Draft Guidance on Preparing a Utility Analysis
July 2009
&EPA
United States
Environmental Protection
Agency
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Draft Guidance on Preparing a Utility Analysis
Acronyms and Abbreviations
BOD biochemical oxygen demand
CEPT chemically enhanced primary treatment
CFR Code of Federal Regulations
CMOM capacity, management, operations and maintenance
CSO combined sewer overflows
EPA U.S. Environmental Protection Agency
l/l infiltration and inflow
IFAS integrated fixed-film activated sludge
LTCP long-term control plan
MBBR moving bed biofilm reactors
MBR membrane bioreactor
MLSS mixed liquor suspended solids
NPDES National Pollutant Discharge Elimination System
POTW publicly owned treatment works
RAS return activated sludge
SRF State Revolving Fund
SSES Sanitary Sewer Evaluation Survey
SSO sanitary sewer overflow
TSS total suspended solids
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Contents
Acknowledgements Error! Bookmark not defined.
Acronyms and Abbreviations i
I. Introduction 1
a. Background 1
b. Draft Peak Flows Policy 2
c. CSO-Related Bypasses 6
II. Description of the Existing Treatment Plant and Collection System 7
a. Summary Narrative 8
b. Process Flow Diagrams 8
c. Approved Design Capacities of Treatment Units 8
d. Determination of Actual Capacities of Treatment Units 8
e. Overview of Collection System
III. Flow Characterization 7
a. Treatment Plant Flows 10
b. Description of Diversions 8
d. Collection System Flows 8
e. Description of Wet Weather SSOs 8
f. Summarize Performance of Past l/l Efforts 11
IV. Projected Peak Wet-Weather Flows 10
V. Identifying and Evaluating Potential Measures to Reduce Diversions 14
a. Operational Measures during Wet-weather flows 14
b. Structural Modifications at the Treatment Plant 16
c. Collection System Improvements to Reduce Flows during Wet-Weather Events 18
d. Additional Alternatives 20
e. Treatment of Diverted Flows 20
f. Emergency Back-Up Equipment 22
VI. Ability to Pay / Financial Capability Assessment 23
a. Review of Current Potable and Wastewater Costs 23
b. Financial Capability Assessment 23
VII. Summary of Public Participation 24
VIII. Selection of Recommended Measures for Implementation 24
a. Financial Plan Error! Bookmark not defined.
b. Proposed Implementation Schedule Error! Bookmark not defined.
c. Estimate of Projected Frequency, Duration and Volume of Diversions 26
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d. Demonstration of No Additional Feasible Alternatives Beyond the Recommended
Measures 25
IX. Proposed Monitoring Protocol 26
X. Proposed Plan for Public Notice of Diversion 26
References 27
Appendix A: Peak Wet-Weather Flows Utility Analysis Completeness Checklist 1
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I. Introduction
The purpose of this document is to provide guidance to permittees that are identifying and evaluating
feasible alternatives to peak wet-weather diversions at a treatment plant. This document considers the
development of a comprehensive Utility Analysis of feasible alternatives to the diversions that may be
submitted to the NPDES authority.
While EPA does not intend that all Utility Analysis be formatted in a certain way or contain the same
information or analysis, the Agency believes that the common components of a comprehensive Utility
Analysis can be identified. This document is organized to reflect the following components of a Utility
Analysis:
• Description of existing treatment plant and collection system.
• Characterization of existing flows
• Projected future flows
• identifying and Evaluating Potential Measures to Reduce Diversions
• Ability to Pay / Financial Capability Assessment
• Summary of Public Participation
• Selection of Recommended Measures for Implementation
• Proposed Monitoring Protocol
• Proposed Plan for Public Notice of Diversion
Appendix A is a completeness checklist that is intended to complement the guidance and support permit
writers in evaluating the completeness and comprehensiveness of a Utility Analysis. The goal of the
checklist is to assist permit writers in efforts to assess and document whether the permit and
administrative record provides a complete, comprehensive and transparent record of permit
development.
a. Background
Many municipal sewage treatment plant experience high peak influent flows during significant wet-
weather events that exceed the treatment capacity of existing secondary treatment units. In such
situations, wet-weather flows are sometimes diverted around secondary treatment units. The diverted
flows are then either discharged directly to receiving waters or recombined with the flows from the
secondary treatment units before discharge. The U.S. Environmental Protection Agency (EPA) interprets
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existing regulations, specifically, the bypass regulation at Title 40 of the Code of Federal Regulations
(CFR) section 122.41(m), to apply in both circumstances.
EPA's National Pollutant Discharge Elimination System (NPDES) regulations prohibit bypass—defined as
the intentional diversion of waste streams from any portion of the treatment facility—except in very
limited circumstances. Section 122.41(m)(4)(i) prohibits bypass, and EPA or the NPDES authority may
take enforcement action against a permittee for bypass, unless:
(A) The bypass was unavoidable to prevent loss of life, personal injury, or severe property damage;
(B) There were no feasible alternatives to the bypass, such as the use of auxiliary treatment
facilities, retention of untreated wastes, or maintenance during normal periods of equipment
downtime; and
(C) The permittee submitted required notices.
