may produce different results. Combining the 

datasets may result in greater noise about the fit for the dose-monitoring equation and, thus, a 

higher uncertainty factor (see Section 5.9.2.2 for a discussion of the uncertainty in interpolation 

factor) 

5.6.4 Quality-control Samples 

Recommended quality-control samples for full-scale reactor testing are listed below. 

• Reactor controls – influent and effluent water samples taken with the UV lamps (in 

the reactor) turned off. The change in log concentration from influent to effluent 

should correspond to a change in RED (from the UV dose-response curve) that is 

within the measurement error of the minimum RED measured during validation 

(typically 3 percent or less). 

• Reactor blanks – influent and effluent water samples taken with no addition of 

challenge microorganism to the flow passing through the reactor. Blanks should be 

collected at least once on each day of testing and the concentration of challenge 

microorganisms should be negligible. 

• Trip controls – one sample bottle of challenge microorganism stock solution should 

travel with the stock solution used for validation testing from the microbiological 

laboratory to the location of reactor testing and back to the laboratory. The change in 

the log concentration of the challenge microorganism in the trip control should be 

within the measurement error. (i.e., the change in concentration over the test run 

should be negligible. This is typically on the order of 3 to 5 percent. 
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• 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, according to 

Standard Methods for the Examination of Water and Wastewater (APHA et al. 1998). 

• Stability samples – influent and effluent samples collected at low and high UVT that 

are used to assess the stability of the challenge microorganism concentration and its 

UV dose-response over the time period from sample collection to completion of 

challenge microorganism assay. The challenge microorganism concentrations in the 

stability samples should be within 5 percent of each other. 

5.7 Guidelines for Conducting Experimental Tests 

 Section 5.7.1 provides general guidelines for preparing the challenge microorganism for 

testing. Sections 5.7.2 and 5.7.3 provide recommendations for conducting full-scale reactor 

testing and collimated beam testing, respectively. Appendix C contains more detail on the 

collimated beam testing methods. Importantly, the recommendations in this section and in 

Appendix C are not step-by-step procedures, but rather an identification of key steps in the 

process. Individuals performing full-scale reactor testing and collimated beam testing should 

work closely with the laboratory personnel and experts in the field of validation testing to ensure 

that appropriate procedures and QA/QC steps are followed. 

5.7.1 Preparing the Challenge Microorganism 

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 specified in Standard Methods for the Examination of Water 

and Wastewater (APHA et al. 1998). Protocols for culturing the challenge microorganism and 

measuring its concentration should be defined and based on published and peer-reviewed 

methods. 

The challenge microorganism concentrations should be stable over the holding time 

between sampling and completion of the assays. If they are not stable, the data collected will be 

unusable because distinguishing the sources of inactivation—exposure to UV light and die-off in 

holding—will be impossible. Instability problems with MS2 phage are well documented in the 

literature (Petri et al. 2000, Swaim et al. 2003, Hargy et al. 2004). Factors that can impact MS2 

phage stability in water include the presence of chlorine, coagulants, ionic strength, surfactants, 

and UV absorbers (Thompson and Yates 1999, Petri et al. 2000, Hargy et al. 2004). Laboratory 

methods can also impact the stability of MS2 phage in water (Thompson and Yates 1999). 

Microbial stability in the test water should be verified before experimental testing begins. 

Stability verification can help ensure that the bioassay and challenge microorganism samples will 

be viable and the data useable. 
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Appendix A provides recommended procedures for preparing stock solutions of MS2 

phage and B. subtilis spores and assaying their concentrations in water samples. Alternative 

procedures and challenge microorganisms can be used if they are acceptable to the state. 

5.7.2 Full-scale UV Reactor Testing 

Three key steps comprise full-scale reactor testing: (1) verifying reactor properties, (2) 

installing the reactor, and (3) conducting the tests. These steps are summarized below. Note that 

key steps are based on UV reactor testing at an off-site validation test facility. Additional steps 

may be necessary for on-site validation. 

Verifying UV Reactor Properties 

For validation, the UV manufacturer should provide the following: 

• A UV reactor that matches the provided specifications. 

• Duty and reference UV sensors that match the provided specifications. 

• UV lamps that have undergone appropriate burn-in. If new lamps are to be used, the 

recommended burn-in period is 100 hours. If aged lamps are to be used, the 

recommended burn-in period is that which will produce lamp output equivalent to the 

fouling/aging factor. More information on aged lamps is provided in Section 5.4.6. 

• For UV reactors with more than one lamp per UV sensor, lamps with the highest 

output positioned closest to the sensor. (See Section 5.4.7 for additional guidance on 

sensor positioning to address lamp variability.) 

• Provisions to reduce lamp output. 

• Provisions to measure the electrical power delivered to the lamps. 

• A temperature sensor and safety cut-off switch to prevent overheating if MP lamps 

are used. 

Installing the UV Reactor 

The UV reactor and the reactor inlet and outlet connections 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 the specifications 

provided by the UV reactor manufacturer. The piping should be inspected to ensure compliance 

with the manufacturer’s specifications. The configuration of inlet and outlet piping to and from 

the reactor and its impact on validation testing is discussed in Sections 3.6 and 5.4.5. Good 

mixing should be confirmed. 
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The physical integrity of the UV reactor and the test train should be verified before 

testing. Personnel who operate the UV reactor during all tests should be familiar with its 

operation and maintenance manual and with any safety requirements. 

Measuring UV Dose Delivery 

During full-scale reactor testing, the reactor is operated at each of the test conditions for 

flow rate, UVT, and lamp power (in accordance with the Validation Test Plan) as described in 

Section 5.6. The following steps should be taken to ensure good results: 

• Confirm steady-state conditions before injecting the challenge microorganism by 

monitoring the UV sensor measurements and the UVT. 

• Inject the challenge microorganism, prepared according to Appendix A, into the flow 

upstream of the reactor. 

• Collect at least three (3) influent and three (3) effluent samples for each test 

condition. Sample volumes should be sufficient for assessing the challenge 

microorganism concentrations in the influent and effluent (typically 10 – 15 mL). 

• Measure and record the flow rate through the reactor, all UV sensor measurements, 

on-line UVT measurements, and any calculated UV dose values both before and after 

the samples are collected. 

• Measure and record the UVT as measured by the UV spectrophotometer with each 

influent sample. 

• Measure and record the electrical power consumed by the lamp ballasts. 

• Repeat the test if the flow rate, UV intensity, lamp power, or UVT changes by more 

than the recommended error of the measurement over the course of sampling (see 

Section 5.5). 

Sample 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. 

1998). Samples should be collected in bottles that have been cleaned and sterilized and should be 

immediately stored on ice, within a cooler, in the dark until analyzed. 

The concentrations of the challenge microorganisms before and after exposure to UV 

light should generally be measured within 24 hours of sample collection, unless stability studies 

indicate that the samples can reliably be considered stable over longer periods of time. Samples 

that are not assayed immediately should be stored in the dark at 4 o

C. Exposure of samples to 

visible light should be avoided. 
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5.7.3 Collimated Beam Testing 

 Collimated beam tests are performed in microbiological laboratories under controlled 

conditions. Recommended test procedures are provided in Section C.2.3. Importantly, all 

collimated beam testing should be conducted using a water sample collected from the influent 

sampling port of the biodosimetry test stand. If the full-scale reactor testing lasts for more than 

one day, at least one collimated beam test should be conducted for each day of testing. A 

minimum of two collimated beam tests is always recommended, one each at the highest and 

lowest UVT values evaluated during full-scale reactor resting. 

5.8 Analyzing Experimental Data 

 Validation testing of UV reactors produces the following types of data for each 

experimental test: 

• Concentration of the challenge microorganism in the influent and effluent sample 

[e.g., plaque forming units per milliliter (pfu/mL) for MS2 phage, colony forming 

units per milliliter (cfu/mL) for B. subtilis spores] 

• UVT of water (percent) 

• Flow rate [gallon per minute (gpm) or mgd] 

• UV intensity as measured by the UV sensor (mW/cm2) 

• Lamp power [watt (W) or kilowatt (kW)] 

• Status (on/off) for each lamp 

All experimental data should be documented, preferably in tabular format, and included in the 

Validation Report. (See Section 5.11.3 for additional guidance on the Validation Report and 

Appendix B for examples.) 

 Section 5.8.1 shows how RED is calculated for each experimental test using a 

combination of full-scale reactor testing data and collimated beam results. Additional analyses of 

RED data depend on the reactor’s UV dose-monitoring strategy. For the UV Intensity Setpoint 

Approach, RED results are averaged for each test condition and evaluated to identify the 

minimum value. For the Calculated Dose Approach, all RED values and associated test 

conditions are used to create a dose-monitoring equation. Sections 5.8.2 and 5.8.3 summarize 

recommended next steps for evaluating RED data for the UV Intensity Setpoint Approach and 

Calculated Dose Approach, respectively. 

 

5.8.1 Calculating the Reduction Equivalent Dose (RED) 

The RED should be calculated for all full-scale reactor test conditions, individually for 

each replicate, using the following method: 
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UV Disinfection Guidance Manual 5-31 November 2006 

1. For each test condition replicate (i.e., influent and effluent sample pairs), calculate the 

log inactivation (log I) using Equation 5.7: 

⎟

⎠

⎞ ⎜

⎝

⎛ = N

N I o log log Equation 5.7 

where: 

 No = Challenge microorganism concentration in influent sample (pfu/mL or 

cfu/mL) 

 N = Challenge microorganism concentration in corresponding effluent sample 

(pfu/mL or cfu/mL) 

2. Determine the RED, in mJ/cm2

 for each test condition replicate pair using the 

measured log inactivation (log I) and the UV dose-response curve developed through 

collimated beam testing (see Appendix C). If individual UV dose-response curves 

cannot be combined, the curve for a given day of testing should be used to determine 

RED for full-scale reactor testing data collected that day. If individual dose-response 

curves developed on the same day of testing cannot be combined, the curve resulting 

in the most conservative (lowest) RED values should be used.

 Note that for the UV Intensity Setpoint Approach, replicates for a given test condition are 

averaged. For the Calculated Dose Approach, replicates are evaluated separately to develop the 

UV dose-monitoring equation. 

 Appendix B shows RED calculations for two hypothetical water systems. Example 5.2 

shows the key inputs and results for the hypothetical water system in Section B.1. 

Example 5.2. Calculating RED Using Validation Test Data. Collimated 

beam testing using a challenge microorganism produces the following UV 

dose-response curve: 

 UV Dose (mJ/cm2

) = 2.18(log I)2 + 15.30(log I) (from Figure B.2)

Full-scale reactor testing produces the following data for each test condition 

and replicate test:

Test 

Condition Replicate 

No 

(pfu/mL) N (pfu/mL) Log I 

RED 

(mJ/cm2

) 

1 1 5.94 4.57 1.37 25.1 

1 2 6.00 4.54 1.46 27.0 

1 3 5.84 4.56 1.28 23.2 

2 1 6.01 4.10 1.91 37.2 

2 2 5.99 4.09 1.9 36.9 

2 3 6.04 4.06 1.98 38.8 

The RED values for each test are shown in the last column. 

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 If the UV reactor uses MP lamps and validation testing is performed using a challenge 

microorganism other than MS2 phage or B. subtilis, an action spectra correction factor (CFas) 

may need to be applied to the RED values to account for differences in the action spectra of the 

target pathogen and challenge microorganism. Section D.4.1 in Appendix D describes this 

concept and presents the correction factors that should be used for the RED adjustment (i.e., 

divide RED by the correction factor). 

 If validation testing is done with two challenge microorganisms whose UV sensitivities 

bracket the UV sensitivity of the target pathogen (i.e., one microorganism is more resistant and 

one is less resistant), the following approach can be used to estimate the RED of the target 

pathogen for each test condition: 

1. For each test condition, calculate the UV sensitivity (mJ/cm2

 per log I) of the 

challenge microorganism using the following equation: 

UV sensitivity = RED / Log I Equation 5.8

 where: 

 RED = The RED for the test replicate as derived by inputting Log I into the 

UV dose-response equation 

 Log I = log inactivation for the test replicate as calculated using Equation 5.7 

2. Create a graph with UV sensitivity on the x-axis and RED (mJ/cm2

) on the y-axis for 

each test condition. 

3. For each challenge microorganism, plot paired UV sensitivity and RED values on the 

graph (2 values). 

4. Draw a straight line between the two points. 

5. Determine the UV sensitivity for the target pathogen by selecting the UV dose from 

Table 1.4 for 1 log inactivation (log I = 1) 

6. Using the straight line in the graph created in Step 4, read the corresponding RED 

value for the UV sensitivity of the target pathogen (as determined in Step 5). 

 Example 5.3 shows this procedure using hypothetical validation test data. As noted in 

Section 5.3, the main advantage of testing two challenge microorganisms whose UV sensitivities 

bracket the sensitivity of the target pathogen is that the factor used to account for challenge 

microorganism bias (the RED Bias factor) can be set to 1.0. (See Section 5.9 for discussion of 

the RED bias factor.) 
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Example 5.3. Validation Testing Using Two Challenge Microorganisms 

Validation testing is performed using MS2 and φx174. The table below summarizes average 

results for three replicates for one test condition (high UVT). 

Challenge 

Microorganism 

Influent 

Conc. 

(pfu/mL) 

Effluent

Conc. 

(pfu/mL)

UV 

Sensitivity 

(mJ/cm2

 per log I)1 Log I2

RED 

(mJ/cm2

)

3

MS2 1×106 1×104 20 2.0 40 

φx174 1×104 0 2 ≥4.0 ≥8.0 

1

 As derived from collimated beam testing data for a log inactivation of 2.0 for MS2 and 4.0 for φx174 

using Equation 5.9. 2

 Based on measured influent and effluent microorganism concentrations from validation testing. 3

 Determined by inputting log I into the UV dose-response equation. 

Paired UV sensitivity and RED values for MS2 and φx174 were plotted on the graph below. 

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25

UV Sensitivity (mJ/cm2

 per log I)

RED (mJ/cm2

)

Straight-line interpolation between the two points yields the following equation: 

78.1 ×= UV RED ySensitivit + 44.4

The equation above predicts that the RED delivered to Cryptosporidium, defined with a UV 

sensitivity of 3.9 mJ/cm2

 per log inactivation (Table 1.4), is: 

2 RED =+×= /4.1144.9.378.1 cmmJ 4 

Because the RED represents the dose delivered to Cryptosporidium, the RED Bias Factor is 

equal to 1.0. 

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5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-34 November 2006 

5.8.2 Selecting the Minimum RED for the UV Intensity Setpoint Approach 

 Replicate RED values (typically 3 – 5 values) should be averaged to produce one RED 

for each test condition. From these average values, the minimum RED should be selected and 

used in the validated dose calculation. If variable-setpoint operations will be used at the water 

treatment plant (i.e., different UV intensity setpoints for different flow rate ranges), the minimum 

RED value should be identified for each flow rate range. 

 Table 5.6 in Section 5.6.1 presents the two test conditions that should be evaluated, at a 

minimum, for the UV Intensity Setpoint Approach. If the UV sensor is in the ideal location (i.e., 

a location that gives UV dose delivery proportional to the UV sensor reading), the two test 

conditions should yield the same RED. If the sensor is located farther from the lamp than the 

ideal location, the minimum RED will be produced under minimum UVT/maximum power 

conditions (Test 1). If the sensor is located closer to the lamp than the ideal position, the 

minimum RED will be produced under maximum UVT /minimum power conditions (Test 2). 

Selecting the minimum RED from these two test conditions accounts for UV reactor designs 

where the sensor is not in the ideal location. See Section D.2 in Appendix D for additional 

discussion on UV sensor positioning. 

5.8.3 Developing the Dose-monitoring Equation for the Calculated Dose 

Approach 

If the reactor uses the Calculated Dose Approach, validation testing results are used to 

develop a dose-monitoring equation for RED. The variables in the dose-monitoring equation are 

typically flow rate, UVT, UV intensity, or some subset thereof. The number of operating banks 

of lamps is also a possible variable for the equation for those water systems that use multiple 

banks. 

EPA recommends using multivariate linear regression to fit an equation to the validation 

test data. Procedures for multivariate linear regression can be found in standard statistical 

textbooks such as Draper and Smith (1998). Software packages, such as Microsoft Excel, can 

also be used to perform the regression analysis and determine the goodness-of-fit. Recommended 

steps for the analysis are summarized below. 

1. Fit an equation for RED as a function of the operating parameters of interest 

(using all the replicate inlet-and-outlet pairs) using multivariate linear 

regression. The equation used for interpolating validation data may have various 

forms depending on how it was derived. An empirical equation that can often provide 

a good fit to validation data has the following form (Wright et al. 2005): 

e

c d

o

a b B QS RED A S ⎟ × ⎠

⎞ ⎜

⎝

⎛ ⎟ × ⎠

⎞ ⎜

⎝

⎛ 10 ××= 1

254 Equation 5.9 

or in linear form, 

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5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-35 November 2006 

( ) () Be Q d S

S cAbaRED o

⎟ ×+⎠

⎞ ⎜

⎝

⎛ ⎟ ×+⎠

⎞ ⎜

⎝

⎛ ×+= ×+ 1 log log 254 log log Equation 5.10 

where: 

 RED = The RED calculated with the dose-monitoring equation, also 

referred to as the “calculated dose” in this guidance manual 

 A254 = UV absorbance at 254 nm 

 S = Measured UV sensor value 

 So = UV intensity at 100 percent lamp power, typically expressed as a 

function of UVT. 

 Q = Flow rate 

 B = Number of operating banks of lamps within the UV reactor 

 a, b, c, d, e = Model coefficients obtained by fitting the equation to the data 

Either the full equation or part of the equation can be used for fitting validation data. 

For example, validation data collected at a constant UVT and lamp power setting can 

be fitted using: 

d

Q aRED ⎟

⎠

⎞ ⎜

⎝

⎛ ×= 1 Equation 5.11 

 or in linear form, 

( ) ⎟

⎠

⎞ ⎜

⎝

⎛ += Q log RED da log)log( 1 Equation 5.12 

The exact form of the relationship will depend on the UV reactor and the functional 

relationships between RED and each variable. 

The equation should pass through the origin (0,0) if the RED is calculated as a 

function of measured UV intensity or inverse flow rate. A zero measured dose should 

correspond to a zero calculated dose. A non-zero intercept would introduce a bias. 

2. Determine the goodness-of-fit. This can be done using procedures found in standard 

statistics books or by reviewing variance tables produced by statistical programs. The 

analysis should determine the p-statistics for the model coefficients. For the fit to be 

acceptable, the p-statistic for each model coefficient should be ≤ 0.05. 

If the p-statistic for a given model coefficient is greater than 0.05, the coefficient is 

not statistically significant. The coefficients are calculated with all the variables 

included. If the p-statistic for any coefficient exceeds 0.05, then, working in reverse, 

the model coefficient with the highest p-statistic should be dropped from the equation 

and the multivariate regression repeated until all p-statistics are less than or equal to 

0.05. Alternatively, the functional form of the equation could be revised to improve 

the relationship between RED and the parameters of interest (e.g., use Equation 5.12 

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5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-36 November 2006 

instead of Equation 5.10). 

3. Verify that there is no significant bias in the fit. One way to do this is to test for 

randomness in residual values (Draper and Smith 1998). The differences between the 

measured and calculated RED values should be randomly distributed around zero and 

not dependent on flow rate, UVT, or lamp status. 

Because both UVT and UV intensity are part of the dose-monitoring equation, it is not 

important that the sensor be in the ideal location. If the UV sensor is in the ideal location, 

however, UVT could be removed from the dose-monitoring equation. See Section D.2 for 

additional discussion of UV sensor positioning. 

5.9 Deriving the Validation Factor (VF) 

Several uncertainties and biases are involved in using experimental testing to define a 

validated dose and validated operating conditions. For example, a challenge microorganism may 

have a different UV sensitivity than the target pathogen. To determine the validated dose, the 

RED (derived in Section 5.8) is divided by a VF to quantitatively account for key areas of 

uncertainty. The equation for the VF is shown below. 

⎟

⎠

⎞ ⎜

⎝

⎛ +×= 100 1 Val

RED

U BVF Equation 5.13 

 where: 

 VF = Validation Factor 

 BRED = RED bias factor 

 UVal = Uncertainty of validation expressed as a percentage 

 In addition to the RED bias factor, a bias factor to account for the influence of nongermicidal light on UV sensor readings (referred to as the “polychromatic bias factor”) should be 

included in Equation 5.13 for MP reactors that meet either of the following criteria: 

• The MP reactor is equipped with a non-germicidal sensor12

• The MP reactor is equipped with a germicidal sensor, but the sensor is mounted 

further than 10 cm from the lamp and the water to be treated has a low UVT (< 80%) 

 

Derivation of the polychromatic bias factor and its inclusion in the VF calculation are addressed 

in Appendix D, Section D.4.3. 

The next two sections provide recommendations for calculating the RED bias factor and 

uncertainty in validation and determining when each should be applied. Appendix D discusses in 

greater detail the basis for the uncertainty and bias terms and how they were derived. 

 

12 EPA recommends that MP reactors be equipped with germicidal sensors to more accurately measure UV light in 

the germicidal range. EPA recognizes, however that reactors with non-germicidal sensors have been installed or 

are about to be installed at water treatment plants prior to the publication of this document.

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Appendix B uses two example case studies to illustrate the calculation of the VF using methods 

described in this section. 

 Some areas of experimental uncertainty are not included in the VF equation. Instead, 

EPA recommends that UV reactor monitoring components meet the performance criteria 

presented in Chapter 6 and validation test results meet the QA/QC criteria as presented 

throughout this chapter and summarized in Section 5.12. Section 5.9.2 includes a method for 

checking key areas of experimental uncertainty and determining when factors should be included 

in the UVal calculation. 

5.9.1 RED Bias Factor 

The RED bias is a correction factor that accounts for the difference between the UV 

sensitivity of the target pathogen and the UV sensitivity of the challenge microorganism. If 

validation testing is performed using two challenge microorganisms whose UV sensitivities 

bracket those of the target pathogen (i.e., one challenge microorganism is less resistant than the 

target pathogen and the other is more resistant than the target pathogen), the RED bias is equal to 

1.0 (i.e., it can be corrected for, see Section 5.8.1 for details). 

If the UV sensitivities of the challenge microorganism and target pathogen are not the 

same, the RED delivered under the same reactor operating conditions will differ. The magnitude 

of this difference depends on the following factors: 

• The dose distribution of the UV reactor 

• The difference between the inactivation kinetics of the challenge microorganism and 

the 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 RED that would be measured for the 

target pathogen. In this case, the RED bias would be greater than 1.0. If the challenge 

microorganism is less resistant (more sensitive) to UV light than the target pathogen, the RED 

measured during validation will be less than the RED that would be measured for the target 

pathogen. In this case, the RED bias should be assigned a value of 1.0. 

The recommended procedure for determining the RED bias is as follows:

1. For the test condition with the lowest UVT, determine the observed UV sensitivity of 

the challenge microorganism for each test replicate using Equation 5.8. 

2. Identify the maximum UV sensitivity for all test replicates. 

3. Use Tables G.1 – G.17 (in Appendix G) to find the RED bias for the target pathogen 

and target log inactivation, the maximum UV sensitivity, and the lowest UVT. Note 

that Tables G.1 – G.17 are for discreet UVT values of 85 percent, 90 percent, and 95 

percent. RED bias can be interpolated for intermediate values of UVT. 
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 EPA recommends calculating one RED Bias for the UV facility, based on the sitespecific application (i.e., minimum operating UVT and target pathogen log inactivation desired), 

which results in a constant VF for all conditions. As an alternative, the RED bias can be defined 

as a function of UVT. This alternative may be advantageous for the Calculated Dose Approach 

where UVT is continually monitored during operations, which means that the VF and the 

validated dose would vary along with UVT. The disadvantage of using a variable VF is that the 

UV reactor control system would need to be designed and programmed to do these calculations 

and that the VF reported to the state will vary (see Section 6.5 for reporting guidance), making 

operations and reporting more complex. 

 Values in Tables G.1 – G.17 are based on theoretical dose distributions (as determined by 

CFD modeling) for several UV reactor designs. Appendix D, Section D.5 provides additional 

information on the derivation of values in Tables G.1 – G.17. Example 5.4 shows how the RED 

bias is determined for hypothetical test conditions. 

Example 5.4. Determining the RED Bias factor. A UV reactor is validated 

using MS2 phage for 3-log Cryptosporidium inactivation credit. The maximum 

MS2 phage UV sensitivity for the validation test condition of lowest UVT (86 

percent) is 18.0 mJ/cm2

 per log inactivation. The RED bias from Table G.3 is 

1.92.

5.9.2 Uncertainty in Validation (UVal) 

The Uncertainty in Validation (UVal), also referred to as the experimental uncertainty, has 

between 1 and 3 input variables based on how well the validation testing adhered to 

recommended QA/QC limits in this guidance manual. At least one input variable, which depends 

on the dose-monitoring strategy of the UV reactor, should be used in all cases. 

 

 Figures 5.4 and 5.5 provide decision trees for selecting the appropriate equation for 

calculating UVal and provide a description of the input variables used for the calculation. The 

next two sections provide guidance for deriving two of the input variables for UVal, which are 

USP (the uncertainty in the setpoint value, which is always calculated for the UV Intensity 

Setpoint Approach) and UIN (the uncertainty in interpolation, which is always calculated for the 

Calculated Dose Approach). US is the uncertainty in UV sensor measurements, expressed as a 

fraction (e.g., 15 percent, or 0.15) as described in Section 5.5.4. UDR is the uncertainty of the 

dose-response fit at a 95-percent confidence level. Note that if individual UV dose-response 

curves cannot be combined and there is more than one UDR value, the maximum value should be 

used in the decision tree. Additional guidelines for estimating UDR are provided in Section C.4. 
5. Validation of UV Reactors 

UV Disinfection Guidance Manual 5-39 November 2006 

Figure 5.4. UVAL Decision Tree for the UV Intensity Setpoint Approach 

Where:

UVal = Uncertainty of validation (representing experimental 

uncertainty)

US = Uncertainty of sensor value, expressed as a fraction (i.e., if

sensor uncertainty = 12%, US = 0.12. See Section 5.5.4 for 

guidance on determining sensor uncertainty.)

UDR = Uncertainty of the fit of the dose-response curve, calculated 

using Equation C.6. If there is more than one UDR value, use the 

maximum value

USP = Uncertainty of setpoint, calculated using Equation 5.14 (See 

Section 5.9.2.1 for additional guidance)

*If UDR is calculated using standard statistical methods instead of using equation C.6, is it > 15%?

UVal = (USP

2 + US2)1/2

UVal = (USP

2 + US2 + UDR2)1/2 UVal = (USP2 + UDR2)1/2

UVal = USP

Is US > 10%

Is UDR > 30% 

using Eq. C.6?*

Yes No

Yes Yes No No

Is UDR > 30% 

using Eq. C.6?*

Where:

UVal = Uncertainty of validation (representing experimental 

uncertainty)

US = Uncertainty of sensor value, expressed as a fraction (i.e., if

sensor uncertainty = 12%, US = 0.12. See Section 5.5.4 for 

guidance on determining sensor uncertainty.)

UDR = Uncertainty of the fit of the dose-response curve, calculated 

using Equation C.6. If there is more than one UDR value, use the 

maximum value

USP = Uncertainty of setpoint, calculated using Equation 5.14 (See 

Section 5.9.2.1 for additional guidance)

*If UDR is calculated using standard statistical methods instead of using equation C.6, is it > 15%?

UVal = (USP

2 + US2)1/2

UVal = (USP

2 + US2 + UDR2)1/2 UVal = (USP2 + UDR2)1/2

UVal = USP

Is US > 10%

Is UDR > 30% 

using Eq. C.6?*

Yes No

Yes Yes No No

Is UDR > 30% 

using Eq. C.6?*

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UV Disinfection Guidance Manual 5-40 November 2006 

Figure 5.5. UVAL Decision Tree for the Calculated Dose Approach 

UVal = (UIN

2 + US2)1/2

UVal = (UIN

2 + US2 + UDR2)1/2 UVal = (UIN2 + UDR2)1/2

UVal = UIN

Is US > 10%

Yes No

Yes No Yes No

Where:

UVal = Uncertainty of validation (representing experimental 

uncertainty)

US = Uncertainty of sensor value, expressed as a fraction (i.e., if 

sensor uncertainty = 12%, US = 0.12. See Section 5.5.4 for

guidance on determining sensor uncertainty.)

UDR = Uncertainty of the fit of the dose-response curve, calculated 

using Equation C.6 If there is more than one UDR value, use the 

maximum value

UIN = Uncertainty of interpolation, calculated using Equation 5.15

(See Section 5.9.2.2 for additional guidance)

*If UDR is calculated using standard statistical methods instead of using equation C.6, is it > 15%?

Is UDR > 30% 

using Eq. C.6?*

Is UDR > 30% 

using Eq. C.6?*

UVal = (UIN

2 + US2)1/2

UVal = (UIN

2 + US2 + UDR2)1/2 UVal = (UIN2 + UDR2)1/2

UVal = UIN

Is US > 10%

Yes No

Yes No Yes No

Where:

UVal = Uncertainty of validation (representing experimental 

uncertainty)

US = Uncertainty of sensor value, expressed as a fraction (i.e., if 

sensor uncertainty = 12%, US = 0.12. See Section 5.5.4 for

guidance on determining sensor uncertainty.)

UDR = Uncertainty of the fit of the dose-response curve, calculated 

using Equation C.6 If there is more than one UDR value, use the 

maximum value

UIN = Uncertainty of interpolation, calculated using Equation 5.15

(See Section 5.9.2.2 for additional guidance)

*If UDR is calculated using standard statistical methods instead of using equation C.6, is it > 15%?

Is UDR > 30% 

using Eq. C.6?*

Is UDR > 30% 

using Eq. C.6?*

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5.9.2.1 Calculating USP for the UV Intensity Setpoint Approach 

The uncertainty in the setpoint value is based on a prediction interval at a 95-percent 

confidence level using the following procedure: 

1. Calculate the average and standard deviation of RED values for each test condition 

(typically at least 3 – 5 replicate pairs are generated for each test condition). 

2. Calculate the uncertainty of the setpoint RED using: 

× %100

× = REDSDt U RED

SP Equation 5.14 

where: 

 RED = Average RED value measured for each test condition 

 SDRED = Standard deviation of the RED values measured for each test condition 

 t = t-statistic for a 95-percent confidence level defined as a function of the 

number of replicate samples using the following: 

Number of Samples t 

3 3.18

4 2.78

5 2.57

3. Select the highest USP from all test conditions for calculating the VF. 

5.9.2.2 Calculating UIN for the Calculated Dose Approach 

For reactors using the Calculated Dose Approach, the uncertainty of interpolation (UIN) is 

calculated as the lower bound of the 95-percent prediction interval for the dose-monitoring 

equation. This prediction interval reflects the noise in the data about that fit. In non-statistical 

terms, the UIN represents the difference between (1) the RED value as derived using measured 

log inactivation and the UV dose-response curve, and (2) the RED value as calculated using the 

dose-monitoring equation (also referred to as the “calculated dose” in this manual). 

UIN is calculated using the following equation: 

× %100 × = RED

SDt UIN Equation 5.15 

where: 

 SD = Standard deviation of the differences between the test RED (based on the 

observed log inactivation and UV dose-response curve), and the RED 

calculated using the dose-monitoring equation for each replicate 

 RED = The RED as calculated using the dose-monitoring equation 

 t = t-statistic at a 95-percent confidence level for a sample size equal to the 

number of test conditions used to define the interpolation: 

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Number of Data Points 

Used to Develop the DoseMonitoring Equation t 

Number of Data Points 

Used to Develop the 

Dose-Monitoring 

Equation t 

3 3.18 14 2.14 

4 2.78 15 2.13 

5 2.57 16 2.12 

6 2.45 17 2.11 

7 2.36 18 2.10 

8 2.31 19-20 2.09 

9 2.26 21 2.08 

10 2.23 22-23 2.07 

11 2.20 24-26 2.06 

12 2.18 27-29 2.05 

13 2.16 ≥30 2.04 

The value of UIN depends on the calculated RED (or calculated dose), increasing at low 

calculated RED values. EPA recommends that one UIN be selected that represents the most 

conservative (largest) uncertainty value calculated for the validated dose operating range (for the 

lowest calculated RED). Alternatively, UIN can be expressed as a function of the calculated RED. 

5.10 Determining the Validated Dose and Validated Operating Conditions 

 As shown in Figure 5.1 in Section 5.2, the last step in the recommended validation 

protocol is to adjust the RED results by the VF to determine the Validated Dose for the UV 

reactor using the following equation: 

Validated Dose = RED / VF Equation 5.16

 Where: 

 RED = the Minimum RED for the UV Intensity Setpoint Approach; or the RED as 

calculated using the dose-monitoring equation (also referred to as the 

calculated dose) for the Calculated Dose Approach 

 VF = the Validation Factor, as calculated using Equation 5.13 

 Because the method and assumptions for this step depend on the dose-monitoring 

strategy of the UV reactor, they are discussed separately below. 

5.10.1 Determining the Validated Dose and Operating Conditions for the UV 

Intensity Setpoint Approach 

 For the UV Intensity Setpoint Approach, Equation 5.16 produces one validated dose for a 

given UV intensity setpoint corresponding to the minimum RED. When the UV reactor is 

operating at a UV intensity level above the setpoint, the true UV dose delivered to 

microorganisms passing through the reactor is always equal to or greater than the validated dose. 
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The inactivation credit for the target pathogen is determined by comparing the validated dose to 

the required dose in Table 1.4. 

 Validated operating conditions are as follows: 

• The UV intensity measured by UV sensors must be greater than the UV intensity 

setpoint. 

• The flow rate must be equal to or less than the flow rate tested. 

• The lamp status for each lamp (i.e., on/off setting) must be equivalent to the settings 

used during validation testing. 

5.10.2 Determining the Validated Dose and Operating Conditions for the 

Calculated Dose Approach 

 For the Calculated Dose Approach, the validated dose varies based on operational 

parameters. Typically, measured values of UVT, UV intensity, and flow rate are entered into the 

dose-monitoring equation to calculate RED. RED is divided by the VF to produce the validated 

dose (Equation 5.16). Although EPA recommends using one VF, an equation may be used for 

the VF if the RED bias factor is expressed as a function of UVT or if UIN is expressed as a 

function of RED. 

 As noted in Section 3.5.2, a key advantage of the Calculated Dose Approach is that water 

systems can reduce power when UVT is high and/or the flow rate is low as long as the Validated 

Dose is greater than or equal to the required dose for the target pathogen and log inactivation 

level. As a reminder, the validated dose must be greater than or equal to the required dose for the 

target pathogen and target log inactivation level to receive treatment credit. 

 Validated operating conditions for the Calculated Dose Approach are as follows: 

• The operating UVT must be equal to or greater than the minimum UVT evaluated 

during validation testing.13

• The operating flow rate must not exceed the flow rate evaluated during validation 

testing (see footnote 13). 

 

13 If the operating UVT measures higher than the maximum UVT evaluated during validation testing, the 

maximum UVT evaluated during validation testing should be used as the default in the dose-monitoring 

equation. Similarly, if the operating flow rate measures less than the minimum flow rate evaluated during 

validation testing, the minimum flow rate evaluated during validation testing should be used as the default in the 

dose-monitoring equation. See Section 6.1.4 for guidance on setting operational controls.
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5.11 Documentation 

 Prior to validation testing, the water system should work with the manufacturers, third 

party reviewers, and engineers assisting with or performing validation testing to prepare the 

following: 

• Documentation for the UV reactor 

• Validation Test Plan 

Once validation testing and the associated data analyses are complete, the UV reactor 

documentation and Validation Test Plan, along with results of validation testing, should be 

incorporated into a Validation Report. 

The next several sections provide more detailed recommendations on validation testing 

documentation. Water systems purchasing a pre-validated reactor will not be preparing 

documentation; however, Sections 5.11.1 through 5.11.3 may be useful as they review validation 

documentation from manufacturers and consulting engineers. State personnel may also find these 

sections helpful when reviewing validation reports. 

5.11.1 UV Reactor Documentation 

Before validation testing, the UV manufacturer should provide the testing party with 

documentation identifying and describing the UV equipment. Documentation should include all 

reactor and component information that impacts UV dose delivery and monitoring, as described 

in Checklist 5.1. 
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Checklist 5.1 UV Reactor Documentation (Page 1 of 2) 

Does UV reactor documentation contain the following elements?

Yes No 

General 

  Technical description of the reactor’s UV dose-monitoring strategy, including the use of 

sensors, signal processing, and calculations (if applicable). 

  Dimensions and placement of all wetted components (e.g., lamps, sleeves, UV sensors, 

baffles, and cleaning mechanisms) within the UV reactor. 

  A technical description of lamp placement within the sleeve. 

 

  Specifications for the UV sensor port indicating all dimensions and tolerances that impact 

the positioning of the sensor relative to the lamps. If the UV sensor port contains a 

monitoring window separate from the sensor, specifications giving the window material, 

thickness, and UV transmittance should be provided. 

Lamp specifications 

  Technical description 

  Lamp manufacturer and product number 

  Electrical power rating 

  Electrode-to-electrode length 

  Spectral output of new and aged lamps (specified for 5 nm intervals or less over a 

wavelength range that includes the germicidal range of 250 – 280 nm and the response 

range of the UV sensors) 

  Mercury content 

  Envelope diameter 

Lamp sleeve specifications

  Technical description including sleeve dimensions 

  Material 

  UV transmittance (at 254 nm for LP and LPHO lamps, and at 200 – 300 nm for MP lamps 

with germicidal sensors) 

Specifications for the reference and the duty UV sensors 

  Manufacturer and product number 

  Technical description including external dimensions 

  Data and calculations showing how the total measurement uncertainty of the UV sensor is 

derived from the individual sensor properties. (See Table D.1 for an example of the 

calculation of UV sensor measurement uncertainty from the uncertainty that arises due to 

each UV sensor property.) 
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Checklist 5.1 UV Reactor Documentation (Page 2 of 2) 

Does UV reactor documentation contain the following elements?

Yes No 

Sensor measurement properties

  Working range 

  Spectral and angular response 

  Linearity 

  Calibration factor 

  Temperature stability 

  Long-term stability 

Installation and operation documentation: 

  Flow rate, head loss, and pressure rating of the reactor 

  Assembly and installation instructions 

  Electrical requirements, including required line frequency, voltage, amperage, 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 lamp breakage. 

5.11.2 Validation Test Plan 

 A validation test plan should document the key components of UV reactor testing. 

Recommended components of a validation test plan are provided in Checklist 5.2. This list is not 

meant to be all-inclusive; engineers should document any factors they believe are important for 

validation testing in their Validation Test Plan. 
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Checklist 5.2 Key Elements of the Validation Test Plan (Page 1 of 1) 

Does the validation test plan contain the following elements? 

Yes No 

  Purpose of Validation Testing. General description of why the tests are being done and how 

the data will be used. 

  Roles and Responsibilities. Key personnel overseeing and performing the full-scale reactor 

testing and collimated beam testing, including their qualifications. This section should 

include contact names and telephone numbers. 

  Locations and Schedule. Location for conducting full-scale reactor testing and collimated 

beam testing. Planned schedule for conducting the tests and performing the data analyses. 

  Challenge Microorganism Specifications. Specifications for the challenge microorganism 

to be used during validation that include the protocols required for growth and 

enumeration, the expected UV dose-response, and suitability for use in validation testing. 

  Plan for state review (if applicable). 

Design of the Biodosimetry Test Stand/On-site Testing Facilities 

  Inlet/outlet piping design, including backflow prevention 

  Mixing 

  Sample ports 

  Pumps 

  Additives (Material Safety Data Sheets for UV-adsorbing chemical, quenching agent) 

Collimated Beam Testing Apparatus 

  Lamp type 

  Collimating tube aperture 

  Distance from light source to sample surface 

  Radiometer make and model 

 

Monitoring Equipment Specifications and Verification of Equipment Accuracy for the following: 

  Flow meters 

  UVT analyzers (if used) 

  UV Spectrophotometers 

  Power measurement 

  UV sensors 

  Radiometer make, model, and calibration certificates 

Experimental Test Conditions including, but not limited to: 

  Number of tests, UVT, flow rate, lamp power, and lamp status for each test condition 

  Lamp fouling factor, use of new or aged lamps 

  Influent concentration of challenge microorganisms for each test condition 

  QA/QC Plan 
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5.11.3 Validation Report 

The validation report should provide detailed documentation of all validation testing 

results. The report should also include all elements of the Validation Test Plan and a summary of 

the field-verified UV reactor properties. 

EPA recommends that the report begin with an executive summary with key information 

that can be used by states and water systems to assess inactivation credit for the target 

pathogen(s). The executive summary should include, at a minimum, 

• The validated dose or range of validated doses, 

• The log credit achieved for the potential target pathogens by the UV reactor, and 

• Validated operating conditions (i.e., flow rate, UVT if the Calculated Dose approach 

is used). 

If the UV Intensity Setpoint approach is used, the executive summary should provide the UV 

intensity setpoint (or setpoints) for the validated dose. If the reactor uses the Calculated Dose 

Approach as its dose monitoring strategy, the dose-monitoring equation should be provided. 

In addition to the items listed above, the executive summary should include the 

following: 

• A brief description of the validated reactor, 

• The assumed fouling/aging factors for the reactor and indication if new or aged lamps 

were used during validation testing, 

• A summary of the validation test conditions, including but not limited to the flow 

rate, UVT, and lamp power for each test condition, 

• Key validation test results used to derive the dose, including but not limited to the 

RED values for each test condition, the UV dose-monitoring equation from 

collimated beam testing, and the VF, 

• A summary of QA/QC checks and results, including UV sensor and radiometer 

reference checks, 

• A description of the validation facilities, 

• The organizations conducting the validation test, and 

• Names and credentials of the individuals/organizations providing third party 

oversight. 

Recommended contents for the detailed validation report are listed in Checklist 5.3. Note 

that these recommendations are not intended to be all-inclusive. Engineers should document any 

test characteristics or outcomes they believe are important in the Validation Report. 
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Checklist 5.3 Key Elements of the Validation Report (Page 1 of 1) 

Does your validation report contain the following elements? 

Yes No 

General 

  Detailed reactor documentation (see Checklist 5.1), including drawings and serial numbers, 

and procedures used to verify reactor properties. 

  Validation test plan (either a summary of key elements, or the test plan can be attached to 

the validation report along with documentation of any deviations to the original test plan) 

Full-scale reactor testing results, with detailed results for each test condition evaluated. Data should 

include, but are not limited to: 

  Flow rate 

  Measured UV intensity 

  UVT 

  Lamp power 

  Lamp statuses 

  Inlet and outlet concentrations of the challenge microorganism 

Collimated beam testing results, including detailed results for each collimated beam test used to create 

the UV dose-response equation: 

  Volume and depth of microbial suspension 

  UV Absorption of the microbial suspension 

  Irradiance measurement before and after each irradiation 

  Petri factor calculations and results 

  Calculations for UV dose 

  Derivation of the UV dose-response equation, including statistical methods and confidence 

intervals (i.e., calculation of UDR) 

QA/QC Checks: 

  Challenge microorganism QA/QC, including blanks, controls, and stability analyses 

  Measurement uncertainty of the radiometer, date of most recent calibration, results of 

reference checks 

  Measurement uncertainty of UV sensors and results of reference checks 

  Measurement uncertainty of the flow meter, UV spectrophotometer, and any other 

measurement equipment used during full-scale testing 

Calculation of the validated dose, log inactivation credit, and validated operating conditions: 

  RED for each test condition 

  Calculation of the VF 

  Setpoints if the reactor uses the UV Intensity Setpoint Approach 

  Dose-monitoring equation if the reactor uses the Calculated Dose Approach 

  Log inactivation credit for target pathogens (e.g., Cryptosporidium, Giardia, and viruses) 

  Validated operating conditions (e.g., flow rate, lamp status, UVT)
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5.12 Guidelines for Reviewing Validation Reports 

 State engineers and water systems purchasing pre-validated reactors should review the 

validation report to confirm the following: 

• Validation testing meets the minimum regulatory requirements as summarized in 

Table 5.1. 

• EPA’s recommended validation protocol was followed and any deviations from the 

protocol are adequately justified. 

• Validated doses achieved by the UV equipment meet or exceed the target pathogen 

log inactivation desired. 

• QA/QC criteria were met during validation testing. 

 

 Checklist 5.4 summarizes the QA/QC recommendations presented throughout this 

chapter and in Appendix C. If a QA/QC plan was prepared prior to validation, reviewers should 

request a copy of the plan and make sure it is consistent with industry standards. 

 Checklist 5.5 contains key elements that should be verified by state or water system 

personnel when reviewing validation reports. States and systems should keep documentation that 

these key validation criteria were met. 
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Checklist 5.4 Review for Quality Assurance/Quality Control (Page 1 of 1) 

Yes No 

Uncertainty in Measurement Equipment (See Section 5.5 and C.2.2 for more information) 

  Flow Meter: Is the measurement uncertainty < 5 percent? 

  UV Spectrophotometer: Is the measurement uncertainty < 10 percent? 

  UV Sensors: Did duty sensors operate within 10 percent of the average of two or more 

reference sensors? If not, was uncertainty in sensor measurement incorporated into the VF? 

  Radiometer: (for collimated beam testing only). Do lamp output measurements vary by no 

more than 5 percent over exposure time? Was the accuracy of the radiometer verified with 

another radiometer? 

QA/QC of Microbial Samples (See Section 5.6.4 for more information)

  Reactor controls: For influent/effluent samples taken with the UV reactor lamps turned 

off, does the change in log concentration correspond to a change in RED that is within the 

measurement error of the minimum RED measured during validation (typically ≤ 3 %)? 

  Reactor blanks: For DAILY influent/effluent samples taken with NO challenge 

microorganisms injected, are the measured concentrations of the challenge microorganism 

negligible? 

  Trip Controls: For an UNTESTED sample bottle of challenge microorganism stock 

solution that travels with tested samples between the laboratory and the reactor, is the 

change in the log concentration of the challenge microorganism within the measurement 

error. (I.e., the change in concentration over the test run should be negligible. This is 

typically on the order of 3 to 5%.)

  Method Blanks: For sterilized reagent grade put through the challenge microorganism 

assay procedure, is the challenge microorganism concentration non-detectable? 

  Stability Samples: For influent/effluent samples at low and high UVT, are the challenge 

microorganism concentrations within 5 percent of each other? 

Uncertainty in Collimated Beam Testing Data (See Appendix C for more information) 

  Do the uncertainties in the terms in the UV dose calculation meet the following criteria: 

• Depth of suspension (d) ≤ 10 percent 

• Incidence irradiance (Es) ≤ 8 percent 

• Petri factor (Pf) ≤ 5 percent 

• L/(d + L) ≤ 1 percent 

• Time (t) ≤ 5 percent 

• (1 – 10-ad)/ad ≤ 5 percent

  Is the uncertainty in dose-response (UDR), as calculated using equation C.6, less than or 

equal to 30 percent? If not, was UDR incorporated into the VF? 
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Checklist 5.5 Review for Key Validation Report Elements (Page 1 of 2) 

Yes No 

  Does the validation testing meet QA/QC criteria (see Checklist 5.4)? 

  For full-scale testing, does the mixing and location of sample ports follow 

recommendations provided in Sections 5.4.3 and 5.4.4, respectively? 

  If the reactor was validated off-site, do inlet/outlet piping conditions at the water treatment 

plant result in a UV dose-delivery that is the same or greater than the UV dose delivery at 

the off-site testing facility? (See Section 3.6 for recommended inlet/outlet piping 

configurations and Section D.6 for considerations for CFD modeling.) 

  Were collimated beam tests and full-scale reactor tests performed on the same day for a 

given test condition and using the same stock solution of challenge microorganisms? (See 

Section 5.7 for experimental testing guidelines.) 

  Is the UV sensitivity of the challenge microorganism and the overall shape of the UV doseresponse curve consistent with the expected inactivation behavior for that challenge 

microorganism? See Appendix A of this manual for published UV dose-response curves 

for MS2 and B. subtilis. 

  Does the validation test design account for lamp fouling and aging, minimum UVT, and 

maximum flow rate expected to occur at the water treatment plant? (See Section 5.6 for 

recommended test design.) 

For UV Reactors Using MP Lamps 

  Is the UV reactor equipped with a germicidal sensor? New UV reactors should have 

germicidal sensors. If an installed reactor uses an MP lamp and a non-germicidal sensor, is 

a polychromatic bias factor incorporated into the derivation of the VF? (See Section D.4.3 

for guidance on the polychromatic bias factor.) 

  Was validation testing conducted using a challenge microorganism other than MS2 or B. 

Subtilis? If yes, was the need for a correction factor assessed and was that factor applied 

based on the outcome? (See Sections 5.3 and D.4.1 for more information) 

For UV Reactors Using the UV Intensity Setpoint Approach 

  Were the minimum test conditions performed as specified in Section 5.6.1? 

  Is the UV intensity setpoint low enough to account for combined conditions of minimum 

UVT and maximum lamp fouling/aging at the water treatment plant (See Section 5.6.1 for 

guidance) 

  Was the minimum RED selected for calculating the validated dose? (See Section 5.8.1 for 

additional guidance.) 

  Does the VF calculation include both the BRED and USP? (See Section 5.9 for additional 

guidance.) 
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Checklist 5.5 Review for Key Validation Report Elements (Page 2 of 2) 

Yes No 

For UV Reactors Using the UV Intensity Setpoint Approach (continued) 

  If US and/or UDR did not meet the QA/QC criteria, were they also included in the VF 

calculation? 

  Is the validated dose greater than or equal to the required dose for the water system’s target 

pathogen and log inactivation level? 

For UV Reactors Using the Calculated Dose Approach 

  Was the minimum number of test conditions evaluated as specified in Section 5.6.2? 

  Was the empirical equation developed using standard statistical methods (e.g., multivariate 

linear regression)? (See Section 5.8.2 for additional guidance.) 

  Does the validation report include an analysis of goodness of fit and bias for the dosemonitoring equation? (See 5.8.2 for additional guidance.) 

  Does the VF calculation include both the BRED and UIN? (See 5.9.) 

  If US and/or UDR did not meet the QA/QC criteria, were they also included in the VF 

calculation? 

  For the range of UVT values and flow rates expected to occur at the water system, is the 

validated dose greater than or equal to the required dose for the system’s target pathogen 

and log inactivation?
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5.13 Evaluating the Need for “Re-validation” 

If a UV reactor is modified in a way that significantly impacts UV dose delivery or 

monitoring (e.g., the wetted geometry changes, the lamp technology changes, the UV sensor 

characteristics, and/or location change), validation testing should be conducted again (i.e., the 

UV reactor has been modified enough to be considered a different reactor with unsubstantiated 

performance). This section discusses some common types of UV reactor modifications and 

provides guidance on when UV reactors should be “re-validated.” 

Lamp Assembly 

The relationship between UV dose delivery and monitoring may be impacted by any 

design change involving modifications to the following lamp components: 

• Lamp arc length 

• Any reflectors, connectors, and spacers used at the lamp ends 

• Lamp envelope diameter 

• Lamp envelope UV transmittance from 185 – 400 nm 

• Mercury content of the lamp 

• Argon content of the lamp 

In many cases, UV dose delivery and UV sensor modeling can be used to assess the 

impacts of changing lamp material and justify the need, or lack of need, for re-validation. 

Changes that will modify the UV output so that emitted intensity is uneven along the 

length of the lamp or around its circumference, however, can have a complex impact on UV dose 

delivery and would likely warrant re-validation. 

Ballasts 

Modifications to lamp ballasts include changing the operating voltage, current, 

frequency, and waveform. Modifications to LP lamps will not impact the relationship between 

UV dose delivery and UV intensity measurements. With MP lamps, changes in lamp operating 

temperature and mercury pressure caused by changes in ballast power will impact the spectral 

distribution of emitted light, resulting in a significant impact on UV reactors with non-germicidal 

sensors. 

If a water system is using non-germicidal sensors, then EPA recommends that the reactor 

be re-validated if there are modifications to the lamp ballasts that change the operating voltage, 

current, frequency, and/or waveform. 

Lamp Sleeves 

Lamp sleeve design changes include changing the sleeve diameter, thickness, and 

material. Changing the sleeve diameter may significantly impact the hydraulics through the 

reactor, the measurement of UV intensity, and/or the ideal location of the UV sensors relative to 
5. Validation of UV Reactors 

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the lamp. Changing the thickness and material of the lamp sleeve will impact its spectral UV 

transmittance, thereby impacting both UV dose delivery and UV intensity measurements. 

UV dose delivery and UV sensor modeling may be used to assess the impact of lamp 

sleeve design changes. For example, a design change from a standard sleeve to the ozone-free 

sleeve described in Figure 5.6 would have a moderate impact on the relationship between UV 

dose delivery and UV sensor readings with a non-germicidal sensor and a negligible impact with 

a germicidal sensor. Modeling can also be used to show that the UV dose delivery at a given 

lamp output, water UVT, and flow rate would be approximately 10 percent greater with the 

standard sleeve than with the ozone-free sleeve. If the modeling indicates a change in dose 

delivery of greater than 10 percent as a result of lamp sleeve design changes, EPA recommends 

that the reactor be re-validated. If it is not possible to model the impact of lamps sleeve design 

changes, EPA also recommends the UV equipment be re-validated. 

Figure 5.6 UVT of Standard and “Ozone-Free” Quartz Assuming 

Air-Quartz and Quartz-Water Interfaces 

UV Reactor and Component Dimensions 

Modifications to the wetted dimensions and positioning of the components within the UV 

reactor will impact the reactor hydraulics and UV dose delivery. Modifications could also impact 

the UV intensity field within the reactor and its measurement. Such changes include altering the 

dimensions of the UV reactor, inlet piping, exit piping, baffles, lamp sleeves, wipers, and/or UV 

sensors. The impact of such modifications on UV dose delivery and UV intensity measurements 

can be large or insignificant. Adding a baffle plate will likely have a large impact on UV dose 

delivery and a small impact on measured UV intensity. Changing the position of a UV sensor 

will likely have a small impact on UV dose delivery and a large impact on the measured UV 

intensity. 

