0.000

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0.400

0.600

0.800

1.000

1.200

1.400

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Log Inactivation

Log Inactivation

LT1ESWTR Disinfection Profiling 

and Benchmarking

Technical Guidance Manual 
Office of Water (4606M) 

EPA 816-R-03-004 

www.epa.gov/safewater 

May 2003 

Printed on Recycled Paper 
This document provides public water systems and States with Environmental Protection Agency’s 

(EPA’s) current technical and policy recommendations for complying with the disinfection profiling 

and benchmarking requirements of the Long Term 1 Enhanced Surface Water Treatment Rule 

(LT1ESWTR). The statutory provisions and EPA regulations described in this document contain 

legally binding requirements. This document is not a regulation itself, nor does it change or substitute 

for those provisions and regulations. Thus, it does not impose legally binding requirements on EPA, 

States, or public water systems. This guidance does not confer legal rights or impose legal obligations 

upon any member of the public. 

While EPA has made every effort to ensure the accuracy of the discussion in this guidance, the 

obligations of the regulated community are determined by statutes, regulations, or other legally binding 

requirements. In the event of a conflict between the discussion in this document and any statute or 

regulation, this document would not be controlling. 

The general description provided here may not apply to a particular situation based upon the 

circumstances. Interested parties are free to raise questions and objections about the substance of this 

guidance and the appropriateness of the application of this guidance to a particular situation. EPA and 

other decisionmakers retain the discretion to adopt approaches on a case-by-case basis that differ from 

those described in this guidance where appropriate. 

 

Mention of trade names or commercial products does not constitute endorsement or recommendation 

for their use. 

This is a living document and may be revised periodically without public notice. EPA welcomes public 

input on this document at any time. 
Acknowledgements 

The Environmental Protection Agency gratefully acknowledges the individual 

contribution of the following: 

*

Mr. Kevin W. Anderson, Pennsylvania Department of Environmental Protection 

 Mr. John E. Brutz, Gallitzin Water Authority 

 Mr. Jerry Biberstine, National Rural Water Association 

 Ms. Alicia Diehl, Texas Natural Resource Conservation Commission *

Mr. Bryce Feighner, Michigan Department of Environmental Quality 

 Mr. J.W. Hellums, Jr., Community Resource Group, Inc. 

 Mr. Allen J. Lamm, New Ulm Public Utilities 

*

Ms. Rebecca Poole, Oklahoma Department of Environmental Quality *

Mr. Jack Schulze, Texas Natural Resource Conservation Commission 

 Mr. Brian Tarbuck, Tolt Treatment Facility, Azurix CDM 

 Mr. Ritchie Taylor, Center for Water Resource Studies, Western Kentucky University 

 Mr. Steve Via, American Water Works Association 

*Participation supported by Association of State Drinking Water Administrators. 
CONTENTS 

May 2003 i EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

1. Introduction....................................................................................................................1 

1.1 Purpose of Document ...........................................................................................1 

1.2 Overview of Long Term 1 Enhanced Surface Water Treatment Rule .................2 

1.3 Overview of Disinfection Profiling and Benchmarking Requirements................3 

1.3.1 Significant Changes to Disinfection Practices.............................................6 

1.3.2 Obtaining State Approval for Significant Changes to Disinfection 

Practices.......................................................................................................6 

1.4 Using Disinfection Profiling and Benchmarking to Balance Rule 

Requirements ........................................................................................................8 

1.5 Contents of this Guidance Document...................................................................9 

2. Disinfection Segment...................................................................................................11 

2.1 Introduction.........................................................................................................11 

2.2 Identifying Disinfection Segments .....................................................................11 

2.2.1 Single Disinfection Segment .....................................................................12 

2.2.2 Multiple Disinfection Segments ................................................................13 

2.2.3 Disinfection Segments for Multiple Treatment Trains..............................16 

2.3 Steps Completed ..................................................................................................18 

2.4 Next Step .............................................................................................................18 

3. Data Collection ............................................................................................................19 

3.1 Introduction..........................................................................................................19 

3.2 Data Needed for the Disinfection Profile ............................................................19 

3.2.1 Peak Hourly Flow Rate..............................................................................20 

3.2.2 Residual Disinfectant Concentration .........................................................21 

3.2.3 Temperature...............................................................................................22 

3.2.4 pH ..............................................................................................................22 

3.3 Data Collection Worksheets ................................................................................25 

3.4 Steps Completed ..................................................................................................25 

3.5 Next Step .............................................................................................................25 

3.6 References............................................................................................................26 

4. Calculating CT.............................................................................................................27 

4.1 Introduction..........................................................................................................27 

4.2 What is CT? .........................................................................................................27 

4.3 Determining “C”..................................................................................................28 

4.4 Determining “T” ..................................................................................................28 

4.4.1 Volume ......................................................................................................29 

4.4.2 Theoretical Detention Time.......................................................................30 

4.4.3 Baffling Factor...........................................................................................31 

4.4.4 Calculate Contact Time .............................................................................32 

4.5 Calculate CTcalc ....................................................................................................35 

4.6 Steps Completed ..................................................................................................38 

4.7 Next Step .............................................................................................................38 

4.8 References............................................................................................................38 
Contents 

EPA Guidance Manual ii May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

5. Calculating Inactivation...............................................................................................39 

5.1 Introduction..........................................................................................................39 

5.2 Log Reduction......................................................................................................39 

5.3 Determining CT Required....................................................................................40 

5.3.1 CT99.9 for Giardia ......................................................................................41 

5.3.2 CT99.99 for Viruses......................................................................................43 

5.4 Calculating Actual Log Inactivation for One Disinfection Segment...................43 

5.5 Calculating Actual Log Inactivation for Multiple Disinfection Segments ..........46 

5.6 Steps Completed ..................................................................................................49 

5.7 Next Step..............................................................................................................49

6. Developing the Disinfection Profile and Benchmark ..................................................51 

6.1 Introduction..........................................................................................................51 

6.2 Constructing a Disinfection Profile .....................................................................52 

6.3 The Disinfection Benchmark ...............................................................................55 

6.4 Significant Changes to Disinfection Practices.....................................................56 

6.5 Benchmark Calculations ......................................................................................56 

6.6 Steps Completed ..................................................................................................59 

6.7 Next Step..............................................................................................................59

7. Evaluating Disinfection Practice Modifications ..........................................................61 

7.1 Introduction..........................................................................................................61 

7.2 System Reporting Requirements .........................................................................63 

7.3 Simultaneous Compliance ...................................................................................63 

7.3.1 Changes to the Point of Disinfection...........................................................65 

7.3.2 Changes to the Disinfectant(s) Used in the Treatment Plant.......................66 

7.3.3 Changes to the Disinfection Process ...........................................................69 

7.3.4 Other Modifications.....................................................................................70 

7.4 How the State will Use the Benchmark...............................................................71 

7.5 Steps Completed ..................................................................................................72 

7.6 References.............................................................................................................72 

8. Treatment Considerations ..............................................................................................73 

8.1 Introduction...........................................................................................................73 

8.2 Alternative Disinfectants and Oxidants ................................................................73 

 8.2.1 Chloramines (NH2Cl)..................................................................................74 

 8.2.2 Ozone (O3) ..................................................................................................75 

 8.2.3 Chlorine Dioxide (ClO2).............................................................................76 

 8.2.4 Potassium Permanganate (KMnO4) ............................................................77 

 8.2.5 Ultraviolet Radiation (UV) .........................................................................77 

 8.2.6 Comparison of Disinfectants.......................................................................78 

8.3 Enhanced Coagulation and Softening..................................................................80 

8.4 Increasing Contact Time......................................................................................82 

8.5 Membranes...........................................................................................................85 

8.6 References............................................................................................................88 
 Contents 

May 2003 iii EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Appendices 

Appendix A. Glossary ...............................................................................................91 

Appendix B. CT Tables...........................................................................................101 

Appendix C. Blank Worksheets..............................................................................113 

Appendix D. Examples............................................................................................121 

Appendix E. Tracer Studies ....................................................................................157 

Appendix F. Calculating the Volume of Each Sub-Unit ........................................167 

Appendix G. Baffling Factors .................................................................................171 

 Appendix H. Conservative Estimate and Interpolation Examples ..........................187 

Figures 

Figure 1-1. Sample Disinfection Profile......................................................................4 

Figure 1-2. Disinfection Profile and Benchmark Decision Tree.................................7 

Figure 2-1. Plant Schematic Showing A Conventional Filtration Plant With One 

Disinfection Segment..............................................................................12 

Figure 2-2. Plant Schematic Showing Two Disinfection Segments..........................13 

Figure 2-3. Plant Schematic Showing One Injection Point with Multiple 

Disinfection Segments............................................................................14 

Figure 2-4. Plant Schematic Showing Two Injection Points with Multiple 

Disinfection Segments............................................................................15 

Figure 2-5. Plant Schematic Showing Identical Treatment Trains and Multiple 

Disinfection Segments............................................................................16 

Figure 2-6. Plant Schematic Showing Multiple Treatment Trains and Multiple 

Disinfection Segments............................................................................17 

Figure 4-1. Baffling Characteristics of a Pipe and Clearwell....................................31 

Figure 6-1. Example of a Completed Disinfection Profile........................................52 

Figure 7-1. Example of Moving the Point of Pre-disinfectant Application ..............65 

Figure 7-2. Example of Changing Disinfectant Type ...............................................68 

Figure 7-3. Changing Pre-disinfection Location and Type of Disinfectant ..............68 

Figure 8-1. Particles Removed Through Membrane Technologies...........................87 
Contents 

EPA Guidance Manual iv May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Tables 

Table 4-1. Volume Equations for Shapes.................................................................30 

Table 4-2. Baffling Factors ......................................................................................32 

Table 7-1. Removal and Inactivation Requirements................................................62 

Table 7-2. Typical Removal Credits and Inactivation Requirements for 

Various Treatment Technologies............................................................62 

Table 8-1. Study Results on Changing Primary and Secondary Disinfectants ........79 
 Contents 

May 2003 v EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

ABBREVIATIONS 

List of common abbreviations and acronyms used in this document: 

AWWA American Water Works Association 

BF Baffling Factor 

C Residual Disinfectant Concentration 

CFR Code of Federal Regulations 

CT The Residual Disinfectant Concentration (mg/l) Multiplied by the 

Contact Time (minutes) 

CWS Community Water System 

DBP Disinfection Byproduct 

DOC Dissolved Organic Carbon 

DOM Dissolved Organic Matter 

EPA Environmental Protection Agency 

FBRR Filter Backwash Recycling Rule 

GAC Granular Activated Carbon 

gal Gallons 

gpm Gallons per Minute 

GWUDI Ground Water Under Direct Influence of Surface Water 

HAA5 Haloacetic Acids 

hrs Hours 

IESWTR Interim Enhanced Surface Water Treatment Rule 

LT1ESWTR Long Term 1 Enhanced Surface Water Treatment Rule 

MCL Maximum Contaminant Level 

MG Million Gallons 

mg/L Milligrams per Liter 

MGD Million Gallons per Day 

m/h Meters per Hour 

MRDL Maximum Residual Disinfectant Level 

NCWS Non-community Water System 

NTNCWS Non-transient Non-community Water System 
Contents 

EPA Guidance Manual vi May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

PWS Public Water System 

PWSID Public Water System Identification 

Q Peak Hourly Flow Rate 

RO Reverse Osmosis 

SCADA Supervisory Control and Data Acquisition 

SDWA Safe Drinking Water Act 

Stage 1 DBPR Stage 1 Disinfectants and Disinfection Byproduct Rule 

SWTR Surface Water Treatment Rule 

T Contact Time (minutes) 

TDT Theoretical Detention Time 

THM Trihalomethanes 

TOC Total Organic Carbon 

TT Treatment Technique 

TTHM Total Trihalomethanes 

UV Ultraviolet 

V Volume 

WTP Water Treatment Plant 

X log inactivation Reduction to 1/10x

 of original concentration by disinfection 

X log removal Reduction to 1/10x of original concentration by physical removal 

μ Micron (10-6 meter) 

μg/L Micrograms per liter 
 Contents 

May 2003 vii EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

MARGIN ICONS

Icons and text in the margins of this document highlight information and additional 

resources. These icons are shown below with brief descriptions of their uses or the types of 

information they may be used to highlight. 

Indicates a reference to the federal regulations. 

Indicates the need to consult with the State. 

Indicates additional references or highlights important 

information. 

Indicates worksheets. 

Indicates sampling or data collection requirements. 

Indicates applicability criteria. 

Indicates a helpful hint or suggestion. 

Highlights a key point or key information. 

Indicates the next step. 
Contents 

EPA Guidance Manual viii May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

This Page Intentionally Left Blank 
1. INTRODUCTION 

May 2003 1 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

In this Chapter: 

• Purpose of Document 

• Overview of 

LT1ESWTR 

• Overview of 

Disinfection Profiling 

and Benchmarking 

Requirements 

• Using Disinfection 

Profiling and 

Benchmarking to 

Balance Rule 

Requirements 

• Contents of this 

Guidance Document 

40 CFR Section 141.501 

1.1 PURPOSE OF DOCUMENT

This guidance manual is intended to help public water systems 

(PWSs) comply with the disinfection profiling and 

benchmarking requirements of the Long Term 1 Enhanced 

Surface Water Treatment Rule (LT1ESWTR). The 

requirements of the LT1ESWTR apply to PWSs that: 

• Serve fewer than 10,000 people; and, 

• Are classified as either surface water or ground water 

under the direct influence of surface water (GWUDI). 

This manual explains disinfection profiling and benchmarking, 

discusses when and why they are necessary, and provides 

guidance as to how to compile a disinfection profile and how to 

calculate the benchmark. This guidance manual also discusses 

how systems and States may use these data to make decisions 

about disinfection practices. Copies of this document and other 

documents that pertain to LT1ESWTR may be obtained by: 

• Contacting the appropriate State office; 

• Calling the Safe Drinking Water Hotline at 

 1-800-426-4791; 

• Downloading from EPA’s website at 

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

• Calling the National Service Center for Environmental 

Publications at 1-800-490-9198 or visiting their website 

at http://www.epa.gov/ncepihom/. 

Systems serving 10,000 people or more have different profiling 

requirements and should refer to the Interim Enhanced Surface 

Water Treatment Rule Guidance Document: Disinfection Profiling 

and Benchmarking, August 1999 (EPA 815-R-99-013).
1. Introduction 

EPA Guidance Manual 2 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Key components of 

LT1ESWTR. 

1.2 OVERVIEW OF LONG TERM 1 ENHANCED 

SURFACE WATER TREATMENT RULE 

The LT1ESWTR is a Federal regulation that establishes a 

treatment technique to control Cryptosporidium. The rule 

applies to public water systems serving fewer than 10,000 

people and classified as either a surface water system or a 

GWUDI system. Key components of the LT1ESWTR are: 

• Systems must provide a minimum of 2-log (99%) 

removal of Cryptosporidium.

• Systems with conventional or direct filtration plants 

must meet more stringent combined filter effluent 

turbidity limits and must meet new requirements for 

individual filter effluent turbidity. 

• Systems using alternative filtration techniques (defined 

as filtration other than conventional, direct, slow sand, 

or diatomaceous earth) must demonstrate to the State the 

ability to consistently achieve 2-log (99%) removal of 

Cryptosporidium and comply with specific Stateestablished combined filter effluent turbidity 

requirements. 

• Systems that meet the filtration avoidance criteria must 

comply with additional watershed control requirements 

to address Cryptosporidium.

• Systems must develop a disinfection profile unless the 

State determines that the disinfection profile is 

unnecessary. The State can only make this determination 

if the system can demonstrate that the levels of Total 

Trihalomethanes (TTHM) and Haloacetic Acids 

(HAA5) are below 0.064 mg/L and 0.048 mg/L, 

respectively. The system must develop a benchmark if 

the system was required to develop a disinfection profile 

and subsequently plans a significant change to 

disinfection practices. 

• New, finished water reservoirs must be covered. 

• Cryptosporidium is now included in the Federal 

definition of GWUDI. 
 1. Introduction

May 2003 3 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking 

40 CFR Sections 141.503 

(c) and 141.503 (d). 

1.3 OVERVIEW OF DISINFECTION PROFILING 

AND BENCHMARKING REQUIREMENTS

The requirements for disinfection profiling and benchmarking 

described in this manual are part of the LT1ESWTR, published 

by EPA on January 14, 2002. The disinfection profiling and 

benchmarking requirements of the LT1ESWTR apply only to 

community and non-transient non-community water systems 

using surface water or GWUDI as a source and serving fewer 

than 10,000 people. 

Transient water systems are not required to create a disinfection 

profile, unless directed by the State. However, transient systems 

are encouraged to use the profile as a tool to help evaluate their 

system. 

40 CFR Section 141.530 

Systems should balance 

disinfection practices with 

Stage 1 Disinfectants and 

Disinfection Byproducts 

Rule requirements. See 

Section 1.4 for more 

information on this topic. 

 A disinfection profile is a graphical representation of a 

system’s level of Giardia lamblia (referred to as Giardia) or 

virus inactivation measured during the course of a year. 

A benchmark is the lowest monthly average microbial 

inactivation during the disinfection profile time period. A 

disinfection benchmark is required only if a system was required 

to develop a disinfection profile and decides to make significant 

changes to its disinfection practices. 

The LT1ESWTR requires systems to analyze their current 

disinfection practices before making changes to these practices. 

This analysis will result in a disinfection profile. Figure 1-1 

depicts a sample disinfection profile. 
1. Introduction 

EPA Guidance Manual 4 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Sample Disinfection 

Profile 

40 CFR Section 141.531 

Systems that believe they 

have data that meet the 

avoidance criteria should 

consult with the State to 

determine whether they are 

required to profile. 

Systems not required to 

profile are encouraged to 

complete and use the 

profile as a tool to help 

evaluate their system. 

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

0 4 8 12 16 20 24 28 32 36 40 44 48 52

Week Tested

Log Inactivation

Figure 1-1. Sample Disinfection Profile 

Each system must complete a disinfection profile unless the 

State determines that the system’s profile is unnecessary. This 

determination will be based on TTHM and HAA5 levels in the 

distribution system. States may determine that a profile is 

unnecessary only if: 

• TTHM and HAA5 samples are collected after January 1, 

1998. 

• The samples are collected in the month with warmest 

water temperature and at the point of maximum 

residence time in the distribution system. 

• TTHM and HAA5 levels in the samples are less than 

80% of the maximum contaminant level (MCL). This 

equates to TTHM <0.064 mg/L and HAA5 <0.048 mg/L. 

Systems that do not have data meeting the avoidance criteria by 

July 1, 2003, for systems serving 500 to 9,999 people and 

January 1, 2004, for systems serving fewer than 500 people 

MUST begin to create a disinfection profile. 
 1. Introduction

May 2003 5 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking 

40 CFR Section 141.532 

40 CFR Section 141.536 

CT = C x T 

C = Residual disinfectant 

concentration, mg/L 

T = Contact time, minutes 

See Chapter 4 for more 

information on CT.

40 CFR Section 141.540 

Systems serving 500 to 9,999 people must begin collecting data 

for the disinfection profile by July 1, 2003. Systems serving 

fewer than 500 people must begin collecting data for the 

disinfection profile by January 1, 2004. 

In order to create a disinfection profile, systems should: 

• Identify disinfection segments; 

• Collect required data for each segment; 

• Calculate CT; and, 

• Calculate inactivation. 

These topics are described in more detail in Chapters 2 - 5 of 

this document. 

Systems must create and retain the profile in graphic form. The 

profile must be made available for review by the State as part of 

a sanitary survey. 

Before any significant changes may be made to the disinfection 

process, a system must calculate the benchmark value based on 

disinfection practices. The benchmark is the lowest monthly 

average microbial inactivation during the disinfection profile 

time period (See Chapter 6 for more detail). The system is 

required to calculate a benchmark if both of the following apply: 

• The system is required to complete a disinfection profile; 

 and, 

• The system plans to make a significant change to 

disinfection practices. 

Systems must also consult with the State for approval before

making any significant change to disinfection practices. 
1. Introduction 

EPA Guidance Manual 6 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

40 CFR Section 141.541 

40 CFR Section 141.542 

1.3.1 Significant Changes to Disinfection 

Practices 

Significant changes to disinfection practice include: 

• Changes to the point of disinfection; 

• Changes to the disinfectant(s) used in the treatment 

plant; 

• Changes to the disinfection process; or, 

• Any other modification identified by the State. 

1.3.2 Obtaining State Approval for Significant 

Changes to Disinfection Practices 

If a system is required to complete a disinfection profile and 

intends to make a change as listed in Section 1.3.1, it must 

consult with the State for approval. The following information 

must be submitted to the State: 

• A description of the proposed change; 

• The disinfection profile for Giardia lamblia (and, if 

necessary, viruses) and disinfection benchmark; 

• An analysis of how the proposed change will affect the 

current levels of disinfection; and, 

• Any additional information requested by the State. 

The flowchart in Figure 1-2 provides information on the 

LT1ESWTR disinfection profiling and benchmarking 

requirements. 
 1. Introduction

May 2003 7 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking 

Figure 1-2. Disinfection Profile and Benchmark Decision Tree 

Is the source

water classified as either surface 

or GWUDI?

Is the system

a transient non-community

water system?

Does the

system serve fewer than 

10,000 people?

YES NO

No disinfection profiling

or benchmarking required

under the LT1ESWTR

provisions.

NO YES

System must comply with

disinfection profiling and 

benchmarking requirements 

under IESWTR.

NO

Has the system tested

TTHM and HAA5 after January 1, 1998

during the month of warmest water temperature 

and at the point of maximum residence time 

in the distribution system? 

YES

Was the annual

TTHM level < 0.064 mg/L and

the annual HAA5 level 

< 0.048 mg/L?

YES

YES

NO

System must profile 

Giardia inactivation.1,2

NO

Did the 

system develop a disinfection 

profile and keep it 

on file?

NO

TT violation3

YES Are there plans 

to modify the existing 

disinfection practice?

System must calculate 

the benchmark for

Giardia inactivation and 

consult with the State.2

YES

TT violation3

Did the system 

calculate the benchmark for Giardia inactivation, 

and consult with the 

State?

NO

No disinfection 

benchmark required.2

NO

System is in compliance 

with disinfection profiling 

and benchmarking 

requirements.

YES

Did the State 

determine that a disinfection profile 

was unnecessary?

YES

NO

1. If using chlorine dioxide, ozone, or chloramines as a primary disinfectant the system must profile and benchmark viral 

inactivation as well. 

2. Disinfection profile must be kept on file for State to review during sanitary survey. 

3. Tier 2 violation. Public notification is required within 30 days. 
1. Introduction 

EPA Guidance Manual 8 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Systems must balance 

disinfection practices with 

Stage 1 DBPR 

requirements. The 

disinfection profile and 

benchmark information 

will assist systems and 

States with achieving this 

balance. 

More information on the 

Stage 1 DBPR is available 

at EPA’s website 

(http://www.epa.gov/safewater/

mdbp/implement.html).

Systems may be 

considering changes to 

disinfection practices 

during the disinfection 

profiling process to 

address Stage 1 DBPR 

requirements. Systems 

should contact the State 

prior to making changes 

to disinfection practices 

and discuss how these 

changes will affect the 

disinfection profiling 

process. 

1.4 USING DISINFECTION PROFILING AND 

BENCHMARKING TO BALANCE RULE 

REQUIREMENTS

The LT1ESWTR disinfection profiling and benchmarking 

requirements will protect public health by assessing the risk of 

exposure to Giardia and viruses as systems begin to take steps 

to comply with Stage 1 Disinfectants and Disinfection 

Byproducts Rule (Stage 1 DBPR) requirements. For systems 

classified as either surface water or GWUDI serving fewer than 

10,000 people, the MCLs for TTHM and HAA5 become 

effective January 1, 2004. The Stage 1 DBPR established an 

MCL of 0.080 mg/L for TTHM and 0.060 mg/L for HAA5. 

TTHM are the sum of the concentrations in milligrams per 

liter of the trihalomethane compounds (trichloromethane 

[chloroform], dibromochloromethane, bromodichloromethane, 

and tribromomethane [bromoform]). The MCL for TTHM is 

0.080 mg/L starting January 1, 2004. Prior to January 1, 2004, 

there is no Federal MCL for TTHM for systems serving fewer 

than 10,000 people. 

HAA5 are the sum of the concentrations in milligrams per liter 

of the haloacetic acid compounds (monochloroacetic acid, 

dichloroacetic acid, trichloroacetic acid, monobromoacetic 

acid, and dibromoacetic acid). The MCL for HAA5 is 0.060 

mg/L starting January 1, 2004. Prior to January 1, 2004, there 

is no Federal MCL for HAA5 for systems serving fewer than 

10,000 people. 

In order to meet the requirements of the Stage 1 DBPR, 

systems may have to consider changes to their disinfection 

practices. Disinfection byproducts (DBPs) such as TTHM and 

HAA5 are formed when organic materials react with 

disinfectants such as chlorine. Therefore, systems with high 

levels of DBPs may need to modify disinfection practices to 

reduce the formation of DBPs. However, changes such as the 

use of lower concentrations of disinfectant will also lessen 

microbial inactivation. Decreasing the amount of disinfectant 

too much may produce water of unsatisfactory microbial 

quality. Disinfection profiling and benchmarking will help to 

ensure that no significant reduction in microbial protection 

results as a system changes disinfection practices to meet the 

TTHM and HAA5 MCLs under Stage 1 DBPR. 
 1. Introduction

May 2003 9 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking 

Contents of Document 

1.5 CONTENTS OF THIS GUIDANCE 

DOCUMENT

This document is organized in the following sections and 

chapters: 

• Chapter 1 - Introduction 

• Chapter 2 –Disinfection Segment 

This chapter defines the term disinfection segment 

and describes how a system would identify the 

disinfection segment(s). 

• Chapter 3 – Data Collection 

This chapter presents the data collection 

requirements for creating a disinfection profile. 

• Chapter 4 – Calculating CT 

This chapter presents information and examples on 

how to calculate CT to be used in the development 

of a disinfection profile. 

• Chapter 5 –Calculating Inactivation 

This chapter presents information and examples on 

how to calculate Giardia and virus inactivation 

values to be used in the development of a 

disinfection profile. 

• Chapter 6 – Developing the Disinfection Profile and 

Benchmark 

This chapter provides information on how to 

develop a disinfection profile using calculated 

inactivation values. This chapter also presents 

information on when the disinfection benchmark 

must be calculated and how to calculate the 

benchmark. 

• Chapter 7 – Evaluating Disinfection Practice 

Modificiations 

This chapter discusses how the disinfection profile 

and benchmark can be used to assess system 

modifications that may be considered for 

compliance. It also discusses the issues associated 

with each modification. 
1. Introduction 

EPA Guidance Manual 10 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Contents of Document 

• Chapter 8 – Treatment Considerations 

This chapter presents case studies and other 

information that may assist systems with 

LT1ESWTR and other rule compliance. 

 Appendices

• Appendix A – Glossary 

• Appendix B – CT Tables 

• Appendix C – Blank Worksheets 

• Appendix D – Examples 

• Appendix E – Tracer Studies 

• Appendix F – Calculating the Volume of each 

 Sub-unit 

• Appendix G – Baffling Factors 

• Appendix H – Conservative Estimate and 

 Interpolation Examples 

 
2. DISINFECTION SEGMENT 

May 2003 11 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

In this Chapter: 

• Identifying 

Disinfection Segments

• Steps Completed

• Next Step

All monitoring points are 

located before or at the 

first customer (40 CFR 

141.533(d)).

Systems need to identify 

disinfection segments. A 

disinfection profile must 

be developed using all 

disinfection segments. 

2.1 INTRODUCTION 

The first step in developing a disinfection profile should be to 

identify the disinfection segments within the plant. A 

disinfection segment is a section of a treatment system 

beginning at one disinfectant injection or monitoring point and 

ending at the next disinfectant injection or monitoring point. 

Every disinfectant injection point is the start of a new 

disinfection segment. Every injection point has an associated 

monitoring point. However, a plant may have only one 

disinfectant point, and choose to monitor at two or more points, 

creating two or more disinfection segments. A system must 

monitor the residual disinfectant before or at the first customer 

(40 CFR Section 141.533(d)). The disinfection segment could 

include distribution pipes and storage tanks located prior to the 

first customer. 

Plants with multiple treatment trains will have multiple 

disinfection segments. If the treatment trains are identical, and 

flow is split equally, the disinfection segments for each train 

should be the same. If the treatment trains are very different, 

the system should identify all disinfection segments and 

develop a disinfection profile for each train separately. 

2.2 IDENTIFYING DISINFECTION SEGMENTS

The suggested starting point for analyzing a plant is to develop 

a summary of the unit processes, disinfectant injection and 

monitoring points. It may be helpful to use a sketch or plan 

drawing of the plant. Drawings like those shown in Figures 2-1 

through 2-6 may help in defining disinfection segments. 
2. Disinfection Segment 

EPA Guidance Manual 12 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example: 

Single Disinfection 

Segment 

2.2.1 Single Disinfection Segment 

Figure 2-1 shows a simple plant, with one injection point and one 

monitoring point, resulting in a single disinfection segment. The 

disinfection segment begins at the chlorine injection point prior to 

the clearwell and ends at the monitoring point after the clearwell. 

Distribution

System

Sedimentation

Chlorine

Injected

Filtration

Clearwell

Monitoring Point

Cl2 residual

Temperature

pH

One Disinfection Segment:

One injection point, one monitoring point

Intake

Coagulation Flocculation

Figure 2-1: Plant Schematic Showing A Conventional 

Filtration Plant With One Disinfection Segment 
 2. Disinfection Segment 

May 2003 13 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example: 

Two Disinfection 

Segments 

2.2.2 Multiple Disinfection Segments 

Figure 2-2 is an example of a system with two injection points 

and two monitoring points, resulting in two disinfection 

segments. Disinfection Segment 1 starts at the chlorine 

injection point (prior to the coagulation basin) and ends at the 

monitoring point after the filters. Disinfection Segment 2 starts 

at the chlorine injection point after the first monitoring point 

(between the filter and the clearwell) and ends at the 

monitoring point after the clearwell and prior to the first 

customer. Even for this simple plant, the analysis of how much 

disinfection takes place in the plant may be complicated. In 

this example, disinfection occurs in the coagulation basin, 

flocculation basin, sedimentation basin, filter, and clearwell, as 

well as in all the associated piping. 

Distribution

System

Sedimentation

Chlorine

Injected

Coagulation

Clearwell

Disinfection Segment 2

Monitoring Point

Cl2 residual

Temperature

pH

Intake

Chlorine

Injected

Disinfection Segment 1

Monitoring Point

Cl2 residual

Temperature

pH

Disinfection

Segment 2

Disinfection Segment 1

Filtration

Flocculation

Figure 2-2: Plant Schematic Showing Two 

Disinfection Segments 
2. Disinfection Segment 

EPA Guidance Manual 14 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example: 

Multiple Disinfection 

Segments

Chlorine residuals tend to 

decline as water moves 

through the treatment 

plant. The benefit of 

monitoring the chlorine 

residual at multiple 

locations is to obtain 

additional credit for the 

higher chlorine levels that 

exist at intermediate 

points in the plant. See 

Chapter 4 for more on the 

benefits of higher 

measured chlorine 

concentrations when 

calculating log 

inactivations. 

Figure 2-3 is an example of a system with one injection point 

and multiple monitoring points. Although the system is 

required to have a minimum of one monitoring point, the 

chlorine is sampled in four locations to obtain higher chlorine 

residual values throughout the treatment train for Surface 

Water Treatment Rule (SWTR) compliance as opposed to 

monitoring at one location after the clearwell where the 

chlorine residual will be much less than measurements prior to 

the clearwell. The first disinfection segment starts at the 

chlorine injection point and ends at the first sampling point 

(between the coagulation and flocculation basins). The next 

three disinfection segments begin at one sampling point and 

end at the following sampling point. Therefore, even though 

there is only one injection point at this system, there are four 

disinfection segments. 

Distribution

System

Sedimentation

Chlorine

Injected

Coagulation

Clearwell

Disinfection Segment 4

Monitoring Point

Cl2 residual

Temperature

pH

Intake

Disinfection

Segment 3

Disinfection

Segment 1

Disinfection

Segment 2

Disinfection Segment 2

Monitoring Point

Cl2 residual

Temperature

pH

Disinfection Segment 1

Monitoring Point

Cl2 residual

Temperature

pH

Filtration

Disinfection

Segment 4

Disinfection Segment 3

Monitoring Point

Cl2 residual

Temperature

pH

Flocculation

Figure 2-3: Plant Schematic Showing One Injection 

Point with Multiple Disinfection Segments 
 2. Disinfection Segment 

May 2003 15 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example: 

Multiple Disinfection 

Segments

Figure 2-4 is an example of a more complicated plant 

schematic where the plant’s multiple disinfection segments 

have been defined. In Figure 2-4 chlorine is sampled in three 

locations to obtain additional credit for higher chlorine residual 

values that exist at intermediate points in the plant. Ammonia 

is added prior to the clearwell to form chloramines. The use of 

a different disinfectant results in a new disinfection segment. 

Distribution

System

Sedimentation

Chlorine

Injected

Coagulation

Clearwell

Disinfection Segment 4

Monitoring Point

Chloramine residual

Temperature

Intake

Ammonia

Injected

Disinfection

Segment 3

Disinfection

Segment 1

Disinfection

Segment 2

Disinfection Segment 2

Monitoring Point

Cl2 residual

Temperature

pH

Disinfection Segment 1

Monitoring Point

Cl2 residual

Temperature

pH

Filtration

Disinfection

Segment 4

Disinfection Segment 3

Monitoring Point

Cl2 residual

Temperature

pH

Flocculation

Figure 2-4: Plant Schematic Showing Two Injection 

Points with Multiple Disinfection Segments
2. Disinfection Segment 

EPA Guidance Manual 16 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example: Disinfection 

Segments for Identical 

Treatment Trains 

2.2.3 Disinfection Segments for Multiple 

Treatment Trains 

For some system configurations, one profile would not 

accurately characterize the entire treatment process. In these 

cases, multiple profiles are suggested. Figure 2-5 shows a 

plant with multiple treatment trains and multiple disinfection 

segments. In this example, the treatment trains are identical in 

that all unit processes in both trains have the same dimensions, 

operating rates, and hydraulic capacity. Since the treatment 

trains are identical, and flow is split equally between the 

treatment trains, the disinfection profile for Disinfection 

Segments 1a and 1b should be identical. Similarly, the 

disinfection profile for Disinfection Segments 2a and 2b should 

be identical. However, systems should check with the State to 

determine if separate disinfection profiles are required for each 

treatment train. 

