LONG TERM 2 ENHANCED SURFACE 

WATER TREATMENT RULE

TOOLBOX GUIDANCE MANUAL

 

 
Office of Water (4606) 

EPA 815-R-09-016

April 2010 

www.epa.gov/safewater 
LT2ESWTR Toolbox Guidance Manual i April 2010

Purpose: 

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

applying the “Toolbox” of Cryptosporidium treatment and management strategies that are part of 

the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR). 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 Toolbox options, the guidance presented here may not be appropriate for all 

situations, and alternative approaches 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 The Cadmus Group, Inc. and Carollo Engineers. 

 Questions should be addressed to: 

Michael Finn

U.S. Environmental Protection Agency

Mail Code 4606M 

1200 Pennsylvania Avenue, NW 

Washington, DC 20460-0001 

Tel: (202) 564-5261 

Fax: (202) 564-3767 

Email: finn.michael@epa.gov 
LT2ESWTR Toolbox Guidance Manual ii April 2010

CONTENTS

Exhibits .....................................................................................................................xi 

 Appendices ................................................................................................................xiii

Acronyms ..................................................................................................................xiv 

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

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

 1.2 Guidance Manual Organization .....................................................................1-2 

 1.3 Regulatory History .........................................................................................1-2 

 1.3.1 Surface Water Treatment Rule ...........................................................1-3 

 1.3.2 Interim Enhanced Surface Water Treatment Rule .............................1-3 

 1.3.3 Stage 1 Disinfectants and Disinfection Byproducts Rule ..................1-4 

 1.3.4 Long Term 1 Enhanced Surface Water Treatment Rule ....................1-4 

 1.3.5 Stage 2 Disinfectant and Disinfection Byproduct Rule .....................1-4 

 1.4 Overview of the Long Term 2 Enhanced Surface Water Treatment Rule .....1-6 

 1.4.1 Monitoring and Treatment Requirements for Filtered Systems .........1-6 

 1.4.2 Monitoring and Treatment Requirements for Unfiltered Systems .....1-7 

 1.4.3 Summary of Microbial Toolbox Options ...........................................1-8 

 1.4.4 Requirements for PWSs with Uncovered Finished Water 

 Reservoirs ..........................................................................................1-10 

 1.4.5 Disinfection Profiling and Benchmarking Requirements ..................1-10 

 1.5 LT2ESWTR Implementation Schedule .........................................................1-11 

2. Watershed Control Program ..................................................................................2-1 

 2.1 Introduction ....................................................................................................2-1 

 2.1.1 Credits Available ...............................................................................2-1 

 2.2 Application Process for the WCP Credit (PWS and 

 State Responsibilities) ....................................................................................2-2 

 2.2.1 Notifying the State of Intention to Participate ...................................2-3 

 2.2.2 Preparation of Watershed Control Program Plan ...............................2-4 

 2.2.2.1 Delineation of Area of Influence ...........................................2-4 

 2.2.2.2 Identification of Cryptosporidium Sources ...........................2-5 

 2.2.2.3 Analysis of Control Measures ...............................................2-5 

 2.2.2.4 Partnerships for Source Water Protection .............................2-5 

 2.2.3 Approval and Continuation of the WCP Credit .................................2-5 

 2.2.3.1 Initial Approval of the WCP Plan .........................................2-5 

 2.2.3.2 Maintenance of the WCP Credit ...........................................2-6 

 2.2.3.2.1 Annual Status Report .............................................2-7 

 2.2.3.2.2 Watershed Sanitary Survey Report ........................2-7 

 2.2.3.3 State Review and Continuation of the WCP Credit ..............2-9 

 2.2.4 PWS and State Checklist for Preparation, Implementation, 

 and Maintenance of the WCP Plan and Associated Credit ................2-9 
LT2ESWTR Toolbox Guidance Manual iii April 2010

 2.3 Benefits and Other Characteristics of the WCP Credit and 

 Related Activities ...........................................................................................2-13 

 2.3.1 Benefits to the PWS and Watershed from a Successful WCP ...........2-13 

 2.3.2 Advantages and Disadvantages of a Watershed Control Program .....2-14 

 2.3.2.1 Advantages ............................................................................2-14 

 2.3.2.2 Disadvantages .......................................................................2-15 

 2.3.3 Incorporation of New Versus Existing Source Water Protection 

 Activities Into a Watershed Control Program ....................................2-17 

 2.4 Tools to Help PWSs Develop the Watershed Control Program Plan ............2-17 

 2.4.1 Identification of the Area of Influence ...............................................2-19 

 2.4.2 Potential and Existing Sources of Cryptosporidium ..........................2-23 

 2.4.2.1 How Do Fate and Transport Affect the Way 

 Cryptosporidium Impacts My Water Supply? ......................2-26 

 2.4.2.2 What Role Should Monitoring Play in the Evaluation 

 of Potential and Existing Sources of Cryptosporidium? ......2-30 

 2.4.3 Analysis of Control Measures ............................................................2-32 

 2.4.3.1 Available Regulatory and Management Strategies ...............2-32 

 2.4.3.2 Partnerships in Watershed Control Plans ..............................2-35 

 2.4.3.3 Addressing Point Sources .....................................................2-36 

 2.4.3.4 Addressing Nonpoint Sources ...............................................2-38 

 2.4.3.5 Is Purchase/Ownership of All or Part of the Watershed 

 a Viable Option? ...................................................................2-43 

 2.5 References ......................................................................................................2-45 

3. Alternative Source/Intake .......................................................................................3-1 

 3.1 Introduction................. ...................................................................................3-1 

 3.2 Changing Sources ..........................................................................................3-1 

 3.2.1 Advantages and Disadvantages ..........................................................3-2 

 3.2.2 Evaluation of Source Water Characteristics for Existing 

 Treatment Requirements ....................................................................3-2 

 3.3 Changing Intake Locations ............................................................................3-2 

 3.3.1 Applicability.......................................................................................3-3 

 3.3.1.1 Advantages and Disadvantages ..............................................3-3 

 3.3.2 Reservoirs and Lakes .........................................................................3-3 

 3.3.2.1 Depth ......................................................................................3-3 

 3.3.2.2 Stratification and Mixing .......................................................3-4 

 3.3.2.3 Proximity to Inflows ..............................................................3-4 

 3.3.3 Streams and Rivers ............................................................................3-4 

 3.3.3.1 Depth ......................................................................................3-4 

 3.3.3.2 Flow and River Hydraulics ....................................................3-5 

 3.3.3.3 Upstream Sources of Contamination .....................................3-5 

 3.3.3.4 Seasonal Effects .....................................................................3-5 
LT2ESWTR Toolbox Guidance Manual iv April 2010

 3.4 Changing Timing of Withdrawals..................................................................3-5 

 3.4.1 Toolbox Selection Considerations .....................................................3-5 

 3.4.1.1 Advantages and Disadvantages ..............................................3-6 

 3.5 References................. .....................................................................................3-6 

4. Bank Filtration .........................................................................................................4-1 

 4.1 Introduction ....................................................................................................4-1 

 4.2 LT2ESWTR Compliance Requirements ........................................................4-2 

 4.2.1 Credits ................................................................................................4-3 

 4.2.2 Monitoring Requirements ..................................................................4-4 

 4.3 Toolbox Selection Considerations .................................................................4-5 

 4.3.1 Advantages and Disadvantages ..........................................................4-5 

4.3.1.1 Removal of Additional Contaminants ..................................4-5 

4.3.1.2 Clogging of Pores..................................................................4-7 

4.3.1.3 Scour .....................................................................................4-8 

4.3.1.4 Additional Treatment Steps ...................................................4-8 

 4.4 Site Selection and Aquifer Requirements ......................................................4-9 

4.4.1 Selected Bank Filtration Sites ............................................................4-10 

4.4.2 Aquifer Type ......................................................................................4-10 

4.4.2.1 Unconsolidated, Granular Aquifers ......................................4-10 

4.4.2.2 Karst, Consolidated Clastic, and Fractured Bedrock 

Aquifers ..................................................................................4-11 

4.4.2.3 Partially Consolidated, Granular Aquifers ............................4-11 

4.4.3 Aquifer Characterization ....................................................................4-12 

4.4.3.1 Coring ...................................................................................4-13 

4.4.3.2 Sieve Analysis .......................................................................4-14 

 4.4.4 Site Selection as it Relates to Scour ...................................................4-15 

4.4.4.1 Stream Channel Erosional Processes ....................................4-15 

4.4.4.2 Unsuitable Sites ....................................................................4-17 

 4.5 Design and Construction ................................................................................4-20 

4.5.1 Well Type ...........................................................................................4-20 

4.5.2 Filtrate Flow Path and Well Location ................................................4-25 

4.5.2.1 Required Separation Distance Between a Well and the Surface 

Water Source ..........................................................................4-25 

4.5.2.2 Locating Wells at Greater than Required Distances from the 

Surface Water Source.............................................................4-25 

4.5.2.3 Delineating the Edge of the Surface Water Source ................4-29 

4.5.2.4 Measuring Separation Distances for Horizontal Wells and Wells 

that are Neither Horizontal Nor Vertical ................................4-30 

 4.6 Operational Considerations ............................................................................4-31 

4.6.1 High River Stage ................................................................................4-31 

4.6.2 Implications of Scour for Bank Filtration System Operations ...........4-31 

4.6.3 Anticipating High Flow Events / Flooding ........................................4-32 
LT2ESWTR Toolbox Guidance Manual v April 2010

4.6.4 Possible Responses to Spill Events and Poor Surface Water 

Quality ................................................................................................4-32 

4.6.5 Maintaining Required Separation Distances ......................................4-32 

 4.7 Demonstration of Performance ......................................................................4-33 

4.7.1 Identification of Collection Devices and Alternative Treatment 

Technologies at the Site.......................................................................4-34 

4.7.2 Source Water Quality and Quantity .....................................................4-35 

4.7.3 Ground Water Travel and Residence Time Calculations and Ambient 

Ground Water Dilution ........................................................................4-35 

4.7.4 Surface and Ground Water Data Collection, Methods and Sampling 

Locations .............................................................................................4-36 

4.7.5 Monitoring Tools ................................................................................4-38 

4.7.6 Tracer Tests and Use of Isotopes ........................................................4-44 

4.7.7 Monitoring Wells Located Along the Shortest Flow Path ..................4-45 

4.7.8 Post-decision Routine Monitoring and Sampling ...............................4-45 

 4.8 Reference .......................................................................................................4-45 

5. Presedimentation................. .....................................................................................5-1 

 5.1 Introduction................. ...................................................................................5-1 

 5.2 LT2ESWTR Compliance Requirements ........................................................5-1 

 5.2.1 Credits................. ...............................................................................5-1 

 5.2.2 Monitoring Requirements ..................................................................5-2 

 5.2.3 Calculations........................................................................................5-2 

 5.3 Toolbox Selection Considerations .................................................................5-3 

 5.3.1 Source Water Quality .........................................................................5-3 

 5.3.2 Advantages and Disadvantages of Installing a 

 Presedimentation Basin ......................................................................5-4 

 5.4 Types of Sedimentation Basins ......................................................................5-4 

 5.4.1 Horizontal Flow .................................................................................5-7 

 5.4.1.1 Rectangular ............................................................................5-7 

 5.4.1.2 Circular ..................................................................................5-7 

 5.4.2 Upflow Clarifier .................................................................................5-7 

 5.4.3 Reactor Clarifier .................................................................................5-7 

 5.4.4 High Flow Rate Designs ....................................................................5-8 

 5.4.5 Ballasted Flocculation ........................................................................5-8 

 5.5 Design and Operational Issues .......................................................................5-8 

 5.5.1 Redundancy ........................................................................................5-8 

 5.5.2 Short Circuiting ..................................................................................5-8 

 5.5.3 Sludge Removal .................................................................................5-9 

 5.5.4 Coagulant Addition and Dose Ranges of Common Coagulants ........5-9 

 5.6 References................. .....................................................................................5-10 
LT2ESWTR Toolbox Guidance Manual vi April 2010

6. Lime Softening.................. .......................................................................................6-1 

 6.1 Introduction................. ...................................................................................6-1 

 6.2 LT2ESWTR Compliance Requirements ........................................................6-1 

 6.2.1 Credit for Cryptosporidium Removal ................................................6-1 

 6.2.2 Reporting Requirements ....................................................................6-2 

 6.3 Split-Flow Processes ......................................................................................6-3 

7. Combined and Individual Filter Performance ......................................................7-1 

 7.1 Introduction................. ...................................................................................7-1 

 7.2 LT2ESWTR Compliance Requirements ........................................................7-2 

 7.2.1 Treatment Credit ................................................................................7-2 

 7.2.2 Monitoring Requirements ..................................................................7-2 

 7.2.2.1 Combined Filter Effluent ......................................................7-2 

 7.2.2.2 Individual Filter Effluent ......................................................7-3 

 7.2.3 Turbidity Monitors .............................................................................7-3 

 7.2.3.1 Methods.................................................................................7-4 

 7.2.3.2 Maintenance and Calibration ................................................7-4 

 7.2.3.3 Quality Assurance / Quality Control (QA/QC) .....................7-5 

 7.3 Reporting Requirements ................................................................................7-5 

 7.3.1 Combined Filter Performance ............................................................7-5 

 7.3.2 Individual Filter Performance ............................................................7-6 

 7.4 Process Control Techniques ...........................................................................7-6 

 7.4.1 Chemical Feed ...................................................................................7-10 

 7.4.1.1 Type of Chemical and Dose ..................................................7-11 

 7.4.1.2 Mixing ...................................................................................7-11 

 7.4.1.3 Streaming Current Detectors and Zeta Potential Monitors ...7-12 

 7.4.2 Flocculation ........................................................................................7-12 

 7.4.3 Sedimentation ....................................................................................7-13 

 7.4.4 Filtration .............................................................................................7-14 

 7.4.4.1 Flow Split ..............................................................................7-14 

 7.4.4.2 Filter Beds .............................................................................7-14 

 7.4.4.3 Backwashing .........................................................................7-15 

 7.4.4.4 Filter to Waste .......................................................................7-16 

 7.4.4.5 Backwash Recycle ................................................................7-16 

 7.4.4.6 Filter Assessments ................................................................7-17 

 7.4.5 Hydraulic Control ..............................................................................7-17

 7.5 Process Management Techniques ..................................................................7-17 

 7.5.1 Standard Operating Procedures (SOPs) .........................................................7-17 

 7.5.2 Prevention and Response Plan for Loss of Chemical Feed............................7-18 

 7.5.3 Adequate Chemical Storage ...........................................................................7-18 

 7.5.4 Voluntary Programs .......................................................................................7-18 

 7.5.4.1 Partnership for Safe Water ................................................................7-19 

 7.5.4.2 Composite Correction Program (CCP) .............................................7-19

 7.6 References................. .....................................................................................7-20 
LT2ESWTR Toolbox Guidance Manual vii April 2010

8. Bag and Cartridge Filters .......................................................................................8-1 

 8.1 Introduction................. ...................................................................................8-1 

 8.2 LT2ESWTR Compliance Requirements ........................................................8-2 

 8.2.1 Credits................. ...............................................................................8-2 

 8.2.2 Reporting Requirements ....................................................................8-2 

 8.2.3 Integration into a Treatment Process Train ........................................8-3 

 8.3 Toolbox Selection Considerations .................................................................8-5 

 8.3.1 Advantages .........................................................................................8-5 

 8.3.2 Disadvantages ....................................................................................8-5 

 8.4 Challenge Testing ..........................................................................................8-5 

 8.4.1 Testing Conditions .............................................................................8-6 

 8.4.1.1 Full Scale Filter Testing ........................................................8-6 

 8.4.1.2 Challenge Particulate ............................................................8-6 

 8.4.1.3 Test Solution Concentration .................................................8-7 

 8.4.1.4 Challenge Test Duration .......................................................8-7 

 8.4.1.5 Water Quality of Test Solution .............................................8-8 

 8.4.1.6 Maximum Design Flow Rate ................................................8-8 

 8.4.1.7 Challenge Particulate Seeding Method .................................8-8 

 8.4.1.8 Challenge Test Solution Volume ..........................................8-9 

 8.4.1.9 Sampling ...............................................................................8-9 

 8.4.2 Calculating Log Removal (141.719(a)(7)-(9)) ...................................8-10 

 8.4.3 Modifications to Filtration Unit after Challenge Testing 

 (141.719(a)(10)) .................................................................................8-11 

 8.5 Design Considerations ...................................................................................8-11 

 8.5.1 Water Quality .....................................................................................8-14 

 8.5.2 Size of Filter System and Redundancy ..............................................8-14 

 8.5.3 Design Layout ....................................................................................8-15 

 8.5.4 Filter Cycling .....................................................................................8-15 

 8.5.5 Pressure Monitoring, Valves, and Appurtenances .............................8-15 

 8.5.6 Air Entrapment ...................................................................................8-16 

 8.5.7 NSF Certification ...............................................................................8-16 

 8.6 Operational Issues ..........................................................................................8-16 

 8.6.1 Pressure Drop (Inlet/Outlet Pressures) ...............................................8-16 

 8.6.2 Water Quality Monitoring ..................................................................8-16 

 8.7 References................................ ......................................................................8-17 

9. Second Stage Filtration ............................................................................................9-1 

 9.1 Introduction................................ ....................................................................9-1 

 9.2 LT2ESWTR Compliance Requirements ........................................................9-1 

 9.2.1 Credits................................ ................................................................9-1 

 9.2.2 Reporting Requirements ....................................................................9-2 

 9.3 Toolbox Selection Considerations .................................................................9-2 

 9.3.1 Advantages .........................................................................................9-3 
LT2ESWTR Toolbox Guidance Manual viii April 2010

 9.3.2 Disadvantages ....................................................................................9-3 

 9.4 Design and Operational Considerations .........................................................9-3 

 9.4.1 Hydraulic Requirements ....................................................................9-4 

 9.4.2 Backwashing ......................................................................................9-4 

 9.4.3 Turbidity Monitoring .........................................................................9-4 

10. Chlorine Dioxide.............................. ........................................................................10-1 

 10.1 Introduction................................ ....................................................................10-1 

 10.2 Log Inactivation Requirements ......................................................................10-2 

 10.2.1 CT Calculation ...................................................................................10-3 

 10.3 Monitoring Requirements ..............................................................................10-6 

 10.3.1 LT2ESWTR .......................................................................................10-6 

 10.3.2 Stage 1 DBPR ....................................................................................10-6 

 10.4 Unfiltered System LT2ESWTR Requirements ..............................................10-7 

 10.5 Disinfection with Chlorine Dioxide ...............................................................10-7 

 10.6 Toolbox Selection Considerations .................................................................10-8 

 10.6.1 Advantages .........................................................................................10-8 

 10.6.2 Disadvantages ....................................................................................10-8 

 10.7 Design Considerations ...................................................................................10-9 

 10.7.1 Designing to Lowest Temperature .....................................................10-9 

 10.7.2 Point of Addition ...............................................................................10-10 

 10.8 Operational Considerations ............................................................................10-10 

 10.9 Safety Issues................................ ...................................................................10-11 

 10.9.1 Chemical Storage ...............................................................................10-11 

 10.9.2 Acute Health Risks of Chlorine Dioxide ...........................................10-11 

 10.10 References................................ ......................................................................10-11 

 

11. Ozone................................ .........................................................................................11-1 

 11.1 Introduction................................ ...................................................................11-1 

 11.2 Credits................................ ............................................................................11-2 

 11.3 CT Determination ..........................................................................................11-4 

 11.3.1 Measuring C for T10 and CSTR Methods ..........................................11-7 

 11.3.2 T10 Method .........................................................................................11-7 

 11.3.3 CSTR Method ....................................................................................11-10 

 11.3.4 Extended T10 and Extended CSTR Methods .....................................11-13 

 11.4 Monitoring Requirements ..............................................................................11-13 

 11.4.1 LT2ESWTR .......................................................................................11-13 

 11.4.2 Stage 1 DBPR and Stage 2 DBPR .....................................................11-13 

 11.5 Unfiltered System LT2ESWTR Requirements ..............................................11-14 

 11.6 Toolbox Selection ..........................................................................................11-14 

 11.6.1 Advantages .........................................................................................11-15 

 11.6.2 Disadvantages ....................................................................................11-15 

 11.7 Disinfection With Ozone ...............................................................................11-16 

 11.7.1 Chemistry ...........................................................................................11-16 
LT2ESWTR Toolbox Guidance Manual ix April 2010

 11.7.2 Byproduct Formation .........................................................................11-18 

 11.7.2.1 Bromate and Brominated Organic Compounds ..................11-18 

 11.7.2.2 Non-Brominated Organic Compounds ...............................11-18 

 11.8 Design................................ ............................................................................11-19 

 11.8.1 Generators and Contactors .................................................................11-19 

 11.8.2 Point of Addition ...............................................................................11-19 

 11.8.3 Biologically Active Filters ................................................................11-20 

 11.8.3.1 Media for Biologically Active Filters .................................11-20 

 11.8.3.2 Operating Biologically Active Filters .................................11-20 

 11.9 Safety Considerations in Design ....................................................................11-21 

 11.10 Operational Considerations ............................................................................11-21 

 11.10.1 Ozone Demand .................................................................................11-21 

 11.10.2 pH .....................................................................................................11-22 

 11.10.3 Temperature ......................................................................................11-22 

 11.10.4 Maintaining Residual Disinfectant in the 

 Distribution System ..........................................................................11-22 

 11.11 References................................ ......................................................................11-23 

12. Demonstration of Performance (DOP) ...................................................................12-1 

 12.1 Introduction................................ ....................................................................12-1 

 12.2 LT2ESWTR Compliance Requirements ........................................................12-2 

 12.2.1 Credits ...............................................................................................12-2 

 12.2.2 Reporting Requirements ...................................................................12-3 

 12.3 Toolbox Selection Considerations .................................................................12-3 

 12.3.1 Overview of the Demonstration Protocol ..........................................12-4 

 12.4 DOP Criteria Development ............................................................................12-5 

 12.4.1 Process Evaluation Criteria ...............................................................12-5 

 12.4.1.1 Treatment Objectives ..........................................................12-5 

 12.4.1.2 Influent Water Quality Characteristics ................................12-6 

 12.4.1.3 System Flow Rate ...............................................................12-6 

 12.4.1.4 Plant Operating Conditions .................................................12-7 

 12.4.2 Selection of Performance Indicators .................................................12-7 

 12.4.2.1 Surrogate Parameters for Cryptosporidium ........................12-7 

 12.4.2.2 Long-Term Performance Indicators ....................................12-9 

 12.4.3 Full-Scale Versus Pilot-Scale Testing...............................................12-9 

 12.5 Demonstration Protocol ................................................................................12-10 

 12.5.1 DOP Test Matrix ...............................................................................12-11 

 12.5.2 DOP Monitoring Plan .......................................................................12-11 

 12.5.2.1 Sampling Location ..............................................................12-14 

 12.5.2.2 Monitoring Parameters ........................................................12-14 

 12.5.2.3 Monitoring Frequency .........................................................12-14 

 12.5.2.4 Quality Assurance/Quality Control (QA/QC) .....................12-14 

 12.5.3 DOP Implementation .........................................................................12-15 

 12.5.3.1 Sample Collection Methods ................................................12-15 
LT2ESWTR Toolbox Guidance Manual x April 2010

 12.5.3.2 Analytical Methods .............................................................12-15 

 12.5.3.3 Microbial Dosing ................................................................12-16 

 12.5.3.4 Documentation of WTP Operating Conditions ...................12-16 

 12.5.4 Data Analysis and Reporting .............................................................12-17 

 12.5.4.1 Evaluation of Performance..................................................12-17 

 12.5.4.2 Reporting for the DOP ........................................................12-17 

 12.5.4.3 Ongoing Reporting ..............................................................12-18 

 12.6 References................................ ......................................................................12-19 

13. Ultraviolet Light............................... ........................................................................13-1 

 13.1 Introduction................................ ....................................................................13-1 

 13.2 UV Disinfection Requirements for Filtered and Unfiltered PWSs ................13-1 

 13.2.1 UV Dose and Validation Testing Requirements ................................13-1 

 13.2.2 UV Disinfection Monitoring Requirements .......................................13-2 

 13.2.3 UV Disinfection Reporting Requirements .........................................13-3 

 13.3.4 Off-specification Operational Requirement for Filtered and 

 Unfiltered Systems .............................................................................13-3 

 13.3 Toolbox Selection Considerations .................................................................13-3 

 13.4 Design and Operational Considerations .........................................................13-4 

 13.5 References................................ ......................................................................13-5 

14. Membrane Filtration................................ ...............................................................14-1 

 14.1 Introduction................................ ....................................................................14-1 

 14.2 Membrane Filtration Requirements under the LT2ESWTR ..........................14-1 

 14.2.1 Challenge Testing ..............................................................................14-2 

 14.2.2 Direct Integrity Testing ......................................................................14-2 

 14.2.3 Continuous Indirect Integrity Monitoring ..........................................14-3 

 14.3 Toolbox Selection Considerations – Advantages and Disadvantages ...........14-3 

 14.4 Design and Operational Considerations .........................................................14-4 

 14.5 References................................ ......................................................................14-6 
LT2ESWTR Toolbox Guidance Manual xi April 2010

EXHIBITS

Exhibit 1.1 Bin Classification and Additional Treatment Requirements for 

 Filtered Systems .............................................................................................1-7 

Exhibit 1.2 LT2ESWTR Treatment Requirements for Unfiltered Systems .....................1-8 

Exhibit 1.3 Microbial Toolbox Options with Available Log Credits ...............................1-9 

Exhibit 1.4 Compliance Schedules ...................................................................................1-12 

Exhibit 1.5 Implementation Timeline for the LT2ESWTR ..............................................1-13 

Exhibit 2.1 Checklist of PWS and State activities during preparation, 

 implementation, and maintenance of WCP plan and associated 

 0.5-log LT2ESWTR treatment credit .............................................................2-3 

Exhibit 2.2 Checklist of PWS and State activities during preparation, 

 implementation, and maintenance of WCP plan and associated 

 0.5-log LT2ESWTR treatment credit .............................................................2-10 

Exhibit 2.3 Ground Water/Surface Water Interaction ......................................................2-22 

Exhibit 4.1 Selected Bank Filtration Systems in Europe and the United States ...............4-10 

Exhibit 4.2 Generalized Depiction of Stream Channel Lateral Migration .......................4-16 

Exhibit 4.3 Schematic Showing Generalized Flow and Required Separation 

 Distance to a Vertical Well ............................................................................4-22 

Exhibit 4.4 Schematic Showing Generalized Flow and Required Separation 

 Distance to a Horizontal Well With Three Laterals .......................................4-23 

Exhibit 4.5 Taking a Water Level Reading ......................................................................4-24 

Exhibit 4.6 The Streambed of a Perched Stream Is Well above the Water Table ............4-26 

Exhibit 4.7 Size of Pathogenic Protozoa and Surrogate Bacteria .....................................4-40 

Exhibit 4.8 Size of Some Common Fresh Water Diatoms ...............................................4-43 

Exhibit 5.1 Example Influent and Effluent Turbidity Values Resulting in 

 0.5 Log Reduction ..........................................................................................5-4 

Exhibit 5.2 Comparison of Sedimentation and Clarifier Types ........................................5-6 

Exhibit 6.1 Typical Two-Stage Lime Softening Process...................................................6-2 

Exhibit 7.1 Maintenance and Calibration Activities for On-line Turbidimeters ..............7-4 

Exhibit 7.2 Maintenance and Calibration Activities for Bench Top Turbidimeters .........7-5 

Exhibit 7.3 Performance Limiting Factors (Adapted from the Composite 

 Correction Program) ......................................................................................7-8 

Exhibit 7.4 Effluent Turbidity Goals for the Sedimentation Process ...............................7-14 
LT2ESWTR Toolbox Guidance Manual xii April 2010

Exhibit 8.1 Schematic of Treatment Process with Bag/Cartridge Filters .........................8-3 

Exhibit 8.2 Bag/Cartridge Filters in Series .......................................................................8-4 

Exhibit 8.3 Bag/Cartridge Filter with UV System ............................................................8-4 

Exhibit 8.4 Single Filter Vessel ........................................................................................8-12 

Exhibit 8.5 Manifold Bag Filter Design ...........................................................................8-13 

Exhibit 8.6 Multiple Filter Vessel ....................................................................................8-13 

Exhibit 10.1 CT Values (mg-min/L) for Cryptosporidium Inactivation 

 by Chlorine Dioxide .......................................................................................10-4 

Exhibit 10.2 CT Calculation Example Schematic ..............................................................10-4 

Exhibit 10.3 Distribution System Monitoring Requirements at Each Plant .......................10-7 

 

Exhibit 11.1 CT Values for Cryptosporidium Inactivation by 

 Ozone (40 CFR 141.730) ...............................................................................11-3 

Exhibit 11.2 Recommended Methods and Terminology for Calculating the 

 Log-Inactivation Credit in an Ozone Contactor .............................................11-6 

Exhibit 11.3 Correlations to Predict C* Based on Ozone Residual Concentrations 

 in the Outlet of a Chamber .............................................................................11-7 

Exhibit 11.4 Inactivation Coefficients for Cryptosporidium, Log base 10 (L/mg-min) .....11-10 

Exhibit 11.5 Reaction Pathways of Ozone in Water ..........................................................11-17 

Exhibit 12.1 Example Filtration Plant Types Eligible for DOP .........................................12-1 

Exhibit 12.2 Flowchart for DOP Protocol ..........................................................................12-4 

Exhibit 12.3 Example DOP Test Matrix ............................................................................12-11 

Exhibit 12.4 Example DOP Monitoring Plan .....................................................................12-13 

Exhibit 13.1 UV Dose Requirements – millijoules per centimeter squared (mJ/cm2

) .......13-2 

 
LT2ESWTR Toolbox Guidance Manual xiii April 2010

APPENDICES

Appendix A. Site Specific Determination of Contact Time for Chlorine Dioxide and Ozone 

Appendix B. Ozone CT Methods 

Appendix C. Measuring Ozone Residual 

Appendix D. Derivation of Extended CSTR Equations 

Appendix E. Tracer Test Data Development & Analysis

Appendix F. Watershed Control Best Management Practices (BMPs) and Case Studies

Appendix G. Review Criteria for Use by States When Reviewing Watershed Control 

(WSC)Program Plans
LT2ESWTR Toolbox Guidance Manual xiv April 2010

ACRONYMS

AFO Animal Feeding Operation

AOC Assimilable Organic Carbon

AOP Advanced Oxidation Process 

ASTM American Society for Testing and Materials

AWOP Area Wide Optimization Program

AWWA American Water Works Association

BMP Best Management Practice

CAFO Concentrated Animal Feeding Operation

CCP Composite Correction Program 

CFC Chlorofluorocarbon 

CFE Combined Filter Effluent 

CMOM Capacity, Management, Operation, and Maintenance

CPE Comprehensive Performance Evaluation

CSO Combined Sewer Overflow 

CSTR Continuously Stirred Tank Reactor 

CT Contact Time

CTE Comprehensive Technical Assistance

CWA Clean Water Act

CWSRF Clean Water State Revolving Fund

DBP Disinfection Byproduct 

DBPR Disinfectants and Disinfection Byproducts Rule 

DEM Digital Elevation Model

DNA Deoxyribonucleic Acid

DOP Demonstration Of Performance 

DWSRF Drinking Water State Revolving Fund 

EDTA Ethylenediamine Tetra-acetic Acid

EM Electromagnetic

EPA United State Environmental Protection Agency 

FBR Filter Backwash Rule

FEMA Federal Emergency Management Agency

GAC Granular Activated Carbon

GIS Geographic Information System 

GPM Gallons Per Minute 

GPR Ground Penetrating Radar 

GROW Geotechnical, Rock and Water Resources Library

GWUDI Ground Water Under the Direct Influence of surface water

HAA Haloacetic Acids

HDT Hydraulic Detention Time

HEC-RAS Hydrologic Engineering Centers River Analysis System 

HUC Hydrologic Unit Code 

IDSE Initial Distribution System Evaluation
LT2ESWTR Toolbox Guidance Manual xv April 2010

IESWTR Interim Enhanced Surface Water Treatment Rule

IFE Individual Filter Effluent 

IP Induced Polarization 

LRAA Locational Running Annual Average 

LRV Log Removal Value 

LT1ESWTR Long Term 1 Enhanced Surface Water Treatment Rule

LT2ESWTR Long Term 2 Enhanced Surface Water Treatment Rule

MCF Membrane Cartridge Filter

MCL Maximum Contaminant Level

MDBP Microbial and Disinfection Byproduct 

MF Microfiltration

MGD Million Gallons per Day

MPA Microscopic Particulate Analysis

MRDL Maximum Residual Disinfectant Level

MS4 Municipal Separate Storm Sewer System

MST Microbial Source Tracking

MTBE Methyltertiarybutylether

NFF National Flood Frequency 

NGWA National Groundwater Association 

NHDES New Hampshire Department of Environmental Services

NMP Nutrient Management Plan

NOM Natural Organic Matter

NPDES National Pollutant Discharge Elimination System

NRCS Natural Resource Conservation Service

NSF National Science Foundation 

NTNCWS Nontransient Noncommunity Water Systems

NTU Nephelometric Turbidity Unit

ORP Oxidation-Reduction Potential 

OSHA Occupational Safety and Health Administration

PCR Polymerase Chain Reaction

PEL Permissible Exposure Limit

POTW Publically Owned Treatment Works 

PWS Public Water System

QA Quality Assurance

QC Quality Control 

RAA Running Annual Average 

RNA Ribonucleic Acid

RO Reverse Osmosis

SCD Streaming Current Detector

SDWA Safe Drinking Water Act

SOC Synthetic Organic Compound 

SOP Standard Operating Procedure 

SP Self Potential

SPDES State Pollutant Discharge Elimination System
LT2ESWTR Toolbox Guidance Manual xvi April 2010

SRF State Revolving Fund 

SSO Sanitary Sewer Overflow

SSRC Spores of Sulfide Reducing Clostridia 

SSS Specific System Study

SWAP Source Water Assessment Program

SWP Source Water Protection

SWTR Surface Water Treatment Rule

TCE Trichloroethylene 

TEM Transient Electromagnetic

THM Trihalomethane 

TMDL Total Maximum Daily Load

TNCWS Transient Noncommunity Water Systems

TNTC Too Numerous To Count 

TOC Total Organic Carbon

TTHM Total Trihalomethanes

UF Ultrafiltration

UFRV Unit Filter Run Volume 

USDA United States Department of Agriculture

USGS United States Geological Survey

UV Ultraviolet light

WCP Watershed Control Program 

WSC Watershed Control 

WTP Water Treatment Plant
LT2ESWTR Toolbox Guidance Manual 1-1 April 2010

1. Introduction

 In establishing drinking water regulations for microbial and disinfection byproduct (MDBP) control, the U.S. Environmental Protection Agency (EPA) is promoting a multi-barrier 

approach for treating drinking water. A multi-barrier treatment process provides a number of 

protective “layers” against contamination by using more than one method of prevention and 

treatment to remove or inactivate microorganisms and minimize disinfection byproducts (DBPs). 

To that end, EPA is publishing this guidance to help public water systems (PWSs) choose 

appropriate combinations of treatment processes for compliance with the Long Term 2 Enhanced 

Surface Water Treatment Rule (LT2ESWTR).

 The LT2ESWTR focuses on improved control of microbial contamination, specifically 

the protozoan parasite Cryptosporidium. Differing from previous drinking water microbial 

regulations, the LT2ESWTR requirements for each system are based on the PWS’s vulnerability 

to contamination, as measured by the occurrence of Cryptosporidium in the source water. This 

“Microbial Framework” strategy stems from a recognition that only some systems may need to 

provide additional protection against Cryptosporidium and that such decisions should be made 

on a system-specific basis.

 With this approach, systems serving 10,000 people or more initially conduct source water 

monitoring to determine average Cryptosporidium concentrations (small filtered systems serving 

less than 10,000 people can first monitor for E. coli to determine if Cryptosporidium monitoring 

is required unless the state notifies them otherwise). Based on their monitoring results, systems 

are classified into different categories (or bins). The bins indicate the additional Cryptosporidium

treatment requirements, if any, that must be met to comply with the rule. Systems required to 

provide additional treatment will choose from a “toolbox” of options consisting of treatment 

technologies, process optimization techniques, and management techniques to meet the 

requirements. Thus, this approach requires enhanced Cryptosporidium treatment for systems with 

higher vulnerability to Cryptosporidium contamination and provides several options for those 

systems to achieve compliance. These options are described in this manual. 

1.1 Guidance Manual Objectives 

 The primary objectives of this manual are to provide guidance to PWSs for selecting 

appropriate microbial toolbox options and achieving compliance for each option. To accomplish 

these objectives, this manual will describe each toolbox option in terms of achieving 

Cryptosporidium treatment credit(s) and discuss design and operational issues. 
1. Introduction

LT2ESWTR Toolbox Guidance Manual 1-2 April 2010

1.2 Guidance Manual Organization 

This manual consists of fourteen chapters and five appendices: 

Chapter 1 Introduction - The remainder of this chapter provides a regulatory history 

and then summarizes key provisions of the LT2ESWTR including 

minimum requirements for each toolbox option. 

Chapters 2 – 14 Toolbox Options - These chapters describe each toolbox option and how 

systems can implement these options to achieve the associated 

Cryptosporidium treatment credit. Where applicable, basic design criteria 

are recommended to achieve a given log removal. Each chapter contains 

its own list of references.

Appendix A Site Specific Determination of Contact Time for Chlorine Dioxide and 

Ozone - describes the different elements of a site specific study to generate

a set of chlorine dioxide or ozone CT values for that site and discusses 

some of the issues involved in the statistical analysis of the results.

Appendix B Ozone CT Methods - describes the Segmented Flow Analysis and 

Extended-CSTR methods to calculate the CT inactivation credits with 

ozone. 

Appendix C Measuring Ozone Residual - discusses ozone residual sample collection, 

measurement, and online ozone residual analyzer calibration. 

Appendix D Derivation of Extended CSTR Equations - provides the derivation of the 

equation used to calculate k*. 

Appendix E Tracer Test Data Development and Analysis – describes how to conduct 

and analyze the results of tracer tests to determine the contact time for CT 

calculations. 

Appendix F Watershed Control Best Management Practices (BMPs) 

and Case Studies - provides a list of programmatic resources and guidance 

available to assist systems in building partnerships and implementing 

watershed protection activities.

Appendix G Review Criteria for Use By States When Reviewing Watershed Control 

(WSC) Program Plans – provides a list of assessment criteria for use by 

states when reviewing WCP plans. 

1.3 Regulatory History 

 The following sections describe the predecessors to the LT2ESWTR. Section 1.3.5 

summarizes key requirements of the Stage 2 Disinfectants and Disinfection Byproducts Rule 
1. Introduction

LT2ESWTR Toolbox Guidance Manual 1-3 April 2010

(DBPR), which was promulgated simultaneously with the LT2ESWTR to balance the risks 

between DBPs and microbial pathogens. 

1.3.1 Surface Water Treatment Rule

 Under the 1989 Surface Water Treatment Rule (SWTR) (54 FR 27486 June 29, 1989),

EPA established treatment requirements for all PWSs using surface water or ground water under 

the direct influence of surface water (GWUDI) as a source. The requirements are intended to 

protect against the adverse health effects associated with Giardia lamblia, viruses, and 

Legionella and include the following: 

• Maintenance of a disinfectant residual in water entering and within the distribution 

system.

• Removal/inactivation of at least 99.9 percent (3-log) of Giardia and 99.99 percent (4-log) 

of viruses. 

• Filtration, unless systems meet specified avoidance criteria.

• For filtered systems, a turbidity limit for the combined filter effluent of 5 nephelometric 

turbidity units (NTUs) at any time and a limit of 0.5 NTU in 95 percent of measurements 

each month for treatment plants using conventional treatment or direct filtration (with 

separate standards for other filtration technologies). These requirements were superseded 

by the 1998 IESWTR and the 2002 LT1ESWTR. 

• Watershed control programs and water quality requirements for unfiltered systems.

1.3.2 Interim Enhanced Surface Water Treatment Rule

 The Interim Enhanced Surface Water Treatment Rule (IESWTR) (63 FR 69478 

December 16, 1998) applies to PWSs serving at least 10,000 people and using surface water or 

GWUDI as a source. These systems were to comply with the IESWTR by January 2002. The 

requirements and guidelines include: 

• Removal of 99 percent (2-log) of Cryptosporidium for systems that provide filtration.

• For treatment plants using conventional treatment or direct filtration, a turbidity 

performance standard for the combined effluent of filters of 1 NTU as a maximum and 

0.3 NTU as a maximum in 95 percent of monthly measurements, based on 4-hour 

monitoring (these limits supersede the SWTR turbidity limits).

• Continuous monitoring of individual filter effluent turbidity in conventional and direct 

filtration plants and recording of turbidity readings every 15 minutes. 
1. Introduction

LT2ESWTR Toolbox Guidance Manual 1-4 April 2010

• A disinfection benchmark to assess the level of microbial protection provided before 

facilities change their disinfection practices to meet the requirements of the Stage 1 

DBPR.

• Inclusion of Cryptosporidium in the definition of GWUDI and in the watershed control 

requirements for unfiltered PWSs.

• All new finished water reservoirs must be covered. 

1.3.3 Stage 1 Disinfectants and Disinfection Byproducts Rule

 Pursuant to requirements under the Safe Drinking Water Act (SDWA), EPA developed 

interrelated regulations to control microbial pathogens and disinfectants/DBPs in drinking water. 

These rules, collectively known as the M-DBP rules, are intended to address complex risk 

trade-offs between the two different types of contaminants. EPA promulgated the IESWTR 

concurrently with the Stage 1 DBPR so that systems could coordinate their responses to the risks 

posed by DBPs and microbial pathogens. 

 The 1998 Stage 1 DBPR (63 FR 69390 December 16, 1998) applies to all community 

water systems (CWSs) and nontransient noncommunity water systems (NTNCWSs) that add a 

chemical disinfectant to their water. Certain requirements in the rule also apply to transient 

noncommunity water systems (TNCWSs). Surface water and GWUDI systems serving at least 

10,000 people were required to comply with the rule by January 2002. All other systems 

(including ground water systems) were required to comply by January 2004. 

The Stage 1 DBPR sets maximum residual disinfectant levels (MRDLs) for chlorine,

chloramines, and chlorine dioxide; and maximum contaminant levels (MCLs) for total 

trihalomethanes (TTHM), haloacetic acids (HAA5), bromate, and chlorite. The MRDLs and 

MCLs, except those for chlorite and chlorine dioxide, are calculated as running annual averages 

(RAAs). 

1.3.4 Long Term 1 Enhanced Surface Water Treatment Rule 

 The Long Term 1 Enhanced Surface Water Treatment Rule (LT1ESWTR) (67 FR 1811 

January 14, 2002) was promulgated in 2002 and extends most of the requirements of the 

IESWTR to surface water and GWUDI systems serving fewer than 10,000 people. 

1.3.5 Stage 2 Disinfectant and Disinfection Byproduct Rule

The requirements of the Stage 2 DBPR (71 FR 388 January 4, 2006) apply to all CWSs 

and NTNCWSs that add a disinfectant other than ultraviolet light (UV), or that deliver water that 

has been treated with a disinfectant other than UV. The Stage 2 DBPR builds on the 1998 Stage 

1 DBPR by requiring reduced levels of DBPs in distribution systems. Major components of the 

rule are described below.
1. Introduction

LT2ESWTR Toolbox Guidance Manual 1-5 April 2010

Initial Distribution System Evaluations

 For many systems, compliance monitoring will be preceded by an Initial Distribution 

System Evaluation (IDSE) to identify Stage 2 DBPR compliance monitoring locations that 

represent distribution system sites with high TTHM and HAA5 levels. The IDSE consists of 

either standard monitoring or a system specific study (SSS). NTNCWSs serving fewer than 

10,000 people are not required to perform an IDSE, and other systems may receive waivers from 

the IDSE requirement.

Compliance with Stage 2 DBPR MCLs 

The numerical MCLs for the Stage 2 DBPR are the same as for the Stage 1 DBPR MCLs: 

0.080 milligrams per liter (mg/L) for TTHM, and 0.060 mg/L for HAA5. The Stage 2 DBPR is 

designed to reduce high TTHM and HAA5 in the distribution system by changing compliance 

monitoring and calculation requirements. Compliance determination for the Stage 2 DBPR is 

based on a locational running annual average (LRAA) (i.e., compliance must be met at each

monitoring location) instead of the system-wide RAA used under the Stage 1 DBPR. 

Routine Monitoring Requirements 

 EPA has adopted a population-based monitoring approach for the Stage 2 DBPR, where 

compliance and IDSE monitoring requirements are based only on source water type and retail 

population served. This is a change from the plant-based approach used in the 1979 TTHM rule 

and the Stage 1 DBPR. 

Operational Evaluations 

 Because Stage 2 DBPR MCL compliance for some systems is based on individual DBP 

measurements at a location averaged over a four-quarter period, a system could find higher 

TTHM or HAA5 levels than the MCL values, while at the same time maintaining compliance 

with the Stage 2 DBPR. This is because the high concentration could be averaged with lower 

concentrations at a given location. For this reason, the Stage 2 DBPR includes a provision for 

“operational evaluations” as follows: 

A system has exceeded an operational evaluation level at any monitoring location when 

the sum of the two previous quarters’ compliance monitoring results plus twice the current 

quarters result, divided by 4, exceeds 0.080 mg/L for TTHM or 0.060 mg/L for HAA5. 

 If an operational evaluation level is exceeded, the system must conduct an “operational 

evaluation” and submit a written report of the evaluation to the state. 
1. Introduction

LT2ESWTR Toolbox Guidance Manual 1-6 April 2010

1.4 Overview of the Long Term 2 Enhanced Surface Water Treatment Rule 

The LT2ESWTR (71 FR 654 January 5, 2006) applies to all PWSs that use surface water 

or GWUDI (referred to collectively as “surface water systems” in this manual). It builds on the 

SWTR, IESWTR, and the LT1ESWTR by improving control of microbial pathogens, specifically 

Cryptosporidium. It requires filtered systems to monitor their source water for Cryptosporidium, 

and based on the results, to meet one of four levels of treatment for Cryptosporidium (with the 

first level requiring no additional treatment). Treatment requirements will be reassessed in the 

future based on a second round of source water monitoring under the current rule. For those 

systems that do not already provide filtration, the LT2ESWTR has specific requirements to 

inactivate two or three logs of Cryptosporidium, depending on source water monitoring results. It 

also requires systems with uncovered finished water reservoirs either to cover the reservoirs or to 

provide additional treatment to the reservoir effluent.

 The next several sections provide a summary of PWS requirements for the LT2ESWTR,

including a summary of microbial toolbox options in Section 1.4.3. Section 1.5 provides the 

implementation timeline for the rule. 

1.4.1 Monitoring and Treatment Requirements for Filtered Systems

 The LT2ESWTR requires most filtered PWSs to conduct source water monitoring to 

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

PWSs must calculate an initial Cryptosporidium bin concentration for each plant for which 

monitoring was required [40 CFR 141.710]. Detailed requirements and guidance on how to 

determine source water Cryptosporidium bin concentrations are provided in the Source Water 

Monitoring Guidance Manual for Public Water Systems for the Final Long Term 2 Enhanced 

Surface Water Treatment Rule, finalized in 2006 and available online at 

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

 Exhibit 1.1 presents the bin classifications and their corresponding additional treatment 

requirements for all filtered systems. The treatment requirements are based on a determination 

that conventional, slow sand, and diatomaceous earth filtration plants in compliance with the 

IESWTR or LT1ESWTR achieve an average of 3-log removal of Cryptosporidium. EPA has 

determined that direct filtration plants achieve an average 2.5-log removal of 

Cryptosporidium (their removal is less than in conventional filtration because they lack a 

sedimentation process). 
1. Introduction

LT2ESWTR Toolbox Guidance Manual 1-7 April 2010

Exhibit 1.1 Bin Classification and Additional Treatment Requirements for Filtered 

Systems1

If your 

Cryptosporidium

concentration

(oocysts/L) is...

Your bin 

classification 

is...

And if you use the following filtration treatment in full 

compliance with existing regulations, then your additional

treatment requirements are...

Conventional 

Filtration 

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 2 treatment

1.5-log 2 treatment1-log 2 treatmentAs determined 

2,4 by the state

> 1.0 and < 3.0 3 2-log 3 treatment

2.5-log 3 treatment2-log 3 treatmentAs determined 

3,5 by the state

> 3.0 4 2.5-log 3 treatment

3-log 3 treatment2.5-log 3 treatmentAs determined 

by the state3,6

1

40 CFR 141.710 and 40 CFR 141.711. 

2

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

3

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

bag/cartridge filters, or bank filtration.

4

Total Cryptosporidium treatment must be at least 4.0-log.

5

Total Cryptosporidium treatment must be at least 5.0-log.

6

Total Cryptosporidium treatment must be at least 5.5-log.

The LT2ESWTR requires systems to comply with additional treatment requirements by 

using one or more management or treatment techniques from the microbial toolbox of options. A 

description of the microbial toolbox options and basic requirements for achieving inactivation 

credit for each are provided in Section 1.4.3. 

1.4.2 Monitoring and Treatment Requirements for Unfiltered Systems

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 and to maintain a 

disinfectant residual in the distribution system. The LT2ESWTR requires 2- or 3- log 

inactivation of Cryptosporidium, depending on the source water concentrations of 

Cryptosporidium as shown in Exhibit 1.2. Detailed requirements and guidance on how to 

determine source water Cryptosporidium concentrations are provided in the Source Water 

Monitoring Guidance Manual for Public Water Systems for the Final Long Term 2 Enhanced 
1. Introduction

LT2ESWTR Toolbox Guidance Manual 1-8 April 2010

Surface Water Treatment Rule, finalized in 2006 and available online at 

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

Exhibit 1.2 LT2ESWTR Treatment Requirements for Unfiltered Systems1

Average Cryptosporidium Concentration

(oocysts/liter)

Additional Cryptosporidium Inactivation 

Requirements

< 0.01 2-log2

 

> 0.01 3-log2 1

40 CFR 141.712. 

2

Overall disinfection requirements must be met with a minimum of two disinfectants.

Unfiltered systems must use chlorine dioxide, ozone, or UV to meet the Cryptosporidium

inactivation requirements in Exhibit 1.2 and must meet overall disinfection requirements (i.e., 

Cryptosporidium, Giardia, and virus inactivation) with a minimum of two disinfectants [40 CFR 

141.712 (d)]. Each of the two disinfectants must achieve by itself the total inactivation required 

for one of the three pathogen types. 

1.4.3 Summary of Microbial Toolbox Options 

Systems receive LT2ESWTR treatment credits by meeting conditions for the microbial 

toolbox options presented in Exhibit 1.3 [40 CFR 141.715]. Systems may use a combination of 

toolbox options to achieve the required log treatment. The intent of the toolbox is to provide 

systems with flexibility in selecting cost-effective LT2ESWTR compliance strategies. Unfiltered 

as well as filtered systems are eligible for treatment credits for the microbial toolbox options 

unless otherwise indicated in the table. Unfiltered systems must use one of the 

inactivation/disinfection tools in the toolbox. 
1. Introduction

LT2ESWTR Toolbox Guidance Manual 1-9 April 2010

Exhibit 1.3 Microbial Toolbox Options with Available Log Credits1

Toolbox Option Cryptosporidium Treatment Credit with Design and Implementation Criteria

Source Toolbox Components

Watershed 

control program

0.5-log credit for state approved program comprising required elements, annual 

program status report to the state, and regular watershed survey. Unfiltered 

systems are not eligible for credit. See 40 CFR 141.716 (a) and Chapter 2 of 

this manual for specific criteria.

Alternative 

source/ intake 

management

No presumptive credit. Systems may conduct simultaneous monitoring for 

treatment bin classification at alternative intake locations or under alternative 

intake management strategies. See 40 CFR 141.716(b) and Chapter 3 of this 

manual for specific criteria.

Pre-Filtration Toolbox Components

Presedimentation 

basin with 

coagulation

0.5-log credit during any month that presedimentation basins achieve a monthly 

mean reduction of 0.5-log or greater in turbidity or alternative state-approved 

performance criteria. To be eligible, basins must be operated continuously with 

coagulant addition and all plant flow must pass through the basin. See 40 CFR 

141.717(a) and Chapter 5 of this manual for specific criteria.

Two-stage lime 

softening 

0.5-log credit for two-stage softening where chemical additional and hardness 

precipitation occur in both stages. All plant flow must pass through both stages. 

Single-stage softening is credited as equivalent to conventional treatment. See 

40 CFR 141.717(b) and Chapter 6 of this manual for specific criteria.

Bank filtration 0.5-log credit for 25-foot setback; 1.0-log credit for 50-foot setback; aquifer must 

be unconsolidated sand containing at least 10 percent fines; average turbidity in 

wells must be less than 1 NTU. Systems using wells followed by filtration when 

conducting source water monitoring must sample the well to determine bin 

classification and are not eligible for additional credit. See 40 CFR 141.717(c) 

and Chapter 4 of this manual for specific criteria

Treatment Performance Toolbox Components

Combined filter 

performance 

0.5-log credit for combined filter effluent turbidity less than or equal to 0.15 NTU 

in at least 95 percent of measurements each month. See 40 CFR 141.718 (a) 

and Chapter 7 of this manual for specific criteria.

Individual filter 

performance

0.5-log credit (in addition to 0.5-log combined filter performance credit) if 

individual filter effluent turbidity is less than or equal to 0.15 NTU in at least 95 

percent of samples each month in each filter and is never greater than 0.3 NTU 

in two consecutive measurements in any filter. See 141.718 (b) and Chapter 7 of 

this manual for specific criteria.

Demonstration of 

performance

Credit awarded to unit process or treatment train based on a demonstration to 

the state with a state-approved protocol. See 40 CFR 141.718 (c) and Chapter 

12 of this manual for specific criteria.
1. Introduction

LT2ESWTR Toolbox Guidance Manual 1-10 April 2010

Additional Filtration Toolbox Options

Bag or cartridge 

filters (individual 

filters)

Up to 2-log credit based on the removal efficiency demonstrated during 

challenge testing with a 1.0-log factor of safety. See 40 CFR 141.719(a) and 

Chapter 8 of this manual for specific criteria.

Bag or cartridge 

filters (in series)

Up to 2.5-log credit based on the removal efficiency demonstrated during 

challenge testing with a 0.5-log factor of safety. See 40 CFR 141.719(a) and 

Chapter 8 of this manual for specific criteria.

Membrane 

filtration

Log credit equivalent to removal efficiency demonstrated in challenge test for 

device if supported by direct integrity testing. See 40 CFR 141.719(b) and 

Chapter 14 of this manual for specific criteria.

Second stage 

filtration

0.5-log credit for second separate granular media filtration stage if treatment 

train includes coagulation prior to first filter. See 40 CFR 141.719 (c) and 

Chapter 9 of this manual for specific criteria.

Slow sand filters 

2.5-log credit as a secondary filtration step; 3.0-log credit as a primary filtration 

process. No prior chlorination for either option. See 40 CFR 141.719(d) and 

Chapter 9 of this manual for specific criteria.

Inactivation Toolbox Components

Chlorine dioxide

Log credit based on measured CT in relation to CT table. See 40 CFR 141.720 

(b) and Chapter 10 of this manual for specific criteria.

Ozone

Log credit based on measured CT in relation to CT table. See 40 CFR 141.720 

(b) and Chapter 11 of this manual for specific criteria.

UV

Log credit based on measured CT in relation to CT table. See 40 CFR 141.720 

(d) and Chapter 13 of this manual for specific criteria. 1 40 CFR 141.715. 

1.4.4 Requirements for PWSs with Uncovered Finished Water Reservoirs

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]. 

1.4.5 Disinfection Profiling and Benchmarking Requirements

 The LT2ESWTR includes a disinfection profile and benchmark requirement to ensure 

that any significant change in disinfection, whether for byproduct control under the Stage 2 

DBPR, improved Cryptosporidium control under the LT2ESWTR, or both, does not significantly 

compromise existing Giardia and virus protection. A disinfection profile is a graphical 

representation of a system’s level of Giardia and viral inactivation measured during the course of 

one or more year(s). A benchmark is the lowest monthly average of microbial inactivation during 

the disinfection profile period. 
1. Introduction

LT2ESWTR Toolbox Guidance Manual 1-11 April 2010

 The profiling and benchmarking requirements under the LT2ESWTR are similar to those 

promulgated under the IESWTR and LT1ESWTR and are applicable to systems making a 

significant change to their disinfection practice. The LT2ESWTR defines significant change as 

follows: 

• Changes to the point of disinfection. 

• Changes to the disinfectant used at the treatment plant. 

• Changes to the disinfection process. 

• Any other modification identified by the state as a significant change to the disinfection 

practice. [40 CFR 141.708(b)]. 

Prior to changing the disinfection practice, the systems must notify the state and include 

the following information: 

• A completed disinfection profile and disinfection benchmark for Giardia lamblia and 

viruses as described in 40 CFR 141.709. 

• A description of the proposed change in disinfection practice.

• An analysis of how the proposed change will affect the current level of disinfection. [40 

CFR 141.708(a)]. 

 Detailed guidance for conducting a disinfection profile and calculating a benchmark is 

provided in the IESWTR Disinfection Profiling and Benchmarking Guidance Manual for systems 

serving at least 10,000 people and the LT1ESWTR Disinfection Profiling and Benchmarking 

Technical Guidance Manual for systems serving less than 10,000 people. Both manuals are 

available on-line at http://www.epa.gov/safewater/mdbp/implement.html and 

http://www.epa.gov/safewater/mdbp/lt1eswtr.html, respectively.

1.5 LT2ESWTR Implementation Schedule 

 The LT2 Rule defines four compliance schedules, which are based on the population 

served by systems as summarized in Exhibit 1.4. Wholesale PWSs must comply with Stage 2 

DBPR and LT2ESWTR requirements based on the population of the largest PWS in the 

combined distribution system. This approach will ensure that PWSs have the same compliance 

schedule under both the LT2ESWTR and Stage 2 DBPR. Although consecutive systems without 

their own source are not required to conduct source water monitoring, they do need to cover any 

uncovered reservoirs or treat the discharge, and meet disinfection profiling and benchmarking 

requirements, if applicable. 
1. Introduction

LT2ESWTR Toolbox Guidance Manual 1-12 April 2010

Exhibit 1.4 Compliance Schedules

If you have a Subpart H source and are this kind of system: You are on schedule number

System serving 100,000 or more people OR a wholesale system

producing surface water in a combined distribution system that 

contains a system serving 100,000 or more people

1 

System serving 50,000 to 99,999 people OR a wholesale system

producing surface water in a combined distribution system with the 

largest system serving 50,000 to 99,999

2 

System serving 10,000 to 49,999 people OR a wholesale system

producing surface water in a combined distribution system with the 

largest system serving 10,000 to 49,999

3 

System serving fewer than 10,000 people. 4 

Source: U.S. EPA 2007. The LT2ESWTR Implementation Guidance. EPA 816-R-07-006, U.S. Environmental 

Protection Agency, Office of Groundwater and Drinking Water, Washington DC. 

 Exhibit 1.5 presents monitoring and treatment deadlines for the LT2ESWTR for systems 

on each of the four schedules defined in Exhibit 1.4. The compliance dates are designed to allow 

systems to comply simultaneously with the Stage 2 DBPR and the LT2ESWTR in order to 

balance risks associated with DBPs with risks associated with microbial pathogens. Compliance 

deadlines for individual microbial toolbox options are presented in Chapters 2 through 14 of this 

manual. 
1. Introduction

LT2ESWTR Toolbox Guidance Manual 1-13 April 2010

Exhibit 1.5 Implementation Timeline for the LT2ESWTR

Treatment

Installation

Crypto monitoring

Treatment 

Installation

Crypto monitoring

Treatment

Installation

Crypto monitoring

E. coli

Possible 

Extension

Possible 

Extension

IDSE

IDSE

IDSE

IDSE

* Includes associated consecutive systems

Possible 

Extension Review

submission

Review

submission

Review

submission

Review

submission

Compliance

1

2

3

4

LT2 Plan or bin classification due

Stage 2 IDSE Plan or report due

2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

2006 2007 2008 2009 2012 2013 2014 2015 2016

Possible

Treatment Installation Extension

Compliance

(if no Crypto Monitoring)

Compliance

(if Crypto Monitoring)

Compliance

Compliance

2010 2011

Crypto monitoring

Source: U. S. EPA 2007. The LT2ESWTR Implementation Guidance. EPA 816-R-07-006, U.S. Environmental 

Protection Agency, Office of Groundwater and Drinking Water, Washington DC. 

Notes:

For systems on Schedules 1 through 4 (see Exhibit 1.4). 

Unfiltered systems must monitor for Cryptosporidium, regardless of size. 
LT2ESWTR Toolbox Guidance Manual 2-1 April 2010

2. Watershed Control Program

2.1 Introduction 

 The watershed control program (WCP) credit provides the opportunity for public water 

systems (PWSs) with surface water sources employing filtration to obtain a 0.5-log credit from the 

Microbial Toolbox by developing and implementing a state-approved WCP plan. The elements of 

a state-approved WCP plan include identification of potential Cryptosporidium sources, 

prioritization of the identified sources, development of control measures to address the prioritized 

sources, and continuation of these efforts in the future. Systems with existing source water 

protection (SWP) efforts that meet these requirements can incorporate them into their stateapproved WCP plan, while systems without existing programs can receive the same credit if they 

develop and implement similar SWP efforts as part of a WCP. 

PWSs in the same watershed typically need to evaluate and control the same 

Cryptosporidium sources. Consequently, in order to pool resources and reduce duplication of 

efforts, in many cases the state and the PWSs in the watershed should work together to develop a 

single joint WCP plan that will allow the state to approve a 0.5-log credit for each PWS that 

participates in the implementation of the plan. This may not be practical or achievable in all cases, 

and in other cases a PWS may have a simpler and smaller watershed that does not include 

upstream PWSs. These systems are still encouraged to work with any downstream PWSs to 

develop joint WCP plans, but PWSs that develop and implement an individual WCP plan 

approved by the state are eligible for the WCP credit. 

 The remainder of this chapter discusses the following in more detail: 

• Required elements for the WCP plan, and the process associated with obtaining and 

maintaining the WCP credit (Section 2.2). 

• Benefits and advantages of the WCP (Section 2.3).

• Guidance and tools to help develop the WCP (Section 2.4). 

2.1.1 Credits Available

 Filtered systems that develop a state-approved WCP designed to reduce the level of 

Cryptosporidium in the watershed can receive a 0.5-log credit towards the Cryptosporidium 

treatment requirements under the LT2ESWTR (40 CFR 141.722). The WCP credit can be added 

to the credit awarded for any other toolbox component.
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-2 April 2010

2.2 Application Process for the WCP Credit (PWS and State Responsibilities) 

The following discussion describes PWS efforts necessary to apply for, implement, and 

maintain the WCP credit. Associated with the PWS efforts are complementary efforts by the state 

or other primacy agency to review and approve the initial award of the credit, and subsequent 

efforts to continue the credit as long as the PWS meets all of their commitments. There are six 

steps associated with obtaining and continuing the WCP credit, some to be performed by the PWS 

to gain the credit and some by the state to approve the credit, including the following (also see 

checklist described in Section 2.2.4): 

• PWS notification to the state indicating that the PWS will be submitting a WCP plan.

• Development and submittal of a WCP plan by the PWS for review by the state.

• State review and approval of the WCP plan submitted by the PWS.

• Implementation of the state-approved WCP plan by the PWS.

• Continued maintenance of the activities outlined in the WCP plan by the PWS.

• Periodic review of progress by the state (annual report prepared by PWS, watershed 

sanitary survey every three years using state guidelines and state-approved personnel).

A PWS intending to utilize the WCP credit must have a WCP plan approved and in place 

within three years after the Cryptosporidium sampling and bin assignment are complete. Exhibit

2.1 outlines the deadlines for key compliance events associated with implementing and 

maintaining the WCP credit. Depending on size, different PWSs will have different deadlines for 

when bin assignment is completed. One year after this deadline (or earlier), the PWS must notify 

its state of its intent to apply for the watershed credit. One year after this deadline, a plan for WCP 

implementation must be prepared by the PWS and submitted to the state. All PWSs requiring 

credits under the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) must 

have these credits in place within three years after the bin assignment deadline. Consequently, the 

state must either approve, conditionally approve, or reject the PWS WCP plan by this date. If the 

state does not respond by this time, the credit will be assumed approved as long as all other 

requirements are met (i.e., WCP implementation and maintenance). In either case, if the state 

determines that the PWS is not implementing or maintaining the activities outlined in the 

approved WCP plan, the state may later withdraw the credit.

 As discussed in the section on the annual WCP status report, if a PWS determines that a 

significant change is needed for a state-approved WCP, the PWS must notify the state, either 

separately or in the body of the annual status report, prior to making any of these changes. The 

notification must list actions the PWS will take in order to mitigate any “likely” (40 CFR 141.716 

(a)(5)(i)) reduction in SWP that might result from the proposed change. 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-3 April 2010

 When a PWS develops a WCP plan for their own system they may choose to consult with 

other water systems to see if they are interested in cooperating with the water system and others to 

develop joint activities or even a common WCP plan. Such collaborative efforts can help increase 

the effectiveness of plan activities both in terms of effectively managing fiscal and technical 

resources but also with respect to the size of the watershed area brought into the plan. Describing 

a multi-PWS watershed control strategy to the state includes the same components as describing a 

single PWS effort. However, it is important to be particularly clear when describing implementing 

mechanisms, lines of authority, responsibilities, and other components that bear on inter-agency 

coordination. Credit may be available for all participants in a joint state-approved WCP plan, 

contingent on all participants following through on their designated roles during implementation 

and maintenance of the approved WCP plan. 

Exhibit 2.1 Checklist of PWS and State activities during preparation, 

implementation, and maintenance of WCP plan and associated 0.5-log LT2ESWTR 

treatment credit

Compliance Event

Compliance Date for Systems of Different Sizes

(by population served)

>100,000 50,000 to 99,999 10,000 to 49,999

Bin assignment deadline April 2009 October 2009 October 2010

Notification to state of PWS intent to prepare 

a WCP plan April 2010† October 2010† October 2011†

PWS submit WCP plan April 2011† October 2011† October 2012†

PWS implement state approved WCP plan‡ April 2012† October 2012† October 2013†

First progress report (annually thereafter) April 2013† October 2013† October 2014†

First sanitary survey report (every three 

years thereafter) April 2015† October 2015† October 2016†

Second round of Cryptosporidium sampling April 2015 October 2015 October 2015

† Can be completed earlier, pending completion of prerequisite events.

‡ If a PWS submits a WCP plan with all required elements by the required due date, and then state does not 

respond by the date in this table, the credit is considered approved and the credit will continue indefinitely into 

the future as long as PWS properly implements the plan and submits required annual progress reports and 

watershed sanitary surveys required every three years.

2.2.1 Notifying the State of Intention to Participate

A system must notify its state of its intention to implement a WCP at least two years prior 

to the applicable compliance date. (40 CFR 141.716(a)(1)). For example, as shown in Exhibit 2.1, 

a system serving 10,000 people has an October 2013 deadline for implementing the WCP plan. 

Therefore, it must inform the state that it intends to develop a WCP plan by October 2011. The 

application and plan must be submitted for approval at least one year prior to the applicable 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-4 April 2010

compliance date (i.e., one year after informing the state of the intent to implement a WCP plan). 

(40 CFR 141.716(a)(2)).

2.2.2 Preparation of Watershed Control Program Plan 

After notifying the state that the PWS intends to prepare a WCP plan, this plan and 

supporting documentation must be submitted to the state for review and approval no later than 

the date indicated in Exhibit 2.1, two years after the bin assignment date. The WCP plan must 

include delineation of the “area of influence,” identification of potential and actual 

Cryptosporidium sources with this delineated area, an analysis of the proposed control 

measures, and identification of an action plan to attempt to reduce Cryptosporidium source 

water levels (40 CFR 141.716(a)(2)). Requirements for systems incorporating new SWP 

efforts into their WCP plan are identical to requirements for systems incorporating existing 

SWP efforts (40 CFR 141.716(a)(3)). The WCP plan must: a) explain how actions are expected 

to contribute to specified goals; b) identify watershed partners and their roles; c) identify 

resource requirements and commitments; and d) outline a schedule, with deadlines, for plan 

implementation and maintenance (40 CFR 141.716(a)(2)(iv)). Each of the activities in the WCP 

should have a timetable for implementation, a budget, and details about how the activity will be 

implemented

 More information on development of the WCP plan is described later in Section 2.4 and 

characteristics of some key elements in the plan are briefly outlined below. 

2.2.2.1 Delineation of Area of Influence

An essential element for the WCP plan is the identification of the “area of influence,” 

outside of which there is not a significant likelihood of Cryptosporidium or fecal contamination 

that affects the treatment plant intake. Identification of Cryptosporidium sources, associated 

control measures, and future watershed surveys (see Section 2.4.1) will be targeted within this 

area. 

 Methods to be used to establish the boundaries of the area of influence are at the 

discretion of the PWS, as long as the state considers it sufficient to approve the area delineated. 

Some methods that could be used include: a) characterization of watershed hydrology, b) 

modeling of Cryptosporidium travel time, or c) when sufficient data exists it can be useful to 

include factors such as fate and/or die-off/inactivation times in natural waters. A PWS could use 

one or more of these methods, or it could use methods that do not include any of the above as 

long as the state considers the results sufficient to adequately establish the boundaries of the area 

of influence. 

 More information on delineation of the area of influence is described later in Section 

2.4.1. 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-5 April 2010

2.2.2.2 Identification of Cryptosporidium Sources

Potential as well as actual sources of Cryptosporidium contamination within the 

delineated area of influence must be identified and the relative impact on source water quality 

assessed (40 CFR 141.716(a)(2)(ii)). More information on watershed Cryptosporidium sources is 

included later in Section 2.4.2. Examples of “potential” sources include various land uses or 

facilities (e.g., publicly owned treatment works (POTWs), concentrated animal feeding 

operations (CAFOs), etc.) for which the PWS lacks specific data on Cryptosporidium occurrence 

in effluent or runoff, but there is a high likelihood of oocysts being present based on published 

research. 

2.2.2.3 Analysis of Control Measures 

 Cryptosporidium control measures included in watershed protection plans may include 

such diverse activities as structural best management practices (BMPs), land use control 

regulations, and public education. The application must present an analysis of control measures 

that address the sources of Cryptosporidium contamination identified for the water treatment plant 

source water. The analysis of control measures must discuss the effectiveness and feasibility of 

each measure in reducing Cryptosporidium loading in the source water (40 CFR 

141.716(a)(2)(iii)).

 More information on Cryptosporidium control measures is described later in Section 

2.4.3. 

2.2.2.4 Partnerships for Source Water Protection

 

PWSs in the same watershed typically need to evaluate and control the same 

Cryptosporidium sources. Consequently, in order to pool resources and reduce duplication of 

efforts, in many cases the state and the PWSs in the watershed should work together to develop a 

single joint WCP plan that will allow the state to approve a 0.5-log credit for each PWS that 

participates in the implementation of the plan. This may not be practical or achievable in all cases, 

and in other cases a PWS may have a simpler and smaller watershed that does not include 

upstream PWSs. These systems are still encouraged to work with any downstream PWSs to 

develop joint WCP plans, but PWSs that develop and implement an individual WCP plan 

approved by the state will get the identical credit as PWSs involved in a state -approved joint 

WCP plan.

2.2.3 Approval and Continuation of the WCP Credit

2.2.3.1 Initial Approval of the WCP Plan 

The state must review each system's proposed WCP plan and either approve, reject, or 

conditionally approve the plan. See Appendix G for review criteria for use by states when 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-6 April 2010

reviewing WCP plans (both required and recommended elements of a WCP are presented in 

Appendix G). If the plan is approved, or if the system agrees to implement the state's conditions 

for approval, the system will be awarded 0.5-log Cryptosporidium removal credit to apply 

toward the LT2ESWTR treatment requirements. The PWS will need a decision from the state

within three years after bin assignment as outlined in Exhibit 2.1 and Figure 2-1 in order to fulfill 

the treatment requirements of the LT2ESWTR. If the state does not respond to a WCP plan by 

the required date, the WCP plan shall be considered “state-approved” and the 0.5-log WCP 

credit shall be awarded to the water system as long as the submitted WCP plan includes all 

required elements (40 CFR 141.716(a)(4)). Under any circumstances, the state can later 

withdraw an approved WCP credit if the state determines that the PWS has not implemented 

and maintained the activities outlined in the approved WCP plan (40 CFR 141.716(a)(5) and

(6)). 

The initial approval will be valid as long as the PWS continues to implement and 

maintain the approved WCP plan, as described in more detail below. 

2.2.3.2 Maintenance of the WCP Credit 

Systems that have obtained state approval of their watershed control programs are eligible 

for the 0.5-log WCP credit as long as they continue to implement and maintain the efforts 

outlined in their state-approved WCP plan, as well as satisfactorily completing the following:

• Submit an annual WCP status report to the state (40 CFR 141.716(a)(5)(i)).

• Conduct a watershed sanitary survey every three years for community water systems (five 

years for non-community systems) using state guidelines and personnel approved by the 

state for this work and submit the survey report to the state (40 CFR 141.716(a)(5)(ii)).

After approval of the WCP plan, if the PWS determines that a change in the plan is 

needed, the PWS must notify the state prior to making the change and must outline any measures 

proposed to mitigate any reduction in SWP that is likely to result from this change (40 CFR 

141.716 (a)(5)(i)). The description of this change must also be included in the next annual status 

report. 

 The annual status reports, WCP plan, and watershed sanitary surveys that are conducted 

every three years must be made available to the public upon request. These documents must be in 

plain language format and include criteria by which to evaluate the success of the program in 

achieving plan goals. The state may withhold portions of the annual status report, WCP plan, and 

watershed sanitary survey as requested by the PWS based on security considerations (40 CFR 

141.716(a)(5)(iii)). To assist the state in this regard, the PWS should clearly indicate the specific 

information that should be held confidential. The system can identify those items for the state, or 

provide parallel “vetted documents” for dissemination to the public. 

Once awarded the 0.5-log WCP credit, water systems will continue to receive the credit 

as long as they continue to implement and maintain the activities outlined in their state-approved 

WCP plan, including preparation and submittal of annual progress reports and sanitary survey 

reports every three years, as required. States may withdraw the credit if they determine that the 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-7 April 2010

PWS is not carrying out the activities outlined in the state-approved WCP (40 CFR 

141.716(a)(6)). 

 More details on preparation and review of the required reports by the PWS (or collection 

of PWSs in a joint WCP) and the state are briefly outlined below. 

2.2.3.2.1 Annual Status Report

 The annual WCP status report must be submitted by the date established by the state. The 

report must describe the PWS’s implementation of the approved plan and assess the adequacy of 

the plan for meeting the system's goals. It also must explain how the system is addressing any 

shortcomings in plan implementation, including those previously identified by the state or by the 

system during a watershed survey. If the system made any substantial changes to its approved 

program, it must describe the changes and explain the reason for making them. If the change is 

likely to reduce the level of SWP, the system must explain what actions it will take to mitigate 

the effects (40 CFR 141.716(a)(5)(i)).

 The annual status report must describe progress being made implementing individual 

control measures (40 CFR 141.716(a)(5)(i)). Progress should be compared with the original 

timetable provided in the WCP plan. Implementation delays should be explained, and actions to 

prevent further delays should be described. 

 The original watershed program plans must include an analysis of the effectiveness and 

feasibility of control measures that could reduce Cryptosporidium loadings from sources of 

contamination to the system’s source water. Annual status reports should provide updates on the 

control measures as they are implemented. The report should address progress being made on 

priority activities and, to the extent possible, evaluate whether projects are achieving their 

objectives. The report should also identify emerging issues and incorporate them into the 

watershed protection program. Since the annual status reports must be made available to the 

public on request, reports must be written in plain language format (40 CFR 141.716(a)(5)(iii)), 

though portions of the report can be withheld for PWS security considerations. To assist the state

in this regard, the PWS should clearly indicate information in the status report that should be 

held confidential. The PWS can identify those items for the state, or provide a parallel “vetted 

report” for dissemination to the public. 

2.2.3.2.2 Watershed Sanitary Survey Report 

A state-approved watershed survey must be conducted once every three years for 

community water systems (five years for non-community water systems), with the first report 

due three years after the WCP approval date (see Exhibit 2.1). The survey must be conducted 

according to state guidelines by persons approved by the state to conduct watershed surveys. A 

report on the results of the survey must be submitted to the state. The survey must meet the 

following criteria (40 CFR 141.716(a)(5)(ii)(A)): 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-8 April 2010

• Cover the area of the watershed that was identified in the approved WCP plan as the area 

of influence.

• Assess the implementation of actions to reduce source water Cryptosporidium levels.

• Identify new sources of Cryptosporidium. 

If the state determines that significant changes may have occurred in the watershed since 

the previous watershed sanitary survey, systems must undergo another watershed sanitary survey 

by a date required by the state, which may be earlier than the regularly scheduled survey (40 

CFR 141.716(a)(5)(ii)(B)). In such an instance, the next survey and subsequent surveys will be 

required three years from this new date.

States developing a watershed sanitary survey program may wish to use the watershed 

sanitary survey manual developed by the California Department of Health Services and the 

California/Nevada Section of American Water Works Association (AWWA). PWSs are required 

to use state-designated persons for the sanitary survey. Conducting a useful watershed survey 

relies in large part on the competence of the individuals responsible for its execution. It is 

expected that the state will designate appropriately trained individuals, including civil or 

environmental engineers, or sanitarians with experience in the operation of water systems and a 

sound understanding of public health principles and waterborne diseases. Although other people 

may be performing the work for these individuals, the people designated by the state will 

supervise and direct the activities conducted for the survey. Efforts performed during the survey 

could include activities such as the following: 

• Review of relevant National Pollutant Discharge Elimination System (NPDES) permits 

and discharge records. 

• Review of pertinent databases (e.g., county geographic information system (GIS) 

systems, etc.). 

• Review of most recent available aerial photography. 

• Interviews with Natural Resources Conservation Service (NRCS), soil conservation 

service, local county planning agencies, regional planning organizations, and other 

organizations as applicable. 

 A final survey report must be submitted to the state for approval (40 CFR 

141.716(a)(5)(ii)). The report should be completed as soon as possible after the survey is 

conducted. The length of the report will depend on the findings of the survey and the size and 

complexity of the watershed. The survey report should include: 1) the date of the survey; 2) who 

was present during the survey; 3) survey findings; 4) recommended improvements to the 

identified problems; and 5) the dates for completion of any improvements. 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-9 April 2010

 The watershed survey reports must be written in a plain language format. Survey results 

must be made available to the public upon request. The state may withhold portions of the survey 

report based on security considerations (40 CFR 141.716(a)(5)(iii)). To assist the state in this 

regard, the PWS should clearly indicate the specific information in the report that should be held 

confidential. The system can identify those items for the state, or provide parallel “vetted 

documents” for dissemination to the public. 

2.2.3.3 State Review and Continuation of the WCP Credit

Once water systems are awarded the 0.5-log WCP credit, they will continue to receive the 

credit as long as they implement and maintain the efforts outlined in their state-approved WCP 

plan. After the WCP plan is approved, ongoing reviews are the annual status report and the report 

from the sanitary surveys conducted and submitted once every three years. 

The initial approval will be valid as long as the PWS continues to implement and 

maintain the approved WCP plan. After approval of the WCP plan, if the PWS determines that a 

change in the plan is needed, the PWS must notify the state prior to making the change and must 

outline any measures proposed to mitigate any reduction in SWP that is likely to result from this 

change (40 CFR 141.716 (a)(5)(i)). The description should include the impact of that change on 

the protection of the watershed so the state and water system will both understand whether the 

assumptions made during the “verbal approval” stage are holding true. 

2.2.4 PWS and State Checklist for Preparation, Implementation, and Maintenance of the 

WCP Plan and Associated Credit 

 Exhibit 2.2 includes a summary of all activities associated with preparing the WCP plan 

by the PWS, review and approval of the plan by the state, implementation and maintenance of 

the plan by the PWS, and assessment of PWS activities by the state in order to allow continuation 

of the credit.
2. Watershed Control Program

Exhibit 2.2 Checklist of PWS and State activities during preparation, 

implementation, and maintenance of WCP plan and associated 0.5-log LT2ESWTR

treatment credit

LT2ESWTR Toolbox Guidance Manual 2-10 April 2010

Description of Task Code of Federal 

Regulations Citation 

(40 CFR)

Section(s) of interest in 

this Guidance Manual

Notification Period 

(due no later than one year following bin determination, see Exhibit 2.1)

The PWS must notify the state one year after bin 

assignment if they intend to later submit a WCP 

plan

141.716(a)(1) 2.2.1

WCP Plan Preparation Period 

(due no later than two years following bin assignment date, see Exhibit 2.1)

The PWS must submit a report containing the WCP 

plan including the following required elements:

141. 716 (a)(2) 2.2, 

2.2.2

Identification of area of influence 141. 716 (a)(2)(i) 2.2.2.1

2.4.1

Identification of potential and actual sources of 

Cryptosporidium contamination in area of influence 

and assessment of relative impact of these sources 

on source water quality

141. 716 (a)(2)(ii) 2.2.2.2

2.4.2

2.4.2.1

2.4.2.2

Analysis of the effectiveness and feasibility of 

control measures that could reduce 

Cryptosporidium loading from sources identified 

within area of influence

141. 716 (a)(2)(iii) 2.2.2.3

2.4.3

2.4.3.1 through 5

Statement of goals and actions to undertake as 

part of the WCP plan implementation and 

maintenance efforts to reduce source water 

Cryptosporidium levels, including an explanation of 

how these actions are expected to contribute to 

achievement of stated goals. 

141. 716 (a)(2)(iv) 2.2.2

Identification of watershed partners and their roles 141. 716 (a)(2)(iv) 2.2.2.4

Identification of resource requirements and 

commitments

141. 716 (a)(2)(iv) 2.2.2

Development of an implementation schedule, 

including deadlines for completing specific actions 

identified in the WCP plan

141. 716 (a)(2)(iv) 2.2.2

Systems can use existing watershed control 

programs to meet requirements of the rule, as long 

as the entire WCP plan includes all the same 

elements required for all systems

141. 716 (a)(3) 2.2.2

2.3.3
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-11 April 2010

Exhibit 2.2 (continued) 

Description of Task Code of Federal 

Regulations Citation 

(40 CFR)

Section(s) of interest in 

this Guidance Manual

State Review of the WCP Plan 

(due no later than three years after bin assignment date, the same time as other treatment credits, see 

Exhibit 2.1)

The state is expected to approve, reject, or 

conditionally approve the WCP plan if it is 

submitted before the due date (see Exhibit 2.1)

141. 716 (a)(2) 2.2

2.2.3.1

2.2.3.3

If a WCP plan containing all required elements (see 

“WCP Plan Preparation Period” items listed above) 

is submitted by the required date, but is not 

formally acted upon by the state prior to the 

required deadline (see Exhibit 2.1), then the WCP 

plan is considered approved by the state and the 

0.5-log WCP credit is allowed. See discussion 

below regarding withdrawal of this or any other 

approval of the credit.

141. 716 (a)(4) 2.2

2.2.3.1

2.2.3.3

Implementation and Maintenance of the 0.5-log WCP Credit 

(occurs after credit is approved by state)

If the PWS or PWSs awarded a 0.5-log WCP credit 

do not carry out the actions outlined in their stateapproved WCP, the state may withdraw the credit

141. 716 (a)(6) 2.2.3

2.2.3.2

In order to maintain the 0.5-log WCP credit the

PWS or PWSs associated with a state-approved 

WCP plan must carry out the actions outlined in the 

plan, assessed by evaluating the annual status 

report and report from the sanitary survey 

conducted every three years

141. 716 (a)(5) 2.2.3

2.2.3.2

The annual status report must describe the PWS’s 

implementation of an approved WCP plan and 

must assess the adequacy of the plan to continue 

to meet its goals

141. 716 (a)(5)(i) 2.2.3

2.2.3.2

2.2.3.2.1

The annual status report must explain how the 

PWS or PWSs associated with the WCP plan are 

addressing any shortcomings in the implementation 

of the plan, including those previously identified by 

the state after the three-year sanitary surveys. 

141. 716 (a)(5)(i) 2.2.3.2.1

The annual status report must describe any 

significant changes that have occurred in the 

watershed since the last sanitary survey.

141. 716 (a)(5)(i) 2.2.3.2.1

The PWS must notify and receive verbal approval 

from the state before implementing any changes to 

a state-approved WCP plan. The PWSs must 

propose actions they will undertake to mitigate any 

changes that appear likely to reduce the level of 

SWP

141. 716 (a)(5)(i)

. These changes must be described in the 

next annual progress report.

2.2.3

2.2.3.2,

2.2.3.2.1
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-12 April 2010

Exhibit 2.2 (continued) 

Description of Task Code of Federal 

Regulations Citation 

(40 CFR)

Section(s) of interest in 

this Guidance Manual

A state-approved watershed survey must be 

conducted once every three years for community 

water systems (five years for non-community water 

systems), with first report due three years after the 

WCP plan is approved by the state. The survey 

must be conducted according to state guidelines by 

persons approved by the state to conduct 

watershed surveys. A report on the results of the 

survey must be submitted to the state once every 

three years. The survey must meet the criteria 

listed below:

141. 716 (a)(5)(ii) 2.2.3

2.2.3.2,

2.2.3.2.2

Encompass the area of influence defined in the 

state-approved WCP plan 

141. 716 (a)(5)(ii)(A) 2.2.3.2.2

Assess actions implemented to reduce 

Cryptosporidium levels within the area of influence

141. 716 (a)(5)(ii)(A) 2.2.3.2.2

Identify any significant new sources of 

Cryptosporidium in the area of influence 

141. 716 (a)(5)(ii)(A) 2.2.3.2.2

If the state determines that significant changes may 

have occurred in the watershed since the previous 

watershed sanitary survey, systems must undergo 

another watershed sanitary survey by a date 

required by the state, which may be earlier than the 

regularly scheduled survey. In such an instance, 

the next survey and subsequent surveys will be 

required three years from this new date.

141. 716 (a)(5)(ii)(B) 2.2.3.2.2

The WCP plan, annual status reports, and 

watershed sanitary survey reports must be written 

in plain language format. All of these documents

must be made available by the state to public, upon 

request. The state can withhold portions of these 

documents identified by the PWS due to security

considerations. To assist the state in this regard, 

the PWS should clearly indicate the information in 

the status report that should be held confidential. 

141. 716 (a)(5)(iii) 2.2.2

2.3.3

2.3.3.2, 

2.2.3.2.1

2.2.3.2.2
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2.3 Benefits and Other Characteristics of the WCP Credit and Related Activities 

2.3.1 Benefits to the PWS and Watershed from a Successful WCP

A well-designed WCP can result in a reduction of overall microbial risk. The risk 

reduction is associated with the implementation of practices that reduce Cryptosporidium as 

well as other pathogens. Further, knowledge of the watershed and factors affecting microbial 

risk, including sources of pathogens, fate and transport of pathogens, and hydrology, can also 

help a system reduce microbial risk. 

There are many potential sources of Cryptosporidium in watersheds, including sewage 

discharges and nonpoint sources associated with animal feces. The feasibility, effectiveness, 

and sustainability of control measures to reduce Cryptosporidium contamination of water 

sources will be site-specific. Consequently, the WCP credit centers on systems working with 

stakeholders in the watershed to develop a site-specific program, and state review and approval 

are required. This section is intended to assist water systems in developing their watershed 

control programs and states in assessing and approving these programs.

PWSs with existing SWP programs, watershed information, partnerships, etc., are 

encouraged to incorporate these into their WCP plans. Whether as a continuation of existing 

efforts or as a result of new efforts specifically initiated for WCP credit, SWP activities for 

identification, prioritization, and control of Cryptosporidium sources are important. These 

activities also provide proactive, preventive steps for reduction of Cryptosporidium risks for 

drinking water consumers. Efforts to identify, prioritize, and control Cryptosporidium sources 

in the watershed offer opportunities for cooperation and collaboration between PWSs who 

will benefit from joint efforts to reduce Cryptosporidium sources in their common 

watershed(s). Consequently, a WCP for one or more PWS can create the potential opportunity 

to extend watershed protection activities until they become watershed-wide, basin-wide, 

regional, state-wide, or even multi-state-wide if enough water systems can cooperate together 

to make it happen. 

A WCP targeting Cryptosporidium reduction is the most advantageous when it is also 

the component of a larger comprehensive SWP program that addresses other chemical and/or 

microbial contaminant threats. For PWSs in many states, much of the background information 

and preparation needed to develop a WCP and comprehensive SWP program are already 

available as a result of the source water assessments required under the 1996 Amendments to the 

Safe Drinking Water Act (SDWA). Section 1453 of the Act required states to conduct source 

water assessments for all public water systems, including delineating the “boundaries of the areas 

providing source waters for PWSs and identifying the origins of regulated and certain 

unregulated contaminants in the delineated area to determine the susceptibility of the PWSs to 

such contaminants.” 
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Information resulting from these assessments should be available from the states. 

Information may also be available for systems where watershed sanitary surveys have been 

performed. These surveys are required as part of the Interim Enhanced Surface Water Treatment 

Rule (IESWTR), and some states have required them for years. The completeness of the 

information contained in these existing resources may need to be supplemented by collecting 

additional background information, particularly information bearing on Cryptosporidium

occurrence.

2.3.2 Advantages and Disadvantages of a Watershed Control Program 

 Section 2.3.2.1, and 2.3.2.2 explain the advantages and disadvantages of a WCP

(respectively). Topics covered include the side impacts on public health and ecological goals, the 

incorporation of a multiple barrier strategy, the availability of analytical methods to track water 

quality progress, the level of commitment required of PWSs, and the potential costs and payoffs 

of implementation efforts. 

2.3.2.1 Advantages 

Measures to control prioritized Cryptosporidium sources in the watershed, as required for 

the WCP credit, will in most instances control other contaminants of concern to PWSs. The 

reduction and prevention of source water contamination by microbial pathogens and other 

contaminants may also serve other public health and ecological goals, such as use of the water 

body for fishing and swimming, reduction of ground water contamination, and protection of 

aquatic habitats and the species that depend on such habitats for survival. The reduction in 

organic material in the source water will make treatment more efficient (and less expensive) and 

reduce the incidence of disinfection byproducts (DBPs). 

While WCPs may be a cost-effective Microbial Toolbox option, the PWS commitment 

needed to initiate and maintain SWP efforts may be substantial. SWP efforts often require many 

years to start seeing measurable results. Furthermore, these efforts must continue to be 

maintained in order for initial improvement to persist. However, the potential payoff of these 

efforts are significant both for the water system and the community they serve. For example, one 

PWS estimates that its current SWP effort has helped the PWS avoid $100 million in capital 

costs and $10 million/yr in operating costs. This savings was accomplished with approximately 

$5 million expended to date on SWP efforts, of which $3 million was recovered through funding 

from grants (Crockett 2005). 

Although the costs associated with implementing a particular Microbial Toolbox option 

are system-specific, a WCP can cost less than other Microbial Toolbox options that require 

installation of additional technology. This is especially the case if other stakeholders contribute 

time and resources to the watershed control program. Stakeholders could include other utilities 

and municipalities, other agencies in the same municipality, county or state agencies, and 

concerned citizens. Though watershed control programs involving land acquisition or purchase 

of conservation easements may be initially more expensive than installing treatment, the longterm benefits (improved quality and stability of source water, reduction in treatment costs, etc.) 

are potentially significant, though prediction of these improvements beforehand and 

measurement afterwards may at times be problematic. 
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Much of the information required to implement a watershed program, such as a 

contaminant source inventory and delineation of the watershed, may already be available in 

some states as a result of the source water assessment conducted under the 1996 SDWA 

Amendments. Although source water assessment programs vary from state to state, they should 

provide much of the basic information required to prepare a plan for a watershed control 

program, allowing systems to incorporate existing information into their WCP plans at minimal 

cost. 

Control of Cryptosporidium sources in the watershed can contribute significantly to 

integrated multiple-barrier treatment strategies. For example, reducing influent Cryptosporidium

loadings will facilitate pre-treatment (e.g., riverbank filtration), conventional treatment (e.g., 

clarification followed by filtration), or post-treatment (e.g., ultraviolet light (UV) irradiation). 

Control of Cryptosporidium sources in the watershed can benefit more than a single water 

treatment facility. In even small watersheds, there may be multiple water intakes that may serve 

other PWSs. Consequently, watershed-based coordination can assist PWSs to develop 

background information and to implement Cryptosporidium control measures. For example, 

since much of the background information needed for the WCP application submitted to the 

state can be very similar for multiple PWSs in the same watershed, working together PWSs can 

be more efficient developing background information and identifying priority actions to manage 

Cryptosporidium sources. 

The use of stakeholder partnerships during development and implementation of a WCP 

can be substantial. In addition to being able to share resources and work together to find funding 

sources, collaboration with other stakeholders can bring important information and resources to 

the PWS. For example, increased contact with upstream or downstream utilities can result in 

sharing of information about source water quality issues, the means that other utilities have used 

to respond to contaminants and water quality changes, and increases the likelihood that other 

utilities will share information about sudden water quality changes that may also affect the other 

utilities using that particular source water.

2.3.2.2 Disadvantages 

Most water systems who have developed and implemented SWP efforts like those needed 

for the state-approved WCP credit have found that these efforts are able to substantially improve 

source water quality (Ashendorff et al. 1997, Vaux 2000). However, seldom will watershed 

activities result in immediate realization of benefits. Many land use policies, wildlife 

management, and public education programs require significant implementation timeframes. 

This challenge is further complicated by the target organism in this rulemaking, 

Cryptosporidium. Cryptosporidium occurs in low concentrations and is difficult to detect using 

existing analytical methods, consequently, it can be hard to discern reductions in 

Cryptosporidium concentrations resulting from watershed control programs even if substantial 

changes are realized in the watershed.
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The PWS commitment needed to initiate and maintain source water protection efforts is 

substantial. SWP efforts often require many years to start seeing measurable results. 

Furthermore, these efforts must continue to be maintained in order for initial improvements to 

persist. Furthermore, it may not be possible to discern the improvements in source water quality 

using these monitoring approaches due to natural environmental variability, the characteristics 

of the source water improvements, and the limitations of the current analytical methods for 

Cryptosporidium or other fecal indicators. However the potential payoff of these efforts are 

significant both for the water system and the community they serve, as noted in the 

“advantages” section. 

As with other treatment options, cost is a significant factor in determining if a WCP will 

be viable for any individual water system. Some federal funding is available to implement some 

aspects of a WCP. For example, the Clean Water Act (CWA) authorizes state revolving fund 

loans to upgrade wastewater treatment plants and provides grants (under Section 319) for 

control of nonpoint source pollution. The Farm Bill of 2002 authorizes several billion dollars for 

management of agricultural pollution. Drinking Water State Revolving Funds (DWSRF) are also 

available to a limited extent for SWP. Each state may set aside as much as 15 percent of its grant 

each year to provide loans for SWP activities, including land or easement acquisition, 

implementation of incentive-based voluntary SWP programs, and implementation of wellhead 

protection programs. A review of potential funding sources is provided in Gullick et al. (2007). 

Like other components of the multiple barrier strategy, the barrier provided by watershed 

protection must be maintained into the future. Consequently, while some SWP efforts can 

produce visible evidence of action in fairly short periods of time (Capacasa 2005, Crockett et al. 

2005), the ongoing maintenance of land use systems and key changes in private actions in the 

watershed will be more difficult to demonstrate. This lack of visibility can lead to challenges for 

long-term fiscal stability and stakeholder engagement, which is compounded when there are 

other competing demands for resources.

A WCP may not be as successful in some circumstances as in others. Potential pitfalls 

are important to consider in deciding whether to undertake a watershed control program. 

Microbiological contaminants are frequently related to nonpoint sources, and control of these 

sources is often highly dependent on changing the behaviors of large groups of people. In a 

voluntary program (e.g., if the water system has no authority to regulate land use and is 

encouraging landowners to voluntarily take action), it is difficult to determine whether individuals 

are making the recommended changes necessary to control contaminants. Although the required 

three-year watershed survey will assist in evaluating progress, systems that implement watershed 

control programs may need to be creative in finding ways to gauge the success of their programs. 

Urban growth and land development can affect the success of a watershed control 

program. The success or failure of a WCP that is based on land use controls will rest in large part 

on how committed the effected communities are to supporting the land use constraints identified 

in the WCP plan. 

A successful WCP requires the cooperation of a variety of stakeholders; however, it may 

be difficult to get agreement or participation from these stakeholders. Alternatively, stakeholder 

groups may agree to perform certain activities, such as outreach; but could lose funding and be 

unable to follow through on their commitments. Systems that have concerns about the likelihood 
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LT2ESWTR Toolbox Guidance Manual 2-17 April 2010

of building strong relationships with their stakeholders may decide that a WCP is not 

appropriate for them. In some watersheds, depending on size of the WCP and the ability to 

share costs with others, significant PWS staff time may be required to oversee a program. These 

costs and staff commitments may be prohibitive for some systems.

2.3.3 Incorporation of New Versus Existing Source Water Protection Activities Into a 

Watershed Control Program

PWSs that already have SWP activities in place that are suitable for incorporation into a 

WCP are encouraged to consider the WCP credit as one of their Microbial Toolbox options since 

continuation of these existing efforts offer the best chance of producing improvements in the 

watershed the most quickly. These systems are also encouraged to cooperate with other utilities 

in their watershed so that information these utilities have in common can be shared, allowing all 

utilities involved to focus on prioritized control measures rather than duplicating these efforts. 