Under section 122.41(m)(4)(ii), the Director may approve an anticipated bypass, after considering its
adverse effects, if the Director determines that the bypass will meet the criteria listed in subsection
(m)(4)(i). An approved anticipated bypass is a recognition that the permitting authority has considered
the adverse impacts of the bypass and has determined that the bypass would or does meet the criteria
of 40 CFR 122.41(m)(4)(i)(A), (B) and (C) and will not take enforcement action against a permittee for the
bypass. Compliance with 40 CFR 122.41(m)(4)(i), in and of itself, would not shield a permittee from
citizen suits for conducting a prohibited bypass. Southern Ohio Coal Company v. Office of Surface
Mining, Reclamation and Enforcement, 20 F.3d 1418, 1427 (6th Cir. 1994).
b. Draft Peak Flows Policy
On December 22, 2005, EPA requested public comments on a draft policy (hereafter called the draft
peak flows policy) regarding peak wet-weather discharges from publicly owned treatment works
(POTW) treatment plants serving separate sanitary sewer collection systems. The 2005 draft peak flows
policy would clarify that the bypass provision applies to wet-weather diversions around secondary
treatment units at POTW treatment plants serving separate sanitary sewers that are recombined prior
to discharge. It also includes an interpretation that under limited circumstances in which anticipated
bypasses meet the requirements of 40 CFT 122.41(m), including implementation of feasible alternatives,
bypasses could be approved in a permit under 40 CFR 122.41(m)(4)(ii).
The December 22, 2005, draft policy would apply only to peak wet-weather diversions around
secondary treatment units that occur at POTW treatment plants that serve separate sanitary sewer
systems. (EPA previously explained the processes by which wet-weather diversions can be approved in
NPDES permits for POTW treatment plants serving combined sewer systems in the 1994 Combined
Sewer Overflow Policy, (CSO Control Policy) 59 Federal Register (FR) 18,693-18,694 (April 19, 1994).) The
2005 draft peak flows policy describes the circumstances under which anticipated bypasses that reflect
the full implementation of feasible alternatives would be approved for the purposes of section
122.41(m)(4)(ii) in the limited context of this policy.
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The draft peak flows policy does not change EPA's interpretation of the bypass regulation as applied in
United States v. City of Toledo, Ohio, 63 F.Supp.2d 834 (N.D. Ohio 1999). Thus, POTW treatment plants
that fail to move forward to meet their obligations under this policy remain subject to the full scope of
enforcement remedies for any violations. Furthermore, nothing in the draft policy provides a basis to
reopen existing enforcement remedies (e.g., orders, decrees, or agreements) that address measures to
reduce or eliminate peak flow wet-weather diversions.
EPA strongly discourages reliance on peak wet-weather flow diversions around secondary treatment
units as a long-term, wet-weather management approach at a POTW treatment plant serving separate
sanitary sewer conveyance systems. Such diversions should be minimized to the extent feasible. EPA
anticipates that, over time, the need to undertake peak wet-weather flow diversions at POTW
treatment plants serving separate sanitary sewer conveyance systems can be eliminated from most
systems in a variety of ways, such as by enhancing and/or expanding storage and treatment capacity
and reducing sources of peak wet-weather flow volume. EPA expects that aggressive efforts by POTW
treatment plants in consultation with NPDES authorities can lead to dramatic reductions in the volume
and duration of peak wet-weather flows; in most cases completely phase out diversions; and improve
the treatment and quality of peak wet-weather flow discharges. EPA also believes that involving the
general public will improve the assessment of various options to minimize peak wet-weather flow
diversions.
The 2005 draft peak flows policy is limited in scope; it applies only (1) to peak flow wet-weather
diversions, (2) from POTW plants, (3) that serve separate sewer collection systems, and (4) that
recombine the diverted peak flows prior to discharge. The draft policy describes the circumstances
under which anticipated bypasses at these plants could be either approved or denied as a result of the
NPDES permitting process. The draft policy identifies the content of an appropriate Utility Analysis that
POTWs should submit with their permit applications to facilitate development of appropriate permit
conditions. The draft policy outlines the decision process involved in reviewing a Utility Analysis and
determining whether approval of peak wet-weather flow diversions is appropriate.
The 2005 draft peak flows policy explains how the NPDES authority should determine whether
anticipated peak wet-weather flow diversions, at POTW treatment plants serving separate sanitary
sewer collection systems, which are recombined with flow from the secondary treatment units prior to
discharge, should be approved or denied under 40 CFR 122.41(m)(4)(ii). Under the draft policy, if the
NPDES authority determines, on the basis of a Utility Analysis and any other available information, that
the criteria of section 122.41(m)(4)(i) will be met and, if the permit includes any more stringent limits
necessary to meet water quality standards (including when an anticipated bypasses occurs (i.e., to take
into account its adverse effects)), the NPDES authority may provide for approval of anticipated bypass of
peak wet-weather flow diversions around secondary treatment units.
The 2005 draft peak flows policy:
• Explains how EPA intends to apply the bypass regulation, specifically, 40 CFR 122.41(m)(4), to
peak wet-weather flow diversions around secondary treatment units at POTW treatment plants
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serving separate sanitary sewer systems where the diverted flow is recombined with flow from
the secondary treatment units prior to discharge;
• Describes a Utility Analysis and relevant steps of the permit review process;
• Identifies a framework within the permit process through which the criteria of the bypass rule
can be evaluated for determining whether anticipated peak wet-weather flow diversions to
which this policy applies could be approved as anticipated bypasses;
• Promotes use of measures to provide the highest possible treatment to the greatest possible
peak wet-weather flow; and
• Promotes reporting and public notification of peak wet-weather diversion events.
The draft policy is limited in scope. The draft policy:
• Does not apply to discharges or overflows prior to the headworks of a POTW treatment plant; or
to dry-weather diversions; or to diversions around primary or diversions that are not
recombined with flow from the secondary treatment units prior to discharge; or to treatment
plants (municipal or industrial) other than those serving separate sanitary sewer collection
systems; and
• Does not address diversions around tertiary treatment units that are separate from secondary
treatment units.