Distribution

System

Sedimentation

Chlorine

Injected

Coagulation

Clearwell

Disinfection Segment 2b

Monitoring Point

Cl2 residual

Temperature

pH

Intake

Chlorine

Injected

Disinfection Segment 1b

Monitoring Point

Cl2 residual

Temperature

pH

Disinfection

Segment 2b

Disinfection Segment 1b

Filtration

Flocculation

Disinfection Segment 1a

Monitoring Point

Cl2 residual

Temperature

pH

Disinfection Segment 2a

Monitoring Point

Cl2 residual

Temperature

pH

Disinfection

Disinfection Segment 1a Segment 2a

Sedimentation

Coagulation

Clearwell

Chlorine

Injected

Filtration

Flocculation

Chlorine

Injected

Distribution

System

1/2 of Flow

1/2 of Flow

Flow

Controller*

*Note: Flow is split equally between treatment trains.

Figure 2-5: Plant Schematic Showing Identical 

Treatment Trains and Multiple Disinfection Segments 
 2. Disinfection Segment 

May 2003 17 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example: Disinfection 

Segments for Multiple 

Treatment Trains 

Figure 2-6 shows a plant with two treatment trains and multiple 

disinfection segments. Although the treatment trains are 

identical, in this example, the flow is not split equally between 

the treatment trains. The disinfection profile for Disinfection 

Segments 1a and 1b may not be identical. Similarly, the 

disinfection profile for Disinfection Segments 2a and 2b may 

not be identical. Therefore, this plant should develop a 

separate disinfection profile for each treatment train. Again, 

the system should check with the State on this issue. 

Distribution

System

Sedimentation

Chlorine

Injected

Coagulation

Clearwell

Disinfection Segment 2b

Monitoring Point

Cl2 residual

Temperature

pH

Intake

Chlorine

Injected

Disinfection Segment 1b

Monitoring Point

Cl2 residual

Temperature

pH

Disinfection

Segment 2b

Disinfection Segment 1b

Filtration

Flocculation

Disinfection Segment 1a

Monitoring Point

Cl2 residual

Temperature

pH

Disinfection Segment 2a

Monitoring Point

Cl2 residual

Temperature

pH

Disinfection

Disinfection Segment 1a Segment 2a

Sedimentation

Coagulation

Clearwell

Chlorine

Injected

Filtration

Flocculation

Chlorine

Injected

Distribution

System

2/3 of Flow

1/3 of Flow

Flow

Controller*

*Note: Flow is NOT split equally between treatment trains.

Figure 2-6: Plant Schematic Showing Multiple 

Treatment Trains and Multiple Disinfection Segments 
2. Disinfection Segment 

EPA Guidance Manual 18 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

2.3 STEPS COMPLETED

2.4 NEXT STEP

After all of the disinfection segments have been identified, data 

must be collected for each disinfection segment. See Chapter 3 

for more information on disinfection profiling data collection 

requirements. 

Identify 

Disinfection 

Segments 

Collect Data 

Calculate CT

Calculate 

Inactivation

Develop the 

Disinfection 

Profile and 

Benchmark

Report and 

Evaluate the 

Disinfection 

Profile and 

Benchmark
3. DATA COLLECTION

May 2003 19 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

In this Chapter: 

• Data Needed for the 

Disinfection Profile 

• Data Collection 

Worksheets

• Steps Completed

• Next Step

• References

40 CFR Section 141.532 

40 CFR Section 141.533 

3.1 INTRODUCTION 

Once a system has identified all disinfection segments, data 

must be collected for each segment to create the disinfection 

profile. For systems serving 500 to 9,999 people, data 

collection must begin no later than July 1, 2003. For systems 

serving fewer than 500 people, data collection must begin no 

later than January 1, 2004. Systems that are required to 

develop a disinfection profile must collect data once a week, on 

the same day of the week, for twelve consecutive months (one 

year). The State may allow the use of a more representative 

data set for disinfection profiling, so systems with sufficient 

historic data should check with the State prior to collecting data 

(40 CFR Section 141.530). 

3.2 DATA NEEDED FOR THE DISINFECTION 

PROFILE 

To develop a disinfection profile, data must be collected once 

per week on the same day of the week for one year. The 

following data are needed for each disinfection segment 

identified (See Chapter 2 for information on disinfection 

segments): 

• Peak Hourly Flow;

• Residual Disinfectant Concentration;

• Temperature; and,

• pH (if chlorine is used).

Measurements must be taken on the same day of the week, 

every week, for one year (52 measurements), during peak 

hourly flow for that day. Data can be measured manually or 

with on-line instrumentation. 

Systems that already have existing data that meet the criteria of 

Section 3.2 may wish to contact the State to determine if the 

existing data can be used to create a disinfection profile in lieu of 

collecting new data. 
3. Data Collection 

EPA Guidance Manual 20 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

All data must be collected 

at peak hourly flow.

Data needed for each 

disinfection segment: 

• Peak Hourly Flow

• Residual Disinfectant 

Concentration

• Temperature

• pH (if chlorine is 

used)

• Volumes and Baffling 

Factors (See Chapter 

4)

3.2.1 Peak Hourly Flow Rate 

The amount of time the water is in contact with the disinfectant 

is a function of flow rate. When the flow rate increases, the 

time the water spends in the plant decreases. Using the peak 

hourly flow rate (required by LT1ESWTR) for analysis 

provides a conservative value for contact time. Some systems 

may be able to use a single peak hourly flow across the plant. 

In some systems, the peak hourly flow may vary across the 

plant. If the system has multiple disinfection segments and 

flow does vary across the plant, the disinfection segments may 

have different peak hourly flows. 

Each system will determine its peak hourly flow rate 

differently. Some possible ways to determine the flow rate are: 

• Flow meter records; 

• Design flow rate; 

• Maximum loading rates to the filters or other treatment 

process units; 

• Raw water pump records; or, 

• Historical maximum flow rate. 

When determining peak hourly flow, systems may want to take 

into consideration the location of their disinfection segment. 

For example, a system with a single disinfection segment with 

disinfection prior to the clearwell may consider using clearwell 

pumping rates versus raw water pump records to determine the 

peak hourly flow rate. 

When compiling data for the disinfection profile, systems will 

monitor once per week on the same calendar day during peak 

hourly flow for residual disinfectant concentration, pH (if 

chlorine is used), and temperature. 

Systems with supervisory control and data acquisition 

(SCADA) systems will be able to review records, identify the 

peak hourly flow, and then obtain the residual disinfectant 

concentration, temperature, and pH (if chlorine is used) that 

were recorded during peak hourly flow. Those systems 

without SCADA will need to coordinate with the State to 

develop a procedure that allows the system to best identify 

peak hourly flow to allow data collection. Some suggested 

approaches are: 
 3. Data Collection 

May 2003 21 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

CT = C x T 

C = Residual disinfectant 

concentration, mg/L 

T = Contact time, minutes 

See Chapter 4 for more 

information on CT.

• Determine when peak hourly flow occurred the day 

before data must be collected. Collect the residual 

disinfectant concentration, temperature, and pH (if 

chlorine is used) on the required day at the time peak 

hourly flow occurred on the previous day. 

• Using the above approach, collect residual disinfectant 

concentration, temperature, and pH (if chlorine is used) 

at three different times (such as before, during, and 

after) near the time peak hourly flow occurred on the 

previous day. Then, based on pump records or other 

information, determine when peak hourly flow actually 

occurred and use the data that were collected nearest to 

the time of peak hourly flow. 

3.2.2 Residual Disinfectant Concentration 

The residual disinfectant concentration is monitored for each 

disinfection segment during peak hourly flow and is measured 

in milligrams per liter (mg/L). At least one monitoring point 

must be associated with each disinfectant injection point. 

However, systems may choose to sample for residual 

disinfectant concentration at more than one location for each 

unique injection point. The residual disinfectant concentration 

must be measured using methods listed in Standard Methods 

for the Examination of Water and Wastewater, 18th (1992), 19th

(1995), or 20th (1998) editions. For those systems using ozone, 

Method 4500-03 B, contained in Standard Methods for the 

Examination of Water and Wastewater, 18th (1992) or 19th

(1995) editions must be used. If approved by the State, 

residual disinfectant concentrations for free chlorine and 

combined chlorine may be measured using DPD colorimetric 

test kits. 

CT is a measure of the strength of the disinfectant for the time 

that the water and disinfectant are in contact. CT is determined 

by multiplying the residual disinfectant concentration (C) by 

contact time (T). Monitoring the residual disinfectant at more 

than one location results in higher CT values since the residual 

disinfectant concentration decreases with each subsequent 

treatment process. For more information on CT refer to 

Chapters 4 and 5. 
3. Data Collection 

EPA Guidance Manual 22 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

If monitoring in oF, use 

the following formula to 

convert from oF to oC: 

 oC = 5 x (oF – 32) 

 9

Example: Collecting data 

for a single disinfection 

segment. 

3.2.3 Temperature 

The temperature is measured at each monitoring point and at 

the same time as the residual disinfectant concentration (during 

peak hourly flow). The temperature should be measured in 

degrees Celsius (o

C) because the CT Tables in Appendix B are 

based on temperature as measured in o

C (See Chapter 5 for an 

explanation of CT tables). Temperature is important since the 

effectiveness of all disinfectants is temperature sensitive. 

Temperature must be measured using Method 2550 in 

Standard Methods for the Examination of Water and 

Wastewater, 18th (1992), 19th (1995), or 20th (1998) editions. 

3.2.4 pH 

If a system uses chlorine as a disinfectant, pH must be 

monitored because chlorine is pH-sensitive and is more 

effective at lower pH values. The pH is sampled at each 

sampling point and at the same time as the residual disinfectant 

concentration (during peak hourly flow). The CT tables in 

Appendix B for chlorine are based on pH. Systems must 

measure pH using EPA Method 150.1 or 150.2, ASTM method 

D1293-95, or Method 4500-H+

 in Standard Methods for the 

Examination of Water and Wastewater, 18th (1992), 19th

(1995), or 20th (1998) editions. 

Example 3-1: Collecting Data for a Disinfection 

Profile 

Collect the data necessary for developing a disinfection profile 

for the system shown below. 

Distribution

System

Sedimentation

Coagulation

Filtration

Clearwell

Monitoring Point

Cl2 residual = 0.8 mg/L

Temperature = 0.5 oC

pH = 6

One Disinfection Segment:

One injection point, one monitoring point

Intake

Chlorine

Injected

Flocculation
 3. Data Collection 

May 2003 23 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Some systems may 

operate the plant at a low 

pH (for instance, a pH of 

6) to achieve enhanced 

coagulation or more 

microbial inactivation (if 

chlorine is used). 

However, systems should 

consider increasing the pH 

prior to sending the 

finished water to the 

distribution system to 

avoid corrosion issues. 

Example 3-1 continued

Step 1. Determine the peak hourly flow. 

From the clearwell pump records the peak hourly flow 

is determined to be 347 gallons per minute (gpm). 

Step 2. Measure the chlorine residual, temperature, and pH 

(since chlorine is used) during peak hourly flow at the same 

monitoring point and at the same time. 

 

During peak hourly flow the following measurements 

are recorded at the same monitoring point at the same 

time: 

Chlorine residual = 0.8 mg/L 

pH = 6 

Temperature = 0.5 oC 

Worksheet #1 in Appendix C can be used to record water 

quality data for the disinfection profile. The worksheet excerpt 

on this page demonstrates how to record the data from this 

example using Worksheet #1 in Appendix C. 

Starting Month: January Year: 2004 PWSID: AA1234567 System/Water Source: XYZ Water Plant

Disinfectant Type: Free Chlorine Prepared by: Joe Operator

Profile Type (check one): X Giardia Viruses

Disinfection Segment/Sequence of Application: Clearwell/1st

3 4 5 6 7 8 9 10 11 12 13 14

Residual Peak Disinf.

Disinf. pH Water Hourly TDT Baffling Contact CTCalc = CT Inactivation Log

Week Conc. Temp. Flow Volume Factor Time (CxT) Req'd Ratio Inactivation*

# C (mg/L) (

o

C) (gpm) (gal) (min.) T (min.) (min-mg/L) (min-mg/L) (Col 11 / Col 12)

1 0.8 6 0.5 347

2

3

4

5

6

*See worksheet #2 to determine total log inactivation if the system has multiple disinfection segments.

WORKSHEET #1

LOG INACTIVATION RATIO DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER
3. Data Collection 

EPA Guidance Manual 24 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example: Collecting data 

for multiple disinfection 

segments. 

Example 3-2: Collecting Data for Multiple 

Disinfection Segments 

Collect the data necessary for developing a disinfection profile 

for the system shown below. 

Distribution

System

Sedimentation

Chlorine

Injected

Coagulation

Clearwell

Disinfection Segment 4

Monitoring Point

Cl2 residual = 0.8 mg/L

Temperature = 5 oC

pH = 7.5

Intake

Disinfection

Segment 3

Disinfection

Segment 1

Disinfection

Segment 2

Disinfection Segment 2

Monitoring Point

Cl2 residual = 0.7 mg/L

Temperature = 5 oC

pH = 7.5

Disinfection Segment 1

Monitoring Point

Cl2 residual = 1.0 mg/L

Temperature = 5 oC

pH = 7.5

Filtration

Disinfection

Segment 4

Disinfection Segment 3

Monitoring Point

Cl2 residual = 0.3 mg/L

Temperature = 5 oC

pH = 7.5

Chlorine

Injected

Step 1. Determine the peak hourly flow for Disinfection 

Segments 1 through 4. 

From the raw water pump records the peak hourly flow is 

determined to be 347 gpm for Disinfection Segments 1, 2, 

and 3. 

From the clearwell pump records the peak hourly flow is 

determined to be 370 gpm for Disinfection Segment 4. 

Step 2. Measure the chlorine residual, temperature, and pH 

(since chlorine is used) during peak hourly flow at the same 

monitoring point and at the same time. 

 

During peak hourly flow the following measurements are 

recorded at the same monitoring point at the same time: 

Disinfection 

Segment 

Chlorine 

Residual (mg/L)

Temperature 

(

o

C) 

pH 

1 1.0 5 7.5 

2 0.7 5 7.5 

3 0.3 5 7.5 

4 0.8 5 7.5 
 3. Data Collection 

May 2003 25 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example 3-2 continued

Again, Worksheet #1 in Appendix C can be used for data 

collection, as shown in Example 3-1. A new copy of 

Worksheet #1 should be used for each disinfection segment for 

systems with multiple segments. Example D-2 in Appendix D 

illustrates how to complete Worksheet #1 for multiple 

disinfection segments. 

3.3 DATA COLLECTION WORKSHEETS

The worksheets in Appendix C are helpful for recording 

collected data. Systems should verify that their State will 

accept the worksheets for recordkeeping and reporting 

purposes. 

3.4 STEPS COMPLETED

3.5 NEXT STEP

Now that data have been collected for each disinfection 

segment, the CT value can be calculated. Chapter 4 explains 

how to calculate CT. 

Identify 

Disinfection 

Segments 

Collect Data 

Calculate CT

Calculate 

Inactivation

Develop the 

Disinfection 

Profile and 

Benchmark 

Report and 

Evaluate the 

Disinfection 

Profile and 

Benchmark
3. Data Collection 

EPA Guidance Manual 26 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

3.6 REFERENCES

APHA, AWWA, WEF. 1998. Standard Methods for the Examination of Water and 

Wastewater, 20th Edition. APHA, Washington, D.C. 

APHA, AWWA, WEF. 1995. Standard Methods for the Examination of Water and 

Wastewater, 19th Edition. APHA, Washington, D.C. 

APHA, AWWA, WEF. 1992. Standard Methods for the Examination of Water and 

Wastewater, 18th Edition. APHA, Washington, D.C. 
4. CALCULATING CT 

May 2003 27 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

In this Chapter: 

• CT 

• Determining “C” 

• Determining “T”

• CTcalc

• Steps Completed 

• Next Step 

• References 

40 CFR Section 141.532 

CTcalc - The CT that is 

calculated for a given 

disinfection segment. 

Equation 4-1 

CTcalc = C x T 

4.1 INTRODUCTION 

If a system is required to complete a disinfection profile, it must 

calculate the CT value for each disinfection segment, known as 

CTcalc. System operational data and other data must be 

collected to determine CTcalc weekly for one year. Systems will 

collect data once a week, on the same day of the week, for 

twelve consecutive months (one year). For systems serving 500 

to 9,999 people, data collection must begin no later than July 1, 

2003. For systems serving fewer than 500 people, data 

collection must begin no later than January 1, 2004 (40 CFR 

Section 141.532). See Chapter 3 for more information on data 

collection. The CTcalc value derived for each disinfection 

segment will be used to calculate the inactivation ratio for each 

disinfection segment on a weekly basis. 

 

4.2 WHAT IS CT? 

CT simply stands for concentration (C) and contact time (T). 

It is the result of multiplying the disinfectant residual 

concentration by the contact time. CT is a measure of 

disinfection effectiveness for the time that the water and 

disinfectant are in contact. “C” is the disinfectant residual 

concentration measured in mg/L at peak hourly flow and “T” is 

the time that the disinfectant is in contact with the water at peak 

hourly flow. The contact time (T) is measured from the point of 

disinfectant injection to a point where the residual is measured 

before the first customer (or the next disinfection application 

point) and is measured in minutes. 

Equation 4-1 

CTcalc (minutes-mg/L) = C x T 

C = Residual disinfectant concentration measured during 

peak hourly flow in mg/L. 

T = Time, measured in minutes, that the water is in 

contact with the disinfectant. 
4. Calculating CT 

EPA Guidance Manual 28 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Contact Time = T 

(Sometimes referred to as 

T10) 

4.3 DETERMINING “C” 

“C” is the residual disinfectant concentration measured during 

peak hourly flow in mg/L. The residual disinfectant 

concentration must be measured for each disinfection segment. 

In addition, the residual disinfectant concentration must be 

measured once per week on the same day of the week during 

peak hourly flow. See Chapter 3 for information on the residual 

disinfectant concentration. 

4.4 DETERMINING “T” 

The disinfectant contact time (T), also referred to as T10 in the 

Guidance Manual for Compliance with the Filtration and 

Disinfection Requirements for Public Water Systems Using 

Surface Water (EPA, 1991), is an estimate of the detention time 

within a basin or treatment unit at which 90 percent of the water 

passing through the unit is retained within the basin or treatment 

unit. T can be determined through a tracer study or estimated 

based on the theoretical detention time and baffling factor. 

Before measuring or calculating T, a system may want to check its 

permits or other documentation that the State may have to see if a 

tracer study has been conduced for its facility. T can be 

determined based on the results of a tracer study. See Appendix E 

for more information on tracer studies. 

T must be calculated 

based on peak hourly 

flow. 

The peak hourly flow rate is used to calculate the contact time 

within the treatment plant. Using the peak hourly flow rate for 

analysis provides a conservative value for the contact time. 

The following steps may be used to calculate T for a treatment 

system: 

• Define the disinfection segments in the system. 

• Determine the peak hourly flow in the disinfection 

segment. 
 4. Calculating CT 

May 2003 29 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

• Calculate the volume of each basin, pipe, or unit 

process in each disinfection segment (See Section 

4.4.1). 

• Calculate the theoretical detention time for each basin, 

pipe, or unit process (See Section 4.4.2). 

• Determine the baffling factor (BF) of each basin, pipe, 

or unit process (See Section 4.4.3). 

• Determine T for each basin, pipe, or unit process based 

on the theoretical detention time and baffling factor 

(See Section 4.4.4). 

• Sum the Ts of each basin, pipe, or unit process for a 

total contact time for the disinfection segment. 

Defining disinfection segments and measuring peak hourly 

flow have already been discussed in Chapters 2 and 3. The 

following sections discuss the remaining topics. 

4.4.1 Volume 

The volume of each basin, pipe, or unit process is used to 

calculate T. Since some treatment units, such as clearwells, 

can have fluctuating levels that affect volume, systems should 

consult the State on what volume should be used for the 

disinfection profile. Systems and States may want to consider 

the following options: 

• Volumes can be based on the minimum volume that can 

occur in the treatment unit. This approach is the most 

conservative. 

• Volumes can be based on the actual volume realized in 

the treatment unit during peak hourly flow if adequate 

information is available to identify the actual volume. 

• Volumes can be based on the lowest volume realized in 

the treatment unit for that day. 

Table 4-1 provides the equations used to find the volume of the 

specific sub-units or segments. See Appendix F for detailed 

examples of sub-units and volume equations. 
4. Calculating CT 

EPA Guidance Manual 30 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Volume Equations 

See Appendix F for more 

information on volume 

calculations. 

1 cubic foot = 7.48 

gallons 

Equation 4-2 

TDT = V/Q 

Table 4-1: Volume Equations for Shapes 

SHAPE EXAMPLE OF UNIT 

WITH THIS SHAPE VOLUME EQUATION 

Cylindrical 

Pipes 

Raw Water Pipe 

Plant Piping 

Finished Water Pipe 

Length x Crosssectional Area (πr

2

)

Rectangular 

Basins 

Rapid Mix, 

Flocculation, and 

Sedimentation 

Basins, Clearwells 

Length x Width x 

Minimum Water Depth 

Cylindrical 

Basins 

Rapid Mix, 

Flocculation, and 

Sedimentation 

Basins, Clearwells 

Minimum Water Depth 

x Cross-sectional Area 

(πr

2

) 

Rectangular 

Filters Filtration 

Surface Area of Filter x 

Depth of Water Above 

Filter Surface (Volume 

of water in the media 

pores may also be 

used.) 

4.4.2 Theoretical Detention Time 

The theoretical detention time (TDT) is the time that the water 

is in a basin, pipe, or unit process assuming perfect plug flow. 

Perfect plug flow assumes no short-circuiting within the basin, 

pipe, or unit process. The TDT is calculated by dividing the 

volume based on low water level by the peak hourly flow 

(Equation 4-2). 

Equation 4-2 

TDT = V / Q 

TDT = Theoretical Detention Time, in minutes 

V = Volume based on low water level, in gallons 

Q = Peak hourly flow, in gpm 
 4. Calculating CT 

May 2003 31 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Plug Flow - The water 

travels through a basin, 

pipe, or unit process in 

such a fashion that the 

entire mass or volume is 

discharged at exactly the 

TDT of the unit and no 

short-circuiting occurs. 

Short-circuiting - A 

hydraulic condition in a 

basin or unit process in 

which the actual flow time 

of water through the basin 

is less than the basin or 

unit process volume 

divided by the peak hourly 

flow. 

4.4.3 Baffling Factor 

The T in each basin, pipe, or unit process is a function of 

configuration and baffling. The flow through a pipe is very 

different than the flow through an unbaffled basin (See Figure 

4-1). The longest path a particle can take through a pipeline is 

not that different from the shortest path. In the case of an 

unbaffled basin, one particle may flow through directly from 

the inlet to the outlet. This short-circuiting particle will be in 

contact with the disinfectant for a relatively short time. 

Figure 4-1: Baffling Characteristics of a Pipe and 

Clearwell

Baffling Factor = 1.0

Baffling Factor = 0.1

Top: This pipe demonstrates a plug flow condition in which all of 

the material sent through the pipe discharges at the theoretical 

hydraulic detention time of the pipe. 

Bottom: This unbaffled basin demonstrates short-circuiting in 

which some of the material entering the basin would come out 

almost immediately, while other material that enters at the same 

time will be detained for a longer period of time. Short-circuiting 

occurs in basins with poor baffling. 
4. Calculating CT 

EPA Guidance Manual 32 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

See Appendix G for more 

information on baffling 

factors. 

Table 4-2 

Baffling Factors 

Equation 4-3 

T = TDT x BF 

Baffling factors (BF) have been developed that allow the 

contact time of a basin, pipe, or unit process to be estimated, 

based on the volume of and flow rate through a basin, pipe, or 

unit process. Baffling factors were developed based on 

numerous tracer studies of basins with different sizes and 

configurations. Table 4-2 and Appendix G provide a summary 

of theoretical baffling factors for various baffling conditions 

and basins. 

Table 4-2: Baffling Factors 

4.4.4 Calculate Contact Time 

T can be calculated once the TDT and baffling factor are 

known (Equation 4-3). 

Baffling 

Condition

Baffling 

Factor Baffling Description 

Unbaffled 

(mixed 

flow) 

0.1 

None, agitated basin, very low 

length to width ratio, high inlet 

and outlet flow velocities. 

Poor 0.3 

Single or multiple unbaffled 

inlets and outlets, no intra-basin 

baffles. 

Average 0.5 Baffled inlet or outlet with some 

intra-basin baffles. 

Superior 0.7 

Perforated inlet baffle, 

serpentine or perforated intrabasin baffles, outlet weir or 

perforated launders. 

Perfect 

(plug flow) 1.0 

Very high length to width ratio 

(pipeline flow), perforated inlet, 

outlet, and intra-basin baffles. 

Equation 4-3 

T = TDT x BF 

T = Time, measured in minutes, that the water is in 

contact with the disinfectant. 

TDT = Theoretical detention time, in minutes 

BF = Baffling factor 
 4. Calculating CT 

May 2003 33 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example 4-1: Determining “T” 

Determine T for the conventional filtration system discussed in Example 3-1. 

Peak Hourly

Flow = 347 gpm

Residual Monitoring

Point

Chlorine

Injected

To

Distribution

System

Side View

Minimum Operating Level

Diameter = 40 ft

Depth = 30 ft

To

Distribution

Peak Hourly System

Flow = 347 gpm

Top View

Step 1. Measure the physical dimensions of the clearwell. 

 

Measure the inner tank diameter to obtain the volume of water in the clearwell rather 

than the volume of the tank itself. 

Diameter = 40 ft 

Measure the minimum operating depth in the clearwell to obtain a conservative 

estimate of the volume of water in the tank. 

 

Minimum Water Depth = 30 ft 
4. Calculating CT 

EPA Guidance Manual 34 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example 4-1 continued 

Step 2. Calculate the volume of the clearwell based on low water level. 

 

From Table 4-1 the equation for calculating the volume of a cylindrical basin is: 

 

 Volume (V) = minimum water depth x cross-sectional area (πr

2

) 

 where 

 π = 3.14 

 radius (r) = diameter / 2 = 40 ft / 2 =20 ft 

 

 V = 30 ft x 3.14 x (20 ft)2 = 37,680 ft3

 V = 37,680 ft3 x (7.48 gal / ft3) 

 V = 282,000 gallons 

The volume of the clearwell = 282,000 gallons 

Note: More information on volume equations and calculations can be found in Appendix F. 

Step 3. Calculate the theoretical detention time. 

 TDT = V / Q (Note: Q = peak hourly flow) (Eq. 4-2) 

 TDT = 282,000 gal / 347 gpm 

 TDT = 813 minutes 

The TDT in the clearwell is 813 minutes 

Step 4. Determine the baffling factor for the clearwell. 

From the diagram shown above there is no baffling in the clearwell. From Table 4-2, 

the baffling factor (BF) for an unbaffled basin is 0.1. 

The baffling factor for the clearwell = 0.1 

Step 5. Calculate the contact time of the disinfectant in the clearwell. 

 Contact Time = TDT x BF (Eq. 4-3) 

 T = 813 min x 0.1 

 T = 81.3 minutes 

 

The contact time in the clearwell = 81.3 minutes 

Worksheet #1 in Appendix C can be used to record data and calculate contact time. The 

worksheet excerpt on the next page demonstrates how data may be recorded from this 

example and previous examples using Worksheet #1 in Appendix C. 
 4. Calculating CT 

May 2003 35 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example 4-1 continued 

Starting Month: January Year: 2004 PWSID: AA1234567 System/Water Source: XYZ Water Plant

Disinfectant Type: Free Chlorine Prepared by: Joe Operator

Profile Type (check one): X Giardia Viruses

Disinfection Segment/Sequence of Application: Clearwell/1st

3 4 5 6 7 8 9 10 11 12 13 14

Residual Peak Disinf.

Disinf. pH Water Hourly TDT Baffling Contact CTCalc = CT Inactivation Log

Week Conc. Temp. Flow Volume Factor Time (CxT) Req'd Ratio Inactivation*

# C (mg/L) (

o

C) (gpm) (gal) (min.) T (min.) (min-mg/L) (min-mg/L) (Col 11 / Col 12)

1 0.8 6 0.5 347 282,000 813 0.1 81.3

2

3

4

5

6

*See worksheet #2 to determine total log inactivation if the system has multiple disinfection segments.

WORKSHEET #1

LOG INACTIVATION RATIO DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER

Equation 4-1 

CTcalc = C x T

4.5 Calculate CTcalc

To calculate CTcalc, a system must monitor the residual 

disinfectant concentration and the amount of time that the 

water is in contact with the disinfectant. A system that is 

required to complete a disinfection profile must determine 

CTcalc values once per week, on the same day of the week, for 

one year. 

The disinfection effectiveness for the time that the water and 

disinfectant are in contact is calculated as follows: 

 

Equation 4-1 

CTcalc (minutes-mg/L) = C x T 

C = Residual disinfectant concentration measured during 

peak hourly flow in mg/L. 

T = Time, measured in minutes, that the water is in 

contact with the disinfectant. 
4. Calculating CT 

EPA Guidance Manual 36 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example 4-2 demonstrates how to determine CTcalc for one 

disinfection segment. If more than one disinfectant is used or 

if residual disinfectants are measured in more than one 

location, then CTcalc must be calculated for each disinfection 

segment. See the examples in Appendix D for more 

illustrations of calculating CTcalc under different operating 

conditions. 

 

Example 4-2: Calculate CTcalc 

Calculate CTcalc for the conventional filtration system in the previous examples. 

Distribution

System

Sedimentation

Coagulation

Filtration

Clearwell

Monitoring Point

Cl2 residual = 0.8 mg/L

One Disinfection Segment:

One injection point, one monitoring point

Intake

Chlorine

Injected

Flocculation

Step 1. Determine “C”. 

From example 3-1, C = 0.8 mg/L

Step 2. Determine “T”. 

From example 4-1, T = 81.3 minutes

Step 3. Calculate CTcalc.

CT = C x T 

CT = 0.8 mg/L x 81.3 minutes 

CT = 65.0 min-mg/L 

CTcalc = 65.0 min-mg/L 
 4. Calculating CT 

May 2003 37 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Worksheet #1 in Appendix C can be used to record data and calculate CTcalc. The worksheet 

excerpt below demonstrates how to record the data from this example and previous 

examples using Worksheet #1 in Appendix C. 

Starting Month: January Year: 2004 PWSID: AA1234567 System/Water Source: XYZ Water Plant

Disinfectant Type: Free Chlorine Prepared by: Joe Operator

Profile Type (check one): X Giardia Viruses

Disinfection Segment/Sequence of Application: Clearwell/1st

3 4 5 6 7 8 9 10 11 12 13 14

Residual Peak Disinf.

Disinf. pH Water Hourly TDT Baffling Contact CTCalc = CT Inactivation Log

Week Conc. Temp. Flow Volume Factor Time (CxT) Req'd Ratio Inactivation*

# C (mg/L) (

o

C) (gpm) (gal) (min.) T (min.) (min-mg/L) (min-mg/L) (Col 11 / Col 12)

1 0.8 6 0.5 347 282,000 813 0.1 81.3 65.0

2

3

4

5

6

*See worksheet #2 to determine total log inactivation if the system has multiple disinfection segments.

WORKSHEET #1

LOG INACTIVATION RATIO DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER
4. Calculating CT 

EPA Guidance Manual 38 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

4.6 STEPS COMPLETED

4.7 NEXT STEP

In addition to CTcalc, CT required must also be determined to 

calculate log inactivation. Chapter 5 describes how to 

determine CT required and calculate log inactivation. 

4.8 REFERENCES

EPA. 1991. Guidance Manual for Compliance with the Filtration and Disinfection 

Requirements for Public Water Systems Using Surface Water. Washington, D.C. 

Identify 

Disinfection 

Segments 

Collect Data 

Calculate CT

Calculate 

Inactivation

Develop the 

Disinfection 

Profile and 

Benchmark 

Report and 

Evaluate the 

Disinfection 

Profile and 

Benchmark
5. CALCULATING INACTIVATION

May 2003 39 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

In this Chapter:

• Log Reduction 

• Determining CT 

Required 

• Calculating Actual 

Log Inactivation for 

One Disinfection 

Segment

• Calculating Actual 

Log Inactivation for 

Multiple Disinfection 

Segments

• Steps Completed

• Next Step

1-log reduction = 90% 

2-log reduction = 99% 

3-log reduction = 99.9% 

4-log reduction = 99.99% 

A spreadsheet (in 

Microsoft® Excel) has 

been developed that can 

be used by systems to 

calculate log inactivations. 

The spreadsheet is 

available on EPA’s 

website at 

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

5.1 INTRODUCTION 

In order to develop a disinfection profile, the Giardia log 

inactivation must be calculated. The log inactivation of viruses 

must also be calculated if the system uses ozone, chloramines, 

or chlorine dioxide for primary disinfection. Ozone, 

chloramines, and chlorine dioxide are not as effective for 

inactivating viruses as for inactivating Giardia, and systems 

must make sure the appropriate virus inactivation is achieved. 

To determine log inactivation achieved through disinfection, a 

series of calculations are completed. First, CTcalc is determined 

(See Chapter 4). Then CTcalc is related to the required CT using 

CT tables (See Section 5.3). The CT required for a desired log 

inactivation is dependent upon pH (if chlorine is used), 

temperature, and residual disinfectant concentration (for 

chlorine). Individual CT tables are used for each type of 

disinfectant because the effectiveness of different disinfectants 

varies with each type of microorganism. For this reason, 

separate CT tables have been developed for chlorine, chlorine 

dioxide, ozone, and chloramines for both Giardia and viruses 

(See Appendix B). 

5.2 LOG REDUCTION

The concept of log reduction (removal and inactivation) is used 

extensively in discussions of compliance with microbiological 

requirements. The term refers to logarithmic theory. 