PWSs with existing SWP programs must incorporate into their WCP implementation plans all 

required elements described earlier in Section 2.2. Some PWSs may already have existing SWP 

programs encompassing all of these elements, while others will need to develop and implement a 

combination of existing and newly designed efforts to meet all of the requirements. PWSs 

without existing SWP programs suitable for the WCP credit are encouraged to develop and 

implement appropriate SWP programs via the WCP credit mechanism. 

2.4 Tools to Help PWSs Develop the Watershed Control Program Plan 

 The following subsections discuss the factors PWSs should consider in developing a WCP 

plan to improve Cryptosporidium reductions in source water, along with descriptions of BMPs 

and other control measures. However, each watershed is different and therefore, each WCP plan 

needs to be tailored for the specific circumstances. 

Systems may be able to use the results of the source water assessments conducted under 

the SDWA Amendments of 1996 to support this effort to develop a WCP plan. These assessments 

establish a foundation for the WCP because they delineate the watershed, providing a starting 

point for defining the area of influence, and they inventory and rank the susceptibility of the water 

supply to actual and potential contamination sources. The assessments covered all priority 

contaminants in a watershed, including Cryptosporidium (U.S. EPA 1997). In some cases, if 

sufficiently detailed, the source water assessments may fully satisfy the analytical needs of the 

WCP plan. However in some cases the information available from the source water assessments is 

quite limited or outdated, and utilities will need to look for or develop other sources for this 

information. 

Other source water and watershed information may be available from sanitary surveys 

conducted for the IESWTR and the Long Term 1 Enhanced Surface Water Treatment Rule

(LT1ESWTR) (these rules require sanitary surveys at least every three years for community water 

systems and at least every five years for noncommunity water systems). Guidance is available at 

www.epa.gov/safewater/mdbp/pdf/sansurv/sansurv.pdf. The California-Nevada section of the 

AWWA and the California Department of Health Services Division of Drinking Water and 

Environmental Management also have developed guidance specifically for watershed sanitary 
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surveys. Coordination of SWP efforts with those of CWA programs such as Total Maximum 

Daily Loads (TMDLs) is beneficial and encouraged. 

Guidance for SWP activities is available from a wide variety of sources, including the 

following: 

• U.S. EPA Source Water Protection webpage (www.epa.gov/safewater/protect.html) 

contains links to a variety of guidance materials. 

• U.S. EPA draft Handbook for Developing Watershed Plans to Restore and Protect our 

Waters (U.S. EPA 2005d). 

• Introduction to EPA’s Drinking Water Source Protection Programs (U.S. EPA, 2003b). 

• Source Water Protection: Best Management Practices and Other Measures for Protecting 

Drinking Water Supplies (U.S. EPA 2002d). 

• State Source Water Assessment and Protection Programs Guidance: Final Guidance (U.S. 

EPA 1997). 

• Protecting and Restoring America's Watersheds: Status, Trends, and Initiatives in 

Watershed Management (U.S. EPA 2001g). 

• Getting in Step: A Guide for Conducting Watershed Outreach Campaigns (Tetra Tech, 

Inc. 2003). 

• U.S. EPA Watershed Academy On-Line Training Modules (available at 

www.epa.gov/watertrain/, e.g., www.epa.gov/watertrain/pdf/ swp.pdf (January 2003) and 

www.epa.gov/watertrain/pdf/swpbmp.pdf (August 2002)). 

• U.S. EPA National Agriculture Compliance Assistance Center website - information on 

animal production practices and BMPs (http://www.epa.gov/agriculture). 

• U.S. EPA Animal Feeding Operations (AFO) Virtual Information Center website 

(http://cfpub.epa.gov/npdes/afo/virtualcenter.cfm). 

• Source Water Protection for Concentrated Animal Feeding Operations: A Guide for 

Drinking Water Utilities (Gullick et al. 2007). 

• AwwaRF (AWWA Research Foundation) Source Water Protection Reference Manual 

(CDM 2002). 

• Source Water Protection: Effective Tools and Techniques You Can Use (1999 Participant 

Manual) (AWWA 1999). 

• Effective Watershed Management for Surface Water Supplies (AwwaRF 1991). 
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• Guidance to Utilities on Building Alliances with Watershed Stakeholders (AwwaRF 

2001). 

• Protecting the Source: Land Conservation and the Future of America’s Drinking Water 

(Ernst 2004). 

• Path to Protection: Ten Strategies for Successful Source Water Protection (Ernst and Hart 

2005). 

• Source Protection Handbook: Using Conservation to Protect Drinking Water Supplies 

(Hopper and Ernst 2005). 

• Source Protection: A National Guidance Manual for Surface Water Supplies (NEIWPCC 

2000). 

• The Center for Watershed Protection provides basic templates to help with design of 

watershed protection programs; see www.stormwatercenter.net. 

• From Source to Tap: Guidance on the Multi-Barrier Approach to Safe Drinking Water 

(CCME 2002). 

 In addition, compilations of successful SWP program case studies are available from the 

following sources: 

• Case Studies of Source Water Protection (U.S. EPA 2005a; 

http://cfpub.epa.gov/safewater/sourcewater/sourcewater.cfm?action=Case_Studies.)

• Section 319 Nonpoint Success Stories (U.S. EPA 2005b; 

www.epa.gov/owow/nps/Success319). 

• Watershed Success Stories – Applying the Principles and Spirit of the Clean Water 

Action Plan (U.S. EPA 2000d; 

 

http://www.blueprintjordanriver.slco.org/docToPdf/WatershedSuccessStor.pdf

• Protecting Sources of Drinking Water: Selected Case Studies in Watershed Management 

(U.S. EPA 1999a, http://www.epa.gov/safewater/sourcewater/pubs/swpcases.pdf) 

• Source Water Collaborative; see www.protectdrinkingwater.org. 

2.4.1 Identification of the Area of Influence

An essential element for the WCP plan is the identification of the “area of influence.” The 

area of influence is the area outside of which the likelihood of Cryptosporidium or fecal 

contamination affecting the treatment plant is not significant. Identification of Cryptosporidium

sources, associated control measures, and future watershed surveys (see Section 2.2.2.1) will be 

targeted within this area. Methods to be used to establish the boundaries of the area of influence 
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are at the discretion of the PWS, as long as the state considers it sufficient to approve the area 

delineated.

Delineation

Systems may develop their own watershed delineation. The starting point for such a 

delineation may be the delineation developed by the state as part of the source water assessment

program (SWAP). In referencing the delineation prepared in the SWAP process, systems 

should be careful to understand how the initial delineation was prepared. Different states 

employed different delineation approaches. To delineate watersheds, some states started with 

watersheds as catalogued by the U.S. Geological Survey (USGS). The USGS has assigned each 

watershed and its subwatersheds in the United States a hydrologic unit code (HUC). Because 

the HUC subwatersheds can be quite large, and a PWS's source may come from only a 

section of the watershed, or portion of a hydrologic unit, sometimes only the part of the 

watershed upstream of the PWS's intake was mapped. Sometimes watersheds were 

further segmented into "critical areas" within which more detailed assessments were 

performed. Some states delineated critical areas based on setbacks from the edge of the source 

water and all tributaries feeding into the source water. Others defined critical areas based on a 

fixed distance or time-of-travel from the intake (upstream of the intake or in all directions from 

the intake).

Systems that need to delineate their watersheds or subwatersheds for the first time and do 

not have GIS available can do so with topographic maps. The first step is to find the source 

(including tributaries) and the water treatment plant intake on the map. Each of the contour 

lines (which is actually not a line but a closed shape) around the source connects points of equal 

elevation. Upstream, the elevation indicated by each contour line increases with distance from the 

source. All precipitation falling within a zone of increasing elevation around the source will flow 

towards the source. Where the contour elevations stop increasing and begin decreasing is the 

break point. On the other side of the break point, water is flowing into a different watershed. The 

area delineated by connecting the break points is the watershed (AWWA 1999). See 

http://www.nh.nrcs.usda.gov/technical/Publications/Topowatershed.pdf for an illustrated fact 

sheet on delineation. If the intake is not at the downstream end of the watershed, it is only 

necessary to delineate the area upstream of the intake. Systems with GIS can follow the same 

process using digital elevation model (DEM) data rather than contour lines. 

PWSs using ground water under the direct influence of surface water (GWUDI) as a 

source can delineate an area of influence by combining a delineation of the watershed influencing 

the ground water source with a delineation of the wellhead protection area. 
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Watershed Hydrology 

Once the watershed has been delineated, PWSs should examine the hydrology of their 

watersheds to help determine the area of influence. The analysis submitted to the state must 

contain information on the watershed's hydrology. Stream discharge can affect the transport of 

sediment and Cryptosporidium oocysts, especially during and after storms. When more rain falls

than can be absorbed immediately by the soil, soil cover, or impervious surface, water will pond 

on the surface. With increasing rainfall, the water will flow to a lower level on the surface, to a 

river, lake, or reservoir, as shown in Exhibit 2.3. As water travels, it may pick up contaminants on 

the soil surface (e.g., Cryptosporidium oocysts from deposited fecal matter). These particles are 

then suspended in the runoff and can be transported to surface water supplies. The 

microorganisms (including parasitic protozoa) associated with the soil can be transported as 

individual organisms, aggregates of organisms, or within an aggregate of soil particles and 

organisms. 
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Exhibit 2.3 Ground Water/Surface Water Interaction

Ground water that is considered to be under the direct influence of surface water is usually 

immediately adjacent to surface water or to the discharge point of a spring. These ground water 

supplies are considered vulnerable to contamination by microbial contaminants like 

Cryptosporidium (consequently, GWUDI sources are treated like surface water sources under the 

SWTR, IESWTR, and LT2ESWTR). GWUDI may be contaminated through direct contamination 

(e.g., an inadequately protected spring), direct infiltration of oocysts from the surface as a result of 

rain, and as a result of the action of pumping wells (see Exhibit 2.3). Given sufficiently high 

pumping rates, wells can locally reverse the direction of ground water flow. In such cases, surface 

water is induced to flow from a river, lake, or reservoir into the adjacent ground water, where it 

may be extracted by pumping wells. If the surface water is contaminated with microbial 

contaminants, the adjacent ground water may also become contaminated. 

 Water quality flow models analyze specific hydrologic, geographic, and water quality 

parameters to estimate the travel time needed for contaminants to reach a drinking water intake 

and the amount of contamination at that intake. Surface runoff models may also be used to assess 

the potential impact of individual Cryptosporidium sources, and to identify watershed areas with 

the greatest potential impact on source water quality. Models should always be validated for the 

settings in which they are used.

PWSs should also consider topography and soil type, which can affect hydrology. Areas 

with steep slopes may experience a higher percentage of overland flow or runoff (as opposed to 

infiltration and subsurface flow) and have faster overland flow rates during rainfall than flat areas. 

Cryptosporidium may be more likely to be transported to water bodies in such areas, although if 

the topography is very steep, livestock that carry Cryptosporidium may not be present. 

Impermeable or compacted soil, impervious surfaces, unvegetated areas, and a high water table 
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can also affect overland flow. Riparian zones can be considered sensitive areas simply due to their 

proximity to streams that feed into source waters. They are also subject to erosion. PWSs should 

also factor soil types into their decisions; areas with high clay content may be more impermeable 

or more subject to erosion and can contribute to high turbidity. 

2.4.2 Potential and Existing Sources of Cryptosporidium

All potential and actual Cryptosporidium sources in the area of influence must be 

identified and reported in the WCP plan (40 CFR 141.716(a)(2)(ii)) and should be evaluated for 

suitable control measures in the plan. Systems may be able to use source inventory data collected 

as part of the source water assessment program. Many states asked systems to assist with 

identifying significant potential contaminant sources, either through field verification or through 

review of inventory databases or other information. Therefore, some PWSs should already have 

this information available. States will also be assessing the risk of each source or category of 

sources, primarily through numerical ranking systems and matrices; systems will have this 

information at their disposal as well. It is possible that the inventory and ranking of potential 

sources may not be detailed enough for a Cryptosporidium watershed control program, but they 

should provide a good starting point. 

 After noting sensitive areas based on topography and geology, it is possible to determine 

whether these areas coincide with land uses that are likely to contribute microbiological 

contamination to the water supply. Reviewing land use and zoning maps can be used to identify 

areas for investigation or for prediction of potential future sources and loadings. Where existing 

mapping does not reflect available databases, investigation of local data sources, such as health 

department data on septic systems and recent sanitary survey results can provide additional 

detailed information. Data on point sources such as wastewater treatment plants that require EPA 

or state permits, e.g., NPDES are also readily available through State and federal data systems. 

NPDES information (also called water discharge permit or PCS data) is available on EPA's 

Envirofacts website at http://www.epa.gov/enviro/index_java.html. Local, state and federal data 

sets are useful for identifying potential sources of contaminants, but these data systems can be out 

of date; actual field surveys may be necessary to confirm the status of existing point and nonpoint 

sources. 

The paragraphs below summarize existing research on Cryptosporidium sources and 

associated land use in watersheds. Because most studies of Cryptosporidium occurrence involve 

sampling at water system intakes, little information is available about occurrence of 

Cryptosporidium within watersheds and transport of oocysts to surface water supplies. When 

possible, developing site-specific relationships can facilitate how to more effectively affect oocyst 

levels released to surface water in the watershed.

Land Use

 Many land uses in a watershed have the potential to introduce Cryptosporidium into water 

supplies. These include point sources—combined sewer overflows (CSOs), wastewater treatment 

plants, and CAFOs—and nonpoint sources, including livestock, wildlife, pets, stormwater runoff, 

and septic systems. 
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 Kaplan et al. (2002) generated a database on pathogen occurrence in watersheds ranging 

from forested to highly urbanized. Corsi et al. (2003) studied the source(s) and magnitude of 

Cryptosporidium by broad characterization of rural, suburban, and urban land uses. In this case 

the primary sources were attributed to urban stormwater and wastewater treatment plant outflows 

during baseflow and stormwater events. 

 The character (topography, plant cover) and land uses (urban, farming) within a watershed 

also influence the occurrence or concentration of Cryptosporidium in surface water (Hansen and 

Ongerth 1991). Oocyst concentrations can be as much as 10 times higher in urban and agricultural 

watersheds (Hansen and Ongerth 1991, Stern 1996) than in undeveloped ones. However, such 

differences may be site-specific—in streams in an agricultural watershed in southern Ontario, no 

connection was found between Cryptosporidium concentration and sources or land use such as 

wastewater treatment plants, CSOs, livestock, crops, houses, wildlife, and campgrounds (Fleming 

et al. 1999). Davies et al. (2005) quantifies information on prevalence and viability of E. coli and 

Cryptosporidium in livestock. Crockett (N.d.) compares prevalence of Cryptosporidium in 

wastewater versus wildlife and livestock, including adult versus neonatal calves. Santin et al. 

(2004) studied genotype and speciation of Cryptosporidium in feces from 1,000 cattle on 15 farms 

in 7 states. Results indicate that Cryptosporidium parvum, which are potentially infectious to 

humans, were detected in feces from pre-weaned calves but nearly all Cryptosporidium in older 

cattle were other non-infectious species or genotypes. 

Point and nonpoint sources of Cryptosporidium are described below.

Point Sources 

Point sources such as CSOoutfalls, which are common in older municipalities, can be a 

significant source of oocysts, depending on the weather and the endemic rate of cryptosporidiosis. 

CSOs contain raw sewage diluted by stormwater. In one study, Cryptosporidium concentrations at 

CSO outfalls on the Allegheny River in Pittsburgh during storms ranged from 0 to 3,000 

oocysts/100 L, with a geometric mean of 2,013 oocysts/100 L (States et al. 1997).

 Wastewater treatment plants may also contribute to oocyst loads, depending on the amount 

of treatment provided. Primary treatment can remove as little as 27 percent of oocysts from 

effluent (Payment et al. 2001); most plants in the United States provide secondary treatment, so 

removal should be better. In the Netherlands, it is estimated that 85 percent of Cryptosporidium 

oocysts occurring in surface water are discharged in wastewater treatment plant effluent 

(Medema and Schijven 2001). In one study in Pittsburgh, oocysts were detected in 33 percent of 

samples with a geometric mean concentration of 924 oocysts/100 L over 24 months of 

sampling (States et al. 1997). In another study near Philadelphia, concentrations ranged from 33 

to 2,490 oocysts per 100 L (67 percent of samples were positive); downstream from the plant, 

concentrations ranged from 325 to 825 oocysts per 100 L (Crockett and Haas 1997). Results from 

more recent research summarized by Crockett (N.d.) indicate that effluent wastewater 

Cryptosporidium concentrations ranges from 0.1 to 1,000 oocysts/L. 

CAFOs can be a significant source of animal waste, which can contaminate source water 

in two ways. If not properly managed, waste can leak or overflow from waste storage lagoons, 

feedlots, or other facilities. In addition, waste applied as fertilizer to fields can run off into 

drinking water sources or source tributaries, especially if over applied. Gullick et al (N.d.) provide 
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a reference guide for drinking water utilities to implement source water protection activities 

related to CAFOs. Finstein (2004) describes impact of ammonia and temperature on 

Cryptosporidium oocyst survival during storage of livestock manure, including capital and 

management intensive treatment processes like anaerobic digestion and less capital intensive 

processes such as aerobic composting. 

Nonpoint Sources

Agriculture can also be a nonpoint source of Cryptosporidium. On a stream running 

through a small dairy farm before feeding into the Allegheny River, Cryptosporidium was 

detected in 82 percent of samples (States et al. 1997), with a geometric mean concentration of 42 

oocysts/100 L. Twice during the 24-month study, concentrations of more than 1,000 oocysts/100 

L were observed. In an agricultural area in Canada, drain tiles contained average concentrations of 

771 oocysts/100 L. Concentrations were high even in tiles on farms without barns (these farms 

were assumed not to have livestock present). Oocysts were also present in some samples in liquid 

swine manure storage lagoons (Fleming et al. 1999).

 Cattle are thought to be significant sources of oocysts. Cryptosporidium infection rates in 

cattle depend on animal age. Calves, particularly those less than one or two months old, have the 

highest rates (infection rates in different studies range from 2 to 39 percent of calves) (Wade et 

al. 2000, Sischo et al. 2000, Huetink et al. 2001). 

 Cryptosporidium may directly enter surface water via waterfowl. Oocysts have been found 

in Canada goose feces collected in the environment (Graczyk et al. 1998). Canada geese, some 

of which no longer migrate, could cause considerable contamination of surface water sources and 

uncovered finished water reservoirs. 

Other wildlife may also be a source of Cryptosporidium, though the impact on source 

water may not be as direct. Deer, muskrat, and other small mammals were shown to carry 

Cryptosporidium in upstate New York (Perz and Le Blancq 2001). In one study of California 

ground squirrels, 16 percent of squirrels sampled were found to shed an average of 50,000 oocysts 

per gram of feces (Atwill et al. 2001). The infection rate in each species and the species present in 

each watershed will vary, so the contribution from wildlife will also differ from watershed to 

watershed.

Although little research has been performed on the overall prevalence of Cryptosporidium 

in pets, Cryptosporidium has been detected in dogs and cats, although pets usually carry strains 

that are rarely detected in humans. Several studies have shown dogs to be significant carriers of 

Giardia, fecal coliform, and other bacteria (Schueler 1999), and these microbes have been found 

in stormwater, suggesting that Cryptosporidium may also be present in urban watersheds and 

stormwater runoff.

Low levels of Cryptosporidium may also enter surface water through septic systems and 

subsequent subsurface transport (Lipp et al. 2001).

“Microbial source tracking” (MST) is a rapidly expanding science designed to identify 

human or animal sources of fecal pollution in the environment. These methods can evaluate E. 

coli, Giardia, Cryptosporidium, and viruses, and identify whether the source is from human, 
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cattle, swine, bird, or other origin. Fecal source tracking using DNA “fingerprinting” can be used 

by water utilities to identify whether or not particular a potential fecal source in their watershed, 

such as an AFO or CAFO, is an actual contributor to wastes identified in the source water. 

 Cryptosporidium sources can be identified through polymerase chain reaction (PCR) 

analysis of Cryptosporidium DNA. PCR can be used to determine the species or genotype of 

Cryptosporidium; many genotypes or species are typically, although not exclusively, found in 

specific hosts, such as cattle, dogs, and humans. In mixed-use watersheds, this information can 

help determine whether Cryptosporidium in the source water could have come from agricultural 

runoff, CSOs, or stormwater runoff.

Numerous references are available that summarize the capabilities and state-of-the-science 

of MST, including the following: 

• U.S. EPA Microbial Source Tracking Guide Document (U.S. EPA 2005c) 

http://www.ces.purdue.edu/waterquality/resources/MSTGuide.pdf. 

• Microbial Source Tracking: Current Methodology and Future Directions (Scott et al. 

2002) http://aem.asm.org/cgi/content/full/68/12/5796. 

• Microbial Indicators of Fecal Contamination: Application to Microbial Source Tracking 

(Bitton 2005) 

http://www.florida-stormwater.org/pdfs/FSAMicrobialSourceTrackingReport.pdf. 

• Microbial Source Tracking and Detection Techniques (USGS website) 

http://water.usgs.gov/owq/microbial.html. 

2.4.2.1 How Do Fate and Transport Affect the Way Cryptosporidium Impacts My 

 Water Supply?

 Transport of oocysts in surface water and ground water and survival of oocysts all affect 

the potential impact of Cryptosporidium on water supplies. A critical review of available research 

on transport of pathogens in watersheds was conducted by Ferguson et al. (2003) and Davies et al. 

(2005). Pyke et al. (2003) summarized the occurrence, sources, and fate of Cryptosporidium in 

agricultural environments. Approaches for reducing overall pathogen loading within an 

agricultural watershed are discussed by Rosen et al. (2001). A summary of the fate of 

Cryptosporidium and the effectiveness for Cryptosporidium inactivation via different agricultural 

BMPs (e.g., composting, anaerobic digestion, manure slurry storage) was provided by Finstein 

(2004). Yeghiazarian et al. (2004) developed a pathogen transport model. Crockett (N.d.) 

discusses the role of wastewater treatment in protecting water supplies against emerging 

pathogens such as Cryptosporidium. 

 The fate and transport of oocysts in the environment is briefly described below. 
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Influence of Precipitation

Sixty-eight percent of waterborne disease outbreaks between 1948 and 1994 were shown 

to be associated with heavy precipitation (Curriero et al. 2001). Cryptosporidium occurrence may 

also be related to seasonal variations in infection among livestock, but any correlation is sitespecific and depends on the source. 

 Crockett and Johnson (2000) noted that Cryptosporidium concentration increased by about 

a factor of 10 during storm events and frequency of detection doubled during these same storm 

events. Research at one New Jersey utility indicated that during storm events, large turbidity 

increases accompanied increases in Cryptosporidium occurrence in the raw source water

(LeChevallier et al. 1998, Atherholt et al. 1998). Consequently, the utility monitors raw water 

turbidity and can shut down the intake for up to 24 hours when raw water turbidity exceeds a 

certain threshold (typically ~15 ntu) during storm events. 

One study showed both Cryptosporidium detection and concentrations at six watersheds 

were highest between the months of October and April, with March experiencing a detection rate 

of more than 30 percent and oocyst concentration of about 0.038 oocysts/L (Sobrinho et al. 2001). 

Other studies have noted a connection between rainfall and "extreme runoff' events in tributaries 

to drinking water sources (Kistemann et al. 2002). One study noted a decrease in farm stream 

concentrations of Cryptosporidium with an increase in 5-day cumulative precipitation (probably 

because continued rainfall washed most of the Cryptosporidium downstream) (Sischo et al. 2000). 

In a study in six watersheds, Sobrinho et al. (2001) found no substantial difference 

between Cryptosporidium detection rates during "event" (rainfall, high turbidity, melting snow 

and spring runoff) and "non-event" sampling when all data were taken together. However, for 

three of the watersheds, when examined individually, detection within each watershed was 

significantly higher during event sampling. 

 One recently completed research project (Sturdevant Rees et al., N.d.) investigated the 

variability of pathogen occurrence and transport through watersheds. Even small rainfall events 

(less than 0.25 inches) were capable of washing Giardia cysts and Cryptosporidium oocysts into 

streams, and saturated or near-saturated ground conditions, and events characterized by rain 

falling on snow, resulted in higher rates of detection. In addition, little correlation was found 

between rainfall event accumulation and Cryptosporidium or Giardia detection. Rather, 

antecedent rainfall and/or remotely sensed soil moisture data indicating saturated or near saturated 

conditions may be important for identifying rainfall events where sampling for Cryptosporidium 

and Giardia is warranted (Sturdevant Rees et al. N.d.). 

Transport in Surface Water

The buoyancy of oocysts in water depends on their attachment to other particles. 

Oocysts that are not bound to particles have a tendency to float, even after being centrifuged 

(Swabby-Cahill et al. 1996). Cryptosporidium oocysts have a very low density (about 1.05 g/cm3

) 

and a very low settling rate (2 mm per hour or less) as noted by Gregory (1994). Oocysts attached 

to wastewater effluent particles may settle more quickly than those that are freely suspended and 

sedimentation velocity increases with particle size (Medema et al. 1998). In source waters, many 
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oocysts are likely to be adsorbed to organic or other suspended material and would probably settle 

more quickly than free-floating oocysts (Medema et al. 1998). 

Transport in Runoff

 Cryptosporidium is thought to be easily transported over land. Because oocysts are 

approximately the size of clay/silt particles, the amount of kinetic energy needed to entrain and 

suspend oocysts in overland flow may be quite small (Walker et al. 1998). Overland flow 

transport models for pathogens were reviewed as part of a U.S. EPA workshop on AFOs

(GeoLogics Corp. 2004). The effects of land slopes, vegetation, and rainfall intensities on 

overland and near-surface transport of Cryptosporidium parvum oocysts were examined by Trask 

et al. (2004). Zhang and coworkers (2001) describe the development and implementation of a 

model for simulating removal of Cryptosporidium parvum oocysts from overland flow, including 

modeling transport of oocysts through vegetative filter strips. 

Transport in Ground Water

Surface water sediments and the aquifer matrix material may play significant roles in 

minimizing oocyst transport to water supply wells; however, it is difficult to isolate the effect of 

these materials on transport. Fractures and dissolution conduits in an aquifer can allow ground 

water and oocysts to effectively bypass the protective action of most of the aquifer matrix. John 

and Rose (2005) provide a review of the factors affecting microbial survival in groundwater. 

 It is known that Cryptosporidium can be transported through soil and ground water 

(Mawdsley et al. 1996, Hurst 1997). Movement of C. parvum through soil and ground water is 

affected by sedimentation and filtration of the surrounding soil and aquifer matrix (Brush et al. 

1999, Harter et al. 2000). Adsorption of oocysts to matrix particles also affects filtration. 

Adsorption depends on the electrical charge of the organism and of the surrounding matrix (Brush 

et al. 1998). 

 Factors other than adsorption and micropore size may influence the oocyst movement. C. 

parvum transport in one study was greater in a silty loam and a clay loam soil than in a loamy 

sand soil (Mawdsley et al. 1996); this contradicts other evidence suggesting that clay soils exhibit 

greater adsorption and smaller micropores than sandy soils. The authors used intact soil cores 

(rather than columns created in the laboratory) to maintain the natural soil structure and 

macropores, and they concluded that the rapid flow of water through macropores largely bypasses 

the filtering and adsorptive effects of the soil and increases the risk of Cryptosporidium transport 

to ground water (Mawdsley et al. 1996). 

Amirtharajah et al. (2002) investigated the transport of a Cryptosporidium surrogate 

(polystyrene microspheres) through unsaturated soils at an undisturbed field site used for cattle 

production. Results showed that the vertical migration of polystyrene microspheres in column 

studies suggests that migration of Cryptosporidium parvum oocysts through fine-textured soils is 

likely to be minimal, and that a small number of these surrogate particles travel through 

preferential flow paths at field sites, especially after rainfall events (Amirtharajah et al. 2002). 
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Survival in the Environment

Several factors influence oocyst survival, including temperature and desiccation. Davies et 

al. (2005) presented results of a series of research experiments designed to identify the key factors 

controlling pathogen transport. Among the many findings summarized is that temperature is a 

very influential factor on the survival of Cryptosporidium oocysts. Both high heat and freezing 

temperatures can result in Cryptosporidium inactivation, while at more moderate temperatures 

(e.g., 4 to 25°C) inactivation rates are relatively slow (Davies et al., 2005, Finstein 2004). Freezethaw cycling is more effective than freezing, perhaps because of increased mechanical damage to 

the oocyst wall during the temperature fluctuations (Walker et al. 2001). 

 Before oocysts enter a water source, they may be vulnerable to desiccation. Robertson et 

al. (1992) reported that air drying an oocyst suspension at room temperature for 4 hours 

eliminated viability. Oocysts in fecal material, however, are protected from desiccation, so their 

viability in the environment is prolonged (Rose 1997). In addition, Cryptosporidium in liquid 

swine manure has been shown to remain viable despite the high ammonia content of the manure 

(Fleming et al. 1999). However, Olson et al. (1999) found that oocyst survival appears to be better 

in soil than in feces.

Once initial contamination has occurred, water can remain a source of viable oocysts for 

days (Heisz 1997, Lisle and Rose 1995). Lisle and Rose reported a duration of 176 days to 

produce die-off rates of 96 percent in tap water and 94 percent in river water under laboratory 

conditions. After 2 days, only 37 percent of the oocysts in tap water were nonviable, suggesting 

that oocysts that reach the distribution system might be viable.

 Olson et al. (1999) compared oocyst survival at temperatures and in media likely to occur 

in the natural environment. They examined survival in -4°, 4°, and 25°C. Unlike Giardia, which 

died off quickly at low temperatures, Cryptosporidium oocyst survival was best at -4°C, with 

close to 50 percent of oocysts remaining viable for 12 weeks. Survival was lowest at 25°C, but 

oocysts were still viable at six weeks. Survival rates were best in water and worst in feces.

Soil texture also can significantly affect inactivation of Cryptosporidium oocysts and 

viruses (CRC 2004). Vegetative filter strips can be effective at removing Cryptosporidium oocysts 

from surface runoff, however, viruses (e.g., PRD1 bacteriophage) and to a lesser extent bacteria 

(E. coli) are more easily transported (Davies et al. 2005, CRC 2004). 
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Loading

 Once you have gathered information about Cryptosporidium sources and the likelihood of 

the oocysts reaching your source water (based on watershed characteristics and fate and 

transport), you should determine the amount and proportion of oocysts that each source is 

expected to contribute to the overall Cryptosporidium load. Loading can be calculated fairly 

easily for constant point sources such as wastewater treatment plants but is more difficult for 

farms and urban runoff. 

2.4.2.2 What Role Should Monitoring Play in the Evaluation of Potential and 

Existing Sources of Cryptosporidium? 

Monitoring of Cryptosporidium is not required to develop the WCP plan or to implement 

it once approved by the state. It may take years to realize measurable improvements in water 

quality after initiating source water protection efforts. Furthermore, discerning improvements in 

source water quality using monitoring can be difficult due to natural environmental variability, the 

nature of the source water improvements, and the limitations of the current analytical methods for 

Cryptosporidium as well as other fecal indicators. However, PWSs that choose to employ this 

option, either separately or in combination with other approaches, may gain some benefit using 

this approach. For example, while the state and/or PWS may already have some knowledge of 

potential Cryptosporidium sources through land use information or discharge permit data, 

monitoring can help determine the extent to which these sources are impacting a source and can 

help target portions of the watershed for extra protection or BMPs implementation. Although 

not required for WCP plan development, implementation, or maintenance, monitoring 

throughout the watershed for Cryptosporidium (or indicators of fecal contamination) can be a 

useful tool in evaluating the success of WCP controls 

 New technologies for MST including deoxyribonucleic acid (DNA) fingerprinting, 

genotyping, and multiple antibiotic resistance may be helpful and more effective at overall 

pathogen source identification (see Section 2.4.2 for references). Presumptive approaches using 

modeling and literature reported in data can also be used to simulate loadings and prioritize areas 

for monitoring or detailed study. 

 Watershed monitoring can help narrow down the locations of some sources and determine 

the load contributed by each source. The Philadelphia Water Department, for example, planned a 

four-tier study to determine why there was such a large difference in protozoan levels at two 

plants using the same source (the Schuylkill River) but located 2.5 miles apart (Crockett and Haas 

1997) (see sidebar).

Because Cryptosporidium occur in low concentrations and are difficult to detect, it may be 

helpful to monitor other parameters in addition to or instead of Cryptosporidium. While E. 

coli concentrations often do not correlate with Cryptosporidium levels, they are good indicators of 

fecal contamination. Fecal coliform bacteria have traditionally been used as water quality 

indicators, but E. coli is thought to be more closely linked to fecal contamination. 

Turbidity does not always indicate fecal contamination; often, increased turbidity is 

simply a product of high sediment levels. However, turbidity may indicate the presence of a 
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water quality problem, where additional research is necessary to determine its cause. Use of a 

surrogate such as turbidity should be supported by evidence of a correlation for that source water. 

Precipitation (rainfall amount and intensity) are important in the release and detachment of 

pathogens from fecal matter, and consequent mobilization in downgradient surface water or 

groundwater. This precipitation also can mobilize and release turbidity. 

 

A New Jersey utility has shown that for their source water increases in raw water 

turbidity are accompanied by increases in source water Cryptosporidium during storm events 

(LeChevallier et al. 1998, Atherholt et al. 1998). As a result of this research for their source 

water, this utility has established a standard operating procedure that when raw water turbidity 

exceeds a threshold of about 15 ntu, intake can be shutdown for up to 24 hours without 

interrupting service in order to let storm related source water flow bypass the treatment plant.

 Monitoring, when implemented, should be conducted regularly. Because nonpoint 

sources of microbiological contamination discharge primarily during wet weather flows 

monitoring during or soon after these events is also important. When combined with stream 

discharge data, rates of storm-related Cryptosporidium transport and loading can be calculated. 

The monitoring frequency should be such that seasonal variability in Cryptosporidium levels is 

observable. 

 There are two types of watershed monitoring for stream networks. First, basinwide 

monitoring involves monitoring just upstream of the confluence of two streams (AwwaRF 1991). 

Conducted at stream junctions throughout the watershed, basinwide monitoring helps give a 

general picture of the water quality and helps isolate the stream reaches contributing to 

contamination. Second, site-specific monitoring involves monitoring just upstream and 

downstream of a suspected or known point or nonpoint source, as the Philadelphia Water 

Department did (Crockett and Haas 1997). Such monitoring is appropriate where impacted 

stream reaches have already been identified. The results of any monitoring should enable the 

system to compare the relative contribution of various sources to the overall Cryptosporidium 

occurrence in the watershed and their effect on water quality. 

Monitoring to Locate Cryptosporidium Sources

To determine the source of Cryptosporidium contamination in the Schuylkill River, the 

Philadelphia Water Department decided to focus on a creek feeding into the Schuylkill just 

upstream of the Queen Lane plant (Crockett and Haas 1997). This creek has several wastewater 

treatment plants in its upper reach and farms and parks along its lower reach. In the first phase, 

the water department tested the Queen Lane intake during dry flow. It then sampled a site along 

the creek downstream of the wastewater treatment plants and one downstream of the farms 

during various weather conditions. In the third phase of sampling, the department sampled 

wastewater effluent and additional sites up- and downstream of some of the wastewater treatment 

plants during different weather events. Lastly, the department planned to focus on the prevalence 

of Cryptosporidium and Giardia in livestock and wildlife along the creek.
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 Monitoring in a reservoir or lake, if applicable, can help systems determine the fate of 

Cryptosporidium once it flows from a stream into the lake, or once it enters the lake directly from 

land immediately adjacent to the lake. Sampling patterns should depend on the shape and depth 

of the lake. A round lake should be sampled at several locations and depths near the center of the 

lake; a long lake should be sampled in a transect along its long axis (AwwaRF 1991). More 

specific monitoring may be needed to answer more detailed questions on fate and transport. For 

instance, does Cryptosporidium concentration decrease due to sedimentation or dilution? How 

long does it take for Cryptosporidium to flow from one end of the reservoir to the intake? 

PWSs may find it helpful to use a GIS to analyze their water quality and contaminant 

source data. For systems that have ArcView software, BASINS 3.0, a software and GIS package 

developed by EPA can assist systems with integrating local data and nationally available preformatted spatial data (e.g., watershed HUCs, DEM data, roads, NPDES permit data, and Clean 

Water Needs Survey data on wastewater treatment plants). BASINS also includes a model for 

determining nonpoint source loading and other models for loading and transport, as well as tools 

for assessing contamination from various sources. 

2.4.3 Analysis of Control Measures 

 The analysis of control measures submitted with the WCP plan must address the relative 

effectiveness of each measure at reducing Cryptosporidium loading to the source water, along 

with the feasibility of each measure (40 CFR 141.716(a)(2)(iii)). This analysis can be based on 

either site-specific experience or information from the peer-reviewed literature. Numerous 

references are available that describe control measures and other BMPs, as listed previously in 

this Section 2.4 of this guidance, and presented below for select topic areas. 

 Control measures may include 1) the elimination, reduction, or treatment of wastewater or 

stormwater discharges, 2) treatment of Cryptosporidium contamination at the sites of the waste 

generation or storage, 3) prevention of Cryptosporidium migration from sources, or 4) any other 

measures that are effective, sustainable, and likely to reduce Cryptosporidium contamination of 

source water. 

2.4.3.1 Available Regulatory and Management Strategies 

For systems in watersheds where most of the land is privately owned, land use regulations 

may be the best way to control pollution, especially in heavily developed or growing areas. 

Examples of possible regulations include septic system requirements, zoning ordinances 

specifying minimum lot sizes or low-impact development, limits on discharge from wastewater 

treatment plants and other facilities, pet waste cleanup ordinances, and requirements for permits 

for certain land uses. Your ability to regulate land use will depend on the authority granted to 

your municipality by the state, the ownership of your system (public or private), and the support 

of your local government and the public. Regulatory authority, steps for designing a regulation 

that can withstand lawsuits, and types of land use regulations are described in the paragraphs 

below.
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Determining Authority to Regulate

 The ability of a municipality to pass a land use ordinance or other law to help reduce 

contamination may depend on the authority the state grants to the local government in the state

constitution or through legislation, although states normally do not interfere with the actual land 

use and zoning rules (AwwaRF 1991). Privately owned water systems may need to ask the 

cooperation of the local government to get source water regulations passed. Publicly owned 

PWSs face less of a hurdle, although winning support of the local government may still be 

difficult.

If a PWS does not own or otherwise have authority over the Cryptosporidium sources in 

the watershed, the analysis will need to reflect developing and maintaining partnerships to assure 

adequate control is in place. This could include coordination with other municipal governments, 

farmers, wastewater treatment plant operators, regional planning agencies, and others. 

 If the area of influence on water quality extends throughout several municipalities, it can 

be difficult to standardize watershed control practices throughout the watershed. The legal 

framework used will depend on who has jurisdiction over land use in the watershed and on the 

authority of the water system (AwwaRF 1991). For example, some states may create agencies 

authorized to promulgate and enforce watershed protection regulations, or interstate agencies may 

be created to regulate watersheds where watersheds cross state boundaries. County 

governments in some states may have some zoning authority and may be able to assist with 

enforcement of some regulations affecting source water (e.g., septic systems).

 Where PWSs do not have regulatory or enforcement authority, they should work with 

other local governments' PWSs and agencies in their watersheds to sign memoranda of agreement 

or understanding, in which each entity agrees to meet certain standards or implement certain 

practices.

Zoning 

 Early zoning laws simply prohibited certain land uses that would be considered nuisances 

in certain areas. Later, zoning ordinances became more specific; further restrictions were imposed 

on the permitted uses, such as limits on building or population density, percentage of impervious 

surface area, building height, and minimum distance of buildings from property boundaries. Most 

zoning ordinances have grandfather clauses that allow nonconforming uses to continue. 

Ordinances may also allow the zoning authority to grant variances if the topography or size of a 

lot make it difficult to comply with a zoning requirement.

To make sure a zoning law can withstand a legal challenge, it is important to make sure 

the appropriate procedures are followed and that the law has sufficient scientific basis (AWWA 

1999). First, be sure you have the authority to regulate. Make sure the rule is specific enough. 

Comply with all administrative procedure requirements; failure to do so is the most common 

reason for rules being revoked. The ordinance should conform to the objectives of the WCP plan, 

which should contain enough data to illustrate how the ordinance will affect water quality. 

 Ordinances should also be designed to withstand a takings lawsuit (AWWA 1999). The 

fifth amendment to the U.S. Constitution states that private property may not be taken for public 
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use without just compensation. Any physical invasion without consent is always considered a 

taking, even if the landowner retains ownership of the land. Installation of a monitoring well or 

stream gauge without consent is an example of a taking.

 To prevent takings claims, the municipality should show the need for the regulation and a 

connection between the ordinance and the expected result (AWWA 1999). This proof should be 

based on a scientific analysis beginning with an accurate delineation of the watershed or wellhead 

protection area/recharge area.

Following the delineation, determine the impact the regulation will have by mapping 

current and projected development under current zoning requirements. Then map current and 

projected development for the proposed ordinance and determine the potential pollutant load 

under each scenario (AWWA 1999). Local groups or universities may be able to provide pollutant 

data and assist with modeling. This "buildout analysis" will help you show that your proposed 

ordinance advances a legitimate government interest and how the effect of the ordinance is 

proportional to the impact of land use in your watershed. 

Types of Ordinances

 Watershed ordinances usually apply within an "overlay district," which may be the area of 

influence you determined for your WCP plan. All existing zoning or land use regulations apply 

within that area, but additional requirements apply within the overlay district. Within your 

watershed, particularly within the area of influence, there are many different kinds of regulatory 

controls you may wish to consider: 

• Large-lot or low-density zoning.

• Limits on certain types of land use except by special permit.

• Impact fees.

• Submission and approval of a watershed protection plan or impact study as a condition for 

development of a subdivision or apartment complex. 

• Performance standards, which permit development but limit the impact of the 

development. 

More detail on each of these types of ordinances is found in Appendix E. Examples of 

source water protection ordinances can be found on EPA's website at 

http://www.epa.gov/owow/nps/ordinance/osm7.htm. 

Land Acquisition/Conservation Easements

 Acquisition of watershed land by the PWS or its affiliated jurisdiction is often the most 

effective approach to protecting the water source. EPA's Drinking Water State Revolving Fund 

allows a percentage of the fund to be set aside for land acquisition associated with watershed 

protection.
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 Land trusts and conservancies can help systems purchase land to protect drinking water 

quality, especially when government agencies are unable to move quickly enough to buy land 

when it becomes available. Trusts can buy and hold land from multiple landowners on behalf of a 

water system until the system can assemble funding to purchase it from the trust. The Trust for 

Public Land (http://www.tpl.org) can provide more information. 

 Trusts also can work with landowners to buy or have landowners donate conservation 

easements. An easement is a legal document that permanently limits the development of a 

piece of land, even after the land is sold or otherwise changes ownership. See 

http://www.landtrust.org/ProtectingLand/EasementInfo.htm for frequently asked questions about 

easements and for an example of a model easement for use in the state of Michigan. The Land 

Trust Alliance (http://www.lta.org), a trade organization for land trusts, has published handbooks 

on designing and managing conservation easement programs. 

 Other government agencies, such as the U.S. Forest Service or state natural resource 

departments, may be able to buy parcels in your watershed if you are unable to afford to purchase 

all the land that needs to be protected.

2.4.3.2 Partnerships in Watershed Control Plans

Many watershed management practices cannot be implemented by water systems alone. 

For example, agricultural BMPs must be implemented by farmers; stormwater BMPs are 

implemented by developers, manufacturers, and government agencies. Parts of your watershed 

may be in different municipalities. Therefore, partnerships with local government and landowners 

is often central to effectively implementing a watershed control program. The WCP can reflect a 

variety of different types of partnerships: 

• Memoranda of agreement or other formalized arrangements with government agencies. 

• Education through technical assistance providers such as cooperative extension agents or 

association representatives. 

• Data collection through local university programs. 

• Agreements to hold conservation easements with state agencies or non-governmental 

organizations. 

• Private agreements with individual property owners. 

Stakeholder participation can be a useful tool in WCP planning. Dialogue with 

stakeholders can be used to identify win-win solutions for both the water supplier and its partners. 

The book Guidance to Utilities on Building Alliances with Watershed Stakeholders (AwwaRF 

2001) explains how to present issues to stakeholders, how to target stakeholders, and how to 

structure your partnership with stakeholders. In addition, developing alliances between water 

utilities and agricultural interests is discussed by Fletcher et al. (2004).
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An important potential partnership opportunity that many water systems should consider 

when pursuing the WCP credit is to develop a cooperative relationship with other PWSs in the 

same watershed. These PWSs in the same watershed will normally have overlapping areas of 

influence and consequently will have some of the same priority Cryptosporidium sources. 

Consequently, each of these PWSs will have an interest in developing control measures for these 

shared Cryptosporidium sources. By cooperating together the PWSs can reduce duplication of 

efforts and thereby collectively focus their energies on prioritized activities. PWSs that cannot 

cooperate together should not pursue joint efforts, because this will create difficulties during 

implementation and maintenance of the WCP plan. However, if they can develop a plan to work 

together to prepare the WCP plan, and can provide the staff and resources to complete their 

designated tasks during implementation and maintenance of the state-approved WCP plan, the 

resulting joint effort can potentially produce a greater benefit (reduction in source water 

Cryptosporidium ) at lower cost than the if each PWS worked separately. 

Watershed control plans must identify watershed partners and their roles (40 CFR 

141.716(a)(2)(iv)). Plans should document the efforts to be made to establish voluntary local 

partnerships, including solicitation of private individuals living within the defined area of 

influence who are likely to be affected by decisions made as part of the watershed protection 

program, and whose participation is important for the success of the program. Plans should also 

document how members of municipal or other local governments or political subdivisions of the 

state that have jurisdiction over the area of influence will participate in the watershed protection 

effort. Watershed protection plans should include descriptions of how the proposed local 

partnership has or will identify and account for any voluntary or other activities already underway 

in the area of influence that may reduce or eliminate the likelihood that Cryptosporidium will 

occur in drinking water. 

2.4.3.3 Addressing Point Sources

Changes in farming practices and in wastewater treatment technologies in the past decade 

have resulted in new management strategies for agricultural and urban point sources. The 

following sections briefly describe solutions for agricultural, wastewater, and stormwater point 

sources; detailed descriptions are provided in Appendix E. As part of your application for WCP

approval, you must submit an analysis of control measures that can mitigate sources of 

Cryptosporidium such as these (40 CFR 141.716(a)(2)(iii)). Loans from the Clean Water State 

Revolving Fund (CWSRF) can be used to fund projects associated with wastewater treatment and 

watershed and estuary management. See www.epa.gov/owm/cwfinance/cwsrf/index.htm for more 

information. 

Concentrated Animal Feeding Operations 

 AFOs are facilities where animals are confined for 45 days or more a year and where no 

vegetation grows in the area used for confinement. This includes farms where animals graze the 

majority of the year but are confined and fed during the winter for at least 45 days. Some AFOs 

are also CAFOs (see Appendix E). EPA recently issued a rule that changed the requirements on 

CAFOs that must apply for NPDES permits (U.S. EPA 2008). The new CAFO rule requires 

CAFOs to implement nutrient management plans that affect manure handling, storage, and land 

application. These plans will include BMPs primarily designed to reduce nitrate and phosphorus 
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contamination but which will at the same time reduce pathogen contamination. Elements of this 

plan may include limiting the manure land application rate, instituting buffer zones where manure 

is applied, ensuring adequate manure and wastewater storage, and others. Gullick et al. (2007 

provide a reference guide for drinking water utilities to implement source water protection 

activities related to CAFOs. 

Wastewater Treatment Plants

All wastewater treatment plants in the United States are required to provide secondary 

treatment (U.S. EPA 2001e). Most plants are also required to disinfect their effluent before 

discharging. However, conventional chlorine disinfection in wastewater plants is ineffective 

against Cryptosporidium. Some wastewater treatment facilities are beginning to implement 

treatment similar to that used for drinking water treatment (e.g., filtration, advanced treatment 

such as UV disinfection). PWSs should identify all wastewater treatment plants in their 

watersheds and determine what their permit effluent limits are and whether the limits are being 

met.

Combined Sewer Overflows (CSOs)

Combined sewers carry both sewage and stormwater to wastewater treatment plants. 

During storms, the volume of water in combined sewers may become too great for wastewater 

plants to treat. As a result, the excess sewage and stormwater are released untreated into surface 

water through CSOs. CSOs are most common in older cities in the northeastern and midwestern 

United States and can be a significant contributor of Cryptosporidium to urban watersheds. 

 There are three major structural solutions to the problem of CSOs: 

• Separate combined sewers into sanitary and storm sewers, where sanitary sewers 

flow to the wastewater treatment plant and storm sewers release to surface water.

• Increase the capacity of the wastewater treatment plant so that it is able to treat 

combined sewage from most storms.

• Build aboveground covered retention basins or construct underground storage 

facilities for combined sewage to hold the sewage until the storm has passed and 

can be treated without overloading the plant. 

 Although CSOs are not regulated directly under their own program, EPA has a CSO 

control policy (U.S. EPA 1994) which encourages minor improvements to optimize CSO 

operation, and CSO management may be written into NPDES or State Pollution Discharge 

Elimination System (SPDES) permits. Minor improvements include maximizing in-line storage 

within the sewer system, reducing inflow, and treatment of CSO outfalls. Stormwater BMPs can 

also reduce the impact of CSOs. 

Sanitary Sewer Overflows

Watersheds with separate sanitary and storm sewer systems may still have water quality 

problems. Sanitary sewer overflows (SSOs) occur when untreated and mostly undiluted 
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sewage backs up into basements, streets, and surface water. SSOs discharging to surface water 

are prohibited under the CWA. Insufficient maintenance and capacity and illegal connections are 

some of the primary causes of SSOs.

SSOs can be reduced by cleaning and maintaining the sewer system; reducing inflow and 

infiltration by repairing leaking or broken service lines; increasing sewer, pumping, and/or 

wastewater treatment plant capacity; and constructing storage for excess wastewater (U.S. EPA 

2001f). 

Municipal Separate Storm Sewer Systems

Municipal separate storm sewer systems (MS4s) in areas with populations of more 

than 100,000 are also required to obtain NPDES permits. Information on storm sewer outfall 

locations, volume discharged, conventional pollutant loads, and existence of illicit discharges is 

submitted as part of the permit application process (U.S. EPA 1996). In addition, these MS4s 

must develop management plans addressing items such as outfall monitoring, structural and 

nonstructural BMPs to be implemented, and identification and elimination of illicit discharges. 

Illicit discharges to MS4s include any non-stormwater discharges, such as discharges that should 

be connected to sanitary sewers (e.g., water from sinks, floor drains, and occasionally toilets), 

illegal dumping of sewage from recreational vehicles, sanitary sewer overflow backing up through 

manhole covers into storm drains, effluent from failing septic systems, water from sump pumps, 

etc.

 Small MS4s (serving areas with populations of less than 100,000), with some exceptions, 

are subject to NPDES permit requirements if they are located in "urbanized areas" as determined 

by the Bureau of the Census. Those MS4s subject to NPDES permits must implement "control 

measures" in six areas, including a plan for eliminating illicit discharges (U.S. EPA 2000b). 

PWSs should work with all MS4 utilities in the area of influence to gather existing 

information about stormwater contamination. MS4 utilities may need to install or retrofit 

structural BMPs, such as retention ponds, to reduce contamination. 

2.4.3.4 Addressing Nonpoint Sources

 The following sections briefly describe BMPs for agricultural, forestry, and urban sources 

of Cryptosporidium; more detailed descriptions are provided in Appendix E. Your WCP plan 

must discuss how these or any other BMPs you choose will be implemented in the area of 

influence. EPA Section 319 grants and Clean Water State Revolving Fund loans can be used for 

nonpoint sources and watershed management purposes.
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Agricultural BMPs

Management Programs

The U.S. Department of Agriculture recommends the following "control points" for 

controlling pathogens (USDA 2000): 

• Preventing initial infection by controlling pathogen import to the farm. 

• Controlling the reproduction and spread of the pathogen throughout the farm. 

• Managing waste. 

• Controlling pathogen export from the farm. 

 PWSs should work with local soil conservation districts or cooperative extensions for 

technical assistance with BMPs.

BMPs that can reduce pathogen loading include the following:

• Composting. 

• Waste management (manure storage and land application). 

• Grazing management. 

• Feedlot runoff diversion. 

• Buffer or filter strips. 

Composting

• Can effectively reduce pathogen concentrations. 

• Entire waste mass should be uniformly treated and there should be no cold spots. 

Buffer Strips

• Provide buffer between area of manure application or grazing and adjacent streams or 

lakes. 

• USDA (2000) recommends that buffer and filter strips be considered secondary practices 

for pathogen control and be used in conjunction with control measures. 
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Grazing Management

• Managed grazing can be cheaper and less environmentally damaging than confined 

feeding and unmanaged grazing. It decreases feed, herbicide, equipment, and fertilizer 

costs, while reducing erosion and increasing runoff infiltration and manure decomposition 

rates (Ohio State University Extension, undated).

• In managed, or rotational, grazing, a sustainable number of cattle or other livestock graze 

for a limited time (usually 2-3 days) on each pasture before being rotated to the next 

pasture. 

Manure Storage

• Manure storage facilities allow farmers to wait until field conditions are more suitable for 

land application.

• Manure storage facilities should be designed to prevent discharge through leaching or 

runoff. They should be lined, and if possible, covered. Facilities that are not covered 

should be designed to contain precipitation and runoff from a 25-year 24-hour storm.

Land Application of Manure

• Several precautions taken in manure application can prevent runoff from entering surface 

water, reducing the likelihood of Cryptosporidium contamination. 

• Manure should not be applied to frozen or saturated ground or before predicted rainfall, or 

near tile drains or dry wells or to land subject to flooding. 

• For pastures to be used for grazing, waste should be stored for at least 60 days and then 

applied at least 30 days before the scheduled grazing period to avoid infection of the 

animals. 

Feedlot Runoff Diversion 

• Diverting clean water before it drains into the feedlot can significantly reduce the amount 

of wastewater that needs to be managed.

• All roofs that could contribute to feedlot runoff should have gutters, downspouts, and 

outlets that discharge away from the feedlot. 
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Forestry BMPs

• Logging can cause increased erosion, leading to increased runoff and making it more 

likely that Cryptosporidium present in wildlife will reach the source water. Logging can 

also cause elevated sediment levels, resulting in high turbidity, which affects water 

treatment efficiency. Examples of forestry BMPs are listed below:

- filter strips

- streamside or riparian management zones

- logging roads should be constructed to minimize runoff 

- road runoff should be diverted away from streams and prevented from channelizing 

- loggers should minimize soil disturbance and compaction on skid trails 

Urban/Suburban BMPs

See http://www.epa.gov/owm/mtb/mtbfact.htm for fact sheets on technologies and BMPs 

municipalities can use to reduce contamination from wastewater and stormwater.

Buffer Zones

• For watersheds in urban areas, buffer zones help to protect development on the floodplain 

from being damaged when the water is high, as well as protect the stream from the effects 

of the development.

• The extent to which buffer zones reduce Cryptosporidium loading is not well understood; 

therefore, they should be used to augment, rather than replace, other watershed 

management practices.

Dry Detention Basins

• Dry detention basins temporarily store stormwater runoff and release the water slowly to 

allow for settling of particulates and the reduction of peak flows. 

Infiltration Devices

• Infiltration devices remove pathogens and particles by adsorption onto soil particles and 

filtration as the water moves through the soil to the ground water. Infiltration devices 

include (NALMS 2000): 

- infiltration basins 

- infiltration trenches

- dry wells
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Sand Filters 

• Sand filters can be used to treat stormwater runoff from large buildings and parking lots. 

Wet Retention Ponds 

• Ponds can effectively reduce suspended particles and, presumably, some pathogens, by 

settling and biological decomposition.

• There is concern, however, that ponds attract wildlife that may contribute additional fecal 

pollution to the water, rather than reducing contamination. 

Constructed Wetlands

• Constructed subsurface flow wetlands (where wetland plants are not submerged) can 

reduce Cryptosporidium and bacteria concentrations in wastewater (Thurston et al. 2001). 

• Wetlands may also be useful for treating stormwater or other polluted water.

Runoff Diversion

• Structures can be installed in urban settings to divert clean water flow before it reaches a 

contamination source. Structures that channel runoff away from contamination sources 

include stormwater conveyances, such as: 

- swales 

- gutters 

- channels 

- drains 

- sewers

Pet Waste Management 

• Municipalities can implement pet waste management programs to encourage pet owners to 

properly collect and dispose of their animals' waste.
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Water Conservation 

• Can help preserve the amount of water available for use, especially during times of 

drought. 

• Can also decrease the amount of wastewater and stormwater generated, thereby protecting 

the quality of the water supply (U.S. EPA 2002d). 

• The following are examples of water conservation methods: 

- low-flow toilets and showerheads

- reducing lawn watering

Low Impact Development

• Low impact development tries to reduce the amount of impervious cover, increase natural 

lands set aside for conservation, and use pervious areas for more effective stormwater 

treatment of residential and commercial developments. 

Septic Systems 

• Failing septic systems can result in clogging and overflow of waste onto land or into 

surface water.

• Water systems should work closely with the local regulatory authority to ensure that septic 

system codes are being properly enforced and to strengthen codes where necessary. 

• PWSs should encourage residents with septic systems in the watershed to understand their 

systems and the proper maintenance that their systems require. Cooperative extensions can 

work with residents on this issue.

Wildlife BMPs

• Steps taken to prevent wildlife from contaminating source water vary with the source and 

type of wildlife. The following are examples of wildlife BMPs:

- boats with noisemakers to scare seagulls and geese away

- fences on the water's edge to keep out larger land animals and humans

2.4.3.5 Is Purchase/Ownership of All or Part of the Watershed a Viable Option?

 PWSs will have the maximum opportunity to realize their watershed protection goals if 

they have complete ownership of the watershed. Where feasible, PWSs should include in their 

watershed protection plan a description of efforts that will be made to obtain ownership, such as 

any special programs or budget. Since complete ownership of the watershed or area of influence is 
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not practical in almost every instance, the system should explain any efforts the PWS will make to 

gain ownership of some critical elements, such as reservoir or stream shoreline and access areas.

 Where ownership of land is not possible, PWSs should describe plans to obtain written 

agreements that recognize the watershed as part of a public water supply. As much as possible, 

maximum flexibility should be given to the PWS to control land uses which could have an 

adverse effect on the water quality. PWSs should include with these descriptions an explanation of 

how they will ensure that landowners will comply with the agreements. 

Utilities can also facilitate the prioritization and purchase of parcels by third parties in 

upstream communities that are already looking to preserve, own, and maintain land. This can be 

done by partnering on grants or other efforts. 
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2.5 References

Amirtharajah, A., M.H. Young, K.D. Pennell, J.L. Steiner, D.S. Fisher, D.M. Endale, and J.P. 

Campbell. 2002. Field Transport of Cryptosporidium Surrogate in a Grazed Catchment. 

Denver: Awwa Research Foundation.

Arora, H., M. LeChevallier, R. Aboytes, E. Bouwer, C. O'Melia, W. Ball, W. Weis, and T. 

Speth. Full-scale evaluation of riverbank filtration at three Midwest water treatment plants. In: 

Proceedings of the AWWA Water Quality Technology Conference, Salt Lake City, Utah, 

November, 2000. Denver: American Water Works Association. 

Ashendorff, A., M. Principe, A. Seeley, J. LaDuca, L. Beckhardt, W. Faber, and J. Mantus. 

1997. Watershed Protection for New York City’s Supply. Jour. AWWA. 89(3):75-88.

Atherholt, T., M. LeChevallier, W. Norton, and J. Rosen. 1998. Effect of rainfall on Giardia 

and Cryptosporidium. Jour. AWWA. 90:9:66-80. 

Atwill, E.R., L. Hou, B.M. Karle, T. Harter, K.W. Tate, and R.A. Dahlgren. 2002. Transport 

of Cryptosporidium parvum oocysts through vegetated buffer strips and estimated filtration 

efficiency. Appl. Environ. Microbiol. 68(11): 5517-27. 

Atwill, E.R., S.M. Camargo, R. Phillips, L.H. Alonso, K.W. Tate, W.A. Jensen, J. Bennet, S. 

Little, T.P. Salmon. 2001. Quantitative shedding of two genotypes of Cryptosporidium parvum 

in California ground squirrels (Spermophilus beecheyi). Appl. Environ. Microbiol. 67(6):2840-

43. 

AWWA, 1999. Source Water Protection: Effective Tools and Techniques You Can Use. 1999 

Participant Manual. Denver: American Water Works Association. Developed for a technical 

training seminar for public water suppliers and local officials. 

AwwaRF. 1991. Effective Watershed Management for Surface Water Supplies. Prepared by 

R.W. Robbins, J.L. Glicker, D.M. Bloem, and B.M. Niss, Portland (OR) Water Bureau. 

Denver: American Water Works Association Research Foundation. 

AwwaRF. 2001. Guidance to Utilities on Building Alliances with Watershed Stakeholders. 

Denver: American Water Works Association Research Foundation. Order No. 90826. 

Bitton, G. 2005. Microbial Indicators of Fecal Contamination: Application to Microbial 

Source Tracking. Gainesville, Florida: University of Florida, 71 pp. 

http://www.florida-stormwater.org/pdfs/FSAMicrobialSourceTrackingReport.pdf. 

Blewett, DA. 1989. Disinfection and oocysts. Cryptosporidiosis Proceedings of Ma Pint 

International Workshop, 1988. Ed K.W. Amur and D.A. Blewett. Edinburgh: The Animal 

Disease Research Association. 107-116. 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-46 April 2010

Brush, C.F., M.F. Walter, L.J. Anguish, and W.C. Ghiorse. 1998. Influence of pretreatment 

and experimental conditions on electrophoretic mobility and hydrophobicity of 

Cryptosporidium parvum oocysts. Appl. Env. Microbiol. 64: 4439-4445. 

Brush, C.F., W.C. Ghiorse, L.J. Anguish, J.-Y. Parlange, and H.G. Grimes. 1999. Transport 

of Cryptosporidium oocysts through saturated columns. Env. Qual. 28: 809-815. 

Capacasa, J. 2005. Partners and Possibilities: Schuylkill Action Network. In Proc. of 2005 

Source Water Protection Symposium. Denver, CO: AWWA.

CCME (Canadian Council of Ministers of the Environment). 2002. From Source to Tap: 

Guidance on the Multi-Barrier Approach to Safe Drinking Water. Developed by the 

Federal-Provincial-Territorial Committee on Drinking Water and the CCME Water Quality 

Task Group. Winnipeg, Manitoba, Canada: CCME, 242 pp. 

http://www.ccme.ca/assets/pdf/mba_eng.pdf

CDM (Camp Dresser & McKee, Inc.). 2002. Source Water Protection Reference Manual 

(CD-ROM). Denver, CO: AWWA Research Foundation and AWWA. 

. 

Center for Watershed Protection 1999. An Introduction to Better Site Design. Watershed

Protection Techniques 3(2): 623-632. 

Chauret, C.; S. Springthorpe, and S. Sattar. 1999. Fate of Cryptosporidium Oocysts, 

Giardia Cysts, and Microbial Indicators During Wastewater Treatment and Anaerobic 

Sludge Digestion. Can. J. Microbiol., 45, 257. 

Corsi, S.; J. Walker; R. Waschbusch, and J. Standridge. 2003. Sources and Variability of 

Cryptosporidium in the Milwaukee River Watershed (Report 99-HHE-2). Alexandria, VA: 

Water Environment Research Foundation. 

Coyne, M.S. and R.L. Blevins. 1995. Fecal bacteria in surface runoff from poultry-manured 

fields. In K.Steele (ed.), Animal Water and the Land-Water Interface, pp. 77-87. Boca 

Raton: Lewis Publishers, CRC Press. 

CRC (Cooperative Research Centre for Water Quality and Treatment). 2004. Pathogen 

Movement and Survival in Catchments, Groundwaters and Raw Water Storages. Salisbury, 

South Australia: The Cooperative Research Centre for Water Quality and Treatment. 17 pp. 

http://www.wqra.com.au/crc_archive/dwfacts/techfact_pathogen_movement.PDF

Crockett, C. 2005. Personal Communication. (December 14, 2005). 

. 

Crockett, C. L. Gaffney, D. Bane, and G. Cavallo. 2005. How to Prioritize and Implement 

Projects to Protect and Improve Water Quality. In Proc. of 2005 Source Water Protection 

Symposium. Denver, CO: AWWA.

Crockett, C. S. [N.d.] The Role Of Wastewater Treatment In Protecting Water Supplies 

Against Emerging Pathogens. Water Environment Research. Forthcoming 2006.
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-47 April 2010

Crockett, C. S., and C. Johnson. 2000. Philadelphia Investigates Cryptosporidium : Five 

Years of Monitoring, Treatment and Epidemiological Studies. In Proc. of the AWWA Water 

Quality Technology Conference. Denver, CO: AWWA.

Crockett, C.S., and C.N. Haas. 1997. Understanding protozoa in your watershed. J 

AWWA 89(9): 62-73. 

Curriero, F.C., J.A. Patz, J.B. Rose, and S. Lele. The association between extreme 

precipitation and waterborne disease outbreaks in the United States, 1948-1994. Am. J. 

Public Health 91(8): 1194-99. 

Davies, C., C. Kaucner, N. Altavilla, N. Ashbolt, C. Ferguson, M. Krogh, W. Hijnen, G. 

Medema, and D. Deere. 2005. Fate and Transport of Surface Water Pathogens in 

Watersheds. Denver, CO: AWWA and AwwaRF, 261 pp. 

Ernst, C. 2004. Protecting the Source: Land Conservation and the Future of America’s 

Drinking Water. Trust for Public Land (Washington, D.C.) and AWWA (Denver, CO), 52 

pp. 

Ernst, C. and K. Hart. 2005. Path to Protection: Ten Strategies for Successful Source Water 

Protection. Washington, D.C.: Trust for Public Land, 23 pp. 

Fact Sheet 2.0. Office of Water. EPA 833-F-00-002. www.epa.gov/npdes/pubs/fact2-0.pdf

Fairfax County. 2001. Wastewater Treatment Plant. 

. 

Website accessed March 2003.

www.co.fairfax.va.us/gov/DPWES/utilities/wwtrmnt_0600.htm

Finstein, M. 2004. Protecting Watersheds from Cryptosporidium in manure: A Literature 

Review. Jour. AWWA. 96(2):114-116. 

. Last modified May 16, 

2001. Website accessed January 2002. 

Fleming, R., D. Hocking, H. Fraser, and D. Alves. 1999. Extent and Magnitude of 

Agricultural Sources of Cryptosporidium in Surface Water. Project #40. National Soil and 

Water Conservation Program. Submitted to Ontario Farm Environmental Coalition, c/o 

Ontario Federation of Agriculture, on behalf of Agricultural Adaptation Council, West 

Guelph, Ontario. Final Report. December 1999. Downloaded January 2002 from 

http://www.ridgetownc.on.ca/research/reports/subject/waterhtrn. 

Fletcher, A., S. Davis, and G. Pyke. 2004. Water Utility/Agricultural Alliances: Working 

Together for Cleaner Water. AWWA Research Foundation, Denver, CO. 163 pp. 

Frankenberger, J.R. et al. 1999. A GIS-based variable source area hydrology model. 

Hydrologic Processes 13:805-822. 

Gburek, W.J. and H.B. Pionke. 1995. Management strategies for land-based disposal of 

animal wastes: Hydrologic implications. pp. 313-323. In K.Steele (ed.), Animal Water and 

the Land-Water Interface, pp. 77-87. Boca Raton: Lewis Publishers, CRC Press. 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-48 April 2010

Gennaccaro, A., M. McLaughlin, W. Quintero-Betancourt, D. Huffman, and J. Rose. 2003. 

Infectious Cryptosporidium parvum Oocysts in Final Reclaimed Effluent. Appl. Environ. 

Microbiol., 69(8), 4983-4984. 

GeoLogics Corporation (Alexandria, VA). 2004. EPA Regional Priority AFO Science 

Question Synthesis Document - Pharmaceuticals and Pathogens. Washington, D.C.: U.S. 

EPA Office of Science Policy and U.S. EPA Office of Research and Development. 

Workshop Review Draft: Supporting Documentation for the EPA Regional Science 

Workshop on Animal Feeding Operations (AFOs) - Science and Technical Support Needs, 

December 6-9, 2004, College Park, Maryland, 60 pp. 

http://www.manure.umn.edu/regulatory/4Pharmaceuticals%20and%20Pathogens.pdf

Gracyk, T, and J. Grace. 2003. Maryland Department of the Environment Cryptosporidium

Occurrence Study in the Potomac River. In Proc. of the AWWA Water Quality Technology 

Conference. Denver, CO: AWWA. 

. 

Graczyk, T.K., R. Fayer, J.M. Trout, E.J. Lewis, C.A. Farley, I. Sulaiman, and A.A. Lal. 1998. 

Giardia sp. cysts and infectious Cryptosporidium parvum oocysts in the feces of migratory 

Canada geese (Branta canadensis). Appl. Env. Microbiol. 64(7): 2736-2738. 

Gregory, J. 1994. Cryptosporidium in water: Treatment and monitoring methods. Filtr. Sep. 

31(3): 283-289. 

Gullick, R., R. Brown, and D. Cornwell. 2007. Source Water Protection for Concentrated 

Animal Feeding Operations: A Guide for Drinking Water Utilities. Report #3020. Denver, CO: 

AWWA and AwwaRF. 

http://www.waterresearchfoundation.org/research/TopicsAndProjects/projectSnapshot.aspx?pn

=3020

Hansen, J.S., and J.E. Ongerth. 1991. Effects of time and watershed characteristics on the 

concentration of Cryptosporidium oocysts in river water. Appl. Environ. Microbiol. 57(10): 

27902795. 

. 

Harter, T., S. Wagner, and E.R. Atwill. 2000. Colloid transport and filtration of 

Cryptosporidium parvum in sandy soils and aquifer sediments. Env. Sci. Tech. 34(1): 62-70. 

Heisz, M. 1997. In vitro survival of Cryptosporidium oocysts in natural waters. International 

Symposium on Waterborne Cryptosporidium. Newport Beach, March 1997. 

Hopper, K. and C. Ernst. 2005. Source Protection Handbook: Using Conservation to Protect 

Drinking Water Supplies. Trust for Public Land (Washington, D.C.) and AWWA (Denver, 

CO), 88 pp. 

Huetink, R.E., J.W. van der Giessen, J.P. Noordhuizen, and H.W. Ploeger. Epidemiology of 

Cryptosporidium spp. and Giardia duodenalis on a dairy farm. Veterinary Parasitology 102(1-

2): 53 -67. 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-49 April 2010

Hurst, C.J. 1997. Modeling the fate of microorganisms in water, wastewater, and soil. Manual 

of Environmental Microbiology. Ed. C.J. Hurst, G.R. Knudsen, M.J. McInerney, L.D. 

Stetzenback, and M.V. Walter. Ch. 22. Washington, DC: American Society for Microbiology. 

John, D.E. and J.B. Rose. 2005. Review of Factors Affecting Microbial Survival in 

Groundwater. Env. Sci. Tech. 39(19): 7345-7356. 

Kaplan, L.A., L.J. Standley, J.D. Newbold, J.H. Standridge, A.L. Mager, S.M. Kluender, L.L. 

Peterson, D.B. Smith, W.C. Hession, and P. Luitweiler. 2002. Evaluation of Sources of 

Pathogens and NOM in Watersheds. Denver: Awwa Research Foundation and AWWA. 

Kistemann, T., T. Classen, C. Koch, F. Dangendorf, R. Fischeder, J. Gebel, V. Vacata, and M. 

Exner. 2002. Microbial load of drinking water reservoir tributaries during extreme rainfall and 

runoff. Appl. Environ. Microbiol. 68(5):2188-97. 

Klett, Brian. 1996. Delineation of a sixty day travel buffer for the protection of the New York 

City Water Supply. In: Proceedings of the AWRA Session on New York City Water Supplies at 

the Symposium on Watershed Restoration Management: Physical, Chemical, and Biological 

Considerations. 103-109. J.J. McDonnell, D.J. Leopold, J.B. Stribling, and L.R. Neville, 

editors. American Water Resources Association, Herndon, Virginia, TPS-96-2. 

LeChevallier, M., W. Norton, M. Abbaszadegan, and T. Atherholt. 1998. Variation in Giardia

and Cryptosporidium Levels in the Delaware River. Voorhees, NJ: American Water Works 

Service Company, 124 pp. 

Lipp, E.K., S.A. Farrah, and J.B. Rose. Assessment and impact of microbial fecal pollution and 

human enteric pathogens in a coastal community. Marine Pollution Bulletin 42(4): 286-93. 

Lisle, J.T., and J.B. Rose. 1995 . Cryptosporidium contamination of water in the U.S. and UK: 

a mini-review. Water SRT—Aqua 44(3): 103-117. 

Mawdsley, J.L., A.E. Brooks, and R.J. Merry. 1996. Movement of the protozoan pathogen 

Cryptosporidium parvum through three contrasting soil types. Biol. Fertil. Soils 21(1-2): 30-36. 

McCuin, R. and J. Clancy. 2004. Cryptosporidium occurrence, removal, and inactivation 

methods for wastewaters. Alexandria, VA: Water Environment Research Foundation. 

Medema, G., F. Schets, P. Teunis, and A. Havelaar. 1998. Sedimentation of free and attached 

Cryptosporidium oocysts and Giardia cysts in water. Appl. Environ. Microbiol. 64(1): 4460-

4466. 

Medema, G.J., and J.F. Schijven. 2001. Modeling the sewage discharge and dispersion of 

Cryptosporidium and Giardia in surface water. Water Res. 35(18): 4307-16. 

Metcalf and Eddy. 1994. Final CSO Conceptual Plan and System Master Plan: Part II CSO 

Strategies. Prepared for the Massachusetts Water Resources Authority. Wakefield, 

Massachusetts.
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-50 April 2010

Moore, J.A. et al. 1988. Evaluating coliform concentrations in runoff from various animal 

waste management systems. Special Report 817. Agricultural Experimental Stations, Oregon 

State University, Corvallis, and the U.S.D.A., Portland, OR. 

MWRD. 1999. Tunnel and Reservoir Plan. Metropolitan Water Reclamation District. 

www.mwrdgc.dst.il.us/plants/tarp.htm. Last modified August 6, 1999. Website accessed 

January 2002. 

NEIWPCC (New England Interstate Water Pollution Control Commission). 2000. Source 

Protection: A National Guidance Manual for Surface Water Supplies. Lowell, MA: NEIWPCC. 

Available at http://www.neiwpcc.org (click on "Publications", then click "Technical Guides" 

and find the report). 

North American Lake Management Society (NALMS). March 2000. Best Management 

Practices to Protect Water Quality.

NRCS. 1992. Agricultural Waste Management Field Handbook . 

NRCS. 1999. National Handbook of Conservation Practices. Natural Resources Conservation 

Service. http://www.ftw.nrcs.usda.gov/nhcp_2.html

Ohio State University Extension. 1992. Ohio Livestock Manure and Wastewater Management 

Guide, Bulletin 604. 

. 

http://ohioline.osu.edu/b604/index.html

Ohio State University Extension. No date. Getting Started Grazing. Edited by Henry 

Bartholomew. 

. Website accessed March 2003.

http://ohioline.osu.edu/gsg/index.html. 

Ohio State University Extension. No date. Vegetation Filter Strips: Application, Installation, 

and Maintenance. AEX-467-94. http://ohioline.osu.edu/aex-fact/0467.html

Olson, M.E., J. Goh, M. Phillips, N. Guselle, and T.A. McAllister. 1999. Giardia cyst and 

Cryptosporidium oocyst survival in water, soil, and cattle feces. J. Environ. Qual. 28(6): 1991-

1996. 

. Website accessed 

March 2003. 

Payment, P., R. Plante, P. Cejka. 2001. Removal of indicator bacteria, human enteric viruses, 

Giardia cysts, and Cryptosporidium oocysts at a large wastewater primary treatment facility. 

Can. Microbiol. 47(3): 188-93. 

Perz, J.F. and S.M. Le Blancq. 2001. Cryptosporidium parvum infection involving novel 

genotypes in wildlife from lower New York State. Appl. Environ. Microbiol. 67(3): 1154-1162. 

Philadelphia Water Department. 2003. Philadelphia Projects. Website. 

http://www.phillywater.org/Schuylkill/projects%20pages/Project Main.htm#Goose%20Project

Robertson, L.J., A.T. Campbell, and H.V. Smith.1992. Survival of Cryptosporidium parvum

oocysts under various environmental pressures. Appl. Environ. Microbiol. 58: 3494-3500. 

. 

Undated. Accessed February 12, 2003. 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-51 April 2010

Rose, J.B. 1997. Environmental ecology of Cryptosporidium and public health implications. 

Annual Rev. Public Health 18:135-161. 

Santin, M., J. Trout, L. Xiao, L. Zhou, E. Greiner, and R. Fayer. 2004. Prevalence And Age 

Related Variation Of Cryptosporidium Species And Genotypes In Dairy Calves. Veterinary 

Parasitology. 122:103-117. 

Schueler, T.R. 1999. Microbes and urban watersheds: concentrations, sources, and pathways. 

Watershed Protection Techniques. 3(1): 554-565. http://www.stormwatercenter.net. 

Scott, T.M., J.B. Rose, T.M. Jenkins, S.R. Farrah, and J. Lukasik. 2002. Microbial Source 

Tracking: Current Methodology and Future Directions. Applied and Environmental 

Microbiology, 68(12): 5796-5803. http://aem.asm.org/cgi/content/full/68/12/5796. 

Sischo, W.M. E.R. Atwill, L.E. Lanyon, and J. George. 2000. Cryptosporidia on dairy farms 

and the role these farms may have in contaminating surface water supplies in the northeastern 

United States. Preventive Veterinary Medicine 43(4): 253-67. 

Sobrinho, J.A.H., J.S. Rosen, M.W. LeChevallier, M.M. Frey, and J.L. Clancy. 2001. 

Variability of Pathogens and Indicators in Source Waters. In: Proceedings of AWWA Water 

Quality Technology Conference, Nov. 11-15, 2001, Nashville, Tennessee. Session M8. 

States, S., K. Stadterman, L. Ammon, P. Vogel, J. Baldizar, D. Wright, L. Conley, J. Sykora. 

1997. Protozoa in river water: sources, occurrence, and treatment. J. AWWA 89(9): 74-83. 

Stern, D. 1996. Initial investigation of the sources and sinks of Cryptosporidium spp. and 

Giardia spp. within the watersheds of the New York City water supply system. In: Proceedings 

of the AWRA Session on New York City Water Supplies at the Symposium on Watershed 

Restoration Management: Physical, Chemical, and Biological Considerations. 111-121. J.J. 

McDonnell, D.J. Leopold, J.B. Stribling, and L.R. Neville, editors. American Water Resources 

Association, Herndon, Virginia, TPS96-2. 

Sturdevant Rees, P.L., S.C. Long, R. Baker, D.H. Bordeau, R. Pei, and P.K. Barten. N.d. 

Development of Event-Based Pathogen Monitoring Strategies for Watersheds. Denver: Awwa 

Research Foundation (forthcoming). 

Swabby-Cahill, K.D., G.W. Clark, and A.R. Cahill. Buoyant qualities of Cryptosporidium 

parvum oocysts. AWWA Water Quality Technology Conference. Boston: AWWA, 1996. 

Tetra Tech, Inc. 2003. Getting in Step: A Guide for Conducting Watershed Outreach 

Campaigns. Washington, D.C.: U.S. EPA. EPA 841-B-03-002, 100+ pp. 

Thurston, J.A., C.P. Gerba, K.E. Foster, M. M. Karpiscak. Fate of indicator microorganisms, 

Giardia, and Cryptosporidium in subsurface flow constructed wetlands. Water Research 35(6): 

1547-1551. 

Trask, J.R., P.K. Kalita, M.S. Kuhlenschmidt, R.D. Smith, and T.L. Funk. 2004. Overland and 

Near-Surface Transport of Cryptosporidium parvum from Vegetated and Nonvegetated 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-52 April 2010

Surfaces. J. Environ. Qual. 33:984-993 (2004). 

http://jeq.scijournals.org/cgi/content/full/33/3/984. 

Tsuchihashi, R., F. Loge, and J. Darby. 2003. Detection of Cryptosporidium parvum in 

Secondary Effluents Using a Most Probable Number – Polymerase Chain Reaction Assay. 

Water Environment Research. 75(4):292-299. 

U.S. Department of Agriculture. 2000. Waterborne Pathogens in Agricultural Watersheds. 

Watershed Science Institute. ftp://ftpfc.sc.egov.usda.gov/WSI/pdffiles/Pathogens_in_Agricultural_Watersheds.pdf

U.S. EPA 1999b. Combined Sewer Overflow Management Fact Sheet: Sewer Separation. 

Office of Water. EPA 832-F-99-041. September. 

. Website 

accessed March 2010. 

http://www.epa.gov/npdes/pubs/sepa.pdf

U.S. EPA 2000c. Wastewater Technology Fact Sheet. Wetlands: Subsurface Flow. Office of 

Water EPA 832-F-00-023. September. 

. 

Website accessed March 2003.

http://www.epa.gov/owm/mtb/wetlandssubsurface_flow.pdf

U.S. EPA 2001f. Sanitary Sewer Overflows Frequently Asked Questions. Office of Wastewater 

Management. Web page updated March 20, 2001. 

. Website accessed March 2003. 

http://cfpub.epa.gov/npdes/faqs.cfm?program_id=4

U.S. EPA 2002c. Polluted Runoff (Nonpoint Source Pollution: Managing Nonpoint Source 

Pollution from Forestry. Pointer No. 8. EPA 841-F-96-004H. Office of Wetlands, Oceans, and 

Watersheds. 

. Website accessed January 2002. 

www.epa.gov/owow/nps/facts/point8.htm

U.S. EPA 2009. Activity Update: Funding Decentralized Wastewater Systems Using the Clean 

Water State Revolving Fund. Office of Water (4204M). EPA 832-F-09-005. 7 pages. 

. Last modified August 28, 2002. 

Website accessed March 2003.

http://www.epa.gov/owm/septic/pubs/arra_septic_fs.pdf

U.S. EPA. 1994. Combined Sewer Overflow (CSO) Policy; Notice. Federal Register 

59(75):1868818698. April 19. 

. Website accessed March 2010. 

U.S. EPA. 1996. Overview of the Storm Water Program. Office of Water. EPA 833-R-96-008. 

June. 42 pp. http://www.epa.gov/npdes/pubs/owm0195.pdf

U.S. EPA. 1997. State Source Water Assessment and Protection Programs. Final Guidance. 

Office of Water. EPA 816-R-97-009. August. 

. Website accessed March 2003.

http://www.epa.gov/safewater/sourcewater/pubs/swpguidance.pdf

U.S. EPA. 1999a. Protecting Sources of Drinking Water: Selected Case Studies in Watershed 

Management. Office of Water. EPA 816-R-98-019, 38 pp. April. 

. 

http://www.epa.gov/safewater/sourcewater/pubs/swpcases.pdf. Accessed December 10, 2002. 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-53 April 2010

U.S. EPA. 2000a. Wastewater Technology Fact Sheet: Granular Activated Carbon Adsorption 

and Regeneration. Office of Water. EPA 832-F-00-017. September. 

http://www.epa.gov/npdes/pubs/carbon_absorption.pdf

U.S. EPA. 2000d. Watershed Success Stories – Applying the Principles and Spirit of the Clean 

Water Action Plan. Washington, DC: U.S. EPA, 68 pp. 

 U.S. EPA 2000b. Storm Water Phase II 

Final Rule: Small MS4 Storm Water Program Overview.

http://www.blueprintjordanriver.slco.org/docToPdf/WatershedSuccessStor.pdf

U.S. EPA. 2001a. Case Study of Local Source Water Protection Program—Burlington, 

Vermont. Web page updated May 11, 2001. 

. 

http://cfpub.epa.gov/safewater/sourcewater/sourcewater.cfm?action=Case_Studies

U.S. EPA. 2001b. Case Study of Local Source Water Protection Program—Manchester, New 

Hampshire. Web page updated November 26, 2002. 

. 

http://cfpub.epa.gov/safewater/sourcewater/sourcewater.cfm?action=Case_Studies

U.S. EPA. 2001c. Case Study of Local Source Water Protection Program—Springfield, 

Missouri. Web page updated November 26, 2002. 

. Accessed 

on December 12, 2002. 

http://cfpub.epa.gov/safewater/sourcewater/sourcewater.cfm?action=Case_Studies

U.S. EPA. 2001d. Proposed Revisions to CAFO Regulations (January 12, 2001; 66 FR 2960): 

Frequently Asked Questions. 

. 

http://www.epa.gov/npdes/pubs/cafo_faq.pdf

U.S. EPA. 2001e. Secondary Treatment Standards. 

. Downloaded 

February, 2002. 

http://cfpub.epa.gov/npdes/techbasedpermitting/sectreat.cfm

U.S. EPA. 2001g. Protecting and Restoring America's Watersheds: Status, Trends, and 

Initiatives in Watershed Management. EPA 840-R-00-001. Washington, D.C.: U.S. EPA Office 

of Water, 56 pp. 

. Last updated February 21, 2001. 

Downloaded January 22, 2002. 

http://www.epa.gov/owow/protecting/. 

U.S. EPA. 2002a. Preamble to Long Term 2 Enhanced Surface Water Treatment Rule. Draft. 

November 6, 2002. 

U.S. EPA. 2002b. Occurrence and Exposure Assessment for Long-Term 2 Enhanced Surface 

Water Treatment Rule. Draft. Prepared by The Cadmus Group, Inc., Arlington, VA. March 

2002. 

U.S. EPA. 2002d. Source Water Protection: Best Management Practices and Other Measures 

for Protecting Drinking Water Supplies. Washington, D.C.: U.S. EPA Office of Ground Water 

and Drinking Water, 125 pp. http://www.epa.gov/watertrain/pdf/swpbmp.pdf. 

U.S. EPA. 2003b. Introduction to EPA’s Drinking Water Source Protection Programs. 

Washington, D.C.: U.S. EPA Office of Ground Water and Drinking Water, 119 pp. 

http://www.epa.gov/watertrain/pdf/swp.pdf. 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-54 April 2010

U.S. EPA. 2005a. Case Studies of Local Source Water Protection. U.S. EPA Office of Ground 

Water and Drinking Water, Washington, D.C. 

http://cfpub.epa.gov/safewater/sourcewater/sourcewater.cfm?action=Case_Studies

U.S. EPA. 2005b. Section 319 Nonpoint Success Stories Website. Washington, DC: U.S. EPA 

Office of Water. EPA 841-F-05-004. 

. 

www.epa.gov/owow/nps/Success319. 

U.S. EPA. 2005c. Microbial Source Tracking Guide Document. Cincinnati, Ohio: U.S. EPA

Office of Research and Development. EPA/600-R-05-064, 133 pp., June 2005. 

http://www.calcoast.org/news/MSTGuide.pdf. 

U.S. EPA. 2005d. Draft - Handbook for Developing Watershed Plans to Restore and Protect 

our Waters. U.S. EPA Office of Water. EPA 841-B-05-005, 414 pp., October 2005. 

http://www.epa.gov/owow/nps/watershed_handbook. 

U.S. EPA. 2006. Water Conservation Practices for Homeowners: Public Education and 

Outreach on Storm Water Impacts. 

http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm?action=browse&Rbutton=detail

&bmp=2&minmeasure=1

U.S. EPA. 2008. National Pollutant Discharge Elimination System Permit Regulation and 

Effluent Limitation Guidelines and Standards for Concentrated Animal Feeding Operations 

(CAFOs). Federal Register 73(225): 70418-70486. November 20. 

. Last updated May 24, 2006. Downloaded March 17, 2010. 

Vaux, H., ed. 2000. Watershed Management for Potable Water Supply: Assessing the New York 

City Strategy. Washington, DC: National Academy of Sciences. 

Vendrall, P.F., K.A. Teague, and D.W. Wolf. 1997. Pathogen indicator organism die-off in soil. 

ASA Annual Meeting, Anaheim, CA.

Wade, S.E., H.O. Mohammed, and S.L. Schaaf. Prevalence of Giardia sp., Cryptosporidium 

parvum and Cryptosporidium andersoni (syn. C. muris) in 109 dairy herds in five counties of 

southeastern New York. Veterinary Parasitology 93(1): 1-11. 

Walker M.J., C.D. Montemagno, and M.B. Jenkins. 1998. Source water assessment and nonpoint 

sources of acutely toxic contaminants: A review of research related to survival and transport of 

Cryptosporidium parvum. Wat. Resour. Res. 34(12): 3383-3392. 

Walker, M, K. Leddy, and E. Hagar. 2001. Effects of Combined Water Potential and 

Temperature Stresses on Cryptosporidium parvum Oocysts. Applied and Environmental 

Microbiology.67(12): 5526–5529. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC93339/. 

Yeghiazarian, L., P. Kalita, M. Kuhlenschmidt, S. McLaughlin, and C. Montemagno. 2004. Field 

Calibration and Verification of a Pathogen Transport Model. Washington, DC: WERF. 
2. Watershed Control Program

LT2ESWTR Toolbox Guidance Manual 2-55 April 2010

Young, R.A., T. Huntrods, and W. Anderson. 1980. Effectiveness of vegetated buffer strips in 

controlling pollution from feedlot runoff. J. Environ. Qual. 9:483-487. 

Zhang, Q., K.R. Mankin, and L.E. Erickson. 2001. Modeling Transport Of Cryptosporidium 

Parvum Oocysts In Overland Flow. Proceedings of the 2001 Conference on Environmental 

Research. http://www.engg.ksu.edu/HSRC/01Proceed/docs/32.pdf. 
LT2ESWTR Toolbox Guidance Manual 3-1 April 2010 

3. Alternative Source/Intake

3.1 Introduction 

 Changing the water source or intake location can improve source water quality and 

reduce treatment requirements for the Long Term 2 Enhanced Surface Water Treatment Rule 

(LT2ESWTR). The rule states that systems may be classified in a bin based on monitoring of an 

alternative source or intake location or monitoring using an alternative procedure for managing 

the timing or level of withdrawal. This monitoring must be conducted in addition to and 

concurrently with monitoring conducted using the existing intake or withdrawal practice. Since 

the LT2ESWTR requires that alternative monitoring be conducted concurrently with source 

water monitoring, this toolbox option needs to be evaluated prior to the start of source water 

monitoring. After monitoring and with state approval, a system would then choose which 

source, intake location, or intake procedure it will use based on bin classification results. (40 

CFR 141.716(b)) 

 This chapter discusses the concurrent monitoring options of changing sources, moving 

the plant intake, and managing the timing or level of withdrawal and is organized as follows: 

3.2 Changing Sources - discusses factors to be considered in changing sources, 

including advantages and disadvantages and influence of source water 

characteristics on existing treatment requirements.

3.3 Changing Intake Locations - discusses the applicability of changing the intake 

locations and variables affecting Cryptosporidium concentrations in reservoirs, 

lakes, streams, and rivers.

3.4 Changing Timing of Withdrawals - describes different approaches, and 

advantages and disadvantages to changing the timing of withdrawals. 

3.2 Changing Sources 

 In order to be able to relocate an intake to a different source, a system would need to 

identify an unallocated source within a reasonable distance of its treatment plant. It is 

recommended that both drinking water programs and state water resource agencies be contacted 

regarding putting new sources into service. The new source would require sufficient unallocated 

flow to meet the system’s needs, including those for peak flow and future growth. The effect of 

the different water quality on the existing treatment process should also be considered. 
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LT2ESWTR Toolbox Guidance Manual 3-2 April 2010 

3.2.1 Advantages and Disadvantages 

 The main advantage of changing sources is avoiding the addition of a new treatment 

process. The capital expense of a new well or new intake may be less than the expenses 

associated with installing and operating a new treatment technology. In addition to having a 

lower Cryptosporidium concentration, the new source may also have better water quality that 

could reduce treatment costs. Systems should assess any potential new source to ensure its 

integrity, quantity, and quality. In addition, switching to a new source often requires approval by 

the state.

A disadvantage associated with changing sources is that the different source water may 

respond differently to the treatment train already existing at the plant. This may require changes 

in plant operating procedures, such as changing the type and amount of coagulant added, the 

length of filtration runs, and the dose of disinfectant added. Another disadvantage is that the 

source may be lower in Cryptosporidium concentration but have higher concentrations of other 

contaminants. There may also be legal and environmental issues associated with tapping a new 

source. Plant standard operating procedures (SOPs) should be updated if a new source is added. 

Finally, the cost of installing a new intake and transmission line should be considered; depending 

on the location of the source or intake in relation to the plant or to existing transmission lines, a 

new source/intake could be more expensive than other toolbox options. 

3.2.2 Evaluation of Source Water Characteristics for Existing Treatment Requirements

If a new source is to be introduced to an existing treatment plant, the treatability of the 

new water by the existing process should be considered. For example, in a conventional 

treatment train consisting of coagulation, sedimentation, and dual media filtration, each source 

water will have unique coagulation properties depending on its characteristics. Organic content, 

alkalinity, and pH all affect the coagulation process. Consequently, water quality parameters 

including pH, alkalinity, total organic carbon (TOC), UV254, turbidity, and iron and manganese 

concentrations should be measured and evaluated against the existing water and the treatment 

train. If coagulation is used as a part of the treatment process, jar tests should be conducted to 

determine the coagulation and settling properties of the new water and to aid in calculating the 

required dose of coagulant. (See American Water Works Association (AWWA) Manual M37, 

Operational Control of Coagulation and Filtration Processes for more information on jar testing.) 

Pilot plant studies can also help determine the treatability of a proposed new source. 

3.3 Changing Intake Locations 

 Another method for reducing Cryptosporidium source concentrations is to move the 

intake within the same source. This could involve relocating an intake within a source or 

changing the depth from which the intake draws. 
3. Alternative Source/Intake

LT2ESWTR Toolbox Guidance Manual 3-3 April 2010 

3.3.1 Applicability

Relocating an intake can be a good strategy if an obvious source of Cryptosporidium is 

present which can easily be avoided by moving the location of the intake. One example of such a 

situation is if an intake could be moved upstream of a municipal wastewater discharge in a river, 

where it had previously been located downstream of the discharge. 

3.3.1.1 Advantages and Disadvantages 

 One advantage of moving the location of an intake is its potentially low relative cost, if 

the distance the intake must be moved is relatively short. This option could be particularly 

attractive if an existing intake structure can be used to withdraw water from a different depth, 

resulting in decreased Cryptosporidium concentrations. 

 Disadvantages could include significant amounts of excavation and piping, as well as 

additional pumping if the intake must be relocated a considerable distance. Also, altering the 

intake may not bring the desired reduction or provide any additional protection against future 

increases or spikes in Cryptosporidium concentration. 

3.3.2 Reservoirs and Lakes

Several variables can affect the concentration of Cryptosporidium at a particular location 

in a reservoir or lake, including the intake depth, the way in which the lake mixes, the thermal 

properties of the lake, and the proximity of the intake to streams and other discharges. It is 

recommended that a water system develop an SOP for water withdrawal based on the specific 

conditions of the waterbody being used as the source. 

3.3.2.1 Depth

 The intake depth can significantly change the quality of the water being drawn and used. 

In general, shallow intakes are more likely to draw water exposed to recreational activity and 

surface water runoff. Deeper intake locations are often more protected from sources of 

Cryptosporidium, unless an intake location is so deep that it draws water containing resuspended material from the lake or reservoir bottom. Water systems are often well-advised to 

draw water from intermediate depths, where they can avoid higher oocyst concentrations that 

may exist near the lake or reservoir surface, and also avoid particles that may be stirred up near 

the bottom. 
3. Alternative Source/Intake

LT2ESWTR Toolbox Guidance Manual 3-4 April 2010 

3.3.2.2 Stratification and Mixing

 Another factor that can affect the depth profile of Cryptosporidium in a lake or reservoir 

is the amount of stratification or mixing present. Larger lakes and reservoirs often stratify, 

especially in the summer months, forming a hypolimnion (a cold lower layer) and an epilimnion 

(a warm upper layer) separated by a thermocline. There is very little mixing between these layers 

when a lake is strongly stratified. Particles may settle through the layers, but there is little other 

mixing. The epilimnion is often well mixed because of the mixing action of wind. Therefore, it is 

likely that Cryptosporidium may be present at uniform concentrations throughout the epilimnion. 

Cryptosporidium oocysts that have attached to particles and settled will have a concentration 

gradient in the hypolimnion. The shape of any concentration gradient will depend on local 

conditions such as temperature, stream inflows, and particle settling rates. Lakes or reservoirs 

that are strongly stratified and have a high input of organics can often develop anoxia in the 

hypolimnion. Therefore, all water quality parameters should be considered before determining 

the depth from which to draw the water. Extremely high withdrawal rates may provide enough 

energy to overcome stratification and draw from the layer outside of where the intake is located. 

3.3.2.3 Proximity to Inflows

 The proximity of the intake to stream inflows may affect the quality of the intake. 

Streams carrying agricultural or urban runoff can cause water quality degradation if located too 

close to a source water intake. States et al. (1998) reported an increase in Cryptosporidium

concentrations with wet weather events, particularly as the sampling location became closer to 

the contamination source. Kortmann (2000) reported a system substantially reduced coliform 

bacteria in their source water by moving their intake further away from a stream which drained 

an agricultural area and by installing an artificial partition in the reservoir to limit the exchange 

of water between the stream input and the rest of the lake. 