A combination of approaches can be used to achieve the goals of the 2005 draft peak flows policy. These
approaches include the following:
• Ensuring full utilization of available secondary treatment capacity;
• Reducing infiltration and inflow (l/l);
• Maximizing the use of the collection system for storage;
• Providing off-line storage; and
• Enhancing and/or expanding secondary treatment capacity.
In cases where these approaches, alone or in combination, are not sufficient to enable a POTW
treatment plant to process its peak wet-weather flows through its secondary treatment units, a POTW
treatment plant might have no feasible alternative to peak wet-weather flow diversions around
secondary treatment units. However, EPA believes the use of diversions around secondary treatment
units at POTW treatment plants serving separate sanitary sewer systems to manage peak wet-weather
flows is not necessary in many cases and cannot be approved for diversions where feasible alternatives
are identified through the Utility Analysis or if the other elements of the bypass regulation are not met.
If feasible alternatives to avoid all anticipated bypasses are available during the permit term, such that
there will be no need for diversions by or before the end of the permit term, approval in the permit
under section 122.41(m)(4)(ii) would not be appropriate; rather, the permit would address only the
requirements to implement the alternatives, including a schedule with specific dates for implementing,
as quickly as feasible, the technologies, upgrades and approaches identified and estimates of the
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associated flow volumes. This implementation schedule would be considered a permit condition as
opposed to a schedule of compliance under 40 CFR 122.47.
The 2005 draft peak flows policy provides that, at the time of application for an NPDES permit for POTW
treatment plants seeking approval of peak wet-weather diversions at a treatment plant as an
anticipated bypass, the plant should submit a comprehensive analysis (Utility Analysis) to the NPDES
authority that does the following:
a. Documents current treatment plant design capacity for all treatment units, the maximum flow
that can be processed through those units, and the feasibility of increasing such treatment
capacity and related costs;
b. Estimates the frequency, duration, and volume of current wet-weather diversions, and
evaluates alternatives to reduce the frequency, duration, and volume of such occurrences and
related costs;
c. Estimates future peak wet-weather flows on the basis of information such as predicted climatic
conditions, anticipated dry-weather flows, projected treatment plant and collection system
changes (e.g., upgrades, extensions, deterioration), and evaluates options for reducing
diversions on the basis of these variables;
d. Assesses existing storage within the collection system or on-site and options for enhanced
utilization or expansion (taking into account physical and technological considerations) of
storage to reduce the frequency, duration, and volume of peak wet-weather diversions, and
the related costs;
e. Assesses other ways to reduce peak wet-weather flow volumes, such as limiting collection
system extensions and slug loadings from indirect dischargers, or water conservation or green
infrastructure techniques;
f. Evaluates technologies, such as supplemental biological treatment, physical/chemical
treatment, (e.g., ballasted flocculation, deep-bed filtration, or membrane technology) that are
or could be used to provide additional treatment to peak wet-weather flows or peak wet-
weather diversions at the POTW treatment plant and the costs of implementing those
technologies;
g. Evaluates the extent to which the permittee is maximizing its ability to reduce l/l throughout
the entire collection system (i.e., not only the portions operated by the utility, but also portions
operated by any municipal satellite community), including the use of existing legal authorities,
potential improvements in the timing or quality of such efforts, and options for obtaining or
expanding legal authorities to reduce l/l from satellite collection systems;
h. Evaluates peak flow reductions obtainable through implementation of existing capacity,
management, operations, and maintenance (CMOM) programs and potential improvements in
the timing or enhancement of those programs and the related costs; or, if no such program
exists, reductions obtainable through the development and implementation of a CMOM
program and the related costs;
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i. Assesses the community's ability to fund the peak wet-weather flow improvements discussed
in the Utility Analysis, taking into consideration current sewer rates; planned rate increases;
other potential sources of federal, state, or local funds; and the costs, schedules, anticipated
financial impacts to the community of other planned water and wastewater expenditures and
other relevant factors affecting the utility's rate base, using as a guide EPA's Combined Sewer
Overflows—Guidance for Financial Capability Assessment and Schedule Development (USEPA
1997);
j. Proposes a protocol for monitoring the total volume diverted, and the duration of each peak
wet-weather diversion event, and a protocol monitoring of the recombined flow at least once
daily during diversions for all parameters for which the POTW treatment plant has daily
effluent limitations or other requirements (e.g., monitoring only requirements) and
representative monitoring for other monitoring requirements of the permit; and
k. Projects the POTW treatment plant effluent improvements and other improvements in the
collection system and treatment plant performance that could be expected if the technologies,
practices or other measures discussed in the Utility Analysis are implemented.
c. CSO-Related Bypasses
EPA has provided guidance on the planning, selection, and implementation of controls to meet
technology- and water quality-based requirements for CSOs under the NPDES program in the National
CSO Control Strategy, 54 FR 37370 (September 8, 1989), and the CSO Control Policy, 59 FR 18688 (April
19, 1994). The 1994 CSO Control Policy provides comprehensive guidance for developing site-specific
NPDES permit requirements for combined sewer systems to address wet-weather CSO discharges from
designed overflow points. The Wet Weather Water Quality Act of 2000 amended the Clean Water Act
(CWA) to provide that each permit, order, or decree issued after December 15, 2000, for a discharge
from a municipal combined sewer must conform to the CSO Control Policy. 33 United States Code
section 1342(q)(l).