UV dose delivery and UV intensity modeling may be used to assess the impacts of these 

modifications. If the modeling indicates a change in dose delivery of greater than 10 percent as a 

result of changes to the wetted dimensions of the reactor and/or changes in the positioning of 

components, EPA recommends that the reactor be re-validated. If it is not possible to model the 

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impact of modification to the wetted dimensions and positioning of components within the UV 

reactor, EPA recommends the UV equipment be re-validated. 

UV Sensors 

 

Modifications to the UV sensors include changes made by the sensor manufacturer to the 

sensor itself, its housing and its associated optical components, or installation within the reactor. 

Any modifications that affect the UV sensor response or the flow within the reactor affect should 

be evaluated to determine their impacts on dose delivery and dose monitoring. For example, if 

the measurement uncertainty of a new sensor is greater than 10 percent, it should be included in 

the VF calculations. If the angular response or spectral response of the UV sensor changes, 

measurements supported by calculations should be used to evaluate the impact of the change on 

UV dose delivery monitoring. 
6. Start-up and Operation of UV Facilities 

This chapter describes the start-up activities and routine operational issues associated 

with a UV disinfection facility. The start-up discussion focuses on the testing performed during 

the start-up process. The rest of the chapter describes requirements and recommendations for 

operation, maintenance, monitoring, recording, and reporting for UV facilities. Figure 6.1 

illustrates the start-up and routine operation. A detailed description of each activity is provided in 

this chapter. 

Chapter 6 covers: 

6.1 UV Facility Start-up 

6.2 Operation of UV Facilities 

6.3 Maintenance of UV Reactors 

6.4 Monitoring and Recording of UV Facility Operation 

6.5 UV Facility Reporting to the State 

6.6 Operational Challenges 

6.7 Staffing, Training, and Safety Issues 

The guidelines provided in this manual are based on industry experience and 

manufacturers’ recommendations. Because of numerous differences among UV facilities and UV 

equipment, this document does not address all start-up and operation and maintenance (O&M) 

issues that may occur. 

6.1 UV Facility Start-up 

For the purposes of this manual, the start-up of the UV facility is considered as the 

transition from the construction phase to the operation phase. A start-up plan should be 

developed in collaboration with the UV facility designer, plant operations staff, and the UV 

manufacturer. The start-up plans should include O&M manual development, state coordination, 

functional testing, determination of validated operating parameters, performance testing, and 

final inspection. 

6.1.1 O&M Manual 

The O&M manual should be site-specific and based on as-built drawings, manufacturer’s 

shop drawings, operating procedures, operational requirements, recommended maintenance 

tasks. If performance testing is completed before the O&M manual is finalized, testing results 

should be included in the manual. If possible, the O&M manual should be developed before 

performance testing and routine operations. At a minimum, O&M manuals should address the 

following items: 

• Federal and state regulatory requirements and guidelines 

• Treatment objectives 

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6. Start-up and Operation of UV Facilities 

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Figure 6.1. Start-up and Operation Flowchart1

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1

 Start-up activities are not necessarily in chronological order. 
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• General description of UV facility 

• Relationship to other unit treatment processes 

• UV reactor design criteria 

• Validated operational parameters 

• Controls and monitoring 

• Compliance monitoring, recording, and reporting 

• Standard operating procedures 

• Start-up procedures 

• Shut-down procedures (manual and automatic) 

• Safety issues 

• Emergency procedures and contingency plan 

• Alarm response plans 

• Preventive 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 

6.1.2 State Coordination during Start-up 

States should be contacted during construction to determine the state-specific 

requirements and submittals. The states may request the record drawings, O&M manual, and an 

engineer’s certificate of completion. In addition, the state may need to visit the site to approve 

the start-up of the UV facility. 

6.1.3 Functional Testing 

Functional testing verifies that each component’s operation is in accordance with the 

specifications in the contract documents. It should include verification of UV equipment 

components, instrumentation and control (I&C) systems, and flow distribution and head loss. 

Items that are not unique to UV facilities (e.g., valves, flow meters, backup generators, or 

uninterruptible power supplies) are not described in this manual; however, their functionality 

should still be verified. 
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6.1.3.1 Verification of UV Equipment Components 

 

Most functional testing is completed through simulations of specific operating conditions 

and monitoring UV reactor operation and response. Functional testing entails flooding and 

energizing UV reactors to confirm the operation of the lamps, ballasts, ballast cooling system, 

cleaning system, UV sensors, and UVT analyzers. 

It is strongly recommended that the UV manufacturer inspect the UV facility before the 

UV reactors are energized 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. 

UV Sensor 

UV sensors must be included in the UV reactor to verify that the reactor is operating 

within validated conditions [40 CFR 141.720(d)]. The calibration of the duty and reference UV 

sensors should be checked during functional testing using the procedure recommended in Section 

6.4.1.1. UV sensors that are not in calibration should be returned to the manufacturer for 

replacement or recalibration. 

Lamps, Ballasts, and Ballast Cooling System 

The lamps, ballasts, and ballast cooling system operation are verified by energizing the 

UV lamps, then verifying lamp and ballast operation via the UV sensor measurements and visual 

verification of the ballast cooling fan operation. In addition, the power [kilowatt (kW)] delivered 

to the lamps should be verified as the same as documented in the validation report for at least 

three power settings. 

On-line UVT Analyzer 

If the dose-monitoring strategy of the UV reactor is the Calculated Dose Approach (see 

Section 3.5.2 for a description of dose-monitoring strategies), the UV reactor should be equipped 

with a UVT analyzer. Calibration of the on-line UVT analyzer should be verified. A 

recommended procedure for verifying calibration is described in Section 6.4.1.2. 

Cleaning System 

 The necessary functional testing depends on the type of cleaning used, and the 

components to be verified for each cleaning system are summarized in Table 6.1. Cleaning 

systems are described in Section 2.4.5. 
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Table 6.1. Functional Testing of Cleaning Systems 

Cleaning System Items to be Verified 

On-line mechanical 

cleaning (OMC) 

• 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 

On-line mechanicalchemical cleaning 

(OMCC) 

• Same as on-line mechanical cleaning (above) 

• The chemical injection point is accessible 

• The seal that contains the chemical solution is intact 

Off-line chemical 

cleaning (OCC) 

• The chemical injection wand should be connected to the chemical 

pump to verify that a proper seal is achieved 

• Outside of the reactor, in a safe location, the chemical pump should 

be initiated to ensure that the wand is operating properly and an 

appropriate amount of pressure is achieved 

• The wand should then be connected to the reactor and turned on to 

make sure the seal is intact and the wand is functioning properly 

6.1.3.2 Verification of Instrumentation and Control Systems 

 

The amount of testing for the instrumentation and control systems depends on the 

complexity of the dose-monitoring strategy and operations approach used. Testing should 

include verifying control loops, checking operation functions, and verifying all control actions. 

As described below, the UV reactors should be run through a series of simulations that represent 

the possible operating scenarios to confirm that the UV reactor responses are appropriate. 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. 

Typically, the packaged UV reactor control panel contains all the components needed to 

control and operate the UV reactor. The panel should provide the operating status, lamp status 

indicators, diagnostic information, and operator interface capability. The panel may also include 

programmable logic controllers (PLC), ballasts, and lamp starters. 

Electronic signal simulations imitate the signals that will be sent to the control system 

during normal operation. The I&C logic programming should be monitored during simulations to 

verify the programming is correct. These “dummy” simulations should be used to confirm that 

UV reactors and all ancillary equipment and instrumentation, including valves, flow meters, and 

UVT analyzers will operate consistent with the I&C programming. The UV reactors should not 

operate during these simulations (i.e., water is not flowing and lamps are not energized). As 

applicable, the following specific operating conditions should be electronically simulated, as 

well as any other conditions the manufacturer recommends: 
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• Cold start of the UV reactors 

• Cool down and restart of the UV reactor 

• Sequencing of the UV reactors in multiple-reactor installations 

• Adjustment of lamp intensity or number of lamps on in response to varying water 

quality and flow rate 

• Shut-down of the UV reactors 

• Operation of the UV reactors during line power failure (when back-up generators or 

UPS are available) 

• Manual override, safety interlocks, and report generation 

• Operation of the UV reactors through the plant SCADA system 

• Incorporation of a sensor correction factor 

In addition to simulating possible operating conditions, each alarm condition and 

monitoring function incorporated in the design should be verified. Possible monitoring functions 

and alarm conditions are discussed in Section 4.3.3 and may include the following conditions: 

• Operation outside the validated conditions 

– Low validated dose or UV intensity 

– Low UVT 

– High flow rate 

• Lamp age 

• Lamp or ballast failure 

• Low water level in the UV reactor 

• High temperature 

• OMC or OMCC system failure 

• Loss of control signals 

 

6.1.3.3 Verification of Flow Distribution and Head Loss 

A minimum of three flow rates that span the range of operating conditions should be 

tested. If possible, one condition should be the maximum design flow rate through the UV 

facility with all duty reactors in operation; the other conditions should consist of combinations of 
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the reactors operating at their design flow rates (e.g., two of five 10-mgd reactors operating at a 

total UV facility flow rate of 20 mgd). Clamp-on type flow meters can be used for field 

verification of the flow split. 

The head loss should be measured at these same test conditions for each reactor and 

compared to the head loss specified in the contract documents (if applicable). Pressure 

transducers or pressure gauges can be used to measure the head loss. 

6.1.4 Determining Validated Operational Conditions and Setting Operational 

Controls 

 

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 141.720(d)(2)]. 

Section 5.10.1 and 5.10.2 describe how the validated dose and validated operating 

conditions are established for two dose-monitoring strategies, the UV Intensity Setpoint 

Approach and the Calculated Dose Approach, respectively. Appendix B supports Sections 5.10.1 

and 5.10.2 by providing examples of validation testing data analyses. Examples 6.1 and 6.2 

expand on guidance in Section 5.10 and Appendix B by showing how the same hypothetical 

water systems in Appendix B established operational alarms to ensure that they operate within 

validated conditions. 
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Example 6.1. Setting Operational Controls for the UV Intensity Setpoint Approach – Single 

Setpoint Operation (Corresponds to the Validation Example in Section B.1) 

Background: System X plans to add UV disinfection to its treatment plant to achieve 

2.5-log Cryptosporidium inactivation credit. Based on LT2ESWTR UV dose requirements 

(summarized in Table 1.4 of this manual), the water system needs to meet a required UV dose of 

8.5 mJ/cm2

 to achieve this level of inactivation. During UV facility planning, the water system 

establishes a design flow of 400 gpm and minimum UVT of 90 percent. 

System X selects two low-pressure high-output (LPHO) reactors (one duty and one standby) with eight lamps each that use the UV Intensity Setpoint Approach. Because their flow rate 

and UVT do not vary much, System X decided to use the single setpoint approach that applies to 

all validated operating conditions. 

Summary of Validation Test Results: Validation testing produced a UV intensity 

setpoint of 11.7 mW/cm2 at a maximum flow rate of 394 gpm with a single reactor operating 

with all lamps turned on. The validated dose at the setpoint is 11.3 mJ/cm2, which is greater than 

the required dose of 8.5 mJ/cm2

. As long as the UV intensity as measured by the UV sensor is 

greater than 11.7 mW/cm2, the validated dose is greater than the required dose and the reactor is 

operating within the validated limits. 

Operational Controls: As shown in the table below, System X set the UV intensity alarm 

at 12.5 mW/cm2

 to provide an operational cushion. System X also set a flow rate alarm at 375 

gpm. Because the validation testing protocol for the UV Intensity Setpoint Approach (as 

described in Chapter 5) accounts for changes in UVT, UVT is not regularly monitored during 

operations. 

Operating Parameter Validated Operating 

Conditions 

Major Alarm 

UV Intensity as measured by 

the UV sensor 

≥ 11.7 mW/cm2 Sounds if < 12.5 mW/cm2

Flow rate through the reactor ≤ 394 mgd Sounds if > 375 gpm 

Although this operating strategy is simple and straightforward, System X could have 

improved efficiency by reducing the UV intensity at lower flow rates, which can only be done if 

the validation data support UV intensity adjustment with flow. To further improve energy 

efficiency using the single setpoint approach, the flow could be maximized through one reactor 

before energizing another reactor for multiple reactor systems. 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-9 November 2006 

Example 6.2. Setting Operational Controls for the Calculated Dose Approach 

(Corresponds to the Validation Example in Section B.2) 

Background: System Y plans to add UV disinfection to their treatment plant to achieve 

2.0-log Cryptosporidium inactivation credit. Based on the LT2ESWTR UV dose requirements 

(summarized in Table 1.4 of this manual), the water system needs to meet a required UV dose of 

5.8 mJ/cm2

 to achieve this level of inactivation. During UV facility planning, System Y 

establishes a design flow rate range of 3 to 10 mgd and a minimum operating UVT of 87 percent. 

 System Y selects three UV reactors (two duty and one stand-by) with six 8-kW mediumpressure (MP) lamps each with power settings ranging from 40 – 100 percent. The reactors have 

one germicidal UV sensor monitoring each lamp. The reactors were validated for flow ranges of 

2.5 – 10 mgd and use the Calculated Dose Approach. 

Summary of Validation Test Results: Validation testing as described in Appendix B 

produced the following dose-monitoring equation (Equation B.14): 

( ) ( ) ( ) S Q RED A S

o

log 51928290 log.. 254 1660 log. 4090 ×+ log. 1 ⎟

⎠

⎞ ⎜

⎝

⎛ ×−−= ×+

 where: 

RED = Calculated dose 

As noted in the validation report, the Validation Factor (VF) is 2.28. The validated dose 

is calculated by dividing the calculated dose by the VF. The validated dose must be greater than 

the required dose of 5.8 mJ/cm2 for System Y to receive treatment credit for 2.0 log-inactivation 

of Cryptosporidium. 

Operational Controls: The table below summarizes the major alarms that System Y 

programmed into their PLC to ensure that they operate within validated conditions 

Operating Parameter 

Validated Operating 

conditions Major Alarm 3

Validated Dose (equal to the 

Calculated Dose / 2.28) 

≥ 5.8 mJ/cm2 Sounds if < 6.3 mJ/cm2

Flow rate through the reactor < 10 mgd 1 Sounds if > 9 mgd 

UVT as measured by an on-line 

UVT analyzer 

 85 - 95% 2 Sounds if <87% 

1

 If the flow rate is less than 2.5 mgd, the PLC will default to 2.5 mgd in the dose-monitoring 

equation. 2 If the UVT measured is higher than 95 percent, which is the highest validated UVT, the PLC will 

default the UVT to 95 percent in the dose-monitoring equation. 3 Note the major alarms are set at a conservative level compared to the validation conditions to give 

the operators more time to respond to low validated dose, high flows, and low UVTs. 

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6.1.5 Performance Testing 

Performance testing is intended to assess the operating performance of the UV facility as 

a whole and is generally accomplished through extensive monitoring during the early stages of 

continuous operation. Note that performance testing is not intended to validate disinfection 

performance, which is completed during validation testing (as described in Chapter 5). However, 

performance testing can be used to confirm that the actual operating conditions are within the 

constraints established during validation testing as described in Section 6.1.4. 

Because performance testing should compare operating conditions to validated 

conditions, the lamps should be operated as they were during validation testing. Therefore, UV 

lamps should be burned-in before performance testing, which typically takes 100 hours of 

continuous operation (Section 5.7.2). The actual required burn-in time should be discussed with 

the manufacturer and confirmed through documented operating experience at other UV facilities. 

The scope and duration of performance testing will be project-specific and should be 

established by the PWS and designer based on the objectives of the performance testing. The 

duration of performance testing should be adequate to demonstrate to the PWS and the state that 

the UV facility can continually perform according to specifications in uninterrupted operation. 

This could be as little as 48 hours, but may be longer, depending upon the nature of the 

installation, the variability of the source, and any specific state and PWS requirements. Similarly, 

the scope of the testing may range from an increased monitoring frequency that confirms 

operation within validated limits to an extensive testing protocol aimed at optimizing reactor 

performance and establishing long-term operating procedures. During performance testing, 

treated water may be sent to the distribution system if upstream treatment has not changed, meets 

existing regulations, and is approved by the state. 

Performance testing may include the following items: 

• Operation of each UV reactor in automatic mode to verify that the control system is 

identical to that established during validation testing 

• Demonstration of UV reactor start-up and switchover sequences that result from 

water quality and/or flow rate changes 

• Observation of operation, including periods of off-specification operation that arise 

from alarm conditions and any power quality problems 

• 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 
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• Confirmation that the programmed cleaning frequency correlates with the actual 

frequency of cleaning 

• Confirmation of duty UV sensor accuracy using reference UV sensors. (See 

Section 6.4.1.1.) 

• Observation of ballast temperature and cooling system performance 

• Verification of the calibration of the on-line UVT analyzer (if applicable). (See 

Section 6.4.1.2.) 

• Confirmation of backup generator and/or UPS power transfer to the UV equipment 

The performance testing should be tailored to the specific UV facility. An example 

monitoring program for a 4-week performance test is shown in Table 6.2. 

Any off-specification time and flow volume should be recorded during all performance 

tests, and these results should be evaluated to verify that off-specification limitations are not 

exceeded. Off-specification volume during performance testing does not need to be reported to 

the state. Recording of the off-specification time and volume is meant to identify operational 

problems to be addressed. During performance testing, any component that is not operating 

properly should be corrected and retested to confirm satisfactory operation. This step may 

require 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 6.4 and as required by the state. 
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Table 6.2. Example Monitoring During a Four Week Performance Test 

Frequency Task Notes 

Confirm the validated 

setpoint(s) 

Monitor reactor operation to confirm compliance with the 

setpoint(s) established during validation (Section 6.1.4). 

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. 

Continuous 

Log off-specification 

occurrences 

Log alarms and indicate whether the reactor is offspecification according to validation criteria. Record offspecification time and volume. 

Monitor UV sensor 

calibration 

Check the duty UV sensor against a reference UV 

sensor, using the recommended protocol (Section 

6.4.1.1) to determine whether the duty UV sensor is in 

calibration. 

Weekly 

Monitor the on-line UVT 

analyzer calibration 

Monitor calibration of the on-line UVT analyzer (Section 

6.4.1.2). 

Switch to standby 

reactor 

Monitor the time necessary to switch to a standby reactor 

to determine if operation will be off-specification during 

switch-over. 

Twice during 

testing period 

Switch to standby 

power or UPS 

Monitor the time necessary to switch to the standby 

power supply to determine if operation will be offspecification because of power transfer. Test the backup 

power supply for a minimum of one hour. 

After 4 weeks, 

100 OMC or 

OMCC cycles, or 

one OCC event 

Inspect lamp sleeves for 

fouling Remove a sleeve from the reactor and inspect as 

recommended in Section 6.3.2.1. 

OMC = on-line mechanical cleaning; OMCC = on-line mechanical chemical cleaning; and OCC = on-line 

chemical cleaning. 

6.1.6 Final Inspection 

As the last step in the start-up process, a detailed inspection of the UV facility should be 

completed. The inspection should include a visual assessment to verify that all components meet 

the technical specifications of the UV equipment specification and validation report and that the 

UV facility was completed in accordance with the construction documents. All UV facility 

components and associated valves and piping should be thoroughly cleaned and disinfected prior 

to service. 

6.2 Operation of UV Facilities 

 

The operation of UV facilities will vary based on the UV manufacturer, the UV reactor 

configuration, and the dose-monitoring strategy. This section discusses required and 

recommended operational and routine start-up and shut-down procedures common to most UV 

equipment and is general in nature. The site-specific operational procedures should be developed 
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in coordination with the manufacturer, UV facility designer, and facility operators, and should be 

described in the O&M manual (Section 6.1.1). Small systems should consider discussing 

operations with the state to determine if simplified operations are possible. 

6.2.1 Operational Requirements 

To receive inactivation credit, the UV reactors must operate within the validated limits 

[40 CFR 141.720(d)]. When a UV reactor is operating outside of these limits, the UV reactor is 

operating off-specification as described in Section 3.4.1. Filtered and 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 141.720(d)(3)]. Guidance on 

determining validated operating conditions is in Section 5.10. The specific monitoring 

requirements associated with off-specification operation are described in Section 6.4.1. 

6.2.2 Recommended Operational Tasks 

UV equipment typically uses automatic control systems and does not need significant 

manual attention for routine operation. Even when UV equipment is operated manually, the only 

parameter that typically can be controlled is lamp power, and some UV reactors can also vary the 

number of lamps energized. Therefore, even manual operation does not result in significant 

operator interaction. Table 6.3 summarizes recommended operational tasks. Recommended 

maintenance tasks are discussed in Section 6.3. 

Table 6.3. Recommended Operational Tasks for the UV Reactor 

Frequency Recommended Tasks 

Daily • Perform overall visual inspection of the UV reactors. 

• Confirm that system control is on automatic mode (if applicable). 

• Check control panel display for status of system components and alarm status and 

history. 

• Verify that all on-line analyzers, flow meters, and data recording equipment are 

operating normally. 

• Review 24-hour monitoring data to confirm that the reactor has been operating within 

validated limits during that period. 

• Verify that ballast cooling fans are operational and that ballasts are not overheated. 

Weekly • Initiate manual operation of wipers (if provided) to verify proper operation. 

Monthly • Check lamp run time values. Consider changing lamps if operating hours exceed 

design life. 

Semiannually • Check ballast cooling fans for unusual noise. 

• Check operation of automatic and manual valves. 
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6.2.3 Start-up and Shut-down of UV Reactors 

 

This section describes start-up procedures, shut-down procedures, and winterization of 

the UV reactors. 

6.2.3.1 Routine Start-up 

The following routine start-up procedure serves as an example approach. The UV 

reactors should be operating within validated conditions once the start-up sequence is complete. 

1. Initiate the UV reactors’ start-up sequence. [Note: Some UV reactors may need 

reduced water flow to cool the lamps during start-up, which would normally be 

initiated automatically. The cooling water exiting the reactor is not disinfected and is 

considered off-specification unless it is diverted to waste.] 

2. Check the SCADA panel or other display to verify that the necessary numbers of 

lamps are on and all of the monitoring parameters are being displayed. 

3. Check and resolve any system alarms being displayed. 

4. Confirm that all on-line analyzers (UV sensors and UVT analyzers, if applicable) and 

flow meters are operating within calibration. 

5. After the lamp warm-up period, increase flow to the validated range (if flow is not 

automatically adjusted with UV reactor control sequence). 

6. Verify correct flow split between parallel UV reactors using flow meters and/or 

differential pressure gauges if these devices are available. 

7. Verify that the UV reactor is operating within validated limits (e.g., flow rate, UV 

intensity, lamp status, validated dose). 

6.2.3.2 Start-up Following Maintenance 

 

The following additional steps should be taken before completing Steps 1 – 7 described 

in the example routine start-up procedure (Section 6.2.3.1) when maintenance has been 

performed on the reactor: 

1. Follow site-specific safety procedures for the power supply and control panel (e.g., 

removing lockouts and tagouts). 

2. Confirm that all lamp and ground connections are properly made. Verify that all 

incoming power conductors, including ground conductors, are properly terminated. 

3. Verify that the lamp ends and all other reactor ports are covered and/or sealed to 

eliminate the potential for operator exposure to UV light. 
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4. Confirm the breakers are turned on, and all electrical cabinets and equipment are clear 

and closed. 

5. Perform Steps 1 – 5 of Section 6.2.3.1. 

6. Verify that all air is purged from the reactors (i.e., the reactor is completely flooded). 

Check the top of the reactor for heat buildup, which indicates an air pocket. 

7. Perform Steps 6 and 7 of Section 6.2.3.1. 

6.2.3.3 Routine Shut-down 

UV reactors are shut down periodically because of 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 close it. 

2. De-energize the reactor immediately after the effluent valve is closed. 

6.2.3.4 Shut-down Prior to Maintenance 

UV reactors are also shut down periodically for maintenance (e.g., cleaning). The 

following steps should be taken following the routine shut-down steps (Section 6.2.3.3) to 

prepare a reactor for maintenance: 

1. Follow lockout and tagout procedures for the facility. 

2. Drain the reactor if necessary for the specific maintenance task. 

3. Inspect and repair or replace any necessary equipment. 

If extended shut-down time is planned, the reactor should be drained to avoid excessive 

fouling. After an extended shut-down period (more than 30 days), the operator should perform a 

cleaning and then inspect the lamp sleeves for fouling. Manual or more extensive cleaning may 

be necessary before start-up, as described in Section 6.3.2.1. 

6.2.3.5 Winterization 

In most drinking water applications, the UV reactors will be located within a building. 

However, in some instances, the reactors may be located in unheated concrete vaults or outside. 

When shutting down a UV reactor for an extended period of time is necessary and damage from 

freezing is possible, the UV reactors should be winterized according to the manufacturer’s 

recommendations. 
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6.3 Maintenance of UV Reactors 

No specific regulatory requirements exist for maintaining a UV reactor; however, the UV 

reactors should be maintained so that disinfection requirements are met. Poor maintenance may 

cause the UV reactors to operate off-specification for extended periods of time. As part of 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. 

6.3.1 Summary of Recommended Maintenance Tasks 

Table 6.4 summarizes the recommended maintenance tasks and refers to the general 

guidelines for those tasks that are discussed in Section 6.3.2. The frequency of performing the 

maintenance tasks in this section are recommendations and likely will be specific to the UV 

equipment installed. Therefore, the UV manufacturer should be contacted to determine the 

appropriate frequency. Items that are not unique to UV facilities (e.g., valves, flow meters, 

uninterruptible or backup power supplies) are not described; however, maintenance on such 

items should also be performed per the manufacturer’s recommendation. Before maintenance is 

performed, the operator should wait at least 5 minutes (or as recommended by the UV 

manufacturer) for the lamps to cool and the energy to dissipate. Lockout and tagout protocol 

should be followed if the main electrical supply to the UV reactors needs to be disconnected for 

the maintenance task. 

Table 6.4. Recommended Maintenance Tasks1

Frequency 

Task 

General Guideline &

Section Reference Action 

Monthly 

(no cleaning or 

OCC) 

Semi-annually 

(OMC or OMCC) 

Check cleaning 

efficiency 

Section 6.3.2.1

• Record UV sensor reading. 

• Extract one sleeve per reactor (or one sleeve per 

bank of lamps) for inspection. 

• If fouling is observed on the first sleeve, check 

remaining sleeves and all UV sensor windows. 

• Manually clean sleeve(s) and UV sensor windows if 

fouling is observed. 

• Record UV sensor reading after cleaning and 

compare to original reading. 

Monthly Check reactor housing, 

sleeves, and wiper 

seals for leaks 

Replace housing, sleeve, or wiper seals if damaged or 

leaking. 

Bimonthly 

(MP lamps) 

Quarterly 

(LP and LPHO 

lamps) 

Check intensity of UV 

lamps 

Section 6.3.2.2

If UV sensors monitor more than one lamp, verify that 

the lamp with the lowest intensity value is closest to the 

UV sensor by replacing the lamp closest to the UV 

sensor with one-forth of the lamps in each row/bank 

(minimum of three). Place the lowest intensity lamp next 

to UV sensor. 

Semi-annually Check cleaning fluid Replenish solution if the reservoir level is low. Drain and 
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Table 6.4. Recommended Maintenance Tasks1

Frequency 

Task 

General Guideline &

Section Reference Action 

(OMCC) reservoir (if provided) 

Section 6.3.2.1

replace solution if the solution is discolored. 

Annually Calibrate reference UV 

sensor 

Section 6.3.2.3

Send the reference UV sensor to a qualified facility 

(e.g., manufacturer) for calibration. Calibration should 

use a traceable standard (e.g., National Institute of 

Standards and Technology (NIST), National Physical 

Laboratory (NPL), Österreichisches Normungsinstitut 

(ÖNORM), or Deutsche Vereinigung des Gas- und 

Wasserfaches (DVGW)). 

Annually Test-trip GFI 

Section 6.3.2.4

Maintain GFI breakers in accordance with 

manufacturer’s recommendations. 

When duty UV 

sensors fail 

calibration 

Replace or recalibrate 

duty UV sensors

Section 6.4.1.1

Send the duty UV sensors to a qualified facility (e.g., 

manufacturer) for calibration, or replace the duty UV 

sensors. 

Manufacturer’s 

recommended 

frequency 

Check thermometer 

and/or water level 

indicator 

Section 6.3.2.5

Visually inspect thermometer and/or water level monitor 

and replace at the manufacturer’s recommended 

frequency. 

Lamp/ 

manufacturer 

specific 

Replace lamp 

Section 6.3.2.6

Replace lamps when any one of the following conditions 

occurs: 

• Initiation of low UV intensity or low validated dose 

alarm (UV intensity or validated dose equal to or less 

than setpoint value) after verifying that this condition 

is caused by low lamp output. 

• Initiation of lamp failure alarm after verifying it is not a 

nuisance alarm. 

When lamps are 

replaced 

Properly dispose of 

lamps 

Section 6.3.2.6

Send spent lamps to a mercury recycling facility or back 

to the manufacturer. 

Sleeve/ 

Manufacturer 

specific 

Replace sleeve 

Section 6.3.2.7

Replace sleeve when damage, cracks, or irreversible 

fouling significantly decreases UV intensity of an 

otherwise acceptable lamp to the minimum validated 

intensity level. Adjust the replacement frequency based 

on operational experience. 

Manufacturer’s 

recommended 

frequency 

Clean UVT analyzer 

and replace parts 

Section 6.3.2.8

Clean and replace parts according to manufacturer’s 

recommended procedure. 

Manufacturer’s 

recommended 

frequency 

Inspect OMC or OMCC 

drive mechanism 

Inspect and maintain OMC or OMCC drive routinely as 

recommended by the manufacturer. 

Manufacturer’s 

recommended 

frequency 

Inspect ballast cooling 

fan 

Section 6.3.2.4

Check the ballast cooling fans for dust buildup and 

damage. Replace if necessary. Replace air filters (if 

applicable). 

OMC = on-line mechanical cleaning; OMCC = on-line mechanical chemical cleaning; and OCC = on-line 

chemical cleaning.

1 Maintenance activities should be consistent with manufacturer’s instructions.
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6.3.2 General Guidelines for UV Reactor Maintenance 

This section describes general guidelines for UV reactor components that relate to 

maintenance tasks summarized in Table 6.4. 

6.3.2.1 Fouling 

As discussed in Chapters 2 and 3, the lamp sleeves and UV sensors/windows will likely 

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. 

Sleeve and UV Sensor Surface/Window Fouling 

Three types of sleeve cleaning techniques, as discussed in Section 2.4.5, are used: off-line 

chemical cleaning (OCC), on-line mechanical cleaning (OMC), and on-line mechanical-chemical 

cleaning (OMCC) methods. 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 over time the UV sensor measurement 

or validated dose (Calculated Dose Approach). For routine operation, the cleaning frequency 

should be increased or decreased based on the amount of fouling left on the sleeves determined 

from the sleeve inspections and the loss of UV intensity before cleaning. 

Sleeves should initially be inspected for fouling every six months if OMC or OMCC is 

used and every month if OCC or no cleaning is used. This frequency should be adjusted, if 

necessary, after operating data are available. A decrease in UV intensity or validated dose at a 

consistent UVT may indicate sleeve fouling, and sleeves should be inspected if fouling is the 

suspected cause of the UV intensity drop (Section 6.6.1). Additionally, the UV sensor windows 

(if applicable) should be inspected for fouling and supplemental cleaning should be conducted if 

necessary, according to the manufacturer’s recommendation. 

For sleeve inspection, one sleeve per reactor (or one sleeve per bank of lamps for reactors 

with multiple rows/banks of lamps) should be inspected. The sleeves should be handled as 

described in Section 6.3.2.7. If damage or fouling is observed, the remaining sleeves should be 

inspected. External sleeve fouling can be difficult to identify. Sleeve discoloration is more easily 

seen by placing the sleeve on a clean, white, lint-free cloth next to a new sleeve. The presence of 

streaks may indicate that the OMC or OMCC wiper material is worn, damaged, or misaligned; 

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. The 

UV reactors need to be drained for sleeve inspection, and the inside of the UV reactor should 

also be inspected. Any algae that has grown on the surface or any other surface fouling that has 

occurred should be manually cleaned according to the UV manufacturer’s recommended 

procedure. 

Manual cleaning (i.e., beyond routine OCC, OMC, or OMCC cleaning) of lamp sleeves, 

if necessary, should be according to manufacturer recommendations and procedures. Abrasive 
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cleaners or pads that might scratch the lamp sleeve should not be used. Also, the inside of the 

sleeve should be dry before 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, no alcohol should remain 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 or OMCC cleaning is used, the wipers should be checked for deformation or 

degradation at the same time the sleeves are checked. The cleaning solution reservoir in OMCC 

systems should be checked every six months to determine whether more solution should be 

added. The solution should be replaced if it is discolored or if the OMCC system is not 

effectively cleaning the sleeve. 

Fouling While Out-of-service 

When the UV reactors are out-of-service and full of water, the sleeves may foul 

(Toivanen 2000). The rate of fouling is site-specific and depends on the water quality. UV 

reactors equipped with OMC or OMCC should continue to clean the sleeves, potentially at a 

lower frequency, even though the UV reactor is off-line, which should prevent fouling of the 

sleeves. For UV reactors that do not include OMC or OMCC, the PWS 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 a shut-down period of more than 30 days, the 

operator should perform a cleaning (OCC, OMC, or OMCC) and then inspect the lamp sleeves 

for fouling. Extraction and manual cleaning of sleeves may be necessary before start-up after 

extended periods of standby. 

6.3.2.2 Lamp Output Variability 

UV lamp output differs for each lamp, depending on lamp age and lot. As discussed in 

Section 2.4.6, a UV sensor measures the changes in UV intensity at its location in the UV 

reactor. However, a UV sensor cannot measure lamp output variability unless each lamp has a 

UV sensor. PWSs that have UV reactors with a UV sensor monitoring more than one lamp 

should assess the UV lamp variability every 2 months for MP lamps or every 3 months for LP 

and LPHO lamps. If all the lamps monitored by a UV sensor are close in age (i.e., their age 

varies by less than 20 percent), it is not necessary to check the output of each lamp. In this case, 

the oldest lamp should be placed in the position nearest the UV sensor. The recommended 

procedure for evaluating the lamp output variability is to: 

1. Identify the lamps that can be used to evaluate the lamp variability (one-fourth of the 

lamps in each row/bank or a minimum of 3 lamps, which ever is greater) 

2. Place each evaluation lamp in the position nearest the UV sensor and record the 

intensity value 

3. Repeat Steps 1 and 2 until one-fourth of the lamps (3 minimum) have been assessed 
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4. Place the lamp with the lowest UV intensity value in the position nearest the UV 

sensor for routine operation 

6.3.2.3 Reference UV Sensors 

 Accurate UV sensors are necessary to verify adequate UV dose delivery during 

operation. Two types of UV sensors are available: duty and reference. Duty UV sensors are online sensors that continuously monitor UV intensity. Reference UV sensors are off-line sensors 

used to assess the duty UV sensor performance. Both types of UV sensors need to be maintained. 

Monitoring of duty UV sensor calibration is described in Section 6.4.1.1. 

The reference UV sensor should be calibrated at least once per year at a qualified facility 

(e.g., manufacturer) to confirm that it is calibrated properly. The reference UV sensor should be 

calibrated against a traceable standard. For example, UV manufactures are currently using NIST, 

NPL, ÖNORM, and DVGW standards. The reference UV sensor should be exposed to UV light 

for a period no longer than necessary to perform the UV intensity measurement. When not in 

use, the reference UV sensor should be stored under conditions that will maintain its integrity 

and accuracy as recommended by the manufacturer. Some PWSs may choose to have multiple 

reference UV sensors to help determine if one reference UV sensor is out of calibration, as a 

replacement reference UV sensor, or to allow multiple duty UV sensors to be checked 

simultaneously. Having multiple reference sensors is helpful if the reference and duty sensor 

measurements do not match because the operator can easily determine which one is in error. If 

the reference UV sensor is found to be out of calibration, the period between calibrations should 

be decreased. 

6.3.2.4 Electrical Concerns 

 

Typically, power to the UV reactors is provided via a distribution transformer, a circuit 

breaker, a disconnect switch at the UV reactor, and related wires and conduits. If maintenance on 

the control panel is necessary, the main electrical supply should be disconnected and the PWS’s 

safety procedures should be followed. 

The power to the lamps is typically delivered through individual GFI circuit breakers and 

ballasts. 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. 

A ballast cooling system is normally provided with LPHO and MP reactors to maintain 

the ballast temperature below the maximum specified limit. LP reactors typically do not need 

ballast cooling. This cooling system should be inspected and maintained as recommended by the 

manufacturer. 
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6.3.2.5 UV Reactor Temperature and Water Level 

The water temperature or water level in the reactor should be monitored because UV 

lamps may break if they become overheated (Appendix E). The thermometer and/or water level 

monitor should be visually inspected and replaced at the manufacturer’s recommended 

frequency. Reactor temperature monitoring and/or water level monitoring are typically included 

in the packaged control systems for MP reactors, although they may not be included in packaged 

control systems for LP and LPHO reactors (due to their much lower operating temperatures). 

6.3.2.6 UV Lamp Replacement 

UV lamp output decreases over time. UV lamps therefore should be replaced periodically 

to maintain sufficient UV dose delivery. Lamp manufacturers should provide documentation of 

lamp output decay characteristics and guaranteed life. This information will help the PWS 

determine the lamp replacement frequency. 

The frequency of UV lamp replacement can be based on a PWS-determined schedule, 

lamp operating hours, or the UV intensity or validated dose reduction. 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 inhibit proper 

operation. 

Because spent UV lamps contain mercury, they are usually considered hazardous waste 

under Subtitle C of the Resource Conservation and Recovery Act (RCRA) (40 CFR parts 260, 

261, 264, and 273). Expended lamps should therefore be sent to a mercury recycling facility 

where the mercury is recovered and lamp components are recycled. Some UV reactor and lamp 

manufacturers will accept spent or broken lamps for recycling or proper disposal (Dinkloh 2001, 

Lienberger 2002, Gump 2002). PWSs should contact the UV manufacturer to determine if they 

accept spent lamps, or contact their state or local agencies for a list of local mercury recycling 

facilities. 

Replacement lamps should be identical to those used during reactor validation with 

respect to arc length, internal and external diameter, spectral output, and placement within the 

quartz sleeve. If the supplied lamps are not equivalent to the lamps used during validation, the 

UV reactor is not operating as validated and will be considered off-specification. The 

manufacturer should provide independent data verifying the lamp aging curve over the entire 

lamp life to show that the new lamps are equal to or better than the validated lamps. However, if 

a PWS replaces the lamps with higher power lamps to receive higher log inactivation credit, 

validation testing should be completed to confirm performance. 

6.3.2.7 Lamp Sleeves 

Lamp sleeves degrade over time due to solarization (Section 2.4.4) and internal sleeve 

fouling, resulting in cloudiness and loss of UV transmittance. Abrasion of the sleeve surface 

during handling or mechanical cleaning may also contribute to the loss of UV transmittance. 

Reduced sleeve transmittance loss is reflected in the UV sensor reading and, therefore, does not 
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have to be monitored. However, a low UV sensor reading may be due to reduced sleeve 

transmittance and should be considered when troubleshooting this problem (as discussed in 

Section 6.6.2). 

Sleeves should be replaced when damage, cracks, or staining diminish UV intensity to 

the point where the minimum validated intensity level or validated dose cannot be met. Sleeves 

in MP equipment should typically be replaced every 3 to 5 years, although sleeves in LP or 

LPHO equipment may not need to be replaced as frequently. This replacement frequency should 

be increased or decreased based on operational experience. Replacement sleeves should be 

identical to the sleeves used during validation in terms of length, inside and outside diameter, 

and UV transmittance, and should meet the design and UV manufacturer’s material and 

construction specifications. If the replacement sleeves differ from those used in validation, UV 

dose delivery and UV sensor modeling can be used to assess the impact of the changes as 

described in Section 5.13. 

The sleeves should be handled in accordance with manufacturer recommendations, using 

clean cotton, powder-free latex, or vinyl gloves because fingerprints can damage the sleeves 

during operation. When sleeves are replaced, the manufacturer’s procedure should be closely 

followed because the lamp sleeve can crack and break from over-tightening of the compression 

nuts that hold it in place. 

6.3.2.8 On-line UVT Analyzer 

 

On-line UVT analyzers should be cleaned and maintained according to the UV 

manufacturer recommendation. On-line UVT analyzer calibration is evaluated periodically as 

part of compliance monitoring (Section 6.4.1.2). 

6.3.3 Spare Parts 

The actual life of a component is a function of many variables, including operating 

conditions, maintenance practices, the quality of the construction materials, and fabrication 

practices. Consequently, estimating the actual life of every component is impossible. 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 equipment 

and to avoid the delivery of off-specification water. 

All UV equipment components have both a design life and a guaranteed life. The design 

life is 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 and are valid under 

specified 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. 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-23 November 2006 

For the Final LT2ESWTR

Table 6.5 provides typical design and guaranteed lives for major UV reactor components. 

These represent current industry trends at the time of publication and are likely to change as 

more O&M information becomes available and technological advances occur. Manufacturers 

should be contacted directly for details specific to their equipment. 

Table 6.5. Typical 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 8,000 hours 4,000 – 8,000 hours 

Sleeve 8 – 10 years 1 – 3 years 

Duty And Reference UV Sensors 3 – 10 years 1 year 

UVT Analyzer 3 – 5 years 1 year 

Cleaning Systems 3 – 5 years 1 – 3 years 

Ballasts 10 – 15 years 1 – 5 years 

1

 Expected duration of operation 

2 Accounts for variability of material quality, production, and operating conditions

 

Following is a suggested minimum inventory of spare parts, expressed as a percentage of 

the installed number. The complete 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 spare MP lamps may be appropriate compared to LP lamps because 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 or OMCC wipers – 5 percent with a minimum of two wipers 

• OMC or OMCC 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 sensor – minimum of 2 units (adjust number based on operating experience) 

• Reference UV sensor – 2 units (more may be needed if wet duty UV sensor are used 

as described in Section 6.4.1.1) 

• On-line UVT analyzer – 1 unit (if used for dose-monitoring strategy) 

 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-24 November 2006 

For the Final LT2ESWTR

6.4 Monitoring and Recording of UV Facility Operation 

This section discusses the required and recommended monitoring and recording activities 

for UV facilities. PWSs should always contact their state to identify any state-specific 

monitoring and reporting requirements and determine when violations of these reporting 

requirements would occur. 

6.4.1 Monitoring and Recording for Compliance Parameters 

PWSs must monitor their UV reactors to determine if the reactors are operating within 

validated conditions. This monitoring must include UV intensity as measured by a UV sensor, 

flow rate, lamp status, and other parameters designated by the state [40 CFR 141.720 (d)(3)]. UV 

reactors should also be regularly monitored to diagnose operating problems, determine when 

maintenance is necessary, and maintain safe operation. In addition to monitoring operational 

parameters, PWSs must verify the calibration of UV sensors in accordance with a protocol that 

the state approves [40 CFR 141.720 (d)(3)]. This section describes the requirements for each of 

these items. 

Because UVT is a critical parameter for the Calculated Dose Approach, EPA believes 

that calibration of UVT analyzers is necessary to determine if reactors are operating within 

validated conditions. Therefore, this section also includes a discussion of calibration of UVT 

analyzers. 

6.4.1.1 Monitoring of Duty UV Sensor Calibration 

Manufacturers will calibrate the UV sensors prior to installation. However, over time the 

UV sensors will drift out of calibration. Because UV sensors are vital to assessing disinfection 

performance, water systems must verify the calibration of UV sensors with a protocol that the 

state approves [40 CFR 141.720 (d)(3)]. If a UV reactor is turned on and the calibration of the 

UV sensors has not been verified, the UV reactor is operating off-specification. 

EPA recommends that calibration of UV sensors be verified with a reference UV sensor 

at least monthly. As noted in Section 6.3.2.3, reference UV sensors are off-line UV sensors that 

should be at least as accurate as the duty UV sensors and should be constructed identically (with 

any exceptions to the reference sensor to make it more accurate). 

Water systems should designate in their protocol whether only the UV sensors in use will 

be monitored, or if all duty and standby sensors will be monitored to confirm calibration. 

Verifying calibration of all duty and stand-by UV reactors has the advantage of rendering all UV 

sensors ready for use at any time if they are needed. 

This section describes the recommended procedure to verify UV sensor calibration and 

the options available if the duty UV sensor fails the recommended calibration criterion. 

Section 6.6 supports this section by presenting a flowchart of the calibration check procedure to 

facilitate decisions if the duty UV sensor fails the calibration criterion. 

 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-25 November 2006 

Duty UV Sensor Calibration Evaluation Procedure 

 

To assess the calibration, the following protocol should be followed: 

1. Because the calibration of the UV sensor is sensitive to the power level of the lamps 

(Swaim et al. 2002), set the lamp power to the level typically used during routine 

operation. 

2. Measure the UV intensity with the duty UV sensor and record the measurement 

result. 

3. Replace the duty UV sensor used in Step 2 with the reference UV sensor in the same 

location (i.e., port). 

4. Measure and record the reference UV sensor measurement. 

5. Calculate the UV sensor calibration ratio (Equation 6.1). If desired, Steps 2-5 can be 

repeated, and a mean calibration ratio can be calculated. 

Calibration Ratio = ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

Ref

D

S

S uty Equation 6.1 

where: 

SDuty = Intensity measured with the duty UV sensor (mW/cm2) 

SRef = Intensity measured with the reference UV sensor (mW/cm2) 

6. Determine if the UV sensor calibration criterion (Equation 6.2) is met for the two UV 

sensor readings or the mean calibration ratio. 

Calibration Ratio ≤ 1.2(see footnote 2) Equation 6.2 

7. If the relationship in Equation 6.2 does not hold true, verify that the reference UV 

sensor is accurate with a different reference UV sensor (i.e., verify that the duty UV 

sensor truly failed the calibration check) by inserting a second reference UV sensor 

and repeating Steps 3 – 6. If a second reference UV sensor is unavailable, the sensor 

calibration can be checked against two duty sensors (as opposed to another reference 

sensor). 

8. If Step 7 confirms the duty UV sensor is out of calibration, replace the duty UV 

sensor with a calibrated UV sensor or apply a UV sensor correction factor (described 

after Example 6.3). 

 

2

 This calibration ratio is higher than the ratio recommended for validation testing (1.1, or 10%, as presented in 

Section 5.5.4). A recommended calibration ratio of 1.2 during operations is based on experience with existing UV 

equipment during routine operations. 

For the Final LT2ESWTR
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-26 November 2006 

9. If a duty UV sensor was replaced, check the replaced UV sensor one hour later by 

repeating steps 2-6 (or based on UV manufacturer’s recommendation) to confirm that 

the replaced duty UV sensor is operating properly. 

Issues to Consider when Monitoring UV Sensor Calibration 

The above UV sensor criteria allow the UV facility to operate out of calibration if duty 

sensor reads conservatively low values compared to the reference sensor. Operating in this 

manner is not energy efficient, however, and the PWS would benefit from having the UV sensor 

recalibrated. 

When re-inserting a duty UV sensor, the rotational alignment of the UV sensor within the 

UV sensor port can affect its sensitivity. This effect may be due to the UV sensor configuration 

(e.g., acceptance angle). The UV sensors should be rotated until the lowest UV intensity reading 

is obtained for routine monitoring purposes with the UV sensor completely inserted into the UV 

sensor port. This affect may not be an issue if the UV sensor is keyed in place or another method 

is used to prevent adjusting the alignment of the sensor. 

Wet UV sensors are in direct contact with the water; therefore, the water in the UV 

reactor needs to be drained before the duty sensors are replaced with reference sensors (Step 3 

above). To reduce the number of times the UV reactor needs to be drained, PWSs should own at 

least the same number of reference sensors as the duty UV sensors in one UV reactor. For 

example, a UV reactor has six duty wet sensors in each UV reactor; therefore, the PWS owns a 

minimum of 6 reference UV sensors to reduce the number of times the UV reactor has to be 

drained during the calibration check procedure. 

Example 6.3. Duty UV Sensors are Verified using Reference Sensor 

(Corresponds to Example 6.1 in Section 6.1.4) 

 System X has one duty UV reactor and one standby reactor. Each reactor has two banks 

of four 200-W LPHO lamps with one germicidal UV sensor per bank (i.e., two UV sensors per 

reactor). The data from a monthly calibration check as presented below show that all of the UV 

sensors meet the UV sensor calibration criterion. Therefore, the duty UV sensors are in 

calibration, and no further action is necessary for the UV sensors this month. 

Within UV Sensor 

Calibration Criterion? Reference UV Calibration Ratio

Sensor 

Reading 

Duty UV 

Sensor 

Reading 

Bank 

Number 

for UV 

Sensor 

For the Final LT2ESWTR

Reactor 

Number (mW/cm2

) (mW/cm2) 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

Ref

D

S

S uty

2.1

S

S

Ref

utyD ⎟ ≤

⎠

⎞ ⎜

⎝

⎛

1 1 13.4 14.6 0.9 Yes 

1 2 12.6 11.8 1.1 Yes 

2 1 11.9 12.5 1.0 Yes 

2 2 15.2 13.7 1.1 Yes 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-27 November 2006 

Use of UV Sensor Correction Factor 

A failed duty UV sensor should be replaced with a calibrated duty UV sensor or the UV 

reactor is off-specification (if operated). However, replacement may not be an option if multiple 

UV sensors fail and/or no additional UV sensors are immediately available. PWSs that cannot 

immediately replace a duty UV sensor that failed the UV sensor calibration criterion 

(Equation 6.2) should implement a UV sensor correction factor (CF). In this approach, a CF is 

selected and applied to either the intensity setpoint or required dose setpoint (depending on the 

dose-monitoring strategy) for the affected UV reactor(s). Operating with a CF is not energy 

efficient; however, this method enables the UV facility to remain in operation while the UV 

sensor problem is resolved. The selected CF should not be changed until the failed UV sensors 

are replaced with factory calibrated UV sensors. This approach is not recommended for longterm operation, and the UV sensor problem should be resolved as quickly as possible. 

The specific steps for the UV sensor CF approach are summarized below: 

1. Use the calibration data to determine the correction factor for each failed UV sensor 

(Equation 6.3). Note that twenty percent is subtracted from the calibration ratio to 

account for the acceptable UV sensor error of 20 percent (i.e., Equation 6.2 shows an 

allowable error of 20 percent). For example, if SDuty = 138 W/m2

 and SRef = 100 

W/m2

, the calibration factor is 1.18. 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ −= 2.0

Re f

Duty

S

S

CFSensor Equation 6.3 

2. Determine the maximum Sensor CF for the failed UV sensors (Equation 6.3) 

(Example 6.4 below presents an example of how to select a Sensor CF). 

3. Multiply the UV intensity setpoint or the required dose (depending on the dosemonitoring strategy) by the UV sensor CF to determine the corrected setpoint or 

required dose (Equations 6.4 and 6.5) that account for the UV sensor errors. 

Corrected UV intensity setpoint = UV intensity setpoint × Sensor CF Equation 6.4 

Corrected required dose = DReq × Sensor CF Equation 6.5 

4. The sensor CF and the corrected UV intensity setpoint or corrected DReq setpoint 

should be included in the report to the state for the affected reactor(s). These 

corrected setpoints are now the basis for off-specification operation until the UV 

sensor calibration problem is resolved. 

5. If the failed UV sensor(s) has not been replaced before the next monthly calibration 

check, the UV reactors with the corrected setpoints should use Equation 6.6 to 

evaluate whether any sensor exceeds the current Sensor CF. The Sensor CF should be 

increased if in any UV sensors fail the previous month’s CF, as described in Equation 

6.6. 

For the Final LT2ESWTR
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-28 November 2006 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

Ref

D

S

S uty < UV Sensor CF+0.2 Equation 6.6 

Example 6.4 shows how a hypothetical water system addressed calibration checks that 

result in multiple UV sensors that are out of calibration. 

Example 6.4. Duty UV Sensors that Do Not Meet Calibration Criteria 

(Corresponds to Example 6.2 in Section 6.1.4)

 

System Y has two duty reactors and one standby reactor. Each reactor has six germicidal 

UV sensors. System Y developed a sensor calibration protocol whereby on a monthly basis, 

system operators verify that sensors are calibrated using Equation 6.2 of this guidance manual. 

Their protocol was approved by the state. 

Data from the UV sensor calibration check for the month of March are presented below:

Correction 

Factor 

Calibration 

Ratio 

Within 

Calibration? Duty UV 

Sensor 

Reading 

Reference 

UV Sensor 

Reading 

Duty UV 

Sensor 

Replaced?

For the Final LT2ESWTR

Reactor 

Number (mW/cm2

) (mW/cm2) ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

Ref

D

S

S uty 20.1

Ref

D ≤⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

S

S uty

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − 2.0

Re f

Duty

S

S

1 259.4 247.8 1.1 Yes NA No 

1 303.8 268.5 1.1 Yes NA No 

1 284.1 303.5 0.9 Yes NA No 

1 400.5 387.1 1.0 Yes NA No 

1 263.2 258.9 1.0 Yes NA No 

1 258.2 266.6 1.0 Yes NA No 

2 368.7 250.6 1.5 No 1.3 No 

2 404.1 311.5 1.3 No 1.1 No 

2 287.9 314.2 0.9 Yes NA No 

2 299.8 214.9 1.4 No 1.2 No 

2 321.3 287.4 1.1 Yes NA No 

2 265.4 347.5 0.8 Yes NA No 

3 379.6 284.6 1.3 No 1.1 No 

3 357.3 303.9 1.2 Yes NA No 

3 258.2 281.5 0.9 Yes NA No 

3 565.5 321.3 1.8 No 1.6 Yes 

3 244.4 147.7 1.7 No 1.5 Yes 

3 238.9 268.1 0.9 Yes NA No 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-29 November 2006 

For the Final LT2ESWTR

Example 6.4. Duty UV Sensors that Do Not Meet Calibration Criteria 

 (continued)

 Six sensors failed calibration with calibration ratios between 1.3 and 1.8; however, 

System Y has only three spare duty UV sensors. The two worst UV sensors were replaced (i.e., 

the UV sensors with a calibration ratio of 1.8 and 1.7), and one of the spare UV sensors was 

retained as a back-up, leaving four UV sensors that failed the calibration criterion. System Y 

applied the UV sensor CF approach to enable their facility to continue operating until the UV 

sensor problem could be resolved with the manufacturer. 