3.3.3 Streams and Rivers

There are several factors to consider when deciding where to locate an intake on a river 

including depth, flow hydraulics, seasonal effects, and upstream sources of contamination. 

3.3.3.1 Depth

Depth is not as likely to affect Cryptosporidium concentrations in small rivers and 

streams as it is in lakes and reservoirs. Fast moving or shallow streams are likely to be fairly 

mixed across all depths. In contrast, deeper and slower moving rivers may be less mixed and 

may show some concentration gradient of Cryptosporidium with unattached oocysts being 

greater near the surface and oocysts attached to particles being greater near the bottom. In rivers 

and streams, intakes located near the bottom are more likely to draw sediment and other particles 

resuspended from the bottom. 
3. Alternative Source/Intake

LT2ESWTR Toolbox Guidance Manual 3-5 April 2010 

3.3.3.2 Flow and River Hydraulics

 Hydraulics of the river and the flow around the intake are extremely important in 

determining the quality of water that enters the system. In general, portions of a stream or river 

with lower velocities and less turbulence will contain less sediment and possibly less 

Cryptosporidium oocysts. Care should also be taken to make sure that the design of the intake 

does not cause turbulence which might stir up sediments. 

3.3.3.3 Upstream Sources of Contamination

 Any potential sources of contamination upstream of a new intake should be identified and 

their impact considered with respect to both biological and chemical contamination. 

Contaminant sources of particular concern for Cryptosporidium include animal feeding 

operations and sewage outfalls. If an intake cannot be located upstream of such a source, then 

locating it as far as possible downstream to allow time for particles to settle may be the next best 

alternative. Analyses of the vulnerability of a stream source should be made on a regular basis. 

Any changes in the vulnerability of a source to Cryptosporidium or other contaminants should be 

reported to the primacy agency. 

3.3.3.4 Seasonal Effects

Cryptosporidium concentrations tend to be higher during runoff events, particularly in the 

spring. Although it is probably not feasible to cease withdrawals during such incidents, it may be 

possible to alter flow rates and coagulant doses to offset the effect of such events.

3.4 Changing Timing of Withdrawals 

 The LT2ESWTR allows the option of changing the timing of withdrawals to obtain a 

lower source water concentration of Cryptosporidium for bin assignment (40 CFR 

141.716(b)(1)). If the system calculates its bin assignment based on this alternative timing, then 

after the compliance deadline, it must continue to draw its source water in the same manner (40 

CFR 141.716(b)(4)). The operating conditions under which the samples were collected for the 

LT2ESWTR must be reported and submitted to the state with the monitoring results (40 CFR 

141.716(b)(3)).

3.4.1 Toolbox Selection Considerations 

 As stated above, the change in timing must be consistent during Cryptosporidium

monitoring and during routine operation after monitoring. Additionally, the LT2ESWTR does 

not allow source water monitoring to deviate from a predetermined schedule by more than 2 
3. Alternative Source/Intake

LT2ESWTR Toolbox Guidance Manual 3-6 April 2010 

days, unless extreme conditions or situations arise that prevent sampling (40 CFR 141.702(b) 

and (c)). Given these limitations, the following provides examples of approaches that are 

recommended and others that are not recommended.

Recommended Approaches 

 Changing the timing of withdrawal on a daily basis (e.g., from the afternoon to morning 

to avoid suspended material stirred up by recreational water use).

 Use a water quality indicator to avoid short-term increases in Cryptosporidium due to 

short-term weather or source water contamination events. For example, if a system routinely 

experiences a spike in turbidity and subsequently, Cryptosporidium, for a 12-24 hour period 

following a storm event, then the system may choose to set up a monitoring plan that delays 

withdrawal for a 24 hour period when detecting a spike in turbidity. 

Approaches Not Recommended 

Limiting withdrawal in response to seasonal effects or weather effects lasting on the 

order of days. This would be a difficult monitoring strategy to follow and stay in compliance 

with the 2 day sampling window. 

3.4.1.1 Advantages and Disadvantages 

 The advantage of changing the timing of withdrawals is it requires no treatment changes, 

only a change in operations. For systems with multiple sources it also allows the greatest 

flexibility in meeting water quality goals.

 A disadvantage of relying on changing withdrawals to lower Cryptosporidium

concentrations is that it may result in decreased flexibility, since systems must follow the same 

withdrawal practices they did during Cryptosporidium source water monitoring. If electing to 

practice a withdrawal approach that defers withdrawal during likely Cryptosporidium events, 

then a system may need some raw water storage capacity.

3.5 References

Gregory, J. 1994. Cryptosporidium in water: Treatment and monitoring methods. Filtr. Sep. 

31(3): 283-289. 

Kortmann, R.W. 2000. Reservoir management approaches exemplified. Proceedings of 

American Water Works Association Water Quality Technology Conference. 

Kortmann, R.W. 1989. Raw water quality control: an overview of reservoir management 

techniques. Journal of the New England Water Works Association. December 1989. pp. 197-220. 
3. Alternative Source/Intake

LT2ESWTR Toolbox Guidance Manual 3-7 April 2010 

Swabby-Cahill, K.D., G.W. Clark, and A.R. Cahill. Buoyant qualities of Cryptosporidium

parvum oocysts. AWWA Water Quality Technology Conference. Boston: AWWA, 1996. 

Walker M.J., C.D. Montemagno, and M.B. Jenkins. 1998. Source water assessment and nonpoint 

sources of acutely toxic contaminants: A review of research related to survival and transport of 

Cryptosporidium parvum. Wat. Resour. Res. 34(12): 3383-3392. 
LT2ESWTR Toolbox Guidance Manual 4-1 April 2010

4. Bank Filtration 

4.1 Introduction 

Bank filtration is a surface water pretreatment process that uses the bed or bank of a river 

(or lake) and the adjacent aquifer as a natural filter. The natural filter performs most efficiently 

when the surface water passes slowly through unconsolidated granular material. In such locations 

and under normal ground water flow conditions, bank filtration is suitable for accomplishing 

sufficient Cryptosporidium removal to partially meet the requirements of the Long Term 2 

Enhanced Surface Water Treatment Rule (LT2ESWTR). To accomplish this, a pumping well 

located in the adjacent aquifer induces surface water infiltration through the bed and bank. 

Bank filtration differs significantly from artificial recharge and from aquifer storage and 

recovery, both of which rely on engineering works to move water into specially constructed and 

maintained recharge basins or wells for infiltration into or replenishment of the aquifer. Although 

microorganism removal can occur in such engineered systems, they are not bank filtration. This 

is because bank filtration relies solely on the natural properties of the surface water bed and 

aquifer, unmodified by engineered works or activity, except for the recovery of ground water via 

a pumping well. Sites with artificial recharge and aquifer storage and recovery operations may 

also receive bank filtration Cryptosporidium removal credit after a suitable site-specific study but 

are not eligible for automatic credit. Slow sand filtration also relies on engineered materials as 

the filter medium and so is not bank filtration.

A significant proportion of microorganisms and other contaminants are removed by 

contact with the aquifer material as the water travels to the well through the subsurface. Flow to 

the well may be horizontal or vertical, but more typically will take a variable path with both 

horizontal and vertical components. The water which has been induced to infiltrate through the 

river’s bed and bank is known as “bank filtrate.” It will be mixed with ambient ground water that 

has taken a different and typically longer path to the well. The ambient ground water may have 

originated as bed or bank infiltration from an upstream portion of the river or from a lake. It may 

have originated from infiltrating precipitation. Regardless, ambient ground water is likely to 

contain different contaminants and contaminant concentrations than bank filtrate because its 

origin and flow pathways differ significantly. Ambient ground water should not be assumed to be 

uncontaminated. 

 

Aquifers suitable for bank filtration are composed of unconsolidated, granular material 

(i.e., grains) and have open, interconnected pores that allow ground water to flow. Pathogen 

removal is enhanced when fine-grained sediment is present along the flow path. Geologic units 

consisting primarily of fine-grained (e.g., clay-sized) materials will have higher removal but will 

be incapable of yielding economically significant water flow rates. In aquifers containing both 

sand-sized and finer grains, the presence of fine grains increases the possibility that pathogens 

will encounter a grain surface. This is because flow is slower and flow paths are longer than they 

would be in aquifers without such fine grains. Microorganisms will be removed from flow as 

they contact and attach to grain surfaces. Although microorganism (e.g., Cryptosporidium) 

detachment can occur, it usually does so at slow rates (Harter et al. 2000). When little or no 
4. Bank Filtration

LT2ESWTR Toolbox Guidance Manual 4-2 April 2010

detachment occurs or when detachment is slow, microorganisms can become non-viable while 

attached to grain surfaces. Thus, bank filtration provides physical removal, and in some cases, 

inactivation, to remove pathogens from water supplies. 

The purposes of this chapter are: 1) to clarify the requirements of the LT2ESWTR related 

to receiving Cryptosporidium removal credit for the use of bank filtration systems; 2) to present 

the current state-of-the-science, advantages and disadvantages of Cryptosporidum removal by 

bank filtration; 3) to explain how local geologic and hydrologic conditions affect the functioning 

and effectiveness of bank filtration systems; 4) to provide suggestions for optimal operation of 

bank filtration systems; and 5) to discuss necessary and sufficient elements of a field and 

laboratory investigation as part of a demonstration of performance (DOP) at a bank filtration site 

(with or without engineered systems) to qualify for additional Cryptosporidium removal credits.

This chapter is organized as follows: 

4.2 LT2ESWTR Compliance Requirements - describes requirements for receiving 

automatic Cryptosporidium removal credits related to the proposed 

installation of bank filtration wells.

4.3 Toolbox Selection Considerations - describes the advantages and disadvantages 

of using bank filtration as a pretreatment technology. 

4.4 Site Selection and Aquifer Requirements - characterizes surface water and aquifer 

types that are suitable for bank filtration.

4.5 Design and Construction - describes the types of wells eligible for bank filtration 

credits and the locations at which such wells are best placed. 

4.6 Operational Considerations - describes issues relevant to the optimal operation of 

bank filtration systems in order to protect public health. 

4.7 Demonstration of Performance

 

 – describes the recommendations for receiving 

additional Cryptosporidium removal credits after a site-specific field and 

laboratory investigation. 

4.2 LT2ESWTR Compliance Requirements 

Systems that propose to install bank filtration wells to meet any additional treatment 

requirements imposed by the LT2ESWTR may be eligible for 0.5 or 1.0 log Cryptosporidium

removal credit (40 CFR 141.717(c)). Systems meeting all regulatory requirements (e.g., systems 

with conventional or direct filtration that meet the well siting and monitoring requirements of 

LT2ESWTR) may receive Cryptosporidium log removal credit. For those systems which already 

use bank filtration as a component of their treatment process and which also have existing 

conventional or direct filtration treatment, the LT2ESWTR requires source water monitoring of 
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LT2ESWTR Toolbox Guidance Manual 4-3 April 2010

produced water from the bank filtration well. This will determine the initial bin classification for 

these systems. Because their source water monitoring accounts for any bank filtration treatment, 

these systems are not eligible for subsequent additional bank filtration credits (40 CFR 

141.703(d)(1)). 

Systems using ground water under the direct influence of surface water (GWUDI) or bank 

filtered water without additional filtration must take source water samples in the surface water to 

determine bin classification (40 CFR 141.703(d)). This applies to systems using an alternative 

filtration demonstration to meet the Cryptosporidium removal requirements of the Interim 

Enhanced Surface Water Treatment Rule (IESWTR) or Long Term 1 Enhanced Surface Water 

Treatment Rule (LT1ESWTR) (40 CFR 141.173(b) and 141.552(a)). The requirements and 

guidance provided in this chapter do not apply to existing primacy agency actions providing 

alternative filtration Cryptosporidium removal credit for IESWTR or LT1ESWTR compliance. 

Alternatively, PWSs may apply to the state for Cryptosporidium treatment credit using a 

DOP (see Chapter 4.7). States may award greater than 1.0-log Cryptosporidium treatment credit 

for bank filtration based on a site-specific demonstration. States may also award DOP

Cryptosporidium treatment credit based on a site-specific study to systems that are unable to 

qualify for the 0.5 or 1.0-log removal credit as described in Chapter 4.2.1. For a bank filtration 

DOP study, the following criteria must be met:

• The study must follow a state-approved protocol and must involve the collection of data 

on the removal of Cryptosporidium or a surrogate for Cryptosporidium and related 

hydrogeologic and water quality parameters during the full range of operating conditions. 

• The study must include sampling both from the production well(s) and from monitoring 

wells that are screened and located along the shortest flow path between the surface water 

source(s) and the production well(s). 

4.2.1 Credits

The LT2ESWTR specifies the following design requirements for systems to receive log 

removal credit for bank filtration (40 CFR 141.717(c)): 

• Wells must draw from granular aquifers that are comprised of clay, silt, sand, or pebbles 

or larger particles. Minor cement may be present.

• Only horizontal and vertical wells are eligible for bank filtration log removal credit. 

• Other ground water collection devices such as infiltration galleries and spring boxes are 

ineligible. 
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LT2ESWTR Toolbox Guidance Manual 4-4 April 2010

• Systems using horizontal or vertical wells located at least 25 feet from the surface water 

source are eligible for a 0.5 log removal credit and those located at least 50 feet from the 

surface water source are eligible for a 1.0 log removal credit. 

- Systems with vertical wells must identify the distance to surface water using the 

floodway boundary or 100 year flood elevation boundary as delineated on Federal 

Emergency Management Agency (FEMA) Flood Insurance Rate maps. 

- Systems with horizontal wells must measure the distance from the normal flow stream 

bed to the closest horizontal well lateral.

• Systems must characterize the aquifer at the proposed production well site to determine 

aquifer properties. 

- At a minimum, the aquifer characterization should include the collection of relatively 

undisturbed continuous core samples from the surface to a depth at least equal to the 

projected bottom of the well screen for the proposed production well. 

- The recovered core length must be at least 90 percent of the total depth to the 

projected bottom of the well screen and each sampled interval should be a composite 

of no more than 2 feet in length. 

- Each composite sample must be examined to determine if at least 10 percent of the 

grains in that interval are less than 1.0 millimeter (mm) in diameter. 

4.2.2 Monitoring Requirements

The LT2ESWTR requires systems to monitor turbidity in bank filtration wells to provide 

assurance that the assigned log removal credit is appropriate. The LT2ESWTR specifically 

requires the following monitoring (40 CFR 141.717(c)(5)):

• Turbidity measurements must be performed on representative water samples from each 

wellhead every four hours that the bank filtration system is in operation or more 

frequently if required by the state.

• Continuous turbidity monitoring at each wellhead may be used. 

• If the monthly average of daily maximum turbidity values at any well exceeds 1 

nephelometric turbidity units (NTU), the system must report this finding to the state 

within 30 days. In addition, within 30 days of the exceedance the system must conduct an 

assessment to determine the cause of the high turbidity levels and submit that assessment 

to the state for a determination of whether any previously allowed credit is still 

appropriate. 
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LT2ESWTR Toolbox Guidance Manual 4-5 April 2010

4.3 Toolbox Selection Considerations 

Bank filtration is best suited to systems that are located adjacent to rivers with consistent 

surface water quality and that plan to use bank filtration as one component of their treatment 

process. For systems that can meet the aquifer requirements (section 4.4) and the design criteria 

(section 4.5), bank filtration can be an efficient, cost-effective pretreatment option to improve 

water quality (Berger 2002). Medema et al. (2000), Medema and Stuyfzand 2002, and Wang et al 

(2000, 2002) documented high removal of Cryptosporidium surrogate organisms at production 

well sites in The Netherlands and in Louisville, Kentucky. There was very little occurrence of 

Cryptosporidium in river water at the Kentucky site and no Cryptosporidium was found in the 

well water at either site. The amount of Cryptosporidium removal at either site is unknown. 

The efficient removal of indicator organisms at the Netherlands site was likely due to the 

relatively impermeable, fine-grained layer of river sediment present, as well as the effect of pyrite 

oxidizing to ironhydroxides for oxidized ground water. Ironhydroxides (at a pH below 7.0) may 

enhance the attachment of microorganisms to riverbed sediments (Medema et al. 2000; Medema 

and Stuyfzand 2002). In Louisville, Kentucky, an alluvial aquifer was chosen for the bank 

filtration site. Wang et al (2000, 2002) found that removal of biological particles increased with 

filtration distance of the riverbank filtration process, although most of the removal occurred at 

the surface of the riverbed, within the first two feet of filtration. Wang et al (2002) attributed the 

removal in their bank filtration system to a combination of mechanical filtering and biological 

activity (e.g., biofiltering) at the surface of the riverbed. 

As will be discussed in section 4.4, only certain sites are suitable for bank filtration. It is 

important to understand the type of bed and aquifer material present, the dynamics of 

groundwater flow, and the potential for scouring of riverbed materials at a potential bank 

filtration site. The degree to which the bed and banks of surface water bodies may effectively 

filter Cryptosporidium may vary not only from site to site, but also at a single site over time. A 

site-specific DOP study requires not only a good understanding of past ground water flow and 

Cryptosporidium surrogate removal efficiency but also ongoing monitoring to identify and to 

take preventive action during poor removal periods. 

4.3.1 Advantages and Disadvantages 

4.3.1.1 Removal of Additional Contaminants 

The two research sites with published data (Medema et al. 2000, Medema and Stuyfzand 

2002, Wang et al. 2000, Wang et al. 2002, Berger 2002) have shown that bank filtration is 

effective at removing Cryptosporidium. Bank filtration has also been shown at some sites to be 

an effective technology for attenuating a variety of additional microorganisms as well as 

particulates, ammonia, nitrate, pesticides (e.g., atrazine), heavy metals, ethylenediamine tetraacetic acid (EDTA), alkylated and chlorinated benzenes and other organic contaminants, and 

disinfection byproducts (DBPs) precursors in the form of Natural Organic Matter (NOM) 

(Schijven et al. 2003, Tufenkji et al. 2002, Ray et al. 2002, Kuehn and Mueller 2000). Bank 
4. Bank Filtration

LT2ESWTR Toolbox Guidance Manual 4-6 April 2010

filtration achieves the removal of these diverse contaminants by facilitating or enhancing 

physical and chemical filtering, sorption, reduction/oxidation, precipitation, ion exchange, and 

biodegradation (Schijven et al. 2003, Ray et al. 2002, Tufenkji et al. 2002). Bank filtration 

further reduces contaminant concentrations and especially shock contaminant loads from spills 

and intentional acts by providing for the multidimensional dispersion and dilution of 

contaminants (Ray et al. 2002). 

The degree to which any particular contaminant will be removed via bank filtration 

depends on site-specific conditions. For example, under aerobic conditions, ammonia is often 

completely transformed to nitrate and nitrite, whereas such removal may not occur under more 

reducing conditions. Oxygen is usually significantly depleted within 5-15 feet of the riverbed, 

due to microbial activity in this zone. As infiltrating water becomes increasingly depleted of 

organic matter due to degradation, microbial activity diminishes, and the aquifer may be 

reaerated at a certain distance from the riverbed (Tufenkji et al. 2002). The anaerobic part of the 

aquifer was observed to remove up to 99 percent of polar organic contaminants at a site in central 

Germany (Juttner 1995). Miettinen et al (1994) found that almost 90 percent of the high 

molecular weight fraction of NOM had been removed at a bank filtration site in Finland. 

Bank filtration can reduce treatment costs by reducing the need for more expensive 

treatment technologies. Particle and microorganism removal during bank filtration allows for 

more efficient filtration, use of membranes, and disinfection during subsequent treatment steps. 

For example, decreasing the concentration of dissolved organic carbon during bank filtration can 

reduce the amount of dissolved organic carbon that needs to be removed in a downstream 

treatment process such as activated carbon filtration. The removal of ammonia means that the 

additional treatment step of oxidizing ammonia with chlorine may be unnecessary. The 

removal of nitrate when water is induced to flow through anaerobic areas may eliminate the need 

for expensive ion exchange or reverse osmosis treatment processes (Kuehn and Mueller 2000). 

Finally, because it is effective at biodegrading many contaminants, including trace organic 

contaminants, bank filtration reduces the need for adding large quantities of flocculants to 

drinking water (Kuehn and Mueller 2000). 

Another advantage of bank filtration as a pretreatment technology is that it acts to 

equalize fluctuations in contaminant concentrations observed in surface waters. This is due to the 

effects of dilution and dispersion which serve to spread peaks in contaminant concentrations over 

space and time by the time they reach wells. Contaminant concentration peaks may be due to 

variations in river water levels, seasonal effects, and runoff, in addition to spills, terrorist acts and 

emissions by municipal and industrial institutions (Kuehn and Mueller 2000). Bank filtration is 

continuously active, and the decreased amplitude of the contaminant peak by the time it reaches a 

well (an inherent result of subsurface transport through porous material) allows for easier and 

less expensive treatment by utilities with limited capabilities. In addition, the time lag between 

contamination of surface water and arrival of contaminant at a well would give utilities more of 

an opportunity to respond to a threat or an accidental spill. Kuehn and Mueller (2000) estimate 

that in many modern bank filtration systems bank filtrate spends anywhere from 5 to 15 days in 

the subsurface before reaching supply wells. At one site in the Netherlands, bank filtrate was 

estimated to spend 45-65 days in the subsurface before reaching the supply well (Medema, et al.
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LT2ESWTR Toolbox Guidance Manual 4-7 April 2010

2000). Residence time depends on site-specific hydrogeology as well as bank filtration system 

design. Bank filtration also smoothes out fluctuations in water temperature. 

The removal of NOM during bank filtration is useful because NOM occurrence can result 

in the production of harmful DBPs, as discussed above. In addition, moderate to high 

concentrations of NOM in drinking water can result in unpleasant taste and odor. Finally, NOM 

removal via bank filtration can also aid in the removal of a large variety of additional organic and 

inorganic contaminants. These contaminants are sometimes made more mobile in surface and 

ground waters due to a partitioning process whereby they are attached to NOM, which is 

relatively mobile, and thereby carried along a flow path. The removal of NOM and associated 

contaminants prior to above-ground treatment is likely to lessen the overall cost of water 

treatment at a given facility.

4.3.1.2 Clogging of Pores

Clogging of the surface water - ground water interface by physical, chemical, and 

biological processes has the potential to be a problem with any riverbank filtration system. Partial 

clogging during riverbank filtration system operation is likely to be unavoidable (Wang et al. 

2001, Goldschneider et al. 2007); however, its effects are not always deleterious. The 

disadvantage of clogging is that it can reduce hydraulic conductivity of the local riverbed and the 

aquifer, thereby temporarily or permanently reducing well yields. On the other hand, a limited 

accumulation of fine-grained sediments and the accompanying development of a biologically 

active zone can enhance pathogen removal. Indeed, this enhanced removal is a basic principle 

behind riverbank filtration as a water treatment technology. An optimal amount of clogging is 

beneficial because it can reduce the size of large pores or reduce entrances to pores in a stream 

bed or aquifer. Pore size reduction and decreased hydraulic conductivity also result in longer 

travel times which can result in additional pathogen inactivation. Transport of fewer pathogens is 

also likely because there are more opportunities for pathogen contact with aquifer grain surfaces. 

Physical clogging of the surface water - ground water interface results from the deposition 

of fine-grained, suspended sediment at the interface and in the near surface pores. The deposition 

and growth of microorganisms also contribute to physical clogging. This clogging may be 

exacerbated during periods of low surface water discharge, and is most apparent near the river’s 

edge where flow velocities are generally lower than at the center of the river. Chemical clogging 

can result from precipitation of dissolved surface water constituents and may occur near the 

interface or anywhere along the flowpath. This is due to the change in geochemical conditions as 

infiltrating water enters the riverbed and aquifer. Factors to be considered when evaluating the 

potential for chemical clogging include electrolyte concentration, pH, redox potential, presence 

of dissolved or colloidal organic matter, and the mineralogy and surface characteristics of stream 

bed and aquifer solids. 

Lastly, biological or microbial clogging can result from the accumulation of bacterial 

cells in pore spaces, the production of extra-cellular polymers, the release of gaseous byproducts 

from denitrifying bacteria and methanogens, and the microbially mediated accumulation of 
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LT2ESWTR Toolbox Guidance Manual 4-8 April 2010

insoluble precipitates (Vandevivere et al. 1995, Baveye et al. 1998). Biogenic gas bubbles have 

the effect of blocking or partially blocking water flow through pores in much the same way that 

solid particles do (Orlob and Radhakrishna 1958, Oberdorfer and Peterson 1985, Sanchez de 

Lozada et al. 1994). Insoluble sulfide salts can cause clogging due to the activity of sulfate 

reducing bacteria, whereas iron hydroxide and manganese oxide deposition can be brought on by 

bacterial iron metabolism (Vandevivere et al. 1995, Baveye et al. 1998). Biological clogging is 

most likely to occur near the surface water - ground water interface where nutrients are most 

available.

Some or all of these processes may act at a particular site to lower hydraulic conductivity 

and thus decrease flow velocities. For example, several months of pumping from a new riverbank 

filtration well in Louisville, Kentucky resulted in a significant decline in well production, 

presumably due to a 70 percent reduction in leakance from the river to the adjacent aquifer. The 

reduced well yields were attributed to the physical clogging of riverbed sediments (Schafer, 

2000). The disadvantage of reduced well yields accompanies the advantages of increased 

microbial inactivation rates due to lower flow velocities (and thus longer residence times in the 

aquifer) as well as increased removal of pathogens due to smaller pores. 

4.3.1.3 Scour

Both the positive and negative effects of clogging on riverbank filtration system 

performance may be diminished following periodic flooding. Scour refers to the erosion of the 

river’s bed and banks. The extent of erosion depends on both flood conditions and the resistance 

of the bed and bank material that has been deposited at a particular site. During flooding, the 

river channel may be scoured and fine sediments at the surface water - ground water interface 

mobilized. 

Much of the removal of the contaminants and microbes discussed previously occurs 

during the first few centimeters of the flow path due to the significant filtering and sorptive 

capabilities of sediments in the riverbed. These sediments are often organic-rich, highly 

biologically active, and fine-grained. The effectiveness of bank filtration, however, may be 

temporarily threatened during high flows if this active layer is washed away or scoured. EPA 

suggests that the potential for stream channel scour be evaluated during riverbank filtration site 

selection (section 4.4). Section 4.5 provides further discussion of scour and its implications for 

riverbank filtration system operation. 

4.3.1.4 Additional Treatment Steps

In addition to clogging and scour, there are several disadvantages to bank filtration which 

utilities may wish to consider and balance against the advantages and cost savings described in 

section 4.3.1. One disadvantage is that an additional aeration step may be required due to the 

possible depletion of oxygen from biological activity during riverbank filtration pretreatment 

(Kuehn et al. 2000). This oxygen depletion may lead to extremely anaerobic conditions over a 
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LT2ESWTR Toolbox Guidance Manual 4-9 April 2010

portion of the flow path, which may result in the release of iron and manganese from the bank 

sediment into the flowing water. This process occurs due to a redox reaction which reduces iron 

and manganese to their water-soluble forms. This condition may necessitate the removal of these 

metals during subsequent treatment steps (Kuehn et al. 2000, Tufenkji et al. 2002). 

On the other hand, if the flow path between the riverbank and the well is long enough, 

iron and manganese may precipitate onto the sediments in the subsurface before ever reaching 

the well (Tufenkji et al. 2002). The aquifer becomes reaerated with increasing distance from the 

riverbed. This is one reason for locating riverbank filtration wells greater than 25 or 50 feet from 

the river, as discussed in section 4.5.2.2. Even though most contaminant removal occurs during 

the first few centimeters of subsurface transport, the reaeration and associated precipitation 

reactions in the aquifer may significantly improve water quality before it reaches the well 

(Tufenkji et al. 2002). The location of the aerated and anaerobic portions of the aquifer vary 

seasonally due to variable recharge, precipitation infiltration, microbial activity and changing 

pumping rates. 

Lastly, riverbank filtration is ineffective at removing a few persistent compounds, 

primarily non-polar organic compounds and highly soluble chemical contaminants such as 

methyltertiarybutylether (MTBE) and trichloroethylene (TCE). These would need to be addressed 

during subsequent treatment steps. In addition, when bank filtration is used to induce infiltration 

of highly contaminated surface water, it may be important to include additional adsorption steps 

during later treatment (Kuehn et al. 2000). 

4.4 Site Selection and Aquifer Requirements 

Unconsolidated, granular aquifers with sufficient amounts of fine-grained material (see 

section 4.4.2) are eligible for Cryptosporidium removal credits under the LT2ESWTR. Partially 

consolidated, granular aquifers may also be eligible for removal credits. Each granular aquifer 

proposed as a bank filtration site should be evaluated on a case-by-case basis with regard to its 

grain size distribution and degree of cementation. For example, a partially consolidated, granular 

aquifer may be too cemented, and thus perhaps too fractured, to provide adequate pathogen 

removal. Geophysical methods, discussed in section 4.5.2.2, may be helpful in determining the 

degree of fracturing of such aquifers. 

This section characterizes river and aquifer types that may be suitable for bank filtration 

surface water treatment. A list of selected sites in the United States and Europe which have used 

bank filtration is provided for reference. No information is available for these sites, however, 

regarding whether they would meet the siting criteria in the LT2ESWTR. Some common aquifer 

types that are clearly not appropriate for this technology are described as well. Finally, sitespecific aquifer criteria which shall be met in order for systems to receive Cryptosporidium

removal credits are outlined in section 4.4.3. 
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4.4.1 Selected Bank Filtration Sites

Exhibit 4.1 Selected Bank Filtration Systems in Europe and the United States

Site Location Well Type*

Wells

mgd (m3/s)

Europe

Number of Maximum Capacity River System

 Torgau, Germany V 42 39.7 (1.737) Elbe

 Mockritz, Germany V 74 28.8 (1.260) Elbe

United States 

 Cincinnati, OH V 10 40.0 (1.750) Great Miami

 Columbus, OH H 4 40.0 (1.750) Scioto/Big Walnut

 Louisville, KY H 1+ 20.0 (0.875) Ohio

 Terra Haute, IN H 1 12.0 (0.525) Wabash

 Jacksonville, IL H 1 8.0 (0.350) Illinois

 Galesburg, IL H 1 10.0 (0.438) Mississippi

 Henry, IL V 1 0.7 (0.030) Illinois

 Mt. Carmel, IL V 1 1.0 (0.044) Wabash

 Quincy, IL H 1+ 10.0 (0.438) Mississippi

 Sacramento, CA H 1 10.0 (0.438) Sacramento

 Sonoma County, CA H, V 5 (H) + 7 (V) 85.0 (3.727) Russian

 Independence, MO H † 1 15.0 (0.656) Missouri

 Lincoln, NB H, V 2 (H) + 44 (V) 35.0 (H) (1.530) Platte

 Kennewick, WA H 1 3.0 (0.130) Columbia

 Kalama, WA H 1 2.6 (0.110) Kalama

 St. Helens, OR H 3 5.0 (0.219) Columbia

 Kansas City, KS H 1 40.0 (1.750) Missouri

 Sioux Falls, OK H 1+ 40.0 (1.750) Missouri

* H–horizontal, V–vertical. 

† Gravel-packed Laterals. 

Reprinted from J AWWA 94(4) (April 2002) by permission. Copyright © 2002. American Water Works Association.

4.4.2 Aquifer Type

4.4.2.1 Unconsolidated, Granular Aquifers

Unconsolidated, granular aquifers can be composed of a wide range of sediment sizes 

including clay, silt, sand, and larger particles. They may also exhibit minor cementation, but 

subsurface samples are typically friable (readily crumbled by hand). To be eligible for bank 

filtration credits under the LT2ESWTR, unconsolidated granular aquifers are expected to contain 

a sufficient amount of fine-grained sediments to achieve adequate pathogen removal and/or 

inactivation (section 4.4.3 prescribes the amount deemed sufficient). In aquifers with these 

characteristics, the flow path is tortuous at the micro-scale, providing many opportunities for 

removal of microorganisms by straining or by their attachment to grain surfaces. 

Many alluvial aquifers contain significant amounts of well-sorted, fine-grained sediments. 

Alluvial aquifers are produced by fluvial depositional processes and are adjacent to modern 

streams. Aquifers formed in glacial deposits may also contain sufficient amounts of fine-grained 

material. These may be “till” deposits, which have a wide range of poorly sorted sediment sizes, 
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LT2ESWTR Toolbox Guidance Manual 4-11 April 2010

or glacial outwash deposits that are formed by meltwater and often contain well-sorted, sandsized sediments. Any of these alluvial or till aquifers would likely be suitable for a bank filtration 

system. On the other hand, coarse gravel aquifers produced by the rapid drainage of glacial lakes, 

or in outwash environments that deposit little fine-grained material, may not be eligible for bank 

filtration credits unless sieve analysis shows sufficient fine-grained material as discussed in 

section 4.4.3.2. 

Alluvial aquifers may be identified on detailed hydrogeologic maps simply as 

“Quaternary alluvium,” indicating both their genesis and relative age. Glacial deposits are 

documented on surficial geology maps and, where aquifer-forming, may be identified on largescale hydrogeologic maps.

4.4.2.2 Karst, Consolidated Clastic, and Fractured Bedrock Aquifers

In karst, consolidated clastic, and fractured bedrock aquifers, ground water velocities are 

fast, and flow paths may be direct, allowing microbial contaminants to travel rapidly to a well 

with little removal or inactivation. Because they do not meet 40 CFR 141.717(c)(2), these aquifer 

types are not eligible for bank filtration treatment credits. 

Karst may be broadly defined as a region where the dissolution of calcitic or other soluble 

bedrock, primarily limestone (calcium carbonate), produces a unique subsurface drainage 

network and associated surface landforms. Ground water movement in karst aquifers differs from 

that in porous, granular aquifers in that flow in the former occurs predominantly in conduits and 

dissolution-enlarged fractures. Consequently, there is little physical removal of microbes and 

other particles by filtration and few opportunities for microbes to come in contact with the 

surfaces of aquifer materials. Furthermore, rapid flow creates conditions where inactivation is 

less likely to occur before ground water reaches a well. 

Although fractures have a role in ground water movement through any aquifer, fractures 

provide the dominant flow-path in fractured consolidated clastic and fractured bedrock aquifers. 

Most consolidated aquifers can be presumed to be fractured. Similar to solution conduits in karst 

aquifers, fractures in consolidated aquifers provide preferential flow paths that may transmit 

ground water at high velocities, and in a relatively direct flow path to a well, with little time or 

opportunity for inactivation or removal of microbial pathogens (e.g., Gaut et al. 2008). Wells 

located in these aquifers would not meet 40 CFR 141.717(c)(2), and therefore would not be 

eligible for bank filtration credit.

4.4.2.3 Partially Consolidated, Granular Aquifers 

Granular aquifers formed by marine processes earlier than Quaternary alluvial or glacial 

deposition may be partially consolidated by natural cement that fills pores, connects grains, and 

makes the aquifer material less friable. Partially consolidated, granular aquifers are present 

within the Atlantic Coastal Plain, Gulf Coast Lowland, Texas Coastal Upland, and Mississippi 
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LT2ESWTR Toolbox Guidance Manual 4-12 April 2010

Embayment aquifer systems (USGS 1998). When significant proportions of cement are present, 

fractures are more likely to exist. As in consolidated aquifers, fractures in partially consolidated, 

granular aquifers create direct paths for microbial contamination that minimize the natural 

filtration capabilities of the aquifer system. EPA suggests that partially consolidated aquifers be 

evaluated at the proposed well location to determine if they may be too cemented, and thus 

perhaps too fractured, to provide sufficient natural filtration. 

The degree of cementation can be evaluated by a variety of methods. Geologic material 

collected from below the aquifer’s weathered zone that is friable upon touch is likely to be 

adequate for bank filtration purposes. Another test for the degree of cementation includes the 

slaking test, which involves alternate wetting and drying of the sample in water, or in salt or 

alcohol solutions. A triaxial compression test can also be used to measure strain in three mutually 

perpendicular directions. Less cemented samples will be more deformable during such tests.

4.4.3 Aquifer Characterization

Systems seeking Cryptosporidium removal credit are required to characterize the aquifer 

properties between their surface water source and their well (40 CFR 141.717(c)(2)). The aquifer 

characterization will include, at a minimum, core sampling to determine grain size distribution. 

This data will establish whether enough fine-grained sediment is present to provide adequate 

filtration. The following procedure outlines the steps necessary under the LT2ESWTR and steps 

recommended by EPA to perform such a characterization, which will ultimately determine 

eligibility for bank filtration treatment credits under the LT2ESWTR.

The necessary steps are:

• Systems must characterize the aquifer at the proposed production well site to 

determine aquifer properties.

- The recovered core length must be at least 90 percent of the total.

- Each composite sample must be examined to determine if at least 10 percent of the 

grains in that interval are less than 1.0 mm in diameter.

EPA also recommends:

• The aquifer characterization should include the collection of relatively undisturbed 

continuous core samples from the surface to a depth at least equal to the projected bottom 

of the well screen for the proposed production well. 

• Each sampled interval should be a composite of no more than 2 feet in length. 

If core recovery is insufficient, another well core should be obtained. 
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LT2ESWTR Toolbox Guidance Manual 4-13 April 2010

• Each 2 foot long composite sample of recovered core should be examined in a laboratory 

using sieve analysis to determine grain size distribution. Core intervals are typically 2 feet 

long for a conventional split-spoon sampler and 3 or 4 feet long for soil probes (e.g., a 

Giddings-type soil probe). 

4.4.3.1 Coring

The collection of relatively undisturbed cores in unconsolidated aquifers can be quite 

difficult, especially when gravel-sized clasts are present. The two most important criteria for 

successful test drilling to obtain a core are sample accuracy and drilling speed. Borehole stability 

is a major problem in drilling in an unconsolidated gravelly formation. Rotary core drilling is 

particularly suited to drilling in unconsolidated formations because the drilling fluid, which cools 

the drill bit and carries up the core, also acts to stabilize the borehole (Driscoll 1986). 

Other drilling methods require the installation of a casing to stabilize the borehole, a 

process which slows down the speed of drilling. Rotary core drilling is the fastest method for 

drilling in an unconsolidated formation. One disadvantage to rotary core drilling is the separation 

of different sized core particles as they rise (smaller particles rise faster) and cross-contamination 

by overlying borehole material. An experienced driller can avoid cross contamination by using 

the dual-wall method of rotary core drilling. In the dual-wall method, the core is pushed up the 

inner pipe of the drill rather than traveling in the space between the drill and the borehole wall 

(Driscoll 1986). Shallow wells will have fewer particle size separation problems than deeper 

wells. The freeze-core method (Balcsak 1995) can be used to obtain in-situ cores from 

streambeds. 

Auger drilling is another method for drilling test wells. In this method an earth auger is 

screwed into the earth by rotation. Auger drilling in an unconsolidated formation is slower than 

rotary core drilling, due to the necessary installation of casing to support the borehole. Sampling 

with augers can provide reliable samples from any depth. A split spoon sampler can be used 

wherein a split spoon is driven to the bottom of the hole. The depth to which an auger can drill is 

dependent on the size of the rig. The maximum drilling depth possible for a small drill rig is 

approximately 250 ft. (Driscoll 1986). 

Information about drilling and finding a driller can be found through the National 

Groundwater Association (NGWA) website: http://www.ngwa.org/. In addition, the 

EnviroDirectory provides listings for laboratories and drillers in New England, the MidAtlantic, and the Great Lakes regions (www.envirodirectory.com).
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LT2ESWTR Toolbox Guidance Manual 4-14 April 2010

4.4.3.2 Sieve Analysis 

 The American Society for Testing and Materials (ASTM) has a published standard for 

conducting sieve analysis, the Standard Test Method for Sieve Analysis of Fine and Coarse 

Aggregates: Standard C 136-1 (ASTM 2003). 

Sieve analysis is used to determine the particle size distribution of a sample of dry 

aggregate of known mass by passing the sample through a series of sieves with progressively 

smaller openings. Sieve analysis requires the following equipment: 

• A balance, accurate to 0.1g or 0.1 percent of test load for fine aggregate, or accurate to 

0.5g or 0.1 percent of test load for a mixture of fine and coarse aggregate. 

• Stackable sieves. 

• A mechanical sieve shaker (for sample sizes greater than 20kg). 

• An oven capable of maintaining 110 ± 5oC (230 ± 9oF). 

In the first step of sieve analysis the sample is dried using the oven. Once dry, its weight 

is measured and recorded. While the sample dries, sieves are selected with suitable openings to 

furnish the information required. For bank filtration related sieve analyses, it is only necessary to 

determine what percentage of the sample is less than 1.0mm; however, it is recommended that 

sieves covering a range of sizes be used so as to prevent the overloading of any one sieve. Once 

the sample is dry and the sieves are stacked in order of decreasing mesh size, the sample is 

placed in the top sieve and sieving either by machine or by hand begins. Sieving should be 

continued until no more than 1 percent by mass of the material retained on an individual sieve 

will pass through that sieve during 1 minute of continuous hand sieving. Lastly the mass on each 

sieve is weighed. The total mass of the material after sieving should correspond closely with the 

original mass of the sample. Using the mass for each size increment and the total mass of the 

sample, the size distribution of the sample can be determined (ASTM 2003). 

Further information about sieve analysis can be found at the ASTM web site 

(www.astm.org). A multi-media sieve analysis demonstration can be found at Geotechnical, 

Rock and Water Resources Library (GROW) http://www.grow.arizona.edu/Grow--

GrowResources.php?ResourceId=139. 

ASTM also provides a search engine which allows the user to search for laboratories that 

perform sieve analyses (http://www.astm.org/LABS/search.html). The EnviroDirectory

provides listings for laboratories and drillers in New England, the Mid-Atlantic, and the Great 

Lakes regions (http://www.envirodirectory.com).
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LT2ESWTR Toolbox Guidance Manual 4-15 April 2010

4.4.4 Site Selection as it Relates to Scour

Stream channel scour may often be an important consideration in choosing sites that are 

suitable for riverbank filtration. This section discusses stream channel erosional processes in 

general, as well as reasons sites with certain characteristics may be unsuitable for riverbank 

filtration. Section 4.6 discusses the implications of periodic scour for riverbank filtration system 

operations. Detailed information on fluvial erosional processes can be obtained from any of a 

number of texts on fluvial geomorphology, hydrology, and river hydraulics (e.g., Leopold et al. 

1964, Ritter et al. 1995, Chow 1964). 

4.4.4.1 Stream Channel Erosional Processes

This discussion focuses on the dominant erosional processes of alluvial rivers because, 

given the LT2ESWTR’s aquifer requirements, such rivers may be among the most suitable for 

bank filtration credits. Although many lake banks are also suitable sites for bank filtration, lakes 

will not be discussed in detail in this section. Lake bank filtration settings typically do not change 

rapidly with time and climate. Their hydrologic properties are not highly variable and thus do not 

require the detailed evaluation discussed here for riverbank filtration settings. 

The width, depth, and gradient of an undisturbed alluvial river has typically adjusted to 

prevailing discharge conditions and sediment loads such that no net erosion or deposition occurs 

over long time periods (Mackin 1948, Leopold and Maddock 1953). The dominant scouring 

process in alluvial rivers is lateral migration (Exhibit 4.2). This process is responsible for the 

stream meanders visible on many floodplains, and is accomplished by the progressive erosion of 

the outside bank of a river bend with concurrent deposition on the inside bank. Because erosion 

is generally matched by deposition in this process, channel dimensions do not change 

significantly over time, and the net result is migration of the channel across the floodplain. 

Stream channel meanders are characteristic of many alluvial rivers and are indicative of a graded 

stream.
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LT2ESWTR Toolbox Guidance Manual 4-16 April 2010

Exhibit 4.2 Generalized Depiction of Stream Channel Lateral Migration

(a) Map of a Stream Meander; (b) Crosssection of the Channel from A-A′ with 

Channel Positions at 2 Successive Times (t0, 

and t1); (c) Map of Stream Meander Showing 

Location After Migration. 
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LT2ESWTR Toolbox Guidance Manual 4-17 April 2010

Downcutting, another type of scour that can occur in fluvial environments, is the vertical 

erosion of the streambed. Downcutting is fairly uncommon in alluvial rivers except during floods 

or if the stream is not graded. The long-term dynamic equilibrium of a graded stream can be 

disrupted by a variety of changing hydrologic and geologic conditions and especially by 

anthropogenic activity. Human activities in a watershed or river channel may alter the conditions 

to which an alluvial river has become adjusted, initiating a period of readjustment marked by 

either progressive downcutting or aggradation (deposition). 

Urbanization generally increases the proportion of impervious surface in a watershed, 

increasing flood volumes during precipitation events because less water is able to infiltrate the 

land surface and recharge ground water (Jacobsen et al. 2001). Increased flood volumes may 

cause higher water levels in a river channel, increasing the shear stress on the channel bed and 

causing scour (Booth 1990). Downcutting may continue until the channel gradient, and/or 

channel dimensions, become adjusted to the new flooding regime. 

Impoundment is another activity that may disrupt the quasi-equilibrium state of a graded 

river and initiate readjustment of the channel. The sharp decrease in sediment supply, which 

commonly occurs subsequent to dam and reservoir construction, may initiate downcutting in the 

reach immediately downstream until the channel adjusts to the lightened sediment load. This has 

been observed downstream of many dams throughout the world. One of the most dramatic 

examples is the 7.5 meters of channel-bed degradation that occurred twelve kilometers 

downstream of the Hoover Dam after its completion in 1935 (Williams and Wolman 1984). 

The construction of artificial levees (raised banks along a stream channel) also may result 

in flooding downstream. Levees allow greater quantities of water to be carried by the stream, thus 

decreasing the probability of flooding in the vicinity of the levee, but increasing flood hazards 

downstream (Montgomery 2000). Even if flooding downstream does not result, the high flows 

downstream may cause downcutting of the river, removal of fine-grained bed material, and thus a 

threat to the protectiveness of a riverbank filtration system. Another possible effect of levees is 

an increase in sedimentation in the channel. Sediment that would otherwise be deposited on the 

floodplain may be trapped within the channel. This can raise the channel bottom and thus raise 

stream stage or the elevation of the water surface in the channel (Montgomery 2000). The 

consequences of this for a riverbank filtration system are variable. Increased sedimentation may 

lead to clogging and/or decreased well yields. On the other hand, higher stream stages may result 

in flooding and scour along certain portions of the river as the channel adjusts to a new 

equilibrium condition. Understanding the impact of current or planned upstream activities can be 

an important part of site selection for a riverbank filtration system. 

4.4.4.2 Unsuitable Sites

Some sites may be unsuitable for bank filtration credit due to the type of aquifer adjacent 

to the river. For example, a stream reach adjacent to a wellfield might be dredged for gravel 

mining. A system may choose to evaluate such situations on a site-by-site basis, however, except 

as specified in the LT2ESWTR, EPA does not require such evaluations or any particular 
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LT2ESWTR Toolbox Guidance Manual 4-18 April 2010

decisions made on the basis of such evaluations. EPA recommends, however, that this 

information be considered in order to ensure that bank filtration systems are protective of public 

health. 

Lower log removals are expected to occur during and shortly after floods because 

protective layers may be removed by flood scour. If such situations are expected to occur very 

frequently, and if a system cannot envision a way of managing the system so as to adequately 

protect its water supply during such events, sites on such rivers may be inappropriate for 

riverbank filtration. EPA recommends that the potential for scouring be considered during site 

selection. If a site that undergoes occasional scour is selected for riverbank filtration, the system 

may wish to locate its wells at greater than the required separation distance from the surface

water body, as discussed in section 4.5.2.2. Such a solution helps to ensure the protection of 

public health. 

The potential for scour can be evaluated first by examining the past frequency of high 

flow and flood events. Data on flood history and discharge is typically available from the U.S. 

Geological Survey, the Army Corps of Engineers, the U.S. Bureau of Reclamation and the 

Department of Homeland Security (formerly FEMA). State and county highway and 

transportation departments typically evaluate river scour to determine the safety of bridge 

supports. A more comprehensive evaluation of the potential for scour can be conducted when the 

effect of past and current human activities (as discussed in section 4.4.4.1) is considered in 

comparison to the history of flood events. 
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LT2ESWTR Toolbox Guidance Manual 4-19 April 2010

Sources of high flow and flood data 

United States Geological Survey (USGS) 

Main Page: http://water.usgs.gov

The National Flood Frequency (NFF) Program: 

http://water.usgs.gov/pubs/wri/wri024168/pdf/entirereport.pdf

A computer program developed by the USGS for estimating the magnitude and frequency of 

floods for ungaged sites. Since 1993, updated equations have been developed by the USGS for 

various areas of the nation. These new equations have been incorporated into an updated 

version of the NFF Program. 

USGS Fact Sheets (listed by state):

http://water.usgs.gov/wid/index-state.html

Includes NFF program methods for estimating flood magnitude and frequency (in rural and 

urban areas) for: AL, AZ, AR, CA, CN, HI, LA, MD, MO, NV, NM, NC, OK, SC, SD, TX UT, 

VT, VA, and WA. These fact sheets describe the application of the updated NFF Program to 

various waterways within the specific state. Includes maps of each of the above state’s 

hydrologic regions, as well as regression equations and statistics. 

WaterWatch: 

http://water.usgs.gov/cgi-bin/dailyMainW?state=us&map_type=flood&web_type=map

Map of current flood and high flow conditions in the United States. The map shows the location 

of streamgages where the water level is currently at or above flood stage () or at high flow 

(•). The high flow conditions are expressed as percentiles that compare the current (i.e., within 

the past several hours) flow value to historical daily mean flow values for all days of the year. 

The real-time data used to produce the maps have not been evaluated or edited.

Army Corps of Engineers 

Main Page: http://www.usace.army.mil/

Flood control and management pages. For example, river and reservoir reports including flood 

level data are available for the St. Louis district of Missouri (see example below) 

(http://mvs-wc.mvs.usace.army.mil/dresriv.html).

Mississippi River:

 

River Mile Gage 

Station

6am 

Levels

24-hr 

Change

National Weather Service 

River Forecast

Flood 

Level

Gage 

Zero

Record 

Level

Record 

Date

Next 3 

Days

Crest Date

309.0 Hannibal 

Dam 22 tw

16.9 -0.2 16.6 16.1 15.6 16.0 449.3 31.80 07/10/93

301.2 15.9 -0.3 15.8 15.3 14.7 16.0 446.1 29.58 07/16/93

 
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LT2ESWTR Toolbox Guidance Manual 4-20 April 2010

U.S. Bureau of Reclamation 

Main Page: http://www.usbr.gov/main/

Dams and Reservoirs Page: http://www.usbr.gov/dataweb/html/dam_selection.html

The project DataWeb provides the most current information on the bureau’s projects, facilities, 

and programs including dam and reservoir information for western states. This data can be 

obtained by selecting a dam or from the state and Region maps.

The Department of Homeland Security (FEMA)

Main Page: http://www.fema.gov/

Flood Hazard Mapping: http://www.fema.gov/fhm/

The flood maps describe where the flood risks are, based on local hydrology, topology, 

precipitation, flood protection measures such as levees, and other scientific data. Fee to obtain 

maps.

 

4.5 Design and Construction 

This section describes the type of wells eligible for bank filtration credits under the 

LT2ESWTR. Because specific well construction requirements (e.g., casing depths) vary by state 

and with geologic conditions, this guidance will address these issues only briefly where 

appropriate. Readers are referred to the agency within their state that makes regulations or 

recommendations regarding well construction for details on issues such as casing depths, annular 

seals, drilling methods, filter packs, etc. Other good general references on well construction 

include Driscoll (1986) and U.S. EPA (1975). 

4.5.1 Well Type 

Only vertical and horizontal wells are eligible for bank filtration credits. Other types of 

ground water collection devices may not provide adequate filtration of pathogens. For example, a 

spring box is a ground water collection device located at the ground surface and is designed to 

contain spring outflow and protect it from surface contamination until the water is used. Spring 

boxes are found where local hydrogeologic conditions have focused ground water discharge into 

a smaller area (i.e., a spring) and at a faster volumetric flow rate than elsewhere. Often, localized 

fracturing or dissolution-enhanced channels are the cause of the focused discharge to the spring. 

As noted in section 4.4.2.2, fractures and dissolution channels have significant potential to 

transport microbial contaminants. Thus, spring boxes are not eligible for bank filtration credit (40 

CFR 141.717(c)(6)). 

Infiltration galleries (or filter cribs) are also not eligible for bank filtration credits. 

Infiltration galleries are designed to collect water infiltrating from the surface, or to intercept 

ground water flowing naturally toward surface water, using a slotted pipe installed horizontally in 

a trench and backfilled with granular material (Symons et al. 2000). An infiltration gallery is not 
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LT2ESWTR Toolbox Guidance Manual 4-21 April 2010

bank filtration even if the material overlying an infiltration gallery is engineered to optimize 

oocyst removal. Bank filtration systems are defined as relying solely on the natural properties of 

the system to remove microbial contaminants. At least one infiltration gallery is associated with 

an outbreak of cryptosporidiosis in the United States (also British Columbia, Canada and Japan). 

The Medford and Talent, Oregon outbreak resulted from treated (filtered) water taken from an 

infiltration gallery intake buried under Bear Creek, Talent, Oregon (Leland et al. 1993). An 

infiltration gallery may, however, be eligible for Cryptosporidium removal credit through a 

demonstration of performance under 40 CFR 141.718(c). 

Horizontal and vertical wells are both eligible for bank filtration credits. They are 

distinguished from each other by the orientations of their well screens, and the important 

implications this has for their well hydraulics (Exhibit 4.3 and 4.4). Collector horizontal wells are 

constructed by the excavation of a central vertical caisson or pipe. One or more laterals (i.e., 

collector lateral well screens) extend horizontally from the caisson bottom and may be very long. 

Laterals may extend radially in all directions - resulting in a radial collector well- or primarily in 

the direction of the river (Driscoll, 1986; Ray, 2001a). The lateral well screens are often installed 

near the bottom of the formation, allowing a greater proportion of the saturated thickness of the 

aquifer to be used. A greater proportion of pathogens and other contaminants are removed when 

the distance between the surface water body and the laterals is increased (Ray 2001a). Section 

4.5.2.2 contains a discussion of when it may be appropriate to locate wells at separation distances 

greater than those required by the LT2ESWTR. Laterals may extend underneath a surface water 

body in the United States. This is generally not how horizontal wells are placed in Europe (Ray 

2001a) because in Europe such wells are required to meet a 55-60 day average travel time 

requirement. An example of a pump house for a horizontal collector well in Louisville, 

Kentucky, is shown in Exhibit 4.5. It is elevated to prevent flood waters from entering it.
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LT2ESWTR Toolbox Guidance Manual 4-22 April 2010

Exhibit 4.3 Schematic Showing Generalized Flow and Required Separation 

Distance to a Vertical Well

Note that the exhibit shows tortuous ground water flow at the micro-scale.
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LT2ESWTR Toolbox Guidance Manual 4-23 April 2010

Exhibit 4.4 Schematic Showing Generalized Flow and Required Separation 

Distance to a Horizontal Well With Three Laterals

 
4. Bank Filtration

LT2ESWTR Toolbox Guidance Manual 4-24 April 2010

Exhibit 4.5 Taking a Water Level Reading

The pump house for the horizontal collector well caisson is in the background.

The choice between using a vertical or horizontal well for a bank filtration system 

depends on the site hydrogeology and the pumping requirements. For systems with large 

production requirements or for pumping in shallow alluvial aquifers, horizontal wells may be 

preferred because they are designed to capture large volumes of surface water recharge with little 

drawdown (Driscoll 1986). Vertical wells with large production requirements are not well suited 

to shallow alluvial aquifers because the necessary low drawdown cannot be sustained (Ray 

2001a). 

Finally, a comparison of construction expense with the costs of well maintenance may 

play a role in the choice of well type. Horizontal collector wells are substantially more costly 

than vertical wells (Driscoll 1986). However, moderately large utilities may need many smaller 

capacity vertical wells to match the capacity of a horizontal well. The maintenance of these 

vertical wells may require significant effort and expense (Ray 2001a). In such cases, horizontal 

collector wells may be preferred.
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LT2ESWTR Toolbox Guidance Manual 4-25 April 2010

4.5.2 Filtrate Flow Path and Well Location 

For systems to receive Cryptosporidium log removal credits, the ground water flow path 

length between the edge of the surface water body and the well should be sufficient for effective 

oocyst removal. This section discusses EPA’s requirements for appropriate flow path lengths and 

associated well locations for the log removal credits available under the LT2ESWTR. The 

ground water flow path length necessary to receive credits is specified for both vertical and 

horizontal wells. A discussion of how to obtain information necessary to define the edge of the 

surface water body is also included. 

4.5.2.1 Required Separation Distance Between a Well and the Surface Water Source

Cryptosporidium oocyst removal may vary significantly throughout the year in many 

bank filtration systems. At most typical bank filtration locations, high log removal rates (e.g., 3.5 

log removal over 13 m) may be expected with the surface water discharges that predominate 

during most of the year. During short flood periods, however, there may be substantially lower 

removal (e.g., 0.5 to 1.0 log removal over 13 m) due to scouring of the surface water–ground 

water interface. A number of factors may contribute to increased risk of Cryptosporidium

reaching wells. These factors include the presence of coarse-grained aquifer or stream bed 

sediments, high river velocities, and frequent scouring of riverbeds. Given the need to protect 

water supplies during periods of high surface water discharge with their potentially lower log 

removal capabilities, the LT2ESWTR rule language (40 CFR 141.717(c)) provides 0.5 log 

removal credit for systems with bank filtration wells located greater than 25 feet from a surface 

water source and 1.0 log removal credit for wells located greater than 50 feet from a surface 

water source. 

4.5.2.2 Locating Wells at Greater than Required Distances from the Surface Water 

 Source

Given the dynamic nature of riverbanks and aquifer systems, including scouring 

processes it may sometimes be advisable to place bank filtration wells at distances greater than 

25 or 50 feet from a surface water source. This extra precaution may also be advisable when a 

system is uncertain as to whether the riverbed and bank contain sufficient fine-grained material to 

provide adequate removal of Cryptosporidium oocysts. That is, EPA is requiring the separation 

distances of 25 feet and 50 feet for the log removal credits discussed above, but greater 

separation distances may result in additional public health protection at some sites. As discussed 

in Section 4.3.1.4, longer flow paths may result in changes in the water oxygen content that may 

be advantageous for iron and manganese removal. The disadvantage of using greater separation 

distances between the surface water source and the bank filtration well is that water yields to the 

well will be decreased. When a system makes a decision to place wells at a greater distance from 

a surface water source than EPA requires, it will need to balance the sacrifice in well yield with 

the added public health protection. 
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LT2ESWTR Toolbox Guidance Manual 4-26 April 2010

The remainder of this section discusses geophysical methods which may be of used to 

construct a conceptual model of subsurface flow conditions in riverbank filtration systems. By 

obtaining hydrogeologic information through geophysical or other means (e.g., pre-existing 

hydrogeologic or geologic maps), systems can determine the degree to which local conditions 

may affect Cryptosporidium removal at the bank filtration site. For example, if mapping the 

bedrock-alluvial interface and the water table at a particular site indicates that the aquifer is fairly 

thin, it is unlikely that infiltrating river water will be diluted by much ambient ground water. In 

such a case it may be advisable to locate wells at greater than the required distance from the 

surface water source. On the other hand, if detailed hydrogeologic investigations indicate that the 

aquifer contains a large proportion of fine-grained sediments, it would not be advisable to locate 

the well at greater than the required distance from the surface water source, because the aquifer is 

already likely to be an efficient pathogen filter, and it would be inadvisable to further sacrifice 

well yields. 

When the aquifer contains fine-grained material, it is possible that well over-pumping 

may break the hydraulic connection between ground water and surface water, yielding a variably 

saturated zone underneath a perched stream, as shown in Exhibit 4.6. Formation of such a 

variably saturated zone during periods of high pumping can greatly alter the existing ground 

water flow paths. New ground water flow paths could result in marked changes in water quality. 

For example, surface water infiltration could occur further upstream, resulting in a longer ground 

water flow path for infiltrating surface water flowing towards the well. The increase in flow pathlength could improve water quality. Alternatively, over-pumping can decrease water quality. This 

may occur because the decreased thickness of the saturated aquifer - due to the formation of a 

large variably saturated zone - may cause faster ground water flow (assuming pumping rates 

remain constant). Faster ground water flow provides less time for contaminant attenuation within 

the aquifer. Finally, the variably saturated zone itself, to the extent that it transmits water, can 

improve water quality because contaminant attenuation is usually increased under variably 

saturated conditions. If possible, the potential for formation of a variably saturated zone should 

be investigated to provide additional information regarding the desirability of locating wells at 

greater than required distances from the surface water source.

Exhibit 4.6 The Streambed of a Perched Stream Is Well above the Water 

Table
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LT2ESWTR Toolbox Guidance Manual 4-27 April 2010

Geophysical methods generally do not disturb subsurface materials. They are often less 

expensive than labor-intensive digging of trial pits or drilling of boreholes. Furthermore, the 

useful information gleaned by using geophysical methods can aid in choosing the best locations 

for wells (Reynolds 1997). Geophysical methods include gravity and magnetic methods, seismic 

methods, electrical methods, and ground penetrating radar (GPR). 

Hydrogeophysical methods can be used in pre-existing boreholes, thereby providing high 

resolution data for a very localized area around the borehole. Alternatively, surface geophysical 

methods can be used to obtain more generalized information over a large area, including 

information on the depth to the water table, the depth to bedrock, and stratigraphy (Hubbard, 

2003). The discussion below provides only a generalized overview of currently available 

geophysical methods. More detailed information can be obtained from texts by Hearst (2000), 

Reynolds (1997), Rubin and Hubbard (2003), Keys (1990) and Burger (1992). 

Gravity surveying measures variations in the acceleration due to the Earth’s gravitational 

field which are caused by density variations in subsurface rocks. Subsurface cavities can be 

detected with this technology; however, sites with such cavities would not be suitable for bank 

filtration. Reynolds (1997) states that gravity methods are fairly uncommon in hydrogeological 

work compared to electrical methods. On the other hand, in the Arizona district of the USGS, 

gravity methods have been in use for over 15 years to evaluate changes in water storage in 

aquifers. These methods can detect water table elevation changes of as little as a few inches 

(Callegary 2003). Thus, gravity methods may be useful at riverbank filtration sites for assessing 

the depth to water table, aquifer thickness, and seasonal effects on the dilution of infiltrating river 

water with ambient groundwater. Magnetic surveying or magnetic anomolies can also be used in 

hydrogeologic investigations. For example, clay infilling bedrock cavities can be detected due to 

slight changes in the magnetic susceptibility of clay and most bedrock (Reynolds 1997). 

Seismic methods are widely used in hydrogeologic investigations. Applied seismology 

involves generating a signal through an explosion or other method at a specific time. The 

generated seismic waves travel through the subsurface, are reflected and refracted back to the 

surface, and the return signals are detected on monitoring instruments. The amount of time that 

elapses is the basis for determining the nature of subsurface layers/materials (Reynolds 1997). 

Reynolds (1997) provides a detailed example of the use of seismic refraction surveying for 

locating the bedrock/alluvial interface at one particular site.

Seismic methods can be used to: 

• Estimate depth to bedrock (ideal for riverbank filtration applications). 

• Determine the nature of bedrock (e.g., cavernous) or location of cavities. Note that karst 

buried by alluvium may contain unexpected ground water flowpaths. 
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• Determine the location of faults that may juxtapose bedrock against alluvial material.

• Determine stratigraphy (useful where sands and clays may be interlayered). 

• Estimate porosity prior to coring. 

• Determine ground water particle velocities (an important parameter for riverbank 

filtration systems). 

Electrical resistivity methods are used extensively in downhole logging to identify 

hydrogeologic units that will produce high flow rates. Electrokinetic surveying makes use of 

electrodes implanted at the ground surface to identify the location of the water table. This may be 

useful at riverbank filtration sites, where water table layer and depth to bedrock can be used to 

determine aquifer thickness - an important parameter in determining how much dilution of bank 

filtrate with ambient groundwater is occurring. A more recent development is the use of 

electrokinetic methods to measure flowrates in boreholes (Reynolds 1997). 

The spontaneous polarisation or self-potential (SP) method is conducted by measuring 

differences in ground electrical potential at different locations, but is still fairly uncommon. 

Another electrical method, the induced polarisation (IP) method can be used to detect ground 

water and water tables, however electromagnetic (EM) induction methods are generally 

considered more practical for these purposes in the field. Contaminated ground water within 

subsurface clays can also sometimes be detected with the IP method (Reynolds 1997). 

EM methods have been used in groundwater investigations to delineate contaminant 

plumes, and thus can be useful in conceptualizing flow systems in a riverbank filtration context if 

there is a significant contrast between river water and ambient ground water. Pulse-transient EM 

(TEM) surveys (a type of EM method) may be useful in conceptualizing flow for riverbank 

filtration. It may also be useful in monitoring the quality of infiltrating water. When data is 

available from both borehole and surface instruments, EM and electrical methods can be used to 

map subsurface geology such as the locations of coarse-grained and fine-grained units. 

GPR has been used as a surface method for contaminant plume mapping and monitoring 

pollutants in groundwater. To operate such a system, a signal generator, transmitting and 

receiving antennae, and a receiver should be used. Generated radiowaves travel in a broad beam 

at high speeds. Energy is lost or attenuated depending on the subsurface materials through which 

the waves travel. GPR has proven valuable in mapping sediment sequences, and can be used to 

investigate sediments through freshwater up to 27 m deep (Reynolds 1997). Thus, it may be of 

use in gaining information about the composition of riverbeds, and for monitoring the effects of 

scour on riverbed composition. GPR can also be used to locate water tables, delineate 

sedimentary structures which may contain pockets of coarse-grained alluvium, and determine the 

spatial extent and continuity of buried clay and peat layers within subsurface deposits. Borehole 

radar can also be used for hydrogeologic investigations.
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Before choosing a specific geophysical method the following should be considered: 

desired level of resolution, area of coverage, site-specific conditions and their influence on the 

applicability of the method, possible non-uniqueness of the geophysical attribute, resources 

needed to interpret the geophysical data, and possible integration with direct measurements. In 

general, mapping the water table and finding the depth to bedrock are considered standard 

hydrogeophysical procedures. Other applications such as estimating permeabilities or porosities 

are at an earlier stage of development and may not yet be appropriate for routine use at riverbank 

filtration sites (Hubbard 2003).

4.5.2.3 Delineating the Edge of the Surface Water Source

The flow paths due to induced infiltration to a vertical well have both vertical and 

horizontal components, and are tortuous at the micro-scale. Such flow will typically have a 

significant horizontal component, especially if the vertical well is screened in a shallow, 

unconsolidated, alluvial aquifer that is eligible for bank filtration credits. Therefore, for the 

purpose of receiving log removal credits, the flow path length to a vertical well is to be 

determined using the measured horizontal distance from the edge of the surface water body to the 

well screen. The edge of the surface water body is defined as the edge of either the 100-year 

floodplain or the floodway, discussed below. The 100-year floodplain is defined by its boundary - 

the flood elevation that has a one percent chance of being equaled or exceeded each year.

As a first step, utilities may use the online maps available at the following website to get 

a general idea of the mapped extent of the 100-year floodplain in their area: 

http://www.esri.com/hazards/makemap.html. In order to satisfy the requirements of the 

LT2ESWTR for the location of the wells of a bank filtration system, however, an official FEMA

(now part of the Department of Homeland Security) flood hazard map must be used. Such maps 

can be ordered in either paper or digital formats from FEMA. The following web site can be used 

to order these maps: http://www.fema.gov/hazard/map/flood.shtm. 

For many areas, the mapped extent of the floodway will also be drawn on the flood 

hazard map obtained by FEMA. The utility may choose to use the edge of the floodway rather 

than the edge of the 100-year floodplain for the purpose of determining the required separation 

distance between a river and a riverbank filtration well. If the mapped extent of the floodway is 

unavailable, the utility may opt to perform the mapping using one of a number of hydraulic 

models approved by FEMA. A list of these approved models is available at 

http://www.fema.gov/plan/prevent/fhm/en_hyala.shtm. EPA recommends using the US Army 

Corps of Engineers’ Hydrologic Engineering Centers River Analysis System (HEC-RAS) model 

for mapping floodway limits. The HEC-RAS software is available for free downloading from 

http://www.hec.usace.army.mil/software/hec-ras/hecras-download.html. The user's manual, 

applications guide, and hydraulic reference manual are available at 

http://www.hec.usace.army.mil/publications/pub_download.html. 

When a utility elects to determine the edge of the floodway and to model the floodway 

boundaries if they are not available from FEMA, the preferred encroachment method within 
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HEC-RAS is Method 4. Method 4 can be summarized as follows, according to FEMA’s Map 

Assistance Center (2003): 

The Method 4 encroachment operates by analyzing the hydraulic conveyance for the 

unencroached one percent annual chance floodplain at each cross section, then 

equally reducing the conveyance from both overbank areas by moving toward the 

stream channel from the edge of the floodplain until the resulting water-surface 

elevation is one foot higher than the unencroached elevation, and the resulting 

encroached conveyance is approximately equal to the unencroached conveyance. The 

new left and right cross-section limits are assumed to be vertical walls. Finally, a 

backwater energy balance is calculated using the new cross sections, which results in 

the encroached or floodway water-surface profile. The floodway modeling process 

requires adjustments and rerunning of the model because the final calculation is the 

backwater energy balance between new cross sections. Many times the 1.0-foot target 

cannot be achieved exactly at each cross section because of energy balance 

considerations. Floodplain geometry, constrictions at culvert and bridge crossings, 

and constrictions from other man-made obstructions in the floodplain may require 

adjustments to the encroachment widths to stay at or below the 1.0-foot maximum 

water-surface increase. Chapter 10 of the HEC-RAS User's Manual includes a 

discussion of performing a floodway encroachment analysis. 

In most areas, however, EPA expects that utilities will find it preferable and simpler to 

use the previously mapped limits of the 100-year floodplain to determine the edge of the river for 

riverbank filtration separation distances.

Although in some areas of the United States the mapped extent of the 100-year floodplain 

may be more easily accessible than the mapped extent of the floodway, some utilities may choose 

to use the edge of the floodway as a starting point for measuring separations distances to wells. 

This is because it typically allows wells to be placed slightly closer to the river and is thus a 

somewhat less restrictive requirement. The floodway is a regulatory concept, and is defined as 

that portion of the overbanks that must be kept free from encroachment to discharge the one 

percent annual chance 

4.5.2.4 Measuring Separation Distances for Horizontal Wells and Wells that are 

 Neither Horizontal Nor Vertical

As noted in section 4.5.1, horizontal wells may have laterals that extend underneath a 

surface water body. The flow direction for induced infiltration to a horizontal well that extends 

under a surface water body is predominately downward. Therefore, the flow path length to a 

horizontal well is the measured vertical distance from the bed of the river under normal flow 

conditions to the closest horizontal well lateral’s intake. 

Some wells may be constructed so that the well is neither truly horizontal nor truly 

vertical. In these cases, there is greater uncertainty about the definition of separation distance 
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from surface water. For simplicity, if the well inclination is closer to being a vertical well than to 

being a horizontal well (i.e., the well is oriented at greater than a 45 degree angle to a horizontal 

line), the separation distance is defined for the purposes of this toolbox option to be the 

horizontal distance from the edge of the river to the closest (in terms of horizontal distance) 

intake on the well. Similarly, if the well is closer to being a horizontal well as opposed to a 

vertical well, separation distance is defined as the shortest possible vertical distance from the 

riverbed to an intake on the well. 

 4.6 Operational Considerations 

4.6.1 High River Stage 

When the river stage (i.e., the elevation of the water surface) is high, the increased head 

gradient between the river and the adjacent aquifer results in increased infiltration and increased 

ground water flow rates. This condition can be expected to occur periodically throughout the year 

at many sites, and will generally be associated with reduced log removals (Gollnitz 1999, Ray 

2001b, Rohns et al. 2006). High river stage is often associated with scouring of riverbed 

sediments. Nevertheless, even when scour does not occur, the high ground water velocities 

associated with high river stage can be a significant threat to a riverbank filtration system. 

One solution to this problem is that pumping rates can be temporarily decreased during 

periods of high river flow (Medema et al. 2000). Decreased pumping rates will in turn decrease 

the head gradient between the river and the well, thereby decreasing subsurface velocities, 

increasing residence times, and facilitating pathogen inactivation. 

4.6.2 Implications of Scour for Bank Filtration System Operations 

Periodic, short-term flood scour can have both negative and positive impacts on the 

performance of a bank filtration system. As noted previously lower log removals of oocysts are 

expected during floods because higher river shear velocities and associated increases in bedload 

transport mobilize fine sediments deposited when discharges were lower. 

Removal of fine sediments opens large pore spaces, increasing the hydraulic conductivity 

across the surface water–ground water interface (Gollnitz 1999, Ray 2001a, Ray 2001b). 

Unfortunately, this potentially increases the number of pathogens transported. Furthermore, the 

microbial activity and unique geochemical environment of the riverbed, which serves to facilitate 

the removal of pathogens via sorption and other processes, may not be present for short periods 

following flood scour. Recent work in Germany (Baveye et al. 2003) suggests that the 

biologically active zone is re-established very quickly after scour, perhaps within 3 days, at least 

when measured in terms of the ability to degrade certain organic compounds. Limited scour can 

reduce clogging at the surface water–ground water interface and improve well yields (Wang et al.

2001). 
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When high river stages or high turbidity levels indicate that flood scour may be occurring 

and compromising the effectiveness of a bank filtration system, pumping rates can be decreased. 

This will lead to lower velocities and longer subsurface residence times, thereby increasing the 

protectiveness of the system (Medema et al. 2000, Juhasz-Holterman 2000). 

4.6.3 Anticipating High Flow Events / Flooding 

Many factors can affect the probability of flood events. Intense rainfall is the most 

apparent factor, however the geomorphology of a watershed is important in determining how 

quickly water will enter a stream system after a rainfall event, as well as how quickly water will 

enter a major river from smaller tributaries. Systems can anticipate that a high flow event will 

occur if a rapid spring thaw follows a winter of unusually heavy snowfall. It is also important to 

be aware of recent changes in vegetation due to wildfires or urbanization. When vegetation is 

removed or decreased, there are fewer barriers to rapid surface runoff, plant roots no longer keep 

soil loose and permeable (thus more compact soils will be less able to decrease surface runoff), 

and plants themselves will be unavailable to take in a certain proportion of precipitation 

(Montgomery 2000). Therefore, systems may wish to monitor for pathogens more frequently or 

change pumping regimes in riverbank filtration systems when high flows are anticipated. 

4.6.4 Possible Responses to Spill Events and Poor Surface Water Quality

One response to a serious water quality threat is to stop pumping from all bank filtration 

production wells. Other pumping regime changes can also be implemented to reduce risks, 

including decreasing the number of hours the system is in operation each day. For systems that 

have a number of wells in operation, it may be advisable to increase pumping rates for wells 

further from the surface water source and decrease pumping rates for wells that are closer 

(Juhasz-Holterman 2000). Juhasz-Holterman (2000) recommended that this kind of change be 

implemented seasonally at a site in the Netherlands. Her study of the site’s hydrology indicated 

that during the winter months, wells were more vulnerable to contamination due to “shortcircuited” flow paths from the polluted river through the subsurface. Her solution involved both 

restricting extraction rates to a few hours a day (which was acceptable due to decreased demand 

during the winter months) as well as an altered pumping regime which relied more on wells 

located further from the river. In general, systems that receive water from multiple pumping 

wells should manage their well field so as to maximize the water residence time in the 

subsurface, to the extent possible, while meeting changing water quantity demand. Methods to 

evaluate subsurface residence time are discussed in Section 4.7, Demonstration of Performance.

4.6.5 Maintaining Required Separation Distances

Alluvial rivers that are experiencing active, progressive erosion as an adjustment to new 

flooding regimes or sediment loads, or in relation to natural lateral migration, may pose serious, 

longer-term challenges to bank filtration systems. For example, significant log removal 
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reductions may be more frequent in an urbanizing basin as a consequence of more frequent 

flooding and associated scouring. In extreme cases, long term degradation of the bed or banks 

may reduce the threshold separation distances between the surface water source and bank 

filtration well. Recall that these separation distances - 25 feet for 0.5 log removal credit and 50 

feet for 1.0 log removal credit - are required to receive log removal credits under the 

LT2ESWTR.

Systems may wish to assess their sites for active, progressive erosion. Lateral migration 

rates can be calculated using sequential aerial photography and/or topographic maps, if available. 

Systems without such data may need to obtain the needed information by conducting sequential 

field surveys of the floodplain area proposed for the site. This will require a far more lengthy 

investigation period. Progressive downcutting could also be measured with sequential field 

surveys of the channel bed elevation over a period of years. Regardless of the method used, the 

threshold separation distances between the surface water source and the bank filtration well must 

be maintained.

4.7 Demonstration of Performance 

PWSs using GWUDI as their source water may receive 0.5 or 1.0-log Cryptosporidium

removal credit based on well siting and aquifer critera as described in section 4.2. Alternatively, 

PWSs may apply to the state for Cryptosporidium treatment credit using a DOP. States may 

award greater than 1.0-log Cryptosporidium treatment credit for bank filtration based on a sitespecific demonstration. States may also award DOP Cryptosporidium treatment credit based on a 

site-specific study to systems that are unable to qualify for the 0.5 or 1.0-log removal credit as 

described in section 4.2.1. 

PWSs using existing bank filtration as pretreatment to a filtration plant are not eligible to 

receive additional treatment credit for bank filtration. In these cases, the performance of the bank 

filtration process in reducing Cryptosporidium levels will be reflected in the monitoring results 

and bin classification under the LT2ESWTR.

For a bank filtration DOP study, the following criteria must be met:

• The study must follow a state-approved protocol and must involve the collection of data 

on the removal of Cryptosporidium or a surrogate for Cryptosporidium and related 

hydrogeologic and water quality parameters during the full range of operating conditions. 

• The study must include sampling both from the production well(s) and from monitoring 

wells that are screened and located along the shortest flow path between the surface water 

source(s) and the production well(s). 

The purpose of this section is to provide additional guidance on the design and conduct of 

a DOP study, as well as guidance on the interpretation of the study data and the award of 
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Cryptosporidium removal credits if warranted. Finally, this section describes the necessity for 

long term performance evaluation monitoring to determine if the log removal credit continues to 

be appropriate. 

4.7.1 Identification of Collection Devices and Alternative Treatment Technologies at the Site

Prior to initiating a DOP study, it is necessary to 1) identify all of the treatment 

technologies and collection devices in use and 2) design a study tailored for each treatment 

technology and collection device as a separate unit process. Evaluating each unit process and 

device separately will enhance knowledge about Cryptosporidium removal at the site.

A PWS may operate one or more vertical wells, horizontal collector wells (caisson wells),

infiltration galleries or spring boxes. Wellfield management operations may include surface 

water diversion into recharge basins or injection of treated or untreated surface water into the 

subsurface. Bank filtration relies upon the nature of the undisturbed (by humans) subsurface 

materials to provide natural filtration, and is relatively unmodified by application of engineered 

structures, flow control or engineering operations such as surface water basin recharge. When 

other technologies, such as artificial recharge, or differing collection devices, such as infiltration 

galleries are used, the treatment processes may vary spatially or temporally. 

The physico-chemical removal processes utilized by the various well field management 

technologies and collection devices will differ. Depending upon the site characteristics and 

recharge operations, an artificial recharge technology may be influenced by intermittent recharge, 

pre-treatment, unsaturated flow and transport, biofiltration, chemical precipitation by oxidation 

and reduction, or other phenomena. The removal processes may operate with significantly 

different removal rates among the differing technologies or collection devices. For example, 1) 

oocyst attachment to the gas-water interface may be a significant removal process in an 

intermittent artificial recharge technology, but will be absent or insignificant in a bank filtration 

system where unsaturated conditions never occur; or 2) a shallow infiltration gallery might 

operate under aerobic conditions only, as compared with a deeper vertical well that might operate 

at times under anaerobic conditions. As discussed in section 4.3.1.4, oxidation and reduction 

reactions may govern Cryptosporidum or especially surrogate transport through the subsurface. 

Thus, each treatment unit process and collection device should be evaluated separately in the 

DOP study. 

 Wells, both horizontal and vertical, that are significantly influenced by surface water 

spreading operations may be considered to be artificial recharge alternative technology if a large

component of their yield results from artificial recharge rather than from bank filtration. As part 

of the study design, a putative assignment of alternative treatment technology type should be 

made for each collection device. Some wells may have high uncertainty about which alternative 

treatment technology (bank filtration or artificial recharge) is appropriate. For these wells, the

study design should include elements suitable to both technologies. For example, spiking studies 

may be appropriate for artificial recharge but not for bank filtration studies. Such spiking studies 
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should be planned for those wells for which there is high uncertainty about the appropriate 

alternative treatment technology.

4.7.2 Source Water Quality and Quantity 

Water quality, flow, and flood data should be compiled to characterize possible worst 

case scenarios. Flooding can, for example, dilute pathogenic contamination in unsewered areas 

with minimal agricultural activity or can increase contamination where combined sewer 

overflows are present. As discussed earlier in this chapter, flood scour can have a deleterious 

effect by removing the protective fine-grained material in the riverbed.

The DOP study should identify and investigate the temporal variability of upstream point 

and non-point source dischargers and pathogen concentrations. The study should compile 

historical data and collect new data to determine the quality of the in-stream flow adjacent to the 

well field and elsewhere upstream, focusing primarily on cyst and oocyst concentrations and their 

most appropriate indicators in surface waters under a variety of surface water flow conditions. 

Surface water samples should be composite samples representative of the water quality in 

differing stream tubes both laterally and vertically within the river (because river water tends to 

remain unmixed, river flow is idealized as flow in a composite of parallel tubes). River water 

samples should be proportionately representative of the actual flow conditions based on the 

historic record and should be collected during both low water and high water stages (if safety 

conditions permit) as well as under normal conditions. 

4.7.3 Ground Water Travel and Residence Time Calculations and Ambient Ground Water 

Dilution

The study should determine the time lag due to travel between the surface water source

and the wells or other collection devices. To accurately calculate pathogen removal, the

approximate lag times are necessary because variable pathogen concentration in surface water 

will affect the removal calculations. Approximate lag times can be determined by collecting 

suitable site-specific parameter data such as surface water and ground water temperature, 

chloride and/or bromide concentrations, and then refined using the most appropriate site-specific 

parameters.

Environmental tracer data (isotopes, Chlorofluorocarbons (CFCs), pharmaceutical 

compounds, etc.) can be collected to verify lag times calculated using temperature, chloride and 

other parameters. Samples from collection devices will typically be mixtures of induced stream 

water and ambient ground water. The ambient ground water may have subsurface residence times 

of months, years or decades and therefore, may have very low concentrations of pathogens and 

indicators. The collection device sample concentrations should be corrected for ambient ground 

water dilution before determining removal by artificial recharge or bank filtration. Ground water 

flow models, water quality data and environmental tracers can be used to determine the amount 
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of ambient ground water dilution for each well or cluster of wells. Because ambient ground water

dilution varies depending on the well pumping rate and/or schedule, the dilution should be 

calculated using several well pumping scenarios. 

The DOP should recognize that it is important to document the ground water flow paths 

contributing to a collection device to improve confidence in the assigned pathogen removal credit 

for each device. For example, a well providing water which is typically 80 percent ambient 

groundwater and 20 percent naturally-filtered surface water may appear to have adequate

pathogen removal when, in fact, dilution by uncontaminated groundwater is the major factor 

resulting in low pathogen concentrations in the produced water. If ambient ground water dilution 

changes significantly (e.g., 20 percent ambient ground water and 80 percent naturally-filtered 

surface water), this well could demonstrate markedly different pathogen removal efficiencies. For 

public health protection, it is essential to ensure that the well will provide 99 percent

Cryptosporidium removal from the surface water through the alternative filtration technologies, 

without dilution by ambient ground water. 

Ground water flow models with particle tracking capability can be used to calculate travel 

times, ground water residence times, lag periods and ambient ground water dilution (e.g., AbdelFattah et al. 2007). Ground water flow model calculations can be improved if site-specific data 

are collected on the hydraulic conductivity of the streambed using seepage meters or other 

technologies and by analysis of well core or cutting samples from the aquifer. Most recently, new 

models have been developed that can explicitly simulate ground water flow to a horizontal 

collector well (e.g., Bakker et al. 2005). If ground water flow models are used, then the study 

design should include the elements appropriate to standard ground water model quality 

assurance, including calibration, history matching, verification and sensitivity analysis. Although 

the dimensionality of the modeling is a site-specific decision, the groundwater flow models 

should have the capability to simulate mutually-interfering pumping wells with non-uniform 

surface recharge over the domain.

The concept of long and short flow paths to a well is illustrated by Gollnitz et al. (2005) 

for a site in Casper, WY, and was determined using a particle tracking ground water flow model. 

However, at the Casper site, the shortest flow path to a collection device is from the surface to 

the infiltration gallery, which is located at a depth of 15-20 feet below the bottom of the recharge 

basins (Gollnitz and Clancy 1994). Comparison of biological particle data among two 30 foot 

deep vertical wells, a 30 foot deep horizontal collector well and the infiltration gallery shows 

poorest removal in all samples collected from the infiltration gallery (Gollnitz et al. 2005, Table 

2), suggesting that there is a correlation between flow path-length and removal efficiency.

4.7.4 Surface and Ground Water Data Collection, Methods and Sampling Locations 

As required by the LT2ESWTR, the DOP study must include the collection of 

hydrogeologic and water quality parameter data during the full range of operating conditions. 

Thus, the paired surface water and ground water samples should be collected, at a minimum, 

monthly for eighteen months to capture both high flow and low flow events over a long time 
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period. Sampling for eighteen months insures that at least one wet season or one dry season is 

sampled twice. Unusual conditions associated with a wet or dry season are less likely to re-occur 

during the second sampling period. 

EPA recognizes that not all wells in a well field are equally at risk from Cryptosporidium. 

Higher risk wells are those that receive surface water that has the shortest ground water residence 

time (or flow path-length). Sampling should be more intensive at higher risk wells (e.g., Gollnitz 

et al. 2005). 

 Data can be collected on an ongoing basis or it can be event based, with the intention of 

capturing the worst case possibilities due to floods. Safe sample collection procedures however, 

should be observed at all times. Ongoing data collection takes place over a longer period of time, 

but tends to provide an average characterization. Event based sample collection is a high 

frequency of monitoring over short periods of time to characterize the worst case scenario. The 

monitoring strategy should try to maximize the possibility of capturing infrequent events without 

sacrificing long-term characterization of average conditions. An appropriate data collection

strategy could include periodic data that is collected on an ongoing basis as well as data collected

during and after flood events (with consideration of appropriate lag time for ground water 

samples), with the intention of capturing the worst case possibilities due to floods. Thus, all data 

should be collected periodically during normal flows but at a higher monitoring frequency over 

short-term, high water periods to characterize the potentially worst case scenario. Late summer or 

drought low flow conditions should also be more intensively sampled if the low water levels 

represent a possible worst case scenario.

 The study design should ensure that the number and location of river water samples 

collected are representative of high and low consumptive use (e.g., pumping for drinking water 

supply, and irrigation) periods. River water samples should be representative of the entire river 

volume, rather than consisting only of samples collected at the surface water intake for the 

treatment plant. If point sources discharge upstream and the stream is not well mixed, then the 

river samples should be proportionate in number and location to the volume of the highly 

concentrated plumes emanating from the point sources. 

 

 As discussed above, the study should be designed to determine proportions of each of the 

unit processes operating at the site (ambient ground water, artificial recharge water and bank

filtrate water) and contributing to a collection device. Typically, identification of ground water

sources is accomplished by compilation of historical ground water geochemical data and 

measurement of major and trace elements, isotopes and other environmental tracers, interpreted

with the assistance of geochemical and ground water flow models. Suitable parameters measured

could include, but are not limited to, organic carbon, chloride, bromide, total dissolved solids 

(TDS), hydrogen, oxygen, uranium, and other isotopes, and CFCs. 

Data collection activities should be designed to ensure that the collected samples are

representative and random. Data analyses should include quantitative assessment of the 

uncertainty associated with each conclusion. Study design should include sufficient sample 
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numbers so as to determine statistical significance for each conclusion to a pre-determined

confidence level.

 The study design should also include a quality assurance project plan, identifying 1) 

reference to the analytical method and laboratory, 2) a reasonable number and percent of blank, 

replicate and spiked samples, 3) detection limits, and 4) sample holding times.

 The presence of multiple data collection wells can serve to increase confidence in the

conclusions. Monitoring well data (preferably from multiple wells) collected along the flow path 

should show a decrease in indicator concentration with distance from surface water to improve 

overall confidence that the measured log removal results are meaningful.

The DOP should determine the capture zone of each collection device and/or conduct dye 

trace studies from local sources such as septic tank leach fields to ensure that indicator organisms

are coming from the source water rather than from land-side septic tanks. Well-water counts of 

E. coli for example, that includes E. coli that originates at a nearby septic tank, rather than at the 

river, will yield lesser calculated removal efficiencies than the actual removal.

4.7.5 Monitoring Tools 

The DOP study should consist of monitoring for Cryptosporidium or a suite of 

Cryptosporidium surrogate organisms at each collection device (or device type cluster) and the 

source river water. Pathogen monitoring could also include Giardia and perhaps members of the 

Microsporidia family (Brusseau et al. 2005). In the absence of Cryptosporidum oocyst removal 

data (calculated using measurable oocyst concentrations in the river and in the collection device),

the DOP should use Cryptosporidium surrogate microorganisms and should demonstrate that 

removal of the recovered surrogate organism(s) would be similar to the removal of 

Cryptosporidium oocysts (to the extent possible using the scientific literature, laboratory and/or 

field studies).

Bank filtration efficiency can be meaningfully demonstrated and is permitted in a DOP 

only in porous media (similar in concept to slow sand filtration but without use of engineered 

materials, flow and flux control and active schmutzdecke management). Log removal calculations 

require counts per volume of the same organism in both surface water and nearby collection 

devices (wells). Comparison of the two values provides information on attenuation during 

subsurface passage. However it is important to calculate log removal only for microorganisms 

that are similar to Cryptosporidium oocysts. Log removal calculations for particles or organisms 

that significantly differ from oocysts in size, shape and porous media transport capability or have 

unknown original or final size and shape (and charge), such as turbidity, standard particle counts, 

and total algae, or larger organisms such as rotifers, crustaceans or fish are less meaningful and 

should not be used. Pumping wells generate turbidity in the aquifer as a result of pumping (van 

Beek et al 2010). Therefore, for groundwater, turbidity data are useful primarily for determining 

disinfectant treatment efficiency. It is not meaningful to count particles not known to originate in 

the surface water, as is the case for turbidity or standard particle counters.
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Cryptosporidium and Cryptosporidium surrogate organism transport in porous media has 

been well studied in both laboratory and field experiments (e.g., Schijven et al. 2003) and will 

not be detailed here. In general, microorganism and porous media grain size and shape are 

important parameters that govern removal efficiency together with grain coatings and water 

chemistry. Predictions without field measurements are highly uncertain. Thus, paired samples 

from surface water and ground water are necessary. 

When subsurface materials are coarse grained (e.g., gravel), ground water flow is 

relatively fast and bank filtration efficiency is significantly reduced. For example, in one study in 

a gravel aquifer (not a bank filtration site), aerobic spores traveled 90 m in about one day (Pang 

et al. 1998, Pang et al. 2005). For coarse grained aquifers, EPA recommends significant 

additional study (increased monitoring frequency of multiple microorganisms from multiple 

monitoring wells) to improve removal efficiency measurement at high ground water velocity. 

Surrogate microorganisms are more likely to be recovered from collection devices when 

the concentration in the surface water is high, such as during a diatomaceous algal bloom (e.g., 

Kearney, Nebraska) (Berger et al. 2002) or during high water stage. Eckert and Irmschser (2006) 

report E. coli recovery in Dusseldorf bank filtration wells only following a flood event. 

The choice of the appropriate suite of Cryptosporidium surrogate organisms is the most 

important element of a DOP study. Favorable surrogate organisms should be 1) equivalent in size 

and shape to Cryptosporidium oocysts (i.e., 4-6 μm and slightly oblate), 2) sufficiently numerous 

in both ground water and surface water so as to be suitable for log removal calculations (log 

removal calculations require counts per volume of the same organism in both surface water and 

nearby collection devices/wells); and 3) sufficiently long-lived in the subsurface (at least as longlived as oocysts) so that inactivation during subsurface passage does not significantly affect the 

calculation. 

The identification of Cryptosporidum oocyst surrogate organisms is based primarily on 

similarity in size and shape. Other factors such as total net charge or charge distribution on the 

outer surface of the microorganism are important elements governing Cryptosporidium transport 

in the subsurface. However, choice of surrogate organisms based on charge or factors other than 

microorganism size and shape is an important research topic (Tufenkji 2007, Tufenkji et al. 

2006) but the available information is insufficient for inclusion in this guidance. Exhibit 4.7 lists 

the size ranges of common pathogenic protozoa and surrogate bacteria. 

No single Cryptosporidium surrogate organism is best. Each organism has strengths and 

weaknesses. Multiple surrogates should be analyzed initially to ascertain which surrogate suite is 

best suited to the DOP at that site. Examples of surrogates include total aerobic bacterial spores 

(e.g., Bacillus subtilus), anaerobic bacterial spores (e.g., Clostridium perfringens and/or 

Clostridium bifermentans), total coliform, E. coli, enterococci bacteria, bacteriophage (e.g., 

Bacteroides phage), coliphage (male-specific and somatic), diatoms (Reilly et al. 2005) at the 

genus or species level, turbidity, particle counting and microscopic particulate analysis (MPA)

(U.S. EPA 1992, AWWA 1990). EPA recommends monitoring for at least three or four surrogate 
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LT2ESWTR Toolbox Guidance Manual 4-40 April 2010

organisms using paired surface water and ground water samples to calculate log removal 

efficiency.

As with any monitoring program, there is a trade-off between monitoring frequency and 

information cost. The cost for each Cryptosporidium surrogate assay varies between $50 and 

$250. EPA recommends that the less expensive assays, such as total aerobic spores, total 

coliform, and enterococci be performed more frequently and the more expensive assays, such as 

MPA, be performed at a lesser frequency. 

Exhibit 4.7 Size of Pathogenic Protozoa and Surrogate Bacteria

Protozoa Size (μm) Surrogate Bacteria Size (μm)

Cryptosporidium parvum

oocyst

(Xiao et al., 2000)

4.2-5.6 Total Coliform

(Holt, 1986)

~0.5-6.0

Giardia lamblia cyst

(WHO, 2004)

8-12 Escherichia coli

(vegetative cell form) 

(Foppen and Schijven, 2006) 

1.1-6.0

Cyclospora sp.

(Mota et al., 2000)

8-10 Clostridium perfringens 

(vegetative cell form)

(Holt, 1986)

2-19

Microsporidia

(Brusseau et al., 2005)

1-5 Clostridium perfingens spore

(Lund and Peck, 1994)

0.3-0.4

Clostridium bifermentans

(vegetative cell form)

(Holt, 1986)

1-11

Clostridium bifermentans spore

(Brock and Madigan, 1991)

1.2

Bacillus subtilus

(vegetative cell form)

(Holt, 1986)

2-5 

Bacillus subtilus spore

(Rice et al., 1996 and P. Payment, 

personal communication 

0.5-0.8

Cryptosporidium oocysts are slightly oblate with a length-to-width ratio that ranged, in 

one study, from 1.04 to 1.33 (Xiao, 2000). Aerobic spores are typically slightly oblate as well but 

smaller than oocysts, ranging from 0.5-0.8 microns in diameter as compared with 4-6 microns for 

oocysts. Bacterial vegetative cells of E. coli are slightly larger than aerobic spores but 

significantly differ in length to width ratios (2.0-6.0 μm × 1.1-1.5 μm, Foppen and Schijven 

2006). Futhermore, vegetative cells produce extracellular polymers, particularly if these cells 

form biofilms, and these polymers may significantly alter passage characteristics in the 

subsurface. The bacterial spore form is significantly longer lived in the environment (and 

especially the subsurface) than the vegetative cell form. 

EPA recently completed a laboratory study (12 laboratories) of the total aerobic spore 

method. The study used natural ground water from a deep confined aquifer in Montana and Ohio 

River surface water. The ground water was analyzed to insure that it was devoid of (but not 
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LT2ESWTR Toolbox Guidance Manual 4-41 April 2010

sterile) aerobic spore forming bacteria. Aerobic spores (from BioBall) with well-defined counts 

but subject to variability were spiked in the ground water samples. Split surface water samples of 

unknown variability were also prepared. One ground water and one surface water sample was 

sent to each laboratory for multiple assay.

Laboratory performance was evaluated by comparing mean assay values separately for 

ground water and surface water. Spiked ground water sample variability among the twelve 

laboratories using Youden’s Laboratory Ranking Test (Youden 1969) did not identify any 

outlying laboratories. Surface water mean values showed two outlying laboratories (one high 

outlying value) but the range of mean values was about a factor of five different between the high 

(23,962 CFU/100 ml) and (low 4,370 CFU/100 ml) values. Spore removal in this guidance is 

typically an assessment of whether surface water spore counts are diminished by a factor of one 

hundred when measured at a well. A factor of five difference is acceptable variability, assuming 

other information does not contradict that assessment.

