United States
Environmental Protection
Agency
Office of Water
(4607)
EPA 815-R-99-013
August 1999
Disinfection Profiling and
Benchmarking
Guidance Manual
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DISCLAIMER
This manual describes the practice of disinfection profiling and benchmarking as required
under the U.S. Environmental Protection Agency's (EPA) Interim Enhanced Surface
Water Treatment Rule (D3SWTR) promulgated December 16, 1998. Disinfection
profiling and benchmarking are procedures to ensure that microbial inactivation is not
significantly reduced due to implementation of the Stage 1 Disinfectant and Disinfection
Byproduct Rule (DBPR) also promulgated on December 16,1998.
This document was issued in support of EPA regulations and policy initiatives involving
development and implementation of the IESWTR and DBPR. This document is EPA
guidance only. It does not establish or affect legal rights or obligation. EPA decisions in
any particular case will be made applying the laws and regulation on the basis of specific
facts when permits are issued or regulations promulgated.
Mention of trade names or commercial products does not constitute an EPA endorsement
or recommendation for use.
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ACKNOWLEDGMENTS
The Environmental Protection Agency gratefully acknowledges the assistance of the
members of the Microbial and Disinfection Byproducts Federal Advisory Committee and
Technical Work Group for their comments and suggestions to improve this document.
EPA also wishes to thank the representatives of drinking water utilities, researchers, and
the American Water Works Association for their review and comment. In particular, the
EPA would like to recognize the following individuals for their contributions:
Sarah Clark, City of Austin
Charlotte Smith, CS&A
Blake Atkins, EPA
Ralph Flournoy, EPA
Thomas Grubbs, EPA
Stig Regli, EPA
Brian Black, HDR Engineering
Faysal Bekdash, SAIC
Jennifer Cohen, SAIC
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CONTENTS
EXECUTIVE SUMMARY . ES-1
1. INTRODUCTION ..;.......... 1-1
1.1 Disinfection Profiling and Benchmarking 1-2'
1.2 Purpose of Disinfection Profiling and Benchmarking 1-3
1.2.1 Disinfection Profiling; Definition and Purpose 1-3
1.2.2 Disinfection Benchmarking: Definition and Purpose 1-3
1.3 State Review :... 1-4
1.4 Primary Information Sources 1-4
2. APPLICABILITY OF DISINFECTION PROFILING AND BENCHMARKING ....2-1
2.1 Systems Subject to the IESWTR 2-1
2.2 Profiling and Benchmarking Applicability 2-1
2.3 Systems Required to Profile Giardia 2-1
2.3.1 Giardia Profile 2-3
2.3.2 TTHM and HAAS Data Requirements 2-3
2.4 Systems Required to Benchmark Giardia 2-5
2.5 Systems Required to Profile and Benchmark Viruses :.. 2-5
3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS 3-1
3.1 Data for Profiling...... .....3-1
3.1.1 Operational Data Required for Profiling : 3-2
3.1.2 Data Quantity 3-2
3.1.3 Data Quality 3-3
3.2 Procedure to Determine Log Inactivation 3-3
3.2.1 Use of CT Values for Disinfection Profiling '. 3-3
3.2.2 Steps to Calculate Log Inactivation 3-4
3.2.3 Determining Disinfectant Residual Concentrations, pH, and Temperature 3-5
3.2.4 Determining Contact Time, Tw 3-8
3.3 Monitoring Procedures 3-14
3.3.1 Defining Disinfection Segments 3-14
3.4 Calculating Estimated Log Inactivation 3-15
3.4.1 SWTR Log Inactivation CT Method 3-15
3.4.2 Determining CT3.iog> Giardia and CT4.log,vinis .....3-16"
3.4.3 Log Inactivation Calculations 3-20
3.4.4 Summing the Estimated Log Inactivations of each Segment to Determine
the Log Inactivation of the Plant 3-21
' 3.5 The Completed Profile 3-21
3.6 Examples of Estimating Log Inactivation of Giardia and Viruses for Conventional Filtration
Plants 3-24
3.6.1 Example of Developing a.Disinfection Profile for a 40 mgd Plant ; 3-25
3.6.2 Example of Developing a Disinfection Profile for a 5 mgd Plant for One Month' 3-37
3.6.3 Determination of Disinfection Profile and Benchmark 3-40
3.6.4 Modification of Disinfection Practice ..3-43
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4. CALCULATING THE BENCHMARK 4-1
4.1 Applicability 4-1
4.2 Benchmark Calculations 4-1
4.3 The Completed Benchmark 4-4
5. USING THE BENCHMARK 5-1
5.1 Definition: Modifying Disinfection Practices 5-1
5.1.1 Moving the Point of Disinfectant Application 5-2
5.1.2 Changing the Disinfectant(s) Used in the Treatment Plant 5-2
5.1.3 Changes to Disinfection Practices , 5-3
5.1.4 Other Modifications Identified by the State : 5-5
5.2 Communicating with the State 5-5
5.3 Calculations to Assess Modification Impact : 5-6
5.4 Alternative Benchmark 5-7
5.5 Illustrative Examples 5-7
5.5.1 DBP Control using Enhanced Coagulation 5-7
5.5.2 Treatment Changes for DBP Control When Enhanced Coagulation is Insufficient 5-11
5.5.3 Summary of Treatment Modification Strategies Impact on Disinfection
and DBP Control 5-19
6. ALTERNATIVE DISINFECTION BENCHMARK 6-1
6.1 Methodology •. 6-4
6.2 Schedule Guidance 6-11
6.3 Source Water Characterization 6-12
6.4 Watershed Control Program 6-14
7. REFERENCES —7-1
APPENDIX A HISTORY
APPENDIX B LOG INACTIVATION METHODS
APPENDIX C CT VALUES FOR BSfACTIVATIONS ACHIEVED BY VARIOUS
DISINFECTANTS
APPENDIX D DETERMINATION OF CONTACT TIME
APPENDED E USING THE REGRESSION METHOD
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FIGURES
Figure 2-1. Profile and Benchmark Decision Tree 2-2
Figure 3-1. Disinfection Profiling Methodology 3-6
Figure 3-2. 1994 Profiling Data '. 3-22
Figure 3-3. 1995 Profiling Data , 3-23
Figure-3-4. 1996 Profiling Data 3-23
Figure 3-5. 40 mgd Conventional Filtration Process Diagram 3-25
Figure 3-6. Log Giardia Inactivation for Existing Disinfection Practice 3-41
Figure 3-7. Log Virus Inactivation for Existing Disinfection Practice 3-42
Figure 3-8. Option 1 Process Diagram 3-45
Figure 3-9. Option 2 Process Diagram 3-46
Figure 3-10. Log Giardia Inactivation for Disinfection Option 1 '. 3-48
Figure 3-11. Log Giardia Inactivation for Disinfection Option 2 3-50
Figure 3-12. Log Virus Inactivation for Disinfection Option 2 : 3-51
Figure 5-1. Impact of DBP Control Strategies on Disinfection and Byproduct Formation 5-21
Figure 6-1. Range for Alternative Disinfection Benchmarks 6-4
Figure 6-2. Impact of Source Water Quality and Filtration Process on Giardia Alternative Disinfection
Benchmark 6-9
Figure 6-3. Impact of Source Water Quality and Filtration Process on Virus Alternative Disinfection
Benchmark 6-10
TABLES
Table 3-1. Acceptable Laboratory Methods for Analyses : 3-7
Table 3-2. Baffling Classifications and Factors 3-11
Table 3-3. Log Inactivations and Percent Inactivations 3-16
Table 3-4. Required CT Values (mg-min/L) for 3-log Inactivation of Giardia Cysts by Free Chlorine,
pH 6.0-9.0 3-19
Table 3-5. Required CT Values (mg-min/L) for 4-Log Inactivation of Viruses by Free Chlorine, pH6.0-
9.0 3-20
Table 3-6. Unit Process Design Conditions Summary 3-26
Table 3-7. Volume Equations '. 3-27
Table 3-8. Actual Readings From a SW Treatment Plant in Missouri 3-38
Table 3-9. Input and Output Data Used to Calculate Log Inactivations 3-39
Table 3-10. Critical Periods for Existing Disinfection Practice 3-40
Table 3-11. Example Log Inactivation Calculations for Multi-Stage Ozone Contactor 3-44
Table 3-12. Critical'Periods for Disinfection Option 1 3-47
Table 3-13. Critical Periods for Disinfection Option 2 3-49
Table 4-1. Daily Log Inactivation for Hypothetical Plant for January 1998'. 4-3
Table 4-2. Monthly Average Log Inactivation Values for Hypothetical Plant 4-4
Table 5-1. Strategies for Primary and Secondary Disinfectants 5-3
Table 5-2. Impacts of Disinfection Practice on DBP Formation 5-4
Table 5-3. Raw Water Quality (Plant A) , 5-8
Table 5-4. Base Condition Unit Processes (Plant A) .• 5-8
Table 5-5. System DBP Concentrations (Plant A) : 5-9
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Table 5-6. Proposed Required Removal of TOC by 5-9
Table 5-7. System DBF Concentrations with 5-10
Table 5-8. Impact of Enhanced Coagulation on Disinfection (Plant A) 5-11
Table 5-9. Raw Water Quality (Plant B) 5-11
Table 5-10. Base Condition Unit Processes (Plant B) 5-12
Table 5-11. System DBP Concentrations (Plant B) 5-12
Table 5-12. System DBP Concentrations with Enhanced Coagulation (Plant B) 5-13
Table 5-13, Impact of Enhanced Coagulation on Disinfection (Plant B) 5-14
Table 5-14. System DBP Concentrations After Enhanced Coagulation and Moving the
Point of Chlorination ; 5-15
Table 5-15. Impact of Moving Chlorine Application Point on Disinfection 5-15
Table 5-16. System DBP Concentrations Seasonal Chlorine Application Points 5-16
Table 5-17. Impact Of Moving Chlorine Application During The Summer Season 5-17
Table 5-18. Cumulative Impact of Settled Water Chlorination, Enhanced Coagulation
and Clearwell Baffling on Disinfection (Plant B) 5-18
Table 5-19. Summary Impacts of DBP Control Strategies Original Practice - Raw Water Chlorination. 5-19
Table 5-20. Impact of DBP Control Strategies on Disinfection and Byproduct Formation 5-20
Table 6-1. Log Removal Credits for Filtration 6-3
Table 6-2. Alternative Disinfection Benchmarks for Systems Not Monitoring 6-6
Table 6-3 Impact of Source Water Quality and Filtration Process on Alternative
DisinfectionBenchmark 6-8
Table 6-4. Example Schedule for Compliance with M/DBP Rules 6-12
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CONTENTS
ACRONYMS
AOC
ASDWA
AWWA
AWWARF
BAG
BAF
BAT
BDOC
BMP
CFR
CFU
CSO
CT
CWS
D/DBP
DBPR
DBF
DBPFP
DOC
DSE
EPA
IESWTR
GAC
gpm
GWR
GWSS
GWUDI
HAAS
ICR
IESWTR
IOA
M-DBP
MCL
Assimilable organic carbon
Association of State Drinking Water Administrators
American Water Works Association
AWWA Research Foundation
Biologically active carbon
Biologically active filtration
Best Available Technology
Biodegradable organic carbon
Best Management Practice
Dimensionless concentration
Code of Federal Regulations
Coliform forming units
Combined Sewer Overflow
Disinfectant residual concentration (C, in mg/L), multiplied by contact time (T, in min);
a measure of disinfection effectiveness.
Community Water System
Disinfectants and disinfection byproducts
Disinfectants and Disinfection Byproducts Rule
Disinfection byproduct
Disinfection byproduct formation potential
Dissolved organic carbon
Distribution system equivalent
United States Environmental Protection Agency
Interim Enhanced Surface Water Treatment Rule
Granular activated carbon
Gallons per minute
Ground Water Rule
Ground Water Supply Survey
Ground water under the direct influence
Five haloacetic acids
Information Collection Rule
Interim Enhanced Surface Water Treatment Rule
International Ozone Association
Microbial/disinfection byproducts
Maximum Contaminant Level
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MCLG
MDL
mg/L
mgd
MRDL
MRDLG
MRL
NIPDWR
NOM
No
NFS
N,
NTU
POE
POU
ppb
ppm
PWS
Q
RSC
SDWA
SM
sso
SWTR
Tio
TDT
THM
THMFP
TOC
TNRCC
TTHM
USEPA
UV
V
WHPA
WIDE
Maximum Contaminant Level Goal
Method Detection Limit
Milligrams per liter
Million Gallons per Day
Maximum Residual Disinfectant Level (as mg/L)
Maximum Residual Disinfectant Level Goal
Minimum Reporting Level
National Interim Primary Drinking Water Regulation
Natural Organic Matter
Influent concentration
Non-point source
Distribution system concentraion
Nepthelometric turbidity units
Point-of-Entry Technologies
Point-of-Use Technologies
Parts per billion
Parts per million
Public water system
Peak hourly flow rate
Relative Source Contribution
Safe Drinking Water Act
Standard Methods
Sanitary Sewer Overflow
Surface Water Treatment Rule
Contact time
Theoretical detention time
Trihalomethane
Trihalomethane formation potential
Total organic carbon
Texas Natural Resource Conservation Commission
Total trihalomethane
United States Environmental Protection Agency
Ultraviolet
Volume
Wellhead protection area
Water Industry Data Base
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EXECUTIVE SUMMARY
The objective of this guidance manual is to help Public Water Systems (PWSs) in
implementing the practice of disinfection profiling and benchmarking-as required under
the Interim Enhanced Surface Water Treatment Rule (IESWTR) promulgated December
16,1998. The IESWTR applies to surface water or Ground Water Under Direct
Influence (GWUDI) of surface water systems serving 10,000 people or more.
This guidance manual describes the applicability of the profiling and benchmarking
provisions to PWSs and details the procedures for generating a disinfection profile and
calculating the disinfection benchmark. Finally, this guidance manual provides guidance
to PWSs on determining "significant changes" to disinfection practices, communicating
with the State, and the use of the disinfection benchmark in modifying disinfection
practices.
The IESWTR defines a disinfection profile as a compilation of daily Giardia and/or virus
log inactivation over a period of a year or more. Disinfection benchmarking is a baseline
or benchmark of historical microbial inactivation practices developed from disinfection
profiling data.
Applicability
Systems are required to develop a disinfection profile for Giardia if their distribution
system DBP running annual average for either TTHM or HAAS concentrations in the
distribution system is greater than or equal to 0.064 mg/L or 0.048 mg/L, respectively.
Systems need one year of TTHM and HAA5 same time period data for disinfection
profile determination.
Systems that are required to profile and intend to "significantly" modify their disinfection
practice are required under the IESWTR to develop disinfection benchmarking for
Giardia. Significant changes to disinfection practices are defined under IESWTR as:
• Moving the point of disinfection
• Changing the type of disinfectant
• Changing the disinfection process
• Making any other change designated as significant by the State.
Systems planing to modify their disinfection practices by adding or switching
disinfectants to ozone or chloramines are required to develop a disinfection profile and
benchmark for viruses. Moreover, EPA strongly recommends that systems switching to
chlorine dioxide also develop a virus profile.
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Creating a Disinfection Profile
Systems required to develop a disinfection profile must:
» Conduct daily monitoring for a minimum period of one year by no later than
March 2001. .
• . And may also use 1 or 2 years of acceptable grandfathered data, in addition to
the 1-year of new operational data.
• Or may use grandfathered data to develop a 3-year disinfection profile.
Systems must coordinate with the State to confirm acceptability of
grandfathered data no later than March 2001, but must conduct the required
monitoring until the State approves the system's request to use grandfathered
data.
Use of CT Values for Disinfection Profiling
The Surface Wa|:er Treatment Rule (SWTR) requires physical removal and/or
inactivation of 3-logs (99.9 percent) ofGiardia and4-logs (99.99 percent) of viruses.
For disinfection profiling and benchmarking, the CT (see p. v for definition) approach
will be used to compute the log inactivation of Giardia or viruses achieved during water
treatment.
To use the SWTR CT tables, disinfectant type, temperature, and pH (for chlorine only)
data are needed. Using this operating information, the CT value corresponding to
inactivation of 3-logs of Giardia (CTs-iog, cianiia) and/or 4-logs of viruses (CT4.iog, virus) can
be read from the SWTR CT tables. Once the CT required to achieve 3-log inactivation of
Giardia and/or 4-log inactivation of viruses is determined, the actual plant CT needs to
be calculated. By determining contact time (Tio) for each treatment unit within a
disinfection segment (based on baffling factors or tracer studies) TIO is multiplied by
residual disinfectant concentration for the disinfection segment.
The plant log inactivation for Giardia and/or viruses is the sum of log inactivation for
each segment. From the daily estimated plant log inactivation data, a disinfection profile
can be created.
Determining the Benchmark
From the daily plant log inactivation records, systems need to compute the average log
inactivation for each calendar month. The lowest monthly average log inactivation values
for each 12-month period are then averaged to determine the benchmark. If one year of
data is available, the lowest monthly average log inactivation is the disinfection
benchmark.
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EXECUTIVE SUMMARY
Systems considering modifications to the disinfection practices can use the benchmark to
assess modification impacts. This assessment is done by calculating the "modification
benchmark" and comparing it to the current benchmark.
If the modification to disinfection practice results in a lower inactivation, an alternative
disinfection benchmark may improve a system's ability to meet the DBPR MCLs without
significantly compromising existing microbial protection.
Systems, under State guidance, may choose to develop an alternative benchmark that is
lower than the existing benchmark. For example, a system may choose to develop an
alternative benchmark when the system cannot simultaneously meet the disinfection
benchmark and the Stage 1 DBPR MCLs. The system may also choose this course of
action because of very high levels of microbial inactivation and/or high quality source
water that has low pathogen occurrence levels.
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1. Introduction
This manual is one in a series of guidance manuals published by EPA to assist both States
and Public Water Systems (PWSs) in complying with the Interim Enhanced Surface Water
Treatment Rule (IESWTR) and Stage 1 Disinfectant and Disinfection Byproduct Rule
(DBPR) drinking water regulations. Other EPA guidance manuals include:
. Alternative Disinfectants and Oxidants Guidance Manual (1999)
• Microbial and Disinfection Byproduct Simultaneous Compliance Guidance
Manual (1999)
• Uncovered Finished Water Reservoirs Guidance Manual (1999)
. Unfiltered Systems Guidance Manual (1999)
. Guidance Manual for Compliance with the Interim Enhanced Surface Water
Treatment Rule: Turbidity Provisions (1999)
. Guidance Manual for Conducting Sanitary Surveys of Public Water Systems;
Surface Water and Ground Water Under the Direct Influence (GWUDI) of
Surf ace Water (1999)
. Guidance Manual for Enhanced Coagulation and Enhanced Precipitative
Softening (1999).
This guidance manual describes the practice of disinfection profiling and benchmarking
as required under the U.S. Environmental Protection Agency's (EPA) IESWTR
promulgated December 16, 1998. This guidance manual will assist PWSs and States
with the implementation of the disinfection profiling and benchmarking provisions of the
IESWTR. As described in the IESWTR, these provisions are intended to ensure that
microbial inactivation is not unduly compromised as public water systems strive to meet
the Stage 1 DBPR.
This guidance manual is organized into several chapters and appendices which are
intended to accomplish the following:
. Defines disinfection profiling and benchmarking, State involvement, and
provides a list of primary resources of information used to develop this
guidance (Chapter 1).
. Describes the applicability of the profiling and benchmarking provisions to
public water systems (Chapter 2).
• Provides a description of the procedures for generating a disinfection profile
and provides an example profile (Chapter 3).
• Provides a description of the procedures for calculating the disinfection
benchmark and provides an example of a benchmark calculation (Chapter 4).
. Discusses the use of the benchmark in modifying disinfection practices,
communicating with the State, and assessing "significant changes" to
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1. INTRODUCTION
disinfection practices (Chapter 5).
• Discusses how a system may use an alternative benchmark in consultation
with the State to remain in compliance with the Stage 1 DBPR MCLs while
still not compromising microbial protection (Chapter 6).
• Provides an overview of the development of profiling and benchmarking
regulations (Appendix A).
• Explains the significance of the log inactivation concept (Appendix B).
• Provides the CT values for inactivations achieved by various disinfectants
(Appendix C).
• Presents discussions on the determination of contact time (Appendix D).
• Provides an example of the Regression Method in determining CT^.\QSjGiardia
(Appendix E).
1.1 Disinfection Profiling and Benchmarking
The IESWTR requires water systems to develop a disinfection profile if they exceed
certain disinfection byproduct (DBP) levels in their distribution system. Water systems
will have to develop a profile if their average total trihalomethane (TTHM) or five
haloacetic acids (HAAS) concentrations in the distribution system exceed specified
concentrations. Thus applicable PWSs must develop a disinfection profile if either of
the following conditions exist:
• The TTHM annual average, based on quarterly samples, is > 0.064 mg/L; or
• The HAAS annual average, based on quarterly samples, is > 0.048 mg/L.
The Microbial and Disinfection Byproduct (M-DBP) Advisory Committee recommended
a value of 80 percent of the maximum contaminant levels (MCLs) because available data
indicated that DBP levels varied from year to year due to many factors (i.e., changes in
source water quality, changes in water demand, etc.). The Advisory Committee targeted
these systems as likely candidates to modify their disinfection practices to comply with
the Stage 1 DBPR. Systems have until March 2000 to complete DBP monitoring if data
are not already available. Precursor removal strategies could be used in lieu of or in
conjunction with changes to existing disinfection practices for Stage 1 DBPR
compliance.
Only systems required to develop a profile and proposing to make significant changes to
disinfection practices are required to develop a benchmark and submit it and other
pertinent information to the State as part of the consultation process. Note that profiling
and benchmarking based on virus inactivation is required only for systems proposing
to add or switch to ozone or chloramines. Virus profiling and benchmarking is
strongly recommended for systems proposing to add or switch to chlorine dioxide.
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1. INTRODUCTION
1.2 Purpose of Disinfection Profiling and
Benchmarking
Under the EESWTR, disinfection profiling and benchmarking are used to determine the
existing levels of disinfection. As water systems comply with the Stage 1 DBPR, they may
make significant modifications to their existing disinfection practices. It is essential that
water-systems understand the impact on microbial protection while making significant
changes in their disinfection practices. Disinfection profiling and benchmarking are
procedures by which systems and States, working together, can ensure that there will be no
significant reduction in microbial protection as the result of modifying disinfection
practices to meet DBF MCLs under the Stage 1 DBPR (USEPA, 1997a).
1.2.1 Disinfection Profiling: Definition and Purpose
The IESWTR defines a disinfection profile as a compilation of daily Giardia and/or virus
log inactivations over a period of a year or more (USEPA, 1997a). Inactivation of
pathogens is typically reported in orders of magnitude inactivation of organisms on a
logarithmic scale. As an illustration, a 2-log inactivation corresponds to a 99 percent
inactivation and a 3-log inactivation corresponds to a 99.9 percent inactivation (see
Appendix B for further discussion). As required under the IESWTR, a disinfection
profile must be developed for a period between one to three years, depending on the
availability and quality of existing data (see Section 2.3).
The daily log inactivation values are calculated based on daily measurements of
operational data (i.e., disinfectant residual concentration, contact time, temperature, and
pH). A plot of daily log inactiyation values versus time provides a visual representation
of the log inactivation that the treatment plant achieved over time. From this plot,
changes in log inactivation due to temperature, flow, disinfectant residual concentrations,
or other changes can be seen. .
The procedures and calculations for disinfection profiling are discussed in detail in
Chapter 3 of this manual.
1.2.2 Disinfection Benchmarking: Definition and Purpose
Disinfection benchmarking is a baseline or benchmark of historical microbial inactivation
practices developed from disinfection profiling data. The benchmark is determined from
interpretation and analysis of the disinfection profile. This benchmark value identifies
the lowest log inactivation that a system has achieved over a period of time. As used
under the IESWTR, the benchmark sets the target disinfection level for alternative
disinfection schemes. A minimum of 3-log Giardia lamblia and 4-log virus removal
and/or inactivation performance must be achieved at all times to comply with the existing
Surface Water Treatment Rule (SWTR) promulgated in 1989; Inactivation levels below
the benchmark may be implemented after State consultation. States should evaluate
inactivation levels below the benchmark by taking source water, watershed, and
treatment factors into consideration.
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The objective of the disinfection benchmark is to facilitate interactions between the States
and PWSs for the purpose of assessing the impact on microbial risk of proposed
significant changes to existing disinfection practices. The disinfection benchmark
provides a criterion for the designs of alternative disinfection strategies. A system that is
required to prepare a disinfection profile will not be allowed to make a significant change
to disinfection practices without first consulting with the State.
1.3 State Review
Under the IESWTR, States will perform the review of disinfection profiles and benchmarks
for water systems. The State will review disinfection profiles as part of periodic sanitary
surveys. If a system is required to develop a disinfection profile and subsequently decides
to make a significant change in disinfection practice, the system must consult with the State
before implementing such a change. Significant changes are defined under IESWTR as
(USEPA, 1998a):
1. Moving the point of disinfection
2, Changing the type of disinfectant
3. Changing the disinfection process
4. Making any other change designated as significant by the State.
Supporting materials for obtaining approval from the State must include a description of
the proposed change, the disinfection profile, and an analysis of how the proposed change
will affect existing levels of microbial protection.
1.4 Primary Information Sources
This document was developed using several primary reference documents previously
developed by EPA. Material from the following publications were used substantially
throughout this document:
• AWWA. 1991. Guidance Manual for Compliance with the Filtration and
Disinfection Requirements for Public Water Systems Using Surface Water
Sources. Washington, D.C. (Also published by USEPA, 1991)
• USEPA. 1997a. "National Primary Drinking Water Regulations; Interim
Enhanced Surface Water Treatment Rule; Notice of Data Availability; Proposed
Rule." 62 FR 59485. November 3.
• USEPA. 1998a. "National Primary Drinking Water Regulations; Interim
Enhanced Surface Water Treatment Rule; Final Rule." 63 FR 69477.
December 16.
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1. INTRODUCTION
Because each of the above documents was previously published by the EPA and provides
substantial reference material throughout this document, specific citations are not provided
when a publication is paraphrased in this document.
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2. APPLICABILITY OF DISINFECTION
PROFILING AND BENCHMARKING
Disinfection profiling and disinfection benchmarking are two separate provisions under the
IESWTR and are triggered by separate criteria, although the benchmarking process
requires profiling. This chapter illustrates the applicability of the disinfection profiling and
benchmarking provisions under the IESWTR to public water systems and how a water
system can make this determination.
2.1 Systems Subject to the IESWTR
The IESWTR applies only to water systems using surface water or ground water under the
direct influence (GWUDf) of surface water, that serve 10,000 or more people. Systems
that serve fewer than 10,000 people are not regulated under the IESWTR and, therefore, the
disinfection profile and benchmark provisions do not apply to these systems at this time,
although the Long-Term 1 Enhanced Surface Water Treatment Rule (LT1ESWTR),
expected to be promulgated in November 2000, will likely require profiling and
benchmarking for these systems. If a system's source water is not defined as surface water
or GWUDI as defined under the IESWTR, the profile and benchmark provisions are not
applicable.
2.2 Profiling and Benchmarking Applicability
The IESWTR specifies that disinfection profiles and benchmarks may be based upon the
inactivation of Giardia and, in some cases, viruses. Disinfection profile and/or benchmark
development must, at a minimum, be based upon the inactivation of Giardia. However,
under certain circumstances, as explained in Section 2.3 (and highlighted in Figure 2-1),
some systems will be required to develop an additional profile and benchmark based on
virus inactivation. The process for determining the applicability of disinfection profiling
and benchmarking to public water systems is described in the following sections and
illustrated in a corresponding decision tree (Figure 2-1).
2.3 Systems Required to Profile Giardia
Systems are required to develop a disinfection profile for Giardia if their distribution
system DBF concentrations exceed certain criteria. Specifically, if the running annual
average for either TTHM or HAAS concentrations in the distribution system are greater
than or equal to 0.064 mg/L or 0.048 mg/L, respectively, water systems must develop a
profile for Giardia. The 12-month profile must be generated by March 2001.
Systems with existing DBF concentrations approaching or exceeding these MCLs are more
likely to modify disinfection practices; therefore, these systems are required to develop a
disinfection profile. Systems with very low DBF concentrations are not likely
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2. APPLICABILITY OF PROFILING AND BENCHMARKING
Monitor TTHM, HAAS
Monitor operation of
disinfection system.
Is the
source classified as
surface water or GWUDI
as defined by
IESWTR?
STOP! No
profiling or
benchmarking
required.
STOP! No
profiling or
benchmarking
required.
Is the system
serving 10,000 or
more people?*
Is the
THM annual
verage >. 0.064 rrig/L or the"
HAAS annual average
>. 0.048 mg/L?
STOP! No
profiling or
benchmarking
required..
Are there
plans to modify the
existing disinfection
practice?
Must profile Giardia
No benchmarking
required.
Does the
modification include
adding or switching to
ozone, chloramines, or
hlorine dioxide?
Must profile Giardia
No benchmarking
required.
Must profile Giardia and viruses.
Must benchmark Giardia and viruses.
Systems serving fewer than 10,000 people will have to comply at a later date.
Figure 2-1. Profile and Benchmark Decision Tree
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2. APPLICABILITY OF PROFILING AND BENCHMARKING
to modify their disinfection practices to control DBFs under the Stage 1 DBPR and are,
therefore, not required to develop a profile. However, these systems may modify
disinfection practices for other reasons and may find profile data useful for design
purposes.
2.3.1 Giardia Profile
As depicted in Figure 2-1, systems meeting the size and source water applicability
requirements must develop a disinfection profile for Giardia if either of the following
conditions exist:
• The TTHM annual average concentration in the distribution system, for the
most recent one-year period, is greater than or equal to 0.064 mg/L; or
• The HAAS annual average concentration in the distribution system, for the
most recent one-year period, is greater than or equal to 0.048 mg/L.
The TTHM and HAAS data used to determine whether disinfection profiling is required
must meet the specifications described in Section 2.3.2. As shown in Figure 2-1, systems
that do not meet either of these criteria would not have to conduct a disinfection profile or
benchmark.
The Advisory Committee selected the TTHM and HAAS criteria listed above for
determining the applicability of disinfection profiling for Giardia based upon the prediction
that water systems not achieving DBF concentrations at least 20 percent below MCLs
would likely change disinfection practices to control DBFs (i.e., apply a 20 percent margin
of safety) to ensure continuing compliance.
2.3.2 TTHM and HAAS Data Requirements
As described above, TTHM and HAAS data are used to make the profiling determination
for Giardia. The IESWTR specifies the TTHM and HAAS data that are to be used for the
disinfection profile determination. In all cases, the following criteria apply:
• One year of TTHM and HAAS data is used to make a profiling determination.
» The TTHM and HAAS data must be from the same time period.
Since the Information Collection Rule (ICR) requires the collection of TTHM and HAAS
data consistent with the profiling applicability determination, the discussion of data
requirements for ICR and non-ICR systems is presented separately.
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ICR Systems
Systems participating in the ICR have the required quarterly TTHM and HAAS data and
are assigned to use these data to determine applicability of benchmarking unless the State
determines otherwise. Therefore, the requirements listed above apply to ICR systems'
TTHM and HAAS data. ICR TTHM and HAAS values are computed as the annual
average, of quarterly averages of the Distribution System Equivalent (DSE) sample, two
average residence time samples and one maximum residence time sample.