System Y applied the CF to the individual reactors, not the entire UV facility. For 

Reactor 2 the highest calibration ratio was 1.5, resulting in a CF of 1.3 (using Equation 6.3). 

The highest calibration ratio for Reactor 3 (after the two sensors with the calibration ratios of 

1.7 and 1.8 were replaced) was 1.3, giving a CF of 1.1. 

System Y’s required dose is 5.8 mJ/cm2

 for 2.0 log inactivation of Cryptosporidium. 

The corrected required doses for Reactors 2 and 3 are as follows: 

• The corrected required dose for Reactor 2 is 7.5 mJ/cm2 (5.8 mJ/cm2 multiplied by a 

CF of 1.3). 

• The corrected required dose for Reactor 3 is 6.4 mJ/cm2 (5.8 mJ/cm2 multiplied by a 

CF of 1.1). 

System Y maintained a validated dose (i.e., the calculated dose from the dosemonitoring equation divided by the Validation Factor) of 7.5 mJ/cm2

 and 6.4 mJ/cm2 for 

Reactors 2 and 3 respectively, until the four duty UV sensors were replaced the following week. 

If the validated dose had fallen below the corrected required dose, the reactors would have been 

off-specification. Any off-specification events and the volume of water treated during the event 

must be reported to the state as described in Section 6.5. 

 In this example, System Y applied a correction factor for two UV reactors with sensors 

that failed calibration. Another option for System Y would have been to move all of the UV 

sensors that require a CF to one UV reactor (i.e., switching out UV sensors between Reactors 2 

and 3). In this case, the CF would have only been applied to one of the UV reactors instead of 

both Reactors 2 and 3. 

6.4.1.2 Monitoring of UVT Analyzer Calibration 

 

Compliance monitoring of UVT analyzer calibration is required only when UVT is an 

integral part of the dose-monitoring strategy, such as with the Calculated Dose Approach. If the 

UV Intensity Setpoint Approach is used, UVT analyzer calibration checks are not required 

because UVT is not used to verify UV dose delivery (Section 6.4.1.4). 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-30 November 2006 

EPA recommends that on-line UVT analyzers be evaluated at least weekly by comparing 

the on-line UVT measurements to UVT measurements using a bench-top spectrophotometer. The 

bench-top spectrophotometer should be maintained and calibrated at the frequency required by 

the manufacturer. The calibration monitoring frequency should be decreased or increased based 

on the performance demonstrated over a one-year period if approved by the state. For example, 

the frequency could be reduced to once per month if the UVT analyzer is consistently within the 

allowable calibration error for more than a month during the first year of monitoring. 

To monitor the calibration, the following UVT calibration check protocol should be 

followed: 

1. Record the reading of the on-line UVT analyzer (UVTon-line). 

2. Collect a grab sample from a location close to the on-line UVT analyzer sampling 

point. 

3. Measure the UVT of the grab sample on a calibrated bench-top spectrophotometer 

(UVTbench). 

4. Compare the on-line UVT (UVTon-line) reading to the bench-top spectrophotometer 

UVT reading using Equation 6.7. 

UVT line-on − bench(%)UVT(%) ≤ 2 percent UVT3

 Equation 6.7 

 

5. Recalibrate the on-line UVT analyzer if Equation 6.7 is not met. If the UVT analyzer 

is not recalibrated, the UV facility is operating off-specification unless mitigation 

steps are taken. 

If recalibration is necessary in four consecutive weeks, water system operators should 

check the calibration daily for 1 week to determine the rate of calibration decay (i.e., the amount 

the UVT analyzer drifts from the UVTbench per day over the week period). Use these data to 

establish a more frequent recalibration frequency that will enable the on-line UVT analyzer to 

stay within the acceptable calibration error. If these data indicate that calibration cannot be 

maintained for at least 24 hours, water systems should consider one of the two options described 

below. The UV facility is off-specification until one of these options is followed or until the 

UVT analyzer meets the criterion shown in Equation 6.7. 

Option 1 - Take manual UVT measurements with a calibrated bench-top 

spectrophotometer every 4 hours and enter the UVT into the PLC. The UVTbench 

entered should be used for the following 4 hours in the monitoring strategy. 

Option 2 - Enter the design UVT value into the PLC and verify daily that the design 

UVT does not exceed the actual UVT with a grab sample. 

 

 

3

 The absolute value of the difference between the UVT analyzer and bench measurement should be used because 

both conservative and non-conservative UVT errors can cause inaccuracies with the dose monitoring strategy. 

For the Final LT2ESWTR
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-31 November 2006 

For the Final LT2ESWTR

Although these options allow the UV facility to continue operating if the calibration 

error is exceeded while the on-line UVT analyzer is being repaired or replaced, these 

options are not intended for long-term operation. These options should not be employed for 

longer than six months. 

Example 6.5 shows how a hypothetical water system confirmed that they met the 

calibration criteria listed above. 

Example 6.5. UVT Analyzer Calibration Check 

(Corresponds to Example 6.2 in Section 6.1.4) 

 

 System Y has a UVT analyzer that shows a UVTon-line of 93.5 percent. The PWS took a 

grab sample from the influent line to the UVT analyzer and brought it back to the laboratory. 

The sample was analyzed for UV absorbance at 254 nm (A254) using a bench-top 

spectrophotometer that has been properly calibrated. The sample A254 is 0.032 cm-1. The grab 

sample A254 was converted to UVT using Equation 2.2, which yields a UVTbench value of 92.9 

percent. The absolute difference between the on-line reading and bench spectrophotometer 

reading was 0.6 percent UVT. This difference was within the calibration error range of 2 

percent UVT, and the UVT analyzer did not need to be recalibrated.

6.4.1.3 Off-specification Events 

Off-specification operation occurs when the UV facility operates outside of the validated 

limits (Section 6.1.4), a UV sensor is not in calibration (Section 6.4.1.1), the UVT analyzer is not 

in calibration (Section 6.4.1.2) (and it is part of the dose-monitoring strategy), or UV equipment 

is not equivalent or better than the equipment validated. 

Validated Parameters 

PWSs must monitor each reactor to determine whether it is operating within validated 

conditions [40 CFR 141.720(d)(3)]. The validated parameters to monitor depend on the dosemonitoring strategy used and the validation results. Table 6.6 presents the monitoring parameters 

for the monitoring approaches and their off-specification triggers. 

Calibration of UV Sensors 

A UV reactor is producing off-specification water if all three of the following conditions 

occur: 

1. Any of the duty UV sensors did not meet the calibration criteria in the state-approved 

protocol (Section 6.4.1.1) and 

2. The duty UV sensors were not replaced with calibrated duty UV sensors and 

3. UV sensor correction factor was not applied. 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-32 November 2006 

Table 6.6. Off-specification Examples for Each Monitoring Approach 

Dosemonitoring 

strategy Parameters Monitored Off-specification Examples 

UV Intensity 

Setpoint 

Approach 

UV intensity, flow rate, 

lamp status 

1. UV intensity below minimum value 

2. Flow rate above validated limit 

Calculated Dose 

Approach 

calculated dose, VF, 

validated dose, flow rate, 

UVT, lamp status 

1. Validated dose below DReq

2. Flow rate above validated limit 

3. UVT below minimum value 

VF = Validation factor 

DReq = Required UV dose (Table 1.4) 

VF

Calculated Dose

Validated Dose =

Calibration of UVT Analyzers 

Similarly, the UV facility is off-specification if the UVT analyzer is found to be out of 

calibration and the remedial actions described in Section 6.4.1.2 are not completed. 

UV Equipment Components 

The LT2ESWTR requires that water systems use reactors that have undergone validation 

testing [40 CFR 141.720 (d)(2)]. It follows, therefore, that installed and replaced components 

should be equal to or better than the components used during validation testing. If not, the UV 

facility is off-specification unless the UV equipment is re-validated. The need for re-validation 

and when the UV facility would be off-specification because of UV equipment components is 

described in Section 5.13. 

6.4.1.4 Monitoring and Recording Frequency of Required Parameters 

The required dose-monitoring parameters (flow rate, UV intensity, number of banks on, 

etc.) should be continuously monitored (i.e., at least every 5 minutes) for each UV reactor, and 

these values should be recorded at least once every 4 hours. Very small systems (e.g., systems 

serving fewer than 500 people) that cannot record reactor status every 4 hours (e.g., manual 

recording is practiced) could consider a reduced recording frequency; however, the frequency 

should not be less than once per day and should be discussed with the state. 

All water systems should record off-specification alarms at a minimum of 5-minute 

intervals until the alarm condition has been corrected. The off-specification volume will start as 

soon as the flow is found to be outside of the validated range. The measurement of offspecification volume will stop as soon as the flow is shown to be within the validated limits. 

The EPA recognizes that the off-specification event may begin before the offspecification alarm is monitored. The off-specification event may also end before the offspeciation alarm is cancelled and recorded. It is assumed that over time the underestimation of 

off-specification water before the alarm is activated and the overestimation of off-specification 

water before the alarm is cancelled will minimize any errors in the calculation of offFor the Final LT2ESWTR
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-33 November 2006 

For the Final LT2ESWTR

specification water. If a facility monitors more frequently than the minimum recommended 5-

minute intervals, the off-specification volume will start as soon as the reactor is monitored as 

operating off-specification and the off-specification volume will stop as soon as the reactor is 

monitored as being on-specification. More frequent off-specification alarm monitoring may more 

accurately account for the off-specification volume. 

These off-specification alarm records should be used to determine the percentage of flow 

volume that is off-specification. The compliance with the off-specification limits is based on the 

off-specification percentage for the UV facility, not for individual reactors. The monitoring 

guidelines are summarized in Table 6.7, and Example 6.6 illustrates the routine and offspecification recording recommendations. 

Table 6.7. Recommended Recording Frequency for 

Required Monitoring Parameters

Parameter 

Recommended 

Recording Frequency Notes 

Off-specification 

Alarm 

Minimum of every 5 

minutes 

Recording should continue until the alarm condition 

has been corrected. 

UV Intensity Every 4 hours The UV intensity must be greater than or equal to the 

validated setpoint. 

UVT1 Every 4 hours The UVT must be greater than or equal to the 

minimum UVT validated. 

Validated Dose1 Every 4 hours The validated dose must be greater than or equal to 

the DReq. 

Lamp Status Every 4 hours Lamps should be energized if water is flowing through 

the UV reactor. 

Flow Rate Every 4 hours The flow rate should be less than or equal to the 

maximum flow tested in validation. 

Production Volume Off-specification events 

and monthly total 

The production volume needs to be recorded so the offspecification compliance calculation can be completed.

Calibration of UV 

Sensors 

Monthly The calibration of the UV sensor should be monitored 

as described in Section 6.4.1.1. 

Calibration of Online UVT Analyzer1Weekly2 The calibration of the UVT analyzer should be 

monitored as described in Section 6.4.1.2.1

1 Required only if necessary for the dose-monitoring strategy (i.e., the Calculated Dose Approach). 

2 Frequency could be reduced as described in Section 6.4.1.2.
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-34 November 2006 

For the Final LT2ESWTR

Example 6.6 Routine and Off-specification Recording 

(Corresponds to Example 6.2 in Section 6.1.4) 

This example illustrates System Y’s daily monitoring and recording of UV equipment 

operation to verify that it is operating within validated limits. The System Y has two duty 

reactors and one standby. Reactor 1 is used only for part of the day; Reactor 2 is used 24 hours 

a day; and Reactor 3 is off-line in the 24-hour period. System Y monitors the off-specification 

alarms every 5 minutes, which is the minimum recommended. 

 At 1:08 PM the flow at System Y went above the validated range because an upstream 

filter was taken off-line for backwashing. At 1:10 PM, the flow through Reactor 2 was 

recorded as being above the validated limit of 10 mgd as an off-specification alarm. This 

resulted in the reactor operating off-specification while the flow split between the reactors was 

adjusted. The flow returned to within the validated range at 1:17 PM when the backwashed 

filter was placed back on-line. System Y recorded that the off-specification alarm was remedied 

at 1:20 PM 

 The off-specification recording started when the first off-specification alarm occurred 

(1:10 PM) and continued at 5-minute intervals until the reactor was monitored as being onspecification again (1:20 PM) when the data recording reverted back to every 4 hours. This 

event is illustrated in the table below. During this 24-hour period, no other off-specification 

events occurred. If System Y monitors the off-specification alarms at 1 minute intervals, the 

off-specification operation would have been more accurately recorded. 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-35 November 2006 

For the Final LT2ESWTR

Example 6.6 Routine and Off-specification Recording (continued) 

Monitoring Reactor 1 Reactor 2 Reactor 3 

Time Reactor 

Status 

Data 

Recorded 

Reactor 

Status 

Data 

Recorded 

Reactor 

Status 

Data 

Recorded 

12:00 AM Off Off-line Onspecification 

Onspecification Off Off-line 

... Off None Onspecification None Off None 

4:00 AM Off Off-line Onspecification 

Onspecification Off Off-line 

... Off None Onspecification None Off None 

8:00 AM Onspecification 

Onspecification Onspecification Onspecification Off Off-line 

... Onspecification None Onspecification None Off None 

12:00 PM Onspecification 

Onspecification Onspecification Onspecification Off Off-line 

... Onspecification None Onspecification None Off None 

1:05 PM Onspecification None Onspecification None Off None 

1:10 PM Onspecification None Offspecification 

Offspecification Off None 

1:15 PM Onspecification None Offspecification 

Offspecification Off None 

1:20 PM Onspecification None Onspecification 

Onspecification Off None 

... Onspecification None Onspecification None Off None 

4:00 PM Onspecification 

Onspecification Onspecification Onspecification Off Off-line 

... Onspecification None Onspecification None Off None 

8:00 PM Onspecification 

Onspecification Onspecification Onspecification Off Off-line 

... Onspecification None Onspecification None Off None 

12:00 AM Onspecification 

Onspecification Onspecification Onspecification Off Off-line 

Daily total offspecification 

events 0 events

1 event 

lasting 10 

minutes 0 events

Note shaded areas indicate data that were recorded.
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-36 November 2006 

For the Final LT2ESWTR

Example 6.6 Routine and Off-specification Recording (continued) 

The off-specification volume of water must be determined for compliance reporting 

(Section 6.5). The table below provides the flow and volume monitoring and recording for this 

example day. The volume was calculated using a flow totalizer in the PLC programming for 

each 5-minute off-specification period. 

Reactor 1 Reactor 2 

Flowrate1 Volume2

Total Daily 

Volume Flowrate1 Volume2

Total Daily 

Volume 

Monitoring Time (mgd) (gal) (gal) (mgd) (gal) (gal) 

12:00 AM 0 - - 9.2 1,526,782 9,144,269

... 

4:00 AM 0 - - 9 1,358,972 1,358,972

... 

8:00 AM 7.2 1,120,254 1,120,254 9.3 1,534,682 2,893,654

... 

12:00 PM 7.4 1,225,897 2,346,151 9.5 1,510,036 4,403,690

... 

1:10 PM NR3 NR3 12.2 4,870,357

1:15 PM NR3 NR3 11.3 38,452 4,908,809

1:20 PM NR3 NR3 9.7 37,522 4,946,331

... 

4:00 PM 7 1,102,564 3,448,715 9.4 1,551,123 5,954,813

... 

8:00 PM 7.2 1,025,951 4,474,666 9.2 1,520,321 7,475,134

... 

12:00 AM 7.3 1,159,951 5,634,617 9.3 1,536,987 9,012,121

Total Daily Offspecification volume (gal) 

- 75,974 

Total Daily Volume (gal) 5,634,617 9,012,121 

Note shaded areas indicate data that were recorded. 

1

 Maximum flow rate was recorded to show the flow was within validated limits. 2

 Volume was estimated in the PLC using the flow rate. 

3

 NR indicates that data were not recorded 

Example 6.6 is based on the flow rate’s increasing beyond the validated range. Offspecification recording would follow the same procedure for any problem resulting in offspecification time (e.g., UV sensor failure or the UVT decreased beyond the validated range). 

6.4.2 Monitoring and Recording for Operational Parameters Not Related to 

Compliance 

To minimize operational problems, facilitate regulatory compliance, and evaluate UV 

reactor performance, parameters in addition to those required for regulatory compliance should 

be monitored. Table 6.8 presents these suggested parameters and the recommended recording 

frequency. These parameters and their monitoring frequency should be adjusted based on site-
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-37 November 2006 

For the Final LT2ESWTR

specific operating experience. For example, if sleeve fouling is a maintenance issue and 

supplemental cleaning is frequent (e.g., monthly), the fouling parameters should be monitored 

daily as shown in Table 6.7 rather than weekly. 

Table 6.8. Recommended Monitoring Parameters and Recording Frequency 

Parameter 

Monitoring 

Frequency 

Recording 

Frequency Notes 

Power Draw Continuous Every 4 hours This information can be used to 

determine the most energy efficient 

operation strategies. 

Water Temperature 

(Only Necessary for MP 

Reactors)

Continuous Daily Monitoring is important to verify that the 

high temperature limit is not exceeded 

(often part of packaged UV control 

system). 

UV Lamp On/Off Cycles Continuous Weekly (Total 

cycles in a week) 

The number of on/off cycles can help 

assess lamp aging. 

Turbidity (In Addition to 

Monitoring Otherwise 

Required Under Subpart 

H) 

Daily Weekly Recommended only if chemicals (e.g., 

lime) are added prior to UV disinfection. 

Monitoring may not be necessary for 

many UV facilities. 

pH, Iron, Calcium, 

Alkalinity, Hardness, 

ORP 

Weekly (reduce if 

fouling is not 

prevalent) 

Weekly These parameters will help assess 

fouling issues if necessary. 

UVT Analyzer 

Calibration1 Weekly (reduce if 

appropriate based 

on operational 

experience) 

Weekly This information can assist in planning 

scheduled maintenance and the O&M 

budget. 

Operational Age2 of the 

Following Equipment: 

• Lamp 

• Ballast 

• Sleeve 

• UV Sensor 

Monthly Monthly This information can assist in planning 

scheduled maintenance and the O&M 

budget. 

Calibration of Flow 

Meter 

Monthly Monthly This information can assist in planning 

scheduled maintenance and the O&M 

budget. 

1 Recommended if not being monitored as discussed in Section 6.4.1.2 2

 Operational age is the amount of time the equipment has been operated (e.g., lamp hours)

6.5 UV Facility Reporting to the State 

Monthly reports must be prepared and submitted to the state (CFR 141.721). This section 

describes the required reporting and provides example reporting forms.
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-38 November 2006 

For the Final LT2ESWTR

6.5.1 Required Reporting 

 

The report must include the percentage of off-specification water for the UV facility and 

the UV sensor calibration monitoring [CFR 141.721(f)]. 

The percentage of off-specification water should be calculated on a volume basis. The 

percentage should be calculated by totaling the 5-minute off-specification alarm records and 

associated volume released during those periods for each reactor. The volume released during the 

off-specification event can be determined by: 

• Using a flow totalizer that automatically records the volume when an offspecification event occurs. 

• The PLC calculating the volume based on flow rate in one-minute (or shorter) 

intervals during the off-specification event. 

• The PLC calculating the volume based on the maximum flow during the offspecification period multiplied by the length of time of the off-specification event. 

Off-specification time can be used a surrogate for off-specification volume only if the 

flow is constant and this method is approved by the state. 

The total off-specification volume for all UV reactors should be divided by the total 

volume produced by the UV facility that month and multiplied by 100 percent (See 

Example 6.7). PWSs with constant flows may use off-specification time as an indicator for offspecification volume (i.e., total off-specification divided by time in operation multiplied by 100 

percent). SCADA and PLC interfaces can be designed to automatically calculate offspecification based on the required monitoring, recording, and reporting. 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-39 November 2006 

For the Final LT2ESWTR

Example 6.7. Off-specification Computation 

(Corresponds to Example 6.6 in Section 6.4.1.4) 

This example illustrates the computation of monthly percent off-specification operation 

for System Y in Example 6.6. System Y had no other off-specification events this month; 

therefore, the table in Example 6.6 captures all off-specification volume for the month. To 

determine percent off-specification volume, monthly production volume totals were obtained 

from the SCADA system. The table below shows the data used for the computation. In this 

example, 0.02 percent (75,974 gal / 305,683,189 gal × 100%) of the volume of water System Y 

treated was off-specification, which is within the allowable regulatory limit of 5 percent. 

Monthly Total Offspecification for UV 

Facility 

Monthly Total Production for 

UV Facility 

Monthly 

Percent Offspecification2

Reactor 

No. 

Time 

(hr) 

Volume

(gal) 1

Time 

(hr) 

Volume 

(gal) 

1 0.00 0 168 17,568,080 

2 0.173 75,974 720 288,115,109 

3 0.00 0 0 0 

Totals 0.173 75,974 888 305,683,189 0.02% 1 Off-specification volumes are from Example 6.6. 2 Total monthly off-specification volume divided by total volume produced in the month 

multiplied by 100 percent 3

 Total monthly off-specification time was shown in Example 6.6 to be 10 minutes (0.17 hr).

The percentage of UV sensors that were checked for calibration must be reported 

monthly. All UV sensors in operation that month should be checked. Additionally, the daily low 

validated dose or daily low UV intensity, depending on the dose-monitoring strategy, should be 

reported to the state monthly. The state may also have additional reporting requirements and 

should be contacted to determine the specific content of the monthly reports and to coordinate 

with other reporting requirements. 

6.5.2 Example Reporting Forms and Calculation Worksheets 

Example forms and calculation sheets are shown in Figures 6.2 through 6.8. The state 

should be contacted to determine whether these forms will be acceptable. The forms are 

described in greater detail below. Two calculation worksheets are also provided that can assist 

with completing the compliance forms; these forms need not be submitted to the state. 

 Figure 6.2 is an example of a summary report that would be completed by the PWS and 

submitted to the state on a monthly basis. 

 Figures 6.3 and 6.4 are example operating logs that would be completed on a daily basis 

for the calculated dose and UV Intensity Setpoint Approach, respectively. The forms would be 

used to record the operating status of the UV equipment and to record the volume of water 

discharged during off-specification operation each day. The state may request that this 

information is submitted on a monthly basis. 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-40 November 2006 

For the Final LT2ESWTR

 Figure 6.5 is an off-specification calculation worksheet that can assist PWSs with 

calculating the off-specification percentage for the daily logs (Figures 6.3 and 6.4); this form 

need not be submitted to the state. 

 Figure 6.6 is an example duty UV sensor calibration log. This log would be completed 

whenever UV sensor calibration checks are performed. The log would be used to record the 

results of the calibration testing and to track any UV sensor recalibration or repair work that was 

completed. The state may request this information to be submitted on a monthly basis. 

 Figure 6.7 is a UV sensor CF calculation worksheet that can help PWSs determine the 

appropriate UV sensor CF when the PWS needs to use this approach to stay in compliance. This 

form need not be submitted to the state. 

 Figure 6.8 is an example on-line UVT analyzer calibration log. This log would be 

completed only by those PWSs that have included on-line UVT analyzers as part of their dosemonitoring strategies. The log would be completed whenever UVT analyzer calibration checks 

are performed. The log would be used to record the results of the calibration testing and to track 

any recalibration or repair work that was completed. The state may request this information to be 

submitted on a monthly basis. 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-41 November 2006 

Figure 6.2. Example Summary Monthly Report 

Reporting Period:

System/Treatment Plant:

PWSID:

Signature of Principal Executive: Date:

Officer or Authorized Agent: Date:

Unit Number Total Run Time (hrs) Total Production (MG) Number of Off- Specification Events Total Off-Specification Volume 

) MG

TotalTotal Volume of Off-Specification Water Produced (MG) [A]Total Volume of Water Produced (MG) [B]Total Off-Specification Water Produced (% of Volume of Water Produced) ([A]/[B]*100)Facility Meets Off-Specification Requirement (< 5% of Volume on a Monthly Basis) (Y/N)Of the _____ sensors, _____ have been checked for calibration and _____ were within the acceptable range of tolerance.The Following Reactors had a Sensor Correction FactorReactor Number Sensor Correction FactorCompliance Certification

(

Off-Specification Data

For the Final LT2ESWTR
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-42 November 2006 

Figure 6.3. Example Daily Operating Log for Calculated Dose Approach 

Reporting Period: Maximum Validated Flow Rate:

System/Treatment Plant: Minimum Validated UVT: Validated Dose = 

PWSID: Target Log Inactivation:

UV Reactor: Target Pathogen: Calculated Dose =Dose that is calculated by validated PLC algorithm

Process Train: Dose Required (Dreq'd): VF = Validation factor

Operator Signature: Validation Factor (VF): CF = UV intensity sensor correction factor. 

Date: The CF is only applied if sensors do not meet recommended criteria 

 (NOTE – a CF will not be needed in most cases)

Dose Requirements UV Dose Adequacy Determination Total OffSpecification 

Day Run Time (hrs) Total Production (MG)

Dreq'd1 

(mJ/cm2

)

 [A]

Sensor Correction 

Factor2 

 [B]

Calculated Dose3 

(mJ/cm2

)

[C]

Daily Minimum 

Validated Dose4 

([C]/[VF]/[B]) 

(mJ/cm2) 

[D]

Flow Rate 

(MGD)

UVT

 (%)

Validated Dose > 

Dreq'd

([D] > [A]) 

(Y/N)

Total OffSpecification Volume

(MG)

123456789

10111213141516171819202122232425262728293031

MinMaxTotal

1 Dre

For the Final LT2ESWTR

q'd is the dose required for the target log inactivation without a VF or Sensor CF applied and can be found in the UVDGM Table 1.4. 2 Sensor CF will be 1 is no CF is used 

3 Calculated dose is calculated using the dose algorithm in the PLC.4 The Validated Dose is the dose based on the calculated dose that is normalized on the Validation Factor and Correction Factor5 Off-specification worksheet (Figure 6.5) should be used to calculate daily off-specification volume. If UVT, flowrate, and/or Validated Dose off-specification occur simultaneously, the off-specification time should only be counted once

Operational Data Data at Daily Minimum Validated Dose

CF VF

Dose Calculated

×
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-43 November 2006 

 Figure 6.4. Example Daily Operating Log for UV Intensity Setpoint Approach 

Reporting Period: Maximum Validated Flow Rate:

System/Treatment Plant: Minimum Validated UVT:

PWSID: Target Log Inactivation:

UV Reactor: Target Pathogen:

Process Train: Intensity Setpoint:

Operator Signature:

Date:

Day

Run 

Time 

(hrs)

Total 

Production 

(MG)

Min (mgd) Ave (mgd) Max (mgd) Intensity Setpoint (W/m2

)

 [A]

Sensor Correction 

Factor1 

 [B]

Adjusted Intensity 

Setpoint

 (W/m2) 

([A] * [B]) 

[C]

Daily Minimum 

Intensity 

(W/m2) 

[D]

Minimum Daily 

Intensity > 

Adjusted Intensity 

Setpoint

([D] > [C]) 

) Y/N (

Total Flow OffSpecification3

(MG)

123456789

10111213141516171819202122232425262728293031

MinMaxTotal

1 Sensor CF will be 1 is no CF is used. 2 UVT measurements are not required but could be useful in addressing operational issues.3 Off-specification worksheet (Figure 6.5) should be used to calculate daily off-specification volume. If UV intensity or flowrate off-specification occur simultaneously, the off-specification time should only be 

 counted once

Total Flow OffSpecification Daily Minimum Intensity Flow Rate Operational Data Intensity Requirements

For the Final LT2ESWTR
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-44 November 2006 

Figure 6.5. Example Off-specification Calculation Worksheet 

For the Final LT2ESWTR

 

Reporting Period: Operator Signature:

System/Treatment Plant: Date:

PWSID:

Off-Specification Calculation Worksheet (Note - This sheet should only be used when an off-specification event occurs):

Date1 Reactor Number Process Train Number Off-Specification Event Description2 Flow Rate3 (MGD) [A] Time (days) [B] Off-Specification Volume (MG) ([A]*[B])

Total Off-Specification Flow for the Day3

1 This workseet should only be used for one date and one reactor.2 This worksheet assumes that the flowrate is constant during the off-specification event. Off-specification volume can also be obtained from a flow totalizer. 3 Off-Specification event can be caused by UVT, flowrate, intensity, or validated dose being out of the validated range.4 The total off-specification flow should be transferred to Figure 6.3 or 6.4 if any off-specification events occurred.
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-45 November 2006 

Figure 6.6. Example Monthly UV Sensor Calibration Check Log 

For the Final LT2ESWTR

 Reporting Period: System/Treatment Plant: Calibration Ratio = PWSID: CF used (if applicable): Operator Signature: Sensor Correction Factor = Date: where S is the measured intensity UV Sensor Calibration Report (Make Additional Copies of Form as Necessary) Date Reactor Number Duty Sensor Number UV Sensor Operating Time (hrs) Reference Sensor Serial Number Duty UV Sensor Reading1 [A] Reference UV Sensor Reading1 [B] Calibration Ratio ([A]/[B]) Calibration Ratio ≤ 1.2 (Y/N) Sensor Correction Factor Used If CF is used, Calibration Ratio - 0.2 ≤ CF (Y/N) rtification: Number of UV sensor calibrated Number of UV sensors out of calibration Number of UV sensor(s) sent to manufacturer to be recalibrated as documented below intensity sensors sent to manufacturer for calibration (Add additional rows as necessary): Sensor Serial Numbe Ce UV 

r Unit No. Date Sent Date Received

hree duty UV sensor and reference UV sensor readings are taken, the mean of the reading can be used. 1 If t

⎞⎟⎟⎠

⎛⎜⎜⎝

− 20 S

S

uty . D

Ref

⎞⎟⎟⎠

⎛⎜⎜⎝ Ref

S

S

Duty
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-46 November 2006 

Figure 6.7. Example UV Sensor CF Calculation Worksheet Reporting Period: CF used (if applicable): System/Treatment Plant: Operator Signature: PWSID: Date: Where: SDuty is the duty UV sensor reading Optional Sensor Correction Factor Worksheet:

For the Final LT2ESWTR

SRef is the duty UV sensor reading

Date Reactor Number Duty Sensor 

Number1

UV Sensor 

Operating Time

(hrs)

Reference Sensor 

Serial Number

Duty UV Sensor

 Reading3

 [A]

Reference UV 

Sensor Reading3

 [B]

Sensor Correction 

Factor

(([A]/[B])-0.2)

Selected UV Sensor 

CF2

1 Only failed sensors need to be input.2 UV Sensor CF should be based on the calibration ratios for the failed sensors and should use the maximum ratio. The CF is reactor specific. 3 If three duty UV sensor and reference UV sensor readings are taken, the mean of the reading can be used.

⎞⎟⎟⎠

⎛⎜⎜⎝

−= 20 S

S

Factor Correction

f

Duty .

Re
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-47 November 2006 

Figure 6.8. Example Monthly UVT Analyzer Calibration Check Log Reporting Period: System/Treatment Plant: |UVTon-line(%) - UVTbench(%)| ≤ 2% UVT PWSID: UVT Analyzer Number: Operator Signature: Date: UVT Analyzer Calibration Report (Make Additional Copies of Form as Necessary) UVT Analyzer Number Week Number Dates On-line Reading (%) [A] Grab Sample Result (%) [B] Difference (%) ( |[A]-[B]| ) Difference ≤ 2% UVT? (Y/N)

12345

Certification:

All calibration checks were within the acceptable tolerance during this month.

Recalibration was required and is documented below.

On-Site Calibration. Manufacturer Calibration.

UVT Analyzer Calibration:

UVT Analyzer Number 

On-site or 

manufacturer 

recalibration?

Date Recalibration 

Performed

Recalibration

 Successful? 

(Y/N)

Initials (On-site Calibration 

Only)

For the Final LT2ESWTR
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-48 November 2006 

For the Final LT2ESWTR

6.6 Operational Challenges 

An excursion from validated operating limits can be caused by low UV intensity, low 

validated dose, low UVT, high flow rate, poor UV sensor calibration, poor UVT analyzer 

calibration, or a combination of these conditions. These conditions should be resolved quickly to 

verify regulatory compliance because they can result in prolonged off-specification operation. 

Additionally, the evaluations described in this section should be initiated before validated criteria 

are exceeded and off-specification occurs. This section discusses some of the potential 

operational challenges and suggests corrective measures. 

6.6.1 Low UV Intensity or Validated Dose Below the Setpoint 

Low UV intensity or validated dose may cause a reactor to operate outside of validated 

limits. Although UV intensity limits are not explicitly set in the Calculated Dose Approach, a 

low UV intensity will reduce the validated dose that is delivered, along with low UVT or high 

flow rate. Therefore, approaches for addressing either a low UV intensity or low validated dose 

readings are often the same. 

The output of the UV lamps, UV transmittance of the sleeves, status of the UV sensor, 

and fouling of both lamp sleeves and UV sensor windows affect UV sensor readings and 

validated dose. Figure 6.9 presents a decision tree for evaluating low UV intensity or low 

validated dose. If strategies in Figure 6.9 cannot be implemented or are not successful in getting 

the UV intensity or validated dose above the required setpoint, the UV manufacturer or UV 

facility designer should be contacted to investigate the problem further. The PWS should activate 

any backup disinfection, shift production to another WTP or source of supply, or consider 

shutting down the WTP until the UV intensity or validated dose is within the validated limits. 

Any time that the UV intensity or validated dose is lower than the validated limit, it should be 

recorded as off-specification (Section 6.4.1.3). 

6.6.2 Low UV Transmittance 

This evaluation of low UVT presumes that either the low intensity evaluation 

(Section 6.6.1) has been completed and either (1) the cause of the low UV intensity was low 

UVT or (2) the operational staff has observed low UVT. If the reactor uses the Calculated Dose 

Approach, it may be programmed to increase lamp output or number of lamps in service to 

accommodate a decrease in UVT if the UVT is still within the validated range. If the UV 

equipment does not sufficiently compensate, or if the UV reactor cannot adjust lamp output, the 

UV intensity or validated dose may fall below the validated limits. 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-49 November 2006 

Figure 6.9. Low UV Intensity or Low Validated Dose Decision Chart 

 

For the Final LT2ESWTR
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-50 November 2006 

For the Final LT2ESWTR

The UVT analyzer or bench-top spectrophotometer equipment should be evaluated to 

determine if the instruments are operating properly as described in Section 6.4.1.2. If the low 

UVT is not due to faulty instruments and is below the validated UVT, the following WTP 

operational changes should be considered: 

• Vary the source water blending ratio (if available) to increase UVT. 

• Where applicable, 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 the oxidant dose prior to the UV facility if possible. However, this strategy 

may increase DBP formation or increase fouling, which should be evaluated before 

this option is used. 

• Investigate potential upstream chemical interferences from a process failure or upset. 

For example, if the ozone quenching system failed, the UVT would decrease. 

A decision tree that summarizes the approach for troubleshooting low UVT is shown in 

Figure 6.10. If the strategies presented in Figure 6.10 and described above cannot be 

implemented or are not successful in correcting the low UVT, the UV manufacturer or UV 

facility designer should be contacted to investigate the problem further. The PWS may consider 

shutting down the WTP or activating a backup disinfection system, if available, until the UVT is 

within the validated limits. The low UVT condition must be recorded as off-specification 

(Section 6.4.1.3) when the UVT is lower than the validated limit and the Calculated Dose 

Approach is used. 

6.6.3 Failure to Meet UV Sensor Calibration Criterion 

Unreliable UV sensor readings can be due to UV sensor malfunction, condensation in the 

UV sensor or between the UV sensor and UV sensor window, lamp malfunction, poor 

grounding, degradation of UV sensor electronics, or electronic short-circuits. Monitoring the UV 

sensor calibration will identify poor performance. 

The integrated procedure of monitoring multiple UV sensor calibrations and evaluating 

failures of the calibration criterion can be complex, especially if multiple UV sensors fail the 

calibration criterion. A decision tree (Figure 6.11) can assist with the monitoring of UV sensor 

calibration and determining whether UV sensors should be replaced or whether a UV sensor CF 

is needed. 

6.7 Staffing, Training, and Safety Issues 

To provide consistent and reliable operation of UV reactors, the PWS must have 

appropriate staffing, training, and safety measures in place. This section discusses these issues. 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-51 November 2006 

Figure 6.10. Low UV Transmittance Decision Chart 

Resample and 

Reanalyze.

UVT is either close 

to or below UVT 

validation limits.

Is on-line UVT 

monitor used?

Record on-line UVT 

measurement and 

check grab 

sample UVT with 

bench-top 

spectrophotometer.

No Yes

Is repeat UVT 

similar and low?

Evaluate 

sampling and 

analytical 

procedures.

No

Check 

spectrophotometer’s 

calibration.

Yes

Is spectrophotometer 

within manufacturer’s

calibration limits?

Recalibrate or repair 

bench-top 

spectrophotometer.

Is the UV facility 

operating off-specification because of 

low UVT?

Recalibrate or 

repair on-line 

UVT monitor.

Yes

No

Yes

Is UVT still below the

 validation limit after the changes?

Is UV intensity

 or validated dose 

below validation 

limits?

Continue 

operation.

Yes

No

No

Contact manufacturer 

or UV facility designer to 

investigate this issue further. Shift 

production demand to another 

supply or WTP if available, or 

consider WTP shutdown.

Yes

See low UV intensity 

decision tree 

(Figure 6.9).

Yes

Can UVT be 

increased through source water changes or 

WTP operation changes?

No

Is the 

│UVTon-line – UVT bench│

≤ 2 % UVT?

No

Yes

Is UVT 

still low?

Yes

No

No

Continue 

operation and 

consider source 

water or WTP 

operational 

changes to 

improve UVT. 

No Continue 

operation.

Start

For the Final LT2ESWTR
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-52 November 2006 

Figure 6.11. Monitoring of UV Sensor Calibration Flowchart 

For the Final LT2ESWTR
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-53 November 2006 

For the Final LT2ESWTR

6.7.1 Staffing Levels 

 

During initial start-up operation, more operator attention will be needed to assist with 

functional and performance testing and to establish site-specific O&M procedures (described in 

Section 6.1.1). However, depending on the level of automation, a typical UV facility requires 

minimal operator attention during normal operation. Generally, UV facilities 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 6.9 describes how various site-specific factors affect staffing needs for a UV facility.

Table 6.9. Factors Impacting Staffing Needs 

Factor Impact on Staffing 

Type of UV Reactor LP and LPHO reactors may require more maintenance than 

an MP reactor because they have more lamps and typically 

are cleaned off-line (i.e., OCC cleaning). However, MP lamps 

generally need to be replaced more often than LP lamps. 

Instrumentation and Monitoring 

Strategy 

More automated control strategies will result in lower staffing 

levels due to enhanced remote operation and monitoring 

capability. 

Water Quality Water quality and UV reactor design affect sleeve fouling and 

cleaning frequency. These factors, in turn, impact the staffing 

needs for manual cleaning for OCC systems and for 

maintaining the OMC or OMCC system. 

6.7.2 Training 

Training is necessary for all personnel who are associated with the UV facility, 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, monitoring, instrumentation, 

and responses to operational issues. 

The UV manufacturer and UV facility designer should provide training on the UV 

reactors, UV facility design, and O&M activities. Training should include both classroom 

instruction and field training. Additionally, actively involving the operators 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 is altered. 

6.7.3 Safety Issues 

This section provides some recommended safety precautions for UV reactor operations. 

The recommended precautions in this section should be considered in addition to manufacturer’s 

recommended safety precautions and procedures, Occupational Safety and Health 
6. Start-up and Operation of UV Facilities 

UV Disinfection Guidance Manual 6-54 November 2006 

For the Final LT2ESWTR

Administration (OSHA) regulations, and state guidance and regulations for UV reactor 

operations. 

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 or sleeves 

• Potential exposure to mercury from broken lamps 

Threshold limit values (TLVs) for UV light apply to occupational exposure to UV 

incident on the skin or eyes. The recommended TLVs depend on the lamp wavelengths emitted 

and the UV intensity (mW/cm2). The PWS can determine the appropriate TLVs for their UV 

reactors, using TLVs for Chemical Substances and Physical Agents and Biological Exposure 

Indices (ACGIH 2006). 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 working in the UV reactor. To minimize the danger of exposure, 

warning signs also should be posted. 

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 and 

the energy to dissipate before maintenance is performed in areas where electric shock may be a 

risk. All safety and operational 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, tagout 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 

temperatures of these components should be checked before touching them. 

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, and 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 E discusses lamp 

breakage and cleanup procedures. 
UV Disinfection Guidance Manual 7-1 November 2006 

For the Final LT2ESWTR 

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FL. 
Appendix A 

Preparing and Assaying 

Challenge Microorganisms 
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Appendix A. Preparing and Assaying Challenge Microorganisms 

UV Disinfection Guidance Manual A-1 November 2006 

For the Final LT2ESWTR

Sections A.1 through A.4 describe procedures that can be used for preparing stock 

solutions of male-specific-2 bacteriophage (MS2 phage) and Bacillus 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. 

A.1 MS2 Phage Stock Preparation 

MS2 phage (American Type Culture Collection [ATCC] 15597-B1) can be propagated 

using a variety of host bacteria, including Escherichia coli C3000 (ATCC 15597); E. coli Famp 

(ATCC 700891); and others (Meng and Gerba 1996, Oppenheimer et al. 1993, NWRI (2003). 

The following propagation method was adapted from NWRI (2003). 

Procedure: 

1. Inoculate sterile tryptic soy broth (TSB) (Difco, Detroit, Michigan) with host bacteria 

transferred from a single colony grown on a nutrient agar plate. Incubate the culture 

with constant stirring at 35 to 37 degrees Centigrade (°C) for 18 to 24 hours. 

2. Transfer 0.5 milliliter (mL) of the host bacterial culture to 50 mL of fresh TSB and 

incubate at 35 to 37 °C for 4 to 6 hours with continuous shaking at 100 Hertz (Hz) to 

obtain a culture in its log growth phase (~3 × 108 cfu/mL, where cfu = colony 

forming unit). 

3. Dilute stock MS2 phage using Tri-buffered saline (pH 7.3) to a concentration of 

~100 pfu/mL (pfu = plaque forming unit). 

4. Add 1 mL of diluted MS2 phage 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 8,000 × G [G = 9.82 meter per second squared 

(m/s2

)] for 10 minutes at 4 °C to remove cellular debris. 

6. Filter the supernatant by passing it through a 0.45-micrometer (μm) low proteinbinding filter. 

7. Assay the concentration of MS2 phage in the stock solution as described in 

Section A.2. 

8. Collect and refrigerate the filtrate at 4 °C, and use within one month. 

Propagation should result in a highly concentrated stock solution of essentially monodispersed phage whose UV dose-response follows second-order kinetics with minimal tailing. 

Figure A.1 presents the UV dose-response of MS2 phage as reported in the literature. Over the 

range of reduction equivalent dose (RED) values demonstrated during validation testing, the 
Appendix A. Preparing and Assaying Challenge Microorganisms 

mean UV dose-response of the MS2 phage stock solution should lie within the 95-percent 

prediction interval of the mean response in Figure A.1. Over a UV dose range of 0 to 

120 millijoule per centimeter squared (mJ/cm2

), the prediction intervals of the data shown in 

Figure A.1 may be defined using the following equations: 

UpperBound I ××−= Dose UV ××+ Dose UV −4 2 −2 104.1log: 106.7 Equation A.1 

Lower Bound I ××−= Dose UV ××+ Dose UV −5 2 −2 106.9log: 105.4 Equation A.2 

Figure A.1. UV Dose-response of MS2 Phage 

0

1

2

3

4

5

6

7

8

0 20 40 60 80 100 120

UV Dose (mJ/cm2

)

Log Inactivation

Havelaar et al., 1990

Meng and Gerba, 1996

Nieuwstad and Havelaar, 1994

Oppenheimer et al., 1993

Sommer et al., 1998

Tree et al., 1997

Mean

Mean Response 90% Prediction Interval

A.2 MS2 Phage Assay 

The concentration of MS2 phage (ATCC 15597-B1) in water samples can be assayed 

using agar overlay technique with E. coli (ATCC 15597) as a host bacterium [(Adams (1959), 

Yahya et al. (1992), Oppenheimer et al. (1993), and Meng and Gerba (1996)]. Each test sample 

should be assayed in triplicate and the sample concentration calculated as the arithmetic average 

of the three measured values. The following procedure can be used. 

UV Disinfection Guidance Manual A-2 November 2006 

For the Final LT2ESWTR
Appendix A. Preparing and Assaying Challenge Microorganisms 

Procedure: 

1. Inoculate sterile TSB (Difco, Detroit, Michigan) 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 host bacterial 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 phage sample using a 0.001-molar (M) phosphatesaline buffer or TSB. 

4. Combine and gently stir 1 mL of host cell solution, 0.1 mL of diluted MS2 phage 

sample, and 2 to 3 mL of molten tryptic soy agar (TSA) (0.7 percent agar, 45 to 

48 °C) (Difco). 

5. Pour the mixture onto solidified TSA (1.5 percent agar) contained in petri dishes. 

The time between mixing the MS2 phage sample with the E. coli host and plating the 

top agar layer should not exceed 10 minutes. After plating, the agar should harden in 

less than 10 minutes. 

6. After the top agar layer hardens, cover and 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 5 millimeter (mm) in diameter in the lawn of host bacteria. 

8. Record the number of plaques per dish and the MS2 phage sample volume and 

dilution. If individual plaques cannot be distinguished because of confluent growth, 

record the plate counts as “TNTC” (too numerous to count). 

9. Calculate the MS2 phage concentration in the water samples: 

= ∑

i

F avgi

V

n

Concentration D , 10 Equation A.3 

where: 

FD = Dilution factor 

ni = Number of counts on each plate (cfu or pfu) 

Vi = Volume of diluted sample used with each plate (mL) 

UV Disinfection Guidance Manual A-3 November 2006 

For the Final LT2ESWTR
Appendix A. Preparing and Assaying Challenge Microorganisms 

Example A.1. A water sample containing MS2 phage 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 

and the average count from these three plate counts is the challenge microorganism 

corresponding to the applied UV dose. Plaque 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: 

mL/pfu1015.4 2

mL 1.0

3

pfu )474032(

10 

mL 1.0

3

pfu )652(

10

. 4

3 2

×=

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎝

⎛ ++

×+

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎝

⎛ ++

×

Conc =

A.3 Bacillus subtilis Spore Preparation 

B. subtilis spores (ATCC 6633) can be propagated using Schaeffer’s medium [Munakata 

and Rupert (1972), Sommer et al. (1995), and DVGW (2006)]. The following propagation 

method was adapted from DVGW (2006). 

Procedure: 

1. Prepare 1 liter (L) of Columbia agar (Oxoid CM 331) using 23.0 grams (g) special 

peptone (Oxoid L 72), 1.0 g starch, 5.0 g NaCl, and 10.0 g agar (Oxoid L 11) in 

phosphate-buffered (to pH 7) water. Autoclave for 15 minutes at 121 oC. 

2. Prepare 1 L of sporulation medium using 280 milligrams (mg) MgSO4·H2O, 1.11 g 

KCl, 3.1 mg FeSO4·7H2O, and 8.9 g nutrient broth (Oxoid CM 67) in phosphatebuffered (to pH 7) water. Autoclave for 15 minutes at 121 oC. 

3. Inoculate Columbia agar (Oxoid CM 331) plates with three smears of B. subtilis and 

incubate 24 hours at 37 oC. 

4. Inoculate 300 mL of sporulation medium with three colonies collected from the agar 

plates that were prepared in Step 3. 

5. Incubate the sporulation medium for 72 hours at 37 oC on a shaker operating at 2 Hz. 

6. Sonicate the resulting culture for 10 minutes at 50,000 Hz and 10 oC. 

7. Harvest the spores by centrifuging 80-mL aliquots at 5,000 × G and 10 oC for 

10 minutes. 

UV Disinfection Guidance Manual A-4 November 2006 

For the Final LT2ESWTR
Appendix A. Preparing and Assaying Challenge Microorganisms 

8. Wash the spores 3 times by re-suspending the pellet in 20 mL of distilled water and 

centrifuging at 5,000 × G for 10 minutes at 10 oC. 

9. Re-suspend the washed spores in 100 mL of 0.001-M phosphate-saline buffer. 

10. Inactivate the vegetative B. subtilis by heating at 80 oC for 10 minutes. 

11. Sonicate the resulting culture for 10 minutes at 50,000 Hz and 10 oC. 

12. Collect the resulting stock solution and assay the B. subtilis spore concentration as 

described in Section A.4. 

13. Refrigerate at 4 oC and use within one month (unless stability over longer periods of 

time can be substantiated). Sonicate for 10 minutes at 50,000 Hz and 10 oC before 

use. 

Propagation should result in a highly concentrated stock solution of mono-dispersed 

B. subtilis spores with a UV dose-response that follows the UV dose-response curves reported in 

the literature and presented in Figure A.2. Over the range of RED values demonstrated during 

validation testing, the mean UV dose-response of the B. subtilis stock solution should lie within 

the 90-percent prediction interval of the mean response provided in Figure A.2. Over a UV dose 

range of 0 to 70 mJ/cm2

, the prediction intervals of the data shown in Figure A.2 are defined 

using the following equations: 

UpperBound I ××−= Dose UV ××+ Dose UV ××− Dose UV −5 3 −3 2 −2 100.2log: 107.2 103.5 

Equation A.4 

LowerBound I ××= Dose UV ××+ Dose UV −4 2 −2 107.5log: 103.4 Equation A.5 

UV Disinfection Guidance Manual A-5 November 2006 

For the Final LT2ESWTR
Appendix A. Preparing and Assaying Challenge Microorganisms 

Figure A.2. UV Dose-Response of B. subtilis Spores 

-1

0

1

2

3

4

5

6

7

0 20 40 60 80 100 120 140

UV Dose (mJ/cm2

)

Log Inactivation

DVGW, 1997

Chang et al, 1985

Hoyer, 2002

Sommer and Cabaj, 1993 (curve 1)

Sommer and Cabaj, 1993 (curve 2)

Sommer et al, 1995

Sommer et al, 1996

Sommer et al, 1998

Mean

90% Prediction Interval of the Mean Response

A.4 Bacillus subtilis Spore Assay 

The concentration of B. subtilis spores (ATCC 6633) in water samples can be assayed 

using plate count agar. As with MS2 phage, each test sample should be assayed in triplicate and 

the sample concentration calculated as the arithmetic average of the three measured values. The 

following procedure was adopted from DVGW (2006). 

Procedure: 

1. Prepare 1 L of plate-count agar (Oxoid CM 325) using 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 the pH to 6.8 ± 0.2 and autoclave for 15 minutes at 121 o 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 0.45-μm membrane filter. 

4. Place filter on a petri dish containing hardened agar and cover plates. 

UV Disinfection Guidance Manual A-6 November 2006 

For the Final LT2ESWTR
Appendix A. Preparing and Assaying Challenge Microorganisms 

UV Disinfection Guidance Manual A-7 November 2006 

For the Final LT2ESWTR

5. Incubate plates 24 ± 2 hours at 37 ± 1 oC. 

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 individual colonies cannot be distinguished because of confluent growth, 

record the plate counts as TNTC. 

8. Calculate the B. subtilis spore concentration in the original samples in units of cfu/mL 

using Equation A.3. 
Appendix A. Preparing and Assaying Challenge Microorganisms 

UV Disinfection Guidance Manual A-8 November 2006 

For the Final LT2ESWTR

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Appendix B 

UV Reactor Testing Examples
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Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-1 November 2006 

For the Final LT2ESWTR 

This appendix presents two validation data analysis examples. Section B.1 presents an 

example for the UV Intensity Setpoint Approach using single-setpoint operations. Section B.2 

presents a more complex example for a UV reactor that uses the Calculated Dose Approach. 

These two examples bracket a wide range of complexities that UV reactor validation testing can 

encompass. All information and data are hypothetical but are representative of real validation 

data in terms of selection of test conditions and variability in measured values. 

B.1 Example 1 – Validation for the UV Intensity Setpoint Approach (a Single 

Setpoint and a Single Disinfection Goal) 

System X plans to add UV disinfection to their treatment process to earn 2.5-log 

Cryptosporidium inactivation credit. Based on the LT2ESWTR dose requirements as 

summarized in Table 1.4 of this manual, System X needs to deliver a minimum required dose of 

8.5 mJ/cm2

 to receive this level of inactivation credit. The hypothetical proposed installation has 

the following design specifications: 

Design flow rate 400 gpm 

Minimum UVT 90 % 

Lamp aging factor 80 % 

Fouling factor 85 % 

Fouling/aging factor 68 % (80 % × 85 %) 

Disinfection goal 2.5-log Cryptosporidium inactivation credit 

The water system’s engineer selected a UV reactor (illustrated in Figure B.1) with the 

following characteristics: 

UV Reactor 

- 2 banks of lamps 

- 4 300-W LPHO lamps per bank 

- Rated for flow rates of 50 – 500 gpm 

- 1 UV sensor/bank positioned equidistant from Lamps 2 & 3

UV Dose-Monitoring Approach UV Intensity Setpoint Approach with one alarm setpoint 
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-2 November 2006 

Figure B.1 Schematic of Hypothetical UV Reactor for Example 1 

B.1.1 Validation Test Plan 

The validation test plan was developed using Checklist 5.2 in Chapter 5. Key elements 

defined in the validation test plan include: 

• The UV manufacturer provided three reference UV sensors for calibrating the duty 

sensors during validation. These reference sensors had been calibrated by an 

independent, qualified sensor testing laboratory before validation and had a 

documented measurement uncertainty of 10 percent (Section 5.5.4) 

• Because the number of UV sensors was less than the number of lamps, the highestoutput lamps were identified prior to testing and were positioned closest to the 

sensors at lamp positions 2 and 3 (Section 5.4.7). New lamps were used during 

validation testing (after a 100 hour burn-in period). 