Essentially, in this context, log reduction relates to the 

percentage of microorganisms physically removed or 

inactivated by a given process. One log reduction means that 

90% of the microorganisms are removed or inactivated. Two 

log corresponds to 99%, three log corresponds to 99.9% and 

four log corresponds to 99.99%. The removal or inactivation 

“log number” coincides with the number of nines in the 

percentage reduction. This chapter will discuss log inactivation 

achieved through disinfection only; however, it should be 

remembered that when determining the total system reduction, 

the physical log removal is added to the log inactivation 

through disinfection for total reduction of microorganisms (See 

Chapter 7). 
5. Calculating Inactivation 

EPA Guidance Manual 40 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

CT99.9 for Giardia = 

3-log inactivation

CT99.99 for viruses = 

4-log inactivation 

If the temperature, pH, or 

residual disinfectant 

concentration falls 

between or outside the 

values listed in the 

columns or rows on the 

table, systems should 

contact the State for the 

appropriate method to 

determine CT required. 

Appendix H presents two 

different interpolation 

methods. 

5.3 DETERMINING CT REQUIRED

After determining CTcalc (See Chapter 4) based on system 

operating parameters and configuration, CT tables are used to 

determine the required CT value for a certain level of 

inactivation. The CT tables in Appendix B give CT values that 

achieve a 3-log inactivation of Giardia and viruses, as a 

function of disinfectant type, temperature, pH and residual 

disinfectant concentration. The following guidelines can be 

used to obtain the required CT value from the CT tables: 

• Find the appropriate table based on the disinfectant 

used. 

• Find the appropriate table based on the microorganism 

of concern (Giardia or viruses). 

• Find the appropriate portion of the table (for chlorine) 

or column based on measured temperature. 

• Find the appropriate column (for chlorine) based on the 

measured pH. Systems should contact the State if the 

pH value is not included in the CT tables in Appendix 

B. 

• Find the appropriate row based on the measured 

disinfectant residual (for chlorine only). 

• Identify the CT value based on the above information. 

The CT tables in Appendix B for chlorine are based on pH. 

The CT tables in Appendix B for Giardia inactivation by 

chloramines and virus inactivation by chlorine dioxide also list 

a range for pH. Although systems are not required to monitor 

the pH for chloramines and chlorine dioxide, systems should 

ensure that the pH falls between the range of 6-9 when this pH 

range is specified in the CT tables. 

The following sections discuss how to obtain 3-log CT required 

for Giardia inactivation (CT99.9) and 4-log CT required for 

virus inactivation (CT99.99). 
 5. Calculating Inactivation 

May 2003 41 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

The CT table for 

inactivation of Giardia by 

chloramines (Table B-7) 

lists values between the 

pH range 6 to 9.

5.3.1 CT99.9 for Giardia

All surface water systems or GWUDI systems are required to 

achieve 3-log (99.9%) reduction of Giardia through removal 

(filtration) and/or inactivation (disinfection) (See 40 CFR 

141.70(a)(1)). States generally grant log removal credits for 

filtration which typically vary depending on the treatment 

process (such as conventional, direct, or alternative filtration). 

For unfiltered systems, all three logs must be achieved through 

disinfection. For filtered systems, refer to Table 7-2 for typical 

log removal credits and resulting inactivation values that must 

be achieved by disinfection. Inactivation through disinfection 

can be achieved by one disinfectant or a combination of 

disinfectants. The method used to calculate log inactivation 

under the LT1ESWTR requires that the CT99.9 value for 

Giardia be determined. Example 5-1 illustrates how to obtain 

CT99.9.

Example 5-1: Determining CT99.9 

The conventional filtration system discussed in Examples 3-1, 4-1, and 4-2 uses chlorine 

disinfectant only; therefore the system only needs to calculate CT99.9 for Giardia because 

chlorine is significantly more effective against viruses than Giardia. Find the required CT 

to achieve 3-log inactivation of Giardia, or CT99.9. 

Distribution

System

Sedimentation

Coagulation

Filtration

Clearwell

Monitoring Point

Cl2 residual = 0.8 mg/L

Temperature = 0.5 oC

pH = 6

One Disinfection Segment:

One injection point, one monitoring point

Intake

Chlorine

Injected

Flocculation
5. Calculating Inactivation 

EPA Guidance Manual 42 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example 5-1 continued

Step 1. Gather required data during peak hourly flow. 

Water temperature = 0.5 °C 

Chlorine residual = 0.8 mg/L 

pH = 6.0

Step 2. Locate appropriate CT table. 

The table for 3-log inactivation of Giardia by free chlorine is Table B-1 in Appendix B. 

Step 3. Identify the appropriate portion of the table based on operating conditions and 

3-log Giardia inactivation. 

The first section of the table is for temperatures less than or equal to 0.5 o

C. The first 

column in that section is for pHs less than or equal to 6.0. The disinfectant residual of 0.8 

mg/L is found in the third row down on the chart. The relevant portion of Table B-1 is 

reprinted below. 

Excerpt from Table B-1:

CT values for 3-Log Inactivation of Giardia Cysts by Free Chlorine (0.5 °C portion of table 

for 0.4 to 1.2 mg/L) 

Step 4. Obtain CT99.9 value. 

From this chart, the value of CT for 3-log inactivation at 0.8 mg/L and pH of 6 is 145 minmg/L. 

CT99.9 for Giardia = 145 min-mg/L 

Worksheet #1 in Appendix C can be used to record data needed to determine CT99.9 and to 

record the value of CT99.9. The worksheet excerpt on the next page demonstrates how to 

record the data from this example and previous examples using Worksheet #1 in 

Appendix C. 

Temperature <= 0.5 oC 

pH 

Chlorine 

Concentration 

(mg/L) 

<=6.0 6.5 7.0 7.5 8.0 8.5 9.0 

<=0.4 137 163 195 237 277 329 390 

0.6 141 169 200 239 286 342 407 

0.8 145 172 205 246 295 354 422 

1.0 148 176 210 253 304 365 437 

1.2 152 180 215 259 313 376 451 
 5. Calculating Inactivation 

May 2003 43 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example 5-1 continued 

Starting Month: January Year: 2004 PWSID: AA1234567 System/Water Source: XYZ Water Plant

Disinfectant Type: Free Chlorine Prepared by: Joe Operator

Profile Type (check one): X Giardia Viruses

Disinfection Segment/Sequence of Application: Clearwell/1st

3 4 5 6 7 8 9 10 11 12 13 14

Residual Peak Disinf.

Disinf. pH Water Hourly TDT Baffling Contact CTCalc = CT Inactivation Log

Week Conc. Temp. Flow Volume Factor Time (CxT) Req'd Ratio Inactivation*

# C (mg/L) (

o

C) (gpm) (gal) (min.) T (min.) (min-mg/L) (min-mg/L) (Col 11 / Col 12)

1 0.8 6 0.5 347 282,000 813 0.1 81.3 65.0 145

2

3

4

5

6

*See worksheet #2 to determine total log inactivation if the system has multiple disinfection segments.

WORKSHEET #1

LOG INACTIVATION RATIO DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER

The CT table for 

inactivation of viruses by 

chlorine dioxide (Table B4) lists values between the 

pH range 6 to 9. 

40 CFR Section 141.534 

5.3.2 CT99.99 for Viruses 

All surface water systems or GWUDI systems are required to 

achieve 4-log (99.99%) reduction of viruses through removal 

(filtration) and/or inactivation (disinfection) (See 40 CFR 

141.70(a)(2)). States generally grant log removal credits for 

filtration which typically vary depending on the treatment 

process (such as conventional, direct, or alternative filtration). 

For unfiltered systems, all four logs must be achieved through 

disinfection. One method used to calculate log inactivation 

uses the CT99.99 value for viruses. Virus inactivation must be 

determined if chloramines, chlorine dioxide, or ozone are used 

for primary disinfection (See 40 CFR 141.535). Example D-3 

in Appendix D illustrates a method for obtaining CT99.99 for a 

system using ozone. 

5.4 CALCULATING ACTUAL LOG 

INACTIVATION FOR ONE DISINFECTION 

SEGMENT

Actual log inactivation can be calculated as a ratio of the CTcalc

value achieved by the system to the CT value required for 3-log 

inactivation of Giardia or 4-log inactivation of viruses. 
5. Calculating Inactivation 

EPA Guidance Manual 44 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Equation 5-1 

The following equation must be used to calculate Giardia log 

inactivation for one disinfection segment: 

Example 5-2 shows how a system may calculate the Giardia

log inactivation achieved in a system with one disinfection 

segment. 

Example 5-2: Determine Actual Log Inactivation for Giardia

The conventional filtration system discussed in Examples 3-1, 4-1, 4-2 and 5-1 uses chlorine 

disinfectant only; therefore the system only needs to calculate actual Giardia log 

inactivation because chlorine is significantly more effective against viruses than Giardia. 

Determine the actual Giardia log inactivation achieved by the system. 

Step 1. Determine CTcalc and CT99.9 for the disinfection segment. 

The following table summarizes the values determined for CTcalc and CT99.9: 

Step 2. Calculate the inactivation ratio for the clearwell. 

Inactivation Ratio = CTcalc / CT99.9

Inactivation Ratio = 65.0 / 145 

Inactivation Ratio = 0.448 

Step 3. Calculate Giardia log inactivation for the clearwell. 

 Giardia log inactivation = 3 x (CTcalc / CT99.9) 

 Giardia log inactivation = 3 x 0.448 

 Giardia log inactivation = 1.34 

Refer to Chapter 7 for more information on interpreting log inactivation values. 

Equation 5-1 

Actual Log Inactivation of Giardia = 3 x (CTcalc / CT99.9) 

Disinfection Segment CTcalc 

min-mg/L 

CT99.9 for Giardia 

min-mg/L 

1-Chlorine 65.0 145 
 5. Calculating Inactivation 

May 2003 45 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example 5-2 continued

Worksheet #1 in Appendix C can be used to record data and calculate log inactivation. The 

worksheet excerpt below demonstrates how data may be recorded from this example and 

previous examples using Worksheet #1 in Appendix C. 

Starting Month: January Year: 2004 PWSID: AA1234567 System/Water Source: XYZ Water Plant

Disinfectant Type: Free Chlorine Prepared by: Joe Operator

Profile Type (check one): X Giardia Viruses

Disinfection Segment/Sequence of Application: Clearwell/1st

3 4 5 6 7 8 9 10 11 12 13 14

Residual Peak Disinf.

Disinf. pH Water Hourly TDT Baffling Contact CTCalc = CT Inactivation Log

Week Conc. Temp. Flow Volume Factor Time (CxT) Req'd Ratio Inactivation*

# C (mg/L) (

o

C) (gpm) (gal) (min.) T (min.) (min-mg/L) (min-mg/L) (Col 11 / Col 12)

1 0.8 6 0.5 347 282,000 813 0.1 81.3 65.0 145 0.448 1.34

2

3

4

5

6

*See worksheet #2 to determine total log inactivation if the system has multiple disinfection segments.

WORKSHEET #1

LOG INACTIVATION RATIO DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER

Equation 5-2 

Check with the State on 

the approved method to 

calculate virus 

inactivation.

Systems should check with their State to determine the 

appropriate method for calculating virus inactivation. The 

following equation was used for the examples presented in this 

document: 

Additional examples of calculating the actual log inactivation 

of Giardia and viruses are contained in Appendix D. A 

spreadsheet has also been developed that can be used by 

systems to calculate log inactivations. The spreadsheet is 

available on EPA’s website

(www.epa.gov/safewater/mdbp/lt1eswtr.html). 

Equation 5-2 

Actual Log Inactivation of Viruses = 4 x (CTcalc / CT99.99)
5. Calculating Inactivation 

EPA Guidance Manual 46 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Equation 5-3 

Σ = Sum of. 

Systems must sum the 

CTcalc / CT99.9 inactivation 

ratios for each 

disinfection segment if the 

system has multiple 

disinfection segments. 

5.5 CALCULATING ACTUAL LOG 

INACTIVATION FOR MULTIPLE 

DISINFECTION SEGMENTS

Actual log inactivation for a system with more than one 

disinfection segment is calculated as a sum of the ratios of the 

CTcalc value achieved by each disinfection segment to the CT 

value required for 3-log inactivation of Giardia or 4-log 

inactivation of viruses in each disinfection segment. 

The following equation must be used to calculate Giardia log 

inactivation for a system with multiple disinfection segments: 

Example 5-3 shows how a system may use the worksheets in 

Appendix C to calculate the Giardia log inactivation achieved 

in a system with multiple disinfection segments. 

Example 5-3: Determine Total Log Inactivation for Giardia

The conventional filtration system discussed in Example D-2 in Appendix D uses chlorine 

as a pre-disinfectant and a primary disinfectant and uses chloramines as a secondary 

disinfectant. Determine the total Giardia log inactivation achieved by the system. 

The worksheets in Appendix C can be used to record data and calculate log inactivation. 

The following table summarizes the calculations for each unit process in Disinfection 

Segment 1 in Example D-2. 

Unit Process Volume (gal) Peak Hourly 

Flow (gpm)

TDT 

(min) BF*

 Contact Time (min)

Coagulation 24,000 5,000 4.8 0.1 0.48 

Flocculation 80,000 5,000 16 0.1 1.6 

Sedimentation 100,000 5,000 20 0.5 10 

Filtration 45,000 5,000 9 0.7 6.3 

Total: 249,000 18.4 

* See Appendix G for baffling factors. 

Equation 5-3 

Actual Log Inactivation of Giardia = 3 x Σ (CTcalc / CT99.9) 
 5. Calculating Inactivation 

May 2003 47 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example 5-3 continued 

The worksheet excerpt below demonstrates how a system may record the data from 

Disinfection Segment 1 in Example D-2 using Worksheet #1 in Appendix C. For this 

example, Worksheet #1 should be copied so the data from each disinfection segment can be 

entered. 

Starting Month: January Year: 2004 PWSID: AA7654321 System/Water Source: ABC Water Plant

Disinfectant Type: Free Chlorine Prepared by: Jon Operator

Profile Type (check one): X Giardia Viruses

Disinfection Segment/Sequence of Application: Coagulation, Flocculation, Sedimentation, Filtration/1st

3 4 5 6 7 8 9 10 11 12 13 14

Residual Peak Disinf.

Disinf. pH Water Hourly TDT Baffling Contact CTCalc = CT Inactivation Log

Week Conc. Temp. Flow Volume Factor Time (CxT) Req'd Ratio Inactivation*

# C (mg/L) (

o

C) (gpm) (gal) (min.) T (min.) (min-mg/L) (min-mg/L) (Col 11 / Col 12)

1 1.0 7.5 10 5,000 249,000 ** ** 18.4 18.4 134 0.137

2

3

4

5

6

*See worksheet #2 to determine total log inactivation if the system has multiple disinfection segments.

**See the previous table showing details of each unit process for theoretical detention times and baffling factors.

WORKSHEET #1

LOG INACTIVATION RATIO DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER

The worksheet excerpt below demonstrates how a system may record the data from 

Disinfection Segment 2 in Example D-2 using Worksheet #1 in Appendix C. 

Starting Month: January Year: 2004 PWSID: AA7654321 System/Water Source: ABC Water Plant

Disinfectant Type: Free Chlorine Prepared by: Jon Operator

Profile Type (check one): X Giardia Viruses

Disinfection Segment/Sequence of Application: Clearwell/2nd

3 4 5 6 7 8 9 10 11 12 13 14

Residual Peak Disinf.

Disinf. pH Water Hourly TDT Baffling Contact CTCalc = CT Inactivation Log

Week Conc. Temp. Flow Volume Factor Time (CxT) Req'd Ratio Inactivation*

# C (mg/L) (

o

C) (gpm) (gal) (min.) T (min.) (min-mg/L) (min-mg/L) (Col 11 / Col 12)

1 1.2 7.5 10 5,000 300,000 60 0.7 42 50 137 0.365

2

3

4

5

6

*See worksheet #2 to determine total log inactivation if the system has multiple disinfection segments.

WORKSHEET #1

LOG INACTIVATION RATIO DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER
5. Calculating Inactivation 

EPA Guidance Manual 48 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example 5-3 continued 

The worksheet excerpt below demonstrates how a system may record the data from 

Disinfection Segment 3 in Example D-2 using Worksheet #1 in Appendix C. 

Starting Month: January Year: 2004 PWSID: AA7654321 System/Water Source: ABC Water Plant

Disinfectant Type: Chloramine Prepared by: Jon Operator

Profile Type (check one): X Giardia Viruses

Disinfection Segment/Sequence of Application: Transmission Pipe/3rd

3 4 5 6 7 8 9 10 11 12 13 14

Residual Peak Disinf.

Disinf. pH Water Hourly TDT Baffling Contact CTCalc = CT Inactivation Log

Week Conc. Temp. Flow Volume Factor Time (CxT) Req'd Ratio Inactivation*

# C (mg/L) (

o

C) (gpm) (gal) (min.) T (min.) (min-mg/L) (min-mg/L) (Col 11 / Col 12)

1 0.6 N/A 10 5,000 31,000 6.2 1.0 6.2 3.7 1,850 0.002

2

3

4

5

6

*See worksheet #2 to determine total log inactivation if the system has multiple disinfection segments.

WORKSHEET #1

LOG INACTIVATION RATIO DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER

The worksheet excerpt below demonstrates how a system may determine total Giardia log 

inactivation for the system in Example D-2 using Worksheet #2 in Appendix C. 

Starting Month: January Year: 2004 PWSID: AA7654321

System/Water Source: ABC Water Plant Prepared by: Jon Operator

Disinfectant Type: Chlorine/Chloramine

Profile Type (check one): X Giardia Viruses

Sum

Disinfection Disinfection Disinfection Disinfection Disinfection of Total

Week Segment Segment Segment Segment Segment Inactivation Log

# 1 2 3 4 5 Ratios Inactivation1

1 0.137 0.365 0.002 0.504 1.51

2

3

4

5

6 1

Giardia : Log Inactivation = 3 x Sum of Inactivation Ratios

 Viruses: Log Inactivation = 4 x Sum of Inactivation Ratios (or a method approved by the State)

Inactivation Ratio for each disinfection segment from Worksheet #1

TOTAL LOG INACTIVATION DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER

WORKSHEET #2

Refer to Chapter 7 for more information on interpreting log inactivation values. 
 5. Calculating Inactivation 

May 2003 49 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Equation 5-4 

Σ = Sum of. 

Systems must sum the 

CTcalc / CT99.99 inactivation 

ratios for each disinfection 

segment if the system has 

multiple disinfection 

segments. 

Check with the State on 

the approved method to 

calculate virus 

inactivation. 

Systems should check with their State to determine the 

appropriate method to use for calculating virus 

inactivation. The following equation was used for the 

examples presented in Appendix D of this document: 

Example D-3 in Appendix D presents one method for 

determining virus log inactivation for a system using ozone. 

5.6 STEPS COMPLETED

5.7 NEXT STEP

Once a system has determined log inactivation values once per 

week for a full year, then a disinfection profile and benchmark 

(if required) can be developed. Chapter 6 presents information 

on how to develop the disinfection profile and calculate a 

benchmark. 

Identify 

Disinfection 

Segments 

Collect Data 

Calculate CT

Calculate 

Inactivation

Develop the 

Disinfection 

Profile and 

Benchmark 

Report and 

Evaluate the 

Disinfection 

Profile and 

Benchmark

Equation 5-4 

Actual Log Inactivation of Viruses = 4 x Σ (CTcalc / CT99.99) 
5. Calculating Inactivation 

EPA Guidance Manual 50 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

This Page Intentionally Left Blank 
6. DEVELOPING THE DISINFECTION PROFILE 

AND BENCHMARK

May 2003 51 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

In this Chapter: 

• Constructing a 

Disinfection Profile 

• The Disinfection 

Benchmark 

• Significant Changes 

to Disinfection 

Practices 

• Benchmark 

Calculations

• Steps Completed

• Next Step

40 CFR Section 141.534 

Systems that use chlorine 

as a disinfectant must 

create a disinfection 

profile for Giardia only. 

40 CFR Section 141.535 

Systems that use 

chloramines, chlorine 

dioxide, or ozone for 

primary disinfection must 

create a disinfection 

profile for Giardia and 

viruses. The method used 

to calculate virus 

inactivation must be 

approved by the State. 

6.1 INTRODUCTION

Once the log inactivation has been calculated, a disinfection 

profile can be developed. A disinfection profile is a graphical 

representation of a system’s level of Giardia or virus 

inactivation measured during the course of a year (Figure 6-1 

provides an example disinfection profile). The disinfection 

profile is the log inactivation (of Giardia or viruses) graphed as 

a function of time. It can be used as a tool in the decision 

making process for a system’s disinfection practices. 

For systems that use chlorine as a disinfectant, Giardia is the 

more difficult organism to treat; therefore it is the limiting 

parameter and the only pathogen for which a disinfection 

profile is required. Viruses may be the limiting parameter for 

systems that use chloramines, chlorine dioxide, or ozone. 

Therefore, systems that use these disinfectants for primary

disinfection must create a disinfection profile for both Giardia

and viruses. The method used to calculate viral log 

inactivations must be approved by the State. 

If a system was required to develop a disinfection profile and 

decides to make a significant change to disinfection practices, 

then a disinfection benchmark must be calculated. The 

disinfection benchmark is the lowest monthly average log 

inactivation. The disinfection benchmark will be used by both 

the system and the State to evaluate proposed modifications to 

disinfection practices. 
6. Developing the Disinfection Profile and Benchmark 

EPA Guidance Manual 52 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

40 CFR Section 141.536 

Example Disinfection 

Profile 

6.2 CONSTRUCTING A DISINFECTION 

PROFILE

After log inactivation values have been calculated once each 

week for one year (using the method presented in Section 5.4), 

the system must produce a disinfection profile. A disinfection 

profile is a graph of log inactivation data. The log inactivations 

may be plotted along the vertical axis of a graph with the 

corresponding weeks of the year plotted along the horizontal 

axis, as shown in Figure 6-1. Systems are required to retain the 

disinfection profile in graphic form and it must be available for 

review by the State as part of a sanitary survey. Example 6-1 

demonstrates how to create a disinfection profile. 

Figure 6-1. Example of a Completed Disinfection 

Profile 

Disinfection Profile for System X, 2004

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

0 4 8 12 16 20 24 28 32 36 40 44 48 52

Week Tested

Log Inactivation

Log Inactivation
 6. Developing the Disinfection Profile and Benchmark 

May 2003 53 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example 6-1: Disinfection Profile for Giardia

Create a disinfection profile for Giardia for the conventional filtration system that was 

discussed in Examples 3-1, 4-1, 4-2, 5-1, and 5-2.. 

Step 1. Calculate the Giardia log inactivations once per week on the same day of the 

week for one year. 

The table below shows the Giardia log inactivations that were calculated each week for one 

year using the methods presented in Section 5.4 and Example 5-2. This information can also 

be obtained from the first and last columns of Worksheet #1 in Appendix C for systems with 

one disinfection segment or Worksheet #2 in Appendix C for systems with multiple 

disinfection segments.

Month Week Log Inact. Month Week Log Inact. 

JAN 1 1.34 JULY 27 1.86 

 2 1.35 28 1.82 

 3 1.38 29 1.76 

 4 1.37 30 1.74 

 5 1.38 31 1.71 

FEB 6 1.38 AUG 32 1.70 

 7 1.39 33 1.66 

 8 1.40 34 1.61 

 9 1.40 35 1.60 

MARCH 10 1.40 SEP 36 1.55 

 11 1.41 37 1.56 

 12 1.42 38 1.52 

 13 1.43 39 1.51 

APRIL 14 1.46 40 1.47 

 15 1.50 OCT 41 1.48 

 16 1.54 42 1.47 

 17 1.57 43 1.47 

 18 1.64 44 1.45 

MAY 19 1.66 NOV 45 1.41 

 20 1.70 46 1.43 

 21 1.72 47 1.41 

 22 1.74 48 1.40 

JUNE 23 1.77 DEC 49 1.40 

 24 1.79 50 1.40 

 25 1.82 51 1.40 

 26 1.81 52 1.37 
6. Developing the Disinfection Profile and Benchmark 

EPA Guidance Manual 54 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example 6-1 continued 

Step 2. Plot the disinfection profile. 

The log inactivations are plotted along the vertical axis with the corresponding weeks of the 

year plotted along the horizontal axis. The log inactivation value for week 1 (1.34) is 

plotted on the vertical axis at a point corresponding to week 1 on the horizontal axis, as 

shown below. The log inactivation value for week 2 (1.35) is plotted on the horizontal axis 

at a point corresponding to week 2 on the horizontal axis. The log inactivation value for 

week 3 (1.38) is plotted on the horizontal axis at a point corresponding to week 3 on the 

horizontal axis. After the points are plotted, lines are drawn to connect the points in order 

by the week tested. 

Week Tested

(Horizontal Axis)

1 2

1.35

1.40

Log Inactivation

(Vertical Axis)

1.30

3

Continue to plot the points for each week until all 52 weeks have been plotted. The 

completed disinfection profile is shown below. 

0.0

0.5

1.0

1.5

2.0

1/1/2004 3/25/2004 6/17/2004 9/9/2004 12/2/2004

Sample Date

Giardia Log Inactivation
 6. Developing the Disinfection Profile and Benchmark 

May 2003 55 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

40 CFR Section 141.540 

System must consult the 

State prior to making 

modifications to 

disinfection practices. 

Once a disinfection profile has been completed for a system, 

the system will have all of the data required to calculate a 

benchmark (which is necessary if the system contemplates 

making a significant change to its disinfection practices). The 

next sections discuss what a benchmark is and how it is 

calculated. 

6.3 THE DISINFECTION BENCHMARK 

The LT1ESWTR requires systems to develop a disinfection 

benchmark if the system is required to create a disinfection 

profile and decides to make a significant change to disinfection 

practices. The system must consult with the State for approval 

prior to making a significant change to disinfection practices 

(See Section 6.4 for a description of significant changes). 

Systems may be considering disinfection modifications for 

compliance with the Stage 1 DBPR requirements or for other 

reasons. The disinfection profile and benchmark information 

will allow the State to assess appropriate modifications to 

disinfection practices, as necessary. As explained in Chapter 1, 

benchmarking is used to characterize the minimum level of 

Giardia, and in some cases, virus log inactivations that are 

provided under current disinfection practices. The benchmark 

calculated under existing conditions can be compared to the 

benchmark calculated under the proposed modifications to 

ensure that changes to disinfection practices do not result in 

inactivation levels lower than the required inactivation values 

without appropriate State consultation and review. 

A benchmark is required if both of the following criteria apply: 

• A disinfection profile is required; 

AND, 

• The system decides to make a significant change(s) to 

disinfection practices. 

Systems that do not use chloramines, ozone, or chlorine 

dioxide as primary disinfection will calculate a profile and 

benchmark for Giardia only. Systems that use chloramines, 

ozone, or chlorine dioxide for primary disinfection must also 

calculate a profile and benchmark based on virus inactivation 

in addition to those for Giardia inactivation. Virus inactivation 

must be determined for these systems to address the possibility 

of reduced inactivation for viruses when using an alternative 

disinfectant. 
6. Developing the Disinfection Profile and Benchmark 

EPA Guidance Manual 56 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

40 CFR Section 141.541 

40 CFR Section 141.543 

Equation 6-1

Monthly Average Log 

Inactivation 

6.4 SIGNIFICANT CHANGES TO DISINFECTION 

PRACTICES 

The LT1ESWTR describes four types of significant 

modifications to disinfection practices: 

• Changes to the point of disinfection; 

• Changes to the disinfectant(s) used in the treatment 

plant; 

• Changes to the disinfection process; or, 

• Any other modification identified by the State. 

Systems may consider one or more of the above-mentioned 

modifications to comply with Stage 1 DBPR. These 

modifications will require the system to calculate its benchmark. 

The benchmark will be used for discussion with the State on 

disinfection modifications. The significant modifications are 

discussed in more detail in Chapter 7. 

6.5 BENCHMARK CALCULATIONS 

A disinfection benchmark is calculated using the following 

steps: 

• Complete a disinfection profile that includes the 

calculation of log inactivation of Giardia and viruses (if 

required) for each week of the profile. 

• Compute the average log inactivation for each calendar 

month of the profile by averaging the weekly log 

inactivation values for each month (See Equation 6-1). 

• Select the month with the lowest average log inactivation 

for the 12-month period. This value is the benchmark.

Example 6-2 demonstrates how to calculate the disinfection 

benchmark. 

Equation 6-1 

 

Monthly Average Sum of Weekly Log Inactivation Values 

Log Inactivation = 

 Number of Weekly Values per Month
 6. Developing the Disinfection Profile and Benchmark 

May 2003 57 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example 6-2: Calculating a Benchmark 

Calculate the disinfection benchmark for the conventional filtration system discussed in 

Examples 3-1, 4-1, 4-2, 5-1, 5-2, and 6-1. 

Step 1. Calculate weekly Giardia log inactivations. 

 This step was completed in Example 6-1. The data is summarized below: 

Month Week Log Inact. Month Week Log Inact. 

JAN 1 1.34 JULY 27 1.86 

 2 1.35 28 1.82 

 3 1.38 29 1.76 

 4 1.37 30 1.74 

 5 1.38 31 1.71 

FEB 6 1.38 AUG 32 1.70 

 7 1.39 33 1.66 

 8 1.40 34 1.61 

 9 1.40 35 1.60 

MARCH 10 1.40 SEP 36 1.55 

 11 1.41 37 1.56 

 12 1.42 38 1.52 

 13 1.43 39 1.51 

APRIL 14 1.46 40 1.47 

 15 1.50 OCT 41 1.48 

 16 1.54 42 1.47 

 17 1.57 43 1.47 

 18 1.64 44 1.45 

MAY 19 1.66 NOV 45 1.41 

 20 1.70 46 1.43 

 21 1.72 47 1.41 

 22 1.74 48 1.40 

JUNE 23 1.77 DEC 49 1.40 

 24 1.79 50 1.40 

 25 1.82 51 1.40 

 26 1.81 52 1.37 
6. Developing the Disinfection Profile and Benchmark 

EPA Guidance Manual 58 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example 6-2 continued 

Step 2. Calculate the monthly average log inactivation for each month. 

Begin by averaging January’s inactivations: 

Average log inactivation for January = ( Sum of Weekly Log Inactivation Values ) 

 (Number of Weekly values in Month) 

 

 = 1.34 + 1.35 + 1.38 + 1.37 + 1.38 

 5 values 

 = (6.82) / (5) = 1.36 

Continue this process for each month. The following are the results for the Example system: 

January 1.36 July 1.78 

 February 1.39 August 1.64 

 March 1.41 September 1.52 

 April 1.54 October 1.47 

 May 1.71 November 1.41 

 June 1.80 December 1.39 

Step 3. Identify the month with the lowest monthly average log inactivation. The log 

inactivation for this month is the disinfection benchmark. 

The month with the lowest monthly average log inactivation is January, with a value of 1.36. 

The benchmark is 1.36. 
 6. Developing the Disinfection Profile and Benchmark 

May 2003 59 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

6.6 STEPS COMPLETED

6.6 NEXT STEP

By calculating the benchmark, the system has identified its 

lowest monthly average inactivation value. This benchmark is 

used as a guide when evaluating disinfection practice 

modifications. Chapter 7 provides information on how to 

evaluate disinfection practice modifications.

Identify 

Disinfection 

Segments 

Collect Data 

Calculate CT

Calculate 

Inactivation

Develop the 

Disinfection 

Profile and 

Benchmark 

Report and 

Evaluate the 

Disinfection 

Profile and 

Benchmark
6. Developing the Disinfection Profile and Benchmark 

EPA Guidance Manual 60 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

This Page Intentionally Left Blank 
7. EVALUATING DISINFECTION PRACTICE 

MODIFICATIONS

May 2003 61 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

In this Chapter:

• System Reporting 

Requirements 

• Simultaneous 

Compliance 

• How the State will 

Use the Benchmark

• Steps Completed

Systems must consult the 

State prior to making 

modifications to 

disinfection practices. 

7.1 INTRODUCTION

The benchmark is a system’s lowest monthly average Giardia

(or virus) inactivation based on the disinfection profile. The 

benchmark must be calculated if a system is required to develop 

a disinfection profile and decides to make a significant 

modification to disinfection practices. The benchmark will help 

in evaluating alternatives to the current disinfection practices. 

Remember, a system must reliably and consistently provide the 

necessary log inactivation through disinfection to achieve 

adequate Giardia and virus log reduction as required by the 

SWTR (See Table 7-1). Table 7-2 provides typical removal 

credits and inactivation requirements for different processes. 

Systems should check with the State on the specific removal 

credits and inactivation requirements since Table 7-2 contains 

typical values. 

If a benchmark value is less than the required log inactivation 

for disinfection, then the system will probably need to modify 

disinfection practices, such as increasing the amount of 

disinfectant or the contact time. Increasing the amount of 

disinfectant will probably require the system to evaluate 

disinfection byproducts more closely. 

If the benchmark is greater than the required inactivation in 

Table 7-2 (or as required by the State), the system can consider 

decreasing the amount of disinfectant added, contact time, or 

altering other disinfection practices. Systems must consult the 

State for approval prior to making a significant change to their 

disinfection practices. The State will work with the system to 

determine if the change is significant and whether it triggers any 

additional requirements under LT1ESWTR. 
7. Evaluating Disinfection Practice Modifications 

EPA Guidance Manual 62 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Microbial removal and 

inactivation requirements. 