Non-ICR Systems
All water systems affected by the IESWTR are currently conducting quarterly monitoring
of TTHMs under the current TTHM regulation. However, only some non-ICR systems
have conducted the necessary HAAS quarterly monitoring. For those water systems with
existing HAAS data, the State will decide the applicability of using that non-ICR data in the
profiling determination based on the following criteria:
« Applicable HAAS Data: These systems have HAAS data that meet the
provisions of 40 Code of Federal Regulations (CFR) §141.72 (a)(2)(ii)
(Disinfection profiling and benchmarking), which stipulates that systems
using "grandfathered" data must use TTHM data collected at the same time
under the provisions of §141.12 (Maximum contaminant levels for total
trihalomethanes) and §141.30 (Total trihalomethanes sampling, analytical and
other requirements). The state must be confident that the sample collection,
handling, and analyses were adequate to provide accurate results. If a system
has made a modification to its treatment train since the HAAS samples were
collected, and this modification would likely have an impact on HAAS
formation, the state must carefully consider whether the data are still
applicable to the modified system.
. No HAAS Data or Data Not Applicable: These systems either do not have
HAAS data or have data that are judged by the State to not be adequate for the
disinfection profile applicability determination (i.e., data may not be
applicable if sample location, handling, and analytical method requirements
currently applied to TTHM monitoring as outlined in 40 CFR §141.12 and
§141.30 are not met). Systems without adequate HAAS data must perform
HAAS quarterly monitoring that meets the requirements specified in 40 CFR
§141.12 and §141.30. The monitoring must be for four quarters; must
completed no later than March 2000; and must be collected during the same
time period as TTHM data.
State Approval of a More Representative Data Set
The State has the authority to approve a more representative data set to determine
profiling applicability if the system makes such a request or if the State determines that a
more representative data set exists. This may occur under a variety of situations,
including, but not limited to:
. A change in treatment or disinfection practice(s)
» A change in source water or source water blending.
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2. APPLICABILITY OF PROFILING AND BENCHMARKING
2.4 Systems Required to Benchmark Giardia
Systems required to profile that intend to significantly modify their disinfection practice
are required under the BESWTR to develop disinfection benchmarking for Giardia. A
more detailed description of what constitutes a significant modification is presented in
Chapter 5.
2.5 Systems Required to Profile and Benchmark
Viruses
Under the IBSWTsR, some systems are required to create a disinfection profile and
benchmark for viruses in addition to Giardia. A system must create a disinfection profile
and benchmark for viruses if all of the following are true:
1. The system is a surface water system or GWUDI serving 10,000 people or
more.
2. The TTHM annual average > 0.064 mg/L or HAAS annual average > 0.048
mg/L.
3. The system plans to modify their disinfection practices by adding or switching
disinfectants to ozone or chloramines. EPA strongly recommends that
systems switching to chlorine dioxide also develop a virus profile.
For systems adding or switching disinfectants to ozone, chloramines, or chlorine dioxide,
meeting a benchmark based on Giardia does not ensure that the inaetivation of viruses
will be maintained. Chlorine is much more effective at inactivating viruses than it is at
inactivating Giardia. Alternative disinfectants such as ozone, chloramines, and chlorine
dioxide are relatively less effective at inactivating viruses as they are inactivating
Giardia. For this reason, systems switching to alternative disinfectants must profile and
benchmark viruses inaetivation.
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3. CREATING A PROFILE: DATA
REQUIREMENTS AND CALCULATIONS
Disinfection profiling is the characterization of a water system's practices (log
inactivation) over a period of time. Appendix B presents a discussion on the
development of log inactivation methods under the SWTR and an example on how to
calculate log inactivations. The disinfection profile is a graphical representation of the
magnitude of daily Giardia or virus inactivations which is developed, in part, based on
daily measurements of the following operational parameters:
• Disinfectant residual concentrations
• Peak hourly flow rate
• Temperature
• pH (chlorine only).
For purposes of complying with the requirements of the IESWTR, a profile can be
prepared from historical treatment plant operating data, if adequate data are available, or
the profile may have to be prepared using data acquired in a new monitoring program.
As noted in Chapter 2, depending on the disinfectant employed, the IESWTR requires
profiles for either Giardia or Giardia and virus. The basic data requirements for creating
a profile based on Giardia or virus are the same. Therefore, if a utility collects operating
data sufficient to profile for Giardia, it can also develop a profile for viruses with only
slight modifications to the calculations described in this chapter.
3.1 Data for Profiling
The IESWTR provides direction on operational data needed for calculating the
disinfection profile. If approved by the State, existing historical (i.e., grandfathered)
operational data may be used for this purpose. If a system does not have three years of
approved grandfathered data, then it must conduct additional monitoring of operational
data to meet the requirements of the IEWSTR. The system may develop a profile using a
combination of both grandfathered data (where less than three years of approved data are
available) and new data. This section provides guidance on the use of grandfathered data,
the need for conducting additional monitoring, the required quality of the existing data,
and the State's role in approving the use of available operational data.
Water systems should not use existing data if these data do not accurately represent the
system's current level of disinfection. For example, existing data should not be used for
systems that have recently made significant modification to their disinfection practices.
A significant modification includes changes in disinfectants or changes in plant
hydraulics or piping schemes that affect disinfection contact time. These treatment train
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
modifications may substantially impact the level of inactivation provided as indicated by
the CT and render existing data unrepresentative of the system's current inactivation
performance. CT, in mg-min/L, is the product of C (the residual disinfectant
concentration in mg/L) and T, (the time that water is in contact with the disinfectant in
minutes).
3.1.1 Operational Data Required for Profiling
The IESWTR requires systems with less than three years of applicable data to conduct
daily monitoring for profiling. As required in the IESWTR, the following data must be
gathered daily at peak hourly flow at each disinfectant residual sampling point in the
treatment plant:
• Disinfectant residual concentration in the treatment plant
• Peak hourly flow rate
• Temperature
• pH (if the system uses chlorine).
For systems with more than one point of disinfectant application, the same data must be
collected at least daily at each of the disinfectant residual sampling points (i.e., segments).
Section 3.2.2 provides a detailed description of acceptable water quality data analysis
methods. Section 3.3.1 and Appendix D contain detailed descriptions of segments.
The time that the disinfectant is in contact with water in the disinfection segment must be
determined on a daily basis to complete the CT calculations. This contact time, measured
as TIO, is determined based on the peak hourly flow rate occurring during the 24-hour
period and the detention time that is equaled or exceeded by 90 percent of the water
passing through the basin. This procedure is detailed in Appendix D. States may allow
systems to use non-peak flow measurements, but EPA is convinced that such
measurements will result in a higher inactivation and may result in a higher benchmark.
3.1.2 Data Quantity
The IESWTR requires systems to create a disinfection profile that covers a minimum of
12 consecutive months. The profile may span a maximum of 36 consecutive months. All
systems will therefore need one- to three- years of data to calculate daily log
inactivations. Existing data may be used if the State determines that the quality of the
data is sufficient. Under the IESWTR, systems without three years of existing acceptable
operational data are required to monitor for one additional year.
Systems required to develop disinfection profiles under this rule must exercise one of the
following three options:
• Option 1 - Systems must conduct daily monitoring as described below. This
monitoring must be completed no later than March 2001 and must cover a
period of one year. The data collected from this monitoring must be used to
develop a one-year disinfection profile.
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
• Option 2 -Systems that conduct monitoring under this rule, as described under
Option 1, may also use one or two years of acceptable grandfathered data, in
addition to the one-year of new operational data, in developing the
disinfection profile.
• Option 3 -Systems that have three years of acceptable existing operational data
are not required to conduct monitoring to develop the disinfection profile
under this rule. Instead, they may use grandfathered data to develop a three-
year disinfection profile. Systems must coordinate with the State to confirm
acceptability of grandfathered data no later than March 2000, but must
conduct the required monitoring until the State approves the system's request
to use grandfathered data.
3.1.3 Data Quality
»
As noted above and in the IESWTR, existing data may be used by systems to calculate
disinfection profiles if the data are approved by the State. For existing data to be
acceptable to the State, the data must be "substantially equivalent" to the quality of CT
data prescribed in the existing SWTR and in this guidance manual.
Substantially equivalent data are data that meet the sampling location, handling, and
analytical method requirements described in this guidance manual and the Guidance
Manual for Compliance with the Filtration and Disinfection Requirements for Public
Water Systems Using Surface Water Sources (AWWA, 1991). The data should
accurately characterize disinfection throughout the treatment plant. Detailed descriptions
of acceptable methods for collecting the required data are provided in Sections 3.2 and
3.3 of this guidance manual. For systems that have recent recorded their daily log
inactivation calculations, the State should verify the accuracy of these calculations as part
of its data review and acceptance process.
3.2 Procedure to Determine Log Inactivation
This section provides an overview of the procedure to calculate CT values to determine
log inactivation as designed under the SWTR and for disinfection profiling.
3.2.1 Use of CT Values for Disinfection Profiling
The CT method is used to evaluate the amount of disinfection a treatment plant achieves
and to determine compliance with the SWTR. The SWTR requires physical removal
and/or inactivation of 3-logs of Giardiaand 4-logs of viruses. For disinfection profiling
and benchmarking, the CT approach will be used to compute the log inactivation of
Giardia or viruses achieved during water treatment.
The CT values corresponding to 3-log Giardia and 4-log viral inactivations are the basis
for determining the estimated log inactivation achieved by the plant on any given day.
Operational information required to use the SWTR CT tables include: disinfectant type,
temperature, pH (for chlorine only), and residual disinfectant concentration. Using this
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operating information, the CT value corresponding to inactivations of 3-logs of Giardia
(CTs-iog, ciardia') and 4-logs of viruses (CT/wog, vims) can be read from the SWTR CT tables.
These CT values are used to determine the estimated log inactivation achieved by
applying a disinfectant to water.
The SWTR CT tables are provided in Appendix C for reference. These tables contain CT
values corresponding to specified log inactivations of Giardia or viruses.
3.2.2 Steps to Calculate Log inactivation
To construct a disinfection profile, actual treatment plant inactivations need to be
determined using the SWTR CT tables. Data must be representative of the entire
treatment plant, from the initial point of disinfectant/oxidant addition to the entrance to
the distribution system; and is not limited to the segments used for compliance with the
inactivation requirements of the SWTR.
Estimated log inactivations are calculated for each disinfection segment of the treatment
train. Once the log inactivations for each segment are calculated, they are summed to
yield the total plant log inactivations. The following steps, which are described in greater
detail in subsequent sections of this chapter and are shown in Figure 3-1, provide the
general procedure for calculating the estimated log inactivations to generate disinfection
profiles:
•> Systems measure the following operational data each day at each disinfectant
residual sampling point (Section 3.3):
— Disinfectant residual concentration (C, in mg/L)
- Water temperature (°C)
— Water pH (for systems using chlorine).
» Systems determine the peak hourly flow rate for each day from flow
monitoring records. The systems calculate contact time (Tjo) for each
disinfection segment based on baffling factors or tracer studies (Section 3.4).
« Systems calculate CTactuai for each disinfection segment under actual operating
conditions (i.e., C x TIO) (Section 3.4).
<• Systems determine the CT required for 3-log Giardia inactivation (CTs-iog,
aardia) and/or 4-log virus inactivation (CT/uog, virus) from the SWTR CT Tables
(Section 3.4 and Appendix C). These required CT values are dependent on
the disinfectant type, residual concentration, temperature, and pH.
« Systems calculate the estimated log inactivation for Giardia and/or viruses for
each disinfection segment (Section 3.4) using:
Segment log inactivation of Giardia = 3.0 * CTactuai / CT3_iog,
- Segment log inactivation of viruses = 4.0 * CTactuai /
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
• Systems sum the segment log inactivations to determine the plant log
inactivations due to chemical disinfection (the segment log inactivation are
additive) (Section 3.4) using:
— Plant log inactivation of Giardia = S (segment log inactivation of
Giardia)
- Plant log inactivation of viruses = £ (segment log inactivation of
viruses)
Figure 3-1 provides a schematic of the disinfection profiling methodology based on the
log inactivation method.
3.2.3 Determining Disinfectant Residual Concentrations,
pH, and Temperature
The disinfectant residual concentration is defined as the concentration of disinfectant
used to protect the distribution system from recontamination. This residual is measured,
along with temperature and pH, at a location referred to as the "residual sampling point."
If a treatment plant has three disinfection segments it will therefore, have three residual
sampling points that must be measured. Disinfection segments are further defined in
Section 3.3.1.
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
Collect Data
Disinfectant Residual
Concentration, Peak Hourly
Flow Rate
Collect Data
Disinfectant Residual,
pH, Temperature
1 Tracer Study or
i V/Q*Baffling Factor
Calculate CT Actual
Look Up CT Required
CT3-log, Glardia
CTi-tog, Viruses
Estimated Segment Log Inactivations
Log Inactivation of Giardia =
Log Inactivation of Viruses =
CT,
'Actual
CT,
3-)og, G/anf/a
CT
4-log, Viruses
Estimated Plant Log Inactivation
By Chemical Disinfection
Plant Log Inactivation of Giardia = (Segment Log Inactivation Giardia)
Plant Log Inactivation of Viruses =^ (Segment Log Inactivation Viruses)
Figure 3-1. Disinfection Profiling Methodology
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
Table 3-1. Acceptable Laboratory Methods for Analyses
Parameter
Temperature3
pH3
Free Chlorine
Chloramine
Chlorine Dioxide
j
Ozone
Acceptable Method(s)1
Thermometric (SM 2550)
Electrometic (SM 4500-H+)
Electrometic (EPAA50.1&2)
Amperometric, Titration
(SM 4500-CI D)
DPD Ferrous, Titration
(SM 4500-CI F)
DPD, Colorimetric
(SM 4500-CI G)
Syringaldizine (FACTS)
(SM 4500-CI H)
Amperometric, Titration
(SM 4500-CI D)
DPD Ferrous, Titration
(SM 4500-CI F)
DPD, Colorimetric
(SM 4500-CI G)
Amperometric, Titration
(SM 4500-CIOa E)
Amperometric, Titration
(SM 4500-CIOa D)
DPD-Glycine
(SM4500-CI02D)
Indigo Method
(SM4500-OsB)
Examples of Commercial
Test Kits/Equipment2
Any good, mercury-filled thermometer but
thermocouples are acceptable
Hach EC series & One series
LaMotte DMA 3000
Orion A series & 300 series
Hach Amperometric Titrator
Fischer-Porter 17T200
Capital Controls 1870E (on-line monitor)
Great Lakes 95CL (on-line monitor)
LaMotte 6806/DT
Hach DR1 00, DR700 & DR/2000
Hach Pocket Colorimeter
LaMotte DC-1100CI
LaMotte SMART Colorimeter
Hach CL17 (on-line monitor)
Hach Amperometric Titrator
Fischer-Porter 17T200
Capital Controls 1870E (on-line monitor)
Great Lakes 95CL (on-line monitor)
LaMotte 6806/DT
Hach DR100, DR700 & DR/2000
Hach Pocket Colorimeter
LaMotte DC-1100CI
LaMotte SMART Colorimeter
Hach CL17 (on-line monitor)
Hach Amperometric Titrator
Fischer-Porter 17T200
(Note: Platinum-Platinum .electrodes are
required.)
LaMotte DC1100-CLO
Hach DR/2000 & DR/4000
(Note: Spectrophotometric procedure is required.)
1 SM - Standard Methods (1995); EPA - EPA Methods, 1995.
2 This is not a complete list of all commercially available test kits nor an endorsement of any specific product.
3 Samples must be analyzed prior to changes in character (e.g., sample allowed to warm prior to taking temperature)
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3.2.4 Determining Contact Time, T10
The contact time or detention time, TIO, is the value estimated using the
theoretical detention time (TDT) and baffling factors or from data collected from
a tracer study.
As discussed in Section 3.3.1, the treatment train may be divided into several
disinfection segments, corresponding to the number of disinfectant application
points. The disinfection segments may include several unit processes of the
treatment train. The total TIO for the disinfection segment is the sum of each TIO
for each unit process within the segment. The T^ can also be calculated for the
whole plant or an entire segment instead of for individual segments, as long as
there are no additional points of disinfectant addition.
The segment TIO is multiplied by the disinfectant residual at the end of the
segment to yield the segment CTactua]- Section 3.4 provides an example of
segmenting the treatment train and the corresponding CT calculations.
There are two methods to determine the contact time for a treatment process. The
first method calculates contact time by utilizing the hydraulic characteristics of the
treatment basin and baffling factors. These baffling factors are shown in
Appendix D or may be available from the State. The second method involves
conducting a tracer study for each disinfection segment. Baffling factors are used
to determine TIO from theoretical detention times in systems when it is impractical
to conduct tracer studies. These two methods and their use are discussed in detail
in Sections 3.2.4.1 and 3.2.4.2.
Tracer Studies versus Baffling Factors
Tracer studies are more accurate than baffling factors as they provide a real
measure of the contact time by measuring the time it takes for the tracer to flow
through each segment in the treatment train. Tracer studies provide a better
understanding of how well the disinfectant is mixing with the water for the
hydraulic conditions of a specific water treatment plant. The disadvantage of the
tracer study is that it is costly to conduct. The baffling factor method is a useful
alternative for determining the contact time. It is less labor intensive,
inexpensive, and easy to perform. The disadvantage, however, is that the baffling
factors may not accurately represent the actual contact time of the system.
A conservative approach to calculating the contact time with baffling factors is to
select the lowest baffling condition that is applicable. Baffling conditions include:
very poor, poor, average, superior, or perfect. If it is not clear whether the baffling
condition for a basin is average or superior, then the conservative approach is to
use the average condition for the TIO calculations.
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Contact Time for Unit Process
The unit processes that comprise each disinfection segment may include
sedimentation, filtration, and pipeline flow, among others. Each of these reactors
has special hydraulic characteristics affecting the contact time. In pipelines, the
contact time can be assumed equivalent to the theoretical detention time and is
calculated by dividing the internal volume of the pipeline by the peak hourly flow
rate through the pipeline. Pipeline flow is assumed to be plug flow with no dead
zones or unutilized volume in the reactor. Therefore, each unit of water is
assumed to spend the same time in the pipeline, referred to as the TDT. For
reactors of other shapes (e.g., a rectangular sedimentation basin) the time spent by
the water in the reactor may vary over a range. For example, some water may
move faster by short-circuiting while other water may spend more time in the
reactor trapped in "dead zones" resulting in little flow. This variation in the time
that water could spend in a particular unit process leads to a distribution of
potential residence times from which TIO can be determined.
Contact Time for Pipe Flow
The contact time calculation for pipe flow is simply the theoretical detention time,
which is the volume (V, in gallons) divided by the peak hourly flow rate (Q, in
gallons per minute (gpm)),
= Contact Time = V/Q (applicable to pipe flow only)
Pipe flow does not require a tracer study to calculate contact time. The baffling
factor for pipe flow is 1 .0.
The following example of pipe flow assumes the pipeline to be 12,800 feet long
and to have a cross-sectional area of 18 square feet (calculated from, its inside
diameter). The peak hourly flow rate in the pipeline is 10,651 gpm. The volume
of water contained within the full pipeline is the length multiplied by the cross-
sectional area. The resulting volume is:
Volume, V = 2800 feet * 18 feet2 = 50,400 ft3
Converting the volume to gallons,
V = 50,400 ft3 * 7.48 gallons/1 ft3 = 376,992 gallons
Calculating the contact time,
= V/Q = 376,992 gallons / 10,651 gpm
=35.4 minutes
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Contact Time in Mixing Basins and Storage Reservoirs
In mixing basins and storage reservoirs, the theoretical detention time generally
does not represent the actual disinfectant contact time because of short-circuiting.
Thus, determining contact time is more complicated with basins.
The time used to compute CTactuai in treatment basins depends on the reservoir
shape, inlets, outlets, and baffling. Most clearwells and some other treatment
basins were not designed to provide optimal hydraulic characteristics for contact
with a disinfectant. Utilities are required to determine the contact time in mixing
basins, storage reservoirs, and other treatment plant unit processes for the
calculation of CTactuai through tracer studies or other methods approved by the
State. For the purpose of determining compliance with the disinfection
requirements of the SWTR, the contact time of mixing basins and storage
reservoirs used in calculating CTactuai should be the detention time in which 90
percent of the water passing through the unit is retained within the basin, (i.e.,
TIO). Information provided by tracer studies is used for estimating the detention
time TIO for the purpose of calculating CTaotuai. If tracer studies are not practical,
the TDT and baffling factor approach can be used. In Appendix D, complete
descriptions of both the TDT and baffling factor method and the tracer test
method to evaluate TIO are provided. A plant with multiple treatment trains and
different operating characteristics should have the critical train identified.
3.2.4.1 Determining Contact Time Using Baffling Factors
The TDT is computed by dividing the volume of a unit process by the peak hourly
flow rate (TDT=V/Q). Baffling factors (Tio/T) selected for a specific unit process
are multiplied by the theoretical detention time to yield an estimate of the contact
time, TIO, as follows:
= Contact Time = V/Q *
Table 3-2 describes baffling classifications and baffling factors (Tio/T ratios).
The baffling factor is a function of design of the basin. A baffling factor of 1.0
represents plug flow characteristics. In plug flow, the TDT is equivalent to the
contact time, TIQ. Design modifications that can increase TIO rnay allow the same
inactivation (CT) with a decreased disinfectant residual.
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
Table 3-2. Baffling Classifications and Factors
Baffling Condition
Unbaffled (mixed flow)
Poor
Average
Superior
Perfect (plug flow)
Tio/T
0.1
0.3
0.5
0.7
1.0
Baffling Description
None, agitated basin, very low length to width ratio, high
inlet and outlet flow velocities
Single or multiple unbaffled inlets and outlets, no intra-
basin baffles
Baffled inlet or outlet with some intra-basin baffles
Perforated inlet baffle, serpentine or perforated intra-
basin baffles, outlet weir or perforated launders
Very high length to width ratio (pipeline flow), perforated
inlet, outlet, and intra-basin baffles
Source: AWWA, 1991.
Using the following example information, the TDT can be calculated:
• Volume of a contact basin = 500,000 gallons
• Peak hourly rate = 10,000 gpm
Contact basin = unbaffled.
The TDT is then calculated as follows:
TDT = V/Q = 500,000 gallons/10,000 gpm = 50 minutes
However, because the contact basin is unbaffled, the Tio/T is 0.1 and the resulting
actual contact time used for determining log inactivation is:
TIO (contact time) = 50 minutes * 0.1 =5 minutes
The CT value for this unit process at 1.2 mg/L residual chlorine is:
CT = 5 minutes * 1.2 mg/L = 6 mg-min/L.
By improving contact conditions through inlet and outlet and some intra-basin
perforated baffles, the Tio/T may improve to 0.7 and, therefore, the new contact
time is:
TIO (contact time) = 50 minutes * 0.7 = 35 minutes.
The new CT value at 1 mg/L of chlorine is:
CT = 35 minutes * 1.2 mg/L = 42 mg-min/L
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* At a pH value of 6.0 and a water temperature of 15°C, the CT value needed to
achieve a 2-log inactivation of Giardia by free chlorine (Table C-4, Appendix C)
is 35 mg-min/L. At a pH value of 6.0 and a water temperature of 15°C the CT
value needed to achieve 2.5-log inactivation of Giardia by free chlorine (Table C-
4, Appendix C) is 44 mg-min/L.
To determine the-estimated Giardia log inactivation for the CT value of 42 mg-
min/L, linear interpolation may be used as follows:
Estimated Log removal = (42 mg-min/L * 2.5 logs) / 44 mg-min/L = 2.4
or
Estimated Log removal = (42 mg-min/L * 2 logs) / 35 mg-min/L = 2.4
In order to determine the contact time using baffling factors, the following steps
ought to be taken:
. Determine peak hourly flow rate, Q, based on operation records;
. Determine the volume of each unit process;
. Calculate the TDT, where TDT = V/Q;
• Determine the baffling factor based on the unit processes baffling
conditions, Tio/T;
. Calculate the contact time, where TIO = TDT * Tio/T; and
• Determine the segment TJQ by summing each TIO of the unit processes
in the segment.
3.2.4.2 Determining Contact Time Using a Tracer Study
A tracer study uses a chemical tracer to determine the detention time of water
flowing through a unit process, segment, or system. Typical chemical tracers
include chloride ions, fluoride ions, and a fluorescent dye Rhodamine WT.
Ideally, the selected tracer chemical should be readily available, easily monitored,
and acceptable for use in potable water supplies. The tracer should also be
conservative (i.e., the tracer is not consumed, or removed during treatment).
Fluoride ions can generally be used in lower concentrations than chloride because
they are typically present in lower concentrations in the water. Rhodamine is a
fluorescent tracer that, if selected, must be used following certain guidelines found
in Appendix D. Selection of a particular chemical tracer may depend on the unit
processes and the salt concentrations present in the water. Specific instructions on
chemical tracers and under what conditions are they most effective are found in
Appendix D. If a tracer study is needed in order to find TIO, a water system
should consult the latest tracer study guidance from the State.
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
The tracer chemical should be added at the same points in the treatment train as
the disinfectant to be used in the CT calculations, since it will be used to
determine TJQ for the disinfection segment. Two common methods of tracer
addition are the step-dose method and the slug-dose method. In the step-dose
method, the tracer chemical is injected at a constant dosage and the endpoint
concentration is monitored. To determine a 90 percent recovery for the tracer,
endpoint sampling should continue until the tracer concentration reaches a steady-
state level. With the slug-dose method, a large dose of tracer chemical is
instantaneously injected. An effective way to achieve instantaneous addition is to
use a gravity-fed tube to release the single dose. The tracer concentration is
monitored at the endpoint, until the entire dose has passed through the system.
Unlike the step-dose method, a mass balance is required to determine whether the
entire tracer dose was recovered. Additional mathematical manipulation is
required to determine TIQ from the concentration versus time profile.
The test procedure for determining the contact time with a tracer study is generally
as follows:
The system determines the flow rate or rates to be used in the study.
• The system selects the tracer chemical and determines the raw water
background concentration of the tracer chemical. The background
level is needed to both determine the quantity of chemical to feed and
to evaluate the data properly.
: . The system determines the tracer addition locations, plans the sample
collection logistics and frequency, and determines the appropriate
tracer dosage. Sampling frequencies depend on the size of the basin—
the larger the basin the easier it is to obtain an adequate profile with
less frequent sampling is needed. Small basins need more frequent
sampling. However, to obtain an adequate profile, large systems may
be more difficult to handle than small basins because sampling events
are longer in duration thus presenting logistical problems in staffing
for sample collection and sample analysis.
• The system conducts the tracer test using either the step-dose or slug-
dose methods.
• The system compiles and analyzes the data.
• The system calculates TIQ.
Additional discussions on tracer studies and determining contact times are
provided in Appendix D. Additional references for information on tracer studies
and details concerning how to conduct one are as follows:
• AWWARF. 1998. "Water Quality Modeling of Distribution System
Storage Facilities." Walter Grayman Consulting Engineer, University
, of Michigan, SSESCO, Charlotte Smith & Associates, and Blue Ridge
Numerics. Denver, CO.
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
• Hudson, H.E., Jr. 1975. Residence Times in Pretreatment. /. AWWA.
45-52.
• Hudson, H.E., Jr. 1981. Water Clarification Processes: Practical
Design and Evaluation. Van Nostrand Reinhold Company, New York.
. Levenspiel, O. 1972. Chemical Reaction Engineering, John Wiley and
Sons, New York. Second Edition.
. Marske, D.M. and J.D. Boyle. 1973." Chlorine Contact Chamber
Design - A Field Evaluation." Water and Sewage Works. 70-77.
• Missouri Department of Natural Resources. 1991. Guidance Manual
for Surface Water System Treatment Requirements. Public Drinking
Water Program.
. Teefy, S.M. 1996. "Tracer Studies in Water Treatment Facilities: A
Protocol and Case Studies." AWWARF.
. Teefy, S.M. and P.C. Singer. 1990. "Performance and Analysis of
Tracer Tests to Determine Compliance of a Disinfection Scheme with
the SWTR." /. AWWA. 82(12):88-98.
• Thirumurthi, D. 1969. "A Breakthrough in the Tracer Studies of
Sedimentation Tanks." J. WPCF. R405-R418.
• TNRCC (Texas Natural Resources Conservation Commission). 1995.
Public Water Supply Technical Guidance Manual. Austin, TX.
3.3 Monitoring Procedures
This section describes the various monitoring procedures for disinfection profiling
as required under the JJESWTR. It addresses the following topics: defining
disinfection segments within a treatment train based on the number of disinfection
application points and determining disinfectant residual concentrations.
3.3.1 Defining Disinfection Segments
The number of disinfection segments within a treatment train must equal or
exceed the number of disinfectant application points in the system. For systems
with multiple points of disinfectant application, such as ozone followed by
chlorine, or chlorine applied at several points in the treatment train, the treatment
train should be divided into multiple disinfection segments. Each segment begins
at the point of disinfection application and ends at the disinfection residual
sampling point. This sampling point is located just prior to the-next disinfection
application point or, for the last disinfection segment, at or before the entrance to
the distribution system or the first customer. As stated before, disinfection
segments may include several unit processes of the treatment train.
For instance, if the treatment train includes two applications of chlorine, then the
treatment train is divided into two disinfection segments. The first segment
begins at the first point of disinfectant application and ends at the residual
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
disinfectant sampling point, just prior to the second disinfectant application point.
The second disinfection segment begins at the second point of disinfectant
application and ends at the second disinfectant residual sampling point. For any
system, the last disinfection segment must end at or before the entrance to the
distribution system or before the first customer. Disinfection segments always
start at the. application point of a disinfectant and end at the residual sampling
point.
Systems may find it useful to divide a single disinfection segment into multiple
segments based on different mixing conditions or treatment units. For example,
in a direct filtration plant where chlorine is applied at the rapid mixing stage and
free chlorine residual is measured at the entrance to the distribution network, the
whole plant is a single disinfection segment. The Tio/T value multiplied by the
free chlorine concentration will give a conservative CT value for the plant (due to
free chlorine volatilization at various treatment stages). Therefore, by measuring
the free chlorine residual at the end of each treatment unit will provide a different
CT value and hence a less conservative estimate of log inactivation.
Section 3.6.1 provides a detailed example of how to define disinfection segments
and then use these segments to compute CT and log inactivation values.
3.4 Calculating Estimated Log Inactivation
The objective of this section is to demonstrate, in greater detail, the calculations
involved in determining the estimated log inactivations. The section describes the
SWTR log inactivation method, procedures to determine minimum regulatory log
inactivations for Giardia (3-log removal) and viruses (4-log removal), procedures
to calculate estimated log inactivations for one disinfection segment of a plant,
and the method to determine the overall estimated plant log inactivation.
3.4.1 SWTR Log Inactivation CT Method
The SWTR requires Giardia and virus inactivations for drinking water systems.
Because of the difficulty in measuring actual microbial inactivations, EPA
developed CT tables (see Appendix C) that can be used to estimate the
inactivations achieved through chemical disinfection. These tables were
developed for approved disinfectants, including chlorine, ozone, chlorine dioxide,
and chloramines.
The tables indicate the log inactivation of Giardia and viruses corresponding to
the operating conditions of temperature, pH, residual disinfectant concentration,
and contact time. These tables are presented in the form of log inactivation
versus operational conditions since the relationship between CT and log
inactivation of Giardia is relatively linear for most disinfectant and organism
combinations. Log inactivation is an expression of the magnitude of
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
microorganisms that are inactivated during the disinfection process. Table 3-3
presents log inactivations and their corresponding percent inactivations.
Table 3-3. Log Inactivations and Percent Inactivations
Log Inactivation
0.0
0.5
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Percent Inactivation
0.000
68.38
90.00
99.00
99.90
99.99
99.999
99.9999
99.99999
Appendix B provides a detailed explanation for the development of the log
inactivation method under the SWTR.
virus
3.4.2 Determining CT3.|0gj Giardia and CT4.|0g,
To calculate the estimated log inactivation of a plant, Equation 3-1 and Equation
3-2 must be used to calculate the log inactivations of each disinfection segment.