• The validation testing was conducted over a one-day period. The UV dose-response 

of the challenge microorganism (measured via a laboratory collimated beam test) was 

evaluated with 1-L influent water samples collected at high and low UVT values 

(Section C.1). 

• All recommended testing protocols as listed in Section 5.7 and Appendix C were 

followed. 

The UV manufacturer had already identified a target setpoint (11.7 mW/cm2

) using 

numerical modeling. System X confirmed with the manufacturer that this setpoint is low enough 

to account for their combined conditions of minimum UVT and maximum lamp fouling and 

aging. The following two UVT–lamp power operating conditions were tested: 

1. The UVT was lowered to produce the target UV sensor setpoint (in this case, the 

resultant UVT was 89.9%), while the lamp power was kept at 100%. 

For the Final LT2ESWTR
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-3 November 2006 

For the Final LT2ESWTR

2. The UVT was raised back to its maximum value (no UV-absorbing chemical added), 

and the lamp power was reduced to produce the same UV sensor value (in this case, 

the resultant lamp power was 66%). 

B.1.2 Test Data 

The data collected during validation testing are presented in Tables B.1 through B.5 and 

are described below: 

• Table B.1 presents the UV dose-response data measured on the influent water 

collected during field validation testing for the laboratory collimated beam test. 

• Tables B.2 through B.4 present the data from full-scale reactor testing 

- Table B.2 presents the flow rate, UVT, lamp power, and UV sensor readings 

measured for each test condition. 

- Table B.3 presents the measured challenge microorganism concentrations for the 

influent and effluent samples collected (in triplicate) from the UV reactor for each 

test condition. 

- Table B.4 presents the UV output of the eight lamps used during validation testing 

measured at the same lamp location (Lamp #2 in Row #1) adjacent to the same 

UV sensor (#1), which was used to identify the highest output lamps. 

• Table B.5 presents data comparing the three reference UV sensor measurements to 

the duty UV sensors used during validation. 

Sections B.1.3 to B.1.8 show how the data will be used to determine whether QA/QC 

criteria are met, to calculate the necessary correction factors, and to determine the validated 

operating conditions for the target log inactivation. 

Table B.1 Challenge Microorganism UV Dose-response Measured 

Using a Collimated Beam Apparatus 

90% UVT 97% UVT 

Replicate #1 Replicate #2 Replicate #1 Replicate #2 UV Dose 

(mJ/cm2) N 

(pfu/mL) 

Log 

N 

N 

(pfu /mL) 

Log 

N 

UV Dose 

(mJ/cm2

) N 

(v/mL) 

Log 

N 

N 

(pfu /mL) 

Log 

N 

0 882329 5.95 944980 5.98 0 1148154 6.06 1300460 6.11 

10 180120 5.26 198394 5.30 10 316328 5.50 257749 5.41 

20 64217 4.81 69438 4.84 20 113644 5.06 74396 4.87 

30 20622 4.31 20100 4.30 30 34679 4.54 25189 4.40 

40 7257 3.86 8145 3.91 40 12624 4.10 9226 3.97 

60 1274 3.11 1399 3.15 60 1980 3.30 1722 3.24 

80 188 2.27 261 2.42 80 387 2.59 211 2.32 

100 80 1.90 90 1.95 100 80 1.90 100 2.00 
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-4 November 2006 

For the Final LT2ESWTR

Table B.2 Flow Rate, UVT, Lamp Power, and UV Sensor Data 

Measured during Validation Testing 

Test ID Banks On Flow Rate 

(gpm) 

UVT

(%) 

Relative Lamp Output

(%) 

Sduty, #1

(mW/cm2) 

Sduty, #2

(mW/cm2) 

1 1, 2 394 89.9 100% 11.7 11.7 

2 1, 2 403 97.0 66% 11.6 11.7 

Table B.3 Measured Influent and Effluent Challenge Microorganism 

Concentrations 

Influent Challenge 

Microorganism 

Log Concentration 

Effluent Challenge 

Microorganism 

Log Concentration 

Replicate # Replicate # 

Test ID 

1 2 3 1 2 3 

1 5.94 6.00 5.84 4.57 4.54 4.56 

2 6.01 5.99 6.04 4.10 4.09 4.06 

Table B.4 Sensor #1 Measurements with Lamp #2 Operated at 100-percent Ballast 

Power 

Lamp ID Sduty #1

(mW/cm2

) Lamp ID Sduty #1

(mW/cm2

)

1 13.6 5 13.9 

2 14.6 6 13.3 

3 14.2 7 14.5 

4 13.4 8 14.3 

Table B.5 Reference UV Sensor Checks

Before/After 

Validation 

Testing 

UVT 

(%) 

Relative 

Lamp Power

(%) 

Sensor

ID 

Sduty

(mW/cm2)

Sref, #1

(mW/cm2) 

Sref, #2

(mW/cm2) 

Sref, #3

(mW/cm2) 

Before 97 100 1 11.3 11.7 12.1 11.4 

Before 97 68 1 5.1 5.5 5.7 5.3 

Before 90 100 2 3.7 4.0 4.1 3.8 

Before 90 68 2 2.0 1.9 1.9 1.8 

After 97 100 1 11.6 11.8 12.2 11.4 

After 97 68 1 5.1 5.4 5.6 5.3 

After 90 100 2 3.9 4.0 4.1 3.9 

After 90 68 2 1.9 1.8 2.0 1.8 

B.1.3 Develop the UV Dose-response Curve from the Collimated Beam Data 

Figures B.1(a) and (b) present the UV dose-response data from Table B.1 at UVT values 

of 90 and 97 percent, respectively. The data have been fitted to quadratic equations that show log 

N as a function of UV dose. The fits were used to identify log No values of 5.91 and 6.05 (i.e., 
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-5 November 2006 

For the Final LT2ESWTR

 

log No where the curves intersect the y-axes) from Figures B.1(a) and (b), respectively. Using 

these values, Table B.6 presents the UV dose-response data defined as UV dose versus log I, 

where log I = log(No/N). 

The UV dose-response data described in Table B.6 were analyzed to determine if the two 

datasets could be combined using the method referred to in Section C.5 (Draper and Smith 

1998).1 A statistical analysis of the collimated beam data is recommended to determine which 

terms are significant, p-value ≤ 0.05, using a standard regression tool. The process is iterative, 

and each time the regression tool is used, one term is dropped until all of the coefficients are 

deemed significant (p-values ≤ 0.05). In this example, three iterations were needed. 

 The regression analysis showed that the two measured UV dose-response curves were 

statistically similar (p < 0.05) and could be combined. Figure B.2 presents the plot of UV dose as 

a function of log inactivation for the combined dataset and the resultant UV dose-response 

equation. 

1 The datasets should be combined whenever possible to develop one UV dose-response equation for calculating all 

RED values. The inability to combine datasets indicates a problem may have occurred with either the calculation 

or the test. Details are provided in Section C.5.
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-6 November 2006 

Figure B.1 Log N Versus UV Dose Using the Data in Table B.1 

at (a) 90 Percent UVT and (b) 97 Percent UVT 

Log N = 0.0002(UV Dose)2 - 0.059(UV Dose) + 5.92

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

0 20 40 60 80 100

UV Dose (mJ/cm2

)

Log N

90% UVT

No = 5.92

(a) 

Log N = 0.0002(UV Dose)2 - 0.057(UV Dose) + 6.05

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

0 20 40 60 80 100

UV Dose (mJ/cm2

)

Log N

97% UVT

No = 6.05

(b) 

For the Final LT2ESWTR
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-7 November 2006 

Table B.6 Challenge Microorganism UV Dose-response Defined as 

UV Dose Versus [Log (No/N)] (i.e., Log I) 

90% UVT 97% UVT 

Replicate Replicate 

#1 #2 #1 #2 UV Dose 

(mJ/cm2

) 5.92 - Log N 

UV Dose 

(mJ/cm2) 6.05 - Log N 

0 -0.03 -0.06 0 -0.01 -0.06 

10 0.66 0.62 10 0.55 0.64 

20 1.11 1.08 20 0.99 1.18 

30 1.61 1.62 30 1.51 1.65 

40 2.06 2.01 40 1.95 2.08 

60 2.81 2.77 60 2.75 2.81 

80 3.65 3.5 80 3.46 3.73 

100 4.02 3.97 100 4.15 4.05 

Figure B.2 Log I Versus UV Dose Using the Data in Table B.6 

RED (mJ/cm2) = 2.18(log I)2 + 15.30(log I)

0

10

20

30

40

50

60

70

80

90

100

0.00 1.00 2.00 3.00 4.00 5.00

Log I

UV Dose (mJ/cm2

)

B.1.4 Verify That QA/QC Criteria Are Met 

Checklist 5.4 was used to ensure that recommended QA/QC criteria were met. 

Calculations of key uncertainties are provided in the next two subsections. 

. 

B.1.4.1 Collimated Beam Data Uncertainty 

The uncertainty in the UV dose calculation using the collimated beam data is calculated 

according to Equation C.6, shown below as Equation B.1: 

For the Final LT2ESWTR
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-8 November 2006 

= × %100

CB

DR DoseUV

SD tU Equation B.1 

where 

UDR = Uncertainty of the UV dose-response fit at a 95-percent confidence level 

UV DoseCB = UV dose calculated from the UV dose-response curve in Figure B.2 

SD = Standard deviation of the difference between the calculated UV doseresponse and the measured values from Table B.1 

t = t-statistic at a 95-percent confidence level for a sample size equal to the 

number of test condition replicates used to define the dose-response 

In this case, SD = 2.2 at 1-log inactivation and t = 2.04 for 32 test condition replicates as 

shown in Table B.6. Equation B.1 can then be used to determine UDR at various log inactivation 

values from Table B.1. The graph below shows the relationship between log inactivation and 

UDR. The value of UDR should not exceed 30 percent at the UV dose corresponding to 1-log 

inactivation of the challenge organism. In this case, UDR = 25 percent at 1.0-log inactivation. 

This value is less than the recommended limit of 30 percent. 

For the Final LT2ESWTR

0%

40%

80%

120%

160%

200%

01234

Log I

UDR

B.1.4.2 UV Sensor Uncertainty 

Guidance in Section 5.5.4 recommends that the manufacturer’s reported sensor 

uncertainty be confirmed as being less than 10 percent. Three reference sensors were used to 

confirm duty sensor measurements. Data analyses are shown in the table on the next page (using 

the data in Table B.5). The two duty UV sensors were within 10 percent of the average readings 

from three reference sensors. 
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-9 November 2006 

Before/ 

After 

Validatio

n Testing 

UVT 

(%) 

Lamp 

Power 

(kW) 

Sensor 

ID 

Sduty 

(W/m2)

SRef,#1

(W/m2)

SRef,#2

(W/m2)

SRef,#3 

(W/m2) 

SRef,avg

(W/m2) 

1

avgRef,

duty − S

S

Before 97 100 1 11.3 11.7 12.1 11.4 11.7 4% 

Before 97 68 1 5.1 5.5 5.7 5.3 5.5 7% 

Before 90 100 2 3.7 4 4.1 3.8 4.0 7% 

Before 90 68 2 2 1.9 1.9 1.8 1.9 7% 

After 97 100 1 11.6 11.8 12.2 11.4 11.8 2% 

After 97 68 1 5.1 5.4 5.6 5.3 5.4 6% 

After 90 100 2 3.9 4 4.1 3.9 4.0 3% 

After 90 68 2 1.9 1.8 2.0 1.8 1.9 2% 

Source of sensor data: Table B.5 

B.1.5 Calculate Log Inactivation and RED 

 Table B.7 shows the measured log inactivation through the UV reactor and the 

associated RED values for each validation test condition. The log inactivation values were 

determined from the field inactivation data in Table B.3. The RED values are determined by 

inputting the log inactivation into the UV dose-response equation. For example, for a log 

inactivation of 1.37 (Test No. 1, Replicate 1), the RED is equal to [2.18 x (1.37)2] + [15.30 x 

1.37], or 25.1. Note that the average and standard deviations for each test condition are also 

calculated for later use in computing the Validation Factor (VF). 

Table B.7 Measured Log I and RED Values for Each Test Condition in Table B.2 

Log I RED (mJ/cm2) 

Test ID # UVT Replicate # Replicate # 

1 2 3 1 2 3 Avg. SDRED

1 89.9 1.37 1.46 1.28 25.1 27.0 23.2 25.1 1.9 

2 97.0 1.91 1.9 1.98 37.2 36.9 38.8 37.6 1.0 

B.1.6 Determine the Validation Factor 

The VF is defined according to Equation 5.132

, shown below as Equation B.2: 

⎟

⎠

⎞ ⎜

⎝

⎛ +×= 100 1 Val

RED

U BVF Equation B.2

B.1.6.1 Calculate the RED Bias 

 2 If the UV reactor was equipped with MP lamps instead of LPHO lamps, the potential for polychromatic bias 

would need to be evaluated using the guidelines in Section 5.9. 

For the Final LT2ESWTR
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-10 November 2006 

 Per guidance provided in Section 5.9.1, The UV sensitivity (RED / Log I) for each test 

replicate was calculated for the test with the lowest UVT value (in this case, Test 1). The RED 

bias for 2.5-log Cryptosporidium inactivation credit at the minimum UVT of 90 percent was 

determined to be 1.79 using Table G.4. Data for this analysis are summarized below. 

Test # RED 

(mJ/cm2

) Log I UVT1 Sensitivity 

(mJ/cm2

 per log I) BB

RED

1.1 25.1 1.37 89.9 18.3 

1.2 27.0 1.46 89.9 18.5 

1.3 23.2 1.28 89.9 18.1 

1.79 

1

 Note: rounding off to match the number of significant figures in Table G.4, 

to determine the value of BRED the UVT value measured for Test #1 

becomes 90 percent. 

B.1.6.2 Calculate the Uncertainty of Validation 

The decision tree in Figure 5.4 was used to identify the correct equation for UVAL. As 

shown in Section B.1.4.1, UDR is less than or equal to 30 percent. As noted in Section B.1.4.2, 

US is less than or equal to 10 percent. Therefore, the equation for UVAL is as follows: 

UVal = USP Equation B.3 

 USP is defined by Equation 5.14 

× %100

× = RED

SDt

U RED

SP Equation B.4 

where 

 t = t-statistic for the number of replicates 

 SDRED = the standard deviation for the RED calculations (Table B.7) 

 RED = the RED at the specific test condition used for the SDRED

The value for t is 3.18 for 3 test replicates. The highest SDRED and associatedRED should be 

used in this calculation. Using data from Table B.7, data from test condition #1 should be used as 

follows: 

%1.24%100

1.25

9.118.3

=×

×

USP =

Using Equation B.3, UVAL = 24.1% 

For the Final LT2ESWTR
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-11 November 2006 

B.1.6.3 Calculate the Validation Factor 

The value of VF can now be calculated using Equation B.2: 

VF ( ) +×= = 22.2100/1.24179.1

B.1.7 Calculate the Validated Dose 

In the step, the minimum RED (in this case, the average RED from test condition #1, 

shown in Table B.7) is divided by the VF to calculate the validated dose using Equation 5.16: 

VF

RED DVal = Equation B.5 

2 /3.11

22.2

1.25 DVal == cmmJ

B.1.8 Assign Log Inactivation Credit Based for the Validated Dose 

The validated dose must be greater than or equal to the required UV dose (Dreq) to 

achieve a given level of pathogen inactivation credit: 

Val ≥ DD req Equation B.6 

In this case, 11.3 mJ/cm2

 is greater than the required dose of 8.5 mJ/cm2for 2.5-log 

inactivation of Cryptosporidium. The UV reactor can receive 2.5-log inactivation credit for an 

installation (with adequate inlet/outlet hydraulics, see Section 5.4.5) that operates under the 

following criteria (Table B.8): 

Table B.8 Validated Dose and Operating Conditions for 

2.5-log Cryptosporidium Inactivation Credit Using the 

Hypothetical UV Reactor Tested in Example 1 

UV Sensor Setpoint Lamp Status Flow Rate Range DVal

11.7 mW/cm2 All lamps should be turned on during 

reactor operations Q ≤ 394 gpm ≥ 11.3 mJ/cm2

B.2 Example 2 – Validation for the Calculated Dose Approach 

System Y plans to add UV disinfection to their treatment plant to earn 2.0-log 

Cryptosporidium inactivation credit. Based on the LT2ESWTR UV dose requirements as 

summarized in Table 1.4 of this manual, System Y needs a minimum germicidal dose of 5.8 

For the Final LT2ESWTR
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-12 November 2006 

For the Final LT2ESWTR

mJ/cm2 to receive this level of inactivation credit. The hypothetical proposed installation has the 

following design specifications: 

Design flow range 3 – 10 mgd 

Design UVT range 87 – 93 % 

Lamp aging factor 80 % 

Fouling factor 85 % 

Fouling/Aging factor 68 % (80 % × 85 %) 

Disinfection goal 2.0-log Cryptosporidium inactivation credit 

The water system’s engineer selected a UV reactor with the following characteristics: 

UV Reactor 

- 1 bank of lamps 

- 6 8-kW MP lamps per bank 

- Ballast power settings range from 40 – 100 %

- Rated for flow rates of 2.5 – 10 mgd 

- 1 germicidal UV sensor per lamp 

UV Dose-Monitoring Approach Calculated Dose Approach with dose pacing 

B.2.1 Validation Test Plan 

The test plan was developed using Checklist 5.2 in Chapter 5 to identify a range of target 

RED values at different flow rate-lamp output-UVT combinations for 1.0- to 3.0-log 

Cryptosporidium inactivation credit (depending on water quality and operating conditions). The 

UV manufacturer used modeled predictions of UV reactor performance to develop the desired 

validation test conditions. The UV manufacturer selected test conditions that target RED values 

ranging from approximately 4 – 43 mJ/cm2

 at UVT values of 85, 90, and 95 percent. Lamp 

power was to be adjusted during testing of the UV reactor to give RED values within the target 

range. Test flow rates of 2.5 – 10 mgd were selected in order to test the full design flow range of 

the UV reactor. This information is summarized in Table B.9 below. 
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-13 November 2006 

For the Final LT2ESWTR

Table B.9 Validation Test Conditions 

Test ID UVT 

(%) 

Flow Rate

(mgd) 

Relative Lamp Output

(%) 

Predicted 

RED3

(mJ/cm2) 

1 95 10 100 14.1 

2 95 5 100 24.6 

3 95 2.5 100 42.8 

4 95 10 70 11.8 

5 95 5 70 20.6 

6 95 2.5 70 35.8 

7 95 10 40 8.9 

8 95 5 40 15.6 

9 95 2.5 40 27.1 

10 90 10 100 7.9 

11 90 5 100 13.8 

12 90 2.5 100 24.1 

13 90 10 70 6.6 

14 90 5 70 11.6 

15 90 2.5 70 20.1 

16 90 10 40 5.0 

17 90 5 40 8.7 

18 90 2.5 40 15.2 

19 85 10 100 4.5 

20 85 5 100 7.8 

21 85 2.5 100 13.5 

22 85 10 70 3.7 

23 85 5 70 6.5 

24 85 2.5 70 11.3 

25 85 10 40 2.8 

26 85 5 40 4.9 

27 85 2.5 40 8.6 

Other key test elements in the validation test plan include: 

• The UV manufacturer provided three reference UV sensors for calibrating the duty 

UV sensors during validation. These reference UV sensors had been calibrated 

previously by an independent, qualified sensor testing laboratory and had a 

documented measurement uncertainty of 10 percent (Section 5.5.4). 

• New lamps were used during validation testing (after a 100 hour burn-in period). 

• The validation testing was conducted over a two-day period (Test Conditions 1 – 18 

on Day 1 and Test Conditions 19 – 27 on Day 2). The UV dose-response of the 

challenge microorganism (measured via a collimated beam test) was evaluated with 

1-L influent water samples at 95 percent UVT, collected on Day 1 of testing and at 85 

percent UVT, collected on Day 2 of testing (Section C.1). 

 

3 From the numerical model predictions developed by the manufacturer. 
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-14 November 2006 

For the Final LT2ESWTR

• All recommended testing protocols as listed in Section 5.7 and Appendix C were 

followed. 

B.2.2 Test Data 

The data collected during validation testing are presented in Tables B.10 through B.13 

and are described below: 

• Table B.10 presents the UV dose-response data measured on the influent water 

collected during field validation testing for the laboratory collimated beam test. 

• Tables B.11 presents the flow rate, UVT, lamp power, and UV sensor readings 

measured for Test Conditions 1 – 9 (95 percent UVT). Testing was also conducted at 

90 and 85 percent UVT. For simplicity, only the results of testing at 95 percent UVT 

are reported here. 

• Table B.12 presents measured challenge microorganism concentrations for the 

influent and effluent samples collected (in triplicate) from the reactor for several test 

conditions. A total of 27 tests (one for each condition described in Table B.9) were 

run. For simplicity, only the first tests at UVT measurements of 95, 90, and 85 

percent, respectively, are shown. 

• Table B.13 presents data comparing the three reference UV sensor measurements to 

UV Duty Sensor #1 used during validation. Comparisons were made to all six duty 

sensors, but for simplicity only the results for Sensor #1 are reported here. 

Sections B.2.3 to B.2.8 show how the data will be manipulated to determine whether 

QA/QC criteria are met, to calculate the necessary correction factors, and to determine the 

validated operating conditions for the target log inactivation. 
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-15 November 2006 

For the Final LT2ESWTR

Table B.10 Challenge Microorganism UV Dose-response Measured 

Using a Collimated Beam Apparatus 

95% UVT 85% UVT 

Replicate #1 Replicate #2 Replicate #1 Replicate #2 

UV Dose 

(mJ/cm2

) 

N 

(PFU/mL) Log N N 

(PFU/mL) Log N UV Dose 

(mJ/cm2

) 

N 

(PFU/mL) Log N N 

(PFU/mL) Log N 

0 65560 4.82 67000 4.83 0 70440 4.85 70000 4.85 

10 13270 4.12 15000 4.18 10 14400 4.16 16120 4.21 

20 2790 3.45 2400 3.38 20 2590 3.41 2560 3.41 

29 640 2.81 591 2.77 30 693 2.84 529 2.72 

39 159 2.20 153 2.18 40 173 2.24 191 2.28 

59 23 1.36 19 1.28 60 28 1.45 20 1.30 

Table B.11 Flow Rate, UVT, Lamp Power, and UV Sensor Data 

Measured during Validation Testing (for UVT = 95% only) 

Test ID Banks On Flow Rate

(mgd) 

UVT

(%) 

Lamp Power

(kW) 

Lowest Measured 

Sduty

(W/m2) 

1 1 10 95 8.0 303.1 

2 1 5 95 8.0 307.9 

3 1 2.5 95 8.0 297.9 

4 1 10 95 5.6 183.1 

5 1 5 95 5.6 180.2 

6 1 2.5 95 5.6 190.3 

7 1 10 95 3.2 91.8 

8 1 5 95 3.2 93.5 

9 1 2.5 95 3.2 89.4 

Table B.12 Measured Influent and Effluent Challenge Microorganism Conc. for 

Three Test Conditions (Three UVTs at 10 mgd and 100% Lamp Power) 

Influent Challenge 

Microorganism 

Log Concentration 

Effluent Challenge 

Microorganism 

Log Concentration 

Replicate Replicate 

Test ID 

#1 #2 #3 #1 #2 #3 

1 4.92 4.8 4.87 3.79 3.69 3.70 

10 4.88 4.89 4.83 3.94 4.01 3.94 

19 4.93 4.90 4.91 4.24 4.28 4.29 
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-16 November 2006 

For the Final LT2ESWTR

Table B.13 Reference UV Sensor Checks for Duty Sensor #1

Before/After 

Validation 

Testing 

UVT 

(%) 

Lamp 

Power 

(kW) 

Sensor

ID 

Sduty #1

(W/m2) 

Sref, #1

(W/m2) 

Sref, #2

(W/m2) 

Sref, #3

(W/m2

) 

Before 95 8 1 304.5 339.5 330.7 339.9 

Before 95 3.2 1 80.2 90.9 86.2 82.7 

Before 85 8 1 100.9 96.1 91.5 91.0 

Before 85 3.2 1 23.2 20.9 21.3 20.9 

After 95 8 1 320.2 301.1 315.0 330.4 

After 95 3.2 1 69.6 76.5 76.3 78.3 

After 85 8 1 99.4 99.4 93.2 91.3 

After 85 3.2 1 19.3 20.4 20.4 20.0 

B.2.3 Develop the UV Dose-response Curve from the Collimated Beam Data 

Figures B.3(a) and (b) present the UV dose-response data from Table B.10 at UVT values 

of 85 and 95 percent, respectively. The data have been fitted to quadratic equations that show log 

N as a function of UV dose. The fits were used to identify log No values for the UV doseresponse curves measured at 85 and 95 percent UVT (4.89 and 4.86, respectively). Table B.14 

presents the UV dose-response data defined as UV dose versus log I (log [No/N]). 

The UV dose-response data described in Table B.14 were analyzed to determine if the 

two datasets could be combined using the method referred to in Section C.5 (Draper and Smith 

1998).4

 A statistical analysis of the collimated beam data is recommended to determine which 

terms are significant, p-value ≤ 0.05, using a standard regression tool. The process is iterative, 

and each time the regression tool is used, one term is dropped until all coefficients are deemed 

significant, p-value ≤ 0.05. In this example, three iterations were required. 

The multiple regression analysis showed that the two measured UV dose-response curves 

were statistically similar and could be combined [i.e., they could each be expressed with only 

two variables, as A log I + B(log I)2

]

5

. Figure B.4 presents the plot of UV dose as a function of 

log inactivation for the combined dataset and the resultant UV dose-response equation. 

 

4 The datasets should be combined whenever possible to develop one dose-response equation for calculating all 

RED values. The inability to combine datasets indicates a problem may have occurred with either the calculation 

or the test. Details are provided in Section C.5 

5 If the regression analysis had shown that the UV dose-response curves could not be combined, separate curves 

would have been used to calculate RED values for data collected on each day of testing (i.e., the curve for 95% 

UVT would be used to calculate RED for full-scale reactor testing data collected on Day 1, and the curve for 85% 

UVT would be used to calculate RED for full-scale reactor testing data collected on Day 2) 
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-17 November 2006 

Figure B.3 Log N Versus UV Dose Using the Data in Table B.10 

at (a) 85 Percent UVT and (b) 95 Percent UVT 

Log N = 0.0003(UV Dose)2 - 0.082(UV Dose) + 4.89

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0 10 20 30 40 50 60 7

UV Dose (mJ/cm2

)

Log N

85% UVT

No = 4.89

0

(a) 

Log N = 0.0002(UV Dose)2 - 0.079(UV Dose) + 4.86

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0 10 20 30 40 50 60 7

UV Dose (mJ/cm2

)

Log N

95% UVT

No = 4.86

0

(b) 

For the Final LT2ESWTR
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-18 November 2006 

Table B.14 Challenge Microorganism UV Dose-Response Defined as 

UV Dose Versus Log(N/No) (i.e., Log I) 

85% UVT 95% UVT 

Replicate Replicate 

#1 #2 #1 #2 UV Dose 

(mJ/cm2

) 4.89 – Log N 

UV Dose 

(mJ/cm2) 4.86- Log N 

0 0.04 0.04 0 0.04 0.03 

10 0.73 0.68 10 0.74 0.68 

20 1.48 1.48 20 1.41 1.48 

29 2.05 2.17 30 2.05 2.09 

39 2.65 2.61 40 2.66 2.68 

59 3.44 3.59 60 3.50 3.58 

Figure B.4 Log I Versus UV Dose Using the Data in Table B.14 

RED = 1.58(log I)2 + 11.10(log I)

0

10

20

30

40

50

60

70

0.00 1.00 2.00 3.00 4.00

Log I

UV Dose (mJ/cm2

)

B.2.4 Verify That QA/QC Criteria Are Met 

Checklist 5.4 was used to ensure that recommended QA/QC criteria were met. 

Calculations of key uncertainties are provided in the next two subsections. 

For the Final LT2ESWTR
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-19 November 2006 

B.2.4.1 Collimated Beam Data Uncertainty 

The uncertainty in the UV dose calculation using the collimated beam data is calculated 

using Equation C.6, shown below as Equation B.7: 

= × %100

CB

DR DoseUV

SD tU Equation B.7 

where 

UDR = Uncertainty of the UV dose-response fit at a 95-percent confidence level 

UV DoseCB = UV dose calculated from the UV dose-response curve in Figure B.4 

SD = Standard deviation of the difference between the calculated UV doseresponse and the measured value from Table B.10 

t = t-statistic at a 95-percent confidence level for a sample size equal to the 

number of test conditions replicates used to define the dose-response 

In this case, SD = 1.2 at 1.0-log inactivation and t = 2.06 for 24 test conditions from 

Table B.14. Equation B.7 can then be used to determine UDR at various log inactivation values 

from Table B.10. The graph below shows the relationship between log inactivation and UDR. As 

shown in the figure below, UDR = 19percent at 1.0-log inactivation. This value is less than the 

recommended limit of 30 percent. 

0%

10%

20%

30%

40%

01234

Log I

UDR

For the Final LT2ESWTR
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-20 November 2006 

B.2.4.2 UV Sensor Uncertainty 

Guidance in Section 5.5.4 recommends that the manufacturer’s reported sensor 

uncertainty be confirmed as being less than 10 percent. Three reference sensors were used to 

confirm duty sensor measurements. Data analyses are shown below. The two duty UV sensors 

were within 10 percent of the average readings from three reference sensors. 

Before/ 

After 

Validation 

Testing 

UVT 

(%) 

Lamp 

Power 

(kW) 

Sduty #1

(W/m2) 

SRef,#1

(W/m2)

SRef,#2

(W/m2)

SRef,#3

(W/m2)

SRef,avg

(W/m2)

1

avgRef,

duty − S

S

Before 95 8.0 304.5 339.5 330.7 339.9 336.7 10% 

Before 95 3.2 80.2 90.9 86.2 82.7 86.6 7% 

Before 85 8.0 100.9 96.1 91.5 91.0 92.9 9% 

Before 85 3.2 23.2 20.9 21.3 20.9 21.0 10% 

After 95 8.0 320.2 301.1 315.0 330.4 315.5 1% 

After 95 3.2 69.6 76.5 76.3 78.3 77.0 10% 

After 85 8.0 99.4 99.4 93.2 91.3 94.6 5% 

After 85 3.2 19.3 20.4 20.4 20.0 20.3 5% 

Source of UV sensor data: Table B.13 

B.2.5 Calculate Log Inactivation and RED 

Table B.15 shows the measured log inactivation through the UV reactor and the 

associated RED values (determined from the UV dose-response equation) for each validation test 

condition. The log inactivation values were determined from the field inactivation data 

(excerpted in Table B.12). 

Table B.15 Measured Log I and RED Values for Each Test Condition in Table B.11 

Log I RED (mJ/cm2) 

Test ID UVT Replicate # Replicate # 

1 2 3 1 2 3 

1 95.1 1.13 1.11 1.17 15.7 15.4 16.3 

2 94.8 1.36 1.35 1.34 19.2 19.0 19.0 

3 94.8 1.66 1.63 1.67 23.9 23.5 24.1 

4 95.4 1.03 1.09 1.12 14.2 15.2 15.6 

5 94.6 1.28 1.27 1.35 18.0 17.8 19.1 

6 94.8 1.55 1.56 1.59 22.2 22.4 22.9 

7 94.7 0.91 0.92 0.90 12.5 12.6 12.4 

8 94.5 1.16 1.17 1.20 16.2 16.3 16.8 

9 95.5 1.39 1.39 1.44 19.7 19.7 20.4 

10 90.0 0.94 0.88 0.89 12.8 12.0 12.1 

11 89.9 1.13 1.11 1.14 15.7 15.3 15.9 

12 90.1 1.35 1.42 1.39 19.1 20.1 19.6 

For the Final LT2ESWTR
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-21 November 2006 

Table B.15 Measured Log I and RED Values for Each Test Condition in Table B.11 

(cont.) 

Log I RED (mJ/cm2) 

Test ID UVT Replicate # Replicate # 

1 2 3 1 2 3 

13 90.1 0.85 0.81 0.86 10.6 10.0 10.7 

14 90.1 1.05 1.03 1.07 13.4 13.1 13.7 

15 89.8 1.30 1.29 1.34 17.1 17.0 17.7 

16 89.6 0.67 0.73 0.74 8.2 8.9 9.0 

17 89.6 0.91 0.94 0.95 11.5 11.8 12.0 

18 90.0 1.21 1.17 1.22 15.7 15.2 15.9 

19 84.6 0.68 0.62 0.62 8.3 7.5 7.5 

20 84.7 0.87 0.91 0.87 10.9 11.4 10.9 

21 84.6 1.17 1.17 1.18 15.1 15.1 15.3 

22 84.7 0.56 0.55 0.54 6.7 6.6 6.4 

23 85.4 0.84 0.76 0.77 10.4 9.4 9.5 

24 85.3 1.07 1.03 1.07 13.7 13.1 13.7 

25 85.1 0.42 0.47 0.44 4.9 5.6 5.1 

26 85.4 0.64 0.67 0.67 7.8 8.1 8.1 

27 85.5 0.93 0.94 0.95 11.7 11.8 11.9 

B.2.6 Develop an Equation to Calculate RED as a Function of the Control 

Variables 

To define an equation to calculate RED as a function of the operating conditions, the 

validation data were fitted for a 1-bank configuration using Equations 5.8 and 5.10, shown below 

as Equations B.8 and B.9 (there is only one bank of lamps, so it is not included as a variable 

here): 

c d

o

a b

QS RED A S ⎟

⎠

⎞ ⎜

⎝

⎛ ⎟ × ⎠

⎞ ⎜

⎝

⎛ 25410 ××= 1 Equation B.8 

Or in linear form, 

() ( ) ⎟

⎠

⎞ ⎜

⎝

⎛ ⎟ ×+⎠

⎞ ⎜

⎝

⎛ ×+= ×+ Q d S

S cAbaRED o

1 log 254log log log Equation B.9 

where 

 RED = The RED calculated with the UV dose-monitoring equation, also 

referred to as the “calculated dose” in this guidance manual 

 A254 = UV absorbance at 254 nm 

 S = Measured UV sensor value 

 So = UV intensity measured at 100 percent lamp power. 

 Q = Flow rate 

a, b, c, d = Model coefficients obtained by fitting the equations to the data 

For the Final LT2ESWTR
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-22 November 2006 

Remember that the validation goal pursued in Example 2 is somewhat different from that 

in Example 1. In Example 1, the simplest possible method (Single Intensity Setpoint Approach) 

was desired. In Example 2, UV dose delivery will be paced to the operating conditions, so the 

goal is to develop an equation that provides the best fit of the data. In some validations, the user 

may try several different equation forms in an effort to find the best fit. The equation forms 

presented here were selected because they have been used successfully at full-scale for numerous 

UV reactors and operating conditions (Wright et al. 2005). 

To develop a best-fit equation for RED in the form shown in Equation B.8 (or B.9 in 

linear form), a theoretical equation for So should first be developed using validation test data to 

capture the variation in So as a function of UVT. A strong goodness-of-fit as determined through 

statistical analysis allows the equation for So to be used in development of the best-fit equation 

for RED. 

1. Develop an expression for So.

The term So is the UV sensor measurement made with a new lamp operating at 

100 percent power in a new, unfouled sleeve being monitored by a calibrated UV sensor through 

a new, unfouled UV sensor port window. So varies with UVT. This relationship can be measured 

during validation or determined from the validation test conditions. In this example, So is 

determined by fitting the validation data using the following relationship. As with Equations B.8 

and B.9, this approach has proven successful for several validation tests (Wright 2005): 

hUVTgf PeeS × = Equation B.10 

Equation B.10 can be expressed as: 

ln( ) ×+×+= ln ( ) PhUVTgfS Equation B.11 

where P is the lamp power in units of kW and f, g, and h are model coefficients to be determined 

in the subsequent analysis. Fitting this equation to the data in Table B.11 at a 95-percent 

confidence level (UVT and P) using the regression tool within spreadsheet software gives the 

following values: 

Term Value p-Statistic 

f -8.402 2.15 × 10-11

g 0.115 1.61 × 10-13

h 1.578 6.97 × 10-16

Inputting these calculated values into Equation B.10 results in the following relationship 

(8 kW is the UV reactor’s maximum power setting, so S = So when P is equal to 8 kW): 

115.0402.8 578.1 eeS P − ×UVT = Equation B.12 

For the Final LT2ESWTR
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-23 November 2006 

and 

115.0402.8 578.18 UVT

o eeS − × = Equation B.13 

The goodness of fit was evaluated by determining the p-statistic for the model 

coefficients in Equation B.13 (see Draper and Smith 1998 or similar for procedure). The pstatistic of each model coefficient was determined and found to be ≤ 0.05. 

2. Calculate log (S/So). 

By defining S as the measured UV sensor readings in Table B.11, Equation B.13 is then 

used to predict So and to produce the data that will be fit to Equation B.9. The UVT (measured as 

A254 values and converted to UVT units), Sduty and Q values are from the measured data (Tables 

B.9 and B.15). 

3. Calculate an expression for RED. 

As with the UV dose-response equations (shown in Figure B.4) and the sensor equations 

(Equations B.12 and B.13), the interpolation equation (Equation B.9) is fitted to the data in Table 

B.15 using a regression tool in spreadsheet software (at a 95-percent confidence level). The 

goodness-of-fit was again evaluated by determining the p-statistic for the model coefficients in 

Equation B.14 (Draper and Smith, 1998). The p-statistic of each model coefficient was 

determined to be ≤ 0.05 (i.e., all were significant). The results of this analysis are shown below 

with the following results: 

Term Value p-Statistic Term Value p-Statistic 

a -0.829 7.76 × 10-16 c 0.166 1.21 × 10-19

b -2.519 3.71 × 10-42 d 0.409 9.35 × 10-42

Inputting these calculated values into Equation B.9 results in the following: 

( ) ( ) ( ) S Q RED A S

o

1 log 51928290 log.. 254 1660 log. ⎟ 4090 ×+ log. ⎠

⎞ ⎜

⎝

⎛ ×−−= ×+

Equation B.14 

Equation B.14 can be used to calculate RED values as a function of the operating 

conditions (measured UVT, flow rate, and UV intensity) provided So is calculated using 

Equation B.13. 

This equation can be programmed within the UV reactor’s program logic controller 

(PLC) to calculate the delivered RED as a function of the current operating conditions for UV 

dose-monitoring. The equation can be used for interpolation over the validated range of flow 

rates (2.5 – 10 mgd), UVT values (85 – 95 percent), and RED values (8.5 – 24.1 mJ/cm2

). If the 

flow rate falls below 2.5 mgd, the PLC should default to 2.5 mgd in the dose-monitoring 

For the Final LT2ESWTR
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-24 November 2006 

equation. If the UVT reads above 85 percent, the PLC should default to 95 percent in the dosemonitoring equation. 

B.2.7 Determine the Validation Factor 

The Validation Factor (VF) is defined according to Equation 5.13, shown below as 

Equation B.15: 

For the Final LT2ESWTR

( 100/1 ) RED +×= UBVF Val Equation B.15 

B.2.7.1 Calculate the RED Bias 

Per guidance provided in Section 5.9.1, the UV sensitivity for the test condition with the 

lowest UVT (85 percent) was calculated to range from13 – 14 mJ/cm2

 per log inactivation. From 

Table G.5 (for a 2.0-log Cryptosporidium inactivation credit), the RED bias for the maximum 

UV sensitivity of 14 mJ/cm2

 is 2.01. 

B.2.7.2 Calculate the Uncertainty of Validation 

The decision tree in Figure 5.5 was used to determine the correct equation for UVAL. As 

shown in B.2.4.1, UDR is less than or equal to 30 percent. As noted in B.2.4.2, US is less than or 

equal to 10 percent. Therefore, the equation for UVAL is as follows: 

UVal = UIN Equation B.16 

UIN is defined by Equation 5.15: 

× %100 × = RED

SDt UIN Equation B.17 

 where 

 SD = Standard deviation of the differences between the test RED (based on the 

observed log inactivation and UV dose-response curve), and the RED 

calculated using the dose-monitoring equation for each replicate 

 RED = The RED as calculated using the dose-monitoring equation 

The value of Uval (i.e., UIN) can be expressed in one of two ways: 

1. As a single value, the most conservative (largest) uncertainty value calculated for the 

validated range. This is typically based on the lowest calculated RED value. 

2. As a function of the calculated RED, that is, as a variable number. 
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-25 November 2006 

In this example, more than 30 test measurements were made for RED, so the t-statistic is 2.04. 

The standard deviation was determined from the RED values recorded during testing and the 

RED values calculated using the dose-monitoring equation (equation B.14). The value of SD was 

determined to be 0.97. 

The water system does not plan to operate below a calculated RED value of 15 mJ/cm2

, 

so a single value of UIN can be calculated as (2.04 x 1.0) / 15 = 0.136. The following equation 

can be used if UIN is calculated as a function of RED at another location (of the same UV 

reactor): 

RED RED RED

t UIN

981970042 100 ... % = × =× σ× = Equation B.18 

where 

 RED = the RED calculated from the dose-monitoring equation (equation B.14) 

B.2.7.3 Calculate the Validation Factor 

If the user prefers to use one UIN value, a single VF is determined using the following 

equation: 

282

15

981 1012 . . . ⎟ =

⎠

⎞ ⎜

⎝

⎛ VF +×= Equation B.19 

If a user at another water system preferred to use UIN as a function of the calculated RED, 

the following equation would be used for calculating the VF: 

RED RED

U VF Val 983 012 981 1012

100

1012 . . . . . ⎟ +=⎠

⎞ ⎜

⎝

⎛ ⎟ +×=⎠⎞ ⎜

⎝

⎛ +×= Equation B.20 

B.2.8 Calculate the Validated Dose 

In the last step, the calculated RED associated with the operating conditions is divided by 

the VF to produce the validated dose: 

VF

RED

DVal = Equation B.21 

where 

 RED = RED calculated from the dose-monitoring equation (Equation B.14) 

 VF = Validation Factor (either Equation B.19 or Equation B.20) 

To calculate DVAL using a point estimate for the VF, use Equation B.22: 

For the Final LT2ESWTR
Appendix B. UV Reactor Testing Examples 

UV Disinfection Guidance Manual B-26 November 2006 

28.2

RED

DVal = Equation B.22 

To calculate a general expression with VF as a function of RED, to determine DVal, 

Equation B.20 would be substituted into Equation B.21: 

RED

RED

DVal 983 012 . . +

= Equation B.23 

B.2.9 Assign Log Inactivation Credit for the Validated Dose(s) 

The validated dose must be greater than or equal to the required UV dose in Table 1.4 

(Dreq) to achieve a given level of pathogen inactivation credit: 

Val ≥ DD req Equation B.24 

or 

2

Val ≥ cm/mJ8.5D Equation B.25 

System Y is in the validated range when the calculated RED from the dose monitoring 

equation is greater than or equal to 5.8 mJ/cm2

 × 2.28 or 13.4 mJ/cm2. (A similar calculation 

would follow if a different water system preferred VF to vary with RED.) 

The UV reactor can receive 2.0-log Cryptosporidium inactivation credit for an 

installation (with adequate inlet/outlet hydraulics, see Section 5.4.5) that operates under the 

criteria outlined in Table B.18: 

Table B.18 Validated Dose and Operating Conditions for 

2.0-log Cryptosporidium Inactivation Credit Using the Hypothetical Reactor 

Tested in Example 2 

Validated Conditions 

Flow Rate Range1 UVT Range2 Lamp Power Range RED3

≤10 mgd ≥ 85% 3.2 – 8 kW ≥ 13.4 mJ/cm2

1 At flow rates below 2.5 mgd, this value (2.5 mgd) should be used as the 

default value in the RED calculation. 

2 At UVT values above 95 %, this value (95% UVT) should be used as the 

default value in the RED calculation. 

3

 Calculated using equations B.13 and B.14. 

For the Final LT2ESWTR
Appendix C 

Collimated Beam Testing to Develop a 

UV Dose-response Curve 
This page intentionally left blank.
Appendix C. Collimated Beam Testing to Develop a UV Dose-response Curve 

UV Disinfection Guidance Manual C-1 November 2006 

For the Final LT2ESWTR 

 The LT2ESWTR requires that validation testing be conducted using “a test 

microorganism whose dose-response characteristics have been identified with a low pressure 

mercury vapor lamp” [40 CFR 141.720 (d)(2)(ii)]. To accomplish this, EPA recommends using a 

collimated beam study of the test (or challenge) microorganism, as described in this appendix. 

The procedure involves placing sample water with the challenge microorganism in an open 

cylindrical container (e.g., a petri dish) and exposing the sample to collimated UV light for a 

predetermined amount of time. The UV dose is calculated using the measured intensity of the 

UV light, UV absorbance of the water, and exposure time. The measured concentration of 

microorganisms before and after exposure provides the “response,” or log inactivation of the 

microorganisms from exposure to UV light. Regression analysis of measured log inactivation for 

a range of UV doses produces the dose-response curve (sometimes expressed as a “doseresponse equation”). 

This appendix describes the recommended collimated beam testing procedure and 

recommended data analyses for developing the UV dose-response curve. Section C.1 provides 

guidelines for identifying test conditions. Section C.2 discusses all aspects of experimental 

testing for the collimated beam study. Data analyses are discussed in Section C.3, followed by a 

discussion of data uncertainty in Section C.4. Specific recommendations for combining doseresponse curves and limitations on applying results when the challenge microorganism exhibits a 

shoulder or tailing are presented in Sections C.5 and C.6, respectively. Documentation of test 

conditions and all results should be included in the Validation Report (see Section 5.11 for 

guidance). 

C.1. Identifying Test Conditions 

At least two water quality conditions should be tested by collimated beam analysis: 

1. The highest UV transmittance (UVT) used in the full-scale reactor test 

2. The lowest UVT used in the full-scale test 

Because UVT is accounted for in the UV dose calculation, the test conditions should produce 

similar results that can be combined to produce one UV dose-response curve. (Performing two 

tests instead of one test verifies that the UV dose is independent of UVT.) 

 

UV doses should be selected to cover the range of targeted values, using a minimum of 

five data points plus a control [zero (0) UV dose]. The selected UV doses should result in 

challenge microorganism inactivation ranging from 0.5 – 1 log unit higher than the highest log 

inactivation to be demonstrated by the UV reactor. Table C.1 shows a sample test matrix. 

At least one collimated beam test should be conducted on each day of full-scale reactor 

testing. 
Appendix C. Collimated Beam Testing to Develop a UV Dose-response Curve 

UV Disinfection Guidance Manual C-2 November 2006 

For the Final LT2ESWTR 

Table C.1. Sample Collimated Beam Test Matrix for Target of 2.0-Logs Inactivation 

of Cryptosporidium and MS2 Phage as the Challenge Microorganism 

Sample 1 Test Condition2 Log Inactivation

Target UV Doses 

(mJ/cm2

) 3

1 0 (control) 0 

2 0.5 10 

3 1.5 30 

4 2.0 40 

5 2.5 50 

Lowest UVT 

6 3.0 60 

7 0 (control) 0 

8 0.5 10 

9 1.5 30 

10 2.0 40 

11 2.5 50 

Highest UVT 

12 3.0 60 1

 The sample should represent the influent water used in full-scale reactor tests. One 

sample should reflect the lowest UVT tested, and one should reflect the highest UVT 

tested. Dose-response curves should be developed separately for each water quality 

condition tested and compared to determine if they can be combined (see Section 

C.5). 2 Each test condition should be repeated at least twice (resulting in a minimum of two 

test condition replicates), and three test condition replicates will likely improve the 

quality of the fit for the dose-response curve. 3 Based on UV sensitivity of MS2 Phage

C.2 Measuring the UV Dose-response of the Challenge Microorganism 

 

The challenge microorganism’s UV dose-response should be measured using a lowpressure (LP) collimated beam apparatus (Figure C.1). This apparatus comprises an enclosed UV 

lamp and a tube with a non-reflective inner surface. The UV light enters the suspension with a 

near zero-degree angle of incidence and is relatively homogenous across the surface area. The 

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. 

Section C.2.1 provides a physical description of the collimated beam apparatus and 

recommendations for operational controls. Accuracy of monitoring equipment is addressed in 

Section C.2.2. Section C.2.3 provides the recommended test procedure, followed by equations 

for UV dose calculations in C.2.4. 
Appendix C. Collimated Beam Testing to Develop a UV Dose-response Curve 

Figure C.1. Collimated Beam Apparatus 

Low-Pressure

Mercury Arc Lamp Lamp Enclosure

Petri Dish Containing

Microbial Suspension

Magnetic Stirrer

UV Light @ 254 nm

Collimating Tube

Note: To measure intensity of UV light, a calibrated radiometer is positioned below the column in place of 

the petri dish. 

C.2.1 Apparatus Design and Operation 

Because UV dose requirements are based on the pathogen inactivation achieved using 

254-nm light, the collimated beam apparatus should use a lamp that emits germicidal UV light 

only at 254 nm (i.e., a LP lamp). To prevent ozone formation, lamps that emit 185-nm light 

should not be used. The output from the lamp measured using a radiometer should vary by no 

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. 

If the line voltage is not sufficiently stable, a voltage regulator may be used to obtain a stable 

power supply. 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 

so 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,” defined as 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 

UV Disinfection Guidance Manual C-3 November 2006 

For the Final LT2ESWTR 
Appendix C. Collimated Beam Testing to Develop a UV Dose-response Curve 

UV Disinfection Guidance Manual C-4 November 2006 

For the Final LT2ESWTR 

window is wider than 5 mm, it should be reduced using a cover slip with a small hole. 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 (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 – 2 cm. The material of the container 

should not adsorb the challenge microorganism enough to impact its measured UV doseresponse. 

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. 

The irradiance at the center of the suspension’s surface before and after exposure to UV 

light should be measured using a UV radiometer calibrated at 254 nm. During measurement, the 

radiometer’s calibration plane should match the height of the suspension’s surface and be 

perpendicular to the incident UV. The calibration plane of the radiometer should be specified in 

the radiometer’s calibration certificate. 

C.2.2 Accuracy of Monitoring Equipment 

 

Similar to the recommended procedures for full-scale reactor testing in Chapter 5, 

spectrophotometer measurements of A254 should be verified using NIST1

-traceable potassium 

dichromate UV absorbance standards and holmium oxide UV wavelength standards. The 

measurement uncertainty of the spectrophotometer should be 10 percent or less. See Section 

5.5.2 for additional guidance on spectrophotometer use and the recommended procedure for 

verifying spectrophotometer measurements. 

 

Radiometers should be calibrated according to the following procedure to ensure that the 

UV intensity is measured with an uncertainty of 8 percent or less at a 95-percent confidence 

level: 

1. The radiometers used in the collimated beam tests should come from the 

manufacturer with a certified uncertainty of 8 percent or less at a 95-percent 

confidence level at the intervals suggested by the manufacturer. 

 

 

1 National Institute of Standards and Technology
Appendix C. Collimated Beam Testing to Develop a UV Dose-response Curve 

UV Disinfection Guidance Manual C-5 November 2006 

For the Final LT2ESWTR 

2. At minimum, the accuracy of the radiometer used to measure the UV intensity should 

be verified at least at the beginning and the end of each collimated beam test session 

using a second radiometer. 

3. The two radiometers should read within 5 percent of each other. If the two 

radiometers do not read within 5 percent of each other, a third radiometer should be 

used to identify which radiometer is out of specification. The two radiometers with 

readings within 5 percent of each other should be used. If none of the radiometer 

readings match, at least two of them are likely out of calibration. 

 If the above criteria are met, the average radiometer measurement can be used in 

calculations. Alternatively, the radiometer that provides the lowest reading could be used. If 

these criteria are not met, the radiometers should be recalibrated. The radiometers should also be 

checked to be sure that the irradiance measurement does not differ by more than 5 percent before 

and after UV exposure. 

C.2.3 Recommended Collimated Beam Test Procedure 

Researchers should collect a sample from the influent sampling port of the biodosimetry 

test stand (or the influent sample for on-site reactor testing to be used for collimated beam 

testing) for collimated beam testing. Typically, a 1 liter sample is sufficient. If the testing extends 

over more than one day, at least one collimated beam test should be conducted for each day of 

testing. If different batches of challenge microorganisms are used, a UV dose-response curve 

should be generated for each batch. 

Personnel who perform collimated beam tests 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. Personnel should follow recommended 

procedures for challenge microorganism preparation and analysis as presented in Appendix A of 

this guidance manual or use an alternative peer-reviewed method. 

The following procedure is recommended for collimated beam testing of a water sample 

containing challenge microorganisms: 

1. Measure the A254 of the sample using a spectrophotometer that has a measurement 

uncertainty of 10 percent of less (see guidance on spectrophotometer measurements 

in Sections C.2.2 and 5.5.2). 

2. Place a known volume from the water sample into a petri dish and add a stir bar. 

Measure the water depth in the petri dish. 

3. Measure the UV intensity delivered by the collimated beam with no sample present 

using a calibrated radiometer (see Section C.2.2 for guidance on calibrating 

monitoring equipment). 

4. Calculate the required exposure time to deliver the target UV dose using 

Equation C.2 (described in the next section). 
Appendix C. Collimated Beam Testing to Develop a UV Dose-response Curve 

5. Block the light from the collimating tube using a shutter or equivalent. 

6. Center the petri dish with the water sample under the collimating tube. 

7. Unblock the light from the collimating tube and start the timer. 

8. When the target exposure time has elapsed, block the light from the collimating tube. 

9. Remove the petri dish and collect the sample for measurement of the challenge 

microorganism concentration. If the sample is not assayed immediately, store in the 

dark at 4 oC. Each sample should be plated in triplicate and the average microbial 

value for the sample calculated from the three plate replicates. 