Under the CSO Control Policy, permittees with combined sewer systems were to immediately undertake
a process to accurately characterize their sewer systems, to demonstrate implementation of nine
minimum controls identified in the policy, and to develop and implement a long-term CSO control plan
that would ultimately provide for compliance with the requirements of the CWA. See 59 FR 18688 (April
19, 1994). The CSO Control Policy identifies EPA's major objectives for long-term control plans (LTCP).
When developing the CSO Control Policy, EPA recognized that some POTW treatment plants might have
primary treatment capacity in excess of their biological treatment capacity. See 59 FR 18693, col. 2. The
policy indicates that one effective strategy to abate pollution resulting from CSOs is to maximize the
delivery of flows during wet weather to the POTW treatment plant for treatment. This strategy can
maximize the use of available POTW facilities for wet-weather flows and ensure that combined sewer
flows receive at least primary treatment prior to discharge. In addition, this strategy might enable the
permittee to eliminate or minimize overflows to sensitive areas. In recognition of the significant water
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quality benefits of maximizing flow to the POTW treatment plant, the CSO Control Policy includes it as a
minimum element of an LTCP.
To further the objective of maximizing treatment at the POTW treatment plant, the CSO Control Policy
provides guidance on the use of an NPDES permit to recognize approval of anticipated bypasses where
the criteria of the bypass provision for such approvals are met. The CSO Control Policy clarifies that it is
the responsibility of the permittee to document, on a case-by-case basis, compliance with 40 CFR
122.41(m) to have an anticipated bypass approved in a permit. The policy indicates that for some CSO-
related permits, the study of feasible alternatives in the LTCP, along with other information in the
permit record, may provide sufficient support for approval of a CSO-related bypass in the permit and to
define the specific parameters under which a bypass can be approved. The policy provides that where a
permit includes an approval of a CSO-related bypass, the permit would define the specific wet-weather
conditions under which a CSO-related bypass would be allowed and would also specify what treatment,
monitoring, and effluent limitations would apply to the bypass flow.
The policy provides that permits with approved bypasses should also make it clear that all wet-weather
flows passing the headworks of the POTW treatment plant will receive at least primary clarification,
solids and floatables removal and disposal, and disinfection where necessary, and any other treatment
that can reasonably be provided.
The CSO Control Policy anticipates that POTW operators will document in their LTCP, or other
documentation, the evaluation of the analysis of feasible alternatives to the diversions. However, where
the analysis of feasible alternatives in the LTCP is incomplete or a facility needs to update its feasibility
analysis before permit reissuance, this guidance document, read in conjunction with the CSO Control
Policy, might provide some technical assistance.
II. Description of the Existing Treatment Plant and Collection
System
A Utility Analysis and permit application should provide a description of the existing treatment plant and
collection system. The description of the treatment plant should include:
• A brief summary narrative of the facility;
• Process flow diagrams;
• A description of design capacities for key unit operations; and
• A summary of efforts to characterize the actual capacity of unit operations where
diversions have occurred.
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a. Summary Narrative
The summary narrative of the facility should provide a brief overview of the existing POTW treatment
plant. It should describe the facility location, size, property boundaries, level and type of treatment
provided and a brief history of major construction at the plant. This information could include the initial
construction date and major expansions or upgrades.
b. Process Flow Diagrams
Schematics showing the process flows during dry- and wet-weather conditions should be included. A
description of the possible flow paths and an indication of when the various flow paths are used should
accompany the process flow diagram(s). At a minimum, a description of the normal dry-weather and
wet-weather flow paths should be provided.
c. Approved Design Capacities of Treatment Units
Section l.a of the December 22, 2005, draft peak flows policy, provides in part that the Utility Analysis
should document current treatment plant design capacity for all treatment units. A summary of the
design capacities for all of the treatment units at the treatment plant should be included. These
processes could include the following:
Preliminary treatment
Equalization
Primary treatment
Secondary treatment
Tertiary Treatment
Disinfection
This information should include the number of units, tank volumes, basis of tank sizing, and rated
capacity. The firm capacity (with the largest unit out of service) and total installed capacity for each
process should be identified. The hydraulic capacity of each process should also be discussed, which will
require a review of the channels, piping, and pumping systems connecting the treatment processes.
The design capacities can be included on the process flow diagrams to show the maximum flows
through each flow path.
d. Determination of Actual Capacities of Treatment Units
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Section l.a of the December 22, 2005, draft peak flows policy, provides in part that a Utility Analysis
should document the maximum flow that can be processed through various unit processes. Treatment
units where diversions associated with limited capacity should be identified. The actual capacity of such
treatment units with limited capacity should be evaluated. The basis of design is often conservative in
terms of process capacities, so the plant might have additional treatment capacity available beyond that
indicated in the basis of design. Conversely, changes in design standards used by regulatory authorities,
operation of the process, or the length of service for a given process could decrease the capacity below
that stated in the basis of design.
An evaluation of the actual capacity of the treatment units can be performed by reviewing operational
data from peak wet-weather events and comparing this information to that contained in the basis of
design, especially during wet-weather conditions, which are drastically different from dry-weather flow
in terms of both hydraulic and organic loadings. In addition, stress testing can be conducted to identify
the peak flows that can be treated under dry or wet-weather conditions for selected treatment
processes. A review of the wet-weather capacities for each process can identify the limiting process. The
total installed capacity, rather than the firm capacity, should be used in this analysis as long as the utility
can provide justification. For example, the equalization basins have no moving parts and the entire
volume should be used for the installed capacity. Another example would be the use of the secondary
process to treat the diluted wastewater, as long as the flow can be transported through the process and
the oxygen supply capability meets the demand. The utility can then evaluate alternatives to increase
the capacity of the bottleneck process through additional modeling and demonstration. The evaluation
can help determine the feasibility of avoiding diversions and the plant improvements that would be
necessary for this goal to be achieved.