The estimated log inactivations for each segment are then summed to calculate the
estimated log inactivations of the plant.
Estimated Log Inactivation of Giardia = 3.0 *
CT
actua Equation 3-1
CT
3-log, Giardia
Estimated Log Inactivation of Viruses = 4.0 *_£•:] Equation 3-2
CT
4-Iog, Virus
Equations 3-1 and 3-2 are derived in Section 3.4.3. To use Equation 3-1 and
Equation 3-2 in order to calculate the estimated log inactivations of a segment the
operator must know the CTactuai and the required CT3ii0g, Giardia or required CT4_iog)
virus- CTactuai is determined based on daily sampling of the residual disinfectant
concentration, C, and calculating the contact time, TIQ- The sampling, and
calculation of contact time, must be performed for each of the disinfectant
segments using the procedures described in Section 3.2. This section describes
how to determine the required CT3.iogr Giardia and the required CT4-i0g, virus for each
of the disinfection segments.
Since plants rarely operate at a pH, temperature and residual disinfection
concentration that exactly matches the CT tables in the Guidance Manual for
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
Compliance with the Filtration and Disinfection Requirements for Public Water
Systems Using Surface Water Sources (AWWA, 1991), the operator may
determine a CT value that lies in between the values. These tables are presented
in Appendix C of this guidance manual.
In addition to linear interpolation (see example in Section 3.2.4.1), two methods
are presented in this manual for determining the CT values, the "Approximation
Method" and the "Regression Method." The PWS should be consistent when
choosing a method to calculate CT.
The Regression Method is an efficient way to calculate CT3.iog, aardia using a
computer spreadsheet when free chlorine is the disinfectant being used. This
method uses empirical regression equations (Smith et al., 1995) to estimate the
CT required to inactivate 3-log Giardia with chlorine. An example of the
Regression Method is found in Appendix E.
The Approximation Method can be used for CTs-iog, Giardia or CT/uog, vims for all
disinfectants. With this method, conservative values of pH, temperature, .and
residual disinfectant concentration are used to select a CT value from the table.
The Approximation Method is more conservative than linear interpolation and the
Regression Method as it approximates the value of the required CT3.iogi Giardia and
the required CT4-iog> virus- Systems with a pH greater than 9.0 should follow
applicable State guidance. The explanation of this method is adapted from a
publication by the Texas Natural Resource Conservation Commission (TNRCC,
1998) and is also discussed in the Guidance Manual for Compliance with the
Filtration and Disinfection Requirements for Public Water Systems Using Surface
Water Sources (AWWA, 1991).
Since it requires no mathematical calculations and reduces errors, the
Approximation Method is usually recommended because it is easier to use.
However, this method is conservative and slightly underestimates the actual
effectiveness of the disinfection process. Also, linear interpolation for all
disinfectants is acceptable.
Procedure (CT3.ioS, Giardia):
• Go to Table 3-4 for Giardia inactivation using free chlorine.
. Find the CT for the temperature that is equal to (or slightly below) the
actual temperature of the water. For example, if the temperature is
19°C, use the 15°C table.
• Go to the section of the table for the pH which is equal to (or slightly
above) the actual pH of the water. For example, if the pH is 7.2, use
the pH=7.5 section.
» Look at the far left side of the table and find the chlorine concentration
that is equal to (or slightly above) the actual free chlorine .
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
concentration. For example, if the chlorine concentration is 1.1 mg/L,
use the 1.2 mg/L row. If the chlorine concentration is above 3mg/L,
use the values corresponding to 3mg/L.
• The value shown at the intersection of the concentration row and the
temperature/pH column is the value of the required-CT3_iog) Giardia. For
example, at pH 7.5, 15°C, and 1.2 mg/L of chlorine, the required CT3.
log, Giardia is 92 mg-min/L.
Example:
Find the value of CT3_iogj Giardia for a water temperature of 10.8°C, a pH of 8.2, and
a residual of 2.5 mg/L for a plant that is using free chlorine as the disinfectant.
Use the next lower temperature, 10°C.
LTsing Table 3-4, look under the pH=8.5 across the 2.6 mg/L row to find that the
is 234 mg-min/L.
Important Note:
The procedure to calculate the required CT3.iog; Giardia when using free chlorine for
water with a pH greater than 9.0 requires the use of the pH 9.0 table or applicable
State guidance. No Giardia disinfection credit is allowed for free chlorine if the
pH in the disinfection segment is above 11.5.
Procedure (CT^iog, virus):
• Go to Table 3-5 for viral inactivation using free chlorine.
• Go to the column for the temperature that is equal to (or slightly
below) the actual temperature of the water. For example, if the
temperature of the water is 10.5°C, use the temperature = 10°C
column.
• The value shown in the 10°C temperature column is the value of CT4-
log, virus-
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
For CT values for the inactivation of Giardia and viruses using chlorine dioxide, ozone,
or chloratnines, use the Tables in Appendix C.
Example:
Find the value of the required CT^iog, virus for a water temperature of JL0.8°C and a pH of
9.0 for a plant that is using free chlorine as the disinfectant.
Using Table 3-5 for free chlorine and using 10°C, the required
g, vims
is 6 mg-min/L.
Table 3-5. Required CT Values (mg-min/L) for 4-Log Inactivation of Viruses
by Free Chlorine, pH 6.0-9.0
Temperature
(°C)
0.5
1
2
3
4
5
6
7
8
9
10
11
12
CT Value
(mg-min/L)
12
11.6
10.7
9.8
8.9
8
7.6
7.2
6.8
6.4
6
5.6
5.2
Temperature
(°C)
13
14
15
16
17
18
19
20
21
22
23
24
25
CT Value
(mg-min/L)
4.8
4.4
4
3.8
3.6
3.4
3.2
3
2.8
2.6
2.4
2.2
2
3.4.3 Log Inactivation Calculations
This section provides the procedures for calculating log inactivations for generating
disinfection profiles. This section provides an example of calculating estimated log
inactivations using the Approximation Method to determine CTs-iog, Giardia and CT4_iog) virus
when using free chlorine at pH less than or equal to 9.0. At pH greater than 9.0, systems
must use the pH 9.0 table or State-approved protocol. The procedure is as follows:
Estimated log inactivation is calculated by assuming the relationship between CT and log
inactivation is linear and can be represented mathematically by the following equation:
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Estimated Log Inactivation _ CTactuai
3 - Log Inactivation CT99.9 (or 3.log, Ciardia)
Rearranging the equation:
Estimated Log Inactivation = 3.0 *.
CT
CT
required
Assuming a base condition of 3-log inactivation for Giardia and 4-log inactivation for
viruses, the general equations are as follows:
Estimated Log Inactivation of Giardia = 3.0 *
CT,
CT
*- i 3-log, Giardia
Equation 3-1
Estimated Log Inactivation of Viruses = 4.0
CT,
actual
CT
Equation 3-2
4-log, Virus
These general equations are actually extrapolations of the SWTR based on the 3-log and
4-log inactivation values. However, they can be used by any surface water treatment
plant, whether practicing filtration or not. The equations remain valid for systems with
lower required inactivations (i.e., filtration plants) because of the linear relationship
between CT and log inactivation.
Summing the Estimated Log Inactivations of each
Segment to Determine the Log Inactivation of the Plant
3.4.4
Once the CT3_iog> Giardia and CT^iog, vims have been determined for a segment in a treatment
plant, this information can be used in Equation 3-1 or Equation 3-2 along with the CTactuai
to calculate the daily log inactivation of Giardia or viruses for a given segment. The
daily log inactivation of the plant is then calculated by summing the log inactivations of
the individual segments into a daily log inactivation for the plant as follows:
Total plant log inactivation = S(segment log inactivation)
3.5 The Completed Profile
The disinfection profile consists of the daily log Giardia (or virus) inactivation levels
plotted against time. The log inactivation calculation methodology was used for a
specific system as an example for developing the BESWTR. Figures 3-2 through 3-4
present the disinfection profiles showing variations in daily log inactiyations of Giardia
at a sample facility from 1994 through 1996. In general, as can be seen from Figures 3-2
and 3-3, seasonal variations in log removal of Giardia can be discerned from the
disinfection profiles. However, as depicted in Figure 3-4, variations to the expected
seasonal disinfection profile pattern may occur in a.year with atypical weather conditions.
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Based on the three years of data/it appears that the lowest inactivation level at this
facility occurred at the end of June 1995.
Systems should keep the completed profile and supporting data on file at the treatment
plant or at its offices in graphical form, as a spreadsheet, or in some other format
approved by the State. A system is not required to submit the profile and supporting data
to the State unless it is requested or if the system intends to make a significant
modification to its disinfection practice. It is important to retain the profile and
supporting data in the event the system decides to modify its disinfection practice and
must therefore, create a benchmark.
16.0
0.0
Jan-94 Feb-94 Mar-94 Apr-94 May-94 Jun-94 Jul-94 Aug-94 Sep-94 Oct-94 Nov-94 Dec-94
Figure 3-2.1994 Profiling Data
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16.0
Jan-95 Feb-95 Mar-95 Apr-95 May-95 Jun-95 Jul-95 Aug-95 Sep-95 Oct-95 Nov-95 Dec-95
Figure 3-3.1995 Profiling Data
16.0
Jan-96 Feb-96 Mar-96 Apr-96 May-96 Jun-96 Jul-96 Aug-96 Sep-96 Oct-96 Nov-96 Dec-96
Figure 3-4.1996 Profiling Data
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3.6 Examples of Estimating Log Inactivation of Giardia
and Viruses for Conventional Fiitration Plants
These examples are intended to enhance the discussion of CT calculations provided
earlier. These examples illustrate the necessary information and computations needed to
perform a complete CT analysis and to determine log inactivation of Giardia and viruses
for a single day. Where applicable, a reference is given to the location within the text
where a more complete description of the topic can be found. Chapter 4 continues these
examples by developing a disinfection benchmark. Chapter 5 also demonstrates the
utility of a disinfection benchmark in designing alternative disinfection strategies to
control DBFs while meeting existing levels of disinfection.
The data required for estimating log inactivation are:
«> pH (chlorine only)
« Water temperature, in °C
» Disinfectant residual, in mg/L
« Peak hourly rate for the day, in gpm
* Volume of water in each segment of treatment plant, in gallons
» Baffling conditions.
The last two data elements, the volume of water in each segment and the baffling
conditions, are set by the treatment plant configuration. pH and water temperature
measurements should be measured at the same time the disinfectant residual sample is
being taken. .Measurements of these parameters should be conducted during or about the
peak hour demand time.
As stated earlier, when calculating estimated log inactivation the following rules are set
as guidance to develop a conservative (when compared to direct linear interpolation of
CT values) log inactivation estimate:
1. Temperature - if the water temperature falls in between what is listed in the
tables the system should use the CT value corresponding to the next lower
temperature.
2. pH - if the water pH falls in between what is listed in the tables, systems
should use the CT value corresponding to the next higher pH value. For pH
values greater than 9.0, systems should use pH 9.0 or apply State guidance.
3. Disinfectant residual - if the disinfectant residual value is in between what is
listed in the tables, the system should use the next higher value to calculate the
CTs-iog, Giardia- If the disinfectant residual is greater than 3 mg/L, the system
must use 3 mg/L for calculating estimated CT and to determine the CT3.iog,
value and for calculation of CTactuai.
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3.6.1 Example of Developing a Disinfection Profile for a 40 mgd
Plant
This example considers disinfection at a 40 mgd conventional filtration plant. The plant
is five years old and is expected to reach capacity in 25 years. A process diagram for the
plant is shown in Figure 3-5. The plant process train is divided into three disinfection
segments. Chlorine is dosed at two locations: to the raw water and immediately prior to
filtration. Ammonia is applied just prior to the clearwell to form chloramines. The three
disinfection segments are shown at the top of the diagram. Each segment begins at the
point of disinfectant application, and ends at the disinfectant residual sampling point. The
diagram indicates information needed to calculate the theoretical detention time using the
peak hourly flow rate and TIO for each unit process determined by the baffling factor
approach discussed earlier and in Appendix D.
^- /\
Segment! J NH4V Segment3
iiii*>n>in»i*i*iitiM _-!••••••••••••»
^
Volume
. (Gallons);
Peak Hourly
Flow(gpm)
Theoretical
Detention
Time (mln)
Baffling
Condition/
Factor
(minj
Disinfectant
Residual
(mfl'L)
CTVdue
(mg/D
. PH
Log
(Qardla)
Log
(Viruses)
jf
Transfer Line
416,374;;
10,651
''••i.t.:'.-':
Plug Flow
1.0
-.:••».[ :
b
W68 •'• "'
10,651
; .'3,24' " "-
Very Poor
0.10
-', *»';;:'"''';
~ I
, mm .
10,651
'• '•"••.«»-• :;
VeiyPoor
0.10
' it :.;.:••;
>B5S/|
I
5 180 947 • •
10,651
<86 ' ; - .
Average
0.50
• 2*3- -';,.:
0.
7
itl Filtration I
-130,870
10,651
tti'; '-.'
Superior
0.70
-';-" m: - '
23
n DOS
V
n,V?
S3 2.
-
s
To
.Distribution
^
Total Plant
23 CTValue
I
282.13
Total Plant
Log Inacfivadon
,1
1.89
Total Plant
Log Inacttvaflon
for Viruses
Je
(RSPJk- Residual Sampling Point
Figure 3-5. 40 mgd Conventional Filtration Process Diagram
August 1999
3-25
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Disinfection Profiling and Benchmarking
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
The plant actually consists of four identical, parallel 10 mgd process trains, so there are
four rapid mix basins, four flocculation basins, etc., all of equivalent size. Because each
of the four trains are identical, the approach to calculating the TDT for a process (e.g.,
rapid mix) is to sum the volumes of the reactors (four times the volume of a single
reactor) and divide by the total plant peak hourly flow rate. Table 3-6-summarizes the
design conditions for each unit process.
Table 3-6. Unit Process Design Conditions Summary
Design Row (mgd)
Theoretical Detention Time
(min)
Hydraulic Loading Rate
eu
c
Li
S
"a
55
i
DC
40
15.0
n/a
X
£
•o
a.
cc
40
1.24
n/a
c
o
IS
3
8
o
u_
40
34.9
n/a
•-—
S
§
£
t>
40
372
0.362 gpm/ft2
c
0
•s
is
u.
40
4.71
4.8 gpm/ft2
0 ""
"o S
1 S
i |
u.
40
5.00
n/a
-------
3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
volume, is used to compute TDT. For example, for filters, the volume of media must be
subtracted to get the volume of the filter process occupied by water. Additionally, for
clearwells or tanks with variable storage volume, the minimum storage volume during the
day is used. The different types of equations used to calculate the volumes are shown in
Table3-7.
Table 3-7. Volume Equations
Volume of Filtration
Volume of Raw Water Pipe
Volume of Rapid Mix Basins
Volume of Water in Clearwells
= Volume of Filters - Volume of Media
= (# of filters) x (Length) x (Width) x (Total depth) -
(# of filters) x (Length) x (Width) x (Depth of media) x (Porosity)
= (Length) x (Cross-sectional Area)
= (# of basins) x (Length) x (Width) x (Depth of water)
= (# of tanks) x (Minimum water depth) x (Cross-sectional Area)
The theoretical detention time is the unit process volume divided by the peak hourly flow
rate. This theoretical detention time must be multiplied by a baffling factor to yield TIQ
(i.e., contact time), if tracer study data are not available. Baffling classifications, TIO
definition, and determination are discussed in detail in Appendix D.
3.6.1.1 Contact Time Computations for 40 mgd Plant
The following pages illustrate detailed calculations to determine contact time for each
unit process, as shown in Figure 3-5.
- ' :- > ' '
Unit Process: RAW WATER PIPE
VOLUME OF RAW WATER PIPE = (Length) x (Cross-Sectional Area)
= (2,835 ft) xnx (2.5 ftf
= 55,665ft3
Convert cubic feet to gallons = 55>665 «* x 7'48g//ons = 4/6,374 gallons
FLOW RATE = Peak hourly flow occurring during the 24-hour period
= 10,651 gpm
THEORETICAL DETENTION TIME Volume of Unit Process
" Flow Rate
TOTRAWWATERP,PE=
as 39. 1 minutes
BAFFLING CONDITION = Perfect flow (Refer to Appendix D for determining baffling
August 1999
3-27
EPA Guidance Manual
Disinfection Profiling and Benchmarking
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
factors).
= 1.0
Tw/
Unit Tio RAW WATER PIPE = TDTx /T
= (39.1 minutes) x(1.0)
= 39 minutes
Unit Process: RAPID MIX BASIN
VOLUME OF RAPID MIX BASINS = (# of basins) x (Length) x (Width) x (Depth of Water)
= (4)x(12ft)x(12ft)x(8ft)
= 4,608 ft3
Convert cubic feet to gallons
_ 4,608 ft3 *7A89a''0nS =34,468 gallons
• 1ft3
THEORETICAL DETENTION TIME (TDT)
FLOW RATE = Peak hourly flow occurring during the 24-hour period
= 10,651 gpm
Volume of Unit Process
Flow Rate
TDT RAPID MIX BASINS = 34,468 gallons
10,651 gpm
= 3.24 minutes
BAFFLING CONDITION = Unbaffled basin (Refer to Appendix D for determining
baffling factors).
T1°/T= 0.10
Unit Tio RAPID MIX BASINS =
= (3.24 minutes) x (0. 10)
= 0.32 minutes
EPA Guidance Manual
Disinfection Profiling and Benchmarking
3-28
August 1999
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
Unit Process: FLOCCULATION
VOLUME OF FLOCCULATION BASINS = (# of basins) x (Length) x (Width) x (Depth of Water)
= (4) x (60 ft) x (30 ft) x (18 ft)
= 129,600ft3
Convert cubic feet to gallons = 129,600 ft3 x 7'48g*//ons = 969,408 gallons
1ft3
FLOW RATE = Peak hourly flow occurring during the 24-hour period
= 10,651 gpm
THEORETICAL DETENTION TIME (TDT) = Volume of Unit Process
Flow Rate
TDT FLOCCULATION BASIN= 969,408 gallons
10,651 gpm
= 91.0 minutes
BAFFLING CONDITION = Unbaffled basin (Refer to Appendix D for determining
baffling factors).
Tw
T = °-10
Unit Tio FLOCCULATION BASIN =
= (91.0 minutes) x (0.10)
= 9.1 minutes
Unit Process: SEDIMENTATION
VOLUME OF SEDIMENTATION BASINS =
(# of basins) x (Length) x (Width) x (Depth of Water)
= 4x234ftx74ftx10ft
= 692,640ft3
Convert cubic feet to gallons = 692>640 tf x 7A8gallons = 4? g
1ft3
FLOW RATE = Peak hourly flow occurring during the 24-hour period
= 10,651 gpm
August 1999
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
THEORETICAL DETENTION TIME (TDT) = Volume of Unit Process
Flow Rate
TDT SEDIMENTATION BASIN = 5,180,947 gallons
10,651 gpm
= 486 minutes
BAFFLING CONDITION = Average baffling conditions (Refer to Appendix D for
determining baffling factors).
= o.50
Unit Tio SEDIMENTATION BASINS= TDTx
= (486 minutes) x (0.50)
= 243 minutes
Unit Process: FILTRATION
VOLUME OF FILTRATION = Volume of Filters - Volume of Media
= (# of filters) x (Length) x (Width) x (Total Depth) ~
(# of filters) x (Length) x (Width) x (Depth of Media) x
(Porosity)
= (9) x(36ft)x(18ft)x(4ft)- (9) x (36 ft) x (18 ft) x (2 ft)
x(0.5)
23,328 ft3 - 5,832 ff = 17,496 if
Total depth is the depth of media plus the minimum depth of water above the media. For this
example, the plant operates with 2 feet of media and a minimum of 2 feet of water above the media.
Convert cubic feet to gallons = 1?i496 # x 7.48gallons = ^^ gaUom
Peak hourly flow occurring during the
FLOW RATE = 24-hour period
= 10,651 gpm
THEORETICAL DETENTION TIME (TDT) = Volume of Unit Process
Flow Rate
TDT FILTRATION BASIN = 130,870 gallons
10,651 gpm
= 72.3 minutes
BAFFLING CONDITION = _ .,_..,. _,.. ,„ , A _,. „
Supenor baffling conditions (Refer to Appendix D
EPA Guidance Manual
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August 1999
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
for determining baffling factors).
Tw.
T = 0.70
Unit Tw FILTRATION= TDTxTl/T
= (12.3 minutes) x (0.70)
= 8.6 minutes
Unit Process: FINISHED WATER PIPE
VOLUME OF FINISHED WATER PIPE = (Length) x (Cross-Sectional Area)
= 946.3 ft xxx (2.5f)f
= 18,581 if
Convert cubic feet to gallons = 18,581 ff x 7-48^a//ons = 133,
FLOW RATE = Peak hourly flow occurring during the 24-hour
period
= 10,651 gpm
THEORETICAL DETENTION TIME (TDT) = Volume of Unit Process
Flow Rate
TDT FINISHED WATER PIPE = 138,983 gallons
10,651 gpm
= 13.0 minutes
BAFFLING CONDITION = Plug flow baffling conditions (Refer to Appendix D
for determining baffling factors).
= 1.0
Unit Tio FINISHED WATER LINE= TDTx Tl//^T.
= (13.0 minutes) x(1.0)
= 13 minutes
August 1999
3-31
EPA Guidance Manual
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
Unit Process: CLEARWELLS
VOLUME OF WATER IN CLEARWELLS* = (# of tanks) x (Minimum water depth) x (Cross-
sectional Area)
= (2) x (20 ft) x (8,394.2 ft)
= 335,768ft3
Convert cubic feet to gallons = 335 768 tf x 7.48ga//ons = 5 ,lons
1ft3
'Volume of clean/veils should reflect a constant minimum storage level that is maintained during
peak hour flows. See Chapter 3 and Appendix D for more discussion.
FLOW RATE = Peak hourly flow occurring during the
24-hour period
= 10,651 gpm
THEORETICAL DETENTION TIME (TDT)= Volume of Unit Process
Flow Rate
TDTCLEARWELLS = 2,511,545 gallons
10,651 gpm
= 236 minutes
BAFFLING CONDITION = Poor baffling conditions (Refer to Appendix D for
determining baffling factors).
Tl%- = 0.30
Unit Tio CLEARWELLS = TDTx
Tw/
= (236 minutes) x (0.30)
= 71 minutes
3.6.1.2 Log Inactivation Computations for 40 mgd Plant
Following the diagram in Figure 3-5, the next step is to compute the estimated log
inactivation of Giardia and viruses for each disinfection segment. Note that profiling
and benchmarking based on virus inactivation is required only for systems proposing
to add or switch to ozone or chloramines. Profiling and benchmarking for virus
inactivation is strongly recommended for systems proposing to add or switch to
chlorine dioxide. This step requires the temperature, pH (only for chlorine), and residual
disinfectant concentration for each segment, as well as the TIO values computed in
Section 3.6.1.1. The segment Tjois the sum of the TIO for each unit process in the
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August 1999
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
segment. To compute CTactuai, multiply the segment TIQ by the residual disinfectant
concentration.
Look up the CT required to inactivate 3-log Giardia (CTs-iog, Giardia) and 4-log viruses
(CT4-iogi vims) in the CT tables (Tables 3-4 and 3-5 or Appendix C). If the temperature, pH,
or residual concentration values fall between those values listed in Tables 3-4 and 3-5 use
the guidelines stated earlier in Section 3.6. Once CTactuai and the CT required for 3-log
Giardia and 4-log virus inactivation are calculated, the estimated log inactivation for the
segment can then be computed:
Estimated Segment log inactivation of Giardia = 3.0 * CTactuai / CT3.iogj Giardia
Estimated Segment log inactivation of Viruses = 4.0 * CTactuai / CT/Hog, vims
Determine log inactivation for each disinfection segment for the 40 mgd plant example:
SEGMENT 1
The concentration of chlorine measured at the end of Segment 1 was 0.23 mg/L.
= (residual disinfection concentration) x (sum of TIO'S for each unit process)
= (0.23 mg/L of chlorine) x (39 + 0.32 + 9. 1 + 243 minutes)
= 67.03 mg-rnin/L
Determine the CTsjog, Giardia (i.e., 3-log inactivation of Giardia) from Table 3-4 or the CT
tables in Appendix C using the appropriate temperature, pH, and residual chlorine
concentration. Assuming:
Temperature = 6.1°C
pH= 8.0
C12= 0.23 mg/L
Using Table 3-4 for 5°C, pH 8.0, and concentration^ 0.4 mg/L to select the appropriate
CT value.
1 s . . " • ' . •
ia — 198 mg-min/L
Determine the CT^og, virus (i-e., 4-log inactivation of viruses) from Table 3-5 or the CT
tables in Appendix C.
August 1999
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
Temperature = 6.1°C
pH= 8.0
C12 = 0.23mg/L
Since .the temperature of 6. 1°C is not covered in the CT table, use the next lower
temperature, 6°C.
g, vims = 7.6 mg-min/L
Determine estimated log inactivation of Giardia and viruses for Segment 1:
Estimated Log inactivation of Giardia = 3.0 x (CTactuai/ CTs^og,
= 3.0 x (67.03 / 198)
= 1.02
Estimated Log inactivation of viruses = 4.0 x (CTactuai/ CT4.iog, virus)
= 4.0 x (67.03 / 7.6)
= 35.3
SEGMENT 2
The concentration of chlorine measured at the end of Segment 2 was 2.63 mg/L.
uai = (residual disinfection concentration) x (sum of TIQ'S for each unit process)
= (2.63 mg/L of chlorine) x (8.6 minutes + 13.0 minutes)
= 56.8 mg-min/L
Determine CTs-iog, Giardia (i-e., 3-log inactivation of Giardia) from Table 3-4 using
temperature, pH, and residual chlorine concentration. Assuming:
Temperature = 6. 1 °C
pH= 7.6
C12= 2.63 mg/L
.iog, ciardia= 263 mg-min/L
Determine required CT4,iog> vims (i-e., 4-log inactivation of viruses) from Table 3-5 or the
CT tables in Appendix C using the following temperature and pH.
EPA Guidance Manual 3-34 August 1999
Disinfection Profiling and Benchmarking
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
Temperature = 6.1°C
pH= 7.6
Since the temperature of 6.1°C is not covered in Table 3-5, use the next lower
temperature.
og, virus = 7.6 mg-min/L
Determine log inactivation of Giardia and viruses for Segment 2:
Estimated Log inactivation of Giardia= 3.0 x (CTactuai / CT3_iog)
= 3.0 x (56.8 7263)
= 0.65
Estimated Log inactivation of viruses = 4.0 x (CTactuai / CT4.iog> vims)
= 4.0 x (56.8 77.6)
= 30.0
SEGMENT 3
The concentration of chloramine measured at the end of Segment 3 was 2.23 mg/L.
CTactuai = (residual disinfection concentration) x (sum of TIO'S for each unit process)
= (2.23 mg/L of chloramine) x (71 minutes)
= 158.3 mg-min/L
Determine CT3_iog> Giardia (i-e., 3-log inactivation of Giardia) from the chloramine tables in
Appendix C. Assuming:
Temperature = 6.1 °C
Since the temperature of 6.1°C is not covered in the Appendix C CT tables for
chloramine, use the next lower temperature.
CTa.iog, aardia = 2, 1 30 mg-min/L
Determine CTViog, vims (i.e., 4-log inactivation of viruses) from the CT tables in Appendix
C using temperature. Assuming:
Temperature = 6.1 °C
August 1999 3-35 EPA Guidance Manual
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
Since the temperature of 6.1 °C is not covered in the CT tables, use the next lower
temperature.
CT4-iog, virus =1,889 mg-min/L
Determine estimated log inactivation of Giardia and viruses for Segment 3:
Estimated Log inactivation of Giardia = 3.0 * (CTactuai / CTs-iog, Giardia)
= 3.0* (158.3 72,130)
= 0.22
Estimated Log inactivation of viruses = 4.0 * (CTactuai/ CT4-iog, virus)
= 4.0* (158.3 71,889)
= 0.34
3.6.1,.3 Estimated Plant Log Inactivation for 40 mgd Plant
The final step is to calculate the estimated log inactivation by chemical disinfection for
the entire plant. The estimated plant log inactivation is simply the sum of the segment
log inactivation for the particular organism (Giardia or viruses).
Estimated Log inactivation for the = Sum of estimated log inactivations of
entire plant by disinfection chemical each disinfection segment
= Estimated Log inactivation Segment 1 +
Estimated Log inactivation Segment 2 +
Estimated Log inactivation Segment 3
Estimated Log inactivation of
Giardia for the entire plant
Estimated Log inactivation of
viruses for the entire plant
= 1.02 + 0.65 + 0.22
= 1.89
= 35.3 + 30.0 + 0.34
= 65.64
EPA guidance suggests that conventional filtration treatment receive a 2.5-log credit for
Giardia removal through sedimentation and filtration. Therefore, to comply with the
SWTR, the plant must achieve at least 0.5-logs inactivation (to achieve at least 3.0-logs
of combined removal and inactivation). This plant is in compliance with the SWTR.
EPA Guidance Manual
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3-36
August 1999
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
3.6.2 Example of Developing a Disinfection Profile for a 5 mgd
Plant for One Month !
This disinfection profile example was developed for a direct filtration treatment plant in
Missouri with a design capacity of 5 mgd. The treatment plant consists of an intake
structure with a pumping station, two units for rapid mixing, two flocculation units, and
three sand filters of equal treatment capacity. Each sand filter is sized for situatuins when
one is out of service, the other two are capable of carrying design flow. The treatment
plant has a clearwell that is used as a contact basin and is used for storage. The volume
of the clear well is equivalent to one-day average production (2.5 million gallons); the
dead storage volume is 1.25 million gallons (storage volume used to calculate contact
time).
Table 3-8 presents the output data of a spreadsheet designed to develop a disinfection
profile for systems using various chemical disinfectants. Because chlorine is applied at
the rapid mixing stage and the free chlorine residual is measured only at the clearwell, the
same value is used for various treatment units.
The data presented in Table 3-8 for pH, temperature and chlorine residual values are
actual readings from the treatment plant. The plant is expected to run at design capacity in
15 years. Currently it serves a population of about 12,000 and runs a maximum peak
hourly rate of 2000 gpm or 2.6 mgd.
The input data needed to calculate daily log inactivation and develop disinfection profile
are: the type of disinfectant, date, daily pH, temperature, peak hourly rate, volume of each
treatment process and disinfectant free residual at each sampling point. Table 3-9
presents the input and output data used for 9/01/96. Using a spreadsheet, Table 3-9 is
developed as an example of automated calculations of estimated log inactivation for
Giardia and viruses using the Approximation Method for the month of September 1996.
Details on how to calculate volume of water in each process unit were provided
previously in a step-by-step detailed example of a 40-mgd treatment plant in Section
3.6.1.
August 1999
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
Table 3-8. Actual Readings From a SW Treatment Plant in Missouri
Date
Disinfectants
Process Name
Volume (gal)
Baffling Condition
(Tic/T)
Peak Hourly Flow
(gpm)
Theoretical Detention
Time (min)
Tio (min)
Free Disinfectant
Concentration (mg/L)1
Plant CT Value (mg-
min/L)
pH
Temperature (°C)
CT3-log,GiaKBa
CT 4-k>g,VTuses
Estimated Plant
Giardia Log
Inactivation
Estimated Plant
Viruses Log
Inactivation
SEGMENT 1
09/01/96
Ck
Rapid Mix
3,500
0.1
1,820
1.92
0.19
0.95
0.18
7.59
23.9
81
2.4
0.01
0.30
Flocculation
130,000
0.3
1,820
71.43
21.43
0.95
20.36
7.59
23.9
81
2.4
0.75
33.93
Sedimentation
0
0.1
1,820
0.00
0.00
0.95
0.00
7.59
23.9
81
2.4
0.00
0.00
Segment 1 Totals
1 Plant only measures residual at discharge from clearwell, therefore, this residual is assumed to
be the residual throughout the plant.