10. Re-measure the UV intensity and calculate the average of this measurement and the 

measurement taken in Step 3. The value should be within 5 percent of the value 

measured in Step 3. 

11. Using Equation C.1 (described in the next section), calculate the UV dose applied to 

the sample based on experimental conditions (this should be similar to the target 

dose). 

12. Repeat steps 1 through 11 for each replicate and target UV dose value (see Table 

C.1). Repeat all steps for each water test condition replicate 

C.2.4 UV Dose Calculation 

 

The UV dose delivered to the sample is calculated using: 

( )

( )t

dALd

L RPED

dA

fsCB )10ln(

101 )1(

254

− 254 −

+ −= Equation C.1 

where: 

 DCB = UV dose (mJ/cm2

) 

 Es = Average UV intensity (measured before and after irradiating the sample) (mW/cm2) 

 Pf = Petri Factor (unitless) 

 R = Reflectance at the air-water interface at 254 nm (unitless) 

 L = Distance from lamp centerline to suspension surface (cm) 

 d = Depth of the suspension (cm) 

 A254 = UV absorbance at 254 nm (unitless) 

 t = Exposure time (s) 

Alternatively, given a target UV dose, the required exposure time may be calculated by rearranging Equation C.1. 

UV Disinfection Guidance Manual C-6 November 2006 

For the Final LT2ESWTR 
Appendix C. Collimated Beam Testing to Develop a UV Dose-response Curve 

( ) dA

fs

dA

L

Ld

RPE

Dt 254 101

)10ln(

)1(

1 254

− −

+

− = Equation C.2 

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 (R) can be 

estimated using Fresnel’s Law as 1.000/1.372, or 0.025 (the index of refraction of air divided by 

the index of refraction for water). 

To control for error in the UV dose measurement, the uncertainties of the terms in the UV 

dose calculation should meet the following criteria: 

Depth of suspension (d) ≤ 10% 

Average incident irradiance (Es) ≤ 8% 

Petri Factor (Pf) ≤ 5% 

L/(d + L) ≤ 1% 

Time (t) ≤ 5 % 

(1 – 10-ad)/ad ≤ 5% 

The uncertainty in incident irradiance can be determined by the procedure for evaluating 

uncertainty of radiometer measurements as presented in Section C.2.2. The remaining 

uncertainties listed above should be estimated by laboratory personnel and documented in the 

Validation Report (See Section 5.11 for guidance). 

C.3 Developing the UV Dose-response Curve 

 Collimated beam tests produce the following types of experimental data: 

• UV Dose in units of mJ/cm2, 

• Concentration of microorganisms in the petri dish prior to UV exposure (No) in units 

of pfu/mL, and 

• Concentration of microorganisms in the petri dish after UV exposure (N) in units of 

pfu/mL. 

One UV dose-response curve should be developed for each UVT condition tested (typically high 

and low). If full-scale reactor testing spans more than one day, at least one UV dose-response 

curve should be developed for each day of testing. 

 EPA recommends using regression analysis to develop each UV dose-response curve 

using the following steps: 

UV Disinfection Guidance Manual C-7 November 2006 

For the Final LT2ESWTR 
Appendix C. Collimated Beam Testing to Develop a UV Dose-response Curve 

1. For each test condition and replicate, plot log N vs. UV dose to identify a common No

as the intercept of the curve at UV dose = 0 (an example is illustrated in Figure C.2).2

Figure C.2. Fitting Effluent Concentration vs. UV Dose to 

Determine a Common Influent Concentration Value 

log N = 0.00010(UV Dose)2 - 0.053(UV Dose) + 5.37

R2

 = 0.997

0

1

2

3

4

5

6

0 20 40 60 80 100

UV Dose (mJ/cm2

)

log N (pfu/mL)

No = 5.37

120

2. Calculate log I for each measured value of N (including zero-dose) and the common 

No identified in Step 1 using the following equation: 

⎟

⎠

⎞ ⎜

⎝

⎛ = N

N I o log log Equation C.3 

where: 

 No = The common No identified in Step 1 (pfu/mL) 

 N = Concentration of challenge microorganisms in the petri dish after 

exposure to UV light (pfu/mL) 

The log inactivation for each replicate should be averaged to produce one value of log 

I per test condition. 

3. Plot UV dose as a function of log I for each test condition. 

4. Use regression analysis to derive an equation that best fits the data, forcing the fit 

through the origin. The equation will have different forms depending on the data. For 

challenge microorganisms exhibiting first-order kinetics, a linear equation should be 

used: 

 

2

 If the measured value of No is used for this calculation, any experimental or analytical error in the measured value 

is carried to all the data points, adding an unrelated bias to each measurement. Therefore, using the y-intercept of 

the curve is recommended.

UV Disinfection Guidance Manual C-8 November 2006 

For the Final LT2ESWTR 
Appendix C. Collimated Beam Testing to Develop a UV Dose-response Curve 

×= log I ADoseUV Equation C.4 

See Figure C.3 for an example of a linear UV dose-response curve. 

Figure C.3. Typical E. coli UV Dose-response Curve 

UV Dose [mJ/cm2] = 1.83 log I

R2

 = 0.994

0

2

4

6

8

10

12

012345

log Inactivation

UV Dose (mJ/cm2

)

6

A quadratic equation can also be used, as illustrated in the example in Figure C.4: 

 ( )

2 log ×+×= log I BI ADoseUV Equation C.5 

Figure C.4. Typical MS2 UV Dose-response Curve 

UV Dose [mJ/cm2] = 1.44(log I)2 + 17.99(log I)

R2

 = 0.998

0

20

40

60

80

100

120

012345

Log Inactivation

UV Dose (mJ/cm2

)

Data points are typical of 

results from two water 

samples

UV Disinfection Guidance Manual C-9 November 2006 

For the Final LT2ESWTR 
Appendix C. Collimated Beam Testing to Develop a UV Dose-response Curve 

5. Evaluate the equation’s goodness-of-fit—the differences between the measured 

UV dose values and those predicted by the equation should be randomly 

distributed around zero and not be dependent on UV dose. The goodness of the fit 

can be examined by standard statistical tests, such as examining the p-statistics for 

the regression coefficients. 

Note that the resulting equation should not be used for extrapolation outside of the measured 

range of UV dose. 

C.4 Collimated Beam Data Uncertainty 

As noted in Section C.3, collimated beam data will often be fit to a linear or a polynomial 

regression. The 95-percent confidence interval (UDR) can be calculated using standard statistical 

methods, such as those described in Draper and Smith 1998, or can be conservatively estimated 

using Equation C.6. 

= ×100%

CB

DR DoseUV

SD tU Equation C.6 

 where: 

 UDR = Uncertainty of the UV dose-response fit at a 95-percent confidence 

level 

 UV DoseCB = UV dose calculated from the UV dose-response curve for the 

challenge microorganism 

 SD = Standard deviation of the difference between the calculated UV doseresponse and the measured value 

 t = t-statistic at a 95-percent confidence level for a sample size equal to 

the number of test condition replicates used to define the doseresponse3

Number of Data Points Used to Develop 

the Dose-Response Equation t 

Number of Data Points Used to Develop 

the Dose-Response Equation t 

10 2.23 17 2.11

11 2.20 18 2.10

12 2.18 19-20 2.09

13 2.16 21 2.08

14 2.14 22-23 2.07

15 2.13 24-26 2.06

16 2.12 27-29 2.05

≥30 2.04

 

 If UV dose-response curves can be combined (as described in the next Section, C.5), the 

combined dataset should be used to calculate UDR. If individual dose-response curves cannot be 

combined, UDR should be calculated separately for each curve. 

 

3

 For example, one test condition evaluated twice (two test condition replicates) with five UV dose points each 

would have a total of ten points. 

UV Disinfection Guidance Manual C-10 November 2006 

For the Final LT2ESWTR 
Appendix C. Collimated Beam Testing to Develop a UV Dose-response Curve 

UV Disinfection Guidance Manual C-11 November 2006 

For the Final LT2ESWTR 

 

EPA recommends that the value of UDR (calculated by Equation C.6) not exceed 

30 percent at the UV dose corresponding to 1-log inactivation of the challenge organism (e.g., 

18 mJ/cm2

 for MS2). If the 95-percent confidence interval is calculated using standard statistical 

methods, UDR should not exceed 15 percent at the UV dose corresponding to 1-log inactivation 

of the challenge organism.4 If there is more than one estimate of UDR (i.e., UV dose-response 

curves cannot be combined), the maximum UDR should be used to determine if it meets this 

criterion. 

If the UDR value calculated by Equation C.6 is greater than 30 percent (15 percent if the 

standard statistical method is used), it should be added to the total uncertainty of validation [e.g., 

UVal = (UIN

2

 + UDR

2

)

1/2, see Section 5.9.2].This allows for a validation plan that is sufficiently 

flexible to continue using the dose-response curve at low values, but will increase the UVal 

accordingly. Similarly, if UV dose-response curves cannot be combined and one or more of the 

individual curves exhibits a UDR value greater than 30%, the maximum value should be used in 

calculating the total uncertainty of validation [UVal = (UIN

2

 + UDR

2

)

1/2 ]. 

C.5 Combining UV Dose-response Curves 

Analysis of regression coefficients indicates whether or not UV dose-response curves 

developed using different water samples can be combined. In order for the UV dose-response 

curves to be combined, differences between the regression coefficients should not be statistically 

significant at a 95-percent confidence level.5

 If differences in the coefficients are statistically 

significant, the reason for this difference should be documented in the Validation Report. 

Differences between measured UV dose-response curves for different water samples could 

indicate one or more of the following: 

1. The UV dose-responses of different batches of the challenge microorganism differ. In 

this case, the UV dose-response curve specific to each cultured batch of the challenge 

microorganism should be used to assess UV dose delivery for the validation test 

conditions using that batch. 

2. Interferences due to water quality, such as coagulation or inactivation of the challenge 

microorganism. In this case, mitigate the cause of the interference or account for the 

interference when assessing UV dose delivery for the validation test conditions. 

3. Errors calculating the UV dose delivered by the collimated beam apparatus. Mismeasuring the incident UV intensity or the UV absorbance of the water sample could 

introduce such errors. 

 If differences between UV dose-response curves cannot be resolved, a single curve 

corresponding to one day’s worth of full scale reactor testing can be used to calculate RED 

values for that day (i.e., there will be one UV dose-response curve per day of full-scale reactor 

validation testing). If two or more UV doe-response curves from the same day of testing cannot 

be combined, the curve resulting in the most conservative (lowest) UV dose should be used for 

4 This criterion (15 percent) differs from the criterion of 30 percent applied to Equation C.6 due to simplifications 

incorporated into Equation C.6. 5 A good description for performing this test is provided in Draper and Smith, 1998.
Appendix C. Collimated Beam Testing to Develop a UV Dose-response Curve 

calculating RED values. If different curves are used for RED calculations, the UV sensitivity of 

the challenge microorganism and shape of each UV dose-response curve should be consistent 

with expected inactivation behavior for that challenge microorganism.

C.6 Using Challenge Microorganisms with Shoulders or Tailing 

In the case of a challenge microorganism with a shoulder or tailing in the UV doseresponse, the UV sensitivity should be defined as the sensitivity over the region of linear log 

inactivation that occurs between the shoulder and the onset of tailing. The shoulder of the UV 

dose-response is defined as the point of intersection of the exponential region with the UV dose 

axis (see Figure C.5). The UV dose-response of the challenge microorganism should not 

demonstrate a shoulder at a UV dose beyond 50 percent of the demonstrated (measured) RED 

range, and should not demonstrate tailing until at least one log inactivation beyond the 

demonstrated (measured) inactivation range. 

Figure C.5. UV Dose-response of B. subtilis Spores 

shoulder

(Adapted from Sommer et al. 1998) 

Example C.1. The UV dose-response of B. subtilis spores has a shoulder at low UV 

dose values (Figure C.5). Because the measured UV dose-response has a shoulder of 

16.5 mJ/cm2

, the B. subtilis spores should only be used to demonstrate RED values 

greater than or equal to 2 × 16.5 mJ/cm2 = 33 mJ/cm2. 

UV Disinfection Guidance Manual C-12 November 2006 

For the Final LT2ESWTR 
Appendix D 

Background to the UV Reactor Validation Protocol 
This page intentionally left blank.
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-1 November 2006 

For the Final LT2ESWTR 

This appendix provides background material for the validation protocol given in 

Chapter 5. The background material is organized into the following six sections. 

• UV dose delivery by UV reactors. Section D.1 describes why a correction factor 

(termed the “RED bias”) should be applied in the Validation Factor calculation to 

account for systematic errors that arise if the challenge microorganism is more 

resistant to UV light than the target pathogen. 

• UV dose monitoring. Section D.2 provides background information on the impact of 

UV sensor placement on UV dose monitoring (whether it is at, closer to, or farther 

from the lamp than the ideal position). It provides a rationale for defining test 

conditions to validate UV reactors using a given UV dose-monitoring approach and 

explains why sensor position is important. 

• UV sensors. Section D.3 provides the basis for the UV sensor calibration criterion 

recommended in Chapter 5. It describes the properties of UV sensors, how those 

properties impact the sensor’s measurement uncertainty, and how that measurement 

uncertainty can be determined. 

• Polychromatic considerations. Section D.4 describes systematic errors that can 

occur with the validation of UV reactors that use medium-pressure (MP) UV lamps 

(1) equipped with non-germicidal UV sensors and/or (2) validated with a challenge 

microorganism that has a UV action spectrum significantly different from that of the 

target pathogen. This section provides a rationale for assessing those errors. 

• Uncertainty of validation. Section D.5 provides a rationale for defining a validation 

factor that accounts for the random uncertainty associated with UV reactor validation 

and monitoring. 

• CFD modeling. Section D.6 provides guidance on using Computational Fluid 

Dynamics (CFD) to model UV dose delivery. 

D.1 UV Dose Delivery by UV Reactors 

UV dose delivery by UV reactors to be used at water treatment plants (WTPs) 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 is termed the “reduction equivalent dose,” or RED. 

D.1.1 Using RED to Demonstrate Target Pathogen Inactivation 

If the UV dose-response of the challenge microorganism does not match the target 

pathogen’s, and the UV dose distribution of the UV reactor is not known, 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 D.1 provides a comparison of the 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-2 November 2006 

UV dose distributions of reactors with ideal and worst-case hydraulics to a UV dose distribution 

that might be seen with a real reactor. 

Figure D.1. UV Dose Distributions of Ideal, Realistic, and 

Worst-case UV Reactors 

0 50 100

UV Dose (mJ/cm2)

Probability

Ideal

0 50 100

UV Dose (mJ/cm2)

Probability

Reality

0 80

UV Dose (mJ/cm2)

Probability

Worst Case

0 Infinity

D.1.1.1 Ideal Reactor Hydraulics 

A UV reactor with ideal hydraulics delivers the same UV dose to all the microorganisms 

passing through the reactor. Its UV dose distribution is represented by a single value. Examples 

of a UV reactor with ideal hydraulics include the stirred suspension irradiated during the 

measurement of UV dose-response with a collimated beam device and an ideal plug-flow 

reactor. In both cases, the delivered dose is the product of the average UV intensity within the 

reactor and the residence time. 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 because all the microorganisms receive the same dose. 

D.1.1.2 Worst-case Hydraulics 

For a reactor with worst-case hydraulics and a measurable RED, an infinite UV dose is 

delivered to one fraction of the flow rate, and zero UV dose is delivered to the other fraction (i.e., 

one of two UV dose values is delivered to each respective microorganism). The net inactivation 

achieved is constant, equal to the fraction receiving the infinite UV dose, and hence independent 

of the microorganism’s inactivation kinetics. With a worst-case UV reactor, the measured 

inactivation is that which would occur with any microorganism regardless of its UV sensitivity. 

D.1.1.3 Real-world Hydraulics 

Using the above definitions of an ideal and a worst-case UV reactor, the log inactivation 

of a pathogen estimated from biodosimetry results will have a value between log(No,c/Nc) and 

[RED/Dp],1

 that is, a “real” UV reactor will have a UV dose distribution that falls somewhere 

between ideal and worst-case (Wright and Lawryshyn 2000). 

 

1 No,c is the influent challenge organism (c) concentration and Nc is the effluent challenge microorganism 

concentration. Dp is the UV sensitivity of the pathogen (p) in units of mJ/cm2

 per log inactivation.

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-3 November 2006 

If the inactivation of the pathogen must be known with absolute confidence, the lower 

bound of that range should be used (assume worst-case hydraulics). When the challenge 

microorganism is more resistant to UV light than the target pathogen, the lower bound is the log 

inactivation of that challenge organism, log [No,c/Nc]. In other words, if a UV reactor is validated 

with a challenge microorganism that is less sensitive to UV light than the target microorganism, 

one cannot know with certainty that the reactor achieved more than the log inactivation 

demonstrated during validation. For example, if 2-log MS2 inactivation is measured, one can 

conclude only that the water system attained ≥ 2-log inactivation for any organism less resistant 

to UV light. 

If the challenge microorganism is less resistant to UV light than the target pathogen, the 

lower bound is the RED measured with the challenge organism divided by the sensitivity of the 

target pathogen, [RED/Dp]. For example, if one measures a φx-174 RED of 12 mJ/cm2

, 

corresponding to 4-log inactivation, one cannot assume that the reactor achieved 4.0-log 

inactivation of Cryptosporidium; one can assume only that the water system attained 

[12 mJ/cm2] / [4.0 mJ/cm2 per log I] = 3.0-log inactivation. 

But both of these assumptions are extreme. For this reason, the “RED Bias” correction 

factor used in the VF is based on the UV dose distribution of a defined “real-world worst-case” 

UV reactor. The RED delivered to a pathogen by a given UV reactor can be estimated from the 

measured RED of the challenge microorganism using Equation D.1: 

RED

c

wcc

wcp

p c B

RED

RED

RED

RED RED ×= =

,

, Equation D.1 

where: 

REDp = Pathogen RED estimated for the UV reactor of interest (mJ/cm2) 

REDc = Challenge microorganism RED measured during biodosimetry (mJ/cm2) 

REDp, wc = Pathogen RED estimated from Figure D.2, the “worst-case” UV reactor 

(mJ/cm2

) 

REDc, wc = Challenge microorganism RED estimated from Figure D.2, the “worstcase” UV reactor (mJ/cm2

) 

BB

RED = RED Bias, the ratio of the RED of the pathogen to the RED of the 

challenge microorganism for a given set of operating conditions 

The development of this factor (RED Bias, or BRED) is discussed below. 

Defining a Realistic Conservative UV Dose Distribution 

Because UV manufacturers strive to optimize the hydraulic design of their UV reactors, 

using the worst-case UV dose distribution represented in Figure D.1 to define the lower bound of 

pathogen inactivation is overly conservative. An alternative approach is to use the UV dose 

distribution of a commercial UV reactor that is representative of plausibly poor UV reactor 

hydraulics. 

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-4 November 2006 

Figure D.2. The UV Dose Distributions Used to Determine 

RED Bias Values Tabulated in Appendix G 

0.00

0.05

0.10

0.15

0.20

0.25

0 20 40 60 80

UV Dose (mJ/cm2

)

Probability

UVT = 98 %

UVT = 95 %

UVT = 90 %

UVT = 85 %

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0 50 100 150

UV Dose (mJ/cm2

)

Probability

UVT = 80 %

UVT = 75 %

UVT = 65 %

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 20 40 60 80

UV Dose (mJ/cm2

)

Fraction Less Than

UVT = 98 %

UVT = 95 %

UVT = 90 %

UVT = 85 %

0.000

0.200

0.400

0.600

0.800

1.000

0 50 100 15

UV Dose (mJ/cm2

)

Fraction Less Than

0

UVT = 80 %

UVT = 75 %

UVT = 65 %

UV dose modeling based on CFD was used to predict dose distributions for commercial 

low-pressure high-output (LPHO) and medium-pressure (MP) UV reactors. Details on the 

approach are provided in Wright and Reddy (2003) and Dzurny et al. (2003). CFD was used to 

predict the trajectories of approximately 3,000 microbes through the UV reactors. UV intensity 

fields within the reactor were modeled using the methods described by Bolton (2000). The UV 

dose delivered to each microbe was predicted by integrating the total UV dose delivered over its 

trajectory through the reactor. The REDs delivered to the target pathogens were calculated 

assuming first-order kinetics with a UV sensitivity defined as the required dose in Table 1.4 

divided by the associated log inactivation credit. The dose distributions were scaled to give 

pathogen REDs equal to the required dose for a given level of log inactivation credit plus an 

uncertainty factor of 25 percent (i.e., for 3-log Cryptosporidium inactivation credit, dose 

distribution was scaled to give a Cryptosporidium RED = 12+ [0.25 × 12] = 15 mJ/cm2

). This 

approach assumes that the UV reactor uses dose pacing2 to deliver the required RED without 

overdosing. REDs were estimated for test microbes of various UV sensitivities assuming firstorder kinetics. The RED bias was calculated as the ratio of the test microbe RED to the pathogen 

RED (BRED = REDtest microbe/REDpathogen). 

The commercial reactor that resulted in the most conservative RED bias values was used 

to develop the RED Bias values in Appendix G. Figure D.2 shows the scaled UV dose 

 

2 The UV reactor maintains the delivered dose at or near the target value by adjusting the lamp power or turning 

"on" or "off" banks of UV lamps or whole UV reactors to respond to changes in UV absorbance, lamp intensity, 

and/or flow rate.

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-5 November 2006 

distributions used to estimate the RED bias for 3-log inactivation credit with Cryptosporidium. 

Figure D.3 shows predictions of RED and log inactivation for those dose distributions as a 

function of the test microbe’s UV sensitivity. Figure D.4 shows the RED bias for 3-log 

Cryptosporidium credit as a function of the test microbe’s UV sensitivity obtained using the data 

in Figure D.2. 

Figure D.3. RED as a Function of Microorganism UV Sensitivity for the 

UV Reactor Represented in Figure D.2

0

10

20

30

40

50

0 10 20 30

UV Sensitivity

 (mJ/cm2

 per log inactivation)

Reduction Equivalent 

Dose (mJ/cm 2

)

98

95

90

85

80

75

65

UVT 

(%)

φx174 MS2

Figure D.4. RED Bias for 3-log Cryptosporidium Credit as a Function of 

Test Microbe UV Sensitivity

0.00

1.00

2.00

3.00

4.00

5.00

0 10 20 30

UV Sensitivity

 (mJ/cm2

 per log inactivation)

RED Bias

98

95

90

85

80

75

65

UVT 

(%)

φx174 MS2

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-6 November 2006 

For the Final LT2ESWTR 

Example D.1. A UV reactor is challenged using MS2 with a UV sensitivity of 18 mJ/cm2 per 

log inactivation. The UVT of the water is 85 percent. Two-log inactivation is measured, 

corresponding to an MS2 RED of 2-log × 18 mJ/cm2-log I = 36 mJ/cm2. These results are used 

to estimate the log inactivation of two pathogens, one with a UV sensitivity of 10 mJ/cm2

 per 

log inactivation and the other with a UV sensitivity of 25 mJ/cm2

 per log inactivation. 

In Figure D.3, the RED delivered to the microorganisms with a UV sensitivity of 10, 18, 

and 25 mJ/cm2 per log inactivation would be 20, 25, and 28 mJ/cm2, respectively. The RED 

Bias values for MS2 relative to the first pathogen is 25/20 = 1.25 while the RED Bias for MS2 

relative to the second pathogen is 25/28 = 0.89. Assuming the reactor has a UV dose 

distribution that is better than the dose distribution used to develop Figure D.2, the RED of the 

first pathogen has a value between 36 and 36/1.25 = 29 mJ/cm2

 and the RED of the second 

pathogen has a value between 36 and 36/0.89 = 40 mJ/cm2

. 

D.2 The Impact of UV Sensor Positioning on UV Dose Monitoring 

This guidance manual focuses on two commonly used UV dose-monitoring strategies, the 

UV Intensity Setpoint Approach and the Calculated Dose Approach, which are summarized 

below. Sections D.2.1 and D.2.2 discuss the impact of UV sensor positioning for the UV 

Intensity Setpoint Approach and Calculated Dose Approach, respectively. 

1. UV Intensity Setpoint Approach. UV dose delivery is indicated by the measured 

flow rate and UV intensity. Minimum UV dose delivery is verified when the 

measured UV intensity is above an alarm (minimum) setpoint value defined as a 

function of the flow rate through the reactor. In a variation of this method, the 

minimum UV dose can be verified when the measured relative UV intensity 

(calculated as a function of UVT) is above an alarm (minimum) setpoint value 

defined as a function of the flow rate through the reactor. 

2. Calculate Dose Approach. Minimum UV dose delivery is verified when the 

calculated UV dose (using an equation dependent on flow rate, relative UV intensity, 

UVT, and sometimes other parameters such as lamp status) is above an alarm 

(minimum) setpoint value. 

D.2.1 UV Intensity Setpoint Approach 

With the UV Intensity Setpoint Approach, dose monitoring is impacted by UV sensor 

positioning (Wright et al. 2002). To illustrate this impact, Figure D.5 presents the relationship 

between UV dose and measured UV intensity for a simple annular reactor containing a single 

low-pressure (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 flow rate of 140 gpm. Simulated UVT values ranged from 

70 to 98 percent, and simulated relative lamp outputs (characterized by relative sensor values) 

ranged from 20 to 100 percent. In each figure, the data are presented as plots of UV dose as a 

function of the UV sensor reading for a range of UVT values. Each point at a given UVT 

represents, in order of increasing UV dose, operation at 20, 40, 60, 80, and 100 percent relative 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-7 November 2006 

For the Final LT2ESWTR 

lamp power. The differences between these figures are due to differences in sensor-to-lamp 

distance (i.e., UV sensor placement). 

1. The UV Sensor Is Located at the “Ideal Position” 

Figure D.5a presents the relationship between delivered UV dose and sensor reading 

obtained when the UV sensor is located at the sensor-to-lamp distance where the relationships 

between UV dose and measured UV intensity at different UVTs overlap. Because these 

relationships overlap, a given UV intensity can be related to a specific level of dose delivery. 

Example D.2. The UV reactor characterized in Figure D.5a is used in a disinfection application 

where the target dose is 20 mJ/cm2

. A UV sensor value S of 18 mW/cm2 is used as an alarm 

setpoint to indicate the UV reactor delivers a dose of 20 mJ/cm2 across the entire operating 

range—the ideal placement of the sensor ensures that an alarm setpoint value of 18 mW/cm2 

will indicate a dose of 20 mJ/cm2.

2. The UV Sensor Is Located Closer to the Lamp Than the “Ideal Position” 

Figure D.5b presents the relationship between delivered UV dose and sensor reading 

when the UV sensor is placed closer to the lamp than the ideal position (i.e., a smaller sensor-tolamp distance than in Figure D.5a). Because the sensor views the lamp through a relatively thin 

water layer, its response to changing UVT is small compared to that in Figure D.5a. 

Accordingly, the relationship between dose delivery and measured UV intensity cannot be 

described by a single relationship for all values of UVT. Unlike the situation depicted in Figure 

D.5a, the delivered dose will decrease at lower UVTs for a given UV sensor reading. 

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 D.3 The UV reactor characterized in Figure D.5.b is used in an application where the 

target dose is 20 mJ/cm2. The UV manufacturer states that a UV sensor value S of 80 mW/cm2

will indicate a dose of 20 mJ/cm2 under design conditions of 85% UVT and 60% relative lamp 

output. However, as shown in Figure D.5.b, a UV intensity of 80 mW/cm2 corresponds to a 

dose ranging from 10 mJ/cm2

 (for 70% UVT) to 37 mJ/cm2 (for 98% UVT). For a UV intensity 

alarm setpoint to ensure a delivered dose of 20 mJ/cm2 under all possible conditions of water 

UVT and lamp output, a sensor setpoint value S’ of 157 mW/cm2

 would need to be used. 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-8 November 2006 

Figure D.5. Relationship between UV Dose and Intensity for a UV Sensor Located 

(a) at the “Ideal Position,” (b) Close to the Lamp, and (c) Far from the Lamp 

0

10

20

30

40

50

60

70

0 20 40 60

Sensor (mW/cm2

)

UV Dose

 (mJ/cm2

)

70%

80%

85%

90%

94%

98% S

UVT

140 gpm 254 nm

(a) 

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120 140 160

Sensor (mW/cm2

)

UV Dose

 (mJ/cm2

)

70%

80%

85%

90%

94%

98% S

UVT

140 gpm 254 nm

S'

(b) 

0

10

20

30

40

50

60

70

0 10 20 30 40

Sensor (mW/cm2

)

UV Dose

 (mJ/cm2

)

70%

80%

85%

90%

94%

98% S

UVT

140 gpm 254 nm

S'

(c) 

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-9 November 2006 

For the Final LT2ESWTR 

3. The UV Sensor Is Located Farther from the Lamp Than the “Ideal Position” 

Figure D.5c presents the relationship between delivered UV dose and sensor reading 

when the UV sensor is located farther from the lamp than the ideal position (i.e., a greater 

sensor-to-lamp distance than in Figure D.5a). Because the sensor views the lamp through a 

relatively thick water layer, its response to changing water transmittance is greater at this 

position than at either the ideal or closer-than-ideal positions. Again, the relationship between 

UV dose delivery and measured UV intensity cannot be described by a single relationship for 

different values of UVT. However, unlike Figure D.5b, the UV dose delivered at a given 

measured UV intensity increases as UVT decreases. Thus, the measured UV intensity should 

only be used to indicate UV dose delivery at the lower end of that range, which occurs 

under conditions of reduced lamp power and maximum UVT (the opposite of what is 

observed with the closer-than-ideal UV sensor position). 

Example D.4. The UV reactor characterized in Figure D.5c is used in an application where the 

target dose is 20 mJ/cm2. A UV intensity alarm setpoint value S of 10 mW/cm2 is proposed 

based on the UV intensity measured under design conditions of 85 percent UVT and 60 percent 

relative lamp output. However, a sensor value of 10 mW/cm2 indicates a UV dose ranging from 

15 to 32 mJ/cm2. To ensure a delivered dose of 20 mJ/cm2 under all possible conditions of 

water UVT and lamp output, a setpoint value S’ of 14 mW/cm2

 would need to be used. 

The manufacturer of the UV reactor selects the location of the UV sensor within a UV 

reactor. If the UV reactor uses the UV Intensity Setpoint Approach for UV dose monitoring, it is 

to the manufacturer’s advantage to optimize the UV sensor’s location to obtain overlapping 

relationships between UV dose delivery and measured UV intensity for different UVT values, 

similar to the example given in Figure D.5a. 

If the UV manufacturer does not optimize the UV sensor’s location, a given UV intensity 

will correspond to a range of UV dose values as opposed to a single value. While this does not 

prevent the UV reactor from using the UV Intensity Setpoint Approach, the monitoring approach 

will be significantly less efficient than with an ideally located UV sensor because the UV reactor 

will be overdosing at many UVT-lamp power combinations that give rise to operation at the 

setpoint. When this occurs, the manufacturer may opt to supplement measurements of UV 

intensity with measurements of UVT to enable more efficient UV dose monitoring (this is 

sometimes referred to in the literature as the “UV Intensity-UVT Setpoint Approach”). The UV 

reactor is verified to be delivering the required UV dose when both the measured UV intensity 

and UVT are above the minimum validated setpoint values (S/So, UVT setpoint, and UVTsetpoint), 

both defined for a specified range of flow rates. With this approach, there are no requirements for 

UV sensor positioning. 

D.2.2 Calculated Dose Approach 

Measurements of flow rate, UV intensity, and UVT can be incorporated into theoretical, 

empirical, or semi-empirical calculations of UV dose delivery. For example, the relationships 

represented in Figure D.5a – c could be defined experimentally and used in an empirical manner 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-10 November 2006 

For the Final LT2ESWTR 

to calculate UV dose (e.g., Equation 5.8). Relationships could also be defined using advanced 

numerical modeling approaches to relate measured intensity to UV dose delivery as a function of 

flow rate and UVT. 

In theory, the UV dose calculation does not necessitate that the UV sensor be placed at 

any one location within the reactor. However, if the UV sensor were placed at the ideal position 

(a location that gives UV dose delivery proportional to the UV sensor reading), the UV dose 

calculation would not require UVT as an input parameter. 

D.3 UV Sensors 

UV sensors are photosensitive detectors that are used to indicate UV dose delivery by 

providing information related to UV intensity at different points in the UV reactor. Reference 

UV sensors are used to check that the measurements made by the on-line, or “duty” sensors are 

valid. 

UV sensors include the following components, arranged as shown in Figure D.6: 

• 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 to slow 

UV sensor degradation. Diffusers also modify the UV sensor’s angular response. 

• Diffusers and apertures reduce the UV light incident on the photodetector to slow 

UV sensor degradation. Diffusers also modify the UV sensor’s angular response. 

• Filters limit the light delivered to the photodiode, typically restricting it to germicidal 

UV wavelengths (~200 – 300 nm). 

• Photodetectors are solid-state devices that produce a current proportional to the 

irradiance on the detector’s active surface. The responsiveness of a typical 

photodetector to UV light is on the order of 0.1 – 0.4 mA/mW. 

• Amplifiers convert the output of the photodetector from a low-level current to a 

standardized output proportional to the incident UV intensity. 

• The housing of the UV 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. 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-11 November 2006 

Figure D.6. Interior UV Sensor Schematics3

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

Housing

UVC Filter

Signal Amplifier

Printed Circuit Boards

Quartz Silica Probe

Digital

Signal

Output

Photodetector

Housing

a. b.

UV Light 

D.3.1 UV Sensor Properties 

The UV sensor should detect germicidal UV radiation and produce a standardized output 

signal proportional to the incident UV irradiance (e.g., 4 – 20 mA). A UV sensor may or may not 

measure the UV light through a monitoring window that is separate from the sensor body. 

Monitoring windows should have a high UVT over the sensor’s spectral response range. 

UV 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 sensor would have the following properties: 

• A linear response to incident UV light, independent of water temperature and stable 

over time. 

• A fixed angular response and a wavelength response that mimics the germicidal 

response of the target microorganism(s). 

 

3

 Figure courtesy of (a) Aquionics and (b) WEDECO UV Technologies; UVC light is 200 – 280 nm (the germicidal 

range). 

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-12 November 2006 

• Has zero measurement noise and bias. 

• Responds only to germicidal UV light. 

• Has zero measurement uncertainty. 

The properties of an ideal UV sensor are presented here to illustrate the benchmark UV 

sensor manufacturers strive to approximate as closely as possible, that is, zero measurement 

error. 

Angular response is a plot of the UV sensor measurement as a function of the incident 

angle of light on the sensor’s window. Angular response is affected by the UV 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 sensor. An ideal 

UV sensor has a “cosine response” (Equation D.2), which results in an accurate measure of the 

light incident on the surface of the photodetector. In practice, UV sensors deviate from the cosine 

response; some potential responses are shown in Figure D.7. 

= SS im cosθ Equation D.2 

 where: 

Sm = Intensity measured by the UV sensor’s photodetector [watt per centimeter 

squared (W/cm2)] 

Si = Intensity incident on the UV sensor’s photodetector’s surface (W/cm2) 

θ = Incident angle at the UV sensor’s photodetector surface (o) 

Figure D.7. Angular Response of Two UV Sensors 

Relative to the 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)

Angular Response 

(Relative Scale)

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)

Angular Response 

(Relative Scale)

Cosine

Sensor 1

Sensor 2

The opening or acceptance angle of the UV 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 value (e.g., 50 percent). The 

acceptance angle is a characteristic of the sensor but does not affect its performance. 

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-13 November 2006 

For the Final LT2ESWTR 

 

The spectral response is a measure of the output of the UV sensor as a function of 

wavelength. It depends on the response of the photodetector and filters and the UV transmittance 

of the monitoring windows, light pipes, and filters. 

The working range of the UV sensor is the intensity 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 working range is limited by the saturation of the photodetector and the 

amplifier. Saturation is the point at which the UV sensor can no longer respond to an increase in 

intensity. 

The detection limit of the UV sensor is the lowest UV intensity that can be detected and 

quantified at a known confidence level. The detection limit is calculated based on 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 level. The detection limit and the resolution depend on the 

measurement noise and on any digitalization of the analog output from the UV sensor by the 

system’s electronics. Measurement bias and noise of a photodetector are increased by 

electromagnetic fields within the UV reactor if the sensor is not properly shielded and grounded. 

An ideal UV sensor (a sensor with an ideal cosine response) responds proportionally to 

the intensity incident on the sensor (Figure D.7). The linearity of the UV sensor is a measure of 

the adherence of the sensor response to that proportional relationship. It is reported as the ratio of 

the measured response to the known incident intensity, usually at a specific confidence level. 

Linearity is affected by bias and saturation. 

D.3.1.1 Calibration and Quantification of UV Sensor Properties 

UV sensors used to monitor monochromatic lamps are often calibrated using the 

substitution method of 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; the measured value is then 

compared to that made using a standard measurement, such as a NIST4

-traceable UV sensor or 

chemical actinometer. The ratio of the standard measurement to the UV sensor output is the 

calibration factor. With UV sensors designed to measure the output of MP lamps, the sensor can 

be calibrated at 254 nm, calibrated as a function of wavelength, or calibrated using 

polychromatic light from an MP lamp with a known spectral output. 

UV sensor linearity is determined by comparing the sensor output as a function of 

incident irradiance to standard measurements of that irradiance. UV 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. The angular response of a UV 

sensor is determined by measuring the dependence of the UV sensor reading on the incident 

angle of a beam of fixed-intensity, collimated UV light. 

4

 National Institute of Science and Technology, Boulder, Colorado.
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-14 November 2006 

For the Final LT2ESWTR 

The spectral response of a UV 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. 

The measurement accuracy of UV sensors changes over time due to mechanical wear 

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. 

Long-term sensor stability is best determined using field data, but may be estimated using 

accelerated life-cycle testing. 

D.3.1.2 Recommendations for Calibration and Quantification of UV Sensor 

Properties 

UV sensors provided by the manufacturer should be individually calibrated. The 

manufacturer should determine linearity and temperature-response over the expected operation 

range of lamp intensity and water temperature expected during operation at WTPs. Because it 

may be affected by infrared transmission of glass filters and fluorescence of diffusers that are 

part of the UV sensor (Larason and Cromer 2001), the sensor spectral response should be 

evaluated from 200 to 1,000 nm. The sensor response should be “germicidal” (see Section 5.4.8 

for the definition of a “germicidal” UV sensor response). 

UV sensor manufacturers should conduct regular testing on their UV sensors to develop a 

database on the effect of long-term use on sensor properties. While some UV sensor properties 

may be measured with each sensor (e.g., calibration), other properties, such as long-term stability 

and angular and spectral response, can practically be measured only on a representative sample 

from a lot. The UV sensor manufacturer should have available for inspection the following 

information: 

• A description of the measured UV sensor properties. 

• A description of the system used to measure each property. 

• A description of the measurement standards used. 

• The documented uncertainty associated with each measurement. 

• A description of the QA/QC procedures used to ensure that the measurements were 

traceable to a standard. 

• Data that demonstrates that the properties of the manufactured UV sensors are within 

specifications over time. 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-15 November 2006 

For the Final LT2ESWTR 

D.3.2 UV Sensor Measurement Uncertainty 

UV sensor measurement uncertainty quantifies how the UV intensity value measured 

with a duty UV sensor (mounted on the UV reactor) compares to the true value. For the purposes 

of this manual, UV sensor uncertainty should be determined by summing the uncertainties that 

arise from calibration, linearity, angular and spectral response, temperature response, and longterm stability (see Table D.1 for an example of this calculation): 

• Uncertainty in the UV sensor calibration arises from the uncertainties associated with 

the standards and instrumentation used to calibrate the sensor (e.g., voltmeters and 

amplifiers). 

• Uncertainty in the UV sensor’s linearity and temperature response arises because 

sensor calibration factors, determined at one temperature and UV irradiance, are used 

over a range of temperatures and irradiances during operations at a WTP. 

• Uncertainty in angular response arises because UV sensors are used in UV reactors to 

measure UV light impacting from different directions but are calibrated with 

collimated light (i.e., light is incident to the surface from only one angle). 

• Variability in spectral and angular response from UV sensor-to-UV sensor results in 

an additional measurement uncertainty not accounted for in calibration. The impact of 

spectral and angular response variability on UV sensor measurement uncertainty can 

be determined either by calculation or by measurement. 

In the calculation approach, UV sensor spectral and angular response, measured on a 

representative sample from a lot, is used as an input to a numerical model that 

predicts sensor readings in a reactor. The variability in the readings predicted by the 

model is used to define an uncertainty term that is included in the calculation of the 

total sensor uncertainty. In the measurement approach, the variability in 

measurements made by a representative number of UV sensors mounted on the 

reactor is used to define the uncertainty. 

• Uncertainty in spectral response arises in MP systems because sensors, calibrated at a 

fixed wavelength, are used in UV reactors equipped with polychromatic lamps. 

• Additional uncertainty arises from long-term UV sensor drift. 

The information described above should be provided by the manufacturer for each duty 

sensor as part of the UV reactor documentation. The purpose of this information is to indicate the 

ability of the manufacturer to quantify the uncertainty for important sensor properties and to 

demonstrate whether the sensor can meet sensor specifications prepared by the system purchaser. 

This information should not be used to verify sensor performance during validation testing or 

operations. Instead, sensor uncertainty should be field-verified by comparing duty sensor 

measurements to calibrated reference sensors, as described in Sections 5.5.4 and 6.4.1.1. 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-16 November 2006 

Table D.1. Example of a UV Sensor 

Uncertainty Calculation Datasheet 

Property Uncertainty (%) 

Spectral response 4 

Angular response 3 

Linearity 3 

Calibration 5 

Temperature response 3 

Long term drift 12 

Total Uncertainty1 15 1

 Total uncertainty is calculated as: 

(12

+32+32+52+32+102)

1/2 = 15%. 

Example D.7. A UV sensor manufacturer calibrates each UV sensor at 20°C with an 

uncertainty of ±1 percent. Linearity, temperature response, angular response, and spectral 

response are evaluated on every tenth sensor manufactured. Linearity ranges from 1 – 5 percent 

over the measurement range of the sensor. Temperature response ranges from 0.1 – 0.2 percent 

per °C—an uncertainty of 5 percent over the temperature range 0 – 40 °C. Models predict that 

the variability in angular and spectral response from sensor-to-sensor will cause uncertainties of 

8 percent and 3 percent, respectively. A laboratory evaluation of UV sensors returned from the 

field indicates that the long-term drift over a one-year period is 11 percent. The measurement 

uncertainty of the UV sensors is calculated as the square root of the sum of the squares of the 

individual percent uncertainties: 

%161138551 222222 Measurement uncertainty =+++++=

D.3.3 Number of UV Sensors 

Lamp-to-lamp variability in UV output impacts both UV dose delivery and monitoring 

(Wright et al. 2004). If a lamp has a lower output than the other lamps in a UV reactor, it will 

deliver lower UV doses to microorganisms passing in its vicinity, thereby shifting the UV dose 

distribution to lower values and reducing the net performance (UV dose delivery) of the reactor. 

The shift in the UV dose distribution will be more pronounced in a reactor with fewer lamps. 

If the number of UV sensors is less than the number of lamps and the sensors do not 

monitor the lamps with the lowest output, the monitoring system will overestimate UV dose 

delivery. Sections 6.3.2.2 and 5.4.7 provide guidance on dealing with this issue in operations and 

validation, respectively. 

D.4 Polychromatic Light Considerations 

LP and LPHO lamps are monochromatic, with UV output at a single wavelength, 254 

nm. MP lamps are polychromatic, with UV output at multiple wavelengths. UV dose delivery 

and monitoring in MP reactors involves UV light from 200 to 320 nm. The output from the UV 

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-17 November 2006 

For the Final LT2ESWTR 

sensor is an integrated response to UV light over wavelengths spanning the sensor’s spectral 

response. If the spectral properties of the UV reactor that influence UV dose delivery and 

monitoring during operation at a WTP are the same as during validation, then the characterized 

UV dose delivery will occur at the WTP. However, if the spectral properties are significantly 

different, UV dose delivery at the WTP can differ substantially from UV dose delivery measured 

during validation for the same measured operating values. The following spectral properties may 

differ: 

• Action spectra of the challenge microorganism and of the target pathogen. 

• Spectral UV absorbance of the water used during validation and at the WTP. 

• UV output of the lamps during validation and at the WTP (see 5.4.6 for details). 

• UVT of the lamp sleeves during validation and at the WTP (see 5.4.6 for details). 

Section D.4.1 describes approaches for assessing the impact of differences in microbial 

action spectrum properties. Section D.4.2 describes an approach for developing a correction 

factor for polychromatic bias for MP reactors when the challenge microorganism is something 

other than MS2 or Bacillus subtilis. Derivation of the polychromatic bias factor for MP UV 

reactors with non-germicidal sensors is presented in Section D.4.3. 

D.4.1 Impact of Microorganism UV Action Spectra Differences 

The dependence of microorganism inactivation kinetics on wavelength can be described 

using an action spectrum – the UV inactivation sensitivity of a microorganism as a function of 

wavelength (Figure D.8). 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. 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-18 November 2006 

Figure D.8. Action Spectra for Various Microorganisms 

0.0

0.5

1.0

1.5

2.0

2.5

225 235 245 255 265 275 285 295 305

Wavelength (nm)

Action 

relative 

to 

254 nm

Herpes simplex

MS2, R-17,fr, 7-S

φx-174

T2

(Adapted by H. Wright from Rauth 1965.) 

The impact of various action spectra on UV dose delivery may be estimated by 

calculating the germicidal lamp output using Equation D.3: 

∑ () () =

= Δ

320

λ 200

G GPP λλλ Equation D.3 

 where: 

PG = Germicidal output of the MP lamp (W/cm)

λ = Wavelength (nm) 

P(λ) = Lamp output at wavelength λ, measured over 1-nm increments [watt per 

nanometer (W/nm)] 

G(λ) = Relative UV sensitivity of the microorganism at wavelength λ (cm-1) 

Δλ = 1-nm wavelength increment (nm) 

Using the published action spectra of fourteen microorganisms (Cabaj et al. 2002, Linden 

et al. 2001, Rauth 1965), Table D.2 presents the germicidal lamp output calculated for a 

commercial MP lamp and the ratio of that output to that of Cryptosporidium. A ratio greater than 

one (1) indicates that the microorganism receives more germicidal output compared to 

Cryptosporidium. If a challenge microorganism with a ratio greater than 1.05 is used to validate 

a MP reactor for Cryptosporidium inactivation, the ratio should be used as a correction factor 

(called the “action spectra correction factor,” or CFas) to account for the greater proportional 

inactivation of the challenge microorganism that arises from the differences in the two action 

spectra. In the case of MS2 and B. subtilis, the ratio is close to one (1) and the correction is small 

(< 0.06). However, based on the data in Table D.2, if φx174 was used to show Cryptosporidium

inactivation, an action spectra correction factor of 1.16 would be needed with MP reactors. In 

other words, the φx174 RED would be divided by 1.16 to determine the RED used to calculate 

the validated dose (see Section 5.8). 

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-19 November 2006 

Table D.2. Germicidal Output Delivered to 14 Microorganisms by an MP Lamp 

Microorganism 

Type / Nucleic acid 

(SS = single strand, 

DS = double strand) 

Germicidal 

Output 

(W/cm) 

Germicidal Output 

Relative to 

Cryptosporidium 

(Action Spectra 

Correction Factor) 

Cryptosporidium oocysts Protozoa / DS DNA 5.64 1.00 

Vaccinia Animal virus / DS DNA 5.46 0.98 

B. subtilis spores Aerobic spore / DS DNA 5.58 0.99 

VSV Animal virus / RNA 5.53 0.99 

MS-2, R-17, fr, 7-S Bacteriophage / SS RNA 5.78 1.04 

T2 Phage / DS DNA 6.05 1.07 

EMC Animal virus / SS RNA 5.98 1.07 

φx-174 Bacteriophage / DS DNA 6.53 1.16 

Polyoma Animal virus / DS DNA 6.74 1.18 

Herpes simplex Human virus / DS DNA 7.00 1.26 

Reovirus-3 Animal virus / DS RNA 7.46 1.32 

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 

D.2. Thus, it is reasonable to assume that these microorganisms are acceptable as challenge 

microorganisms for many pathogens whose action spectra are 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, the correction factor can also 

be estimated by comparing the UV dose-response of the challenge microorganism to that of MS2 

measured with a LP and a MP lamp. The correction factor would be defined as: 

⎟

⎟

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎜

⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

=

2

04.1

LP MS

MP

LP challenge

MP

as

k

k

k

k

CF Equation D.4

 Where: 

CFas = Correction factor for the difference in action spectra between the challenge 

microorganism and MS2 (unitless) 

kMP = Slope of the UV dose-response measured with a MP collimated beam (cm2

/mJ) 

kLP = Slope of the UV dose-response measured with a LP collimated beam (cm2

/mJ) 

1.04 = Germicidal output of MS2 relative to Cryptosporidium, from Table D.2 

(unitless) 

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-20 November 2006 

For the Final LT2ESWTR 

Example D.8. A UV reactor is validated with the virus φx-174 (germicidal output relative to 

Cryptosporidium is 1.14). The reactor uses the UV Intensity Setpoint Approach and measures 

an RED of 25 mJ/cm2

 at the setpoint. In calculating the validated dose for Cryptosporidium or 

Giardia inactivation credit, the measured RED used to calculate the Validated Dose (discussed 

in Section 5.8.2) would have to be adjusted to 25 mJ/cm2

/1.14 = 22 mJ/cm2. 

NOTE: The correction factor described in this section is applicable only to MP reactors. 

It should be used if CFas ≥ 1.06. The correction factor that accounts for differences in the action 

spectra is not the same correction factor that accounts for differences in the microorganism UV 

sensitivities described in Section D.1 (UV dose-distribution impacts). The correction factor 

described in Section D.1, the “RED Bias,” applies to all UV reactors regardless of lamp type. 

D.4.2 Water Absorption of UV Light 

 

During UV reactor validation, a UV-absorbing chemical is added to the bulk flow passing 

through the reactor in order to simulate high-UV absorbance (low-UVT) events that could occur 

at a WTP. Common UV-absorbing chemicals currently in use for validation testing include 

lignin sulfonate, sodium thiosulfate, fluorescein, coffee, concentrated humic acids, 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 UV dose delivery and monitoring. Figure D.9 

illustrates UV spectra measured in waters at several different WTPs. 

Figure D.10 compares the UV absorbance spectra of coffee and lignin sulfonate to those 

of two drinking water sources (“Water A” and “Water B”). For a given UVT, the UV absorption 

at wavelengths above and below 254 nm is greater with coffee and lignin sulfonate than with the 

drinking water sources. If those chemicals are used during validation of a MP reactor, the RED 

and UV intensity values measured at a given flow rate, lamp output, and water UVT will be 

lower during validation than at the WTP (Wright et al. 2002). 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-21 November 2006 

Figure D.9. Spectral UV Absorption of Water at Various WTPs 

Figure D.10. Comparison of the UV Absorbance Spectra of Additives Used during 

UV Reactor Validation to the UV Absorbance of Two Finished Waters 

Lignin Sulfonic Acid 

The magnitude of the impact of this difference in the UV absorbance spectra on the 

measured UV intensity will depend on the location of the UV sensors relative to the lamps 

(Wright et al. 2002). Any new or alternative chemical used to alter UVT in the validation test 

should have its UV absorbance spectra evaluated and compared to natural waters. Significant 

differences will result in a proportionally large impact on RED and intensity readings during 

validation. 

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-22 November 2006 

D.4.3 Determining the Polychromatic Bias Factor (BPoly) 

 The term “polychromatic bias” refers to polychromatic differences between validation 

and operation of a UV reactor. UV reactors with MP lamps that were installed prior to the 

publication of this document may use non-germicidal sensors and, thus, may exhibit 

polychromatic bias. To account for polychromatic bias during validation testing, a polychromatic 

bias factor (Bpoly) should be incorporated into the Validation Factor: 

⎟

⎠

⎞ ⎜

⎝

⎛ +××= 100 1 Val

RED Poly

U BBVF Equation D.5 

See Section 5.9 for a complete discussion of the Validation Factor. 

Tables D.2 and D.3 should be used to estimate BPoly using the following validation testing 

information: 

• The UV-absorbing compound (coffee or LSA) 

• The minimum UVT tested (for all validation tests) 

• The lamp sleeve-to-sensor distance (i.e., water layer) 

Note that Tables D.2 and D.3 are for discreet values of UVT and lamp sleeve-to-sensor distance. 

The Polychromatic Bias Factor can be interpolated for intermediate values. 

Example D.9. An MP UV reactor with a non-germicidal sensor located 5 cm from the lamp 

sleeve is validated using coffee as a UV-absorbing chemical. The UV reactor is validated at a 

minimum UVT value of 85%. Using Table D.4, the polychromatic bias values at 85% UVT 

values is 1.12. 

Table D.3. Polychromatic Bias Values for an MP UV Reactor Using a 

Non-germicidal UV Sensor and Validated with LSA 

Water Layer Polychromatic Bias Values for a UVT of: 

(cm) 70% 80% 85% 90% 95% 98% 

2 1.00 1.00 1.00 1.00 1.00 1.00 

5 1.22 1.08 1.04 1.00 1.00 1.00 

10 1.74 1.38 1.25 1.13 1.05 1.02 

15 2.28 1.71 1.48 1.27 1.12 1.05 

20 2.76 2.07 1.74 1.42 1.18 1.07 

25 3.19 2.41 1.99 1.58 1.25 1.10 

Note: water layer = sensor to lamp distance

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-23 November 2006 

Table D.4. Polychromatic Bias Values for an MP UV Reactor Using a 

Non-Germicidal UV Sensor and Validated with Coffee 

Water Layer Polychromatic Bias Values for a UVT of: 

(cm) 70% 80% 85% 90% 95% 98% 

2 1.01 1.00 1.00 1.00 1.00 1.00 

5 1.57 1.22 1.12 1.05 1.01 1.00 

10 3.70 1.99 1.56 1.29 1.11 1.04 

15 9.42 3.42 2.25 1.61 1.22 1.08 

20 24.6 6.11 3.34 2.04 1.35 1.12 

25 64.3 11.0 5.11 2.61 1.50 1.16 

Note: water layer = sensor to lamp distance 

 In addition to the polychromatic bias that can occur from the use of non-germicidal 

sensors, polychromatic bias can occur when a germicidal sensor in an MP UV reactor is farther 

away from the lamp than the ideal location. In this case, the water layer can act as an optical 

filter, preferentially absorbing lower wavelength light and introducing polychromatic bias. The 

polychromatic bias exhibited by germicidal UV sensors that are further away from the lamp than 

the ideal location is not expected to be significant as long as the sensor is 10 cm or closer to the 

lamp (or further if the water being tested exhibits a UVT greater than 90%). This criterion is met 

for most MP reactors on the market at the time of manual publication and thus is not addressed in 

this manual. 