Table 7-1: Removal and Inactivation Requirements 

Table 7-2: Typical Removal Credits and Inactivation 

Requirements for Various Treatment Technologies 

Microorganism

 

Required Log 

Reduction

Treatment

Giardia

 

3-log (99.9%)

 

Removal and/or 

Inactivation 

Viruses 

 

4-log (99.99%)

 

Removal and/or 

Inactivation 

Cryptosporidium 2-log (99%) Removal 

Typical Log 

Removal Credits 

Resulting 

Disinfection Log 

Inactivation 

Requirements

Process

Giardia Viruses Giardia Viruses

Conventional 

Treatment 

2.5 2.0 0.5 2.0 

Direct 

Filtration 

2.0 1.0 1.0 3.0 

Slow Sand 

Filtration 

2.0 2.0 1.0 2.0 

Diatomaceous 

Earth Filtration 

2.0 1.0 1.0 3.0 

Alternative 

(membranes, 

bag filters, 

cartridges) 

* * * * 

Unfiltered 0 0 3.0 4.0 

* Systems must demonstrate to the State by pilot study or other means 

that the alternative filtration technology provides the required log 

removal and inactivation shown in Table 7-1. 
 7. Evaluating Disinfection Practice Modifications 

May 2003 63 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

40 CFR Section 141.542 

Stage 1 DBPR = Stage 1 

Disinfectants and 

Disinfection Byproducts 

Rule 

7.2 SYSTEM REPORTING REQUIREMENTS 

A system that is considering a significant change to its 

disinfection practice must calculate the disinfection benchmark, 

provide the benchmark to the State, and consult with the State 

for approval before making the significant change. The 

LT1ESWTR describes four types of significant changes to 

disinfection practices: 

• Changes to the point of disinfection; 

• Changes to the disinfectant(s) used in the treatment 

plant; 

• Changes to the disinfection process; or, 

• Any other modification identified by the State. 

As part of the consultation and approval process, a system must 

submit the following information to the State: 

• A description of the proposed change. 

• The disinfection profile and disinfection benchmark for 

Giardia. If the system uses chloramines, ozone, or 

chlorine dioxide for primary disinfection, the system 

must also submit a profile and benchmark for viruses. 

• An analysis of how the proposed change will affect the 

current levels of disinfection. 

• Any additional information requested by the State. 

Disinfection profiling and benchmarking will help ensure that 

microbial protection is not compromised by any modifications 

to disinfection practices. These modifications are discussed in 

more detail in Section 7.3. 

7.3 SIMULTANEOUS COMPLIANCE

The LT1ESWTR is not the only rule that affects or dictates 

disinfection practices. The Stage 1 DBPR applies to some or 

all PWSs (depending on the disinfectant used) that add 

chlorine, chloramines, chlorine dioxide, or ozone. This rule 

establishes MCLs for TTHM and HAA5, in addition to other 

byproducts (depending on the disinfectant used). The MCLs 

are 0.080 mg/L for TTHM and 0.060 mg/L for HAA5 under 
7. Evaluating Disinfection Practice Modifications 

EPA Guidance Manual 64 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Disinfection Byproducts = 

DBP 

the Stage 1 DBPR. Surface water systems and systems 

classified as GWUDI that serve less than 10,000 people must 

comply with this rule by January 1, 2004. The Stage 1 DBPR 

also establishes maximum residual disinfectant levels 

(MRDLs) for systems using chlorine, chloramines, and 

chlorine dioxide. 

The following terms may be helpful for understanding 

disinfection practices: 

• Disinfection Byproduct (DBP) Precursors – DBP 

precursors are constituents naturally occuring in source 

water that react with a disinfectant to form DBPs. The 

primary DBP precursor is natural organic matter, which 

is monitored as total organic carbon (TOC). Organic 

matter reacts with the disinfectant to form TTHM, 

HAA5, and other DBPs. The Alternative Disinfectants 

and Oxidants Guidance Manual (EPA, 1999a) provides 

more detailed information on DBP formation. 

• Pre-disinfection – Pre-disinfection occurs when a 

disinfectant is added to the treatment train prior to the 

primary disinfectant injection location. The purpose of 

pre-disinfection is to obtain additional inactivation 

credits, to control microbiological growth in subsequent 

treatment processes, to improve coagulation, and/or to 

reduce tastes and odors. 

• Primary Disinfection – The disinfectant used in a 

treatment system with the primary objective to achieve 

the necessary microbial inactivation. 

• Secondary Disinfection – The disinfectant applied 

following primary disinfection in a treatment system 

with the primary objective to maintain the residual 

disinfectant throughout the distribution system. 
 7. Evaluating Disinfection Practice Modifications 

May 2003 65 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example: 

Moving the point of predisinfectant application

7.3.1 Changes to the Point of Disinfection 

Any change in the location of the disinfectant application 

constitutes a significant change to disinfection practices. For 

instance, a water system that uses pre-disinfection may consider 

moving the point of disinfectant application further into the 

treatment train (See Figure 7-1). This modification will result in 

a reduction of contact time between DBP precursors and the 

disinfectant(s) with corresponding reduction (typically) in the 

production of DBPs. Also, moving the pre-disinfection to a 

location after some of the treatment processes where organics 

have been removed will result in less contact between organic 

matter (precursors) and disinfectant; therefore, fewer DBPs 

should be created. 

Figure 7-1. Example of Moving the Point of Predisinfectant Application 

Distribution

System

Sedimentation

Coagulation

Intake Clearwell

Predisinfection

Location

1 1 1

Cl2

Feed

Filtration

Flocculation

1 Potential locations for pre-disinfection. For example, 

the system may consider relocating the pre-disinfection 

location from the intake to one of three other possible 

locations. The potential for DBP formations decreases 

further down the treatment train for two reasons: 

1. Contact time between DBP precursors and 

disinfectants is reduced. 

2. DBP precursors are removed with each subsequent 

treatment process. 

A system that is considering moving the point of 

disinfectant application further into the treatment process 

should make sure that it can maintain adequate contact time 

and meet required log inactivations. 
7. Evaluating Disinfection Practice Modifications 

EPA Guidance Manual 66 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Systems should identify all 

impacts on the treatment 

process and maintain 

compliance with all rules 

when moving the point of 

disinfectant application.

When moving the point of disinfection further into the treatment 

process, a system should consider whether adequate contact time 

will still be available to achieve sufficient disinfection. This 

type of modification may affect the amount of inactivation 

achieved by the system. Systems may find that seasonal use of 

this type of modification is helpful in reducing summertime 

DBP levels, which are typically the highest. 

In conventional treatment, DBP precursors are removed through 

coagulation, sedimentation, and filtration. Moving the point of 

disinfectant application to another point downstream from these 

processes can reduce the concentration of DBP precursors that 

come in contact with the disinfectant. However, moving the 

disinfectant application point downstream also reduces the time 

that the disinfectant is in contact with the water and the CTcalc

for the disinfectant. Increasing the disinfectant concentration 

can help maintain a higher CTcalc, but greater disinfectant 

concentrations also lead to increased DBP formation. In 

addition, the disinfectant concentration may be limited 

(depending on the disinfectant used) by the MRDL for the 

disinfectant. Another alternative for maintaining CT is to add 

baffling to the clearwell or to storage tanks downstream in order 

to increase the disinfectant contact time. An increase in the 

contact time value may allow a lower disinfectant concentration 

and may result in fewer disinfection byproducts. However, the 

system must make sure it provides enough disinfectant to 

achieve the required microbial inactivation values. 

7.3.2 Changes to the Disinfectant(s) Used in 

the Treatment Plant 

Water systems typically use one or more of the following 

disinfectants: 

• Chlorine; 

• Chloramines; 

• Chlorine Dioxide; 

• Ozone; or, 

• Ultraviolet (UV). 

A water system may consider changing the disinfectant used in 

its treatment plant to comply with the Stage 1 DBPR MCLs.
 7. Evaluating Disinfection Practice Modifications 

May 2003 67 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Considerations for 

alternative disinfectants. 

Systems may also consider changing both the disinfectant and 

point of disinfection application. For example, a system may 

shift from chlorine as the sole source of disinfection to chlorine 

prior to the clearwell (primary disinfection) and ammonia 

added after the clearwell for chloramine (secondary 

disinfection) (See Figure 7-2). This configuration allows for 

chlorine to achieve Giardia and virus inactivation in the 

clearwell. The addition of ammonia after the clearwell to 

produce chloramines can reduce the formation of DBPs in the 

distribution system. In another example, a system may move 

the pre-disinfection location from a point prior to the 

presedimentation basin to a point prior to coagulation. In 

addition, the system may change the pre-disinfectant from 

chlorine to ozone to reduce TTHM and HAA5 formation (See 

Figure 7-3). 

As a system considers different disinfectants, it should evaluate 

the following: 

• What DBPs are created by the disinfectant? 

• What concentrations and contact times are required to 

provide adequate microbial inactivation? 

• Where is the best point of application in the treatment 

train to minimize DBP’s and maximize inactivations? 

For more information on disinfectants, refer to Chapter 8 of 

this guidance manual and the Alternative Disinfectants and 

Oxidants Guidance Manual (EPA, 1999a), available on EPA’s 

website http://www.epa.gov/safewater/mdbp/implement.html. 
7. Evaluating Disinfection Practice Modifications 

EPA Guidance Manual 68 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example: 

Changing disinfectant 

type 

Example:

Changing pre-disinfection 

location and type of 

disinfectant

Figure 7-2. Example of Changing Disinfectant Type 

Figure 7-3. Changing Pre-disinfection Location and 

Type of Disinfectant 

Distribution

System

Clearwell

Primary Disinfectant

(Existing)

Cl2

Feed

Secondary Disinfectant (New)

Ammonia Feed for

Monochloramine Formation

From Filtration

Chlorine is used as the sole disinfectant and is added prior 

to the clearwell to obtain Giardia and virus inactivation. 

The system decides to add ammonia after the clearwell to 

produce chloramine. Using chloramine as a secondary 

disinfectant has two advantages: 

1) Chloramine typically has a lower potential for 

TTHM and HAA5 formation than chlorine. 

Chloramine should result in lower TTHM and 

HAA5 formation in the distribution system. 

2) Chloramine residuals last longer than chlorine. 

Distribution

System

Pre- Sedimentation

Sedimentation

Coagulation

Intake Clearwell

Cl

Feed

Chlorine

(point 1)

Ozone

(point 2)

Filtration

Change pre-disinfection location from prior to the presedimentation basin (point 1) to prior to coagulation (point 

2). Changing from chlorine to ozone may also be 

considered to reduce TTHM and HAA5 formation. 
 7. Evaluating Disinfection Practice Modifications 

May 2003 69 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Chlorine is pH sensitive 

and more effective at 

lower pHs. 

7.3.3 Changes to the Disinfection Process 

Other changes to the disinfection process also require water 

systems to consult with the State before making the treatment 

change. Some modifications to the disinfection process include 

the following: 

• Changing the contact basin geometry and baffling 

conditions to provide additional contact time; 

• Increasing or decreasing the pH during disinfection; or, 

• Decreasing the disinfectant dose during warmer 

temperatures. 

The LT1ESWTR requires water systems to submit information 

to the State prior to making a change (See Section 7.2). 

Effects of Basin Geometry and Baffling Conditions on 

Contact Time

Changing the contact basin geometry or baffling conditions 

may result in more inactivation by changing the T value in the 

CTcalc value. With this modification, additional inactivation is 

achieved without increasing the disinfectant concentration. 

pH Effects on Chlorine

Chlorine is very sensitive to pH. Decreases in pH provide 

increased inactivation of Giardia and viruses. Therefore, at 

lower pHs a lower chlorine dose or contact time can be applied 

while achieving a sufficient level of inactivation of both 

Giardia and viruses. This in turn can reduce the potential for 

DBP formation. However, decreasing the pH is a processsensitive issue and could result in other system changes, such as 

increased coagulant demand for proper floc formation, 

distribution system corrosion problems, or precipitation of 

certain inorganics. Extensive jar tests and pilot scale studies 

may be necessary before adjusting the pH. 
7. Evaluating Disinfection Practice Modifications 

EPA Guidance Manual 70 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Chlorine is temperature 

sensitive and more 

effective at higher 

temperatures. 

Temperature Effects on Chlorine and DBP Formation

Chlorine is more effective at higher temperatures, which results 

in faster chemical reactions and consequently, a higher 

potential for DBP formation. Warmer surface waters also 

support more organic growth, increasing the potential for DBP 

formation. However, since chlorine is more reactive at higher 

temperatures, it is also more effective against microorganisms 

such as Giardia and viruses. Thus, when water temperatures 

are warmer the chlorine dose or contact time can be decreased 

while achieving the same amount of microbial inactivation as 

in cooler temperatures. However, if a system decreases the 

chlorine dose or contact time, the system should ensure that it 

is maintaining sufficient log inactivations of both Giardia and 

viruses.

7.3.4 Other Modifications 

The State may determine that other changes in water system 

operations should be considered significant changes in 

disinfection practices. If the State makes such a determination, 

systems that make these other significant changes must develop 

and submit a disinfection benchmark. 

The modifications listed in Sections 7.3.1 through 7.3.3 are not 

an exhaustive list. States may determine that other types of 

changes are also significant. Therefore, a water system should 

check with the State program office for assistance in 

determining whether a proposed change triggers the 

disinfection benchmarking procedure. Other modifications that 

may require State consultation and approval are enhanced 

coagulation, enhanced softening, or oxidation. Water systems 

can refer to the Alternative Disinfectants and Oxidants 

Guidance Manual (EPA, 1999a) and the Enhanced 

Coagulation and Enhanced Precipitative Softening Guidance 

Manual (EPA, 1999b) for additional information. Copies of 

these guidance manuals can be obtained by downloading from 

EPA’s website at 

http://www.epa.gov/safewater/mdbp/implement.html. 
 7. Evaluating Disinfection Practice Modifications 

May 2003 71 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

The State may use the 

benchmark to make sure 

that modifications to 

disinfection practices still 

maintain required 

microbial inactivation. 

7.4 HOW THE STATE WILL USE THE 

BENCHMARK

The State is expected to use the benchmark to evaluate the 

microbial inactivation the system has achieved over time and 

compare this with the expected microbial inactivation the 

system will achieve by disinfection practice modifications. 

Systems with a benchmark that is less than the inactivation 

requirements in Table 7-2 or those required by the State will 

probably need to modify disinfection practices in order to 

provide the necessary level of disinfection. For instance, a 

conventional treatment plant has calculated a benchmark of 0.3 

for Giardia but is required to achieve 0.5-log Giardia

inactivation through disinfection. This system would need to 

provide additional disinfection to achieve the required 0.5-log 

Giardia inactivation. At the same time, systems must also 

make sure that they maintain compliance with the Stage 1 

DBPR. The system must consult with the State for approval 

and provide all necessary information prior to any significant 

modification, as described in Section 7.2. 

Systems may consider modifying disinfection practices if the 

benchmark is greater than the inactivation requirements in 

Table 7-2 or the inactivation required by the State. For 

instance, a conventional treatment plant has calculated a 

benchmark of 1.3 for Giardia but is only required to achieve a 

0.5-log Giardia inactivation through disinfection. The system 

uses chlorine and is having difficulties complying with TTHM 

and HAA5 MCLs established by the Stage 1 DBPR. The 

system considers decreasing the amount of chlorine, but must 

determine what level of chlorine is needed to meet the 0.5-log 

Giardia inactivation and still maintain compliance with the 

Stage 1 DBPR. Again, the system must consult with the State 

for approval prior to making any significant modifications and 

must provide all necessary information. 

The benchmark may be used by the State as a minimum level 

of inactivation of Giardia and viruses that must be maintained 

by water systems when modifying disinfection practices. The 

State may also use the disinfection profile and benchmark to 

determine an appropriate alternative benchmark under different 

disinfection scenarios. The LT1ESWTR Implementation 

Guidance Manual (under development at this time) will 

provide additional information on how States will use the 

disinfection profile and benchmark. 
7. Evaluating Disinfection Practice Modifications 

EPA Guidance Manual 72 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

7.5 STEPS COMPLETED

7.6 REFERENCES

EPA. 1999a. Alternative Disinfectants and Oxidants Guidance Manual (EPA 815-R-99-

014). Washington, D.C. 

EPA. 1999b. Enhanced Coagulation and Enhanced Precipitative Softening Guidance 

Manual (EPA 815-R-99-012). Washington, D.C. 

Identify 

Disinfection 

Segments 

Collect Data 

Calculate CT

Calculate 

Inactivation

Develop the 

Disinfection 

Profile and 

Benchmark

Report and 

Evaluate the 

Disinfection 

Profile and 

Benchmark
8. TREATMENT CONSIDERATIONS

May 2003 73 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

In this Chapter: 

• Alternative 

Disinfectants and 

Oxidants 

• Enhanced 

Coagulation and 

Softening 

• Increasing the Contact 

Time 

• Membranes

For more information on 

alternative disinfectants 

and oxidants, see the 

Alternative Disinfectants 

and Oxidants Guidance 

Manual (EPA, 1999a), 

available on EPA’s 

website 

http://www.epa.gov/safewater/

mdbp/implement.html.

8.1 INTRODUCTION

In order to comply with the requirements of the LT1ESWTR 

and the Stage 1 DBPR, water systems may decide to make 

changes to their disinfection practices or other treatment 

processes. Disinfection profiling and benchmarking will help 

ensure that microbial protection is not compromised by any of 

these modifications. Since DBPs are formed when organic 

material reacts with disinfectants such as chlorine, water 

systems may choose to use less chlorine or modify treatment 

processes to reduce the formation of DBPs. Some methods 

that water systems may use to control disinfection byproducts, 

while meeting the required inactivation levels for Giardia and 

viruses, include the use of alternative disinfectants and 

oxidants, enhanced coagulation and softening, increasing the 

contact time, or the use of alternative filtration techniques, such 

as membranes. Pilot testing is generally recommended before 

any of these major modifications are made to plant processes. 

The State must also be consulted for approval prior to any 

modifications. 

8.2 ALTERNATIVE DISINFECTANTS AND 

OXIDANTS

This section discusses various alternative disinfectants and 

oxidants that may be considered for meeting both inactivation 

and Stage 1 DBPR requirements. A more complete discussion 

of this topic is provided in the Alternative Disinfectants and 

Oxidants Guidance Manual (EPA, 1999a). 

Chlorine has long been considered an effective disinfectant in 

water systems and is the most widely used disinfectant by 

small systems. Chlorine is typically used in one of three forms: 

chlorine gas, sodium hypochlorite (typically liquid), and 

calcium hypochlorite (typically solid). Chlorine effectively 

inactivates a wide range of pathogens, including Giardia and 

viruses. Chlorine residuals are generally carried into the 

distribution system for further protection. The Stage 1 DBPR 

established an MRDL of 4.0 mg/L measured as chlorine if 

chlorine is used (community and non-transient non-community 

water systems). Remember, surface water and GWUDI 

systems must maintain a residual disinfectant concentration of 

0.2 mg/L at the entry point to the distribution system and must 
8. Treatment Considerations 

EPA Guidance Manual 74 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Alternative disinfectants 

and oxidants to consider 

are chloramines, ozone, 

chlorine dioxide, ultraviolet radiation, and 

potassium permanganate. 

Stage 1 DBPR 

For community and nontransient non-community 

water systems using 

chloramines: 

MRDL of 4.0 mg/L as 

chlorine.

maintain a detectable residual disinfectant in the distribution 

system (40 CFR Section 141.72). 

The use of chlorine as a disinfectant, particularly as a predisinfectant, has typically been found to increase the formation 

of DBPs. One option to resolve this problem is to use a 

different pre-disinfectant or an oxidant such as chlorine 

dioxide, ozone, or potassium permanganate. The type of 

oxidant used and its concentration have significant effects on 

DBP formation. Consideration should also be given to the pH 

of the water, since lowering the pH decreases TTHM formation 

but increases formation of other chlorinated organic species 

(Dowbiggin and Thompson, 1990). In addition, higher 

temperatures speed up the reaction between chlorine and 

organic material, thus increasing finished water TTHM and 

HAA5 levels (Singer, 1999). 

Retaining adequate disinfectant residual at all points in the 

water distribution system is important to inhibit bacteriological 

growth, and using chlorine to achieve this has been a widely 

accepted practice. However, the long detention time for water 

at the ends of water mains promotes DBP formation when 

chlorine is used. An alternative disinfectant, such as 

chloramines, may then be an option. 

8.2.1 Chloramines (NH2Cl)

Chloramines are formed when chlorine and ammonia are added 

together, either simultaneously or sequentially. The ammonia 

can be applied before or after the chlorine. However, applying 

ammonia after the chlorine has been found to inactivate 

pathogens more effectively (AWWA, 1999). Chloramination 

is normally practiced as a ratio of approximately 1 part of 

ammonia to 4 parts of chlorine (on a mg/L basis) to ensure 

monochloramine formation (Kawamura, 2000). Chloramines 

are typically used as a secondary disinfectant since they are 

more stable than chlorine and can provide better protection in 

the distribution system (Kawamura, 2000). To meet required 

inactivations of Giardia and viruses, chloramines require more 

contact time than chlorine. Chloramines typically have a lower 

potential than chlorine for producing DBPs. The Stage 1 

DBPR established an MRDL of 4.0 mg/L as chlorine for 

systems using chloramines. The CT tables presented in 

Appendix B of this guidance document assume ammonia is 

added after chlorine to form chloramines. 
 8. Treatment Considerations 

May 2003 75 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Case Study – Schneider and Tobiason (2000) 

Jar-testing was used to study the effects of preozonation on interactions among 

coagulants, particles, and natural organic matter. Synthetic water (deionized, distilled 

water with organic matter, particles, and background ions added) and waters from 

Lake Gaillard in Branford, Connecticut; the Oradell reservoir in Oradell, New Jersey; 

and the Passaic River in Little Falls, New Jersey, were tested. Experiments were run 

with ozone only and with ozone followed by coagulation. The research found that 

when alum was used as a coagulant, preozonation hindered the removal of turbidity 

and dissolved organic matter (DOM) at the conditions tested. Cationic polymers, 

however, allowed small increases in the removal of turbidity and DOM. It was found 

that varying the preozone contact time from 4 to 28 minutes had little effect on settled 

water turbidity, TOC, and dissolved organic carbon for the conditions tested.

Stage 1 DBPR 

For community and nontransient non-community 

water systems using 

ozone: 

MCL for bromate 

= 0.010 mg/L

8.2.2 Ozone (O3) 

Over the last fifteen years, the most widely studied alternative 

to chlorine as a disinfectant has been ozone (Schneider and 

Tobiason, 2000). Ozone is used for both oxidation and 

disinfection. It must be generated at the point of application, 

since it is an unstable molecule. Ozone is a powerful oxidant 

and is more effective than chlorine, chloramines, and chlorine 

dioxide for inactivation of viruses, Cryptosporidium, and 

Giardia (EPA, 1999a). 

Ozone effectively oxidizes DBP precursors, but its 

effectiveness is pH and temperature dependent. It can only be 

used as a primary disinfectant, since it is unable to maintain a 

residual in the distribution system. Chlorine or chloramines 

should also be applied as a secondary disinfectant to maintain 

the residual. 

The use of ozone poses some health and safety concerns that 

should be addressed by a utility considering its use. 

Instrumentation should be provided for ozone systems to 

protect both personnel and the equipment. Ozone is highly 

corrosive and toxic, and ozonation systems are relatively 

complex. While ozone does not form halogenated DBPs 

except in bromide-rich waters, it does form a variety of organic 

and inorganic byproducts, such as bromate. Bromate is 

regulated by the Stage 1 DBPR at 0.010 mg/L.
8. Treatment Considerations 

EPA Guidance Manual 76 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

8.2.3 Chlorine Dioxide (ClO2) 

Chlorine dioxide is a powerful oxidant and disinfectant that is 

effective at inactivating bacterial, viral, and protozoan 

pathogens. Chlorine dioxide is generated on-site and is equal 

or superior to chlorine in its disinfection ability. Chlorine 

dioxide is primarily used in the United States as a means of 

taste and odor control, oxidation of iron and manganese, and 

control of TTHM and HAA5 (Kawamura, 2000). Chlorine 

dioxide doses are limited due to production of chlorite. The 

Stage 1 DBPR regulates chlorite. Utilities using chlorine 

dioxide may have to use granular activated carbon (GAC) or a 

chemical reducing agent, such as sulfur dioxide, to remove the 

chlorite residual. 

Case Study – Ashe, et al. (1994) 

The Brewer Water District, serving 9,100 customers, operates pumping and treatment 

facilities in Eddington, Maine. The water system draws its water from Hatcase Pond, 

disinfects with sodium hypochlorite, adjusts the pH with caustic soda, adds sodium fluoride 

for fluoridation, and sends the treated water to a 50,000-gallon clearwell. In order to 

comply with new and pending regulations, the system needed to reduce TTHMs and 

achieve a 3-log inactivation of Giardia cysts and a 4-log inactivation of viruses. The 

District began addressing its disinfection concerns by studying its present water quality and 

disinfection practices. The raw water supply was found to exhibit a high chlorine demand 

and a rapid rate of TTHM formation. The use of ozone, chlorine dioxide, and chloramines 

were considered in place of chlorine disinfection. The use of chloramines was determined 

to be economically unfeasible due to the large volume of additional storage required in 

order to meet CT criteria. Pending legislation for the regulation of chlorine dioxide 

byproducts (chlorite is now regulated) eliminated it from further consideration. Ozone was 

therefore chosen as a primary disinfectant for pilot plant study. Results showed that 

ozonation at a dose of 2.0 mg/L to 3.5 mg/L and a contact time of 6 to 9 minutes would 

provide the required CT value for this system under all water temperatures and pH 

conditions, and adequately destroy the organic compounds that form DBPs. Chloramines 

were chosen for use as a secondary disinfectant in order to maintain a residual throughout 

the distribution system while reducing trihalomethane formation in the distribution system. 

Hatcase Pond

40,000 Gallon Ozone

Contact Tank

250,000 Gallon

Storage Tank

Caustic Soda

Hypochlorite

Ammonia

Fluoride

To Distribution

Pump

Brewer Water District proposed treatment process. 
 8. Treatment Considerations 

May 2003 77 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Stage 1 DBPR 

For water systems using 

chorine dioxide:

MRDL of 0.8 mg/L as 

ClO2. 

MCL of chlorite of 1.0 

mg/L (only applies to 

community and nontransient non-community 

systems). 

EPA is currently 

developing a UV 

Guidance Manual. 

Chlorine dioxide is a strong oxidant and aids in reducing 

TTHM and HAA5 by oxidizing precursors. Chlorine dioxide 

can also be used to reduce taste and odors or as a primary or 

secondary disinfectant. Chlorine dioxide has the ability to 

maintain a residual in the distribution system for an extended 

period of time (Kawamura, 2000). The Stage 1 DBPR 

establishes an MRDL of 0.8 mg/L as ClO2 for systems 

(community, non-transient non-community, and transient noncommunity) using chlorine dioxide. 

8.2.4 Potassium Permanganate (KMnO4) 

Potassium permanganate is primarily used as a pre-oxidant to 

control algal growth, tastes, and odors, and to remove iron, 

manganese, and color. It may also be used to control DBP 

formation by oxidizing organic precursors and reducing the 

demand for other disinfectants (EPA, 1999a). It is not allowed 

under the Surface Water Treatment Rule to be used as a 

disinfectant to achieve microbial inactivation. A water 

treatment plant may choose to use potassium permanganate as a 

pre-oxidant, in lieu of chlorine, and then move the chlorination 

point further into the treatment train. This configuration may 

help in the control of DBPs by reducing the concentration of 

natural organic matter and delaying the introduction of chlorine 

until after the majority of precursors have been removed in the 

treatment process. 

There are some disadvantages to using potassium 

permanganate. Potassium permanganate must be handled 

carefully when preparing the feed solution, since it can cause 

serious eye injury, irritate the skin and respiratory system, and 

can be fatal if swallowed. It also tends to turn the water a pink 

color. 

8.2.5 Ultraviolet Radiation (UV) 

UV light is becoming an increasingly popular method of 

disinfecting drinking water. One advantage of UV is that it 

does not cause the formation of harmful disinfection 

byproducts. Recent studies also show that UV can inactivate 

Cryptosporidium and Giardia. An additional advantage is that 

there are fewer safety concerns with using UV than with 

chemical disinfectants such as chlorine gas or chlorine dioxide. 

UV rays inactivate microorganisms by penetrating their cell 
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LT1ESWTR Disinfection Profiling and Benchmarking 

Systems should conduct a 

pilot study prior to 

eliminating or adding a 

disinfectant. 

walls to damage essential cell components. UV is not effective 

on all pathogens, and studies are being done to identify UV’s 

effectiveness. 

The UV dose is the product of the irradiance and the time that 

an organism is exposed to that irradiance. The dose is 

expressed in millijoules per square centimeter (mJ/cm2

) or the 

equivalent, milliwatt-seconds per square centimeter 

(mW·s/cm2

). A common UV dose used in drinking water 

disinfection is 38-40 mJ/cm2

 (Cotton, et al., 2001). Data from 

recent research indicate that a dose of 40 mJ/cm2 will achieve 

at least a 2-log inactivation of Cryptosporidium (Cotton, et al., 

2001). Giardia is thought to be equally as sensitive as or more 

sensitive than Cryptosporidium to UV light. Bacteria are more 

susceptible to UV disinfection than Cryptosporidium. Some 

viruses are significantly less susceptible to UV disinfection 

than Cryptosporidium and bacteria. Thus, virus inactivation is 

likely to control the dose when UV is used as the only 

disinfectant in drinking water treatment. 

Despite the many advantages of UV systems, these systems 

also have some shortcomings. Since UV is a physical 

disinfectant, not a chemical disinfectant, it does not leave a 

residual in the water. Thus, a secondary disinfectant must be 

applied to maintain distribution system residuals. Another 

disadvantage is that higher turbidity may shield organisms and 

prevent them from being exposed to UV. Additionally, organic 

material absorbs UV light and can increase the UV demand of 

the water. Therefore, it is recommended that systems apply UV 

light as a disinfectant after filtration, where turbidity and 

organics in the water are reduced. Another potential problem is 

that scale can form on the quartz sleeves that house the UV 

lamps, depending on the ions, hardness, alkalinity, and pH of 

the water. This in turn causes a reduction in the amount of UV 

energy that is transmitted to the water. However, regular 

cleaning of the sleeves can reduce the effects of scaling. 

Finally, the operation of the UV lamps may be temperature 

dependent. 

8.2.6 Comparison of Disinfectants 

EPA and the Association of Metropolitan Water Agencies 

(AMWA) funded a two-year study of 35 water treatment 

facilities to evaluate DBP production based on various 

combinations of primary and secondary disinfectants. Among 

four of the facilities, alternative disinfection strategies were 
 8. Treatment Considerations 

May 2003 79 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

investigated to evaluate the difference in DBP production from 

the systems’ previous disinfection strategies (or base 

disinfection conditions). The results were analyzed in three 

reports (Metropolitan and Montgomery, 1989; Jacangelo, et al., 

1989; Malcolm Pirnie, Inc., 1992) that documented different 

aspects of the study. Table 8-1 summarizes the results of the 

study. This study illustrates that a change in primary 

disinfectant from chlorine to ozone or to chloramines may help 

reduce TTHM and HAA5. 

Table 8-1. Study Results on Changing Primary and Secondary Disinfectants 

DBP 

Change in Disinfection Practice Concentration Change 1

(Primary Disinfectant/Secondary Disinfectant) TTHM HAA5 

Chlorine/Chlorine 

To 

Chlorine/Chloramines2

Utility #7 Decrease Decrease 

Chlorine/Chlorine Utility #19 Decrease Decrease 

To 

Ozone/Chlorine Utility #36 No change No change 

Chlorine/Chloramines 

To 

Ozone/Chloramines 

Utility #7 Decrease Decrease 

Chlorine/Chlorine 

To 

Chloramines/Chloramines 

Utility #36 Decrease Decrease 

Ozone/Chlorine 

To 

Ozone/Chloramines 

Utility #36 Decrease Decrease 

Chloramines/Chloramines Utility #25 Decrease Decrease To 

Ozone/Chloramines Utility #36 No change No change 

Chlorine/Chlorine Utility #7 Decrease Decrease 

To 

Ozone/Chloramines Utility #36 Decrease Decrease 

1

Several studies were conducted to examine the effects of changing primary and secondary disinfectants on 

DBP levels. For instance, changing the secondary disinfectant from chlorine to chloramines resulted in a 

decrease in both TTHM and HAA5. Results are based on full-scale evaluations at Utilities #19 and #25 and on 

pilot scale evaluations at Utilities #7 and #36. 

2

Free chlorine contact time was 4 hours for Utility #7 during use of chlorine/chloramine strategy. 

Source: Malcolm Pirnie, Inc., 1992; Jacangelo, et al., 1989. 
8. Treatment Considerations 

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LT1ESWTR Disinfection Profiling and Benchmarking 

Enhanced softening

means the improved 

removal of DBP precursors 

by precipitative softening. 

Enhanced coagulation 

means the addition of 

sufficient coagulant for 

improved removal of DBP 

precursors by conventional 

filtration treatment. 

Guidance for 

implementing enhanced 

coagulation or enhanced 

precipitative softening is 

provided in the Enhanced 

Coagulation and 

Enhanced Precipitative 

Softening Guidance 

Manual (EPA, 1999b).

8.3 CHANGES IN ENHANCED COAGULATION 

AND SOFTENING

In conventional water treatment plants, precursors of DBPs 

may be removed through the coagulation process with 

aluminum or ferric salts and/or polymers. If a greater reduction 

in the DBP level is required, then the treatment techniques of 

either enhanced coagulation or enhanced precipitative softening 

can be employed. 

With fewer precursors present, the formation of DBPs is 

thereby reduced. Enhanced coagulation also allows for more 

effective disinfection, since the chlorine demand is lower in 

water treated by enhanced coagulation. In addition, the lower 

pH resulting from enhanced coagulation allows chlorine to 

inactivate Giardia more effectively, since chlorine is more 

effective at lower pHs. 

One way to implement enhanced coagulation is to change the 

type or dose of coagulant and/or polymer aid. However, before 

either enhanced coagulation or enhanced softening is 

implemented at a water treatment plant, the proposed changes 

should be evaluated through pilot-testing or bench-scale 

studies. Jar testing is commonly used to simulate coagulant 

dose changes and its effectiveness. A water treatment plant 

should first determine the present status of the coagulation 

process by taking TOC samples from the raw water and the 

finished water. With this data, the percent removal of TOC 

may be calculated and a desired TOC removal level may be 

determined. 

Changes to the coagulation and softening processes may have 

secondary effects on a water treatment plant. The pH of the 

water may be altered by the changes, thus affecting the 

disinfection process. Over the typical plant pH operating range 

of 5.5 to 9.5, decreasing the pH values improves the 

disinfection characteristics of chlorine and ozone and decreases 

the effectiveness of chlorine dioxide (EPA, 1999b). Another 

secondary effect of enhanced coagulation or softening may be 

the production of a lighter and more fragile floc that can carry 

over into the filters, thus shortening filter runs and increasing 

the amount of filter backwash water produced. Efficient 

sedimentation is extremely important prior to the filters to 

prevent filter overload. More sludge may also result from 

enhanced coagulation and enhanced softening, because of 
 8. Treatment Considerations 

May 2003 81 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

increased coagulant and lime dosages and more TOC removal. 