Filtration
80,000
0.3
1,820
43.96
1.3.19
0.95
12.53
7.59
23.9
81
2.4
0.46
20.88
Tio
CT
CTa-log.Giardia
CT4-log,vruses
Giardia Log
Inactivation
Virus Log
Inactivation
Clear Well
1,250,000
0.1
1-;820
686.81
68.68
0.95
65.25
7.59
23.9
81
2.4
2.42
108.75
103.49
98.31
81
2.4
3.64
163.86
EPA Guidance Manual
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August 1999
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
Table 3-9. Input and Output Data Used to Calculate Log Inactivations
Date
09/01/96 •
09/02/96
09/03/96
09/04/96
09/05/96 .
09/06/96
09/07/96
09/08/96
09/09/96
09/10/96
09/11/96
09/12/96
09/13/96
09/14/96
09/15/96
09/16/96
09/17/96
09/18/96
09/19/96
09/20/96
09/21/96
09/22/96
09/23/96
09/24/96
09/25/96
09/26/96
09/27/96
09/28/96
09/29/96
09/30/96
Peak Hourly
Flow Rate
(gpm)
1,820
1,880
1,855
1,840
1,840
1,830 .
1,810
1,820
1,875
1,834
1,867
1,811
1,847
1,869
1,839
1,846
1,828 ,
1,823
1,820
1,845
1,860
1,852
1,855
1,843
1,859
1,835
1,845
1,860
1,855
1,824
SEGMENT 1
pH Temperature Disinfectant Segment 3-log 4-log Estimated Estimated
Residual CT Actual Giardia Viruses Segment Segment Virus
(mg/L) CT CT Giardia Inactivation2
Inactivation1
7.59 23.9
7.85 22.8
7.87 21.5
7.81 21
7.86 21
7.94 20.3
8.11 19.4
7.89 18.9
7.67 19.6
7.64 19.7
6.75 19.8
6.65 18.9
6.73 18.5
6.85 19
6.72 20.3
6.92 21.1
6.71 19.4
6.96 18
6.89 16.4
7.00 15.6
700 15.7
7.06 15.8
6.62 15.5
7.43 15.1
7.27 14.9
7.38 14.1
7.41 13.3
7.28 13
7.43 13.3
7.42 14
0.95
1.17
1.02
1.23
1.03
1.04
1.1
1.03
1.29
1.24
1.03
1.0
1.03
1.01
1.1
1.16
1.08
0.61
1.29
1.17
1.03
0.96
1.18
1.12
1.3
1.12
1.05
1.31
1.58
1.45
98.31
117.22
103.57
125.91
105.44
107.04
114.47
106.59
129.58
127.32
103.93
103.98 '
105.04
101.77
112.64
118.33
111.26
63.02
133.47
119.47
104.31
97.65
119.84
114.49
131.72
114.97
107.19
132.69
160.47
149.73
81
83
83
85
83
83
134
111
114
114
76.
76
76
76
57
57
76
73
78
92
92
90
76
92
140
137
137
140
144
144
2.4
2.6
2.8
2.8
2.8
3.0
3.2
3.4
3.2
3.2
3.2
3.4
3.4
3.2
3.0
2.8
3.2
3.4
3.8
4.0
40
4.0
4.0
4.0
4.4
4.4
4.8
4.8
4.8
4.4
3.64
4.24
3.74
4.44
3.81
3.87
2.56
2.88
3.41
3.35
4.10
4.10
4.15
4.02
5.93
6.23
4.39
2.59
5.13
3.90
3.40
3.26
4.73
3.73
2.82
2.52
2.35
2.84
3.34
3.12
163.86
180.34
147.95
179.87
150.62
142.72
143.08
125.40
161.98
159.15
129.91
122.33
123.58
127.21
150.19
169.04
139.07
74.14
140.50
119.47
104.31
97.65
119.84
114.49
119.74
104.52
89.32
110.57
133.73
136.11
1 3.0 X CT actual
CT 3-log, GianSa
2 4.0 XCT actual
CT4-tog, viruses
August 1999
3-39
EPA Guidance Manual
Disinfection Profiling and Benchmarking
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
3.6.3 Determination of Disinfection Profile and Benchmark
Listed below are tasks needed to develop the disinfection profile and set the benchmark:
. Repeat the above calculations for 1, 2, or 3 years of available or collected data.
. Arrange total plant estimated log inactivation in chronological order,
beginning with the earliest data.
. Develop a graphical plot of estimated log inactivation versus time (i.e.,
disinfection profile). Inactivation should be on the y-axis and time (days)
should be on the x-axis.
. Calculate the average (arithmetic mean) estimated disinfection log inactivation
for each calendar month.
. Determine the calendar month in a year with the lowest average log
inactivation. The lowest average month becomes the "critical period" for that
year.
Table 3-10 lists the critical periods for this plant in each year and the corresponding log
inactivation.
Table 3-10. Critical Periods for Existing Disinfection Practice
Year
1994
1995
1996
Month of
Critical Period for
Giardia Inactivation
February
February
January
Log
Inactivation
of Giardia
2.0
1.5
1.6
Month of
Critical Period for
Viral Inactivation
February
February
February
Log
Inactivation
of Viruses
63.3
50.7
50.8
The benchmark is the lowest monthly average log inactivation and is calculated as the
average of the three critical periods. For the plant illustrated in Table 3-10, the
benchmarks for Giardia and viruses are calculated as follows:
= Average Log Inactivation of Critical Periods
(2+1.5 + 1.6)73
1-7
= Average Log Inactivation of Critical Periods
(63.3 + 50.7 + 50.8)73
54.9
The disinfection profiles and benchmarks based on Giardia and viruses are illustrated in
Figures 3-6 and 3-7.
Benchmarky
EPA Guidance Manual
Disinfection Profiling and Benchmarking
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August 1999
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
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Disinfection Profiling and Benchmarking
3-42
August 1999
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
3.6.4 Modification of Disinfection Practice
In this example for a 40 mgd plant, the utility.has determined that DBF concentrations
exceed profiling applicability triggers and has developed a profile. It then intends to
modify its disinfection practice to control DBFs. The plant is considering two options for
control:
Option 1
• Replace pre-oxidation using chlorine with potassium permanganate
preoxidation. Although no disinfection credit is available for using potassium
permanganate, the utility staff believes that it would effectively control tastes
and odors. The point of chlorination is moved downstream of sedimentation
to assist in the control of DBFs.
• Apply the chlorine dose after sedimentation to increase the chlorine residual
by 20 percent to offset the loss in disinfection contact time.
. Add ammonia prior to the clearwell as in the original disinfection scheme.
A process diagram of Option 1 proposed modifications is shown in Figure 3-8.
Option 2
• Replace pre-oxidation using chlorine with potassium permanganate
preoxidation.
• Add an ozone contactor just prior to rapid mix to compensate for the loss in
disinfection credit associated with eliminating prechlorination. The ozone
contactor would have a theoretical detention time of 1.3 minutes under the
design flow of 40 mgd. The utility plans to operate under conditions
providing the ozone residuals presented in Table 3-11. Table 3-11 illustrates
CT calculations and log inactivation calculations under specific assumptions.
Also, biologically active filtration to control AOC produced by ozonation will.
be used to control distribution system regrowth. Refer to the Alternative
Disinfectants and Oxidants Guidance Manual (USEPA, 1999a) for more
information.
• Move the point of chlorination just downstream of filtration to assist with the
control of DBFs and virus inactivation.
• Add ammonia prior to the clearwell as in the original disinfection scheme.
A process diagram of Option 2 proposed modifications is shown in Figure 3-9.
August 1999
3-43
EPA Guidance Manual
Disinfection Profiling and Benchmarking
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
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Disinfection Profiling and
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS t
A disinfection profile and alternative disinfection benchmark were developed for the first
disinfection option (i.e., using potassium permanganate for pre-oxidation and using a
chlorination point downstream for post-sedimentation). The proposed modification to
disinfection does not include adding or switching to ozone, chloramines, or chlorine
dioxide. Therefore, developing a profile and benchmark based on virus inactivation is
not required. Table 3-12 lists the critical periods for each year and the corresponding log
inactivation values.
Table 3-12. Critical Periods for Disinfection Option 1
Year
1994
1995
1996
Month of
Critical Period for
Giardia Inactivation
February
February
January
Log
Inactivation
Of Giardia
0.7
0.5
0.5
Modification Benchmarko^/a = Average Log Inactivation of Critical
Periods
= (0.7 + 0.5 + 0.5)73
= 0.6
The daily log inactivations and modification benchmark for Giardia are illustrated in
Figure 3-10. Note that the modification Benchmarkc/ar<&z for Option 1 is 0.6-log
inactivation, which is lower than the existing Benchmarkc/a^/a of 1.7-log inactivation.
The system realizes that a higher free chlorine residual will improve the alternative
benchmark level by about 0.2-log inactivation (say from 0.6 mg/L to 1.2 mg/L of free
chlorine at 5°C and a pH of 8). These results indicate that Option 1 would not provide an
equivalent degree of protection against Giardia as compared to the existing disinfection
scheme.
A system is not prohibited from making a change that will result in a lower benchmark.
Either the chlorine dose or contact time could be increased for this option to meet the
current disinfection benchmark. A long-term option could involve increasing contact
time by improving baffling conditions in the contact basin. The system may consult with
the State on how to change its disinfection practice that will result in a lower inactivation
level and at the same time protect public health as detailed in Chapter 5 (Using the
Benchmark) and 6 (Alternative Disinfection Benchmark) of this guidance manual.
August 1999
Disinfection Profiling and Benchmarking
3-47
EPA Guidance Manual
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
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EPA Guidance Manual
Disinfection Profiling and Benchmarking
3-48
August 1999
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
A disinfection profile and benchmark were also developed for the second disinfection
option using the same methods as Option 1. Because Option 2 would add ozone to the
disinfection system, profiling and benchmarking based on virus inactivation is also
required. Table 3-13 lists the critical periods for each year and the corresponding log
inactivation values
Table 3-13. Critical Periods for Disinfection Option 2
Year
1994
1995 :
1996
Month of
Critical Period for
Giardia Inactivation
February
February
January
Log
Inactivation
of Giardia
2.6
2.1
2.1
Month of
Critical Period for
Virus Inactivation
February
February
February
Log
Inactivation
of Viruses
19.3
15.4
15.5
Modification Benchmarkc/a^/a - Average Log Inactivation of Critical
Periods
= (2.6 + 2.1+2.1)73
= 2:3
Modification Benchmarkvimses = Average Log Inactivation of Critical
Periods
= (19.3 + 15.4 + 15.5)73
= 16.7
The daily log inactivations and benchmarks of Giardia and viruses are illustrated in
Figures 3-11 and 3-12. Note that the Modification Benchmarkc/ar^a for Option 2
achieves 2.3-log inactivation, which is higher than the existing Benchmarkaa^ja of 1.7-
log inactivation. This indicates that Option 2 would provide equivalent or better
microbial protection against Giardia when compared with the existing disinfection
strategy.
However, the Modification Benchmarkyimses for Option 2 achieves a log inactivation of
16.7, which is lower than the existing Benchmarkvjruses of 54.9-log inactivation.
Consequently, Option 2 would not provide an equivalent degree of microbial protection
against viruses when compared with the existing disinfection strategy, although 16.7-log
inactivation of viruses would provide excellent protection against these pathogens. This
indicates that the proposed disinfection strategy works against Giardia, but the Utility
would need to consult with the State prior to implementing an alternative benchmark for
virus inactivation. See Chapter 6 for more information.
August 1999
Disinfection Profiling and Benchmarking
3-49
EPA Guidance Manual
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
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3-50
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
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Benchmarking
3-51
EPA Guidance Manual
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3. CREATING A PROFILE: DATA REQUIREMENTS AND CALCULATIONS
THIS PAGE INTENTIONALLY LEFT BLANK
EPA Guidance Manual
Disinfection Profiling and Benchmarking
3-52
July 1999
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4. CALCULATING THE BENCHMARK
The IESWTR requires systems to use disinfection benchmarking to determine whether
there may be a significant reduction in microbial inactivation as a result of modifying
disinfection practices to meet the Stage 1 DBPR MCLs for TTHMs and HAAS. This
determination will allow for an informed consultation with 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 eases,
virus log inactivations that are provided under current disinfection practices to ensure that
changes to disinfection practices do not result in inactivation levels lower than the
calculated benchmark without appropriate State consultation and review. The
disinfection benchmark quantifies a lower bound of the existing disinfection practices so
that alternative disinfection strategies can be compared to current minimum levels of
disinfection. This chapter describes the procedure to calculate a disinfection benchmark.
4.1 Applicability
Water systems required to develop a disinfection profile are required to develop a
benchmark based on Giardia inactivation if they are planning to "significantly modify"
their disinfection practices.
Systems that are planning to add or switch primary disinfectants to include ozone,
chloramines, or chloride dioxide must also calculate a profile and benchmark based on
virus inactivation in addition to Giardia. Virus inactivation must be determined for these
systems to address the possibility of reduced protection against viruses when using an
alternative disinfectant.
4.2 Benchmark Calculations
The calculation of disinfection profiling, including the estimated log inactivation of
Giardia and viruses, is described in Chapter 3 of this guidance manual. Once the
disinfection profile is calculated, the methodology for determining the benchmark is the
same for viruses as it is for Giardia.
As described in the IESWTR, a disinfection benchmark is calculated using the following
steps: .
• Complete a disinfection profile that includes the calculation of log inactivation
of Giardia and/or viruses for each day of the profile.
• Compute the average log inactivation for each calendar month of the profile
by averaging the daily log inactivation values.
August 1999
4-1
EPA Guidance Manual
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4. CALCINATING THE BENCHMARK
For each 12-month period the profile covers (i.e., 0-12 months, 12-24 months,
and 24-36 months), select the month with the lowest average log inactivation
for each 12-month period. This month is the "lowest average month" for the
12-month period (LowestAverageMontht, where i designates the first, second,
or third year and is known as the "critical period").
— If data from only one year are available, the critical period for that year
becomes the benchmark.
— If data from multiple years are available, systems must calculate their
benchmarks as the average of the lowest monthly averages for each year.
Using three years of data as an example, the benchmark would be
calculated as follows:
Benchmark =
( LowestAverageMonth i + LowestAverageMonth 2 + LowestAverageMonth 3 )
The following example demonstrates how a benchmark is calculated using three years of
log inactivation data.
Disinfection Benchmark Example Calculation:
Step 1. Calculate the monthly average log inactivations for each month of disinfection
profiling data. In this example, three years of data are available. Table 4-1
presents the daily log inactivation values of a hypothetical system for the month of
January 1998.
The monthly average log inactivation is calculated by summing the daily values
and dividing by the number of days in the month as follows:
V Daily Log Inactivation Values
Monthly Average Log Inactivation = —
Days per Month
For this example, the monthly average log inactivation for January 1998 is 3.94,
calculated as follows:
^T Daily Log Inactivation Values \ 20.10
Days per Month
31
3.94
EPA Guidance Manual
Disinfection Profiling and Benchmarking
4-2.
August 1999
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4. CALCULATING THE BENCHMARK
Monthly average log inactivations are then calculated in a similar manner for the
other 35 months in the three-year period.
Table 4-1. Daily Log Inactivation for Hypothetical Plant for January 1998
Date
1/1/98
1/2/98
1/3/98
1/4/98
1/5/98
1/6/98
1/7/98
1/8/98
1/9/98
1/10/98
1/11/98
1/12/98
1/13/98
1/14/98
1/15/98
1/16/98
Log Inactivation
3.26
3.17
3.36
4.82
3.65
3.22
4.03
4.97
4.77 •
4.31
4.57
3.89
4.11
4.30
3.10
4.89
Date
1/17/98
1/18/98
1/19/98
1/20/98
1/21/98
1/22/98
1/23/98
1/24/98
1/25/98
1/26/98
1/27/98
1/28/98
1/29/98
1/30/98
1/31/98
Log inactivation
3.62
4.31
4.73
4.19
3.23
4.22
3.34
3.63
4.35
3.24
3.04
3.07
3.68
4.54
4.48
Step 2. Next, the minimum monthly average log inactivation values for .each year (each
12-month period) should be identified. Table 4-2 provides the average monthly
log inactivations for the hypothetical system in this example. The minimum
values for each year (i.e., January 1996, January 1997, and February 1998) are
highlighted.
This example is typical in that lowest monthly average log inactivation values
often occur during the winter due to the reduced effectiveness of disinfection at
lower temperatures. Note that the three minimum monthly values for each year
are not the minimum three values for the entire three-year record (i.e., although
the average log inactivation of 3.09 for February 1997 is less than the average log
inactivation 3.23 for January 1996, the January 1996 value is used). That is, the
minimum monthly average for each of the three years is used to calculate the
benchmark, not the three lowest values.
August 1999
4-3
EPA Guidance Manual
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4. CALCULATING THE BENCHMARK
Table 4-2. Monthly Average Log Inactivation Values for Hypothetical Plant
January-96
February-96
March-96
April-96
May-96
June-96
July-96
August-96
September-96
October-96
November-96
December-96
3.23
3.42
3.62
4.31
4.73
4.19
4.56
4.22
3.34
3.63
4.35
3.65
January-97
February-97
March-97
Aprii-97
May-97
June-97
July-97
August-97
September-97
October-97
November-97
December-97
3.04
3.09
3.68
4.54
4.48
3.26
3.17
3.36
4.82
3.65
3.22
4.03
January-98
February-98
March-98
Apri!-98
May-98
June-98
July-98
August-98
September-98
October-98
November-98
December-98
3.94
3.07
4.31
4.27
3.45
4.11
.4.30
3.62
4.77
3.68
4.54
3.52
Step 3. Finally, the benchmark is calculated as an average of the minimum monthly
average values for each of the three years. For this example, the benchmark is
calculated as follows:
^T LowestAverageMonths i _ (3.23 + 3.04 + 3.07) _
Number of years
= 3.11
If the plant has only two years of log inactivation data (i.e., 1997 and 1998), the average
of the minimum values for 1997 and 1998 are used and the benchmark is equal to 3.06
(i.e., [3.04+3.07]/2). Likewise, if the plant has only one year of acceptable data (i.e.,
1998), the single lowest average month is used and the benchmark is 3.07.
Several detailed examples are provided in Chapter 5 to further illustrate the calculation of
benchmarks when modifications to disinfection practices are being considered.
4.3 The Completed Benchmark
As required in the IESWTR, water systems must work with their states when calculating
benchmarks. Once the benchmarking calculations are completed, water systems must
submit the calculations and supporting data to the State for consultation prior to changing
disinfection practices. The State will use the benchmark to evaluate the microbial
inactivation the system has achieved over time and compare this with the modified
disinfection system. The use of the benchmark is discussed further in Chapter 5.
EPA Guidance Manual
Disinfection Profiling and Benchmarking
4-4
August 1999
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5. Using the Benchmark
The IESWTR establishes the disinfection benchmark as the lower bound on disinfection
effectiveness of ah existing water system. 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.their disinfection practices. The State would then require that
all proposed modifications to existing disinfection practices be designed to meet current
disinfection benchmarks. The State may also use the profile and benchmark to determine
an appropriate alternative benchmark (see Chapter 6). Disinfection benchmarks provide a
reference point for States to evaluate whether systems will compromise microbial
protection when complying with the Stage 1 DBPR provisions to control disinfection
byproducts.
This chapter provides a definition of significant modifications to disinfection practices,
and describes State involvement in the process. Chapter 6 includes a discussion on how a
State may set alternative disinfection benchmarks for systems that cannot maintain their
current Giardia or virus benchmark.
5.1 Definition: Modifying Disinfection Practices
This section describes example modifications to disinfection practice that may trigger the
benchmarking process required under the IESWTR. Although this section summarizes
several DBF control alternatives as illustrative examples, it is not meant to provide a
comprehensive discussion of this subject. A more complete discussion of certain DBF
control alternatives is provided in the Alternative Disinfectants and Oxidants Guidance
Manual (USEPA, 1999a). , ,
A public water system may consider modifying their disinfection practices to comply with
provisions of the Stage 1 DBPR. Significant modifications to disinfection practices
trigger disinfection benchmarking requirements under the IESWTR. As described in the
IESWTR, significant modifications to disinfection practices are defined as the following:
• Moving the point of disinfectant application
• Changing the disinfectant(s) used in the treatment plant
• Changing disinfection practices
• Any other modification identified by the State as significant.
A brief description of each of these four types of modifications is presented below.
August 1999
5-1
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5. USING THE BENCHMARK
5.1.1 Moving the Point of Disinfectant Application
Water systems using pre-disinfection might consider moving the point of disinfectant
application further into the plant treatment train to reduce the contact time between DBF
precursors and the disinfectant(s). The TTHM formation potential may be reduced by as
much as 50 percent through conventional coagulation and settling (Singer and Chang,
1989; Summers et al., 1997).
Conventional water treatment plants that apply chlorine to raw water generally have
adequate contact time for disinfection. Many water systems have eliminated or changed
their pre-disinfection practices to control DBFs. Pre-disinfection practices involve using
chemical or physical processes to remove precursors from the source water. Moving the
point of disinfection after clarification with enhanced coagulation allows for greater
removal of DBF precursors before disinfectant is added and also reduces the disinfectant
demand of the water. When moving the point of disinfection further into the.treatment
process, a system must consider whether adequate contact time is available to achieve
sufficient disinfection and how this modification will affect the benchmark. Systems may
find that seasonal use of this modification is helpful in reducing summer DBF levels,
which are typically the highest.
5.1.2 Changing the Disinfectant(s) Used lin the
Treatment Plant
Water systems may consider changing the disinfectant used in their treatment plant to
comply with the Stage 1 DBPR MCLs. Several studies have evaluated the implications
of changing the disinfection practices in water treatment plants. EPA and the Association
of Metropolitan Water Agencies (AMWA) funded a two-year study of 35 water treatment
facilities to evaluate DBF production. Among four of the facilities, alternative
disinfection strategies were investigated to evaluate the difference in DBF production
from the plants' 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 5-1 presents the 10 potential strategies often considered for primary and secondary
disinfection. Table 5-2 fists the changes in DBF production observed in the four plants
after eight of these new strategies were implemented.
As shown in Table 5-2, employing different and more carefully selected primary and
secondary disinfectants reduced the amount of DBFs produced. In general, the results
followed the characteristics of the DBFs associated with the primary disinfectant used
(i.e., halogenated DBFs with chlorine compounds). Organic oxidation products form
when strong oxidants such as ozone are used. However, by carefully selecting the
primary and secondary disinfectants, and avoiding long contact times and high dosages of
halogens, the total DBF formation declined. It is important to note that the study did not
evaluate bromate formation.
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5. USING THE BENCHMARK
Table 5-1. Strategies for Primary and Secondary Disinfectants
Base Disinfection Condition
Chlorine/Chlorine
Chlorine/Chlorine
Chlorine/Chlorine
Chlorine/Chlorine
Chlorine/Chlorine
Chlorine/Chlorine
Chlorine/Chloramine
Chlorine/Chloramine
Ozone/Chlorine
Chloramine/Chloramine
Modified Disinfection Practice
Chlorine/Chloramine
Chloramine/Chloramine
Chlorine dioxide/Chloramine
Ozone/Chlorine
Ozone/Chloramine
Chlorine dioxide/Chlorine
Ozone/Chloramine
Chlorine dioxide/Chloramine
Ozone/Chloramine
Ozone/Chloramine
Note: Disinfectants are listed as primary disinfectant/secondary disinfectant
Since systems can initially determine what is considered a significant change in
disinfection practice (including those specifically identified by the State), they may also
consider changing the disinfectant and point of disinfectant application. For example, a
system shifting from chlorine/chlorine to chlorine dioxide/chloramine may want to
consider shifting the ammonia application point after the point of chlorine application to
allow for some chlorine contact time for virus inactivation.
5.1.3 Changes to Disinfection Practices
Other significant changes to disinfection practices also require water systems to consult
with the State before making the treatment change. Types of modifications considered
significant include, but are not limited to, the following:
• Changes in the contact basin geometry and baffling conditions
. Increases in the pH during disinfection by greater than 1 unit (for chlorine
only)
• Changes in the raw water source.
The IESWTR requires that water systems provide information to the State supporting the
rationale for the potential treatment change. Types of supporting materials include a
description of the proposed change, the disinfection profile, and an analysis of how the
proposed change will affect the current disinfection benchmark.
August 1999
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5. USING THE BENCHMARK
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5. USING THE BENCHMARK
5.1.4 Other Modifications Identified by the State
The State may ultimately determine what changes in water system operations constitute a
change in disinfection practices. If the State concludes that a change in disinfection
practice is a significant modification, the water system must develop and submit a
disinfection benchmark.
The modifications listed in Sections 5.1.1 through 5.1.3 are not an exhaustive list and
may be amended at the State's discretion. Therefore, a water system should check with
the State program office for assistance in determining whether the proposed change
triggers the disinfection benchmarking procedure. Water systems can refer to Alternative
Disinfectants and Oxidants Guidance Manual for additional information and references
on disinfectant capabilities and the potential implications of modifying disinfection
practices (USEPA, 1999a).
5.2 Communicating with the State
The IESWTR requires public water systems to consult with the State in order to assess
the impact that disinfection modifications may have on their current log inactivatjon
levels. Using the disinfection benchmarking method, the State may determine if the
change in disinfection practice is acceptable (e.g., meets the current disinfection
benchmark). However, there is no federal requirement for State approval of disinfection
modifications.
As required under the IESWTR, the system must submit profiling information to the
State. Profiling information includes:
• Detailed plans (schematic) and operating strategy of the proposed
modifications to disinfection practices.
• The disinfection profile and supporting calculations and data for both the
existing practice and the proposed change.
. The current disinfection benchmark value and supporting calculations.
• Detailed calculations that assess the potential impact of the intended changes
in disinfection practice (i.e., with regard to anticipating changes in log
inactivation to achieve modifications on current log inactivation (discussed in
Section 5.3)).
Note that systems adding or switching to ozone or chloramines must provide the above
information for both Giardia and viruses. EPA strongly recommends that systems also
calculate a virus profile and benchmark if they are switching to chlorine dioxide.
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5.3 Calculations to Identify Modification Impact
To assess the impact of modifications on current log inactivation, systems need to
perform several additional benchmarking calculations. Specifically, water systems
should calculate "modification benchmarks," based on the current operating conditions
before the process change is made. These modification benchmarks should be compared
to the original benchmark to evaluate the expected inactivation level of the modified
disinfection practice.
The steps to calculate these modification benchmarks are as follows:
. Identify the lowest average months from the original profile (i.e., the one to
three months that were averaged to calculate the original benchmark).
• Using the temperature, pH, and contact times (unless the modification
significantly changes these values) from the original profile calculations,
systems calculate the daily log inactivation for Giardia (and/or viruses) for
each day of the month under the proposed modification (i.e., for conditions
after the modification is complete). The water system will need to assume
reasonable values for the disinfectant residuals. It may also need to calculate
or estimate contact times, or identify new points of disinfectant residual
sampling to reflect the modification.
. Calculate the average log Giardia and/or virus inactivation for the months
identified in the first bullet.
. Calculate the average of the monthly values. This value is the modification
benchmark.
. Compare the original benchmark to the modification benchmark. If the
modification benchmark is greater than the original benchmark, the
modification will likely be acceptable after consultation with the State.
Modification benchmarks lower than the original benchmark should be
evaluated by the State to determine whether the resulting level of disinfection
is still considered adequate based on source water quality and watershed
conditions (discussed further in Chapter 6).
The system and State should discuss the reasons for any modification and whether better
options exist, and assess the modification's impact on log inactivation. The State and the
system should jointly assess the impact that the proposed modification will have on log
inactivation levels of Giardia and/or viruses.
A detailed example of calculating the impact of changes in disinfection practices,
including the comparison of original and modification benchmarks, is provided in Section
5.5.
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5. USING THE BENCHMARK
5.4 Alternative Benchmark
As addressed in the EESWTR, situations will exist when a system may need to develop an
alternative benchmark to comply with the Stage 1 DBPR provisions. These situations are
detailed in Chapter 6.
The disinfection benchmark can also be met by a combination of inactivation with a
chemical disinfectant and an improvement in the physical removal of pathogens after
consultation with the State. Consider an unfiltered system with a disinfection benchmark
of 4-logs for Giardia. If this system were to implement conventional filtration and
receive 2.5-log Giardia removal credit, the chemical disinfection required to meet the
existing disinfection benchmark could be reduced to 1.5-log Giardia inactivation.
Likewise, a utility that makes a process enhancement to improve pathogen removal could
receive credit toward achieving its existing disinfection benchmark. Consider a
conventional filtration plant that upgrades its process to include ultrafiltration using
membranes. Because ultrafiltration has been demonstrated to achieve greater than 6-logS
of Giardia removal, the existing Giardia disinfection benchmark could be reduced by an
amount deemed acceptable by the State (AWWARF, 1997). The remainder of the
existing disinfection benchmark could be accomplished with chemical disinfection.
5.5 Illustrative Examples
This section considers simple examples of disinfection byproduct control. These
examples are applicable to conventional filtration plants that are considering additional
control of DBFs to comply with the Stage 1 DBPR. The examples include process
changes that may accomplish the goals of controlling DBF levels and disinfection
benchmarking. This section does not discuss major process changes, such as alternative
primary disinfectants, since they require extensive engineering evaluation! As discussed
previously, the system should only implement significant changes to a disinfection
practice after careful consideration and consultation with the State. In most
circumstances, the system should seek the assistance of a qualified professional engineer
to develop and implement a process change. The Microbial and Disinfection Byproducts
Simultaneous Compliance Guidance Manual (USEPA, 1999b) presents case studies and
scenarios involving solutions to some of the potential conflicting compliance issues.
5.5.1 DBF Control using Enhanced Coagulation
5.5.1.1 Base Conditions (Plant A)
This section considers the base condition to be a conventional filtration plant (Plant A)
that practices prechlorination. Table 5-3 lists the important raw water characteristics,
while Table 5-4 describes the important unit processes of Plant A.
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5. USING THE BENCHMARK
Table 5-3. Raw Water Quality (Plant A)
Parameter
PH
TOG (mg/L)
UV-254 (1/cm)
Bromide (mg/L)
Temperature (°C)
Alkalinity (mg/L as CaCOs)
SUVA (L/mg-m)
Value
7.5-8.0
3.8-5.0
0.1-0.15
0.15-0.2
6-20
50-60
-2.5-3.7
Table 5-4. Base Condition Unit Processes (Plant A)
Process
Influent
Chlorine
Alum
Rapid Mix
Flocculation
Settling
Filtration
Clearwell
Distribution
Characteristics
Raw Water Characteristics above
Dose 4 mg/L
Dose 20 mg/L
5 minutes detention, 0.1 baffling factor
20 minutes detention, 0.3 baffling factor
90 minutes detention, 0.3 baffling factor
15 minutes detention, 0.5 baffling factor
60 minutes detention, 0.1 baffling factor
3 days maximum detention time
The disinfection benchmark for Giardia for this conventional filtration plant is 0.75-logs.