Because the information gained from stress testing is likely to be used to attempt to re-rate the
capacities of some or all of the process units, EPA suggests that a plan be developed and shared with the
NPDES permitting authority and other applicable state or federal authorities that would be involved in
the approval of capacity changes. The results of the stress testing can be used to modify the maximum
flows through a given unit operation.
e. Overview of the Collection System
The description of flows in the collection system should be prefaced with a brief overview of the
collection system that includes:
• A general description of the size of the collection system and population served;
• Identification of major sewersheds within the collection system;
• Identification of all municipal satellite collection systems in the collection system;
• The number and location of pump stations.
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Appropriate maps should be used in this description.
III. Flow Characterization
Characterization of dry-weather and peak wet-weather flows reaching the treatment plant and at key
locations in the collection system is a major goal of the Utility Analysis. The applicant should provide
detailed information to characterize the following:
• Dry-weather flows at the treatment plant
• Peak wet-weather flows reaching the treatment plant
• Bypasses at the treatment plant
• Overflows in the collection system
This portion of the analysis should describe:
• existing flows at the treatment facility;
• the history of diversions at the treatment plant;
• existing wet weather flows at critical locations in the collection system; and
• the history of wet weather sanitary sewer overflows in the collection system.
a. Treatment Plant Flows
One important objective of the Utility Analysis is to characterize wet- and dry-weather flows at the
treatment plant. This is reflected in Section l.b of the December 22, 2005, draft peak flows policy, which
in part provides that the Utility Analysis should provide estimates of the frequency, duration, and
volume of current wet-weather diversions. The NPDES permit application regulations require that an
applicant provide the design flow rate of the treatment plant (i.e., the wastewater flow rate that the
plant was built to handle), along with the average daily flow rate and maximum daily flow rate for each
of the past 3 years. The Utility Analysis should provide additional information on flow rates, particularly
those related to peak flow conditions.
The location and method used to monitor flow should be briefly described. The Utility Analysis should
describe the various measures and terms used as different definitions for flows are sometimes used
industry-wide.
If significant changes in flow rates have occurred in recent years, an explanation for the change should
be included. For example, flow rate changes associated with l/l elimination projects, expansion of
service areas, and capacity increases should be identified.
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b. Description of Diversions
Information should be provided for any process bypass that has occurred at the plant including the
following:
• Date of bypass event
• Process(es) bypassed
• Amount of precipitation and snowmelt, if applicable, associated with wet-weather diversion
• Volume of flow diversion
• Duration of the diversion
• Cause of bypass (e.g., wet weather, mechanical failure, power loss)
A discussion of the secondary treatment processes and the precipitation events that have
resulted in flows greater than the available capacity should be included. The seasons when
diversions are most likely to occur should also be discussed. The average frequency, duration,
and volume of the diversions should be included. In addition, the range of values observed at
the plant for these parameters should also be presented.
c. Collection System Flows
The Utility Analysis should identify sewersheds with high levels of l/l. For sewersheds that have been
identified as having high levels of l/l, the Utility Analysis should provide a brief description of studies that
have been conducted to identify and quantify sources of l/l within the sewershed and identify and
quantify wet weather SSOs. The summary should include a description of when the studies were
conducted, provide an overview of methods used and summarize major results of the studies and the
corrective actions that were taken in response to the findings.
The peak capacity of major interceptors and main trunk lines feeding into the treatment plant should be
identified, at a minimum. The capacities for each pump station should also be included, and any
deficiencies noted.
Sewerage authorities can accept flow from tributary customers and might not have comprehensive
information regarding the tributary customer system behavior and overflows. In all cases, the data
should be characterized to clarify whether the entire system to the POTW is being characterized, or if
the characterization is limited to that portion of the system that is under the jurisdiction of the POTW
authority.
e. Description of Wet Weather Sanitary Sewer Overflows
The following information should be tabulated for each wet-weather sanitary sewer overflows (SSOs)
that have occurred in the collection system in at least the last five years:
• Date of overflow event
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• Location of overflows by sewershed
• Estimated volume of overflow
• Rainfall/ snowmelt volume associated with the event
• Duration of overflow event
• Cause of overflow, if known (i.e., I/I)
• Methodology used for identifying the frequency, volume, and duration of wet-weather
overflows (e.g., installed monitoring equipment, operator observations, public complaints)
• Steps taken to reduce, eliminate, and prevent recurrence of the overflow
Data should allow for an understanding of how varying precipitation conditions affect system behavior
and help to clarify year-to-year variability in system behavior relative to frequency, duration, and
amount of rain or snowmelt that occurs. The collection system might respond differently depending on
the amount of rain received and the period between precipitation events. If the precipitation data for
the SSOs are not available at the plant, a nearby airport might have precipitation records. Local
monitoring station data should be used if available, when reporting a specific wet-weather event,
because precipitation distribution can vary widely for a given area. When making projections of future
flows, an assumption should be made that wet-weather SSO discharges are eliminated with overflow
volumes either directed to the plant or eliminated from the system.
/. Summarize Performance (Success/Limitations) of Past l/l Efforts
A description of prior projects to characterize and control wet-weather flows should be identified. This
could include such projects as Sanitary Sewer Evaluation Survey (SSES) studies and subsequent
corrective actions, increased transport capacity to address limited capacities, construction of sanitary
equalization basins in the collection system or private property l/l elimination programs. Completion
dates of these projects and impacts of these projects on the overflow history as contained in the
previous item should be described in a narrative.