D.5 Analytical Foundation for UV Dose Monitoring and Defining 

Uncertainty 

 UV installations should be sized and operated in a manner that accounts for the 

measurement uncertainty associated with UV dose delivery monitoring. The objective of UV 

dose delivery monitoring is to indicate the level of inactivation of the target pathogen. This 

section derives a measurement equation for UV dose monitoring (Wright and Mackey 2003). 

This equation is used in this manual as the analytical foundation for defining the uncertainty of 

UV 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 D.6: 

p

p

p D

RED

I

10

log = Equation D.6 

 where: 

log Ip = Log inactivation of the pathogen 

REDp = RED of the pathogen (mJ/cm2) 

D10p = UV sensitivity of the pathogen (mJ/cm2 per log I) 

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-24 November 2006 

If the UV reactor delivers a UV dose distribution, the log inactivation of the pathogen is 

related to the inactivation of a challenge microorganism by substituting Equation D.1 into 

Equation D.6: 

p

c

RED

p D

RED

B

I

10

1 log = Equation D.7 

where: 

REDc = RED of the challenge microorganism (mJ/cm2

) 

Assuming the challenge microorganism RED is proportional to the measured UV 

intensity (REDc ∝ S), the log inactivation of the pathogen can be expressed according to 

Equation D.8: 

RED p

p D

S

B

I

10

1 log α = Equation D.8 

 where: 

S = UV intensity measured at the WTP with a duty UV sensor (mW/cm2

) 

α = Constant relating challenge microorganism inactivation to measured intensity 

(mJ/mW) 

The constant α is determined during validation as the ratio of the measured RED of the 

challenge microorganism to the measured UV intensity (REDc/S). Assuming that inactivation is 

proportional to flow rate (log Np ∝ Q), Equation D.9 can be used: 

Q

Q

S

S

D

RED

B

I v

vp

c

RED

p

10

1 log = Equation D.9 

 where: 

Sv = UV intensity measured during validation (W/cm2

) 

Qv = Flow rate measured during validation (mgd) 

Q = Flow rate measured at the WTP (mgd) 

Assuming the UV 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: 

⎟

⎠

⎞ ⎜

⎝

⎛ = c

oc c c N

N DRED 10 log Equation D.10 

 where: 

D10c = UV sensitivity of the challenge microorganism (mJ/cm2

 per log inactivation) 

No,c = Challenge microorganism concentration measured at the reactor influent 

(organisms/mL) 

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-25 November 2006 

Nc = Challenge microorganism concentration measured at the reactor effluent 

(organisms/mL) 

The UV sensitivity of the challenge microorganism (D10c) can be calculated according to 

Equation D.11 from the UV dose-response measured using the collimated beam apparatus: 

C

CB

c I

D

D

log 10 = Equation D.11 

 where: 

DCB = UV dose delivered by the collimated beam apparatus (mJ/cm2

) 

log IC = Log inactivation of the challenge microorganism observed with a UV dose of 

DCB

Substitution of Equations D.10 and D.11 into equation D.9 gives the equation for dose 

monitoring using the UV Intensity Setpoint Approach: 

( )

C

v CC

vp

CB

RED

p I

NN

Q

Q

S

S

D

D

B

I

log

1 log log ,0

10

= Equation D.12 

The uncertainty of dose monitoring arises from the uncertainties associated with each 

term in the measurement equation (Wright and Mackey 2003, Taylor 1982) and is accounted for 

by application of the validation factor (VF) in Section 5.9 and the application of quality 

assurance/quality control during operation of the reactor at the WTP. 

D.6 Considerations for CFD Modeling 

 

CFD UV dose modeling of the impact of UV reactor inlet and outlet conditions on RED 

could be used in conjunction with one of the approaches outlined in Section 3.6. to assess 

whether UV dose delivery at the WTP installation is equal to or better than UV dose delivery 

achieved during validation. However, several issues with a CFD-based approach should be 

considered: 

• There is little agreement on appropriate procedures for assessing the credibility of 

CFD models. 

• CFD models for prediction of UV dose delivered by a reactor comprise coupled submodels for turbulent flow, microbial transport, UV intensity, and microbial 

inactivation. Many options and approaches are available for each sub-model. 

Currently, no consensus has been reached for which approaches are most suitable for 

predicting UV dose delivery in a full-scale reactor. 

• CFD modeling of UV dose delivery requires a multi-disciplinary approach. 

Knowledge of fluid mechanics, light physics, microbial inactivation, numerical 

modeling, and UV process engineering is essential for credible CFD modeling of UV 

For the Final LT2ESWTR 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-26 November 2006 

For the Final LT2ESWTR 

dose delivery. The pool of this type of integrated expertise is currently limited, which 

presents a challenge for states tasked to review CFD modeling reports. 

A generalized modeling approach for predicting UV dose delivery involves the 

following: 

1. Construct a 3-D computational model of the UV system, including all major 

components that influence the flow patterns in the reactor. This includes resolution of 

all wetted surfaces in the reactor and the upstream/downstream piping systems. 

2. Perform a steady-state CFD simulation by solving governing flow equations 

(i.e., Navier-Stokes and turbulence equations). This results in a prediction of point 

velocities across the interior of the UV system for the specified inlet flow rate. 

3. Perform a UV intensity simulation for the UV system using a UV light intensity 

model. This results in a prediction of point UV intensity values across the interior of 

the UV system for specified values of UV lamp intensity and UVT. 

4. Perform a particle tracking simulation using the combined numerical flow/UV 

intensity field. A random walk or particle physics model may be employed. Hundreds 

of numerical particles are randomly “injected” at the model inlet, and their x,y,zcoordinates are predicted as a function of time. The result is a predicted path line for 

each injected particle, which represents a random microbial path through the reactor. 

5. Calculate the estimated UV dose for each injected particle by summing the 

cumulative UV dose at a series of points along the predicted particle path. The result 

is a UV dose distribution. 

6. Determine the log inactivation and RED for a microorganism with known UV 

inactivation kinetics based on the UV dose distribution calculated in Step 5. 

If CFD is applied for simulation of UV dose delivery, it should adhere to the following 

guidelines: 

1. Only a qualified party with appropriate expertise should develop a CFD-based 

hydraulic or full UV reactor performance model. Such parties could include a 

professional engineer with extensive modeling experience, a CFD consulting firm, or 

a manufacturer with review by an independent CFD consultant. 

2. The same overall modeling approach and sub-models should be used for both the 

validation site model and the WTP model. At a minimum, the following QA/QC 

procedures should be used during CFD model development and execution: 

• The density of the numerical grid and size of the time step used in simulations 

affect CFD results. In general, results become more accurate as the grid becomes 

finer and the time step becomes smaller. Grid and time-step convergence analysis 

should be performed to verify that grid and time-step sizes are sufficiently 

resolved such that smaller grid and time step sizes do not change predicted results. 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-27 November 2006 

For the Final LT2ESWTR 

Procedures for this analysis are presented in the Guide for the Verification and 

Validation of Computational Fluid Dynamics Simulations (AIAA 1998). 

• Numerical convergence and consistency of the CFD models should be verified 

and documented. Procedures for this analysis are presented in the abovereferenced AIAA guide. 

• A sensitivity analysis of the major parameters that affect UV-dose prediction 

should be conducted. Examples include (but are not limited to ) boundary 

conditions for lamp UV output and reactor wall reflection, number of particles 

used in a microbial transport simulation, and UV dose-response inactivation 

constants. 

3. CFD models should not be calibrated with experimental RED data for the purposes of 

obtaining agreement between model predictions and field measured values. 

Calibration to RED data for a limited set of conditions does not necessarily improve 

the accuracy of future predictions, particularly because hydraulic conditions can 

greatly differ between the validation site and the WTP installation. 

4. Error estimates and confidence intervals for the CFD model predictions should be 

developed for both the validation site and the WTP installation. This could be 

performed by comparing CFD model predictions and experimental data for the 

validation site, then assuming the same level of error for the CFD model prediction 

for the WTP installation. 

Following the above guidelines, CFD can be used to predict the relative difference in 

RED between a validation site and a WTP installation. If analysis indicates that UV dose 

delivery is better at the WTP, RED credit should only be granted for the experimentally 

measured RED from the validation site. 

CFD-based UV dose modeling should not be used in lieu of validation for prediction of 

the actual RED magnitude as a means of granting pathogen inactivation credit. As discussed 

previously, CFD is still an emerging technology, and CFD models for UV dose delivery are 

complex. Uncertainty and error ranges for these models are not known. CFD-based UV dose 

delivery models would need to undergo a formal industry-wide verification and validation 

process before they could be considered suitable for extrapolation of data for establishing 

inactivation credit. A possible approach for verification and validation of hydraulic CFD models 

is outlined in the AIAA CFD guide (1998). 

It is anticipated that CFD models for UV dose prediction will develop and improve in the 

future. This manual is not intended to be the final word on CFD modeling for UV disinfection. 

Engineers, regulators, and manufacturers should also consult with the AIAA manual and future 

CFD guidance that may arise in the water industry. 
Appendix D. Background to the UV Reactor Validation Protocol 

UV Disinfection Guidance Manual D-28 November 2006 

For the Final LT2ESWTR 

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Appendix E 

UV Lamp Break Issues 
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Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-1 November 2006 

For the Final LT2ESWTR 

The lamps in most UV reactors designed for water disinfection contain mercury or an 

amalgam of mercury and another element, such as indium or gallium. Mercury has properties 

that allow for cost-effective generation of UV light. These properties include a sufficient vapor 

pressure at ambient temperatures to provide for efficient production of resonance radiation and a 

low ionization energy to facilitate starting a lamp (Phillips 1983). Lamp manufacturers are 

continuing to reduce the mercury content of UV lamps (USEPA 1997b; Walitsky 2001). 

However, mercury-free lamps, such as pulsed UV lamps containing xenon, are not widely used 

for water disinfection at present. 

The mercury contained within a UV lamp is isolated from exposure to water by the lamp 

envelope (referred to as the “lamp” in this appendix for simplification) and a surrounding lamp 

sleeve (Figure 2.13). However, breakage of a UV lamp creates the risk of exposure to mercury, 

which can cause adverse health effects (USEPA 2006). 

Although UV disinfection utilizes UV lamps with mercury, UV disinfection is an 

important disinfection technology that provides additional public health protection. To date, 

there have been few lamp breaks at existing UV facilities. The risk to human health and the 

environment from the mercury in UV lamps used in the treatment of drinking water is very 

small. It can be addressed through engineering and administrative methods used to prevent UV 

lamp breaks and exposure to mercury if breaks do occur, as described in this appendix. 

This appendix discusses the issues associated with breaks of UV lamps used for drinking 

water disinfection. Lamp breaks are divided into off-line and on-line breaks. Off-line breaks 

occur when the lamps are not installed in the reactor or when the reactor is not in operation. Online lamp breaks occur when the lamp and lamp sleeve break during reactor operation. 

Sections E.1.1, E.2.1, and E.2.2 address potential causes of lamp breaks (including 

known occurrences) and corresponding preventive measures. Sections E.2.3 and E.2.4 address 

containment of mercury after a break and suggest components of a lamp-break response plan. 

Regulatory issues associated with lamp breaks, including lamp disposal, are discussed in 

Section E.3. Mercury in UV disinfection facilities and documented mercury reactions in PWS, is 

discussed in Sections E.4 and E.5. A summary of the information presented in this appendix is 

located in Section E.6. References for this appendix are presented in Chapter 7. 

E.1 Off-line Lamp Breaks 

Off-line breaks occur when a lamp breaks during shipping, handling, storage, or 

maintenance. Off-line breaks also can occur when the lamp and the lamp sleeve break in a UV 

reactor that is not in operation. Because water is not flowing through the reactor, off-line breaks 

do not pose a hazard to the water consumer but may be a hazard to operators or employees in the 

vicinity of the break. 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-2 November 2006 

For the Final LT2ESWTR 

E.1.1 Potential Causes of Off-line Lamp Breaks and Corresponding Prevention 

Measures 

Off-line lamp breaks are caused by improper handling. The UV manufacturer should 

train operators in proper handling and maintenance of UV lamps. In addition, lamps should be 

stored horizontally in individual packaging to reduce the potential for lamp breaks. Lamps 

should not be stacked unpackaged or propped vertically in corners (Dinkloh 2001). 

E.1.2 Off-line Mercury Release Cleanup Procedures 

Water systems should have a lamp break response plan for containing and cleaning the 

off-line spills. The local poison control center, fire department, or public health board can assist 

in determining appropriate responses for different spill sizes and in developing a plan. 

Small spills can be contained and collected with commercially available mercury spill 

kits. Small spills are defined as the amount of mercury in a broken thermometer, or less than 2.25 

grams (g) (USEPA 1992, USEPA 1997a). Given that the mercury content in a single UV lamp 

typically ranges from 0.005 – 0.4 g (discussed in Section E.4.2), a single lamp break and 

multiple lamp breaks that result in release of less than 2.25 g are categorized as small spills. 

Mercury and materials used during the cleanup procedure are regulated as hazardous 

wastes and should be disposed of properly as described in Section E.3.3. EPA’s Office of 

Superfund Remediation and Technology Innovation (formerly the Office of Emergency and 

Remedial Response) recommends that “...[in] 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). 

E.2 On-line Lamp Breaks 

On-line lamp breaks occur when a lamp and lamp sleeve break while water is flowing 

through the reactor. These breaks may have the potential to pose a hazard to the water consumer, 

as well as to operators or employees in the vicinity of the break. This section discusses potential 

causes of on-line lamp breaks, prevention measures, and documented occurrences of on-line 

lamp breaks and mercury release. 

E.2.1 Potential Causes of On-line Lamp Breaks and Corresponding Prevention 

Measures 

Lamp breaks can be caused by debris in the water, improper UV reactor orientation, 

water temperature variations, exceeding positive or negative pressure limits (water hammer), 

electrical surges, or improper maintenance. Lamps may also break as a result of manufacturing 

defects in the lamp or improper selection of the lamp or sleeve material. 
Appendix E. UV Lamp Break Issues 

E.2.1.1 Debris 

Debris may originate from the raw water or from treatment process equipment. Although 

most UV reactors will be installed after the filters in the treatment train, upstream equipment 

may release parts or fragments such as nuts or bolts that can break the lamp sleeves or UV 

lamps. Unfiltered systems may be more prone to debris because there is minimal upstream 

treatment to UV disinfection, which should be considered in the UV facility design. 

Groundwater systems have the potential to pull stones or gravel from wells that can enter UV 

reactors and break lamps (Malley 2001, Roberts 2000). 

The consideration of prevention measures may be beneficial if debris historically has 

occurred prior to installation of the UV facility. Placement of screens, baffles, or low velocity 

collection areas upstream of UV reactors or vertical orientation (i.e., vertical flow of water 

through the UV reactor) may reduce the risk of debris entering the reactor (Cairns 2000, Malley 

2001, McClean 2001b). Note that the lamps should be oriented horizontally relative to the 

ground even if the UV reactor is installed with water flowing vertically as discussed in 

Section E.2.1.2. The extent of containment these safety measures provide is unknown. Water 

systems and designers should determine the applicability of these techniques on a site-specific 

basis. 

E.2.1.2 Improper UV Lamp Orientation 

The orientation of UV lamps within a UV reactor can also increase the potential for lamp 

breaks. Orienting lamps perpendicular to the ground can result in differential heating of the lamp 

and the sleeve, which can lead to eventual cracking of the lamp and sleeve. As such, regardless 

of the direction of water flow relative to the ground, UV lamps should be oriented parallel and 

not perpendicular to the ground (Figure E.1). 

Figure E.1. Example of Proper Horizontal and Improper Vertical 

UV Lamp Orientation in Reactors Relative to the Ground Water Flow Water Flow

Horizontal 

Orientation

(recommended)

Vertical

Orientation

(not recommended)

Lamp 

Sleeve

Lamp 

Installation

Piping

UV 

Reactor

Water

Flow

Water

Flow

Horizontal 

Orientation

(recommended)

Vertical

Orientation

(not recommended)

Lamp 

Sleeve

Lamp 

Installation

Piping

UV 

Reactor

UV Disinfection Guidance Manual E-3 November 2006 

For the Final LT2ESWTR 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-4 November 2006 

For the Final LT2ESWTR 

E.2.1.3 Loss of Water Flow and Temperature Increases 

UV lamps are designed to operate within a specific temperature range to maximize UV 

light output. Without flowing water to cool the lamp, the lamp temperature can rise above the 

maximum operating temperature and break (Dinkloh 2001, Malley 2001, Srikanth 2001a, 

Srikanth 2001b). There are two conditions that may cause overheating of lamps: 

• Operating UV lamps while there is no water in the UV reactor (i.e., the lamp is in air) 

• Operating UV lamps while water is not flowing through the UV reactor (i.e., the 

water in the UV reactor is stagnant) 

Overheating occurs much faster in air than in stagnant water and is more likely to occur 

with medium-pressure (MP) than low-pressure (LP) lamps (due to lamp operating temperatures). 

If UV lamps are energized in air, the lower temperature water entering the reactor may cause the 

lamp sleeve and the lamp to break due to temperature differentials (Dinkloh 2001, Malley 2001), 

even if upper temperature levels are not exceeded. Lamp overheating and temperature 

differentials could, therefore, break all the lamps within the affected reactor. 

Operating a UV lamp in stagnant water can also cause lamps to overheat and break. 

Water flow during UV start-up (i.e., cooling water) cools the lamps and prevents lamps from 

overheating and breaking. However, cooling water may not be necessary with low-pressure 

high-output (LPHO) lamps (Haubner 2005). Whether cooling water is needed depends on the 

specific MP reactor manufacturer, and the manufacturer should be contacted to determine this 

(Leinberger 2005, Larner 2005, Bircher 2005). 

To prevent lamp breaks, operating procedures should ensure that the following conditions 

are met: 

• The lamps are not operating while the reactor is not full (i.e., while air is in the 

reactor). 

• If recommended by the UV manufacturer, water should be flowing through the UV 

reactor if the UV lamps are operating. 

Hydraulics should be designed so that lamps are submerged at all times during reactor 

operation. UV facility designs should also 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. UV equipment should and typically does include temperature sensors and alarms that 

automatically shut down the reactor before critical temperatures are exceeded (Leinberger 2005, 

Larner 2005, Bircher 2005, Dinkloh 2001, Malley 2001, Srikanth 2001b). 

E.2.1.4 Pressure-related Events 

Hydraulic pressures that exceed the operating limits of the lamp sleeves may break them. 

Although breaking the lamp sleeve does not automatically break the lamp, the lamp is more 

vulnerable when the sleeve has been damaged, potentially allowing the hot lamp to come into 

direct contact with colder surrounding water. 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-5 November 2006 

For the Final LT2ESWTR 

Most lamp sleeves are designed to withstand continuous positive pressures of at least 

120 pounds-force per square inch gauge (psig) (Roberts 2000, Aquafine 2001, Dinkloh 2001, 

Srikanth 2001a, Srikanth 2001b). However, negative gauge pressures below −1.5 psig have been 

shown to adversely affect lamp sleeve integrity (Dinkloh 2001). The pressure tolerance of the 

quartz lamp sleeve varies and depends on the quality, thickness, and length of the sleeve. 

Positive and negative pressures that exceed these levels, such as those associated with water 

hammer, may cause the lamp sleeve to crack or break. 

Thus, water hammer can potentially break all lamps within an affected reactor. The water 

system should perform a surge analysis to determine if water hammer is a potential problem, and 

the UV facility designer should specify the pressure and flow ranges expected. The manufacturer 

should provide lamp sleeves with the appropriate material, thickness, geometry, and seals for the 

specified pressure and should provide the water system with the lamp sleeve pressure tolerances. 

E.2.1.5 Handling and Maintenance Errors 

A lamp or lamp sleeve damaged by improper off-line handling or maintenance may break 

when the UV reactor is returned to service. For example, over-tightening compression nuts when 

securing the lamp sleeve can cause a fracture of the lamp sleeve or a leak around the sleeve or 

compression nut cavity (Aquafine 2001, Dinkloh 2001, Srikanth 2001a, Srikanth 2001b, Swaim 

et al. 2002). This problem may not become apparent until after start-up of the UV reactor and 

may cause a lamp break. Operation and maintenance training can help prevent these types of 

lamp breaks. 

E.2.1.6 UV Reactor Manufacturing Problems 

The UV reactor manufacturer should design the UV reactor to reduce the possibility of 

lamp breaks. This section describes UV manufacturing problems that may cause lamp breaks if 

not properly addressed. Addressing these UV reactor manufacturing issues is typically the 

responsibility of the manufacturer. However, some causes of lamp breaks can be mitigated 

during the design of the UV facility. 

Electrical Considerations 

If the UV facility 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 produce voltages exceeding lamp ratings (overdriving lamps) may also cause the lamp to 

break (Malley 2001). 

To reduce the likelihood of these problems, the UV facility designer should specify 

circuit/ground fault interrupters (GFI) in the UV facility electrical design. In addition, 

replacement UV lamps should be electrically compatible with the UV equipment. 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-6 November 2006 

For the Final LT2ESWTR 

Cleaning Mechanism Considerations 

The cleaning mechanism may break the lamp sleeve and lamp if the mechanism 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. 

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 2001). Some UV reactors are not subject to 

this problem because the wipers rest away from the lamp sleeve when not in use and an alarm 

sounds when the wiper stops along the lamp sleeve. 

 Once the UV facility is in operation, the operators should perform routine inspections of 

the inside of the UV reactors to ensure that the cleaning mechanism is not fused to the sleeve. 

Thermal Expansion and Contraction 

Other potential causes of lamp breaks include improper matching of lamp materials with 

respect to thermal expansion characteristics. Manufacturers should use compatible materials 

within the lamp to avoid stress and damage from thermal expansion and contraction differences 

between materials that can occur under various operating, shipping, or handling conditions 

(Cairns 2000). In addition, improper seal design or lamp swelling can cause water leaks around 

the seals that can result in electrical shorts and cracking of lamps (Cairns 2000). 

The UV facility designer should specify the temperature ranges likely to be encountered 

during shipping, storage, and lamp operation in the UV equipment procurement documents so 

the manufacturer can select the appropriate materials. 

E.2.1.7 Summary of Potential Causes and Methods of Prevention of On-line UV 

Lamp Breaks 

Table E.1 summarizes the potential causes of on-line lamp breaks and briefly describes 

the preventive measures that UV facility designers and operators can implement to reduce each 

risk. Documented cases of on-line lamp breaks are discussed in Section E.2.2. 

E.2.2 On-line Lamp Break Incidents 

Relatively few incidents of on-line lamp breaks with mercury release have been 

documented. A literature review was conducted to compile information on UV lamp breaks in 

operating UV facilities. Several facilities were contacted for more information about the 

incidents. Although all documented lamp breaks involved MP lamps, some of the causes 

reported for the lamp breaks are independent of the lamp type (Malley 2001). The documented 

lamp break incidents, categorized according to the cause of the incident, are summarized in 

Table E.2. 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-7 November 2006 

For the Final LT2ESWTR 

Table E.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. 

Lamp Orientation • Vertical installation relative to the 

ground may cause overheating and 

lamp breaks. 

• Install reactors with lamps oriented parallel 

to the ground to reduce differential heating. 

Loss of Water 

Flow and 

Temperature 

Increases 

• Lamps may overheat and break. 

• The temperature differential 

between stagnant water or air and 

flowing water (upon resumption of 

flow) may cause lamp breaks. 

• Reactors should always be completely 

flooded and flowing during lamp operation. 

Temperature and flow sensors that are 

linked to an alarm and automatic shutoff 

system can be used to indicate irregular 

temperature or flow conditions. 

Pressure-related 

Events 

• Excessive positive or negative 

pressures may exceed lamp sleeve 

tolerances and break the lamp 

sleeve. 

• A surge analysis should be completed 

during design 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. 

Maintenance and 

Handling 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. 

• Electrical surges can cause shortcircuiting and lamp socket damage. • Adequate circuit breakers/GFI should be 

specified to prevent damage to the reactor. 

UV Reactor 

Manufacturing 

Problems • Applying power that exceeds 

design rating of lamps can cause 

lamps to burst from within. 

• Replacement lamps should be electrically 

compatible with reactor design. 

• Misaligned or heat-fused cleaning 

mechanism may break or damage 

the lamp sleeve and lamp. 

• 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. 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-8 November 2006 

For the Final LT2ESWTR 

Table E.2. Mercury Release Incidents Involving UV Lamp Breaks 

Identified 

Cause 

Number of 

Incidents Description of Incident 

Debris 5 (4) Stones entered the reactors and struck the lamps.1

(1) Gravel entered the reactor through the booster pump and struck 

the lamp.2

Loss of Water 

Flow and 

Temperature 

2 (2) Lamps were left on and allowed to reach high temperatures 

[600 degrees Centigrade (o

C)] in empty non-operating reactors.1 

Restoration of flow caused cooler water (20 o

C) to break the lamps. 

Operator Error 1 (1) Forklift collided with on-line reactor.3

Manufacturer 

Design 

7 (1) Applied power exceeded the tolerances of the lamp, causing the 

lamp to burst from within.1

(2) Vertical orientation of lamps in the reactor resulted in differential 

heating and eventual cracking of the lamp and sleeve because heat 

accumulated at the tops of the lamp and sleeve.1

 

(1) High operating temperatures resulted in deformation of the lamp 

sleeve. The lamp sleeve sagged and on contact with the lamp, both 

the lamp and lamp sleeve broke.4

(1) Manufacturing defect. Lamps exploded after approximately 300 

hours of operation.5

(2) Contaminated quartz material used by lamp manufacturer.6

1

 Survey of European water and domestic wastewater and hazardous waste treatment systems (Malley 2001) 2

 European drinking water systems (Roberts 2000) 

3

 European brewery (Roberts 2000) 

4

 UV-peroxide groundwater remediation reactor (Moss 2002a) 

5

 Drinking water system (Region of Waterloo 2004) 

6

 Drinking water system (Wright 2005) 

Impacts from debris caused five of the documented lamp breaks. In four of the five 

incidents reported, UV lamps were oriented perpendicular to the flow of water, indicating that 

lamps in this orientation may be more vulnerable to lamp breaks. However, the lamps in one 

instance were parallel to the flow of water, so orientation alone will not prevent lamp breaks. 

An additional incident involving debris occurred when a bolt from the filter underdrain 

broke a lamp sleeve. The lamp was not broken by the bolt, and mercury was not released because 

the UV equipment was immediately shut down to respond to the sleeve break (McClean 2001a). 

Because no mercury was released, the incident is not included in Table E.2; however, this 

incident indicates that equipment debris can also be hazardous. 

Loss of water flow and the resulting increase in lamp temperature caused two of the 

documented lamp breaks. In these cases, the operating lamps reached extremely high 

temperatures (> 600 °C) in air. When water flow resumed, the cooler water (20 °C) caused the 

lamps to break (Malley 2001). These incidents can be prevented if UV equipment has a safety 

mechanism that will shut down the UV lamps if flow decreases or lamp temperature significantly 

increases (Malley 2001). 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-9 November 2006 

For the Final LT2ESWTR 

Operator error caused one of the documented lamp breaks. 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 mercury release (Roberts 2000). 

The seven remaining lamp breaks are attributed to improper manufacturer design. In one 

of the lamp breaks, 30-kilowatt (kW) power was specified for the application. However, a 

manufacturing error resulted in a higher power being applied and caused the lamp to burst from 

within (Malley 2001). 

Another manufacturer design problem that resulted in two breaks was vertical orientation 

of the lamps within the UV reactor. The vertical orientation allowed heat to accumulate at the 

tops of the lamp and sleeve, which caused them to break (Malley 2001). It is worth noting that 

modern UV reactors do not mount lamps vertically, even in vertically oriented reactors such as 

those discussed in Section E.2.1.2. 

Another lamp break attributed to a manufacturer design flaw resulted from deformation 

of the lamp sleeve at operating temperatures. The incident occurred in a UV-peroxide reactor 

designed for well-head treatment of tetrachloroethene-contaminated groundwater (Moss 2002a). 

The UV reactor was positioned between the groundwater extraction pump and the distribution 

system booster pumps. The 7-foot long MP lamp sleeve sagged and came into contact with the 

lamp. The lamp and lamp sleeve broke, releasing mercury. The lamp failure triggered an alarm, 

shutting down both the groundwater extraction and distribution system booster pumps. Liquid 

mercury was found on the bottom of the reactor. Water samples taken at a nearby fire hydrant 

were positive for mercury but were below the maximum contaminant level (MCL) of 

2 micrograms per liter (μg/L) (Moss 2002a, Moss 2002b). 

Similar to the prior incident, a manufacturing defect in an MP lamp caused several lamps 

at a WTP to explode after approximately 300 hours of operation, releasing mercury into the 

water (Tramposch 2004). The break occurred in a large 47-inch diameter horizontal reactor. The 

lamp failure alarm triggered closure of the reactor isolation valves within 90 seconds and 

initiated automatic flushing of the clearwell. The quartz fragments and 64 percent of the mercury 

were recovered in the bottom of the reactor. The baffles within the reactor appear to have 

prevented the mercury and quartz from leaving the reactor after the break. The flushed clearwell 

water was sent to an on-site holding tank where it was tested for mercury, which was not 

detected. Mercury was also not detected in the piping between the UV reactors and the clearwell 

and in the UV reactor drain water. The water system believes that the remainder of the mercury 

was fused to the reactor walls because mercury was not discovered downstream, and mercury 

vapor was detected in the UV reactor when the hazardous materials (HazMat) contractor was 

cleaning the UV reactor. 

Twenty four hours after the reactor was drained in response to the lamp break, mercury 

vapor concentrations within the reactor exceeded health and safety limits (Section E.3.2), 

although mercury vapor concentrations in the ambient air surrounding the UV reactor were not 

above these safety limits. During all stages of the cleanup operation, the HazMat contractor 

ensured that the area was well-ventilated and monitored for mercury vapor. As a result of this 

incident, the water system observed that cleaning the reactor quickly is imperative because 

mercury vapor can accumulate in the reactor (Region of Waterloo 2004). 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-10 November 2006 

For the Final LT2ESWTR 

The two final documented lamp breaks occurred because of the use of contaminated 

quartz material by the lamp manufacturer. The contamination weakened the protective quartz 

sleeve and the lamp, resulting in breaks at two water treatment plants (WTP). 

E.2.3 Design Considerations for Containment after a Lamp and Sleeve Break 

This section briefly describes potential methods to contain mercury from a lamp break. 

However, the extent of containment provided by these measures is unknown. Water systems and 

designers should determine the applicability of these isolation techniques on a site-specific basis 

and include the specific steps to be taken in the water system’s response plan. 

To isolate the mercury in the reactor or downstream, water systems may install springreturn actuated valves with a short closure time on the reactor inlet and outlet piping (McClean 

2001b). Given the short residence time of many MP reactors, the outlet-side valve should be 

located far enough downstream so that the valve has time to close and isolate the mercury 

upstream. UV facility designers should evaluate valve closure times with respect to the potential 

for water hammer. 

Condensed mercury and quartz fragments may be contained and collected in areas of low 

water velocity such as the bottom of a shut-down reactor, sumps, or a clearwell. To prevent 

quartz fragments from entering the water system, a strainer can be installed on the reactor outlet 

piping (McClean 2001b, Srikanth 2001a, Srikanth 2001b). Another option is to include a 

mercury trap in the design (Figure E.2). A mercury trap could include a tee fitting after the UV 

reactor. Flow will enter the tee and flow upward. The tee may also include an elbow that is 

sealed but accessible. If the water velocity in the tee fitting and following pipe is low enough, 

some of the mercury and quartz fragments may settle out in bottom of the elbow (Mutti 2004). 

The head loss associated with such measures should be considered in the hydraulic profile. 

Designers should also consider installation of drains and piping to allow disposal of potentially 

contaminated water from the reactor or trap to a waste container or truck. The effectiveness of a 

strainer and mercury trap has not been evaluated and is unknown.

E.2.4 On-line Lamp Break Response Plan 

On-line lamp breaks should be preventable with appropriate design and operation of UV 

reactors. However, water systems should develop a written lamp break response plan in case an 

on-line UV lamp break occurs. Water systems should coordinate with their state when 

developing the following plan components: 

• Identification of a lamp break 

• Site-specific containment measures 

• Mercury sampling and compliance monitoring 

• Site-specific cleanup procedures 

• Reporting requirements 

• Public notification requirements 
Appendix E. UV Lamp Break Issues 

Figure E.2. Example of a Potential Mercury Trap* 

Water

Flow

Mercury and quartz 

may collect in the 

bottom of the elbow*

Blind flange allows for 

the removal of the 

mercury and quartz

UV Reactor

Isolation 

Valve

*The effectiveness of the mercury trap has not been evaluated and is unknown.

Water

Flow

Mercury and quartz 

may collect in the 

bottom of the elbow*

Blind flange allows for 

the removal of the 

mercury and quartz

UV Reactor

Isolation 

Valve

*The effectiveness of the mercury trap has not been evaluated and is unknown.

Identification of a Lamp Break

UV reactors should be equipped with alarms that are activated when a lamp fails. A lamp 

failure alarm may be due to a lamp break or to another problem. Because alarms associated with 

lamp failure and GFIs may be due to lamp and sleeve breaks, the UV equipment should be shut 

down, isolated in response to these alarms, and inspected to determine whether a lamp break 

occurred. 

Site-specific Containment Measures

In the event of a lamp failure alarm, the UV reactor should be immediately shut down, 

and operators should assume a lamp break has occurred and implement the procedures to contain 

the mercury while determining the cause of the alarm. The containment procedures should be 

outlined in detail in the water system’s response plan based on the specific UV facility and any 

containment measures included in the design. 

Mercury Sampling 

 Mercury sampling should be implemented after an on-line UV lamp break. Sampling 

procedures should specify sample locations, frequencies, and analysis methods. Sampling 

frequencies should consider flow rate, detention time, and travel time to the first potential 

consumer. Sample locations should be chosen based on where the mercury may settle (e.g., low 

velocity areas) and where mercury vapor may accumulate (e.g., a drained UV reactor). Table E.3 

lists some possible sample locations (Region of Waterloo 2004, Stantec 2004). 

UV Disinfection Guidance Manual E-11 November 2006 

For the Final LT2ESWTR 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-12 November 2006 

For the Final LT2ESWTR 

Table E.3. Mercury Sampling Locations 

Media Location Purpose 

Water • Reactor drain 

• Piping downstream of the UV 

reactor, including the distribution 

system entry point at a minimum 

• Low velocity areas, such as 

clearwells 

• Assess the extent of mercury 

contamination and identify areas requiring 

cleanup. 

Air1 • Reactor or other locations where 

mercury vapor may collect 

• Ambient air 

• Assess whether it is safe to access 

mercury-contaminated equipment and 

piping for cleanup. The UV reactor interior 

may be accessible through an air vent. 

• Assess whether adequate ventilation is 

provided to safely proceed with mercury 

cleanup. 

1

 Methods for air sampling are available from the Occupational Safety and Health Administration (OSHA) at 

http://www.osha.gov/dts/sltc/methods/inorganic/id140/id140.html. 

Site-specific Cleanup Procedures

Site-specific cleanup procedures should be incorporated into the water system’s response 

plan. Issues to consider are assessment of mercury contamination in the air, water, or on 

surfaces, disposal of any isolated or condensed mercury, potential disposal or treatment of 

contaminated water, cleanup responsibilities (by water system staff or contracted hazardous 

materials team), and Federal or state cleanup or disposal requirements. 

An example of a currently operating UV facility’s site-specific clean-up procedures is 

summarized below. The procedure includes the following major steps (Stantec 2004): 

1. Hydraulically isolate the UV reactor. 

2. Ventilate the area and shut down ventilation equipment that circulates air to other 

parts of the building. 

3. Wear personal protective equipment, including gloves, eye protection, suits, shoe 

covers, and breathing protection. 

4. Drain water from the reactor through a mesh filter into a tank for disposal. 

5. Measure the mercury vapor concentration within the reactor and ensure that it is at an 

acceptable level (limits shown in Section E.3.2). 

6. Open the reactor and remove quartz and mercury from the reactor using a mercury 

spill kit. 

7. Perform a mass balance to assess how much mercury has been recovered. 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-13 November 2006 

For the Final LT2ESWTR 

Reporting and Public Notification Requirements 

The water system should determine any reporting and public notification requirements by 

coordinating with the state. If reporting or public notification is required, the response plan 

should include the information that must be reported to the state and the notification procedures. 

Reporting requirements may include a description of the release, estimated quantity of the 

release, shut-down or containment procedures, cleanup or disposal methods, and sampling 

procedures (including sampling locations, frequencies, and results). 

E.3 Regulatory Review 

This section presents a review of regulations that may apply if UV lamp breaks occur at a 

WTP. 

E.3.1 Safe Drinking Water Act 

Under the Safe Drinking Water Act (SDWA), EPA established a primary MCL of 2 μg/L 

for inorganic mercury [40 CFR 141.62(b)] and the associated monitoring requirements. The limit 

was designed to protect against mercury contamination in the source water and not a transient 

event like lamp breaks. Consequently, the water system should contact the state to determine 

whether additional mercury monitoring will be required in response to lamp breaks. 

E.3.2 Operator Health and Safety – Exposure Limits 

Mercury exposure to employees in WTPs falls under the regulatory authority of OSHA. 

The exposure limits set by OSHA focus on exposure through 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 that is not to be exceeded 

for an 8-hour workday during a 40-hour workweek. 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 E.4 lists the PELs, 

cPELs, and IDLHs for mercury compounds and organo alkyls containing mercury. 

In the event of a spill, the volatilization and the resultant mercury vapor concentration depends 

on air currents, temperature, surface area/dispersion of mercury droplets, and time. If a mercury 

spill is not cleaned up promptly, the levels in Table E.4 may be exceeded where mercury vapor 

collects (e.g., drained UV reactor). For example, in the lamp break described in Section E.2.2, 

these limits were exceeded within the reactor 24 hours after the reactor was drained. However, 

prompt response and proper cleanup procedures (e.g., ventilation and other measures described 

in Section E.2.4) should prevent exposure levels over these standards.
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-14 November 2006 

For the Final LT2ESWTR 

Table E.4. Health and Safety Standards for Mercury Compounds in Air 

Compound 

OSHA PEL 

(mg-Hg/m3

)

1

OSHA cPEL 

(mg-Hg/m3

) 

NIOSH IDLH 

(mg-Hg/m3

) 

Mercury compounds NR 0.1 10 

Organo alkyls containing 

mercury 

0.01 0.04 2 

NR – not reported 

1

 milligrams mercury per meter cubed 

E.3.3 UV Lamp Disposal Regulations 

UV lamps must be disposed of properly, as described in Section 6.3.2.6 and should be 

recycled. Some UV reactor and lamp manufacturers will accept spent or broken lamps for 

recycling or proper disposal (Dinkloh 2001, Leinberger 2002, Gump 2002). Alternatively, water 

systems should contact their state primacy agency or other local or state resource agencies for a 

list of local mercury recycling facilities. 

E.3.4 Clean Water Act 

Mercury discharges to water bodies in the United States are regulated under the Clean 

Water Act. Mercury-contaminated water from a lamp break should not be discharged to the 

environment through storm sewers or other means; discharges should be coordinated with the 

state and the local wastewater authority for proper treatment and disposal. 

E.4 Mercury in UV Disinfection Facilities 

Understanding the type of mercury and amount of mercury present in UV disinfection 

facilities can help determine the potential dispersion and transport of mercury through a WTP. 

However, the fate and transport of mercury after a lamp break has not been assessed by the 

drinking water industry. 

E.4.1 Type of Mercury in UV Disinfection Facilities 

Characterizing the form of mercury in an operating lamp is important because this form 

represents the starting point for mercury dispersion, speciation, and reaction chemistry in the 

water following a lamp break. Mercury in an LP or MP UV lamp is pure elemental mercury 

while LPHO lamps use a mercury amalgam, which typically is an alloy with indium. 

Elemental mercury is usually a liquid at ambient temperature and pressure. However, 

given its vapor pressure (Table E.5), elemental mercury can vaporize at ambient temperatures. 

Other physical and chemical properties of elemental mercury that affect its fate and transport are 

given in Table E.5. 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-15 November 2006 

For the Final LT2ESWTR 

Table E.5. Physical and Chemical Properties of 

Elemental Mercury (Merck & Co., Inc. 1983) 

Property1 Value 

Density (g/mL1 at 25 oC) 13.534 

Solubility (g/L2 at 25 oC) 0.062

Vapor pressure (mm Hg at 25 oC) 0.002 

1

 grams per milliliter; grams per liter 

2 Further information regarding mercury solubility in water 

can be found in Glew and Hames (1971).

In operating lamps, elemental mercury (from pure or amalgamated mercury) is vaporized 

in the presence of an inert gas. The concentration of mercury in the vapor phase is controlled 

predominantly by temperature. At typical LP and LPHO lamp operating temperatures, only a 

small portion of the liquid (pure) or solid (amalgam) mercury is vaporized. However, at typical 

MP lamp temperatures (600 to 900 oC; Table 2.1), mercury is present primarily in the vapor 

phase due to the high operating temperatures (Phillips 1983). 

The relative proportion of mercury in the liquid/amalgam phase and in the vapor phase in 

an operating lamp may affect the fate of the mercury. (See Section E.5.) Liquid-phase elemental 

mercury is considerably denser than water (density = 13.5 g/mL; Table E.5). 

As the UV lamp is operating, mercury-containing compounds can be formed on the 

internal lamp surface (Altena et al. 2001). After a break, these deposits may dissolve in water, 

releasing mercury into the water (Merck & Co. 1983). 

Figure E.3 illustrates the expected forms of mercury in an operating lamp. Note that 

much of the elemental mercury will volatilize in an operating MP lamp and that amalgams are 

only used in LPHO lamps. 

E.4.2 Amount of Mercury in UV Disinfection Facilities 

The amount of mercury in a UV disinfection facility is site-specific and can be calculated 

using the amount of mercury per lamp, the number of lamps per reactor, and the number of 

reactors in the facility. This section contains information on the amount of mercury in UV lamps 

and uses this information in example calculations showing the amount of mercury contained in 

hypothetical UV facilities. This information is provided as an order of magnitude range of 

mercury levels that could be present in UV facilities. 

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, longer lamps and 

lamps with higher pressures and power ratings typically contain more mercury. Table E.6 

summarizes the quantities of elemental mercury added to lamps during manufacturing based on 

information provided by manufacturers and published literature values. 
Appendix E. UV Lamp Break Issues 

Figure E.3. Mercury Speciation in Operating UV Lamps 

Table E.6. Elemental Mercury Content in UV Lamps 

Mercury Content (mg per lamp) 

Lamp 

Type 

Electrical 

Power Rating 

[Watt (W)] 

Phillips 

(1983) 

Clear and 

Berman (1994) 

Manufacturer 

Survey 

LP 15 – 70 “a single drop”(1) 20(2) 5 – 50 

120 – 260 NR 26,(3) 36(4) LPHO 150 

400 NR 75.5 NR 

MP 1000 NR 250 NR 

1 – 25 kW 1.4 – 14.5 

mg/cm(5)

NR 200 – 400, 

0.3 – 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

 milligram per centimeter (mg per cm) of lamp length, reported lamp lengths are 6 – 300 cm 

(Primarc Limited 2001) 

NR – Not Reported 

The amount of mercury in a UV facility can be estimated using the values in Table E.6 as 

a guide. In order to develop these estimates, 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 required UV dose and validation reduction equivalent dose (RED) 

target (Table E.7). Design parameters included the number of lamps needed to obtain an MS2 

phage RED of 40 millijoule per centimeter squared (mJ/cm2

)

1

 during validation testing and the 

total number of reactors for each of the three design flows. Calculations assume 50, 150, and 400 

milligrams (mg) of mercury per LP, LPHO, and MP lamp, respectively. When determining the 

amount of mercury at a specific UV facility, water systems should contact the lamp manufacturer 

for updated information because mercury content varies with lamp type and manufacturer. 

 

UV Disinfection Guidance Manual E-16 November 2006 

1

 Corresponds approximately to a 3 log Cryptosporidium inactivation (depending on validation testing and 

associated Validation Factor) 

For the Final LT2ESWTR 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-17 November 2006 

For the Final LT2ESWTR 

Table E.7. Mercury Quantity in Example UV Facilities1, 2

Design 

Flow 

(mgd) 

Average 

Flow 

(mgd) 

Lamp 

Type 

Average 

Number of 

Reactors 

Average Number 

of Lamps 

(per reactor) 

Total Hg in UV 

Facility3

(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

 Target MS2 phage RED of 40 mJ/cm2 , which corresponds approximately to a 3 log 

Cryptosporidium inactivation (depending on validation testing and associated Validation Factor) 2

 Water quality criteria: Ultraviolet transmittance (UVT) = 89% (A254 = 0.05 cm-1), Turbidity = 

0.1 nephelometric turbidity units (NTU), 

Alkalinity = 60 mg/L as CaCO3, Hardness = 100 mg/L as CaCO3 3

 Values given represent the amount of elemental mercury added to lamps during manufacturing.

E.5 Documented Mercury Reactions in PWSs 

Currently the fate of mercury following a lamp break has not been experimentally 

determined. This section describes documented mercury reactions in water systems. 

Liquid elemental mercury and solid mercury amalgams have high densities (Table E.5) 

and will probably settle in areas of low water velocity, providing an opportunity for containment 

and removal. In prior cases when liquid mercury was released from water treatment equipment, 

such as manometers, flow instrumentation, or pump seals, mercury was found to have settled in 

the clearwell, but whether all of the released mercury was recovered is not known (Cotton 2002). 

Smaller particles from the vapor phase mercury may be transported farther or be more readily 

dissolved in water than liquid elemental mercury and solid mercury amalgams. However, in 

sampling following a recent on-line MP lamp break (described in Section E.2.2), mercury was 

not detected in any of the downstream sample locations, which could indicate that the mercury 

was contained by the UV reactor. The water system theorized that the remaining mercury was 

potentially attached to the UV reactor walls. 

Liquid-phase elemental mercury does not readily dissolve in water. Kolch (2001) 

monitored the mercury concentrations in a 50-L batch reactor following the destruction of one 

LPHO lamp (containing approximately 150 mg Hg). Mercury concentrations reached 

approximately 2.5 μg/L in the batch reactor water, and amalgamated mercury was found settled 

on the bottom of the reactor (Dinkloh 2001). The low concentration of dissolved mercury in the 

water is likely an indication that little, if any, of the mercury amalgam dissolved into the water. 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-18 November 2006 

For the Final LT2ESWTR 

E.6 Summary and Conclusions 

UV disinfection is an important disinfection technology that provides additional public 

health protection. To date, there have been few lamp breaks at existing UV facilities. The risk to 

human health and the environment from the mercury in UV lamps used in the treatment of 

drinking water is very small. Procedures and actions can be taken to reduce the chances of a 

lamp break and mitigate mercury release that UV lamp breaks cause. In addition, monitoring of 

mercury after known lamp breaks indicates that most of the mercury is contained, and 

concentrations in the water downstream of the UV reactor do not exceed the SDWA MCL. 

However, more research is needed to understand the fate of mercury in a drinking water 

environment following a UV lamp break and to evaluate the dispersion and transport of mercury 

through a WTP and distribution system. 

Lamp breaks are divided into off-line and on-line breaks. Off-line lamp breaks typically 

occur during storage, handling, or maintenance and cause small spills. Small spills should be 

contained, cleaned up, and disposed of properly. Monitoring of mercury vapor concentration in 

the ambient air is important to protect personnel during the clean-up procedures. 

On-line breaks occur when the lamp sleeve and lamp break while the UV reactor is in 

operation. Incidents have been reported of on-line UV lamp breaks associated with impact from 

debris, improper UV reactor orientation, loss of water flow, temperature differentials, faulty UV 

equipment design, procedural errors, and manufacturing defects. However, on-line lamp breaks 

are largely preventable with appropriate design, operation, maintenance, and operator care. The 

following engineering and administrative methods may help prevent UV lamp breaks: 

• Screens, baffles, or low-velocity collection areas prior to the reactor influent to 

prevent entrance of debris 

• UV reactor installation with lamps oriented parallel to the ground to reduce 

differential heating 

• Temperature and flow sensors and alarms to detect critical conditions and to shut 

down the UV reactors and water flow 

• Surge analysis to determine if water hammer may be a potential problem or whether 

pressure relief values need to be installed 

• Comprehensive operator training and UV equipment maintenance program 

• Adequate circuit breakers/GFIs should be specified to prevent damage to the reactor. 

• Operators and maintenance staff should perform routine inspection and maintenance 

according to manufacturers’ recommendations. 

• 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. 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-19 November 2006 

For the Final LT2ESWTR 

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 these measures provide is unknown. Water 

systems and designers should consider the applicability of these isolation techniques on a sitespecific basis. Water systems should prepare a lamp break response plan in preparation for a 

potential UV lamp break and mercury release. This plan should address sampling and cleanup 

procedures as well as compliance with the SDWA, OSHA health and safety standards, and Clean 

Water Act. Water systems are encouraged to recycle or return all mercury-containing lamps to 

mercury re-generating facilities or the lamp manufacturer. 
Appendix E. UV Lamp Break Issues 

UV Disinfection Guidance Manual E-20 November 2006 

For the Final LT2ESWTR 

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Appendix F 

Case Studies 
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Appendix F. Case Studies 

UV Disinfection Guidance Manual F-1 November 2006 

For the Final LT2ESWTR 

This appendix provides examples of how various utilities have implemented UV 

disinfection in their water systems. The UV facilities described in the following case studies 

were selected because they represent a broad range of UV facility conditions. They represent 

medium-pressure (MP) and low-pressure high-output (LPHO) reactor installations; on-site and 

off-site validation testing; installation on filtered water, unfiltered water, and an uncovered 

reservoir; and other varying goals and design issues. 

The purpose of this appendix is to provide an overview of the manner in which UV 

disinfection has recently been implemented for drinking water disinfection in North America. 

The case studies describe issues and approaches used to implement UV disinfection technology. 

Specific step-by-step procedures for selecting design criteria and validation are not described. 

Rather, each case study provides a summary of the reasons for implementing UV disinfection, 

the design issues that were considered, and how implementation was approached. They are 

meant to be instructive as examples of how UV disinfection can be applied across a range of 

source waters, equipment types, and retrofit locations. It is important to follow the specific stepby-step guidance and examples provided in the previous sections of this manual to ensure that 

the final guidance is appropriately applied to any new installations. 

The organization of each case study generally follows the organization of this manual. 

Each study provides introductory information about the water system and a discussion of the 

planning, design, validation, and operation and maintenance steps completed by each public 

water system (PWS). The first two case studies (Albany, New York and Weber Basin Water 

Conservancy District, Utah) feature in-depth descriptions, and the remaining three case studies 

contain briefer summaries of similar information. 

When reading these case studies, it is important to recognize that these facilities were 

implemented before the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) 

and the guidance provided in this manual were finalized. Although drafts of this manual were 

available, some of the guidance has changed over time. In particular, the validation approaches 

and testing programs have changed since these projects were implemented. 

Following are some of the highlights from each case study: 

• Section F.1 – Albany, New York. MP reactors installed on an uncovered finished 

water reservoir that experiences bi-directional flow. 

• Section F.2 – Weber Basin Water Conservancy District, Utah. LPHO reactors 

validated off-site at the Portland Validation Facility. 

• Section F.3 – Clayton County Water Authority, Georgia. LPHO reactors in which 

challenge microorganism die-off problems were resolved to allow for on-site 

validation to proceed. 

• Section F.4 – Newark, Ohio. MP reactors installed at a lime-softening facility on 

individual filter effluent pipes. 

• Section F.5 – Winnipeg, Manitoba. MP reactors installed on an unfiltered source. 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-2 November 2006 

For the Final LT2ESWTR 

F.1 Albany, New York – MP Facility on a Finished Water Reservoir with 

On-site Validation 

The City of Albany (City) owns and operates a 32-million gallons per day (mgd) 

conventional surface water treatment plant (WTP), serving over 100,000 people. The Feura Bush 

WTP operates at a relatively constant treatment rate (typically about 20 mgd). The Loudonville 

Reservoir, a finished water reservoir on the opposite side of the City, floats on the distribution 

system, filling and emptying throughout the day as distribution system demand fluctuates. 

Loudonville Reservoir has two functions—distribution storage and emergency/backup 

supply. The reservoir is a 200-million gallon, uncovered, finished water storage facility, 

consisting of three basins. The reservoir has two inlets/outlets to the distribution system, and 

reservoir effluent water is automatically rechlorinated before delivery to the distribution system. 

In addition to rechlorinating the water as it re-enters the City’s distribution system, the City 

periodically batch chlorinates the reservoir to maintain water quality. 

The City expanded its water quality enhancement program at the Loudonville Reservoir, 

which consisted of a series of water system improvements, including UV disinfection, being 

made under the direction of Albany’s Mayor Gerald Jennings to ensure that customers receive 

the best possible water quality at all times. 