This system applies chlorine to the raw water for disinfection to achieve at least a 0.2
mg/L distribution system residual. Since chlorine and alum are both acids, the pH is
reduced from about 7.5 in the influent to 7.1 in the finished water. Total organic carbon
is removed in the coagulation/settling process from 5.0 mg/L in the raw water to 3.7 mg/L
in the finished water (which is inadequate to meet Stage 1 DBPR requirements for
enhanced coagulation). This results in a concurrent decline in SUVA.
The TTHM and HAAS concentrations experienced by this system with its three-day
detention time in the distribution system are listed in Table 5-5. The running annual
average (RAA) TTHM and HAAS values are 87 and 58 (JLg/L, respectively. Because the
TTHM value exceeds the Stage 1 MCL, this system must implement a strategy for TTHM
control. Also, since the HAAS concentration is close to the MCL, the system should
implement a HAAS control strategy.
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Table 5-5. System DBF Concentrations (Plant A)
Parameter
TTHM (ng/L)
HAAS Oig/L)
Summer
145
71
Winter
29
44
RAA
87
58
Note: Running annual average is based on quarterly sampling (not shown).
The plant examines making four modifications to its disinfection practices to control
DBFs. These modifications include:
1. Practicing enhanced coagulation as required by the Stage 1 DBPR
2. Installing chloramination to provide residual disinfection
3. Moving the point of chlorine application after settling (possibly a seasonal
change)
4. Improving hydraulic characteristics of clearwell.
The system operator assesses whether practicing enhanced coagulation is likely to achieve
the desired TTHM and HAAS reductions. Based on UV absorbance, TOC
concentrations, and DBF levels, the plant's management decides to employ enhanced
coagulation as a first step to control DBF levels.
5.5.1.2 Enhanced Coagulation for DBF Control (Plant A)
Enhanced coagulation improves the removal of organic carbon in the coagulation and
settling processes. Because the system is not exempt from enhanced coagulation
requirements, it must achieve TOC removal requirements as stated in Table 5-6. Because
waters with greater alkalinity and lower TOC concentrations are more difficult to
coagulate, performance requirements in these categories are lower than for other
categories.
Table 5-6. Proposed Required Removal of TOC by Enhanced
Coagulation/Enhanced Softening for Surface Water Systems Using
Conventional Treatment
Source Water TOC
(mg/L)
>2.0-4.0
>4.0-8.0
>8.0
Source Water Alkalinity
(mg/L as CaCOa)
0-60
35.0%
45.0% ,
50.0%
>60-120
25.0%
35.0%
40.0%
>1201
15.0%
25.0%
30.0%
Enhanced coagulation alternative compliance criteria applicable to waters with raw-water SUVA < 2.0 L/mg-m.
1 Systems practicing precipitative softening must meet the TOC removal requirements
in this column.
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The system in question has a raw water alkalinity of 50-60 mg/L as CaCOs and a raw
water TOC of 4.5-5.0 mg/L. Based on Table 5-6, these conditions require the utility to
remove 45 percent or more TOC through the coagulation and settling process as an
annual average (refer to the Guidance Manual for Enhanced Coagulation and Enhanced
Precipitative Softening for additional information (USEPA, 1999g))." The utility currently
adds 20 mg/L of alum. This alum dose reduces the TOC from 5.0 to 3.7 mg/L through
settling. This is equivalent to 26 percent removal ([5.0-3.7]/5.0*100%). Through jar
testing, the plant operators determine that it needs to add 40 mg/L alum to achieve 45
percent, removal of TOC (i.e.; to achieve 2.7 mg/L TOC in its settled water). Practicing
enhanced coagulation in settled water is expected to result in the following DBF
concentrations in the distribution system (Table 5-7).
Table 5-7. System DBF Concentrations with Enhanced Coagulation,
Settled Water Chlorination (Plant A)
Parameter
TTHM (ng/L)
HAAS fttg/L)
Summer
Before EC
145
71
After EC
99
54
Winter
Before EC
29
44
After EC
20
33
RAA
Before EC
87
58
After EC
60
44
Note: Running Annual Average (RAA) is based on quarterly sampling (not shown).
EC = Enhanced Coagulation
In addition to controlling DBFs, enhanced coagulation allows for more effective
disinfection. This occurs by two mechanisms:
• A greater residual is provided for the same chlorine dose since the chlorine
demand is lower in water treated by enhanced coagulation.
• Chlorine is more effective at inactivating Giardia at the lower pH resulting
from enhanced coagulation.
The disinfectant residual achieved by a given dose is a function of contact time and
disinfectant demand of the water, among other factors. Because TOC exerts a
disinfectant demand, the disinfectant residual will be greater when practicing enhanced
coagulation (for the same chlorine dose).
The addition of alum to water decreases the pH of the water. For instance, the pH of the
settled water under the original 20 mg/L alum dose was 7.1, whereas the pH of the settled
water under the 40 mg/L alum dose is 6.6. This drop in pH with enhanced coagulation
may adversely impact corrosion in the distribution system and should be mitigated
appropriately. The drop in pH actually improves disinfection, because chlorine is more
effective at inactivating Giardia at lower pH. Acids, such as hydrochloric acid, are used
in treatment plants to lower pH levels to enhance coagulation and improve filter
performance. Table 5-8 indicates the improved disinfection occurring due to enhanced
coagulation and'disinfection of settled water. The system also maintains a disinfection
level above its current benchmark. The system also may reduce its chlorine dose to
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5. USING THE BENCHMARK
maintain its pre-enhanced coagulation chlorine residual levels of 0.8mg/L and to conserve
financial reserves.
Table 5-8. Impact of Enhanced Coagulation on Disinfection (Plant A)
Coagulation
Practice
Existing (20 mg/L
Alum)
Enhanced (40 mg/L
Alum)
Chlorine Residual
in Finished Water
(mg/L)
0.8
1.2
Contact Time
(minutes)
47
47
CT (mg-
min/L)
37.6
56.6
pHat-
Residual
Sampling
Point
7.1
6.6
Log
Inactivation of
G/ard/aat5°C
0.75
1.3
5.5.2 Treatment Changes for DBF Control When Enhanced
Coagulation is Insufficient
5.5.2.1 Base Conditions (Plant B)
The base condition considered for this example, Plant B, is a conventional filtration plant
that practices prechlorination. Table 5-9 lists the important raw water characteristics for
this plant, while Table 5-10 describes the important unit processes of Plant B.
Table 5-9. Raw Water Quality (Plant B)
Parameter
pH
TOC (mg/L)
UV-254 (1/cm)
Bromide (mg/L)
Temperature (°C)
Alkalinity (mg/L as CaCOS)
Value
7.6-7.9
4.0-5.0
0.15-0.2
0.15-0.2
5.0-24
50-60
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Table 5-10. Base Condition Unit Processes (Plant B)
Process
Influent
Chlorine
Alum
Rapid Mix
Flocculation
Settling
Filtration
Clean/veil
Distribution
Characteristics
Raw Water Characteristics above
Dose4mg/L
Dose 20 mg/L
5 minutes detention, 0.1 baffling factor
20 minutes detention, 0.3 baffling factor
80 minutes detention, 0.3 baffling factor
15 minutes detention, 0.5 baffling factor
60 minutes detention, 0.1 baffling factor
3 days maximum detention time
The disinfection benchmark for Giardia for this conventional filtration plant is 1.0 log.
This system applies chlorine to the raw water for disinfection and maintains a detectable
residual throughout the distribution system. The effects of both,chlorine and alum on pH
is evident in the decrease in pH levels from about 7.6 in the influent to 6.9 in the finished
water. TOC is removed in the coagulation/settling process from 5.0 mg/L in the raw
water to 3.7 mg/L in the finished water. This results in a concurrent decline in UV
absorbance.
The TTHM and HAA5 concentrations experienced by this system with its 3-day detention
time in the distribution system are listed in Table 5-11. The running annual average
(RAA) TTHM and HAAS values are 99 and 65 jag/L, Because the TTHM value exceeds
the Stage 1 MCL, this system must implement a strategy for DBF control.
Table 5-11. System DBP Concentrations (Plant B)
Parameter
TTHM (ng/L)
HAA5 (jig/L)
Summer
165
85
Winter
39
55
RAA
99
65
Note: Running annual average is based on quarterly sampling (not shown).
The plant examines making four modifications to its disinfection practices to control
DBFs. These modifications include:
1. Practicing enhanced coagulation as required by the Stage 1 DBPR
2. Installing chloramination to provide residual disinfection
3. Moving the point of chlorine application after settling (possibly a seasonal
change)
4. Improving hydraulic characteristics of the clearwell.
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5.5.2.2 Enhanced Coagulation for DBP Control (Plant B)
Because the system is not exempt from enhanced coagulation requirements, it must
achieve the TOC removal requirements stated in Table 5-6.
The system in question has a raw water alkalinity of 50-60 mg/L as CaCO3 and a raw
water TOC of 4.5-5.0 mg/L. Based on Table 5-6, these conditions require the utility to
remove 45 percent or more TOC through the coagulation and settling process as an
annual average. The utility currently adds 20 mg/L of alum. This alum dose reduces the
TOC from 5,0 to 3.7 mg/L through settling. This is equivalent to 26 percent removal
([5.0-3.7]/5.0*100%). Through jar testing, the plant operators determine that they need to
add 40 mg/L alum to achieve 45 percent removal of TOC (i.e., to achieve 2.7 mg/L TOC
in its settled water). Practicing enhanced coagulation results in the following DBP
concentrations in the distribution system (Table 5-12).
Table 5-12. System DBP Concentrations with Enhanced Coagulation
(Plant B)
Parameter
TTHM (ug/L)
HAAS (ug/L)
Summer
Before EC
165
85
After EC
99
65
Winter
Before EC
39
55
After EC
25.
38
RAA
Before EC
99
65
After EC
73
57
Note: Running annual average is based on quarterly sampling (not shown).
In addition to reducing DBPs, enhanced coagulation allows for more effective
disinfection and some TOC removal. Because TOC exerts a disinfectant demand, the
disinfectant residual will be greater (for the same chlorine dose).
The addition of alum to water decreases the pH of the water. For instance, when the pH
of the settled water under the original 20 mg/L alum dose was 7.1, the pH of the settled
water under the 40 mg/L dose was 6.5. This drop in pH with enhanced coagulation may
adversely impact corrosion in the distribution system and should be mitigated
appropriately. The drop in pH actually improves disinfection, however, since chlorine is
more effective at inactivating Giardia at lower pH. Table 5-13 indicates the improved
coagulation occurring due to enhanced coagulation. The system also maintains a
disinfection level above its current benchmark.
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Table 5-13. Impact of Enhanced Coagulation on Disinfection (Plant B)
Coagulation
Practice
Existing (10mg/L
Alum)
Enhanced (40 mg/L
Alum)
Chlorine Residual
in Finished Water
(mg/L)
1.4
1.8
Contact Time
(minutes)
44
44
CT (mg-
min/L)
61.6
79.2
pHat
Residual
Sampling
Point
7.1
6.5
Log
Inactivation of
Giardiaat5°C
1
1.7
While improving its level of Giardia inactivation, the system fails to reach the desired
reductions in TTHM and HAA5 levels (see Section 2.5). The system considers switching
to chloramines for a secondary disinfectant in order to reduce DBF levels.
5.5.2.3
Chloramines
Chloramines can be used as a secondary disinfectant to control DBF formation in the
distribution system. This system is considering the application of free chlorine to its raw
water, with application of ammonia to the suction line of the high service pumps. This
allows disinfection using free chlorine, while quenching the free chlorine residual with
ammonia to limit formation of regulated DBFs in the distribution system. The use of
chloramines for residual disinfection is dicussed extensively in the Alternative
Disinfectants and Oxidants Guidance Manual (USEPA, 1999a).
The use of chloramines by this system will not affect its primary disinfection because
ammonia is applied following the clearwell. Therefore, the disinfection level listed in
Table 5-13 for enhanced coagulation (1.7-log Giardia inactivation) is still applicable for
this system using chloramines for residual disinfection.
Chloramines will effectively control DBF formation in the distribution system. For
systems that exceed DBF MCLs within the plant, rather than the distribution system,
ammonia would need to be applied prior to the clearwell for effective DBF control. For
this system, application of ammonia at the suction line of the high service pumps (after
clearwell) allows disinfection levels to be maintained while further controlling DBFs.
For this system, use of chloramines combined with enhanced coagulation and settled
water chlorination results in TTHM and HAAS concentrations of 66 |ig/L and 51 u,g/L
running annual average, respectively.
5.5.2,4 Moving the Point of Chlorine Application after Settling
The purpose of this modification is to reduce the concentration of DBF precursors prior to
the addition of chlorine. TOC is removed during the coagulation/settling process. For
this system, the TOC level declines from about 5.0 to 3.7 mg/L after settling, with the
addition of 20 mg/L of alum. Moving the point of chlorination, therefore, results in the
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5. USING THE BENCHMARK
chlorination of water with significantly lower TOC. Because TOC is a surrogate measure
for natural organic material (a principal DBF precursor), and the TOC level has been
reduced, this should reduce the formation of DBFs.
Moving the point of chlorine application from raw water to settled water results in DBF
formation shown in Table 5-14. The chlorine dose is not changed from the baseline
condition which is 4.0 mg/L. This modification results in a decrease in TTHM
concentration of about 20 percent and HAAS concentration of about 30 percent.
Table 5-14. System DBF Concentrations After Enhanced Coagulation
and Moving the Point of Chlorination
Parameter
TTHM (ng/L)
HAAS (u.g/L)
Summer
Only EC .
99
65
After
moving
POC
80
46
Winter
Only EC
25
38
After
moving
POC
20
27
RAA
Only EC
73 .
57
After
moving
POC
55
35
Note: Running annual average is based on quarterly sampling (not shown).
POC = Point of Ghlorination
EC = Enhanced Coagulation
Under baseline conditions, the system added chlorine to the raw water and used the
detention time available in the rapid mix, flocculation, and sedimentation basins. This
contact time is about 31 minutes at peak hourly flow (i.e., 70 percent of total contact tune
available). Once the system moves chlorine application to settled water, it loses the
benefit of this contact time.
The achieved chlorine residual is a function of chlorine dose and decay. Chlorine decay
depends on the chlorine demand of the water and contact time, among other factors.
Organic carbon exerts chlorine demand. Because settled water contains less TOC and
because chlorine is in contact with water for a shorter duration, the chlorine residual in
the finished water is greater when chlorine is applied to settled water (Table 5-15). For
application of chlorine to settled water, the chlorine residual is greater but the contact
time is shorter. This results in an overall decrease in disinfection level (i.e., the CT) by
about 50 percent.
Table 5-15. Impact of Moving Chlorine Application Point on Disinfection
Chorine Application
Point
Raw Water
Settled Water
Contact Time
(minutes)
44
13.5
Chlorine Residual in
Finished Water
(mg/L)
1.8
2.8
CT (mg-min/L)
79.2
37.8
Log Inactivation
of Giardia at 5°C
and pH 6.5
1.7
0.8
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5. USING THE BENCHMARK
Moving the point of chlorine application from raw to settled water does assist in
controlling DBF formation but is less than the disinfection benchmark. However, if the
chlorine application point is moved seasonally, this may not be an issue. This is
discussed further in the next section.
5.5.2*5 Seasonal Chlorine Application Points
The plant operators consider changing the point of disinfectant application only during
summer when DBF formation is highest, and the CTs required for pathogen inactivation
are at their lowest. A seasonal change in the point of chlorine application can assist in
controlling DBFs and meeting disinfection benchmarking goals.
The disinfection benchmark characterizes the minimum disinfection achieved based on
historic plant operating data. Because the effectiveness of disinfection is significantly
reduced at lower temperatures, the benchmark is typically determined during the winter
months (i.e., December, January, and February). Therefore, the existing disinfection level
in these months should be maintained. However, disinfection is more effective in
summer, and therefore does not require as high a CT as in winter. This may allow a
utility to move the point of chlorine application downstream in the treatment train when
less contact tune is needed.
Disinfection byproduct formation is typically greatest in summer, since the rate of DBF
formation is greater at higher temperatures and in the presence of DBF precursors (e.g.,
when algae may be at their highest concentrations.) These contrasting issues of needing
to maintain disinfection levels in winter and needing to control DBFs primarily during
summer lead to the concept of seasonal DBF application points. That is, apply chlorine
early in the process train in winter to maximize contact time and apply chlorine later in
the process train in summer to control DBFs.
The plant operators decide to use the existing raw water chlorination point from
December through February, and move the point of chlorination to settled water from
March through November. The winter chlorination point and dose will be the same as
historic practices, so the existing benchmark will be maintained. The impact of seasonal
chlorine application points on DBF concentrations is summarized in Table 5-16. The
seasonal chlorine application points evaluated at this utility satisfy the existing
disinfection benchmark (1.0) by maintaining critical winter disinfection.
Table 5-16. System DBF Concentrations After Enhanced Coagulation and
Moving of Chlorine Application Points
Parameter
TTHM (ng/L)
HAAS (ug/L)
Summer
80
46
Winter
25
38
RAA
57
42
Note: Running annual average is based on quarterly sampling (not shown).
RAA = Running Annual Average
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5. USING THE BENCHMARK
Table 5-17 shows the impact of moving the disinfection point during the summer season
on Giardia inactivation. By moving the point of chlorine application to settled water
during warmer periods, the DBF concentrations were controlled below the Stage 1 MCLs.
This was accomplished using the same chlorine dose. A utility considering this
alternative must ensure that the minimum disinfection requirements of the SWTR are met
at all times and that an adequate disinfectant residual is provided for distribution.
Table 5-17. Impact Of Moving Chlorine Application During
The Summer Season
Chorine
Application
Point
Raw Water
(Winter)
Settled Water
(Summer)
Contact
Time
(minutes)
44
13.5
Chlorine Residual
in
Finished Water (mg/L)
1k
2.8
CT
(mg-min/L)
79.2
37.8
Log
Inactivation of
Giardia at
20°CandpH
6.5
—
2.0
Log
Inactivation of
Giardia at 5°C
and pH 6.5
1.7
..
5.5.2.6
Clearwell Baffling
Moving the point of chlorination to settled water combined with practicing enhanced
coagulation will allow plants to comfortably meet Stage 1 DBF MCLs. Enhanced
coagulation also improves disinfection, but it cannot make up for the reduced contact
time associated with moving chlorine application from raw to settled water. Compare the
Log inactivation values for raw water (1.0) with enhanced coagulation (1.7) presented on
Table 5-18. For this system^ moving the point of chlorination combined with enhanced
coagulation results in a 50 percent decrease in disinfection level. Although seasonal
chlorination point strategy could meet disinfection benchmarking goals by maintaining
existing winter disinfection, another method to meet benchmarking goals would be to
improve the hydraulics of the clearwell using baffles.
Baffling and disinfection contact time are discussed extensively in Appendix D. The
clearwell for the system being discussed is not baffled and has been estimated to have a
, baffling factor (Tjo/T) of 0.1. This is the worst classification of baffling for disinfection
contact time and the system only receives credit for 10 percent of the theoretical detention
time (60 minutes). Therefore, opportunity exists to substantially improve disinfection by
improving the hydraulic characteristics of the clearwell for disinfection contact time.
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5. USING THE BENCHMARK
Table 5-18. Cumulative Impact of Settled Water Chlorination, Enhanced
Coagulation and Clearwell Baffling on Disinfection (Plant B)
Modification
1. Original Raw Water
Chlorination at 5°C
2. Enhanced
Coagulation at 5°C
3. Seasonal Settled
Water Chlorination at
20°C
4. Regular Settled Water
Chk>rinationat50C
5. Enhanced
Coagulation, Settled
Water Chlorination at
5°C
6. Enhanced
Coagulation, Settled
Water Chlorination,
Clearwell Baffling at
5°C
Disinfection
Contact Time
(minutes)
- 44
44
13.5
13.5
13.5
37.5
Disinfectant
Residual (mg/L)
1.4
1.8
2.8
2.8
2.8
2.8
CT(mg-
min/L)
61.6
79.2
37.8
37.8
37.8
105
Finished
Water pH
7.1
6.5
6.5
6.9
6.5
6.5
Log
Inactivation of
Giardia
1.0 (benchmark)
1.7
' 2.0
0.64
0.8
2.7
The system has developed a design to baffle the Clearwell and improve its baffling factor
from 0.1 to 0.5 (average conditions). The baffling design includes inlet and outlet baffles,
with some intra-basin baffles. Using the theoretical detention time of 60 minutes, a
baffling factor of 0.1 yields 6 minutes of contact time (Tio) while a factor of 0.5 yields 30
minutes of contact time. Please review other sections of this manual for calculations
using baffling factors and guidance on baffling the clearwell or other basins. Table 5-18
compares the cumulative impact on disinfection of the modifications presented above:
moving point of Chlorination (regular or during summer season only), enhanced
coagulation, and clearwell baffling.
Table 5-18 indicates that enhanced coagulation, seasonal settled water Chlorination, and
clearwell baffling together provide greater disinfection than the original practice of
chlorinating raw water, using a chlorine dose of 4 mg/L for both situations. Baffling the
clearwell is not expected to significantly impact DBF formation. Therefore, RAA TTHM
and HAAS concentrations are expected to be 57 ug/L and 46 |ig/L, respectively. The
greater disinfection provided through baffling modification, enhanced coagulation and
settled water Chlorination, would allow the utility to reduce its chlorine dose to less than 3
mg/L and still meet or exceed its disinfection benchmark, further controlling DBF
concentrations.
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5. USING THE BENCHMARK
5.5.3 Summary of Treatment Modification Strategies
Impact on Disinfection and DBP Control
The system described as Plant B had running annual average DBP concentrations greater
than the Stage 1 DBPR MCLs. The system considered four strategies for DBP control.
These strategies and their impacts on disinfection and byproduct formation are
summarized in Table 5-19. This experience demonstrates how a single change did not
allow simultaneous compliance. Rather, several carefully selected components were
integrated for DBP control while maintaining the historical disinfection benchmark:
Table 5-19. Summary Impacts of DBP Control Strategies
Original Practice - Raw Water Chlorination
Strategy
Settled Water Chlorination
Enhanced Coagulation
Clean/veil Baffling
Chloramines for residual
disinfection
Disinfection
-
+
. +
0
Byproduct Control
+
+
0
+
Note: + for improvement, - for degradation, 0 for no impact
Table 5-20 and Figure 5-1 summarizes the experience of "Plant B" in selecting a DBP
control strategy that maintains historical critical period disinfection levels. No single
component solved these problems. Instead, several carefully .selected components were
required to meet DBP MCLs while maintaining historical critical period disinfection.
Moving the point of Chlorination to settled water combined with enhanced coagulation
allowed the utility to meet Stage 1 DBP MCLs, but sacrificed disinfection due to the
shorter chlorine contact time. Historical disinfection levels were achieved by also
baffling the clearwell to recover some of the lost disinfection contact time. Another
alternative for meeting the disinfection benchmark would be to maintain seasonal
chlorine application points. This strategy would chlorinate raw water during critical
period disinfection months used to calculate the benchmark (i.e.; winter conditions).
During warmer conditions, chlorine would be applied to settled water to control DBPs.
Seasonal chlorine application points combined with enhanced coagulation would have
also met the Stage 1 DBP MCLs and disinfection benchmarking goals for the system
under consideration.
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5. USING THE BENCHMARK
Table 5-20. Impact of DBF Control Strategies on Disinfection and
Byproduct Formation
Treatment Type
Raw Water Chlorination
Settled Water Chlorination
without Enhanced
Coagulation
Chloramines without
Enhanced Coagulation
Enhanced Coagulation
Chloramines with Enhanced
Coagulation
Settled Water Chlorination
with Enhanced Coagulation
Seasonal Chlorination with
Enhanced Coagulation
Enhanced Coagulation,
Settled Water Chlorination,
Clean/veil Baffling,
Chloramines
TTHM
Concentration1
(HS/L)
99
92
89
73
66
55
57
45
HAAS Concentration1
(WL)
65
62
59
57
51
35
42
30
Critical Log
Inactivation of G/ard/a2
1.0
0.8
1.0
1.7
1.7
0.8
1.7
2.7
1 as running annual average
2at5°CandpH6.5
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5. USING THE BENCHMARK
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5. USING THE BENCHMARK
THIS PAGE INTENTIONALLY LEFT BLANK
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6. Alternative Disinfection
Benchmark
Some'systems may not be able to meet Stage 1 DBPR MCLs while maintaining their
existing disinfection practices and benchmark. Under these conditions, the system must
consult with the State to discuss appropriate compliance strategies, including an
alternative disinfection benchmark. The alternative disinfection benchmark would be
lower than the calculated disinfection benchmark,' allowing the utility greater flexibility to
achieve compliance with DBPR MCLs while still not significantly compromising
microbial protection. However, the alternative disinfection benchmark must not be lower
than the disinfection requirements of the SWTR.
Each State will formulate its own plan for evaluating inactivation data and setting
alternative disinfection benchmarks. The plan should foster cooperation between the
State and water systems. The goal of an alternative disinfection benchmark is to improve
a system's ability to meet the DBPR MCLs without significantly compromising existing
microbial protection. The system and State should consider source water quality, existing
physical barriers to pathogens, and the risk of waterborne disease to set an alternative
disinfection benchmark. The information and examples presented here are intended as
guidance. Each State should develop its own plan for evaluating and setting alternative
disinfection benchmarks.
The following examples describe characteristics of systems that may choose to develop
an alternative benchmark:
• Systems that cannot simultaneously meet the disinfection benchmark and the
Stage 1 DBPR MCLs and which have:
- very high levels of microbial inactivation and/or
- high quality source water that has low pathogen occurrence levels.
These examples are not meant to be exhaustive. If a system has circumstances similar to
the above examples, it may want to consult the State to set an alternative disinfection
benchmark to gain greater flexibility for complying with the provisions of the Stage 1
DBPR.
Systems with Very High Levels of Microbial Inactivation
Some water systems'have very high existing levels of inactivation. These high values may
be the result of the following:
. The disinfectant dose is controlled by the need to maintain a residual in the
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6. ALTERNATIVE DISINFECTION BENCHMARK
distribution system rather than by the need to provide the primary disinfection
required by the SWTR. The dose required to provide a distribution system
residual often determines in-plant disinfection practices.
• To simplify compliance with the SWTR, a system may operate with a
"minimum specified residual" under worst case operating"conditions. Because
the worst case-conditions may not occur simultaneously (i.e., lowest
temperature and greatest peak hourly flow rate), the utility may be achieving
much greater disinfection levels than required by the SWTR.
• The disinfectant in use may be much more effective against a particular
pathogen. For example, chlorine is much more effective at inactivating
viruses than it is Giardia. For this reason, systems that inactivate Giardia
with chlorine may be achieving very high logs inactivation of viruses (e.g.,
greater than 10 logs) as indicated by extrapolation using the CT concept. A
system may want to apply for an alternative disinfection benchmark for
viruses, if it is considering switching to another disinfectant or improving its
physical removal processes.
• The treatment plant is operating well below design flow and, therefore,
disinfection contact time is extremely long.
In the above examples, the benchmark inactivation for Giardia and/or viruses may be so
high that the log inactivation levels would be well in excess of treatment needed.
Therefore, there may be an opportunity to reduce the level of calculated inactivation
without significantly increasing the risk of waterborne disease.
Systems Exceeding the Stage 1 DBF MCLs
It may be very difficult for some systems to maintain current levels of Giardia or virus
inactivation and simultaneously comply with Stage 1 DBPR MCLs (0.080 mg/L and
0.060 mg/L for TTHM and HAA, respectively). These systems may want to set an
alternative benchmark to obtain greater flexibility for DBPR compliance.
Consider a system that has been using free chlorine for primary,disinfection and
maintenance of a distribution system residual. The system is interested in switching to
chloramines for residual disinfection in order to limit free chlorine contact time and
control DBF formation. Chloramines are less effective for inactivating both Giardia and
viruses. Therefore, if ammonia is added prior to the historical point of chlorine residual
measurement, the level of primary disinfection would be diminished from historical
practices (i.e., the system would fall below its existing disinfection benchmark). In this
example the system could either increase the free chlorine residual to meet the existing
benchmark or apply to the State for an alternative disinfection benchmark. Another
option, presented earlier, is the seasonal use of chloramines, which may not require an
alternative benchmark.
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6. ALTERNATIVE DISINFECTION BENCHMARK
Systems with High Quality Source Water
Water systems with very stable and high quality source water (usually in well-protected
watersheds) may have a lower risk of microbial occurrence. Disinfection of high quality
water with low pathogen occurrence, beyond the requirements of the SWTR, may not be
warranted provided that, filtration is well operated and watershed control is practiced.
The SWTR requires all plants to provide at least 4-log inactivation and/or removal of
viruses and 3-log inactivation and/or removal of Giardia. Because SWTR allows states
to give credit for filtration, the log inactivation required by chemical disinfection can be
significantly lower. The EPA recommends that the State allow more credits for Giardia
and virus removal by filtration if the following applies (AWWA, 1991):
1. It is determined that the system is not currently at significant risk of
microbiological contamination at the existing level of disinfection.
2. Less stringent interim disinfection conditions are necessary for the system to
modify its disinfection process to optimally achieve compliance with the
SWTR as well as forthcoming DBF regulations.
Table 6-1 presents the different log removal credits allocated for different types of
filtration.
Table 6-1. Log Removal .Credits for Filtration
Filtration
Conventional
Direct Filtration
Slow-Sand Filtration
Diatomaceous Earth
Filtration
Giardia Log
Removal
2.5
2.0
2.0
2.0
Virus Log
Removal
2.0
.1.0
2.0
1.0
Conditions for Credit Allocation •
Meets the following:
A)Total treatment train achieves
1 ) at least 99% turbidity removal or filtered water turbidities
are less than 0.5 NTU or
2) 99.9% particle removal in size ranges of 5 to 15 um is
demonstrated; and
B)The level of HPC bacteria in the filtered water entering
the distribution system is consistently less than 10/mL
Same conditions as above.
Same conditions as above.
Same conditions as above.
Source: AWWA, 1991. • '
Figure 6-1 illustrates the potential range for alternative disinfection benchmarks. The
daily log inactivation of Giardia or viruses over a period of time constitutes the
disinfection profile. The disinfection benchmark, shown as a solid horizontal line on the
profile, is the average of the lowest month of each year. Therefore, the benchmark is
typically determined by the disinfection practiced in winter months (January and February
in the profile shown). The level of inactivation required by the SWTR (assuming States
grant a removal credit of 2.5-logs for conventional treatment and 2-logs for direct
filtration) is shown as horizontal dashed lines on the figure for conventional and direct
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6. ALTERNATIVE DISINFECTION BENCHMARK
filtration. This log inactivation removal is determined by subtracting the physical
removal credit for filtration from the total log inactivation/removal required by the
SWTR. The bold arrows denote the range for alternative disinfection benchmarks.
Alternative disinfection benchmarks are lower than existing disinfection benchmarks, but
always must be equal to or greater than requirements of the SWTR.
18
16 ••
ID-
g, 6-
2 ••
N ^
Existing Disinfection Benchmark
RANGE FOR ALTERNATIVE
DISINFECTION BENCHMARKS
Regulatory Floor - 0.5 for conventional filtration
1.0 for direct filtration N^
Date
Figure 6-1. Range for Alternative Disinfection Benchmarks
6.1 Methodology
Options for developing the alternative disinfection benchmark are described below.
These options are guidance only. The State may choose to adopt a methodology for
setting alternative benchmarks based on this guidance or develop other methodologies.
However, under no circumstances may the State set an alternative disinfection benchmark
lower than disinfection level required by the SWTR.