The laboratory study shows that the aerobic spore laboratory method can be reproducibly 

performed by different laboratories and also provides acceptable recoveries of spores from spiked 

water samples. Because aerobic spores are: 1) relatively cheap and easy to measure, 2) 

identifiable without unacceptable laboratory error, 3) long-lived in the environment, 4) similar in 

shape to oocysts (albeit slightly smaller), and 5) do not produce extracellular polymers, EPA 

recommends aerobic spores, if present in large numbers in the surface water at the DOP site, as 

the most useful Cryptosporidium surrogate organism. Aerobic spores have long been recognized 

as a useful measure of surface water influence on and hygienic quality of ground water (e.g., 

Schubert 1975). Aerobic spores are also commonly used to assess the performance of engineered 

filtration systems (e.g., Mazoua and Chauveheid 2005). 

The MPA method counts spores but these are fungal spores and not bacterial spores. 

Bacterial spore assay requires, at present, a culture step that is not currently part of the MPA 

method. MPA simply concentrates particulates and counts them. An aerobic spore assay standard 

method is available (APHA 2004). EPA recommends that unused aerobic spore sample be 

refrigerated (4 degrees C.). These refrigerated samples may then be re-assayed up to 24 hours 

post-receipt of the sample at the laboratory so that additional and differing dilutions can be 

conducted to reanalyze samples that are reported as “Too Numerous to Count” (TNTC).

Aerobic spore data have been collected from several recent studies at potential bank 

filtration sites (e.g., Weiss et al. 2005, Vogel et al. 2005, Gollnitz et al. 2004, Gollnitz et al. 

2005, Partinoudi and Collins 2007, Gollnitz et al. 2007). It is important to differentiate sites that 

may be described as riverbank filtration sites but are not recognized as GWUDI by the state. For 

example, both Lincoln, NE (Vogel et al. 2005) and Cincinnati, OH (Gollnitz et al. 2004) field 

sites were studied in great detail using very sophisticated methods despite not being regulated as 

GWUDI. Thus, high aerobic spore log removal at these sites is expected because they are 

regulated as ground water rather than as surface water. Finally, at least one laboratory counted 

aerobic spore colonies in ground water without use of a dissecting microscope (in contrast to 

APHA 2004) (Partinoudi and Collins 2007). Partinoudi and Collins (2007) did not use a 

microscope so they report a high aerobic spore detection limit (<30 CFU/100 ml). Based on 
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LT2ESWTR Toolbox Guidance Manual 4-42 April 2010

unpublished data from Casper, WY and results reported in Locas et al. (2008), Schubert (1975) 

and Rice et al (1999), the aerobic spore natural background concentration is about 10 CFU/100 

ml or less. Based on these studies, values higher than 10 CFU/100 ml may be considered to have 

some surface water influence. Thus, a high detection limit makes it difficult to differentiate 

native and surface water-influenced ground water. 

The ability to produce environmentally-resistant aerobic spores is fairly limited in the 

bacterial world. Current methodology recovers almost exclusively those aerobic spores from the 

genus Bacillus. There are many species of Bacillus and all produce aerobic spores. Members of 

this genus are found naturally in all environments, including surface waters and especially in soil. 

It is assumed that, when found in surface water, aerobic spores found naturally in soil are washed 

into surface water by natural processes. As soil bacteria, aerobic spore populations in surface 

water are expected to comprise a diverse bacterial population, representing all aerobic spore taxa 

within the surface water watershed. The aerobic spore population in a ground water sample may 

be 1) similarly as taxonomically diverse as the surface water population, 2) taxonomically less 

diverse than the surface water population because some spore taxa have favorable properties 

(e.g., charge) for subsurface passage while other taxa are more likely to attach to aquifer solids, 

or 3) taxonomically diverse or not but representative only of the spore population in the soil in 

the immediate vicinity of the wellhead.

Gollnitz et al. (2005) suggest that ground water aerobic spore samples exhibit “endospore 

monocultures” and also suggest that these “monocultures” explain the instances when collection 

devices exhibit negative (low) removal efficiency. (Given the high uncertainty in log removal 

calculations, the difference between negative and low removal efficiency is not significant.)

“Monocultures” implies that all of the cells recovered on the growth medium are of the same 

strain and possible clonal, being genetically identical. This would ordinarily imply that the 

organisms were growing either in the groundwater itself or in the sample once it was collected. 

Some type of genetic profiling analysis would need to be run in order to document all of the cells 

recovered as clonal. To date no such data has been reported. A more likely explanation for a 

sample yielding Bacillus colonies that are morphologically similar is laboratory contamination. 

Thus, any suggestion that, in the absence of genetic profile or speciation data, colonies are 

“monocultures,” should be recognized as premature and possibly incorrect. Finally, as discussed 

above, low taxa diversity in a ground water sample (when recognized by speciation data) 

provides no information on log removal by subsurface passage. 

Anaerobic spores are also recommended as surrogate microorganisms because these 

microorganisms, like aerobic spores, are small (0.3-1.2 μm), spherical, and long-lived. Riverbank 

filtration studies in the Netherlands (e.g., Shijven et al. 2003, Medema and Stuyfzand 2002) used

spores of sulfide-reducing Clostridia (SSRC) and Clostridium bifermentans spores as 

Cryptosporidium surrogates in studies of the Rhine and Meuse Rivers. However, anaerobic 

spores probably do not have a significant presence in surface water unless there are significant 

upstream sewage discharges to surface water, as there are in the Rhine and Meuse. Thus, the 

utility of anaerobic spores at a DOP site where surface water quality is typically very good is 

limited. 
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LT2ESWTR Toolbox Guidance Manual 4-43 April 2010

Total coliform bacteria are vegetative cells that, like Bacillus sp., originate largely as soil 

bacteria, and are found at high density in surface water. Total coliform density in wells has been 

used for GWUDI determination in bank filtration settings. Price et al., 1999 show that higher 

total coliform density occurs in horizontal collector well #5 from January to April, during highest

flow conditions in the river. Thus, well #5 is used primarily during summer months when water 

use demand is high and river flow is low. However, using measurement of aerobic spores (or 

carboxylated microspheres, Metge et al. 2007) in addition to measurement of total coliform 

vegetative cells might provide a differing assessment because aerobic spores should more 

efficiently passage through the subsurface and may be present in significant density before 

January or after April.

Diatoms are a specialized group of marine and freshwater algae that all produce a rigid 

cell wall (frustule) composed of silica. There are 58 freshwater diatom genera (AWWA 1995). 

Diatoms are counted separately from other algae in a MPA (U.S. EPA 1992). The MPA method 

only counts whole diatoms; diatom fragments are not considered. Exhibit 4.8 shows the size and 

shape of some common freshwater diatoms. Diatoms vary in size (e.g., from 4-10 microns to 60 

or 70 microns). Smaller diatoms may be transported through sand and other porous media at rates 

similar to oocysts. Larger diatoms may, if they have large length to width ratios, orient 

themselves in the ground water flow field so that the long axis is parallel to the flow direction, 

which also may allow them to pass through sand and other porous media. Diatoms are 

photosynthesizing algae that require light to maintain their green chlorophyll. After about 6 

months residence time in the subsurface, the green color will fade. (Susan Boutros, EPA GWUDI 

Determination Presentation, Denver CO, verbal communication on unpublished laboratory 

experiments with diatoms placed in a refrigerator).

Exhibit 4.8 Size of Some Common Fresh Water Diatoms

Diatom Size (length x width) (μm) Shape

Stephanodiscus hantzchii 10 × 5-8 Cylindrical (Hendricks et al., 2000)

Synedra acus 60-70 × 3-4 Needle (Hendricks et al., 2000)

Cyclotella meneghiniana 5 × 3 Cylindrical (Hendricks et al., 2000)

Cyclotella pseudostelligera 4-10 Centric (Reilly et al., 2005)

Fragilaria crotonensis 40-170 × 2-4 Pennate (Reilly et al., 2005)

Aulacoseira granulata 4-30 Centric (Reilly et al., 2005)

Asterionella formosa 40-80 × 1.3-6 Pennate (Reilly et al., 2005)

Nitzschiia palea 15-70 × 2.5-5 Pennate (Reilly et al., 2005)

Because most diatoms are larger than Cryptosporidium oocysts (see section 4.7.2), 

diatom occurrence in a well signals that oocysts, like diatoms, could also be present in 

inadequately filtered drinking water from a well. Diatoms occurrence is subject to less 

uncertainty because the rigid frustule is not likely to be sufficiently deformable to pass through 

smaller pores, unlike most biological particles. Thus, one or more whole diatom tests, identified 

in well water, and counted using the MPA or another method, are particularly meaningful data. 

Some diatom species are also identifiable using immunoassay methods (Walker et al. 2005), 
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LT2ESWTR Toolbox Guidance Manual 4-44 April 2010

although the detection limit is high (500 cells per liter) and thus not well suited for porous media 

groundwater sites where the diatom count is expected to be significantly lower. 

EPA recommends weekly or biweekly aerobic spore samples plotted on a graph together 

with monthly diatom data from MPA and river stage values to evaluate bank filtration efficiency. 

The spore data are a measure of bank filtration efficiency which should decrease with increasing 

river stage (i.e., high spore occurrence in a well at high river stage). For a regulated river (e.g., 

upstream dams and reservoirs) the correlation between aerobic spore recovery in wells and river 

stage might be muted or non-existent. Diatom data are used to validate the aerobic spore data; 

weeks or months with high spore recovery in wells should also have accompanying diatom 

occurrence in well water.

4.7.6 Tracer Tests and Use of Isotopes

 It may be useful for the study design to include a pilot test at one or more collection

devices taken off-line. The pilot test could consist of adding a known quantity of inactivated 

cysts or indicator organism (including microspheres) and conservative tracer ("spiking") to the 

source and collecting and analyzing samples from the collection device. This type of pilot test 

may assist in the assessment of actual removal through the alternative filtration system. If a 

correlation between cysts and indicator organism(s) can be established, this correlation could be 

used to focus and expedite monitoring. Spiking studies are best suited for artificial recharge 

studies but can also be used in bank filtration studies if an injection well is drilled to insert the 

spiked samples. If an injection well is used, it would be preferable to drill it as a slant well under 

the riverbed that bottoms just below the riverbed. 

 As a supplement to special spiking studies, the DOP could conduct pilot laboratory 

column or tank studies of relatively undisturbed natural or engineered materials to evaluate their 

performance when challenged with a cyst, oocyst, indicator(s) and conservative tracer spike. 

These studies are especially important for demonstrating that the porous media transport of the 

indicator(s) identified in field studies is similar to the transport of cysts and oocysts. 

At least three studies (Coplen et al. 1999, Vogel et al. 2005, Hunt et al. 2005) have used 

stable isotopes in a North American bank filtration study, although isotopes are commonly used 

in less specialized ground water and surface water interaction studies in the United States and 

elsewhere. Coplen et al. showed that, for Portland, OR, municipal field well #1, increasing 

Columbia River contribution to well yield with pumping, with about half of the yield from 

surface water at day 7 and culminating at 82 percent surface water on day 23 of the pump test. 

Hunt et al. showed that the travel time of surface flood water to the municipal wells in La Crosse, 

WI was approximately 2 months as compared to inter-flood periods with about 9 month travel 

times. Age dating using 3

H-3He and tracers such as CFCs and SF6 were less useful at the site. 
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LT2ESWTR Toolbox Guidance Manual 4-45 April 2010

4.7.7 Monitoring Wells Located Along the Shortest Flow Path

The LT2ESWTR allows DOP credit for a site-specific study ONLY if one or more 

monitoring wells are screened and located along the shortest flow path between the surface water 

source(s) and the production device(s) (well). As discussed in section 4.7.3, particle-tracking 

ground water flow models are best suited for identifying the shortest flow path and the 

appropriate depth of the screened interval. However, optimally located, existing monitoring wells 

may also be used if they are directly located between the river and the well and are screened at a 

depth intermediate between the river channel bottom and the production well screen. 

4.7.8 Post-decision Routine Monitoring and Sampling

Any DOP study should include a component to develop a routine sampling and 

monitoring program that would validate the continuation of any removal credit granted. EPA 

recommends continued post-decision bi-weekly or monthly aerobic spore monitoring to insure 

that any approved Cryptosporidium log removal credit is maintained over time. Post-decision 

routine monitoring is similar to filtration plant performance testing and will allow comparison to 

previously-collected data to determine if degradation of alternative filtration performance is 

occurring and to document improvements in alternative treatment removals based on improved 

wellfield operation and maintenance.

 

4.8 Reference

Abdel-Fattah, A., R. Langford, D. Schulze-Makuch. 2007. Application of particle-tracking 

techniques to bank infiltration: a case study from El Paso, Texas, USA. Environmental Geology 

(in press).

APHA (American Public Health Association). 2004. 9218 Aerobic Endospores in Standard 

Methods for the Examination of Water and Wastewater, p. 9-47-9-48. 

ASTM (American Society for Testing and Materials). 2003. Standard Test Method for Sieve 

Analysis of Fine and Coarse Aggregates - Standard C 136-1. 

AWWA (American Water Works Association). 1990. Guidance Manual for Compliance with the 

Filtration and Disinfection Requirements for Public Water Systems Using Surface Water 

Sources. American Water Works Association, Denver, 573 p. 

AWWA (American Water Works Association). 1995. Problem Organisms in Water: 

Identification and Treatment, AWWA Manual of Water Supply Practices M7, AWWA, Denver, 

Colorado, 153 p. 

Bakker, M., V.A. Kelson and K.H. Luther. 2005. Multilayer Analytical Element Modeling of 

Radial Collector Wells. Ground Water 43(6):926-934. 
4. Bank Filtration

LT2ESWTR Toolbox Guidance Manual 4-46 April 2010

Baveye, P., Berger, P., Schijven, J., and Grischek, T. 2003. Research needs to improve 

knowledge of bank filtration removal of pathogens, in Riverbank Filtration: Improving Source 

Water Quality, edited by Ray, C., Melin, G. and Linsky, R., Kluwer, Dordrecht. 

Baveye, P., P. Vandevivere, B. L. Hoyle, P.C. DeLeo, and D. Sanchez de Lozada. 1998. 

Environmental impact and mechanisms of the biological clogging of saturated soils and aquifer 

materials. Critical Reviews in Environmental Science and Technology. 28(2): 123-191. 

Berger, P. 2002. Removal of Cryptosporidium Using Bank Filtration in Riverbank Filtration: 

Understanding Contaminant Biogeochemistry and Pathogen Removal, C. Ray (ed.). The 

Netherlands: Kluwer Academic Publishers. p. 85-121. 

Booth, D.B. 1990. Stream-channel incision following drainage basin urbanization. Water 

Resources Bulletin. 26(3): 407–417. 

Brock, T.D. and M.T. Madigan. 1991. Biology of Microorganisms. Prentice Hall, Englewood 

Cliffs, New Jersey, 776 p. 

Brusseau, M.L., J.K. Oleen, J. Santamaria, L. Cheng, P. Orosz-Coghlan, A.S. Chetochine, W.J. 

Blandford, P. Rykwalder, and C.P. Gerba. 2005. Transport of microsporidium Encephalitozoon 

intestinales spores in sandy porous media. 39:3636-3642. 

Burger, H.R., D.C. Burger, and R.H. Burger, 1992. Exploration Geophysics of the Shallow 

Subsurface. Upper Saddle River, NJ: Prentice Hall.

Callegary, James, United States Geological Survey, personal communication, 3/03. 

Chow, V.T. 1964. Handbook of Applied Hydrology. New York: McGraw Hill.

Coplen, T.B., A.L. Herczeg and C. Barnes. 1999. Isotope Engineering – Using Stable Isotopes of 

the Water Molecule to Solve Practical Problems in P.G. Cook and A. L. Herczeg (eds.) 

Environmental Tracers in Subsurface Hydrology, Kluwer, Boston. 

Driscoll, F.G. 1986. Groundwater and Wells. 2nd Edition. St. Paul, Minnesota: Johnson Division. 

Dunne, T., and Leopold, L.B. 1978. Water in Environmental Planning. New York: W.H. 

Freeman and Company. 818 pp. 

Eckert, P. and R. Irmscher. 2006. Over 130 years of experience with riverbank filtration in 

Dusseldorf, Germany. Journal of Water Supply: Research and Technology-AQUA, 55(4):283-

291. 

FEMA Map Assistance Center. 2003. Personal communication. 
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LT2ESWTR Toolbox Guidance Manual 4-47 April 2010

Foppen, J.W.A and J.F. Schijven. 2006. Evaluation of data from the literature on the transport 

and survival of Escherichia coli and thermotolerant coliforms in aquifers under saturated 

conditions. Water Research 40:401-426. 

Gaut, S., L. Robertson, B. Gjerde, A. Dagestad and B. Brattli. 2008. Occurrence of 

Cryptosporidium oocysts and Giardia cysts in Norwegian groundwater wells in bedrock. Journal 

of Water and Health, 9(3):383-388. 

Goldenberg, L.C., I. Hutcheon, N. Wardlaw, and A.J. Melloul. 1993. Rearrangement of fine 

particles in porous media causing reduction of permeability and formation of preferred pathways 

of flow: experimental findings and a conceptual model. Transport in Porous Media 13: 221-237. 

Goldschneider, A.A., K.A. Haralampides, K.T.B. MacQuarrie. 2007. River sediment and flow 

characteristics near a bank filtration water supply: Implications for riverbed clogging. Jour. 

Hydrology, 344:55-69. 

Gollnitz, W.D. 1999. Induced infiltration rate variability and water quality sampling issues. 

Proceedings of the International Riverbank Filtration Conference. November 4-6, 1999. 

Louisville, Kentucky. 

Gollnitz, W.D. and J.L. Clancy. 1994. Evaluation of the Casper Aquifer and Wellfield for 

Ground Water Under the Direct Influence of Surface Water, unpublished report prepared for the 

Casper Public Utilities, Casper, Wyoming, 35 p. 

Gollnitz, W.D., B.L.Whitteberry and J.A. Vogt. 2004. Riverbank filtration: Induced infiltration 

and groundwater quality. Journal AWWA 96(12):98-110. 

Gollnitz, W.D., J.L. Clancy, J.B. McEwen and S.C. Garner. 2005. Riverbank filtration for 

IESWTR Compliance. Journal AWWA 97(12):64-76. 

Gollnitz, W.D., J.L. Clancy, M. Cunnane, and B. Beauchene. 2007. Riverbank filtration for 

SWTR Compliance-Kennewick, Washington in Proceedings Water Quality Technology 

Conference, Charlotte, North Carolina, American Water Works Association, Denver Colorado. 

Harter, T., S. Wagner, and E. R. Atwill, 2000. Colloid Transport and Filtration of 

Cryptosporidium parvum in Sandy Soils and Aquifer Sediments, Environmental Science and 

Technology, 34(1), pp. 62-70. 

Hearst, J.R., P.H. Nelson, and F.L. Paillet, 2000. Well Logging for Physical Properties: A 

Handbook for Geophysicists, Geologists, and Engineers, 2nd Edition. New York: Wiley. 

Hendricks, D.W., 2001. Biological Particle Surrogates for Filtration Performance Evaluation, 

American Water Works Association Research Foundation Report #181, Denver, Colorado. 
4. Bank Filtration

LT2ESWTR Toolbox Guidance Manual 4-48 April 2010

Holt, J.G. 1986. Bergey’s Manual of Systematic Bacteriology, vol. 2, Lippincott, Williams and 

Wilkins, Baltimore, Maryland.

Hubbard, Susan M., Lawrence Berkeley National Laboratory, personal communication, 3/03. 

Hubbs, S., J.Z. Wang, and R. Song. 2001. Use of Riverbank Filtration to Meet the Requirements 

of SWTR and DBP Rules. Presentation at the American Water Works Association Water Quality 

Technology Conference, November 11-15, Nashville, TN. 

Hunt, R.L., T.B. Coplen, N.L. Haas, D.A. Saad, M.A. Borchardt. 2005. Investigating surface 

water-well interaction using stable isotope ratios of water. Jour. Hydrology 302:154-172. 

Jacobson, R.B., S.R. Femmer, and R.A. McKenney. 2001. Land-use Changes and the Physical 

Habitat of Streams : a review with emphasis on studies within the U.S. Geological Survey 

Federal-State cooperative program. U.S. Geological Survey Circular 1175. Reston, VA.: U.S. 

Geological Survey. 63 pp. 

Juhasz-Hoterman, M.H.A. 2000. Reliable drinking water by bank filtration along the river Maas 

(Meuse), by knowledge of the system combined with simple resources, in Proceedings of 

International Riverbank Filtration Conference, Nov. 2-4, Duesseldorf, W. Julich and J. Schubert 

(eds.), International Arbeitgemeinschaft der Wasserwerke im Rheineinzugsgebiet, Amsterdam.

Juttner, F. 1995. Elimination of terpenoid odorous compounds by slow sand and river bank 

filtration of the Ruhr River, Germany. Water Science & Technology. 31: 211-217. 

Jüttner, F. 1999. Efficacy of bank filtration for the removal of fragrance compounds and aromatic 

hydrocarbons. Water Science Technology 40(6): 123-128. 

Keys, W.S., 1990, Borehole geophysics applied to groundwater investigations: U.S. Geological 

Survey, Techniques of Water-Resources Investigations of the U.S. Geological Survey, Book 2, 

Collection of Environmental Data, Chapter E2, 150 p. 

Kuehn, W. and U. Mueller. 2000. Riverbank filtration: an overview. AWWA Journal. 92(12): 

60-69. 

Leland, D., J. McAnulty, W. Keene, and G. Stevens. 1993. A cryptosporidiosis outbreak in a 

filtered-water supply. AWWA Journal. 85(6):34-42. 

Leopold, L.B. and Maddock, T. 1953. The Hydraulic Geometry of Stream Channels and Some 

Physiographic Implications. U.S. Geological Survey Professional Paper 252. Washington: U.S. 

Government Printing Office. 

Leopold, L.B., M.G. Wolman, and J.P. Miller. 1964. Fluvial Processes in Geomorphology. San 

Francisco: W H Freeman and Co.
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LT2ESWTR Toolbox Guidance Manual 4-49 April 2010

Locas, A., C. Barthe, A.B. Margolin, and P. Payment. 2008. Groundwater microbiological 

quality in Canadian drinking water municipal wells. Can. J. Microbiol. 54:1-7. 

Lund, B.M. and Peck, M.W. 1994. Heat resistance and recovery of spores of non-proteolytic 

Clostridium botulinum in relation to refrigerated, processed foods with an extended shelf-life. 

Journal of Applied Bacteriology Symposium Supplement 76:115S-128S. 

Mazoua, S. and D. Chauveheid. 2005. Aerobic spore-forming bacteria for assessing quality of 

drinking water produced from surface water. Water Research 39:5186-5198. 

Medema, G.J. and P.J. Stuyfzand. 2002. Removal of micro-organisms upon basin recharge, deep 

well injection and river bank filtration in the Netherlands, in Management of Aquifer Recharge 

for Sustainability, P. Dillon (ed.), Swets & Zeitlinger, Lisse, p. 125-131. 

Medema, G.J., M.H.A. Juhasz-Hoterman, and J.A. Luitjen. 2000. Removal of micro-organisms 

by bank filtration in a gravel-sand soil, in Proceedings of International Riverbank Filtration 

Conference, Nov. 2-4, Duesseldorf, W. Julich and J. Schubert (eds.), International 

Arbeitgemeinschaft der Wasserwerke im Rheineinzugsgebiet, Amsterdam.

Metge, D.W., R.W. Harvey, R. Anders, D. O. Rosenberry, D. Seymour and J. Jasperse. 2007. 

Use of carboxylated microspheres to assess transport potential of Cryptosporidium parvum

oocysts at the Russian River water supply facility, Sonoma County, California, Geomicrobiology 

Journal, 24:231-245. 

Miettinen, I.T., P.J. Martikainen, T. Vartiainen. 1994. Humus transformation at the bank 

filtration water plant. Water Science & Technology, 30 (10): 179. 

Montgomery, Carla W., 2000. Environmental Geology, updated 5th edition. Boston: McGraw 

Hill.

Mota, P., C.A. Rauch and S.C. Edberg. 2000. Microsporidia and Cyclospora: Epidemiology and 

assessment of risk from the environment. Critical Reviews in Microbiology 26(2):69-90. 

Oberdorfer, J.A. and F.L. Peterson. 1985. Waste-water injection: geochemical and 

biogeochemical clogging processes. Ground Water 23: 753-761. 

Orlob, G.T. and G.N, Radhakrishna. 1958. The effects of entrapped gases on the hydraulic 

characteristics of porous media. Transactions of the American Geophysical Union 39: 648-659. 

Palcsak, B.B. 1995. Using the Freeze-Core Method to Collect Streambed Samples for 

Determination of Particle-Size Distribution. US Geological Survey Open-File Report 95-466. 

14p. 

Pang, L., M. Close, and M. Noonan. 1998. Rhodamine WT and Bacillus subtilis transport 

through an alluvial gravel aquifer. Ground Water 36:112-122. 
4. Bank Filtration

LT2ESWTR Toolbox Guidance Manual 4-50 April 2010

Pang, L., M. Close, M. Goltz, M. Noonan, and L. Sinton. 2005. Filtration of Bacillus subtilus 

spores and the F-RNA phage MS2 in a coarse alluvial gravel aquifer: Implications in the 

estimation of setback distances. Journal of Contaminant Hydrology 77:165-194. 

Partinoudi, V. and M.R. Colins. 2007. Assessing RBF reduction/removal mechanisms for 

microbial and organic DBP precursors. Journal AWWA 99(12):61-71. 

Price, M.L., J. Flugum, P. Jeane and L. Tribbett-Peelen. 1999. Sonoma County finds 

groundwater under the direct influence of surface water depends on river conditions in Abstracts, 

International Riverbank Filtration Conference, November 4-6, 1999, Louisville, Kentucky, 

National Water Research Institute, Fountain Valley, CA.

Purdue University. 2001. Well Water Location and Condition on the Farm. Available on the 

Internet at: http://www.epa.gov/seahome/welloc.html. accessed November 27, 2002. 

Ray, C. 2001a. Riverbank filtration: an analysis of parameters for optimal performance. 

Proceedings of the Annual Conference of the American Water Works Association. June 17-21, 

2001. Washington, DC. 

Ray, C. 2001b. Modeling riverbank filtration systems to attenuate shock loads in rivers. 

Proceedings of the Annual Conference of the American Water Works Association. June 17-21, 

2001. Washington, DC. 

Ray, C., T. Grischek, J. Schubert, J. Wang, and T. Speth. 2002. A perspective of riverbank 

filtration, J. AWWA, 94(4): 149-160. 

Reilly, T.J., C.E. Walker, A. L. Baehr, R. M. Schrock, and J.R. Reinfelder. 2005. Occurrence of 

Diatoms in Lakeside Wells in Northern New Jersey as an Indicator of the Effect of Surface Water 

on Ground-Water Quality. U.S. Geological Survey Scientific Investigations Report 20005-5263, 

13 p. 

Reynolds, J.M., 1997. An Introduction to Applied and Environmental Geophysics. New York: 

Wiley.

Rice, E.W., C.H. Johnson, M.C. Meckes, K.C. Kelty and R. Moore. 1999. Microbial indicators 

for monitoring ground water quality in Proceedings, Annual Meeting of the American Water 

Works Association, Chicago, Illinois, American Water Works Association, Denver Colorado. 

Ritter, D.F., C.R. Kochel, and J.R. Miller. 1995. Process Geomorphology. 3rd edition. Dubuque, 

Iowa: Wm. C. Brown Publishers. 

Rohns, H.-P., C. Forner, P. Exkert and R. Irmscher. 2006. Efficiency of riverbank filtration 

considering the removal of pathogenic microorganisms of the River Rhine in R. Gimbel, N. 
4. Bank Filtration

LT2ESWTR Toolbox Guidance Manual 4-51 April 2010

Graham and R. Collins (eds.), Recent Progress in Slow Sand and Alternative Biofiltration 

Processes. IWA Publishers, London UK, 539-546. 

Rubin Y. and S. Hubbard, 2003. Hydrogeophysics. Kluwer.

Sanchez de Lozada, D., P. Vandevivere, P. Baveye, and S. Zinder. 1994. Decrease of the 

hydraulic conductivity of sand columns by Methanosarcina barkeri. World Journal of 

Microbiology and Biotechnology 10: 325-333. 

Schafer, D. 2000. Groundwater modeling in support of riverbank infiltration for Louisville Water 

Company, in Proceedings of International Riverbank Filtration Conference, Nov. 2-4, 

Duesseldorf, W. Julich and J. Schubert (eds.), International Arbeitgemeinschaft der Wasserwerke 

im Rheineinzugsgebiet, Amsterdam.

Schijven, J., Berger, P., Miettenen, I. 2003. Removal of viruses, bacteria, protozoa and toxins 

using bank filtration, in Riverbank Filtration: Improving Source Water Quality, edited by Ray, 

C., Melin, G. and Linsky, R., Kluwer, Dordrecht. 

Schubert, R.H.W. 1975. The detection of spores of the Bacillus-species within the scope of the 

hygeienic control of water pollution. Zbl. Bakt. Hyg. Abt. Orig. B 160:155-162 (in German with 

and English Abstract). 

State Coordinating Committee on Ground Water (SCCGW). 2000. State of Ohio Technical 

Guidance for Well Construction and Ground Water Protection. Available on the Internet at: 

http://www.dnr.state.oh.us/portals/7/pubs/pdfs/wellconguide.pdf. accessed November 27, 2002. 

Symons, J.M., L.C. Bradley, Jr., T.C. Cleveland, eds. 2000. The Drinking Water Dictionary. 

American Water Works Association, Denver, CO. 

Tufenkji, N. 2007. Modeling microbial transport in porous media: Traditional approaches and 

recent developments. Advances in Water Resources 30:1455-1469. 

Tufenkji, N., D.R. Dixon, R. Considine, C. J. Drummond. 2006. Multi-scale 

Cryptosporidium/sand interactions in water treatment. Water Research 40:3315-3331. 

Tufenkji, N., J.N. Ryan, and M. Elimelech. 2002. The promise of bank filtration: a simple 

technology may inexpensively clean up poor-quality raw surface water. Environmental Science 

and Technology. 36: 422A - 428A. 

United States Enviromental Protection Agency (U.S. EPA). 1975. Manual of Water Well 

Construction Practices. Office of Water Supply. EPA/ 570/9-75-001. Washington, D.C. 156 pp. 

United States Environmental Protection Agency (U.S. EPA). 1992. Consensus Method for 

Determining Groundwaters Under the Direct Influence of Surface Water Using Microscopic 

Particulate Analysis (MPA). EPA 910/9-92-029. 
4. Bank Filtration

LT2ESWTR Toolbox Guidance Manual 4-52 April 2010

United States Environmental Protection Agency (U.S. EPA). 2006. Long Term 2 Enhanced 

Surface Water Treatment Rule (LT2ESWTR). Federal Register. Thursday, January 5, 2006. 

71(3):654-786 (US EPA 815-Z-06-001). 

United States Geological Survey (USGS). 1998. The National Atlas of the United States of 

America. Principal Aquifers. [Map]. Reston, VA: U.S. Department of the Interior, U.S. 

Geological Survey.

Van Beek, C.G.E.M., A.H. de Zwart, M. Balemans, J.W. Kooiman, C. van Rosmalen, H. timmer, 

J. Vandersluys and P.J. Stuyfzand. Concentration and size distribution of particles in abstracted 

groundwater. Water Research 44:868-878. 

Vandevivere, P., P. Baveye, D. Sanchez de Lozada, and P. DeLeo. 1995. Microbial clogging of 

saturated soils and aquifer materials: Evaluation of mathematical models. Water Resources 

Research 31(9): 2173-2180. 

Vogel, J.R., S.I. Harris, T.B. Coplen, E.W. Rice and I.M. Verstraeten. 2005. Microbe 

Concentrations, Laser particle counts, and Stable Hydrogen and Oxygen Isotope Ratios in 

Samples from a Riverbank Filtration Study, Platte River, Nebraska, 2002 to 2004. US Geological 

Survey Data Series 133, 61 p. 

Walker, C.E., R.M. Schrock, T.J. Reilly and A.L. Baehr. 2005. A direct immunoassay for 

detecting diatoms in groundwater as a indicator of the direct influence of surface water. Journal 

of Applied Phycology 17:81-90. 

Wang, J.Z., Hubbs, S.A. and Song, R. 2002. Evaluation of Riverbank Filtration as a Drinking 

Water Treatment Process, American Water Works Association Research Foundation Report 

90922, 145 p. 

Wang, J.Z., R. Song, and S.A. Hubbs. 2000. Particle removal through riverbank filtration 

process, in Proceedings of the International Riverbank Filtration Conference, Nov. 2-4, 

Duesseldorf, W. Julich and J. Schubert (eds.), International Arbeitgemeinschaft der Wasserwerke 

im Rheineinzugsgebiet, Amsterdam.

Wang, J.Z., S.A. Hubbs, and M. Unthank. 2001. Factors Impacting the Yield of Riverbank 

Filtration Systems. Presentation at the American Water Works Association Water Quality 

Technology Conference, November 11-15, Nashville, TN. 

Warner, J.W., T.K. Gates, R., Namvargolian, P. Miller, and G. Comes. 1994. Sediment and 

microbial fouling of experimental groundwater recharge trenches. Journal of Contaminant 

Hydrology 15: 321-344. 
4. Bank Filtration

LT2ESWTR Toolbox Guidance Manual 4-53 April 2010

Weiss, W.J., E.J. Bouwer, R. Aboytes, M.W. LeChevallier, C.R. O’Melia, B.T. Le, K.J. Schwab. 

2005. Riverbank filtration for control of microorganisms: Results from field monitoring. Water 

Research 39:1990-2001. 

Weiss, W.J., E.J. Bouwer, W.P. Ball, C.R. O’Melia, H. Arora, T.F. Speth. 2003. Reduction in 

DBP precursors and pathogens during riverbank filtration at three midwestern drinking water 

utilities, in Riverbank Filtration for Water Supply, C. Ray and R. Linsky (eds). Kluwer Academic 

Publishers. 

WHO. 2004. Guidelines for Drinking Water Quality, Third Edition, Volume 1, 

Recommendations, World Health Organization, Geneva, 515 p. 

Williams, G.P., and Wolman, M.G. 1984. Downstream Effects of Dams on Alluvial Rivers. U.S. 

Geological Survey Professional Paper 1286. Washington: U.S. Government Printing Office. 

Xiao, L., U.M. Morgan, R. Fayer, R.C.A. Thompson, and A.A. Lal. 2000. Cryptosporidium

sytematics and implications for public health. Parsitology Today 16(7):287-292. 

Youden, W.J. 1969. Ranking Laboratories by Round-Robin Tests in Precision Measurement and 

Calibration, Statistical Concepts and Procedures, US Department of Commerce, National Bureau 

of Standards, Special Publication 300, 1:165-169. 

 
LT2ESWTR Toolbox Guidance Manual 5-1 April 2010

5. Presedimentation

5.1 Introduction 

 Presedimentation is a preliminary treatment process used to remove gravel, sand, and 

other material from the raw water and dampen particle loading fluctuations to the rest of the 

treatment plant. A plant may receive credit for a presedimentation basin for any month the basin 

meets the requirements as described in 40 CFR 141.717(a). 

 

Sedimentation processes are common in the water treatment process and much design and 

operational information is available. However, the use of an additional sedimentation basin in 

series, or a pre-sedimentation basin at the head of the treatment plant is not as common as the 

standard sedimentation basin, and little information is available. Therefore, the guidance 

provided in this chapter is based on the design and operational principles of sedimentation 

processes. 

 This chapter on presedimentation is organized as follows: 

5.2 Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) Compliance 

Requirements - This section describes the criteria presedimentation basins must 

achieve in order to receive Cryptosporidium removal credit. 

5.3 Toolbox Selection Considerations - This section assists systems in determining 

whether the presedimentation toolbox option is a viable and beneficial option for 

meeting the LT2ESWTR bin requirements. 

5.4 Types of Presedimentation Basins - This section compares several sedimentation 

basins and clarifiers in terms of structure and factors affecting settling efficiency.

5.5 Design and Operating Issues - This section discusses typical design and 

operational issues including redundancy, short circuiting, sludge removal, and 

coagulant addition. 

5.2 LT2ESWTR Compliance Requirements 

5.2.1 Credits

 Presedimentation basins with coagulant addition may receive 0.5 log Cryptosporidium

removal credit under the LT2ESWTR if they meet the following criteria (40 CFR 141.717(a)):

• The presedimentation basin must be in continuous operation and must treat all of the flow 

taken from a surface water or ground water under the direct influence of surface water 

(GWUDI) source. 
5. Presedimentation

LT2ESWTR Toolbox Guidance Manual 5-2 April 2010

• A coagulant must be continuously added to the presedimentation basin while the plant is 

in operation. 

• The presedimentation basin must achieve a monthly mean reduction of 0.5 log (68 

percent) or greater in turbidity or alternative state-approved performance criteria that 

demonstrate at least 0.5 log mean removal of micron sized particulate material through 

the presedimentation process. 

5.2.2 Monitoring Requirements

Systems must measure presedimentation basin influent and effluent turbidity at least once 

per day or meet state-specified performance criteria (40 CFR 141.717(a)). State-specified criteria 

could include aerobic spore removal (see Chapter 12, Section 12.4.2.1 of this guidance manual) 

or particle count reduction. Laboratory support would be needed for spore counts and grab 

sampling and dilution would be needed to assess particle count reduction. 

5.2.3 Calculations 

 For compliance with the LT2ESWTR, the log turbidity reduction must be calculated as a 

monthly mean, from readings collected daily, according to the following equation (40 CFR 

141.717(a)(3)(i)).

 Log Reduction = 

Log10(Monthly Average Influent Turbidity) - Log10(Monthly Average Effluent Turbidity) 

Or if calculated as a percent,

Percent Reduction = 

 (Monthly Average Influent Turbidity) - (Monthly Average Effluent Turbidity

 (Monthly Average Influent Turbidity) 

) x 100 

Example Calculation 

 Average influent turbidity = 16.3 nephelometric turbidity units (NTU) 

Average effluent turbidity = 4.2 NTU 

 Log Reduction = Log10(16.3) - Log10(4.2) = 0.59 

 Percent Reduction = (16.3-4.2)/16.3 = 74.2% 
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LT2ESWTR Toolbox Guidance Manual 5-3 April 2010

5.3 Toolbox Selection Considerations 

 The purpose of this section is to assist systems in determining whether the 

presedimentation toolbox option is a viable and beneficial option for meeting the LT2ESWTR

bin requirements. There are two general aspects for systems to evaluate when considering this 

toolbox option: 

1) Can the turbidity removal requirements be met consistently over the expected 

range of raw water conditions? 

2) What are the advantages and disadvantages of installing a presedimentation basin? 

 For presedimentation, the first question is driven by source water particle load and how 

much of that load a proposed sedimentation basin would remove. Before researching potential 

presedimentation designs, a system should determine if their source water has a high enough 

turbidity on a consistent basis. Section 5.3.1 discusses the source water characteristics necessary 

to meet the compliance requirements. Section 5.3.2 discusses the advantages and disadvantages 

of adding a presedimentation process to the treatment train. 

5.3.1 Source Water Quality 

 To meet the 0.5 log turbidity removal requirement, the source water should have 

consistently high turbidity, and comply with state specified performance criteria. When source 

water turbidity is seasonally or consistently low, most presedimentation basins will have 

difficulty achieving 0.5 log reduction, and systems may need to use another tool in the toolbox to 

meet state-specified criteria such as aerobic spore removal or particle count reduction. For 

example, if a system has an average of 10 NTU source water turbidity for a few months of the 

year, the average effluent turbidity would have to be 3.2 NTU for those months, which could be 

difficult for some systems to achieve. Exhibit 5.1 lists influent and effluent turbidity values that 

yield 0.5 log reduction. These are example values to help when considering using the tool. 
5. Presedimentation

LT2ESWTR Toolbox Guidance Manual 5-4 April 2010

Exhibit 5.1 Example Influent and Effluent Turbidity Values Resulting in 0.5 Log 

Reduction

Monthly Average Turbidity (NTU)

Influent Effluent

2 0.6

10 3.2

30 9.5

50 15.8

70 22.1

80 25.3

100 31.6

5.3.2 Advantages and Disadvantages of Installing a Presedimentation Basin 

 The presedimentation process can reduce influent fluctuations in particle loading, flow, 

and other water quality parameters. An additional sedimentation process in series provides 

increased operational flexibility to handle rapid changes in influent turbidity. It also allows for 

enhanced performance of subsequent processes in the treatment plant. 

 As with the addition of many unit processes, the two major disadvantages are capital 

costs and land requirements. The requirement of coagulant addition may increase chemical costs, 

although the amount added in the next stage could be reduced. Whether these chemical costs 

offset each other is site-specific.

5.4 Types of Sedimentation Basins 

There are several types of sedimentation basins (also called clarifiers) used for drinking 

water treatment. Selection of a basin for presedimentation should be based on turbidity removal 

capability and meeting the flow and space requirements of the facility. The focus of this chapter 

is on guidance for complying with the LT2ESWTR, therefore the discussion in this section is 

limited to factors affecting settling efficiency, as measured by turbidity removal. Further 

information on design can be found in the following literature: 

• Water Quality and Treatment—A Handbook of Community Water Supplies, 5th ed. 

(AWWA 1999). 

• Integrated Design and Operation of Water Treatment Facilities, 2nd ed. (Kawamura 

2000). 
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LT2ESWTR Toolbox Guidance Manual 5-5 April 2010

 Exhibit 5.2 provides a comparison of several sedimentation basins and clarifiers. It is 

likely that only horizontal clarifiers would be chosen for presedimentation, since they are less 

complex in operation compared to the others (i.e., upflow, high rate, reactor, and ballasted sand 

clarifiers). The table includes the additional types since some plants that choose to employ the 

presedimentation toolbox option may elect to use their current sedimentation basin for 

presedimentation and construct a new basin for primary sedimentation. The performance 

advantages and disadvantages listed in the table relate to settling efficiency or indications for 

potential process upset. These were derived from Integrated Design and Operation of Water 

Treatment Facilities (Kawamura 2000) and are characteristic of sedimentation processes, not 

specifically presedimentation processes. The remainder of this section provides short descriptions 

of different clarifier types.
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LT2ESWTR Toolbox Guidance Manual 5-6 April 2010

Exhibit 5.2 Comparison of Sedimentation and Clarifier Types

Type Performance Advantages Performance Disadvantages

Applicable for Presedimentation and Sedimentation

Horizontal Flow (general) -Easy to operate and maintain 

Rectangular Basin -Tolerant to shock loads 

-Good for handling large flows

-Subject to wind and density 

currents (causing short-circuiting)

-Designs with trays have shown 

poor settling efficiency

Circular Basin -Easy sludge removal

-Can obtain high clarification 

efficiency

-Greater potential for hydraulic 

imbalance in comparison to 

rectangular basin (not good for 

removing alum flocs) 

Applicable for Sedimentation

Upflow Clarifier (general) -High clarification efficiency -Need constant flow rate and 

water quality 

-Limitations on size

Center Feed -Easy sludge removal -Short circuiting

Peripheral Feed -Good for source water with high 

solids

-Potential short-circuiting

High Rate Settlers 

(horizontal flow or upflow)

-Increases the hydraulic load 

capability and settling efficiency 

of horizontal flow basins and 

clarifiers

-Can form scales (calcium 

carbonate) which clog flow

-Poor flocculation possible

Reactor Clarifiers (general) -Good clarification due to seeding 

effect

-Need constant flow rate and 

water quality

-Requires greater operator skill

High recirculation and 

mechanical sludge plow

-Tolerant to shock loads -Dependent on one drive unit

-Limitations on size

Sludge blanket zone and 

mechanical sludge plow

-Good turbidity removal -Very sensitive to shock loads

-Requires 2-4 days to build 

sludge blanket

Ballasted sand -Can handle higher flows with 

very low detention times (on the 

order of minutes)

-Can handle shock particle loads 

without increasing coagulant 

dose

-Quick process startup

-Short detention time means not 

much time for process 

adjustments

Note: Adapted from “Integrated Design and Operation of Water Treatment Facilities.” Kawamura (2000).

Sedimentation processes can be categorized in three general types: horizontal flow basins 

or clarifiers, upflow clarifiers, and reactor clarifiers. High rate settlers are modified horizontal or 

upflow clarifiers with plate or tube modules placed into the basin to increase the settling area. An 
5. Presedimentation

LT2ESWTR Toolbox Guidance Manual 5-7 April 2010

additional design described in this chapter that differs from the three general types is ballasted 

sand or high-rate microsand process (a proprietary design).

5.4.1 Horizontal Flow 

5.4.1.1 Rectangular 

In rectangular sedimentation tanks the water flows in one end and ideally proceeds 

through the basin in a plug flow manner. A uniform distribution at the inlet is an important 

design factor. Rectangular basins can be susceptible to density currents that cause short 

circuiting. These basins are easy to operate, have low maintenance costs, offer predictable 

performance under most conditions, and are most tolerant to shock loads. High rate settlers can 

be easily installed to improve settling efficiency. Rectangular basins are particularly well suited 

for large systems compared to circular basins that require additional space and yard piping for 

equivalent flow. 

5.4.1.2 Circular

 The flow in circular basins is commonly from a center feed well, radially outward to the 

peripheral weirs. In comparison to rectangular basins, circular basins will have more land 

between the basins and also require more yard piping. Circular basins have easy sludge removal, 

can obtain high clarification efficiency, and are adaptable to high rate settling modules. However, 

if flow distribution from the inlet is not uniform, the settling efficiency will be hindered. These 

basins are not as hydraulically stable as rectangular basins.

5.4.2 Upflow Clarifier

In upflow clarifiers the influent enters at the bottom and clarified water flows upward 

while the solids settle to the bottom. As with horizontal flow basins, upflow clarifiers can also be 

modified with high rate settling modules. Upflow clarifiers can provide higher clarification 

efficiency than horizontal flow; however, they are more sensitive to shock loads than horizontal 

flow basins. 

5.4.3 Reactor Clarifier

Reactor clarifiers use the seeding concept to improve settling. The water flows through 

the sludge layer so particles can coalesce with already formed flocs. Two common designs of 

reactor clarifiers are slurry recirculation and sludge blanket clarifiers. Both operate on a center

feed system with built-in flocculation zones. The process is more complex than traditional 

horizontal or upflow clarifiers. Reactor clarifiers can provide high clarification efficiency but at 

the cost of flexibility—the source water quality and hydraulic loads must be constant. 
5. Presedimentation

LT2ESWTR Toolbox Guidance Manual 5-8 April 2010

5.4.4 High Flow Rate Designs 

High rate settlers are modules of inclined tubes or plates that are installed in horizontal 

flow (plates only) or simple upflow clarifiers. They provide increased surface area for particles to 

settle and reduce settling time. Kawamura (2000) noted poor performance occurred when flow 

distribution was uneven and flocculation was poor. 

5.4.5 Ballasted Flocculation 

Ballasted flocculation is a high-rate, physical-chemical clarification process that uses 

sand to improve the settling of flocculated particles. The floc attaches to the surface of a sand 

particle, which has a settling time 20 to 60 times faster than an alum floc (Kawamura 2002), thus 

creating a high-rate settling process. Because of the increased settling rate, the space required is 

much less than other clarifiers.

5.5 Design and Operational Issues 

5.5.1 Redundancy

As stated earlier, for compliance with the LT2ESWTR, all flow must be treated by the 

presedimentation process to receive Cryptosporidium treatment credit (40 CFR 141.717(a)). 

Systems should consider the need for redundancy in the design of a presedimentation process. 

Smaller systems or systems with a demand much lower than the design capacity may be able to 

shut down the water treatment plant for presedimentation basin maintenance activities and, thus, 

not require additional basins for redundancy. However, systems that operate on a continuous 

basis do not have that flexibility and should have a plan for staying in compliance while a basin 

is shut down. 

5.5.2 Short Circuiting

 A common issue that must be considered in the design and operation of presedimentation 

basins is short-circuiting. If a portion of flow does not have the adequate detention time, then the 

effluent turbidity is likely to be higher than anticipated. Several factors affect short-circuiting 

including even distribution of flow at the inlet, density or temperature differentials between 

influent and basin water, surface currents, and basin cleaning and sludge removal. 

 A proper design of the inlet is one of the most important design factors. In addition to 

flow short-circuiting, a poorly designed inlet can lead to overall hydraulic instability in the settle 
5. Presedimentation

LT2ESWTR Toolbox Guidance Manual 5-9 April 2010

zone. Installation of perforated baffles is a simple and effective method for even flow distribution 

from the inlet to the basin.

 Temperature differentials and high wind velocities could induce circular currents in the 

vertical direction of the basin. Influent water warmer than the basin water will rise to the surface 

and reach the outlet of the sedimentation basin much faster than the intended detention time of 

the basin. Influent water colder than the basin water will dive to the bottom of the basin and flow 

along the bottom of the basin and rise to the top of the basin at the outlet, thereby reaching the 

outlet of the sedimentation basin much faster than the intended detention time of the basin. 

Above ground tanks built of steel are more susceptible to temperature differentials from exposure 

to the sun and heat transfer. 

 The degradation of effluent water quality due to wind is more noticeable in circular or 

square sedimentation basins of diameters greater than 100 – 115 feet. When using long, shallow 

rectangular settling basins, effects of wind induced currents can be minimized by ensuring that 

the longitudinal axis of the basin is perpendicular to the prevailing wind direction. In addition to 

causing flow short-circuiting, currents can also scour settled solids, causing resuspension of 

settled solids and increasing effluent turbidity.

5.5.3 Sludge Removal 

 Sludge build-up in the tank decreases the volume of the sedimentation basin and reduces 

the settling time in the basin. Additionally, as sludge builds up, particles become more 

susceptible to resuspension during sludge removal, increasing the effluent turbidity. 

Sedimentation basins with high rate settlers accumulate sludge rapidly, and therefore require 

continuous sludge removal. 

5.5.4 Coagulant Addition and Dose Ranges of Common Coagulants 

Current operational practices of presedimention processes often focus on mitigating 

shock loads in the raw water supply (such as turbidity spikes due to precipitation in river source 

waters). However, during periods of low influent turbidity less attention may be given to the 

actual performance of the basin, resulting in less than 0.5 log turbidity reduction through the 

basin. To receive the credit, the presedimentation basin may need to be operated more 

stringently, including the addition of coagulant. The coagulant dose required to treat an influent 

stream depends on the chemical composition of the influent, the characteristics of the colloids 

and suspended matter in the influent, the addition of a coagulant aid, the water temperature, and 

mixing conditions. Coagulant dose and other water chemistry parameters of the coagulation and 

sedimentation processes are system-specific. Jar test procedures for evaluating the appropriate 

coagulants, dosages, and other chemical attributes for a treatment train are provided in AWWA’s 

Operational Control of Coagulation and Filtration Processes. 
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LT2ESWTR Toolbox Guidance Manual 5-10 April 2010

5.6 References

AWWA. 2000. Operational Control of Coagulation and Filtration Processes, AWWA Manual 

M37, Second Edition, pp. 1-34. 

Kawamura, Susumu. 2000. Integrated Design and Operation of Water Treatment Facilities. John 

Wiley & Sons, Inc. 

U.S. EPA, 1998. Optimizing Water Treatment Plant Performance Using the Composite 

Correction Program, pp. 233-236. 
LT2ESWTR Toolbox Guidance Manual 6-1 April 2010

6. Lime Softening

6.1 Introduction 

Lime softening is a drinking water treatment process that uses chemical precipitation with 

lime and other chemicals to reduce hardness and to enhance clarification prior to filtration. Lime 

softening can be categorized into two general types: (1) single stage softening that is used to 

remove calcium hardness and (2) two-stage softening that is used to remove magnesium hardness 

and high levels of calcium hardness. A single stage softening plant includes a primary clarifier 

and filtration components. A two stage softening plant has an additional clarifier located between 

the primary clarifier and filter. Within these general categories there are several possible 

treatment schemes; however, describing each is beyond the scope of this chapter. 

This toolbox option is practical for lime softening plants that either have a two stage 

process or could upgrade to a two stage process. The advantage of using this toolbox option to 

achieve compliance with the Long Term 2 Enhanced Surface Water Treatment Rule

(LT2ESWTR) is that systems will have the treatment process in place or if an upgrade or 

modification is needed, it could benefit the treatment of other contaminants. A disadvantage for 

softening plants is a potential reduced flexibility in the treatment train since all water must be 

treated by both stages. 

Since the water systems considering this toolbox option will most likely have a lime 

softening process in place, this section does not provide design or operational information. 

Instead, this section focuses on the requirements that lime softening systems must meet to receive 

Cryptosporidium removal credit and how those requirements can be met with general process 

modifications. The chapter is organized into two sections: 

6.2 LT2ESWTR Compliance Requirements - describes the criteria that plants must 

meet in order to receive additional credit for Cryptosporidum removal, and 

reporting requirements to maintain compliance.

6.3 Split Flow Processes - addresses compliance issues for split flow processes.

6.2 LT2ESWTR Compliance Requirements 

6.2.1 Credit for Cryptosporidium Removal 

The LT2ESWTR requires plants to meet the following criteria in order to receive 0.5-log 

credit towards additional Cryptosporidium treatment requirements (40 CFR 141.717(b)): 
6. Lime Softening

LT2ESWTR Toolbox Guidance Manual 6-2 April 2010

• The plant must have a second clarification step between the primary clarifier1 and filter 

which is operated continuously. For split treatment processes, only the portion of flow 

going through two clarification stages can receive credit. If a portion of flow bypasses one 

stage, additional treatment must be provided to the bypassed portion (see section 6.3). 

• Chemical addition and hardness removal must occur in two separate and sequential 

stages.

Exhibit 6.1 shows a typical two stage lime softening process. 

Exhibit 6.1 Typical Two-Stage Lime Softening Process

6.2.2 Reporting Requirements

The LT2ESWTR requires monthly verification and reporting of the following conditions 

for systems using the lime softening option (40 CFR 141.721):

• Chemical addition and hardness precipitation occurred in two separate and sequential 

softening stages prior to filtration. 

 

1 For purposes of compliance with the lime-softening toolbox option, “clarifier” is used as a general term 

for processes with settling and solids removal. 

• Both clarifiers treat 100 percent of the plant flow. 

A schematic of the treatment processes, clearly identifying the two stages of clarification, 

will assist the state in evaluating the process for the purposes of LT2ESWTR removal credit. 

Monitoring of chemical doses in the secondary clarifier over the expected range of seasonal raw 

water quality and recording of minimum and average chemical concentration will assist the state

in evaluating the process and the system in determining operating criteria. 
6. Lime Softening

LT2ESWTR Toolbox Guidance Manual 6-3 April 2010

6.3 Split-Flow Processes 

Split-flow processes divert a portion of the flow from either the first or second stage of 

the process and then blend the two streams together further downstream. Only the portion of flow 

that receives the two stages of treatment would be eligible for the 0.5 log credit. In these 

situations, systems would either have to: 1) eliminate the bypass and direct the entire flow 

through both stages, or 2) treat the bypassed portion with another toolbox option, such as 

chlorine dioxide, membranes, or ozone to receive Cryptosporidium inactivation/removal credit

for that stream. 
LT2ESWTR Toolbox Guidance Manual 7-1 April 2010

7. Combined and Individual Filter Performance

7.1 Introduction 

 Turbidity is an optical property that measures the amount of light scattered by suspended 

particles in a solution. It can detect a wide variety of particles in water (e.g., clay, silt, mineral 

particles, organic and inorganic matter, and microorganisms), but cannot provide specific 

information on particle type, number, or size. Therefore, the U.S. Environmental Protection 

Agency (EPA) recognizes that turbidity reduction is not a direct indication of pathogen removal, 

but is an effective indicator of process control. 

The Surface Water Treatment Rule (SWTR), Interim Enhanced SWTR (IESWTR), and 

Long Term 1 Enhanced SWTR (LT1ESWTR) all motivate public water systems to achieve a 

certain level of finished water quality by requiring them to meet specified filtered water turbidity 

limits. Under the IESWTR and LT1ESWTR, combined filter effluent (CFE) turbidity in 

conventional and direct filtration plants must be less than or equal to 0.3 nephelometric turbidity 

units (NTU) in 95 percent of samples taken each month and must never exceed 1 NTU. These 

plants are also required to conduct continuous monitoring of turbidity for each individual filter, 

and provide an exceptions report to the state or regulating agency when certain criteria for 

individual filter effluent (IFE) turbidity are exceeded. 

The Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) awards 

additional Cryptosporidium treatment credit to certain plants that maintain finished water

turbidity at levels significantly lower than currently required. This credit is not available to 

membrane, bag/cartridge, slow sand, or diatomaceous earth plants, due to the lack of 

documented correlation between effluent turbidity and Cryptosporidium removal in these 

processes.

The remainder of this chapter is organized as follows:

7.2 LT2ESWTR Compliance Requirements - describes the conditions for receiving 

Cryptosporidium removal credit and monitoring requirements for maintaining 

compliance.

7.3 Reporting Requirements - describes the routine reporting requirements that 

systems must follow to receive credit.

7.4 Process Control Techniques - discusses modifications or operational aspects that 

provide the tightened process control needed to meet the turbidity requirements 

for this toolbox option. 

7.5 Process Management Techniques - describes standard operating procedures 

(SOPs), response plans for loss of chemical feed, adequate chemical storage, and 

voluntary programs that encourage full process control from administration to 

operation and maintenance.
7. Combined and Individual Filter Performance

LT2ESWTR Toolbox Guidance Manual 7-2 April 2010

7.2 LT2ESWTR Compliance Requirements 

7.2.1 Treatment Credit

 For systems using conventional or direct filtration treatment to obtain an additional 0.5 

log Cryptosporidium removal credit, the LT2ESWTR requires the CFE turbidity measurements 

taken for any month at each plant are less than or equal to 0.15 NTU in at least 95 percent of the 

measurements (40 CFR 141.718(a)).

 The LT2ESWTR also allows systems using conventional or direct filtration treatment to 

claim an additional 0.5 log Cryptosporidium removal credit for any month at each plant that 

meet both of the following IFE turbidity requirements (40 CFR 141.718(b)):

1) IFE turbidity must be less than 0.15 NTU in at least 95 percent of values recorded at each 

filter in each month, excluding the 15 minute period following return to service from a 

filter backwash. 

AND

2) No individual filter may have a measured turbidity greater than 0.3 NTU in two 

consecutive measurements taken 15 minutes apart.

Systems may claim credit for combined filter performance AND individual filter 

performance in the same month (40 CFR 141.718(b)) for 1.0 log total. 

7.2.2 Monitoring Requirements

 For both the CFE and IFE options, compliance with the LT2ESWTR is determined by 

sample measurements taken for the IESWTR and LT1ESWTR (40 CFR 141.718). In other 

words, the LT2ESWTR does not require any additional monitoring from the IESWTR and 

LT1ESWTR.

7.2.2.1 Combined Filter Effluent

 The monitoring frequency and compliance calculation requirements for the CFE option 

are that CFE turbidity must be measured at 4-hour intervals (or more frequently) and 95 percent 

of the measurements from each month must be less than or equal to 0.15 NTU (40 CFR 

141.721).
7. Combined and Individual Filter Performance

LT2ESWTR Toolbox Guidance Manual 7-3 April 2010

7.2.2.2 Individual Filter Effluent

 The LT2ESWTR has specific reporting requirements. The monitoring frequency and 

compliance calculation requirements for the IFE option are that IFE turbidity must be measured 

every 15 minutes (excluding the 15 minute period following return to service from a filter 

backwash) and 95 percent of the measurements from each month must be less than or equal to 

0.15 NTU (40 CFR 141.721). 

 The LT2ESWTR specifies no individual filter may have a measured turbidity greater 

than 0.3 NTU in two consecutive measurements taken 15 minutes apart (40 CFR 141.721). If the 

individual filter is not providing water which contributes to the CFE (i.e., it is not operating, is 

filtering to waste, or its filtrate is being recycled) the system does not need to report the turbidity 

for that specific filter.

7.2.3 Turbidity Monitors

An important aspect of awarding additional removal credit for lower finished water 

turbidity is the performance of turbidimeters in measuring turbidity below 0.3 NTU. EPA 

believes that currently available turbidity monitoring equipment is capable of reliably assessing 

turbidity at levels below 0.1 NTU, provided instruments are well calibrated and maintained. EPA 

strongly recommends systems that pursue additional treatment credit for lower finished water 

turbidity develop the procedures necessary to ensure accurate and reliable measurement of 

turbidity at levels of 0.1 NTU and less, and believes these procedures to be essential to maintain 

toolbox credit. 

Turbidimeter maintenance should include frequent calibration by the manufacturer’s 

methods as well as frequent verification, in order to measure accurately in the low turbidity 

ranges required for this toolbox option. Chapter 3 of the LT1ESWTR Turbidity Provisions 

Guidance Manual describes the sampling methods, operation, maintenance, and calibration for 

turbidimeters and discusses quality assurance and quality control measures. This section 

summarizes the information from that chapter, including the approved methods, commonly used 

turbidimeters, calibration standards, and important factors of maintaining turbidimeters. Systems 

are encouraged to review Chapter 3 of the LT1ESWTR Turbidity Provisions Guidance Manual to 

ensure their operation, maintenance, and calibration practices meet or exceed those 

recommended by EPA. For a full copy of this document see: 

The LT1ESWTR guidance manuals are available on EPA’s website at:

http://www.epa.gov/ogwdw000/mdbp/lt1eswtr/guidance_lt1_turb.pdf. 

 
7. Combined and Individual Filter Performance

LT2ESWTR Toolbox Guidance Manual 7-4 April 2010

7.2.3.1 Methods

 Currently, EPA has approved four methods for the measure of turbidity (described in 40 

CFR 141.74): 

• EPA Method 180.1. 

• Standard Method 2130B. 

• Great Lakes Instrument Method 2. 

• Hach Filter Trak. 

 

7.2.3.2 Maintenance and Calibration

 Maintenance and calibration of both benchtop and on-line turbidimeters are fully 

described in the LT1ESWTR Turbidity Provisions Guidance Manual. It is very important to 

follow the manufactures procedures for maintenance and calibration of turbidimeters, as they 

vary between manufacturers. Exhibits 7.1 and 7.2 list several maintenance and calibration 

activities common among manufacturers for on-line and bench top turbidimeters. These 

activities should be conducted for all turbidimeters to ensure proper operation on a consistent 

basis.

Exhibit 7.1 Maintenance and Calibration Activities for On-line Turbidimeters

Activity Recommended Frequency

Inspect for cleanliness Weekly

Verify sample flow rate Weekly

Verify calibration with primary standard, secondary 

standard or by comparison with bench-top Weekly on CFE turbidimeter and monthly on all IFE 

turbidimeters

Clean and calibrate with primary standard

1

Quarterly

Replace lamp Annually 

1

Clean and recalibrate with primary standard if verification indicates greater than a +/-10 percent deviation from 

secondary standard.
7. Combined and Individual Filter Performance

LT2ESWTR Toolbox Guidance Manual 7-5 April 2010

Exhibit 7.2 Maintenance and Calibration Activities for Bench Top Turbidimeters

Activity Recommended Frequency

Inspect for cleanliness of bulbs and lenses Regularly, such as monthly or quarterly

Verify calibration with secondary standard Daily

Clean and calibrate with primary standard

1

Quarterly

Replace lamp Annually or according to manufacturer’s 

recommendations 1 Instrument calibration should be verified on a daily basis 

(http://www.epa.gov/ogwdw000/mdbp/pdf/turbidity/chap_03.pdf). Clean and recalibrate with primary standard if 

verification indicates greater than a +/-10 percent deviation from secondary standard.

 In addition to those activities listed in the tables, the following documentation or record 

keeping items should be developed and kept up to date: 

• Log of turbidimeter maintenance and calibration. 

• Quality assurance/quality control (QA/QC) plan for accuracy and consistency. 

• SOPs

7.2.3.3 Quality Assurance / Quality Control (QA/QC)

 Systems should develop a QA/QC plan for measuring turbidity. This plan should include 

written SOPs to ensure that operation, maintenance, and calibration activities are carried out in a 

consistent manner, and that each activity is understood by all that are involved. At a minimum, 

systems should develop SOPs for cleaning turbidimeters, creating Formazin Standards, 

calibrating turbidimeters, and referencing index samples.

 For bench top turbidimeters, measurement errors can be introduced by dirt, scratches, or 

condensation on the glassware, air bubbles in the sample, and particle settling. Operators should 

strictly follow manufactures procedures for sampling and maintenance.

7.3 Reporting Requirements 

7.3.1 Combined Filter Performance

 In order to receive the 0.5 log removal credit for the LT2ESWTR, a water system must 

submit monthly verification of CFE turbidity levels less than or equal to 0.15 NTU in at least 95 

percent of the 4-hour CFE measurements taken each month (40 CFR 141.721). 
7. Combined and Individual Filter Performance

LT2ESWTR Toolbox Guidance Manual 7-6 April 2010

7.3.2 Individual Filter Performance

 For the 0.5 log IFE removal credit under the LT2ESWTR, a water system must report 

monthly verification of IFE turbidity levels less than or equal to 0.15 NTU in at least 95 percent 

of all maximum daily IFE measurements taken each month for each filter (excluding the 15 

minute period following startup after backwash), and monthly verification that there were no IFE 

measurements greater than 0.3 NTU in two consecutive readings 15 minutes apart for any filter 

(40 CFR 141.721).

As requirements of the IESWTR and the LT1ESWTR, water systems must report 

monthly that they have conducted individual filter turbidity monitoring. Systems are required to 

report actual IFE measurements only if they have exceeded one of the IFE turbidity triggers. 

Systems that would apply successfully for the 0. 5 log Cryptosporidium IFE removal credit for 

LT2ESWTR compliance would not, by definition, be systems that were required to report IFE 

measurements under the earlier regulations. A system must, therefore, submit additional 

information about IFE turbidity measurements in order to receive the 0.5 log credit.

7.4 Process Control Techniques 

To meet the lower finished water turbidity requirements, systems will need a high level 

of process control from the source water intake to the filters. The Guidance Manual for 

Compliance with the IESWTR: Turbidity Provisions (U.S. EPA 1999) discusses many design 

and operational aspects water systems should consider for achieving low effluent turbidity. 

Chapter 4 of that manual provides design and operational modifications systems can use to 

optimize their process for compliance with the LT2ESWTR toolbox requirements. This chapter 

of the Toolbox Guidance Manual builds on that information, by highlighting those modifications 

or operational aspects that provide the tightened process control needed to meet the turbidity 

requirements for this toolbox option. To meet the lower finished water turbidity requirements of 

the CFE or IFE performance standards, systems will need consistent process performance and 

the ability to maintain the high filtered water quality under sub-optimal conditions and changing 

water quality.

 

 Design and operational factors are not the only considerations for maintaining the high 

filtered water quality standards; all areas of a water system must be dedicated towards the 

process optimization goal, including administration and maintenance. This toolbox option will 

require continuing effort and commitment from management and operations staff. Exhibit 7.3 

lists several factors in the areas of administration, design, operation, and maintenance that may 

The IESWTR guidance manuals are available on EPA’s website at:

http://www.epa.gov/OGWDW/mdbp/mdbptg.html. 
7. Combined and Individual Filter Performance

LT2ESWTR Toolbox Guidance Manual 7-7 April 2010

limit a system’s ability to continually meet the LT2ESWTR lower finished water turbidity 

requirements. This table demonstrates the importance of considering the capabilities of the entire 

water system. This table was adapted from the Composite Correction Program (CCP), an EPA 

program for optimizing water treatment plant performance (discussed in section 7.5.4.2). 
7. Combined and Individual Filter Performance

LT2ESWTR Toolbox Guidance Manual 7-8 April 2010

Exhibit 7.3 Performance Limiting Factors

(Adapted from the Composite Correction Program)

ADMINISTRATION

Plant Administrators

Policies

Do existing policies or the lack of policies discourage staff members from making 

required operation, maintenance, and management decision to support plant 

performance and reliability?

Familiarity with Plant Needs Do administrators lack first-hand knowledge of plant needs?

Supervision

Do management styles, organizational capabilities, budgeting skills, or 

communication practices at any management level adversely impact the plant to the 

extent that performance is affected?

Planning Does the lack of long range planning for facility replacement or alternative source 

water quantity or quality adversely impact performance?

Complacency Does the presence of consistent, high quality source water result in complacency 

within the water utility?

Reliability

Do inadequate facilities or equipment, or the depth of staff capability, present a 

potential weak link within the water utility to achieve and sustain optimized 

performance?

Source Water Protection Does the water utility lack an active source water protection program?

Plant Staff

Number Does a limited number of staff have a detrimental effect on plant operations or 

maintenance?

Plant Coverage

Does the lack of plant coverage result in inadequate time to complete necessary 

operational activities? (Note: This factor could have significant impact if no 

alarm/shutdown capability exists - see design factors).

Personnel Turnover Does high personnel turnover cause operation and maintenance problems that affect 

process performance or reliability?

Compensation

Does a low pay scale or benefit package discourage more highly qualified persons 

from applying for operator positions or cause operators to leave after they are 

trained?

Work Environment Does a poor work environment create a condition for “sloppy work habits” and lower 

operator morale?

Certification Does the lack of certified personnel result in poor O&M decisions?

Financial

Operating Ratio Does the utility have inadequate revenues to cover operation, maintenance, and 

replacement of necessary equipment (i.e., operating ratio less than 1.0)?

Coverage Ratio Does the utility have inadequate net operating profit to cover debt service 

requirements (i.e., coverage ratio less than 1.25)?

Reserves Does the utility have inadequate reserves to cover unexpected expenses or future 

facility replacement? 

DESIGN

Source Water Quality 

Microbial

Contamination 

Does the presence of microbial contamination sources in close proximity to the water 

treatment plant intake impact the plant’s ability to produce an adequate treatment 

barrier?
7. Combined and Individual Filter Performance

LT2ESWTR Toolbox Guidance Manual 7-9 April 2010

Unit Process Adequacy

Intake Structure Does the design of the intake structure result in excessive clogging of screens, buildup of silt, or passage of material that affects plant equipment?

Presedimentation Basin Does the design of an existing presedimentation basin or the lack of a 

presedimentation basin contribute to degraded plant performance?

Raw Water Pumping Does the use of constant speed pumps cause undesirable hydraulic loading on 

downstream unit processes?

Flow Measurement Does the lack of flow measurement devices or their accuracy limit plant control or 

impact process control adjustments? 

Chemical Storage and Feed 

Facilities Do inadequate chemical storage and feed facilities limit process needs in a plant? 

Flash Mix Does an inadequate mixing result in excessive chemical use or insufficient 

coagulation to the extent that it impacts plant performance?

Flocculation Does a lack of flocculation time, inadequate equipment, or lack of multiple flocculation 

stages result in poor floc formation and degrade plant performance?

Sedimentation Does the sedimentation basin configuration or equipment cause inadequate solids 

removal that negatively impact filter performance? 

Filtration Do filter or filter media characteristics limit the filtration process performance?

Disinfection

Do the disinfection facilities have limitations, such as inadequate detention time, 

improper mixing, feed rates, proportional feeds, or baffling, that contribute to poor 

disinfection?

Sludge/Backwash Water 

Treatment and Disposal

Do inadequate sludge or backwash water treatment facilities negatively influence 

plant performance?

Plant Operability

Process Flexibility

Does the lack of flexibility to feed chemicals at desired process locations or the lack 

of flexibility to operate equipment or processes in an optimized mode limit the plant’s 

ability to achieve desired performance goals?

Process Controllability Do existing process controls or lack of specific controls limit the adjustment and 

control of a process over the desired operating range?

Process Instrumentation 

/Automation

Does the lack of process instrumentation or automation cause excessive operator 

time for process control and monitoring?

Standby Units

Does the lack of standby units for key equipment cause degraded process 

performance during breakdown or during necessary preventive maintenance 

activities? 

Flow Proportioning Does inadequate flow splitting to parallel process units cause individual unit 

overloads that degrade process performance?

Alarm Systems Does the absence or inadequacy of an alarm system for critical equipment or 

processes cause degraded process performance?

Alternate Power Source Does the absence of an alternative power source cause reliability problems leading to 

degraded plant performance?

Laboratory Space and 

Equipment Does the absence of an adequately equipped laboratory limit plant performance?

Sample Taps Does the lack of sample taps on process flow streams prevent needed information 

from being obtained to optimized performance?
7. Combined and Individual Filter Performance

LT2ESWTR Toolbox Guidance Manual 7-10 April 2010

OPERATION

Testing

Process Control Testing Does the absence or wrong type of process control testing cause improper 

operational control decisions to be made? 

Representative Sampling Do monitoring results inaccurately represent plant performance or are samples 

collected improperly?

Process Control

Time on the Job Does staff’s short time on the job and associated unfamiliarity with process control 

and plant needs result in inadequate or improper control adjustments?

Water Treatment 

Understanding

Does the operator’s lack of basic water treatment understanding contribute to 

improper operational decisions and poor plant performance or reliability?

Application of Concepts and 

Testing to Process Control

Is the staff deficient in the application of their knowledge of water treatment and 

interpretation of process control testing such that improper process control 

adjustments are made?

Operational Resources

Training Program Does inadequate training result in improper process control decisions by plant staff?

Technical Guidance

Does inappropriate information received from a technical resource (e.g., design 

engineer, equipment representative, regulator, peer) cause improper decision or 

priorities to be implemented?

Operational 

Guidelines/Procedures

Does the lack of plant-specific operating guidelines and procedures result in 

inconsistent operational decision that impact performance?

MAINTENANCE

Maintenance Program

Preventive

Does the absence or lack of an effective preventive maintenance program cause 

unnecessary equipment failures or excessive downtime that results in plant 

performance or reliability problems?

Corrective Does the lack of corrective maintenance procedures affect the completion of 

emergency equipment maintenance?

Housekeeping Does a lack of good housekeeping procedures detract from the professional image of 

the water treatment plant?

Maintenance Resources

Materials and Equipment Does the lack of necessary materials and tools delay the response time to correct 

plant equipment problems?

Skills or Contract Services Do plant maintenance staff have inadequate skills to correct equipment problems or 

do the maintenance staff have limited access to contact maintenance services?

7.4.1 Chemical Feed

 There are two main considerations for the chemical application of a coagulation and 

flocculation treatment process:

• Are the chemicals and their dose optimum for the treatment process?

• Are they properly mixed or dispersed at the right point in the system? 
7. Combined and Individual Filter Performance

LT2ESWTR Toolbox Guidance Manual 7-11 April 2010

7.4.1.1 Type of Chemical and Dose 

 Optimizing the coagulation and flocculation for the range of water quality and demand 

experienced by the plant is a key factor in improving the overall treatment performance and 

ensuring process control. One method commonly used to evaluate the type and dose of coagulant 

and other chemical additives is the jar test (AWWA 2000a). 

 To provide the process control necessary for producing consistently low filter water 

turbidity, systems should establish SOPs for changing chemical additions when raw water 

quality changes significantly. The SOPs should list the appropriate chemicals to be added and 

the dose according to specified raw water conditions. Jar tests or other chemical evaluations 

should be conducted with raw water samples representing conditions from high water quality to 

the worst-case scenario and should reasonably represent the treatment process.

7.4.1.2 Mixing

 Adding coagulants at the proper location and providing the right amount of mixing is 

critical to the coagulation and flocculation processes.

• Metal salts such as alum and ferric chloride should be added at the point of highest 

mixing. 

• Low weight polymers can be added with the metal salts or at a second stage mixing 

process.

• High weight polymers should be added at a point of gentle mixing. 

 The coagulation process occurs rapidly; therefore, it is important that the coagulant is 

well-dispersed and distributed across the width of the flow stream at the point of addition. Flash 

mixers are necessary for coagulants requiring instantaneous mixing. Systems with mechanical 

mixers for these types of coagulants should consider changing to a design that provides more 

uniform dispersion as studies have indicated that mechanical mixers experience short circuiting 

and frequent maintenance requirements (Kawamura 2000). Kawamura rated several flash mixer 

designs according to (in order of importance) effectiveness, reliability, minimal maintenance, 

and cost: 

 1) Diffusion mixing by pressured water jets. 

 2) In-line static mixing. 

 3) In-line mechanical mixing. 

 4) Hydraulic mixing. 
7. Combined and Individual Filter Performance

LT2ESWTR Toolbox Guidance Manual 7-12 April 2010

 5) Mechanical flash mixing. 

 6) Diffusion by pipe grid. 

 The mixing speed should be adjustable and changed with flow and raw water conditions 

as necessary. Cold water is more viscous and may require a higher mixing energy. Highly turbid 

or colored water may also require more mixing power to properly disperse the coagulant. For 

flash mixing, Kawamura (2000) recommends G × t values of 300 to 1600, where G is the mixing 

energy (expressed in seconds-1) and t is time (seconds).

7.4.1.3 Streaming Current Detectors and Zeta Potential Monitors 

 The coagulation process should be monitored continuously, with real time output. 

Streaming current detectors (SCDs) can provide on-line coagulation control, by measuring the 

net surface charge of the particle and ionic species in a sample of water. Through jar testing or 

other coagulant studies, the charge measurement is correlated to the optimal coagulation 

conditions. The SCDs are typically located directly after coagulant addition to allow the operator 

time to adjust the dose of the coagulant before filtration. This quick response can prevent process 

upsets due to fluctuations in influent water quality. Zeta potential monitors also indicate particle 

surface charge and can be used in the same manner as SCDs.

 Source waters high in iron or manganese concentrations and the use of treatment 

chemicals with iron salts or potassium permanganate can extensively increase maintenance 

requirements (AWWA 2000a). Additionally, use of powdered activated carbon can increase 

maintenance requirements. AWWA recommends comparing SCD and zeta potential monitoring 

results to jar tests on a regular basis (AWWA 2000a). 

7.4.2 Flocculation

 The purpose of the flocculation process is to aggregate the particles into larger groups of 

particles or “flocs” that will settle in the subsequent sedimentation process. Through gentle and 

prolonged agitation, the suspended particles collide with each other and form flocs. The mixing 

must be thorough enough to provide opportunities for the particles to collide but also gentle 

enough to prevent the flocculated particles from breaking apart. It is likely, however, that some 

floc breakup will occur. As aggregates grow in size, they are more likely to break up due to the 

shearing forces in the mixing chamber. In this situation the aggregation and breakup can occur 

simultaneously leading to a steady-state distribution of floc sizes.

 The key factors of an effective flocculation process include: adequate mixing, low floc 

breakup, and plug flow conditions. The following guidance can help to achieve these conditions: 
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LT2ESWTR Toolbox Guidance Manual 7-13 April 2010

• Tapered mixing is most appropriate with variable G values ranging from 70 sec-1 to 15 

sec-1. 

• If flow is split between two flocculators, they should be mixing at the same speed. 

Coagulant dosages are most likely optimized to one speed.

• Basin inlet and outlet conditions should prevent floc breakup. 

• Baffling should be adequate to provide plug flow conditions. 

7.4.3 Sedimentation

 The purpose of the sedimentation process is to enhance filtration by removing the 

flocculated particles. As with other unit processes, the sedimentation process should be 

optimized and provide a consistent settled water quality. The key factors of a good settling 

process include:

• Minimization of short circuiting.

• Sludge removal equipment should not re-suspend particles or produce currents in the 

water.

• Surface loading rate, or overflow rate, needs to provide enough settling time. If 

flocculated particles are not settling, it could be a function of particle density or the 

surface loading rate.

• Continuous or frequent turbidity monitoring of settled water. 

To provide consistent, well-clarified water from the sedimentation basin, the operating 

parameters of the sedimentation basin may need to be adjusted with significant fluctuations in 

raw water quality. For example, if a runoff event causes a spike in turbidity, the particles may 

need more time to settle, and by decreasing the flow through the basin it is possible to achieve 

the desired level of clarification. Exhibit 7.4 lists sedimentation basin effluent turbidity goals for 

several state and industry optimization programs, such as the Area-Wide Optimization Program 

(AWOP). This is a multi-state effort whose goal is to help conventional surface water treatment 

plants optimize their existing particle removal and disinfection capabilities. For information on 

AWOP, including state contacts, please visit this site: 

http://www.asdwa.org/index.cfm?fuseaction=Page.viewPage&pageId=481&parentID=473&nod

eID=1. Operators need knowledge and authority to modify the coagulation and flocculation 

processes or reduce the flow to the plant when settled water quality goals are not being met. For 

long-term process control, tracking seasonal raw water quality changes and their impacts on the 
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LT2ESWTR Toolbox Guidance Manual 7-14 April 2010

settling process can provide valuable information for optimizing the overall sedimentation 

process.

Exhibit 7.4 Effluent Turbidity Goals for the Sedimentation Process

Optimization Program

Sedimentation Basin or Clarifier Effluent 

Turbidity Goal

California - Cryptosporidium Action Plan 1 to 2 NTU

Texas < 2 NTU

Partnership for Safe Water / EPA Composite 

Correction Program (CCP)

1 NTU for raw water conditions of < 10 NTU

2 NTU for raw water conditions of > 10 NTU

Note: For information on the Partnership for Safe Water, please visit this site: 

http://www.awwa.org/Resources/PartnershipMain.cfm?ItemNumber=51227&navItemNumber=51231. 

 The sludge blanket level is also an important factor for optimum settling conditions. A 

water system should have SOPs for sludge draw-off that include routine checks of the sludge 

pumping lines. Sludge pumping lines can plug, causing disruption of the sludge blanket and 

consequently disrupting the settling process. 

7.4.4 Filtration

 

Filtration is the last step in the particle removal process. Although filter performance is a 

function of the coagulation, flocculation, and sedimentation processes, proper filter operation is 

needed to provide the high quality finished water required for this toolbox option. The following 

factors should be considered when optimizing or evaluating filtration performance.

7.4.4.1 Flow Split

 Systems should evaluate the flow distribution to the filters to ensure there is an even load 

across all filters under the range of expected operating conditions (e.g., filter out of service, 

backwash).

7.4.4.2 Filter Beds

The filters should be operated with a design capacity that considers at least one filter as a 

reserve. The reserve filter is put on-line to maintain flow stability to the filters; if this is not 

possible, flow to the filters should be reduced. This will allow consistent flow when one filter is 

backwashed or taken out of service for maintenance.

Media loss or disturbance can lead to particles passing through the filters. The filter 

should be inspected on a regular basis to detect changes in the media. Media should be inspected 

to ensure depths of media are proper, the media are evenly distributed, and the size distribution 
7. Combined and Individual Filter Performance

LT2ESWTR Toolbox Guidance Manual 7-15 April 2010

of the media are still to specifications. Media samples can be taken with a coring device or by 

excavation for the inspection. If media are lost or damaged, they should be replaced. 

7.4.4.3 Backwashing

Backwashing is an integral part of the filtration process. Two important operating 

parameters for backwashing are the backwash flow rate and frequency of cycles. Other factors 

relating to backwash that affect filter effluent quality are hydraulic surges and filter start-up or 

“ripening.” 

Flow rate 

Systems should determine the appropriate flow that will clean the filter and prevent 

mudball formation, but will not upset the filter media and subject the underdrain to sudden 

momentary pressure increases. Typical flow rates are 15 to 20 gpm/ft2

 which result in 15 to 30 

percent bed expansion. 

Frequency

Although the filter effluent turbidity is the indicator for pathogen control and the 

determining factor for compliance, other operating parameters should be used to determine when 

backwash is needed. Emelko et al. (2000) performed filtration studies where pathogen 

breakthrough occurred towards the end of the filter cycle before an increase in turbidity was 

detected. Their studies emphasize the need to evaluate and optimize backwashing cycles with 

respect to filter effluent water quality. Most systems use filtration time, headloss, effluent 

turbidity, or effluent particle counts to indicate when backwashing is needed. For improved 

process control, it may be beneficial to use all indicators.

Systems with multiple filters also should evaluate the hydraulic surges resulting from 

backwashing. The timing of individual filter backwash cycles should be considered with respect 

to the other filters, particularly adjacent filters. Consider the following two examples: 

• If a large system with 50 filters backwashed 10 filters at the same time, this would cause 

a 20 percent increase in flow to the other filters. In this situation, the system could 

backwash fewer filters at one time or reduce the flow to the filters to avoid the filter 

overload. 

• When one filter is backwashed, a hydraulic surge can be experienced by an adjacent 

filter.
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Improving filter effluent during start-up 

 It is very important for systems to conduct a full evaluation of their backwashing process 

and operational variations to optimize the process. At the process optimization level, systems 

should minimize turbidity spikes in the filter effluent resulting from the backwashing process—it 

only takes a few high turbidity readings to cause non-compliance. The following operational 

practices may provide improved filter effluent during start-up: 

• Ramping the backwash rate down in increments to allow better media gradation. 

• Resting a filter after backwash for several minutes or up to several hours before putting 

the filter in service. 

• Adding a polymer to the backwash water. 

• Slowly increasing the hydraulic load on the filter as it is brought back on line. 

7.4.4.4 Filter to Waste

During the beginning of a filter cycle the filter is “ripening” and the effluent turbidity is 

usually higher. To avoid sending this poorer quality water to the CFE stream, the filter effluent 

produced during the first few minutes of a filter cycle can be sent to waste (filter to waste) or 

recycled to the head of the plant. Some systems filter to waste or recycle until the filter effluent 

reaches the desired level of turbidity. Practicing filter to waste produces an overall higher quality 

water and may be necessary to maintain a CFE or IFE below 0.15 NTU. 

7.4.4.5 Backwash Recycle

Plants that recycle the backwash water to the head of the plant should evaluate the 

impacts the backwash stream has on the coagulation, flocculation, and sedimentation processes. 

For example, the operator should know how the coagulation and flocculation processes need 

adjusting when there is a change in recycle flow. Ideally, the impacts of the recycle flow on 

these processes should be minimized.

For systems that recycle, the Filter Backwash Rule (FBR) requires spent filter backwash, 

thickener supernatant, or liquids from dewatering processes to be returned through all the 

processes of a system’s existing conventional or direct filtration treatment train (40 CFR 

141.76(c)). The rule allows for alternative recycle locations with state approval (40 CFR 

141.76(c)). 
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7.4.4.6 Filter Assessments

Filter assessments can provide valuable information for optimizing the performance of a 

filter. The IESTWR and LT1ESWTR require systems to conduct an individual filter selfassessment if a filter exceeds specified effluent turbidity criteria. However, systems seeking 

Cryptosporidium treatment credit for lower finished water turbidity should also consider 

conducting filter assessments to evaluate operating parameters and optimize filter performance. 

Chapter 5 of the IESWTR Turbidity Guidance Manual describes how to conduct an individual 

filter self- assessment. 

7.4.5 Hydraulic Control 

 Proper hydraulic control throughout the treatment process is essential. In the coagulation 

and sedimentation processes it is important to minimize short circuiting so the majority of the 

water receives the designed coagulation and sedimentation treatment. Hydraulic surges can cause 

greater turbulence that may break up flocculating particles and resuspend settling particles. In 

the subsequent filtration process, hydraulic surges can cause particle breakthrough anytime 

during the filtration cycle. Systems should look at historical water demand data and other 

conditions that adversely affect the system’s ability to control filter performance (e.g., 

backwashing, changes in flow). With these data, they should develop operating plans to address 

the condition and allow control of the filter effluent quality. 

7.5 Process Management Techniques 

7.5.1 Standard Operating Procedures (SOPs)

Developing SOPs for all aspects of the operation and maintenance of a water system is 

essential for running a high quality system. SOPs provide the basis for ensuring that activities are

accomplished in a consistent manner. They should be kept as simple as possible in order to 

ensure that each operator is consistent in carrying out the task at hand. The title of the procedure 

should be clear, concise, and descriptive of the equipment, process, or activity. SOPs should be 

developed with input from staff, thus enabling them to understand and implement procedures in 

compliance with applicable requirements.
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7.5.2 Prevention and Response Plan for Loss of Chemical Feed

Loss of chemical feed is a common cause of increased turbidity through the treatment 

processes. Plants should have equipment and SOPs for preventing such occurrences or reacting 

to them rapidly if they do occur. The following items are necessary to prevent an upset in water 

quality due to a chemical feed failure.

• SOPs to verify doses with feed response time (lag time) accounted for. 

• Redundant feeds. 

• Routine maintenance of all chemical feed parts (e.g., pump, feed line). 

• Inventory of spare parts available so repairs can be made quickly. 

• Pump or feed failure alarms. 

• Process monitors detecting chemical feed failure (e.g., streaming current, zeta potential, 

and pH monitors). 

7.5.3 Adequate Chemical Storage 

Sufficient chemical storage is necessary to ensure continued operation of the plant at 

proper dosages, including enough to run at higher dosages if an unexpected turbidity spike 

should occur in the raw water. Care must also be taken, however, to follow manufacturer’s 

suggestions on the useful life of the chemical. Many coagulants will degrade over time and will 

not perform properly and may even cause increased turbidity if allowed to age too long. Storage 

tanks should also be designed so that there are no dead spaces where chemicals may accumulate 

with much longer residence times than the hydraulic residence time of the tank. 

7.5.4 Voluntary Programs 

EPA, state regulatory agencies, AWWA, and other drinking water organizations have 

established voluntary programs for systems to ensure the delivery of safe water to their 

customers. These programs often focus on optimizing the treatment process and identifying the 

limiting factors of performance. Consequently, they are excellent aids for systems considering 

this toolbox option. This section discusses two programs, the Partnership for Safe Water and the 

CCP. (The CCP is also promoted as part of the Partnership for Safe Water).
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7.5.4.1 Partnership for Safe Water 

The Partnership for Safe Water is a voluntary cooperative effort between EPA, AWWA, 

and surface water systems. The goal of the program is to “provide a new measure of safety to 

millions of Americans by implementing prevention programs where legislation or regulation 

does not exist. The preventive measures are based around optimizing treatment plant 

performance, and thus increasing protection against microbial contamination in America’s 

drinking water supply.” (http://www.awwa.org/partner/partner1.htm).

For further information about the Partnership for Safe Water and how to join, see AWWA’s 

website: 

http://www.awwa.org/Resources/PartnershipMain.cfm?ItemNumber=51227&navItemNumber=5

1231. 

Water systems that participate in the program go through four phases: 

Phase I: Commitment – operators and management indicate their willingness to complete the 

program through phase III. 

Phase II: Data Collection and Analysis – the water system must collect one year of raw, settled, 

and filter effluent turbidity data and submit to AWWA for analysis. 

Phase III: Self Assessment – allows the system to examine the capabilities of the existing plant’s 

operation and administration and identify factors that limit performance.

Phase IV: Procedures and Applications Package – systems demonstrate they addressed areas of 

limited performance and produce high quality water as measured by filter effluent turbidity. 

 Through the efforts of monitoring, data analysis, and evaluating the capabilities of unit 

processes, significant improvements in water quality can be achieved. In the Partnership’s 2001 

Annual report, AWWA reported an increase from 20 percent to 32 percent of plants completing 

Phase II with finished water turbidity levels less than 0.1 NTU (based on 95th percentiles). At 

the beginning of Phase III, approximately 51 percent of plants reported 95th percentile turbidity 

less than 0.1 NTU, and after completing Phase III approximately 70 percent of plants achieved 

less than 0.1 NTU. 

7.5.4.2 Composite Correction Program (CCP)

 The CCP was developed in 1988 to optimize surface water treatment plant performance 

with respect to protection from microbial pathogens. The program consists of two parts, the 

comprehensive performance evaluation (CPE) and comprehensive technical assistance (CTA). 

The CPE is a thorough review and analysis of a facility’s design capabilities and associated 

administrative, operational, and maintenance practices as they relate to achieving optimum 
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LT2ESWTR Toolbox Guidance Manual 7-20 April 2010

performance from the facility. It can be conducted by the system or by a third party over a period 

of roughly 3 to 4 days. The CTA builds on the results of the CPE by addressing the combination 

of factors that limit a facility’s performance. If conducted by a third party, it should be 

implemented by a third party who is in a position to pursue corrective actions in all areas, 

including politically sensitive, administrative, or operational limitations.

 EPA published a handbook, Optimizing Water Treatment Plant Performance Using the 

Composite Correction Program (1998), that fully describes the goals, methods, and procedures 

of the CCP. To obtain a copy, call the EPA Safe Drinking Water Hotline at 1-800-426-4791. 

7.6 References

American Water Works Association. 2000a. Operational Control of Coagulation and Filtration 

Processes, 2nd Edition. American Water Works Association.

American Water Works Association. 2000. Water Quality and Treatment 5th Edition. McGraw 

Hill.

Kawamura, Susumu. 2000. Integrated Design and Operation of Water Treatment Facilities. John 

Wiley & Sons, Inc. 

U.S. EPA. 1998. Optimizing Water Treatment Plant Performance Using the Composite 

Correction Program. Office of Water and Office of Research and Development. EPA 625/6-

91/027. 
LT2ESWTR Toolbox Guidance Manual 8-1 April 2010

8. Bag and Cartridge Filters

8.1 Introduction 

 Under the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR), bag and 

cartridge filters are defined as pressure driven separation devices that remove particles larger 

than 1 micrometer (μm) using an engineered porous filtration media. Bag filters are typically 

constructed of non-rigid, fabric filtration media housed in a pressure vessel in which the 

direction of flow is from the inside of the bag to the outside. Cartridge filters are typically 

constructed as rigid or semi-rigid self supporting filter elements housed in pressure vessels in 

which flow is from the outside of the cartridge to the inside (40 CFR 141.2). 

A pressure vessel may contain either single or multiple filters in a series or in parallel. As 

the water flows through a bag or cartridge filter, particles collect on the filter and the difference 

in pressure from the inlet to the outlet, termed “pressure drop,” increases. Once a “terminal 

pressure drop” is reached, the bag or cartridge filter is replaced.

 

 Typically, bag and cartridge filters are used by small systems for protozoa or other 

particle removal. The pore sizes in the filter bags and cartridges designed for protozoa removal 

are small enough to remove protozoan cysts and oocysts but generally large enough that viruses, 

bacteria, and fine colloidal clays could pass through. 

 This chapter provides background information on the treatment performance, design, and 

operation of bag and cartridge filters, with emphasis on those issues that a system should 

consider for integrating bag or cartridge filters into its treatment process to comply with the 

LT2ESWTR. This chapter is organized as follows:

8.2 LT2ESWTR Compliance Requirements - describes criteria and reporting 

requirements that systems must meet to receive Cryptosporidium treatment credit. 

8.3 Toolbox Selection Considerations - describes the advantages and disadvantages 

of integrating a bag and cartridge filtration process for compliance with the 

LT2ESWTR.

8.4 Challenge Testing - describes the challenge testing that a bag or cartridge filter 

must pass to be awarded Cryptosporidium treatment credit for the LT2ESWTR. 

8.5 Design Considerations - discusses influent water quality, size of filter system and 

redundancy, layout features, filter cycling, pressure monitoring, valves and 

appurtenances, air entrapment, and National Science Foundation (NSF) 

certification. 

8.6 Operational Issues - discusses pressure drop across the filter, and monitoring to 

assess performance and indicate possible process upsets with the bag or cartridge 

filter or other upstream processes.
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8.2 LT2ESWTR Compliance Requirements 

8.2.1 Credits

Bag and cartridge filtration processes that meet the EPA definition and demonstrate 

Cryptosporidium removal through challenge testing may receive the following Cryptosporidium

removal credit for the LT2ESWTR (40 CFR 141.719(a)): 

• Up to 2.0-log removal for individual bag or cartridge filters showing a minimum of 3.0-

log removal in challenge testing. 

• Up to 2.5-log removal for bag or cartridge filters in series showing a minimum of 3.0-log 

removal in challenge testing. 

 Challenge testing must be conducted according to the LT2ESWTR requirements outlined 

in section 8.4 of this chapter. A 1-log factor of safety for a single filter and 0.5-log factor of 

safety for multiple filters in series is applied to the allowable removal credit over that 

demonstrated by challenge testing because bag and cartridge filters cannot have their integrity 

directly tested; hence, there are no means of verifying their removal efficiency during routine 

use.

 Recently, some cartridge filtration devices have been developed for drinking water 

treatment using membrane media, which can be direct integrity tested. These membrane 

cartridge filters (MCFs) could be considered a membrane filtration process for the purpose of 

compliance with the LT2ESWTR treatment requirements for Cryptosporidium (i.e., the MCF 

process would be eligible for the same credit, and subject to the same requirements, as a 

membrane filtration process). A direct integrity test is a physical test applied to a membrane unit 

to identify and isolate integrity breaches (i.e., one or more leaks that could result in 

contamination of the filtrate). Manufacturers can provide information on direct integrity testing 

and whether it is feasible with their products. Refer to the EPA Membrane Filtration Guidance 

Manual (U.S. EPA 2005) for direct integrity testing and other membrane filtration requirements.

8.2.2 Reporting Requirements

All reporting requirements for the Surface Water Treatment Rule (SWTR), Interim 

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

Treatment Rule (LT1ESWTR) are still applicable; the LT2ESWTR does not modify or replace 

any previous rule requirements. The location of filter effluent turbidity monitoring for 

compliance with the IESWTR and LT1ESWTR does not change with the installation of a bag or 

cartridge filter as a secondary filtration process. That is, a system would still monitor filter 

effluent turbidity after the primary filters for compliance with the IESWTR and LT1ESWTR.

When bag and/or cartridge filters are used to comply with treatment requirements, the 

LT2ESWTR requires systems to submit an initial report that demonstrates the following (40 

CFR 141.721(f)): 
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LT2ESWTR Toolbox Guidance Manual 8-3 April 2010

• Process meets the definition of a bag or cartridge filter. 

• Removal efficiency from challenge testing (described in section 8.4). The removal 

demonstrated must be 1.0-log greater than the credit awarded for a single and 0.5-log 

greater than the credit awarded for multiple filters in series. 

 This initial report must be submitted by April 1, 2012 for systems serving more than 

100,000, October 1, 2012 for systems serving between 50,000 and 99,999, October 1, 2013 for 

systems serving between 10,000 and 49,999 and October 1, 2014 for small systems serving 

fewer than 10,000 people. 