In addition, collection system rehabilitation efforts that improve system reliability and transport capacity
should be described. This information should include planned efforts and recently completed
rehabilitation. Information from past studies, such as those required under the Clean Water Act
Construction Grants program to assure proper and efficient operation and maintenance of treatment
works and their associated collection systems should be considered, if available. These provisions
required the development of operation and maintenance manuals, emergency operating programs,
personnel training, adequate budget, and operational reports. In addition, past trends of collection
system rehabilitation should be considered, including a summary of completed projects performed in
accordance with an SSES.
Ensuring that the system has adequate capacity, and providing proper management, operation and
maintenance, can reduce the occurrence of collection system failures. Effective utility management can
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therefore be described in terms of CMOM and is necessary to maintain the capacity of the collection
system, to reduce the occurrence of temporary problem situations such as blockages, to protect the
structural integrity and capacity of the system, and to anticipate potential problems and take preventive
measures. An ancillary effect of a CMOM program controlling remote overflows might be more flow
reaching the treatment plant. This is reflected in Section l.h of the December 22, 2005, draft peak flows
policy, which states, in part, that the utility should evaluate peak flow reductions obtainable through
implementation of existing CMOM programs and potential improvements in the timing or enhancement
of those programs and the related costs; or, if no such program exists, reductions obtainable through
the development and implementation of a CMOM program and the related costs.
IV. Projected Peak Wet-Weather Flows
An objective of the Utility Analysis is to provide estimates of future peak wet-weather flows. This is
reflected in Section l.c of the December 22, 2005, draft peak flows policy which, in part provides that
the Utility Analysis should provide estimates of future peak wet-weather flows on the basis of
information such as predicted climatic conditions, anticipated dry-weather flows, projected treatment
plant and collection system changes (e.g., upgrades, extensions, deterioration).
Projections of future flows within the collection system and entering the treatment plant are to be
included as part of the Utility Analysis. Plans to expand the service area geographically or consolidate
existing treatment plants should be described. Information on population projections within the service
area should be included. Data from National Weather Service Stations can be used to develop long-term
weather predictions that could affect future flows. The anticipated effect of the service area expansion,
population projections, and future climate predictions on average and peak flows should be described.
In addition, an analysis of water use trends within the utility's service area for the past several years
should be included. Results of such an analysis could be quantitative or qualitative.
The applicant should describe efforts to replace or rehabilitate the system, including projections on how
such activities could increase or decrease flows to the plant. A description and schedule for
rehabilitation projects at the treatment plant and in the collection system should be provided. The
schedule for l/l removal should also be included. The impact of these improvements on the average and
wet weather treatment plant flows should be summarized. Although l/l removal projects often decrease
the amount of flow that enters the treatment plant, rehabilitation within the collection system could
increase the amount of flow needing treatment. The current l/l projections should also be summarized.
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V. Identifying and Evaluating Potential Measures to Reduce
Diversions
The Utility Analysis should identify and evaluate a comprehensive set of potential alternatives that
would reduce or eliminate wet-weather diversions at the treatment plant. Possible measures that
should be evaluated can be grouped into the following general categories:
• Operational changes at the treatment plant
• Structural modifications at the treatment plant
• Collection system improvements
• Additional alternatives
• Treatment of diverted flows
• Consideration of emergency back-up equipment
For each alternative identified and evaluated in the Utility Analysis, the following information should be
included (to the degree applicable):
• For structural modifications (such as storage or expansion of treatment units), sizing of the
equipment or process or a description of the level of effort associated with incorporating the
alternative into the existing treatment plant.
• Anticipated performance of the equipment during the diversion of flow.
• Projected plant effluent expected if the alternative is implemented.
• Operational requirements for the process and additional labor hours required annually.
• The capital and operational costs associated with the process. Order of magnitude capital costs
might be sufficient for initial screening of options. For example, if a connection to the collection
system of the nearest treatment plant does not exist, providing a rough estimate of the piping
required might be sufficient.
• Non-monetary factors that might affect the evaluation. Examples might include siting
requirements in terms of land availability, topography and the ability to provide sufficient
biomass for secondary treatment systems that will be used only during wet-weather events.
a. Operational Measures during Wet-weather flows
Developing an alternate or wet weather mode of operation can minimize or prevent bypasses. The wet
weather mode of operation will be specific to the facility in terms of the flow patterns and the process
parameters. Some common practices are described below.
Maximum Units in Operation
Maximizing the number of treatment units available during wet-weather events can increase plant
capacity. When a wet-weather event is anticipated, all primary clarifiers, aeration basins, secondary
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clarifiers, tertiary treatment units, and disinfection units should be placed in service. This might require
that routine maintenance be scheduled for periods when wet-weather events are less likely to occur.
Unscheduled repairs should be performed as expeditiously as possible, particularly when wet-weather
events are likely to occur. These actions can help keep the maximum number of treatment units
available for use during wet-weather events. If an emergency repair is required during a wet-weather
event, the ability to allow wastewater to flow through the unit should be considered, to provide
hydraulic capacity even though treatment would be limited.
Buffer Flow
To increase the likelihood of providing treatment for the peak flows (or providing as much treatment as
possible during the peak flow period), it is recommended that a small buffer capacity be maintained, on
the order of 5 to 10 percent of the design peak flow when wet-weather events are anticipated. As the
plant influent increases with the wet weather progression, this buffer flow would be reduced gradually
down to zero, and thus maintain stability of the biomass in the secondary system.