The goal, of the SWTR is to ensure that the annual risk of Giardia lamblia infection for an
individual is less than 10"4 cases/person/year. The SWTR used an exponential risk
assessment model (Rose, 1988) to calculate the logs of treatment necessary to keep the
annual risk of infection below 10"4 cases/person/year for different concentrations of
Giardia lamblia cysts in source water. EPA developed two options, or methodologies,
for setting an alternative benchmark from this risk paradigm.
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6. ALTERNATIVE DISINFECTION BENCHMARK
Cryptosporidiumwas not used as a reference for establishing alternative disinfection
benchmarks because most systems currently employ disinfection which is assumed to
provide little or no inactivation of this pathogen. Therefore, any change in disinfection
practice is not addressed with respect to Cryptosporidium. These options are provided as
guidance or recommendations only. Systems and States may use or modify these options
or develop their own options.
Option 1 - No Monitoring
This option allows a utility to set an alternative disinfection benchmark without
characterizing the quality of its source water. The lack of monitoring data requires the
assumption that high levels of disinfection be provided. This option may be attractive to
systems that have average source water quality, have high existing disinfection
benchmarks, and do not need flexibility to meet the DBPR MCLs.
The goal of the SWTR is to limit infections by Giardia to one per year per 10,000 people
(10"4 cases/person/year). This is assumed to be the maximum acceptable risk of infection.
For source water having an average of 1 Giardia cyst per 100 L (very good quality water)
and receiving 3-logs of treatment for Giardia, the risk of infection is about
10"4 cases/person/year. If one assumes a maximum Giardia concentration for source
water of 100,000 per 100 L, then an 8-log removal/inactivation would be needed to
maintain a 10"4 cases/person/year risk for Giardia. The 100,000 cysts per 100L
concentration is approximately one order of magnitude higher than the highest Giardia
cyst concentration known to be measured in source waters of drinking water supplies
(LeChevallier et al., 1991b). The value of 8-logs is -calculated by assuming that a finished
water cyst concentration of 10"3 per 100L would be needed to achieve about a lO^risk of
infection (cases/person/year) (Regli et al., 1991).,
Table 6-2 applies to systems that need to set an alternative disinfection benchmark
without the benefit of monitoring data. All systems that choose this option should ,
achieve an 8-log treatment (combination of physical removal and chemical inactivation)
for Giardia to meet the minimum acceptable risk. Assuming a 2.5-log physical removal
,by conventional filtration, 5.5-logs Giardia inactivation is the minimum alternative
disinfection benchmark.
Table 6-2 also indicates minimum alternative disinfection benchmarks for viruses. These
were derived assuming a maximum virus concentration in source waters of 10,000 per
100L and assuming that a viral concentration of 10"7 L would be needed to achieve a 10"4
risk level (Regli et al., 1991).
Credits for the physical removal of pathogens by filtration should be subtracted, from the
total treatment requirements to derive the level of treatment needed by chemical
disinfection. The removal of pathogens is dependent on the organism of interest and the
filtration process. Guidance for removal credits for filtration are provided in the
Filtration Credit (logs) columns of Table 6-2, reprinted from the Guidance Manual for
Compliance with the Filtration and Disinfection Requirements for Public Water Systems
Using Surface Water Sources (AWWA, 1991).
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6. ALTERNATIVE DISINFECTION BENCHMARK
Table 6-2. Alternative Disinfection Benchmarks for Systems Not Monitoring
Filtration Process
Conventional
Direct
Slow Sand
Diatomaceous Earth
Giardia
Total
Treatment
Required
(logs)*
8.0
8.0
8.0
8.0
Filtration
Credit (logs)
2.5
2.0
2.0
2.0
Alternative
Disinfection
Benchmark
(logs)
5.5
6.0
6.0
6.0
Virus
Total
Treatment
Required ;
(logs)*
9.0
9.0
9.0
9.0
Filtration
Credit (logs)
2.0
1.0
2.0
1.0
Alternative
Disinfection
Benchmark
(logs)
7.0
8.0
7.0
8.0
* Assuming source water Giardia concentration of 100,000/100 L and viral concentration of 10,000/1 OOL.
Source: AWWA, 1991.
Option 2 - Source Water Characterization
For this option, a system monitors its source water quality for one year. The alternative
benchmark is developed based on the quality of the source water. Source water is
characterized by monitoring either E. coli or fecal coliform. Unfiltered systems already
monitor for fecal coliforms as a requirement to avoid filtration and therefore could
continue to monitor for fecal coliform to help set an alternative benchmark. Guidelines
for source water characterization are presented later in this section. At the end of the
sampling duration, the system determines the 90th percentile value for E. coli or fecal
coliform concentration, and uses these measurements for the alternative disinfection
benchmark.
Until better analytical methods are developed and tested for protozoa, EPA believes that
E. coli or fecal coliforms are the best available indicator at this time since these
parameters can be practically measured and indicate the potential for pathogen
contamination in the source water. EPA also believes that guidelines for prescribing
minimum level of total treatment, for purposes of establishing alternative disinfection
benchmarks, can be reasonably prescribed based on E. coli or fecal coliform levels in the
source water.
The SWTR specifies that unfiltered systems must have a running six month 90th
percentile source water fecal coliform levels of less than 20/100 mL as one of the criteria
for avoiding filtration. Similarly, such systems must also provide at least 3-log
inactivation of Giardia through disinfection each day that water is delivered to customers.
If the system fails to achieve 3-log inactivation any two or more days per month, the
system is in violation of a treatment technique requirement for that month. If the
violation occurs during a second month in any 12 consecutive months the system serves
water to the public, then the system must install filtration unless the State decides that one
of the violations was unusual and unpredictable. Filtration is triggered, regardless of the
cause, after a third violation.
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6. ALTERNATIVE DISINFECTION BENCHMARK
EPA believes that this minimum level of inactivation, as prescribed under the SWTR, is
an appropriate alternative benchmark for unfiltered systems having an excess of 3-logs of
inactivation for Giardia or 4-logs of inactivation for viruses.
EPA recommends a minimum alternative benchmark of 1-log inactivation of Giardia for
systems using conventional treatment and 1.5-log inactivation of Giardia for systems
using direct, slow sand, or diatomaceous earth filtration for filtered systems that want to
lower'their disinfection level below the benchmark. This is recommended if the source
water 90th percentile for either E. coli or fecal coliforms is less than 20/100 mL based on
one year of water with at least five samples taken each week. Similarly, EPA
recommends a minimum alternative benchmark of 2.5-log virus inactivation for systems
using conventional treatment or slow sand filtration and 3.5-log virus inactivation for
systems using direct or diatomaceous earth filtration.
EPA believes that plant operations to meet the minimum alternative benchmark as
described above and the new turbidity performance criteria in the IESWTR should
prevent significant increases in microbial risk for systems choosing to change their
disinfection practices while complying with the Stage 1 DBPR.
Systems with higher source water E. coli or fecal coliform concentrations should provide
alternative benchmarks as indicated in Tables 6-2 and 6-3 and Figures 6-2 and 6-3. EPA
developed the recommended proportions, presented in the above mentioned tables and
figures, by first assuming the worst case source water concentrations (i.e., the 90th
percentile) E. coli or fecal coliform concentrations of 20,000/100 mL would correspond
to worst case Giardia concentrations of 100,000 per 100 L, and treat at such
contamination levels, including 5.5-log Giardia inactivation for systems using
conventional treatment, and 6-log Giardia inactivation for systems using direct, slow
sand, or diatomaceous earth filtration. These inactivation levels would be needed to
achieve the SWTR's 10"4 annual risk of infection goal, assuming the minimum Giardia
physical removal credits recommended for filtration under the SWTR. EPA then
assumed that proportional levels of disinfection treatment between the two sample points
should provide a reasonable barrier of protection against microbial risk if systems wish to
change their disinfection practices to comply with the Stage 1 DBPR.
Table 6-3 presents the recommended alternative disinfection benchmarks as a function of
source water quality and the physical removal process employed. The values in the table
have been interpolated between the two endpoints of poor and good water quality, and
include the credits mentioned above for sedimentation and filtration. Once the system
has determined its 90th percentile value of indicator organism in source water, it may use
Table 6-3 to select the recommended minimum alternative disinfection benchmark.
A graphical representation of Table 6-3 is presented in Figures 6-2 and 6-3. These figures
display the 90th percentile indicator concentrations on the y-axis, with recommended
alternative disinfection benchmarks on the x-axis. The two lines on each figure represent
the different filtration processes.
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6. ALTERNATIVE DISINFECTION BENCHMARK
Table 6-3. Impact of Source Water Quality and Filtration Process on
Alternative Disinfection Benchmark
90th Percentile Indicator
Concentration*
(cfu/100ml)
<20
30
40
50
60
70
80
90
100
200
300
400
500
600
700
800
900
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
£20,000
Giardia Alternative Disinfection
Benchmark
(log inactivation)
Conventional
1.0
1.3
1.5
1.6
1.7
1.8
1.9
2.0
2.0
2.5
2.8
3.0
3.1
3.2
3.3
3.4
3.5
3.5
4.0
4.3
4.5
4.6
4.7
4.8
4.9
5.0
5.0
5.5
Direct, Slow Sand,
or Diatomaceous
Earth
1.5
1.8
2.0
2.1
2.2
2.3
2.4
2.5
2.5
3.0
3.3
3.5
3.6
3.7
3.8 .
3.9
4.0
4.0
4.5
4.8
5.0
5.1
5.2
5.3
5.4
5.5
5.5
6.0
Virus Alternative Disinfection Benchmark
(loglnactivation)
Conventional or Slow
Sand
2.5
2.8
3.0
3.1.
3.2
3.3
3.4
3.5
3.5
4.0
4.3
4.5
4.6
4.7
4.8
4.9
5.0
5.0
5.5
5.8
6.0
6.1
6.2
6.3
• 6.4:
6.5
6.5'
7.0
Direct or
Diatomaceous Earth
3.5
'3.8
4.0
4.1
4.2
4.3
4.4
4.5
4.5
5.0
5.3
5.5
5.6
5.7
5.8
5.9
6.0
6.0
6.5
6.8
7.0
7.1
7.2
7.3
7.4
7.5
7.5
8.0
* Indicator concentration refers to either E. coli or fecal coliform.
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6. ALTERNATIVE DISINFECTION BENCHMARK
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6. ALTERNATIVE DISINFECTION BENCHMARK
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August 1999
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6. ALTERNATIVE DISINFECTION BENCHMARK
Adjustment Factors
It may be appropriate for the State and system to consider adjusting the alternative
disinfection benchmark based on qualitative factors. These factors would allow the State
and system to increase or decrease the alternative disinfection benchmark based on
information not considered in the methodology.
Examples of conditions that might be used by the State and system to increase the
alternative disinfection benchmark:
• Upstream sewage discharge, combined sewer overflow (CSO), sanitary sewer
overflow (SSO), contaminated stormwater, feedlots upstream
. Operational issues (e.g., variability of finished water quality)
• Variable source water quality
• Previous waterborne disease outbreaks
• Noncompliance with Total Coliform Rule.
Examples of conditions that might be used by the State and system to decrease the
alternative benchmark:
. Excellent filter effluent quality (less than 0.1 NTU), especially with average
raw water turbidities greater than 10 NTU
. Two-stages of physical treatment (e.g., conventional treatment and
nanofiltration)
. Exceptionally low fecal coliform or E. coli levels (i.e., substantially less than
the 20/100 mL cutoff) if the system is at the minimum indicated alternative
disinfection benchmark
• Occasional use of ozone or other oxidants for taste and odor, iron, and
manganese control
• Large credits for long contact times with water transported through
transmission lines prior to treatment plant.
6.2 Schedule Guidance
The date for complying with Stage 1 DBPR and IESWTR is December 2001 (3 years
after promulgation) for subpart H systems serving at least 10,000 people. Therefore, EPA
recommends that a one-year source water monitoring program to support the
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6. ALTERNATIVE DISINFECTION BENCHMARK
development of an alternative disinfection benchmark begin in, or before, the last quarter
of 2000. Waiting until the last quarter of 2000 would not be prudent, since it would not
allow lime to develop the alternative disinfection benchmark and implement and select a
strategy to meet DBPR MCLs and the alternative benchmark. A system may want to
proceed with TTHM, HAAS monitoring and source water monitoring simultaneously
rather than sequentially to provide the greatest flexibility for complying with all
applicable rules. Table 6-4 shows a schedule that may allow systems to use Option 2 to
develop an alternative disinfection benchmark and still provide time for a utility to
implement a DBF control strategy that will meet the alternative disinfection benchmark
by the compliance deadline.
Table 6-4. Example Schedule for Compliance with M-DBP Rules
1999
2000
2001
2002
DBPR and IESWTR
Compliance Task
Source Water
Characterization
Profile/benchmark/State
consultation
Apply State-approved
Alternative Disinfection
Benchmark
Implement Improvements/
changes (if needed)
6.3 Source Water Characterization
Source water characterization used to develop an alternative disinfection benchmark
includes sample collection, sample analysis, data evaluation and reporting. The objective
is to characterize the source water, prior to any treatment, in terms of either fecal coliform
or E. coli concentrations. Elevated concentrations of fecal coliform and E. coli in surface
water indicate a greater probability of contamination by pathogens. Understanding the
quality of the source water allows the State and water system to select an appropriate
level for the alternative disinfection benchmark.
Sample Collection. Water systems collect five water samples per week, on different days,
for one year. The one-year monitoring period will assess seasonal differences in source
water character. If five samples per week are collected and analyzed over a 52-week
calendar year, the water system will have 260 data values at the end of the year.
Source water samples should be collected at a location prior to treatment. At this
location, the water should not be subject to surface runoff. It is not appropriate for
systems to collect samples downstream from the addition of a disinfectant or oxidant. In
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6. ALTERNATIVE DISINFECTION BENCHMARK
addition, it is not appropriate for systems to collect samples downstream of
coagulation/sedimentation or filtration.
The samples should be collected by the grab method using sterile whirlpack bags, sterile
plastic, or sterile glass containers. The volume required is less than 100 ml (120 ml
bottles are standard bacteriological sampling bottles), but the laboratory should be
contacted for verification. No chemical preservative is required, but the sample should be
stored"in an iced cooler. Sample temperature should be between 1 and 4.4°C during
transportation and samples should be stored in the dark. The sample must not be held
more than 6 hours prior to laboratory analysis (Standard Methods, 1995).
Sample Analysis. The fecal coliforin and E. coll samples should be analyzed using one of
four analytical methods identified in EPA National Primary Drinking Water Regulations,
40 CFR I4l.21(f)(6)(i-iv). The methods include:
1. An extension of Method 922IE described in Standard Methods (1995)
2. An extension of Method 922 IB using nutrient agar
3. Minimal medium ONPG-MUG Test documented by Edberg,etal. (1988).
4. The Colisure Test by Milipore Corporation, Technical Services Department,
80 Ashby Road, Bedford, MA 01730.
Data Evaluation. In any week, the system should obtain five values for indicator
organism concentrations corresponding to five different days of that week. If a system
misses the collection of a value, the system should record the letter "M" for missing data,
for the day of the week that the data value was not collected. Therefore, in any week, the
utility will obtain five values, some of which will be the letter "M" if data are missing.
Systems are encouraged to collect all 260 values and not to have missing values. Values
that are missed are assumed to have poor water quality and count against the system when
'developing the alternative disinfection benchmark.
In general, data on concentrations of microbiological organisms in water from streams,
lakes, and reservoirs often exhibit a large number of samples with very low
concentrations and a few samples with high concentrations. Thus, the average or mean
concentration is not a very good measure on the expected concentrations because of the
few large values. For this reason, a distribution frequency (percent of samples above or
below a specified value) is more meaningful. For setting the alternative disinfection
benchmark, EPA recommends the 90th percentile value.
To determine the 90th percentile value the data should be sorted from the largest value to
the smallest value recorded (regardless of the date of collection). All of the "M," or
missing values, should be placed at the top of the list. The result of this action should be
a list of the top 26 data values of the 260 total values with missing values at the top of the
list followed by the largest numerical values that decrease to the smallest value at the
bottom of the list. The 90th percentile value is found by locating the 26th number of the
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6. ALTERNATIVE DISINFECTION BENCHMARK
list. It is this 90th percentile value that characterizes the quality of the source water for
developing the alternative disinfection benchmark.
As part of the consultation with the State, the system may want to explain why samples
were missed (e.g., sample container lost or samples not analyzed in atimely manner).
The system may then be able to develop a different 90th percentile by dropping missed
samples from the calculation.
Use of Historical Database. Some systems may already monitor their source water for
fecal coliform and E. coli. The resulting historical database may be sufficient for the
State and system to develop an alternative disinfection benchmark. The historical
database is considered sufficient for making this determination if:
• The raw water sampling location is upstream from the point of any treatment
. At least five samples per week are collected on different days
• The sampling period covers at least one year
• Methods of analysis are consistent with those presented herein.
6.4 Watershed Control Program
A watershed control program is a surveillance and monitoring program that is conducted
to protect the quality of a surface water source. An aggressive and detailed watershed
control program is desirable to effectively limit or eliminate potential contamination by
microbial pathogens. A watershed program may impact parameters such as turbidity,
certain organic compounds, viruses, total and fecal coliforms, Giardia, Cryptosporidium,
and areas of wildlife habitation. However, the program is expected to have little or no
impact on parameters such as naturally occurring inorganic chemicals. Limiting human
activity in the watershed may reduce the likelihood of animals becoming infected with
pathogens and thereby reduce the transmission of pathogens by wildlife. Preventing
animal activity near the source water intake prior to disinfection may also reduce
pathogen occurrence at the intake.
The effect of a watershed program is difficult to quantify since many variables that
influence water quality are beyond the control or knowledge of the water supplier. As a
result, the benefit of a watershed control program or specific control measures must in
many cases be based on accumulated cause and effect data and on the general knowledge
of the impact of control measures rather than on actual quantification. The effectiveness
of a program to limit or eliminate potential contamination by microbial pathogens will be
determined based on: the comprehensiveness of the watershed review; the ability of the
water system to effectively carry out and monitor the management decisions regarding
control of detrimental activities occurring in the watershed; and the potential for the water
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6. ALTERNATIVE DISINFECTION BENCHMARK
system to maximize land ownership and/or control of land use within the watershed.
Under the SWTR, a watershed control program should include as a minimum:
A description of the watershed including its hydrology and land ownership
Identification, monitoring and control of watershed characteristics and
activities in the watershed which may have an adverse effect on the source
water quality
A program to gain ownership or control of the land within the watershed
through written agreements with landowners, for the purpose of controlling
activities which will adversely affect the microbiological quality of the water
• An annual report which identifies special concerns in the watershed and how
they are being handled, identifies activities in the watershed, projects adverse
activities expected to occur in the future and how the utility expects to address
them.
Appendix J of the Guidance Manual for Compliance with the Filtration and Disinfection
Requirements for Public Water Systems Using Surface Water Sources (AWWA, 1991)
contains a more detailed guide to a comprehensive watershed program.
In preparing a watershed control program, surface water systems should draw upon the
State watershed assessments and non-point source (NFS) pollution management
programs required by §319 of the Clean Water Act. Information on these programs is
available from State water quality agencies or EPA's regional offices. Assessments
identify NFS pollutants in water and assess the water quality. Utilities should use the
assessments when evaluating pollutants in their watershed. Surface water quality
assessments can also be obtained from the lists of waters prepared under §304(1) of the
Clean Water Act, and State biennially prepared §305(b) reports.
State NFS management programs identify best management practices (BMPs) to be
employed in reducing NFS pollution. These management programs can be incorporated
in the watershed program to protect against degradation of the source water quality.
For systems using ground water sources under the influence of surface water, the control
measures delineated in the Wellhead Protection (WHP) program encompass the
requirements of the watershed control program, and can be used to fulfill the
requirements of the watershed control program. Guidance on the content of Wellhead
Protection Programs and the delineation of wellhead protection areas is given in
Guidance for Applicants for State Wellhead Protection Program Assistance Funds Under
the Safe Drinking Water Act (USEPA, 1987a) and Guidelines for Delineation of
Wellhead Protection Areas (USEPA, 1987b), available at www.epa.gov/OGWDWOOO/
whpnp.html.
As a minimum, the WHP program must:
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Specify the duties of State agencies, local governmental entities and public
water supply systems with respect to the development and implementation of
Programs.
Determine the wellhead protection area (WHPA) for each wellhead as defined
in subsection 1428(e) based on all reasonably available hydrogeologic
information, ground water flow, recharge and discharge and other information
the State deems necessary to adequately determine the WHPA.
Identify within each WHPA all potential anthropogenic sources of
contaminants which may have any adverse effect on the health of persons.
Describe a program that contains, as appropriate, technical assistance,
financial assistance, implementation of control measures, education, training
and demonstration projects to protect the water supply within WHPAs from
such contaminants. . ]
Present contingency plans for locating and providing alternate drinking water
supplies for each public water system in the event of well or wellfield
contamination by such contaminants.
Consider all potential sources of such contaminants within the expected
wellhead area of a new water well which serves a public water supply system.
Provide for public participation.
EPA Guidance Manual
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7. REFERENCES
AWWA (American Water Works Association). 1991. Guidance Manual for Compliance
with the Filtration and Disinfection Requirements for Public Water Systems Using
Surface Water Sources. Denver, CO.
AWWARF (American Water Works Association Research Foundation). 1997. Membrane
Filtration for Microbial Removal. Denver, CO.
Edberg, et al. 1988. "National Field Evaluation of a Defined Substrate Method for the
Simultaneous Enumeration of Total Coliforms and Esherichia coli from Drinking Water:
Comparison with the Standard Multiple Tube Fermentation Method." Applied and
Environmental Microbiology. 54:3197.
Grabbs, W.D., B. Macler, and S. Regli. 1992. Modeling Giardia Occurrence and Risk.
EPA-81 l-B-92-005. Office of Water Resource Center, U.S. Environmental Protection
Agency, 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." J. AWWA. 81(8):74.
LeChevallier, M.W., D.N. Norton, and R.G. Lee. 1991a. "Occurrence of Giardia and
Cryptosporidium spp. in Surface Water Supplies." Appl. Environ. Microbiol. 57:2610-
2616.
LeChevallier, M.W., D.N. Norton, and R.G. Lee. 1991b. "Giardia and Cryptosporidium
spp. in Filtered Drinking Water Supplies." Appl. Environ. Microbiol. 57(9):2617-2621.
Malcolm Pirnie, Inc. 1992. Technologies and Costs for Control of Disinfection
Byproducts. Prepared for the U.S. Environmental Protection Agency, Office of Ground
Water and Drinking Water, Washington, D.C. PB93-162998.
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 the U.S. Environmental Protection Agency,
Technical Support Division, Office of Drinking Water, Washington, D.C.
Regli S., B.A. Macler, J.E. Cromwell, X. Zhang, A.B. Gelderoos, W.D. Grabbs, and F.
Letkiewicz. 1993. "Framework for Decision Making: EPA Perspective." Safety of Water
Disinfection: Balancing Chemical and Microbial Risk. G.F. Craun, editor. International
Life Sciences Institute Press, Washington, D.C.
August 1999
7-1
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Disnfection Profiling and Benchmarking
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7. REFERENCES
Rose, J. 1988. Cryptosporidium in Water; Risk of Protozoan Waterborne Transmission.
Report prepared for the USEPA, Office of Water. ;
Singer, P.C. and S.D. Chang. 1989. "Correlations Between Trihalomethanes and Total
Organic Halides Formed During Water Treatment." J. AWWA. 81(8):61.
Smith D.B., R.M. Clark, B.K. Pierce, and S. Regli. 1995. "An Empirical Model for
Interpolating C*T Values for Chlorine Inactivation of Giardia lamblia." J. Water SRT-
Aqua.44(5):2Q3-2U.
Standard Methods. 1995. Standard Methods for the Examination of Water and
Wastewater 19th Edition, American Public Health Association, AWWA, Water
Environment Federation, Washington, D.C.
Summers, R.S., G. Solarik, V.A. Hatcher, R.S. Isabel, and J.F. Stile. 1997. "Analyzing
the Impacts of Predisinfection Through Jar Testing." Conference proceedings, AWWA
Water Quality Technology Conference, Denver, CO.
TNRCC (Texas Natural Resources Conservation Commission); 1998. Monthly Reporting
Requirements for Surface Water Treatment Plants. Austin, TX.
TNRCC. 1996. Public Water Supply Technical Guidance Manual. Chapter 27, Austin,
TX.
USEPA (U.S. Environmental Protection Agency). 1999a. Alternative Disinfectants and
Oxidants Guidance Manual. Prepared by Science Applications International Corporation
(SAIC) for the USEPA, Office of Ground Water and Drinking Water, Washington, D.C.
USEPA. 1999b. Microbial and Disinfection Byproduct Simultaneous Compliance
Guidance Manual. Prepared by SAIC for the USEPA, Office of Ground Water and
Drinking Water, Washington, D.C.
USEPA. 1999c. Uncovered Finished Water Reservoirs Guidance Manual. Prepared by
SAIC for the USEPA, Office of Ground Water and Drinking Water, Washington, D.C.
USEPA. 1999d. Unfiltered Systems Guidance Manual. Prepared by SAIC for the
USEPA, Office of Ground Water and Drinking Water, Washington, D.C.
USEPA. 1999e. Guidance Manual for Compliance with the Interim Enhanced Surface
Water Treatment Rule: Turbidity Provisions. Prepared by SAIC for the USEPA, Office
of Ground Water and Drinking Water, Washington, D.C.
USEPA. 1999f. Guidance Manual for Conducting Sanitary Surveys of Public Water
Systems; Surface Water and Ground Water Under the Direct Influence (GWUDI) of
Surface Water. Prepared by SAIC for the USEPA, Office of Ground Water and Drinking
Water, Washington, D.C.
EPA Guidance Manual
Disinfection Profiling and Benchmarking
7-2
August 1999
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7. REFERENCES
USEPA. 1999g. Guidance Manual for Enhanced Coagulation and Enhanced
Precipitative Softening. Prepared by Malcolm Pirnie, Inc. for the USEPA, Office of
Ground Water and Drinking Water, Washington, D.C.
USEPA 1998a. "National Primary Drinking Water Regulations; Interim Enhanced
Surface Water Treatment Rule; Final Rule." 63 FR 69477. December 16.
USEPA. 1998b. "Disinfection Benchmark Guidance Manual Outline." Received by
SAIC and HDR Engineering, Inc. through personal communication with the U.S.
Environmental Protection Agency. February 16.
USEPA. 1997a. "National Primary Drinking Water Regulations: Interim Enhanced
Surface Water Treatment Rule; Notice of Data Availability; Proposed Rule." 62 FR
212:59485. November 3.
USEPA. 1997b. "National Primary Drinking Water Regulations: Disinfectants and
Disinfection Byproducts; Notice of Data Availability; Proposed Rule." 62 FR 212:59388.
November 3.
USEPA. 1994. "National Primary Drinking Water Regulations: Enhanced Surface Water
Treatment Requirements; Proposed Rule." 59 FR 38668. EPA/81 l-Z-94-004. July 29.
USEPA. 1987a. Guidance for Applicants for State Wellhead Protection Program
Assistance Funds Under the Safe Drinking Water Act.
USEPA. 1987b. Guidelines for Delineation ofWellhead Protection Areas.
Van der Kooij, D., L.W.C.A. van Breeman, F. Houtepen, and J. Williamsen-Zwaagstra.
1995. Removal of Microorganisms in Surface Water Treatment in the Netherlands.
Conference proceedings, AWWA Water Quality Technology.
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APPENDIX A. HISTORY
This section describes the historical development of disinfection profiling and
benchmarking procedures and is important in understanding the purpose and intent of
these procedures under the IESWTR.
Regulatory Background
The Safe Drinking Water Act (SDWA) Amendments of 1996 mandate that EPA develop
interrelated regulations to control microbial pathogens and disinfectants/disinfection
byproducts (D/DBPs) in drinking water. These rules are collectively known as the
microbial/disinfection byproducts (M-DBP) rules and are intended to address complex
risk trade-offs between the desire to inactivate pathogens found in water and the need to
reduce chemical compounds formed as byproducts during disinfection.
To address the complex risk trade-offs between chronic DBP health risks and acute
pathogenic health risks, EPA promulgated the ICR in May 1996 as a means to obtain data
from large systems (i.e., surface water systems serving more than 100,000 people and
groundwater systems serving more than 50,000 people). Information requested in the
ICR addresses source water quality, byproduct formation, and drinking water treatment
plant design arid operations. Since promulgation and implementation of the ICR was
delayed, information from the ICR was unavailable for two rulings, therefore the
profiling and benchmarking procedures were developed.
EPA is promulgating the M-DBP cluster of rules in two phases. The rules in the first
phase, the Stage 1 DBPR and the IESWTR, were promulgated December 16, 1998. The
Stage 1 DBPR applies to all community water systems and nontransient noncommunity
water systems that treat their water with a chemical disinfectant for either primary or
residual treatment and addresses the formation of DBPs during water treatment. The
IESWTR applies to all public water systems that use surface water or GWUDI, and serve
greater than 10,000 people. The IESWTR amends the Surface Water Treatment Rule
(SWTR) and includes new .and more stringent requirements for controlling waterborne
pathogens including Giardia, viruses, and Cryptosporidium.
A Long-Term 1 ESWTR will be promulgated in December 2000 and will address
treatment requirements for surface water systems serving fewer than 10,000 people. EPA
had hoped to use ICR data for the IESWTR and Stage 1 DBPR, but delays in
promulgation eliminated this potential data source.
The second phase, the Stage 2 DBPR and the Long-Term 2 ESWTR, will be promulgated
in 2002 and will revisit the regulations for the formation of byproducts during
disinfection for all systems and the inactivation and removal of pathogens for surface
water systems, respectively. The key dates for these regulatory activities are provided in
Table A-l.
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APPENDIX A. HISTORY
Table A-1. Key Dates for Regulatory Activities
Date
Regulatory Action
December 2000
Promulgate Long-Term 1 Enhanced Surface Water Treatment Rule
May 2002
Promulgate Stage 2 Disinfectants and Disinfection Byproduct Rule
May 2002
Promulgate Long-Term 2 Enhanced Surface Water Treatment Rule
Convening of the Federal Advisory Committee
In May 1996, EPA initiated a series of public meetings to exchange information on issues
related to M-DBP regulations. In 1997, the EPA established the M-DBP Advisory
Committee under the Federal Advisory Committee Act (FACA) to facilitate stakeholder
participation and to help meet the deadlines for the ffiSWTR and Stage 1 DBPR
established by Congress in the 1996 SDWA Amendments. The purpose of this Advisory
Committee was to collect, share, and analyze new information and data, as well as to
build consensus on the regulatory implications of this new information.
The Advisory Committee was concerned that water systems would reduce disinfection
(e.g., logs of Giardia inactivation) to meet Stage 1 DBPR requirements for DBPs. At the
time the SWTR was issued, EPA had limited data concerning Giardia and
Cryptosporidium occurrence in source waters and treatment efficiencies. The 3-log
removal/inactivation of Giardia and 4-log removal/inactivation of enteric viruses
required by the SWTR were developed to provide protection from most pathogens in
source waters. However, additional data have become available since promulgation of
the SWTR concerning source water occurrence and treatment efficiencies for Giardia, as
well as for Cryptosporidium (LeChevallier et al., 1991a; 1991b).
The Advisory Committee was concerned that if water systems currently provide four or
more logs of removal/inactivation for Giardia, such systems might reduce existing levels
of disinfection to meet the DBP requirements of the Stage 1 DBPR. This change in
disinfection practices could result in systems only marginally meeting the 3-log
removal/inactivation requirement for Giardia specified in the current SWTR. Depending
upon source water Giardia concentrations, such treatment changes could lead to
significant increases in microbial risk (Regli et al., 1993; Grubbs et al., 1992; USEPA,
1994b).