In addition, inorganic contaminant levels for iron, manganese, 

aluminum, sulfate, chloride, and sodium in finished water may 

increase with increased coagulant dosages (depending on type 

of coagulant used). A recent study by Carlson, et al. (2000) 

presents secondary effects of enhanced coagulation and 

softening. 

Case Study - Bell-Ajy, et al. (2000) 

Research, including jar tests using raw water from 16 water utilities throughout the United 

States and two full-scale evaluations, was conducted to evaluate the optimal coagulation 

conditions for removal of TOC and DBPs. Jar test results showed that when optimized 

coagulation was implemented, treatment effectiveness seemed pH dependent. Jar tests using 

alum, ferric chloride, and polyaluminum chloride coagulants with sulfuric acid for pH

reduction removed more TOC than those at higher pH levels. In the full-scale applications, 

enhanced coagulation effectively increased TOC removal and reduced trihalomethanes and 

trihalomethane formation potentials. With a lowering of pH during the coagulation process, 

turbidity and particle removals were improved. The researchers recommended that sludge 

generation, floc carryover, and dewatering, along with the point of chlorine addition and 

alkalinity consumption, be considered in the treatment scheme before enhanced coagulation is 

implemented.

Case Study – Kramer and Horger (1998) 

The Samuel S. Baxter Water Treatment Plant in Philadelphia conducted a series of jar tests to 

look at enhanced coagulation and its impact on TOC removal from the source water. At the 

time of the study, the 200 MGD Baxter plant used pre-treatment, flocculation/sedimentation, 

filtration, and disinfection to treat its water. It seasonally used potassium permanganate as a 

preoxidant to control algae and its associated tastes and odors. Ferric chloride was used as the 

primary coagulant. This enhanced coagulation study considered the pH of coagulation as well

as the coagulant dose. Jar tests using treatment plant water showed that the optimal pH was 

significantly less than the pH that was being used in the plant for coagulation and that the 

coagulant dose could be reduced by 10-30%. Full scale testing at the water treatment plant 

showed that by reducing the pH of coagulation, TTHM formation was significantly reduced 

and TOC removal increased significantly. Further investigation is necessary to determine the 

impact of lower pH on the formation of haloacetic acids. 
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8.4 INCREASING CONTACT TIME

Increasing either C (the disinfectant residual concentration) or 

T (the contact time for the disinfectant) will increase the CT 

value and provide additional credit for Giardia cyst and virus 

reductions. The value of CT can be increased by constructing 

additional storage, increasing the disinfectant residual, 

changing the disinfectant, lowering the pH, increasing the 

minimum clearwell depth, lowering high service peak flows, or 

improving clearwell hydraulics to allow for a greater detention 

time (Bishop, 1993). Increasing disinfectant concentrations to 

improve CT poses the problem of increasing the formation of 

DBPs, particularly with chlorine. One way to gain additional 

disinfection credit without increasing the disinfectant dosage is 

to increase the detention time in the clearwell. Increased 

detention time serves to allow more contact time, thus 

providing more opportunity for the destruction of 

microorganisms. 

Another way to increase CT is to construct additional storage 

prior to high service pumping. However, the cost and 

utilization of available space makes this option less preferred 

(Bishop, 1993). These suggested operating scenarios may limit 

DBP formations if the majority of DBP precursors have been 

removed. If a significant amount of DBP precursors, such as 

organic matter, is present when the disinfectant is added, these 

scenarios may not be advantageous. 

As discussed in Chapter 4 of this manual, the detention time 

used in the CT calculation is not the theoretical detention time 

(basin volume divided by flow rate), but rather the amount of 

time in which 10 percent of the fluid passes through a basin, 

process, or system in which a disinfectant residual is 

maintained. This value is determined from tracer tests or is 

estimated with the use of a baffling factor. Certain basin 

shapes and designs allow good mixing, while others allow 

short-circuiting. The baffling factors listed in Table 4-2 

account for various baffling conditions, inlet/outlet designs, and 

basin configurations. A water system desiring more contact 

time in order to increase its CT value may improve the 

hydraulics of its existing clearwell by improving the detention 

time within the unit through baffling or inlet/outlet changes. 
 8. Treatment Considerations 

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Possible clearwell changes are: 

• Relocating the inlet and/or outlet to maximize the 

separation distance between them; 

• Perforating the distribution and collection piping to 

disperse flow across the clearwell; 

• Using overflow inlets to disperse existing horizontal 

inlet flows; 

• Using baffles to disperse inlet flow; 

• Perforating baffle walls to disperse flows into and out of 

basins; and, 

• Using inlet or outlet weirs or launderers to distribute 

flow (Bishop, 1993). 

Case Study – Pinsky, et al. (1991) 

The South Central Connecticut Regional Water Authority (RWA) hired a consulting firm 

to conduct a comprehensive study of current disinfection practices at three of its surface 

water treatment plants. Two of these plants (Lake Gaillard and West River) are direct 

filtration plants and the third one (Lake Saltonstall) is a conventional water treatment 

plant. Where current disinfection practices were found to be inadequate, disinfection 

strategies and alternatives for satisfying CT requirements were investigated and 

recommendations were developed. Tracer studies confirmed that the filtered water 

reservoirs experienced short-circuiting. After research of various physical and chemical 

alternatives, the recommendation for the West River plant was the construction of a 

single vertical baffle in the finished water reservoirs at a total cost of $147,000. At the 

Lake Gaillard Plant, it was determined that a single vertical baffle in each of the two 

finished water reservoirs, at a total project cost of $120,000, would meet the CT 

requirement. 
8. Treatment Considerations 

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Case Study – Teefy, et al. (1995) 

The Alameda County Water District (ACWD) in Fremont, California, was not meeting the 

disinfection requirements of the SWTR or its DBP requirements. The existing water 

treatment plant was a conventional surface water treatment plant that used free chlorine for 

disinfection, alum and cationic polymer for coagulation and filtration, and chloramines for 

secondary disinfection. In order to receive additional disinfection credit, it was decided that 

the plant’s 750,000-gallon reservoir should be modified to obtain more detention time for 

the chlorine. The common inlet/outlet configuration of the tank did not allow for any 

contact time credit. In order to determine the best type and location of the new inlet and 

outlet structures, more than 50 configurations were tested in a scale modeling study. 

Twenty minutes was determined to be the desired T10 after the improvements were 

completed. Based on the model results, a spiral-type arrangement with the inlet coming in 

tangential to the side of the tank and the outlet line coming directly out of the bottom center 

of the tank was chosen. In addition to these changes to the reservoir, the point of sodium 

hydroxide and aqua ammonia addition was moved from immediately after the filters to 

downstream of the newly-modified reservoir. This move was made to slow the formation of 

trihalomethanes and to make the required CT requirement easier to achieve. The total 

project cost of $1,800,000 included the modeling study, engineering design, and actual 

construction of the improvements. The full-scale results were close to the model 

predictions, but agreement was not always good. Tests at the mid-range operating depth 

agreed best with the model predictions. Alum Chlorine

Southbay

Aqueduct Filters

Clarifiers

Cationic

Polymer

Clearwell

To Distribution

Treated

Water

Reservoir

Sodium

Hydroxide

Ammonia

Fluoride

Chlorine

Chlorine
 8. Treatment Considerations 

May 2003 85 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

For more information on 

membranes, refer to 

Membrane Practices for 

Water Treatment (AWWA, 

2001).

8.5 MEMBRANES

Four basic classes of membrane technology are currently used 

in the water treatment industry: reverse osmosis, nanofiltration, 

ultrafiltration, and microfiltration. Figure 8-1 presents the 

typical pore size range and removal capabilities for these 

membrane process classes. Membranes have a distribution of 

pore sizes, and this distribution will vary according to the 

membrane material and manufacturing process. When a pore 

size is stated, it can be presented as either nominal (i.e., the 

average pore size) or absolute (i.e., the maximum pore size) in 

terms of microns (μm). The removal capabilities of reverse 

osmosis and nanofiltration membranes are typically not stated 

in terms of pore size, but instead as a molecular weight cutoff 

representing the approximate size of the smallest molecule that 

can be removed by the membrane. 

All of these membrane processes are effective at removing 

Giardia, Cryptosporidium, and most bacteria (provided the 

membrane has no leaks). The amount of removal will depend 

on the type of membrane used. Reverse osmosis, 

nanofiltration, and ultrafiltration should remove viruses, 

assuming there are no leaks in the membranes. Reverse 

osmosis and nanofiltration are capable of removing inorganic 

and organic contaminants, including DBP precursors (AWWA, 

1999). 

Membranes can be effective in decreasing the amount of DBPs 

formed: 

• The removal of pathogens by membranes should reduce 

the amount of disinfectant required for inactivation and 

should result in lower finished water DBP 

concentrations; and, 

• The removal of DBP precursors should result in lower 

finished water DBP concentrations (when reverse 

osmosis or nanofiltration is used). 

It is important to remember that these membrane processes are 

physical barriers only, and must be followed by disinfection to 

ensure inactivation of pathogens not removed by the membrane 

barrier, control of bacterial regrowth in downstream system 

plumbing, and an adequate distribution system residual. 

Membranes can also be used to achieve other treatment 
8. Treatment Considerations 

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objectives. More information on membranes can be obtained 

from the Guidance Manual for Membrane Filtration (under 

development by EPA-OGWDW).

Case Study – Bing, et al. (2001) 

The Delta Water Treatment Plant in Delta, Ohio, which serves approximately 

3,200 people, treats river water with lime-soda softening and filtration. In order 

to meet increasing demand and upcoming regulations the plant needed to 

upgrade the facility. An integrated membrane system, consisting of 

microfiltration and reverse osmosis, was chosen for a pilot study. The 

microfiltration filtrate was blended with the reverse osmosis permeate to reduce 

the demand on the reverse osmosis system while still meeting water quality 

objectives. The dissolved organic carbon, trihalomethane formation potential, 

and haloacetic acid formation potential were substantially reduced by reverse 

osmosis and in the blended water were well within the compliance levels of the 

Stage 1 DBPR. Turbidity, hardness, and particulate removal goals of 0.05 NTU, 

110-150 mg/L, and 2 logs, respectively, were also surpassed in the blended 

water. An additional benefit of this system was that the pH of the finished water 

was lower than in the existing system, meaning that a lower chlorine dose could 

be used to meet CT requirements, further reducing the formation of DBPs. This 

study showed that this integrated membrane system is suitable for small systems 

using surface water sources. 
 8. Treatment Considerations 

May 2003 87 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Figure 8-1. Particles Removed Through Membrane Technologies 

Micron Scale

Approximate

Molecular

Weight

Typical Size

Range of

Selected

Water

Constituents

Membrane

Process*

Ionic Range Molecular Range Macro Molecular

Range Micro Particle Range Macro Particle Range

0.001 0.01 0.1 1.0 10 100 1000

100 1000 10,000 100,000 500,000

Dissolved Organics

Bacteria

Sand

Salts Colloids Cryptosporidium

Viruses

Particle Filtration

Microfiltration

Ultrafiltration

Nanofiltration

Reverse Osmosis

* Particle Filtration is shown for reference only. It is not a membrane separation process.

Giardia

Source: AWWA/ASCE, 1998. 
8. Treatment Considerations 

EPA Guidance Manual 88 May 2003 

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8.6 REFERENCES

Ashe, C.R., V.F. Coletti, and R.A. Stoops. 1994. Small System Compliance: CT versus 

DBPs. 1993 AWWA Annual Conference Proceedings. 

AWWA. 2001. Membrane Practices for Water Treatment. Denver, CO. 

AWWA. 1999. Water Quality & Treatment - Handbook of Community Water Supplies. 

Fifth Edition. McGraw-Hill. New York, NY. 

AWWA/ASCE. 1998. Water Treatment Plant Design. Third Edition. McGraw-Hill. New 

York, NY. 

Bell-Ajy, K., M. Abbaszadegan, E. Ibrahim, D. Verges, and M. LeChevallier. 2000. 

Conventional and Optimized Coagulation for NOM Removal. Journal AWWA, 92(10):44-

58. 

Bing, J., R. Jackson, D. Heyman, L. Born, and R. Huehmer. 2001. Compliance with 

M/DBP Rules Using Integrated Membrane Systems. 2001 AWWA Annual Conference 

Proceedings. 

Bishop, M.M. 1993. The CT Concept and Modifications to Improve Detention Times. 1992 

AWWA Annual Conference Proceedings. 

Bishop, M.M., J.M. Morgan, B. Cornwell, and D.K. Jamison. 1993. Improving the 

Disinfection Time of a Water Plant Clearwell. Journal AWWA, 85(3):68-75. 

Carlson, K., S. Via, B. Bellamy, M. Carlson. 2000. Secondary Effects of Enhanced 

Coagulation and Softening. Journal AWWA, 92(6):63-75. 

Cotton, C.A., D.M. Owen, G.C. Cline, T. P. Brodeur. 2001. UV Disinfection Costs for 

Inactivating Cryptosporidium. Journal AWWA, 93(6):82-94. 

Dowbiggin, W.B. and J.C. Thompson. 1990. Preparing for the Disinfection Byproducts 

Regulations Case Studies. 1989 AWWA Annual Conference Proceedings. 

EPA. 1999a. Alternative Disinfectants and Oxidants Guidance Manual (EPA 815-R-99-

014). Washington, D.C. 

EPA. 1999b. Enhanced Coagulation and Enhanced Precipitative Softening Guidance 

Manual (EPA 815-R-99-012). Washington, D.C. 

Jacangelo, J.G., N.L. Patania, K.M. Reagan, E.M. Aieta, S.W. Krasner, and M.J. Mcguire. 

1989. Impact of Ozonation on the Formation and Control of Disinfection Byproducts in 

Drinking Water. Journal AWWA, 81(8):74. 
 8. Treatment Considerations 

May 2003 89 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Kawamura, Susumu. 2000. Integrated Design and Operation of Water Treatment 

Facilities. Second Edition. John Wiley & Sons. New York, NY. 

Kramer, L.A. and J. Horger. 1998. The Optimization of Coagulation Using Dual pH 

Adjustment: The Philadelphia Water Department Prepares to Meet Stage 1 D/DBP. 1997 

AWWA Water Quality Technology Conference Proceedings. 

Malcolm Pirnie, Inc. 1992. Technologies and Costs for Control of Disinfection Byproducts. 

Prepared for EPA, Washington, D.C. 

Metropolitan and Montgomery. 1989. Disinfection Byproducts in United States Drinking 

Waters. Metropolitan Water District of Southern California and James M. Montgomery 

Consulting Engineers, Inc. Prepared for EPA, Washington, D.C. 

 

Pinsky, D.E., H.J. Dunn, and A.F. Hess. 1991. CT Compliance Strategies: Three Case 

Studies. 1990 AWWA Annual Conference Proceedings. 

Schneider, O.D. and J.E. Tobiason. 2000. Preozonation Effects on Coagulation. Journal 

AWWA, 92(10):74-87. 

Singer, P.C., editor. 1999. Formation and Control of Disinfection By-Products in Drinking 

Water. AWWA. Denver, CO. 

Teefy, S., R. Shaver, and Almeda County Water District. 1995. From Model to Full Scale: 

Modification of a Reservoir to Improve Contact Time. 1994 AWWA Annual Conference 

Proceedings. 

 
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Appendix A 

Glossary
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 Appendix A. Glossary 

May 2003 93 EPA Guidance Manual 

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A.1 GLOSSARY

baffle. A flat board or plate, deflector, guide or similar device constructed or placed in 

flowing water or slurry systems to cause more uniform flow velocities, to absorb energy, 

and to divert, guide, or agitate liquids (water, chemical solutions, slurry). 

baffling factor (BF). The ratio of the actual contact time to the theoretical detention time. 

clarifier. A large circular or rectangular tank or basin in which water is held for a period of 

time, during which the heavier suspended solids settle to the bottom by gravity. Clarifiers 

are also called settling basins and sedimentation basins. 

clearwell. A reservoir for the storage of filtered water with sufficient capacity to prevent 

the need to vary the filtration rate in response to short-term changes in customer demand. 

Also used to provide chlorine contact time for disinfection. 

coagulant. A chemical added to water that has suspended and colloidal solids to destabilize 

particles, allowing subsequent floc formation and removal by sedimentation, filtration, or 

both. 

coagulation. As defined in 40 CFR 141.2, a process using coagulant chemicals and mixing 

by which colloidal and suspended materials are destabilized and agglomerated into flocs. 

community water system (CWS). A public water system which serves at least 15 service 

connections used by year-round residents or regularly serves at least 25 year-round 

residents. 

conventional filtration treatment. As defined in 40 CFR 141.2, a series of processes 

including coagulation, flocculation, sedimentation, and filtration resulting in substantial 

particulate removal. 

Cryptosporidium. A disease-causing protozoan widely found in surface water sources. 

Cryptosporidium is spread by the fecal-oral route as a dormant oocyst from human and 

animal feces. In its dormant stage, Cryptosporidium is housed in a very small, hard-shelled 

oocyst form that is resistant to chlorine and chloramine disinfectants. When water 

containing these oocysts is ingested, the protozoan may cause a severe gastrointestinal 

disease called cryptosporidiosis. 

CT or CTcalc. As defined in 40 CFR 141.2, the product of “residual disinfectant 

concentration” (C) in mg/l determined before or at the first customer, and the corresponding 

“disinfectant contact time” (T) in minutes, i.e., “C” x “T”. If a public water system applies 

disinfectants at more than one point prior to the first customer, it must determine the CT of 

each disinfectant sequence before or at the first customer to determine the total percent 

inactivation or “total inactivation ratio”. In determining the total inactivation ratio, the 

public water system must determine the residual disinfectant concentration of each 

disinfection sequence and corresponding contact time before any subsequent disinfection 

application point(s). “CT99.9” is the CT value required for 99.9 percent (3-log) inactivation 

of Giardia lamblia cysts. CT99.9 for a variety of disinfectants and conditions appear in 

Tables 1.l- 1.6, 2.1, and 3.1 of §141.74(b)(3) in the Code of Federal Regulations. 

CTcalc/CT99.9 is the inactivation ratio. The sum of the inactivation ratios, or total inactivation 

ratio shown as Σ [(CTcalc) / (CT99.9)] is calculated by adding together the inactivation ratio 
Appendix A. Glossary 

EPA Guidance Manual 94 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

for each disinfection sequence. A total inactivation ratio equal to or greater than 1.0 is 

assumed to provide a 3-log inactivation of Giardia lamblia cysts. 

detention time. The average length of time a drop of water or a suspended particle remains 

in a tank or chamber. Mathematically, it may be determined by dividing the volume of 

water in the tank by the flow rate through the tank.

diatomaceous earth filtration. As defined in 40 CFR 141.2, a process resulting in 

substantial particulate removal, that uses a process in which: (1) a “precoat” cake of 

diatomaceous earth filter media is deposited on a support membrane (septum), and (2) while 

the water is filtered by passing through the cake on the septum, additional filter media, 

known as “body feed,” is continuously added to the feed water to maintain the permeability 

of the filter cake. 

direct filtration. As defined in 40 CFR 141.2, a series of processes including coagulation 

and filtration, but excluding sedimentation, and resulting in substantial particulate removal. 

disinfectant. As defined in 40 CFR 141.2, any oxidant, including but not limited to 

chlorine, chlorine dioxide, chloramines, and ozone added to water in any part of the 

treatment or distribution process, that is intended to kill or inactivate pathogenic 

microorganisms. 

disinfectant contact time. As defined in 40 CFR 141.2, the time in minutes that it takes for 

water to move from the point of disinfectant application or the previous point of disinfectant 

residual measurement to a point before or at the point where residual disinfectant 

concentration (“C”) is measured. Where only one “C” is measured, “T” is the time in 

minutes that it takes for water to move from the point of disinfectant application to a point 

before or at where residual disinfectant concentration (“C”) is measured. Where more than 

one “C” is measured, “T” is (a) for the first measurement of “C”, the time in minutes that it 

takes for water to move from the first or only point of disinfectant application to a point 

before or at the point where the first “C” is measured and (b) for subsequent measurements 

of “C”, the time in minutes that it takes for water to move from the previous “C” 

measurement point to the “C” measurement point for which the particular “T” is being 

calculated. Disinfectant contact time in pipelines must be calculated based on “plug flow” 

by dividing the internal volume of the pipe by the maximum hourly flow rate through that 

pipe. Disinfectant contact time within mixing basins and storage reservoirs must be 

determined by tracer studies or an equivalent demonstration. 

disinfection. As defined in 40 CFR 141.2, a process which inactivates pathogenic 

organisms in water by chemical oxidants or equivalent agents. 

disinfection benchmark. The lowest monthly average microbial inactivation during the 

disinfection profile time period.

disinfection byproduct precursors. Substances that can be converted into disinfection 

byproducts during disinfection. Typically, most of these precursors are constituents of 

natural organic matter. In addition, the bromide ion (Br-

) is a precursor material.

disinfection byproducts (DBPs). Inorganic and organic compounds formed by the reaction 

of the disinfectant, natural organic matter, and the bromide ion during water disinfection 

processes. Regulated DBPs include trihalomethanes, haloacetic acids, bromate, and chlorite. 
 Appendix A. Glossary 

May 2003 95 EPA Guidance Manual 

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disinfection profile. As stated in 40 CFR 141.530, a graphical representation of your 

system’s level of Giardia lamblia or virus inactivation measured during the course of a year.

disinfection segment. A section of the system beginning at one disinfectant injection or 

monitoring point and ending at the next disinfectant injection or monitoring point. 

effluent. Water or some other liquid that is raw, partially or completely treated that is 

flowing from a reservoir, basin, treatment process or treatment plant. 

enhanced coagulation. As defined in 40 CFR 141.2, the addition of sufficient coagulant 

for improved removal of disinfection byproduct precursors by conventional filtration 

treatment. 

enhanced softening. As defined in 40 CFR 141.2, the improved removal of disinfection 

byproduct precursors by precipitative softening. 

filtration. As defined in 40 CFR 141.2, a process for removing particulate matter from 

water by passage through porous media. 

finished water. Water that has passed through a water treatment plant such that all the 

treatment processes are completed or “finished” and ready to be delivered to consumers. 

Also called product water. 

flocculation. As defined in 40 CFR 141.2, a process to enhance agglomeration or collection 

of smaller floc particles into larger, more easily settleable particles through gentle stirring by 

hydraulic or mechanical means. 

Giardia lamblia. Flagellated protozoan, which is shed during its cyst-stage with the feces of 

man and animals. When water containing these cysts is ingested, the protozoan causes a 

severe gastrointestinal disease called giardiasis. 

ground water under the direct influence of surface water (GWUDI). As defined in 40 

CFR 141.2, any water beneath the surface of the ground with significant occurrence of 

insects or other macroorganisms, algae, or large-diameter pathogens such as Giardia 

lamblia or Cryptosporidium, or significant and relatively rapid shifts in water characteristics 

such as turbidity, temperature, conductivity, or pH which closely correlate to climatological 

or surface water conditions. Direct influence must be determined for individual sources in 

accordance with criteria established by the State. The State determination of direct influence 

may be based on site-specific measurements of water quality and/or documentation of well 

construction characteristics and geology with field evaluation. 

haloacetic acids five (HAA5). As defined in 40 CFR 141.2, the sum of the concentrations 

in milligrams per liter of the haloacetic acid compounds (monochloroacetic acid, 

dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid), 

rounded to two significant figures after addition. 

influent water. Raw water plus recycle streams. 

interpolation. A technique used to determine values that fall between the marked intervals 

on a scale. 

log inactivation. The percentage of microorganisms inactivated through disinfection by a 

given process. 
Appendix A. Glossary 

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log reduction. The percentage of microorganisms reduced through log removal added to 

the log inactivation. One log reduction means that 90% of the microorganisms are removed 

or inactivated. Two log corresponds to 99%, three log is 99.9% and four log corresponds to 

99.99%. 

log removal. The percentage of microorganisms physically removed by a given process. 

maximum contaminant level (MCL). As defined in 40 CFR 141.2, the maximum 

permissible level of a contaminant in water which is delivered to any user of a public water 

system. 

membrane filtration. A filtration process (e.g., reverse osmosis, nanofiltration, 

ultrafiltration, and microfiltration) using tubular or spiral-wound elements that exhibits the 

ability to mechanically separate water from other ions and solids by creating a pressure 

differential and flow across a membrane. 

micrograms per liter (μg/L). One microgram of a substance dissolved in each liter of 

water. This unit is equal to parts per billion (ppb) since one liter of water is equal in weight 

to one billion micrograms. 

micron. A unit of length equal to one micrometer (μm). One millionth of a meter or one 

thousandth of a millimeter. One micron equals 0.00004 of an inch. 

milligrams per liter (mg/L). A measure of concentration of a dissolved substance. A 

concentration of one mg/L means that one milligram of a substance is dissolved in each liter 

of water. For practical purposes, this unit is equal to parts per million (ppm) since one liter 

of water is equal in weight to one million milligrams. Thus a liter of water containing 10 

milligrams of calcium has 10 parts of calcium per one million parts of water, or 10 parts per 

million (10 ppm). 

non-community water system (NCWS). As defined in 40 CFR 141.2, a public water 

system that is not a community water system. A non-community water system is either a 

“transient non-community water system (TWS)” or a non-transient non-community water 

system (NTNCWS).” 

non-transient non-community water system (NTNCWS). As defined in 40 CFR 141.2, a 

public water system that is not a community water system and that regularly serves at least 

25 of the same persons over six months per year. 

organics. Carbon-containing compounds that are derived from living organisms. 

oxidant. Any oxidizing agent; a substance that readily oxidizes (removes electrons from) 

something chemically. Common drinking water oxidants are chlorine, chlorine dioxide, 

ozone, and potassium permanganate. Some oxidants also act as disinfectants. 

oxidation. A process in which a molecule, atom, or ion loses electrons to an oxidant. The 

oxidized substance (which lost the electrons) increases in positive valence. Oxidation never 

occurs alone, but always as part of an oxidation-reduction (redox) reaction. 

pathogens, or pathogenic organisms. Microorganisms that can cause disease (such as 

typhoid, cholera, or dysentery) in other organisms or in humans, animals and plants. They 

may be bacteria, viruses, or protozoans and are found in sewage, in runoff from animal 
 Appendix A. Glossary 

May 2003 97 EPA Guidance Manual 

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farms or rural areas populated with domestic and/or wild animals, and in water used for 

swimming. There are many types of microorganisms which do not cause disease. These 

microorganisms are called non-pathogens. 

pH. pH is an expression of the intensity of the basic or acid condition of a solution. 

Mathematically, pH is the negative logarithm (base 10) of the hydrogen ion concentration, 

[H+]. [pH = log (1/H+)]. The pH may range from 0 to 14, where 0 is most acidic, 14 most 

basic, and 7 neutral. Natural waters usually have a pH between 6.5 and 8.5. 

plug flow. The water travels through a basin, pipe, or unit process in such a fashion that the 

entire mass or volume is discharged at exactly the theoretical detention time of the unit. 

pre-disinfection. The addition of a disinfectant to the treatment train prior to the primary 

disinfectant injection location. Generally, the purpose of pre-disinfection is to obtain 

additional inactivation credits, to control microbiological growth in subsequent treatment 

processes, to improve coagulation, or to reduce tastes and odors. 

primary disinfection. The disinfectant used in a treatment system to achieve the necessary 

microbial inactivation. 

public water system (PWS). As defined in 40 CFR 141.2, a system for the provision to the 

public of water for human consumption through pipes or, after August 5, 1998, other 

constructed conveyances, if such system has at least fifteen service connections or regularly 

serves an average of at least twenty-five individuals daily at least 60 days out of the year. 

Such term includes: any collection, treatment, storage, and distribution facilities under 

control of the operator of such system and used primarily in connection with such system; 

and any collection or pretreatment storage facilities not under such control which are used 

primarily in connection with such system. Such term does not include any “special irrigation 

district.” A public water system is either a “community water system” or a “non-community 

water system”. 

reservoir. Any natural or artificial holding area used to store, regulate, or control water. 

secondary disinfection. The disinfectant application in a treatment system to maintain the 

disinfection residual throughout the distribution system. 

sedimentation. As defined in 40 CFR 141.2, a process for removal of solids before 

filtration by gravity or separation. 

short-circuiting. A hydraulic condition in a basin or unit process that occurs when the 

actual flow time of water through the basin is less than the basin or unit process volume 

divided by the peak hourly flow.

State. As defined in 40 CFR 141.2, the agency of the State or Tribal government which has 

jurisdiction over public water systems. During any period when a State or Tribal 

government does not have primary enforcement responsibility pursuant to Section 1413 of 

the Safe Drinking Water Act, the term “State” means the Regional Administrator, U.S. 

Environmental Protection Agency. 

surface water. As defined in 40 CFR 141.2, all water which is open to the atmosphere and 

subject to surface runoff. 
Appendix A. Glossary 

EPA Guidance Manual 98 May 2003 

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total organic carbon (TOC). As defined in 40 CFR 141.2, total organic carbon in mg/L 

measured using heat, oxygen, ultraviolet irradiation, chemical oxidants, or combinations of 

these oxidants that convert organic carbon to carbon dioxide, rounded to two significant 

figures. 

total trihalomethanes (TTHM). As defined in 40 CFR 141.2, the sum of the concentration 

in milligrams per liter of the trihalomethane compounds (trichloromethane [chloroform], 

dibromochloromethane, bromodichloromethane and tribromomethane [bromoform]), 

rounded to two significant figures. 

trihalomethane (THM). As defined in 40 CFR 141.2, one of the family of organic 

compounds, named as derivatives of methane, wherein three of the four hydrogen atoms in 

methane are each substituted by a halogen atom in the molecular structure. 

tracer. A foreign substance mixed with or attached to a given substance for subsequent 

determination of the location or distribution of the foreign substance. 

tracer study. A study using a substance that can readily be identified in water (such as a 

dye) to determine the distribution and rate of flow in a basin, pipe, ground water, or stream 

channel. 

transient non-community water system. As defined in 40 CFR 141.2, means a noncommunity water system that does not regularly serve at least 25 of the same persons over 

six months per year. 

virus. As defined in 40 CFR 141.2, a virus of fecal origin which is infectious to humans by 

waterborne transmission. 

water supply system. The collection, treatment, storage, and distribution of potable water 

from source to consumer. 
 Appendix A. Glossary 

May 2003 99 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

A.1 REFERENCES

Calabrese, E.J., C.E. Gilbert, and H. Pastides, editors. 1988. Safe Drinking Water Act 

Amendments, Regulations and Standards. Lewis Publishers. Chelsea, MI. 

California State University. 1988. Water Treatment Plant Operation. School of Engineering, 

Applied Research and Design Center, Sacramento, CA. 

Dzurik, A.A., Rowman, and Littlefield. 1990. Water Resources Planning. Savage, MD. 

Symons, J., L. Bradley, Jr., and T. Cleveland, editors. 2000. The Drinking Water 

Dictionary. AWWA. Denver, CO. 