 

 For routine compliance reporting, systems must verify each month that 100 percent of 

plant flow was treated by the bag or cartridge filter (40 CFR 141.721(f)). One possible approach 

states may elect to use for flow verification is to have operators certify each month that all flow 

was treated by the filter. States may require additional reporting at their discretion. Section 8.6 

provides recommendations for filter effluent and process monitoring. 

8.2.3 Integration into a Treatment Process Train

 To achieve compliance with the IESWTR and LT1ESWTR, all plants (except those 

meeting the filter avoidance criteria in 40 CFR 141.71) must have a filtration process approved 

by the state. Approved processes receive 2-log Cryptosporidium removal credit under the 

IESWTR and LT1ESWTR. For compliance with additional treatment requirements for the 

LT2ESWTR, bag and cartridge filters should be added as an additional filtration process 

following the existing primary filtration (see Exhibits 8.1 and 8.2). The bag and cartridge filters 

provide additional removal of the smaller contaminants and any contaminants that break through 

the granular media filters during the end of a run cycle or process upsets. 

Exhibit 8.1 Schematic of Treatment Process with Bag/Cartridge Filters

For those systems using a bag or cartridge filter process to meet LT1ESWTR

requirements, thus serving as the primary filtration process, it may be possible to configure the 

bag or cartridge filters in a series (see Exhibit 8.2).
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LT2ESWTR Toolbox Guidance Manual 8-4 April 2010

Exhibit 8.2 Bag/Cartridge Filters in Series

 Another possible configuration is a bag or cartridge filter followed by an ultraviolet light 

(UV) system (see Exhibit 8.3). This configuration would allow removal of particles and 

microbial pathogens as well as inactivation of Cryptosporidium, Giardia, and viruses. In this 

case, the bag or cartridge filter would serve as the primary filter and thus, be subject to SWTR, 

IESWTR, and LT1ESWTR requirements, while the UV system would be subject to the 

LT2ESWTR requirements. Refer to EPA’s UV Disinfection Guidance Manual (U.S. EPA 2006) 

for information regarding UV systems and associated requirements with LT2ESWTR.

Exhibit 8.3 Bag/Cartridge Filter with UV System

Factors that should be considered when developing a treatment process scheme include 

available space, hydraulic profile, and point of disinfection. Space requirements are small for bag 

and cartridge filter systems, but extra space for maintenance activities should be considered in 

the planning process. Because a significant headloss is associated with an additional filtration 

process, systems should consider their hydraulic profile when integrating new filters into an 

existing process sequence. Although the addition of a new bag filtration process does not 

necessarily require that the point of primary disinfection be changed, some bag filtration 

installations chlorinate prior to the bag filtration process to minimize biofilm growth on the bags. 

However, if systems are considering using a bag or cartridge filter as the primary filter as in 

Exhibit 8.3, chlorinating prior to filtration will likely cause higher disinfection byproduct 

formation compared to post-filter chlorination since the filtration process will remove some 

organic material.
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LT2ESWTR Toolbox Guidance Manual 8-5 April 2010

8.3 Toolbox Selection Considerations 

This section describes the advantages and disadvantages of integrating a bag and 

cartridge filtration process for compliance with the LT2ESWTR.

8.3.1 Advantages 

The advantages of bag and cartridge filtration processes include low maintenance 

requirements, relatively low capital cost, minimal operator training, and low space requirements. 

The only routine maintenance required is filter replacement when a defined terminal pressure 

drop or other operating parameter, such as filter age or volume treated, is reached. The operation 

of these systems is straightforward and requires little technical skill. In addition, the filter 

materials are relatively inexpensive and the housing system is not complex, resulting in 

relatively low capital costs.

8.3.2 Disadvantages 

A disadvantage of bag and cartridge filtration processes is most filters must be replaced 

instead of regenerated. For larger flows, or water with higher particle loads, frequent filter 

replacement increases operation and maintenance costs. Additional pumps may be required to 

provide needed pressure. Also, redundancy should be built into the process design, increasing 

costs. Bag and cartridge filters can also be subject to clogging by biofilm growth or excess 

coagulants. Maintaining a residual through the filter is one possible way to prevent biofilm 

growth. See the Simultaneous Compliance Guidance Manual (U.S. EPA 2007) for additional 

recommendations. 

8.4 Challenge Testing 

Manufacturers commonly rate fabric filters by pore size or pore distribution. However, 

there is no industry standard for measuring or reporting these characteristics. This lack of 

standardization causes problems for establishing design criteria to ensure that a given bag or 

cartridge filter will effectively remove a given percentage of Cryptosporidium. Furthermore, an 

oocyst has different structural characteristics than the markers used to determine pore size; thus, 

the rate of rejection may differ for an oocyst versus the test markers used to determine pore size 

or molecular weight cutoff. To compensate for these factors of uncertainty for Cryptosporidium

removal, the LT2ESWTR requires bag or cartridge filters to be challenge tested to determine 

removal credit. 

 Challenge testing is a process in which a known quantity of Cryptosporidium oocysts (or 

an acceptable surrogate) is added to the filter influent and the effluent concentration is measured 

to determine the removal capabilities of the filter. This testing is product-specific, not sitespecific, meaning it does not have to be tested at every water system seeking removal credit. 

Instead, a manufacturer (or independent third party) would challenge test each of its products in 

order to obtain a 2.0- or 2.5-log Cryptosporidium removal rating. Bag or cartridge filters must be 
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LT2ESWTR Toolbox Guidance Manual 8-6 April 2010

challenge tested, however, in the same configuration that the system will use, either as individual 

filters or as a series of filters.

 For compliance with the LT2ESWTR, EPA defined a set of test conditions that must be 

met for an acceptable challenge test. These conditions provide only a framework for the 

challenge test; states may develop additional testing requirements. The EPA Membrane 

Filtration Guidance Manual (U.S. EPA 2005) contains detailed guidance on developing 

challenge test protocol and conducting the test for membrane processes that relate to these 

requirements. Additionally, NSF International, in cooperation with EPA, developed the Protocol 

for Equipment Verification Testing for Physical Removal of Microbiological and Particulate 

Contaminants (NSF International 2005) with a chapter for testing bag and cartridge filters 

(Chapter 4). Although the protocol was developed for compliance with the SWTR, some testing 

principles still apply1

. 

 Section 8.4.1 describes the test conditions required by the LT2ESWTR (40 CFR 

141.719(a)(2)-(8)). Section 8.4.2 shows how to calculate the log removal value (LRV) for 

challenge testing results. Section 8.4.3 discusses modifications to the filter unit (e.g., change in 

filter media) occurring after challenge testing that may require additional challenge testing.

8.4.1 Testing Conditions

8.4.1.1 Full Scale Filter Testing

Challenge testing must be conducted on full-scale bag or cartridge filters and the 

associated filter housing or pressure vessel that are identical in material and construction to the 

filters and housing the system will use for removal of Cryptosporidium. 

8.4.1.2 Challenge Particulate 

 Challenge testing must be conducted using Cryptosporidium or a surrogate which is 

removed no more efficiently than Cryptosporidium. The microorganism or surrogate used during 

challenge testing is referred to as the “challenge particulate.” The concentration of the challenge 

particulate must be determined using a method capable of discreetly quantifying the specific 

organism or surrogate used in the test; gross measurements such as turbidity may not be used (40 

CFR 141.719(a)(3)). Key physical characteristics to be considered for identifying an acceptable 

surrogate include size, shape, and surface charge. Other factors include ease of measurement and 

cost. Chapter 3 of EPA’s Membrane Filtration Guidance Manual (U.S. EPA 2005) describes the 

characteristics of acceptable surrogates and lists potential and inert surrogates for 

Cryptosporidium. Examples of possible microbial surrogates are P. dimunita and S. marcessans. 

 

 

1 Specific sections of the EPA/NSF ETV Protocol that provide guidance for developing and conducting a challenge 

test for LT2ESWTR include: section 7.0, Characterization of Feed Water; section 11.0, Operating Conditions; 

section 12.3, Work Plan; section 13.0, Data Management; and section 14.0, QA/QC.
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8.4.1.3 Test Solution Concentration 

 In order to demonstrate a removal efficiency of at least 3-log for bag or cartridge 

filters, it may be necessary to seed the challenge particulate into the test solution. A criticism of 

this approach is that the seeded levels are orders of magnitude higher than those encountered in 

natural waters, which could lead to artificially high estimates of removal efficiency. To address 

this issue, EPA set a limit on the maximum feed concentration applied to a filter during the 

challenge study. The limit is based on the detection limit of the challenge particulate:

Equation 8-1 

Maximum Feed Concentration = 1.0 × 104

 × Filtrate Detection Limit 

 These concentrations allow the demonstration of up to 4.0-log removal for bag filters and 

cartridge filters during challenge testing if the challenge particulate is removed to the detection 

limit.

Example 8-1 - Determining maximum allowable feed concentration 

If the detection limit of the surrogate test is 2 units/L, then the maximum feed 

concentration is 1 × 104

 × (2) = 2 × 104

8.4.1.4 Challenge Test Duration

Each filter must be tested for a duration sufficient to reach “terminal pressure drop” (40 

CFR 141.719(a)(6)). Terminal pressure drop is a parameter specified by the manufacturer that 

establishes the end of the useful life of the filter. Continuous challenge particulate feed is not 

required (i.e., intermittent seeding is permitted). At a minimum, removal efficiency must be 

determined during three periods over the filtration cycle: 

• Within 2 hours of start-up of a new filter. 

• When the pressure drop is between 45 and 55 percent of the terminal pressure drop. 

• At the end of the run after the pressure drop has reached 100 percent of the terminal 

pressure drop. 

 The rule does not specify the number of samples that must be collected during each of the 

three periods. Because the effluent concentration is often very low and near the detection limit, it 

may be beneficial to collect more effluent than influent samples to obtain a more accurate 

removal efficiency. 
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8.4.1.5 Water Quality of Test Solution

Water quality can have a significant impact on the removal of particulate contaminants, 

such as Cryptosporidium. In general, bag and cartridge filters in water treatment do not 

experience influent turbidity concentrations much greater than 10 nephelometric turbidity units

(NTU). For the application of the LT2ESWTR, they typically will receive filtered water and 

thus, very low turbidity. 

A clean-water challenge test will generally provide the most conservative estimate of 

removal efficiency. However, since the challenge test must run until terminal head loss is 

reached, the challenge test solution should contain some solids to cause the head loss build-up 

across the filter, but not an excessive amount that will cause a rapid build-up. Particulate foulants 

that may be appropriate to add to the test solution include clay particles (such as bentonite or 

kaolin) or carbon powder, as long as they are not excessively fine-sized. 

 The following are recommended for the challenge test solution: 

• High quality water with a low to moderate concentration of suspended solids should be 

used as the challenge solution. Suspended solids concentration should be high enough to 

achieve a reasonable rate of headloss buildup, but not so high that the headloss builds up 

too rapidly to conduct the challenges at the various headloss levels. 

• No oxidants, disinfectants, or other pretreatment chemicals should be added to the test 

solution. 

• Test water should be characterized with respect to basic water quality parameters, such as 

pH, turbidity, temperature, and total dissolved solids. 

8.4.1.6 Maximum Design Flow Rate

The challenge test must be conducted at the maximum design flow rate for the filter as 

specified by the manufacturer (40 CFR 141.719(a)(5)). 

8.4.1.7 Challenge Particulate Seeding Method

There are two basic approaches to seeding: batch seeding and in-line injection. In batch 

seeding, all of the challenge particulates are introduced into the entire volume of test solution 

and mixed to a uniform concentration. Batch seeding requires the entire test solution to be 

contained in a reservoir and for the reservoir to be well mixed to ensure a uniform concentration 

of the seeded particles. Generally, batch seeding is used for small scale systems that only require 

relatively small amounts of feed solution for testing. 

In-line injection is the most common seeding approach used in challenge testing, 

allowing challenge particulates to be introduced into the feed on either a continuous or 

intermittent basis. While either could be used, intermittent seeding may be preferable to 
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LT2ESWTR Toolbox Guidance Manual 8-9 April 2010

continuous seeding for conducting the challenge test at the required intervals (i.e., a minimum of 

beginning, middle, and end-of-run). If intermittent injection is used, equilibrium should be 

achieved during each seeding event prior to the collection of feed and filtrate samples. 

In-line injection delivers the challenge particles from a concentrated stock solution with a 

known feed concentration. Guidelines and examples for determining challenge test feed 

concentration and stock solution delivery rates are provided in Chapter 3 of the Membrane 

Filtration Guidance Manual (U.S. EPA 2005). 

In-line injection requires additional equipment, such as chemical feed pumps, injection 

ports, and in-line mixers. These components should be designed to ensure a consistent challenge 

particulate concentration in the feed. A chemical metering pump that delivers a steady flow is 

recommended (pumps that create a pulsing action should be avoided). The injection port should 

introduce the challenge material directly into the bulk feed stream to aid in dispersion. An in-line 

static mixer should be placed downstream of the injection port, and a feed sample tap should be 

located approximately ten pipe diameters downstream of the mixer (U.S. EPA 2005). 

8.4.1.8 Challenge Test Solution Volume

 The volume of the test solution depends on filtrate flow rate, test duration, and hold-up 

volume of the test system. For intermittent, in-line injection, the seeded test solution volume can 

be considerably less than that required for batch seeding. Formulas for calculating test solution 

volume and examples are provided in Chapter 3 of the Membrane Filtration Guidance Manual

(U.S. EPA 2005). 

8.4.1.9 Sampling

 An effective sampling program depends on a detailed sampling plan and the use of 

appropriate sampling methods, locations, and quality assurance/quality control (QA/QC) 

measures.

 Samples can be collected using either grab or composite sampling methods. Grab 

samples consist of pre-determined amounts of water taken from the feed or filtrate streams, while 

composite samples are of the entire process stream. Grab sampling is commonly used to 

determine the concentration of challenge particulates in the feed solution, while grab or 

composite sampling is used to analyze the filtrate stream. Good sampling practices include 

flushing samples taps, using clean sample containers, and preventing cross contamination of 

samples. QA/QC measures include clearly identifying samples, collecting duplicates, and using 

blanks. 

In many cases, it may be advantageous to collect more filtrate samples than feed samples, 

since the concentration of the challenge microorganism in the filtrate samples is expected to be 

very low and error of just a few particles could have significant impact on the demonstrated 

removal efficiency. 
8. Bag and Cartridge Filters

LT2ESWTR Toolbox Guidance Manual 8-10 April 2010

 Sample port design is an important consideration and should ensure that a representative 

sample is obtained. Poorly designed ports contain large volumes where stagnation may occur 

(e.g., large valves and long sample tubes) and pull the sample from the edge of the pipe. A well 

designed port has a sample quill that extends into the center of the pipe to draw a more 

representative sample.

 Chapter 3 of the Membrane Filtration Guidance Manual (U.S. EPA 2005) contains 

additional information on developing sampling plans and provides schematics of typical 

sampling apparatuses.

8.4.2 Calculating Log Removal (141.719(a)(7)-(9))

Removal efficiency of a filter must be determined from the results of challenge testing 

and calculated using Equation 8-2. 

Equation 8-2 

LRV = Log10(Cf) - Log10(CP) 

Where:

LRV = log removal value demonstrated during challenge testing 

Cf

C

= feed concentration measured during the challenge test

p = filtrate concentration measured during the challenge test 

The feed and filtrate concentrations must be expressed in the same units (number of 

challenge particulate per unit volume). If the challenge particulate is not detected in the filtrate, 

then the filtrate concentration (CP) must be set equal to the detection limit.

Example 8-2 - Calculating the LRV

Feed Concentration (Cf

Filtrate Concentration (C

) 20,000 units/L 

P) 3 units/L 

LRV = Log(20,000) - Log(3) 

LRV = 4.30 - 0.48 = 3.82 

 The LT2ESWTR does not specify how the feed and effluent concentration must be 

determined. A conservative approach would be to use the lowest feed concentration and highest 

filtrate concentration from each filter run. 

A challenge test will likely evaluate multiple filters. An LRV must be calculated for each 

filter tested. The final log removal efficiency assigned to the filter process tested depends on the 

number of filters tested:
8. Bag and Cartridge Filters

LT2ESWTR Toolbox Guidance Manual 8-11 April 2010

• If fewer than 20 filters were tested during a challenge study, the overall removal 

efficiency for the filter product line must be set equal to the lowest LRV observed among 

the filters.

• If 20 or more filters were tested during challenge testing, the overall removal efficiency 

for the product line must be set equal to the 10th percentile of the LRVs observed during 

the challenge study. (The percentile is defined by [i/(n+1)] where i is the rank of n

individual data points ordered lowest to highest. If necessary the system may calculate 

the 10th percentile using linear interpolation). 

8.4.3 Modifications to Filtration Unit after Challenge Testing (141.719(a)(10))

 If a previously tested filter is modified in a manner that could change the removal 

efficiency of the filter product line, challenge testing to demonstrate the removal efficiency of 

the modified filter must be conducted and submitted to the state. Significant modifications may 

include, but not limited to: 

• Changes to the filtration media (e.g., different fabric, change in the filter manufacturing 

process). 

• Changes to the configuration of the filtration media. 

• Modifications to the sealing system.

8.5 Design Considerations 

 Bag and cartridge filter systems may contain anywhere from one to over twenty filter 

units. There is no maximum number of filters a system can include; however, membrane or other 

filtration processes become more practical for larger flows since bag and cartridge filters are 

generally replaced instead of backwashed or regenerated. A single filter unit is comprised of the 

filter media (bag or cartridge), housing, and associated piping and valves. Exhibit 8.4 shows a 

typical single filter vessel (housing).
8. Bag and Cartridge Filters

Source: U.F. Strainrite

Systems with multiple filters may be designed as a manifold with connective piping 

between the individual filters in separate housing or alternatively as multiple filters in a single 

housing. Exhibits 8.5 and 8.6 show the manifold design and multiple filter vessel design, 

respectively.

Exhibit 8.4 Single Filter Vessel

LT2ESWTR Toolbox Guidance Manual 8-12 April 2010
8. Bag and Cartridge Filters

Exhibit 8.5 Manifold Bag Filter Design

Exhibit 8.6 Multiple Filter Vessel

LT2ESWTR Toolbox Guidance Manual 8-13 April 2010

Source: U.F. Strainrite
8. Bag and Cartridge Filters

LT2ESWTR Toolbox Guidance Manual 8-14 April 2010

 The designs of bag and cartridge filters are not complex; however, there are a couple of 

key issues that should be taken into consideration. First, the filter units should be designed 

integrally with their respective housing systems. Poor fittings can cause leaks and premature 

failure. Manufacturers can provide individual filter units that can be retrofitted into the existing 

process or complete filter houses that are skid mounted. It is important to adhere to the 

manufacturer’s instructions on filter installation. 

 Second, the overall water treatment process design should minimize sudden changes in 

pressures applied to the bag or cartridge filters. Each time the flow to the filter is interrupted and 

then restarted, a sudden increase in pressure can occur across the filter unit unless steps are taken 

to allow for gradual pressure ramp-up. The particle load in the filter effluent often increases 

when the filter cycle begins. A study by McMeen (2001) reported that the increase in particle 

load could be occurring due to the seal at the top of the filter failing when the pressure suddenly 

increases. Bag filters are especially susceptible to cycling because these pressure fluctuations 

also increase wear on the fabric and seams, causing premature failure. Section 8.5.4 provides 

recommendations for reducing filter cycling. 

8.5.1 Water Quality

 As previously described, systems seeking compliance with the LT2ESWTR will most 

likely integrate a bag or cartridge filter process after the primary filtration process. As a result, 

influent water quality, with respect to high particulate levels, should not be an issue. However, 

for systems with existing processes that use coagulants, the presence of residual coagulant in the 

primary filter effluent may clog the pores of a bag or cartridge filter. Although this will not 

impair removal efficiency for Cryptosporidium, it will shorten the time until the terminal 

pressure drop is reached, thus reducing filter life. 

Another water quality issue is the potential for biofilm growth on the bag or cartridge 

filter media. Systems can add a disinfectant prior to the bag or cartridge filters to prevent biofilm 

growth. (The filters must be compatible with the disinfectant.) 

8.5.2 Size of Filter System and Redundancy

 Systems should be adequately designed to handle maximum day or maximum 

instantaneous flow, depending on the existing treatment process design. Prolonged operation at 

maximum flow velocity clogs the filter media faster than operating at lower flow velocities. The 

total volume throughout is greater when operating at a flow velocity lower than maximum flow 

velocity rated for the filter.

 A minimum of two bag or cartridge filter housings should be provided to ensure 

continuous water treatment in the event of failure in the filter operation and to allow for filter 

maintenance and replacement. For water systems that do not require continuous operation, a 

state may approve a single filter housing operation. Redundancy in pumps is also recommended 

to ensure continuous operation. 
8. Bag and Cartridge Filters

LT2ESWTR Toolbox Guidance Manual 8-15 April 2010

8.5.3 Design Layout 

 Design layout features that should be considered for most designs are as follows: 

• Piping should be designed to allow isolation of the individual filter units or vessels for 

maintenance and filter replacement. 

• Common inlet and outlet headers for the filter units. 

• Sufficient available head to meet the terminal pressure drop and system demand.

8.5.4 Filter Cycling

Filter cycling refers to the starting and stopping of the pump or filter operation. This can 

be problematic with bag filter processes (cartridge filters are not known to have this problem) in 

which water is pumped directly from the source to the filter, and then out to the distribution 

system. In these situations, the filters operate on demand, similar to wells for small systems, and 

the sudden increase in pressure across the filter causes premature wear and filter failure. For 

LT2ESWTR compliance, systems with bag filters in a series or followed by UV disinfection 

should consider the following recommendations for controlling the flow into the filter process to 

minimize filter cycling.

• Lengthen the filter runs by reducing the flow as much as possible through the filter. 

• Install or divert the flow to a storage facility (e.g., pressure tank, clearwell) after the bag 

filtration process. The stored water can supply the frequent surges in demand and thus 

reduce the bag or cartridge filter cycling.

During filter start-up and other hydraulic surges, bag and cartridge filters often experience an 

increase in filter effluent turbidity. Systems should consider the following options to improve 

filtered water quality.

• Design for filter to waste capability. EPA strongly recommends filtering to waste for 

the first few minutes of the filter cycle.

• Install a slow opening and closing valve ahead of the filter to reduce flow surges. 

8.5.5 Pressure Monitoring, Valves, and Appurtenances

 As previously mentioned, once the terminal pressure drop has been reached, the filter 

should be replaced. At a minimum, pressure gauges should be located before and after the bag or 

cartridge filter system and should be monitored at least daily. A valve or flow restricter should 

be installed on the inlet header pipe of the filters to maintain flows below the maximum 

operating flow for the filters.
8. Bag and Cartridge Filters

LT2ESWTR Toolbox Guidance Manual 8-16 April 2010

8.5.6 Air Entrapment

 An automatic air release valve should be installed on the top of the filter housing to 

release any air trapped in the filter. These valves should be checked routinely and properly 

maintained. 

8.5.7 NSF Certification 

 All components used in the drinking water treatment process should be evaluated for 

contaminant leaching and certified under ANSI/NSF Standard 61. 

8.6 Operational Issues 

This section discusses two key issues associated with operating bag or cartridge filters, 

pressure changes and water quality monitoring. 

8.6.1 Pressure Drop (Inlet/Outlet Pressures)

The pressure drop across the filter directly relates to the amount of particle build-up on 

the filter material and to the time when the filter should be replaced. Typical pressure drops 

across a clean filter are 1 to 2 psig (pounds per square inch-gauge) and can increase to a 

differential of 20 to 30 psig when the terminal pressure drop is achieved. The pressure 

differential does not increase linearly with run time; the differential pressure increases at a faster 

rate with the duration of the run or as more material accumulates on the filter. The time between 

filter replacement is primarily dependent on flow rate, but also on influent water quality and 

filter material (i.e., size of pores).

 The differential pressure between the inlet and the outlet header should be monitored to 

determine when the filter needs replacement. Also the differential pressure should be monitored 

immediately after replacing a filter and placing the unit back in service to verify that the filter 

was properly installed to prevent bypassing around the seals. An alarm could also be linked to 

the pressure gauges to ensure the operator is alerted when a filter needs to be replaced due to 

fouling or failure of the filer or associated seals. 

8.6.2 Water Quality Monitoring

 In addition to monitoring the pressure drop across the filter, the influent and effluent 

turbidity or particle count can be monitored to assess performance and indicate possible process 

upsets with the bag or cartridge filter or other upstream processes. The recommended monitoring 

frequency depends on the influent water quality and its variability. At a minimum, the pressure 

differential and effluent turbidity can be checked daily. If the filter is used to meet the treatment 

requirements of IESWTR/LT1ESTWR, turbidity monitoring is required and the state will set a 

turbidity performance standard. During the initial start-up phase of a newly integrated bag or 
8. Bag and Cartridge Filters

LT2ESWTR Toolbox Guidance Manual 8-17 April 2010

cartridge filtration system, monitoring can be more frequent and then can be reduced once the 

operator becomes familiar with the system. If continuous monitoring of turbidity and/or pressure 

differential is employed, the output from the sensors should be sent to an alarm to warn operators 

of sudden changes in operation, or if the filter element needs replacing. 

EPA recognizes turbidity has limitations as an indicator of filter failure or pathogen 

breakthrough. However, in the absence of a better indicator, monitoring both influent and 

effluent turbidity over a full run (i.e., from start to end of the filter life) can provide a 

performance baseline. The baseline can then be used to indicate process upsets. This method 

may not be applicable to systems with very low raw water turbidity or where the influent has 

been filtered; the difference between influent and effluent turbidity may be too low to provide 

meaningful data. 

 Particle counters can be another valuable monitoring tool. If available, periodic checks of 

influent and effluent particle counts are also recommended to ensure the filter is removing 

particles in the appropriate size range (i.e., 4-6 microns). 

8.7 References

McMeen. 2001. Alternate Filtration: Placing New Technology in an Old Regulatory Box. 

American Water Works Association, Membrane Conference Proceedings.

NSF International. 2005. Protocol for Equipment Verification Testing for Physical Removal of 

Microbiological and Particulate Contaminants. 40 CFR 35.6450. 

http://www.epa.gov/etv/pubs/059205epadwctr.pdf. 

U.S. EPA. 2005. Membrane Filtration Guidance Manual. Office of Water. EPA 815-R-06-009. 

November, 2005. http://www.epa.gov/ogwdw/disinfection/lt2/compliance.html. 

U.S. EPA. 2006. Ultraviolet Disinfection Guidance Manual for the Final Long Term 2 

Enhanced Surface Water Treatment Rule. Office of Water. EPA 815-R-06-007. November, 

2006. http://www.epa.gov/safewater/disinfection/lt2/compliance.html. 

U.S. EPA. 2007. Simultaneous Compliance Guidance Manual for the Long Term 2 and Stage 2 

DBP Rules. EPA 815-R-07-017. March, 2007. 

http://www.epa.gov/safewater/disinfection/stage2/compliance.html. 
LT2ESWTR Toolbox Guidance Manual 9-1 April 2010

9. Second Stage Filtration 

9.1 Introduction 

The Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) 40 CFR 

141.719(c) describes second stage filtration as the use of a rapid sand, dual media, granular 

activated carbon (GAC), or other fine grain media unit process applied in a separate stage 

following rapid sand or dual media filtration. Applying an additional layer of media, such as a 

GAC cap, on an existing single stage filtration unit does not qualify for this credit. 

 This chapter is organized as follows: 

9.2 LT2ESWTR Compliance Requirements - discusses criteria and reporting 

requirements that systems must meet to receive Cryptosporidium removal. 

9.3 Toolbox Selection Considerations - discusses issues specific to second stage 

filtration that water systems should consider when selecting toolbox options. 

9.4 Design and Operational Considerations - discusses hydraulic issues, backwashing, 

and turbidity monitoring for systems that integrate a second stage filtration in their 

treatment train.

9.2 LT2ESWTR Compliance Requirements 

9.2.1 Credits

Under the LT2ESWTR, a system that employs a second, separate filtration stage may 

receive 0.5 log credit for Cryptosporidium removal (40 CFR 141.719(c)) under the following 

conditions. 

• The first stage of filtration is preceded by a coagulation step. 

• The second stage of filtration is comprised of rapid sand, dual media, GAC, or other fine 

grain media. 

• Both filtration stages treat 100 percent of plant flow.

• The state must approve the treatment credit based on an assessment of the design 

characteristics of the filtration process.

Under the LT2ESWTR, a system integrating a slow sand filtration process for the second 

stage of filtration can receive 2.5 log credit for Cryptosporidium removal (40 CFR 141719(d))

under the following conditions. 
9. Second Stage Filtration

LT2ESWTR Toolbox Guidance Manual 9-2 April 2010

• No disinfectant residual is present in the influent to the slow sand filtration process. 

• Both filtration stages treat 100 percent of plant flow. 

• The state must approve the treatment credit based on an assessment of the design 

characteristics of the filtration process.

9.2.2 Reporting Requirements

To receive Cryptosporidium removal credit for compliance with the LT2ESWTR,

systems must verify that 100 percent of the flow was filtered through both stages and that the 

first stage was preceded by a coagulation step (40 CFR 141.721(f)). 

 Reporting for LT2ESWTR does not take the place of the IESWTR and LT1ESWTR

reporting requirements. Specifically, the turbidity of the combined and individual filter effluent 

from the first filtration stage must be reported as required by the IESWTR and LT1ESWTR (40 

CFR 141.74, 40 CFR 141.174(a), 40 CFR 141.551, and 40 CFR 141.560). 

9.3 Toolbox Selection Considerations 

 Plants already employing a second unit process that meets the requirements for this 

toolbox option (e.g., GAC columns to meet dissolved organic or taste and odor treatment goals) 

are in the ideal position to seek credit. Other plants that have enough excess filtration capacity or 

unused filter beds (e.g., built in anticipation of unrealized plant expansions), may be able to 

convert piping to enable these filters to operate in series for relatively low cost. However, many 

plants will find that integrating second stage filtration into an existing treatment train poses 

significant additional space, capital, and hydraulic requirements. These systems may want to 

consider this option if the additional treatment provides other benefits. For example, systems that 

use chloramination and/or ozone could run the second stage under biological filtration conditions 

to reduce assimilable organic carbon (AOC), which promotes biofilm growth and nitrification 

(for chloraminating systems) in the distribution system.

 Additionally, plants experiencing taste and odor problems or dissolved organic 

contaminants in their raw water might consider installing GAC columns to alleviate these 

problems and also receive the Cryptosporidium removal credit.

 

Slow sand filtration plants who wish to consider this toolbox option should either have 

sufficient excess filtration capacity to allow filters to operate in series (with possible piping 

modifications) or have sufficient land area to build additional filters. 
9. Second Stage Filtration

LT2ESWTR Toolbox Guidance Manual 9-3 April 2010

9.3.1 Advantages 

The advantages of a second stage filtration process are the same for both rapid and slow 

sand plants and include operator familiarity with the process, ease of operation, and potential to 

reduce disinfection byproducts. For plants with existing processes and infrastructure meeting the 

two-stage requirements, implementation costs are likely to be relatively low.

9.3.2 Disadvantages 

 The disadvantages associated with second stage filtration apply primarily to those plants 

that do not have existing processes in place or cannot easily convert built-in infrastructure. In 

addition to the capital cost for new filters, these plants may need the following improvements to 

integrate a second stage of filtration:

• Space if there is currently no room for expansion in the existing plant grounds. 

• Additional pumping to compensate for head loss associated with an additional filtration 

process. 

• Increased backwash supply and treatment. 

 For those plants that have existing infrastructure available for a second stage of filtration, 

they still may have to account for an increased volume of backwash and loss of head due to the 

second stage.

Systems with rapid sand filtration plants that are considering integrating slow sand 

filtration into their treatment process should be aware of the following differences in operation 

and performance of slow sand plants compared to rapid sand plants: 

• More space required for slow sand plants. 

• Decreased filtering performance with cold temperatures. 

• Maintenance of filters requires draining and scraping a thin layer off the top of the filter. 

9.4 Design and Operational Considerations 

 The design of the second stage is site-specific and depends on existing infrastructure 

(e.g., some systems may have enough filtration capacity to operate filters in series) and space and 

hydraulic requirements. EPA does not specify or restrict certain configurations, beyond the 

requirement that all flow must be treated by both stages. Systems that have existing filters not in 

use or not used to capacity may reconfigure the piping to operate in series. Media sizing for the 
9. Second Stage Filtration

LT2ESWTR Toolbox Guidance Manual 9-4 April 2010

second stage is also not specified; however, typical design standards for regular or deep bed 

filters should be followed. 

9.4.1 Hydraulic Requirements

 Additional pumps may be needed to provide the necessary head between the first and 

second stages of filtration. The number of pumps and total number of filters should allow for 

redundancy, to ensure that sufficient treatment capacity is in place to treat all the plant flow in the 

event of equipment breakdown or maintenance. However, the filter loading rate to the second 

stage does not necessarily need to be the same as for the first stage. The water influent to the 

second stage should be significantly cleaner, and may enable higher loadings. Final design 

loading rates should be determined in consultation with the state. 

If the filter effluent from the first stage filters is not combined and sent to the second 

stage filters via a distribution box or other flow equalization device, plant operation may be more 

complex. For example, if the effluent from one first stage filter is sent to just one second stage 

filter, then as the flow from first filter decreases (or headloss through it increases), flow through 

the second filter will also decrease, unless automatic effluent control valves are installed on the 

second stage filter. Also, in this case, whenever the first stage filter is backwashed, the second 

stage filter will also be out of service.

9.4.2 Backwashing

 Consistent with the Filter Backwash Recycling Rule, the filter backwash from the second 

stage (as well as the first stage) must be recycled to the head of the plant if it is recycled. The 

existing backwashing capacity may be limited and need to be increased. There may be 

insufficient finished water storage to supply backwash water or there may need to be additional 

pumping capacity, depending upon the design of the additional filtration stage (e.g., if the 

existing filters have a small area and the new filters are significantly larger, the existing 

backwash pumps may not be able to supply water at a high enough flow to properly expand the 

filter bed). It is likely that the second stage filters would need to be backwashed less frequently 

than the first stage ones, due to the lower solids loading. 

Filter ripening and/or filter-to-waste times for the second filtration stage will most likely 

differ from the first stage.

9.4.3 Turbidity Monitoring

 Depending on the removal performance of the first stage filtration process, it may be 

difficult to see differences in second stage removal performance if monitoring of the second stage 

process is limited to the combined filter effluent (CFE) of the second stage. Individual filter 
9. Second Stage Filtration

LT2ESWTR Toolbox Guidance Manual 9-5 April 2010

effluent (IFE) monitoring of the second stage filters on a continuous or routine basis may identify 

performance issues that can be addressed proactively.
LT2ESWTR Toolbox Guidance Manual 10-1 April 2010

10. Chlorine Dioxide

10.1 Introduction 

 Chlorine dioxide (ClO2) is used for disinfection, taste and odor control, and iron and 

manganese removal. Chlorine dioxide is effective for inactivation of bacteria, viruses, and 

protozoa, including Cryptosporidium while forming fewer halogenated byproducts than chlorine. 

It is stable only in dilute aqueous solutions and must be generated on-site. It can be generated 

using a variety of starting materials including chloride, chlorite, or chlorate. 

The Surface Water Treatment Rule (SWTR), Stage 1 Disinfection Byproducts Rule 

(Stage 1 DBPR), and Interim Enhanced Surface Water Treatment Rule (IESWTR) all recognize 

the ability of chlorine dioxide to inactivate pathogens. As a result, there is much information and 

guidance available on the application of chlorine dioxide for disinfection, particularly in the 

following two guidance manuals:

• Guidance Manual for Compliance with the Filtration and Disinfection Requirements for 

Public Water Systems Using Surface Water Sources (U.S. EPA 1991) (commonly 

referred to as the Surface Water Treatment Rule Guidance Manual). 

- Describes how to calculate the contact time (CT) value for a given disinfectant, 

including methodologies for determining the residual concentration (C) and contact 

time (T).

- Includes CT values for log-inactivation of Giardia and viruses. 

• Alternative Disinfectants and Oxidants Guidance Manual (U.S. EPA 1999) provides full 

descriptions of:

- Chlorine dioxide chemistry. 

- On-site generation. 

- Primary uses and points of applications. 

- Pathogen inactivation and disinfection efficiency. 

- Byproduct production. 

- Analytical methods. 

- Operational considerations. 

The Alternative Disinfectants and Oxidants Guidance Manual is available on EPA’s 

website, http://www.epa.gov/safewater/mdbp/implement.html.
10. Chlorine Dioxide

LT2ESWTR Toolbox Guidance Manual 10-2 April 2010

 The purpose of this chapter is to (1) describe what systems need to do to achieve 

Cryptosporidium inactivation treatment credit for disinfecting with chlorine dioxide, (2) discuss 

design and operational considerations that will assist water systems in deciding whether this 

toolbox option is a practical option for them, and (3) discuss key issues associated with using 

chlorine dioxide as a disinfectant. This chapter is organized as follows: 

10.2 Log Inactivation Requirements - describes the concentration and time variables of 

the CT parameter, presents the chlorine dioxide CT table for Cryptosporidium, 

and provides a sample CT calculation. 

10.3 Monitoring Requirements - describes monitoring requirements of both 

LT2ESWTR and Stage 1 DBPR. 

10.4 Unfiltered Systems LT2ESWTR Requirements - describes the level of 

Cryptosporidium inactivation unfiltered systems must provide and monitoring 

requirements that must be met. 

10.5 Disinfection with chlorine dioxide - describes chlorine dioxide chemistry and 

disinfection with chlorine dioxide. 

10.6 Toolbox Selection Considerations - discusses the advantages and disadvantages of 

disinfection with chlorine dioxide. 

10.7 Design Considerations - discusses effects of temperature and the point of chlorine 

dioxide addition on achieving the required CT value. 

10.8 Operational Considerations - discusses water quality parameters that affect the 

disinfection ability of chlorine dioxide. 

10.9 Safety Issues - describes considerations for chemical storage and discusses the 

acute health risks of chlorine dioxide. 

10.2 Log Inactivation Requirements 

Systems can achieve anywhere from 0.25- to 3.0-log Cryptosporidium inactivation with 

the addition of chlorine dioxide. The amount of Cryptosporidium inactivation credit a system 

may receive is determined by the CT provided in the treatment process (40 CFR 141.720(b)). 

This methodology provides a conservative characterization of the dose of chlorine dioxide 

necessary to achieve a specified inactivation level of Cryptosporidium. CT is the product of the 

disinfectant concentration and disinfectant CT and is defined in the LT2ESWTR (40 CFR 

141.720(a)): 
10. Chlorine Dioxide

LT2ESWTR Toolbox Guidance Manual 10-3 April 2010

 CT = Disinfectant (mg/L) × Contact Time (minutes)

“T” is the time (in minutes) it takes the water, during peak hourly flow, to move from the point 

of disinfectant application to a point where, C, residual concentration is measured at or prior to 

the first customer, or between points of residual measurement. 

“C” is the concentration of chlorine dioxide present in the system, expressed in mg/L. 

 The concept of regulating surface water treatment disinfection processes through CT was 

first introduced in the SWTR. Tables of Giardia and virus log-inactivations correlated to CT 

values, commonly referred to as CT tables, were presented in the SWTR Guidance Manual. For 

the LT2ESWTR, EPA developed CT tables for the inactivation of Cryptosporidium. 

Alternatively, a system may conduct a site-specific study to determine the CT values necessary 

to meet a specified log-inactivation, using state approval (40 CFR 141.720(c)). Appendix A 

provides guidance for conducting a site-specific study.

10.2.1 CT Calculation 

 The methodology and calculations for determining CT have not changed from the SWTR 

to the LT2ESWTR requirements. This section briefly reviews how CT is used to determine loginactivation for the SWTR and presents the chlorine dioxide CT table for Cryptosporidium

inactivation. Refer to the SWTR Guidance Manual for descriptions of measuring C and 

determining T. 

 CT can be calculated for an entire treatment process or broken into segments and 

summed for a total CT value. C is measured at the end of a given segment. T is generally 

estimated by methods involving established criteria (flow, volume, and contactor geometry) or 

tracer studies. The following steps describe the CT calculation from measured C and T values for 

a segment of the entire treatment process:

1. Calculate CTcalc by multiplying the measured C and T values.

2. From the CT table (see Exhibit 10.1 for the CT table for Cryptosporidium), find the 

CT value for the log-inactivation desired, this is CTtable. 

3. Calculate the ratio of CTcalc/CTtable for each segment.

4. If a system has multiple segments, sum the CTcalc/CTtable ratios for a total inactivation 

ratio. 

5. If the ratio of CTcalc/CTtable is at least 1, then the treatment process provides the loginactivation that the CTtable

 

represents (log-inactivation desired from step #2). 
10. Chlorine Dioxide

LT2ESWTR Toolbox Guidance Manual 10-4 April 2010

Exhibit 10.1 CT Values (mg-min/L) for Cryptosporidium Inactivation by Chlorine 

Dioxide 1

Log 

credit

Water Temperature, °C 

<=0.5 1 2 3 5 7 10 15 20 25 30

0.25 159 153 140 128 107 90 69 45 29 19 12

0.5 319 305 279 256 214 180 138 89 58 38 24

1.0 637 610 558 511 429 360 277 179 116 75 49

1.5 956 915 838 767 643 539 415 268 174 113 73

2.0 1275 1220 1117 1023 858 719 553 357 232 150 98

2.5 1594 1525 1396 1278 1072 899 691 447 289 188 122

3.0 1912 1830 1675 1534 1286 1079 830 536 347 226 147

1

Log credit=(0.001506×(1.09116)

Systems may use this equation to determine log credit between the indicated values: Temp

Source: 141.720 (b)(1)

)× CT

Example CT Calculation

 A plant draws 1.5 million gallons per day (MGD) of 5 degrees Celsius water from a 

stream, adding 1.8 mg/L of chlorine dioxide at the intake. The water travels through 2 miles of 

12 inch pipe to a settling tank. The detention time in the tank, as determined by a tracer study, is 

150 minutes. After the tank, it travels through another 12-inch pipe to the plant. Exhibit 10.2 

provides a schematic of an intake, piping, and tank. The concentration of chlorine dioxide at each 

point is measured as follows: 

 Cinitial = 1.8 mg/L 

 Centering tank = 1.6 mg/L

 Cleaving tank = 0.8 mg/L 

 Cleaving 2nd pipe = 0.2 mg/L

Exhibit 10.2 CT Calculation Example Schematic
10. Chlorine Dioxide

LT2ESWTR Toolbox Guidance Manual 10-5 April 2010

 The residence times of the two sections of pipe are determined assuming plug flow. 

Therefore, the time for each section is calculated as follows:

T1 = (A1*L1/Q1) = (πr

2

L1/Q1)*(7.48 gal/1 ft3)*(MG/1,000,000 gal)*(1,440 min/day)

where:

• A is the cross-sectional area of the pipe in square feet; 

• Q is the volumetric flow rate in MGD; 

• L is the length of pipe in feet; and

• r is the radius of the pipe in feet. 

Therefore the times for the two sections of the pipe are as follows:

 T1 = 2 mi*(5,280 ft/mi)* π*(0.5 ft)2

*(0.0108 MG*sec/ft3

 T

*day)/(1.5 MGD) = 59.7 min 

3 = 0.25 mi*(5,280 ft/mi)*π*(0.5 ft)2

*(0.0108 MG*sec/ft3*day)/(1.5 MGD) = 7.4 min 

The T10, or time for 90 percent of a tracer to pass through the section for the tank is as 

follows: 

 T2 = 150 minutes 

CT Calculation:

Step 1. Calculate CT for each segment.

The concentrations and times for each segment are known. The T’s are calculated above and the 

C is the concentration measured at the end of each segment. The CT for each segment is 

calculated as follows:

CT1

CT

 = (1.6 mg/L) × (59.5 min) = 95.2 mg×min/l

2

CT

 = (0.8 mg/L) × (150 min) = 120 mg×min/l

3 = (0.2 mg/L) × (7.4 min) = 1.5 mg×min/l

Step 2. Look up CTtable in Exhibit 10.1. For 5°C and 0.5-log inactivation,

CTtable = 214 mg×min/L 

Step 3. Calculate the ratio of CTcalc/CTtable for each segment.

(CTcalc/CTtable)1

(CT

 = 95.2/214 = 0.44 

calc/CTtable)2

(CT

 = 120 /214 = 0.56 

calc/CTtable)3 = 1.5/214 = 0.01 

Step 4. Sum the CTcalc/CTtable for each segment.

(CTcalc/CTtable)total = 0.44 + 0.56 + 0.01 = 1.01 
10. Chlorine Dioxide

LT2ESWTR Toolbox Guidance Manual 10-6 April 2010

Determine Log Inactivation:

If the result of Step 4 is greater than 1, the log-inactivation associated with the CTtable values is 

achieved. If the result is less than 1, that level of log-inactivation is not achieved (if the loginactivation was less than 1.0, the calculations should be repeated at a lower log-inactivation). In 

this example, the sum of the CTcalc/CTtable for all the segments is greater than 1, so the system 

qualifies for a 0.5-log Cryptosporidium inactivation.

This example is intended to show a CT determination. Meeting log inactivation requirements 

using a disinfectant in a raw water supply may not be appropriate for raw water conditions (e.g., 

high turbidity) that may interfere with disinfectant efficacy. 

10.3 Monitoring Requirements 

10.3.1 LT2ESWTR

The LT2ESWTR requires CT calculation at least once per day with both C and T 

measured during peak hourly flow (40 CFR 141.720 (a)). Since systems may not know when 

the peak hourly flow is, EPA recommends monitoring flow on an hourly basis. Continuous flow 

monitoring and recording can also be used to determine peak flow. Systems should reevaluate 

CT whenever they modify a process and the hydraulics are affected (e.g., add a pump for 

increased flow, reconfigure piping). 

The chlorine dioxide concentration should be measured using approved analytical 

methods, either DPD, (Standard Method 4500-ClO2 D) or Amperometric Method I or II, 

(Standard Method 4500-ClO2 C or E, respectively). Details on these methods can be found in 

Standard Methods for the Examination of Water and Wastewater, 20th edition, American Public 

Health Association, 1998. 

Note, if a system changes its disinfection process, the LT2ESWTR requires the system to 

calculate a disinfection profile and benchmark (40 CFR 141.708) (see Chapter 1, section 1.4.5 

for details).

10.3.2 Stage 1 DBPR 

The Stage 1 DBPR requires all systems using chlorine dioxide for disinfection or 

oxidation to monitor daily for chlorine dioxide and chlorite at the distribution system entry point. 

In addition, systems must take monthly chlorite samples at three locations in the distribution 

system. Exhibit 10.3 lists the chlorine dioxide and chlorite distribution system monitoring 

requirements.
10. Chlorine Dioxide

LT2ESWTR Toolbox Guidance Manual 10-7 April 2010

Exhibit 10.3 Distribution System Monitoring Requirements at Each Plant

Location Frequency

Chlorite

Distribution System Entry Point Daily

Distribution System Sample Set of 3:

 1 Near First Customer

 1 At Average Residence Time

 1 At Maximum Residence Time

Monthly

Chlorine Dioxide

Distribution System Entry Point Daily

If the chlorine dioxide maximum residual disinfectant level (MRDL) of 0.8 mg/L or the 

chlorite maximum contaminant level (MCL) of 1.0 mg/L is exceeded in any of the samples, 

additional monitoring is required (see the Stage 1 DBPR, 40 CFR141.132(b) for further 

information). Depending on the results of the additional samples, the system could have an acute 

violation with more serious public notification requirements than for a chlorine MRDL violation. 

The monthly monitoring requirements for chlorite may be reduced if all chlorite samples are 

below the MCL for a 1-year period. 

10.4 Unfiltered System LT2ESWTR Requirements 

 The LT2ESWTR requires unfiltered systems to provide at least 2.0-log Cryptosporidium

inactivation (40 CFR 141.712(b)). If their source water Cryptosporidium concentration is greater 

than 0.01 oocyst/liter, then systems must provide 3.0-log Cryptosporidium inactivation (40 CFR 

141.712(b)). The requirements of the previous SWTR regulations still apply— achieve 3-log

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

the distribution system (e.g., free chlorine or chloramines). LT2ESWTR also requires that a 

minimum of two disinfectants be used to meet overall disinfection requirements.

 The monitoring requirements described in section 10.3 apply to unfiltered systems. 

Additionally, the LT2ESWTR requires unfiltered systems to meet the Cryptosporidium loginactivation requirements determined by the daily CT value every day the system serves water to 

the public, except one day per calendar month (40 CFR 141.712(c)). Therefore, if an unfiltered 

system fails to meet Cryptosporidium log-inactivation two days in a month, it is in violation of 

the treatment technique requirement.

10.5 Disinfection with Chlorine Dioxide 

 Chlorine dioxide (ClO2) is an uncharged compound of chlorine in the +IV oxidation 

state. It is a relatively small, volatile, and highly energetic molecule, and a free radical even in 

dilute aqueous solutions. At high concentrations, it reacts violently with reducing agents. 

However, it is stable in dilute solution in a closed container in the absence of light. When an 

aqueous solution is open to the atmosphere, chlorine dioxide readily comes out of solution. 

Aqueous solutions of chlorine dioxide are also susceptible to photolytic decomposition, 

depending on the time of exposure and intensity of ultraviolet light (UV) light.
10. Chlorine Dioxide

LT2ESWTR Toolbox Guidance Manual 10-8 April 2010

 Disinfection of protozoa is believed to occur by oxidation reactions disrupting the 

permeability of the cell wall (Aieta and Berg 1986). Chlorine dioxide functions as a highly 

selective oxidant due to its unique, one-electron transfer mechanism where it is reduced to 

chlorite (ClO2

-

) (Hoehn et al. 1996). 

 In drinking water, chlorite (ClO2

-

) is the predominant reaction end product, with 

approximately 50 to 70 percent of the chlorine dioxide converted to chlorite and 30 percent to 

chlorate (ClO3

-

) and chloride (Cl-) (Werdehoff and Singer 1987). This has a significant impact 

on disinfection capabilities for drinking water, since chlorite is a regulated drinking water 

contaminant with an MCL of 1.0 mg/L. Based on a 50 to 70 percent conversion of chlorine 

dioxide to chlorite, the maximum dose is limited to 1.4 to 2.0 mg/L unless the chlorite is 

removed through subsequent treatment processes. 

10.6 Toolbox Selection Considerations 

10.6.1 Advantages 

There are several advantages to using chlorine dioxide as a primary disinfectant. Chlorine 

dioxide is approximately four times as effective as chlorine for the inactivation of Giardia and is 

a stronger disinfectant than chlorine for bacteria (White 1999). However, free chlorine is more 

effective for the inactivation of viruses. Other advantages of disinfection with chlorine dioxide 

include: 

• A high oxidizing potential allows it to oxidize other compounds such as manganese and 

some taste and odor compounds. 

• Chlorine dioxide does not form regulated halogenated organic byproducts.

• The effect of pH on the disinfection ability of chlorine dioxide is much smaller than for 

other disinfectants. 

• Chlorine dioxide has shown a synergistic effect when combined with other disinfectants 

such as ozone, chlorine, and chloramines that leads to greater inactivation with the 

disinfectants added in series than by either disinfectant individually. 

• Chlorine dioxide can be used in the control of zebra mussels. 

10.6.2 Disadvantages 

 A major disadvantage of chlorine dioxide is the byproduct formation of chlorite and 

chlorate. Section 10.6 describes the dose limits of chlorine dioxide due to the formation of 

chlorite. Other disadvantages of disinfection with chlorine dioxide include: 
10. Chlorine Dioxide

LT2ESWTR Toolbox Guidance Manual 10-9 April 2010

• Difficulty in maintaining an effective residual. Additionally, residual will be lost in the 

filters.

• It decomposes upon exposure to sunlight, flourescent light bulbs, and UV disinfection 

systems.

• Ability to disinfect is reduced under colder temperatures. 

• It can form brominated disinfection byproducts (DBPs) in the presence of bromide. 

• If the ratio of reactants in the chlorine dioxide generator is incorrect, excess aqueous 

chlorine can remain, which can form halogenated disinfection byproducts. 

• Chlorine dioxide must be generated on-site.

• There may be a need for three-phase power which may not be compatible with some 

water systems.

• Chlorine dioxide can be explosive at high temperatures or pressures.

• Storage of sodium chlorite solution can be problematic due to crystallization at low 

temperatures or high concentrations and stratification at temperatures below 40°F (or 

4°C).

• Dialysis patients are sensitive to higher chlorite levels and should be notified if chlorine 

dioxide is going to be added where it has not routinely been used. 

• Training, sampling, and analysis costs are high. 

• If used together with granular activated carbon (GAC) it can react to form chlorate.

Systems considering using chlorine dioxide as a disinfectant should perform chlorine 

dioxide demand/decay tests on the water being considered for disinfection (raw water or filter 

effluent) under normal and poor water quality conditions. If chlorine dioxide is added where the 

demand is 1.4 mg/L or greater, the system may have difficulty complying with the chlorite MCL. 

If the raw water has a chlorine dioxide requirement greater than 1.4 mg/L, chlorine dioxide 

might still be able to be used for post disinfection since the oxidant demand will be less after the 

filters. The Simultaneous Compliance Guidance Manual can be consulted on the how to offset 

the disadvantages of chlorine dioxide use.

10.7 Design Considerations 

10.7.1 Designing to Lowest Temperature 

As the water temperature declines, chlorine dioxide becomes less effective as a 

disinfectant. LeChevallier et al. (1997) found that reducing the temperature from 20 degrees 

Celsius to 10 degrees Celsius reduced disinfection effectiveness by 40 percent. Since the 
10. Chlorine Dioxide

LT2ESWTR Toolbox Guidance Manual 10-10 April 2010

treatment achieved for chlorine dioxide addition is temperature dependent, systems need to 

consider the variability in water temperature to ensure they meet the CT level for the minimum 

treatment needed for compliance. For example, if a system is required to provide an additional 1-

log Cryptosporidium treatment and plans to achieve that with chlorine dioxide alone, then it 

should determine the CT required for the lowest water temperature experienced and ensure it can 

meet those CT requirements.

10.7.2 Point of Addition 

 There are two main considerations for determining locations of chlorine dioxide addition 

for the purpose of Cryptosporidium inactivation—CT and chlorine dioxide demand. 

Additionally, systems using ozone should consider that ozone will degrade chlorine dioxide. The 

application point for chlorine dioxide should be well upstream of the ozone process or just after 

the ozone process. 

Contact Time 

 The CT requirements for Cryptosporidium are much higher than for Giardia and viruses 

and when designing to the lowest water temperatures, the resulting CT requirements are 

relatively high for even the 0.5- and 1.0-log inactivation. Chlorine dioxide readily degrades when 

exposed to light from flourescent lamps or the sun; therefore, all the available concentration in 

open basins will most likely not be utilized for disinfection. For most systems, the point of 

application will be either at the raw water intake or after the filters, whichever can provide the 

necessary CT. 

Oxidant Demand 

The oxidant demand of the water affects chlorite and chlorate byproduct formation. If the 

chlorine dioxide requirement of the raw water is greater than 1.4 mg/L then chlorite 

concentration will likely exceed the MCL. However, chlorine dioxide could be added after the 

filters where the oxidant demand is frequently lower and, therefore, a lower dose of chlorine 

dioxide would result in a lower byproduct concentration of chlorite. 

10.8 Operational Considerations 

Of all the water quality parameters, water temperature has the strongest effect on the 

disinfection ability of chlorine dioxide. The concentration of suspended matter and pH also have 

an effect, but to a lesser extent than temperature. Although the disinfection potential of chlorine 

dioxide is not strongly affected by pH, studies have shown that chlorine dioxide disinfection is 

better under higher pH (LeChevallier et al. 1997). 

 Suspended matter and pathogen aggregation affect the disinfection efficiency of chlorine 

dioxide. Protection from chlorine dioxide inactivation due to bentonite was determined to be 

approximately 11 percent for water with turbidity values less than or equal to 5 NTU and 25 

percent for turbidity between 5 and 17 NTUs (Chen et al. 1984). 
10. Chlorine Dioxide

LT2ESWTR Toolbox Guidance Manual 10-11 April 2010

 Based on the research discussed above, the optimal conditions for Cryptosporidium

disinfection with chlorine dioxide are low turbidity, high pH, and high temperature.

10.9 Safety Issues 

Because chlorine dioxide can be explosive and pose 

acute health risks to those exposed to gaseous chlorine 

dioxide, a safety plan should be developed that includes 

precautions for generation, handling, storage, and emergency 

response. 

10.9.1 Chemical Storage 

Most chlorine dioxide generators use sodium chlorite solutions as a raw material. If 

sodium chlorite solutions are accidently acidified or exposed to a reducing agent, uncontrolled 

production and release of gaseous chlorine dioxide can result. In addition to being toxic, if the 

gaseous chlorine dioxide reaches concentrations greater than 10 percent, it can spontaneously 

explode. 

 Sodium chlorite should be stored away from other chemicals, particularly any acid 

solutions or chemicals that could act as reducing agents. Construction materials in sodium 

chlorite storage areas, as well as chlorine dioxide generating areas, should be fire resistant such 

as concrete. Sodium chlorite fires burn especially hot and produce oxygen as a byproduct, so 

special fire fighting techniques are required to extinguish the fire. These firefighting techniques 

should be part of the safety plan and proper equipment and supplies should be stored nearby. 

Temperatures in storage and generation areas should be kept below 30 degrees Celsius. 

10.9.2 Acute Health Risks of Chlorine Dioxide 

Exposure to gaseous chlorine dioxide can cause shortness of breath, coughing, respiratory 

distress, and pulmonary edema. The Occupational Safety and Health Administration (OSHA) 

permissible exposure limit (PEL) is 0.1 ppm. Areas where chlorine dioxide is generated and 

stored should have appropriate monitoring to detect leaks of chlorine dioxide or other chlorine 

containing chemicals into the air. Proper ventilation and scrubbing systems should be installed. 

First aid kits and respirators should also be accessible outside the building. Operators should be 

trained to use the respirators. 

10.10 References

Aieta, E., and J.D.Berg. 1986. A Review of Chlorine Dioxide in Drinking Water Treatment. J. 

AWWA. 78(6):62-72. 

Andrews, R.C., Z. Alam, R. Hofmann, L. Lachuta, R. Cantwell, S. Andrews, E. Moffet, G.A. 

Gagnon, J. Rand, and C. Chauret, 2005. Impact of Chlorine Dioxide on Transmission, 

Treatment, and Distribution System Performance. AWWARF. Denver, CO. 

Airborne concentrations 

greater than 10 percent 

may cause explosions.
10. Chlorine Dioxide

LT2ESWTR Toolbox Guidance Manual 10-12 April 2010

APHA. 1998. Standard Methods for the Examination of Water and Wastewater, 20th edition, 

American Public Health Association.

Chen, Y.S.R., O.J. Sproul, and A.J. Rubin. 1984. Inactivation of Naegleria Gruberi cysts by 

Chlorine Dioxide. EPA Grant R808150-02-0, Department of Civil Engineering, Ohio State 

University. 

Hoehn, R.C., A.A. Rosenblatt, and D.J. Gates. 1996. Considerations for Chlorine Dioxide 

Treatment of Drinking Water. Conference proceedings, AWWA Water Quality Technology 

Conference, Boston, MA. 

LeChevallier, M.W., et al. 1997. Chlorine Dioxide for Control of Cryptosporidium and 

Disinfection Byproducts. Conference proceedings, 1996 AWWA Water Quality Technology 

Conference Part II, Boston, Massachusetts.

U.S. EPA. 1999. Alternative Disinfectants and Oxidants Guidance Manual. EPA 815-R-99-014. 

April, 1999. http://www.epa.gov/safewater/mdbp/mdbptg.html. 

U.S. EPA. 1991. Guidance Manual for Compliance with the Filtration and Disinfection 

Requirements for Public Water Systems Using Surface Water Sources. Washington, D.C. 

U.S. EPA. 2007. Simultaneous Compliance Guidance Manual for the Long Term 2 and Stage 2 

DBP Rules. EPA 815-R-07-017. March, 2007. 

http://www.epa.gov/safewater/disinfection/stage2/compliance.html. 

Werdehoff, K.S, and P.C. Singer. 1987. Chlorine Dioxide Effects on THMFP, TOXFP and the 

Formation of Inorganic By-Products. J. AWWA. 79(9):107. 

White, Geo. Clifford. 1999. Handbook of Chlorination and Alternative Disinfectants, 4th edition, 

John Wiley & Sons, Inc.

Zhou, P., and J. Neemann, 2004. Use of Chlorine Dioxide and Ozone for Control of Disinfectant 

By-Products. AWWARF Denver, CO.
LT2ESWTR Toolbox Guidance Manual 11-1 April 2010

11. Ozone

11.1 Introduction

 Ozone is commonly used in drinking water treatment for disinfection as well as taste and 

odor control. Ozone is a strong oxidant that can inactivate microorganisms, including 

Cryptosporidium, Giardia, and viruses, as well as oxidize and break down natural organic matter 

(NOM). It exists as a gas at room temperature and must be generated on-site. Ozone reacts 

rapidly with organic and inorganic compounds and does not maintain a residual over the time 

scales associated with secondary disinfection. 

The Surface Water Treatment Rule (SWTR), subsequent Stage 1 Disinfectants and 

Disinfection Byproducts Rule (DBPR), and Interim Enhanced SWTR (IESWTR) all recognize 

the capability of ozone to inactivate pathogens. As a result, there is much information and 

guidance available on the application of ozone for disinfection, particularly in the following two 

guidance manuals: 

• Guidance Manual for Compliance with the Filtration and Disinfection Requirements for 

Public Water Systems Using Surface Water Sources (U.S. EPA 1991) (commonly 

referred to as the SWTR Guidance Manual). 

- Describes how to calculate the contact time (CT) value for ozone (CT is described in 

the next section), including methodologies for determining the residual concentration 

(C) and contact time (T).

- Includes ozone CT values for log-inactivation of Giardia and viruses.

• Alternative Disinfectants and Oxidants Guidance Manual (U. S. EPA 1999) provides full 

descriptions of:

- Ozone chemistry

- On-site generation

- Primary uses and points of application

- Byproduct production 

- Analytical methods

- Operational considerations

The SWTR Guidance Manual and Alternative Disinfectants and Oxidants 

Guidance Manual are available on EPA’s website:

http://www.epa.gov/safewater/mdbp/implement.html
11. Ozone

LT2ESWTR Toolbox Guidance Manual 11-2 April 2010

The purpose of this chapter is to (1) describe what systems need to do to receive 

Cryptosporidium treatment credit for disinfecting with ozone, (2) discuss design and operational 

considerations that will assist water systems in deciding whether this toolbox option is a practical 

option for their system, and (3) discuss key issues associated with using ozone as a disinfectant. 

This chapter is organized as follows:

 

11.2 Credits

 

 - discusses Cryptosporidium inactivation credit systems can receive with 

the addition of ozone, and relates CT to Cryptosporidium inactivation credit.

11.3 CT Determination - summarizes how CT is used to determine log inactivation 

credit for the SWTR and highlights the changes in CT calculation methodologies 

from the SWTR to the LT2ESWTR.

11.4 Monitoring Requirements - discusses monitoring requirements of both Long Term 

2 Enhanced Surface Water Treatment Rule (LT2ESWTR) and Stage 1 DBPR.

11.5 Unfiltered Systems LT2ESWTR Requirements - discusses Cryptosporidium

inactivation requirements that unfiltered systems must meet.

11.6 Toolbox Selection - discusses the potential advantages and disadvantages of 

ozone processes. 

11.7 Disinfection with Ozone - describes reaction pathways of ozone in water as well 

as inorganic and organic byproduct formation.

11.8 Design

11.9 

 - discusses similarities and differences of different types of ozone 

generators and contactors, general considerations in determining the locations of 

ozone addition, and filter media and operating conditions of biologically active 

filters.

Safety Considerations in Design - discusses various safety considerations that 

should be taken into account in the design of ozone generators. 

11.10 Operational Issues - discusses how ozone disinfection and CT calculation are 

affected by ozone demand, pH, temperature, and residual disinfectant in the 

distribution system.

11.2 Credits 

Systems can receive between a 0.25 to 3.0 log Cryptosporidium inactivation credit with 

the addition of ozone, depending on the ozone dose applied. The value of the Cryptosporidium

inactivation credit that a system receives is determined by the CT or inactivation provided in the 

treatment process. CT values are established to provide a conservative characterization of the 

dose of ozone necessary to achieve a specified inactivation of Cryptosporidium. CT is defined as 

the product of the disinfectant concentration and disinfectant contact time:
11. Ozone

LT2ESWTR Toolbox Guidance Manual 11-3 April 2010

 CT = Disinfectant (mg/L) x Contact Time (minutes)

• “T” is the time it takes the water to move from the point where the initial disinfectant 

residual concentration is measured to the point where the final disinfectant residual 

concentration is measured in a specified disinfectant segment. 

• “C” is the concentration of dissolved ozone in mg/L. 

The concept of regulating surface water treatment disinfection through CT was first 

introduced in the SWTR. Tables relating Giardia and virus log inactivation with associated CT 

values, commonly referred to as CT tables, were presented in the SWTR Guidance Manual. For 

the LT2ESWTR, EPA developed CT values for Cryptosporidium inactivation by ozone (Exhibit 

11.1). 

 

Exhibit 11.1 CT Values for Cryptosporidium Inactivation by Ozone 

(40 CFR 141.730)

Log 

credit 

Water Temperature, ∘C1

<=0.5 1 2 3 5 7 10 15 20 25 ≥30

0.25 6.0 5.8 5.2 4.8 4.0 3.3 2.5 1.6 1.0 0.6 0.39

0.5 12 12 10 9.5 7.9 6.5 4.9 3.1 2.0 1.2 0.78

1.0 24 23 21 19 16 13 9.9 6.2 3.9 2.5 1.6

1.5 36 35 31 29 24 20 15 9.3 5.9 3.7 2.4

2.0 48 46 42 38 32 26 20 12 7.8 4.9 3.1

2.5 60 58 52 48 40 33 25 16 9.8 6.2 3.9

3.0 72 69 63 57 47 39 30 19 12 7.4 4.7

1

CT values between the indicated temperatures may be determined by linear interpolation.

The CT values reported in Exhibit 11.1 were developed from the following equation:

Cryptosporidium Log Credit = 0.0397×(1.09757)

Temp ×CT Equation 11-1 

where “Temp” is the water temperature expressed in degrees Celsius between 0.5 and 25 o

C. A 

water system may use the above equation in lieu of the table. Equations for Giardia and virus 

shown below for ozone were derived from the k10 values presented in Appendix O to the SWTR

Guidance Manual for Giardia and virus. 

Giardia Log Credit = 1.0380×(1.0741)

Temp ×CT Equation 11-2 

virus Log Credit = 2.1744×(1.0726)

Temp ×CT Equation 11-3 
11. Ozone

LT2ESWTR Toolbox Guidance Manual 11-4 April 2010

If a utility believes that the CT values for Cryptosporidium presented in Exhibit 11.1 or 

calculated by Equation 11-1 do not accurately represent the conditions needed to achieve the 

desired level of inactivation in their system, they have the option of conducting a site specific 

study to generate a set of CT tables for their facility. The study would involve measuring actual 

Cryptosporidium inactivation performance under site conditions. If accepted by the state, the CT 

tables generated by the site study would replace the tables given in this guidance for the site at 

which the study was performed. Guidance on site specific studies of Cryptosporidium

inactivation is presented in Appendix A. 

11.3 CT Determination

 The recommended methodologies and calculations for determining CT have two 

modifications from the SWTR to the LT2ESWTR. 

For Cryptosporidium, EPA recommends that no inactivation credit be granted for the 

first dissolution chamber due to the higher CT requirements of Cryptosporidium compared to 

Giardia and virus. This differs from the SWTR guidance manual, where EPA recommends 

granting Giardia and virus inactivation credit for a "first chamber" (i.e. dissolution chamber; see 

section O.3.3 of Appendix O of the SWTR guidance manual ) of an ozone contactor, provided 

that the residual ozone concentration measured at the outlet from the first chamber met minimum 

concentration levels. For Cryptosporidium, the relatively small CT values normally achieved due 

to oxidant demand in the first dissolution chamber and the resources required for routine ozone 

monitoring would likely offset the benefit from the small Cryptosporidium credit achieved.

If no tracer study data are available for determining T10, EPA recommends using the 

Continuous Stirred Tank Reactor (CSTR) approach (described below) or the Extended CSTR 

approach (described in Appendix B). In the SWTR Guidance Manual methods were presented 

for determining the ratio of T10 to the theoretical hydraulic detention time (HDT) based on 

baffling characteristics (see Table C-5 of the SWTR Guidance Manual). However, these T10/ 

(HDT) ratios based on baffling characteristics were based on baffling characteristics of 

clearwells and basins. At this time, EPA is not aware of similar studies for ozone contactors that 

could be used to develop comparable recommendations and hence tracer studies are 

recommended for determining T10/HDT ratios. 

This guidance manual presents four methods for calculating CT in an ozone contactor: 

• T10

• CSTR Method. 

 Method. 

• Extended T10

• Extended CSTR Method. 

 Method. 

These methods differ in the level of effort associated with them. Selecting the appropriate 

method(s) to use depends on the configuration of the ozone contactor, the availability of stateapproved tracer testing results, and the amount of process evaluation and monitoring that a 
11. Ozone

LT2ESWTR Toolbox Guidance Manual 11-5 April 2010

system wishes to undertake. The T10 and CSTR methods are the simplest methods and are 

described in this chapter. Appendix B provides more information for choosing the appropriate 

method and detailed guidance for the Extended T10 and Extended CSTR methods. Exhibit 11.2 

summarizes the current methods, including a description of the situations when their use is 

appropriate. It should be noted that, while this Manual is focused on Cryptosporidium

inactivation, any of the four CT calculation methods discussed herein can also be applied to 

calculate the CT for obtaining credits for Giardia or virus inactivation in an ozone contactor 

under the requirements of the SWTR, recognizing that the first chamber credit for Giardia and 

virus inactivation provided under the SWTR remain valid, while no such credit is recommended 

for Cryptosporidium. 
11. Ozone

LT2ESWTR Toolbox Guidance Manual 11-6 April 2010

Exhibit 11.2 Recommended Methods and Terminology for Calculating the 

Log-Inactivation Credit in an Ozone Contactor

Section 

Description 

Terminology Method for Calculating 

Log-Inactivation

Recommended Restrictions

Without Tracer Data

Chambers where ozone is added 

First chamber First Dissolution 

Chamber

No Cryptosporidium loginactivation credit is 

recommended

The SWTR criteria for 1st

Other chambers

chamber 

credit should still be used if 

calculating inactivation of Giardia

and virus

Co-Current or 

Counter-Current 

Dissolution 

Chambers 

CSTR Method in each 

chamber with a 

measured effluent ozone 

residual concentration

No credit should be given to a 

dissolution chamber unless a 

detectable ozone residual has 

been measured upstream of this 

chamber

Reactive Chambers

≥ 3 consecutive 

chambers

Extended 

Reactive Zone

Extended CSTR Method 

in each chamber

Detectable ozone residual should 

be present in at least 3 chambers 

in this zone, measured via in-situ 

sample ports. Otherwise, the 

CSTR method should be applied 

individually to each chamber 

having a measured ozone residual

< 3 consecutive 

chambers

Reactive 

Chamber(s)

CSTR Method in each 

chamber

The SWTR criteria for 1st

With Approved Tracer Data

chamber 

credit should still be used if 

calculating inactivation of Giardia

and virus

Chambers where ozone is added 

First chamber First Dissolution 

Chamber

No Cryptosporidium loginactivation is credited to 

this section

The SWTR criteria for 1st

Other chambers

chamber 

credit should still be used if 

calculating inactivation of Giardia

and virus

Co-Current or 

Counter-Current 

Dissolution 

Chambers

T10 No credit should be given to a 

dissolution chamber unless a 

detectable ozone residual has 

been measured upstream of this 

chamber

,or CSTR Method in 

each chamber with a 

measured effluent ozone 

residual concentration

Reactive Chambers

≥ 3 consecutive 

chambers

Extended

Reactive Zone

Extended T10 Detectable ozone residual should 

be present in at least 3 chambers 

in this zone, measured via in-situ 

sample ports. Otherwise, the T

or

Extended CSTR Method 

in each chamber. The 

Extended CSTR method 

is not appropriate for 

non-conventional 

contactors.

10 

< 3 consecutive 

chambers

or 

CSTR method should be applied to 

each chamber having a measured 

ozone residual

Reactive 

Chamber(s)

T10 or CSTR Method in None

each chamber
11. Ozone

LT2ESWTR Toolbox Guidance Manual 11-7 April 2010

The remainder of this section describes how to calculate C for the T10 and CSTR 

methods and then describes the T10 and CSTR methodologies. 

11.3.1 Measuring C for T10 and CSTR Methods

The recommended methods for determining C have not been modified from those 

presented in the SWTR Guidance Manual. The two methods for determining C are:

1) Direct measurement of the concentration profile of dissolved ozone in each contact 

chamber (described in section O.3.2 of the SWTR Guidance Manual). 

2) Indirect prediction of the characteristic C based on dissolved ozone measurements at the 

contact chamber outlet (described in section O.3.3 of the SWTR Guidance Manual). 

 

 For the second method that involves predicting the characteristic C based on outlet 

measurements, the correlations presented in Exhibit 11.3 should be used based on Cin and Cout

measurements. To be granted inactivation credit for a chamber, its final ozone concentration 

should be above the detection limit (i.e., have a positive Cout value). 

Exhibit 11.3 Correlations to Predict C* Based on Ozone Residual Concentrations 

in the Outlet of a Chamber

Method Turbine 

Dissolution Chamber 

Co-Current Flow

Dissolution 

Chamber CounterCurrent Flow Reactive Chamber 

T10 Cout Cout or (Cin+Cout)/2 Cout

C /2 out

CSTR Cout Cout or (Cin+Cout)/2 Cout/2 C

C* - Characteristic concentration, used for CT calculation. 

out

Cout

C

 - Ozone residual concentration at the outlet from the chamber. 

in - Ozone residual concentration at the inlet to the chamber, which can be Cout of the immediate upstream 

chamber. 

11.3.2 T10 Method

 Using the T10 approach, the T10 is the time at which 10 percent of the water in the 

contactor or segment has passed through the contactor or segment. EPA recommends that tracer 

studies be used to determine the T10/HDT ratio for ozone contactors. The SWTR Guidance 

Manual and Tracer Studies in Water Treatment Facilities: A Protocol and Case Studies describe 

how to conduct a tracer test. 

Appendix C of the SWTR Guidance Manual provides a description of tracer studies and 

tracer study methods. Appendix E of this guidance provides a description of tracer test 

development and analysis. In general, tracer studies should represent the range of flow and 
 11. Ozone

operational conditions expected for the ozone process and should include data quality criteria 

(i.e. percent tracer recovered). Tracer chemicals should be conservative (high percent recovery) 

and should be acceptable to the primacy agency for use in public water systems. 

CT can be calculated for an entire treatment process (e.g., an entire ozone contactor) or 

broken into segments (e.g., individual contact chambers) and summed for a total CT value for all 

segments. C is measured either at the end of a given segment or both the beginning and end of 

the segment. 

The following steps describe the CT calculation from measured C and T values for a 

segment or the entire treatment process: 

1)	 Calculate CTcalc by multiplying the measured C and T values. Sum CTcalc for individual 

segments to obtain CTcalc for the entire ozone contactor. 

2)	 Calculate log inactivation credit using CT tables, or Equations 11.1, 11.2 or 11.3. 

3)	 Calculate the Inactivation Ratio as Log-Credit Achieved / Log-Credit Required. 

4)	 If the Inactivation Ratio is at least 1.0, then the treatment process provides the level of log 

inactivation desired. 

Example CT Calculation and Log Credit Determination using the T10 Method 

A water system employs a four chamber ozone contactor to achieve a 0.5-log 

Cryptosporidium inactivation credit. The contactor is designed and operated as shown in the 

following diagram. 

C4out = 0.0 mg/L C2out = 0.8 mg/L 

Chamber 1 

Counter-Current 

Chamber 2 

Co-Current 

C1out = 1.2 mg/L = C2in 

Chamber 3 

Counter-Current 

Chamber 4 

Reactive Flow 

C3out = 0.9 mg/L = C4in 

The water temperature is 5 degrees Celsius. Each chamber has a volume of 1,000 gallons. 

The flowrate through the contactor is 100 gpm. Results from a tracer test showed the T10/HDT 

ratio for the contactor is 0.60. 

LT2ESWTR Toolbox Guidance Manual 11-8 April 2010 
11. Ozone

LT2ESWTR Toolbox Guidance Manual 11-9 April 2010

The first step is to determine the ozone concentration for each chamber (segment). EPA 

recommends that inactivation credit not be granted for the first chamber, therefore concentrations 

are only calculated for Chambers 2, 3, and 4. Using Exhibit 11.3, C can be determined with the 

following equations: 

 Chamber 2: C = (Cin + Cout) / 2 or C = Cout 

 Chamber 3: C = Cout/2 

 Chamber 4: C = Cout 

 Therefore for: 

 Chamber 2: C = (1.2 + 0.8) / 2 = 1.0 mg/L (this equation gives the higher C value) 

 Chamber 3: C = 0.9 / 2 = 0.45 mg/L 

 Chamber 4: C = 0.0 mg/L 

2) Calculate the T for each chamber.

The T10 measured across all four chambers is divided proportionally by volume among 

the four chambers. This method should not be used if the sum of the volumes of the chambers 

with effluent ozone concentrations of zero (non-detectable) is 50 percent or greater than the 

entire volume of the chambers. In this example, only the last chamber had a non-detectable final 

concentration and that chamber is 25 percent the volume of all the chambers. Therefore the 

overall T10 can be extrapolated among the chambers to estimate individual chamber T10 values. 

 

 

 T10   T  Vol  1000 T10 of each chamber = ×HDT = 10

r × chamber = ×   chambe     [0.6]   = 6 HDT  HDT  Flowrate   100 

min. 

 (In this example, the volume of each chamber is the same. Therefore, the T10 of each 

chamber is also 6 minutes) 

 3) Calculate the CT for each chamber. CT for the total contactor is the sum of the CT for 

individual chambers. 

 Chamber 1: not calculated

 Chamber 2: CT = 1.0 mg/L × 6 min = 6.0 mg-min/L

 Chamber 3: CT = 0.45 mg/L × 6 min = 2.7 mg-min/L

 Chamber 4: CT = 0 mg/L × 6 min = 0 mg-min/L

 Total CT = 8.7 mg-min/L

 

 4) Calculate the Cryptosporidium log credit using Equation 11-1: 

Cryptosporidium Log Credit = 0.0397×(1.09757)

5

×8.7 = 0.55 logs 
11. Ozone

LT2ESWTR Toolbox Guidance Manual 11-10 April 2010

5) Calculate Cryptosporidium inactivation Ratio as 0.55/0.5 = 1.1. 

The inactivation ratio is greater than 1.0, and therefore this system achieves 0.5 log 

Cryptosporidium inactivation credit. 

 

 

11.3.3 CSTR Method

 The CSTR method is recommended for contactors that experience significant back 

mixing (T10/HDT ≤ 0.5) or when no tracer data is available. This method uses the HDT of the 

ozone contactor, as described below, for estimating the contact time. The CSTR method should 

be applied to the individual chambers in the contactor. 

For the CSTR approach, log inactivation is calculated with the following equation: 

 -Log (I/I0) = Log (1 + 2.303 × k10 × C × HDT) Equation 11-4 

where:

 -Log (I/I0

 k

) = the log inactivation

10 = log base 10 inactivation coefficient (L/mg-min)1

 C = Concentration from Exhibit 11-3 (mg/L)

 HDT = Hydraulic detention time (minutes) 

 

Exhibit 11.4 presents the k10 values for Cryptosporidium (k10 values are calculated from the CT 

table).

Exhibit 11.4 Inactivation Coefficients for Cryptosporidium, Log base 10 

(L/mg-min)

Water Temperature, ∘

≤0.5

C 

1 2 3 5 7 10 15 20 25 ≥30

k10 0.0417 0.0430 0.0482 0.0524 0.0629 0.0764 0.101 0.161 0.254 0.407 0.648

 

To interpolate between the temperatures in the table, Equation 11-5 can be used. 

 k10 = 0.0397 × (1.09757)Temp Equation 11-5 

 

1

 k10 is calculated from the CT table with the following equation: Log inactivation = k10 x CT
11. Ozone

LT2ESWTR Toolbox Guidance Manual 11-11 April 2010

 In order to apply Equation 11-4, both C and HDT must be known. These two parameters 

can be determined for individual chambers or for zones consisting of multiple, adjacent 

chambers. In general, if the concentration is measured at 3 or more points in the contactor, the 

Extended CSTR method will provide a larger CT credit than CSTR method alone, so the basic 

CSRT method will likely not be used. 

 EPA recognizes that, for many situations, either the CSTR or the T10 method can be used 

to calculate inactivation credit, and that they may generate two different estimates of log 

inactivation. EPA recommends that systems use, and states accept, the higher estimate of the log 

inactivation credit. However, systems should select one method to be used and use that method 

consistently. 

Example - CT Calculation and Log Credit Determination using the CSTR Method with the 

concentration measured for each chamber

 

A system employs a three-chamber ozone contactor, with ozone addition in the first two 

chambers. The second chamber is a counter-current flow dissolution chamber with influent and 

effluent ozone concentrations of Cin = 0.3 mg/L and Cout = 0.3 mg/L. The effluent ozone 

concentration in the third, reactive chamber is Cout = 0.2 mg/L. At 10°C, Exhibit 11-4 shows the 

k10 value at 0.101 L/mg-min. The HDT for each chamber is 20 minutes.

Chamber 1

Counter-Current

Chamber 2

Counter-Current

Chamber 3

Reactive Flow

C1out = 0.3 mg/L = C2in C2out = 0.3 mg/L

C3out = 0.2 mg/L 

 1) Determine the C values for each chamber for the CSTR calculation

 Chamber 1: No Cryptosporidium inactivation credit recommended

 Chamber 2: C = C2out

 Chamber 3: C = C

/2 = 0.3 / 2 = 0.15 mg/L 

3out = 0.2 mg/L 
11. Ozone

LT2ESWTR Toolbox Guidance Manual 11-12 April 2010

 2) Calculate the log inactivation for each chamber using Equation 11-4 

 Chamber 2: Log inactivation = Log(1 + 2.303×0.101×0.15×20) = 0.23 

 Chamber 3: Log inactivation = Log(1 + 2.303×0.101×0.20×20) = 0.28 

 

 3) Sum the log inactivation values to determine the total log credit achieved.

The total log-inactivation across the contactor is 0.23 + 0.28 = 0.51 log inactivation, 

which is greater than the target of 0.5 logs. Therefore, 0.5 log credit is achieved. 

Example - CT Calculation and Log Credit Determination using the CSTR Method with the 

concentration not measured for each chamber

 A system employs a four chamber ozone contactor, with ozone addition in the first two 

chambers. The second chamber is a counter-current flow dissolution chamber with influent and 

effluent ozone concentrations of Cin = 0.3 mg/L and Cout = 0.3 mg/L. The effluent ozone 

concentration in the third, reactive chamber is unknown, and in the fourth, reactive chamber is 

0.1 mg/L. At 10 °C, k10 = 0.101 L/mg-min. The HDT for each chamber = 20 minutes. Chambers 

3 and 4 are considered a single zone, and the effluent concentration of Chamber 3 is assumed to 

be equal to that of Chamber 4. 

 1) Determine the C values for each chamber

 Chamber 1: No Cryptosporidium inactivation credit recommended

 Chamber 2: C = C2out

 Chamber 3: C = C

/2 = 0.3 / 2 = 0.15 mg/L 

4out

 Chamber 4: C = C

= 0.1 mg/L 

4out = 0.1 mg/L

 2) Calculate the log inactivation for each chamber using Equation 11-4 
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LT2ESWTR Toolbox Guidance Manual 11-13 April 2010

 Chamber 2: Log inactivation = Log(1 + 2.303×0.101×0.15×20) = 0.23 

 Chamber 3: Log inactivation = Log(1 + 2.303×0.101×0.10×20) = 0.17 

 Chamber 4: Log inactivation = Log(1 + 2.303×0.101×0.1×20) = 0.17 

 3) Sum the log inactivation values to determine the log credit achieved. 

The total log-inactivation across the contactor is 0.23 + 0.17 + 0.17 = 0.57 log 

inactivation. Therefore, the minimum 0.5 log credit is achieved. 

11.3.4 Extended T10 and Extended CSTR Methods

 The Extended T10 and Extended CSTR methods require the measurement of the ozone 

concentration at a minimum of three points within the contactor. These data are used to predict 

an ozone concentration profile through the contactor. The Extended methods generally result in 

greater CT credit and hence lower doses of ozone needed to achieve the same calculated level of 

inactivation, when compared to the direct T10 or CSTR method. Appendix B provides a complete 

description of the extended methods and how they are applied to an ozone contactor. 

11.4 Monitoring Requirements

11.4.1 LT2ESWTR

 The LT2ESWTR (40 CFR 141.720(a)) requires daily CT monitoring conducted during 

peak hourly flow. Since systems may not know when the peak hour flow will occur, EPA 

recommends monitoring on an hourly basis. Systems should re-evaluate contact time whenever 

they modify a process and the hydraulics are affected (e.g., add a pump for increased flow, 

reconfigure piping).

 The concentration of ozone is measured with the indigo methods, Standard Method 4500-

O3 B and Standard Method 4500 O3 B-97. Details on these methods can be found in Standard 

Methods for the Examination of Water and Wastewater, 20th edition, American Public Health 

Association, 1998. Appendix C provides information on sample collection, preparation and 

stability of reagent, and calibration and maintenance of online monitors. 

11.4.2 Stage 1 DBPR and Stage 2 DBPR

 The Stage 1 DBPR requires all systems using ozone for disinfection or oxidation to take 

at least one bromate sample per month for each treatment plant using ozone (See the Stage 1 

DBPR, 40 CFR 141.132(b) for further information). Samples must be taken at the distribution 

system entry point when the ozone system is operating under normal conditions. Under the Stage 

2 DBPR (40 CFR 141.132 (b) (3) (ii) (B), beginning April 1, 2009, systems may reduce 

monitoring from monthly to quarterly if the system demonstrates that the running annual average 
11. Ozone

LT2ESWTR Toolbox Guidance Manual 11-14 April 2010

raw water bromide concentration is less than 0.0025 mg/L, based on monthly measurements for 

the most recent four quarters. Systems that were allowed to reduce bromate monitoring to 

quarterly prior to April 1, 2009, may remain on quarterly monitoring if the running annual 

average raw water bromide concentration is less than 0.0025 mg/L. The MCL for bromate is 10 

μg/L based on a running annual average. 

11.5 Unfiltered System LT2ESWTR Requirements

 

 The LT2ESWTR requires unfiltered systems to meet the following requirements (40 CFR 

141.712(b), (c) and (d)): 

• Provide at least 2.0 log Cryptosporidium inactivation. 

• If their source water Cryptosporidium concentration is greater than 0.01 oocyst/liter, then 

the system must provide 3.0 log Cryptosporidium inactivation. 

• Use a minimum of two disinfectants to meet overall disinfection requirements. 

 The requirements of the previous SWTR regulations still apply—achieve 3 log 

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

the distribution system (e.g., free chlorine or chloramines). 

 The monitoring requirements described in section 11.4 apply to unfiltered systems. 

Additionally, unfiltered systems must meet the Cryptosporidium log-inactivation requirements 

every day the system serves water to the public, except one day per calendar month (40 CFR 

141.712(c)). Therefore, if an unfiltered system fails to meet Cryptosporidium log-inactivation 

two days in a month, it is in violation of the treatment technique requirement. 

11.6 Toolbox Selection

 Selecting ozone disinfection to receive Cryptosporidium inactivation credit for 

compliance with the LT2ESWTR has cost, operational, and upstream and downstream process 

implications. The ozone CT requirements for Cryptosporidium inactivation are significantly 

higher than those for Giardia and virus, and capital requirements could be substantial for a 

system seeking higher than 0.5 log credit. As a result, ozone is likely a better option for systems 

that will benefit from its other treatment effects. This section discusses the potential advantages 

and disadvantages of ozone processes. 
11. Ozone

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11.6.1 Advantages 

 Ozonation reduces many other contaminants and improves process performance, both 

directly and indirectly. The indirect benefits are those where other aspects of the treatment 

process can be improved or changed, resulting in a higher finished water quality. The advantages 

of ozone use include: 

• Total organic carbon (TOC) reduction. 

• Iron, manganese, and sulfide oxidation. 

• Taste, odor, and color control. 

• Lower formation of trihalomethanes (THMs) and haloacetic acids (HAAs) upon post 

chlorination due to precursor removal and generally lower chlorine doses. 

• Biological stability when ozonation is followed with biological filtration. 

11.6.2 Disadvantages 

 Considering only benefits from Cryptosporidium inactivation credit, the capital, 

operational, and maintenance costs are relatively high compared to other toolbox options for 

similar credit, especially for systems treating colder water. Other disadvantages include:

 

• Higher level of maintenance and operator skill required.

• Additional safety and containment issues with ozone contactors. 

• Possible need for three-phase power which may not be compatible with some water 

systems.

• Bromate formation (bromate is a regulated disinfection byproduct (DBP)). 

• Potential enhanced formation of other unregulated DBPs either from ozonation alone 

(e.g. formaldehyde) or upon secondary chlorination/chloramination (e.g. chloropicrin). 

• Upstream processes can cause fluctuations in ozone demand, thus affecting ozone 

residual control.

• Assimilable organic carbon (AOC) production, which can contribute to biofilm growth in 

the distribution system if not removed. 

• High capital requirements to achieve CT requirements with low water temperatures 

(below 10 oC).
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LT2ESWTR Toolbox Guidance Manual 11-16 April 2010

11.7 Disinfection With Ozone

11.7.1 Chemistry

 The stability of ozone upon dissolution into natural waters is governed by both the direct 

reaction of ozone with various constituents in the water, as well as the so-called “auto catalytic 

chain decomposition” reaction. The general behavior in the dissolved ozone concentration over 

time has been described by numerous investigators (e.g. Park et al. 2001; Elovitz et al. 1999, 

2000) as a two-stage process: an initial rapid consumption step (ozone consumed after a few 

seconds to 30 seconds) followed by a slower ozone decay step which can often be described by 

first-order kinetics. Various water quality parameters, including temperature, pH, DOC 

concentration, DOC character, and alkalinity, as well as the ozone dose can affect the amount 

and rate of ozone consumed in these two stages (Elovitz et al. 2000; Park et al. 2001; Buffle et al. 

2006). For example, increasing ozone dose can increase the amount of ozone consumed in the 

initial reaction phase; however, the rate of ozone decay in the second phase is slower. This is of 

importance for considerations to ozone dose requirements in order to maintain dissolved ozone 

for sufficient disinfection (i.e. CT requirements).

One of the consequences of the spontaneous decomposition of ozone during water 

treatment is the generation of hydroxyl free radicals (Hoigné and Bader 1983a, 1983b; Glaze et 

al. 1987). The hydroxyl free radicals are among the most reactive oxidizing agents in water, with 

most reaction rates on the order of 108

 - 1010 M-1 s

-1. Because of their high reactivity, the half-life 

of hydroxyl free radicals is on the order of microseconds. The formation of hydroxyl radicals is 

affected by the same water quality parameters that affect ozone decomposition (see above), and 

is also vastly different in the initial reaction phase versus the secondary reaction phase (Elovitz et 

al. 1999, 2000; Buffle et al. 2006). Under typical conditions for ozonation of drinking water 

source waters, the transient concentration of the hydroxyl free radicals can reach as high as 10-11

M during the first 20 milliseconds of the initial reaction phase (Buffle et al. 2006). During the 

secondary phase, hydroxyl radical concentrations are typically on the order of 10-13 to 10-14 M 

(Elovitz et al. 1999, 2000) and reach levels above 10-12 M under typical ozonation conditions 

(Glaze and Kang 1988). Despite these low transient concentrations, the hydroxyl free radical can 

be a very important reactant for the oxidation of constituents that are slow reacting with respect 

to molecular ozone. Under certain conditions, an ozonation process can be operated with the 

intent of creating an “Advanced Oxidation Process” (AOP) which purposefully enhances the 

formation of hydroxyl free radicals from the decomposition of ozone. While the enhanced 

formation of hydroxyl radicals can lead to enhanced oxidation of certain micropollutants, it will 

also lower the overall ozone CT. The application of AOPs is discussed further in Chapter 7 of the 

Alternative Disinfectants Guidance Manual for information on advanced oxidation processes). 
11. Ozone

LT2ESWTR Toolbox Guidance Manual 11-17 April 2010

Exhibit 11.5 Reaction Pathways of Ozone in Water

 

 Exhibit 11.5 illustrates the major pathways that develop during ozonation: 

• Direct oxidation of compounds by molecular ozone in the aqueous phase. 

• Oxidation of compounds by hydroxyl free radicals produced during the decomposition of 

ozone. 

As indicated in Exhibit 11.5, the direct reaction with molecular ozone is relatively slow 

compared to the hydroxyl reaction. However, the reaction with many aqueous species is still very 

rapid compared to other disinfectants. In addition, the concentration of ozone is orders of 

magnitude greater then the concentration of hydroxyl radicals. Hence, when assessing the role of 

the two pathways, both the intrinsic reaction rate as well as the concentration of the two oxidants 

must be considered. 

The reaction of hydroxyl free radicals with carbonate and bicarbonate produces carbonate 

and bicarbonate radicals. These free radicals may also participate in the oxidation of chemical 

and microbial species. However, these reactions tend to be selective and, with some exception, 

their contribution to micropollutant oxidation is negligible compared to the two main pathways. 
11. Ozone

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11.7.2 Byproduct Formation 

Reactions between ozone and NOM can form a variety of organic byproducts including 

aldehydes, ketones, and acids. Inorganic byproducts are also formed. Bromide reacts with ozone 

and hydroxyl radicals to form bromate, a regulated drinking water contaminant with an MCL of 

10 μg/l. Oxidation of bromide to hypobromous acid during ozonation, and subsequent reaction of 

he hypobromous acid with natural organic matter can lead to the formation of brominated 

organic compounds, such as bromoform and dibromoacetic acid, which are also regulated 

through the total trihalomethanes (TTHMs) and haloacetic acids (HAA5) MCLs under the Stage 

2 DBPR.

11.7.2.1 Bromate and Brominated Organic Compounds 

 Bromate and brominated organic compound formation is dependent on water quality and 

treatment conditions, and only occurs in waters with bromide ion present. Bromate formation 

occurs through a very complex, nonlinear mechanism involving both ozone and hydroxyl free 

radical pathways. Bromate formation generally increases with increasing pH, carbonate 

alkalinity, ozone dose, and temperature, and perhaps most importantly, bromide concentration. 

However, attempts at reducing bromate formation by lowering pH may increase the formation of 

brominated organic byproducts. Other methods for minimizing bromate formation during 

ozonation include ammonia addition (Hoffman et al. 2001), and more recently the so-called 

Chlorine-Ammonia Process (Buffle et al. 2004). Overall, the source water bromide concentration 

is a very important factor when considering adding ozone to a treatment process. Source waters 

with bromide concentrations greater than 50 ppb likely need to consider the possibility of 

significant bromate formation (von Gunten, 2003) 

11.7.2.2 Non-Brominated Organic Compounds

Ozone reacts with NOM and breaks larger organic molecules down into simpler, more 

biodegradable compounds such as aldehydes, ketones, and acids. These biodegradable organic 

molecules are a food source for microorganisms and can affect biological growth in the 

distribution system. Escobar and Randall (2001) conducted a case study at a ground water 

treatment plant that was adding ozone to improve the aesthetic quality of the water. They found 

that the AOC (the fraction of TOC that is most readily utilized by bacteria) concentrations 

significantly increased in the distribution system however, with diligent maintenance of chlorine 

residual biological growth was suppressed. Biofilters can be used to reduce the AOC entering the 

distribution system. (Section 11.8.3 describes biofilters and their operation.) 
11. Ozone

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11.8 Design

11.8.1 Generators and Contactors

There are several types of ozone generators and contactors. All generators use oxygen as 

a raw material and convert it to ozone using electrochemical reactions. They differ from each 

other in the source of oxygen used and the configuration of generator elements. Generators can 

use either air or pure oxygen as an oxygen source. The Alternative Disinfectants and Oxidants 

Guidance Manual describes the type of generators and contactors in detail.

11.8.2 Point of Addition

 Raw water quality, turbidity, and ozone demand are commonly used to assess the 

possible locations for adding ozone. The Alternative Disinfectants and Oxidants Guidance 

Manual describes the water quality characteristics, advantages, and disadvantages of feed points 

at a raw water location, after sedimentation, and after first-stage filtration of a two-stage process. 

The general considerations are:

• Integrating ozone into overall disinfection strategy for the treatment facility.

• Placing the ozone addition point further downstream, particularly after physical removal 

processes, generally reduces both the ozone demand and byproduct formation. 

• Adding ozone ahead of filtration allows any biodegradable organics, formed from the 

ozonation of more recalcitrant TOC, to be removed by subsequent biological activity in 

the filters. Also, solid-phase manganese and iron formed through oxidation by ozone can 

be removed by the filters. 

 

 In general, applying ozone prior to coagulation can enhance clarification. Applying prior 

to filtration can also improve filtration performance; however these effects are site-specific and 

are likely to depend on ozone dose. 