The M-DBP Advisory Committee's recommendations to the EPA included tighter
turbidity performance criteria and individual filter monitoring requirements as part of the
TJESWTR. The revised turbidity performance criteria would contribute to a key EESWTR
objective, that is to establish a microbial backstop to prevent significant increases in
microbial risk when systems implement the DBP standards under the Stage 1 DBPR.
The Advisory Committee also agreed that another major component of a microbial
backstop would be provisions for disinfection profiling and benchmarking.
EPA Guidance Manual
Disinfection Profiling and Benchmarking
A-2
August 1999
-------
APPENDIX A. HISTORY
Profiling and Benchmarking Procedures
The M-DBP Advisory Committee made recommendations to EPA on disinfection
profiling and benchmarking procedures to assure that pathogen control is maintained
while the Stage 1 DBPR provisions are implemented. In developing the profiling and
benchmarking procedures, the Advisory Committee evaluated the following issues; what
information systems should be gathered to evaluate current disinfection practices, how
the profiling and benchmark procedures should operate, and how systems and States
should work together to assure that microbial control is maintained.
Based on data provided by systems and reviewed by the Advisory Committee, the
microbial inactivation baseline, expressed as logs of Giardia inactivation, demonstrated
high variability. Inactivation varied by several logs on a day-to-day basis at any
particular treatment plant and by 10 or more ten logs over a year due to changes in water
temperature, flow rate (and consequently contact time), seasonal changes in residual
disinfectant, pH, and disinfectant demand (and consequently disinfectant residual). There
were also differences between years at individual plants.
To address these variations, the Advisory Committee recommended a disinfection
profiling approach for a system to characterize their existing disinfection practices. In
essence, this approach allows a plant to chart or plot its daily levels of Giardia
inactivation on a graph that, when viewed on a seasonal or annual basis, represents a
"profile" of the plant's inactivation performance. The system can use the profile to
develop a baseline or "benchmark" of inactivation against which to measure possible
changes in disinfection practices.
This approach makes it possible for a plant to change its. disinfection practices to meet the
Stage 1 DBPR maximum contaminant levels (MCLs), without a significant increase in
microbial risk. The benchmarking approach and guidance in this manual provide tools
for plants to understand potential impacts of modifying disinfection practices.
August 1999
A-3
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Disinfection Profiling and Benchmarking
-------
APPENDIX A. HISTORY
THIS PAGE INTENTIONALLY LEFT BLANK.
EPA Guidance Manual
Disinfection Profiling and Benchmarking
A-4
August 1999
-------
APPENDIX B. LOG INACTIVATION
METHODS
Development of the Log Inactivation Method under the SWTR
The disinfection profile is based on microbial inactivation. As part of the SWTR, EPA
developed a method to calculate microbial inactivation for evaluating the effectiveness of
disinfection in a water system. Chemical disinfection does not remove microorganisms
from water but inactivates them so they can no longer infect consumers. Under the
method developed for the SWTR, the actual plant disinfection conditions are converted to
a theoretical level of inactivation of specific microorganisms.
The conversion from plant conditions to microbial inactivation is accomplished based on
"CT tables" developed for the SWTR, where C is the residual disinfectant concentration
(mg/L) and T is the time (in minutes) that water is in contact with the disinfectant.
These tables relate CT values to levels of inactivation under various operating conditions.
Different tables exist for different disinfectants. As the CT value is increased, a greater
percentage of microorganisms are inactivated by chemical disinfection. The CT, and
therefor the level of inactivation, can be increased by applying greater doses of the
disinfectant or by increasing the time that the water is in contact with the disinfectant.
The level of inactivation is generally referred to in terms of "log inactivation" since
inactivation is measured on a logarithmic scale (i.e., orders of magnitude reduction). For
example, a 2-log inactivation and/or removal of Giardia corresponds to inactivating 99
percent of the Giardia cysts through the disinfection process while a 3-log inactivation
and/or removal corresponds to a 99.9 percent inactivation.
Log inactivation is a measure of the percent of microorganisms that are inactivated
during the disinfection process and is defined as:
Log Inactivation = Log
NT
Where,
N0 = initial (influent) concentration of viable microorganisms
NT = concentration of surviving microorganisms
Log = logarithm to base 10
Log inactivation is related to the percent inactivation, defined as:
( NT"l
Percent Inactivation =1 — * 100
NL
August 1999
B-1
EPA Guidance Manual
Disinfection Profiling and Benchmarking
-------
APPENDIXB. LOG INACTIVATION METHODS
Therefore, the relationship between log inactivation and percent inactivation is as follows:
i
1
Percent Inactivation = 1—
or,
Log Inactivation = Log
i (\Log Inactivation
100
*100
100 - Percent Inactivation
The following two examples illustrate the relationship between influent and effluent
concentrations, percent inactivation, and log inactivation.
Example 1
A utility has an influent concentration (N0) of active Giardia of 10,000 cysts/lOOL and a
concentration of surviving microorganisms at the first point in the distribution system
(NT) of 10 cysts/1 OOL. What is the log inactivation of this treatment process?
• (N "i
Log Inactivation = Log ——
\ i j
^10,000^
. 10 J
Log Inactivation = Log
Log Inactivation = Log 1,000
Log Inactivation = 3
Example 2
Given that the utility has a 3-Log Inactivation of Giardia, what is the percent inactivation
o/Giardia?
Percent Inactivation =1 —:
1 rvLog Inactivation
100
Percent Inactivation = 1 r * 100
I io3J
'ercent Inactivation =1 * 100
i ( 1,000 J
Percent Inactivation = (1-.001)* 100
Percent Inactivation = 99.9
EPA Guidance Manual
Disinfection Profiling and Benchmarking
B-2
August 1999
-------
APPENDIXB. LOG INACTIVATION METHODS
As the two examples show, a 3-log inactivation equals 99.9 percent inactivation. Table
B-l presents similar calculations for different log inactivations and corresponding percent
inactivations.
Table B-1. Log Inactivation and Percent Inactivation
Log Inactivation
0.0
0.5
1.0
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3.0
4.0
5.0
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7.0 '
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0.00
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90.00
99.00
99.90
99.99
99.999
99.9999
99.99999
August 1999
B-3
EPA Guidance Manual
Disinfection Profiling and Benchmarking
-------
APPENDIX B. LOG INACTIVATION METHODS
THIS PAGE INTENTIONALLY LEFT BLANK
EPA Guidance Manual
Disinfection Profiling and Benchmarking
B-4
August 1999
-------
APPENDIX C. CT VALUES FOR
INACTIVATIONS ACHIEVED BY VARIOUS
DISINFECTANTS
This appendix provides a reprint of the CT tables for determining inactivations achieved
by various disinfectants. These tables were originally provided in EPA's Guidance
Manual for Compliance with the Filtration and Disinfection Requirements for Public
Water Sources (AWWA, 1991).
August 1999
C-1
EPA Guidance Manual
Disinfection Profiling and Benchmarking
-------
APPENDIX C. CT VALUES FOR INACTIVATIONS ACHIEVED BY VARIOUS DISINFECTANTS
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August 1999
C-3
EPA Guidance Manual
Disinfection Profiling and Benchmarking
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APPENDIX C. CT VALUES FOR INACTIVATIONS ACHIEVED BY VARIOUS DISINFECTANTS
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EPA Guidance Manual
Disinfection Profiling and Benchmarking
C-4
August 1999
-------
APPENDIX C. CT VALUES FOB INACTIVATIONS ACHIEVED BY VARIOUS DISINFECTANTS
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C-5
EPA Guidance Manual
Disinfection Profiling and Benchmarking
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APPENDIX C. CT VALUES FOR INACTIVATIONS ACHIEVED BY VARIOUS DISINFECTANTS
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EPA Guidance Manual
Disinfection Profiling and Benchmarking
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August 1999
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APPENDIX C. CT VALUES FOR INACTIVATIONS ACHIEVED BY VARIOUS DISINFECTANTS
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APPENDIX C. CT VALUES FOR INACTIVATIONS ACHIEVED BY VARIOUS DISINFECTANTS
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EPA Guidance Manual
Disinfection Profiling and Benchmarking
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August 1999
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APPENDIX C. CT VALUES FOR INACTIVATIONS ACHIEVED BY VARIOUS DISINFECTANTS
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-------
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APPENDIX C. CT VALUES FOR INACTIVATIONS ACHIEVED BY VARIOUS DISINFECTANTS
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August 1999
C-11
EPA Guidance Manual
Disinfection Profiling and Benchmarking
-------
-------
APPENDIX D. DETERMINATION OF
DISINFECTANT CONTACT TIME
This, appendix originally "appeared as Appendix C in the Guidance Manual for
Compliance with the Filtration and Disinfection Requirements for Public Water Systems
Using Surface Water Sources (AWWA, 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 Guidance Manual.
As indicated in Chapter 3, 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 Primacy Agency.
For the purpose of determining compliance with the disinfection requirements of the
SWTR, 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 TIO 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 is used for estimating
the detention time, TIO, for the purpose of calculating CT.
This appendix is divided into three sections. The first section presents a brief synopsis of
tracer study methods, procedures, and data evaluation. In addition, examples are
presented for conducting hypothetical tracer studies to determine the TIO contact time in a
clearwell. The second section presents a method of determining T10 from theoretical
detention times in systems where it is impractical to conduct tracer studies. The third
section provides examples on how to incorporate baffling classification and factors into
CT calculations and provides detailed practical examples on the use of tracer studies and
baffling conditions to calculate Tio/T.
D.1 Tracer Studies
D.1.1 Flow conditions
Although detention time is proportional to flow, it is not generally a linear function.
Therefore, tracer studies are needed to establish detention times for the range of flow
rates experienced within each disinfectant segment.
As discussed in Section 3.4.2, a single flow rate may 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.
August 1999
D-1
EPA Guidance Manual
Disinfection Profiling and Benchmarking
-------
APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
In filter plants, the plant flow is relatively uniform from the intake through the filters An
increase or reduction in the intake pumping capacity will impart a proportional change in
flow through each process unit prior to and including the filters. Therefore, at a constant
intake pumping rate flow variations between disinfectant segments within a treatment
plant, excluding clearwells, are likely to be small, and the design capacity of the plant, or
plant flow, can be considered the nominal flow rate through each individual process unit
within the plant. Clearwells may operate at a different flow rate than the rest of the plant,
depending on the pumping capacity.
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 highest test flow rate is at least 91 percent of the highest flow rate
expected to ever occur in that segment. Four data points will 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 TIO vs. Q for each segment in the system. A smooth line is drawn
through the points on each graph to create a curve from which TIO may be read for the
corresponding Q at peak hourly flow conditions. This procedure is presented in Section
D.I.8.
i p j
It may not be practical for all systems to conduct studies at four flow rates. The number
of tracer tests that are practical to conduct is dependent on site-specific restrictions and
resources available to the system. Systems with limited resources can conduct a
minimum of one tracer test for each disinfectant segment at a flow rate of not less than 91
percent of the highest flow rate experienced at that segment. If only one tracer test is
performed, the detention time determined by the test may be used to provide a
conservative estimate in CT calculations for that segment for all flow rates less than or
equal to the tracer test flow rate. Tw is inversely proportional to flow rate, therefore, the
TIO at a flow rate other than that which the tracer study was conducted (T10S) can be'
approximated by multiplying the Tw from the tracer study (T10T) by the ratio of the tracer
study flow rate to the desired flow rate, (i.e., TIOS = T1OT • Qr/Qo).
Where:
TIOS = TIO at system flow rate
TIOT = TIO at tracer flow rate
QT = tracer study flow rate
QD = system flow rate
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
EPA Guidance Manual
Disinfection Profiling and Benchmarking
D-2
August 1999
-------
APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
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, TIO should be recorded at the average flow rate over the
course of the test.
D.i.2 Other Tracer Study Considerations
In addition to flow conditions, detention times determined by tracer studies are dependent
on the water level in the contact basin. 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 which are operated at a near
constant level (that is, flow in equals flow out), the detention time determined by tracer
tests is valid for calculating CT when the basin is operating at water levels greater than or
equal to the level at which the test was performed. If the water level during testing is
higher than the normal operating level, the resulting concentration profile will predict an
erroneously high detention time. Conversely, extremely low water levels during testing
may lead to an overly conservative detention time. Therefore, 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). This
procedure will provide a detention time for the contact basin, which is also valid when
the water level is rising (flow out less than flow in) from a level that is at or above the
level when the TIO was determined by the tracer study. Whether the water level is
constant or variable, the tracer study for each segment should be repeated for several
different flows, as described in the previous segment.
For clearwells that are operated with extreme variations in water level, maintaining a CT
to comply with inactivation requirements may be impractical. Under such operating
conditions, a reliable detention time is not provided for disinfection. However, the
system may install a weir to ensure a minimum water level and provide a reliable
detention time.
Systems comprised of storage reservoirs that experience seasonal variations in water
levels might perform tracer studies during the various seasonal conditions. For these
systems, tracer tests should be conducted at several flow rates and representative water
levels that occur for each seasonal condition. The results of these tests can be used to
develop hydraulic profiles of the reservoir for each water level. These profiles can be
August 1999
D-3
EPA Guidance Manual
Disinfection Profiling and Benchmarking
-------
APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
plotted on the same axis of TIO vs. Q and may be used for calculating CT for different
water levels and flow rates.
Detention time may also be influenced by differences in water temperature within the
system. For plants with potential for thermal stratification, additional tracer studies are
suggested under the various seasonal conditions that are likely to occur. The contact
times determined by the tracer studies under the various seasonal conditions should
remain valid as long as no physical changes are made to the mixing basin(s) or storage
reservoir(s).
The portion of the system with a measurable contact time between two points of
disinfection or residual monitoring is referred to as a segment. 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 TIO for each segment
containing process unit(s). The TIO 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 segment is then determined. The total inactivation and log
inactivation achieved in the system can then be determined by summing the inactivation
ratios for all segments as explained in Section 3.5.
I .. .
For systems that have two or more units of identical size and configuration, tracer studies
only need to be conducted on one of the units. The resulting graph of T10 vs. flow can be
used to determine TIO for all identical units.
Systems with more than one segment in the treatment plant may determine TIO for each
segment:
i i
i !
• By individual tracer studies through each segment, or
• By one tracer study across the system. l
If possible, tracer studies should be conducted on each segment to determine the TIO 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.
i i
However, it may not always be practical for systems to conduct tracer studies for each
segment because of time and manpower constraints. In these cases, one tracer study may
be used to determine the TIO values for all of the segments at one flow rate. This
procedure involves the following steps:
• Add tracer at the beginning of the furthest upstream disinfection segment.
• Measure the tracer concentratipn at the end of each disinfection segment.
EPA Guidance Manual
Disinfection Profiling and Benchmarking
D-4
August 1999
-------
APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
• Determine the TIO to each monitoring point, as outlined in the data evaluation
examples presented in Section D.I.7.
« Subtract TIO values of each of the upstream segments from the overall TIO
value to determine the TIO of each downstream segment.
This approach is valid for a series of two or more consecutive segments as long as all
process units within the segments experience the same flow condition. This approach is
illustrated by Hudson (1975) in which step-dose tracer tests were employed to evaluate
the baffling characteristics of flocculators and settling basins at six water treatment
plants. At one plant, tracer chemical was added to the rapid mix, which represented the
beginning of the furthest upstream disinfection segment in the system. Samples were
collected from the flocculator and settling basin outlets, and analyzed to determine the
residence-time characteristics for each segment. Tracer measurements at the flocculator
outlet indicated an approximate TIO of 5 minutes through the rapid mix, interbasin piping,
and floeculator. Based on tracer concentration monitoring at the settling basin outlet, an
approximate TIO of 70 minutes was determined for the combined segments, including the
rapid mix, interbasin piping, flocculator, and settling basin. The flocculator TIO of 5
minutes was subtracted from the combined segments' TIO of 70 minutes, to determine the
TIO for the settling basin alone (65 minutes).
This approach may also be applied in cases where disinfectant application and/or residual
monitoring is discontinued at any point between two or more segments with known TIO
values. These TIO values may be summed to obtain an equivalent TIO for the combined
segments.
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.
D.1. 3 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,
While both tracer test methods can use the same tracer materials and involve measuring
the cpncentration of tracer with time, each has distinct advantages and disadvantages with
respect to tracer addition procedures and analysis of results.
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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 ievel. Step-dose tracer
studies are frequently employed in drinking water applications for the following reasons:
! . '
• The resulting normalized concentration vs. time profile-is directly used to
determine TIQ, 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.
Alternatively, with 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 pver time as the tracer
passes through the unit. A disadvantage of this technique is that very concentrated
solutions are needed for the dose in order to adequately define the concentration versus
time profile. Intensive mixing is therefore required 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 must be
carefully computed to provide an adequate tracer profile at the effluent of the
basin;
: j
* The resulting concentration vs. time profile catmot be used to directly
determine TIQ without further manipulation; and
» A mass balance on the treatment segment is required 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 is acceptable for
conducting drinking water tracer studies, and the choice of the method may be
determined by site-specific constraints or the system's experience.
D.1.4 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. Historically, many chemicals have been used in tracer
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studies that do not satisfy all of these criteria, including potassium permanganate, alum,
chlorine, and sodium carbonate. However, 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
lOmg/L;
• Drinking water concentrations should not exceed 0.1 ug/L;
• Studies that result in human exposure to the dye must be brief and infrequent;
and
• Concentrations as low as 2 mg/L can be used in tracer studies because of the
low detection level in the range of 0.1 to 0.2 ug/L.
The use of Rhodamine B as a tracer in water flow studies is not recommended by the
EPA.
The choice of a tracer chemical can be made based, in part, on the selected dosing
method and also 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 floe 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.
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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.
D.1.5 Tracer Addition
The tracer chemical should be added at the same point(s) in the treatment train as the
disinfectant to be used in the CT calculations.
D.1.S.1 Step-dose Method
The duration of tracer addition is dependent on the volume of the basin, and hence, it's
theoretical detention time. In order to approach a steady-state concentration in the water
exiting the basin, tracer addition and sampling should usually be continued for a period of
two to three times the theoretical detention time (Hudson, 1981). It is not necessary to
reach a steady-state concentration in the exiting water to determine TIQ; however, it is
necessary to determine tracer recovery. It is recommended that the tracer recovery be
determined to identify hydraulic characteristics or density problems. Generally, a 90
percent recovery is considered to provide reliable results for the evaluation
In all cases, the tracer chemical should be dosed in sufficient concentration to easily
monitor a residual at the basin outlet throughout the test. The required tracer chemical
concentration is generally dependent upon the nature of the chosen tracer chemical
including its background concentration, and the mixing characteristics of the basin to be
tested. Recommended chloride doses on the order of 20 mg/L (Hudson, 1975) should be
used for step-method tracer studies where the background chloride level is less than 10
mg/L. Also, fluoride concentrations as low as 1.0 to 1.5 mg/L are practical when the raw
water fluoride level is not significant (Hudson, 1975). However, tracer studies conducted
on systems suffering from serious short-circuiting of flow may require substantially
larger step-doses. This would be necessary to detect the tracer chemical and to
adequately define the effluent tracer concentration profile.
D.1.5.2 Slug-dose Method
The duration of tracer measurements using the slug-dose method is also dependent on the
volume of the basin, and hence, it's theoretical detention time. In general, samples
should be collected for at least twice the basin's theoretical detention time, or until tracer
concentrations are detected near background levels. In order to get reliable results for TIO
values using the slug-dose method it is recommended that the total mass of tracer
recovered be approximately 90 percent of the mass applied. This guideline requires
sampling until the tracer concentration recedes to the background level. The total mass
recovered during testing will not be known until completion of the testing and analysis of
the data collected. The sampling period needed is very site specific. Therefore, it may be
helpful to conduct a first run tracer test as a screen to identify the appropriate sampling
period for gathering data to determine
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Tracer addition for slug-dose method tests should be instantaneous and provide uniformly
mixed distribution of the chemical. Tracer addition is considered instantaneous if the
dosing time does not exceed 2 percent of the basin's theoretical detention time (Marske
and Boyle, 1973). One recommended procedure for achieving instantaneous tracer
dosing is to apply the chemical by gravity flow through a funnel and hose apparatus. This
method is also beneficial because it provides a means of standardization, which is
necessary to obtain reproducible results.
The mass of tracer chemical to be added is determined by the desired theoretical
concentration and basin size. The mass of tracer added in slug-dose tracer tests should be
the minimum mass needed to obtain detectable residual measurements to generate a
concentration profile. As a guideline, the theoretical concentration for the slug-dose
method should be comparable to the constant dose applied in step-dose tracer tests, (i.e.,
10 to 20 mg/L and 1 to 2 mg/L for chloride and fluoride, respectively). The maximum
mass of tracer chemical needed is calculated by multiplying the theoretical concentration
by the total basin volume. This is appropriate for systems with high dispersion and/or
mixing. This quantity is diluted as required to apply an instantaneous dose, and minimize
density effects. It should be noted that the mass applied is not likely to get completely
mixed throughout the total volume of the basin. Therefore, the detected concentration
might exceed theoretical concentrations based on the total volume of the basin. For these
cases, the mass of chemical to be added can be determined by multiplying the theoretical
concentration by only a portion of the basin volume. An example of this is shown in
Section D. 1.7.2 for a slug-dose tracer study. In cases where the tracer concentration in
the effluent must be maintained below a specified level, it may be necessary to conduct a
preliminary test run with a minimum tracer dose to identify the appropriate dose for
determining TIO without exceeding this level.
D.1.6 Test Procedure
In preparation for beginning a tracer study, the raw water background concentration of
the chosen tracer chemical must be established. The background concentration is
essential, not only for aiding 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. The monitoring procedure is outlined in the following steps:
If the tracer chemical is normally added for treatment, discontinue its addition to the
water in sufficient time to permit the tracer concentration to recede to its background
level before the test is begun.
• 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
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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.
D.1.6.1 Step-dose Method
At time zero, the tracer chemical feed will be 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 well-defined plot of tracer concentration vs. time. If
on-site analysis is available, less frequent residual monitoring may be possible until a
change in residual concentration is first detected. As a guideline, in systems with a
theoretical detention time greater than 4 hours, sampling may be conducted every 10
minutes for the first 30 minutes, or until a tracer concentration above the baseline level is
first detected. In general, shorter sampling intervals enable better characterization of
concentration changes; therefore, sampling should be conducted at 2 to 5-minute intervals
from the time that a concentration change is first observed until the residual concentration
reaches a steady-state value. A reasonable sampling interval should be chosen based on
the overall detention time of the unit being tested.
If verification of the test is desired, the tracer feed should be discontinued, and the
receding tracer concentration at the effluent should be monitored at the same frequency
until tracer concentrations corresponding to the background level are detected. The time
at which tracer feed is stopped is time zero for the receding tracer test and must be noted.
The receding tracer test will provide a replicate set of measurements that can be
compared with data derived from the rising tracer concentration versus time curve. For
systems which currently feed the tracer chemical, the receding curve may be generated
from the time the feed is turned off to determine the background concentration level.
D.1.6.2 Slug-dose Method
At time zero for the slug-dose method, a large instantaneous dose of tracer will be 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
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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.
D.1.7 Data Evaluation
Data from tracer studies should be summarized in tables of time and residual
concentration. • These data are then analyzed to determine the detention time, TIO, to be
used in calculating CT. Tracer test data from either the step-dose or slug-dose method
can be evaluated graphically, numerically, or by a combination of these techniques.
D. 1.7.1 Step-dose Method
The graphical method of evaluating step-dose test data involves plotting a graph of
dimensionless concentration (C/Co) versus time and reading the value for TIO 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 TIO 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.
An illustration of the TIO determination will be presented in an example of the data
evaluation required for a clearwell tracer study.
D.1.7.2 Slug-dose Method
Data from slug-dose tracer tests is analyzed by converting it to the mathematically
equivalent step-dose data and using techniques discussed in Section D.I.7.1 to determine
TIQ. 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
graphically or numerically integrating the curve. The conversion to step-dose data is then
completed in several mathematical steps involving the total area.
A graphical technique for converting the slug-dose data involves physically measuring
the area using a planimeter. The planimeter is an instrument used to measure the area of
a plane closed curve by tracing its boundary. Calibration of this instrument to the scale
of the graph is required to obtain meaningful readings.
The rectangle rule is a simple numerical integration method that approximates the total
area under the curve as the sum of the areas of individual rectangles. These rectangles
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have heights and widths equal to the residual concentration and sampling interval (time)
for each data point on the curve, respectively. Once the data has been converted, TIO may
be determined in the same manner as data from step-dose tracer tests.
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. The steps and
specific details involved with evaluating data from both tracer study methods are
illustrated in the following examples.
Example for Determining TIO in a Clearwell :
Two tracer studies employing the step-dose and slug-dose methods of tracer addition
were conducted for a clearwell with a theoretical detention time, T, of 30 minutes at an
average flow of 2.5 MOD. Because fluoride is added at the inlet to the clearwell as a
water treatment chemical, necessary feed equipment was in place for dosing a constant
concentration of fluoride throughout the step-dose tracer test. Based on this convenience,
fluoride was chosen as the tracer chemical for the step-dose method test. Fluoride was
also selected as the tracer chemical for the slug-dose method test. Prior to the start of
testing, a fluoride baseline concentration of 0.2 mg/L was established for the water
exiting the clearwell.
Step-close Method Test
For the step-dose test a constant fluoride dosage of 2.0 mg/L was added to the clearwell
inlet. Fluoride levels in the clearwell effluent were monitored and recorded every
3 minutes. The raw tracer study data, along with the results of further analyses are shown
in Table D-l.
The steps in evaluating the raw data shown in the first column of Table D-l are as
follows. First, the baseline fluoride concentration, 0.2 mg/L, is subtracted from the
measured concentration to give the fluoride concentration resulting from the tracer study
addition alone. For example, at elapsed time = 39 minutes, the tracer fluoride
concentration, C, is obtained as follows:
C — Qmeasured ~ M>aseline
= 1.85 mg/L-0.2 mg/L
= 1.65 mg/L
This calculation was repeated at each time interval to obtain the data shown in the third
column of Table D-l. As indicated, the fluoride concentration rises from 0 mg/L at t = 0
minutes to the applied fluoride dosage of 2 mg/L, at t = 63 minutes.
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The next step is to develop, dimensionless concentrations by dividing the tracer
concentrations in the second column of Table D-l by the applied fluoride dosage, Co = 2
mg/L. For time = 39 minutes, C/Co is calculated as follows:
C/Co = (1.65 mg/L)/(2.0 mg/L)
= 0.82
The resulting dimensionless data, presented in the fourth column of Table D-l, is the
basis for completing the determination of TIO by either the graphical or numerical
method.
TABLE D-1. CLEARWELL DATA - STEP-DOSE TRACER TEST(1>2'3)
t
(minutes)
0
3
6
9
12
15
18
21
24
27
30
33
36 ,
39
42
45
48
51
54
57
60
63
Fluoride Concentration
Measured
(mg/L)
0.20
. . 0.20
0.20
0.20
0.29
0.67
0.94
1.04
1.44
1.55
1.52
1.73
1.93
1.85
1.92
2.02
1.97
1.84
2.06
2.05
2.10
2.14
Tracer
(mg/L)
0
0
0
0
0.09
0.47
0.74
0.84
1,24
1.35
1.32
1.53
1.73
1.65
1.72
1.82
1.77
1.64
1.86
1.85
1.90
1.94
Dimensionless
(C/Co)
0
0
0
0
0.045
0.24
0.37
0.42
0.62
0.68
0.66
0.76
0.86
0.82
0.86
0.91
0.88
0.82
0.93
0.92
0.95
0.96
1. Baseline cone. = 0.2 mg/L, fluoride dose = 2.0 mg/L
2. Measured cone. = Tracer cone. + Baseline cone.
3. Tracer cone. = Measured cone. - Baseline cone.
In order to determine Tioby the graphical method, a plot of C/Co vs. time should be
generated using the data in Table D-l. A smooth curve should be drawn through the data
as shown on Figure D-l.
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is read directly from the graph at a dimensionless concentration (C/Co) corresponding
to the time for which 10 percent of the tracer has passed at the effluent end of the contact
basin (Tio). For step-dose method tracer studies, this dimensionless concentration is
C/Co = 0.10 (Levenspiel, 1972).
o
10
20
30
40
f IME (MINUTES)
50
60
Figure D-1. C/Co vs. Time — Graphical Analysis for T10
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TIO should be read directly from Figure D-l at C/Co = 0.1 by first drawing a horizontal
line (C/Co = 0.1) from the Y-axis (t = 0) to its intersection with the smooth curve drawn
through the data. At this point of intersection, the time read from the X-axis is TIO and
may be found by extending a vertical line downward to the X-axis. These steps were
performed as illustrated on Figure D-l, resulting in a value for TIO of approximately 13
minutes.
For the numerical method of data analysis, several additional steps are required to obtain
TIO from the data in the fourth column of Table D-l. The forms of data necessary for
determining TIO through a numerical solution are logio (1-C/Co) and t/T, the elapsed time
divided by the theoretical residence time. These are obtained by performing the required
mathematical operations on the data in the fourth column of Table D-l. For example,
recalling that the theoretical detention time, T, is 30 minutes, the values for logio (1-
C/Co) and t/T are computed as follows for the data at t = 39 minutes:
loglo (1-C/Co) = logio (1-0.82)
= log10(0.18)
. =-0.757
t/T = 39 min/30 min= 1.3
This calculation was repeated at each time interval to obtain the data shown in Table D-2.
These data should be linearly regressed as logio (1-C/Co) versus t/T to obtain the fitted
straight-line parameters to the following equation:
(D
Iog10 (1-C/Co) = m(t/T) -I- b
In equation 1, m and b are the slope and intercept, respectively, for a plot of logio
(1-C/Co) vs. t/T. This equation can be used to calculate TIO, assuming that the correlation
coefficient for the fitted data indicates a good statistical fit (0.9 or .above).
A linear regression analysis was performed on the data in Table D-2, resulting in the
following straight-line parameters:
slope = m = -0.774
intercept =b =0.251
correlation coefficient = 0.93
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Table D-2. Data For Numerical Determination Of T10
t/T
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
- 1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
Logio(1-C/Co)
0
0
0
0
-0.020
-0.116
-0.201
-0.237
-0.420
-0.488
-0.468
-0.629
-0.870
-0.757
-0.854
-1.046
-0.939
-0.745
-1.155
-1.125
-1.301 :
-1.532
Although these numbers were obtained numerically, a plot of logic (1-G/Co) versus t/T is
shown for illustrative purposes on Figure D-2 for the data in Table D-2. In this analysis,
data for time = 0 through 9 minutes were excluded because fluoride concentrations above
the baseline level were not observed in the clearwell effluent until t = 12 minutes.
Equation 1 is then rearranged in the following form to facilitate a solution for
(2)
In equation 2, as with graphical method, TIO is determined at the time for which C/Co =,
0.1. Therefore, in equation 2, C/Co has been replaced by 0.1 and t (time) by TIQ. To
obtain a solution for TIO, the values of the slope, intercept, and theoretical detention time
are substituted as follows:
T10/30 min. = (log,0 (1 - 0.1) - 0.251)/(-0.774)
= 12 minutes
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A A, A
o
O
O
0.01
0
2.5
Slope, mi-0.774
Intercept, b» 0.251
Correlation Coefficient - 0.93
Figure D-2. 1-C/Co vs. tTT — Numerical Analysis for
In summary both the graphical and numerical methods of data reduction resulted in
comparable, but not identical values for TIQ. With the numerical method, TIO was
determined as the solution to an equation based on the straight-line parameters to a linear
regression analysis of the tracer study data instead of an "eyeball1 estimate from a data
plot.