USEPA. 2002. Code of Federal Regulations, Title 40, Chapter I, Section 141.2. July 1. 

von Huben, H. 1991. Surface Water Treatment: The New Rules. AWWA. 
Appendix A. Glossary 

EPA Guidance Manual 100 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

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Appendix B 

CT Tables
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 Ap

May 2003 103 E

 LT1ESWTR Disinfection Profilin

TABLE B-1 

CT VALUES* FOR 3-LOG INACTIVATION 

OF GIARDIA CYSTS BY FREE CHLORINE 

Temperature <=0.5°C Temperature =5°C Tempe

Chlorine Concentration pH pH (mg/L) 

<=6.0 6.5 7.0 7.5 8.0 8.5 9.0 <=6.0 6.5 7.0 7.5 8.0 8.5 9.0 <=6.0 6.5 7.0 

<=0.4 137 163 195 237 277 329 390 97 117 139 166 198 236 279 73 88 104 

0.6 141 168 200 239 286 342 407 100 120 143 171 204 244 291 75 90 107 0.8 145 172 205 246 295 354 422 103 122 146 175 210 252 301 78 92 110 1.0 148 176 210 253 304 365 437 105 125 149 179 216 260 312 79 94 112 1.2 152 180 215 259 313 376 451 107 127 152 183 221 267 320 80 95 114 1.4 155 184 221 266 321 387 464 109 130 155 187 227 274 329 82 98 116 1.6 157 189 226 273 329 397 477 111 132 158 192 232 281 337 83 99 119 1.8 162 193 231 279 338 407 489 114 135 162 196 238 287 345 86 101 122 2.0 165 197 236 286 346 417 500 116 138 165 200 243 294 353 87 104 124 2.2 169 201 242 297 353 426 511 118 140 169 204 248 300 361 89 105 127 2.4 172 205 247 298 361 435 522 120 143 172 209 253 306 368 90 107 129 2.6 175 209 252 304 368 444 533 122 146 175 213 258 312 375 92 110 131 2.8 178 213 257 310 375 452 543 124 148 178 217 263 318 382 93 111 134 3.0 181 217 261 316 382 460 552 126 151 182 221 268 324 389 95 113 137 

Temperature = 15°C Temperature = 20°C Tempe

Chlorine Concentration pH pH (mg/L) 

<=6.0 6.5 7.0 7.5 8.0 8.5 9.0 <=6.0 6.5 7.0 7.5 8.0 8.5 9.0 <=6.0 6.5 7.0 

<=0.4 49 59 70 83 99 118 140 36 44 52 62 74 89 105 24 29 35 

0.6 50 60 72 86 102 122 146 38 45 54 64 77 92 109 25 30 36 0.8 52 61 73 88 105 126 151 39 46 55 66 79 95 113 26 31 37 1.0 53 63 75 90 108 130 156 39 47 56 67 81 98 117 26 31 37 1.2 54 64 76 92 111 134 160 40 48 57 69 83 100 120 27 32 38 1.4 55 65 78 94 114 137 165 41 49 58 70 85 103 123 27 33 39 1.6 56 66 79 96 116 141 169 42 50 59 72 87 105 126 28 33 40 1.8 57 68 81 98 119 144 173 43 51 61 74 89 108 129 29 34 41 2.0 58 69 83 100 122 147 177 44 52 62 75 91 110 132 29 35 41 2.2 59 70 85 102 124 150 181 44 53 63 77 93 113 135 30 35 42 2.4 60 72 86 105 127 153 184 45 54 65 78 95 115 138 30 36 43 2.6 61 73 88 107 129 156 188 46 55 66 80 97 117 141 31 37 44 2.8 62 74 89 109 132 159 191 47 56 67 81 99 119 143 31 37 45 3.0 63 76 91 111 134 162 195 47 57 68 83 101 122 146 32 38 46 

*Although units did not appear in the original tables, units are min-mg/L. 
Appendix B. CT Tables 

EPA Guidance Manual 104 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

TABLE B-2 

CT VALUES* FOR 

4- LOG INACTIVATION OF VIRUSES BY FREE CHLORINE

 pH 

Temperature (o

C) 6-9 10

0.5 12 90 

5 8 60 

10 6 45 

15 4 30 

20 3 22 

25 2 15 

*Although units did not appear in the original tables, units are min-mg/L. 
 Appendix B. CT Tables 

May 2003 105 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking 

TABLE B-3 

CT VALUES* FOR 

3-LOG INACTIVATION OF GIARDIA CYSTS 

BY CHLORINE DIOXIDE

Temperature (o

C) 

< = 1 5 10 15 20 25

63 26 23 19 15 11 

*Although units did not appear in the original tables, units are min-mg/L. 
Appendix B. CT Tables 

EPA Guidance Manual 106 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

TABLE B-4 

CT VALUES* FOR 

4-LOG INACTIVATION OF VIRUSES 

BY CHLORINE DIOXIDE pH 6-9

Temperature (o

C) 

< = 1 5 10 15 20 25

50.1 33.4 25.1 16.7 12.5 8.4 

*Although units did not appear in the original tables, units are min-mg/L. 
 Appendix B. CT Tables 

May 2003 107 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking 

TABLE B-5 

CT VALUES* FOR 

3-LOG INACTIVATION OF GIARDIA CYSTS 

BY OZONE

Temperature (o

C) 

< = 1 5 10 15 20 25

2.9 1.90 1.43 0.95 0.72 0.48 

*Although units did not appear in the original tables, units are min-mg/L. 
Appendix B. CT Tables 

EPA Guidance Manual 108 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

TABLE B-6 

CT VALUES* FOR 

4-LOG INACTIVATION OF VIRUSES BY OZONE

Temperature (o

C) 

< = 1 5 10 15 20 25

1.8 1.2 1.0 0.6 0.5 0.3 

*Although units did not appear in the original tables, units are min-mg/L. 
 Appendix B. CT Tables 

May 2003 109 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking 

TABLE B-7 

CT VALUES* FOR 

3-LOG INACTIVATION OF GIARDIA CYSTS 

BY CHLORAMINE pH 6-9

Temperature (o

C) 

< = 1 5 10 15 20 25

3,800 2,200 1,850 1,500 1,100 750 

 

*Although units did not appear in the original tables, units are min-mg/L. 
Appendix B. CT Tables 

EPA Guidance Manual 110 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

TABLE B-8 

CT VALUES* FOR 

4-LOG INACTIVATION OF VIRUSES BY CHLORAMINE

Temperature (o

C) 

< = 1 5 10 15 20 25

2,883 1,988 1,491 994 746 497 

*Although units did not appear in the original tables, units are min-mg/L. 
 Appendix B. CT Tables 

May 2003 111 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking 

TABLE B-9 

CT VALUE* FOR 

INACTIVATION OF VIRUSES BY UV

Log Inactivation

2.0 3.0

21 36 

*Although units did not appear in the original tables, units are min-mg/L. 
Appendix B. CT Tables 

EPA Guidance Manual 112 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

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Appendix C 

Blank Worksheets
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 Appendix C. Blank Worksheets 

May 2003 115 EPA Technical Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

The following worksheets can be used to record and report information to the State on 

Giardia or virus inactivation. Systems should check with their State prior to using these 

worksheets for acceptability. 

A completed example of the Log Inactivation Ratio Determination worksheet 

(Worksheet#1) can be found in Chapters 3 through 5 and Appendix D of this Guidance 

Manual. 

A completed example of the Total Log Inactivation Determination worksheet (Worksheet 

#2) can be found in Chapter 5 and Appendix D of this Guidance Manual. 
Appendix C. Blank Worksheets

EPA Technical Guidance Manual 116 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

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 Appendix C. Blank Worksheets 

May 2003 117 EPA Technical Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Starting Month: Year: PWSID: System/Water Source:

Disinfectant Type: ________________ Prepared by:

Profile Type (check one): Giardia Viruses

Disinfection Segment/Sequence of Application2

:

3 4 5 6 7 8 9 10 11 12 13 14

Residual Peak Disinf.

Disinf. pH Water Hourly TDT Baffling Contact CTCalc = CT Inactivation Log

Week Conc. Temp. Flow Volume Factor Time (CxT) Req'd Ratio Inactivation*

# C (mg/L) (

o

C) (gpm) (gal) (min.) T (min.) (min-mg/L) (min-mg/L) (Col 11 / Col 12)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

*See worksheet #2 to determine total log inactivation if the system has multiple disinfection segments.

WORKSHEET #1

LOG INACTIVATION RATIO DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER1
Appendix C. Blank Worksheets

EPA Technical Guidance Manual 118 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Notes:

1. The system is only required to calculate log inactivation values once per week on the same day 

of the week. For instance, the system may choose to calculate log inactivation values on 

Wednesday of every week. If the system has more than one point of disinfectant application or 

uses more than one type of disinfectant, then the system can calculate log inactivation ratios on 

separate sheets and sum the log inactivation ratios to obtain the total inactivation achieved by 

the plant using Worksheet #2 in Appendix C of the LT1ESWTR Disinfection Profiling and 

Benchmarking Technical Guidance Manual. 

2. Use a separate form for each disinfectant application point and related residual sample site. 

Enter the disinfectant and sequence position, e.g., "ozone/1st" or "chlorine dioxide/3rd". 

3. Disinfectant concentration must be measured during peak hourly flow. 

4. If the system uses chlorine, the pH of the disinfected water must be measured at the same 

location and time the chlorine residual disinfectant concentration is measured during peak hourly 

flow. 

5. The water temperature must be measured at the same location and time the residual disinfectant 

concentration is measured during peak hourly flow. Temperature must be in degrees Celsius 

(

o

C). 

6. Peak hourly flow for the day must be provided for the disinfection segment. 

7. The volume is the operating volume in gallons realized by the pipe, basin, or treatment unit 

process during peak hourly flow. 

8. Theoretical detention time in minutes equals the volume in gallons in column 7 divided by the 

peak hourly flow in gpm in column 6. 

9. Enter the baffling factor for the system's pipe, basin(s), or treatment unit process as determined 

by a tracer study or assigned by the State. 

10. Disinfectant contact time in minutes is determined by multiplying the theoretical detention time in 

minutes in column 8 by the baffling factor in column 9. 

11. CTcalc is determined by multiplying the residual disinfectant concentration in mg/L in column 3 by 

the disinfectant contact time in minutes in column 10. 

12. The CTrequired value should be determined based on the tables contained in Appendix B of the 

LT1ESWTR Disinfection Profiling and Benchmarking Technical Guidance Manual or tables in the 

Surface Water Treatment Rule Guidance Manual. CTrequired for Giardia is CT99.9 (or 3-log 

inactivation) and CTrequired for viruses is CT99.99 (or 4-log inactivation). 

13. Inactivation ratio equals CTcalc in column 11 divided by CTrequired in column 12. 

14. Log Inactivation for Giardia = 3 x Inactivation ratio in column 13. 

Log Inactivation for viruses = 4 x Inactivation ratio in column 13. 

For multiple disinfection segments, Worksheet #2 should be used to sum inactivation ratios for 

each disinfection segment to calculate system log inactivation.
 Appendix C. Blank Worksheets 

May 2003 119 EPA Technical Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Starting Month: Year: PWSID:

System/Water Source: Prepared by:

Disinfectant Type: ________________

Profile Type (check one): Giardia Viruses

Sum

Disinfection Disinfection Disinfection Disinfection Disinfection of Total

Week Segment Segment Segment Segment Segment Inactivation Log

# 1 2 3 4 5 Ratios Inactivation1

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52 1 Giardia : Log Inactivation = 3 x Sum of Inactivation Ratios

 Viruses: Log Inactivation = 4 x Sum of Inactivation Ratios (or a method approved by the State)

Inactivation Ratio for each disinfection segment from Worksheet #1

TOTAL LOG INACTIVATION DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER

WORKSHEET #2
Appendix C. Blank Worksheets

EPA Technical Guidance Manual 120 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

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Appendix D 

Examples
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 Appendix D. Examples

May 2003 123 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

This appendix provides example of ways a system may comply with the regulations for a 

disinfection profile and a disinfection benchmark. This appendix does not establish any 

additional requirements for completing a disinfection profile or a disinfection benchmark 

beyond the regulations established in the LT1ESWTR. 

The following examples are presented in this appendix: 

• Example D-1: Calculate Actual Log Inactivation for One Disinfectant..............Page 125 

• Example D-2: Calculate Actual Log Inactivation for Three Disinfection Segments 

and Two Disinfectants .........................................................................................Page 129 

• Example D-3: Develop a Disinfection Profile and Benchmark for a System 

with Multiple Disinfection Segments ..................................................................Page 143 
Appendix D. Examples

EPA Guidance Manual 124 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

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 Appendix D. Examples

May 2003 125 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-1: Calculate Actual Log Inactivation for One Disinfectant 

Distribution

System

PreSedimentation

Coagulation

Filtration

Clearwell

CT Monitoring Point

Cl2 residual = 1.0 mg/L

Temperature = 10 oC

pH = 6

One Disinfection Segment:

One injection point, one monitoring point

Intake

Chlorine

Injected

Flocculation

In this example, the direct filtration treatment system added chlorine prior to the clearwell 

and it was required to create a disinfection profile. The system must determine the log 

inactivation for Giardia achieved through disinfection. 

Step 1. Determine the peak hourly flow. 

 

From the raw water pump records the peak hourly flow (Q) is determined to be 5,000 

gallons per minute (gpm). 

Step 2. Measure the chlorine residual, temperature, and pH (since chlorine is used) 

during peak hourly flow at the monitoring point and at the same time. 

 Temperature = 10oC 

pH = 6 

Chlorine residual = Cchlorine = 1.0 mg/L 

Step 3. Measure the physical dimensions of the clearwell. 15.3 ft

35 ft

75 ft

Minimum Operating Depth
Appendix D. Examples

EPA Guidance Manual 126 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-1 continued

Measure the inner tank length and width to obtain the volume of water in the 

clearwell rather than the volume of the tank itself. 

Length = 75 ft

Width = 35 ft 

Measure the minimum operating depth in the clearwell to obtain a conservative 

estimate of the volume of water in the tank. 

Minimum Operating Depth = 15.3 ft

Step 4. Calculate the volume of the water in the clearwell based on low water level. 

 Volume (V) = minimum water depth x length x width 

 V = 15.3 ft x 75 ft x 35 ft = 40,160 ft3 

 V = 40,160 ft3 x (7.48 gal / ft3) 

 V = 300,000 gal 

 

Step 5. Calculate the Theoretical Detention Time (TDT) in the clearwell. 

 

 TDT = V / Q 

 TDT = 300,000 gal / 5,000 gpm 

 TDT = 60 minutes 

Step 6. Determine the baffling factor (BF) for the clearwell. 

Clearwell BF = 0.5 (from Table G-1 in Appendix G for average baffling condition 

as shown below.) 

Step 7. Calculate the contact time of the disinfectant in the clearwell. 

Contact Time (T) = TDT x BF 

 T = 60 min x 0.5 

 T = 30 minutes 
 Appendix D. Examples

May 2003 127 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-1 continued 

Step 8. Calculate the CT for the disinfection segment. 

 

 CTcalc = Cchlorine x T 

 CTcalc = 1.0 mg/L x 30 min 

 CTcalc = 30 min-mg/L 

Step 9. Determine the required CT99.9 necessary to obtain 3-log Giardia inactivation. 

The required CT value for 3-log Giardia inactivation (CT99.9) may be obtained by 

using CT Table B-1 in Appendix B, CT Values for 3-Log Inactivation of Giardia

Cysts by Free Chlorine. In this example, the required CT99.9 is 79 min-mg/L for a 

pH of 6, temperature of 10 oC, and Cchlorine of 1.0 mg/L. The relevant section of 

Table B-1 is reprinted below and the pertinent section of the table is highlighted. 

Excerpt from Table B-1: 

CT Values for 3-Log Inactivation of Giardia Cysts by Free Chlorine 

(10 o

C portion of table, for concentrations from 0.4 to 1.2) 

Temperature = 10 oC 

pH 

Chlorine 

Concentration 

(mg/L) <=6.0 6.5 7.0 7.5 8.0 8.5 9.0 

<=0.4 73 88 104 125 149 177 209 

0.6 75 90 107 128 153 183 218 

0.8 78 92 110 131 158 189 226 

1.0 79 94 112 134 162 195 234 

1.2 80 95 114 137 166 200 240 

Step 10. Calculate the inactivation ratio for the clearwell. 

 Inactivation ratio = CTcalc / CT99.9

 = (30 min-mg/L) / (79 min-mg/L) 

 Inactivation ratio = 0.380 

Step 11. Calculate the actual Giardia log inactivation for the clearwell. 

 Log inactivation = 3 x CTcalc / CT99.9

 Log inactivation = 3 x 0.380 

 Log inactivation = 1.14 

The Giardia log inactivation for this system is 1.14. 
Appendix D. Examples

EPA Guidance Manual 128 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-1 continued 

Assuming the system received a 2.0 log Giardia removal credit from the State for direct 

filtration, it must achieve at least 1.0 log Giardia inactivation for a total 3.0 log Giardia

reduction as required in the Surface Water Treatment Rule (40 CFR Section 

141.70(a)(1)). The value of 1.14 log Giardia inactivation exceeds the required 1.0 log 

Giardia inactivation. A calculation for virus inactivation does not need to be 

performed since only free chlorine is used as a disinfectant. 

The worksheets in Appendix C can be used to record data and calculate log inactivation. 

The table below demonstrates how to record the data from this example using Worksheet #1 

in Appendix C. 

Starting Month: January Year: 2004 PWSID: AA6543210 System/Water Source: LMN Water Plant

Disinfectant Type: Free Chlorine Prepared by: Jim Operator

Profile Type (check one): X Giardia Viruses

Disinfection Segment/Sequence of Application: Clearwell/1st

3 4 5 6 7 8 9 10 11 12 13 14

Residual Peak Disinf.

Disinf. pH Water Hourly TDT Baffling Contact CTCalc = CT Inactivation Log

Week Conc. Temp. Flow Volume Factor Time (CxT) Req'd Ratio Inactivation*

# C (mg/L) (

o

C) (gpm) (gal) (min.) T (min.) (min-mg/L) (min-mg/L) (Col 11 / Col 12)

1 1.0 6 10 5,000 300,000 60 0.5 30 30.0 79 0.38 1.14

2

3

4

5

6

*See worksheet #2 to determine total log inactivation if the system has multiple disinfection segments.

WORKSHEET #1

LOG INACTIVATION RATIO DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER
 Appendix D. Examples

May 2003 129 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-2: Calculate Actual Log Inactivation for Three Disinfection 

Segments and Two Disinfectants 

Distribution

System

Sedimentation PreSedimentation

Coagulation

Clearwell

Disinfection Segment 3

Monitoring Point

Chloramine residual = 0.6 mg/L

Temperature = 10 oC

Intake

Filtration

Chlorine

Disinfection

Segment 2

Chlorine

Disinfection Segment 1

Monitoring Point

Cl2 residual = 1.0 mg/L

Temperature = 10 oC

pH = 7.5

Disinfection Segment 1

Flocculation Ammonia

Disinfection Segment 2

Monitoring Point

Cl2 residual = 1.2 mg/L

Temperature = 10 oC

pH = 7.5

Disinfection

Segment 3

In this example chlorine is added to the conventional treatment system before coagulation as 

a predisinfectant and again prior to the clearwell as a primary disinfectant. Ammonia is 

added to the system after the clearwell to create chloramines as the secondary disinfectant to 

maintain a residual throughout the distribution system. The system was required to create a 

disinfection profile. Therefore, the system must determine the actual log inactivation for 

Giardia (Note: In this example virus log inactivations do not need to be calculated because 

chloramine is being used as a secondary disinfectant). 

Since there are three points where the disinfectant is added, the inactivation ratio must be 

calculated for each disinfection segment. 

A. Determine the Giardia Inactivation Ratio for Disinfection Segment 1 

Disinfection Segment 1 begins at the chlorine injection location just prior to coagulation and 

ends at the chlorine monitoring point just after the filters. 

Step 1. Determine the peak hourly flow. 

From the raw water pump records the peak hourly flow (Q) is determined to be 5,000 

gpm. 

Step 2. Measure the chlorine residual, temperature, and pH (since chlorine is used) 

during peak hourly flow at the chlorine monitoring point and at the same time. 

 Temperature = 10oC 

pH = 7.5 

Chlorine residual = Cchlorine = 1.0 mg/L 
Appendix D. Examples

EPA Guidance Manual 130 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-2 continued 

Step 3. Measure the physical dimensions of the sub-units in Disinfection Segment 1. 

Measure inner tank diameter or length and width to obtain the volume of water in the 

tanks rather than the volume of the tanks themselves. 

Measure the minimum operating depth in the tanks, where applicable, to obtain 

conservative estimates of the volume of water in the tanks. 

Coagulation:

13.7 ft

13.7 ft

17.1 ft

Length = 13.7 ft 

Width = 13.7 ft 

Depth = 17.1 ft 

Flocculation: 14.0 ft

11.5 ft

66.4 ft

Length = 66.4 ft 

Width = 11.5 ft 

Depth = 14.0 ft 
 Appendix D. Examples

May 2003 131 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-2 continued 

Sedimentation:

39.9 ft

10.7 ft

Diameter = 39.9 ft 

Depth = 10.7 ft 

Filtration: 4 ft

9.4 ft

20 ft

Top of Filter Media

Depth above filter media = 4 ft 

Length = 20 ft 

Width = 9.4 ft 

Number of filters = 8 

Step 4. Calculate the volume of the water in each sub-unit in Disinfection Segment 1. 

Coagulation:

 

 Volume (V) = Length x Width x Depth 

 V = 13.7 ft x 13.7 ft x 17.1 ft = 3,210 ft3

 V = 3,210 ft 3 x (7.48 gal / ft3) 

 V = 24,000 gallons 

Flocculation:

 Volume (V) = Length x Width x Depth 

 V = 66.4 ft x 11.5 ft x 14.0 ft = 10,690 ft3

 V = 10,690 ft3

 x (7.48 gal / ft3) 

 V = 80,000 gallons
Appendix D. Examples

EPA Guidance Manual 132 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-2 continued 

Sedimentation:

 Volume (V) = π x Radius2 x Depth 

 π = 3.14 (constant) 

 Radius = Diameter / 2 = 39.9 / 2 = 19.95 ft 

 V = 3.14 x (19.95 ft)2 x 10.7 ft = 13,370 ft3

 V = 13,370 ft3 x (7.48 gal / ft3) 

 V = 100,000 gallons 

Filtration:

 Volume (V) = Length x Width x Depth of Water Above Media x # of Filters 

 V = 20 ft x 9.4 ft x 4 ft x 8 filters = 6,020 ft3

 V = 6,020 ft3

 x (7.48 gal / ft3) 

 V = 45,000 gallons 

Step 5. Calculate the Theoretical Detention Time (TDT) in the sub-units in Disinfection 

Segment 1. 

TDT = V / Q 

Coagulation:

 TDT = 24,000 gal / 5,000 gpm 

 TDT = 4.8 minutes 

Flocculation:

 TDT = 80,000 gal / 5,000 gpm 

 TDT = 16 minutes

Sedimentation:

 TDT = 100,000 gal / 5,000 gpm 

 TDT = 20 minutes

Filtration:

 TDT = 45,000 gal / 5,000 gpm 

 TDT = 9 minutes
 Appendix D. Examples

May 2003 133 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-2 continued

Step 6. Determine the baffling factors (BF) for the sub-units in Disinfection Segment 1. 

The table below summarizes the baffling factors in this example for the sub-units in 

Disinfection Segment 1. 

Unit Process BF *

(1) Coagulation 0.1 

(2) Flocculation 0.1 

(3) Sedimentation 0.5 

(4) Filtration 0.7 

*See Appendix G for Baffling Factors 

Step 7. Calculate the contact time (T) in the sub-units in Disinfection Segment 1. 

 

 T = TDT x BF 

Coagulation:

 T = 4.8 min x 0.1 

 T = 0.48 minutes 

Flocculation:

 T = 16 min x 0.1 

 T = 1.6 minutes 

Sedimentation:

 T = 20 min x 0.5 

 T = 10 minutes 

 

Filtration:

 T = 9 min x 0.7 

 T = 6.3 minutes 

Step 8. Calculate the total contact time in Disinfection Segment 1. 

Total Contact Time (Ttotal) = Sum of T in each sub-unit 

 Ttotal = 0.48 min + 1.6 min + 10 min + 6.3 min 

 Ttotal = 18.4 minutes 
Appendix D. Examples

EPA Guidance Manual 134 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-2 continued 

Step 9. Calculate the CT for Disinfection Segment 1 (CTcalc) 

 CTcalc = Cchlorine x Ttotal

 CTcalc = 1.0 mg/L x 18.4 min 

 CTcalc = 18.4 min-mg/L 

The CTcalc for Disinfection Segment 1 = 18.4 min-mg/L 

Step 10. Determine the required CT99.9 necessary to obtain 3-log Giardia inactivation. 

The required CT value for 3-log Giardia inactivation (CT99.9) may be obtained by 

using CT Table B-1 in Appendix B, CT Values for 3-Log Inactivation of Giardia

Cysts by Free Chlorine. The CT99.9 in this example is 134 min-mg/L for a pH of 7.5, 

temperature of 10 o

C, and Cchlorine of 1.0 mg/L. The relevant section of Table B-1 is 

reprinted below and the pertinent section of the table is highlighted. 

Excerpt from Table B-1: 

CT Values for 3-Log Inactivation of Giardia Cysts by Free Chlorine 

(10 oC portion of table, for concentrations from 0.6 to 1.4) 

Temperature = 10 oC 

pH 

Chlorine 

Concentration 

(mg/L) <=6.0 6.5 7.0 7.5 8.0 8.5 9.0 

0.6 75 90 107 128 153 183 218 

0.8 78 92 110 131 158 189 226 

1.0 79 94 112 134 162 195 234 

1.2 80 95 114 137 166 200 240 

1.4 82 98 116 140 170 206 247 

Step 11. Calculate the inactivation ratio for Disinfection Segment 1. 

 

 Inactivation ratio = CTcalc / CT99.9

 Inactivation ratio = (18.4 min-mg/L) / (134 min-mg/L) 

 Inactivation ratio = 0.137 
 Appendix D. Examples

May 2003 135 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-2 continued

B. Determine the Giardia Inactivation Ratio for Disinfection Segment 2 

Disinfection Segment 2 in this example begins at the chlorine injection location just prior to 

the clearwell and ends just after the clearwell. 

Step 1. Determine the peak hourly flow. 

The peak hourly flow (Q) for Disinfection Segment 2 is the same as the peak hourly 

flow in Disinfection Segment 1. 

Peak hourly flow = 5,000 gpm. 

Step 2. Measure the chlorine residual, temperature, and pH (since chlorine is used) 

during peak hourly flow at the chlorine monitoring point and at the same time. 

Temperature = 10oC 

Chlorine residual = Cchlorine = 1.2 mg/L 

pH = 7.5 

Step 3. Measure the physical dimensions of the clearwell. 15.3 ft

35 ft

75 ft

Minimum Operating Depth

Measure the inner tank length and width to obtain the volume of water in the 

clearwell rather than the volume of the tank itself. 

Length = 75 ft

Width = 35 ft 

Measure the minimum operating depth in the clearwell to obtain a conservative 

estimate of the volume of water in the tank. 

Minimum Operating Depth = 15.3 ft
Appendix D. Examples

EPA Guidance Manual 136 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-2 continued

Step 4. Calculate the volume of the water in the clearwell based on low water level. 

 Volume (V) = minimum water depth x length x width 

 V = 15.3 ft x 75 ft x 35 ft = 40,160 ft3

 

 V = 40,160 ft3

 x (7.48 gal / ft3) 

 V = 300,000 gal 

 

Step 5. Calculate the Theoretical Detention Time in the clearwell. 

 

Theoretical Detention Time (TDT) = V / Q 

 TDT = 300,000 gal / 5,000 gpm 

 TDT = 60 minutes 

Step 6. Determine the baffling factor for the clearwell. 

 

Clearwell Baffling Factor (BF) = 0.7 (from Table G-1 for superior baffling 

condition as shown below.) 

Step 7. Calculate the contact time of the disinfectant in the clearwell. 

 Contact Time (T) = TDT x BF 

 T = 60 min x 0.7 

 T = 42 minutes 

Step 8. Calculate the CT for the disinfection segment. 

 

 CTcalc = Cchlorine x T 

 CTcalc = 1.2 mg/L x 42 min 

 CTcalc = 50 min-mg/L 
 Appendix D. Examples

May 2003 137 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-2 continued 

Step 9. Determine the required CT99.9 necessary to obtain 3-log Giardia inactivation. 

The required CT value for 3-log Giardia inactivation (CT99.9) may be obtained by 

using CT Table B-1 in Appendix B, CT Values for 3-Log Inactivation of Giardia

Cysts by Free Chlorine. The CT99.9 in this example is 137 min-mg/L for a pH of 7.5, 

temperature of 10 oC, and Cchlorine of 1.2 mg/L. The relevant section of Table B-1 is 

reprinted below and the pertinent section of the table is highlighted. 

Excerpt from Table B-1: 

CT Values for 3-Log Inactivation of Giardia Cysts by Free Chlorine 

(10 oC portion of table, for concentrations from 0.8 to 1.6) 

Temperature = 10 o

C 

pH 

Chlorine 

Concentration 

(mg/L) <=6.0 6.5 7.0 7.5 8.0 8.5 9.0 

0.8 78 92 110 131 158 189 226 

1.0 79 94 112 134 162 195 234 

1.2 80 95 114 137 166 200 240 

1.4 82 98 116 140 170 206 247 

1.6 83 99 119 144 174 211 253 

Step 10. Calculate the inactivation ratio for the clearwell. 

Inactivation ratio = CTcalc / CT99.9

 Inactivation ratio = (50 min-mg/L) / (137 min-mg/L) 

 Inactivation ratio = 0.365 

C. Determine the Giardia Inactivation Ratio for Disinfection Segment 3 

Disinfection Segment 3 in this example begins at the chloramine injection location after the 

clearwell and ends at the monitoring point in the transmission pipe, which is prior to the first 

customer. 

Step 1. Determine the peak hourly flow. 

The peak hourly flow (Q) for Disinfection Segment 3 is the same as the peak hourly 

flow in Disinfection Segment 1. 

Peak hourly flow = 5,000 gpm. 
Appendix D. Examples

EPA Guidance Manual 138 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-2 continued 

Step 2. Measure the chloramine residual and temperature during peak hourly flow at the 

chlorine monitoring point and at the same time. 

Temperature = 10oC 

Chloramine residual = Cchloramine = 0.6 mg/L 

 

Step 3. Measure the physical dimensions of the pipe. 

5,280 Feet

Side View

End View

(Closeup)

Diameter = 12 in

Measure the length of the pipe and the inner pipe diameter to obtain the volume of 

water in the pipe rather than the volume of the pipe itself. 

Diameter = 12 in x (1 ft / 12 in) = 1 ft 

Length = 5,280 ft 

Step 4. Calculate the volume of the water in the pipe. 

 Volume (V) = π x Radius2

 x Length 

 π = 3.14 (constant) 

 Radius = Diameter / 2 = 1.0 / 2 = 0.5 ft 

 V = 3.14 x (0.5 ft)2

 x 5,280 ft = 4,140 ft3 

 V= 4,140 ft3

 x (7.48 gal / ft3) 

 V = 31,000 gallons 

 

Step 5. Calculate the Theoretical Detention Time in the pipe. 

 

Theoretical Detention Time (TDT) = V / Q 

 TDT = 31,000 gal / 5,000 gpm 

 TDT = 6.2 minutes 
 Appendix D. Examples

May 2003 139 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-2 continued

Step 6. Determine the baffling factor for the pipe. 

 

Baffling Factor (BF) = 1.0 (from Table G-1 in Appendix G for a pipe) 

Step 7. Calculate the contact time of the disinfectant in the pipe. 

 Contact Time (T) = TDT x BF 

 T = 6.2 min x 1.0 

 T = 6.2 minutes 

Step 8. Calculate the CT for the disinfection segment. 

 

 CTcalc = Cchloramine x T 

 CTcalc = 0.6 mg/L x 6.2 min 

 CTcalc = 3.7 min-mg/L 

Step 9. Determine the required CT99.9 necessary to obtain 3-log Giardia inactivation. 

The required CT value for 3-log Giardia inactivation (CT99.9) may be obtained by 

using CT Table B-7 in Appendix B, CT Values for 3-Log Inactivation of Giardia

Cysts by Chloramine pH 6-9. The CT99.9 in this example is 1,850 min-mg/L for a 

temperature of 10 o

C. Table B-7 is reprinted below and the pertinent section of the 

table is highlighted. 

Table B-7: 

CT Values for 3-Log Inactivation of Giardia Cysts by Chloramine pH 6-9 

Temperature (oC) 

< = 1 5 10 15 20 25

3,800 2,200 1,850 1,500 1,100 750 

Step 10. Calculate the inactivation ratio for the pipe. 

 Inactivation ratio = CTcalc / CT99.9

 Inactivation ratio = (3.7 min-mg/L) / (1,850 min-mg/L) 

 Inactivation ratio = 0.002
Appendix D. Examples

EPA Guidance Manual 140 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-2 continued 

D. Determine Total Giardia Log Inactivation for the System. 

Step 1. Determine the total Giardia inactivation ratio for the system. 

Total Inactivation ratio = Σ (CTcalc / CT99.9) = 0.137 + 0.365 + 0.002 = 0.504 

Step 2. Determine the total Giardia log inactivation for the system. 

 Total log inactivation = 3 x Σ (CTcalc / CT99.9) 

 Total log inactivation = 3 x (0.504) 

 Total log inactivation = 1.51 

The total Giardia log inactivation for the system is 1.51. 

Assuming the system received a 2.5 log Giardia removal credit from the State for 

conventional filtration, it must achieve at least 0.5 log Giardia inactivation for a total 

3.0 log Giardia reduction as required in the Surface Water Treatment Rule (40 CFR 

Section 141.70(a)(1)). The value of 1.51 log Giardia inactivation exceeds the required 

0.5 log Giardia inactivation. A calculation for virus inactivation does not need to be 

performed since only free chlorine is used as the primary disinfectant. 

E. Worksheets 

The worksheets in Appendix C can be used to record data and calculate log inactivation. 

The table below summarizes the calculations for each unit process in Disinfection 

Segment 1. 

Unit Process Volume (gal) Peak Hourly Flow (gpm) BF* Contact Time (min)

Coagulation 24,000 5,000 0.1 0.48 

Flocculation 80,000 5,000 0.1 1.6 

Sedimentation 100,000 5,000 0.5 10 

Filtration 45,000 5,000 0.7 6.3 

Total: 249,000 18.4 

* See Appendix G for baffling factors. 
 Appendix D. Examples

May 2003 141 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-2 continued 

The worksheet excerpt below demonstrates how data may be recorded from Disinfection 

Segment 1 using Worksheet #1 in Appendix C. For this example, Worksheet #1 needs to be 

copied so the data from each disinfection segment can be entered. 

Starting Month: January Year: 2004 PWSID: AA7654321 System/Water Source: ABC Water Plant

Disinfectant Type: Free Chlorine Prepared by: Jon Operator

Profile Type (check one): X Giardia Viruses

Disinfection Segment/Sequence of Application: Coagulation, Flocculation, Sedimentation, Filtration/1st

3 4 5 6 7 8 9 10 11 12 13 14

Residual Peak Disinf.

Disinf. pH Water Hourly TDT Baffling Contact CTCalc = CT Inactivation Log

Week Conc. Temp. Flow Volume Factor Time (CxT) Req'd Ratio Inactivation*

# C (mg/L) (

o

C) (gpm) (gal) (min.) T (min.) (min-mg/L) (min-mg/L) (Col 11 / Col 12)

1 1.0 7.5 10 5,000 249,000 ** ** 18.4 18.4 134 0.137

2

3

4

5

6

*See worksheet #2 to determine total log inactivation if the system has multiple disinfection segments.

**See the previous table showing details of each unit process for theoretical detention times and baffling factors.

WORKSHEET #1

LOG INACTIVATION RATIO DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER

The worksheet excerpt below demonstrates how data may be recorded from Disinfection 

Segment 2 using Worksheet #1 in Appendix C. 

Starting Month: January Year: 2004 PWSID: AA7654321 System/Water Source: ABC Water Plant

Disinfectant Type: Free Chlorine Prepared by: Jon Operator

Profile Type (check one): X Giardia Viruses

Disinfection Segment/Sequence of Application: Clearwell/2nd

3 4 5 6 7 8 9 10 11 12 13 14

Residual Peak Disinf.

Disinf. pH Water Hourly TDT Baffling Contact CTCalc = CT Inactivation Log

Week Conc. Temp. Flow Volume Factor Time (CxT) Req'd Ratio Inactivation*

# C (mg/L) (

o

C) (gpm) (gal) (min.) T (min.) (min-mg/L) (min-mg/L) (Col 11 / Col 12)

1 1.2 7.5 10 5,000 300,000 60 0.7 42 50 137 0.365

2

3

4

5

6

*See worksheet #2 to determine total log inactivation if the system has multiple disinfection segments.