Detrimental impacts on filtration operation have also been reported. Bishop et al. (2001) 

investigated the effects of ozone on filtration with a raw water of moderate turbidity, TOC, iron, 

and manganese concentrations. With ozone doses of 0.5 to 1.0 mg/L, turbidity increased in the 

contactors with visible floc formation. At lower ozone doses, 0.16 to 0.35 mg/L, the turbidity 

still increased, but not as much as the higher ozone dose. Because of the higher filter loadings, 

the duration of filter cycles decreased. The authors believed the increased turbidity was partially 

due to solid-phase manganese formation, and also likely due to the organic matter and residual 

metals. 
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11.8.3 Biologically Active Filters

 When ozone oxidizes organic matter, the AOC in the water typically increases. Some 

systems use biologically active filters to remove the AOC prior to chlorination and entry to the 

distribution system. Microbes present in the upper portion of the filters consume the AOC, 

mineralizing them to carbon dioxide and water, and reducing the amount available to 

microorganisms in the distribution system (e.g., microorganisms in pipeline biofilm) and for 

DBP formation. 

 

 

11.8.3.1 Media for Biologically Active Filters

Any filter media that has sufficient surface area for microbes to attach to can be used for 

biological filtration. Slow sand, rapid sand, and granular activated carbon (GAC) filters have all 

been successfully used for biologically active filtration. Research indicates that both 

sand/anthracite and sand/GAC filters can support the total amount of biomass to sufficiently 

remove organic components (LeChevallier et al. 1992, Krasner et al. 1993, Coffey et al. 1995). 

Wang and Summers (1996) and Zhang and Huck (1996) have shown that the contact time with 

the biofilm is more important than the mass of biofilm above a minimum level of biomass. 

Generally, the longer the contact time the greater the removal of AOC. However, the increase in 

removal is not a linear relationship—the removal rate decreases at extended contact times 

(Zhang & Huck 1996). DBP precursors most often take longer to biodegrade, making extended 

contact times necessary if this is the process goal. This can be achieved with deep anthracite 

filter beds or GAC filters (Prevost et al. 1990). The adsorption capacity of GAC provides a 

longer time for the organic compounds to be consumed by the biomass as the particles are 

adsorbed by the GAC (LeChevallier et al. 1992). 

11.8.3.2 Operating Biologically Active Filters 

It is not necessary to seed a biological filter in order to obtain the necessary biological 

growth. The organisms naturally present in the system are sufficient to obtain the needed growth. 

The only additional requirement is to provide the conditions for biological growth. These 

conditions include necessary food sources, sufficient dissolved oxygen, nutrients, proper pH and 

temperature. The products from ozone and NOM reactions will provide the needed food for the 

microorganisms to grow. The reaction of ozone also produces oxygen as one of its products, so 

the dissolved oxygen concentration should be sufficiently high. Generally, the pH and nutrient 

levels in most waters will also be sufficient to allow the necessary growth. Organic removal will 

generally be higher at higher temperatures. Several studies have found significantly decreased 

removal at temperatures below 15 degrees Celsius (Krasner et al. 1993, Coffey et al. 1995, 

Daniel and Teefy 1995). 

 In order to maintain biological growth, a disinfectant other than ozone cannot be added 

prior to the filters. GAC filters can reduce small disinfectant residuals through reaction with the 

carbon; however, this can lead to physical breakdown of the GAC and more frequent media 

replacement. Using chlorinated or chloraminated backwash water can also be a concern. Studies 
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LT2ESWTR Toolbox Guidance Manual 11-21 April 2010

have shown mixed results with chlorinated backwash water, with some showing no effect and 

others showing significantly reduced removal (Miltner et al. 1996, Miltner et al. 1995, Hacker et 

al. 1994, Reckhow et al. 1992, McGuire et al. 1991). Short vigorous backwashes with a 

relatively low chlorine dose may be more effective in maintaining biological filtration than less 

vigorous backwashes at longer times with higher chlorine doses (Urfer et al. 1997). 

11.9 Safety Considerations in Design

Ozone is a corrosive gas and according to Occupational Safety and Health Administration 

(OSHA) Standards, exposure to airborne concentrations should not exceed 0.1 ppm (by volume) 

averaged over an eight-hour work shift. 

 Ozone generators should be housed indoors for protection from the environment, and to 

protect personnel from leaking ozone in the case of a malfunction. Ventilation should be 

provided to prevent excess temperature rise in the generator room, and to exhaust the room in the 

case of a leak. Adequate space should be provided to remove the tubes from the generator shell 

and to service the generator power supplies. Off-gas destruct units can be located outside if the 

climate is not too extreme. If placed inside, an ambient ozone detector should be provided in the 

enclosure. All rooms should be properly ventilated, heated, and cooled to match the equipmentoperating environment. 

11.10 Operational Considerations

 When using ozone for disinfection, it is important to evaluate all the factors that could 

affect the CT achieved. For example, if raw water quality fluctuates and ozone demand 

increases, the residual concentrations will decrease unless ozone dose is adjusted. The system is 

now at risk of not achieving the required level of CT. The ozone demand, pH, and temperature of 

the raw water, under worst-case to best-case conditions, should be evaluated to determine their 

effect on ozone disinfection. Systems should develop standard operating procedures (SOPs) for 

addressing changes in raw water quality. The remainder of this section discusses how these 

factors affect ozone disinfection and the CT calculation.

11.10.1 Ozone Demand

 The following water quality constituents contribute to ozone demand: 

• Natural organic matter (NOM)—Ozone will oxidize organic matter, which includes 

compounds causing taste and odor. As discussed in section 11.8.2 organic byproducts are 

also produced. 

• Synthetic organic compounds (SOCs)—Some SOCs can be oxidized and mineralized 

under favorable conditions. 
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LT2ESWTR Toolbox Guidance Manual 11-22 April 2010

• Bromide—Ozone will oxidize bromide forming, hypobromous acid, hypobromite ion, 

bromate ion, brominated organics, and bromamines. 

• Bicarbonate or carbonate ions—The hydroxyl radical reacts with bicarbonate and 

carbonate ions and form carbonate radicals. 

 Ozone demand is particularly important to the CT calculation since it directly affects the 

residual ozone used in the CT calculation. Ozone concentrations in water are generally 

monitored continuously using an on-line residual monitor, and confirmed periodically with a 

bench top instrument. As the ozone demand changes, the amount of ozone applied can be 

adjusted to maintain the desired CT. 

 

11.10.2 pH

The pH of water does not have a significant effect on ozone disinfection capabilities. 

However, there is strong impact of pH on ozone demand and decay rate. As pH increases, ozone 

decomposition increases, and there is a concomitant increase in the formation of the hydroxyl 

radical. Under the pH range typically used for drinking water treatment (e.g. pH 6 – 9), the initial 

demand may be only increased by increasing pH due to changing pH speciation of compounds 

that react directly with ozone (Buffle et al, 2006b). However, increasing pH has a significant 

effect on the secondary reaction phase due to increased contribution of the autocatalytic chain 

decomposition (Elovitz et al., 2000; Buffle et al, 2006b). 

11.10.3 Temperature

Like all chemical reactions, the reaction rate between ozone and a pathogen increases 

with increasing temperature. The CT requirements are based on temperature; as temperature 

decreases, the CT required to achieve a given level of inactivation increases. Conversely, the rate 

of ozone decay decreases as temperature decreases, generally resulting in a higher CT for a given 

ozone dose and hydraulic resident time. The ozone process should be designed to provide the 

necessary log inactivation under all conditions. Standard operating procedures (SOPs) should 

also describe process adjustments required to operate at the lowest water temperatures 

experienced by the system in the past 10 years. 

11.10.4 Maintaining Residual Disinfectant in the Distribution System

It is necessary to maintain a residual in the distribution system to prevent microbial 

regrowth. Because of the reactive nature of ozone, its residual tends to dissipate within minutes 

and cannot be relied upon to maintain a disinfectant throughout the distribution system. 

Therefore, a secondary disinfectant should be used, usually either chlorine or chloramine. 
11. Ozone

LT2ESWTR Toolbox Guidance Manual 11-23 April 2010

11.11 References

Bishop, M. M., Qiao, F., Iversen, G., and Carter, G.C. 2001. Intermediate Ozonation for 

Cryptosporidium Inactivation and Effects on Filtration. AWWA WQTC Proceedings.

Buffle, M.-O., Galli, S. and von Gunten, U. 2004. Enhanced Bromate Control During Ozonation: 

The Chlorine-Ammonia Process. Environ. Sci. Technol., 38, 5187-5195. 

Buffle, M.-O., J. Schumacher, Meylan, S., Jekel, M., von Gunten, U. 2006. Ozonation and 

Advanced Oxidation of Wastewater: Effect of O3 Dose, pH, DOM and HO•-scavengers on 

Ozone Decomposition and HO• Generation. Ozone Sci. Eng. 28:247-259. 

Buffle, M.-O. and von Gunten, U. 2006b. Phenol and Amine Induced HO• Generation during 

Initial Phase of Natural Water Ozonation. Environ. Sci. Technol. 40, 3057-3063. 

Coffey, B.M., S.W. Krasner, M.J. Sclimenti, P.A. Hacker, and J.T. Gramith. 1995. A 

Comparison of Biologically Active Filters for the Removal of Ozone By-Products, Turbidity and 

Particles. In Proc. AWWA WQTC. Denver, CO: AWWA. 

Daniel, P., and S. Teefy. 1995. Biological Filtration: Media, Quality, Operations, and Cost. In 

Proc. AWWA Annual Conf. Denver, CO: AWWA. 

Elovitz, M. S. and U. von Gunten. 1999. Hydroxyl Radical/Ozone Ratios during Ozonation 

Processes. I. The Rct Concept. Ozone Sci. Eng. 21(3): 239-260. 

Elovitz, M. S., U. von Gunten, H.-P. Kaiser. 2000. Hydroxyl Radical/Ozone Ratios during 

Ozonation Processes. II. The Effect Of Temperature, pH, Alkalinity and DOM Properties. Ozone 

Sci. Eng. 22: 123-150. 

Escobar I.C. and A.Randall. 2001. Case Study: Ozonation and Distribution System Biostability. 

J. AWWA. 93(10):77-89. 

Glaze, W.H., et al. 1987. The Chemistry of Water Treatment Processes Involving Ozone, 

Hydrogen Peroxide, and Ultraviolet Radiation. Ozone Sci. Eng. 9(4):335. 

Glaze, W.H., and J.W. Kang. 1988. Advanced Oxidation Processes for Treating Groundwater 

contaminated with TCE and PCE: Laboratory Studies. J. AWWA. 88(5):57- 63. 

Hacker, P.A., C. Paszko-Kolva, M.H. Stewart, R.L. Wolfe, and E.G. Means. 1994. Production 

and Removal of Assimilable Organic Carbon Under Pilot-Plant Conditions through the Use of 

Ozone and PEROXONE. Ozone Sci. Eng., 16(3): 197-212. 

Hofmann, R. and R. C. Andrews. 2001. Ammoniacal Bromamines: A Review of their Influence 

on Bromate Formation during Ozonation. Water Res. 35(3): 599-604. 
11. Ozone

LT2ESWTR Toolbox Guidance Manual 11-24 April 2010

Hoigné J., and H. Bader. 1983a. Rate Constants of Reaction of Ozone with Organic and 

Inorganic Compounds in Water - I. Non-dissociating Organic Compounds. Water Res. 17:173-

183. 

Hoigné J., and H. Bader. 1983b. Rate Constants of Reaction of Ozone with Organic and 

Inorganic Compounds in Water - II. Dissociating Organic Compounds. Water Res. 17:185-194. 

Krasner, S.W., W.H. Glaze, H.S. Weinberg, et al. 1993. Formation of Control of Bromate During 

Ozonation of Water Containing Bromide. J. AWWA. 85(5):62. 

LeChevallier, M.W., W.C. Becker, P. Schorr, and R.G. Lee. 1992. Evaluating the Performance 

of Biologically Active Rapid Filters. J. AWWA. 84(4):136-146. 

McGuire, M.J. et al. 1991. Pilot-scale Evaluation of Ozone and PEROXONE (90951). 

AWWARF. Denver, CO.

Miltner, R.J., R.S. Summers, N.R. Dugan, M. Koechling, and D.M. Moll. 1996. A Comparative 

Evaluation of Biological Filters. In Proc. AWWA WQTC. Denver, CO: AWWA. 

Miltner, R.J., R.S. Summers, and J.Z. Wang. 1995. Biofiltration Performance: Part 2, Effect of 

Backwashing. Jour. AWWA, 87(12):64. 

Park, H., Hwang, T., Kang, J., Choi, H, and Oh, H. 2001. Characterization of Raw Water for the 

Ozone Application Measuring Ozone Consumption Rate. Water Res. (35) (11) p. 2607-2614. 

Prevost, M., R. Desjardins, D. Duchesne, and C. Poirier. 1990. Chlorine Demand Removal by 

Biological Activated Carbon Filtration in Cold Water. In Proc. AWWA, WQTC. Denver, CO: 

AWWA. 

Reckhow, D.A., J.E. Tobiason, M.S. Switzenbaum, R. McEnroe, Y. Xie, X. Zhou, P. 

McLaughlin, and H.J. Dunn. 1992. Control of Disinfection Byproducts and AOC by PreOzonation and Biologically Active In-Line Direct Filtration. Conference proceedings, AWWA 

Annual Conference, Vancouver, British Columbia. 

Teefy, S. 1996. Tracer Studies in Water Treatment Facilities: A Protocol and Case Studies. 

AWWARF.

Urfer, D., P.M. Huck, S.D.J. Booth, and B.M. Coffey. 1997. Biological Filtration for BOM and 

Particle Removal: A Critical Review. Jour. AWWA, 89(12):83. 

von Gunten, U. 2003. Ozonation Of Drinking Water: Part II. Disinfection and By-Product 

Formation in Presence of Bromide, Iodide or Chlorine. Water Res. 37(7): 1469-1487. 

Wang, J., and R.S. Summers. 1996. Biodegradation Behavior of Ozonated Natural Organic 

Matter in Sand Filters. Rev. Sci. Eau, 1:3. 

Zhang, S., and P.M. Huck. 1996. Biological Water Treatment: A Kinetic Modeling Approach. 

Wat. Res., 30(5):1195. 
LT2ESWTR Toolbox Guidance Manual 12-1 April 2010

12. Demonstration of Performance (DOP)

12.1 Introduction 

 The purpose of the “demonstration of performance” (DOP) toolbox component is to 

allow a system to demonstrate that a plant, or a unit process1

 within a plant, should receive a 

higher Cryptosporidium treatment credit than is presumptively awarded under the Long Term 2 

Enhanced Surface Water Treatment Rule (LT2ESWTR). Presumptive treatment credits are 

applicable to any physical removal process that complies with the provisions of the Interim 

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

Treatment Rule (LT1ESWTR) (40 CFR 141.552). Exhibit 12.1 gives some examples of typical 

plants that could be eligible for a DOP credit. DOP credits can be granted for any process,

including inactivation processes; however, this chapter is limited to a discussion of physical 

removal processes. Membrane processes receiving DOP credit must still meet challenge testing 

and direct integrity testing requirements as specified in 40 CFR 141.719. 

Exhibit 12.1 Example Filtration Plant Types Eligible for DOP

Plant Type Minimum Elements of Process Train

Conventional Coagulation/Flocculation, Sedimentation, 

High Rate Granular Media Filtration

Slow Sand Filtration Slow Sand Filtration

Diatomaceous Earth Diatomaceous Earth Filtration

Softening/Granular Media Filtration Single Stage Lime Softening, High Rate 

Granular Media Filtration

Direct Filtration Coagulation/Flocculation, High Rate 

Granular Media Filtration

Where a system can demonstrate that a plant, or a unit process within a plant, 

consistently achieves a Cryptosporidium treatment efficiency greater than the presumptive credit 

specified in the LT2ESWTR, the state may allow the system to receive a higher Cryptosporidium

treatment credit for compliance with the LT2ESWTR (40 CFR 141.718(c)). To demonstrate the 

higher level of Cryptosporidium treatment, systems must conduct a site-specific study using a 

protocol approved by the state. This study must account for all expected operating conditions 

and, at the discretion of the state, determine ongoing monitoring and/or performance 

requirements to ensure conditions under which the DOP was awarded are maintained during 

routine operations. 

In general, the term “treatment” in the LT2ESWTR refers to both physical removal and 

inactivation of Cryptosporidium. Treatment credits discussed in this chapter pertain to physical 

removal. 

 

1

 A system would conduct a DOP of a unit process while ensuring the other parts of the treatment process were 

achieving their assumed Cryptosporidium treatment. For example, maximizing removal in a pre-sedimentation basin 

can cause reduced removal in the subsequent sedimentation basin and filters.
12. Demonstration of Performance (DOP): Microbial Removal

LT2ESWTR Toolbox Guidance Manual 12-2 April 2010

This chapter provides guidance for implementing the DOP toolbox option and is organized as 

follows: 

12.2 LT2ESWTR Compliance Requirements - discusses DOP treatment credit with 

respect to other toolbox options and reporting requirements. 

12.3 Toolbox Selection Considerations - describes selection considerations for plants 

to consider before conducting a DOP study, the duration of a DOP study, and an 

approach for conducting a DOP study. 

12.4 DOP Criteria Development - discusses key issues of DOP design including 

process evaluation criteria, selection of performance indicators, and full-scale 

versus pilot-scale testing.

12.5 Demonstration Protocol - discusses the minimum elements that should be 

included in the DOP protocol - DOP test matrix, DOP monitoring plan, DOP 

implementation, and data analysis and reporting. 

12.2 LT2ESWTR Compliance Requirements 

12.2.1 Credits

 The LT2ESWTR does not specify how treatment performance must be demonstrated; 

however the protocol used must be approved by the state (40 CFR 141.718(c)). Determination of 

an increased Cryptosporidium treatment credit will be made by the state. 

The LT2ESWTR does not allow systems to claim presumptive credit for the toolbox 

options listed below, if that component is included in the DOP credit (40 CFR 141.718(c)(1)). 

• Presedimentation

• Two-stage lime softening

• Bank filtration

• Combined or individual filter performance

• Membrane filters

• Bag and cartridge filters

• Second stage filtration
12. Demonstration of Performance (DOP): Microbial Removal

LT2ESWTR Toolbox Guidance Manual 12-3 April 2010

For example, if a plant receives a DOP credit for a treatment train, the system may not 

also receive credit for a presedimentation basin or achieving the lower finished water turbidity of 

the combined filter performance option. 

States may award a lower level of Cryptosporidium treatment credit towards compliance 

for the LT2ESWTR to a system where, based on site-specific information, a plant or a unit 

process achieves a Cryptosporidium treatment efficiency less than a presumptive credit specified 

in the LT2ESWTR (40 CFR 141.718(c)).

12.2.2 Reporting Requirements

The LT2ESWTR requires results from the testing be submitted no later than the 

Cryptosporidium compliance date (40 CFR 141.721): 

• Schedule 1 – April 1, 2012 

• Schedule 2 – October 1, 2012 

• Schedule 3 – October 1, 2013 

• Schedule 4 – October 1, 2014 

The state may require systems to report operational data on a monthly basis to verify that 

conditions under which DOP credit was awarded are maintained during routine operation (40 

CFR 141.721).

12.3 Toolbox Selection Considerations 

 The DOP toolbox option is intended for plants that operate at a high level of 

performance. A system should review existing performance data to verify that it can meet high 

performance levels under a range of operating conditions (including filters out of service, 

returning to service, and flow rate changes) before conducting a DOP study. EPA recommends 

systems achieve less than 0.1 nephelometric turbidity units (NTU) in each individual filter 

effluent as an indicator for considering whether the DOP option is practical. 

 Before applying the DOP approach to an individual unit process, facilities should 

carefully consider the potential advantages and disadvantages of such an approach. The 

microbial toolbox allows for treatment credits for unit processes based on specified design 

and/or operational criteria described in other chapters of this manual. It is possible that a detailed 

DOP program may result in a lower credit than already granted by the LT2ESWTR.

 A DOP study should address the range of operating conditions (e.g., flow rates, chemical 

and disinfection practices and dosages) and seasonal raw water quality variations based on a 
12. Demonstration of Performance (DOP): Microbial Removal

LT2ESWTR Toolbox Guidance Manual 12-4 April 2010

review of plant operating records and historical water quality records. If source or operating 

conditions are expected to change (e.g., turbidity events, high watershed runoff, increased 

system demands) these should be addressed in the DOP study. Systems should have a 

contingency plan for achieving compliance with the LT2ESWTR if the DOP does not provide 

the anticipated credit.

12.3.1 Overview of the Demonstration Protocol 

 This chapter presents one approach for conducting a DOP study. Other approaches or 

modifications to this approach may be approved by the state. Major elements of the DOP 

protocol include the following: 

• Development of DOP evaluation criteria and test matrix. 

• DOP implementation. 

• Data analysis and reporting. 

 

 Exhibit 12.2 presents a flowchart relating these elements to the overall microbial toolbox 

framework. Each of these topics is discussed in detail in this chapter.

Exhibit 12.2 Flowchart for DOP Protocol
12. Demonstration of Performance (DOP): Microbial Removal

LT2ESWTR Toolbox Guidance Manual 12-5 April 2010

12.4 DOP Criteria Development 

Source water Cryptosporidium levels and water quality characteristics vary from system 

to system. Accordingly, DOP programs should be tailored to address site-specific process issues 

associated with each water treatment plant (WTP). Major questions that should be resolved 

during the design of the DOP include (but are not limited to) the following:

• What are the governing process evaluation criteria and treatment objectives?

• What microorganism or surrogate parameter(s) should be used to demonstrate removal 

efficiency of Cryptosporidium? 

• Should the DOP be conducted at full-scale or pilot-scale?

Each of these questions is addressed in the following sections.

12.4.1 Process Evaluation Criteria

Process evaluation encompasses the treatment objectives of the plant, influent water 

quality, system demand, and operating conditions or treatment techniques. The DOP plan should 

address all critical operating conditions, whether conducted in full-scale or pilot-scale. Influent 

water quality, flow rates, process configurations, and operating conditions need to be clearly 

defined during the development of the DOP plan. Common process evaluation criteria are 

discussed in this section.

12.4.1.1 Treatment Objectives

 The DOP toolbox option primarily relates to Cryptosporidium removal by physical 

methods such as clarification and filtration. However, WTPs are tasked to remove or control 

multiple contaminants in the source water besides Cryptosporidium. The impact of operational 

strategies and treatment methods for other contaminants on the efficiency of Cryptosporidium

removal should be considered during the DOP criteria development stage. The system should not 

change its operational strategy between the DOP study conditions and routine operation after the 

study has ended—the DOP credit is based on the operational strategy used in the study. For 

example, a system that uses enhanced coagulation throughout the study period should also use it 

during routine operation for compliance with the LT2ESWTR. 

Other examples of treatment techniques that can affect Cryptosporidium removal and 

thus should be considered in the development stage include the following: 
12. Demonstration of Performance (DOP): Microbial Removal

LT2ESWTR Toolbox Guidance Manual 12-6 April 2010

• Prechlorination may be used to enhance floc formation (and Cryptosporidium removal) in 

filtration trains. However, prechlorination may also promote trihalomethane (THM) and 

haloacetic acid (HAA) formation. Therefore, prechlorination doses used during the DOP 

study should be set to balance floc and disinfection byproduct formation. Operational 

guidelines should be documented in the DOP plan. 

• Granular media filter run times may be extended to increase unit filter run volumes 

(UFRVs) and filter efficiency. However, increased UFRVs also increase the potential for 

Cryptosporidium breakthrough. Maximum UFRVs should be established to minimize 

Cryptosporidium breakthrough. 

• Alternative coagulation strategies may be used to enhance Cryptosporidium removal in 

granular media filters but may also result in post-filtration flocculation that can cause 

deposition or scaling in water distribution systems. Coagulant dosing rates should be set 

during the DOP study to minimize downstream floc formation. 

Additionally, if a treatment process or plant technique is used intermittently for a 

seasonal or sporadically occurring contaminant, this treatment should also be used as needed 

during the DOP study, consistent with routine operation. 

12.4.1.2 Influent Water Quality Characteristics

Source water quality characteristics that may affect Cryptosporidium removal 

efficiencies should be identified. These will depend on the treatment processes employed and 

may include (but are not limited to) turbidity, pH, alkalinity and temperature. Critical (or worstcase) ranges for these parameters that are anticipated over the plant design life or permit period 

should be clearly defined. The demonstration study should include tests run under the worst-case 

source water conditions. In pilot-scale DOP studies, raw source water can be modified to 

simulate worst-case water qualities.

12.4.1.3 System Flow Rate

The system flow rate or range of flow rates to be evaluated during the DOP should be 

clearly defined. Where possible, plant performance should be demonstrated for the critical flow 

condition that defines permitted plant capacity (e.g., peak instantaneous flow or peak daily flow). 

For full-scale studies, this may not be feasible for facilities that operate significantly below 

permitted or maximum capacity. For pilot-scale studies, the range of system unit process flow 

rates should replicate the full-scale low, intermediate, and maximum flow and recycles rates.
12. Demonstration of Performance (DOP): Microbial Removal

LT2ESWTR Toolbox Guidance Manual 12-7 April 2010

12.4.1.4 Plant Operating Conditions

 WTP operations can vary significantly over the course of the demonstration period due to 

various factors including, but not limited to, raw water quality, system flow rate, and 

maintenance activities. The critical operating conditions that may impact Cryptosporidium

removal at the WTP should be defined. Issues to consider include the following: 

• What are the normal and worst-case operating conditions for each unit process with 

respect to Cryptosporidium removal? 

• How many process trains or elements are normally in service? How will the plant 

perform when units are out of service for maintenance and repair, thereby increasing unit 

process flow rates (particularly in filters)?

• What is the process control strategy for chemical addition? How does this relate to 

Cryptosporidium removal? 

• What is the process control strategy for filter operations? How does this relate to 

Cryptosporidium removal? 

• How will the plant’s recycle, backwash, and filter-to-waste schemes affect 

Cryptosporidium removal? 

 In the case of pilot-scale studies, performance demonstrations should replicate full-scale 

operating conditions in any respect that may influence Cryptosporidium removal. 

12.4.2 Selection of Performance Indicators

 Although the LT2ESWTR mandates treatment controls for Cryptosporidium, it is not 

currently feasible to demonstrate actual Cryptosporidium removal at full-scale facilities. In most 

cases, influent Cryptosporidium levels are not consistently high enough to demonstrate 

significant (such as 4 log) removal across the process train. Raw water spiking of 

Cryptosporidium is not a feasible option at full-scale facilities due to the potential health risk to 

system users and the number of oocysts required. Consequently, alternative indicators of 

Cryptosporidium removal will be needed for facilities that plan to conduct DOP studies at fullscale.

12.4.2.1 Surrogate Parameters for Cryptosporidium

EPA has reviewed a number of studies that suggest aerobic bacteria spores are a suitable 

indicator of Cryptosporidium removal in conventional treatment trains (coagulation, 

flocculation, sedimentation and filtration). Some characteristics of aerobic spores (as 

summarized by Cornwell et al. 2001) are: 
12. Demonstration of Performance (DOP): Microbial Removal

LT2ESWTR Toolbox Guidance Manual 12-8 April 2010

• Naturally occurring (Nieminski and Bellamy 2000, Jakubowski et al. 1996). 

• Do not pose health risks (Jakubowski et al. 1996, Rice et al. 1996). 

• Can be detected at low concentrations (< 1 cfu/100 mL). 

• Are slightly smaller than Cryptosporidium oocysts (Rice et al. 1996). 

• Spore removal by water treatment is a conservative indicator of Cryptosporidium

removal (Rice et al. 1996, Dugan et al. 1999, Nieminski and Bellamy 2000, Emelko 

2001). 

• Reduction of indigenous spores by inactivation is expected to be negligible in 

comparison with removal of spores by physical processes (Jakubowski et al. 1996, Rice 

et al. 1996). 

• Aerobic spores do not undergo re-growth during treatment. 

 Although aerobic spores appear to be a suitable indicator for Cryptosporidium removal in 

filtration plants, raw source water spore concentrations will likely not be high enough throughout 

the study period to demonstrate high log removal across a full-scale treatment train.

Microspheres may also be used as an alternative indicator for Cryptosporidium removal. 

Microspheres are chemically inert, easy to handle, and relatively inexpensive. They can be 

manufactured with a uniform particle size and smooth surface, making them appropriate as a 

conservative indicator. They can also be manufactured without a significant surface charge to 

minimize particle interaction and loss to processes such as adsorption. Microspheres are easily 

obtainable in concentrations of 107

 to 109 particles per mL, which should be adequate to prove 

desired removal rates. 

 The biggest disadvantage of the use of microspheres is in detection methods. Particle 

counters are effective in counting microspheres. In fact, microspheres are often used to calibrate 

particle counters. However, particle counters may not be able to distinguish between 

microspheres and other particles and may not be able to distinguish conglomerated particles. 

Other problems with particle counters such as coincidence error and the limited dynamic range 

can also skew results. A more effective method of measurement involves capturing the 

microspheres by filtering the sample and then counting microspheres. Use of fluorescent 

microspheres can aid in the counting process (Abbaszadegan et al. 1997, Li et al. 1997). 

Microspheres also tend to have lower zeta potentials than live Cryptosporidium oocysts (Dai and 

Hozalski 2003). Recent work, however, has found ways to adjust the surface charge on 

microspheres to more closely mimic natural pathogens (Pang et al. 2009). 

If appropriate detection methods are used and the microspheres are conservative 

representatives of Cryptosporidium oocysts, microspheres can be a good surrogate for 
12. Demonstration of Performance (DOP): Microbial Removal

LT2ESWTR Toolbox Guidance Manual 12-9 April 2010

Cryptosporidium. Emelko and Huck (2004) found that 4.6 micron carboxylated fluorescent dyed 

polystyrene microspheres acted as a good indicator for Cryptosporidium over a wide range of log 

removals. For example; neutrally charged, 1 micron, spherical latex microspheres could provide 

an acceptable conservative indicator for Cryptosporidium removal. 

The state may accept alternative indicators for Cryptosporidium; however, they should 

not be more easily removed than Cryptosporidium. The surrogate parameter should give a direct 

view of removal and should be an element that is not created in the plant (e.g., particle counts 

caused by chemical precipitation). Furthermore, the method of measurement should be sensitive 

enough to detect temporal variations in the parameter. Parameters such as turbidity or particle 

counts may be used in the DOP study, but are not suitable as stand-alone surrogates.

12.4.2.2 Long-Term Performance Indicators

 As discussed previously, plants that implement a DOP plan should document long-term 

performance of filtration facilities for turbidity and/or particle count reduction. While turbidity 

and particle counts are not suitable as stand-alone indicators for full-scale Cryptosporidium

removal, such data can be used to identify changes in the filtration performance. 

It is recommended that individual filter efficiency be monitored frequently to identify 

differences in individual filter performance. This will allow the plant to assess temporal 

variations in filter effluent quality and will provide improved process control.

12.4.3 Full-Scale Versus Pilot-Scale Testing

 In general, full-scale testing is preferred over pilot-scale testing since the performance of 

existing process trains is demonstrated directly. However, full-scale studies may not be feasible 

for many facilities for the following reasons:

• Influent Cryptosporidium levels will not be high enough to demonstrate high log 

removal. Likewise, influent aerobic spore concentrations may not be high enough to 

demonstrate significant log removal. 

• Full-scale spiking with aerobic spores may not be feasible due to larger flows.

• Facilities may operate well below design or permitted flow capacity for the entire study 

period. 

• Demonstration of worst-case operating conditions at full-scale may be difficult to plan, 

especially with regard to raw water quality and flow rates.
12. Demonstration of Performance (DOP): Microbial Removal

LT2ESWTR Toolbox Guidance Manual 12-10 April 2010

The major concern with the use of pilot-scale testing is the uncertainty associated with 

scale-up of pilot results to predict the performance of full-scale systems. Other potential 

limitations of pilot-scale studies are:

• Pilot-scale data generally represent steady-state conditions; however, sudden changes in 

flow or water quality may have a significant effect on Cryptosporidium removal; such 

changes are difficult to capture in a pilot-scale plant.

• Pilot-scale plants generally have much tighter process controls and higher levels of 

attention than full-scale plants; and thus, may not be indicative of actual full-scale 

performance.

• A pilot-scale plant cannot represent expected individual differences between multiple 

filters in a full-scale plant.

• Particle loadings to the treatment process in a pilot-scale study may be much higher than 

actual full-scale loadings, and thus, may not represent actual operating conditions. 

• It may be too difficult to construct a pilot plant that represents the entire full-scale 

process train.

 Pilot system dimensions and flow rates should be sufficiently large to minimize scale-up 

issues. Some recommended guidelines for pilot filter sizing include the following (U.S. EPA 

1991): 

• Unit filtration rate in the pilot system should be identical to that of the full-scale plant.

• Pilot filter diameter should be greater than or equal to 100 times the media diameter.

• Media diameter and depth should be identical to that of the full-scale system.

Pilot systems should also incorporate all major process elements of the full-scale process 

train, including chemical addition systems and recycle streams. Such systems must be able to 

simulate flow rate and water quality perturbations (i.e., temporal disturbances to steady state 

conditions). 

12.5 Demonstration Protocol 

Once the DOP criteria have been developed, the DOP protocol can be formulated. This 

section outlines the minimum elements that should be included in the DOP protocol. 

Participation from the governing regulatory agency should be solicited during the DOP protocol 

development phase. 
12. Demonstration of Performance (DOP): Microbial Removal

LT2ESWTR Toolbox Guidance Manual 12-11 April 2010

12.5.1 DOP Test Matrix 

 The first step in the formulation of the specific DOP protocol is the development of a 

matrix of test conditions to be evaluated during the DOP period. These test conditions should be 

formulated to assess Cryptosporidium removal (or other suitable parameters) under a range of 

normal and worst-case scenarios. The DOP matrix should clearly define specific test scenarios to 

be evaluated, incorporating the following criteria: 

• Source water quality ranges– including minimum/maximum limits for critical water 

quality parameters that influence Cryptosporidium removal in the plant. 

• Influent flow rates– including the maximum flow rate that defines plant capacity. 

• Operating scenarios– including all operations that may cause process upset in the 

treatment train (e.g., events that cause temporal changes in water quality, and flow 

loadings to process units). These operations include, but are not limited to: filter 

backwashing, filter-to-waste practices, intermittent recycles, returning filters to service, 

and routine maintenance practices.

 Critical influent flow ranges and operating conditions should be identified during the 

DOP criteria development phase, as described in section 12.2. The demonstration period should 

be at least one year, and should encompass all critical operating conditions. An example test 

matrix format is presented in Exhibit 12.3. 

Exhibit 12.3 Example DOP Test Matrix

Scenario

Condition

(Normal or 

Worst-Case)

Influent 

Concentration 

Range Flow

Influent 

Concentration Range

Flow 

Rate 

Range

Units in 

Service

Backwash 

Conditions

Date of 

Scenario 

Test

Surrogate Turbidity 

S1 Normal Average Average Average 4 (All)

S2 Normal Average Average Average 3 

S3 Worst Case A Average Average High 3 

S4 Worst Case B High High Average 3 

S5 Worst Case C Low Low Average 3 

12.5.2 DOP Monitoring Plan

 The DOP involves sampling and analysis of Cryptosporidium indicators in the raw 

source water and filtration train effluent over the course of a demonstration period defined by the 

DOP test matrix. Once the test matrix is established, the DOP monitoring plan should be 

formulated to define the following protocol details: 
12. Demonstration of Performance (DOP): Microbial Removal

LT2ESWTR Toolbox Guidance Manual 12-12 April 2010

•

•

•

•

Monitoring locations. 

Test parameters (field and laboratory). 

Monitoring frequency. 

Quality assurance/quality control (QA/QC) procedure for/during sampling. 

 

A sample DOP monitoring plan is presented in Exhibit 12.4 
Chapter 12 – Demonstration of Performance (DOP): Microbial Removal

LT2ESWTR Toolbox Guidance Manual 12-13 April 2010 

Exhibit 12.4 Example DOP Monitoring Plan

Monitor 

Event 

Number

Date Test 

Scenario 

ID (See 

Exhibit 

12.2)

Effluent Sample Locations Number of Samples per Location A

Filter 1 Filter 2 Filter 3 Filter 4 Crypto/ 

Aerobic 

Spores

Particle 

Count

pH Temp.

1 Week 1 S1 X X X X 2 1 B 1 1 

2 Week 2 S3 X X X X 1 1 1 1 

3 Week 3 S2 X X X X 1 1 1 1 

... ... ... ... ... ... ... ... ... ... ... 

52 Week 52 S4 X X X X 2 1 B 1 1 

A - Influent sample location identical for all test scenarios. 

B - Duplicate samples. 
12. Demonstration of Performance (DOP)

LT2ESWTR Toolbox Guidance Manual 12-14 April 2010

12.5.2.1 Sampling Location 

 Paired samples should be collected from the plant influent (raw source sample) and the 

combined filter effluent for a DOP study of an entire plant. The plant influent location should be 

before the pre-sedimentation basins and off-stream storage facilities and follow any process 

recycles added prior to the first major unit process element of the treatment train. For pilot 

studies involving microbial dosing, the influent monitoring point should follow complete mixing 

of the source water and injection stream. The plant effluent sample should be comprised of 

composite samples from the effluent of all operating filters. It is recommended that at least five 

sample pairs (influent/effluent) be collected during each test run to capture temporal changes in 

filter and effluent quality.

12.5.2.2 Monitoring Parameters

Samples should be analyzed for all parameters required to assess Cryptosporidium

removal in the treatment trains, as discussed in section 12.2. Parameters such as pH, alkalinity, 

temperature, and turbidity should be measured and recorded in the field. 

12.5.2.3 Monitoring Frequency

 A monitoring event is defined as a paired (concurrent) sampling of plant influent and 

filter effluent samples. At a minimum, monitoring should be performed once per week for 52 

consecutive weeks. More frequent monitoring may be required to capture all critical operating 

scenarios defined by the DOP Test Matrix. The DOP database should be sufficiently large to 

allow for statistical analysis.

 If a DOP credit is issued by the state, the credit will be conditional on continuing 

demonstration of a higher level of performance. The DOP Monitoring Plan can be modified to 

document continuing performance at a reduced sampling frequency. However, sampling events 

should still capture critical operating scenarios.

12.5.2.4 Quality Assurance/Quality Control (QA/QC)

Quality assurance/quality control (QA/QC) sampling should be performed to allow 

assessment of data variability and quantification errors due to sample collection procedures and 

analytical methods. At a minimum, duplicate samples should be collected during one monitoring 

event per month. 
12. Demonstration of Performance (DOP)

LT2ESWTR Toolbox Guidance Manual 12-15 April 2010

12.5.3 DOP Implementation

 The DOP should commence only after the state approves the DOP test matrix and 

monitoring protocol. The DOP plan should be administered by a qualified WTP operator or 

water process engineer. Data review and QA/QC practices should be conducted routinely to 

ensure that the objectives of the DOP program are met. Particular attention should be given to 

verification of the plant operating conditions (influent loadings, unit process loadings, etc.) to 

confirm that all critical operating scenarios identified in the DOP test matrix are evaluated during

the demonstration period. 

 Personnel responsible for implementing the DOP monitoring plan should be properly 

trained in sample collection techniques, QA/QC procedures and operational data acquisition. 

Specific procedures should be used to collect and analyze samples as described in the following 

sections:

• Sample collection and preservation methods. 

• Analytical methods. 

• Microbial dosing methods (for pilot tests). 

• Documentation procedures. 

12.5.3.1 Sample Collection Methods

Influent and effluent samples should be collected in a manner that is representative of the 

entire cross sectional flow at each monitoring point. If possible, monitoring points should be 

located in straight sections of pipe or channel well downstream of bends. For open channel 

flows, samples should be collected from mid-depth and mid-width of the channel. For pipe flow, 

samples should be collected from the tap directly into the sample containers. In each case, the 

sampling method should not reduce or prevent transfer of suspended solids from the process 

stream to the sample container. Parameters such as pH, turbidity, alkalinity and temperature 

should be directly measured in the field. 

12.5.3.2 Analytical Methods

The analytical methods for monitoring Cryptosporidium under the LT2ESWTR are 

prescribed at 40 CFR 141.704 and described in the Public Water System Guidance Manual for 

Source Water Monitoring under the LT2ESWTR. Analytical methods for all other water quality 

parameters should be performed in accordance with Standard Methods for the Examination of 

Water and Wastewater, 20th edition, or the most recent edition. 
12. Demonstration of Performance (DOP)

LT2ESWTR Toolbox Guidance Manual 12-16 April 2010

12.5.3.3 Microbial Dosing

 For pilot testing that involves spiking of Cryptosporidium, aerobic spores or other 

indicators, microbial dosing procedures should be clearly established. Guidelines for microbial 

stock preparation and dosing are presented in this section. 

 A concentrated mixture of microorganisms should be prepared and fed to the raw source 

stream at a known feed rate, based on the microbial density in the concentrated stock, the flow 

rate of the pilot system, and the desired microorganism concentration in the pilot system. An 

equation that describes this relationship is:

Equation 12-1 

where: 

 Cpilot

 C

 = the microbial concentration in the pilot system

feed

Q

 = the microbial concentration in the concentrated stock solution 

pilot

 Q

 = the flow rate of the pilot system (includes all process recycles present at 

the influent feed point, if applicable) 

feed = the flow rate of the concentrated stock solution 

 For each trial, the test microorganisms should be completely mixed in a volume of raw 

water sufficient to supply the pilot plant for the duration of the experiment. The tank containing 

the suspension of test microorganisms should be continuously mixed for the duration of each 

experiment to promote homogeneity of the mixture. The concentrated stock should be delivered 

by a positive displacement pump (e.g., peristaltic) to the main process flow at a flow rate 

dictated by Equation 12-1. Cpilot and Cfeed should be selected to provide a high enough influent 

microbial concentration to demonstrate at least 4 log removal in the pilot system. Based on this 

approach, Cpilot should be set at least 104 higher than the method detection limit for the test 

microorganism. The microbial density in the stock solution should be sampled at least twice, and 

preferably three times, during a feeding interval to verify consistent densities.

12.5.3.4 Documentation of WTP Operating Conditions

 It is important to document WTP operating conditions during monitoring events to 

evaluate the effect of varying operating scenarios on Cryptosporidium removal. Standardized 

reporting forms should be developed to provide, at a minimum, the following information: 

• System flow rate (instantaneous/flow chart, hourly and daily average). 

• Operating mode (process scheme, number of trains, number of units in service). 
12. Demonstration of Performance (DOP)

LT2ESWTR Toolbox Guidance Manual 12-17 April 2010

• Water pH, alkalinity, turbidity and temperature. 

• Performance data. 

• Chemical addition rates/doses. 

• Mechanical equipment in operation, with flow rates (major pumps, blowers, etc.). 

• Recycle and backwash flows/rates. 

• Related maintenance activities occurring prior to or during sampling event. 

12.5.4 Data Analysis and Reporting 

12.5.4.1 Evaluation of Performance

To receive DOP treatment credits above presumptive credits in the LT2ESWTR, a plant 

should demonstrate consistent attainment of a specific log reduction of Cryptosporidium (or 

suitable indicators). To meet this objective, log reduction should first be computed for each 

monitoring event according to: 

 

 Log Removal = - log (Cinf/Ceff) Equation 12-2 

where: Cinf

 C

 = influent Cryptosporidium or indicator concentration

eff = effluent Cryptosporidium or indicator concentration

 For effluent samples in which no Cryptosporidium, spores, or other indicators are 

detected, the concentration should be set to the method detection limit. 

The state will determine the level of DOP credit a facility receives based on review of the 

log removal data. 

 For the case of pilot testing and the use of multiple indicators for Cryptosporidium

removal calculations will be site specific.

12.5.4.2 Reporting for the DOP

 At the conclusion of the DOP test period, a detailed report summarizing the major 

findings of the DOP program must be submitted to the governing regulatory agency. At a 

minimum, the DOP report should include the following information: 
12. Demonstration of Performance (DOP)

LT2ESWTR Toolbox Guidance Manual 12-18 April 2010

• Detailed description of full-scale WTP, including process flow schematics. 

• Summary of treatment objectives and WTP design criteria. 

• DOP test matrix and monitoring plan. 

• DOP data summary. 

• Detailed pilot plant design data (if applicable). 

• Data analysis for estimate of Cryptosporidium log reduction. 

• Appendices for raw full-scale/pilot-scale analytical and operational data. 

• Monitoring plan to verify that on-going performance is equivalent to treatment credit. 

Source water indicators used in the study should be monitored to ensure performance is 

met.

• Plan for addressing operating conditions (e.g., influent water turbidity) out of the range 

tested in the study. The DOP test matrix generally sets the range of operating conditions 

under which the LT2ESWTR treatment credit is applicable. Therefore, it is advisable to 

develop a plan for addressing potential out-of compliance conditions. For example, if the 

influent source water quality conditions ranged from 5 NTU to 25 NTU during the study, 

the system may plan to make operational adjustments for influent water with turbidity 

greater than 25 NTU and increase filter effluent monitoring. Any such deviations would 

be reported to the state. 

12.5.4.3 Ongoing Reporting

As discussed previously, if a DOP credit is issued by the state, the credit will be 

conditional on continuing demonstration of a high level of performance. The DOP Monitoring 

Plan should be modified to document continuing performance at a reduced sampling frequency, 

while still capturing critical operating conditions. States may require systems receiving a DOP 

credit to report operational and progress monitoring data on a routine basis. Operational data 

should verify that continuous process control and optimization procedures are in place. 

 The DOP credit is applicable to minimum and maximum raw source water and finished 

water quality limits defined in the DOP Test Matrix. Routine reporting should be performed to 

verify that plants operate within these limits. If an exception occurs, it should be reported to the 

state in a timely manner. Frequent exceptions may prompt the state to require the plant to 

conduct a comprehensive performance evaluation (CPE) or similar operational evaluation to 

identify causes and solutions for exceptions. 
12. Demonstration of Performance (DOP)

LT2ESWTR Toolbox Guidance Manual 12-19 April 2010

12.6 References 

Abbaszadegan, M., M.N. Hansan, C.P. Gerba, P.F. Roessler, B.R. Wilson, R. Kuennen, and E.

Van Dellen. 1997. The disinfection efficacy of a point-of-use water treatment system

against bacterial, viral, and protozoan waterborne pathogens. Water Research. 

31(3):574-582. 

American Public Health Association, American Water Works Association, and Water 

Environment Federation. 1998. Standard Methods for the Examination of Water and 

Wastewater. Washington D.C. 

Cornwell, D.A., MacPhee, M., Brown, R. 2001. Cryptosporidium Removal Credit Assignable in 

the LT2ESWTR Toolbox, Report to AWWA Government Affairs Office, Washington, D.C. 

Dai, X., Hozalski, R.M. 2003. Evaluation of microspheres as Surrogates for Cryptosporidium 

Parvum Oocysts in Filtration Experiments. Environmental Science and Technology. 37(5):1037-

1042. 

Dugan, N., Fox, K, Miltner, R., Lytle, D., Williams, C., Parrett, C., Feld, Owens, J. 1999. 

Control of Cryptosporidium oocysts by steady-state conventional treatment. Proc. of 1999 

AWWA Annual Conference and Exposition. Denver, CO: AWWA.

Dugan, N., Fox, K., Owens, J., Miltner, R. 2001. Controlling Cryptosporidium oocysts through 

conventional treatment. Journal AWWA, 93(12):64-76. 

Emelko, M., Huck, P., Slawson, R. 1999. Design and operational strategies for optimizing 

Cryptosporidium removal by filters. Proceedings of the 1999 AWWA Water Quality Technology 

Conference. Denver, CO: AWWA.

Emelko, M., Huck, P. 2004. Microspheres as Surrogates for Cryptosporidium Filtration. Journal 

AWWA. 96(3):77-91. 

Emelko, M. 2001. Removal of Cryptosporidium parvum by Granular Media Filtration. Ph.D. 

Dissertation. University of Waterloo, Waterloo, Ontario, Canada. 

Jakubowski, W., Boutros, S., Faber, W., Fayer, R., Ghiorse, W., LeChevallier, M., Rose, J., 

Schaub, S., Singh, A., Stewart, M. 1996. Environmental methods for Cryptosporidium. Journal 

AWWA. 88(9):107-121. 

Li, S.Y., J.A. Goodrich, J.H. Owens, G.E. Willeke, F.W. Schaefer III, and R.M. Clark. 1997. 

Reliability of non-hazardous surrogates for determining Cryptosporidium removal in bag 

filters. Journal AWWA. 89(5):90-99. 
12. Demonstration of Performance (DOP)

LT2ESWTR Toolbox Guidance Manual 12-20 April 2010

Mazounie, P., Bernazeau, F. Alla, P. 2000. Removal of Cryptosporidium by high rate contact 

filtration: The Performance of the Prospect Water Filtration Plant During the Sydney Water 

Crisis. Water Science and Technology. 41(7):93-101. 

Nieminski, E., Bellamy, W. 2000. Application of Surrogate Measures to Improve Treatment 

Plant Performance. Denver, CO: AwwaRF and AWWA.

Pang, L., Nawostawska, V., Ryan, J.N., Williamson, W. M., Walshe, G., Hunter, K.A. 2009. 

Modifying the Surface Charge of Pathogen Sized Microspheres for Studying Pathogen Transport 

in Groundwater. Journal of Environmental Quality 38 2210-2217. 

Rice, E., Fox, K., Miltner, R., Lytle, D., Johnson, C. 1996. Evaluating plant performance with 

endospores. Journal AWWA. 88(9):122-130. 

U.S. EPA. 1991. Guidance Manual for Compliance with the Filtration and Disinfection 

Requirements for Public Water Systems Using Surface Water Sources. Washington, D.C. 

 

Yates, R., Scott, K., Green, J., Bruno, J.,De Leon, R. 1998. Using aerobic spores to evaluate 

treatment plant performance. Proceedings of the 1998 AWWA Annual Conference and 

Exposition. Denver, CO: AWWA.
LT2ESWTR Toolbox Guidance Manual 13-1 April 2010

13. Ultraviolet Light

13.1 Introduction 

The use of ultraviolet (UV) light for disinfection of drinking water is a relatively new 

application in the United States, although used for decades in the wastewater industry. UV 

disinfection is the process of irradiating water with UV light. The UV light is absorbed by the 

genetic material of microorganisms, damaging it, and preventing the microorganisms from 

reproducing. UV disinfection has been found to be particularly effective against protozoa and 

bacteria. 

This chapter summarizes the requirements for water systems using UV disinfection to 

achieve compliance with the Long Term 2 Enhanced Surface Water Treatment Rule 

(LT2ESWTR) and provides considerations for toolbox selection. Water systems and states 

should refer to the UV Disinfection Guidance Manual (U.S. EPA 2006) for detailed guidance on 

design and operation of UV systems and the validation testing that must be conducted for 

compliance with the LT2ESWTR.

13.2 UV Disinfection Requirements for Filtered and Unfiltered PWSs 

 

The LT2ESWTR has several requirements related to the use of UV disinfection, they 

address:

• UV doses for different levels of inactivation credit. 

• Performance validation testing of UV reactors. 

• Monitoring. 

• Reporting. 

• Off-specification operation. 

13.2.1 UV Dose and Validation Testing Requirements 

 

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

Cryptosporidium, Giardia, and viruses (Exhibit 13.1). The UV dose values in Exhibit 13.1 are 

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

systems meeting turbidity requirements. 

Unlike chemical disinfectants, UV light does not leave a chemical 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 absorbance. To determine the 

operating conditions under which the reactor delivers the required dose for treatment credit, the 
13. Ultraviolet Light

LT2ESWTR Toolbox Guidance Manual 13-2 April 2010

LT2ESWTR requires PWSs to use UV reactors that have undergone validation testing [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. 

Exhibit 13.1 UV Dose Requirements – millijoules per centimeter squared 

(mJ/cm2)

1

Target

Pathogens

Log Inactivation

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Cryptosporidium 1.6 2.5 3.9 5.8 8.5 12 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). 

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 lowpressure mercury vapor lamp [40 CFR 141.720(d)(2)(ii)]. 

• Using the above requirements as a basis, EPA developed a recommended validation 

protocol, presented in Chapter 5 of the UV Disinfection Guidance Manual (U.S. EPA 

2006). 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 

LT2ESWTR. 

13.2.2 UV Disinfection Monitoring Requirements

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 [40 CFR 

141.720(d)(3)(i)]. 
13. Ultraviolet Light

LT2ESWTR Toolbox Guidance Manual 13-3 April 2010

13.2.3 UV Disinfection Reporting Requirements

 

The LT2ESWTR requires PWSs to report the following items [40 CFR 141.721(f)(15]: 

• 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. 

13.3.4 Off-specification Operational Requirement for Filtered and Unfiltered Systems 

To receive disinfection credit for UV disinfection, 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 [40 CFR 141.720(d)(3)(ii)]. 

13.3 Toolbox Selection Considerations 

UV disinfection is a relatively simple to use and highly effective technology for 

inactivating Cryptosporidium. Its main advantages include:

• It can inactivate chlorine-resistant pathogens such as Cryptosporidium oocysts and 

Giardia cysts at relatively low doses. It is often the lowest cost treatment option for 

inactivating Cryptosporidium. 

• It does not produce regulated disinfection byproducts (DBPs). 

• Its effectiveness is not pH or temperature dependent. 

The disadvantages of UV disinfection include: 

• UV disinfection effectiveness cannot be measured in “real-time” like chemical 

disinfectants.

• UV disinfection provides no distribution system residual.

• Much higher UV doses are required for virus inactivation. 

• Power quality problems can disrupt disinfection in some cases. 
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LT2ESWTR Toolbox Guidance Manual 13-4 April 2010

13.4 Design and Operational Considerations 

UV reactors for drinking water treatment typically consist of a closed-vessel containing 

UV lamps, UV sensors, and temperature sensors. UV lamps are usually housed within lamp 

sleeves to protect and insulate them. Some reactors include automatic cleaning mechanisms to 

keep the lamp sleeves free of deposits. UV sensors, flow meters, and in some cases, analyzers for 

UV absorbance (or a related parameter, UV transmittance) are used to determine the dose 

delivered by the reactor. UV lamps can be low pressure, low pressure-high output, or medium 

pressure mercury vapor lamps. Low pressure lamps emit light at one wavelength (i.e., 

monochromatic) and operate with the mercury under low vapor pressures. Low pressure highoutput lamps are similar to low pressure lamps but operate at higher temperatures and have a 

higher UV light output. Medium-pressure lamps are polychromatic and operate at higher 

temperatures and mercury vapor pressures.

Below is an example of some key design and operational issues that should be considered 

when evaluating UV treatment options. Refer to the UV Disinfection Guidance Manual (U.S. 

EPA 2006) for more detailed guidance on UV facility design and operation. 

• Water quality – The UV absorbance of the water to be treated is very important in the 

design of UV facilities. This is because UV absorbance influences UV dose delivery and 

therefore affects the UV reactor selection, validation requirements, and the UV facility 

size and cost. Compounds in the water can also foul lamp sleeves and other UV reactor 

components. Fouling is dependent on calcium, hardness, alkalinity, lamp temperature, 

pH, oxidation-reduction potential (ORP) and certain inorganic constituents (e.g., iron and 

manganese). UV facilities are typically equipped with cleaning systems to prevent 

fouling. 

• Power quality – UV lamps can turn off if a voltage fluctuation, power quality anomaly, or 

power interruption occurs. Power quality tolerances depend on the UV equipment design 

and vary significantly among UV manufacturers. If power quality may be a problem at the 

intended installation location, a power quality assessment may be needed to quantify and 

understand the potential for off-specification operations. 

• Hydraulic needs and limitations - Headloss through a UV reactor depends on the specific 

reactor, piping configuration, and flow rate. Typical headloss ranges from 0.5 to 3.0 feet 

for a reactor. 

• Maintenance - UV reactors will need to be periodically shut down for regular 

maintenance. Typical maintenance tasks include checking UV sensor calibration, 

checking lamp cleaning efficiency, and replacing lamps and sleeves.

• LT2 Reporting Requirements - Systems are required to submit validation test results 

demonstrating operating conditions that achieve required UV dose as well as with a 

monthly report summarizing the percentage of water entering the distribution system that 

was not treated by UV reactors operating within validated conditions for the required 

dose. 
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LT2ESWTR Toolbox Guidance Manual 13-5 April 2010

13.5 References

U.S. EPA. 2006. Ultraviolet Disinfection Guidance Manual for the Final Long Term 2 Enhanced 

Surface Water Treatment Rule. Office of Water. EPA 815-R-06-007. November, 2006. 

http://www.epa.gov/safewater/disinfection/lt2/compliance.html. 
LT2ESWTR Toolbox Guidance Manual 14-1 April 2010

14. Membrane Filtration

14.1 Introduction 

The Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) defines 

membrane filtration as follows:

Membrane filtration is a pressure or vacuum driven separation process in which 

particulate matter larger than 1 micrometer is rejected by an engineered barrier, 

primarily through a size-exclusion mechanism, and which has a measurable 

removal efficiency of a target organism that can be verified through the 

application of a direct integrity test. This definition includes the common 

membrane technologies of microfiltration, ultrafiltration, nanofiltration, and 

reverse osmosis. [40 CFR 141.2] 

Membrane processes that meet the requirements of LT2ESWTR will receive 

Cryptosporidium removal credit. 

The U.S. Environmental Protection Agency (EPA) recently published the Membrane 

Filtration Guidance Manual for systems considering using membranes to comply with the 

requirements of the LT2ESWTR (U.S. EPA 2005). Readers interested in detailed information on 

membrane filtration should consult the Membrane Filtration Guidance Manual. This chapter 

summarizes rule requirements and lists advantages and disadvantages of membrane filtration 

compared with other toolbox technologies. 

14.2 Membrane Filtration Requirements under the LT2ESWTR

In order to receive removal credit for Cryptosporidium under the LT2ESWTR, a 

membrane filtration system must meet the following three criteria:

1. The process must comply with the definition of membrane filtration as stipulated by the 

rule.

2. The removal efficiency of a membrane filtration process must be established through a 

product-specific challenge test and ongoing, site-specific direct integrity testing during 

system operation. 

3. The membrane filtration system must undergo periodic direct integrity testing and 

continuous indirect integrity monitoring during operation. 

The rule does not prescribe a specific removal credit for membrane filtration processes. 

Instead, removal credit is based on system performance as determined by challenge testing and 

verified by direct integrity testing. Thus, the maximum removal credit that a membrane filtration 

process may receive is the lower value of either [40 CFR 141.719(b)(1)]: 
14. Membrane Filtration

LT2ESWTR Toolbox Guidance Manual 14-2 April 2010

• The removal efficiency demonstrated during challenge testing. 

OR

• The maximum log removal value that can be verified by the direct integrity test used to 

monitor the membrane filtration process. 

 Based on this framework, a membrane filtration process could potentially meet the Bin 4 

Cryptosporidium treatment requirements, as shown in Exhibit 1.1 of this guidance manual. 

These primary elements of the regulatory requirements for membrane filtration under the 

LT2ESWTR, including challenge testing, direct integrity testing, and continuous indirect 

integrity monitoring, are summarized in the following sections. 

14.2.1 Challenge Testing

 Since there are no uniform design criteria that can be used to ensure the removal 

efficiency of a membrane process, challenge testing is required to demonstrate the ability of a 

membrane process to remove a specific target organism. The removal efficiency demonstrated 

during challenge testing establishes the maximum removal credit that a membrane process would 

be eligible to receive, provided that this value is less than or equal to the maximum log removal 

value that can be verified by the direct integrity test [40 CFR 141.719(b)(1)], as described in the 

next section. The LT2ESWTR only requires product-specific challenge testing; once the removal 

efficiency has been demonstrated, additional testing is not required unless the product is 

significantly modified. 

14.2.2 Direct Integrity Testing

While challenge testing can demonstrate the ability of an integral membrane process to 

remove the target organism, integrity breaches can develop in the membrane during routine 

operation that could allow the passage of microorganisms. In order to verify the removal 

efficiency of a membrane process during operation, direct integrity testing is required for all 

membrane filtration processes used to comply with the LT2ESWTR [40 CFR 141.719(b)(3)]. A 

direct integrity test is defined as a physical test applied to a membrane unit in order to identify 

and isolate integrity breaches. The rule does not mandate the use of a specific type of direct 

integrity test, but rather performance criteria that any direct integrity test must meet. These 

criteria include requirements for resolution, sensitivity, and frequency [40 CFR 141.719(b)(3)]: 

• Resolution: The direct integrity test must be applied in a manner such that a 3 

micrometer breach contributes to the response from the test. 

• Sensitivity: The direct integrity test must be capable of verifying the ability of a 

membrane filtration system to achieve the log removal value awarded to the process by 

the state.
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LT2ESWTR Toolbox Guidance Manual 14-3 April 2010

• Frequency: The direct integrity test must be applied at a frequency of at least once per 

day, although less frequent testing may be permitted by the state at its discretion if 

appropriate safety factors are incorporated.

A control limit must also be established for a direct integrity test, representing a threshold 

response which, if exceeded, indicates a potential integrity problem and triggers subsequent 

corrective action. For the purposes of LT2ESWTR compliance, this threshold response must be 

indicative of an integral membrane unit capable of achieving the Cryptosporidium removal credit 

awarded by the state. 

14.2.3 Continuous Indirect Integrity Monitoring

 Systems must conduct continuous indirect integrity monitoring on each membrane unit 

[40 CFR 141.719(b)(4)]. For the purposes of the LT2ESWTR, indirect integrity monitoring is 

defined as monitoring some filtrate water parameter that is indicative of the removal of 

particulate matter, and “continuous” is defined as monitoring at a frequency of no less than once 

every 15 minutes [40 CFR 141.719(b)(4)(ii)]. Although turbidity monitoring is specified as the 

default method of continuous indirect integrity monitoring under the rule, other methods, such as 

particle counting or particle monitoring, may be used in lieu of turbidity monitoring at the 

discretion of the state [40 CFR 141.719(b)(4)(i)]. For any indirect method used, a control limit 

must be established that is indicative of acceptable performance. Monitoring results exceeding 

the control limit for a period of more than 15 minutes must trigger immediate direct integrity 

testing [40 CFR 141.719(b)(4)(iv)]. 

14.3 Toolbox Selection Considerations – Advantages and Disadvantages

 Membrane filtration is a highly efficient technology for removing pathogens and other 

particulates from drinking water. Its main advantages are:

• Removes bacteria and protozoa. 

• Can lower DBPs by allowing lower disinfectant doses and removing DBP precursors. 

• Can remove arsenic. Microfiltration (MF) and ultrafiltration (UF) can remove particulate 

arsenic (dissolved arsenic can be converted to particulate arsenic by coagulation prior to 

the MF/UF system). NF and reverse osmosis (RO) can remove dissolved arsenic. 

• UF, NF, and RO can remove viruses, however it is very difficult to perform a direct 

integrity test that can detect a defect as small as a virus. 
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LT2ESWTR Toolbox Guidance Manual 14-4 April 2010

 Membrane filtration is an advanced technology and can be more expensive than 

conventional technologies. Its major disadvantages are: 

• Total cost may exceed that of conventional technologies. 

• Can be fouled by organics and minerals. 

• Increased loss of process water.

14.4 Design and Operational Considerations 

There are a number of different types of membrane materials and module system designs 

for different classes of membranes. In general, MF and UF use hollow-fiber membranes, while 

NF and RO use spiral-wound membranes. Hollow-fiber membrane systems may be either 

pressure-driven (i.e., positive pressure as a driving force for filtration) or vacuum-driven (i.e., 

utilizes negative pressure as a driving force for filtration). In pressure-driven systems, upstream 

pumps are employed to push water across the membrane barrier. Vacuum-driven systems 

employ downstream pumps to induce suction on the inside the membrane fibers, pulling water 

across the barrier. Membrane systems using spiral-wound modules are pressure-driven, with six 

to eight modules usually arranged in series inside a containment vessel. 

Membrane systems are typically designed and constructed in one or more discrete water 

production units, also called racks, trains, or skids. Production unit design varies widely by 

manufacturer and type of system (i.e., hollow-fiber vs. spiral-wound) but typically contains the 

membrane treatment system, associated piping, appurtenances, and other features. A typical 

membrane treatment system is composed of a number of identical units that combine to produce 

the total filtrate flow. 

 A major design variable for membrane systems is the flux, or the flow per unit of 

membrane area. Membranes are most often designed to operate at constant flux (or within a 

specific range fluxes, with the applied pressure (i.e., positive or negative) varying with the 

degree of resistance to flow. This resistance may be caused by fouling or changes in temperature, 

which affect water viscosity. Because the flux can vary significantly with temperature, the 

average, minimum, and maximum temperature of the water to be treated should be considered 

when designing the system. Pilot studies are often performed to optimize the flux, pretreatment, 

and cleaning regime (chemicals, doses, and intervals) for a particular application. 

 Core membrane process operations include backwashing, chemical cleaning, and 

integrity testing. The frequency of these processes is usually determined during pilot testing, but 

in the case of integrity testing may also be dictated by regulatory requirements. Backwashing is 

similar in principle to that for conventional media filters and is intended to remove contaminants 

accumulated on the membrane surface. Note that backwashing is only applicable to the 

microporous MF/UF membranes, but does not apply to the semi-permeable NF/RO membranes, 

which cannot be backwashed. Chemical cleaning is periodically conducted to remove any 

accumulated foulants; for MF/UF systems, this constitutes any fouling that is not removed on a 

routine basis via backwashing. Integrity testing is conducted to ensure that the membrane is free 

of any breaches, leaks, or defects that might allow unfiltered water to bypass the membrane 
14. Membrane Filtration

LT2ESWTR Toolbox Guidance Manual 14-5 April 2010

barrier. This testing is required by many states and at the federal level for applications in which 

membrane filtration is used to comply with the Cryptosporidium removal requirements of the 

LT2ESWTR.

Feed water quality is also a primary design consideration for membrane systems, as this 

can affect both the flux and rate of membrane fouling. For MF/UF systems, high levels of 

turbidity and TOC can increase backwashing requirements and chemical cleaning frequencies, 

causing poor performance and shortening membrane life. Additionally for NF/RO systems, high 

levels of scaling ions can increase energy consumption and chemical cleaning frequency and can

result in poor performance and shortened membrane life. In many cases, pretreatment may 

improve feed water quality at lower cost than incorporating additional membrane area. Other 

important issues to consider in the design of membrane filtration systems include cross 

connection control, system reliability, chemical cleaning and residuals management.
14. Membrane Filtration

LT2ESWTR Toolbox Guidance Manual 14-6 April 2010

14.5 References

U.S. EPA. 2005. Membrane Filtration Guidance Manual. Office of Water. EPA 815-R-06-009. 

November, 2005. http://www.epa.gov/ogwdw/disinfection/lt2/compliance.htm. 
LT2ESWTR Toolbox Guidance Manual A-1 April 2010

Appendix A

Site Specific Determination of Contact Time for Chlorine Dioxide and Ozone

A water system may perform a site specific study to generate a set of chlorine dioxide or 

ozone contact time (CT) values for that site if it believes those developed by the U.S. 

Environmental Protection Agency (EPA) do not reflect the true inactivation achieved. Such a 

study would involve measuring actual Cryptosporidium inactivation under site conditions, with a 

full range of temperature and contact times. If accepted by the state, the CT values may be used 

instead of those developed by EPA. 

The Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) does not 

specify any requirements for the chlorine dioxide or ozone site-specific study, only that it be 

approved by the state (40 CFR 141.720(c)). This appendix describes the different recommended 

elements of a study and discusses some of the issues involved in the statistical analysis of the 

results.

A.1 Experimental Design 

 

 Experiments should be conducted with water that is representative of the water to be 

treated with respect to all conditions that can affect Cryptosporidium inactivation. Inactivation 

experiments should be performed with water exerting the highest oxidant demand (i.e., spring 

run-off or summer conditions) at high temperature to obtain the worst-case scenario in terms of 

chlorine dioxide or ozone demand/decay rate. In addition, experiments should also be conducted 

with water obtained during the winter months at the lowest temperatures observed at the 

treatment plant. These experiments would allow for the determination of the highest CTs that 

would be necessary to achieve the required level of inactivation. Additional experiments may be 

necessary to characterize the effects of other water quality parameters. 

In order to obtain the most challenging water to assess the chlorine dioxide or ozone 

process, a predetermined testing schedule should be established based on source water 

TOC and UV254 levels. Testing can occur when source water values for these parameters 

fall within defined worst-case ranges. Experiments should then be performed in the 

laboratory at worst-case temperatures for a given month. 

 In order to obtain a complete data set, testing should occur at least every other month over 

the course of an entire year. Each sample date should be determined by the first time the total 

organic carbon (TOC) or UV254 levels are within 75 percent of the maximum historical value for 

that month. At the time of sampling, sufficient water should be acquired to allow for three sets of 

experiments to be conducted, with each experiment having six data points (CT values) and a 

control. Two independent sets of experiments should be conducted with the water. Should 

significant discrepancies develop between the data sets, a third set of experiments would need to 

be conducted. An example experimental matrix is provided in Exhibit A.1 .
Appendix A - Site Specific Determination of Contact Time for Chlorine Dioxide and Ozone

LT2ESWTR Toolbox Guidance Manual A-2 April 2010

Exhibit A.1 Example Experimental Test Matrix

Date

Temperature to be 

Tested

(Historical Record)

Water Quality 

Criteria

Schedule of Experiments

Test 1 Test 2 Test 3

February 

Lowest Annual 

TOC or UV254 > 

75% of max 

historical value

X X If Required

April

Highest in April Same X 

X If Required

June Highest in June Same X X If Required

August Highest Annual Same X X If Required

October Highest in October Same X X If Required

December Lowest in December Same X X If Required

A.2 Experimental Procedure 

 

A.2.1 Preparation of Oocysts 

High oocyst quality is imperative to the success of the study because sub-standard oocysts could 

dramatically affect the data in a way that would underestimate the CT required to achieve a desired level of 

inactivation. Traditionally, Cryptosporidium parvum oocysts are derived from two host sources, bovine 

and rodent. The most common strain of Cryptosporidium parvum used to date is the Iowa strain, 

developed by Dr. Harley Moon. It is recommended that the utility perform all experiments using fresh (< 1 

month old) Iowa-strain oocysts obtained from a reputable supplier. The utility should ensure that after 

purification the supplier stores the oocysts at 4o

C in a solution of dichromate or 0.01 M phosphate buffer 

saline solution (pH 7.4) containing two antibiotics (1,000 U/mL penicillin, and 1,000 mg/mL 

streptomycin), and an antimycotic (2.5 mg/mL amphotericin B). The oocysts should be shipped in a cooler 

on ice to the utility via next-day service. Upon arrival, the oocysts should be placed in a refrigerator and 

stored at 4 degrees C until needed.

When ready for use, the oocysts should be suspended in 0.01 M pH 7 buffer and centrifuged at a 

relative centrifugal force of approximately 1,100 for at least 10 minutes. Following centrifugation, the 

oocysts should be aspirated and re-suspended in the buffer, then centrifuged again at the same conditions. 

This step should be repeated once more to remove as much of the antibiotic or dichromate solution as 

possible. Following the last aspiration, the oocysts should be re-suspended in approximately 10 mL of the 

pH 7 buffer. The oocysts should then be stored at 4o

C until the experiment is initiated. The oocysts should 

be vortexed thoroughly prior to initiation of the experiment. Additional details regarding this procedure 

can be found in Rennecker et al. 1999.
Appendix A - Site Specific Determination of Contact Time for Chlorine Dioxide and Ozone

LT2ESWTR Toolbox Guidance Manual A-3 April 2010

A.2.2 Source Water Preservation 

Testing should be conducted as close as possible to the date that the experimental water is 

collected. If testing is to be performed at a location other than the utility where the water was 

collected, the water should be sent to the laboratory via an overnight delivery service and stored 

at 4 degrees Celsius until the start of testing. 

A.2.3 Experimental Apparatus

A.2.3.1 Chlorine Dioxide

It is recommended that chlorine dioxide be generated using the equipment and procedures 

outlined in Standard Methods for the Examination of Water and Wastewater (APHA 1998). With 

this as a basis, all inactivation experiments using chlorine dioxide should be performed using a 

batch-reactor configuration. An example of such a system is provided by Ruffell et al. 2000. This 

system uses an enclosed recirculating water bath to maintain the desired temperature inside the 

reactor vessels, which consist of 2-liter amber glass bottles. During the experiment, care should 

be taken to minimize the exposure of the reactors to light. Mixing of the reactor contents is 

provided with a magnetic stir bar and stir plate.

A.2.3.2 Ozone

Inactivation experiments can be performed with either a semi-batch or batch reactor 

configuration. When performing experiments with a semi-batch system, it is recommended that 

analytical components similar to those described by Hunt and Mariñas (1997) be used. Using this 

system, the reactor vessel containing the experimental water is maintained at the experimental 

temperature by immersion in a water bath. Ozone can be generated from either compressed air or 

oxygen and passed through a continuously-stirred glass bottle, which serves to dampen the effect 

of fluctuating ozone concentration. The ozonated gas leaving the dampening bottle is then 

introduced to the experimental water via a fine-bubble diffuser. The ozonated water is stirred 

continuously using a magnetic stirring plate and a stir bar. 

It is recommended that inactivation experiments performed using a batch reactor 

configuration use analytical components similar to those described by Kim (2002). This reactor 

used a 100-mL gas-tight syringe to prevent ozone in solution from volatilizing into the 

atmosphere. The temperature inside the reactor is held constant by immersion in a recirculating 

water bath, and mixing is provided by a stir bar in the syringe controlled by a magnetic stir plate. 

Ozone can be produced from either compressed air or oxygen. A concentrated ozone stock 

solution should be prepared using distilled de-ionized or reverse osmosis-filtered water.
Appendix A - Site Specific Determination of Contact Time for Chlorine Dioxide and Ozone

LT2ESWTR Toolbox Guidance Manual A-4 April 2010

Other, less complex, batch reactor systems are also available which simply use an open 

vessel such as an Erlenmeyer flask or beaker (Finch et al. 1993a). With these systems, the reactor 

containing the experimental water is typically maintained at the desired temperature using a 

water bath. An ozonated solution, prepared with distilled de-ionized or reverse osmosis water, is 

added to the experimental water, and the ozone dose is measured from the diluted experimental 

water. When using this type of batch-reactor configuration that is open to the atmosphere, the 

user should take into account that ozone is lost to volatilization. This loss of ozone should be 

considered and minimized when performing any inactivation or demand/decay experiment.

A.2.4 Inactivation experiments

The CT values obtained from each of the site-specific inactivation experiments are 

expected to be similar to those provided in the standard LT2ESWTR tables. Therefore, utilities 

wishing to determine site-specific inactivation data are advised to use the standard tables as a 

baseline. Each experiment should be designed such that six data points span the range of the 

standard inactivation curve for a given temperature. One control point with no disinfectant 

should also be taken.

A.2.4.1 Chlorine Dioxide

An experimental protocol developed from Ruffell et al. 2000 is provided here as an 

example. The reactor bottle should be filled with experimental water to a total volume 

corresponding to the desired sample volume times the number of samples expected per bottle (6 

is recommended). The bottle is then placed in the water bath and allowed to equilibrate to the 

target experimental temperature. At this point, chlorine dioxide stock solution is added to the 

reactor bottle at the target dose. The reactor bottle is then capped to minimize chlorine dioxide 

volatilization. The chlorine dioxide concentration is measured approximately 10 min after 

dosing. An experiment was conducted by adding approximately a pre-determined number of 

oocysts to the reactor that will be sufficient for at least six data points. Note the volume of the 

oocyst aliquot should be less than 1 mL. Samples are then taken periodically at the contact times 

that correspond to the desired CT. The samples are immediately filtered through a 1μm filter. 

The filter is then placed in a clean 50 mL beaker and rinsed with approximately 15 mL of the 

dilute surfactant. The resulting oocyst suspension is transferred into a sterile 15 mL centrifuge 

tube.

These steps are repeated at various contact times corresponding to target CT parameters. 

After the last sample is taken, the chlorine dioxide dose is measured again. “Control” samples are 

also taken for each experiment by placing a sample of oocysts inside a similar reactor containing 

the experimental water minus the disinfectant at the target temperature. The oocysts are typically 

exposed to this condition for the duration of the experiment and subsequently processed for 

viability assessment with methods similar to those for the disinfected samples.
Appendix A - Site Specific Determination of Contact Time for Chlorine Dioxide and Ozone

LT2ESWTR Toolbox Guidance Manual A-5 April 2010

A.2.4.2 Ozone

If a semi-batch reactor configuration is used, the protocol described by Rennecker et al. 

(1999) is recommended. The protocol is described briefly as follows. Ozonated gas is applied to 

the temperature-acclimated experimental water via a fine bubble diffuser. The ozone gas 

concentration is adjusted to achieve steady-state at dissolved ozone concentrations representative 

of what would be observed at the facility. The actual dissolved ozone concentration achieved for 

each experiment is measured. Mixing of the ozonated water is performed with a magnetic stir bar 

and stirring plate. An inactivation experiment is initiated by injecting a suspension containing a 

sufficient number of oocysts into the reactor, and ends by simultaneously removing the bubble 

diffuser and injecting a quenching agent. It should be noted that the number of oocysts necessary 

for each data point is dependent on the viability assessment method selected. Oocysts are then 

removed from the quenched solution by filtration through a 1 μm filter. The reactor is then rinsed 

with approximately 50 mL of a dilute surfactant, and then again with approximately 100 mL of 

the experimental water to remove any residual surfactant. Both eluents are passed through the 

filter that is then placed in a clean 50 mL beaker and rinsed with approximately 15 mL of the 

dilute surfactant. The resulting oocyst suspension is transferred into a sterile 15 mL centrifuge 

tube. These steps are repeated at various contact times corresponding to target CT parameters 

(i.e., the product of dissolved ozone concentration and contact time).

Control samples are prepared with each daily experimental set by shutting off the ozone 

generator, but allowing the oxygen gas to flow through the system. Oxygen gas is allowed to bypass the semi-batch reactor after shutting off the generator to purge residual ozone gas from the 

system. All other conditions used for the control are consistent with the experimental conditions 

previously described. The contact time for control samples is 1 minute. After completion of the 

experiment, the samples are generally centrifuged at 1,100g for 10 minutes and stored in a 

phosphate buffer solution for a period of time not to exceed 48 hours prior to viability assessment 

procedures.

Experiments performed with a head-space free reactor can follow the following protocol 

(described previously in Kim 2002). The experimental temperature is maintained by immersing 

the 100-mL syringe, which serves as the reactor in a water bath. Mixing inside the reactor is 

provided using a stir bar and magnetic stir plate. The syringe is filled with the experimental water 

containing enough oocysts for all six data points. At this point, an aliquot of temperatureadjusted ozone stock solution of known concentration is added. Samples are then taken at time 

intervals corresponding to the pre-determined estimated CT using a syringe containing a 

quenching reagent. The samples are then processed using filtration and centrifugation, similar to 

those described above. A control should be performed for each experiment by placing the sample 

number of oocysts in the experimental water at the desired temperature. The oocysts should 

remain there for a period of time equal to the duration of the inactivation experiment. After this 

time, the oocysts should be processed in a manner consistent with the disinfected samples.
Appendix A - Site Specific Determination of Contact Time for Chlorine Dioxide and Ozone

LT2ESWTR Toolbox Guidance Manual A-6 April 2010

Experiments performed with batch reactor components that are not head-space free 

typically follow a similar, although less complex protocol. An example of such a system and the 

associated experimental protocol can be obtained from Finch et al. 1993a. 

It should be noted that for all batch-reactor systems, a careful characterization of the 

ozone demand and decay kinetics of the experimental water should be performed prior to any 

disinfection testing. In addition, it is also recommended that ozone concentration samples be 

procured alternately between inactivation samples to verify ozone concentrations observed 

during the disinfection study. 

A.2.5 Sample Processing

After procuring each sample point, the samples should be stored at 4oC until the end of 

the experiment. At the end of each experiment, the samples should be centrifuged at a relative 

centrifugal force of 1,100 for at least 10 minutes to remove quenching agents or surfactants. 

Following centrifugation, the samples should be carefully aspirated and re-suspended in 0.01 M 

pH 7 buffer solution. The samples should be stored at 4 degrees until the time of viability 

assessment.

A.2.6 Viability Assessment

Determining the viability of oocysts for varying levels of disinfection is one of the most 

critical components of the inactivation experiments. At present, there are three methods available 

to assess Cryptosporidium parvum viability, each presenting unique advantages and 

disadvantages. These methods include the following techniques: 

1) Animal infectivity. 

2) Cell culture (in vitro infectivity). 

3) In vitro excystation. 

The most established of these methods is animal infectivity. This viability assessment 

method typically involves inoculating immuno-suppressed neonatal mice with varying numbers 

of oocysts exposed to a particular CT. After a certain incubation period, the mice are then 

sacrificed and their intestinal tracts are examined for signs of Cryptosporidium-induced infection 

(cryptosporidiosis). The primary benefit of this method is that it demonstrates that the treated 

oocysts are capable of reproduction inside a mammalian host and therefore are able to induce an 

infection. One criticism of this method is that although an infection is capable of being observed, 

mouse infectivity has not been correlated to human infectivity. In addition, the protocol 

associated with this method is difficult and expensive. It requires specialized laboratory training, 

facilities, and equipment. An example of this protocol can be found in Finch et al. 1993b. 
Appendix A - Site Specific Determination of Contact Time for Chlorine Dioxide and Ozone

LT2ESWTR Toolbox Guidance Manual A-7 April 2010

A second method used to assess the viability of Cryptosporidium parvum is known as in 

vitro infectivity or cell culture. At present, cell culture methodologies used for this purpose are 

based on either microscopic evaluation (Slifko et al. 1997) or polymerase chain reaction (PCR) 

(Rochelle et al. 1997). The first step in using cell culture to assess oocyst viability involves 

applying the treated oocysts to a lawn of cells (typically derived from human or canine cell lines). 

After an incubation period, using microscopic evaluation-based culture methods, the cells are 

stained with fluorescent chemicals and then examined microscopically for various 

Cryptosporidium life stages. The presence of these life stages suggests that the oocysts were 

capable of reproduction and thus were viable and likely able to cause an infection in humans. 

When using a PCR-based technique, after incubation the cells are processed and the 

Cryptosporidium parvum Ribonucleic acid (RNA) is extracted. Infectivity is then determined by 

targeting specific genetic sequences in the RNA. The primary advantage of using cell culture to 

assess Cryptosporidium parvum infectivity is that it can measure very low concentrations of 

oocysts. Therefore, cell culture is capable of demonstrating high levels of inactivation. In 

contrast, the disadvantages associated with using cell culture include a lack of agreement over the 

preferred cell lines and viability assessment technique. In addition, there has been no 

extrapolation between cell culture techniques and human infectivity. Lastly, cell culture 

techniques are complex and typically require specialized equipment and rigorous training, which 

makes this procedure somewhat expensive. 

A third method known as in vitro excystation has also been developed to assess the 

viability of Cryptosporidium parvum (Rennecker et al. 1999). This method involves exposing 

oocysts to a simulation of a mammalian digestive tract. Following the simulation, the oocysts are 

then examined microscopically for oocyst life stages that are indicative of viability. The 

advantages of this method are that it is cost-effective, offers the ability to rapidly develop data, 

and requires minimal training. The main disadvantage of the method is that of the three methods 

described, in vitro excystation has the least similarity to an actual infection. However, it should 

be noted that in spite of this fact, two published studies have shown that inactivation data 

obtained with in vitro excystation closely matches animal infectivity and/or cell culture data 

(Rennecker et al. 2000, Owens et al. 1999). 
Appendix A - Site Specific Determination of Contact Time for Chlorine Dioxide and Ozone

LT2ESWTR Toolbox Guidance Manual A-8 April 2010

A.3 Statistical Analysis 

A general approach for calculating a set of CT values involves the following steps: 

1) Fitting an inactivation model(s) to the experimental inactivation data (for the

entire year).

2) Calculating the predicted average CT requirements from the best fit model.

1) Calculating and applying a factor of safety for the average predicted CT 

requirement.

One approach by Clark et al. (2002) used a one-parameter Chick-Watson model to fit 

experimental data sets and develop standard CT curves, relative to inactivation level and

temperature. As described in the LT2ESWTR Preamble, EPA used the Clark et al. approach for 

developing CT values but adjusted the analysis to account for different types of uncertainties and 

variability inherent in the data. EPA wanted to account for variability among different water 

matrices and oocyst strains, but not variability within the same group (i.e., same oocyst lot and 

water), and uncertainty in the regression. While such a complex approach may not be necessary 

for a site-specific study, the water system should be aware of the uncertainties and variability of 

the experimental data and use a statistical method that builds in a reasonable safety factor to 

ensure public health is protected. 

Two types of confidence bounds that are commonly used when assessing relationships 

between variables, such as disinfectant dose (CT) and log inactivation, are confidence in the 

regression and confidence in the prediction. Confidence in the regression accounts for uncertainty 

in the regression line (e.g., a linear relationship between temperature and the log of the ratio of 

CT to log inactivation). Confidence in the prediction accounts for both uncertainty in the 

regression line and variability in experimental observations it describes the likelihood of a single 

future data point falling within a range. Bounds for confidence in prediction are wider (i.e., more 

conservative) than those for confidence in the regression. Depending on the degree of confidence 

applied, most points in a data set typically will fall within the bounds for confidence in the 

prediction, while a significant fraction will fall outside the bounds for confidence in the 

regression. 
Appendix A - Site Specific Determination of Contact Time for Chlorine Dioxide and Ozone

LT2ESWTR Toolbox Guidance Manual A-9 April 2010

A.4 References

American Public Health Association, American Water Works Association, and Water 

Environment Federation. 1998. Standard Methods for the Examination of Water and Wastewater. 

Washington D.C. 

Clark, R.M.; Sivagenesan, M., Rice; E.W.; and Chen, J. (2002). Development of a Ct equation 

for the inactivation of Cryptosporidium oocysts with ozone. Wat. Res. 36, 3141-3149. 

Finch, G. R.; Black E. K.; Gyurek, L.; and Belosevic, M. (1993a). Ozone inactivation of C. 

parvum in demand-free phosphate buffer determined by in vitro excystation and animal 

infectivity. J.Appl. Environ. Microbiol. 59(12),4203-4210. 

Finch, G.R.; Daniels, C.W.; Black, E.K.; Schaefer III, F.W.; and Belosevic, M. .(1993b). Dose 

response of C. parvum in outbred neonatal CD-1 mice. J.Appl. Environ. Microbiol. 59(11), 3661-

3665. 

Hunt, N. K.; and Mariñas, B.J. (1997) Kinetics of Escherichia coli inactivation with ozone. 

Wat. Res. 31(6), 1355-1362. 

Kim, J. H.; Tomiak, R. B.; Rennecker, J. L.; Mariñas, B. J.; Miltner, R.J.; and Owens, J. H. 

(2002). AInactivation of Cryptosporidium in a Pilot-Scale Ozone Bubble-Diffuser Contactor. Part 

I I: Model Verification and Application.@ ASCE Journal of Environmental Engineering, 128(6), 

522-532. 

Li, H.; Finch, G.R.; Smith, D.W.; and Belosevic, M. (2000). Chemical inactivation of 

Cryptosporidium in water treatment. AWWA Research Foundation, Denver, CO. 

Owens, J.H.; Miltner, R.J.; Slifko, T.R.; and Rose J.B. (1999). In vitro excystation and infectivity 

in mice and cell culture to assess chlorine dioxide inactivation of Cryptosporidium oocysts. 

Proceedings of the AWWA WQTC Conference, Tampa.

Rennecker, J. L.; Mariñas B. J.; Owens J. H.; and Rice E. W. (1999) Inactivation of C. parvum

oocysts with Ozone. Water Res. 33 (11), 2481 - 2488. 

Rochelle, P.A.; Ferguson, D.M.; Handojo, T.J.; De Leon, R.; Stewart, M.H.; and Wolfe, R.L. 

(1997). An assay combing cell culture with reverse transcriptase PCR to detect and determine the 

infectivity of waterborne C. parvum. J.Appl. Environ. Microbiol. 63(5) 2029 - 2037. 

Ruffell, K.M; Rennecker, J.L.; and Mariñas, B.J.(2000). Inactivation of C. Parvum oocysts with 

chlorine dioxide. Wat. Res. 34 (3), 868 - 876. 
Appendix A - Site Specific Determination of Contact Time for Chlorine Dioxide and Ozone

LT2ESWTR Toolbox Guidance Manual A-10 April 2010

Slifko, T.R.; Friedman, D.; Rose, J.B.; and Jakubowski, W. (1997). An in vitro method for 

detecting infectious Cryptosporidium oocysts with cell culture. J.Appl. Environ. Microbiol. 63(9) 

3669 - 3675. 
LT2ESWTR Toolbox Guidance Manual B-1 April 2010

Appendix B 

Ozone CT Methods

Abbreviations and Glossary 

BrO -

3 Bromate ion 

CeffT10 Chamber effluent ozone residual in mg/L times chamber T10 time in minutes

Co-current A chamber in an ozone contactor where the water is flowing upward and the 

chamber ozone gas bubbles are rising. The direction of flow of the water and the gas 

is the same.

Counter- A chamber in an ozone contactor where the water is flowing downward and 

current the ozone gas bubbles are rising. The direction of flow of the water is in the 

chamber opposite direction of the gas flow. 

CSTR Completely Stirred Tank Reactor – fully mixed volume 

CT The product of Concentration and Time in mg/L-min

DBP Disinfection byproduct 

gpm Gallons per minute

HDT Hydraulic detention time calculated as the volume divided by the flow. 

When volume is expressed in gallons, and flow expressed in gallons/minute, 

then the calculated HDT is in minutes. 

In-situ sample Sample ports that take a sample from the flow of the chamber, typically 

ports through tubing that projects into the flow. 

k

* The first-order ozone decay coefficient, min-1 

k10 Log-base-10 value of the lethality coefficient for the inactivation of 

Cryptosporidium, Giardia or virus with ozone. The units of k10 in this 

document are L/mg-min.

-Log (I/Io) Log inactivation. Negative log-base-10 of the survival rate (N/No) of the 

microorganisms, where Io is the number of viable organisms entering the 

contactor, and I is the number of viable organisms leaving the contactor. 

PQL Practical Quantitation Limit: The minimum concentration of an analyte 

(substance) that can be measured with a high degree of confidence that the 

analyte is present at or above that concentration. 

Q Water flow – usually expressed in gallons per minute (gpm) or million 

gallons per day (MGD). 

RTD Residence Time Distribution probability distribution function describing 

the residence time of a fluid element within a contactor.

segment A theoretical or physically real chamber within a contactor. Used 

predominantly to denote a theoretical chamber for non-conventional 

contactors. 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-2 April 2010

T The time at which 10 percent of the water in the contactor or segment has 

passed through the contactor or segment. EPA recommends that tracer 

studies be used to determine the T

10

10

Up flow 

chamber

/HDT ratio for ozone contactors. The 

SWTR Guidance Manual and Tracer Studies in Water Treatment Facilities: 

A Protocol and Case Studies describe how to conduct a tracer test. 

A chamber within an over-under baffled bubble-diffuser ozone contactor in 

which the direction of water flow is upward. 

V Volume of the contacting zone in question – usually expressed in gallons or 

million gallons. 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-3 April 2010

B.1 Introduction

B.1.1 Background 

Appendix O of the Surface Water Treatment Rule (SWTR) Guidance Manual (U.S. EPA 

1991) includes a description of different methods for determining inactivation credit using an 

ozone contactor. These recommended methods differ in the level of effort associated with them 

and, in general, the ozone dose needed to achieve a given level of inactivation. This appendix 

provides guidance to help water systems select the more appropriate methods for their ozone 

process. More importantly, it builds on the information presented in the SWTR Guidance 

Manual with detailed descriptions of the extended T10

The four methods for calculating LT2ESWTR ozone inactivation credit, presented in 

Chapter 11 and this appendix, are described below. 

 method and extended continuous stirred 

tank reactor (CSTR) method. Appendices C, D and E compliment this appendix with 

descriptions of ozone residual sampling and laboratory analysis (Appendix D) and derivations of 

equations used in the extended CSTR approach (Appendix E).

1. T10 --calculates CT through a contactor assuming hydraulic conditions similar to plug 

flow and can only be used with tracer study data. Using the T10 approach, the contact 

time (T) is the time at which 10 percent of the water in the contactor or segment has 

passed through the contactor or segment. Even in well-baffled contactors, the T10 is most 

often less than 65 percent of the average hydraulic detention time (HDT) through the 

contactor, and generally underestimates the true CT achieved. (The T10

2. CSTR--calculates log inactivation credit in a chamber by assuming it to be completely 

mixed. It is applicable to contactors that experience significant back mixing or when no 

tracer study data are available. EPA recommends using this method (or the Extended 

CSTR) when no tracer study data are available. (The CSTR approach is described in 

Chapter 11, section 11.3.) 

 approach is 

described in Chapter 11, section 11.3.) 

3. Extended T10--a method that utilizes three measured ozone residuals at three chamber 

effluents in a reactive zone to predict the ozone residual concentrations at the effluents of 

the non-monitored chambers in the zone. It then uses the standard T10

4. Extended CSTR--a method that utilizes three measured ozone residuals at three chamber 

effluents in a reactive zone to predict the ozone residual concentrations at the effluents of 

the non-monitored chambers in the zone. It then uses the standard CSTR method to 

calculate the CT from all chambers using both measured and predicted ozone residuals. 

This method is only applied to reactive chambers and not to dissolution chambers, and 

does not require tracer study data. 

 method to 

calculate the CT from all chambers using both measured and predicted ozone residuals. 

This method is only applied to reactive chambers and not to dissolution chambers, and 

requires tracer study data.
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-4 April 2010

While this guidance manual describes four methods, other methods or modifications to 

these methods may be used at the discretion of the state. 

B.2 Selection of Methods for Calculating Inactivation Credit

Selecting the appropriate methods to use depends on the configuration of the ozone 

contactor and amount of process evaluation and monitoring that a water system is willing to 

undertake. It is also possible that combinations methods can be used. For contactors with 

multiple segments it is likely that the CT of one or two segments would be calculated using 

either the T10 or CSTR methods, while the CT for the remaining segments would be calculated 

with the Extended T10 or the Extended CSTR method. 

Of the four methods described in the previous section, the two Extended methods are 

more complex. The Extended methods require measurements of the ozone concentration at a 

minimum of three points within this portion of the contactor. The residual measurements are then 

used to develop a predicted ozone concentration profile through this portion of the contactor. 

While many mathematical principles are discussed in these methods, their implementation is 

fairly straightforward. In fact, the methods presented in this appendix can be programmed into a 

conventional spreadsheet or a plant computer control system. 

The following exhibits define the types of chambers potentially present in an ozone 

contactor and show the recommended methods for calculating the inactivation credit achieved. 

Only the T10 or CSTR methods can be applied to dissolution chambers. However, they can be 

applied to the reactive chambers as well. If no tracer test data are available, it is recommended 

that the CSTR method be used. The Extended methods are applied over a minimum of three 

consecutive reactive chambers. Exhibit B.1 shows the recommended methods.
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-5 April 2010

Exhibit B.1 Recommended Methods and Terminology for Calculating the Log 

Inactivation Credit

Section 

Description Terminology 

Method for Calculating 

Log Inactivation Recommended Restrictions

Without Tracer Data

Chambers where ozone is added

First chamber First 

Dissolution 

Chamber

No log Cryptosporidium

inactivation credit is 

recommended

The SWTR criteria for 1st

Other chambers 

chamber 

credit should still be used if 

calculating inactivation of Giardia

and virus

Co-Current or 

CounterCurrent 

Dissolution 

Chambers

CSTR Method in each 

chamber with a 

measured effluent 

ozone residual 

concentration

No credit should be given to a 

dissolution chamber unless a 

detectable ozone residual has been 

measured upstream of this chamber

Reactive Chambers

> 3 consecutive 

reactive chambers

ExtendedCSTR Zone

Extended CSTR 

Method in each 

chamber

Detectable ozone residual should 

be present in at least 3 chambers in 

this zone, measured via in-situ 

sample ports. Otherwise, the CSTR 

method should be applied 

individually to each chamber having 

a measured ozone residual

< 3 consecutive 

reactive chambers

CSTR 

Reactive 

Chamber(s)

CSTR Method in each 

chamber with a 

measured effluent 

ozone residual 

concentration

None

With Tracer Data

CounterCurrent 

Dissolution 

Chambers

dissolution chamber unless a 

detectable ozone residual has been 

measured upstream of this chamber

each chamber

Reactive Chambers

> 3 consecutive 

chambers with insitu sample ports

ExtendedCSTR Zone 

Extended T10 Detectable ozone residual should 

be present in at least 3 chambers in 

this zone, measured via in-situ 

sample ports. Otherwise, the T

 or 

Extended CSTR 

Method in each 

chamber. The Extended 

CSTR method is not 

appropriate for nonconventional contactors.

10 or 

CSTR method should be applied to 

each chamber having a measured 

ozone residual

< 3 consecutive 

chambers 

T10 or CSTR T

Reactive 

Chamber(s)

10 or CSTR Method in None

each chamber 

Chambers where ozone is added

First chamber First 

Dissolution 

Chamber

No log Cryptosporidium

inactivation is credited 

to this section

The SWTR criteria for 1st

Other chambers 

chamber 

credit should still be used if 

calculating inactivation of Giardia

and virus

Co-Current or T10 or CSTR Method in No credit should be given to a 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-6 April 2010

B.3 Dissecting an Ozone Contactor

B.3.1 Ozone Contactor Configurations

Ozone contactors are designed in a wide variety of configurations. Different 

configurations are adaptable to the Extended T10

Exhibit B.2 shows configurations with multiple, consecutive well-defined reactive 

chambers. The water flow pattern in such contactors can be an “over-under” pattern, a 

“serpentine” pattern, or a combination of both. Gaseous ozone is added to the water by one of 

two procedures. Gaseous ozone can be injected into the influent water before the water enters the 

contactor, a process often called “in-line” ozone addition (see schematic B & D in Exhibit B.2). 