Early in the project planning phase, the City and its consultant determined that UV 

disinfection offers the most flexible and holistic solution for improving the reservoir water 

quality. UV disinfection provides the City with an additional disinfection barrier that is compact, 

relatively simple to operate, free from regulated disinfection byproducts (DBP), and effective 

against chlorine-resistant pathogens. 

Few data were available on the water quality at the reservoir. Therefore, the reservoir 

water quality was assumed to be similar to the Feura Bush WTP finished water quality, which is 

summarized in Table F.1. 

F.1.1 Planning 

This section discusses key planning decisions made for Albany’s UV facility. Figure F.1 

is a timeline of the process the City used to implement UV disinfection. 
Appendix F. Case Studies 

Table F.1. Summary of Feura Bush WTP Finished Water Quality (2000) 

Parameter Units Average Minimum Maximum 

UV Absorbance(1) cm-1 0.03 0.011 0.054 

UV Transmittance(1) percent 93 88 98 

Turbidity NTU(2) 0.23 0.12 0.54 

pH – 8.40 7.80 9.20 

Alkalinity mg/L(3)-CaCO3 40.9 35.7 48.3 

Temperature °C 10.4 1.1 20.0 

Total Hardness mg/L-CaCO3 54.2 50.0 58.0 

Iron mg/L <0.03 <0.03 0.03 

Manganese mg/L <0.03 <0.03 <0.03 

Aluminum mg/L 0.07 0.07 0.07 

Specific Conductance m-mhos/cm(4) 176 148 211 

1

 Data collected January 2001 – September 2001. 

2

 nephelometric turbidity units 

3

 milligrams per liter 

4

 millimhos per centimeter 

Figure F.1. Albany UV Implementation Timeline 

F.1.1.1 UV Disinfection Goals 

 The City chose a multiple-barrier approach for disinfection at the reservoir, incorporating 

both UV disinfection and chlorination, to provide a greater level of protection. The City’s UV 

UV Disinfection Guidance Manual F-3 November 2006 

For the Final LT2ESWTR 
Appendix F. Case Studies 

light and chlorine disinfection systems provide more effective inactivation of viruses and 

chlorine-resistant pathogens than the former chlorine-only system, while minimizing DBP 

formation. Additionally, the UV facility provides an additional level of protection to facilitate the 

City’s compliance with the LT2ESWTR inactivation requirements for uncovered storage 

(Section 1.3.3). 

F.1.1.2 UV Retrofit Location 

The UV facility is located at the reservoir rather than at the WTP. Flow from each of the 

three reservoir basins is routed through the UV disinfection facility (Figure F.2) before it enters 

the City’s distribution system. 

Figure F.2. UV Retrofit Location at Loudonville Reservoir 

Basin

A 

Loudonville 

Reservoir 

UV 

Disinfection

Facility Basin

B 

Basin 

C 

City 

Distribution 

System 

Feura 

Bush 

WTP 

F.1.1.3 Key Design Parameters 

Water quality, the fouling/aging factor, and flow rate are key parameters to be considered 

during the planning phase. Table F.2 summarizes key preliminary design parameters for the UV 

facility design. 

Table F.2. UV Facility Preliminary Design Parameters 

Criterion Unit Value 

UV Transmittance percent 88 

Fouling/Aging Factor percent 60 

Peak Flow Rate mgd 40 

UV Disinfection Guidance Manual F-4 November 2006 

For the Final LT2ESWTR 
Appendix F. Case Studies 

Water Quality 

Several water quality parameters affect UV dose delivery and, therefore, UV equipment 

design (Table F.1). The most important is ultraviolet transmittance (UVT), which is calculated 

from A254 as described in Section 3.4.4.1. Reservoir UVT data were collected for approximately 

3 months prior to design, but long-term UVT data were not available for the reservoir. The 

minimum UVT of 88 percent (A254 of 0.054) measured in the WTP finished water, therefore, was 

used to conservatively estimate UVT at the reservoir (Figures F.3 and F.4). 

Figure F.3. UVT Data for Feura Bush WTP Finished Water 

figure 

86

88

90

92

94

96

98

100

102

1/1/2001

1/16/2001

1/31/2001

2/15/2001

3/2/2001

3/17/2001

4/1/2001

4/16/2001

5/1/2001

5/16/2001

5/31/2001

6/15/2001

6/30/2001

7/15/2001

7/30/2001

8/14/2001

8/29/2001

9/13/2001

9/28/2001

10/13/2001

10/28/2001

11/12/2001

11/27/2001

UV254 Transmittance

Finished Water

Basin A

Basin B

Basin C

2

Fouling/Aging Factor 

A fouling/aging factor of 0.6 was selected for Albany’s UV equipment to be 

conservative. Because of the low hardness, iron, and manganese concentrations, fouling was not 

considered a significant issue. Nevertheless, the selected fouling/aging factor does reduce the 

necessary frequency of lamp replacement. 

UV Disinfection Guidance Manual F-5 November 2006 

For the Final LT2ESWTR 
Appendix F. Case Studies 

Figure F.4. Cumulative UVT Data for Feura Bush WTP Finished Water 

86%

88%

90%

92%

94%

96%

98%

0% 20% 40% 60% 80% 100%

Cumulative Frequency Percentile

UV Transmittance (%)

Finished Water

Design UVT

(Conservative value chosen 

because few reservoir data were 

available)

1

 Note: 190 samples collected between January 2001 and November 2001.

Flow Rate 

The UV facility was sized for 40 mgd (10 mgd per unit) for emergency or backup 

conditions when the reservoir and UV facility must be able to satisfy the entire system demand 

(30 mgd maximum). Under normal operating conditions, the UV facility maximum flow is 

10 mgd. 

Power Quality 

The electric service provider for the proposed location of the UV facility was contacted 

regarding the availability and quality of power at the site. It was determined that high quality 

power was available, and power conditioning equipment was therefore unnecessary. 

F.1.1.4 Equipment and Monitoring Strategy 

Both MP and LPHO UV equipment were considered during the planning phase. MP UV 

equipment was selected for the design because of the smaller footprint and because, at that time, 

there was more experience with MP equipment in the United States. Another benefit of MP 

equipment was the use of the calculated dose-monitoring strategy, which allowed the City to 

address the anticipated variability in flow rate and direction. 

UV Disinfection Guidance Manual F-6 November 2006 

For the Final LT2ESWTR 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-7 November 2006 

For the Final LT2ESWTR 

F.1.1.5 UV Equipment Validation Options 

The City of Albany chose on-site validation testing because no UV validation centers 

were operating in the United States at that time and because on-site testing would allow for: 

• Testing under the exact piping configuration of the UV facility. 

• Optimizing UV facility operations throughout the life of the facility. 

Space requirements, injection and sample ports, and other elements required for on-site 

validation were coordinated with the UV equipment supplier and included in the UV facility 

design. 

F.1.1.6 Hydraulics 

 

No modifications to existing hydraulics were required for the UV facility at the reservoir. 

Installation of the UV facility did slightly reduce the full operating capacity of the reservoir. 

However, during an extended emergency condition, the UV facility can be bypassed to allow use 

of the entire reservoir volume. 

F.1.1.7 Selected Configuration 

 

The selected configuration of the UV equipment is summarized in Table F.3. The target 

male-specific-2 bacteriophage (MS2) reduction equivalent dose (RED) to be verified during 

validation was 40 millijoules per centimeter squared (mJ/cm2) and was chosen to target highlevel inactivation of various pathogens during emergency operation. The MS2 RED was based 

on best practices in North America and Europe at the time. 

Table F.3. UV Equipment Configuration 

Criterion Unit Value 

UV Lamp Type – MP 

Target MS2 RED mJ/cm2 40(1)

Number of UV Units (Duty + Standby) number 4 + 0(2)

Design Flow Rate per Unit mgd 10 

Number of Lamps per Unit number 8 

Lamp Power (Each) kW(3) 10 

1

 40 mJ/cm2 is the target MS2 RED to be proven during validation testing 

and to be used when the reservoir is operating in its emergency mode. 

The target MS2 RED used during the normal distribution function of the 

reservoir is 240 mJ/cm2

 for virus inactivation. 2

 40 mgd is needed for emergency and not normal operation; therefore, a 

standby unit was not provided. 3

 kilowatt (kW)
Appendix F. Case Studies 

F.1.2 Design 

Given the wide range of UV equipment available, pre-purchase of the UV equipment by 

the owner was selected as the best way to proceed. Pre-purchase documents were issued in 

November 2001. Two suppliers bid on the UV equipment, and the contract was awarded to the 

low bidder, Trojan Technologies, Inc., in December 2001. By selecting the equipment early in 

the project, the project team was able to work closely with the manufacturer during the design of 

the system and support facilities (e.g., instrumentation and control) for the 24-inch UVSwiftTM 

units. The following sections describe Albany’s UV facility design. 

F.1.2.1 Facility Hydraulics 

 

Four parallel treatment trains of equal capacity (10 mgd) comprise the UV facility 

(Figure F.5). Water enters and exits the UV facility via 48-inch diameter influent and effluent 

manifolds. Each parallel treatment train consists of a 24-inch diameter lateral, influent and 

effluent modulating isolation valves, strap-on ultrasonic flow meter, and an MP UV unit. The 

UV units are installed in vertical piping to minimize the footprint of the UV facility and to 

promote settling of debris, if any, in the inlet piping to protect the lamps. 

Figure F.5. UV Disinfection Facility at the Loudonville Reservoir 

Although water hammer and surge conditions were determined to be minimal, a 

combination air/vacuum release valve was incorporated into each UV treatment train. The valves 

provide protection from adverse pressure conditions and facilitate the release of trapped air 

during start-up. 

The UV facility was designed to handle large flow variations. The facility can treat 

typical daily flows with one or two 10-mgd units in service. With all four units in service, the 

UV Disinfection Guidance Manual F-8 November 2006 

For the Final LT2ESWTR 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-9 November 2006 

For the Final LT2ESWTR 

facility can also treat the City’s full system demand during WTP or transmission main shutdowns or during an emergency condition. 

The UV facility was also designed to handle bi-directional flow through the UV 

equipment because water passes through the UV units during both the reservoir fill and the draw 

cycles. The bi-directional flow design (described below) enables the City to maintain the UV 

facility in a constant state of readiness to deliver disinfected water to the distribution system 

whenever the reservoir switches to a draw cycle (i.e., when treated water is sent to customers). 

Operation in Fill Mode 

When WTP production exceeds distribution system demand, the reservoir fills (fill 

mode). All flow passes through the UV facility to the reservoir, and the UV equipment operates 

at minimum intensity because disinfection of the influent water is not needed. The primary 

objective of operating the UV facility during the fill mode is to ensure that the UV equipment is 

on and ready to provide adequate disinfection when the reservoir switches to draw mode. 

Operation in Draw Mode 

When distribution system demand is greater than WTP production, water drains from the 

reservoir to the distribution system (draw mode). Because a UV unit is always on, there is no 

time delay for disinfection of outgoing water when the flow direction changes. 

F.1.2.2 Operational Strategy, Instrumentation and Control 

The UV equipment was designed so that at least one UV train is in service at all times to 

ensure that a UV unit is ready to disinfect the reservoir water whenever flow into the distribution 

system occurs. When system demand matches the WTP production rate, however, very little 

flow into or out of the reservoir occurs. To prevent high lamp temperature (and automatic shutdown), a cooling water bypass line was installed downstream of the UV equipment and upstream 

of each isolation valve to allow a nominal flow through the unit [approximately 80 gallons per 

minute (gpm)]. The cooling water line is equipped with a motor-actuated valve for automatic 

opening when the water temperature exceeds a set value [90 degrees Fahrenheit (°F)] or when a 

start-up or shut-down signal is received. During start-up of a UV unit, the cooling water flow is 

discharged to waste. Following start-up, all flow enters the distribution system. 

The UV equipment is controlled by a central programmable logic controller (PLC) using 

the calculated dose-monitoring strategy. The central PLC uses flow rate and direction data from 

each of the four treatment trains to control the overall operation of the UV equipment and to 

sequence the operation of individual UV units. Input for controlling the UV equipment is 

provided by a strap-on flow meter on each UV treatment train, two on-line UVT analyzers in the 

piping header, and eight UV sensors in each UV unit. The individual control panel for each UV 

unit adjusts the lamp power and calculated dose of each UV unit in response to the flow rate, 

UVT, and UV intensity, to ensure an appropriate level of disinfection. 

As the distribution system demand increases, the central PLC initiates start-up of the next 

UV unit once the flow rate through the first unit reaches a manually entered percentage of its 

rated capacity. After the second unit has warmed up (approximately 5 minutes), the central PLC 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-10 November 2006 

For the Final LT2ESWTR 

opens the modulating valve for that train and brings the unit on-line. Flow is then split between 

the two active UV trains. This scenario continues with the other two UV units as necessary based 

upon distribution system demand. 

F.1.2.3 Electrical Power Configuration 

Power quality was not expected to be an issue at the reservoir. Therefore, power 

conditioning equipment for the UV equipment was not necessary. An uninterruptible power 

supply (UPS) was included for the UV control panel to convey alarms and other critical UV 

facility information. A backup diesel generator, capable of providing backup power for all 

elements of the UV facility, and an automatic transfer switch were also included in the design. 

Because the UV equipment is not on a UPS, a brief interruption of the UV disinfection occurs 

when the UV facility switches to the backup generator. Disinfection is reinitiated once backup 

power is active and the UV lamps have restarted. To minimize the number of power transfers 

and resulting UV lamp power interruptions, retransfer of the facility back to grid power is done 

manually to allow an operator to determine when conditions are appropriate for a transfer back to 

grid power (e.g., when the reservoir is inflowing).

F.1.2.4 Capital Costs 

Bids were received upon completion of the final design, and the construction contract 

was awarded. The approximate cost of the UV facility at the time of construction was 

$3,125,000, which, when adjusted to 2006 dollars (ENR BCI = 4356), equates to approximately 

$3,805,000. Major cost components (in 2006 dollars) included: 

• $680,000 for the UV equipment 

• $2,410,000 for a new UV building and yard piping 

• $360,000 for ancillary piping, valves, and controls 

• $355,000 for electrical. 

F.1.3 Validation 

On-site validation of the UV equipment was completed in October 2003, which was prior 

to the promulgation of the LT2ESWTR. The validation was based on a previous draft of this 

guidance document. Because the guidance has changed, any new installations should follow the 

validation protocol described previously in this manual and not follow the example given in this 

section. 

Two validation tests were performed at this facility. Contract validation testing was 

conducted to confirm that the equipment met the design criteria specified in the UV equipment 

procurement document. Expanded validation testing was also conducted to assess whether 

energy efficiency could be improved by modifying the lamp operating strategies and UV facility 

maintenance. The expanded validation testing was co-funded by the City of Albany and the New 

York State Energy Research and Development Authority (NYSERDA). 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-11 November 2006 

For the Final LT2ESWTR 

2

Trojan Technologies, Inc. conducted the validation testing with Dr. James Malley, Jr. 

(University of New Hampshire, Durham) providing third-party oversight for the contract 

validation testing. Christine Cotton of Malcolm Pirnie, Inc., provided third-party oversight for 

the NYSERDA validation testing. 

F.1.3.1 Contract Validation Conditions 

The validation testing procedures were based on the UV Disinfection Guidance Manual 

(UVDGM) Proposal Draft (USEPA 2003). The validation testing procedures in this manual and 

the protocol followed for Albany’s on-site validation testing did not significantly differ. 

However, the data analysis and Validation Factor calculations do differ from those in this 

manual. 

The validation testing was conducted at the reservoir. The challenge microorganism used 

in the testing was MS2 phage. Dissolved instant coffee, a UV absorber, was used to adjust the 

UV transmittance to the desired test conditions. There was no residual chlorine in the water; 

therefore, the water did not need to be dechlorinated. 

Following is a summary of the range of validation conditions: 

• Lamps were operated at 0 percent (off), 60 percent (to account for lamp aging and 

fouling effects), or 100 percent (full power) of their nominal output during the 

validation testing. 

• Flow rates ranged from 2.0 – 10.3 mgd. 

• UVT ranged from 88 – 99 percent. 

• Target MS2 RED values ranged from 0 to 150 mJ/cm2. 

The test conditions and results for the validation tests are summarized in Table F.4. The 

UV equipment passed the contract validation testing. 

F.1.3.2 NYSERDA Validation Conditions 

The validation testing procedures used in the NYSERDA validation testing were the 

same as in the contract validation testing. The NYSERDA testing conditions follow: 

• Four or six (of eight possible) lamps in the unit were energized. 

• Lamps were operated at 0 percent, 60 percent, 80 percent, or 100 percent of their 

nominal output. 

• Flow rates ranged from 2.0 – 10.0 mgd. 

• UVT ranged from 87 – 100 percent. 

• Target MS2 RED values ranged from 0 – 150 mJ/cm . 
Appendix F. Case Studies 

Table F.4. Validation Testing Conditions and Results for Contract Validation 

Run 

No. 

Flow 

(mgd) 

Configuration UVT 

Modifier 

Test 

Organism

Lamp 

Power 

(%) 

UVT 

(%) 

Influent MS2(1)

(log 

PFU/mL(2)) 

Effluent MS21

(log PFU/mL) 

Log 

Reduction

MS2 

RED 

(mJ/cm2

)

1 9.8 8 lamps on None None 0 98.5 0 3.15 0.00 0 

2 9.7 8 lamps on None MS2 0 98.3 6.16 6.25 -0.09 -9.4 

3 9.9 8 lamps on None MS2 100 98.5 6.14 0 6.14 150.3 

4 9.8 8 lamps on None MS2 60 98.6 6.08 0 6.08 148.7 

5 9.7 8 lamps on Coffee MS2 60 87.4 6.19 4.37 1.82 39.4 

6 9.9 8 lamps on Coffee MS2 100 87.5 6.21 3.28 2.93 68.0 

7 5.0 8 lamps on None MS2 60 98.5 5.78 0 5.78 141.0 

8 4.9 8 lamps on Coffee MS2 60 87.8 5.66 3.10 2.56 58.4 

9 2.0 8 lamps on None MS2 60 98.1 5.83 0.37 5.44 132.5 

10 2.0 8 lamps on Coffee MS2 60 88.0 5.79 1.11 4.68 112.7 

11 10.3 8 lamps on None None 0 98.6 0 2.99 0.00 0 

1

 The value shown represents the average of three replicate samples. 2

 plaque forming units per milliliter 

The collimated beam dose-response results for an example day of testing (Day 1) are 

shown in Figure F.6. 

Figure F.6. Collimated Beam UV Dose-response Curve 

UVT Adjusted with Coffee:

y = 0.038x + 0.250

R2

 = 0.987

No UVT Adjustment:

y = 0.040x + 0.308

R2

 = 0.988

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 20 40 60 80 100 120 140

UV Dose (mJ/cm2

)

Log Inactivation

No UVT Adjustment

UVT Adjusted with Coffee

UV Disinfection Guidance Manual F-12 November 2006 

For the Final LT2ESWTR 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-13 November 2006 

For the Final LT2ESWTR 

The test conditions and results for the validation tests are summarized in Table F.5. The 

implications of the NYSERDA validation testing results on operation and maintenance (O&M) 

are described in Section F.1.3.4. Additional validation data analysis may be completed in the 

future to determine the validation factor and validated dose in accordance with the validation 

data analyses described in this manual (Chapter 5). 

Table F.5. Validation Testing Conditions and Results for NYSERDA Validation 

Run 

No. 

Flow 

(mgd) 

Configuration UVT 

Modifier 

Test 

Organism

Lamp 

Power 

(%) 

UVT 

(%) 

Influent MS21

(log PFU/mL)

Effluent MS21

(log PFU/mL) 

Log 

Reduction

MS2 

RED 

(mJ/cm2

)

1 10.0 6 lamps on None None 0 99.0 0 0 0.00 0.00 

2 9.9 6 lamps on None MS2 0 99.2 6.25 6.18 0.06 -4.7 

3 9.9 6 lamps on None MS2 60 99.1 5.85 0.30 5.59 140.8 

4 9.9 6 lamps on Coffee MS2 80 87.5 5.98 3.93 2.04 47.4 

5 2.0 6 lamps on Coffee MS2 60 87.2 6.55 2.40 4.12 102.1 

6 2.0 4 lamps on Coffee MS2 60 87.4 6.34 3.89 2.45 58.1 

7 10.1 4 lamps on None MS2 80 99.0 5.83 0.37 5.83 147.1 

8 10.0 4 lamps on Coffee None 100 88.7 0 0.602 0.00 0.00 

9 10.0 4 lamps on None None 0 99.9 0 2.752 0.00 0.00 

1 The value shown represents the average of three replicate samples. 2

 Test microorganisms injected in prior tests had pooled in a deadspace upstream of the UV reactor and later bled 

back into the main flow stream. 

F.1.3.3 Issues Encountered During Validation Testing 

During validation testing, the following issues were encountered: 

• Ultrasonic flow meter uncertainty. Before the start of the contract validation 

testing, a discrepancy between a flow meter installed on the UV unit to be tested and 

an existing flow meter farther downstream was noted. The meter manufacturer was 

contacted, and a representative was sent to the site. A portable strap-on flow meter 

was installed next to the test unit flow meter; the portable meter was consistent with 

the test unit meter. The downstream flow meter was determined to be in error, having 

been set to the diameter of the casing pipe and not the diameter of the internal carrier 

pipe. Upon resolution of the investigation, the validation testing continued using the 

test unit flow meter to measure the flow rate. 

• Test organism not injected. In the test plan, Run No. 8 of the NYSERDA validation 

testing should have had organisms injected. However, during the testing, no 

organisms were injected. Instead of re-doing the testing, Run No. 8 was used as an 

additional control. 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-14 November 2006 

For the Final LT2ESWTR 

F.1.3.4 Validation Implications for Operation and Maintenance 

The NYSERDA validation testing results indicated that the UV equipment can achieve 

(and exceed) the 40 mJ/cm2

 target MS2 RED when operating in a power saving mode with only 

4 or 6 (of a possible 8) lamps on and with UVT between 87 and 99 percent. When the data were 

analyzed in accordance with the UVDGM Proposal Draft (USEPA 2003) guidelines that were 

available at the time, the testing indicated that 3-log Cryptosporidium inactivation and 1.5-log 

virus inactivation, if desired, could be achieved under expanded lamp control conditions. 

Validation of the alternative lamp operating configuration is expected to result in cost savings 

from reduced power usage. 

F.1.4 Start-up and Operation of the UV Facility 

Construction of the facility was completed in February 2003. Full-scale operation began 

in March 2003. 

F.1.4.1 Start-up and Construction Issues 

Although some problems occurred upon initial start-up of the UV equipment, all parties 

involved worked to resolve the issues to the City’s satisfaction. The problems and resolutions are 

briefly discussed below: 

• Control system. The manufacturer’s control system does not calculate the UV dose 

correctly during the “fill mode” conditions. Because flow is bi-directional in the 

inlet/outlet pipe, a negative value for flow was used in the fill condition (a positive 

value for the “draw mode”). It could be corrected by changing the programming to 

use an absolute value of the flow input in the calculation. 

• UVT analyzers. The on-line UVT analyzers initially reported inconsistent readings. 

Samples were taken at the midpoint and top of the pipe. The samples taken at the top 

of the pipe were found to be occasionally erroneous due to air bubbles in the sample. 

To correct the problem, the sample ports for the on-line UVT analyzers were adjusted 

so that all samples were taken from the midpoint of the pipe. 

• Cleaning system. The wiper cleaning mechanism originally provided with the UV 

equipment frequently jammed due to grit entering and binding the threads of a wiper 

system rod. The UV manufacturer refined the design and provided a replacement 

wiper drive system with a self-cleaning traversing nut and a rod with a larger thread 

pitch and depth. 

• Intensity sensors. In several instances, the coating on the intensity sensors degraded. 

The UV manufacturer improved the design and provided new UV sensors for the UV 

reactors. The new sensors were provided prior to validation testing, so re-validation 

was not necessary. 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-15 November 2006 

For the Final LT2ESWTR 

• UV lamp failure. At start-up, approximately thirty percent of the lamps failed to 

energize. A similar percentage also failed to start in a second shipment of lamps. The 

UV manufacturer tracked the problem to a batch of lamps with manufacturing 

defects. The manufacturer corrected the problem and installed a new batch of lamps. 

F.1.4.2 Operation and Maintenance 

The following operational tasks are regularly performed at Albany’s UV facility. These 

tasks take approximately one hour per day, seven days per week. 

• Daily overall visual inspection of the UV equipment. 

• Daily check of the control system to ensure it is in automatic mode. 

• Daily check of the control panel display for status of UV equipment components and 

alarms. 

• Daily check of on-line analyzers, flow meters, and data recording equipment. 

• Daily review of 24-hour monitoring data to ensure that the equipment has been 

operating properly. 

• Daily check of cleaning mechanism operation. 

• Daily check of lamp run time. 

• Daily check of ballast cooling fans for unusual noise. 

• Weekly check of valve operation. 

The City of Albany also performs regular maintenance tasks at the UV facility. Due to 

budget cut-backs, the original maintenance frequencies for several tasks have been reduced. The 

current scheduled maintenance tasks include the following: 

• Monthly calibration check of UV sensors. 

• As-needed calibration check of UVT analyzers. Due to the sensitivity of these 

analyzers and re-calibration difficulties described in Section F.1.4.3, the calibration of 

these analyzers is checked only when problems are evident (every few months). The 

calibration was formerly checked weekly. 

• Quarterly to annual check of equipment housing, sleeves, and wiper seals for leaks. 

• As-needed replacement of the duty sensor with a calibrated back-up sensor. To date, 

replacement has been unnecessary. 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-16 November 2006 

For the Final LT2ESWTR 

• Annual check of the cleaning system efficiency by inspecting and manually cleaning 

the sleeves. This maintenance was previously performed quarterly. 

• Quarterly check of the cleaning fluid reservoir. 

• Annual calibration of the reference sensor by the manufacturer. 

• As-needed replacement of lamps that have broken or are at the end of their lamp life 

(currently replaced after 4,000 hours). Approximately 20 lamps have been replaced in 

the first 2 years of operation. 

• As-needed replacement of sleeves that have broken or fouled. To date, approximately 

3 sleeves have been broken and replaced. 

• As-needed cleaning of UVT analyzers. 

• As-needed inspection of cleaning system drive mechanism. 

• As-needed inspection of ballast cooling fan. 

Performance of these tasks is currently estimated to take approximately two hours per 

week per unit (8 hours per week total). An additional 8 hours per month is spent on 

troubleshooting. Before the cut-backs, performance of these tasks at the recommended frequency 

(Sections 6.3.1 and 6.4.1.4) took an estimated eight hours per week per unit (32 hours per week 

total). 

F.1.4.3 Operational Challenges 

Although the City generally has found the facility to be relatively simple to operate, a few 

challenging conditions have been encountered: 

• UVT analyzers. Due to the very high UVT of the water (typically 95 percent), the 

City’s maintenance personnel have difficulty calibrating the UVT analyzers. 

• Wiping collar maintenance. The City’s maintenance personnel have had problems 

re-aligning the cleaning system’s wiper collars on the sleeves when the collars have 

been completely removed from the equipment for maintenance. Difficulties with this 

maintenance task have resulted in several broken sleeves. 

• UV equipment draining. Inadequately sized drains in the UV unit delay 

maintenance because of the excessive time needed to drain water. 

F.1.5 Future UV Facility Plans 

Because very few data were available on the water quality at the reservoir, a conservative 

UVT of 88 percent was selected for the design of the UV equipment. However, full-scale 

operating data indicate that the UVT of the water at the reservoir is actually much greater 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-17 November 2006 

For the Final LT2ESWTR 

(approximately 95 percent), which enables the City of Albany to target virus inactivation with its 

UV facility over a larger range of flow rates when the reservoir is functioning as distribution 

storage. Albany’s validation data indicate that the facility can achieve 1.5-log virus inactivation 

and could likely be validated for greater virus inactivation if an appropriate challenge organism 

can be identified. Therefore, credit for greater than 1.5-log virus inactivation may be sought in 

the future. 

F.2 Weber Basin Water Conservancy District – LPHO Facility with Off-site 

Validation 

The Weber Basin Water Conservancy District (District) was established in 1950 to 

provide for the conservation and development of the water resources within the District 

boundaries and to use these resources to the greatest benefit of the public. The District is 

currently a drinking water wholesaler, serving a total population of approximately 400,000 

people. 

The District’s Weber WTP No. 3 is a 46-mgd conventional WTP with settled water 

ozonation for taste and odor control and UV light for enhanced disinfection (Figure F.7). The 

plant was expanded to its present capacity and other improvements were made in 2001. 
Appendix F. Case Studies 

Figure F.7. Weber WTP No. 3 Process Flow Diagram 

The primary raw water supply for the Weber WTP No. 3 is the Weber River, delivered 

through the Davis Aqueduct to the plant. Miscellaneous side creeks can also be used by the 

District to supplement irrigation water or for a raw water supply for the Weber WTP No. 3. The 

creeks are diverted directly to the plant intake without any upstream storage. There are no 

independent water quality data on these two supplies. 

The raw water quality at the Weber WTP No. 3 can vary significantly throughout the 

year. Storm events and spring run-off can increase turbidity rapidly. Algal blooms cause taste 

and odor problems, especially in the late summer and fall. Raw water quality data for 1996 to 

1998 is summarized in Table F.6, and Table F.7 summarizes Weber WTP No. 3 filtered water 

quality. 

 

 

Ferric Chloride 

Sodium Hypochlorite 

Sodium Bisulfate 

Sodium Hypochlorite 

PAC 

Polymer 

Polymer 

Ozone 

UV Disinfection Guidance Manual F-18 November 2006 

For the Final LT2ESWTR 

Distribution 

System

Raw 

Water Filters 

Intermediate 

Ozone 

Contactors 

Sedimentation 

Basins 

Flocculation 

Basins 

Inlet 

Structure/

Flash Mix 

Polymer 

Ozone 

Finished Water 

Storage 

Reservoir 

Chlorine 

Contact Basin 

UV 

Reactors 

Sodium Hypochlorite 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-19 November 2006 

For the Final LT2ESWTR 

Table F.6. Summary of Raw Water Quality (1996 – 1998) 

Parameter Units Average Minimum Maximum 

Turbidity NTU 29.2 0.3 3,800 

pH - 7.5 6.6 8.5 

Alkalinity mg/L as CaCO3 177 58 268 

Temperature °C 10.4 2.0 19.6 

Total Hardness mg/L as CaCO3 215 142 264 

Calcium Hardness mg/L as CaCO3 N/A N/A N/A 

Iron mg/L N/A N/A N/A 

Manganese mg/L N/A N/A N/A 

Total Organic Carbon mg/L 3.2(1) 1.1 7.6 

1

 Data collected at Weber WTP No. 3. 

Table F.7. Summary of Filtered Water Quality (2002 – 2004) 

Parameter Units Average 

Turbidity NTU < 0.15 

pH – 7.40 

Alkalinity mg/L-CaCO3 180 

Temperature °C 10.7 

Total Hardness mg/L-CaCO3 225 

Calcium Hardness mg/L-CaCO3 N/A 

Iron mg/L < 0.02 

Manganese mg/L Below detection limit 

Total Organic Carbon mg/L < 3 

F.2.1 Planning 

This section discusses key planning decisions made for Weber Basin’s UV facility. 

Figure F.8 is a timeline of the process the District used to implement UV disinfection. 
Appendix F. Case Studies 

Figure F.8. Weber WTP No. 3 UV Implementation Timeline 

F.2.1.1 UV Disinfection Goals 

Raw water monitoring by the District indicated that the water system could be classified 

as a Bin 2 or Bin 3 system under the LT2ESWTR. (See Section 1.3.1.) For the purposes of the 

preliminary design, it was assumed that the water system could either initially or ultimately be 

classified as a Bin 3 system, which would require the UV facility to provide 2.0-log additional 

Cryptosporidium inactivation. 

As part of the preliminary design process for the UV facility and other plant 

improvements, the District reevaluated its Giardia treatment. Before the 2001 improvements, the 

Weber WTP No. 3 used free chlorine for disinfection of Giardia and viruses. Following the 2001 

improvements, the finished water reservoirs had sufficient capacity to continue to provide the 

required level of Giardia and virus inactivation. However, the use of free chlorine for 3-log 

Giardia inactivation in the finished water reservoirs was discontinued for the following reasons: 

• The UV reactors could easily be designed for both Giardia and Cryptosporidium 

inactivation. 

• Incorporating Giardia disinfection with the proposed Cryptosporidium disinfection 

process would make a significant portion of the existing finished water reservoirs 

available for operational storage to benefit the distribution system. 

Table F.8 summarizes the treatment goals for the Weber WTP No. 3. 

F.2.1.2 UV Retrofit Location 

Because of existing hydraulic constraints, the UV reactors could be installed only 

downstream of the filters (see Figure F.7). 

UV Disinfection Guidance Manual F-20 November 2006 

For the Final LT2ESWTR 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-21 November 2006 

For the Final LT2ESWTR 

Table F.8. Disinfection Goals 

Process Cryptosporidium Giardia Virus 

Filters 3.5(1) 2.5 2.0 

UV Light 2.0 2.0 – 

Chlorine – 1.0 4.0 

Total Provided 5.5 5.5 6.0 

1

 Combined filter effluent turbidity 0.15 NTU in 95 percent of samples each 

month to provide a second barrier. 

F.2.1.3 Key Design Parameters 

Water quality, fouling/aging factor, and flow rate are critical parameters to be considered 

during the planning phase. Table F.9 summarizes key preliminary design parameters for the UV 

reactor design. 

Table F.9. UV Facility Preliminary Design Parameters 

Criterion Unit Value 

UV Transmittance percent 90 

Fouling/Aging Factor percent 67 

Flow Rate mgd 15.3 

Water Quality 

Several water quality parameters affect UV dose delivery and, therefore, UV reactor 

design. (See Table F.7.) The most important is UVT, which is calculated from the UV 

absorbance at 254 nm (A254) as described in Section 3.4.4.1. Based on the available UV 

absorbance data, a design UVT of 90 percent (A254 of 0.046 cm-1) was selected (Figures F.9 and 

F.10). 

Fouling/Aging Factor 

A fouling/aging factor of 67 percent was selected during planning. The factor was 

incorporated into the design to account for the reduction in lamp output at the end of lamp life 

and the reduction in lamp output due to irreversible sleeve fouling. 

Flow Rate 

The UV facility capacity was designed to match the 46-mgd WTP capacity with three 

units in operation and one unit out of service. Therefore, the design flow rate through each 

reactor was 15.3 mgd. 

Power Quality 

To ensure operation of the UV equipment, standby power was provided with a new 

backup generator. No other power conditioning equipment was needed. 
Appendix F. Case Studies 

Figure F.9. UVT Data for Weber WTP No. 3 Finished Water 

86

88

90

92

94

96

98

100

6/29/96

7/29/96

8/28/96

9/27/96

10/27/96

11/26/96

12/26/96

1/25/97

2/24/97

3/26/97

4/25/97

UV254 Transmittance

Average

95% Confidence

Interval

99% Confidence

Interval

Figure F.10. Cumulative UVT Data for Weber WTP No. 3 Finished Water 

86%

88%

90%

92%

94%

96%

98%

100%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Cumulative Frequency Percentile

UV Transmittance (%)

Finished Water

Design UVT 

UV Disinfection Guidance Manual F-22 November 2006 

For the Final LT2ESWTR 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-23 November 2006 

For the Final LT2ESWTR 

F.2.1.4 Equipment and Monitoring Strategy 

Because both MP and LPHO reactors were considered for the design, the footprint for the 

larger LPHO reactors was used for planning. The District selected variable setpoint operation 

based on flow rate for the UV equipment to conserve power. 

F.2.1.5 UV Reactor Validation Options 

Off-site validation was selected for the following reasons: 

• Discharge of validation test water from on-site validation testing was not feasible. 

• At the time, UV disinfection was an innovative process without references for potable 

water applications in the United States. Therefore, off-site validation prior to delivery 

was deemed a reasonable validation option. 

F.2.1.6 Hydraulics 

Changes to the plant’s hydraulic profile were made as part of the overall plant expansion 

and improvements in 2001. The UV reactors fit into the plant’s new hydraulic profile at the 

combined filter effluent without need for additional modifications or intermediate pumping.

F.2.1.7 Selected Configuration 

Only UV reactor manufacturers that had validated a reactor were considered for the 

design, but pre-validation of the proposed UV reactor was not required. A bid specification was 

written before design of the UV facility. The base bid was for LPHO reactors with alternate bids 

allowed for MP reactors. Although the UV equipment was not pre-purchased, bids were 

evaluated, and the detailed design was based on the selected manufacturer (WEDECO Inc.). The 

selected configuration of the UV reactors is summarized in Table F.10. The target MS2 RED to 

be verified in validation was selected to target high-level inactivation of various pathogens based 

on best practices in North America and Europe at the time. The selected configuration has one 

sensor for every bank of lamps and has one UVT analyzer. 

Table F.10. UV Reactor Configuration 

Criterion Unit Value 

UV Lamp Type – LPHO 

MS2 RED1 mJ/cm2 40 

Number of UV Units (Duty + Standby) number 3 + 1 

Design Flow Rate per Unit mgd 15.3 

Number of Banks per Unit number 6 

Number of Lamps per Bank number 12 

Lamp Power (Each) W 360 1 The MS2 RED to be proven during validation testing. 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-24 November 2006 

For the Final LT2ESWTR 

.2.2 Design 

he following sections describe the UV facility design in more detail. 

F.2.2.1 Facility Hydraulics 

he UV reactors were installed on the combined filter effluent to fit into the plant’s 

hydrau h 

e 

F.2.2.2 Operational Strategy, Instrumentation and Control 

he UV supplier provided a variable setpoint control strategy based on a UV Intensity 

Setpoin

ent 

in 

low meters and flow control valves were not provided for each reactor. However, each 

reactor

tor 

F.2.2.3 Electrical Power Configuration 

n electrical engineer reviewed power fluctuations and quality at the WTP and 

determ r was 

F.2.2.4 Capital Costs 

he UV facility construction was part of a larger expansion and improvement project. 

The po

$1,210,000 for the UV equipment. 

F

T

T

lic profile. Filtered water from a common header is passively divided into four 30-inc

influent pipes to the UV reactors. To compensate for a possible uneven flow split, upstream 

isolation valves can be manually throttled. The head loss through the UV reactors, ancillary 

piping, and valves is 2.3 feet of water at maximum plant capacity (46 mgd). Effluent weirs ar

installed to ensure that the reactors remain submerged. 

T

t Approach. In this approach, the minimum UV intensity values determined during 

validation can be set for several flow rate ranges. The UV ballast system has 50 to 100 perc

variable power capabilities, allowing the UV reactor to automatically adjust based on relative 

sensor intensity and flow rate to conserve power (see section F.2.3.1 for details). For the UV 

reactor to stay in compliance (i.e., to ensure minimum UV dose delivery), the UVT must rema

at or above the minimum value (90 percent), the flow rate through the reactor must be less than 

or equal to the maximum validated flow rate, and the UV sensor values must all be at or above 

the UV intensity setpoint for that flow rate and number of lamp banks on as determined by the 

validation data (See Section F.2.3.). 

F

 was provided with UV sensors. Additionally, a motorized valve downstream of each 

reactor remains closed during start-up until the reactor is on and warmed up. Therefore, offspecification water is not delivered to consumers and does not need to be wasted during reac

start-up. 

A

ined that power conditioning equipment was not needed. However, standby powe

provided with a new backup generator to ensure continuous UV equipment operation. 

T

rtion of the construction cost attributed to the UV facility was $2,230,000 in 2006 (ENR

BCI = 4356) dollars. Major cost components included: 

•
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-25 November 2006 

For the Final LT2ESWTR 

• $400,000 for a new UV building. 

• $250,000 for ancillary piping, valves, and controls. 

• $370,000 for electrical improvements. 

F.2.3 Validation 

Off-site validation testing was originally conducted at the Deutsche Vereinigung des Gasund Wasserfaches (DVGW) facility in Germany. However, validation testing at the DVGW 

facility proved problematic because the strict DVGW requirements do not allow flexibility in 

validation or operation setpoints. As such, further validation testing at the Portland Validation 

Facility was conducted in July 2003. The second validation was in accordance with the U.S. 

guidelines based on the UVDGM Proposal Draft (USEPA 2003). 

The Portland Validation Facility is located at the City of Portland, Oregon Bureau of 

Water’s Groundwater Pumping Station of the Columbia Southshore Wellfield, Portland, Oregon. 

The Columbia Wellfield is a 90-mgd supplemental drinking water supply that the Portland Water 

Bureau owns and operates. The wellfield provides up to 43 mgd of continuous flow to the UV 

reactor test train. Typical water quality of the groundwater is shown in Table F.11 

Table F.11. Southshore Wellfield Water Quality 

Characteristics 

Parameter Unit Value 

UVT % 96.8 – 98.6 (98.3 average) 

Hardness mg/L 38 – 144 

Alkalinity mg/L CaCO3 34 – 169 

pH unitless 5.8 – 8.8 

Chlorine mg/L none 

The high UVT allowed testing of the full range of operating UVT conditions, and the 

zero chlorine residual eliminated the need to quench the chlorine prior to adding chlorinesensitive challenge microorganisms. 

Carollo Engineers conducted the validation testing. Clancy Environmental Consultants 

(CEC), St. Albans, Vermont, supervised the injection and sampling of the challenge 

microorganism. CEC prepared all stock solutions of the challenge microorganism, measured 

challenge microorganism UV dose-response using the collimated beam apparatus, and assayed 

challenge microorganism concentrations. WEDECO Inc. (Charlotte, North Carolina) operated 

the UV reactor during biodosimetry testing with oversight by Carollo. 
Appendix F. Case Studies 

F.2.3.1 Validation Conditions 

 

The Portland Validation Facility allowed testing conditions (e.g., piping configuration) to 

be defined for each validation test, which allowed the testing to be optimized to US guidelines 

based on the UVDGM Proposal Draft (USEPA 2003). The Weber WTP No. 3 reactor was 

validated with inlet piping that included a 90-degree bend located three pipe diameters upstream 

of the reactor and another 90-degree bend less than three pipe diameters downstream of the 

reactor. This configuration did not represent the actual piping arrangement at the Weber WTP 

No. 3 but, instead, represented the “worst case” flow conditions through the UV reactor. 

The high UVT of the Columbia Wellfield water allowed the full range of UVT conditions 

to be tested. Lignin sulfonic acid (LSA), a UV absorber, was used to reduce the UVT as needed. 

The challenge microorganism used in the validation testing was MS2 phage. Static mixers were 

used to ensure that additives were well mixed upstream of the reactor inlet sampling port and the 

reactor exit sampling port. The testing configuration is shown in Figure F.11. 

Figure F.11. Validation Testing Configuration 

The UV reactor was tested using a range of flow rate, UVT, and operating lamp 

combinations to validate UV dose delivery and UV sensor measurements. The experimental 

matrix was designed to validate the vendor’s UV intensity setpoint approach with variable 

setpoint operation for a range of water quality conditions within the defined design criteria. The 

experimental matrix also was intended to enable the PWS to optimize performance (i.e., deliver 

the target MS2 RED with a minimal number of lamp banks in operation at a minimum power 

level). 

Following is a summary of the range of validation conditions: 

• All lamps were operated at 67 percent of their nominal output during the validation 

testing to account for lamp aging and fouling effects. 

• Flow rates ranged from 0.94 – 20 mgd. 

UV Disinfection Guidance Manual F-26 November 2006 

For the Final LT2ESWTR 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-27 November 2006 

For the Final LT2ESWTR 

• UVT ranged from 85 – 95 percent. 

• The number of lamp banks that were on was 1, 2, 4, 5, and 6 (all banks). 

• Target MS2 RED values ranged from 20 – 100 mJ/cm2. 

The test conditions and results for several of the 34 validation tests that were conducted 

are summarized in Table F.12. 

 

Table F.12. Excerpt of Test Conditions for Validation Testing at the 

Portland Validation Facility (Total No. of Tests = 34) 

Run No. 

Flow Rate 

(mgd) 

No. 

Banks On 

UVT

(%) 

UVT 

Modifier 

Test 

Organism 

Lamp Power

(%) 

12 2.07 2 84.7 LSA MS2 67 

9 2.36 1 95.0 LSA MS2 67 

3 0.94 1 84.7 LSA MS2 67 

16 17.61 2 94.8 LSA MS2 67 

36 19.57 6 94.8 LSA MS2 67 

22 20.00 4 89.9 LSA MS2 67 

38 14.49 5 90.1 LSA MS2 67 

19 15.65 4 85.0 LSA MS2 67 

29 12.07 6 85.3 LSA MS2 67 

The collimated beam dose-response results for an example day of testing are shown in 

Figure F.12, and Table F.13 summarizes validation testing results. 

UV transmittance measurements were checked using National Institute of Standards and 

Technology (NIST)-traceable UV absorbance standards. Flow measurements were checked by 

comparison of manufacturer calibration to internal settings. The UV dose-response of the MS2 

phage met bounds described by the UVDGM Proposal Draft (2003) and NWRI (2003). 

Biodosimetry and sensor testing and data analysis were based on the June 2003 Draft UVDGM 

recommendations (Tier 2 analysis). 
Appendix F. Case Studies 

Figure F.12. Collimated Beam UV Dose-response Data 

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

0 20 40 60 80 100 120

UV Dose (mJ/cm2

)

Log Inactivation

Sample 15

Sample 3

Sample 36

Sample 24

Table F.13. Validation Testing Results 

Run 

No. 

Flow 

Rate 

(mgd) 

No. 

Banks On 

Lamp 

Power 

(%) 

UVT 

(%) 

Log 

Infl. MS2 

(pfu/mL) 

Log 

Effl. MS2

(pfu/mL) 

MS2 RED 

(mJ/cm2 Log I ) 

12 2.07 2 67 84.7 5.04 2.16 2.88 57.9 

9 2.36 1 67 95.0 5.29 0.97 4.32 96.0 

3 0.94 1 67 84.7 5.38 2.41 2.97 66.9 

16 17.61 2 67 94.8 4.69 3.23 1.46 28.4 

36 19.57 6 67 94.8 5.32 2.00 3.32 76.7 

22 20 4 67 89.9 4.69 3.45 1.24 23.6 

38 14.49 5 67 90.1 4.81 2.73 2.07 42.9 

19 15.65 4 67 85.0 4.76 3.53 1.23 23.4 

29 12.07 6 67 85.3 4.67 2.57 2.09 41.6 

The measured relationship between UV sensor measurements and UVT at the 80 percent 

intensity turn-down reflecting end-of-lamp-life (EOLL) was described using a power function (A 

and B are constants): 

UVTBA SensorUV eeEOLLUVT × ),( = Equation F.1 

The functions describing the UV sensor measurements as a function of ballast power 

setting and UVT were obtained using new lamps, sleeves, and UV sensors in a clean UV reactor. 

The functions can be compared to measurements made at a WTP to assess the relative output of 

the lamps compared to the data measured during validation. 

The test results were evaluated by plotting measured MS2 RED (mJ/cm2

) as a function of 

number of operating banks divided by flow rate (Q in mgd), again based on a specific UVT value 

UV Disinfection Guidance Manual F-28 November 2006 

For the Final LT2ESWTR 
Appendix F. Case Studies 

and at the intensity turn-down reflecting end-of-lamp-life. Statistical analysis was used to 

determine if data sets obtained with different rows could be combined. The relationship was 

fitted by a polynomial function (A and B are constants): 

2

),( ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = Q

Banks B

Q

Banks AEOLLUVTRED Equation F.2 

Based on these equations, an automatic UV dose-monitoring strategy was developed that 

determined the necessary number of banks/rows in operation and the ballast power, so that a 

selected target MS2 RED (e.g., 40 mJ/cm2) is met. 

F.2.3.2 Validation Implications for O&M 

The validation testing data were used to develop equations that would automatically 

determine the needed number of banks in operation and ballast power so that the selected target 

MS2 RED (e.g., 40 mJ/cm2

) could be provided. 

F.2.4 Start-Up and Operation of the UV Facility 

Full-scale operation of the facility began in June 2004. Photos of the UV equipment are 

shown in Figures F.13 and F.14. 

Figure F.13. UV Reactors at the Weber WTP No. 3 

(3 duty + 1 standby reactor in parallel) 

 

UV Disinfection Guidance Manual F-29 November 2006 

For the Final LT2ESWTR 
Appendix F. Case Studies 

Figure F.14. UV Reactor Electrical Cabinets at the Weber WTP No. 3 

(the floor above the UV reactors) 

F.2.4.1 Start-up and Construction Issues 

Since start-up, the UV facility has been operating as intended (no frequent UV unit shutdowns, lamp failures, or similar mechanical problems). However, as with any new unit process, 

some problems have been experienced, including the following: 

• UV-monitoring system. Not all of the low-level alarm settings and controls in the 

monitoring system software worked properly in the first version of software provided. 

The vendor subsequently updated the software. 

• Manganese fouling. Sleeve and sensor fouling was a serious problem when the UV 

equipment was first started. Although dissolved manganese concentrations were not 

measured, the problem began when the District began adding ferric chloride for 

coagulation. Analysis of the foulant indicated that manganese, an impurity present in 

the coagulant, caused the fouling. To control this problem, hypochlorite was fed 

upstream of the filters to oxidize the manganese, which was then removed by the 

filters. The District plans to discontinue the hypochlorite feed once the intermediate 

ozonation system is fully operational. 

• Cleaning system. The phosphoric acid chemical cleaning system originally intended 

for use with the UV reactors was installed on a cart on the upper level of the UV 

disinfection room. A long hose with a spray-nozzle attachment was to be hand-carried 

to the lower level and inserted into the UV reactors for cleaning. However, the 

cleaning system could not provide enough suction to work properly with the cart 

located on the upper level, and the cart could not be moved to the lower level due to 

mobility constraints. Therefore, a new chemical cleaning system that could provide 

appropriate pumping power was constructed on the lower level adjacent to the UV 

reactor. 

UV Disinfection Guidance Manual F-30 November 2006 

For the Final LT2ESWTR 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-31 November 2006 

For the Final LT2ESWTR 

• Control panel. The UV equipment was designed with one transformer for each 

ballast enclosure. During the first summer of operation, the enclosures overheated and 

had to be opened so the transformers could be cooled with box fans. The 

manufacturer recommends that the temperature in the control room not exceed 

100 oF. The temperature in this room was not measured during the first summer of 

operation, so whether the overheating was due to an inadequate or faulty control 

panel cooling system or to high temperatures in the control room is unknown. The 

source of this problem remains under investigation. 

• Training. The manufacturer did not provide on-site operator training on UV reactor 

O&M until after the UV equipment had been operating for several months. 

 

F.2.4.2 Operation and Maintenance Requirements 

The UV sensors are calibrated monthly, and no sensor drift has been observed since the 

facility has been operational. Similarly, the online UVT analyzer is also checked monthly and no 

drift has been observed. No lamps have been replaced since the UV facility began operations, 

and no chemical cleaning has been performed. Inspections of some of the lamps, however, reveal 

no signs of fouling. Information on the amount of labor required to perform these O&M tasks 

was not readily available. No information was readily available on power usage. 

F.2.4.3 Operational Challenges 

Except for resolving the start-up and construction issues (Section F.2.4.1), the District has 

generally found the facility to be relatively simple to operate. 

F.2.5 Future UV Facility Plans 

The District plans to apply to the Utah Department of Environmental Quality for 

approval of the UV disinfection system for Cryptosporidium and Giardia credit in the future. 

The District may target different Cryptosporidium and Giardia inactivation levels in the 

future to respond to future regulatory requirements. The UV manufacturer has provided curves 

showing flow rate versus number of UV lamp banks in operation for three different MS2 RED 

(20, 30, and 40 mJ/cm2

) to enable future operational flexibility. 

F.3 Clayton County Water Authority – LPHO Facility with On-site 

Validation 

The Clayton County Water Authority (CCWA) owns and operates three WTPs, which 

serve more than 250,000 people in Clayton County, Georgia. The Freeman Road WTP is a 12-

mgd conventional surface WTP. Chlorine dioxide is applied prior to the rapid mix process to 

oxidize taste and odor compounds and iron and manganese, and free chlorine is applied to the 

filtered water for disinfection. In 2002 the plant was upgraded to include a UV disinfection 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-32 November 2006 

For the Final LT2ESWTR 

facility. The filtered water quality characteristics that were the basis for the UV reactor design 

are summarized in Table F.14. 

Table F.14. Summary of Freeman Road WPP 

Filtered Water Quality 

Parameter Units Design Value 

Turbidity NTU 0.18 

pH – 8.1 

Alkalinity mg/L-CaCO3 29 

Iron mg/L 0.1 

Manganese mg/L 0.02 

Total Organic Carbon mg/L < 2 

F.3.1 Planning and Design 

This section discusses the key planning and design decisions made for CCWA’s UV 

facility. 

F.3.1.1 UV Disinfection Goals 

The UV equipment was installed at the Freeman Road WTP to provide an additional 

pathogen barrier. The basis for the facility design was 2.5-log Cryptosporidium inactivation. UV 

disinfection was selected over other disinfection technologies because of its effectiveness against 

pathogens and its cost-effectiveness. 

F.3.1.2 UV Retrofit Location 

The UV reactors were installed on the combined filter effluent piping in a new standalone building. As part of the UV retrofit, chemical feeds for lime, fluoride, phosphoric acid, and 

chlorine were relocated to follow the UV reactors. 

F.3.1.3 Key Design Parameters 

The UV reactors for the Freeman Road WTP were bid and selected before detailed design 

of the facility. The bid was open to LPHO and MP reactors. A life cycle cost analysis that 

incorporated the capital costs and the anticipated energy and maintenance costs was used to 

select the UV reactors. Ultimately, LPHO reactors were selected for the Freeman Road WTP. 

After selecting the reactors, one set of plans and specifications was developed for the design. The 

UV facility consisted of three WEDECO Series K reactors. Key design parameters for the UV 

reactors are shown in Table F.15. A conservative MS2 RED (50 mJ/cm2

) to be verified during 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-33 November 2006 

For the Final LT2ESWTR 

validation testing was selected because the design was to be completed while the LT2ESWTR 

and this manual were still under development. 