Existing Equalization Basins
Existing equalization basins that are typically used to store the diurnal fluctuations in flow should be
emptied when wet-weather events are anticipated. This will allow the entire basin volume to be
available to store wet-weather flows.
Protection of Biomass Inventory for Activated Sludge
Loss of the biomass (i.e., washout of the mixed liquor suspended solids [MLSS]) from the aeration
system is another operational challenge that should be addressed during wet-weather events. Biomass
can be lost when high flows cause the mixed liquor to overwhelm the secondary clarifiers and results in
the biomass being discharged through the outfall or captured in the tertiary treatment system without a
method of returning the mixed liquor to the aeration tank(s).
An operational strategy that can reduce the loss of biomass is to reduce the return activated sludge
(RAS) rate from the secondary clarifiers to the head of the aeration basin during a wet-weather event.
This would cause the biomass to accumulate in the secondary clarifiers. To use this strategy, the
secondary clarifiers should have sufficient capacity to treat the anticipated peak wet-weather flows.
There is a risk that the sludge blanket could rise too high and be resuspended, possibly releasing mixed
liquor over the weirs. The operational changes that can be made also include reducing aeration in
certain parts of the aeration basin during peak hours (WEF 2006).
Good Settling Biomass
Maintaining a mixed liquor that settles quickly can improve the performance of the secondary clarifiers.
If settling is a problem during dry-weather operation, it will be even more difficult to maintain biomass
during wet weather periods. There are many causes for poor settling (Jenkins et al. 2003). One possible
cause is a young sludge age. A young sludge might not settle properly, which can be prevented by
reducing the wasting rate in the secondary clarifier. However, a wasting rate that is too low could lead
to anoxic conditions in the sludge blanket in the secondary clarifier, which could lead to sludge bulking,
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when the release of nitrogen gas brings settled sludge to the surface of the clarifier. The proper sludge
age and wasting rates must be determined by each plant. Increasing the settling velocity of the biomass
can increase the efficiency of the secondary clarifiers during wet-weather events. The guidelines for
surface overflow rates and solids loading rates were based on assuming an average settling velocity for
the particles anticipated to be in the biomass. If the settling velocity is increased, compared to dry-
weather operation, a portion of the effect of higher flows can be diminished. The settling velocity can be
increased by forming larger, denser, and thus heavier particles. All other causes and cures should be
pursued to maintain well settling biomass at the facility.
Chemical Addition during the Wet-Weather Period
The mixed liquor settling velocity can be increased by feeding or increasing the feed rate of metal salts
(such as ferric chloride or alum) in the aeration basin or immediately upstream of the secondary
clarifiers. This strategy would be most effective when biochemical oxygen demand (BOD) and total
suspended solids (TSS) concentrations are relatively high, early in a wet-weather event. Dilute flows that
often occur later in the wet-weather event might not have a sufficient density of particles to form a floe
large enough to settle even with the addition of metal salts.
Minimal Sludge Blanket in the Secondary Clarifiers
Minimizing the sludge blanket (the amount of sludge stored in the bottom of the secondary clarifier)
when wet-weather events are anticipated can decrease the likelihood of biomass loss over the weirs.
During high flows, the solids that were previously settled in the secondary clarifiers could be
resuspended and lost over the weir. Therefore, returning the sludge to the aeration basin will prevent
the resuspension and possible biomass loss.
Management of Recycle Flows
To reduce the hydraulic loading rate during the peak wet-weather flows, the return of in-plant recycle
flows from tertiary filters or sludge handling processes should be delayed until flows begin to subside.
This can be carried out manually by the plant personnel or by automatic control measures at the facility.
b. Structural Modifications at the Treatment Plant
Equalization Basin
Additional storage during wet-weather events can be provided at the treatment plant (additional
storage in the collection system is discussed below). Existing storage units could be expanded, or new
storage units could be constructed. Equalization basin(s) can be provided at the treatment plant.
Existing storage or equalization tanks could be expanded by building additional compartments adjacent
to the existing tanks. If space is not available adjacent to the existing tanks, new storage or equalization
could be provided at an available location within the system. Constructing the tanks at a separate
location from the existing tanks might require pumping to avoid hydraulic problems depending on the
topography of the treatment plant site. Staff at sites with more elevation changes will have to evaluate
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the placement of the tank(s) more carefully. Placing a tank that is not adjacent to the collection system
or in between the processes at the plant could require additional piping and pumping, which would
increase the costs associated with this alternative.
Implement a Step-Feed Activated Sludge Process
Decreasing the MLSS concentration entering the secondary clarifiers will result in a lower suspended
solids loading rate, which will improve the performance in the secondary clarifiers during wet-weather
events. Using a step-feed approach maintains biomass inventory in the aeration basin and decreases the
solids loading rate to the secondary clarifiers. In a step-feed system, the primary effluent can enter the
secondary process at the head of the aeration basin or at intermediary points within the aeration basin.
During wet-weather flows, a majority of the flow would enter the aeration basin in the downstream
feed locations. All the RAS would be pumped to the head of the aeration basin. The result is a higher
mixed liquor concentration in the head of the aeration basin, with lower concentrations in the middle of
the basin. The lowest MLSS concentrations would be found at the end of the aeration basin, which
would then be settled in the secondary clarifiers.