WORKSHEET #1

LOG INACTIVATION RATIO DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER
Appendix D. Examples

EPA Guidance Manual 142 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-2 continued 

The worksheet excerpt below demonstrates how data may be recorded from Disinfection 

Segment 3 using Worksheet #1 in Appendix C. 

Starting Month: January Year: 2004 PWSID: AA7654321 System/Water Source: ABC Water Plant

Disinfectant Type: Chloramine Prepared by: Jon Operator

Profile Type (check one): X Giardia Viruses

Disinfection Segment/Sequence of Application: Transmission Pipe/3rd

3 4 5 6 7 8 9 10 11 12 13 14

Residual Peak Disinf.

Disinf. pH Water Hourly TDT Baffling Contact CTCalc = CT Inactivation Log

Week Conc. Temp. Flow Volume Factor Time (CxT) Req'd Ratio Inactivation*

# C (mg/L) (

o

C) (gpm) (gal) (min.) T (min.) (min-mg/L) (min-mg/L) (Col 11 / Col 12)

1 0.6 N/A 10 5,000 31,000 6.2 1.0 6.2 3.7 1,850 0.002

2

3

4

5

6

*See worksheet #2 to determine total log inactivation if the system has multiple disinfection segments.

WORKSHEET #1

LOG INACTIVATION RATIO DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER

The worksheet excerpt below demonstrates how to determine total Giardia log inactivation 

for the system using Worksheet #2 in Appendix C. 

Starting Month: January Year: 2004 PWSID: AA7654321

System/Water Source: ABC Water Plant Prepared by: Jon Operator

Disinfectant Type: Chlorine/Chloramine

Profile Type (check one): X Giardia Viruses

Sum

Disinfection Disinfection Disinfection Disinfection Disinfection of Total

Week Segment Segment Segment Segment Segment Inactivation Log

# 1 2 3 4 5 Ratios Inactivation1

1 0.137 0.365 0.002 0.504 1.51

2

3

4

5

6 1

Giardia : Log Inactivation = 3 x Sum of Inactivation Ratios

 Viruses: Log Inactivation = 4 x Sum of Inactivation Ratios (or a method approved by the State)

Inactivation Ratio for each disinfection segment from Worksheet #1

TOTAL LOG INACTIVATION DETERMINATION FOR SURFACE WATER SYSTEMS OR

GROUND WATER SYSTEMS UNDER THE DIRECT INFLUENCE OF SURFACE WATER

WORKSHEET #2
 Appendix D. Examples

May 2003 143 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-3: Develop a Disinfection Profile and Benchmark for a System 

with Multiple Disinfection Segments 

In this example a conventional filtration treatment plant added ozone in contact chambers at 

the front of the plant as a primary disinfectant and used chlorine as the secondary 

disinfectant after the clearwell. The ozone residual was measured at each ozone contact 

chamber and the chlorine residual was measured in the transmission pipe. The system was 

required to create a disinfection profile. Since ozone is used as a primary disinfectant, the 

system must calculate both Giardia and virus log inactivations. 

Sedimentation

Flocculation Coagulation Filtration

Clearwell

To

Distribution

System

Disinfection Segment 4

Monitoring Point

Chlorine Residual = 0.8 mg/L

Temperature = 0.5 oC

pH = 7.0

Disinfection

Segment 4

Ozone Contact Chambers

Disinfection

Segment 1

Disinfection

Segment 2

Disinfection

Segment 3

Chlorine See enlarged drawing below

C1out = 0.5 mg/L

C2in = 0.4 mg/L

C2out = 0.6 mg/L C3in = 0.6 mg/L

C3out C = 0.1 mg/L 1in = 0.0 mg/L

Chamber 1 Chamber 3 Chamber 2

Temp = 0.5 oC Temp = 0.5 oC

Temp = 0.5 oC Temp = 0.5 oC

Disinfection

Segment 1

Disinfection

Segment 2

Disinfection

Segment 3

NOTE: Following is one method for calculating Giardia inactivation for ozone using 

the procedure presented in Guidance Manual for Compliance with the Filtration and 

Disinfection Requirements for Public Water Systems Using Surface Water Sources (EPA, 

March 1991). Systems must use a method approved by the State; therefore, systems 

should check with the State to determine if it approves this method or if another 

method is required. 
Appendix D. Examples

EPA Guidance Manual 144 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-3 continued

A. Determine the Giardia Log Inactivation for Disinfection Segment 1

Step 1. Measure the ozone residual at the inlet and outlet of Contact Chamber 1 during 

peak hourly flow. 

C1in = 0.0 mg/L 

C1out = 0.5 mg/L 

Table D-1. Correlations to Predict C Based on Inlet and Outlet Ozone 

Concentrations 

Flow Configuration 

Turbine Co-Current Flow Counter-Current 

Flow Reactive Flow 

First Chamber C Partial Credit1 Partial Credit1 Not Applicable 

All Other 

Chambers C = Cout

C = Cout

or 

C = (Cout + Cin) / 2 

C = Cout / 2 C = Cout

1. 1-log of virus inactivation providing that Cout > 0.1 mg/L and 0.50 log Giardia inactivation providing that 

Cout > 0.3 mg/L. 

(Source: Guidance Manual for Compliance with the Filtration and Disinfection 

Requirements for Public Water Systems Using Surface Water Sources, EPA, March 1991) 

Step 2. Determine the Giardia log inactivation in Contact Chamber 1. 

In Contact Chamber 1 the flow is counter-current since the water flows in the 

opposite direction that the ozone flows (Note: Ozone is introduced in the bottom of 

the chamber and bubbles upward). According to Table D-1, since the outlet ozone 

concentration is 0.5 mg/L, which is greater than 0.3 mg/L, the Giardia log 

inactivation in Contact Chamber 1 is 0.50. 

B. Determine the Giardia Log Inactivation for Disinfection Segment 2 

Step 1. Measure the temperature and the ozone residual at the inlet and outlet of Contact 

Chamber 2 during peak hourly flow. 

Temperature = 0.5 o

C 

C2in = 0.4 mg/L 

C2out = 0.6 mg/L 
 Appendix D. Examples

May 2003 145 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-3 continued

Step 2. Determine C in Contact Chamber 2. 

Table D-1. Correlations to Predict C Based on Inlet and Outlet Ozone 

Concentrations 

Flow Configuration 

Turbine Co-Current Flow Counter-Current 

Flow Reactive Flow 

First Chamber C Partial Credit1 Partial Credit1 Not Applicable 

All Other 

Chambers C = Cout

C = Cout

or 

C = (Cout + Cin) / 2 

C = Cout / 2 C = Cout

1. 1-log of virus inactivation providing that Cout > 0.1 mg/L and 0.50 log Giardia inactivation providing that 

Cout > 0.3 mg/L. 

(Source: Guidance Manual for Compliance with the Filtration and Disinfection 

Requirements for Public Water Systems Using Surface Water Sources, EPA, March 1991) 

In Contact Chamber 2 the flow is counter-current since the water flows in the 

opposite direction that the ozone flows. According to Table D-1, C = Cout / 2 for 

contact chambers with counter-current flow. 

C = C2out / 2 

C = 0.6 mg/L / 2 

C = 0.3 mg/L 

Step 3. Determine the contact time in Contact Chamber 2.

The contact time for all of the ozone contact chambers taken together was 

determined by a tracer study to be 15 minutes at peak hourly flow. The total contact 

time can be divided proportionally by volume between all three chambers if the 

chambers with final concentrations of zero (non-detectable) do not make up 50% or 

greater of the total volume of the chambers. Since the final concentration in all 

chambers is greater than zero and since the contact chambers are all identical with 

equal volumes the contact time can be divided equally between all three chambers: 

T = Ttot / 3 chambers = 15 min / 3 chambers = 5 minutes per chamber
Appendix D. Examples

EPA Guidance Manual 146 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-3 continued 

Step 4. Calculate CTcalc in Contact Chamber 2. 

 CTcalc = C x T 

 CTcalc = 0.3 mg/L x 5 min 

CTcalc = 1.5 min-mg/L 

Step 5. Locate appropriate CT table. 

The table for 3-log inactivation of Giardia by ozone is Table B-5 in Appendix B. 

Step 6. Identify the appropriate portion of the table based on operating conditions. 

Locate the column for 0.5 °C (< = 1 o

C). 

Table B-5 

CT Values for 3-Log Inactivation of Giardia Cysts by Ozone 

Temperature (oC) 

< = 1 5 10 15 20 25

2.9 1.90 1.43 0.95 0.72 0.48 

Step 7. Obtain CT99.9 value. 

From this chart it is determined that the value of CT for 3-log inactivation by ozone at 0.5°C 

is 2.9 min-mg/L. 

 CT99.9 = 2.9 min-mg/L 

Step 8. Calculate the Giardia inactivation ratio for Disinfection Segment 2. 

Inactivation ratio = CTcalc / CT99.9

 Inactivation ratio = (1.5 min-mg/L / 2.9 min-mg/L) 

 Inactivation ratio = 0.517 

Step 9. Calculate Giardia inactivation for Disinfection Segment 2. 

 Giardia log inactivation = 3 x (CTcalc / CT99.9) 

 Giardia log inactivation = 3 x 0.517 

 Giardia log inactivation = 1.55 
 Appendix D. Examples

May 2003 147 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-3 continued 

C. Determine the Giardia Log Inactivation for Disinfection Segment 3 

Step 1. Measure the temperature and the ozone residual at the inlet and outlet of Contact 

Chamber 3 during peak hourly flow. 

Temperature = 0.5 o

C 

C3in = 0.6 mg/L 

C3out = 0.1 mg/L 

Step 2. Determine C in Contact Chamber 3. 

Table D-1. Correlations to Predict C Based on Inlet and Outlet Ozone 

Concentrations 

Flow Configuration 

Turbine Co-Current Flow Counter-Current 

Flow Reactive Flow 

First Chamber C Partial Credit1 Partial Credit1 Not Applicable 

All Other 

Chambers C = Cout

C = Cout

or 

C = (Cout + Cin) / 2 

C = Cout / 2 C = Cout

1. 1-log of virus inactivation providing that Cout > 0.1 mg/L and 0.50 log Giardia inactivation providing that 

Cout > 0.3 mg/L. 

(Source: Guidance Manual for Compliance with the Filtration and Disinfection 

Requirements for Public Water Systems Using Surface Water Sources, EPA, March 1991) 

In Contact Chamber 3 the flow is co-current since the water flows in the same 

direction that the ozone flows. According to Table D-1, C = (Cout + Cin) / 2 for 

contact chambers with co-current flow. 

C = (C3in + C3out) / 2 

C = (0.6 mg/L + 0.1 mg/L) / 2 

C = 0.35 mg/L 

Step 3. Determine the contact time in Contact Chamber 3.

It was determined in Part B, Step 3 of this example that the contact time in each 

chamber is 5 minutes. 

T = 5 minutes 
Appendix D. Examples

EPA Guidance Manual 148 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-3 continued

Step 4. Calculate CTcalc in Contact Chamber 3. 

 CTcalc = C x T 

 CTcalc = 0.35 mg/L x 5 min 

CTcalc = 1.75 min-mg/L 

Step 5. Locate appropriate CT table. 

The table for 3-log inactivation of Giardia by ozone is Table B-5 in Appendix B. 

Step 6. Identify the appropriate portion of the table based on operating conditions. 

Locate the column for 0.5 °C (< = 1 o

C). 

Table B-5 

CT Values for 3-Log Inactivation of Giardia Cysts by Ozone 

Temperature (oC) 

< = 1 5 10 15 20 25

2.9 1.90 1.43 0.95 0.72 0.48 

Step 7. Obtain CT99.9 value. 

From this chart it is determined that the value of CT for 3-log inactivation by ozone at 0.5°C 

is 2.9 min-mg/L. 

 CT99.9 = 2.9 min-mg/L 

Step 8. Calculate the Giardia inactivation ratio for Disinfection Segment 3. 

Inactivation ratio = CTcalc / CT99.9

 Inactivation ratio = (1.75 min-mg/L / 2.9 min-mg/L) 

 Inactivation ratio = 0.603

Step 9. Calculate Giardia inactivation for Disinfection Segment 3. 

 Giardia log inactivation = 3 x (CTcalc / CT99.9) 

 Giardia log inactivation = 3 x 0.603 

 Giardia log inactivation = 1.81 
 Appendix D. Examples

May 2003 149 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-3 continued 

D. Determine Giardia Log Inactivation for Disinfection Segment 4 

Step 1. Determine the peak hourly flow. 

From the raw water pump records the peak hourly flow (Q) is determined to be 5,000 

gpm. 

Step 2. Measure chlorine residual, temperature, and pH during peak hourly flow at the 

chlorine monitoring point and at the same time. 

Temperature = 0.5 o

C 

pH = 7.0 

Chlorine residual = Cchlorine = 0.8 mg/L 

Step 3. Measure the physical dimensions of the pipe. 

5,280 Feet

Side View

End View

(Closeup)

Diameter = 12 in

Measure the length of the pipe and the inner pipe diameter to obtain the volume of 

water in the pipe rather than the volume of the pipe itself. 

Diameter = 12 in x (1 ft / 12 in) = 1.0 ft 

Length = 5,280 ft 

Step 4. Calculate the volume of the water in the pipe. 

 Volume (V) = π x Radius2

 x Length 

 π = 3.14 (constant) 

 Radius = Diameter / 2 = 1.0 / 2 = 0.5 ft 

 V = 3.14 x (0.5 ft)2

 x 5,280 ft = 4,140 ft3

 V= 4,140 ft3

 x (7.48 gal / ft3) 

 V = 31,000 gallons 
Appendix D. Examples

EPA Guidance Manual 150 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-3 continued

Step 5. Calculate the Theoretical Detention Time in the pipe. 

 

Theoretical Detention Time (TDT) = V / Q 

 TDT = 31,000 gal / 5,000 gpm 

 TDT = 6.2 minutes 

Step 6. Determine the baffling factor for the pipe. 

 

Baffling Factor (BF) = 1.0 (from Table G-1 in Appendix G for a pipe) 

Step 7. Calculate the contact time of the disinfectant in the pipe. 

 Contact Time (T) = TDT x BF 

 T = 6.2 min x 1.0 

 T = 6.2 minutes 

Step 8. Calculate the CT for the disinfection segment. 

 

 CTcalc = Cchlorine x T 

 CTcalc = 0.8 mg/L x 6.2 min 

 CTcalc = 5.0 min-mg/L 

Step 9. Determine the required CT99.9 necessary to obtain 3-log Giardia inactivation. 

The required CT value for 3-log Giardia inactivation (CT99.9) is obtained by using 

CT Table B-1 in Appendix B, CT Values for 3-Log Inactivation of Giardia Cysts by 

Free Chlorine. The CT99.9 is 205 min-mg/L for a pH of 7.0, temperature of 0.5 oC, 

and Cchlorine of 0.8 mg/L. The relevant section of Table B-1 is reprinted below and 

the pertinent section of the table is highlighted. 

Excerpt from Table B-1: 

CT Values for 3-Log Inactivation of Giardia Cysts by Free Chlorine 

(0.5 o

C portion of table, for concentrations from 0.4 to 1.2 mg/L) 

Temperature = 0.5 oC 

pH 

Chlorine 

Concentration 

(mg/L) <=6.0 6.5 7.0 7.5 8.0 8.5 9.0 

<=0.4 137 163 195 237 277 329 390 

0.6 141 169 200 239 286 342 407 

0.8 145 172 205 246 295 354 422 

1.0 148 176 210 253 304 365 437 

1.2 152 180 215 259 313 376 451 
 Appendix D. Examples

May 2003 151 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-3 continued

Step 10. Calculate the Giardia inactivation ratio for the pipe. 

 Inactivation ratio = CTcalc / CT99.9

 Inactivation ratio = (5.0 min-mg/L / 205 min-mg/L) 

 Inactivation ratio = 0.024 

Step 11. Calculate the actual Giardia log inactivation for the pipe. 

Log inactivation = 3 x CTcalc / CT99.9

Log inactivation = 3 x 0.024 

 Log inactivation = 0.07

The log inactivation of Giardia for Disinfection Segment 4 is 0.07.

E. Calculate the Total Giardia Inactivation for the System 

Step 1. Sum the Giardia log inactivations for all of the disinfection segments to 

determine the total Giardia log inactivation achieved by the system. 

 From Disinfection Segment 1: 

 Giardia log inactivation = 0.50 

From Disinfection Segment 2: 

 Giardia log inactivation = 1.55 

From Disinfection Segment 3: 

 Giardia log inactivation = 1.81

From Disinfection Segment 4: 

 Giardia log inactivation = 0.07 

 

Total Giardia log inactivation = 0.50 + 1.55 + 1.81 + 0.07 = 3.93 

Assuming the system received a 2.5 log Giardia removal credit from the State for 

conventional filtration, it must achieve at least 0.5 log Giardia inactivation for a total 

3.0 log Giardia reduction as required in the Surface Water Treatment Rule (40 CFR 

Section 141.70(a)(1)). The value of 3.93 log Giardia inactivation exceeds the required 

0.5 log Giardia inactivation. 
Appendix D. Examples

EPA Guidance Manual 152 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-3 continued 

F. Determine Virus Log Inactivation for Disinfection Segment 1 

Since ozone is used as a primary disinfectant in this system, the log inactivation for viruses 

must also be calculated. 

Step 1. Determine the virus log inactivation in Contact Chamber 1. 

Table D-1. Correlations to Predict C Based on Inlet and Outlet Ozone 

Concentrations 

Flow Configuration 

Turbine Co-Current Flow Counter-Current 

Flow Reactive Flow 

First Chamber C Partial Credit1 Partial Credit1 Not Applicable 

All Other 

Chambers C = Cout

C = Cout

or 

C = (Cout + Cin) / 2 

C = Cout / 2 C = Cout

1. 1-log of virus inactivation providing that Cout > 0.1 mg/L and 0.50 log Giardia inactivation providing that 

Cout > 0.3 mg/L. 

(Source: Guidance Manual for Compliance with the Filtration and Disinfection 

Requirements for Public Water Systems Using Surface Water Sources, EPA, March 1991) 

In Contact Chamber 1 the flow is counter-current since the water flows in the 

opposite direction that the ozone flows (Note: Ozone is introduced in the bottom of 

the chamber and bubbles upward). According to Table D-1, since the outlet ozone 

concentration is 0.5 mg/L (determined in Part A of this Example), which is greater 

than 0.1 mg/L, the virus log inactivation in Disinfection Segment 1 (Contact 

Chamber 1) is 1.0. 

G. Determine Virus Log Inactivation for Disinfection Segment 2 

Step 1. Determine the required CT99.99 necessary to obtain 4-log virus inactivation for 

Contact Chamber 2. 

The required CT value for 4-log virus inactivation (CT99.99) is obtained by using CT Table 

B-6 in Appendix B, CT Values for 4-Log Inactivation of Viruses by Ozone. In this example 

the required CT99.99 is 1.8 min-mg/L for a temperature of 0.5 o

C. 
 Appendix D. Examples

May 2003 153 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-3 continued

Table B-6 

CT Values for 4-Log Inactivation of Viruses by Ozone 

Temperature (oC) 

< = 1 5 10 15 20 25

1.8 1.2 1.0 0.6 0.5 0.3 

Step 2. Calculate the virus inactivation ratio for Contact Chamber 2. 

CTcalc has already been calculated for Disinfection Segment 2. 

CTcalc = 1.5 min-mg/L 

 Inactivation ratio = CTcalc / CT99.9

 Inactivation ratio = (1.5 min-mg/L / 1.8 min-mg/L) 

 Inactivation ratio = 0.833 

Step 3. Calculate the virus inactivation for Contact Chamber 2. 

Virus log inactivation = 4 x CTcalc / CT99.99

Virus log inactivation = 4 x 0.833 

 Virus log inactivation = 3.3 

The log inactivation of viruses for Disinfection Segment 2 is 3.3.

H. Determine Virus Log Inactivation for Disinfection Segment 3 

Step 1. Determine the required CT99.99 necessary to obtain 4-log virus inactivation for 

Contact Chamber 3. 

The required CT value for 4-log virus inactivation (CT99.99) is obtained by using CT Table 

B-6 in Appendix B, CT Values for 4-Log Inactivation of Viruses by Ozone. The required 

CT99.99 is 1.8 min-mg/L for a temperature of 0.5 oC. 

Table B-6 

CT Values for 4-Log Inactivation of Viruses by Ozone 

Temperature (oC) 

< = 1 5 10 15 20 25

1.8 1.2 1.0 0.6 0.5 0.3 
Appendix D. Examples

EPA Guidance Manual 154 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example D-3 continued

Step 2. Calculate the virus inactivation ratio for Contact Chamber 3. 

CTcalc has already been calculated for Disinfection Segment 3. 

CTcalc = 1.75 min-mg/L 

 Inactivation ratio = CTcalc / CT99.9

 Inactivation ratio = (1.75 min-mg/L / 1.8 min-mg/L) 

 Inactivation ratio = 0.972 

Step 3. Calculate the actual virus log inactivation for Contact Chamber 3. 

Log inactivation = 4 x CTcalc / CT99.99

Log inactivation = 4 x 0.972 

 Log inactivation = 3.9 

The log inactivation of viruses for Disinfection Segment 3 is 3.9.

I. Determine Virus Log Inactivation for Disinfection Segment 4. 

Even though chlorine is the only disinfectant used in Disinfection Segment 4, the virus 

inactivation for Disinfection Segment 4 must also be calculated to determine the virus 

inactivation for the whole system. 

Step 1. Determine the required CT99.99 necessary to obtain 4-log virus inactivation for 

Disinfection Segment 4. 

The required CT value for 4-log virus inactivation (CT99.99) is obtained by using CT Table 

B-2 in Appendix B, CT Values for 4-log Inactivation of Viruses by Free Chlorine. The 

required CT99.99 is 12 min-mg/L for a pH of 7.0 and temperature of 0.5 o

C. Table B-2 is 

reprinted on the next page and the pertinent section of the table is highlighted. 

Table B-2: 

CT Values for 4-Log Inactivation of Viruses by Free Chlorine 

 pH 

Temperature (o

C) 6-9 10

0.5 12 90 

5 8 60 

10 6 45 

15 4 30 

20 3 22 

25 2 15 
 Appendix D. Examples

May 2003 155 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example D-3 continued 

Step 2. Calculate the virus inactivation ratio for Disinfection Segment 4. 

CTcalc has already been calculated for Disinfection Segment 4. 

CTcalc = 5.0 min-mg/L 

 Inactivation ratio = CTcalc / CT99.9

 Inactivation ratio = (5.0 min-mg/L / 12 min-mg/L) 

 Inactivation ratio = 0.417

Step 3. Calculate the actual virus log inactivation for Disinfection Segment 4. 

Log inactivation = 4 x CTcalc / CT99.99

Log inactivation = 4 x 0.417 

 Log inactivation = 1.7

J. Calculate the Total Virus Inactivation for the System 

Sum the virus log inactivations for all of the Disinfection Segments to determine the total 

virus log inactivation achieved by the system. 

 From Disinfection Segment 1: 

 virus log inactivation = 1.0 

From Disinfection Segment 2: 

 virus log inactivation = 3.3 

From Disinfection Segment 3: 

 virus log inactivation = 3.9

From Disinfection Segment 4: 

 virus log inactivation = 1.7 

 

Total virus log inactivation = 1.0 + 3.3 + 3.9 + 1.7 = 9.9 

Assuming the system received a 2.0 log virus removal credit from the State for 

conventional filtration, it must achieve at least 2.0 log virus inactivation for a total 4.0 

log virus reduction as required in the Surface Water Treatment Rule (40 CFR Section 

141.70(a)(2)). The value of 9.9 log virus inactivation exceeds the required 2.0 log virus 

inactivation. 
Appendix D. Examples

EPA Guidance Manual 156 May 2003 

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Appendix E 

Tracer Studies
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 Appendix E. Tracer Studies 

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E.1 INTRODUCTION

Information in this appendix is based on Appendix C in the Guidance Manual for 

Compliance with the Filtration and Disinfection Requirements for Public Water Systems 

Using Surface Water Sources (EPA, 1991). For more information on tracer studies, readers 

are encouraged to consult Appendix C in the Guidance Manual for Compliance with the 

Filtration and Disinfection Requirements for Public Water Systems Using Surface Water 

Sources (EPA, 1991) or Tracer Studies in Water Treatment Facilities: A Protocol and Case 

Studies (Teefy, 1996). 

As indicated in Chapter 4, fluid passing through a pipe is assumed to have a detention time 

equal to the theoretical or mean residence time at a particular flow rate. However, in mixing 

basins, storage reservoirs, and other treatment plant process units, utilities will be required to 

determine the contact time for the calculation of CT through tracer studies or other methods 

approved by the State. 

The contact time of mixing basins and storage reservoirs used in calculating CT should be 

the minimum detention time experienced by 90 percent of the water passing through the 

unit. This detention time was designated as T10 according to the convention adopted by 

Thirumurthi (1969). A profile of the flow through the basin over time can be generated by 

tracer studies. Information provided by these studies may be used for estimating the 

detention time, T10, for the purpose of calculating CT. (Note: T10 is referred to as “T” 

elsewhere in this document. However, for consistency with the Guidance Manual for 

Compliance with the Filtration and Disinfection Requirements for Public Water Systems 

Using Surface Water Sources (EPA, 1991), T10 is used in this appendix.) 

This appendix presents a brief synopsis of tracer study methods, procedures, and data 

evaluation. More detailed information about conducting tracer studies is available in 

Appendix C of the lengthier Guidance Manual for Compliance with the Filtration and 

Disinfection Requirements for Public Water Systems Using Surface Water Sources (EPA, 

1991). It is important to obtain assistance from the State before conducting a tracer study to 

ensure State approval of the results. 

E.2 FLOW EVALUATION

Although detention time is proportional to flow, it is not generally a linear function. Tracer 

studies may establish detention times for the range of flow rates experienced within each 

disinfectant segment. Systems should note that a single flow rate might not characterize the 

flow through the entire system. With a series of reservoirs, clearwells, and storage tanks, 

flow will vary between each portion of the system. 

Ideally, tracer tests should be performed for at least four flow rates that span the entire range 

of flow for the segment being tested. The flow rates should be separated by approximately 

equal intervals to span the range of operation, with one near average flow, two greater than 

average, and one less than average flow. The flows should also be selected so that the 
Appendix E. Tracer Studies 

EPA Guidance Manual 160 May 2003 

Disinfection Profiling and Benchmarking 

highest test flow rate is at least 91 percent of the highest flow rate expected to ever occur in 

that segment. Four data points should assure a good definition of the segment’s hydraulic 

profile. 

The results of the tracer tests performed for different flow rates should be used to generate 

plots of T10 versus flow (Q) for each segment in the system. A smooth line is drawn through 

the points on each graph to create a curve from which T10 may be read for the corresponding 

flow at peak hourly flow conditions. Refer to Appendix C, section C.1.7 of the Guidance 

Manual for Compliance with the Filtration and Disinfection Requirements for Public Water 

Systems Using Surface Water Sources (EPA, 1991), for an illustration of this procedure. 

The most accurate tracer test results are obtained when flow is constant through the segment 

during the course of the test. Therefore, the tracer study should be conducted at a constant 

flow whenever practical. For a treatment plant consisting of two or more equivalent process 

trains, a constant flow tracer test can be performed on a segment of the plant by holding the 

flow through one of the trains constant while operating the parallel train(s) to absorb any 

flow variations. Flow variations during tracer tests in systems without parallel trains or with 

single clearwells and storage reservoirs are more difficult to avoid. In these instances, T10 

should be recorded at the average flow rate over the course of the test. 

E.3 VOLUME EVALUATION

In addition to flow conditions, detention times determined by tracer studies depend on the 

water level and subsequent volume in treatment units. This is particularly pertinent to 

storage tanks, reservoirs, and clearwells, which, in addition to being contact basins for 

disinfection are also often used as equalization storage for distribution system demands and 

storage for backwashing. In such instances, the water levels in the reservoirs vary to meet 

the system demands. The actual detention time of these contact basins will also vary 

depending on whether they are emptying or filling. 

For some process units, especially sedimentation basins that are operated at a near constant 

level (that is, flow in equals flow out), the detention time determined by tracer tests should 

be sufficient for calculating CT when the basin is operating at water levels greater than or 

equal to the level at which the test was performed. When conducting a tracer study to 

determine the detention time, a water level at or slightly below, but not above, the normal 

minimum operating level is recommended. For many plants, the water level in a clearwell 

or storage tank varies between high and low levels in response to distribution system 

demands. In such instances, in order to obtain a conservative estimate of the contact time, 

the tracer study should be conducted during a period when the tank level is falling (flow out 

greater than flow in). 
 Appendix E. Tracer Studies 

May 2003 161 EPA Guidance Manual 

 Disinfection Profiling and Benchmarking

E.4 DISINFECTION SEGMENTS 

For systems that apply disinfectant(s) at more than one point, or choose to profile the 

residual from one point of application, tracer studies should be conducted to determine T10

for each segment containing process unit(s). The T10 for a segment may or may not include 

a length of pipe and is used along with the residual disinfectant concentration prior to the 

next disinfectant application or monitoring point to determine the CTcalc for that segment. 

The inactivation ratio for the section is then determined. The total log inactivation achieved 

in the system can then be determined by summing the inactivation ratios for all sections as 

explained in Chapter 5 of this document. 

For systems that have two or more units of identical size and configuration, tracer studies 

could be conducted on one of the units but applied to both. The resulting graph of T10

versus flow can be used to determine T10 for all identical units. 

Systems with more than one segment in the treatment plant that are conducting a tracer 

study may determine T10 for each segment: 

• By individual tracer studies through each segment; or, 

• By one tracer study across the system. 

If possible, tracer studies should be conducted on each segment to determine the T10 for each 

segment. In order to minimize the time needed to conduct studies on each segment, the 

tracer studies should be started at the last segment of the treatment train prior to the first 

customer and completed with the first segment of the system. Conducting the tracer studies 

in this order will prevent the interference of residual tracer material with subsequent studies. 

For ozone contactors, flocculators, or any basin containing mixing, tracer studies should be 

conducted for the range of mixing used in the process. In ozone contactors, air or oxygen 

should be added in lieu of ozone to prevent degradation of the tracer. The flow rate of air or 

oxygen used for the contactor should be applied during the study to simulate actual 

operation. Tracer studies should then be conducted at several air/oxygen to water ratios to 

provide data for the complete range of ratios used at the plant. For flocculators, tracer 

studies should be conducted for various mixing intensities to provide data for the complete 

range of operations. 

E.5 TRACER STUDY METHODS

This section discusses the two most common methods of tracer addition employed in water 

treatment evaluations, the step-dose method and the slug-dose method. Tracer study 

methods involve the application of chemical dosages to a system, and tracking the resulting 

effluent concentration as a function of time. The effluent concentration profile is evaluated 

to determine the detention time, T10. 

In preparation for beginning a tracer study, the raw water background concentration of the 
Appendix E. Tracer Studies 

EPA Guidance Manual 162 May 2003 

Disinfection Profiling and Benchmarking 

chosen tracer chemical should be established. The background concentration is important, 

not only to aid in the selection of the tracer dosage, but also to facilitate proper evaluation of 

the data. 

The background tracer concentration should be determined by monitoring for the tracer 

chemical prior to beginning the test. The sampling point(s) for the pre-tracer study 

monitoring should be the same as the points to be used for residual monitoring to determine 

CT values. Systems should use the following monitoring procedure: 

• Prior to the start of the test, regardless of whether the chosen tracer material is a 

treatment chemical, the tracer concentration in the water is monitored at the sampling 

point where the disinfectant residual will be measured for CT calculations. 

• If a background tracer concentration is detected, monitor it until a constant 

concentration, at or below the raw water background level, is achieved. This 

measured concentration is the baseline tracer concentration. 

Following the determination of the tracer dosage, feed and monitoring point(s), and a 

baseline tracer concentration, tracer testing can begin. 

Equal sampling intervals, as could be obtained from automatic sampling, are not required for 

either tracer study method. However, using equal sample intervals for the slug-dose method 

can simplify the analysis of the data. During testing, the time and tracer residual of each 

measurement should also be recorded on a data sheet. In addition, the water level, flow, and 

temperature should be recorded during the test. 

E.5.1 Step-Dose Method 

The step-dose method entails introduction of a tracer chemical at a constant dosage until the 

concentration at the desired end point reaches a steady-state level. At time zero, the tracer 

chemical feed is started and left at a constant rate for the duration of the test. Over the 

course of the test, the tracer residual should be monitored at the required sampling point(s) 

at a frequency determined by the overall detention time and site-specific considerations. As 

a general guideline, sampling at intervals of 2 to 5 minutes should provide data for a welldefined plot of tracer concentration versus time. If on-site analysis is available, less frequent 

residual monitoring may be possible until a change in residual concentration is first detected. 

Regular sampling is continued until the residual concentration reaches a steady-state value. 

One graphical method of evaluating step-dose test data involves plotting a graph of 

dimensionless concentration (tracer concentration (C) / applied tracer concentration (Co)) 

versus time and reading the value for T10 directly from the graph at the appropriate 

dimensionless concentration. Alternatively, the data from step-dose tracer studies may be 

evaluated numerically by developing a semi-logarithmic plot of the dimensionless data. The 

semi-logarithmic plot allows a straight line to be drawn through the data. The resulting 

equation of the line is used to calculate the T10 value, assuming that the correlation 

coefficient indicates a good statistical fit (0.9 or above). Drawing a smooth curve through 

the data discredits scattered data points from step-dose tracer tests. 
 Appendix E. Tracer Studies 

May 2003 163 EPA Guidance Manual 

 Disinfection Profiling and Benchmarking

Step-dose tracer studies are frequently employed in drinking water applications for the 

following reasons: 

• The resulting normalized concentration versus time profile is directly used to 

determine T10, the detention time required for calculating CT; and, 

• Very often, the necessary feed equipment is available to provide a constant rate of 

application of the tracer chemical. 

One other advantage of the step-dose method is that the data may be verified by comparing 

the concentration versus elapsed time profile for samples collected at the start of dosing with 

the profile obtained when the tracer feed is discontinued. 

E.5.2 Slug-Dose Method 

In the slug-dose method, a large instantaneous dose of tracer is added to the incoming water 

and samples are taken at the exit of the unit over time as the tracer passes through the unit. 