Alternatively, ozone enriched gas can be bubbled into one or more chambers, a process called 

“in-chamber” ozone addition (see schematic A & C in Exhibit B.2). In-chamber ozone addition 

takes place in chambers that have an over-under flow pattern and not in chambers that have a 

serpentine flow pattern (Exhibit B.2-C) in order to ensure full and complete ozone dissolution 

into all the water flow. These so-called bubble columns can be counter-current or co-current, 

describing the directional flow of the water with respect to the upward flowing bubbles. Note, 

Exhibit B.2 only shows example configurations; size and geometry of the chambers will vary. 

or Extended CSTR methods, but 

implementation details vary with contactor configuration. It is important for a water system to 

identify the type of configuration and become familiar with the terminology used in this 

guidance manual. 

In contrast to the multi-chamber configuration, ozone contactors may also be comprised 

of only one or two reactive zones. Examples of such contactors are shown in Exhibit B.3, which 

include a closed-pipe contactor (see schematic A) and two open-channel contactors (see 

schematics B & C). All three contactors depict a long and narrow water flow path that may 

promote more plug-flow hydrodynamics within the majority of the chamber. As with multichamber contactors, ozone can be added in-line, or in-chamber. Contactors A and B illustrate inline ozone addition. Contactor C illustrates in-chamber ozone addition. 
Appendix B - Ozone CT Methods

Exhibit B.2 Schematics of Typical Conventional Configurations of Ozone 

Contactors with Multiple Chambers

Top View of a Serpentine

Contactor with In-Line Ozone

Addition

(D)

Side View of an Over-Under

Contactor with In-Line Ozone

Addition

(B)

LT2ESWTR Toolbox Guidance Manual B-7 April 2010

Exhibit B.3 Example Schematics of Non-conventional Configurations of Ozone 

Contactors 

90

Pipe Ozone Contactor

(A)

Ozone

Side View of an Open-Channel Ozone

Contactor with In-Line Ozone Addition

(B)

Ozone

Side View of an Open-Channel

Contactor with In-Chamber

Ozone Addition

(C)

Ozone

Ozone

Side View of an Over-Under

Contactor with In-Chamber

Ozone Addition

Ozone

(A)

Ozone

Top View of a Combined OverUnder & Serpentine Contactor

with In-Chamber Ozone Addition

Over-Under

Chambers

(C)
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-8 April 2010

Whether the contactor is configured with multiple chambers or predominantly a single 

long and narrow chamber, a major consideration of determining inactivation credit is 

characterizing the hydrodynamics within the contactor. The EPA recommends performing a 

tracer study to ascertain a description of the hydrodynamics. A tracer study will afford the 

necessary information for utilizing the T10 or Extended T10 methods. However, as with SWTR 

guidance manual, this guidance offers possibilities to apply an assumed, theoretical 

hydrodynamic condition to the contactor if a tracer study is not performed. However, since this 

option involves a major assumption, the guidance recommends assuming a hydrodynamic 

condition that is somewhat conservative with respect to the efficiency of disinfection kinetics. In 

particular, for utilities that opt to not perform a tracer study, this guidance offers the CSTR and 

Extended CSTR methods, which assumes that each chamber within a contactor has a high degree 

of mixing equivalent to an ideal CSTR. 

B.3.2 Classification of the Chambers and Contactor Zones 

To properly apply the methods discussed in this manual, the contactor should be divided 

into specific sections or zones. To ensure clarity, certain terminology is adopted for unique 

sections of an ozone contactor, as presented in Exhibit B.1. 

Exhibit B.4 shows example schematics of conventional and non-conventional 

configurations of ozone contactors. Schematic A is that of a conventional configuration of a 10-

chamber over-under baffled ozone contactor with in-chamber ozone addition. Ozone is being 

added only in Chambers 1 and 4 in this example. Chamber 1 is classified as a “First Dissolution 

Chamber” and it is recommended that no disinfection credit be granted for this chamber. Rapid, 

initial ozone reactions and the transitional development of the ozone residual occur in the first 

dissolution chamber. As such, a representative dissolved ozone profile is difficult to estimate 

without multiple sample ports along the bubble column. The second and third chambers in the 

contactor shown in schematic A of Exhibit B.4 are reactive chambers through which ozone is 

decaying. These chambers are called “Reactive Chambers.” The T10 or CSTR method could be 

used to calculate the log inactivation across such Reactive Chambers when ozone residual values 

are available from the effluent of the chamber. The T10 and CSTR methods are described in 

Chapter 11. 

The fourth chamber in the contactor shown in schematic A of Exhibit B.4 includes ozone 

addition. This chamber is called a Co-Current "Dissolution Chamber.” It should be emphasized 

that there is a distinction between a "Dissolution Chamber" and "First Dissolution Chamber." A 

chamber is given the “Dissolution Chamber” notation only when ozone residual has been 

detected at any point upstream of the influent to that chamber, thus signifying that the initial (i.e. 

instantaneous) ozone demand has been met. In other words, chamber 4 in schematic A of Exhibit 

B.4 can be classified as a Dissolution Chamber only if ozone residual has been detected at the 

effluent of either chamber 1, 2, or 3. The T10 or CSTR method could be used to calculate the log 

inactivation credit across a Dissolution Chamber. If no ozone residual was detected upstream of 

this chamber location, then chamber 4 takes on the classification of, and is treated as, a “First 

Dissolution Chamber” and as with chamber 1, no log inactivation credit is granted. 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-9 April 2010

Chambers 5 through 10 in schematic A in Exhibit B.4 represent the “Extended zone” 

since they meet the criterion of containing a minimum of three consecutive reactive chambers. If

tracer data are unavailable, the Extended-CSTR approach is used to calculate the log inactivation 

across each chamber in this zone. If tracer data are available and can be used to calculate the 

T10/HDT ratio of the contactor, then the Extended T10 approach or the Extended CSTR approach 

could be used to calculate the log inactivation across each chamber in this zone. Modeling is 

used to calculate the ozone residual concentration at the effluent of each chamber within the 

Extended zone. Either Extended method requires an accurate estimation of the ozone decay 

coefficient, k

*

, and the initial ozone residual at the entrance to the zone, Cin. Estimation of these 

two parameters, which is discussed in sections B.4.3.1 and B.4.3.2, requires the measurement of 

three ozone residual values across the minimum span of three chambers. 

In the case of a contactor with in-line ozone addition, the entire contactor potentially 

becomes an Extended zone. If the contactor has at least three chambers equipped with in-situ 

sample ports and a measurable ozone residual, then the requirements for calculating k* and Cin

have been met and the entire contactor can be treated as an Extended zone. Care should be taken 

in locating the first ozone sample port such that enough reaction time is allowed for the 

immediate ozone demand to be fully met before the sample port. 
Appendix B - Ozone CT Methods

Exhibit B.4 Names of Various Sections of Different Types of Ozone Contactors

Ozone

(A)

Ozone

(B)

Ozone

90

Extended Zone

[minimum Length:Width Ratio of 5:1 & minimum Length:Height Ratio of 5:1]

(C)

1 2 3 4 First Dissolution Chamber Reactive Chamber Dissolution Chamber

Extended Zone

[min. Length:Width Ratio of 5:1 & min. Length:Height Ratio of 5:1]

1 2 3 4 5 6 7 8 9 10

Extended Zone

First Dissolution ChamberReactive Chambers

Co-Current Dissolution Chamber

[minimum of 3 consecutive reactive chambers]

LT2ESWTR Toolbox Guidance Manual B-10 April 2010

Schematics B and C in Exhibit B.4 represent non-conventional configurations of ozone 

contactors used in water treatment. While these contactors do not necessarily have clearly 

defined chambers divided by baffle walls, this guidance provides for methods such that they can 

be evaluated in a similar manner as conventional contactors with chambers are evaluated under 

the Extended T10 method. For such contactors, the Extended zone could be divided into 

segments that represent theoretical chambers; the number of theoretical chambers determined by 

a tracer test. Subsequent sections of this Appendix and Appendix E include examples and 

calculations that illustrate this approach. 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-11 April 2010

B.4 Extended T10 or Extended-CSTR Approaches for Ozone Contactors

B.4.1 Introduction

The methods described in this chapter represent a more sophisticated approach to 

calculating inactivation credit in an ozone contactor as compared to the T10 and CSTR 

approaches. This approach could potentially provide a higher and more accurate estimate of the 

level of microbial inactivation than that obtained using the T10

The approach described in this chapter is called the Extended Approach. Certain aspects 

of this methodology were introduced in Appendix O of the SWTR Guidance Manual. However, 

the material presented here greatly expands upon the SWTR Guidance Manual, and may provide 

beneficial new tools for the utility. As indicated in Chapter 11, the approached described herein 

can be used for calculating CT for Cryptosporidium, Giardia, or virus inactivation credit. 

 approach. The potential benefits 

of using these more sophisticated measures are lower ozone doses and consequently lower

formation of some ozonation disinfection byproducts, (e.g., bromate). However, as a 

consequence of this added sophistication, a higher degree of system evaluation and monitoring is 

needed for a given inactivation credit. Whether use of these more sophisticated approaches 

actually benefit the utility depends on many factors including the sought-after level of 

inactivation, the reactor configuration, and the water quality. 

The Extended method relies on measured ozone residuals across the Extended zone to 

model the ozone decay through the zone. The outcome is then used to project the ozone residual 

at any location in the Extended zone. This approach, which is applied only to an Extended zone 

as defined in the earlier section, includes the following four steps: 

Step 1 – Measure the ozone residual at the effluent of at least three chambers in the Extended 

zone. 

Step 2 – Utilizing the measured residuals and flowrate through the contactor, calculate the 

empirical ozone decay coefficient, k*, and the ozone residual in the influent of the 

Extended zone, Cin

Step 3 – Utilize the calculated k* and C

. 

in

Step 4 – Utilize the calculated ozone residual values to calculate the CT and subsequent log 

inactivation across each chamber in the Extended zone. 

to predict the ozone residual concentration at the 

influent or effluent of any chamber in the Extended zone. 

Ozone residual measurement at the three locations might be conducted manually using 

the Indigo Trisulfonate method, or continuously using on-line ozone analyzers. The Quality 

Assurance protocols discussed in Appendix C should be implemented to ensure that the ozone 

residual measurements are accurate. For on-line control systems that utilize continuous residual 

monitoring, instantaneous disinfection calculation will not be possible because of general 

fluctuations in the residual monitor’s responses to small changes in system operation. Most 

control systems include a function to conduct a rolling average of monitor readings at a preset 

interval. If this approach is used, EPA recommends that the averaging interval not exceed the 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-12 April 2010

HDT of the contactor at design flowrate. For example, if the contactor is designed with an HDT 

of 10 minutes at its design flowrate, then the control system’s averaging interval should not 

exceed 10 minutes. Therefore, if the monitor collects an ozone residual reading every two (2) 

minutes, then the control system would report an average of the previous five (5) consecutive 

readings every two minutes. The k* and Cin

B.4.2 Extended T

 values for the Extended zone are calculated using 

these rolling average residual values.

10 Method

To utilize this method for calculating the CT across an Extended zone of an ozone 

contactor, the contactor must have a representative set of tracer test results that have been used to 

set the T10/HDT ratio for the contactor. Guidance on determining the T10/HDT ratio for the 

contactor is found in SWTR Guidance Manual, Tracer Studies in Water Treatment Facilities: A 

Protocol and Case Studies, and Appendix E. The Extended zone comprises three or more 

individual chambers. Inactivation within each individual chamber is calculated in accordance 

with the T10 method described in Section 11.3.2. The sum of CT and log inactivation values for 

individual chambers gives the CT and log inactivation across the entire zone. The distinction 

between a standard Reactive Chamber and a chamber that is a component of an Extended zone is 

the manner in which each chamber’s Cout value is obtained. In the case of a standard Reactive 

Chamber, Cout is obtained from an actual measurement of the dissolved ozone residual at the exit 

of the chamber. In contrast, Cout for a chamber in an Extended T10 zone is a calculated value. It 

is important to note that the calculation of Cout

In addition to enabling the calculation of C

for a chamber is performed only on as many 

physical chambers (as in the case of multi-chamber contactors) or theoretical segments (as in the 

case on non-conventional contactors) within the Extended zone. 

out for all individual chambers, the Extended 

the T10 method follows the guidance established in Appendix O of the SWTR Guidance Manual 

in allowing the overall the T10/HDT ratio calculated for the entire contactor to be applied to each 

individual chamber – the so-called linear extrapolation of the T10/HDT ratio. As noted in 

Appendix O of the SWTR Guidance Manual and in Lev and Regli (1992), this allowance is not 

theoretically correct, and except for atypical contactor designs where a more plug-flow region is 

followed by a highly mixed zone of similar volume, this allowance will lead to a higher 

calculation of CT credit than if the actual T10/HDT ratio of each chamber was used. As described 

in Appendix O (and also alluded to in Section 11.3.2. of this guidance), this allowance comes 

with the proviso that the linear extrapolation of the overall T10/HDT ratio is not appropriate if 

more than 50% of the volume of the contactor has an ozone concentration of “zero.” In the 

context of the SWTR, a measurement of “zero” ozone residual is equivalent to “below the ozone 

method practical quantitation limit (PQL).” Because the Extended T10 method allows ozone 

concentrations to be calculated to values that may be below the PQL of the allowable ozone 

methods (see Appendix C), essentially a value of “zero”, the proviso in Appendix O needs to be 

revised. As such, the EPA recommends that the linear extrapolation of the overall T10/HDT ratio 

is not applicable if more than 50% of the contactor volume has a measured or calculated (using 

the more conservative k* value) ozone residual of less than 0.05 mg/L. If the proviso is not met, 

the system may estimate its CT credit using other methods, or it may conduct a tracer test to 

evaluate the T10/HDT ratio for a shorter section of the contactor, and use that ratio in the same 

manner. Because of the possibility of the 50% criteria not being met, the EPA recommends that 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-13 April 2010

if a system is conducting its first or new tracer tests to characterize the contactor, that they collect 

tracer data at intermediate points in the contactor in addition to the exit of the entire contactor. 

The procedure for calculating Cout for a chamber in an Extended T10 zone is described in 

this section. The value of Cout for a chamber in an Extended T10 zone is calculated using an 

empirical ozone decay coefficient, k*, and the ozone residual concentration at the entrance to the 

zone, Cin. Equation B-1 shows how to calculate the ozone residual at any location X along the 

Extended zone: 

 * [Volume] C  0

X C E k × − 

= × xp − X 

 in      (B-1) 

  Q 



where: k

*

 = Empirical ozone decay coefficient, min-1, calculated as described in section 

B.4.2.1. 

Cin = Calculated ozone residual concentration at the entrance to the Extended zone, 

mg/L, calculated as described in section B.4.2.2. 

 [Volume]0–X = Volume, in gallons, from the beginning of the Extended zone to a location X 

along the water path in the Extended T10 zone. 

Q = Water flow through the contactor, gpm 

Equation B-1 describes the Extended zone as a plug flow reactor for the purpose of calculating 

the profile of the ozone residual, C, along the zone. 

Once the values of the ozone residual concentrations at the effluent of each chamber in 

the Extended T10 zone are calculated, Equation 11-4 can then be used to calculate the log 

inactivation achieved across that chamber. The total log inactivation achieved across the entire 

contactor is equal to the sum of the log inactivation values calculated for each chamber. 

 I   T10  − Log   = × × ×  chambe    k10 C HDT r Equation 11-4    I 0   HDT 

where:

 -Log (I/I0) = the log inactivation

 k10 = log base-ten inactivation coefficient for the target organism (L/mg-min)

 C = Ozone residual concentration from Exhibit 11.3 (mg/L)

HDTchamber = Hydraulic detention time through the chamber (minutes)

(T10/HDT) = approved T10/HDT ratio for the contactor

The values of k10 for the inactivation of Cryptosporidium, Giardia, and virus with ozone 

can be expressed by the following equations (Temp = water temperature in o

C): 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-14 April 2010

Inactivation of Cryptosporidium with Ozone: k10 = 0.0397 × (1.09757)Temp

Inactivation of Giardia with Ozone: k10 = 1.0380 × (1.0741)Temp

Inactivation of virus with Ozone: k10 = 2.1744 × (1.0726)Temp

The values of k10 for the inactivation of Giardia and virus were derived from the k10

values for Giardia and virus inactivation listed in Appendix O of the SWTR Guidance Manual. 

The values of k

*

 and Cin should be determined every time log inactivation credit is 

calculated (i.e., at least daily). The following sub sections describe the procedures for estimating 

the values of k* and Cin across the Extended zone. 

B. 4.2.1 Determining the Value of k* 

The empirical ozone decay coefficient, k* is calculated using ozone sample 

measurements, taken from in-situ sample ports, and an assumed theoretical model of the 

chamber’s hydrodynamics. In the application of the Extended T10 method, the Extended zone is

modeled as a plug-flow reactor. It is important to note that the empirical ozone decay coefficient, 

k*, will unlikely be the same value as the first-order decay rate constant that would be measured 

in a batch or plug-flow reactor. Only when the Extended zone hydrodynamics are that of a plugflow system will the empirical k* be equivalent to the true first-order rate constant. Nonetheless, 

the simplifying assumption of plug-flow hydrodynamics and use of the two-parameter 

exponential (i.e. first-order kinetic) equation to fit two or three ozone residual measurements and 

subsequently predict the ozone concentration at other points in an Extended zone has been shown 

to be useful (Rakness, Najm et al. 2005). 

The steps outlined below pertain to an Extended zone with a minimum of three 

consecutive chambers with measurable ozone residuals. That is, there should be at least three insitu sample ports from the Extended zone with measurable ozone residual. The three ozone 

residual measurements, C1, C2, and C3, are needed to estimate the value of the ozone decay 

coefficient, k*. For example, the Extended zone in the contactor shown in Exhibit B.4(A)

includes chambers 5 through 10. The ozone residual values at any three chambers in that span 

can be used to represent C1, C2, and C3 in this analysis. The following steps should be followed 

to calculate the k

*

 value: 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-15 April 2010

Step 1 – Use Equation B-2 and residual measurements C1 and C2 to calculate the k value 

representing the ozone decay between locations 1 and 2, k*

1−2 . (A derivation and 

explanation of Equation B-2 is presented in Appendix E): 

*

* Q C  k 1

1−2 = Ln [ ] ×   (B-2) Volume 1−2 C2 

where: k*

1−2 = Empirical ozone decay coefficient between sampling locations 1 & 2, min-1

C1 = Measured ozone residual at location 1, mg/L 

C2 = Measured ozone residual at location 2, mg/L

 [Volume]1-2 = Volume between sampling locations 1 and 2, gallons 

Q = Water flow through the contactor, gpm 

Step 2 – Use residual measurements C1 and C3 along with Equation B-3 to calculate the k

*

 valu

representing ozone decay between sampling locations 1 and 3, k*

1−3 : 

e 

* Q C1  k1−3 = Ln [ ] ×   (B-3) Volume 1−3  3 

where: k*

1−3 = Empricial ozone decay coefficient between sampling locations 1 & 3, min-1

C1 = Measured ozone residual at location 1, mg/L 

C3 = Measured ozone residual at location 3, mg/L

 [Volume]1-3 = Volume between sampling locations 1 and 3, gallons 

Q = Water flow through the contactor, gpm 

It should be emphasized that sampling location 1 should not be at the entrance to the 

Extended zone, but should be at least one chamber into the zone. For example, in Exhibit B.4(A), 

C1 should not be measured at the entrance to chamber 5, since that is the entrance to the 

Extended zone (notice in Figure B.4(A) that ozone is added to chamber 4). Instead, the first 

Extended zone sampling location should be located at the effluent of chamber 5, or downstream 

of that location. Section O.3.2 of Appendix O of the SWTR Guidance Manual provides guidance 

on the use of in-situ sample ports for direct ozone measurements.

Step 3 – The value of k* that is to be used in Equation B-1 will be calculated as the average of 

k* * 1−2 and k1−3 as shown in Equation B-4. 

 * + *  * k =  1−2 k k 1−3  (B-4)  2 

C
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-16 April 2010

It is normal for the individual values of k*

1−2 and k*1−3 to be somewhat different. 

However, it is recommended that they be within the range of 80 percent to 120 percent of the 

average k* value calculated in Step 3. That is, 

abs k * − k1−i

* ≤ 20%

k

If the two k* values do not meet this criterion, the utility may 1) reject the measured residual 

values and collect new samples until this quality assurance (QA) criterion is met, or 2) select the 

higher of the two k* values as the more conservative estimate. 

B.4.2.2 Determining the Value of Cin 

While it is possible to measure the ozone residual at the entrance to the Extended zone 

(e.g., an in-situ sample port), it is not recommended that the measured value be used because it is 

usually higher than the residual predicted by the first-order decay profile (Amy et al., 1997; 

Carlson et al., 1997; Hoigné and Bader, 1994; Rakness and Hunter, 2000; Rouston et al., 1998). 

This phenomenon is commonly attributed to the more rapid initial ozone decay, which is 

followed by a somewhat slower first-order decay profile. For this reason, the Cin representing the 

ozone decay in the Extended zone should be back-extrapolated using the three downstream 

rolling average ozone residual values. 

The value of Cin can be calculated once the value of k* is estimated from the three rolling 

average residual ozone values. Maintaining the assumption of a first-order decay rate, Equations 

B-5 through B-7 can be used to estimate the value of Cin from the three rolling average ozone 

residual concentrations: 

 [Volume]  Cin = C ×  × −

k * 0 1

1 exp 

 (B-5) 

 Q 

 [Volume]  Cin,2 = C2 ×exp *

× 0−2   k  (B-6) 

 Q 

 [Volume]  Cin,3 = C3 ×exp × −3   k * 0

 (B-7) 

 Q 

where: k

*

 = Average decay coefficient from Equation B-4, min-1

C1 = Measured ozone residual at location 1, mg/L

C2 = Measured ozone residual at location 2, mg/L

C3 = Measured ozone residual at location 3, mg/L

 [Volume] 0–1 = Volume, in gallons, between the entrance of the Extended zone and sampling 

location 1 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-17 April 2010

 [Volume] 0–2 = Volume, in gallons, between the entrance of the Extended zone and sampling 

location 2 

 [Volume] 0–3 = Volume, in gallons, between the entrance of the Extended zone and sampling 

location 3 

Q = Water flow through the contactor, gpm 

The Cin value is then calculated as the average of the three values determined by 

Equations B-5 through B-7: 

C + C + C  C in

in = ,1 in, 2 in,3   (B-8) 

 3 

A systematic example of the Extended T10 approach is presented in section B.4.5 

B.4.3 Extended CSTR Approach 

The direct CSTR approach for calculating log inactivation across a chamber in an ozone 

contactor is discussed in Section 11.3.3. This section includes a discussion of the application of 

the Extended CSTR method to an Extended zone as defined in Sections B.3.2 and B.4.1. The 

Extended CSTR approach can be used whether or not tracer test results are available for the 

ozone contactor. However, it was developed primarily to afford a method for systems that choose 

not to perform a tracer study. In particular, in cases where a system chooses not to perform a 

tracer study, the Extended CSTR method makes an assumption that the hydrodynamics of each 

individual chamber is equivalent to an ideal CSTR. The assumption of CSTR hydrodynamics is 

considered somewhat conservative in terms of the efficiency towards chemical conversion for 

first-order and higher-order reactions. That is, the predicted disinfection in an ideal CSTR will be 

less than for an equivalent-volume, ideal plug-flow chamber. However, the CSTR assumption is 

not the most conservative assumption. Recent studies have indicated that in typical over-under 

baffled contactors, the hydrodynamics of an individual chamber can be worse in terms of 

reaction efficiency than a single CSTR due to recirculation patterns, dead-volumes, and short 

circuiting (Kim, Kim, et al. 2010; Kim, Nemlioglu, et al. 2010; Kim, Elovitz, et al. 2010). EPA 

therefore considers the CSTR assumption inherent in the Extended CSTR method as a 

reasonable balance between conservatism and pragmatism. 

As discussed earlier, the Extended CSTR approach applies to a zone that includes at least 

three consecutive reactive chambers. Inactivation within each chamber is calculated according to 

Equation 11-1, exactly as it is for the CSTR chamber, and the sum of the log inactivation values 

for individual chambers gives the inactivation across the whole zone. The distinction between a 

CSTR Reactive Chamber and a chamber that is a component of an Extended CSTR zone is the 

manner in which the value for C is obtained. In the case of the CSTR Reactive Chamber, C is 

obtained from an actual measurement of the dissolved ozone residual at the exit of the chamber 

(i.e., Cout). In contrast, C for a chamber in an Extended CSTR zone is a calculated value. The 

procedure for calculating C for an Extended CSTR zone is described in this section. 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-18 April 2010

The value of C for an Extended CSTR is also calculated using an empirical ozone decay 

coefficient, k*, and the ozone residual concentration at the entrance to the zone, Cin. Equation B9 shows how to calculate the ozone residual at the effluent of chamber “X” in an Extended 

CSTR zone:

[ ] N X

X

X

in

X

QN

Volume

k

C C −





 





×

×+

=

−

−

0

0

* 0 1

(B-9) 

where: k

*

 = Empricial ozone decay coefficient, min-1

C

, calculated as described in section 

B.4.3.1 

in

 [Volume]

 = Calculated ozone residual concentration at the entrance to the Extended CSTR 

zone, mg/L, calculated as described in section B.4.3.2 

0–X

N

 = Volume, in gallons, from the beginning of the Extended CSTR zone to the 

effluent of chamber “X”

0–X

Q = Water flow through the contactor, gpm 

 = Number of chambers from the beginning of the Extended CSTR zone to the 

effluent of chamber “X”

Equation B-9 describes the Extended-CSTR zone between the first chamber (subscript 0) and 

chamber X as a series of equal-volume CSTR reactors. This is a simplifying assumption that is 

based on a balance between ease of implementation and consistency with other provisions within 

this guidance manual. 

Once the values of the ozone residual concentrations at the effluent of each chamber in 

the Extended CSTR zone are calculated, Equation 11-4 can then be used to calculate the log 

inactivation achieved across that chamber. The total log inactivation achieved across the entire 

contactor is equal to the sum of the log inactivation values calculated for each chamber. 

 -Log (I/I0) = Log (1 + 2.303 × k10

where:

 × C × HDT) Equation 11-4 

 -Log (I/I0

 k

) = the log inactivation

10

 C = Concentration from Exhibit 11-2 (mg/L)

= log base ten inactivation coefficient (L/mg-min)

 HDT = Hydraulic detention time (minutes) 

The k10 can be determined using the equations as presented in Section B.4.2 or calculated 

from the CT table with the following equation: Log inactivation = k10 × CT. 

The values of k

*

 and Cin should be determined every time log inactivation credit is 

calculated (i.e., at least daily). These parameters are calculated using three measured ozone 

residuals from three locations within the Extended-CSTR zone. 
Appendix B - Ozone CT Methods

B.4.3.1 Determining the Value of k* 

The ozone decay coefficient, k* is calculated using ozone sample measurements, taken 

from in-situ sample ports, and an assumed theoretical model of the chamber’s hydrodynamics. 

The Extended CSTR approach assumes that each individual chamber in the Extended zone is as 

a CSTR, and hence the Extended zone can be modeled as a group of CSTRs in series. It is 

important to note that the empirical ozone decay coefficient, k*, will unlikely be the same value 

as the first-order decay rate constant that would be measured in a batch or plug-flow reactor. 

Only when the each chamber of the Extended zone behaves as an ideal CSTR and, consequently, 

the series of chambers acts as CSTRs-in-series, will the empirical k* be equivalent to the true 

first-order rate constant. Nonetheless, the simplifying assumption of CSTR hydrodynamics and 

use of the CSTR-in-series equation (e.g. Equation B.10) enables a reasonable prediction of the 

ozone concentration at other points in an Extended (Rakness, Najm et al. 2005). 

The steps outlined below pertain to a contactor with a minimum of three consecutive 

chambers with measurable ozone residuals. That is, there should be at least three in-situ sample 

ports from the Extended CSTR zone with measurable ozone residual. The three ozone residual 

measurements, C1, C2, and C3, are needed to estimate the value of the ozone decay coefficient, 

k*. For example, the Extended CSTR zone in the contactor shown in Exhibit B.5 includes 

chambers 5 through 10. The ozone residual values at any three chambers in that span can be used 

to represent C1, C2, and C3 in this analysis. The following steps should be followed to calculate 

the k

*

 value: 

Step 1 – Use Equation B-10 and residual measurements C1 and C * 2 to calculate the k value 

representing the ozone decay between locations 1 and 2, k*

1−2 . (A derivation and 

explanation of Equation B-10 is presented in Appendix D):

  1      2 − ×   N1−  * N  k − = 1 2 Q C

1 2  1  [ −   1 (B-10) Volume]  1−2  C  2     

where: k*

1−2 = Empirical ozone decay coefficient between sampling locations 1 & 2, min-1

C1 = Measured ozone residual at location 1, mg/L 

C2 = Measured ozone residual at location 2, mg/L 

 [Volume]1-2 = Volume between sampling locations 1 and 2, gallons 

N1-2 = Number of chambers between sampling locations 1 and 2 

Q = Water flow through the contactor, gpm 

Step 2 – Use residual measurements C1 and C3 along with Equation B-11 to calculate the k

*

value representing ozone decay between sampling locations 1 and 3, k*

1−3 : 

LT2ESWTR Toolbox Guidance Manual B-19 April 2010
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-20 April 2010

 

 1   * N1− Q  3×  C  N1−3   k −   1 = 1

3 [ 1

V ] −   (B-11) olume  1−3  C  3    

where: k*

1−3 = Empirical ozone decay coefficient between sampling locations 1 & 3, min-1

C1 = Measured ozone residual at location 1, mg/L 

C3 = Measured ozone residual at location 3, mg/L

 [Volume]1-3 = Volume between sampling locations 1 and 3, gallons 

N1-3 = Number of chambers between sampling locations 1 and 3 

Q = Water flow through the contactor, gpm 

It should be emphasized that sampling location 1 should not be at the entrance to the 

Extended zone, but should be at least one chamber into the zone. For example, in Exhibit B.4(A), 

C1 should not be measured at the entrance to chamber 5, since that is the entrance to the 

Extended zone. Instead, the first Extended CSTR zone sampling location should be located at the 

effluent of chamber 5, or downstream of that location. Section O.3.2 of Appendix O of the 

SWTR Guidance Manual provides guidance on the use of in-situ sample ports for direct ozone 

measurements.

Step 3 – The value of k* that is to be used in Equation B-1 will be calculated as the average of 

k*

1−2 and k*

1−3 as shown in Equation B-12. 

 * + * 

k* k =  1−2 k1−3  (B-12)  2 

It is normal for the individual values of k* * 1−2 and k1−3 to be somewhat different. 

However, it is recommended that they be within the range of 80 percent to 120 percent of the 

average k* value calculated in Step 3. That is, 

abs k * − k1−i

* ≤ 20%

k

If the two k* values do not meet this criterion, the utility may 1) reject the measured residual 

values and collect new samples until this quality assurance (QA) criterion is met, or 2) select the 

higher of the two k* values as the more conservative estimate. 

B.4.3.2 Determining the Value of Cin 

While it is possible to measure the ozone residual at the entrance to the Extended CSTR 

zone (e.g., an in-situ sample port), it is not recommended that the measured value be used 

because it is usually higher than the residual predicted by the first-order decay profile (Amy et 

al., 1997; Carlson et al., 1997; Hoigné and Bader, 1994; Rakness and Hunter, 2000; Rouston et 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-21 April 2010

al., 1998). This phenomenon is commonly attributed to the more rapid initial ozone decay, which 

is followed by a somewhat slower first-order decay profile. For this reason, the Cin

The value of C

 representing 

the ozone decay in the Extended CSTR Zone should be extrapolated using the downstream 

measured ozone residual values. 

in can be calculated once the value of k* is estimated from the three 

residual ozone measurements. Maintaining the assumption of first-order decay rate, and again 

using the CSTR (or equal-volume CSTR-in-series if there are more than one chamber between 

sample ports) assumption, Equations B-13 through B-15 can be used to estimate the value of Cin

from the three measured ozone residual concentrations: 

[ ] 10

10

* 10

11, 1

−





 





×

×+×=

−

−

N

in QN

Volume

kCC (B-13) 

[ ] 20

20

* 20

22, 1

−





 





×

×+×=

−

−

N

in QN

Volume

kCC (B-14) 

[ ] 30

30

* 30

33, 1

−





 





×

×+×=

−

−

N

in QN

Volume

kCC (B-15) 

where: k

*

 = Ozone first-order decay coefficient, min

C

-1

1

C

 = Measured ozone residual at location 1, mg/L 

2

C

 = Measured ozone residual at location 2, mg/L 

3

N

 = Measured ozone residual at location 3, mg/L 

0-1

N

 = Number of chambers between the entrance to the Extended CSTR Zone and 

sampling location 1 

0-2

N

 = Number of chambers between the entrance to the Extended- CSTR Zone and 

sampling location 2 

0-3

 [Volume]

 = Number of chambers between the entrance to the Extended- CSTR Zone and 

sampling location 3 

 0–1

 [Volume]

 = Volume, in gallons, between the entrance of the Extended CSTR Zone and 

sampling location 1 

 0–2

 [Volume]

 = Volume, in gallons, between the entrance of the Extended- CSTR Zone and 

sampling location 2 

 0–3

Q = Water flow through the contactor, gpm 

 = Volume, in gallons, between the entrance of the Extended- CSTR Zone and 

sampling location 3 

The Cin value is then calculated as the average of the three values determined by 

Equations B-13 through B-15: 





 



 ++ = 3

in in 2,1, in 3,

in

CCC

C 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-22 April 2010

A systematic example of the Extended CSTR approach is presented in section B.4.5. 

B.4.4 Quality Assurance for Extended Method Calculations

The Extended method depends on ozone residual measurements and assumed contactor 

hydrodynamics in order to predict ozone concentrations through the contactor. This section 

includes recommended QA controls intended to verify the validity of the residual predictions. 

Other considerations that have an important impact on characterizing the hydrodynamics and the 

ozone profile are discussed in Appendices C and E. 

The predicted ozone residual concentration, the parameter C in Equation 11-1, 

encompasses both the hydrodynamic assumption and ozone measurements. The principal QA 

issues focus on the prediction of the value of C. As seen in equation B-1, C depends on the 

parameters k* and Cin. In section B.4.3.1, as part of the discussion on the calculation of k*, it is 

stipulated that the individual k* values (i.e., k*1-2 and k*1-3) should be within 20 percent of the 

average value. This QA control is meant to ensure that ozone residual measurements used to 

calculate the ozone decay profile are consistent with the calculated profile. Since the calculation 

of Cin (Equations B-13 through B-15) depends on k*, as well as the measured ozone 

concentrations, the QA criteria for k* is sufficient for Cin

Finally, one of the most important aspects of any application of a model towards 

predicting reactor performance is the confirmation of the model’s prediction. This is, in essence, 

“model validation.” Appendix O of the SWTR Guidance Manual makes several points to this 

effect. Ideally, model validation would take the form of measuring the actual disinfection of the 

target microorganism. A more practical alternative is to compare the predicted ozone 

concentrations to measured values. The general recommendation is that the predicted ozone 

residual should not be more than 20 percent higher than the measured value. Note that this is a 

one-sided QA control. In other words, an under-prediction of the ozone residual is acceptable 

since it results in a conservative CT value. However, an over-prediction by more than 20 percent 

is not desirable. 

. Therefore, no additional QA criteria 

are necessary for it.

The ozone concentration measurements used to calculate k* and Cin cannot be compared 

to the predicted ozone residuals, since they are interdependent. It is recommended that ozone 

samples be taken from other sampling locations in the contactor, and those values compared to 

the calculated C at those locations. 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-23 April 2010

B.4.5 Examples of Extended Method Application

This section provides examples calculating the log inactivation credits across 

conventional and non-conventional contactors using the Extended T10 method and the Extended 

CSTR method. The calculations are completed for Cryptosporidium log inactivation using k10

for Cryptosporidium. Calculations for Giardia and virus log inactivation are completed similarly 

by using k10 for Giardia or virus in place of k10

Example 1 – Conventional Multi-chamber Contactor with In-situ Sample Ports and One 

Dissolution Chamber

for Cryptosporidium. 

Exhibit B.5 shows a schematic of a 12-chamber ozone contactor. The contactor is treating 

50 MGD of water at a temperature of 20°C. The volumes of the individual chambers are noted 

on the schematic. Ozone is added to the first chamber only. The bottom graph in Exhibit B.5 

shows the values of the ozone residual measured at the effluents of chambers 2, 5, and 8. 
Appendix B - Ozone CT Methods

Exhibit B.5 Schematic of the Ozone Contactor and the Measured Ozone Residual 

Values in Example 1

Ozone

1 2 3 4 5 6 7 8 9 12

SFA Reactive Zone

No Credit

0

0.2

0.4

0.6

0.8 C1 = 0.71 mg/L

C2

 = 0.41 mg/L

C3 = 0.20 mg/L

Ozone Residual, mg/L

3 6 9 12 15 18 21

HDT, min

50 MGD

[34,720 gpm]

104,000 gal.

Cinitial

10 11

104,000 gal.104,000 gal.104,000 gal.104,000 gal.104,000 gal.104,000 gal.104,000 gal.104,000 gal.104,000 gal.104,000 gal.104,000 gal.

24 27 30 33 36

The goal is to calculate the Cryptosporidium inactivation credit across the contactor using 

the Extended T10 and Extended CSTR methods. 

Extended T10 Method

Chamber 1 (First Dissolution Chamber) – No Cryptosporidium inactivation credit is given to 

the first dissolution chamber.

Chambers 2 through 12 (Extended T10 zone) – This zone is classified as an Extended zone. 

The Extended T10 zone calculations are applied to determine the log inactivation across each of 

the 11 chambers. Since the third ozone residual measurement, C3, is above 0.05 mg/L, and it is 

made at point in the contactor more than 50% of the volume, it is permissible to use the overall 

T10/HDT ratio for each individual chamber. The following steps are implemented: 

Step 1: Calculate k*

 value – The k

* value is calculated as described in section B.3.2.1 using the 

three ozone residual measurements, C1, C2, and C3 that are shown in Figure B.6. The values of 

k*

1−2 and k*

1−3 can be calculated using Equations B-2 and B-3 as follows:

LT2ESWTR Toolbox Guidance Manual B-24 April 2010
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-25 April 2010

* Q C  34 0  k1 2 = Ln [ ] ×  1 ,720 .71 -1

−  = ×Ln   = 0.0611 min 

Volume 1−2 C2  3×104,000 0.41

* Q C1  34,720  0.71 k1−3 = ] ×Ln   = ×Ln [   = 0.0705 min-1 Volume 1−3 C3  6×104,000 0.20

The k

* value is then calculated as the average of k*

1−2 and k*1−3 as follows:

* k * + *

k  1−2 k = 1−3  0.0611 + 0.0705 = = 0.0658 min-1    

 2   2 

A QA check shows that the values of k*

1−2 and k*1−3 are within 7.1 percent of the average k

*

value of 0.0658 min-1. This value of k

*

 is within the recommended maximum variability of 20 

percent. If this criterion were not met, then the k* value could be set at 0.0705 min-1, which is the 

higher (i.e., more rapid decay rate) of the two values. 

Step 2: Calculate Cin value – The value of Cin is calculated using the approach described in 

Section B.4.2.2. With the value of k

* calculated at 0.0658 min-1, Equations B-5 to B-7 can be 

used to calculate the Cin value as follows:

 * [Volume]  −   104,000 C 0 1

in,1 = C1×exp ×  =  k  0.71×exp0.0658×  = 0.86 mg/L 

 Q   34,720 

 * [Volume]0−2   4×104,000  Cin,2 = C2 ×exp  = ×  ×   k ×  0.41 exp 0.0658 = 0.90 mg/L 

 Q   34,720 

 * [Volume]0−3   7×104,000  Cin,3 = C3 ×exp k × = 0.20×exp0.0658×    = 0.79 mg/L 

 Q   34,720 

Therefore, 

Cin,1 + Cin, 2 + Cin,3  0.86 + 0.90+ 0.79 Cin =   =   = 0.85 mg/L

 3   3 

Step 3: Calculate the value of k10 – The value of k10 for the inactivation of Cryptosporidium

with ozone at the measured temperature of 20°C can be obtained from equation 11-2 directly and 

equals 0.2555 L/mg-min.
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-26 April 2010

Step 4: Calculate the Ozone Residual at the Effluent of Each Chamber – Knowing the values of 

C and k

* in , the ozone concentration at the effluent of each chamber within the Extended T10 zone 

can be calculated. These values are calculated using Equation B-1: 

[ ] 































×−×= −

Q

Volume CC Exp k X

X in

* 0

where CX is the calculated concentration at a location “X” along the water path through the 

Extended T10 zone. For example, the residual concentration at the effluent of chamber 4, C4,out, 

is calculated as:















 



 



 ×

×−×=

720,34

000,1043 C ,4 out 85.0 Exp 0658.0 = 0.47 mg/L 

Note that the Extended T10 zone begins at the effluent of Chamber 1, which makes the subscript 

to [Volume] in the equation above depicted as “1-4.” Exhibit B.6 lists the calculated residual 

values for each chamber using the same approach, beginning with chamber 2. 

Exhibit B.6 Application of the Extended T10 Method to the Example 

Vol./Chamber = 104,000 gallons

Flowrate = 34,720 gpm

Cin = 0.85 mg/L

k

*

 = 0.0658 min-1

k10 = 0.2555 L/mg-min

T10/HDT = 0.65

(1) (2) (3) (4) (5) (6)

HDT from Calculated Integrated

Entrance of Zone Cout Log Residual Log

Chamber HDT, min mg/L Inactivation mg/L Inactivation

2 3.0 0.70 0.35 0.77 0.38

3 6.0 0.57 0.29 0.63 0.32

4 9.0 0.47 0.23 0.52 0.26

5 12 0.39 0.19 0.43 0.21

6 15 0.32 0.16 0.35 0.17

7 18 0.26 0.13 0.29 0.14

8 21 0.21 0.11 0.24 0.12

9 24 0.18 0.09 0.19 0.10

10 27 0.14 0.07 0.16 0.08

11 30 0.12 0.06 0.13 0.07

12 33 0.10 0.05 0.11 0.05

Sum = 1.7 Sum = 1.9

C = Cout C = Cint
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-27 April 2010

Step 4: Calculate Log Inactivation – To calculate the log inactivation across a chamber using the 

T10 method, the values of C, T10 and k10 are required. The value of k10 for the inactivation of 

Cryptosporidium with ozone at the water temperature of 20oC was determined earlier at 

0.2555 L/mg-min. Using Exhibit 11.5, the value of C used in calculating CT in a reactive 

chamber can be set equal to Cout or to the integrated residual, Cint. Exhibit B.6 lists the Cout and 

Cint values for each chamber in the Extended T10 zone, as well as the associated log inactivation 

using the following equation: 

















××= ×

HDT

T

Log CkonInactivati HDTchamber

10

10

For example, if C is set to Cout, the log inactivation achieved in chamber 4 is calculated as:

logs23.065.0347.02555.0 10

4 10 4, 4 =×××=















×××=

HDT

T

Log CkonInactivati out HDT

On the other hand, if C is set to the Cint, then Cint is calculated using the following equation: 

( )

















− =

outX

X in

X in outX

X

C

C

Ln

CC

C

,

,

, ,

,int

For chamber 4, Cint is calculated as:

( ) 52.0 mg/L

47.0

57.0

47.057.0

,4 int =





 





− =

Ln

C

The log inactivation achieved in chamber 4 is then calculated as:

logs26.065.0352.02555.0 10

4 10 4, 4 =×××=















×××=

HDT

T

Log CkonInactivati out HDT

Column (4) in Exhibit B.6 lists the log inactivation values calculated for chambers 2 through 12 

after setting C equal to Cout. Column (6) in Exhibit B.6 lists the log inactivation values calculated 

for chambers 2 through 12 after setting C equal to Cint. The sum of the log inactivation achieved 

(totals of Columns 4 and 6 in Exhibit B.6) is 1.7 logs using the Cout approach, and 1.9 logs using 

the Cint approach. 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-28 April 2010

Extended CSTR Method

Chamber 1 (First Dissolution Chamber) – No Cryptosporidium inactivation credit is given to 

the first dissolution chamber.

Chambers 2 through 12 (Extended CSTR zone) – This zone is classified as an ExtendedCSTR zone. The Extended CSTR calculations (Section 4.3) are applied to determine the log 

inactivation across each of the eleven chambers. The following steps are implemented 

Step 1: Calculate k* value – The k

*

 value is calculated as described in section B.4.3.2.1 using the 

three ozone-residual measurements, C1, C2, and C3 that are shown in Figure B.6. The values of 

k*

1−2 and k*1−3 can be calculated using Equations B-2 and B-3 as follows:

 

 1    1  * N Q C N

1−   1−  2 × 1 2  3×34,720  0.71 3

k  2 =   1− [ ] −   −   1 = [ ] 1 = 0.0670 min-1 Volume  

1−2  C2  3×104,000  0.41       

  1      1  * N Q  N

1 ×   −3  C1  1−3  6×34,720  0.71 k =   [ − 1 =  6

1−3   −  1 -1  = 0.0785 min 

Volume]  

1−3  C3  [6×104,000] 0.2        

The k

*

 value is then calculated as the average of k*

1−2 and k*1−3 as follows:

k * + *  * 1−2 k1−3 0.0670 + 0.0785 k =   = -1   = 0.0728 min 

2   2   

A QA check shows that the values of k* and k* 1−2 1−3 are within 8 percent of the average k

*

 value 

of 0.0728 min-1. This value of k

*

 is within the recommended maximum variability of 20 percent. 

If this criterion were not met, then the k* value could be set at 0.0785 min-1, which is the higher 

(i.e., most rapid decay rate) of the two values. 

Also note that the k* value determined by the Extended T10 method is not the same as that 

determined by here by the Extended CSTR method. This is to be expected since the two methods 

use different conceptual hydrodynamic models to determine the ozone profile. 

Step 2: Calculate Cin value – The value of Cin is calculated using the approach described in 

Section 4.2.2. With the value of k

* calculated at 0.0728 min-1, Equations B-5 to B-7 can be used 

to calculate the Cin value as follows: 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-29 April 2010

 [Volume] 

N0−1

 , = *  [104,000] 1

Cin 1 C1×1+k ×

0−1

 = 0.71×1+0.0728×  = 0.86 mg/L 

 N0−1×Q   1×34,720

 [ e]  0 2

* Volum N −

0−2  [4×104,000]

4

Cin, 2 = C2 ×1+k ×  = 0.41×1+0.0728×  = 0.90 mg/L 

 N0−2 ×Q   4×34,720 

 * [Volume] 

N0−3

0−3  [7×104,000]

7

Cin,3 = C3 ×1+ k ×  = 0.20×1+0.0728×  = 0.80 mg/L

 N0−3 ×Q   7×34,720 

Therefore, 

Cin,1 + Cin, 2 + Cin,3  0.86+ 0.90+ 0.80 Cin =   =   = 0.85 mg/L

 3   3 

Step 3: Calculate the value of k10 – The value of k10 for the inactivation of Cryptosporidium

with ozone at the measured temperature of 20°C can be obtained from Equation 11-5 directly 

and equals 0.2555 L/mg-min. Otherwise the value for k10 could be determined using Equation 

11-4. 

Step 4: Calculate the Ozone Residual at the Effluent of Each Chamber – Knowing the values of 

C and k

* in , the ozone concentration at the effluent of each chamber within the Extended CSTR 

zone can be calculated. These values are calculated using Equation B-1:

C C = in

X

 * [Volume] N0 X

+ × 0−X  −

1 k   N0−X ×Q 

where CX is the calculated concentration at the effluent of chamber “X”. For example, the 

residual concentration at the effluent of chamber 4 is calculated as:

0.85 C4 = = 0.47 mg/L 

 [3×104,000]

3

1+0.0728×   3×34,720 

Note that the Extended CSTR zone begins at the effluent of Chamber 1, which makes the 

subscript to [Volume] in the equation above depicted as “1-4”. Exhibit B.7 lists the calculated 

residual values for each chamber using the same approach, beginning with chamber 2. 
Appendix B - Ozone CT Methods

Exhibit B.7 Application of the Extended CSTR Method to the Example 

Vol./Chamber = 104,000 gallons

Flowrate = 34,720 gpm

Cin = 0.854 mg/L

k

*

 = 0.0728 min-1

k10 = 0.2555 L/mg-min

(1) (2) (3) (4)

HDT from Calculated

Entrance of Zone Cout Log

Chamber HDT, min mg/L Inactivation

2 3.0 0.70 0.35

3 6.0 0.58 0.30

4 9.0 0.47 0.26

5 12 0.39 0.23

6 15 0.32 0.19

7 18 0.26 0.16

8 21 0.21 0.14

9 24 0.18 0.12

10 27 0.14 0.10

0.12 0.08

0.10 0.07

Sum = 2.0

11 30

12 33

LT2ESWTR Toolbox Guidance Manual B-30 April 2010

Step 4: Calculate Log Inactivation – Knowing the values of C, k10, and k

*

, Equation 11-4 is used 

to calculate the log inactivation achieved in each chamber in the Extended CSTR Zone:

[ ] 



 



 − +=

Q

Volume

Log inactivati Logon Ck X

10 X 303.21

where CX is the effluent residual concentration from Chamber X, CX,out (see Exhibit 11-

3), while [Volume]X is the volume of that chamber. For example, for chamber 4, if the CX is set 

at CX,out, then the log inactivation achieved in chamber 4 is calculated as: 





 



 − ×××+=

720,34

000,104 Log inactivati Logon 47.02555.0303.21 = 0.26 logs 

Column (4) in Exhibit B.7 lists the log inactivation values calculated for chambers 2 

through 12 under this approach. The sum of the log inactivation achieved (total of Column 4 in 

Exhibit B.6) is 2.0 logs. 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-31 April 2010

Exhibit B.8 shows a schematic of a non-conventional ozone contactor. The contactor is 

treating 12.5 MGD of water at a temperature of 8°C. The contactor is comprised of a long and 

narrow channel with no baffle walls. Ozone is added into the raw water line upstream of the 

contactor. Ozone analyzers are installed at three locations along the length of the contactor. The 

bottom graph in Exhibit B.8 shows the values of the rolling average ozone residuals reported by 

the ozone analyzers at the three monitored locations. It is noted that the contactor length:width 

ratio is 90:12 (7.5:1), while the length:height ratio is 90:9 (10:1), both of which are greater than 

the minimum desired value of 5:1. The goal is to calculate the Giardia inactivation credit across 

the contactor using the Extended T10 and Extended CSTR methods. 

Because non-conventional contactors do not possess specific zones, such as the baffle 

gaps in conventional over-under contactors, where a high degree of mixing can create 

homogeneity of solute concentrations, they pose additional considerations for measuring 

representative ozone concentrations at specific HDTs in the contactor. That is, measuring ozone 

concentrations at the baffle gaps of a conventional contactor is generally regarded as providing a 

representative average ozone concentration exiting the specific chamber. In contrast, it is not as 

well understood what the exact hydrodynamics are within the non-conventional contactors, and 

consequently whether an ozone sample taken at one point along the length of the contactor is 

representative of the average concentration across the cross-section at that theoretical HDT. The 

issue of a representative ozone measurement is also true for calculated ozone residuals. 

Furthermore, since the non-conventional contactor does not have clearly defined chambers, it is 

necessary to subdivide the Extended zone of non-conventional contactors into an appropriate 

number of conceptual segments such that characteristic ozone concentrations can be calculated 

and the Extended CT10 method applied. The term “segment” is chosen here to avoid confusion 

with the term “chamber,” which may connote an actual physical chamber in a baffled contactor, 

as opposed to a conceptual or theoretical segment denoted here. 

This guidance utilizes a calculation described in numerous chemical reactor design texts 

(e.g. Levenspiel 1999; Fogler 2005) for defining the hydrodynamics measured by a tracer test in 

terms of a series of ideal CSTRs. The so-called Tanks-in-Series (sometimes called CSTR-inseries) model, as applied to non-conventional contactors, affords a method for subdividing the 

single large chamber of the contactor into multiple equal-sized conceptual segments based on an 

analysis of a tracer test. The calculations necessary are similar to those required for calculating 

the T10 value. Appendix E provides guidance on performing this calculation for both pulse input 

and step input tracer tests. 

The tanks-in-series model is typically used in reaction engineering by employing CSTRspecific reaction equations (e.g. Equations B-11 and B-13) to each of the theoretical tanks. 

Application of the tanks-in-series calculation here is used only to calculate a theoretical number 

of segments in which to subdivide the single large chamber. Once the chamber is divided into 

segments with theoretical HDTs for each, the Extended T10 method can be applied accordingly. 

This calculation is viewed as a pragmatic method to determine a reasonable number of segments. 

Taken with the discussion regarding the linear extrapolation of the overall T10/HDT ratio across 

all segments (see section B.4.2 above), it is reasonable to establish some recommendation for 

limiting the extrapolation of T10/HDT and calculation of Cx to what could otherwise be an 

infinite number of conceptual segments. 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-32 April 2010

Extended T10 Method

The entire contactor shown in Exhibit B.8 is treated as an Extended zone. Tracer analysis 

performed as per Appendix E determined a T10/HDT ratio of 0.67 Furthermore, the tanks-inseries tracer analysis determined that the tracer output corresponded to a series of 12 equal-sized

segments. The Extended zone, which is 90 ft long, has ozone sample points at approximately 

16%, 33% and 50% of the contactor volume. Since the third ozone residual measurement, C3, is 

above 0.05 mg/L, and it is measured at point in the contactor that is at least 50% of the volume, it 

is permissible to use the overall T10/HDT ratio for each of the 12 theoretical chambers, or 

segments. The Extended T10 zone calculations (Section B.4.2) are applied to determine the log 

inactivation across each segment, and thus the entire contactor. The following steps are 

implemented: 

Step 1: Calculate k* value – The k

* value is calculated as described in section B.4.2.1 using the 

three ozone residual measurements, C1, C2, and C3 that are shown in Exhibit B.8. With 12 equal 

segments, each segment would have a volume of approximately 6059 gallons. Ozone sample 

ports depicted in Exhibit B.8 are located at approximately the effluents of segments 2, 4, and 6. 

With a flowrate of 8,680 gpm, values of k*

1−2 and k*1−3 can be calculated using Equations B-2 

and B-3 as follows:

* Q C1  8,680 0.54 k1−2 = ×Ln [   ×  = 0.375 min

Volume] = Ln -1  

1−2 C2  2× 6,059 0.32

* Q C1  8,680 0.54 k1−3 = ] ×Ln [   = ×Ln   = 0.276 min-1 Volume 1−3 C3  4× 6,059 0.25

*

The k

* k k*

value is then calculated as the average of 1−2 and 1−3 as follows:

k * *

* 1−2 + k1−3  0.375 + 0.276 k =   =     2  2  = 0.325 min-1 

k* k*

A QA check shows that the values of 1−2 and 1−3 are within 15 percent of the average 

k

*

 value of 0.325 min-1. This value of k

* is within the recommended maximum variability of 20

percent. If this criterion were not met, then the k* value could be set at 0.375 min-1, which is the 

higher of the two values. 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-33 April 2010

Exhibit B.8 Schematic of the Ozone Contactor and the Measured 

Ozone Residual Values in Example 2 

90 1st Sampling Location 3rd Sampling Location

2nd Sampling Location

15 ft 15 ft 15 ft 15 ft 15 ft 15 ft

1 2 3 4 5 6

Ozone

9 ft

90 ft

0

0.2

0.4

0.6

0.8

C1 = 0.54 mg/L

C2 = 0.32 mg/L

C3 = 0.25 mg/L

Ozone Residual, mg/L

1.0

Side View

12 ft

12.5 MGD

HDT, min

Top View

Step 2: Calculate Cin value – The value of Cin is calculated using the approach described in 

Section 4.2.2. With the value of k

* calculated at 0.325 min-1, Equations B-5 to B-7 can be used to 

calculate the Cin value as follows: 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-34 April 2010

 [Volume]   2× 6,059  Cin,1 = C1 ×exp k * × 0−1  = 0.54×exp0.325×    = 0.85 mg/L

 Q   8,680 

 [Volume]   4× 6,059  Cin,2 = C p −

2 ×ex  * × 0 2  =  k  0.32×exp0.325×  = 0.79 mg/L

 Q   8,680 

 [Volume] C = C ×exp k * × 0−3   6× 6,059 

in,3 3 =   0.25×exp0.325×  = 0.98 mg/L

 Q   8,680 

Therefore, 

Cin,1 + Cin, 2 + Cin, 3  0.85 + 0.79 + 0.98 Cin =   =   = 0.87 mg/L

 3   3 

Step 3: Calculate the value of k10 – The value of k10 for the inactivation of Giardia with ozone 

at the measured temperature of 8°C can be obtained using the equation presented in B.4.2, which 

states:

Giardia k10 = 1.0380×(1.0741)

Temp

Using the above equation, the k10 value for the inactivation of Giardia with ozone at 8 o

C is 

calculated at 1.839 L/mg-min. 

Step 4: Calculate the Ozone Residual at the Effluent of Each Segment – As with a conventional 

contactor composed of multiple chambers, knowing the values of C * in and k , the ozone 

concentration at the effluent of each chamber within the Extended T10 zone can be calculated. In 

the case of non-conventional contactors, the tanks-in-series tracer analysis (see Appendix E) is 

employed to subdivide the single large chamber into multiple theoretical segments, and the 

concentration at the effluent of each segment determined as with conventional contactors. These 

values are calculated using Equation B-1: 

 [Volume]  C = C ×Exp− 0

X in k *

× −X       Q 

where CX is the calculated concentration at a location “X” along the water path through the 

Extended T10 zone. For example, the residual concentration at the effluent of the 8th theoretical 

segment, C8,out, is calculated as:

  8× 6,059  C8,out = 0.87×Exp− 0.325× 

 = 0.14 mg/L 

  8,680 

Note that the Extended T10 zone begins at the effluent of segment 1, which makes the subscript 

to [Volume] in the equation above depicted as “1-8”. Exhibit B.6 lists the calculated residual 
Appendix B - Ozone CT Methods

values for each segment using the same approach, beginning with segment 2. In many cases, the 

ozone sampling ports will be located at HDTs that do not coincide with the HDT of a theoretical 

segment. However, since values of Cx will be calculated for all the theoretical segments, it is 

problematic that the actual ozone measurements are not used as the characteristic Cx value. 

Exhibit B.9 Application of the Extended T10 Method to the Example 

LT2ESWTR Toolbox Guidance Manual B-35 April 2010

Vol./Segment =

Flowrate =

6,059 gallons

8,680 gpm

Cin =

k*

 =

k10 =

0.87 mg/L

0.325 min-1

1.839 L/mg-min

T10/HDT = 0.67

(1) (2) (3) (4)

HDT from Calculated

Entrance of Zone C Log

C = Cout

out

Segment HDT, min mg/L Inactivation

1 0.7 0.69 0.60

2 1.4 0.55 0.48

3 2.1 0.44 0.38

4 2.8 0.35 0.30

5 3.5 0.28 0.24

6 4.2 0.22 0.19

7 4.9 0.18 0.15

8 5.6 0.14 0.12

9 6.3 0.11 0.10

10 7.0 0.09 0.08

11 7.7 0.07 0.06

12 8.4 0.06 0.05

Sum = 2.7

Step 4: Calculate Log Inactivation – To calculate the log inactivation across a segment using the 

T10 method, the values of C and k10 are required. The value of k10 for the inactivation of Giardia

with ozone at the water temperature of 8 oC was determined earlier at 1.839 L/mg-min. Using 

Exhibit 11.2, the value of Cout is used in calculating CT for each segment in the Extended zone. 

Exhibit B.9 lists the Cout values for each segment in the Extended T10 zone, as well as the 

associated log inactivation using the following equation: 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-36 April 2010

 T 

Log Inactivation = k × × segment × 

 10

10 C HDT 

 HDT 



For example, the log inactivation achieved in segment 8 is calculated as:

 T

Log Inactivation = k ×C ×HDT8 ×  10 

10 ,8 = 8 out 1.839×0.14× (5.6 − 4.9)× 0.67 = 0.12 logs  HDT   

Column (4) in Exhibit B.9 lists the log inactivation values calculated for segments 1 through 12 

The sum of the log inactivation achieved (total of Column 4 in Exhibit B.9) is 2.7 logs. 

Extended CSTR Method

The Extended CSTR method is presented in this guidance primarily as a method for 

systems which choose not to perform a tracer study. For conventional multi-chamber contactors, 

the utility has the option of using the Extended CSTR method even when a tracer is available. 

This method may afford a better inactivation credit than the Extended T10 method when the 

T10/HDT ratio is considerably low (e.g. below 0.5). However, as explained in section B.4.3, the 

CSTR assumption may be somewhat conservative with regards to a multi-chamber contactor’s 

hydrodynamics, whose design is to promote plug-flow as opposed to highly mixed flow. In the 

context of the non-conventional contactor, the guidance described immediately above and in 

Appendix E uses a tracer study to closely characterize the hydrodynamics in terms of tanks-inseries. In as much as the tanks-in-series analysis calculates the exact number of segments 

corresponding to the contactor’s residence time distribution, there is virtually no conservatism 

with respect to the hydrodynamic model. There is minor conservatism in rounding down the 

tanks-in-series calculation to the nearest integer. Additionally, the CSTR assumption inherently 

leads to a lower reaction efficiency as compared to the complete segregation assumption. 

Nonetheless, the EPA feels that the overall lack of conservatism related to the hydrodynamic 

assumption precludes the use of the Extended CSTR method to non-conventional contactors. 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-37 April 2010

References 

Amy, G.L., P. Westerhoff, R.A. Minear, and R. Song. 1997. Formation and Control of 

Brominated Ozone By-Products. AWWA Research Foundation, Denver, CO. 

Carlson, K., K. Rakness, and S. MacMillan. 1997. Batch Testing Protocol for Optimizing Ozone 

System Design. Presented at AWWA Annual Conference in Atlanta, GA, June 15-19, 1997. 

Fogler, H.S. (2005). Elements of Chemical Reaction Engineering. Upper Saddle River, NJ, 

Prentice Hall.

Froment, G.F. and K.B. Bischoff. 2nd ed. 1990. Chemical Reactor Analysis and Design. New 

York: John Wiley & Sons. 

Gordon, G., R.D. Gauw, Y. Miyahara, B. Walters, and B. Bubnis. 2000A. Using Indigo 

Absorbance to Calculate the Indigo Sensitivity Coefficient, Journal AWWA, 92(12): 96-100. 

Gordon, G., B. Walters, and B. Bubnis. 2000B. The Effect of Indigo Purity on Measuring the 

Concentration of Aqueous Ozone, Conference Proceedings: Advances in Ozone Technology, 

Orlando, FL. International Ozone Association, Pan American Group. 

Guidance Manual for Compliance With the Filtration and Disinfection Requirements for Public 

Water Systems Using Surface Water Sources. March 1991 Edition. U.S. EPA Office of Drinking 

Water, Cincinnati, OH. 

Hoigné, J. and H. Bader. 1994. Characterization of Water Quality Criteria for Ozonation 

Processes. Part II: Lifetime of Added Ozone. Ozone Science & Engineering, Vol. 16, No. 2: pp. 

121-134. 

Kim, D.-I., M. Elovitz, P.J.W. Roberts, J.-H. Kim. 2010. Investigating and enhancing 

performance of a multi-chamber ozone contactor using 3D laser induced fluorescence. Journal 

AWWA, 102(10): 61-70. 

Kim, D., D.-I. Kim, et al. 2010. Large eddy simulation of flow and tracer transport in 

multichamber ozone contactors. Journal of Environmental Engineering 136(1): 22-31. 

Kim, D., S. Nemlioglu, et al. 2010. Ozone contactor flow visualization and quantification using 

3-dimensional laser induced fluorescence. Journal AWWA 102: 90-99. 

Lev, O. and S. Regli 1992. Evaluation of Ozone Disinfection Systems: Characteristic

Time T. J. Envir. Engrg. 118: 268-285. 

Levenspiel, O., 3rd ed. 1999. Chemical Reaction Engineering. New York: John Wiley & Sons. 

Rakness, K.L. G. Gordon, B. Bubnis, D.J. Rexing, E.C. Wert, and M. Tremel. 2001. 
Appendix B - Ozone CT Methods

LT2ESWTR Toolbox Guidance Manual B-38 April 2010

Underestimating Dissolved Ozone Residual Using Outdated or Impure Indigo, Conference 

Proceedings: International Ozone Association 15th World Congress; London, England; 

International Ozone Association - September 2001. 

Rakness, K.L. and G.F. Hunter. 2000. Advancing Ozone Optimization During Pre-Design, 

Design and Operation. AWWA Research Foundation, Denver, CO, and Electric Power Research 

Institute-Community Environmental Center, St. Louis, MO. 

Rakness, K.L., and G.F. Hunter. 2001. Monitoring and Control of Ozone Disinfection for 

Crypto, Giardia, and Virus Inactivation. Conference Proceedings of International Ozone 

Association World Congress; London, England - September 2001. 

Rakness, K.L., G. Gordon, D.J. Rexing, and E.C. Wert. 2002. Reported Ozone Residual Data 

Might Be Undervalued. Conference Proceedings: American Water Works Association Annual 

Conference; New Orleans, LA - June 2002. 

Rakness, K. L., I. Najm, et al. 2005. Cryptosporidium log-inactivation with ozone using Effluent 

CT10, Geometric Mean CT10, Extended Integrated CT10, and Extended CSTR calculations. 

Ozone Science and Engineering 27(5): 335-350. 

Roustan, M., H. Debellfontaine, Z. Do-Quang, and J. Duguet. 1998. Development of a Method 

for the Determination of Ozone Demand of Water. Ozone Science & Engineering. Vol. 20, No. 

6: pp. 513-520. 

Standard Methods for the Examination of Water and Wastewater, 20th Edition. 1998. American 

Public Health Association, American Water Works Association, and Water Environment 

Federation, pp. 4-137 and 4-138. 

Teefy, S. and P. Singer. 1990. Performance and Analysis of Tracer Tests to Determine 

Compliance of a Disinfection Scheme with the SWTR. Journal AWWA, 82(12):88-89. 

Teefy, S. et al., 1996. Tracer Studies in Water Treatment Facilities: A Protocol and Case 

Studies. Final Report. American Water Works Association Research Foundation. American 

Water Works Association, Denver, CO. 
LT2ESWTR Toolbox Guidance Manual C-1 April 2010

Appendix C

Measuring Ozone Residual

 

Accurate ozone residual data will allow the calculation of correct log-inactivation values 

and maintain optimized performance. Ozone residual measurements might be inaccurate if 

sampled or measured incorrectly. Residual measurement Quality Assurance (QA) issues include:

• Configuration of the ozone sample collection lines within the contactor.

• Stability of the indigo trisulfonate reagent when analyzing grab samples. 

• Standardization and maintenance of on-line ozone analyzers.

C.1 Sample Collection

Ozone contactors are sealed vessels that may have a single large chamber, referred to as 

non-conventional contactors, or multiple chambers (typically separated by walls or baffles with 

somewhat small openings separating the chambers) referred to as conventional contactors. Water 

samples from the interior of the contactor are collected via sample lines that penetrate the walls 

or roof structure of the contactor. Since dissolved ozone decays in water with a half-life ranging 

from less than a minute to 30 minutes for typical drinking water treatment applications, the 

ozone profile (the concentration of ozone along the general flow-path of a contactor) will vary 

significantly depending on the water quality, the method of operation, and water flow conditions 

(e.g. hydrodynamics and HDT). Consequently, the location, number, and design of sample tubes 

plays an important role in operating the ozonation process and calculating CT credit. This 

guidance, and additional information found in the SWTR, provides recommendations for 

measuring ozone residual including sample ports considerations and analytical methods. 

Placement of sample ports depends on the configuration of the contactor – conventional 

versus non-conventional. For conventional contactors, considerable experience of practitioners, 

as well as more recent studies (Shiono and Teixeira 2000; Kim, Kim, et al. 2010; Kim, 

Nemlioglu, et al. 2010; Kim, Elovitz, et al. 2010), suggest that the water flowing through the 

baffle gap (i.e. exiting one chamber and flowing into the next, see Exhibit C.1) is reasonably well 

mixed. Whether the water arrives that way from the previous chamber, or slip-streams or 

“parcels” of different water with greatly varying age distributions arrive simultaneously and mix 

thoroughly in the constricted region of the baffle gap is not well understood. However, it appears 

that for typical baffle gaps (i.e. not too wide), placement of the sample tube within the (3-

dimensional) center of the gap (1⁄4 to 1⁄2 the distance), can afford a representative sample of water 

exiting the chamber. 

Once again, because the rate of ozone decay varies considerably with operating 

conditions, a separate sample port located at the outlet of each chamber within the contactor 

allows maximum flexibility for sampling ozone residual over variable operating conditions. 

Sample ports located at the outlets of diffusion chambers should be placed to ensure the diffusers 

do not interfere with the collected sample. Gas bubbles might be carried into the sample inlet and 
Appendix C – Measuring Ozone Residual

LT2ESWTR Toolbox Guidance Manual C-2 April 2010

cause errors in the residual measurement. A sample inlet tube that is flared and that is turned 

either upward or opposite the flow of the water (depending on the location) reduces the potential 

for entrapment of gas bubbles. However in highly turbid waters, a vertical inlet and flared 

configuration might result in clogging due to solids deposition inside the line. In these cases a 

compromise is to position the sample line such that the inlet is horizontal rather than vertical. 

Exhibit C.1 Example Sample Locations in an Over/Under Baffled Bubble 

Diffuser Contactor

R

r-Cur

tion C

eac

ssol

erer

u

t

-Cur

tion C

eac

ssol i

er

R

er

r

Ozone

1 2 3 4 5 6 7 8 9 10

SFA Reactive Zone

First DChambCSTR Chamb

CounteDissolu

[minimum of 4 consecutive chambers]

Ozone

1 2 3 4 5 6 7 8 9 10

SFA Reactive Zone

First DChambCSTR Chamb

CounteDissolu

[minimum of 4 consecutive chambers]

Sample location in flow stream (typical). 

Inlet is located at a distance of 1⁄4 to 1⁄2 of 

the contactor width. Inlet might be upward 

and flared, or might be horizontal.

Minimizing the travel time through the sample line is also important, especially when the 

ozone decay rate is high (i.e., ozone half-life is short). It is desirable to minimize the travel time 

so that the ozone decay during travel from the inside of the contactor to end of the tube is <10 

percent. Exhibit C.2 shows the relationship between simulated sample line travel time and ozone 

residual loss for various ozone half-life values. For example, the travel time in the sample line 

should be less than 10 seconds if the ozone half-life is one minute, in order to maintain the ozone 

residual loss at or below 10 percent. 
Appendix C – Measuring Ozone Residual

LT2ESWTR Toolbox Guidance Manual C-3 April 2010

Exhibit C.2 Relationship Between Ozone Residual Loss and Detention 

Time through the Ozone Sample Line for Various Ozone Half-Life Values

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100 110 120

Detention Time in Sample Line (sec)

Ozone Residual (% of Initial Residual)

HL = 30 sec

HL = 1 min

HL = 2 min

HL = 5 min

Ozone Half-live

The sample line diameter should be large enough (minimum 3/8th inch inside diameter 

and preferably 1⁄2-in to 3⁄4-inch) to minimize clogging of the line with suspended solids. Sample 

pipe diameter and flow rate should be selected in order to: 

• Maintain consistent flow without plugging. 

• Minimize detention time in the sample line. 

• Meet flow rate requirements of an on-line analyzer installed at that location. 

Gravity flow is all that is necessary to meet sample flow requirements in most locations. 

In other cases, pumping is necessary. Sample lines might contain some gas bubbles as well as 

liquid. It is important to ensure that lines are vented in high spots where gas binding might occur. 

Gaseous ozone in high concentrations is hazardous to breathe. Sample line vents and drains 

should be directed away from occupied areas. Section O.3.2 of Appendix O of the Guidance 

Manual for Compliance with the Filtration and Disinfection Requirements for Public Water 

Systems Using Surface Water Sources (U.S. EPA 1991) (commonly referred to as the SWTR 

Guidance Manual) includes further information regarding direct measurement of dissolved 

ozone. 

This guidance also addresses the use of the Extended T10 method for non-conventional 

contactors. Because these contactors are typically, by definition, dominated by a single large 

chamber, there are no distinct physical structures (e.g. baffle gaps) that tend to provide a wellmixed environment from which to collect a representative water sample with an estimable HDT. 

Consequently, the question arises as to how to collect a representative sample, and whether a 

basic measure of the distance along the longitudinal transect of the theoretical flow-path is a 

good measure of the HDT associated with a sample port. There is not sufficient knowledge 
Appendix C – Measuring Ozone Residual

LT2ESWTR Toolbox Guidance Manual C-4 April 2010

regarding the range of hydrodynamic conditions that might be present at any point within the 

array of non-conventional contactor geometries. However, in this guidance, the EPA has taken a 

pragmatic approach, based on considerable input from stakeholders, that for contactors with 

sufficiently long chambers (relative to the height and width), the distance along the longitudinal 

transect is a reasonable proxy for HDT. In addition, water samples representative of the crosssection of the contactor at that HDT can be collected with proper placement of a sample tube.

In this consideration, it is recognized that the inlet and outlet to most non-conventional 

contactors, regardless of the overall length, are not designed for optimal hydrodynamic 

efficiency. As a result, the hydrodynamics in the regions of the inlet and outlet may be prone to 

dead volumes, short-circuiting and large eddy effects. In contrast, if the length-scale of the 

chamber is large with respect to the cross-sectional dimensions, the hydrodynamics may become 

more developed, more plug-flow-like and homogeneous (across the cross-section) within the 

majority of the length of the contactor. Once again, in a pragmatic approach to develop a useful 

method for calculating CT credit in such contactors, the EPA proposes that representative water 

(ozone) samples can be collected in the same manner as samples from conventional contactors 

provided the contactor meets certain geometric criteria. In the context of assuming potentially 

poor hydrodynamics at the inlet and outlet regions, but more homogenous flow elsewhere, nonconventional contactors should have high length-to-width (L:W) and length-to-height (L:H) 

ratios. 

To employ the Extended T10

All other considerations regarding sample port design follow those of 

conventional contactors. 

 method, a minimum of three sample ports is needed. 

Because of the potential to have considerable inlet/outlet effects, placement of the sample tubes 

is important, and consideration should be given to placement in the areas believed to have more 

developed, steady-state flow. The EPA suggests the installation of additional ports to allow 

flexibility in monitoring and control. Moreover, the EPA suggests an assessment of the shortterm variability in the ozone measurement from any sample port to help determine if the 

hydrodynamics in that region are irregular (unsteady and non-ideal) or relatively stabile (more 

plug-flow-like). For example, if multiple ozone samples collected over a short time-span during 

steady operating conditions demonstrate high variability (e.g. > 20%) in ozone concentration, 

then this may indicate that the hydrodynamics of the contactor in that region are not conducive to 

affording a representative ozone sample. 

C.2 Ozone Residual Measurement

Ozone residual should be determined using the Indigo Method (Standard Methods 4500-

Ozone – 20th Edition 1998) when analyzing grab samples. The method assumes that high-purity 

reagents are used. Since the publication of the 20th Edition, several reports (Gordon et al. 2000a 

and 2000b, Rakness et al. 2001, Rakness and Hunter 2001, and Rakness et al. 2002) have been 

published discussing a potential biasing in the Indigo Method. The potential biasing involves the 

value of the so-called “sensitivity factor,” f, as defined in the Standard Method. In short, these 
Appendix C – Measuring Ozone Residual

LT2ESWTR Toolbox Guidance Manual C-5 April 2010

reports suggest that the actual sensitivity factor might be lower than the Standard Method’s 

value, and hence the calculated ozone concentration will be undervalued.

The Standard method’s proportionality constant, f, (0.42 L mg-1

cm

-1) that is used to 

calculate the ozone residual is based on an indigo trisulfonate molar absorbance, ε, of 20,000 M-1

cm

-1

• The source and age of the neat indigo trisulfonate solid. 

. These recent reports suggest that f may not be constant and may depend on: 

• The age and handling of the indigo stock solution that is prepared as part of the method. 

Briefly, these reports indicate that, due to either of the above aspects, f can be 

substantially lower than 0.42 L mg-1cm

-1. In other words, the molar absorbance can be much 

lower than 20,000 M-1

cm

-1. Gordon et al. (2000a and 2000b), Rakness et al. (2001), Rakness and 

Hunter (2001), and Rakness et al. (2002) reported that the apparent molar absorbance of some 

indigo stock solutions might be as low as 11,000 M-1

cm

-1, and in an extreme case 6,000 M-1

cm

-1. 

The authors suggest that the ramifications of applying an f value of 0.42 L mg-1cm

-1

The gravimetric indigo trisulfonate method is fairly easy to apply in the field and is 

accurate. It should be noted that the method described herein is somewhat different than the 20

 when the 

solution has a lower true f value are the underestimation of the ozone concentration. EPA does 

not have information that these issues are resolved at the time of issuing the guidance manual. 

th

• Prepare indigo stock solution as described in Standard Methods. 

Edition of Standard Methods in that the volume of both the blank and the samples are determined 

gravimetrically. The procedural steps include:

• Prepare Reagent II solution (for ozone residuals greater than 0.05 mg/L), as described in 

Standard Methods. 

• Prepare flasks for sampling. 

• Clean, dry and label several 125 mL Erlenmeyer flasks (enough for each sample plus one 

blank). 

• Obtain the tare weight of each flask.

• Add 10.0 mL of Reagent II solution to each flask. 

• Add approximately 90 mL of distilled water to one or two flasks and use these flasks as 

the blank (i.e., use value from one blank or average of values from two blanks). 

• Collect ozone sample.

• Thoroughly flush sample line to be used. 
Appendix C – Measuring Ozone Residual

LT2ESWTR Toolbox Guidance Manual C-6 April 2010

• Do not run sample down the side of the flask, as this will cause ozone off-gassing. 

• Fill flask with sample, gently swirling flask until a light blue color remains. Do not 

bleach completely or the residual value will be incorrect.

• Wipe-dry the outside of sample and blank flasks.

• Weigh sample and blank flasks.

• Total weight for sample is tare weight of flask plus 10 mL indigo plus added sample.

• Total weight for blank is tare weight of flask plus 10 mL indigo plus added distilled 

water.

• Prepare the spectrophotometer for measuring absorbance.

• Identify the cell path length (e.g., 1 cm, 5 cm, etc.). 

• Set the wavelength to 600 nanometers. 

• Measure absorbance of blank and samples within four hours. 

• Follow instructions for spectrophotometer concerning zeroing the instrument. 

• Record absorbance of each sample and each blank.

• Complete calculations – see example below.

Example: 

A 10 mL aliquot of Reagent II solution was added to a 125 mL Erlenmeyer flask that was 

used for the blank. The flask had a tare weight of 83.62 g. The final weight of the flask, plus the 

10 mL aliquot of reagent, plus the added distilled water was 179.77 g. The total volume of the 10 

mL Reagent II aliquot plus added distilled water was determined by subtracting the bottle’s tare 

weight from the total weight, assuming that 1 mL of liquid weighs 1 g (96.15 mL = [179.77 g – 

83.62 g] * 1 mL / 1 g). 

The spectrophotometer had a path length of 1 cm. The absorbance reading of the 

gravimetric blank was measured as 0.234 cm

-1 at wavelength of 600 nm. This reading must be 

corrected for the difference in the volume of the blank used in order to check the quality of the 

reagent. The calculated absorbance of a 1:100 blank dilution can be determined using Equation 

C-1. In this case, the 1:100 absorbance value was 0.225 cm-1, which is greater than or equal to 

0.225 cm-1. This means that the indigo trisulfonate solution was considered acceptable. 
Appendix C – Measuring Ozone Residual

LT2ESWTR Toolbox Guidance Manual C-7 April 2010

 Blank of Volume in Absorbance cm 100 @ mL

 100 mL

Path Length

Absorbance

1- × =

















 (C-1) 

0.234 

 

 1 cm  1- 96.15 mL 0.225 cm

 100 mL × =

The 125 mL flask that was used for the ozone sample had a tare weight of 94.10 g. 

Sample water was directed into the 10 mL of Reagent II solution until a light blue color 

remained. The final weight of the flask, plus the 10 mL aliquot plus the sample, was 167.39 g. 

The absorbance reading at a path length of 1 cm was 0.159. The volume of the water sample was 

63.29 mL (63.29 mL = [167.39 g – 94.10 g – 10 g] * 1 mL / 1 g). The ozone residual was 

calculated using Equation C-2, which resulted in a value of 0.41 mg/L. 

( ) ( )

b V f

V A - V A mg/L = 

S

TSBB

××

× ×

(C-2) 

where AB = absorbance of the blank (as measured, not as corrected by equation C-1) 

 AS = absorbance of the sample

 VB = volume of the blank plus indigo, mL 

 VT = total volume of the sample plus indigo, mL 

 VS = volume of the sample (total weight – tare weight – 10)

 f = 0.42 

 b = path length of cell, cm 

( ) ( ) 0.41 mg/L 1 63.29 0.42

73.29 0.159 - 96.15 0.234 = ××× ×

C.3 On-line Ozone Residual Analyzer Calibration

On-line ozone residual analyzers are available that can continuously monitor ozone 

residual in the water. This makes it possible to automate the disinfection credit calculation using 

the plant’s computer-control system. However, the analyzers must be maintained properly and 

their calibration must be checked periodically so that readings match grab-sample results that are 

based on the indigo trisulfonate procedure. Generally, probe-type monitor readings tend to drift 

downward over time due to weakening of the electrolyte solution. Calibration checks should be 
Appendix C – Measuring Ozone Residual

LT2ESWTR Toolbox Guidance Manual C-8 April 2010

conducted regularly, such as at least once per week. This section describes a calibration check 

protocol which involves collecting grab-samples and analyzer readings simultaneously and 

comparing the values. 

The calibration check should consist of collecting at least three, and preferably five, 

ozone residual grab samples and corresponding analyzer readings. The following calibration 

protocol has been used successfully at operating ozone facilities. 

• Collect three to five grab-sample ozone residuals. Obtain an analyzer reading while the 

grab sample is being collected. Wait 15 seconds to 30 seconds between each pair of grab 

sample and analyzer reading. 

• Measure the ozone residual concentration in the grab samples using the indigo 

trisulfonate method. 

• Calculate the average grab-sample ozone residual value and the average analyzer ozone 

residual value. 

• Compare the average of the on-line analyzer to that of the indigo grab-samples. The 

average of the on-line analyzer should not deviate more than 10 percent or 0.05 mg/L 

(which ever is largest) from the grab-sample average. If the average of the on-line 

analyzer deviates more than this, then adjust the meter reading per the manufacturer’s 

instructions. Note that this QA control is two-sided. It is especially important that the online analyzer not record more than 10 percent or 0.05 mg/L greater

• Allow the analyzer to stabilize for a period of 30 minutes after adjusting the meter 

reading and repeat steps 1 through 4 until the difference calculated in step 4 is <10

percent of the grab-sample average and <0.05 mg/L. 

 than the grab samples. 

However, a negative deviation (negative bias), while not effecting public health, may also 

be useful as an indication of a malfunctioning unit.
Appendix C – Measuring Ozone Residual

LT2ESWTR Toolbox Guidance Manual C-9 April 2010

C.4 References

Kim, D.-I., M. Elovitz, P.J.W. Roberts, J.-H. Kim. 2010. Investigating and enhancing 

performance of a multi-chamber ozone contactor using 3D laser induced fluorescence. 

JAWWA, 102(10): 61-70.

Kim, D., D.-I. Kim, et al. (2010). Large eddy simulation of flow and tracer transport in 

multichamber ozone contactors. Journal of Environmental Engineering 136(1): 22-31. 

Kim, D., S. Nemlioglu, et al. (2010). Ozone contactor flow visualization and quantification 

using 3-dimensional laser induced fluorescence. JAWWA 102: 90-99. 

Shiono, K. and E. C. Teixeira (2000). Turbulent characteristics in a baffled contact tank. J. 

Hydraul. Res. 38(4): 271-416. 
LT2ESWTR Toolbox Guidance Manual D-1 April 2010

Appendix D

Derivation of Extended CSTR Equations

 The discussion presented in the document used some key equations and relied on specific 

assumptions. In this appendix, one key equation is derived, and one key assumption is discussed 

and justified. 

D.1 Derivation of the Equation Used to Calculate k*

 In Appendix B, Equation B-2 expressed the value of k

*

 between two points 1 and 2 as 

shown by Equation D-1:

[ ] 



















− 













×  =





 





−

− −

−

1

21

1

2

1

21

* 21

21

N

C

C

Volume

QN k (D-1) 

 Equation D-1 is a transformation from the equation of first-order decay across a series of 

N equal-size continuous stirred tank reactors (CSTRs): 

21

21

* 21

1

2

1

1

−





 





















+

= 















−

−

N

N

HDT k

C

C (D-2) 

 The derivation of this equation can be found in many reference texts on modeling 

chemical reactors (e.g., Froment et al., 1990, Levenspiel 1999). Since hydraulic detention time 

(HDT) is equal to the volume between locations 1 and 2, [Volume]1-2, divided by the flowrate, Q, 

then Equation D-2 is transformed to Equation D-3: 

[ ] 21

21

* 21

21

1

2

1

1

−

































×

+

= 















−

−

−

N

NQ

Volume

k

C

C (D-3) 

Therefore,

[ ]















 =

































×

+

−

−

−

−

2

1

21

* 21

21

21

1

C

C

NQ

Volume

k

N

(D-4) 
Appendix D – Derivation of Extended CSTR Equations

LT2ESWTR Toolbox Guidance Manual D-2 April 2010

then, 

[ ] 21

1

2

1

21

* 21

21 1 −















 =

































×

+

−

−

−

N

C

C

NQ

Volume

k (D-5) 

then, 

[ ]

















− 













 =















×

−

−

−

− 1

21

1

2

1

21

* 21

21

N

C

C

NQ

Volume

k (D-6) 

and then, 

[ ] 















− 













×  = −

−

− − 1

21

1

2

1

21

* 21

21

N

C

C

Volume

NQ k (D-7) 

 As noted, Equation D-2 is based on the fundamental assumption that the hydrodynamic 

profile through the volume separating locations 1 and 2 can be approximated by a series of N

equal-size CSTRs. If equal-size chambers separate locations 1 and 2, then each chamber is 

somewhat conservatively assumed to be an ideal CSTR, with HDT = [Volume]/Q, and the value 

of N in the above derivation is set equal to the number of chambers between locations 1 and 2. 

However, it was recognized that not all ozone contactors are configured with equal-size 

chambers in series. It is possible to treat each chamber as its own CSTR and have a series of 

unequal-size CSTRs. An expression of C2/C1 similar to that shown in Equation D-2 is still 

possible. For example, if locations 1 and 2 were separated by three CSTRs with HDT values of 

HDTa, HDTb, and HDTc, the ratio of C2/C1 for a first-order decay reaction can still be expressed 

as:

[ ( )] [ ( )] [ ( )] a b HDTc C k HDT k HDT k

C

*

21

*

21

*

21 1

2

1

1

1

1

1

1

− − + −

×

+

×

+

= 













 (D-8) 

Or in general terms,

∏[ ] ∗ + =















i HDT i C k

C

)(1

1

1

2 (D-9) 

 Unfortunately, it is not possible to transform Equation D-9 to derive a simple linear 

expression of k* as a function of the other measured parameters when the number of chambers is 

greater than three. To maintain a singular methodology for any number of chambers, and to 
Appendix D – Derivation of Extended CSTR Equations

LT2ESWTR Toolbox Guidance Manual D-3 April 2010

allow the calculation to be performed in conventional spreadsheets and plant computer control 

systems, a compromise was to assume equal-volume CSTRs. With this assumption, Equation 

D-1 is used to calculate the value of k* between two sampling locations regardless of the number 

and sizes of chambers between the two locations. 

 The simplifying assumption of equal-size CSTRs for calculating k* is non-conservative 

relative to a k* value calculated by allowing for unequal-sized chambers. That is, for first-order 

ozone decay reaction, unequal-sized CSTR reactors in series would be the least efficient (ideal) 

reactor configuration for promoting ozone decay. Hence, calculating k* based on equation D-9 

gives the largest, or most conservative, value of k*. The model of equal-sized CSTR reactors in 

series is a more efficient configuration for promoting ozone decay. Hence, calculating k* from 

Equation D-1 (based on equation D-2) gives a less conservative estimate of k*. To take the 

comparison to the opposite extreme, calculating k* based on a plug-flow assumption (e.g., 

Equation 4-7) gives the smallest, or a non-conservative, estimate of k*. 

The impact of the simplifying equal-sized CSTR assumption on the estimate of k* and 

Cin involves several considerations. The first issue is the quantitative difference between the 

most conservative estimate, based on Equation D-9, and the recommended approach based on 

Equation D-2. This is essentially an issue of what chemical and hydrodynamic conditions affect 

the efficiency of the ozone decay reaction. This is a somewhat complex issue dependent on the 

reaction rate (represented by the Damköhler I Number, Da1 [Da1= k*×HDT]), the number of 

chambers considered, and the disparity in volumes among the unequal-sized chambers. In 

principle, as the reaction rate increases, the number of chambers approaches two (the minimum), 

and the volume differences among the chambers increases, the difference between the reaction 

efficiencies of the two reactor configurations increases. Some situations could result in 

approximately 30% differences between k* values. Other situations could results in negligible 

differences. Because of the many factors involved, it is difficult to establish qualitative rules for 

all possible cases. However, the utility and the primacy agency may consider further analysis for 

contactors with 2-3 chambers with a large volume difference and a large Da1. 

The second, and perhaps overriding, issue concerning the impact of the simplifying 

assumption is whether or not it still provides a certain element of conservatism over the true 

contactor performance. That is, an actual contactor with unequal-sized chambers might have 

reasonably good hydrodynamics such that even the equal-size CSTR assumption is conservative. 

This too, however, is very system specific, and is a difficult issue to resolve due to the numerous 

factors involved. 
LT2ESWTR Toolbox Guidance Manual E-1 April 2010

Appendix E 

Tracer Test Data Development & Analysis

E.1 Overview & Quality Assurance 

Tracer test data are required to implement the Extended T10 method. The tracer test is 

conducted only once, and its results are applicable as long as the contactor geometry remains 

unchanged. Tracer tests can be conducted in the same manner as those described in Appendix C 

of the Surface Water Treatment Rule Guidance Manual (SWTR Guidance Manual) (USEPA 

1991). Either a “step input” or a “pulse input” tracer test could be used. The step input tracer 

test consists of applying a constant dosage of a conservative chemical (tracer) at the head of the 

ozone contactor and measuring the concentration of the tracer chemical at the outlet of the 

contactor at selected time intervals, while maintaining constant flowrate through the contactor. 

The pulse input tracer test (also called a slug dose test) consists of a rapid injection of a specific 

mass of a tracer chemical at the head of the ozone contactor over a very short period of time, and 

then measuring the tracer concentration at the outlet of the contactor at selected time intervals, 

again while maintaining a relatively constant flowrate through the contactor. 

Appendix C of the SWTR Guidance Manual recommends that at least four tracers be 

performed at a range of expected operational flowrates. This Guidance supports the earlier 

recommendation and further strongly recommends that at least two tracer tests be conducted at 

the lowest and highest expected operational flowrates. This recommendation is based on 

possibility that hydrodynamic efficiency of the contactor could vary as a function of flowrate. 

For example, in terms of dispersion, it could be expected that the overall dispersion in a 

contactor could vary with the flowrate. For the ozone-Cryptosporidium reaction system, 

increasing dispersion leads to an effective decrease in efficiency of the relevant reactions in the 

contactor. Therefore, it is important to consider the effect of flowrate on the hydrodynamic 

efficiency. Once all the tracer tests have been evaluated, it is recommended that the utility use 

the tracer data that demonstrates the greatest “spread” (i.e. dispersion or variance) in the tracer. 

This is a measure of conservatism applied to the Extended T10 approach, and perhaps more 

important for the newly developed approach for non-conventional contactors. The added safety 

factor it provides depends strongly on the specific contactor geometry and the range of the 

expected flowrates. 

Systems considering tracer studies should contact their state regulatory agencies 

regarding the use of tracer chemicals. Commonly used tracer chemicals are fluoride or lithium 

ions. Fluoride might be added as sodium fluoride (NaF) or as fluosilicic acid (F6H2Si). Lithium 

is typically added as lithium chloride (LiCl). At the time of preparation of this document, only 

sodium fluoride and fluosilicic acid were NSF-certified additives for drinking water. As such, a 

fluoride-based chemical has been the tracer of choice in many applications. 

Lithium chloride is not yet NSF-approved, but some State regulatory agencies allow the 

use of lithium for tracer testing because of the following three primary advantages it has over 

fluoride: 
Appendix E – Tracer Test Data Development & Analysis

LT2ESWTR Toolbox Guidance Manual E-2 April 2010

1. Lithium is present at very low background concentrations (about 5 to 10 ppb) in most 

natural waters compared to the fluoride background concentration, which might range 

from 0.2 to 0.5 mg/L. 

2. Lithium can be analyzed reliably at concentrations as low as 5 ppb, which is much lower 

than the typical minimum-reporting limit for fluoride (which is about 0.1 mg/L). 

3. There is no health-based limit for lithium in drinking water. The fluoride limit is 4 mg/L. 

 The range of lithium concentration during a tracer test is typically between 5 and 250 

ppb, which is a 50-fold range. The range of fluoride concentration in the tracer test is between 

about 0.5-mg/L to 3.5 mg/L, which is a 5-fold range. A broader range provides for a better 

resolution in the tracer test results. The collection of reliable tracer test data is important for 

accurate assessment of the variance as well as T10 values. The Quality Assurance criteria 

outlined below are applicable for obtaining high-quality tracer test results. Once completed, 

follow the steps concerning preparing test results for calculation of the variance of the data. 

1. At least two tracer tests should be performed for each contactor. The tracer tests 

should be conducted at the lowest and highest expected flowrates through the 

contactor. The expected flowrate is that which the utility plans on operating the 

system. If there are multiple identical parallel contactors, tracer tests can be 

conducted on one contactor and applied to the other identical contactors. Of the RTDs 

developed from the tracer tests, it is recommended that, for further use in the 

Extended T10

2. During the tracer test, the flow rate through the contactor should remain as constant 

as possible, with the maximum or minimum value remaining within 90 percent to 110 

percent of the average flowrate during the test. That is, the flow rates should be ± 10 

percent of the set flow rate.

 method, the utility use the RTD demonstrating the greatest tracer 

spread. 

3. The tracer test should be conducted over a minimum period of three hydraulic 

detention times (HDT) of the contactor. For example, if the HDT of the contactor is 

eight (8) minutes, then the minimum test duration (during which effluent samples are 

being collected) is 24 minutes. 

4. The number of samples collected during a tracer test should be maximized to generate 

the most accurate estimate of the RTD as possible. There are practical limitations to 

the number of samples that can be taken and analyzed. However, it is recommended 

that a minimum of thirty samples (30) should be collected during a tracer test. 

Samples can be collected at unequal time intervals. Sampling density should be 

focused on the inflection and peak (for pulse-input) portions of the breakthrough 

curve to minimize the numerical errors due to approximations during the RTD

calculations. This would benefit the utility by decreasing the estimated spread of the 

tracer and hence the inefficiency of the contactor hydrodynamics.
Appendix E – Tracer Test Data Development & Analysis

LT2ESWTR Toolbox Guidance Manual E-3 April 2010

5. The background concentration of the tracer chemical upstream of the point of tracer 

injection should be measured over the duration of the tracer test. The number of 

background samples collected should be no less than 20 percent of the number of 

effluent samples collected during the test.

6. For pulse input tests, the total mass of the tracer ion or chemical recovered should be 

calculated and should be between 85 percent and 115 percent of the mass added. It 

should be emphasized that this refers to the tracer ion being measured (i.e., Li+ or F-) 

and not the chemical being added (i.e., LiCl or NaF). Therefore, the mass of Li+ or Fadded is calculated and then compared to the total mass of Li+ or F7. For step-input tests, the average tracer ion or chemical concentration in the samples 

collected during the last 10 percent of the sampling period should be between 85 

percent and 115 percent of the tracer dose being added. It is also noted here that the 

concentration referenced is that of the tracer ion being measured (such as Li

 recovered from 

the contactor effluent over the course of the tracer test.

+ or FE.2 Development & Analysis of Tracer Test Data for Variance Calculation 

), 

and not the tracer chemical being added (i.e., LiCl or NaF). 

The tracer test characterizes the hydrodynamic patterns inside a flow-through contactor. 

Details concerning tracer tests are described in Appendices C and O of the SWTR Guidance 

Manual, in Teefy & Singer (1990), and Teefy (1996). The two types of tracer tests - step tracer 

test and pulse (or slug) tracer test - differ in how the test is conducted, but they result in nearly 

the same final set of values that are required for the implementation of the extended T10

The tracer data analysis presented here outlines the procedure for transforming the tracer 

data to a residence time distribution (RTD), or exit age distribution E. In addition, the 

procedures allow for calculation of the mean residence time (t

approach. The guidance outlined here generally follows precedence set in Appendices C and O

of the SWTR Guidance Manual. In particular, Appendix C describes the technique for 

spreadsheet-style numerical integration of tracer data using right rectangle rule integration step 

(described on page C-21, Appendix C simply as the “rectangle rule”). Appendix O in turn 

describes a technique for numerical differentiation of the tracer data using the forward 

differentiation technique. These two techniques take slightly different approaches to step-wise 

treatment of the data; an aspect that is apparent only for treatment of the step dose data. 

m) (which is defined as the firstmoment of the RTD) and the variance of the RTD (also called the second moment about the 

mean of the RTD). Finally, calculations are outlined for determining the cumulative distribution 

function F (also called F-curve, which is useful in determining the T10

E.2.1 Pulse-Input Tracer Test Data Development & Analysis 

value.