Table F.15. UV Reactor Design Parameters 

Criterion Unit Value 

UV Transmittance percent 91 

Fouling/Aging Factor percent 60 

Total Flow Rate mgd 12 

Target MS2 RED1 mJ/cm2 50 

Number of UV Units (Duty + Standby) number 2 + 1 

Design Flow Rate per Unit mgd 6 

Number of Lamps per Unit number 30 

Lamp Power (Each) W 275 

1 The MS2 RED to be proven during validation testing. 

Facility Hydraulics 

No modifications to the plant hydraulics were required at the Freeman Road WTP 

because the available head between the filter effluent control weir and the clearwell was 

sufficient for the UV reactors. 

Operational Strategy, Instrumentation and Control 

A magnetic flow meter is installed in the piping upstream of each UV reactor to monitor 

the flow split between the reactors. Each UV reactor is equipped with three UV sensors, one for 

each bank of lamps. Additionally, an on-line UVT analyzer enables the UV reactors to be 

operated with either a (1) UV Intensity Setpoint Approach or (2) Calculated Dose Approach. 

However, the UV equipment has not yet been validated for operation in calculated dose mode. 

Electrical Power Configuration and Power Quality 

A UPS was provided at the Freeman Road WTP to ensure that the UV equipment 

remains in continuous operation. No power quality or outage issues have been experienced at the 

facility. 

Capital Cost 

The total capital cost for the UV facility at the Freeman Road WTP was approximately 

$2,170,000 in 2006 (ENR BCI = 4356) dollars. The cost includes all elements related to the UV 

facility, the building, UV reactors, piping, valves, electrical system, instrumentation and controls, 

and other ancillary equipment. 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-34 November 2006 

For the Final LT2ESWTR 

F.3.2 Validation 

Validation testing was conducted in February 2003. On-site rather than off-site validation 

was selected to maximize flexibility in selecting the specific operating conditions for testing and 

in allowing potential future testing as EPA requirements are established. 

One of the three reactors was designed to serve as a test reactor for on-site validation. 

The inlet and outlet piping to the test reactor can be isolated, and the outlet piping allows flow to 

be routed to waste. The other two UV reactors and their upstream and downstream piping are 

identical in design to the test reactor, so the testing was representative of each of the other 

reactors. 

Preliminary testing before validation indicated that nearly complete die-off of the MS2 

phage had occurred in both the influent and effluent samples from the UV reactor. Although the 

chlorine dioxide preoxidation system had been shut down several days before testing, jar test 

results indicated that low levels of chlorate or chlorine dioxide caused the die-off. The jar tests 

also indicated that the effect of the chlorate or chlorine dioxide on the MS2 could be alleviated 

by adding LSA, a compound commonly used during validation to reduce UVT. The problem was 

therefore resolved by spiking the microbial samples with LSA before shipping them to the 

laboratory for analysis. 

The challenge microorganism used in the testing was MS2 phage. A dilute LSA solution 

was used to reduce the UVT as needed in the filtered water and to prevent die-off in the MS2 

samples. The results of the validation testing are shown in Table F.16. 

Table F.16. Validation Testing Conditions and Results 

for CCWA’s Freeman Road WTP 

Run 

No. 

Flow 

(mgd) Configuration1 UVT Modifier 

Test 

Organism

Lamp 

Power 

(%) 

UVT

 (%) 

Influent MS2 

(log PFU/mL)

Effluent MS2 

(log PFU/mL) 

Log 

Reduction

MS2 

RED 

(mJ/cm2

)

1F 5.41 3 banks on LSA MS2 50 91.2 5.35 2.76 2.59 57.2 

2F 5.95 3 banks on LSA MS2 50 90.8 5.44 2.96 2.48 54.1 

3F 6.49 3 banks on LSA MS2 50 90.7 5.43 3.30 2.13 44.6 

4F 7.29 3 banks on LSA MS2 50 91.9 5.48 3.46 2.02 41.7 

5F2 7.39 3 banks on LSA MS2 – – 5.54 5.57 -0.04 – 

1

 Each reactor contains 3 banks with 10 lamps per bank. 

2

 Control run 

F.3.3 Start-up and Operation of the UV Facility 

Construction of the UV facility at the Freeman Road WTP was completed in December 

2002, and full-scale operation began in April 2003. At the time of publication, operations and 

maintenance data were not made available for the Freeman Road WTP. 

Although the UV equipment has generally operated well since start-up, an issue requiring 

a minor change did arise in the first year. In October 2003 after several months of operation, 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-35 November 2006 

For the Final LT2ESWTR 

WEDECO replaced the UV lamps in the reactors with a new production model. The new lamps 

were tested to ensure that the UV intensity of the replacement lamps (following a 100-hour burnin period) was equal to or better than the intensity of the lamps that had been replaced. However, 

comparison of the intensity data for the replacement lamps to the data that had been collected 

during validation indicated that the intensity of the replacement lamps was less than that of the 

previous lamps. 

An investigation of the problem determined that microbubbles in the water passing 

through the UV reactor were causing the measured decrease in UV intensity, not the replacement 

lamps. The existing air release valve on each reactor did not sufficiently release entrained air 

from the water, particularly in colder months due to the higher dissolved oxygen concentration in 

the water. To alleviate the problem, two additional air release valves were installed on the 

influent header between the filter control weir (the source of the entrained air) and the UV 

reactors. The additional air release capability minimized the formation of microbubbles during 

periods of low water temperature. 

F.4 Newark Water Treatment Plant – MP Reactors on Each Filter Effluent 

Pipe 

The Newark WTP, located in Newark, Ohio, is a 15-mgd surface WTP. The Newark 

WTP has an average daily flow rate of approximately 8 mgd and serves a population of more 

than 47,500 people. Treatment processes at the Newark WTP include preoxidation with 

potassium permanganate and powdered activated carbon for removal of taste- and odor-causing 

contaminants, lime softening, sedimentation, recarbonation, rapid sand filtration, and disinfection 

with UV light and chlorine. 

The filtered water quality characteristics that were the basis for the UV reactor design are 

summarized in Table F.17. 

Table F.17. Summary of Newark WTP Filtered Water Quality 

Parameter Units Average Minimum Maximum 

pH – 7.6 7.2 8.2 

Turbidity NTU 0.23 0.18 0.53 

Total Alkalinity mg/L-CaCO3 60 40 100 

Total Hardness mg/L-CaCO3 120 90 160 

Calcium Hardness mg/L-CaCO3 67.4 75 60 

Iron mg/L 0.03 0.01 0.10 

Manganese mg/L 0.02 0.01 0.03 

Temperature oF 60 33 80 

Total Organic Carbon mg/L 1 – 2 No data No data 
Appendix F. Case Studies 

F.4.1 Planning and Design 

This section discusses the key planning and design decisions made for Newark’s UV 

facility. 

F.4.1.1 UV Disinfection Goals 

UV disinfection was installed at the Newark WTP to provide an additional treatment 

barrier against pathogens and to ensure public health protection in the event of high turbidity in 

the raw water. The City’s water source has historically experienced turbidity spikes following 

rainfall events. 

F.4.1.2 UV Retrofit Location 

Three locations were considered for the UV reactors. Two locations were on the 

combined filter effluent at the plant’s chlorine contact basin that is used to achieve chlorine 

disinfection requirements. This basin is located prior to the clearwell and finished water pump 

station. The third location considered was on each of the ten individual filter effluents (IFE). The 

IFE location was selected because both the capital and O&M costs were less than the costs for 

the other alternatives. Additionally, the IFE location provided a high degree of redundancy and 

O&M enhancements due to the number of reactors. 

To accommodate the retrofit, filter effluent piping had to be rearranged on four filters to 

provide the desired straight piping runs upstream and downstream of the reactor. Also, existing 

valves on two filters had to be rotated 90 degrees to provide sufficient clearance to service the 

reactors. Figure F.15 illustrates the UV reactor installation on one of the filter effluent pipes. 

Figure F.15. UV Reactor at the Newark WTP 

UV Disinfection Guidance Manual F-36 November 2006 

For the Final LT2ESWTR 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-37 November 2006 

For the Final LT2ESWTR 

F.4.1.3 Key Design Parameters 

The UV reactor specification allowed MP reactors only because LPHO reactors could not 

meet the space constraints of this application. The competitive bid resulted in the selection of 

Trojan’s 12-inch UVSwiftTM reactor because the other bidder could not meet the head loss 

requirement. The UV equipment was selected prior to design and purchased as part of the UV 

facility construction contract. 

Key design parameters for the UV reactor are shown in Table F.18. The target MS2 RED 

of 40 mJ/cm2

 to be verified in validation was selected based on best practices in North America 

and Europe at the time to inactivate a range of pathogens. 

Table F.18. UV Reactor Design Parameters 

 

Criterion Unit Value 

UV Transmittance percent 85 

Fouling/Aging Factor percent 80 

Total Flow Rate mgd 15 

MS2 RED1 mJ/cm2 37 

Number of UV Units number 10 

Design Flow Rate per Unit mgd 1.5 

Number of Lamps per Unit number 4 

Lamp Power (Each) kW 1.26 

1

 The MS2 RED to be proven during validation testing.

Facility Hydraulics 

The head losses created by the UV reactors and the necessary piping modifications were 

less than 6 inches at the maximum flow rate through the reactors. Therefore, no additional 

pumping or other hydraulic modifications were required for the addition of the UV reactors. 

Each reactor was rated for a maximum flow rate of 1.5 mgd, which corresponded to the 

rated maximum filter capacity of 4 gallons per minute per square foot (gpm/sf). 

Operational Strategy, Instrumentation, and Control 

The UV equipment operates using the Calculated Dose Approach. The UV reactors 

automatically adjust to changing conditions to ensure that the calculated dose does not fall below 

the dose setpoint. Each reactor normally operates for 4 days followed by a day out of operation, 

corresponding to the normal filter service times. When UV reactors are returned to operation, an 

isolation valve located upstream of the UV reactor is closed, and plant service water flowing at 

approximately 20 gpm is used to cool the lamps while the reactor is started up and the lamps 

return to full power (approximately 10 to 15 minutes). After passing through the reactor, the 

cooling water enters the process train and is sent to the contact time (CT) basin for primary 

disinfection and then to the finished water clearwell. Once the lamps reach full power, the 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-38 November 2006 

For the Final LT2ESWTR 

upstream isolation valve is opened, and filtered water flows through the UV reactors. The 

advantage of using plant service water as cooling water during reactor start-up is that because it 

has previously been treated by UV disinfection, the cooling water does not have to be included in 

the calculation of off-specification water. 

Each reactor was located downstream of an existing flow control valve and flow meter. 

New isolation valves were installed downstream of each reactor. In addition to UV sensors, each 

reactor has a level sensor and temperature sensor. The level and temperature sensors protect the 

UV reactor from running dry or overheating or both. Each reactor was also provided with an 

auxiliary potable water supply connection to maintain a minimum flow rate of 15 gpm to prevent 

overheating during reactor start-up and shut-down. Each reactor has a UVT analyzer located on 

the filter effluent pipe to provide UVT measurements for dose monitoring. 

Electrical Power Configuration and Power Quality 

Power quality at Newark WTP was assessed over a fifteen-month period (March 2003 –

June 2004) as part of an American Water Works Association Research Foundation Project 

(Cotton et al. 2005). During this time, 240 power quality events occurred. The frequency and 

classification of the power quality events at the WTP for this fifteen-month period are shown in 

Table F.19. 

Table F.19. Power Quality at Newark WTP 

 

City of Newark 

Power Quality Event Total 

Monthly 

Average 

Maximum 

Month 

Instantaneous Voltage Sag 215 14.33 75 

Momentary Voltage Sag 6 0.4 2 

Temporary Voltage Sag 0 0 0 

Instantaneous Swell 0 0 0 

Instantaneous Interruption 0 0 0 

Momentary Interruption 8 0.53 4 

Temporary Interruption 3 0.2 1 

Sustained Deep Undervoltage 1 0.07 1 

Sustained Interruption 7 0.47 4 

Total Estimated Time (minutes) NA 168 775 

Estimated % Off-specification Time NA 0.38 1.74 

Approximately 90 percent of the power quality events were instantaneous voltage sags 

(i.e., voltage sags lasting between 0.5 and 30 cycles). The UV reactors were not operational until 

May 2004, so the off-specification time shown in Table 4.19 is an estimate calculated by 

assuming 10 minutes of off-specification time for each voltage sag lasting more than 2 cycles. 

Although a UPS system was not installed at the Newark WTP, the UV equipment’s ballast and 

electrical design prevents the UV reactors from losing power in many cases. As a result, the 

WTP is not having trouble meeting the off-specification requirements proposed in this guidance 

manual. 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-39 November 2006 

For the Final LT2ESWTR 

Capital Cost 

The capital cost for adding the UV equipment to the Newark WTP was $1,135,000 (2006 

dollars – ENR BCI = 4356). The cost includes modifications to the existing building and piping, 

UV reactors, piping, valves, instrumentation and controls, and other ancillary equipment. 

F.4.2 Validation 

No validation testing had been performed at the time of publication because no 

disinfection credit was needed. Newark may choose to validate the reactors in the future. 

F.4.3 Start-up and Operation of the UV Facility 

Construction of the UV facility was substantially complete and full-scale operation began 

in May 2004. The project was completed in July 2004. Overall, the UV equipment has operated 

smoothly, and only minor issues were encountered during start-up. 

Minor issues with the control panels and wiring were resolved by the factory 

representative and the contractor. Additionally, the automatic backwash sequence programming 

had to be rewritten to accommodate the UV reactor cooling water. During the reprogramming, 

problems with existing valve actuators were uncovered that required some actuator limit 

switches to be adjusted. 

Operations and maintenance costs were not readily available at the time of publication. 

F.5 City of Winnipeg Water Treatment Plant – MP Facility with On-site 

Validation 

The City of Winnipeg’s water supply is obtained from a surface water source and is 

currently unfiltered. Water is chlorinated, and fluoride and phosphate are also added before it is 

distributed to the 630,000 people served by the water system. 

A new WTP is currently under construction for the City of Winnipeg, which will use the 

following processes: rapid mix, coagulation, flocculation, dissolved air flotation, ozone, 

biological activated carbon filtration, UV disinfection, and chloramination. The UV facility was 

constructed before the rest of the treatment plant (scheduled for completion in 2007) to minimize 

the risk posed by Cryptosporidium and other waterborne pathogens. The UV facility will be 

integrated within the new WTP when it is constructed. 

The raw water quality characteristics that were the basis for the UV reactor design are 

summarized in Table F.22. 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-40 November 2006 

For the Final LT2ESWTR 

Table F.22. Summary of Raw Water Quality (1989 – 1994) 

Parameter Units Average Minimum Maximum 

pH – 8.2 7.4 9.1 

Turbidity NTU 1.0 0.3 5.3 

Total Organic Carbon mg/L 9.3 5 17 

Dissolved Organic Carbon mg/L 8.3 4 15 

Plankton cells/mL 39,700 200 666,000 

Total Alkalinity mg/L-CaCO3 81 72 95 

Total Hardness mg/L-CaCO3 83 68 97 

Color (true) TCU1 < 5 < 5 10 

1

 True color units 

F.5.1 Planning and Design 

This section discusses the key planning and design decisions made for the City of 

Winnipeg’s UV facility. 

F.5.1.1 UV Disinfection Goals 

The UV reactors were designed to provide 2-log Cryptosporidium inactivation in the 

Deacon Reservoir raw water. The goal will remain unchanged when the UV facility is later used 

to treat filtered water, even though the facility will be able to treat higher flow rates (at higher 

UVT). 

F.5.1.2 UV Retrofit Location 

The UV facility currently treats unfiltered raw water. In 2007 when the WTP is expected 

to be complete, the UV facility will be located downstream of the combined filter effluent. The 

equipment was installed in an existing pump station building on the site. 
Appendix F. Case Studies 

F.5.1.3 UV Reactor Selection 

Because of space limitations in the existing building, MP reactors were selected. The MP 

reactors were selected in a competitive pre-selection/proposal process prior to completion of the 

final design. A cost/benefit model was used to evaluate the two pre-selected UV equipment 

alternatives (a typical model summary is shown in Figure F.16). Benefit scores (example values 

shown in stacked bars in Figure F.16) for non-monetary evaluation criteria were developed in 

advance of bids for each alternative by assigning relative weights to each criterion and then 

scoring each alternative against the criteria. The present worth costs for each alternative 

(example values shown in the line plot in Figure F.16) were then divided by the corresponding 

benefit score to calculate the cost/benefit ratio (example values shown in line plot in Figure 

F.16). The supplier that had the lowest cost/benefit ratio (i.e., low cost and high benefit), was 

then selected. 

F.5.1.4 Key Design Parameters 

Six Calgon Sentinel® 48-inch UV reactors comprise the UV facility. Key design 

parameters for the UV reactors are shown in Table F.23. The target MS2 RED to be verified in 

validation was based on criteria from the UVDGM Proposal Draft (USEPA 2003) for 2-log 

inactivation of Cryptosporidium. 

Following construction of Winnipeg’s WTP, the design UVT will be increased to 

90 percent. The design flow rate per reactor will also be increased; however, the total design 

flow rate through the facility will be reduced to 106 mgd as reactors are changed to stand-by and 

other measures are taken to improve UV facility redundancy. 

Figure F.16. Cost-Benefit Comparison for Winnipeg’s UV Reactors 

 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Supplier 1 Supplier 2 Supplier 3

UV Equipment Suppliers

Relative Benefit

-$8.0

-$6.0

-$4.0

-$2.0

$0.0

$2.0

$4.0

$6.0

$8.0

$10.0

Costs ($ M)

Equipment

characteristics

Maintenance

Manufacturer’s

experience

Operation

Equipment validation

Cost/Benefit ($M)

Present Worth ($m)

UV Disinfection Guidance Manual F-41 November 2006 

For the Final LT2ESWTR 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-42 November 2006 

For the Final LT2ESWTR 

Table F.23. UV Reactor Design Parameters 

Criterion Unit Value 

UV Transmittance percent 75 

Fouling/Aging Factor percent 70 

Total Flow Rate mgd 130 

MS2 RED1 mJ/cm2 28 

Number of UV Units (All Duty) number 6 

Design Flow Rate per Unit mgd 22 

Number of Lamps per Unit number 9 

Lamp Power (Each) kW 21.6 

1

 The MS2 RED to be proven during validation testing.

Facility Hydraulics 

No modifications to the facility hydraulics were required for the addition of the UV 

facility at the existing building. Furthermore, the hydraulics of the future WTP will be designed 

to incorporate the UV facility. 

Operational Strategy, Instrumentation, and Control 

Control of the UV reactors is based on the UV intensity setpoint (i.e., UV Intensity 

Setpoint Approach). A UV sensor was provided for each lamp in each reactor to monitor 

performance. Flow meters and modulating valves on each reactor are used to distribute the water 

among operating reactors, and isolation valves are located upstream of each reactor. 

As the system demand increases and reactors approach their maximum capacities, 

additional reactors are started up as needed. The procedure is followed in reverse as system 

demand decreases. 

Electrical Power Configuration and Power Quality 

Power quality problems are not common at the location of Winnipeg’s UV facility, so a 

UPS was not provided. For initial (unfiltered) operation, a back-up generation system was not 

provided for the UV facility; therefore, the UV facility is not operational during power outages. 

A back-up power system will be provided for long-term operation when the WTP is constructed. 

Capital Cost 

The total capital cost for the City of Winnipeg’s UV facility was approximately 

$5,885,000 in 2006 U.S. dollars (ENR BCI = 4356). The cost includes the UV reactors, piping, 

valves, instrumentation and controls, and other ancillary equipment. 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-43 November 2006 

For the Final LT2ESWTR 

F.5.2 Validation 

Although the UV reactors had been validated off-site before installation, the off-site 

testing had focused mainly on typical UVT levels, and only a limited number of runs had tested 

UVT levels below 80 percent. Therefore, the City of Winnipeg made on-site validation testing a 

bidding requirement. The on-site validation testing focused on lower UVT levels (70 – 

78 percent), consistent with the raw water to be treated by the UV facility. The on-site testing 

included tests at a range of flow rates (6 – 25 mgd) and lamp settings. 

The on-site testing was conducted in February 2005. The challenge microorganism was 

MS2, and SuperHumeTM (potassium humate salts) was used to adjust the UVT of the test water. 

Thirty-eight tests were run, and additional blanks and other quality control samples were also 

taken. An excerpt of the validation testing conditions and results are shown in Table F.24. 

F.5.3 Start-up and Operation of the UV Facility 

Construction of the UV facility was completed in December 2004. Although validation 

has been completed, full-scale operation of the facility will be started up after functional testing 

has been completed. 

Table F.24. Excerpt of the Validation Testing Conditions and Results 

for the City of Winnipeg 

Run 

No. 

Flow 

(mgd) 

Configuration1 UVT 

Modifier 

Test 

Organism

UV 

Intensity 

(W/m2

) 

UVT

(%) 

Influent MS2 

(log PFU/mL)

Effluent MS2 

(log PFU/mL) 

Log 

Reduction

MS2 

RED 

(mJ/cm2

)

1 24.9 3 banks on SH MS2 138.0 74.9 4.94 3.18 1.76 32.6 

2 25.1 3 banks on SH MS2 97.0 74.9 5.02 3.21 1.80 33.4 

3 25.0 3 banks on SH MS2 155.0 77.5 5.26 3.31 1.94 36.5 

4 24.9 3 banks on SH MS2 63.0 77.5 5.27 4.27 1.00 17.3 

13 25.3 3 banks on SH MS2 30.0 70.2 5.35 4.77 0.58 9.4 

14 25.0 2 banks on SH MS2 87.0 70.0 5.44 4.24 1.21 20.5 

16 12.5 3 banks on SH MS2 117.0 77.5 5.25 2.84 2.41 48.3 

17 12.6 2 banks on SH MS2 63.0 77.8 5.26 3.67 1.59 29.7 

30 6.3 2 banks on SH MS2 63.0 77.7 5.28 3.59 1.70 31.9 

31 6.3 3 banks on SH MS2 48.0 74.8 5.36 3.22 2.14 41.9 

1

 Each reactor contains 3 banks with 3 lamps per bank. 

SH – SuperHumeTM 
Appendix F. Case Studies 

UV Disinfection Guidance Manual F-44 November 2006 

For the Final LT2ESWTR 

This page intentionally left blank. 
Appendix G 

Reduction Equivalent Dose Bias Tables 
This page intentionally left blank. 
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-1 November 2006 

For the Final LT2ESWTR 

Tables G.1 – G.17 present RED Bias as a function of water ultraviolet transmittance 

(UVT) and challenge microorganism UV sensitivity for various log inactivation levels (ranging 

from 4.0 – 0.5) for Cryptosporidium, Giardia, and viruses. Tables G.1 – G.8 present RED Bias 

values for Cryptosporidium, Tables G.9 – G.16 present RED Bias values for Giardia, and Table 

G.17 presents RED Bias values for viruses. The RED Bias values for intermediate UVT values 

(e.g., UVT between 85 and 90 percent) can be interpolated from the values in the table, if 

desired. 

Index of Tables 

Table No. Table Title Page 

G.1 RED Bias Values for 4.0-log Cryptosporidium Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

2 

G.2 RED Bias Values for 3.5-log Cryptosporidium Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

3 

G.3 RED Bias Values for 3.0-log Cryptosporidium Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

4 

G.4 RED Bias Values for 2.5-log Cryptosporidium Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

5 

G.5 RED Bias Values for 2.0-log Cryptosporidium Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

6 

G.6 RED Bias Values for 1.5-log Cryptosporidium Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

7 

G.7 RED Bias Values for 1.0-log Cryptosporidium Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

8 

G.8 RED Bias Values for 0.5-log Cryptosporidium Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

9 

G.9 RED Bias Values for 4.0-log Giardia Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

10 

G10. RED Bias Values for 3.5-log Giardia Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

11 

G.11 RED Bias Values for 3.0-log Giardia Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

12 

G.12 RED Bias Values for 2.5-log Giardia Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

13 

G.13 RED Bias Values for 2.0-log Giardia Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

14 

G.14 RED Bias Values for 1.5-log Giardia Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

15 

G.15 RED Bias Values for 1.0-log Giardia Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

16 

G.16 RED Bias Values for 0.5-log Giardia Inactivation Credit as a Function of UVT and UV 

Challenge Microorganism Sensitivity 

17 

G.17 RED Bias Values for Virus Inactivation Credit as a Function of UV Challenge Microorganism 

Sensitivity 

18 
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-2 November 2006 

For the Final LT2ESWTR 

Table G.1. RED Bias Values for 4.0-log Cryptosporidium Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Cryptosporidium log inactivation credit 4.0 

Required UV dose (mJ/cm2) 22 

Cryptosporidium UV sensitivity (mJ/cm2/log I) 5.5 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 4 ≤ 6 1.01 1.02 1.03 1.03 1.03 1.03 1.04

> 6 ≤ 8 1.05 1.09 1.12 1.14 1.16 1.17 1.18

> 8 ≤ 10 1.08 1.15 1.21 1.25 1.27 1.30 1.33

> 10 ≤ 12 1.11 1.20 1.29 1.34 1.38 1.42 1.46

> 12 ≤ 14 1.13 1.24 1.37 1.44 1.49 1.53 1.60

> 14 ≤ 16 1.15 1.28 1.44 1.53 1.59 1.65 1.73

> 16 ≤ 18 1.16 1.31 1.50 1.61 1.69 1.76 1.86

> 18 ≤ 20 1.17 1.34 1.55 1.69 1.78 1.87 1.99

> 20 ≤ 22 1.18 1.36 1.61 1.77 1.87 1.97 2.11

> 22 ≤ 24 1.19 1.38 1.66 1.84 1.96 2.08 2.24

> 24 ≤ 26 1.20 1.40 1.70 1.91 2.05 2.18 2.36

> 26 ≤ 28 1.21 1.41 1.74 1.98 2.14 2.28 2.48

> 28 ≤ 30 1.22 1.43 1.78 2.04 2.22 2.38 2.60

> 30 ≤ 32 1.22 1.44 1.81 2.10 2.30 2.47 2.73

> 32 ≤ 34 1.23 1.45 1.85 2.16 2.38 2.57 2.84
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-3 November 2006 

For the Final LT2ESWTR 

Table G.2. RED Bias Values for 3.5-log Cryptosporidium Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Cryptosporidium log inactivation credit 3.5 

Required UV dose (mJ/cm2) 15 

Cryptosporidium UV sensitivity (mJ/cm2/log I) 4.3 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 4 ≤ 6 1.05 1.08 1.11 1.13 1.14 1.16 1.17

> 6 ≤ 8 1.09 1.16 1.23 1.27 1.30 1.33 1.36

> 8 ≤ 10 1.11 1.22 1.33 1.40 1.44 1.49 1.55

> 10 ≤ 12 1.14 1.27 1.43 1.52 1.58 1.64 1.73

> 12 ≤ 14 1.15 1.30 1.51 1.63 1.71 1.79 1.90

> 14 ≤ 16 1.17 1.33 1.58 1.73 1.84 1.94 2.07

> 16 ≤ 18 1.18 1.36 1.64 1.83 1.96 2.08 2.24

> 18 ≤ 20 1.19 1.38 1.70 1.92 2.08 2.21 2.41

> 20 ≤ 22 1.20 1.40 1.75 2.01 2.19 2.35 2.58

> 22 ≤ 24 1.21 1.42 1.79 2.09 2.30 2.48 2.74

> 24 ≤ 26 1.22 1.43 1.83 2.16 2.40 2.61 2.90

> 26 ≤ 28 1.23 1.45 1.87 2.23 2.50 2.73 3.07

> 28 ≤ 30 1.23 1.46 1.91 2.30 2.60 2.86 3.23

> 30 ≤ 32 1.24 1.47 1.94 2.36 2.69 2.98 3.38

> 32 ≤ 34 1.24 1.48 1.97 2.42 2.78 3.09 3.54
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-4 November 2006 

For the Final LT2ESWTR 

Table G.3. RED Bias Values for 3.0-log Cryptosporidium Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Cryptosporidium log inactivation credit 3.0 

Required UV dose (mJ/cm2) 12 

Cryptosporidium UV sensitivity (mJ/cm2/log I) 4.0 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 4 ≤ 6 1.05 1.10 1.15 1.17 1.19 1.21 1.23

> 6 ≤ 8 1.09 1.18 1.27 1.32 1.36 1.40 1.45

> 8 ≤ 10 1.12 1.23 1.38 1.47 1.52 1.58 1.66

> 10 ≤ 12 1.14 1.27 1.47 1.59 1.68 1.75 1.86

> 12 ≤ 14 1.16 1.31 1.55 1.71 1.82 1.92 2.06

> 14 ≤ 16 1.17 1.33 1.62 1.82 1.96 2.08 2.26

> 16 ≤ 18 1.18 1.36 1.68 1.92 2.09 2.24 2.45

> 18 ≤ 20 1.19 1.38 1.73 2.01 2.22 2.39 2.65

> 20 ≤ 22 1.20 1.39 1.78 2.10 2.34 2.54 2.84

> 22 ≤ 24 1.21 1.41 1.82 2.18 2.45 2.69 3.03

> 24 ≤ 26 1.22 1.42 1.85 2.25 2.56 2.83 3.21

> 26 ≤ 28 1.22 1.43 1.89 2.32 2.66 2.96 3.40

> 28 ≤ 30 1.23 1.44 1.92 2.38 2.76 3.10 3.58

> 30 ≤ 32 1.23 1.45 1.95 2.44 2.86 3.23 3.76

> 32 ≤ 34 1.24 1.46 1.97 2.50 2.95 3.35 3.94
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-5 November 2006 

For the Final LT2ESWTR 

Table G.4. RED Bias Values for 2.5-log Cryptosporidium Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Cryptosporidium log inactivation credit 2.5 

Required UV dose (mJ/cm2) 8.5 

Cryptosporidium UV sensitivity (mJ/cm2/log I) 3.4 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.02 1.04 1.06 1.07 1.07 1.08 1.09

> 4 ≤ 6 1.07 1.14 1.22 1.27 1.30 1.33 1.37

> 6 ≤ 8 1.11 1.21 1.36 1.44 1.50 1.56 1.63

> 8 ≤ 10 1.13 1.26 1.46 1.60 1.69 1.77 1.89

> 10 ≤ 12 1.15 1.30 1.55 1.74 1.87 1.98 2.15

> 12 ≤ 14 1.17 1.32 1.63 1.87 2.03 2.18 2.39

> 14 ≤ 16 1.18 1.35 1.69 1.98 2.19 2.37 2.64

> 16 ≤ 18 1.19 1.37 1.74 2.08 2.34 2.56 2.88

> 18 ≤ 20 1.20 1.38 1.79 2.17 2.47 2.74 3.12

> 20 ≤ 22 1.21 1.40 1.83 2.26 2.60 2.91 3.35

> 22 ≤ 24 1.21 1.41 1.87 2.33 2.72 3.07 3.58

> 24 ≤ 26 1.22 1.42 1.90 2.40 2.84 3.23 3.81

> 26 ≤ 28 1.23 1.43 1.93 2.47 2.95 3.39 4.03

> 28 ≤ 30 1.23 1.44 1.95 2.53 3.05 3.54 4.26

> 30 ≤ 32 1.23 1.45 1.97 2.58 3.15 3.68 4.48

> 32 ≤ 34 1.24 1.45 1.99 2.63 3.24 3.82 4.70
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-6 November 2006 

For the Final LT2ESWTR 

Table G.5. RED Bias Values for 2.0-log Cryptosporidium Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Cryptosporidium log inactivation credit 2.0 

Required UV dose (mJ/cm2) 5.8 

Cryptosporidium UV sensitivity (mJ/cm2/log I) 2.9 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.04 1.08 1.12 1.14 1.16 1.18 1.20

> 4 ≤ 6 1.09 1.17 1.30 1.37 1.42 1.47 1.54

> 6 ≤ 8 1.12 1.23 1.43 1.57 1.66 1.75 1.87

> 8 ≤ 10 1.14 1.27 1.53 1.74 1.88 2.00 2.19

> 10 ≤ 12 1.16 1.31 1.62 1.88 2.08 2.25 2.50

> 12 ≤ 14 1.17 1.33 1.68 2.01 2.26 2.48 2.80

> 14 ≤ 16 1.18 1.35 1.74 2.12 2.43 2.70 3.10

> 16 ≤ 18 1.19 1.37 1.78 2.22 2.58 2.91 3.40

> 18 ≤ 20 1.20 1.38 1.82 2.30 2.73 3.11 3.69

> 20 ≤ 22 1.21 1.39 1.85 2.38 2.86 3.31 3.97

> 22 ≤ 24 1.21 1.40 1.88 2.45 2.98 3.49 4.25

> 24 ≤ 26 1.22 1.41 1.91 2.51 3.09 3.66 4.53

> 26 ≤ 28 1.23 1.42 1.93 2.57 3.20 3.83 4.80

> 28 ≤ 30 1.23 1.42 1.95 2.62 3.30 3.99 5.06

> 30 ≤ 32 1.23 1.43 1.97 2.67 3.39 4.14 5.33

> 32 ≤ 34 1.24 1.44 1.99 2.71 3.48 4.29 5.59
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-7 November 2006 

For the Final LT2ESWTR 

Table G.6. RED Bias Values for 1.5-log Cryptosporidium Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Cryptosporidium log inactivation credit 1.5 

Required UV dose (mJ/cm2) 3.9 

Cryptosporidium UV sensitivity (mJ/cm2/log I) 2.6 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.05 1.10 1.17 1.21 1.24 1.26 1.30

> 4 ≤ 6 1.10 1.18 1.34 1.46 1.54 1.60 1.71

> 6 ≤ 8 1.12 1.23 1.47 1.66 1.80 1.92 2.10

> 8 ≤ 10 1.14 1.27 1.56 1.83 2.04 2.21 2.48

> 10 ≤ 12 1.16 1.30 1.63 1.97 2.25 2.49 2.85

> 12 ≤ 14 1.17 1.32 1.68 2.09 2.43 2.74 3.21

> 14 ≤ 16 1.18 1.33 1.73 2.18 2.60 2.98 3.56

> 16 ≤ 18 1.19 1.35 1.77 2.27 2.75 3.21 3.90

> 18 ≤ 20 1.20 1.36 1.80 2.35 2.89 3.42 4.24

> 20 ≤ 22 1.20 1.37 1.83 2.41 3.01 3.62 4.57

> 22 ≤ 24 1.21 1.37 1.85 2.47 3.13 3.80 4.89

> 24 ≤ 26 1.21 1.38 1.87 2.53 3.23 3.98 5.21

> 26 ≤ 28 1.22 1.39 1.89 2.57 3.33 4.15 5.52

> 28 ≤ 30 1.22 1.39 1.90 2.62 3.42 4.31 5.82

> 30 ≤ 32 1.22 1.40 1.92 2.66 3.51 4.46 6.12

> 32 ≤ 34 1.23 1.40 1.93 2.69 3.59 4.60 6.41
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-8 November 2006 

For the Final LT2ESWTR 

 

Table G.7. RED Bias Values for 1.0-log Cryptosporidium Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Cryptosporidium log inactivation credit 1.0 

Required UV dose (mJ/cm2) 2.5 

Cryptosporidium UV sensitivity (mJ/cm2/log I) 2.5 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.05 1.10 1.18 1.24 1.28 1.32 1.37

> 4 ≤ 6 1.09 1.17 1.34 1.49 1.60 1.69 1.83

> 6 ≤ 8 1.12 1.21 1.45 1.68 1.87 2.04 2.28

> 8 ≤ 10 1.14 1.24 1.52 1.83 2.10 2.35 2.71

> 10 ≤ 12 1.15 1.26 1.58 1.95 2.30 2.63 3.12

> 12 ≤ 14 1.16 1.28 1.62 2.05 2.47 2.89 3.53

> 14 ≤ 16 1.17 1.29 1.66 2.13 2.63 3.12 3.91

> 16 ≤ 18 1.18 1.30 1.69 2.20 2.76 3.34 4.29

> 18 ≤ 20 1.18 1.31 1.71 2.26 2.88 3.54 4.66

> 20 ≤ 22 1.19 1.32 1.73 2.32 2.99 3.73 5.01

> 22 ≤ 24 1.19 1.33 1.75 2.36 3.09 3.90 5.36

> 24 ≤ 26 1.20 1.33 1.76 2.40 3.18 4.06 5.69

> 26 ≤ 28 1.20 1.34 1.78 2.44 3.26 4.22 6.02

> 28 ≤ 30 1.20 1.34 1.79 2.47 3.33 4.36 6.33

> 30 ≤ 32 1.21 1.35 1.80 2.50 3.40 4.49 6.64

> 32 ≤ 34 1.21 1.35 1.81 2.53 3.47 4.62 6.94
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-9 November 2006 

For the Final LT2ESWTR 

Table G.8. RED Bias Values for 0.5-log Cryptosporidium Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Cryptosporidium log inactivation credit 0.5 

Required UV dose (mJ/cm2) 1.6 

Cryptosporidium UV sensitivity (mJ/cm2/log I) 3.2 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.02 1.04 1.07 1.10 1.13 1.15 1.18

> 4 ≤ 6 1.06 1.10 1.19 1.30 1.40 1.49 1.62

> 6 ≤ 8 1.08 1.13 1.27 1.44 1.61 1.78 2.04

> 8 ≤ 10 1.10 1.15 1.32 1.55 1.79 2.03 2.42

> 10 ≤ 12 1.11 1.17 1.36 1.63 1.93 2.24 2.79

> 12 ≤ 14 1.12 1.18 1.39 1.69 2.04 2.43 3.13

> 14 ≤ 16 1.12 1.19 1.41 1.74 2.15 2.60 3.45

> 16 ≤ 18 1.13 1.20 1.43 1.79 2.23 2.75 3.76

> 18 ≤ 20 1.14 1.21 1.45 1.83 2.31 2.89 4.06

> 20 ≤ 22 1.14 1.21 1.46 1.86 2.38 3.02 4.33

> 22 ≤ 24 1.14 1.22 1.47 1.89 2.44 3.13 4.60

> 24 ≤ 26 1.15 1.22 1.48 1.91 2.49 3.24 4.86

> 26 ≤ 28 1.15 1.23 1.49 1.93 2.54 3.33 5.10

> 28 ≤ 30 1.15 1.23 1.50 1.95 2.58 3.43 5.34

> 30 ≤ 32 1.16 1.23 1.51 1.97 2.62 3.51 5.56

> 32 ≤ 34 1.16 1.23 1.51 1.98 2.66 3.59 5.78
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-10 November 2006 

For the Final LT2ESWTR 

Table G.9. RED Bias Values for 4.0-log Giardia Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Giardia log inactivation credit 4.0 

Required UV dose (mJ/cm2) 22 

Giardia UV sensitivity (mJ/cm2/log I) 5.5 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 4 ≤ 6 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 6 ≤ 8 1.01 1.02 1.03 1.03 1.03 1.03 1.04

> 8 ≤ 10 1.05 1.09 1.12 1.14 1.16 1.17 1.18

> 10 ≤ 12 1.08 1.15 1.21 1.25 1.27 1.30 1.33

> 12 ≤ 14 1.11 1.20 1.29 1.34 1.38 1.42 1.46

> 14 ≤ 16 1.13 1.24 1.37 1.44 1.49 1.53 1.60

> 16 ≤ 18 1.15 1.28 1.44 1.53 1.59 1.65 1.73

> 18 ≤ 20 1.16 1.31 1.50 1.61 1.69 1.76 1.86

> 20 ≤ 22 1.17 1.34 1.55 1.69 1.78 1.87 1.99

> 22 ≤ 24 1.18 1.36 1.61 1.77 1.87 1.97 2.11

> 24 ≤ 26 1.19 1.38 1.66 1.84 1.96 2.08 2.24

> 26 ≤ 28 1.20 1.40 1.70 1.91 2.05 2.18 2.36

> 28 ≤ 30 1.21 1.41 1.74 1.98 2.14 2.28 2.48

> 30 ≤ 32 1.22 1.43 1.78 2.04 2.22 2.38 2.60

> 32 ≤ 34 1.22 1.44 1.81 2.10 2.30 2.47 2.73
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-11 November 2006 

For the Final LT2ESWTR 

Table G.10. RED Bias Values for 3.5-log Giardia Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Giardia log inactivation credit 3.5 

Required UV dose (mJ/cm2) 15 

Giardia UV sensitivity (mJ/cm2/log I) 4.3 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 4 ≤ 6 1.05 1.08 1.11 1.13 1.14 1.16 1.17

> 6 ≤ 8 1.09 1.16 1.23 1.27 1.30 1.33 1.36

> 8 ≤ 10 1.11 1.22 1.33 1.40 1.44 1.49 1.55

> 10 ≤ 12 1.14 1.27 1.43 1.52 1.58 1.64 1.73

> 12 ≤ 14 1.15 1.30 1.51 1.63 1.71 1.79 1.90

> 14 ≤ 16 1.17 1.33 1.58 1.73 1.84 1.94 2.07

> 16 ≤ 18 1.18 1.36 1.64 1.83 1.96 2.08 2.24

> 18 ≤ 20 1.19 1.38 1.70 1.92 2.08 2.21 2.41

> 20 ≤ 22 1.20 1.40 1.75 2.01 2.19 2.35 2.58

> 22 ≤ 24 1.21 1.42 1.79 2.09 2.30 2.48 2.74

> 24 ≤ 26 1.22 1.43 1.83 2.16 2.40 2.61 2.90

> 26 ≤ 28 1.23 1.45 1.87 2.23 2.50 2.73 3.07

> 28 ≤ 30 1.23 1.46 1.91 2.30 2.60 2.86 3.23

> 30 ≤ 32 1.24 1.47 1.94 2.36 2.69 2.98 3.38

> 32 ≤ 34 1.24 1.48 1.97 2.42 2.78 3.09 3.54
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-12 November 2006 

For the Final LT2ESWTR 

Table G.11. RED Bias Values for 3.0-log Giardia Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Giardia log inactivation credit 3.0 

Required UV dose (mJ/cm2) 11 

Giardia UV sensitivity (mJ/cm2/log I) 3.7 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.01 1.02 1.03 1.03 1.03 1.03 1.04

> 4 ≤ 6 1.07 1.12 1.18 1.21 1.23 1.25 1.28

> 6 ≤ 8 1.10 1.20 1.31 1.37 1.41 1.45 1.51

> 8 ≤ 10 1.13 1.25 1.42 1.52 1.58 1.65 1.74

> 10 ≤ 12 1.15 1.29 1.51 1.65 1.74 1.83 1.95

> 12 ≤ 14 1.16 1.32 1.59 1.77 1.90 2.01 2.17

> 14 ≤ 16 1.18 1.35 1.66 1.89 2.04 2.18 2.38

> 16 ≤ 18 1.19 1.37 1.72 1.99 2.18 2.34 2.59

> 18 ≤ 20 1.20 1.39 1.77 2.08 2.31 2.51 2.79

> 20 ≤ 22 1.21 1.41 1.81 2.17 2.43 2.66 2.99

> 22 ≤ 24 1.22 1.42 1.85 2.25 2.55 2.81 3.19

> 24 ≤ 26 1.22 1.44 1.89 2.32 2.66 2.96 3.39

> 26 ≤ 28 1.23 1.45 1.92 2.39 2.77 3.11 3.59

> 28 ≤ 30 1.24 1.46 1.95 2.46 2.87 3.25 3.78

> 30 ≤ 32 1.24 1.47 1.98 2.52 2.97 3.38 3.98

> 32 ≤ 34 1.25 1.47 2.00 2.57 3.06 3.52 4.17
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-13 November 2006 

For the Final LT2ESWTR 

Table G.12. RED Bias Values for 2.5-log Giardia Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Giardia log inactivation credit 2.5 

Required UV dose (mJ/cm2) 7.7 

Giardia UV sensitivity (mJ/cm2/log I) 3.1 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.03 1.06 1.09 1.11 1.12 1.13 1.14

> 4 ≤ 6 1.08 1.16 1.26 1.32 1.35 1.39 1.44

> 6 ≤ 8 1.12 1.23 1.40 1.50 1.57 1.63 1.73

> 8 ≤ 10 1.14 1.28 1.51 1.67 1.77 1.87 2.00

> 10 ≤ 12 1.16 1.32 1.60 1.81 1.96 2.09 2.27

> 12 ≤ 14 1.18 1.34 1.67 1.94 2.13 2.30 2.54

> 14 ≤ 16 1.19 1.37 1.74 2.06 2.29 2.50 2.80

> 16 ≤ 18 1.20 1.39 1.79 2.16 2.45 2.70 3.06

> 18 ≤ 20 1.21 1.40 1.83 2.25 2.59 2.88 3.31

> 20 ≤ 22 1.22 1.42 1.87 2.34 2.72 3.07 3.56

> 22 ≤ 24 1.22 1.43 1.91 2.41 2.85 3.24 3.81

> 24 ≤ 26 1.23 1.44 1.94 2.48 2.97 3.41 4.05

> 26 ≤ 28 1.23 1.45 1.97 2.55 3.08 3.57 4.30

> 28 ≤ 30 1.24 1.46 1.99 2.61 3.18 3.73 4.53

> 30 ≤ 32 1.24 1.46 2.01 2.66 3.28 3.88 4.77

> 32 ≤ 34 1.25 1.47 2.03 2.71 3.38 4.02 5.00
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-14 November 2006 

For the Final LT2ESWTR 

Table G.13. RED Bias Values for 2.0-log Giardia Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Giardia log inactivation credit 2.0 

Required UV dose (mJ/cm2) 5.2 

Giardia UV sensitivity (mJ/cm2/log I) 2.6 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.05 1.11 1.17 1.20 1.22 1.24 1.27

> 4 ≤ 6 1.10 1.20 1.35 1.44 1.50 1.56 1.64

> 6 ≤ 8 1.13 1.26 1.48 1.65 1.76 1.85 1.99

> 8 ≤ 10 1.15 1.30 1.59 1.82 1.99 2.13 2.34

> 10 ≤ 12 1.17 1.33 1.67 1.97 2.19 2.39 2.67

> 12 ≤ 14 1.18 1.36 1.74 2.10 2.39 2.64 3.00

> 14 ≤ 16 1.20 1.37 1.79 2.21 2.56 2.87 3.32

> 16 ≤ 18 1.21 1.39 1.84 2.31 2.72 3.09 3.64

> 18 ≤ 20 1.21 1.40 1.87 2.40 2.87 3.30 3.95

> 20 ≤ 22 1.22 1.41 1.91 2.48 3.01 3.51 4.25

> 22 ≤ 24 1.23 1.42 1.94 2.55 3.13 3.70 4.55

> 24 ≤ 26 1.23 1.43 1.96 2.61 3.25 3.88 4.85

> 26 ≤ 28 1.24 1.44 1.98 2.67 3.36 4.06 5.14

> 28 ≤ 30 1.24 1.45 2.01 2.72 3.46 4.22 5.43

> 30 ≤ 32 1.24 1.45 2.02 2.77 3.56 4.38 5.71

> 32 ≤ 34 1.25 1.46 2.04 2.81 3.65 4.54 5.98
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-15 November 2006 

For the Final LT2ESWTR 

Table G.14. RED Bias Values for 1.5-log Giardia Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Giardia log inactivation credit 1.5 

Required UV dose (mJ/cm2) 3.0 

Giardia UV sensitivity (mJ/cm2/log I) 2.0 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.08 1.16 1.28 1.35 1.40 1.45 1.51

> 4 ≤ 6 1.13 1.25 1.47 1.63 1.75 1.84 1.99

> 6 ≤ 8 1.15 1.30 1.60 1.86 2.05 2.21 2.45

> 8 ≤ 10 1.17 1.33 1.69 2.04 2.31 2.55 2.89

> 10 ≤ 12 1.19 1.36 1.77 2.19 2.54 2.86 3.32

> 12 ≤ 14 1.20 1.38 1.82 2.31 2.75 3.15 3.75

> 14 ≤ 16 1.21 1.39 1.87 2.42 2.93 3.42 4.16

> 16 ≤ 18 1.22 1.41 1.91 2.51 3.10 3.68 4.56

> 18 ≤ 20 1.23 1.42 1.94 2.59 3.25 3.92 4.96

> 20 ≤ 22 1.23 1.43 1.96 2.66 3.39 4.14 5.34

> 22 ≤ 24 1.24 1.43 1.99 2.72 3.51 4.35 5.72

> 24 ≤ 26 1.24 1.44 2.01 2.78 3.63 4.55 6.08

> 26 ≤ 28 1.25 1.45 2.03 2.83 3.74 4.74 6.44

> 28 ≤ 30 1.25 1.45 2.04 2.87 3.84 4.92 6.80

> 30 ≤ 32 1.25 1.46 2.06 2.91 3.93 5.09 7.14

> 32 ≤ 34 1.26 1.46 2.07 2.95 4.01 5.25 7.48
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-16 November 2006 

For the Final LT2ESWTR 

Table G.15. RED Bias Values for 1.0-log Giardia Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Giardia log inactivation credit 1.0 

Required UV dose (mJ/cm2) 2.1 

Giardia UV sensitivity (mJ/cm2/log I) 2.1 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.07 1.14 1.26 1.34 1.40 1.45 1.52

> 4 ≤ 6 1.11 1.21 1.42 1.61 1.75 1.86 2.04

> 6 ≤ 8 1.14 1.25 1.53 1.82 2.04 2.24 2.53

> 8 ≤ 10 1.16 1.28 1.61 1.98 2.29 2.58 3.01

> 10 ≤ 12 1.17 1.30 1.67 2.10 2.51 2.89 3.47

> 12 ≤ 14 1.18 1.32 1.72 2.21 2.70 3.17 3.91

> 14 ≤ 16 1.19 1.33 1.75 2.30 2.86 3.43 4.35

> 16 ≤ 18 1.20 1.34 1.78 2.37 3.01 3.67 4.77

> 18 ≤ 20 1.20 1.35 1.81 2.44 3.14 3.89 5.17

> 20 ≤ 22 1.21 1.36 1.83 2.49 3.25 4.10 5.57

> 22 ≤ 24 1.21 1.37 1.84 2.54 3.36 4.29 5.95

> 24 ≤ 26 1.22 1.37 1.86 2.58 3.46 4.47 6.32

> 26 ≤ 28 1.22 1.38 1.87 2.62 3.55 4.63 6.68

> 28 ≤ 30 1.22 1.38 1.89 2.66 3.63 4.79 7.03

> 30 ≤ 32 1.22 1.38 1.90 2.69 3.70 4.94 7.38

> 32 ≤ 34 1.23 1.39 1.91 2.72 3.77 5.08 7.71
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-17 November 2006 

For the Final LT2ESWTR 

Table G.16. RED Bias Values for 0.5-log Giardia Inactivation Credit as a 

Function of UVT and UV Challenge Microorganism Sensitivity 

Giardia log inactivation credit 0.5 

Required UV dose (mJ/cm2) 1.5 

Giardia UV sensitivity (mJ/cm2/log I) 3.0 

UVT (%) ≥ 98 ≥ 95 ≥ 90 ≥ 85 ≥ 80 ≥ 75 ≥ 65

Challenge UV sensitivity (mJ/cm2/log I) 

Lower Upper 

RED Bias 

0 ≤ 2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 2 ≤ 4 1.03 1.05 1.09 1.14 1.17 1.20 1.24

> 4 ≤ 6 1.06 1.11 1.22 1.34 1.45 1.55 1.70

> 6 ≤ 8 1.09 1.14 1.29 1.49 1.67 1.85 2.13

> 8 ≤ 10 1.10 1.16 1.35 1.59 1.85 2.11 2.54

> 10 ≤ 12 1.11 1.18 1.39 1.68 2.00 2.34 2.92

> 12 ≤ 14 1.12 1.19 1.42 1.74 2.12 2.54 3.28

> 14 ≤ 16 1.13 1.20 1.44 1.80 2.23 2.72 3.63

> 16 ≤ 18 1.14 1.21 1.46 1.84 2.32 2.88 3.95

> 18 ≤ 20 1.14 1.22 1.48 1.88 2.40 3.02 4.26

> 20 ≤ 22 1.15 1.22 1.49 1.91 2.47 3.15 4.56

> 22 ≤ 24 1.15 1.23 1.50 1.94 2.53 3.28 4.84

> 24 ≤ 26 1.15 1.23 1.51 1.97 2.59 3.39 5.11

> 26 ≤ 28 1.16 1.24 1.52 1.99 2.64 3.49 5.37

> 28 ≤ 30 1.16 1.24 1.53 2.01 2.69 3.58 5.62

> 30 ≤ 32 1.16 1.24 1.53 2.03 2.73 3.67 5.86

> 32 ≤ 34 1.16 1.25 1.54 2.04 2.77 3.76 6.09
Appendix G. Reduction Equivalent Dose Bias Tables 

UV Disinfection Guidance Manual G-18 November 2006 

For the Final LT2ESWTR 

Table G.17. RED Bias Values for Virus Inactivation Credit as a 

Function of UV Challenge Microorganism Sensitivity 

Virus log inactivation credit 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 

Required UV dose (mJ/cm2) 39 58 79 100 121 143 163 186 

Virus UV sensitivity (mJ/cm2/log I) 78 58 53 50 48 48 47 47 

Challenge UV sensitivity (mJ/cm2/log I)

Lower Upper 

RED Bias 

> 1 ≤ 25 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

> 25 ≤ 50 1.00 1.00 1.00 1.00 1.01 1.01 1.01 1.01

> 50 ≤ 60 1.00 1.00 1.02 1.03 1.03 1.03 1.04 1.04

> 60 ≤ 70 1.00 1.02 1.04 1.05 1.05 1.05 1.06 1.06

> 70 ≤ 80 1.00 1.04 1.05 1.06 1.07 1.07 1.07 1.07

> 80 ≤ 90 1.01 1.05 1.06 1.07 1.08 1.08 1.08 1.08

> 90 ≤ 100 1.02 1.06 1.07 1.08 1.09 1.09 1.09 1.09

> 90 ≤ 100 1.02 1.06 1.07 1.08 1.09 1.09 1.09 1.09