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Slug-dose Method Test
A slug-dose tracer test was also performed on the clearwell at a flow rate of 2.5 mgd. A
theoretical clearwell fluoride concentration of 2.2 mg/L was selected. The fluoride
dosing volume and concentration were determined from the following considerations:
Dosing Volume
» The fluoride injection apparatus consisted of a funnel and a length of copper
tubing. This apparatus provided a constant volumetric feeding rate of 7.5
liters per minute (L/min) under gravity flow conditions.
«> At a flow rate of 2.5 mgd, the clearwell has a theoretical detention time of
30 minutes. Since the duration of tracer injection should be less than 2
percent of the clearwell's theoretical detention time for an instantaneous dose,
the maximum duration of fluoride injection was:
Max. dosing time = 30 minutes x .02 = 0.6 minutes
«• At a dosing rate of 7.5 L/min, the maximum fluoride dosing volume is
calculated to be:
Max. dosing volume = 7.5 L/min. x 0.6 minutes = 4.5 L
For this tracer test, a dosing volume of 4 liters was selected, providing an instantaneous
fluoride dose in 1.8 percent of the theoretical detention time.
; i
i [
Fluoride Concentration
• The theoretical detention time of the clearwell, 30 minutes, was calculated by
dividing the clearwell volume, 52,100 gallons or 197,200 liters, by the average
flow rate through the clearwell, 2.5 mgd.
• Assuming the tracer is completely dispersed throughout the total volume of
the clearwell, the mass of fluoride required to achieve a theoretical
concentration of 2.2 mg/L is calculated as follows:
Fluoride mass (initial) = 2.2 mg/L x 197,200 L x
lOOOmg
= 434g
The concentration of the instantaneous fluoride dose is determined by dividing
this mass by the dosing volume, 4 liters: \
434 g '•
Fluoride concentration = — — — = 109 g/L
Fluoride levels in the exit to the clearwell were monitored and recorded every 3 -minutes.
The raw slug-dose tracer test data are shown in Table D-3.
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
The first step in evaluating the data for different times is to subtract the baseline fluoride
concentration, 0.2 mg/L, from the measured concentration at each sampling interval
(Table D-3). This is the same as the first step used to evaluate step-dose method data and
gives the fluoride concentrations resulting from the tracer addition alone, shown in the
third column of Table D-3. As indicated, the fluoride concentration rises from 0 mg/L at
t = 0 minutes to the peak concentration of 3.6 mg/L at t = 18 minutes. The exiting
fluoride concentration gradually recedes to near zero at t = 63 minutes. It should be
noted that a maximum fluoride concentration of 2.2 mg/L is based on assuming complete
mixing of the tracer added throughout the total clearwell volume. However, as shown in
Table D-3, the fluoride concentrations in the clearwell effluent exceeded 2.2 mg/L for
about 6 minutes between 14 and 20 minutes. These higher peak concentrations are
caused by the dispersion of tracer throughout only a portion of the total clearwell volume.
If a lower tracer concentration is needed in the effluent because of local or federal
regulations, the mass to be added should be decreased accordingly.
The dimensionless concentrations in the fourth column of Table D-3 were obtained by
dividing the tracer concentrations in the third column by the clearwell's theoretical
concentration, Co = 2.2 mg/L. These dimensionless concentrations were then plotted as a
function of time, as is shown by the slug-dose data on Figure D-3. These data points
were connected by straight lines, resulting in a somewhat jagged curve.
The next step in evaluating slug-dose data is to determine the total area under the slug-
dose data curve on Figure D-3. Two methods exist for finding this area - graphical and
numerical. The graphical,method is based on a physical measurement of the area using a
planimeter. This involves calibration of the instrument to define the units' conversion
and tracing the outline of the curve to determine the area. The results of performing this
procedure may vary depending on instrument accuracy and measurement technique.
Therefore, only an illustration of the numerical technique for finding the area under the
slug-dose curve will be presented for this example.
The area obtained by either the graphical or numerical method would be similar.
Furthermore, once the area is found, the remaining steps involved with converting the
data to the step-dose response are the same.
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
Table D-3. Clean/veil Data — Slug-Dose Tracer Test(1>2'3)
T
(Minutes)
0
' -3
6
9
12
15
18
21
24
27
30
33
36
39
42
45
48
51
54
57
60 •
63
Fluoride Concentration
Measured
(mg/L)
0.2
0.2
0.2
0.2
1.2
3.6
3.8
2.0
2.1
1.4
1.3
1.5
1.0
0.6
1.0
0.6
0.8
0.6
0.4
0.5
0.6
0.4
Tracer
(mg/L)
0
0
0
0
1
3.4
3.6
1.8
1.9
1.2
1.1
1.3
0.8
0.4
0.8
0.4
0.6
0.4
0.2
0.3
0.4
0.2
Dimensionless
(C/Co)
0
0
0
0
0.45
1.55
1.64
0.82
0.86
0.55
0.50
0.59
' 0.36
0.18
0.36
0.18
0.27
0.18
0.09
0.14
0.18
0.09
1. Measured cone.=Tracer cone. + Baseline cone.
2. Baseline cone. = 0.2 mg/L, fluoride slug dose cone. = 109 g/L, theoretical cone. = 2.2 mg/L.
3. Tracer cone. = Measured cone. - Baseline cone.
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
O
o
O
Slug-dose data
—a—
Step—dose data
.A
10 20 30 40 50 60 70-
TIME (MINUTES) '"-'...
Figure D-3. C/Co vs. Time — Conversion of Slug- to Step-Dose Data
August 1999
D-21
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
Table D-4 summarizes the results of determining the total area using a numerical
integration technique called the rectangle rule. The first and second columns in Table D-
4 are the sampling time and fluoride concentration resulting from tracer addition alone,
respectively. The steps in applying these data are as follows. First, the sampling time
interval, 3 minutes, is multiplied by the fluoride concentration at the end of the 3-minute
interval to give the incremental area, in units of milligram minutes per liter. For example,
at elapsed time, t = 39 minutes, the incremental area is obtained as follows:
Incremental area = sampling time interval x fluoride cone.
= 39-36) minutes x 0.4 mg/L
= 0.2 mg-min/L ..
This calculation was repeated at each time interval to obtain the data shown in the third
column of Table D-4.
If the data had been obtained at unequal sampling intervals, then the incremental area for
each interval would be obtained by multiplying the fluoride concentration at the end of
each interval by the time duration of the interval. This convention also requires that the
incremental area be zero at the first sampling point, regardless of the fluoride
concentration at that time.
As is shown in Table D-4, all incremental areas were summed to obtain
59.4 mg-min/L, the total area under the slug-dose tracer test curve. This number
represents the total mass of fluoride that was detected during the course of the tracer test
divided by the average flow rate through the clearwell.
To complete the conversion of slug-dose data to its equivalent step-dose response
requires two additional steps. -The first involves summing, consecutively, the incremental
areas in the third column of Table D-4 to obtain the cumulative .area at the end of each
sampling interval. For examplej the cumulative area at time, t = 27 minutes is found as
follows:
Cumulative area = 0 + 0 + 0 + 0 + 3 + 10.2 + 10.8 + 5.4 + 5.7 + 3.6 ;
- 38.7 mg-min/L
The cumulative areas for each interval are recorded in the fourth column of Table D-4.
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
Table D-4. Evaluation of Slug-Dose Data
T
(Minutes)
0
3
6
9
12
15
18
21
' 24
27
30
33
36
39
42
45
48
51
54
57
60
63
Fluoride
(mg/L)
0
0
0
'0
1
3.4
3.6
1.8
1.9
1.2
1.1
1.3
0.8
0.4
0.8
0.4
0.6
0.4
0.2
0.3
0.4
0.2
Incremental Area
(mg-min/L)
0
0
0
0
3
10.2
10.8
5.4
5.7
3.6
3.3
3.9
2.4
1.2
2.4
1.2
1.8
1.2
0.6
0.9 ;'
1.2
0.6
Cumulative Area
(mg-min/L)
0
0
0
0
3
13.2
24.0
29.4
35.1
38.7
42.0
45.9
48.3
49.5
51.9
53.1
54.9
56.1
56.7
57.6
58.8
59.4
Equivalent
Step-Dose Data
.0
0
0
0
0.05
. 0.22
0.40
0.49
0.59
0.65
0.71
0.77
0.81
0.83
0.87
0.89
0.92
0.94
0.95
0.97
0.99
. 1.00
Total Area = 59.4
The final step in converting slug-dose data involves dividing the cumulative area at each
interval by the total mass applied. Total area based on applied mass is calculated as
follows:
Total area mass applied/average flow =
mg L
434 g x 1000—- / 6,570——
g mm
mg - min
66.1—2-
For time = 39 minutes, the resulting step-dose data point is calculated as follows:
C/Co = 49.5 mg-min/L / 59.4 mg-min/L
0.83
The result of performing this operation at each sampling interval is the equivalent step-
dose data. These data points are shown in the fifth column of Table D-4 and are also
plotted on Figure D-3 to facilitate a graphical determination of TIQ. A smooth curve was
fitted to the step-dose data as shown on the figure.
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
TIO can be determined by the methods illustrated previously in this example for
evaluating step-dose tracer test data. The graphical method illustrated on Figure D-3
results in a reading of TI o = 15 minutes.
D.1.7.3 Additional Considerations
In addition to determining TIO for use in CT calculations, slug-dose tracer tests provide a
more general measure of the basin's hydraulics in terms of the fraction of tracer recovery.
This number is representative of short-circuiting and dead space in the unit resulting from
poor baffling conditions and density currents induced by the tracer chemical. A low
tracer recovery is generally indicative of inadequate hydraulics. However, inadequate
sampling in which peaks in tracer passage are not measured will also result in an under
estimate, of tracer recovery. The tracer recovery is calculated by dividing the mass of
fluoride detected by the mass of fluoride dosed. .
The dosed fluoride mass was calculated previously and was 434 grams. The mass of
detected fluoride can be calculated by multiplying the total area under the slug-dose
curve by the average flow, in appropriate units, at the time of the test. The average flow
in the clearwell during the test was 2.5 mgd or 6,570 L/min. Therefore, the mass of
fluoride tracer that was detected is calculated as follows:
Detected fluoride mass = total area x average flow
lg
me - mm
= 59.4 —2— x
= 390g
1000 mg
x 6,570
mm
Tracer recovery is then calculated as follows:
Fluoride recovery = detected mass/dosed mass x 100
= 390g/434gxlOO
= 90 %
This is a typical tracer recovery percentage for a slug-dose test, based on the experiences
of Hudson (1975) and Thirumurthi (1969).
D.1.8 Flow Dependency of T10
For systems conducting tracer studies at four or more flows, the TIO detention time should
be determined by the above procedures for each of the desired flows. The detention
times should then be plotted versus flow. For the example presented in the previous
section, tracer studies were conducted at additional flows of 1.1,4.2, and 5.6 MGD. The
TIO values at the various flows were:
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
Flow
l.l
2.5
4.2
5.6
25
13
7
4
data for these tracer studies were plotted as a function of the flow, Q, as shown in
Figure D-4.
If only one tracer test is performed, the flow rate for the tracer study should be not less
than 91 percent of the highest flow rate experienced for the segment. The hydraulic
profile to be used for calculating CT would then be generated by drawing a line through
points obtained by multiplying the TIO at the tested flow rate by the ratio of the tracer
study flow rate to each of several different flows in the desired flow range.
For the example presented in the previous section, the clearwell experiences a maximum
flow at peak hourly conditions of 6.0 mgd. The highest tested flow rate was 5.6 mgd, or
93 percent of the maximum flow. Therefore, the detention time, TIO = 4 minutes,
determined by the tracer test at a flow rate of 5.6 mgd may be used to provide a
conservative estimate of TIO for all flow rates less than or equal to the maximum flow
rate, 6.0 mgd. The line drawn through points found by multiplying TIO = 4 minutes by
the ratio of 5.6 mgd to each of several flows less than 5.6 mgd is also shown in Figure D-
4 for comparative purposes with the hydraulic profile obtained from performing four
tracer studies at different flow rates.
August 1999
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
CO"
35
30
25
20
15
10
4-Flow profile
1-Fiow profile
Average
Maximum
Extrapolation
-J 1 U
J 1 L
4
FLOW (MGD)
Figure D-4. Detention Time vs. Flow
8
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
D.2 Determination of T10 without Conducting a Tracer
Study
In some situations, conducting tracer studies for determining the disinfectant contact
time, TIO, may be impractical or prohibitively expensive. The limitations may include a
lack of funds, manpower-or equipment necessary to conduct the study. For these cases,
the Primacy Agency may allow the use of "rule of thumb" fractions representing the ratio
of TIO to T, and the theoretical detention time, to determine the detention time, TIO, to be
used for calculating CT values. This method for finding TIO involves multiplying the
theoretical detention time by the rule of thumb fraction, T.io/T, that is representative of
the particular basin configuration for which Tip is desired. These fractions provide rough
estimates of the actual TIO and are recommended to be used only on a limited basis.
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 Tio/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.
D.2.1 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.
The ratio Tio/T was calculated from the data presented in the studies and compared to its
associated hydraulic flow characteristics. Both studies resulted in Tio/T values that
ranged from 0.3 to 0.7. The results of the studies indicate how basin baffling conditions
can influence the Tio/T ratio, particularly baffling at the inlet and outlet to the basin: As
the basin baffling conditions improved, higher Tio/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 Tio/T fraction
is more related to the geometry and baffling of the basin than the function of the basin.
August 1999 D-27 EPA Guidance Manual
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
For this reason, Ti
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
a baffled inlet and outlet, and intra-basin baffling to redistribute the flow throughout the
basin's cross-section.
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
basiri(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.
D.2.3 Examples of Baffling
Examples of these levels of baffling conditions for rectangular and circular basins are
explained and illustrated in the following 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, is illustrated on Figure D-5. 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 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 on Figure D-6. 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.
August 1999
D-29
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
PLAN
t
SECTION
XX
Figure D-5. Poor Baffling Conditions — Rectangular Contact Basin
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
I/I
/ !
! ' -
/ 1 A r
PLAN
/"
'
-
/
;V ^
! k
'
/ X
'
/ x x.
„ i
I
m>" j
--> 1
__^- -~"^
•^ .,, — .../...../
X
X
Id
SECTION
Figure D-6. Average Baffling Conditions — Rectangular Contact
Basin
August 1999
D-31
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
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.
Superior baffling conditions are exemplified by the flow pattern and physical
characteristics of the basin shown on Figure D-7. The inlet to the basin consists of
submerged, target-baffled 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 sharp-crested 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. These inlet and outlet structures are in some cases by
themselves sufficient to attain superior baffling conditions; however, maze-type intra-
basin baffling was included as an example of how this type of baffling aids in flow
redistribution within a contact basin.
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
is shown on Figure D-8. 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 on Figure D-9 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.
Superior baffling conditions are attained in the basin configuration shown on Figure D-10
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.
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
/
.1 /
Ld
y
\
Y
; S
s
X
i -
A
/ /
1/r
M
/
^\
/
-"\
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
. ~~~ ™~~ """* ~"~ ~^x—•/
PLAN
SECTION
Figure D-8. Poor Baffling Conditions — Circular Contact Basin
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August 1999
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
PLAN
0J
(XTz!
-S-
B&F—
/f*
/S//////S/////S/SSS s777A
SECTION
Figure D-9. Average Baffling Conditions — Circular Contact Basin
August 199.9
D-35
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
PLAN
SECTION
Figure D-10. Superior Baffling Conditions — Circular Contact Basin
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August 1999
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
D.2.4 Additional Considerations
Flocculation basins and ozone contactors represent water treatment processes with
slightly different characteristics from those presented in Figures D-5 through D-10
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 -Tio/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
Tio/T values in the range of 0.5 to 0.7. This observation indicates that not only will
compartmentation result in higher Tio/T values through be,tter flow distribution, but also
that the effects of agitation intensity on Tio/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 (Tio/T =
0.5), whereas unbaffled, single-compartment flocculation basins are characteristic of poor
baffling conditions (Tio/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 Tio/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 aTio/T of 0.3, regardless of the outlet conditions, unless mtra-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 Tio/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 TIQ. As such, the TO value can be obtained by
subtracting the volume of the filter media, support gravel, and underdrains from the total
volume and calculating the theoretical detention time by dividing this volume by the flow
through the filter. The theoretical detention time is then multiplied by a factor of 0.7,
corresponding to superior baffling conditions, to determine the TIO value.
D.2.5 Conclusions
The recommended Tio/T values and examples are presented as a guideline for use by the
Primacy Agency in determining TIO values in site specific conditions and when tracer
studies cannot be performed because of practical considerations. Selection of Tio/T
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
values in the absence of tracer studies was restricted to a qualitative assessment based on
currently available data for the relationship between basin baffling conditions and their
associated Tio/T values. Conditions which are combinations or yariations of the above
examples may exist and warrant the use of intermediate Tio/T values such as 0.4 or 0.6.
As 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 Tio/T fractions and definitions of baffling conditions may be
appropriate.
D.3 Use of Baffling Conditions and Tracer Studies to
Determine Contact Time
i j
This section provides further discussion and practical examples for using baffling factors
and tracer studies to determine the contact time.
I i
Use of Baffling Conditions to Determine Contact Time
To determine a contact time using baffling factors, data about the treatment system are
needed. These data include volumes of the unit processes, the peak hourly flow rate, and
the baffling factors of each unit process based on the baffling condition. The volume of
the unit process is the volume of water in that portion of the treatment system. This
volume does not include equipment such as filter media that take up a portion of the basin
volume. Thus, the volume of a filtration process used in determining contact time will be
the volume of filtration basin beneath the minimum water level rninus the volume
occupied by the filter media and underdrain. The peak hourly flow rate is the maximum
quantity of water passing through the process during a one-hour period within the 24-
hour duration. The peak hourly flow rate should be determined from the system
operation records.
For example, suppose a unit process within a disinfection segment is composed of a
flocculation basin with unbaffled conditions. Thus, from Table 3-2 the Tio/T value is 0.1.
In this example the volume of the basin is 969,500 gallons and the peak hourly flow rate
is 10,651 gpm. The TDT can be calculated as follows:
TDT= V/Q = 969,500 gallons / 10,651 gpm = 91.0 minutes
If the theoretical detention time for the unit process is 91.0 minutes, then the resulting
contact time is 9.1 minutes. That is,
TIO (contact time) = 91.0 minutes * 0.1 = 9.1 minutes
If the disinfection segment consists of several unit processes, then the theoretical
detention time should be calculated for each unit process. The TIO should be determined
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August 1999
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
from the TDT and baffling factor for each unit process in the segment. The segment
is the sum of the TIQS from each unit process.
The following list is a summary of the steps required to determine the contact time with baffling factors:
• Determine peak hourly flow rate, Q, based on operation records;
• Determine the volume of each unit process;
• Calculate the Theoretical Detention Time, where TDT = V/Q;
• Determine the Baffling Factor based on the unit processes baffling conditions;
• Calculate the Contact Time, where TIO = TDT * Tio/T; and
• Determine the segment TIO by summing the TIQS of the unit processes in the
segment.
Determining Contact Time Using a Tracer Study
A tracer study uses a chemical tracer to determine the detention time of water flowing
through a unit process, segment, or system as stated earlier in Chapter 3. Typical
chemical tracers include chloride ions, fluoride ions, and Rhodamine WT. Ideally, the
selected tracer chemical should be readily available, conservative, easily monitored, and
acceptable for use in potable water supplies. By conservative it is meant that the tracer is
not consumed or removed during treatment. Fluoride ions can generally be used in lower
concentrations than chloride because they are typically present in lower concentrations in
the water. Rhodamine is a fluorescent tracer that if selected must be used following
guidelines presented earlier in this appendix. Selection of a particular chemical tracer
may depend on the unit processes and the salt concentrations present in the water. If a
tracer study is needed in order to find TIO, a water system should consult the latest tracer
study guidance from the state.
The tracer chemical should be added at the same points in the treatment train as the
disinfectant to be used in the CT calculations, since it will be used to determine TIO for
the disinfection segment. Two common methods of tracer addition are the step-dose
method and the slug-dose method. In the step-dose method, the tracer chemical is
injected at a constant dosage and the endpoint concentration is monitored. To determine
a 90 percent recovery for the tracer, endpoint sampling should continue until the tracer
concentration reaches a steady-state level. With the slug-dose method, a large dose of
tracer chemical is injected, instantaneously. An effective way to achieve instantaneous
addition is to use a gravity-fed tube to release the single dose. The tracer concentration is
monitored at the endpoint, until the entire dose has passed through the system. Unlike
the step-dose method, a mass balance is required to determine whether the entire tracer
dose was recovered. Additional mathematical manipulation is required to determine TIO
from the concentration versus time profile.
Data from tracer studies should be summarized in tables of time and residual
concentration. These data are then analyzed to determine the detention time, TIO, to be
used in calculating CT. Tracer test data from either the step or slug-dose method can be
evaluated graphically, numerically, or by a combination of these techniques. The
graphical method of evaluating step-dose test data involves plotting a graph of
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
dimensionless concentration (C/Co) versus time and reading the value for TIO directly
from the graph at the appropriate dimensionless concentration. C0 is the dosage,
concentration injected into the system and C is the tracer concentration at any time during
the test. Alternatively, the data from step-dose tracer studies may be evaluated
numerically by developing a semi-logarithmic plot of the dimensionless data (see Section
D.I). 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 TIO value, assuming there is a good
statistical fit. That is, the data points are not too scattered and the line drawn is a
reasonable approximation of the data points. The slug-dose method, however, requires
data to be analyzed by converting it to the mathematically equivalent step-dose data and
using techniques discussed above for step-dose data evaluation. This procedure is more
complicated and the details to evaluate the slug-dose data are found in Section D.I.7.2.
i !
Several other considerations when conducting a tracer study are the temperature, flow
rates, and water levels in the basins. Detention time may be influenced by differences in
water temperature within the system. For plants with potential for thermal stratification,
additional tracer studies are suggested under the various seasonal conditions that are
likely to occur. The contact times determined by the tracer studies under the various
seasonal conditions should remain valid as long as no physical changes are made to the
mixing basin(s) or storage reservoir(s).
i i
Detention time is proportional to flow. However, it is not always a linear relationship.
Therefore, it is best to conduct tracer studies over a range of flow rates typical of the
disinfectant segment. Flow rates may vary throughout the treatment system as the water
travels through the unit processes. The goal of the tracer tests is to determine an accurate
portrayal of the contact time within each unit process. Thus, it is important to select the
flows carefully. Ideally, tracer tests should be performed for at least four flow rates that
span the entire range of flow for the section being tested. The flow rates should be
separated by approximately equal intervals to span the range of operation. The four flow
rates should be one near the average flow, two greater than average, and one less than
average flow. The flows should also be selected so that the highest test flow rate is at
least 91 percent of the highest flow rate expected to ever occur in that section.
It may not be practical for all systems to conduct studies at four flow rates. The number
of tracer tests that are practical to conduct is dependent on site-specific restrictions and
resources available to the system. Systems with limited resources can conduct a
minimum of one tracer test for each disinfectant segment at a flow rate of not less than 91
percent of the highest flow rate experienced at that section. If only one tracer test is
performed, the detention time determined by the test may be used to provide a
conservative estimate in CT calculations for that section for flow rates less than or equal
to the tracer test flow rate. See Section D.I. 1 for calculating a TIO at a different flow rate
than the tracer test flow rate.
Tracer studies should be conducted during periods when the water level is maintained in
accordance with normal plant operation. For basins that have constant water level, the
recommended procedure is to maintain the basin's water level at: or slightly below, but
not above, the normal level. For basins that are operated at extreme water levels,
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
particularly clearwells, disinfectant contact time should not be used to compute the total
CT value because reliable detention time is not provided for disinfection. The
recommended water levels during the tracer study for several unit processes are
summarized in Table D-6.
Table D-6. Recommended Water Levels during a Tracer Study
Unit Process
Sedimentation Basins - Operating at a Near
Constant Level
Clean/veil and Storage Tanks
Clearwells Operated with Extreme Variation in
Water Level
Storage Reservoirs - Experiencing Seasonal
Variations
Recommended Water Levels
Water levels at or slightly below, but not above, the normal minimum
operating level.
Conduct study during a period when tank level is falling.
Does not provide a reliable detention time. However, the system may install
a weir to ensure a minimum water level and provide a reliable detention
time.
Perform studies during various seasonal conditions by using representative
water levels for each seasonal condition.
As stated earlier in Chapter 3, the tracer must be added at the same locations in the plant
where the disinfectant is added. The duration of tracer addition should be sufficient to
approach steady-state conditions which is usually two to three times the theoretical
detention time. Tracer dosage should be in sufficient concentration to easily monitor the
concentration in the effluent. If there is low background tracer concentration, the dosage
can be fairly low (i.e., in the range of 1 to 2 mg/L for fluoride ions). However, for basins
with serious short-circuiting, substantially larger dosages are necessary to detect the
tracer and to define the effluent tracer profile adequately. The test procedure for
determining the Contact Time with a tracer study is generally as follows:
• The system determines the flow rate or rates to be used in the study.
• The system selects the tracer chemical and determine the raw water background concentration
of the tracer chemical. The background level is needed to both determine the quantity of
chemical to feed and to evaluate the data properly.
• The system determines the tracer addition locations, plan the sample
collection logistics and frequency, and determine the appropriate tracer
dosage. Sampling frequencies depend on the size of the basin—the larger the
basin the easier it is to obtain an adequate profile with less frequent sampling.
Small basins need more frequent sampling.
» The system conducts the tracer test using either the step-dose or slug-dose
methods.
• The system compiles and analyzes the data.
• The system calculates TIO.
Additional references for information on tracer studies and details concerning how to
conduct one are listed below:
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APPENDIX D. DETERMINATION OF DISINFECTANT CONTACT TIME
Hudson, H.E., Jr. 1975. "Residence Times in Pretreatment." J. AWWA.
January:45-52. ,
Hudson, H.E., Jr. 1981. Water Clarification Processes: Practical Design and
Evaluation. Van Nostrand Reinhold Company, New York.-
i
_ i
J-^venspiel, O. 1972. Chemical Reaction Engineering, second edition. John
Wiley and Sons, New York.
Marske, D.M. and J.D. Boyle. 1973. "Chlorine Contact Chamber Design - A
Field Evaluation." Water and Sewage Works. January:70-77.
i
Missouri Department of Natural Resources, Public Drinking Water Program.
1991. Guidance Manual for Surface Water System Treatment Requirements.
:
Teefy S.M. and P.C. Singer. 1990. "Performance and Analysis of Tracer Tests
to Determine Compliance of a Disinfection Scheme with the SWTR." J.
AWWA. 82(12):88-98.
Thirumurthi, D. 1969. "A Breakthrough in the Tracer Studies of
Sedimentation Tanks." /. WPCF. R405-R418. November.
TNRCC. 1995. Public Water Supply Technical Guidance Manual, Chapt. 27,
Texas Natural Resources Conservation Commission, Austin, TX.
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APPENDIX E. USING THE
REGRESSION METHOD
E.1 Using the Regression Method to Find CT3
log, Giardia When Using Chlorine
Plants may choose to use the Regression Method to determine the value
when using free chlorine. This method is useful to calculate the CTs-iog, Giardia for a long
historical data set of pH, temperature and residual disinfection concentrations. Unlike the
Approximation Method, the operator is not required to manually look up values in a table
for each day of the historical record. (Recall that systems that are required to create a
disinfection profile must do so fcr one to three years of daily data.) Instead of having to
look up CT values for each day in the record, the Regression Method allows the operator
to simply use a formula that is a function of pH, temperature and residual disinfection
concentration. Using this formula in a spreadsheet should greatly reduce the time
required to calculate the disinfection profile. The following section presents the
equations and demonstrates its utility in calculating CT3-iogj) Gia
An empirical model was developed by Smith et al. (1995), that directly predicts CT
values that are equal to or greater than the original CT values in the SWTR over the
entire range of variables covered in the SWTR Guidance Manual. The equations below
can be used to directly compute CT values for chlorine inactivation:
CT = (0.353*J0(12.006+e(2-46-a073*temp+ai25*C+0-389*pH)) Equation 3-3
(for temperature < 12.5 °C)
CT = (0.361*/)(-2.261+e(2-69-0-065*temp+aill*C+a361*pH)) Equation 3-4
(for temperature > 12.5 °C)
Where:
I = 3, the number of logs inactivation required
Temp= temperature in degrees Celsius
C = residual chlorine concentration in mg/L
pH = the negative log concentration of hydrogen ion
e= 2.7183, the base for the natural logarithm
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APPENDIX E. USING THE REGRESSION METHOD
The SWTR did not include log inactivation credit for waters with pH greater than 9.0. As
such, if the plant operates at a pH level higher than 9.0, the Approximation Method
described above should be used to calculate the CT3_i0g, oiardia- Systems should apply State
requirements, however, in the absence of state regulations, the utility should default to
using CT values calculated for a pH less than 9.0.
Procedure:
• Determine whether the temperature is above or below 12.5 °C to select
between Equations 3-3 and 3-4 to directly compute the CT values for Giardia
inactivation. using chlorine (If using a spreadsheet an "IF' statement can be
used to select the correct equation based on the temperature.)
• Use daily temperature (°C,) residual disinfectant concentration (mg/L), pH,
and I =* 3 in the appropriate equation to calculate the
Find the value of CT3_iog> Giardia for a water temperature of 11°C, a pH of 8.2, and a
residual of 2.5 mg/L for a plant that is using free chlorine as the disinfectant.
I I
Using Equation 3-3 since temperature is less than 12.5 °C, then:
CT = (0.3537)(12.006+e(2-46-a073temp+0-125C+0^89pH))
CT = (1.059)(12.006+e(2-46-0-073*11+ai25*2-5+0^89*8-2))
CT = (1.059)(12.006+e(2-46-803+3125+3-189))
i
CT = (1.059)(12.006+e(5'1585))
CT = (1.059)(12.006+173.90)
! !
CT= 196.87
•
The CTs-iog, Giarfia of 197 as calculated by the Regression Method more closely
approximates the actual CTs.jog, Giardia than the values calculated using the Approximation
Method that estimates the CT3_iogi Giardia at 234 (see Section 3.5).
E.2 Calculation of Estimated Log Inactivation
Using the Regression Method
Required CT values for 3-log inactivation of Giardia using chlorine can be determined
using CT tables as provided hi Appendix C, or can be calculated using disinfectant-
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APPENDIX E. USING THE REGRESSION METHOD
specific equations, such as the chlorine equations developed by Smith et al (1995). These
equations predict required CT values for 3 -log inactivation that are greater than or equal
to the original values in the SWTR over the entire range of independent variables covered
in the Guidance Manual for Compliance with the Filtration and Disinfection
Requirements for Public Water Systems Using Surface Water Sources (AWWA, 1991).
Using these equations, CT values for inactivation of Giardia using chlorine can be
computed.
For Temperature < 12.5 °C:
CT = (0.353 7)(12.006+e(2-46-a°73
c+ °389 PH)
For Temperature > 12.5 °C:
CT = (0.361 7)(-2.261+e(2-69-a065 temP+a111 c+ °'361
Where:
7=3, log removal of Giardia
e = 2.7183, the base of the natural logarithm
C = chlorine residual concentration (mg/L)
Temp = temperature in °C
Once the CT required for inactivation of 3-log Giardia and 4-log viruses is determined,
the actual log inactivation for that segment can be estimated as:
Estimated Segment Log Inactivation of Giardia = 3.0 * CTactuai /
Estimated Segment Log Inactivation of viruses = 4.0 * CTactuai / CT4.bg, virus
The total plant estimated log inactivation due to chemical disinfection is:
Total Plant Estimated Inactivation = E segment inactivation
due to chemical disinfection
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