An example of a pulse input tracer study test and the RTD analysis results are shown in 

Exhibit E.1. The tracer test corresponds to the Extended T10 example shown in Appendix B, 

section B.4.5, Exhibits B.8 and B.9 for the non-conventional contactor. The tracer test was 

conducted at a flow rate that resulted in a theoretical hydraulic detention time (HDT) of 8.4 

minutes. This HDT value was calculated as the total volume (in gallons) of the main large 

chamber shown in Exhibit B.8 divided by the water flow rate during the tracer test (in gpm). 
Appendix E – Tracer Test Data Development & Analysis

LT2ESWTR Toolbox Guidance Manual E-4 April 2010

In this example, a 5-gallon solution containing 10 lbs of fluoride (F-) was injected as 

quickly as possible (<15 seconds) into the influent water flow to the contactor. Samples were 

collected frequently from the feed water upstream of the point of tracer injection (background 

samples) and from the contactor effluent. The background fluoride concentration was monitored 

throughout the test duration and the average was calculated at 0.20 mg/L. The HDT of the water 

volume between the tracer addition point and the effluent tracer sampling location was 8.35 

minutes. The contactor effluent samples were collected every minute for 25 minutes. 

The first four columns in Exhibit E.1 represent the actual tracer test results. The 

subsequent columns include calculated values based on the tracer test results. Columns 5 

through 10 contain information that is required for the Extended T10 calculation for the nonconventional contactors. Column 11 contains additional information. A description of the 

contents and calculations for each column is presented below: 

Column 1: Datapoint counter. 

Column 2: Time of tracer chemical sample collection from the start of the test.

Column 3: Background tracer chemical concentration measured in the influent water during the 

test.

Column 4: Tracer chemical concentration measured at the effluent of the contactor during the 

test.

Column 5: Average background tracer chemical concentration calculated as the average of all 

the background values listed in Column 3. 

Column 6: “Effective Concentration”, which refers to the tracer concentration measured in the 

effluent of the contactor (Column 4) minus the average background tracer 

concentration (Column 5). The value of the effective concentration must be 

positive. At the beginning and at the end of the step-tracer test, the value of the 

effective concentration may be calculated as a negative value because the effluent 

tracer concentration may be slightly lower than the background concentration due to 

minor analytical errors. This is the case, for example, for the first two time points 

with measured concentrations of 0.15 and 0.19 mg/L. In such cases, the value of 

effective concentration must be set to zero instead of a negative value. 

Column 7: The values in Column 7 are the product of the effective concentration (Column 6) 

and the preceding timestep (i.e. ti – ti-1). For example, the effective concentration at 

datapoint #8 (2.27 mg/L) is multiplied by the difference between the elapsed time 

for datapoint #8 (7 minutes) and the elapsed time for datapoint #7 (6 minutes). 

All the values Column 7 are summed at the bottom Column 7. This sum calculates 

the area under the pulse response curve according the equation 

area = ∫Cdt ≅ ∑Ci∆ti

i
Appendix E – Tracer Test Data Development & Analysis

LT2ESWTR Toolbox Guidance Manual E-5 April 2010

where ∆ti = ti − ti−1 . 

It has units of [mass × time/volume] and is used to calculate the total mass 

recovered during the tracer test. This is examined further below. 

This summation is an example of the right rectangle rule for integration established 

in Appendix C of the SWTR Guidance Manual. Note, that due to the process of 

using the previous timestep, the first cell in this column is blank (or “0”). 

Column 8: The values in Column 8 are a simple multiplication of the time (Column 2) and the 

value in Column 7. 

All the values Column 8 are summed at the bottom Column 8. This sum represents 

the integral

sum = ∫tCdt ≅ ∑tiCi∆ti

i

where ∆ti = ti − ti−1

 In addition, this value is used in the calculation of the mean residence time, tm, by 

dividing the sum of Column 8 (128.27) by the sum of Column 7 (14.29). 

∫tCdt sum column 8

tm = ≅ ∫Cdt sum column 7

 In this example, a value of 8.97 minutes is calculated. This compares well with the 

theoretical HDT of 8.35 minutes. Differences may arise due to experimental error 

in conducting the tracer test or numerical dispersion related to the quality and timeresolution of the tracer data. 

 

Column 9: The values in Column 9 represent the values for the residence time distribution 

(RTD) or exit age distribution E. These values are calculated by dividing the 

effective concentration (Column 6) by the sum of the values of Column 7 (i.e. 

14.29, shown at the bottom of Column 7). For example, the value for E for data 

point #11 is calculated by dividing the concentration (1.81; Column 6) by 14.29. 

Column 10: The values in Column 10 are calculated as the product of several terms according to 

the equation 

(t )

2

i − tm Ei∆ti

where ∆ti = ti − ti−1 , and tm is taken as the value calculated at the bottom of the 

Exhibit. Note, one should use the calculated value of the mean residence time 

rather than the theoretical HDT.

Once the values for each term in Column 10 are calculated, the sum of all values is 

made at the bottom of Column 10. ∫(t − tm )

2

Edt ≅ ∑(ti − tm )

2

Ei∆ti
Appendix E – Tracer Test Data Development & Analysis

LT2ESWTR Toolbox Guidance Manual E-6 April 2010

The summed value at the bottom of Column 10 represents the variance of the RTD 

about the mean, typically denoted by the term σ

2

. The significance of this value is 

discussed below. 

Note, this summation is again an example of the right rectangle established in 

Appendix C of the SWTR Guidance Manual. Due to the process of using the 

previous timestep, the first cell in this column is blank (or “0”). 

 

Column 11: Finally, the values in Column 11 represent the F-curve. They are calculated 

according to the equation 

F(ti) = ∫ Edt ≅ ∑Ei∆ti

i i

The value for each datapoint is calculated by multiplying the value in Column 10 by 

the preceding time step (i.e.<ti = ti – ti-1) and then adding this value to the 

preceding value of F (i.e. in the cell above). For example, for datapoint #12, 

 F12 = E12∆t12 + F11 = 0.09(11−10) + 0.76 = 0.85

The F-curve is useful for determining the T10 value as described the Appendix O of 

the SWTR Guidance Manual. 
Appendix E – Tracer Test Data Development & Analysis

LT2ESWTR Toolbox Guidance Manual E-7 April 2010

Exhibit E.1 Example of Pulse-Tracer Test Results & RTD Analysis

theoretical HDT = 8.35 min

Flow Rate = 12.5 MGD

Mass Added = 10 lbs

% 

Recovered 

Mass Recovered (tm-basis) = 9.3 lbs 93%

Mass Recovered (HDT-basis) = 8.6 lbs 86%

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)

Tracer Test Data: Avg.

Background Background Effective

Data Time Conc. Conc. Conc. Conc.

Point min mg/L mg/L mg/L mg/L Ci

∆t ti

Ci

∆t E(t) (t-tm)

2

xEi

∆t F(ti

)

1 0 0.200 0.150 0.20 0.00 0.00

2 1 0.190 0.20 0.00 0.00 0.00 0.00 0.00 0.00

3 2 0.230 0.200 0.20 0.00 0.00 0.00 0.00 0.00 0.00

4 3 0.203 0.20 0.00 0.00 0.00 0.00 0.00 0.00

5 4 0.210 0.293 0.20 0.09 0.09 0.36 0.01 0.16 0.01

6 5 0.793 0.20 0.59 0.59 2.95 0.04 0.65 0.05

7 6 0.220 1.700 0.20 1.50 1.50 8.98 0.10 0.93 0.15

8 7 2.469 0.20 2.27 2.27 15.86 0.16 0.62 0.31

9 8 0.180 2.530 0.20 2.33 2.33 18.62 0.16 0.15 0.47

10 9 2.475 0.20 2.27 2.27 20.45 0.16 0.00 0.63

11 10 0.190 2.0150 0.20 1.81 1.81 18.12 0.13 0.13 0.76

12 11 1.470 0.20 1.27 1.27 13.94 0.09 0.36 0.85

13 12 0.200 1.0931 0.20 0.89 0.89 10.68 0.06 0.57 0.91

14 13 0.780 0.20 0.58 0.58 7.50 0.04 0.65 0.95

15 14 0.200 0.490 0.20 0.29 0.29 4.02 0.02 0.51 0.97

16 15 0.380 0.20 0.18 0.18 2.65 0.01 0.45 0.98

17 16 0.200 0.312 0.20 0.11 0.11 1.74 0.01 0.38 0.99

18 17 0.262 0.20 0.06 0.06 1.00 0.00 0.27 0.99

19 18 0.220 0.250 0.20 0.05 0.05 0.84 0.00 0.27 1.00

20 19 0.218 0.20 0.02 0.02 0.29 0.00 0.11 1.00

21 20 0.200 0.210 0.20 0.01 0.01 0.13 0.00 0.06 1.00

22 21 0.210 0.20 0.01 0.01 0.15 0.00 0.07 1.00

23 22 0.190 0.203 0.20 0.00 0.00 0.00 0.00 0.00 1.00

24 23 0.195 0.20 0.00 0.00 0.00 0.00 0.00 1.00

25 24 0.200 0.201 0.20 0.00 0.00 0.00 0.00 0.00 1.00

26 25 0.200 0.20 0.00 0.00 0.00 0.00 0.00 1.00

note, all 

values must 

be ≥0

14.29 128.27 6.32

t m = 8.97

σ2 = 6.32

N = 12.7

σθ

2

 = 0.079

In Exhibit E.1, the summations at the bottom of specific columns are the information 

needed to implement the Extended T10 method for non-conventional contactors. The principal 

calculation needed is the equivalent number of tanks-in-series represented by the RTD. 

According to the tanks-in-series theory, the number of theoretical number of tanks, N, is 

determined according to
Appendix E – Tracer Test Data Development & Analysis

LT2ESWTR Toolbox Guidance Manual E-8 April 2010

t

2

N = m

σ2

As shown in Exhibit E.1, 

t

2 2

N = m (8.97) = = 2 12.7

σ 6.32

Therefore, the tracer test determined that the contactor has a hydrodynamic character similar to a 

series of 12.7 CSTRs in series. Implementation of the Extended T10 method rounds the value of 

N down to the nearest integer, and uses this value for the number of segments for subdividing the 

non-conventional contactor. In this example, the Extended T10 calculation would use 12 

segments.

Exhibit E.1 contains additional calculations that are not necessary for implementing the 

Extended T10 method, but are needed for quality control of the tracer test, or for further general 

information concerning the contactor hydrodynamics. 

The mass of the tracer chemical recovered is calculated according to: 

Tracer Mass, lbs = 0.005792×Q×tm ×∑Ci∆ti

where: Q = water flow rate during the test, MGD

tm = mean residence time determined by the RTD analysis, minutes 

∑Ci∆ti = total sum of the area under the tracer response curve, calculated as the sum of 

the terms in Column 7. 

0.005792 = conversion factor, which is 8.34 lbs/gallon divided-by 1440 min/day 

The mass of tracer chemical recovered was calculated at 8.6 lbs using the above equation, which 

is 93 percent of the tracer mass added (10 lbs). This value is within the acceptable range of 85

percent – 115 percent. For comparison, the mass of tracer recovered if the calculation is based 

on HDT instead of tm is 8.6 lbs for a 86 percent recovery.

 Exhibit E.1 also shows the value for the dimensionless variance denoted by the term, σ 2

θ , 

which is the dimensionless form of the variance, σ2. The variance represents the second moment 

abut the mean of the RTD. It is calculated according to

2

σ2 σ

θ = t

2

m

which is the inverse of the equation for determining N, such that 

2 1

σ θ = N

Aside from an alternate form of calculating N, the value of σ 2

θ is useful for comparing 

the results of different tracer studies. For example, a higher value of σ 2

θ measured at a tracer 

study of a different flowrate suggests that there is greater mixing in the contactor under that 

flowrate. When more than one tracer test is performed, such as at various flowrates as 
Appendix E – Tracer Test Data Development & Analysis

LT2ESWTR Toolbox Guidance Manual E-9 April 2010

recommended, the conservative recommendation is to use the RTD data set associated with the 

greatest variance, σθ

2

. 

 Finally, Column 11 shows the calculation of the F-curve which is one method for 

determining the T10 value for the contactor. The T10 value for this tracer study appears to be 

between 5 and 6 minutes. When interpolated properly, and divided by the mean residence time 

tm, the T10

E.2.2 Step-Input Tracer Data Development & Analysis 

ratio is obtained.

An example of a step-input tracer test data and the RTD analysis results are shown in 

Exhibit E.2. As with the pulse test above the step tracer test was conducted in the nonconventional contactor shown for the Extended T10 example in Appendix B (section B.4.5, 

Exhibits B.8 and B.9). This tracer test was also conducted at a flow rate that resulted in a 

theoretical hydraulic detention time (HDT) of 8.35 minutes. This theoretical HDT value was 

calculated as the total volume (in gallons) of the main large chamber shown in Exhibit B.8 

divided by the water flow rate during the tracer test (in gpm). The chemical feed solution 

concentration and flowrate achieved a theoretical tracer concentration of 1.7 mg/L. This tracer 

dose value is denoted as Co in the header of Exhibit E.2. 
Appendix E – Tracer Test Data Development & Analysis

12 12 0.19 1.60 0.20 1.40 0.09 0.911 1.10 0.0378 0.3425397

13 13 1.66 0.20 1.46 0.06 0.948 0.75 0.0228 0.3667422

14 14 0.18 1.70 0.20 1.49 0.04 0.971 0.49 0.0125 0.3130403

15 15 1.72 0.20 1.51 0.02 0.984 0.29 0.0066 0.2383962

16 16 0.21 1.73 0.20 1.52 0.01 0.990 0.16 0.0046 0.2238288

Exhibit E.2 Example of Step-Tracer Test Results and RTD Analysis

Co = 1.7 mg/L

Average Final Effective Conc. = 1.54 mg/L

% Steady State = 90%

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)

Tracer Test Data: Avg.

Background Background Effective

Data Time Conc. Conc. Conc. Conc.

Point min mg/L mg/L mg/L mg/L <Ci

F(ti) = 

Ci

/Cavg_max ti<Ci

E i = 

<Fi

/<ti

(t-tm)

2

xEi 

<t

0 0 0.20 0.20 0.20 0.00 0.000 0.0000

1 1 0.20 0.20 0.00 0.00 0.000 0.00 0.0000 0

2 2 0.25 0.20 0.20 0.00 0.00 0.000 0.00 0.0000 0

3 3 0.20 0.20 0.00 0.00 0.000 0.00 0.0044 0.1580832

4 4 0.22 0.21 0.20 0.01 0.01 0.004 0.03 0.0397 0.9870264

5 5 0.27 0.20 0.07 0.06 0.044 0.30 0.1017 1.6171139

6 6 0.18 0.43 0.20 0.22 0.16 0.146 0.94 0.1563 1.3955149

7 7 0.67 0.20 0.46 0.24 0.302 1.68 0.1747 0.6902496

8 8 0.21 0.94 0.20 0.73 0.27 0.477 2.15 0.1582 0.1544181

9 9 1.18 0.20 0.98 0.24 0.635 2.19 0.1252 1.811E-05

10 10 0.20 1.37 0.20 1.17 0.19 0.760 1.92 0.0907 0.092923

11 11 1.51 0.20 1.31 0.14 0.851 1.53 0.0599 0.2424492

LT2ESWTR Toolbox Guidance Manual E-10 April 2010

17 17 1.73 0.20 1.53 0.01 0.995 0.12 0.0020 0.1252382

18 18 0.20 1.74 0.20 1.53 0.00 0.997 0.05 0.0013 0.1056345

19 19 1.74 0.20 1.53 0.00 0.998 0.04 0.0018 0.1786096

20 20 0.20 1.74 0.20 1.54 0.00 1.000 0.05 -0.0011 -0.137209

21 21 1.74 0.20 1.54 0.00 0.999 -0.04 0.0018 0.2571887

22 22 0.20 1.74 0.20 1.54 0.00 1.001 0.06 -0.0011 -0.191685

23 23 1.74 0.20 1.54 0.00 0.999 -0.04 0.0006 0.1111397

24 24 0.20 1.74 0.20 1.54 0.00 1.000 0.02 0.0000 0

25 25 1.74 0.20 1.54 0.00 1.000 0.00

1.54

note, all 

values must 

be ≥ 0

13.81 7.27

tm = 8.99

σ2

 = 7.27

N = 11.1

σθ

2

 = 0.090

The first four columns in Exhibit E.2 represent the actual tracer test results. The 

subsequent columns include calculated values based on the tracer test results and are required for 

the Extended T10 calculation for the non-conventional contactors. Column 11 contains 

additional information. A description of the contents and calculations for each column is 

presented below:
Appendix E – Tracer Test Data Development & Analysis

LT2ESWTR Toolbox Guidance Manual E-11 April 2010

Column 1: Datapoint counter. 

Column 2: Time of tracer chemical sample collection from the start of the test.

Column 3: Background tracer chemical concentration measured in the influent water during the 

test.

Column 4: Tracer chemical concentration measured at the effluent of the contactor during the 

test.

Column 5: Average background tracer chemical concentration calculated as the average of all 

the background values listed in Column 3. 

Column 6: “Effective Concentration”, which refers to the tracer concentration measured in the 

effluent of the contactor (Column 4) minus the average background tracer 

concentration (Column 5). The value of the effective concentration must be 

positive. At the beginning of the step-tracer test, the value of the effective 

concentration may be calculated as a negative value because the effluent tracer 

concentration may be slightly lower than the background concentration due to 

minor analytical errors. This is the case, for example, for the first two time points 

with measured concentrations of 0.15 and 0.19 mg/L. In such cases, the value of 

effective concentration must be set to zero instead of a negative value. 

. 

Column 7: The values in Column 7 are calculated as the change between the effective 

concentration (Column 6) at this timestep and the value of the effective 

concentration of the preceding time step (i.e. <Ci = Ci – Ci-1). For example, to 

calculate the <C for datapoint #16, the effective concentration at datapoint #15 

(1.49 mg/L) is subtracted from the concentration at datapoint #16 (1.51) to obtain 

0.02 mg/L. 

All the values Column 7 are summed at the bottom Column 7. This sum calculates 

a value for the maximum concentration reached at the end of the tracer experiment. 

This value (1.54 mg/L) is shown at the top of the Exhibit as “Average Final 

Effective Conc.” This value is often determined from averaging the last few data 

points in the Column 7. This value is compared to the theoretical chemical dose 

(Co) of 1.7 mg/L. This result is discussed below in terms of quality control. 

This summation is an example of the right rectangle rule for integration, established 

in Appendix C of the SWTR Guidance Manual, for the term

∫ dC ≅ ∑∆Ci

i

where Ci = Ci − Ci−1 . 

Note, that due to the process of using the previous timestep, the first cell in this 

column is blank (or “0”). 
Appendix E – Tracer Test Data Development & Analysis

LT2ESWTR Toolbox Guidance Manual E-12 April 2010

Column 8: Column 8 determines the values for the F-curve, or the cumulative distribution 

function, F, calculated as the ratio of the effective concentration (Column 6) to the 

calculated tracer chemical dose (1.54 mg/L) determined at the bottom of Column 7. 

It is noted that all the F values must be positive. The value of F might be returned 

as negative at the beginning of the step tracer test, because the effluent tracer 

concentration is slightly lower than the background concentration due to minor 

analytical errors. In such cases, the value of F must be set to zero instead of the 

negative value. The F-curve can be used for calculating the T10 value. It is also 

used further for calculating the E-curve (Column 10). 

 

Column 9: The values in Column 9 are the product between the time at that datapoint (Column 

2) and the change in the effective concentration (Column 6) at this timestep and the 

value of the effective concentration of the preceding timestep (i.e. <Ci = Ci – Ci-1). 

This is also equivalent to multiplying the value in Column 2 and that in Column 7. 

For example, to calculate the t<C for datapoint #16, the effective concentration at 

datapoint #15 (1.49 mg/L) is subtracted from the concentration at datapoint #16 

(1.51) to obtain 0.02 mg/L, and that value is multiplied by the time for datapoint 

#16 (15 min) to obtain 0.29 for Column 9. 

 

All the values Column 9 are summed at the bottom Column 9, and the value is used 

further in the calculation of the mean residence time, tm, according to

∫tdC ∑ti∆Ci sum column 9

tm = ≅ = ∫ dC ∫ ∆Ci sum column 7

In this example, a value of 8.99 minutes is calculated. This compares well with the 

theoretical HDT of 8.35 minutes. Differences may arise due to experimental error 

in conducting the tracer test or numerical dispersion related to the quality and timeresolution of the tracer data.

As before, this summation is an example of the right rectangle rule for integration 

established in Appendix C of the SWTR Guidance Manual. Note, that due to the 

process of using the previous timestep, the first cell in this column is blank (or “0”). 

Column 10: Column 10 calculates the values for the residence time distribution (RTD) or exit 

age distribution E. These values are calculated by a process of numerical 

differentiation based on the relationship 

dF E = dt

In accord with the procedure demonstrated in Appendix O of the SWTR (page O.2-

19), a forward differentiation is used according to 

∆F F − F

E = i i i

i ≈ +1

∆t t t i i+1 − i
Appendix E – Tracer Test Data Development & Analysis

LT2ESWTR Toolbox Guidance Manual E-13 April 2010

As noted, this forward differentiation maintains a slight conservatism with regard to 

determining the E function. Because the process is a forward process, the last value 

in the column is blank (or “0”). 

As an example, to calculate the E value for datapoint #5, 

F6 − F

E = 5 0.044 − 0.004

5 = = 0.04

t6 − t5 5 − 4

The E function is used further in Column 11. 

Column 11: The values in Column 10 are calculated as the product of several terms according to 

the equation 

(ti − tm )

2

Ei∆ti

where ∆ti = ti − ti−1 , and tm is taken as the value calculated at the bottom of the 

Exhibit. Note, one should use the calculated value of the mean residence, tm, time 

rather than the theoretical HDT.

Once the values for each term in Column 11 are calculated, the sum of all values is

made at the bottom of Column 11. ∫(t − t )

2 )2

m Edt ≅ ∑(ti − tm Ei∆ti

The summed value at the bottom of Column 11 represents the variance of the RTD

about the mean, typically denoted by the term σ2. The significance of this value is 

discussed below. 

Note, this summation is again an example of the right rectangle established in 

Appendix C of the SWTR Guidance Manual. Due to the process of using the 

previous timestep, the first cell in this column is blank (or “0”).

 In Exhibit E.2, the summations at the bottom of specific columns are the information 

needed to implement the Extended T10 method for non-conventional contactors. The principal 

calculation needed is the equivalent number of tanks-in-series represented by the RTD. 

According to the tanks-in-series theory, the number of theoretical number of tanks, N, is 

determined according to

t

2

N = m

σ2

As shown in the example above, 

t

2 (8.99)

2

N = m = = 2 11.1

σ 7.27

Therefore, the tracer test determined that the contactor has a hydrodynamic character 

similar to a series of 11.1 CSTRs in series. Implementation of the Extended T10 method rounds 
Appendix E – Tracer Test Data Development & Analysis

the value of N down to the nearest integer, and uses this value for the number of segments for 

subdividing the non-conventional contactor. In this example, the Extended T10 calculation 

would use 11 segments.

It is interesting to note that the results of the step dose tracer test analyzed in Exhibit E.2 

differed somewhat from the results of the pulse dose tracer test analyzed previously in Exhibit 

E.1. Some of this difference may be attributed to basic errors in dosing and analytical 

determinations in the tracer tests. In addition, the step dose tracer test involves a numerical 

differentiation (Column 10, step dose) that is typically less accurate than numerical integration 

(used several times in each analysis). This could be a source for numeric dispersion which leads

to higher estimated hydrodynamic variance. This is not to imply that one procedure is better 

than another. There are advantages and disadvantages to both dose methods. Details of those 

issues can be found in Bellamy, Finch et al (1998) and the previous referenced sources. 

Exhibit E.2 contains additional calculations that are not necessary for implementing the 

Extended T10 method, but are needed for quality control of the tracer test, or for further general 

information concerning the contactor hydrodynamics. 

The average effective concentration during the last 10 percent of the test duration (ca. 3 

minutes) was 1.54 mg/L. That is also the maximum concentration calculated according to 

Column 7. That maximum was within was 90 percent of the dose of 1.7 mg/L added to the 

influent water. This value is within the acceptable range of 85 percent to 115 percent. 

 Exhibit E.2 also shows the value for the dimensionless variance denoted by the term, σ 2

θ , 

which is the dimensionless form of the variance, σ2

. The variance represents the second moment 

abut the mean of the RTD. It is calculated according to

2

σ2 σ

θ = t

2

m

which is the inverse of the equation for determining N, such that 

1

σ2

θ = N

Aside from an alternate form of calculating N, the value of σ 2

θ is useful for comparing 

the results of different tracer studies. For example, a higher value of σ 2

θ measured at a tracer 

study of a different flowrate suggests that there is greater mixing in the contactor under that 

flowrate. When more than one tracer test is performed, such as at various flowrates as 

recommended, the conservative recommendation is to use the RTD data set associated with the 

greatest variance, σ 2

θ . 

LT2ESWTR Toolbox Guidance Manual E-14 April 2010
Appendix E – Tracer Test Data Development & Analysis

LT2ESWTR Toolbox Guidance Manual E-15 April 2010

E.3 References

Bellamy, W. D., G. R. Finch, et al. (1998). Integrated Disinfection Design Framework. Denver, 

CO, AWWA Research Foundation. 

Fogler, H. S., Elements of Chemical Reaction Engineering. 4th ed.; Prentice Hall: Upper Saddle 

River, NJ, 2005. 

Levenspiel, O., Chemical Reaction Engineering. 3rd ed.; John Wiley & Sons: New York, 1999. 

Teefy, S. 1996. Tracer Studies in Water Treatment Facilities: A Protocol and Case Studies. 

Final Report. American Water Works Association Research Foundation. American Water Works 

Association, Denver, CO. 

Teefy, S. and P. Singer. 1990. Performance and Analysis of Tracer Tests to Determine 

Compliance of a Disinfection Scheme with the SWTR. Journal AWWA, 82(12):88-89. 

Wen, C. Y.; Fan, L. T., Models for Flow Systems and Chemical Reactors. Decker: New York, 

1975. 
LT2ESWTR Toolbox Guidance Manual F-1 April 2010

Appendix F 

Watershed Control Best Management Practices (BMPs) and Case Studies

This appendix provides a list of programmatic resources and guidance available to assist 

systems in building partnerships and implementing watershed protection activities. Examples of 

partnerships and possible control measures for different sources are summarized in Chapter 2, 

Section 2.4.3.2; this appendix provides further detail to the control measures described in chapter 

2. 

F.1 Regulatory and Other Management Strategies 

For systems in watersheds where most of the land is privately owned, land use regulations 

may be the best way to control pollution, especially in heavily developed or growing areas. 

Examples of possible regulations include septic system requirements, zoning ordinances 

specifying minimum lot sizes or low-impact development, limits on discharge from wastewater 

treatment plants and other facilities, pet waste cleanup ordinances, and requirements for permits 

for certain land uses. Your ability to regulate land use will depend on the authority granted to 

your municipality by the state, the ownership of your system (public or private), and the support 

of your local government and the public. Regulatory authority, steps for designing a regulation 

that can withstand lawsuits, and types of land use regulations are described in the paragraphs 

below. 

F.1.1 Determining Authority to Regulate 

Where a water system is privately owned, it may be necessary to ask the cooperation of 

the local government to get source water regulations passed. For a municipal water system whose 

watershed is located entirely within the municipality, issuing zoning or land use ordinances 

should be less of a hurdle. The ability of a municipality to pass a land use ordinance or other law 

to help reduce contamination may depend on the authority the state grants to the local 

government in the state constitution or through legislation, although states normally do not 

interfere with the actual land use and zoning rules (AWWARF 1991). States generally permit 

zoning for the purposes of protecting public health or general welfare. However, some states may 

prevent local governments from passing laws that are more stringent than state law or that 

conflict with state laws. State laws in other states may prevent municipal governments from 

passing certain local laws that are not expressly permitted elsewhere in state law. 

If the watershed or the area of influence on water quality extends throughout several 

municipalities, it can be difficult to standardize watershed control practices throughout the 

watershed. The legal framework used will depend on who has jurisdiction over land use in the 

watershed and on the authority of the water system (AWWARF 1991). New York State law, for 

instance, authorizes municipalities to draft watershed regulations, which are then approved and 

adopted by the state. This gives the municipalities the authority to enforce the watershed rules 

within their watersheds even if the watershed is outside municipal boundaries. For instance, New 
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York City sets water quality standards, land use restrictions, and approves wastewater treatment 

plant designs in its watersheds in upstate New York. The City of Syracuse conducts watershed 

inspections on Skaneateles Lake, its source of supply, several miles outside of Syracuse. Both of 

these systems are filtration avoidance systems, so it is especially important that they have some 

control over areas outside their jurisdictions.

The Metropolitan District Commission, although not a PWS, was created by the state of 

Massachusetts and is authorized to promulgate and enforce watershed protection regulations in 

watersheds used by the Massachusetts Water Resources Authority to provide water to the Boston 

metropolitan area. Some watersheds which extend across state boundaries have governing bodies 

authorized by Congress. The formation of the Tahoe Regional Planning Agency was the result of 

a compact between the states of California and Nevada and was approved by Congress. The 

agency is authorized to pass ordinances, including source water protection rules that regulate land 

use in the area around Lake Tahoe. 

County governments in some states may have some zoning authority and may be able to 

assist with enforcement of some regulations affecting source water (e.g., septic systems). In most 

cases where watersheds cross jurisdictions, however, public water systems (PWSs) will not have 

regulatory or enforcement authority. PWSs in this situation should work with other local 

governments, PWSs and agencies in their watersheds to sign memoranda of agreement or 

understanding, in which each entity agrees to meet certain standards or implement certain 

practices. 

The City of New York signed a memorandum of agreement in 1997 with the state of New 

York, U.S. Environmental Protection Agency (EPA), and 79 municipalities within its 

watersheds. The agreement calls for the creation of local and regional watershed protection 

programs and, for New York City, funding for water quality and infrastructure improvement 

projects in upstate New York. Other cities, such as Salem and The Dalles, both in Oregon, have 

signed memoranda of understanding with the U.S. Forest Service, which owns most of the land 

in the cities’ watersheds. These memoranda define the management responsibilities of each PWS 

and the Forest Service.

F.1.2 Zoning

 This section describes the steps you should follow to make a zoning law that is likely to 

withstand a legal challenge. Basically, it is important to make sure the appropriate procedures are 

followed and that the law has sufficient scientific basis (AWWA 1999). First, be sure you have 

the authority to regulate, especially if you are proposing something besides a simple zoning law. 

Make sure the rule is specific enough; if a map of an overlay district is not drawn to a small 

enough scale, it may be difficult to tell which properties are affected. Comply with all 

administrative procedure requirements, such as notifying the public of the proposed changes and 

holding a public hearing; failure to do so is the most common reason for rules being revoked. 
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Follow substantive due process, which means that the regulation should promote the 

municipality’s public health goals. In practice, this means the ordinance should conform to the 

objectives of the watershed control program plan. The plan should contain enough data to 

illustrate how the ordinance will affect water quality. 

Ordinances should also be designed to withstand a takings lawsuit (AWWA 1999). The 

Fifth Amendment to the U.S. Constitution states that private property may not be taken for public 

use without just compensation. Any physical invasion without consent is usually considered a 

taking, even if the landowner retains ownership of the land. Installation of a monitoring well or 

stream gauge without consent is an example of a taking. 

In addition, ordinances that fail to advance a legitimate government interest or deny a 

landowner economically viable use of his land can be viewed as takings, even if the landowner 

retains full ownership (AWWA 1999). The first criterion means that there should be a need for 

the ordinance; for example, if a planned development’s storm sewers and wastewater treatment 

plant will discharge into an area outside a municipality’s wellhead protection area, the 

municipality cannot cite impacts on the drinking water as a factor in the decision to restrict 

development without compensating landowners. Under the second criterion, if property values 

decrease but still retain some value (e.g., due to a decrease in permitted building density), the 

ordinance does not result in a taking. A regulation that restricts all development would probably 

be considered a taking. In keeping with these two criteria, the effect of an ordinance should be 

proportional to the predicted impact of development. Thus, if a municipality determines that halfacre zoning is sufficient to protect a drinking water source, it may not zone for five acres. 

To prevent takings claims, the municipality should show the need for the regulation and a 

connection between the ordinance and the expected result (AWWA 1999). This proof should be 

based on a scientific analysis beginning with an accurate delineation of the watershed or 

wellhead protection area/recharge area. A zoning district based on an arbitrary fixed radius 

around a well or lake would probably be considered insufficient in court unless it is characterized 

as an interim boundary. A court challenge could claim that such a district protects an area that 

does not contribute to the watershed or that land that is part of the watershed is not being 

protected (failing to advance the government’s interests).

Following the delineation, determine the impact the regulation will have by mapping 

current and projected residential, commercial, and industrial development under current zoning 

requirements. Then map current and projected development for existing regulations and for the 

proposed ordinance, and determine the potential pollutant load under each scenario (AWWA 

1999). You may not be able to determine Cryptosporidium loading if you have not monitored, 

but there may be data available on fecal coliform bacteria from different sources in your 

watershed (e.g., agriculture, septic system failure, pets and wildlife). If your PWS has not 

collected such data, other local agencies, such as sewer authorities, non-profit groups, 

universities, or planning commissions, as well as the U.S. Geological Survey, may have water 

quality data. Water quality models can help you determine pollutant loading. This buildout 
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analysis will help you show how your proposed ordinance advances a legitimate government 

interest and how the effect of the ordinance is proportional to the impact of land use in your 

watershed.

Types of Ordinances 

Watershed ordinances usually apply within an overlay district, which may be the area of 

influence you determined for your watershed control plan. All existing zoning or land use 

regulations apply within that area, but additional requirements apply within the overlay district. 

Following are some land use ordinances you may wish to consider: 

• Large-lot or low-density zoning. Unless lots are very large (such lots can use septic 

systems and wells), large-lot zoning may be inefficient, as it increases costs for sewer, 

water, and road development. This type of zoning also may go against affordable housing 

requirements. However, it may be useful in agricultural areas for preserving rural 

character and preventing subdivision of farms. 

• Limits on certain types of land use except by special permit. Such ordinances should 

specify criteria for granting special permits and designate an authority that may grant 

permits. The authority should present findings that back up its decision to grant the 

permit. Special permits are granted for a particular lot, not for the owner of that lot. 

• Impact fees. The regulating authority must be sure it has authority to impose such fees. 

Impact fees collected can be used to pay for mitigation of pollution caused by 

development, e.g., for preventing runoff or buying land elsewhere in the watershed. Fees 

should be proportional to the impact and the cost of mitigation, and the purpose of the 

fees should be specified in the regulation. A disadvantage to impact fees is that they may 

in some cases be considered taxes, and local government’s authority to impose taxes may 

be limited. Fees are more likely to withstand challenge if they are framed as optional 

services provided to the developer (i.e., the developer can choose not to develop) and if 

the fees are set aside for the PWS or stormwater utility rather than put into general funds. 

• Submission and approval of a watershed protection plan or impact study as a condition 

for development of a subdivision or apartment complex. This type of ordinance requires 

that watershed protection plan or stormwater control be implemented before a building 

certificate of compliance is issued. Plans should be required to designate the party 

responsible for maintaining stormwater facilities after construction is complete.

• Performance standards. A performance standard permits development but limits impact 

of the development. For example, the regulation could specify that permits require that 

the pollutant loading rate of the development is no more than a certain percentage of the 

pre-development loading rate of the area. This would require enforcement or monitoring 

to make sure the development continues to comply. In its permit application, the 
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developer would also be required to list mitigation steps it would take if it exceeded the 

pollutant loading requirements. 

Most zoning ordinances have grandfather clauses that allow nonconforming land uses to 

continue. Ordinances may also allow the zoning authority to grant variances if the topography or 

size of a lot make it difficult to comply with a zoning requirement.

Examples of source water protection ordinances can be found at: 

http://www.epa.gov/owow/nps/ordinance/osm7.htm. 

F.1.3 Land Acquisition and Conservation Easements 

Acquisition of watershed land by the utility or its affiliated jurisdiction is often the most 

effective approach to protecting the water source. Landowners usually consider acquisition as 

fair, since it compensates them for their property while protecting the watercourses nearby. Land 

conservation has also been found to provide multiple benefits aside from controlling pathogen 

contamination, such as flood control, limited recreational use, and the protection of historic and 

environmental resources. EPA’s Drinking Water State Revolving Fund (SRF) allows a 

percentage of the fund to be set aside for land acquisition associated with watershed protection. 

Note that some states may not allow SRF funds to be used in this manner. 

Several organizations exist that can help systems purchase watershed land to protect 

drinking water quality, especially when government agencies are unable to move quickly enough 

to buy land when it becomes available. The Trust for Public Land (http://www.tpl.org) and small 

local land trusts and conservancies can facilitate the land acquisition process. Trusts can buy and 

hold land from multiple landowners on behalf of a water system until the system can assemble 

funding to purchase it from the trust. Trusts may also maintain land ownership themselves. The 

Trust for Public Land also can assist with development of financing strategies for land purchases. 

Trusts also can work with landowners to buy or have landowners donate conservation 

easements. An easement is a legal document that permanently limits the development of a piece 

of land, even after the land is sold or otherwise changes ownership. The landowner selling or 

donating the easement specifies the development restrictions to apply to the land. The law varies 

from state to state, but the owner of the easement (the government agency or land trust) has the 

authority to determine if the requirements of the easement are being followed. If not, the owner 

of the easement make take legal action. Easements donated to government agencies or to land 

trusts may be eligible for tax deductions. See 

http://www.landtrust.org/ProtectingLand/EasementInfo.htm for frequently asked questions about 

easements and for an example of a model easement for use in the state of Michigan. The Land 

Trust Alliance (http://www.lta.org), a trade organization for land trusts, has published handbooks 

on designing and managing conservation easement programs.
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Other government agencies, such as the U.S. Forest Service or state natural resource 

departments, may be able to buy parcels in your watershed if you are unable to afford to purchase 

all the land that needs to be protected. 

F.2 Addressing Point Sources 

F.2.1 Concentrated Animal Feeding Operations 

Some animal feeding operations (AFOs) may be considered concentrated animal feeding 

operations (CAFOs) if they have more than a specified number of animals and/or if they 

discharge pollutants into navigable waters through a manmade ditch or other device or if they 

discharge directly into waters of the United States. Possible sources of pollutants at CAFOs 

include runoff that flows through feedlots; failure of pumps, pipes, or retaining walls of manure 

storage lagoons; runoff from areas where manure is applied to the soil; and direct contact of 

animals with surface water. CAFOs are located primarily in the South and Midwest, but the 

number of such facilities is increasing as farms consolidate their operations. 

EPA recently issued a rule that changed the requirements on CAFOs that must apply for 

National (or state) Pollutant Discharge Elimination System (NPDES or SPDES) permits (U.S. 

EPA 2008). CAFOs that discharge or propose to discharge must apply for NPDES permits and 

submit Nutrient Management Plans (NMPs) at the time that they submit a permit application. 

Permitting authorities are required to review the NMPs and provide the public with an 

opportunity for meaningful public review and comment. The permitting authorities are required 

to incorporate the terms of NMPs as NPDES permit conditions. If an unpermitted CAFO can 

certify to the permitting authority that they do not discharge or propose to discharge, then they 

are not required to apply to an NPDES permit. In the final rule, EPA provided clarification on 

how operators should evaluate whether they discharge or propose to discharge. The CAFO owner 

or operator must determine on a case-by-case basis whether the CAFO will discharge based on 

the CAFO design, construction, operation, and maintenance. 

Many CAFOs do not currently have permits due to limited state resources for compliance 

(medium and small AFOs may be designated as CAFOs only by state or regional staff after onsite 

inspection). For CAFOs (and other NPDES permittees) that do have individual permits, you may 

want to attend the public hearing required as part of the permit renewal process, especially if you 

have any concerns about the adequacy of the existing permit requirements to prevent 

Cryptosporidium or other drinking water contamination. 

F.2.2 Wastewater Treatment Plants 

All wastewater treatment plants in the United States are required to provide secondary 

treatment (primary treatment consists of sedimentation, while in secondary treatment, aeration 
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provides oxygen to bacteria that take in nutrients and digest organic material) (U.S. EPA 2001b). 

Most plants are also required to disinfect their effluent before discharging. However, 

conventional chlorine disinfection may be ineffective against Cryptosporidium. 

Some wastewater treatment facilities are beginning to implement treatment similar to that 

used for drinking water treatment. The Robbins Plant of the Upper Occoquan Sewerage 

Authority in Centreville, Virginia, discharges into a stream that feeds into a reservoir in northern 

Virginia. Following secondary treatment using activated sludge, the facility provides other 

treatment, including clarification, multimedia filtration, and disinfection (U.S. EPA 2000a). The 

Cole Pollution Control Plant in Fairfax County, Virginia, which discharges into a creek flowing 

into the Potomac River, also uses advanced treatment, including chlorine disinfection, filtration, 

and dechlorination (Fairfax County 2001). 

PWSs should identify all wastewater treatment plants in their watersheds and determine 

what their permit effluent limits are and whether the limits are being met. Some of this 

information may already be available through the source water assessment program. PWSs may 

wish to work with the wastewater utilities and appropriate government agencies to get them to 

voluntarily upgrade the treatment provided. PWSs with the appropriate legal authority may wish 

to require wastewater plants to use certain technologies. An example might be switching from 

chlorine to ozone or ultraviolet radiation disinfection before discharging. 

F.2.3 Combined Sewer Overflows 

Combined sewer overflows (CSOs) are most common in older cities in the northeastern 

and midwestern United States and can be a significant contributor of Cryptosporidium to urban 

watersheds. 

There are three major structural solutions to the problem of CSOs. The first is to separate 

combined sewers into sanitary and storm sewers, where sanitary sewers flow to the wastewater 

treatment plant and storm sewers release to surface water. This separation may cause the 

unwanted side effect of increasing overall contamination due to the fact that storm water is no 

longer being treated. For example, separating sewers resulted in only an estimated 45-percent 

reduction in fecal coliform removal in a bay in Boston (Metcalf and Eddy 1994, cited in U.S. 

EPA 1999c). Separating sewers is also very expensive and often impractical. The second option 

is to increase the capacity of the wastewater treatment plant so that it is able to treat combined 

sewage from most storms. The third, very expensive solution is to build aboveground open or 

covered retention basins or to construct underground storage facilities for combined sewage to 

hold the sewage until the storm has passed and can be treated without overloading the plant. The 

Metropolitan Water Reclamation District in Cook County, Illinois, chose the third option, 

building 109 miles of tunnel up to 35 feet wide and several underground reservoirs underneath 

Chicago and its suburbs, with most funding from EPA (MWRD 1999). In addition to reducing 

CSOs, the tunnels eliminated flooding that had previously affected the area due to its flat 
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topography. The project also eliminated the need for individual municipalities to implement their 

own CSO programs. 

CSOs are not regulated directly under their own program, but EPA has a CSO control 

policy (U.S. EPA 1994) which encourages minor improvements to optimize CSO operation, and 

CSO management may be written into NPDES or SPDES permits. The CSO policy also 

encourages development of long term control plans for each CSO system; such plans would 

require significant construction, and few utilities have drafted or implemented them yet. Planned 

construction projects can be included as control measures in watershed control plans. PWSs 

should determine the extent of the CSO programs in place in municipalities within their 

watersheds. They may be able to work with other utilities to address overflow sites of particular 

concern. Many municipalities with CSOs made major structural changes to their systems in the 

1980s and 1990s; current improvements are more likely to involve streamlining operation and 

management.

Many large cities have already addressed a significant portion of their CSOs, but there are 

additional smaller steps they can take to reduce the amount of sewage released during a wet 

weather event. These include maximizing in-line storage (storage available in the sewer pipes 

themselves) through regular inspection and removal of obstructions and sediment, installation 

and maintenance of flow regulators, upgrading pumping capacity (assuming the treatment plant 

can handle the increased volume); raising weirs at CSO outfalls; and installing computerized 

sensors to control flow during storms. Stormwater Best Management Practices (BMPs) can also 

reduce the impact of CSOs. 

Additionally, reducing inflow (entry of storm water into the combined sewers) and 

infiltration (entry of storm water through cracks and manholes) is important. Inflow can be 

reduced by disconnecting roof drains and sump pumps from sewers, restricting flow into storm 

drains, and constructing storm water detention ponds and infiltration devices. If overflow events 

can be reduced, it may be possible to eliminate some outfalls. Some sewer systems also have 

installed some treatment of CSOs including disinfection and screening; this treatment may be 

required as part of a NPDES permit.

F.2.4 Sanitary Sewer Overflows 

Sanitary sewer systems normally feed into wastewater treatment plants but can still cause 

water quality problems. Sanitary sewer overflows (SSOs) occur when untreated and mostly 

undiluted sewage backs up into basements, streets, and surface water. SSOs discharging to 

surface water are prohibited under the Clean Water Act. Insufficient maintenance and capacity 

and illegal connections are some of the primary causes of SSOs. Many sanitary sewers are 

subject to inflow and infiltration, just as combined sewers are, caused by cracks in pipes or bad 

connections to service lines. They may receive water they were not designed to receive, such as 

storm water from roof drains that should be connected to storm sewers, or wastewater from new 
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developments that did not exist when the wastewater treatment plant was designed. SSOs can be 

reduced by cleaning and maintaining the sewer system; reducing inflow and infiltration by 

repairing leaking or broken service lines; increasing sewer, pumping, and/or wastewater 

treatment plant capacity; and constructing storage for excess wastewater (U.S. EPA 2001c).

EPA and the states will continue to address these problems using various aspects of the 

capacity, management, operation and maintenance (CMOM) concept. The CMOM concept 

encourages the use of self-assessments and pro-active correction of system deficiencies to avoid 

further deterioration of the sanitary sewer infrastructure and resultant SSOs. In some cases, EPA 

and the states will use a combination of administrative and civil judicial enforcement action to 

achieve these goals (U.S. EPA 2009). 

F.2.5 Municipal Separate Storm Sewer Systems 

Municipal separate storm sewer systems (MS4s) in areas with populations of more than 

100,000 are also required to obtain NPDES permits. Information on storm sewer outfall 

locations, volume discharged, conventional pollutant loads, and existence of illicit discharges is 

submitted as part of the permit application process (U.S. EPA 1996). In addition, these MS4s 

must develop management plans addressing items such as outfall monitoring, structural and 

nonstructural BMPs to be implemented, and identification and elimination of illicit discharges. 

Illicit discharges to MS4s include any non-stormwater discharges, such as discharges that should 

be connected to sanitary sewers (e.g., water from sinks, floor drains, and occasionally toilets), 

illegal dumping of sewage from recreational vehicles, sanitary sewer overflow backing up 

through manhole covers into storm drains, effluent from failing septic systems, water from sump 

pumps, etc. 

Small MS4s (serving areas with populations of less than 100,000) are subject to NPDES 

permit requirements if they are located in urbanized areas as determined by the Bureau of the 

Census. Some small MS4s in urbanized areas may be eligible for waivers from the NPDES 

requirement. Those MS4s subject to NPDES permits must implement control measures in six 

areas, including a plan for eliminating illicit discharges (U.S. EPA 2000b). 

PWSs should work with all MS4 utilities in the area of influence to gather existing 

information about storm water contamination. MS4 utilities may need to install or retrofit 

structural BMPs, such as retention ponds, to reduce contamination. Most studies of structural 

stormwater BMPs focus on nutrient or sediment removal, so almost no information is available 

on Cryptosporidium removal, and limited information is available on bacterial removal. 

However, a few studies of bacteria in structural BMPs show that bacteria survive for weeks to 

months in retention pond sediments and natural lake environments. In addition, other studies 

showed higher bacteria levels in retention pond effluent than in influent. This suggests that 

stormwater pond sediments resuspended during storms can be a source of pathogens (Schueler 

1999). 
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F.3 What BMPs Can Help Alleviate Nonpoint Sources? 

The following sections describe BMPs for agricultural, forestry, and urban sources of 

Cryptosporidium. Your watershed control program plan must discuss how these or any other 

BMPs you choose will be implemented in the area of influence. EPA Section 319 grants and 

Clean Water State Revolving Fund (CWSRF) loans can be used for nonpoint sources and 

watershed management purposes.

F.3.1 Agricultural BMPs 

F.3.1.1 Management Programs 

The U.S. Department of Agriculture (USDA) (2000) recommends a multiple-barrier 

approach to controlling pathogen transport and proliferation on farms and in agricultural 

watersheds. It recommends the following control points: 

• Preventing initial infection by controlling pathogen import to the farm. 

• Controlling the reproduction and spread of the pathogen throughout the farm. 

• Managing waste. 

• Controlling pathogen export from the farm. 

These control points should not be treated separately. For example, waste management 

affects reproduction and spread of the pathogen if feed becomes contaminated with waste. Waste 

management is also related to pathogen export; composting can kill Cryptosporidium oocysts 

before they leave the farm.

BMPs that can reduce pathogen loading include composting, waste management (manure 

storage and land application), grazing management, feedlot runoff diversion, and buffer or filter 

strips. PWSs should work with their local soil and water conservation districts and agricultural or 

cooperative extensions, which can help farmers design and implement pollution management 

plans and BMPs. Details about these conservation practices are provided in the USDA Natural 

Resources Conservation Service’s (NRCS) National Handbook of Conservation Practices (NRCS 

1999) at http://www.ftw.nrcs.usda.gov/nhcp_2.html.

Management strategies designed to minimize direct livestock contamination of surface 

water with Cryptosporidium should focus primarily on young animals (those less than 3 months 

old) and their waste, since calves are more likely to shed Cryptosporidium. Efforts should also 

focus on cow herds as a whole when calves are present.
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Several NRCS programs provide technical assistance to farmers and subsidize the cost of 

implementing BMPs. These include Agricultural Management Assistance, the Environmental 

Quality Incentives Program, the Conservation Reserve Program, and the Conservation Reserve 

Enhancement Program (see www.nrcs.usda.gov/programs). The last two programs also pay 

farmers rent on erodible cropland taken out of production. More information is available at 

http://www.fsa.usda.gov/dafp/cepd/crpinfo.htm. The 2002 Farm Bill increased funding for these 

programs and created new ones as well. For example, the new Conservation Security Program 

will recognize and reward farmers who are leaders in environmental management.

F.3.1.2 Composting 

Composting can effectively reduce pathogen concentrations. Temperatures greater than 

55 degrees Celsius (131Ε F) can be easily attained and maintained long enough to inactivate 

most oocysts (Blewett 1989). To reliably achieve Cryptosporidium inactivation, however, the 

entire waste mass should be uniformly treated and there should be no cold spots. Intense 

management may be needed to completely mix the composted material.

A study was conducted to determine the effectiveness of compost piles in inactivating 

Cryptosporidium oocysts. Four compost piles were used. Two compost piles consisted of manure 

while the other two compost piles consisted of surface soil. Each compost pile was injected with 

two million oocysts in an aqueous suspension. Every two to four weeks, Cryptosporidium

oocysts were extracted and tested from both sets compost piles. Both experiments show that 

inactivation of Cryptosporidium oocysts occurred after 40 days of composting. However, the 

compost pile with manure fared slightly better after 150 days (Jenkins et al. 1999). 

F.3.1.3 Buffer Strips

Buffer strips, or filter strips, provide a buffer between the area of manure application or 

grazing and adjacent streams or lakes. Filter strips have been studied primarily with regard to 

their effectiveness at sediment and nutrient removal. Nutrient removal has been shown to be 

extremely variable, while agricultural grass filter strips consistently remove 65 percent or more 

of sediment (Ohio State University Extension undated). How sediment removal relates to 

Cryptosporidium removal is not known. Cryptosporidium often adsorbs to suspended material 

the size of clay and silt particles, which is the type of sediment that is likely to pass through the 

filter strip, especially at high flow velocities.

Few studies have evaluated the ability of buffer strips to remove Cryptosporidium. 

However, one study found that grass filter strips with slopes of 20 percent or less and widths of at 

least 3 meters resulted in removal of 1 to 3 log (90 to 99.9 percent) during mild to moderate 

precipitation (Atwill et al. 2002). More data are available on removal of bacteria. Moore et al. 

(1988) reviewed the work of several investigators and concluded that vegetative filters are most 
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reliable at removing bacteria at high concentrations from waste effluent. Bacterial populations in 

runoff from buffer areas seem to equilibrate at approximately 104 to 105 organisms per 100 

milliliters, regardless of experimental conditions. For this reason, USDA (2000) recommends 

that buffers and filter strips be considered secondary practices for pathogen control and be used 

in conjunction with other source, proliferation, and waste treatment and control measures to form 

an integrated, comprehensive pathogen management system.

The NRCS encourages the use of riparian forest buffers of at least 35 to 100 feet 

(depending on floodplain width) for stream restoration purposes but recommends additional 

width in high sediment and animal waste application areas. Grass filter strips may be added 

upgradient of the forest buffers or may be used alone. The NRCS (1999) recommends grass filter 

strip widths of at least 20 feet, but width should be determined based on the slopes of the strip 

and the field being drained, the area being drained, the erosion rate, sediment grain size 

distribution, runoff volume, and the vegetation in the strip. Filter strips should follow contours as 

much as possible to promote sheet flow. The area being drained should have a slope of less than 

10 percent. Grazing should not generally be permitted within the filter strip. Maintenance 

activities should include mowing to prevent woody growth, inspection after storm events, repair 

of any gullies, reseeding of disturbed areas, and any other steps needed to maintain overland 

sheet flow.

Vegetated buffer strips were tested to see if they were effective at removing 

Cryptosporidium during rainfall rates of 15 or 40 mm/h for four hours. Buffers were set on a 

slope of 5 to 20 percent and soil textures consisted of silty clay, loam, or sandy loam. 

It was found that vegetated buffer strips consisting of sandy loam or higher soil bulk 

densities had a 1 to 2 log reduction/m. Buffers consisting of silty clay, loam, or lower bulk 

densities had a 2 to 3 log reduction/m. Also, it was found that vegetated buffer strip made of 

similar soils removed at least 99.9 percent of Cryptosporidium oocysts from agricultural runoff 

when slopes were less than or equal to 20 percent and had a length of at least 3 meters (Atwill et 

al. 2002). 

F.3.1.4 Grazing Management

Managed grazing can be cheaper and less environmentally damaging than confined 

feeding and unmanaged grazing. It decreases feed, herbicide, equipment, and fertilizer costs, 

while reducing erosion and increasing runoff infiltration and manure decomposition rates (Ohio 

State University Extension, undated). In managed, or rotational, grazing, a sustainable number of 

cattle or other livestock graze for a limited time (usually 2-3 days) on each pasture before being 

rotated to the next pasture. This allows vegetation regrowth and prevents overgrazing, which can 

contribute to erosion and runoff, and helps distribute manure evenly over the grazed area. It also 

prevents soil compaction, thereby increasing infiltration. One of the best ways to prevent surface 

water contamination during rotational grazing is to prevent grazing along streams (through 
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-13 April 2010

fencing and use of a buffer strip) and to provide alternative water sources for livestock. Providing 

water in each paddock can increase the number of cattle the pasture is able to support. Even 

where rotational grazing is not used, surface water contamination can be reduced by keeping 

cattle, especially calves, out of streams. 

Vermont wanted to reduce the concentration of E. coli, fecal streptococcus, and fecal 

coliform bacteria as well as total phosphorus found in the Lake Champlain Basin Watershed. It 

was assumed that a significant concentration of bacteria originated from nonpoint sources located 

in agricultural lands. To reduce the bacterial concentration entering the watershed, Vermont 

decided to improve animal waste management with dairy cows. In nearby areas, pastures 

containing dairy cows near streams and streambanks were found. It was believed that these 

pastures were one of the major sources of contamination due to bacterial excretion and 

streambank erosion caused by dairy cows. Minimizing erosion along streambanks allows for 

healthy vegetation, which will help filter nutrients. To prevent dairy cows from getting near or in 

streams and streambanks, bridges were constructed across streams. Fences along streambanks 

were also constructed to keep dairy cows from eroding streambanks. 

When construction was complete, the watershed was monitored for three years. Exhibit 

F-1 represents the average reduction in concentration for the specific contaminant.

Exhibit F-1 Average Reduction of Specific Contaminant

E. coli 29%

Fecal streptococcus 40%

Fecal coliform bacteria 38%

Total phosphorus 15%

These numbers show that the construction of bridges and fences had a significant impact 

on the reduction of bacterial and total phosphorous concentrations. Only minimal fence 

maintenance was required. Therefore, keeping dairy cows away from streams and streambanks 

may significantly reduce bacterial and phosphorus concentration in the watershed with minimal 

hassles (U.S. EPA 2002c). 

F.3.1.5 Manure Storage

Manure storage facilities allow farmers to wait until field conditions are more suitable 

for land application. Without manure storage facilities, farmers must distribute manure on 

adjacent fields daily. However, weather conditions are not always appropriate for manure 

application. During the winter, for example, frozen soil conditions allow Cryptosporidium

oocysts to be washed into watercourses, and oocysts survive longer at cold temperatures. 
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-14 April 2010

Manure storage facilities should be designed to prevent discharge through leaching or 

runoff. They should be lined and, if possible, covered. Facilities that are not covered should be 

designed to contain precipitation and runoff from a 25-year 24-hour storm. Storage areas should 

have embankments to prevent overflow and collapse of the storage facility and to divert runoff 

from outside the facility from contamination. Facilities should be sited outside of flood plains. 

Manure should be stored for a time period sufficient for microorganisms to die off. 

F.3.1.6 Land Application of Manure

Several precautions taken in manure application can prevent runoff from entering surface 

water, reducing the likelihood of Cryptosporidium contamination. Buffer strips should be 

situated between the water body and area of manure application. Manure should not be applied to 

frozen ground or before predicted rainfall. Manure should not be applied near tile drains or dry 

wells or to land subject to flooding. If soil is dry and cracked, fields should be tilled before 

application. Soil and manure should be tested for nutrient levels, and the application rate should 

be tailored to the soil and specific crop needs. To minimize runoff, waste should be injected 

(injection creates holes 6-14 inches deep and does not turn soil over) or applied to the surface and 

then plowed under. Applying manure to land with crop residue or new crops rather than bare soil 

also minimizes erosion. Surface application without plowing under may be acceptable if 

conditions are warm and dry this enables significant pathogen die-off (Vendrall et al. 1997) by 

exposure to ultraviolet light (UV) light and desiccation. The Agricultural Waste Management 

Field Handbook (NRCS 1992), Chapter 5, Table 5-3 contains a detailed review of restricting 

features that should be considered during manure spreading. 

For pastures to be used for grazing, waste should be stored for at least 60 days and then 

applied at least 30 days before the scheduled grazing period, to avoid infection of the animals. 

Use of these areas for grazing should be limited to mature animals. Manure spreading on pastures 

used for grazing or on hayfields should take place when minimal amounts of vegetation are 

present, just after harvesting or grazing. This allows sunlight and desiccation to destroy the most 

pathogens and reduces the chance of pathogen adherence to the forage. 

Critical source areas are defined as saturated areas that can expand and contract rapidly, 

based on soil, hydrological, and slope characteristics (Gburek and Poinke 1995). These areas are 

dominated by saturated overland flow and rapidly respond to subsurface flow. Therefore, 

watershed managers should identify the boundaries of potential saturated areas and ensure that 

waste is only applied outside of those boundaries to minimize Cryptosporidium oocyst runoff. 

Some tools have been developed to delineate critical source areas (e.g., Cornell Soil Moisture 

Routing Model; Frankenberger 1999). Less detailed delineations can also be made using 

information such as soil drainage class, flooding frequency, wetland mapping, areas of 

concentrated flow, and aerial photo interpretations. 
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-15 April 2010

F.3.1.7 Feedlot Runoff Diversion 

Clean roof and surface water can be diverted away from feedlots to a drainage system that 

is independent of a farm’s waste management system (Ohio State University Extension 1992). 

All roofs that could contribute to feedlot runoff should have gutters, downspouts, and outlets that 

discharge away from the feedlot. Berms around the feedlot can divert surface runoff. Diverting 

clean water before it drains into the feedlot can significantly reduce the amount of wastewater 

that needs to be managed. Runoff within the feedlot should be contained and treated in the waste 

management system for the lot.

F.3.2 Forestry BMPs 

Forestry practices are not likely to significantly contribute to Cryptosporidium sources, 

since wildlife levels decrease or, at most, remain constant after logging. However, logging can 

cause increased erosion, leading to increased runoff and making it more likely that 

Cryptosporidium present in wildlife will reach the source water. In addition, logging can cause 

elevated sediment levels, resulting in high turbidity, which affects water treatment efficiency.

Filter strips, where ground cover is maintained around lakes, permanent and intermittent 

streams, and wetlands, help trap sediment. Filter strip width should increase with slope of the 

area being logged. Streamside or riparian management zones are intended to stabilize stream 

banks and maintain shade over streams to minimize water temperature fluctuations. Streamside 

management zones and filter strips often overlap, but limited logging is often permitted within 

streamside management zones (NRCS 1999). 

Logging roads should be constructed to minimize runoff through proper grading and 

drainage. Road runoff should be diverted away from streams and prevented from channelizing. 

Loggers should minimize soil disturbance and compaction on skid trails, the trails used to drag 

logs to trucks for loading (U.S. EPA 2002a). 

F.3.3 Urban/Suburban BMPs

Urban/Suburban BMPs can reduce burden on sewage infrastructure and address CSOs 

and non-point sources. See http://www.epa.gov/owm/mtb/mtbfact.htm for fact sheets on 

technologies and BMPs municipalities can use to reduce contamination from wastewater and 

stormwater.
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-16 April 2010

F.3.3.1 Buffer Zones

For watersheds in urban areas, buffer zones help to protect development on the floodplain 

from being damaged when the water is high, as well as protect the stream from the effects of the 

development. 

The utility, municipality, or cooperating jurisdictions may acquire land bordering the 

reservoir and/or its tributaries. Alternatively, buffers can be required by zoning ordinances, 

conservation easements, or subdivision regulations. Buffer zones can be fixed width or 

variable width. In a fixed-width zone, the buffer zone encompasses a certain distance from the 

stream bank or some other hydrological reference point (e.g., the high water mark of a stream). 

The widths of fixed buffer zones vary considerably among water sources, frequently ranging 

from 50 feet to 250 feet of buffer from the stream edge. Another form of buffer zone, the 

variable-width buffer, can vary in width depending on the hydrological sensitivity, stream size, 

and character of the land adjacent to the watercourse.

Considerations for developing local buffer requirements are the size and location of the 

stream, the nature of existing or potential development, and the financial and political feasibility 

of establishing protected zones around the streams and reservoir of the watershed. Although 

buffer zones have been found to trap fecal waste (Coyne and Blevins 1995, Young et al. 1980), 

the extent to which they reduce Cryptosporidium loading is not well understood. For this reason, 

buffer zones should be used to augment, rather than replace, other watershed management 

practices to help protect overall source water quality.

Buffer zones should be routinely inspected to ensure that sources of contamination have 

not been introduced to the area and that the buffer is being maintained (e.g., that buffers are kept 

unmowed). Watershed managers should also be aware of storm sewers and culverts that may be 

draining into the waterways and bypassing the buffer zones altogether. 

F.3.3.2 Dry Detention Basins

Dry detention basins temporarily store stormwater runoff and release the water slowly to 

allow for settling of particulates and the reduction of peak flows. These structures hold a certain 

amount of water from a storm and release the water through a controlled outlet over a specified 

time period based on design criteria. Most basins dry out completely between storm events. The 

major failure of these basins is that some are not designed or maintained properly, resulting in 

too slow a release of water to empty the basin before the next storm. If the basin remains partially 

full, only a portion of the design runoff volume from the next storm will be retained. With 

inadequate detention, pollutants are not removed from the runoff. Dry detention basins also risk 

the possibility of resuspension of pathogens from the basin sediments if hydraulic retention times 

are compromised by poor design or failure to keep the outlets open. 
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-17 April 2010

F.3.3.3 Infiltration Devices

Infiltration devices remove pathogens and particles by adsorption onto soil particles and 

filtration as the water moves through the soil to the ground water. Infiltration devices include 

infiltration basins, infiltration trenches, and dry wells (NALMS 2000). Properly designed devices 

can reproduce hydrological conditions that existed before urban development, and provide 

ground water recharge and control of peak storm water flows. In order for them to function 

effectively, infiltration devices must be used only where the soil is porous and can readily absorb 

storm water at an adequate rate. An advantage of infiltration devices over many other urban 

BMPs is that they provide significant ground water recharge in areas with a high percentage of 

impervious surface. 

F.3.3.4 Sand Filters

Sand filters can be used to treat storm water runoff from large buildings and parking lots. 

As the name implies, storm water is filtered through beds of sand, which may be located above 

ground in self-contained beds, or can be installed underground in trenches or concrete boxes. 

Underground sand filters can be installed in urban settings where space is restricted and the 

filters are not visible. Pathogens and particles are removed by filtering storm water through 

approximately 18 inches of sand. Above-ground filters may be preceded by grassed filter strips or 

swales to pre-treat the incoming storm water and prevent the sand filters from clogging. 

Sand filters are often more expensive to construct than infiltration trenches (NALMS 

2000). They do not provide a significant amount of storm water detention, and their ability to 

remove pathogens is limited. They require little maintenance; the sand surface should be raked 

and a few inches of dirty sand on the filter surface should be removed and replaced periodically, 

so that the filters do not clog. 

F.3.3.5 Wet Retention Ponds

Wet retention ponds maintain a permanent pool of water that is augmented by storm 

water runoff. The ponds fill with storm water, which they slowly release over several days until 

the pond returns to its normal depth. Ponds can effectively reduce suspended particles and, 

presumably, some pathogens, by settling and biological decomposition. There is concern, 

however, that ponds attract wildlife that may contribute additional fecal pollution to the water, 

rather than reducing contamination. Bacteria may also survive in pond sediment. 

Many people find wet ponds aesthetically pleasing, and welcome their use for storm 

water control. Some maintenance of the ponds is required in order for them to continue to 

function effectively and to avoid nuisance odors and insect problems. Wetland plants should be 
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-18 April 2010

periodically harvested, and the pond inlets and outlets should be kept clear so that flow is not 

impeded. Wet ponds can be an appealing play area for children, so safety measures should also 

be taken to restrict access.

F.3.3.6 Constructed Wetlands

Constructed subsurface flow wetlands (where wetland plants are not submerged) can 

reduce Cryptosporidium and bacteria concentrations in wastewater (Thurston et al. 2001). 

Subsurface flow prevents the public from coming into contact with wastewater and prevents 

mosquitos problems. Wetlands may also be useful for treating storm water or other polluted 

water. However, the matrix material of a constructed subsurface flow wetland (gravel is often 

used) may provide an environment for bacterial growth, and animals living in the wetlands may 

contribute microbes to the effluent (Thurston et al. 2001). Animals are probably less significant 

than they would be in a free water surface wetland. The growth of bacteria in the wetland 

medium is both positive and negative; bacteria that help break down materials in wastewater are 

more plentiful, but fecal coliform also can survive in such environments. Constructed wetlands 

are relatively inexpensive and are often used on small scales to treat water at small facilities such 

as schools, apartment complexes, and parks (U.S. EPA 2000c). 

A wetland was constructed in Tucson, Arizona to help remove Cryptosporidium from the 

secondary sewage effluent. The wetland had a maximum depth of 1.4 meter, length of 61 meters, 

and width of 8.2 meters. The wetland was designed to have a retention time of approximately 

four days with an average flow rate of 58 liters/minute. The wetland was planted with cattail, 

bulrush, black willow, and cottonwood. It was found that the wetland effectively removed 64.2 

percent of Cryptosporidium oocysts from the secondary sewage effluent (Thurston et al. 2001). 

Two wetlands were constructed to determine if they could effectively remove 

Cryptosporidium from untreated domestic wastewater. One wetland was planted with bulrush 

and the other wetland was made of bare sand. The influent domestic water flowed directly into 

two setting tanks in series. Then the flow split into the two wetlands in parallel. The wetlands’ 

detention time was 1 to 2 days. The results of this study showed that both planted and unplanted 

wetlands removed about 90 percent Cryptosporidium oocysts. Slightly more oocysts were 

removed in the planted wetland. The test shows that planted and unplanted wetlands are effective 

in removing Cryptosporidium oocysts (Quinez-Daz et al. 2001). 

F.3.3.7 Runoff Diversion 

As with feedlot runoff diversion, structures can be installed in more urban settings to 

divert clean water flow before it reaches a contamination source. Structures that channel runoff 

away from contamination sources include stormwater conveyances such as swales, gutters, 

channels, drains, and sewers. Graded surfaces can also be used to re-direct sheet flow, and 
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-19 April 2010

diversion dikes or berms can be installed to route sheet flow around areas that are being protected 

from runoff.

F.3.3.8 Pet Waste Management

Municipalities can implement pet waste management programs to encourage pet owners 

to properly collect and dispose of their animal’s waste. Many communities have pet waste 

ordinances that require pet owners to clean up after their pets on public property or anywhere 

outside their own yards; however, compliance is limited, and enforcement is usually not a 

priority. In addition, most ordinances do not require pet owners to clean up pet waste in their own 

yards (this problem can usually be addressed, though only reactively, through nuisance or pet 

neglect laws). Some communities have ordinances that govern the cleanup process by requiring 

disposal of pet waste with regular trash, burial, or flushing it down the toilet. Enforcement of 

these ordinances with fines for noncompliance is probably the best way to increase compliance. 

To increase public awareness about pet waste, you can distribute educational materials 

through emails, letters, public service announcements, and signs. Posting is the most common 

outreach strategy for managing pet waste. Pet waste stations containing waste receptacles for 

public use are another popular solution. Public works departments have also formed voluntary 

commitment and partnership programs with pet owners and local pet stores in the community to 

promote good pet waste management. 

F.3.3.9 Water Conservation 

Water conservation is usually presented as a practice that can help preserve the amount of 

water available for use, especially in times of drought. However, water conservation can also 

decrease the amount of wastewater and stormwater generated, thereby protecting the quality of 

the water supply (U.S. EPA 2002b). Use of low-flow toilets and showerheads, for example, can 

allow wastewater treatment plants to treat wastewater from more customers without having to 

increase capacity, reducing the occurrence of combined or sanitary sewer overflows. The reduced 

load on wastewater treatment plants can also decrease the need for rate increases. Reducing lawn 

watering decreases the amount of runoff entering storm sewers, combined sewers, and surface 

water.

F.3.3.10 Low Impact Development

Low impact development, or better site design, is a watershed practice that reduces 

pollutant loads, conserves natural areas, saves money, and increases property values (Center for 

Watershed Protection 1999). A fundamentally different approach to residential and commercial 

development, site design tries to reduce the amount of impervious cover, increase natural lands 
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-20 April 2010

set aside for conservation, and use pervious areas for more effective stormwater treatment. Low 

impact development involves changing traditional practices for residential street and parking lot 

design, lot development, and conservation of natural areas. Some specific steps for better site 

design include the following (Center for Watershed Protection 1999): 

• Design residential streets based on the minimum width needed to support travel 

lanes, on-street parking, and emergency and maintenance vehicle access. For 

example, a street with single family houses with driveways does not need two 

lanes for parking. Construct sidewalks on only one side of the street. 

• Minimize the number of cul-de-sacs. Where cul-de-sacs are built, place 

landscaped islands to reduce their impervious cover. 

• Advocate open space or cluster design subdivisions on smaller lots. 

• Reduce imperviousness by promoting alternative driveway surfaces and shared 

driveways that connect two or more homes together. Reduce driveway length by 

allowing decreased front setbacks.

• Direct rooftop runoff to pervious areas such as yards, open channels, or vegetated 

areas rather than the roadway and stormwater sewers. Better yet, install open 

vegetated channels instead of storm sewers.

• Reduce the imperviousness and size of parking lots by minimizing stall 

dimensions, incorporating efficient parking lanes, and using pervious materials in 

the spillover parking areas where possible. Use lower parking ratios where 

possible (e.g., where mass transit is available and codes permit). 

• Provide stormwater treatment for parking lot runoff using bioretention areas, filter 

strips, and/or other practices. 

• Create a naturally vegetated buffer system along all perennial streams that 

encompasses critical environmental features such as the 100-year floodplain, steep 

slopes, and wetlands. 

• Clearing and grading of forests and native vegetation at a site should be limited to 

the minimum amount needed to build lots, allow access, and provide fire 

protection. Specify a party legally responsible for maintaining the vegetated area.

Some aspects of low impact development may be prohibited outright under traditional 

zoning and development regulations, so low impact development practices may need to be 

codified. Where such practices remain voluntary or require exemptions from existing regulations, 

water systems should work with local planners to encourage the switch to better site design. 
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-21 April 2010

F.3.3.11 Septic Systems

Failing septic systems can be a major source of microbial contamination in a watershed. 

Poor placement of leachfields can feed partially treated waste directly into a drinking water 

source. Poorly constructed percolation systems may allow wastewater to escape before it has 

been properly treated. Failing systems can result in clogging and overflow of waste onto land or 

into surface water.

Most septic system regulations require construction permits and an inspection before the 

system begins operating, but few require any follow-up. Where failing systems are a serious 

problem or are close to a drinking water source, however, some municipalities have maintenance 

or inspection requirements. For example, the Portland (Maine) Water District requires permits 

for all septic systems within 200 feet of Sebago Lake, its primary source (U.S. EPA 1999a). 

These septic systems are subject to regular inspection and may face stricter design requirements 

than systems outside the boundary. Portland also has the authority to inspect systems within 

1,000 feet of Sebago Lake tributaries. Similarly, the Onondaga County Water Authority in New 

York visually inspects every septic system in the water system annually. Every three years each 

septic system is subject to a dye tracer test. Enforcement cases are referred to the county health 

department (U.S. EPA 1999a). 

Although water systems rarely have enforcement authority over septic systems, they 

should work closely with the local regulatory authority to ensure that septic system codes are 

being properly enforced and to strengthen codes where necessary. Utilities should also encourage 

residents with septic systems in the watershed to understand their systems and the proper 

maintenance that their systems require. Programs run by many state cooperative extensions 

provide educational material and checklists for septic system owners about proper siting and 

maintenance. Utilities may also want to encourage residents to hook up to a sanitary sewer 

system where feasible. CWSRF loans, USDA Rural Utilities Service funds, and Department of 

Housing and Urban Development Community Development Block Grants are available for septic 

system rehabilitation or replacement. Individual homeowners may be eligible for some of these 

loans (U.S. EPA 1999b). Some of these funds may also be used to build centralized wastewater 

treatment.

F.3.3.12 Wildlife BMPs

Steps taken to prevent wildlife from contaminating source water vary with the source and 

type of wildlife. Some reservoirs and lakes employ boats with noisemakers to scare seagulls and 

geese away. Many systems with control of the land around their reservoirs place fences on the 

water’s edge to keep out larger land animals and humans. To keep geese from feeding along the 

river bank just upstream from one of its intakes, the Philadelphia Water Department planted a 

riparian buffer and wildflower meadow and conducted a public education program to prevent 

people from feeding the geese (Philadelphia Water Department 2003). 
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-22 April 2010

F. 4 Case Studies of Existing Watershed Control Programs 

Many types of systems can benefit from a watershed control program. This section 

contains case studies of watershed control programs in place at different PWSs around the 

United States. These studies show how systems of different sizes and source water types and 

with varying regulatory authority have adopted watershed control programs to fit their specific 

needs. This section also describes advantages and disadvantages of implementing a watershed 

control program. 

As shown by the case studies below, successful watershed control programs will vary 

significantly in their approach to source protection. The systems in the case studies did not 

focus specifically on Cryptosporidium but on controlling microbial point and non-point 

sources and other contaminants. However, many of the elements noted in these case studies 

may be useful in watershed control programs addressing Cryptosporidium. However, since 

each watershed is different, the appropriate watershed control program (WCP) plan for each 

watershed will also be different. Some PWSs may need to develop efforts or measures 

completely different than those outlined in these examples. Furthermore, some of the 

approaches outlined in the referenced examples may not be suitable for other watersheds due 

to different site-specific conditions, and hence may not be used by PWSs developing a 

successful state-approved WCP for these watersheds. 

For more case studies, see the following sources: 

• Case Studies of Source Water Protection (U.S. EPA 2005a; 

http://cfpub.epa.gov/safewater/sourcewater/sourcewater.cfm?action=Case_Studies). 

• Section 319 Nonpoint Success Stories (U.S. EPA 2005b; 

www.epa.gov/owow/nps/Success319). 

• Watershed Success Stories – Applying the Principles and Spirit of the Clean Water 

Action Plan (U.S. EPA 2000d; 

http://nepis.epa.gov/EPA/html/DLwait.htm?url=/Exe/ZyNET.exe/20001P7N.PDF?ZyAct

ionP=PDF&Client=EPA&Index=2000 Thru 

2005&File=D%3A%5CZYFILES%5CINDEX%20DATA%5C00THRU05%5CTXT%5C

00000001%5C20001P7N.txt&Query=%E2%80%A2%09Watershed%20Success%20Stor

ies%20%E2%80%93%20Applying%20the%20Principles%20and%20Spirit%20of%20th

e%20Clean%20Water%20Action%20Plan&SearchMethod=3&FuzzyDegree=0&User=A

NONYMOUS&Password=anonymous&QField=pubnumber%5E%22800R00003%22&U

seQField=pubnumber&IntQFieldOp=1&ExtQFieldOp=1&Docs= ). 

• Protecting Sources of Drinking Water: Selected Case Studies in Watershed Management 

(U.S. EPA 1999a; http://www.epa.gov/safewater/sourcewater/pubs/swpcases.pdf). 
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-23 April 2010

Burlington, Vermont 

Medium Surface Water PWS, Watershed Located in Multiple Jurisdictions 

The City of Burlington has a population of 40,000 and is located on the shore of Lake 

Champlain, a 120-mile long, 12-mile wide lake that is the source of drinking water for the city 

and other municipalities. In such a large watershed with multiple landowners, it is difficult to 

control activities that affect water quality. Burlington addresses microbial pollution through a 

combination of land use control, reduction in combined sewer overflow, watershed 

restoration, and outreach. 

Through Act 250, the state of Vermont regulates land use near lake shores and rivers, 

accounting for new wastewater treatment plants and sewer systems, timber management, 

impervious surface area, water withdrawal by ski areas for snowmaking, and other issues. To 

address combined sewer overflow problems that were affecting Lake Champlain water quality, 

the city increased the capacity of its main wastewater treatment plant and extended the outfall 

far into the lake to dilute the effluent. The city separated the sanitary and storm sewers at its 

smaller plants. Two streams feeding into the lake that suffer from poor water quality are 

currently undergoing restoration, including retrofitting of existing storm water detention ponds, 

channel stabilization to prevent erosion, and outreach to change pet waste management, lawn 

care, and other practices (U.S. EPA 2001a). 

Manchester, New Hampshire 

Large Surface Water System Where State Plays an Active Role

The City of Manchester receives its water from Lake Massabesic, which is located 

approximately three miles east of downtown Manchester. Management of the water supply is 

primarily under the jurisdiction of the Manchester Water Works. The lake has a surface area 

of about 2,500 acres and a gross storage capacity of nearly 15 billion gallons. For more than 

120 years, this reservoir has served Manchester and five other communities. The Lake 

Massabesic water supply is supplemented by Tower Hill Pond, which has a gross storage 

capacity of 1.3 billion gallons. Manchester controls microbial pollution by restricting land use 

in the portions of the watershed controlled by the water works and the state.

The watershed area for the supply covers 42 square miles with over 25 percent owned 

and managed by the New Hampshire Department of Environmental Services (NHDES). The 

NHDES monitors these areas and controls recreational use through regulations posted in the 

surrounding area, which are enforced by a staff of watershed patrol officers. These regulations 

strictly prohibit such activities as waste disposal, horseback riding, boating, or any other 

activity that would immediately or indirectly endanger the surface water quality. Other areas 

of the watershed are primarily monitored by the Manchester Water Works and have regulated 

levels of outdoor recreation. Activities such as mountain biking or the establishment of docks 

and moorings are subject to review and permitting by this agency. Parts of Lake Massabesic 

closest to the intake are closed to all activity.
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LT2ESWTR Toolbox Guidance Manual F-24 April 2010

The NHDES has provided funding to the Manchester Water Works for the protection of 

its watershed, specifically funding the installation of a storm water treatment facility and a 

project to address erosion and sedimentation. DES also provided funding for emergency 

planning, wellhead protection management plans, drainage mapping, storm water best 

management practices, and public outreach and education. The source of all this funding was 

the source water protection-related set-asides from the Drinking Water State Revolving Fund 

(U.S. EPA 2001b). 

Springfield, Missouri

Large GWUDI and Surface Water System with Rapidly Urbanizing Watershed 

Springfield is a city of approximately 150,000 residents located in southwestern 

Missouri. Much of Springfield's bedrock is limestone and dolomite, and karst features are very 

pronounced. There are numerous streams, springs, and large concentrations of sinkholes in the 

area. The city's drinking water is provided by City Utilities of Springfield, a municipallyowned utility. The city uses a combination of springs, wells, reservoirs, and the James River 

to supply its daily demand of approximately 30 mgd. 

The three primary threats to Springfield's water quality that have been identified by its 

watershed committee are: 1) urbanization in the watershed; 2) wastewater treatment in 

suburban and rural areas, which consists primarily of septic systems on karst terrain; and 3) 

agriculture, especially animal waste from concentrated beef and dairy cattle operations. 

Agricultural and urban BMPs are the primary methods used to address microbial contaminants.

In 1984 a citizen-based Watershed Management Coordinating Committee was 

established to guide and oversee water protection efforts. The group later incorporated as a 

non-profit organization and renamed itself the Watershed Committee of the Ozarks. The 

committee's operating budget is provided by Greene County (in which much of the watershed 

lies), the City of Springfield (containing the bulk of the water users), and City Utilities (U.S. 

EPA 2001c). 

In 2001, the Committee hosted a workshop on conservation development and better site 

design for Springfield and Greene County planning and zoning staff members, hosted a 

workshop on agricultural BMPs for farmers, helped local developers incorporate stormwater 

BMPs and better site design into their developments, and helped local farmers install 

alternative watering facilities. The Committee currently has grants under Section 319 of the 

Clean Water Act to restore several of the area's watersheds. One of these grants involves a 

study of the current and future loading rates of sediment and nutrients and future construction 

of a wetland or forebay to treat runoff from the Valley Water Mill watershed as it enters the 

reservoir. Another project for the Little Sac River Watershed, which provides 85 percent of 

Springfield's water, has just gotten underway (Watershed Committee of the Ozarks 2001). 
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-25 April 2010

F.5 References

Atwill, E.R., L. Hou, B.M. Karle, T. Harter, K.W. Tate, and R.A. Dahlgren. 2002. Transport of 

Cryptosporidium parvum oocysts through vegetated buffer strips and estimated filtration 

efficiency. Appl. Environ. Microbiol. 68(11): 5517-27. 

AWWARF. 1991. Effective Watershed Management for Surface Water Supplies. Prepared by 

R.W. Robbins, J.L. Glicker, D.M. Bloem, and B.M. Niss, Portland (OR) Water Bureau. Denver: 

American Water Works Association Research Foundation. 

AWWA. 1999. Source Water Protection: Effective Tools and Techniques You Can Use. 1999 

Participant Manual. Denver: American Water Works Association. Developed for a technical 

training seminar for public water suppliers and local officials. 

Blewett, D.A. 1989. Disinfection and oocysts. Cryptosporidiosis. Proceedings of the First 

International Workshop, 1988. Ed. K.W. Angus and D.A. Blewett. Edinburgh: The Animal 

Disease Research Association. 107-116. 

Center for Watershed Protection. 1999. An Introduction to Better Site Design. Watershed 

Protection Techniques 3(2): 623-632. 

Coyne, M.S. and R.L. Blevins. 1995. Fecal bacteria in surface runoff from poultry-manured 

fields. In K.Steele (ed.), Animal Water and the Land-Water Interface, pp. 77-87. Boca Raton: 

Lewis Publishers, CRC Press.

Fairfax County. 2001. Wastewater Treatment Plant. 

http://www.co.fairfax.va.us/gov/DPWES/utilities/wwtrmnt_0600.htm. Last modified May 16, 

2001. Website accessed January 2002. 

Frankenberger, J.R. et al. 1999. A GIS-based variable source area hydrology model. Hydrologic 

Processes 13:805-822.

Gburek, W.J. and H.B. Pionke. 1995. Management strategies for land-based disposal of animal 

wastes: Hydrologic implications. pp. 313-323. In K.Steele (ed.), Animal Water and the LandWater Interface, pp. 77-87. Boca Raton: Lewis Publishers, CRC Press.

Jenkins, M.B, M. J. Walker, D. D. Bowman, L. C. Anthony, and W. C. Ghiorse. Use of a 

Sentinel System for Field Measurements of Cryptosporidium parvum Oocyst Inactivation in Soil 

and Animal Waste. Appl. Environ. Microbiol. 1999(65):1998-2005. 

Metcalf and Eddy. 1994. Final CSO Conceptual Plan and System Master Plan: Part II CSO 

Strategies. Prepared for the Massachusetts Water Resources Authority. Wakefield, 

Massachusetts.
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-26 April 2010

Moore, J.A. et al. 1988. Evaluating coliform concentrations in runoff from various animal waste 

management systems. Special Report 817. Agricultural Experimental Stations, Oregon State 

University, Corvallis, and the U.S.D.A., Portland, OR.

MWRD. 1999. Tunnel and Reservoir Plan. Metropolitan Water Reclamation District. 

http://www.mwrd.org/irj/portal/anonymous/tarp. Last modified August 6, 1999. Website 

accessed January 2002.

NALMS (North American Lake Management Society). March 2000. Best Management Practices 

to Protect Water Quality. 

NRCS. 1999. National Handbook of Conservation Practices. Natural Resources Conservation 

Service. http://www.nrcs.usda.gov/technical/standards/nhcp.html.

NRCS. 1992. Agricultural Waste Management Field Handbook. 

Ohio State University Extension. 1992. Ohio Livestock Manure and Wastewater Management 

Guide, Bulletin 604. http://ohioline.osu.edu/b604/index.html. Website accessed March 2003.

Ohio State University Extension. No date. Vegetation Filter Strips: Application, Installation, and 

Maintenance. AEX-467-94. http://ohioline.osu.edu/aex-fact/0467.html. Website accessed March 

2003. 

Ohio State University Extension. No date. Getting Started Grazing. Edited by Henry 

Bartholomew. http://ohioline.osu.edu/gsg/index.html.

Philadelphia Water Department. 2003. Philadelphia Projects. Website. 

http://www.phila.gov/water/index.html. Undated. Accessed February 12, 2003. 

Quinez-Daz, M. de J., M.M. Karpiscak; E. D. Ellman, and C.P. Gerba. Removal of Pathogenic 

and Indicator Microorganisms by a Constructed Wetland Receiving Untreated Domestic 

Wastewater. 2001. Environmental Sci. health Part A Tox Hazard Subst Environ Eng. 36(7): 

1311-1320. 

Schueler, T.R. 1999. Microbes and urban watersheds: concentrations, sources, and pathways. 

Watershed Protection Techniques. 3(1): 554-565. http://www.stormwatercenter.net. 

Thurston, J.A., C.P. Gerba, K.E. Foster, and M.M. Karpiscak. 2001. Water Res. 35(6):1547-

1551. Fate of Indicator Microorganisms, Giardia and Cryptosporidium in Subsurface Flow 

Constructed Wetlands. 
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-27 April 2010

U.S. Department of Agriculture. 2000. Waterborne Pathogens in Agricultural Watersheds. 

Watershed Science Institute. 

ftp://ftp-fc.sc.egov.usda.gov/WSI/pdffiles/Pathogens_in_Agricultural_Watersheds.pdf. Website 

accessed March 2003. 

 U.S. EPA. 2009. The Clean Water Act: Sanitary Sewer Overflow Strategy Summary of 2008 – 

2010. 

http://www.epa.gov/compliance/resources/publications/data/planning/priorities/fy2008prioritycw

asso.pdf. Website access May 2009.

U.S. EPA. 2008. National Pollutant Discharge Elimination System Permit Regulation and 

Effluent Limitation Guidelines and Standards for Concentrated Animal Feeding Operations 

(CAFOs). Federal Register 73(225): 70418-70486. November 20. 

U.S. EPA 2002a. Polluted Runoff (Nonpoint Source Pollution: Managing Nonpoint Source 

Pollution from Forestry. Pointer No. 8. EPA 841-F-96-004H. Office of Wetlands, Oceans, and 

Watersheds. http://www.epa.gov/owow/nps/facts/point8.htm. Last modified August 28, 2002. 

Website accessed March 2003. 

U.S. EPA. 2002b. Public Education and Outreach on Storm Water Impacts: Water Conservation 

Practices for Homeowners. http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm. Last 

updated November 25, 2002. Downloaded December 10, 2002. 

U.S. EPA. 2002c. Section 319 Success Stories. 3:154-155. Available online: 

http://www.epa.gov/owow/nps/Section319III/. 

U.S. EPA. 2001a. Proposed Revisions to CAFO Regulations (January 12, 2001; 66 FR 2960): 

Frequently Asked Questions. http://www.epa.gov/npdes/pubs/cafo_faq.pdf. Downloaded 

February, 2002. 

U.S. EPA. 2001b. Secondary Treatment Standards. 

http://cfpub.epa.gov/npdes/techbasedpermitting/sectreat.cfm. Last updated April 9, 2007. 

Downloaded January 22, 2002. 

U.S. EPA 2001c. Sanitary Sewer Overflows Frequently Asked Questions. Office of Wastewater 

Management. Web page updated March 20, 2001. 

http://cfpub.epa.gov/npdes/faqs.cfm?program_id=4. Website accessed January 2002. 

U.S. EPA. 2000a. Wastewater Technology Fact Sheet: Granular Activated Carbon Adsorption 

and Regeneration. Office of Water. EPA 832-F-00-017. September. 

http://www.epa.gov/owmitnet/mtb/carbon_absorption.pdf. 
Appendix F - Watershed Control Best Management Practices (BMPs) and Case Studies

LT2ESWTR Toolbox Guidance Manual F-28 April 2010

U.S. EPA 2000b. Storm Water Phase II Final Rule: Small MS4 Storm Water Program Overview. 

Fact Sheet 2.0. Office of Water. EPA 833-F-00-002. http://www.epa.gov/npdes/pubs/fact2-0.pdf. 

Website accessed March 2003.

U.S. EPA 2000c. Wastewater Technology Fact Sheet. Wetlands: Subsurface Flow. Office of 

Water EPA 832-F-00-023. September. 

http://www.epa.gov/owm/mtb/wetlands-subsurface_flow.pdf. Website accessed March 2003. 

U.S. EPA. 1999a. Protecting Sources of Drinking Water: Selected Case Studies in Watershed 

Management. Office of Water. EPA 816-R-98-016. April. 

http://www.epa.gov/enviroed/pdf/swpcases.pdf. Accessed December 10, 2002. 

U.S. EPA 1999b. Funding Decentralized Wastewater Systems Using the Clean Water State 

Revolving Fund. Office of Water (4204). EPA 832-F-09-005. 4 pages. 

http://www.epa.gov/owm/septic/pubs/arra_septic_fs.pdf. Website accessed March 2003.

U.S. EPA 1999c. Combined Sewer Overflow Management Fact Sheet: Sewer Separation. Office 

of Water. EPA 832-F-99-041. September. http://www.epa.gov/npdes/pubs/sepa.pdf. Website 

accessed March 2003.

U.S. EPA. 1996. Overview of the Storm Water Program. Office of Water. EPA 833-R-96-008. 

June. 42 pp. www.epa.gov/npdes/pubs/owm0195.pdf. Website accessed March 2003.

U.S. EPA. 1994. Combined Sewer Overflow (CSO) Policy; Notice. Federal Register

59(75):18688-18698. April 19. 

Vendrall, P.F., K.A. Teague, and D.W. Wolf. 1997. Pathogen indicator organism die-off in soil. 

ASA Annual Meeting, Anaheim, CA. 

Young, R.A. et al. 1980. Effectiveness of vegetated buffer strips in controlling pollution from 

feedlot runoff. J. Environ. Qual. 9:483-487. 
LT2ESWTR Toolbox Guidance Manual G-1 April 2010

Appendix G 

Review Criteria for Use by States When Reviewing Watershed Control (WSC) 

Program Plans

LT2 WSC 

Requirement Assessment Criteria Addressed in Sufficient Detail?

Watershed Control Program Plan

* Does the plan specifically address potential and 

existing Cryptosporidium sources in the watershed? 

* Have the proposed actions in the plan been clearly 

defined and sufficiently addressed? 

* Does the plan explain how the actions described are 

expected to contribute to specified goals? 

Does the plan prioritize its proposed efforts? Does it 

define short-term and long-term actions and prioritize 

them?

* 

Does the plan include, in detail, what other resources 

will be required to implement the watershed control 

measures? Does it identify the source(s) of those 

resources?

Review of Potential Sources 

* 

Has the area of influence been delineated in 

appropriate detail, taking into consideration available 

information about Cryptosporidium fate, transport 

and local hydrogeological characteristics? Have 

sensitive areas been identified?

Is the scale of the delineation appropriate for the 

watershed plan? Does it provide a level of detail 

sufficient for effective decisions to be made?

Has the intake location been identified relative to the 

water body?

Is any information available about time of travel in 

the watershed?

* 

Does it seem that all activities within the watershed 

that could result in Cryptosporidium contamination of 

the water supply have been identified and located?

* 

Have contaminant sources been located and 

described relative to the drinking water source intake 

location?

Have the likelihood and timing of releases of 

contamination been addressed?
Appendix G - Review Criteria for Use by States When Reviewing Watershed Control (WSC) Program Plans 

LT2 WSC 

Requirement Assessment Criteria Addressed in Sufficient Detail?

Are there permitted wastewater discharges (NPDES) 

of concern? If there are wastewater treatment plants 

* in the area of influence, systems should include 

information about their size, discharge quantity, and 

whether there has been any recent significant 

noncompliance with permit conditions. 

Are sludge disposal areas identified and 

characterized? Are there any locations in the 

* watershed where biosolids have been applied? 

Have they been identified? When in the year are 

they applied?

Have stormwater discharges been located? Are 

* there any discharges directly into the surface water 

supply?

Have septic systems been identified and located? Is 

* information available about their age, condition, 

design, and siting? 

Has land use been characterized and mapped? Are 

areas subject to zoning requirements or changes in 

LT2ESWTR Toolbox Guidance Manual G-2 April 2010

zoning? 

* 

If land uses in the watershed include agriculture, 

have the types of farming been identified? Are 

feedlots located? Are fields where manure is spread 

identified?

* Have Concentrated Animal Feeding Operations 

(CAFOs) been identified and located?

* Have natural sources of Cryptosporidium been 

identified and located?

* Have recreational areas (e.g., campgrounds, trailer 

parks) been identified and located?

Has any on-site landfilling, land treating, or surface 

impounding of waste other than landscape waste or 

construction and demolition debris taken place, and 

will such circumstances continue?

Does the analysis address the effectiveness of 

physical barriers (e.g., geology, hydraulic conditions, 

intake structure and location) at preventing the 

movement of contaminants to the drinking water 

source?
Appendix G - Review Criteria for Use by States When Reviewing Watershed Control (WSC) Program Plans 

LT2ESWTR Toolbox Guidance Manual G-3 April 2010

LT2 WSC 

Requirement Assessment Criteria Addressed in Sufficient Detail?

Have tributaries or areas of the reservoir with 

* evidence of high levels of microbial contamination 

been identified? If so where are they located relative 

to the intake?

If Cryptosporidium monitoring data exist for the 

watershed, have results been addressed and 

discussed?

Have recreational uses of the surface water supply 

* been identified? Has the effect of those uses on 

Cryptosporidium loading been addressed?

Are there portions of the watershed with high 

* percentages of impervious surfaces which might lead 

to increased stormwater runoff?

Is water quality monitoring and assessment 

information (305(b) Report available?

Have existing best management practices or controls 

been identified and located?

Is there any information available about the 

effectiveness of current pollution prevention 

activities? 

Potential Control Measures to Control Cryptosporidium 

Contamination 

* 

Do the control measures proposed specifically 

address the reduction of Cryptosporidium

contamination?

* 

Would the implementation of the proposed control 

measures take place in areas where there would be 

an impact on Cryptosporidium loading into the water 

supply?

If the proposed control measures are ongoing, has 

the utility explained how they would be sustained?

* 

Is the water utility in a position where it could 

implement the control measures itself, or would other 

parties be responsible?

* 

Are there implementation agreements between the 

utility and other parties responsible for 

implementation?

How does the utility track control measures 

implemented by other parties?
LT2 WSC 

Requirement Assessment Criteria Addressed in Sufficient Detail?

Has the water system responded adequately to 

concerns expressed about the source or watershed 

area in past inspections and sanitary surveys?

Appendix G - Review Criteria for Use by States When Reviewing Watershed Control (WSC) Program Plans 

LT2ESWTR Toolbox Guidance Manual G-4 April 2010