At time zero for the slug-dose method, a large instantaneous dose of tracer is added to the 

influent of the unit. The same sampling locations and frequencies described for step-dose 

method tests also apply to slug-dose method tracer studies. One exception with this method 

is that the tracer concentration profile will not equilibrate to a steady-state concentration. 

Because of this, the tracer should be monitored frequently enough to ensure acquisition of 

data needed to identify the peak tracer concentration. 

Slug-dose method tests should be checked by performing a material balance to ensure that 

all of the tracer fed is recovered, or mass applied equals mass discharged. 

Data from slug-dose tracer tests may be analyzed by converting it to the mathematically 

equivalent step-dose data and using the techniques discussed above for the step-dose method 

to determine T10. A graph of dimensionless concentration versus time should be drawn 

which represents the results of a slug-dose tracer test. The key to converting between the 

data forms is obtaining the total area under the slug-dose data curve. This area is found by 

integrating the curve graphically or numerically. The conversion to step-dose data is then 

completed in several mathematical steps involving the total area. 

Slug-dose concentration profiles can have many shapes, depending on the hydraulics of the 

basin. Therefore, slug-dose data points should not be discredited by drawing a smooth curve 

through the data prior to its conversion to step-dose data. 
Appendix E. Tracer Studies 

EPA Guidance Manual 164 May 2003 

Disinfection Profiling and Benchmarking 

A disadvantage of the slug-dose method is that very concentrated solutions are needed for 

the dose in order to adequately define the concentration versus time profile. Intensive 

mixing is therefore necessary to minimize potential density-current effects and to obtain a 

uniform distribution of the instantaneous tracer dose across the basin. This is inherently 

difficult under water flow conditions often existing at inlets to basins. Other disadvantages 

of using the slug-dose method include: 

• The concentration and volume of the instantaneous tracer dose needs to be carefully 

computed to provide an adequate tracer profile at the effluent of the basin;

• The resulting concentration versus time profile should not be used to directly 

determine T10 without further manipulation; and,

• A mass balance on the treatment segment should be used to determine whether the 

tracer was completely recovered.

One advantage of this method is that it may be applied where chemical feed equipment is 

not available at the desired point of addition, or where the equipment available does not 

have the capacity to provide the necessary concentration of the chosen tracer chemical. 

Although, in general, the step-dose procedure offers the greatest simplicity, both methods 

are theoretically equivalent for determining T10. Either method or another method may be 

used for conducting drinking water tracer studies, and the choice of method may be 

determined by site-specific constraints or the system’s experience. 

E.6 TRACER SELECTION

An important step in any tracer study is the selection of a chemical to be used as the tracer. 

Ideally, the selected tracer chemical should be readily available, conservative (that is, not 

consumed or removed during treatment), easily monitored, and acceptable for use in potable 

water supplies. Chloride and fluoride are the most common tracer chemicals employed in 

drinking water plants that are nontoxic and approved for potable water use. Rhodamine WT 

can be used as a fluorescent tracer in water flow studies in accordance with the following 

guidelines: 

• Raw water concentrations should be limited to a maximum concentration of 10 

mg/L; 

• Drinking water concentrations should not exceed 0.1 μg/L; 

• Studies that result in human exposure to the dye should be brief and infrequent; and, 

• Concentrations as low as 2 μg/L can be used in tracer studies because of the low 

detection level in the range of 0.1 to 0.2 μg/L. 

The use of Rhodamine B as a tracer in water flow studies is not recommended by the EPA. 
 Appendix E. Tracer Studies 

May 2003 165 EPA Guidance Manual 

 Disinfection Profiling and Benchmarking

The choice of a tracer chemical can be made based, in part, on the selected dosing method 

and on the availability of chemical feeding equipment. For example, the high density of 

concentrated salt solutions and their potential for inducing density currents usually precludes 

chloride and fluoride as the selected chemical for slug-dose tracer tests. 

Fluoride can be a convenient tracer chemical for step-dose tracer tests of clearwells because 

it is frequently applied for finished water treatment. However, when fluoride is used in 

tracer tests on clarifiers, allowances should be made for fluoride that is absorbed on floc and 

settles out of water (Hudson, 1975). Additional considerations when using fluoride in tracer 

studies include: 

• It is difficult to detect at low levels, 

• Many states impose a finished water limitation of 1 mg/L; and, 

• The federal secondary and primary drinking water standards (i.e., the MCLs) for 

fluoride are 2 and 4 mg/L, respectively. 

For safety reasons, particularly for people on dialysis, fluoride is not recommended for use 

as a tracer in systems that normally do not fluoridate their water. The use of fluoride is only 

recommended in cases where the feed equipment is already in place. The system may wish 

to turn off the fluoride feed in the plant for 12 or more hours prior to beginning the fluoride 

feed for the tracer study. Flushing out fluoride residuals from the system prior to conducting 

the tracer study is recommended to reduce background levels and avoid spiked levels of 

fluoride that might exceed EPA’s MCL or SMCL for fluoride in drinking water. In 

instances where only one of two or more parallel units is tested, flow from the other units 

would dilute the tracer concentration prior to leaving the plant and entering the distribution 

system. Therefore, the impact of drinking water standards on the use of fluoride and other 

tracer chemicals can be alleviated in some cases. 
Appendix E. Tracer Studies 

EPA Guidance Manual 166 May 2003 

Disinfection Profiling and Benchmarking 

E.7 REFERENCES

EPA. 1991. Guidance Manual for Compliance with the Filtration and Disinfection 

Requirements for Public Water Systems using Surface Water Sources. Washington, D.C. 

Hudson, H.E., Jr. 1975. Residence Times in Pretreatment. Journal AWWA, 67(1): 45-52. 

Teefy, Susan. 1996. Tracer Studies in Water Treatment Facilities: A Protocol and Case 

Studies. American Water Works Association Research Foundation. Denver, CO. 
Appendix F 

Calculating the Volume of 

Each Sub-Unit
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Appendix F. Calculating the Volume of Each Sub-Unit 

May 2003 169 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Note: If dimensions are in feet and the volume is calculated in cubic feet, then the volume 

should be converted to gallons by using the conversion: 1 ft3

 = 7.48 gal. 

Water Pipe (raw or treated): 

Fluid Volume = Length x Cross-Sectional Area (Assumes full-pipe flow) 

Rectangular Basin: 

Fluid Volume = Length x Width x

Minimum Water Depth 

Cylindrical Basin: 

Fluid Volume = Minimum Water Depth x Cross-Sectional Area 

 

Minimum Water Depth

Water Level

Length 

Side View 

Length 

Width

Cross-Sectional Area = 3.1416 * r2

 r = inner radius = d / 2 

d = inner diameter

Top View 

d 

r

d 

Cross-Sectional Area = 3.1416 * r2

 r = inner radius = d / 2 

d = inner diameter

Cross-Section View 

r

Side View 

Minimum 

Water Depth

Water Level
Appendix F. Calculating the Volume of Each Sub-Unit

EPA Technical Guidance Manual 170 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

 Filters 

 Fluid Volume = Volume of Water Above Filter Surface 

= Length x Width x Depth of Water Above Filter Surface 

Note: Some States may give credit for volume in media. Check with the State for the 

appropriate method to use for calculating volume in media. 

Width 

Length 

Depth of 

Water Above 

Filter Surface
Appendix G 

Baffling Factors
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G.1 INTRODUCTION

Information in this appendix is based on Appendix C in the Guidance Manual for 

Compliance with the Filtration and Disinfection Requirements for Public Water Systems 

Using Surface Water Sources (EPA, 1991). References to the main body of the report, 

section headers, and some terminology have been modified to relate better to the content of 

this Disinfection Profiling and Benchmarking Technical Guidance Manual. (Note: T10 is 

referred to as “T” elsewhere in this document. However, for consistency with the 

Guidance Manual for Compliance with the Filtration and Disinfection Requirements for 

Public Water Systems Using Surface Water Sources (EPA, 1991), T10 is used in this 

appendix.) 

In some situations, conducting tracer studies for determining the disinfectant contact time, 

T10, may be impractical or prohibitively expensive. The limitations may include a lack of 

funds, personnel, or equipment necessary to conduct the study. States may allow the use of 

“rule of thumb” fractions representing the ratio of T10 to T, and the theoretical detention 

time (TDT), to determine the detention time, T10, to be used for calculating CT values. This 

method for finding T10 involves multiplying the TDT by the rule of thumb fraction, T10/T, 

which is representative of the particular basin configuration for which T10 is desired. These 

fractions provide rough estimates of the actual T10 and systems should coordinate with their 

State when selecting a baffling factor. 

Tracer studies conducted by Marske and Boyle (1973) and Hudson (1975) on chlorine 

contact chambers and flocculators/settling basins, respectively, were used as a basis in 

determining representative T10/T values for various basin configurations. Marske and Boyle 

(1973) performed tracer studies on 15 distinctly different types of full-scale chlorine contact 

chambers to evaluate design characteristics that affect the actual detention time. Hudson 

(1975) conducted 16 tracer tests on several flocculation and settling basins at six water 

treatment plants to identify the effect of flocculator baffling and settling basin inlet and 

outlet design characteristics on the actual detention time. 

G.2 IMPACT OF DESIGN CHARACTERISTICS

The significant design characteristics include length-to-width ratio, the degree of baffling 

within the basins, and the effect of inlet baffling and outlet weir configuration. These 

physical characteristics of the contact basins affect their hydraulic efficiencies in terms of 

dead space, plug flow, and mixed flow proportions. The dead space zone of a basin is basin 

volume through which no flow occurs. The remaining volume where flow occurs is 

comprised of plug flow and mixed flow zones. The plug flow zone is the portion of the 

remaining volume in which no mixing occurs in the direction of flow. The mixed flow zone 

is characterized by complete mixing in the flow direction and is the complement to the plug 

flow zone. All of these zones were identified in the studies for each contact basin. 

Comparisons were then made between the basin configurations and the observed flow 

conditions and design characteristics. 
Appendix G. Baffling Factors 

EPA Guidance Manual 174 May 2003 

Disinfection Profiling and Benchmarking 

The ratio T10/T was calculated from the data presented in the studies and compared to its 

associated hydraulic flow characteristics. Both studies resulted in T10/T values that ranged 

from 0.3 to 0.7. The results of the studies indicate how basin baffling conditions can 

influence the T10/T ratio, particularly baffling at the inlet and outlet to the basin. As the 

basin baffling conditions improved, higher T10/T values were observed, with the outlet 

conditions generally having a greater impact than the inlet conditions. 

As discovered from the results of the tracer studies performed by Marske and Boyle (1973) 

and Hudson (1975), the effectiveness of baffling in achieving a high T10/T fraction is more 

related to the geometry and baffling of the basin than the function of the basin. For this 

reason, T10/T values may be defined for five levels of baffling conditions rather than for 

particular types of contact basins. General guidelines were developed relating the T10/T 

values from these studies to the respective baffling characteristics. These guidelines can be 

used to determine the T10 values for specific basins. 

G.3 BAFFLING CLASSIFICATIONS

The purpose of baffling is to maximize utilization of basin volume, increase the plug flow 

zone in the basin, and minimize short circuiting. Some form of baffling at the inlet and 

outlet of the basins is used to evenly distribute flow across the basin. Additional baffling 

may be provided within the interior of the basin (intra-basin) in circumstances requiring a 

greater degree of flow distribution. Ideal baffling design reduces the inlet and outlet flow 

velocities, distributes the water as uniformly as practical over the cross section of the basin, 

minimizes mixing with the water already in the basin, and prevents entering water from 

short circuiting to the basin outlet as the result of wind or density current effects. Five 

general classifications of baffling conditions – unbaffled, poor, average, superior, and 

perfect (plug flow) - were developed to categorize the results of the tracer studies for use in 

determining T10 from the TDT of a specific basin. The T10/T fractions associated with each 

degree of baffling are summarized in Table G-1. Factors representing the ratio between T10

and the TDT for plug flow in pipelines and flow in a completely mixed chamber have been 

included in Table G-1 for comparative purposes. However, in practice the theoretical T10/T 

values of 1.0 for plug flow and 0.1 for mixed flow are seldom achieved because of the effect 

of dead space. Conversely, the T10/T values shown for the intermediate baffling conditions 

already incorporate the effect of the dead space zone, as well as the plug flow zone, because 

they were derived empirically rather than from theory. 
 Appendix G. Baffling Factors

May 2003 175 EPA Guidance Manual 

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Table G-1. Baffling Classifications 

Baffling Condition T10/T Baffling Description 

Unbaffled (mixed flow) 0.1 

None, agitated basin, very low length to 

width ratio, high inlet and outlet flow 

velocities. 

Poor 0.3 Single or multiple unbaffled inlets and 

outlets, no intra-basin baffles. 

Average 0.5 Baffled inlet or outlet with some intra-basin 

baffles. 

Superior 0.7 

Perforated inlet baffle, serpentine or 

perforated intra-basin baffles, outlet weir or 

perforated launders. 

Perfect (plug flow) 1.0 

Very high length to width ratio (pipeline 

flow), perforated inlet, outlet, and intrabasin baffles. 

As indicated in Table G-1, poor baffling conditions consist of an unbaffled inlet and outlet 

with no intra-basin baffling. Average baffling conditions consist of intra-basin baffling and 

either a baffled inlet or outlet. Superior baffling conditions consist of at least a baffled inlet 

and outlet, and intra-basin baffling to redistribute the flow throughout the basin’s crosssection. 

The three basic types of basin inlet baffling configurations are a target-baffled pipe inlet, an 

overflow weir entrance, and a baffled submerged orifice or port inlet. Typical intra-basin 

baffling structures include diffuser (perforated) walls; launders; cross, longitudinal, or maze 

baffling to cause horizontal and/or vertical serpentine flow; and longitudinal divider walls, 

which prevent mixing by increasing the length-to-width ratio of the basin(s). Commonly 

used baffled outlet structures include free-discharging weirs, such as sharp-crested and 

multiple V-notch, and submerged ports or weirs. Weirs that do not span the width of the 

contact basin, such as Cipolleti weirs, should not be considered baffling as their use may 

substantially increase weir overflow rates and the dead space zone of the basin. 

G.4 EXAMPLES OF BAFFLING

Examples of these levels of baffling conditions for rectangular and circular basins are 

explained and illustrated in this section. Typical uses of various forms of baffled and 

unbaffled inlet and outlet structures are also illustrated. 

The plan and section of a rectangular basin with poor baffling conditions, which can be 

attributed to the unbaffled inlet and outlet pipes, are illustrated in Figure G-1. The flow 

pattern shown in the plan view indicates straight-through flow with dead space occurring in 

the regions between the individual pipe inlets and outlets. The section view reveals 

additional dead space from a vertical perspective in the upper inlet and lower outlet corners 
Appendix G. Baffling Factors 

EPA Guidance Manual 176 May 2003 

Disinfection Profiling and Benchmarking 

of the contact basin. Vertical mixing also occurs as bottom density currents induce a 

counter-clockwise flow in the upper water layers. 

The inlet flow distribution is markedly improved by the addition of an inlet diffuser wall and 

intra-basin baffling as shown in Figure G-2. However, only average baffling conditions are 

achieved for the basin as a whole because of the inadequate outlet structure - a Cipolleti 

weir. The width of the weir is short in comparison with the width of the basin. 

Consequently, dead space exists in the corners of the basin, as shown by the plan view. In 

addition, the small weir width causes a high weir overflow rate, which results in short 

circuiting in the center of the basin. 
 Appendix G. Baffling Factors

May 2003 177 EPA Guidance Manual 

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Figure G-1. Poor Baffling Conditions- Rectangular Contact Basin 

Plan View

Section View
Appendix G. Baffling Factors 

EPA Guidance Manual 178 May 2003 

Disinfection Profiling and Benchmarking 

Figure G-2. Average Baffling Conditions- Rectangular Contact Basin 

Plan View

Section View
 Appendix G. Baffling Factors

May 2003 179 EPA Guidance Manual 

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Superior baffling conditions are exemplified by the flow pattern and physical characteristics 

of the basin shown in Figure G-3. The inlet to the basin consists of submerged, targetbaffled ports. This inlet design serves to reduce the velocity of the incoming water and 

distribute it uniformly throughout the basin’s cross-section. The outlet structure is a sharpcrested weir that extends for the entire width of the contact basin. This type of outlet 

structure will reduce short circuiting and decrease the dead space fraction of the basin, 

although the overflow weir does create some dead space at the lower corners of the effluent 

end. 

Figure G-3. Superior Baffling Conditions- Rectangular Contact Basin 

Plan View

Section View
Appendix G. Baffling Factors 

EPA Guidance Manual 180 May 2003 

Disinfection Profiling and Benchmarking 

The plan and section of a circular basin with poor baffling conditions, which can be 

attributed to flow short circuiting from the center feed well directly to the effluent trough are 

shown in Figure G-4. Short circuiting occurs in spite of the outlet weir configuration 

because the center feed inlet is not baffled. The inlet flow distribution is improved 

somewhat in Figure G-5 by the addition of an annular ring baffle at the inlet which causes 

the inlet flow to be distributed throughout a greater portion of the basin’s available volume. 

However, the baffling conditions in this contact basin are only average because the inlet 

center feed arrangement does not entirely prevent short circuiting through the upper levels of 

the basin. 

Figure G-4. Poor Baffling Conditions- Circular Contact Basin 

Plan View

Section View
 Appendix G. Baffling Factors

May 2003 181 EPA Guidance Manual 

 Disinfection Profiling and Benchmarking

Figure G-5. Average Baffling Conditions- Circular Contact Basin

Plan View

Section View
Appendix G. Baffling Factors 

EPA Guidance Manual 182 May 2003 

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Superior baffling conditions are attained in the basin configuration shown on Figure G-6 

through the addition of a perforated inlet baffle and submerged orifice outlet ports. As 

indicated by the flow pattern, more of the basin’s volume is utilized due to uniform flow 

distribution created by the perforated baffle. Short circuiting is also minimized because only 

a small portion of flow passes directly through the perforated baffle wall from the inlet to 

the outlet ports. 

Figure G-6. Superior Baffling Conditions- Circular Contact Basin 

Plan View

Section View
 Appendix G. Baffling Factors

May 2003 183 EPA Guidance Manual 

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G.5 ADDITIONAL CONSIDERATIONS

Flocculation basins and ozone contactors represent water treatment processes with slightly 

different characteristics from those presented in Figures G-1 through G-6 because of the 

additional effects of mechanical agitation and mixing from ozone addition, respectively. 

Studies by Hudson (1975) indicated that a single-compartment flocculator had a T10/T value 

less than 0.3, corresponding to a dead space zone of about 20 percent and a very high mixed 

flow zone of greater than 90 percent. In this study, two four-compartment flocculators, one 

with and the other without mechanical agitation, exhibited T10/T values in the range of 0.5 to 

0.7. This observation indicates that not only will compartmentation result in higher T10/T 

values through better flow distribution, but also that the effects of agitation intensity on 

T10/T are reduced where sufficient baffling exists. Therefore, regardless of the extent of 

agitation, baffled flocculation basins with two or more compartments should be considered 

to possess average baffling conditions (T10/T = 0.5), whereas unbaffled, single-compartment 

flocculation basins are characteristic of poor baffling conditions (T10/T = 0.3). 

Similarly, multiple stage ozone contactors are baffled contact basins which show 

characteristics of average baffling conditions. Single stage ozone contactors should be 

considered as being poorly baffled. However, circular turbine ozone contactors may exhibit 

flow distribution characteristics that approach those of completely mixed basins, with a 

T10/T of 0.1, as a result of the intense mixing. 

In many cases, settling basins are integrated with flocculators. Data from Hudson (1975) 

indicates that poor baffling conditions at the flocculator/settling basin interface can result in 

backmixing from the settling basin to the flocculator. Therefore, settling basins that have 

integrated flocculators without effective inlet baffling should be considered as poorly 

baffled, with a T10/T of 0.3, regardless of the outlet conditions, unless intra-basin baffling is 

employed to redistribute flow. If intra-basin and outlet baffling is utilized, then the baffling 

conditions should be considered average with a T10/T of 0.5. 

Filters are special treatment units because their design and function is dependent on flow 

distribution that is completely uniform. Except for a small portion of flow that short circuits 

the filter media by channeling along the walls of the filter, filter media baffling provides a 

high percentage of flow uniformity and can be considered superior baffling conditions for 

the purpose of determining T10. As such, the T value can be obtained by subtracting the 

volume of the filter media, support gravel, and underdrains from the total volume and 

calculating the TDT by dividing this volume by the flow through the filter (Check with the 

State on what volume may be allowed in a filter). The TDT may then be multiplied by a 

factor of 0.7, corresponding to superior baffling conditions, to determine the T10 value. 

G.6 CONCLUSIONS

The recommended T10/T values and examples are presented as a guideline for use by the 

State in determining T10. Conditions that are combinations or variations of the above 

examples may exist and warrant the use of intermediate T10/T values such as 0.4 or 0.6. As 
Appendix G. Baffling Factors 

EPA Guidance Manual 184 May 2003 

Disinfection Profiling and Benchmarking 

more data on tracer studies become available, specifically correlations between other 

physical characteristics of basins and the flow distribution efficiency parameters, further 

refinements to the T10/T fractions and definitions of baffling conditions may be appropriate. 
 Appendix G. Baffling Factors

May 2003 185 EPA Guidance Manual 

 Disinfection Profiling and Benchmarking

G.7 REFERENCES

EPA. 1991. Guidance Manual for Compliance with the Filtration and Disinfection 

Requirements for Public Water Systems using Surface Water Sources. Washington, D.C. 

Hudson, H.E., Jr. 1975. Residence Times in Pretreatment. Journal AWWA, 67(1): 45-52. 

Marske, D.M. and J.D. Boyle. 1973. Chlorine Contact Chamber Design – A Field 

Evaluation. Water and Sewage Works, January, pp. 70-77. 
Appendix G. Baffling Factors 

EPA Guidance Manual 186 May 2003 

Disinfection Profiling and Benchmarking 

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Appendix H 

Conservative Estimate and 

Interpolation Examples
This Page Intentionally Left Blank
Appendix H. Conservative Estimate and Interpolation Examples

May 2003 189 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

In some instances, the collected data for the disinfection profile will not coincide exactly 

with the values in the CT tables. The following examples present two methods on how to 

obtain CT99.9 values. Systems should check with the State if these methods are acceptable 

for obtaining CT99.9.
Appendix H. Conservative Estimate and Interpolation Examples

EPA Guidance Manual 190 May 2003 

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Appendix H. Conservative Estimate and Interpolation Examples

May 2003 191 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example H-1: Conservative Estimate Example for Obtaining CT99.9

Distribution

System

Sedimentation

Coagulation

Filtration

Clearwell

Monitoring Point

Cl2 residual = 0.9 mg/L

Temperature = 6 oC

pH = 6.7

One Disinfection Segment:

One injection point, one monitoring point

Intake

Chlorine

Injected

Flocculation

This example will demonstrate one method, Conservative Estimate, for obtaining CT99.9 

when collected data values are between values on the CT table. In this example a 

conventional filtration treatment system added chlorine prior to the clearwell and it was 

required to create a profile. The system must determine the Giardia log inactivation 

achieved through disinfection. 

A. Determine the required CT99.9 necessary to obtain 3-log Giardia 

inactivation. 

The required CT value for 3-log Giardia inactivation (CT99.9) may be obtained using CT 

Table B-1 in Appendix B, CT Values for 3-Log Inactivation of Giardia Cysts by Free 

Chlorine. 

Step 1. Round the temperature value. 

Since the temperature of 6 oC is not shown in the table, the next lowest temperature 

on the table, 5 o

C, is used to obtain a conservative estimate of CT99.9. The lower 

temperature value was chosen since chlorine is less effective at lower temperatures.

Step 2. Round the pH value. 

Since the pH of 6.7 is not shown in the table, the next highest pH, 7.0, is used to 

obtain a conservative estimate of CT99.9. The higher pH value was chosen since 

chlorine is less effective at a higher pH.
Appendix H. Conservative Estimate and Interpolation Examples

EPA Guidance Manual 192 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example H-1 continued 

Step 3. Round the residual chlorine concentration value. 

Since the residual chlorine concentration of 0.9 mg/L is not shown on the table, the 

next highest residual chlorine concentration, 1.0 mg/L, is used to obtain a 

conservative estimate of CT99.9. A higher residual chlorine concentration is used to 

obtain a higher required CT99.9 value, which will result in a lower calculated log 

inactivation ratio value. 

Step 4. Determine CT99.9. 

In this example the CT99.9 is 149 min-mg/L for a pH of 7.0, temperature of 5 oC, and Cchlorine

of 1.0 mg/L. The relevant section of Table B-1 is reprinted below and the pertinent section 

of the table is highlighted. 

Excerpt from Table B-1:

CT values for 3-Log Inactivation of Giardia Cysts by Free Chlorine (5°C portion of table for 

0.4 to 1.2 mg/L) 

Temperature 5 oC 

pH 

Chlorine 

Concentration 

(mg/L) 

<=6.0 6.5 7.0 7.5 8.0 8.5 9.0 

<=0.4 97 117 139 166 198 236 279 

0.6 100 120 143 171 204 244 291 

0.8 103 122 146 175 210 252 301 

1.0 105 125 149 179 216 260 312 

1.2 107 127 152 183 221 267 320 
Appendix H. Conservative Estimate and Interpolation Examples

May 2003 193 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example H-2: Interpolation Example for Obtaining CT99.9

Distribution

System

Sedimentation

Coagulation

Filtration

Clearwell

Monitoring Point

Cl2 residual = 0.9 mg/L

Temperature = 6 oC

pH = 6.7

One Disinfection Segment:

One injection point, one monitoring point

Intake

Chlorine

Injected

Flocculation

This example will demonstrate another method, interpolation, for obtaining CT99.9 when 

collected data values are between values on the CT table. In this example a conventional 

filtration treatment system added chlorine prior to the clearwell and it was required to create 

a profile. The system must determine the Giardia log inactivation achieved through 

disinfection. 

A. Determine the required CT99.9 necessary to obtain 3-log Giardia 

inactivation. 

The required CT value for 3-log Giardia inactivation (CT99.9) may be obtained using CT 

Table B-1 in Appendix B, CT Values for 3-Log Inactivation of Giardia Cysts by Free 

Chlorine. Since the temperature of 6 o

C, the pH of 6.7, and the residual chlorine 

concentration of 0.9 mg/L are not shown on the table, interpolation is used to determine the 

CT99.9 value. 
Appendix H. Conservative Estimate and Interpolation Examples

EPA Guidance Manual 194 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example H-2 continued 

Step 1. Interpolate for CT99.9 at pH of 6.7 at the next lowest temperature of 5 oC and the 

next lowest residual chlorine concentration of 0.8 mg/L. 

Excerpt from Table B-1:

CT Values for 3-Log Inactivation of Giardia Cysts by Free Chlorine (5°C portion of 

table for 0.4 to 1.2 mg/L) 

Temperature 5 oC 

pH 

Chlorine 

Concentration 

(mg/L) 

<=6.0 6.5 7.0 7.5 8.0 8.5 9.0 

<=0.4 97 117 139 166 198 236 279 

0.6 100 120 143 171 204 244 291 

0.8 103 122 146 175 210 252 301 

1.0 105 125 149 179 216 260 312 

1.2 107 127 152 183 221 267 320 

 (CT99.9 at pH 7.0) – (CT99.9 at pH 6.5) = (CT99.9 at pH 6.7) – (CT99.9 at pH 6.5) 

 pH 7.0 – pH 6.5 pH 6.7 – pH 6.5 

 146 min-mg/L – 122 min-mg/L = (CT99.9 at pH 6.7) – 122 min-mg/L 

 7.0 – 6.5 6.7 – 6.5 

 24 min-mg/L = (CT99.9 at pH 6.7) – 122 min-mg/L 

 0.5 0.2 

 24 min-mg/L x 0.2 = (CT99.9 at pH 6.7) –122 min-mg/L 

 0.5 

 9.6 min-mg/L = (CT99.9 at pH 6.7) – 122 min-mg/L 

 CT99.9 at pH 6.7 = 9.6 min-mg/L + 122 min-mg/L 

 

CT99.9 at pH 6.7 = 131.6 min-mg/L 
Appendix H. Conservative Estimate and Interpolation Examples

May 2003 195 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example H-2 continued

Step 2. Interpolate for CT99.9 at pH of 6.7 at the next highest temperature of 10 oC and the 

next lowest residual chlorine concentration of 0.8 mg/L. 

Excerpt from Table B-1:

CT Values for 3-Log Inactivation of Giardia Cysts by Free Chlorine (10°C portion of 

table for 0.4 to 1.2 mg/L) 

Temperature 10 oC 

pH 

Chlorine 

Concentration 

(mg/L) 

<=6.0 6.5 7.0 7.5 8.0 8.5 9.0 

<=0.4 73 88 104 125 149 177 209 

0.6 75 90 107 128 153 183 218 

0.8 78 92 110 131 158 189 226 

1.0 79 94 112 134 162 195 234 

1.2 80 95 114 137 166 200 240 

 (CT99.9 at pH 7.0) – (CT99.9 at pH 6.5) = (CT99.9 at pH 6.7) – (CT99.9 at pH 6.5) 

 pH 7.0 – pH 6.5 pH 6.7 – pH 6.5 

 110 min-mg/L – 92 min-mg/L = (CT99.9 at pH 6.7) – 92 min-mg/L 

 7.0 – 6.5 6.7 – 6.5 

 18 min-mg/L = (CT99.9 at pH 6.7) – 92 min-mg/L 

 0.5 0.2 

 18 min-mg/L x 0.2 = (CT99.9 at pH 6.7) – 92 min-mg/L 

 0.5 

 7.2 min-mg/L = (CT99.9 at pH 6.7) – 92 min-mg/L 

 CT99.9 at pH 6.7 = 7.2 min-mg/L + 92 min-mg/L 

 

CT99.9 at pH 6.7 = 99.2 min-mg/L 
Appendix H. Conservative Estimate and Interpolation Examples

EPA Guidance Manual 196 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example H-2 continued

Step 3. Interpolate for CT99.9 at pH of 6.7, temperature of 6oC, and the next lowest 

residual chlorine concentration of 0.8 mg/L. 

The table below summarizes the CT99.9 values determined at a pH of 6.7, residual 

chlorine concentration of 0.8 mg/L, and temperatures of 5 o

C and 10 oC. 

pH = 6.7 

Temperature Chlorine 

Concentration 5 o

C 10 oC 

0.8 mg/L 131.6 min-mg/L 99.2 min-mg/L 

 (CT99.9 at 10 oC) – (CT99.9 at 5 oC) = (CT99.9 at 6 oC) – (CT99.9 at 5 oC) 

 10 o

C – 5 oC 6 oC – 5 oC 

99.2 min-mg/L – 131.6 min-mg/L = (CT99.9 at 6 oC) – 131.6 min-mg/L 

 10 oC – 5 oC 6 oC – 5 oC 

-32.4 min-mg/L = (CT99.9 at 6 oC) – 131.6 min-mg/L 

 5 o

C 1 oC 

-32.4 min-mg/L x 1 o

C = (CT99.9 at 6 oC) – 131.6 min-mg/L 

 5 o

C 

-6.48 min-mg/L = (CT99.9 at 6 oC) – 131.6 min-mg/L 

CT99.9 at 6 o

C = -6.48 min-mg/L + 131.6 min-mg/L 

CT99.9 at 6 o

C = 125.1 min-mg/L

CT99.9 at a pH of 6.7, temperature of 6o

C, and residual chlorine concentration of 

0.8 mg/L is 125.1 min-mg/L. 
Appendix H. Conservative Estimate and Interpolation Examples

May 2003 197 EPA Guidance Manual 

 LT1ESWTR Disinfection Profiling and Benchmarking

Example H-2 continued

Step 4. Repeat steps 1 through 3 at the same pH and temperatures, but with a residual 

chlorine concentration of 1.0 mg/L.

The results are summarized in the table, below. 

pH Temperature 

(

o

C) 

Residual Chlorine 

Conc. (mg/L) 

CT99.9

(min-mg/L) 

6.7 5 1.0 134.6 

6.7 10 1.0 101.2 

6.7 6 1.0 127.9 

CT99.9 at a pH of 6.7, temperature of 6oC, and residual chlorine concentration of 

1.0 mg/L is 127.9 min-mg/L.

Step 5. Interpolate for CT99.9 at pH of 6.7, temperature of 6o

C, and residual chlorine 

concentration of 0.9 mg/L.

The table below summarizes the CT99.9 values determined at a pH of 6.7, temperature 

of 6o

C, and residual chlorine concentrations of 0.8 mg/L and 1.0 mg/L. 

pH = 6.7 

Chlorine Residual Conc. 

Temperature 0.8 mg/L 1.0 mg/L 

6oC 125.1 min-mg/L 127.9 min-mg/L 

(CT99.9 at 1.0 mg/L) – (CT99.9 at 0.8 mg/L) = (CT99.9 at 0.9 mg/L) – (CT99.9 at 0.8 mg/L) 

 1.0 mg/L – 0.8 mg/L 0.9 mg/L – 0.8 mg/L 

 

127.9 min-mg/L – 125.1 min-mg/L = (CT99.9 at 0.9 mg/L) – 125.1 min-mg/L 

 1.0 mg/L – 0.8 mg/L 0.9 mg/L – 0.8 mg/L 

2.8 min-mg/L = (CT99.9 at 0.9 mg/L) – 125.1 min-mg/L 

 0.2 mg/L 0.1 mg/L 

2.8 min-mg/L x 0.1 mg/L = (CT99.9 at 0.9 mg/L) – 125.1 min-mg/L 

 0.2 mg/L 

1.4 min-mg/L = (CT99.9 at 0.9 mg/L) – 125.1 min-mg/L 

CT99.9 at 0.9 mg/L = 1.4 min-mg/L + 125.1 min-mg/L 
Appendix H. Conservative Estimate and Interpolation Examples

EPA Guidance Manual 198 May 2003 

LT1ESWTR Disinfection Profiling and Benchmarking 

Example H-2 continued 

CT99.9 at 0.9 mg/L = 126.5 min-mg/L 

CT99.9 at a temperature of 6o

C, pH of 6.7, and residual chlorine concentration of 

0.9 mg/L is 126.5 min-mg/L.