Many secondary treatment processes can be adapted to a step-feed system via the addition of piping. A
similar process could be used for an oxidation ditch system with concentric rings, which are used in
smaller treatment plants. The applicability of oxidation ditches might be limited, as primary clarifiers are
typically not constructed upstream of the process. Any diversions around the oxidation ditch would
likely require a separate treatment process to meet NPDES permit limits. One possible strategy would be
to operate the outer ring as a separate unit that is isolated from the middle and inner rings. No mixed
liquor from the outer ring would be discharged to the secondary clarifiers during the wet-weather
event. RAS would be pumped to the outer ring of the oxidation ditch. The middle and inner rings would
receive flow during the wet-weather event. In this way, the biomass is preserved in the outer ring of the
oxidation ditch, which can be distributed to the middle and inner rings after the wet-weather event has
ended.
Tank for Storage of Biomass
A separate storage tank could be built near the aeration basins and secondary clarifiers that could be
used to store the biomass during wet-weather events. The primary effluent would be fed to the head of
the aeration basin, as under the normal dry-weather operation. A line could be provided to the new
storage tank to allow primary effluent containing higher BOD and TSS concentrations to be stored for
later treatment. The RAS would be split between the aeration basin and the new storage tank. Once the
peak wet-weather flows have passed, the wastewater in the storage tank could be directed to the
secondary clarifiers and all RAS could be returned to the aeration basins. The storage tank could be
flushed and placed on standby until the next wet-weather event.
Increase Capacity of Individual Treatment Units
The review of the design and actual capacities of the treatment plant processes might have identified
treatment or hydraulic bottlenecks. By adding additional treatment units for selected processes or
upgrading transfer capacities, additional flow could be treated and might result in fewer diversions.
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If the actual capacity of a treatment process is less than the design capacity, the piping between the
previous process and the process in question should be reviewed. This could involve a hydraulic analysis
of the design or an inspection of the pipe to see if sediments or a blockage has developed during the
operation of the plant.
The amount of capacity that will be gained by the planned improvements should be stated, and any
changes to the process flow diagram should be explained.
c. Collection System Improvements to Reduce Flows during Wet-
Weather Events
Section l.g of the December 22, 2005, draft peak flows policy, states in part, that utilities should
evaluate the extent to which the permittee is maximizing its ability to reduce l/l throughout the entire
collection system (i.e., not only the portions operated by the utility, but also portions operated by any
municipal satellite community), including the use of existing legal authorities, potential improvements in
the timing or quality of such efforts, and options for obtaining or expanding legal authorities to reduce
l/l from satellite collection systems.
Increased Transport Capacity
To reduce SSOs in the collection system, the transport capacity can be increased. The actual capacity of
a given pipe might be less than the design capacity because of the accumulation of sediment or other
materials, such as oil and grease. A maintenance program to clean sections of the collection system that
are subject to sedimentation could provide a relatively low-cost process to increase the transport
capacity of wet-weather flows. In situations where the design capacity is insufficient to transport the
wet-weather flows, replacing a section of pipe with a larger diameter than the existing pipe, installing a
parallel pipe, or increasing the pump station capacity should be considered to alleviate the capacity
problem. Increased maintenance or expanding the capacity of the collection system could reduce the
number of overflows at the designated location, but plant personnel should take care to ensure that the
downstream sections of the collection system can accommodate the additional flows. In addition, the
changes would result in greater flows entering the treatment plant. The ability of the plant to treat
these additional flows should be described in the Utility Analysis.
Remote Storage
Equalization or storage basins or tunnels for wet-weather flow in the collection system should be
considered. Storage in the collection system can have added the added benefit of being a component in
a comprehensive effort to control of sanitary system overflows. Once flows decreased in the collection
system, the water in the storage unit could be released at a rate selected during the design.
Flow Reductions Accomplished by Reducing l/l
A description of the efforts to quantify the l/l entering the collection system should be included. The
information could include individual studies that have been performed or a summary of a SSES, if one
has been completed recently (i.e., within the past five years). The completed and planned l/l removal
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projects should be listed, including the amount of wet-weather flow that will be removed by each
project. A review of the alternatives that were considered and the justification for the selected projects
should also be provided. If an SSES was completed, the executive summary of the document should be
included as an attachment to the Utility Analysis.
If the party preparing the Utility Analysis does not control the entire service area for the treatment
plant, a description of the agreements in place that establish how the collection system is operated and
controlled should be provided. For example, a description of any intergovernmental agreements in place
that require ordinances to be adopted by the member communities within the service area or give
regulatory authority to certain parties should be described. Information on l/l reductions in the entire
service area should be provided.
I/I can originate from individual residences and businesses, which would then enter the sanitary
collection system through the individual building laterals. These l/l sources might include footing drain
connections from older homes or aging pipes. If an SSES was conducted, estimates on the number of
houses that could have footing drain connections to the sanitary sewer might have been calculated. In
addition, tests could have been done at selected residences to predict whether l/l was entering through
the building laterals because of the pipe being compromised by tree roots or decay due to age. A
summary of this information should be included in the Utility Analysis, if available.
Green Infrastructure
Green infrastructure practices should be evaluated for use in conjunction with l/l reduction efforts. It
might be possible to use green infrastructure practices to remove stormwater flows from the sanitary
collection system, such as footing drain, downspout, and roof drain disconnections. Green infrastructure
projects can include constructing green roofs, rain gardens or larger bioretention facilities, and
vegetated swales. These approaches can also be used in conjunction with other methods to capture
stormwater, such as rain barrels. Rather than transferring these sources to the stormwater collection
system, a green infrastructure practice could allow the water to slowly infiltrate and regenerate
groundwater resources. However, when implementing any stormwater storage practice, personnel
should ensure that it is not resulting in locally elevated groundwater tables that drain to public or
private sewers through cracks or joints.
Flow Reductions by Controlling Slug Loadings during Wet Weather
Section l.e of the December 22, 2005, draft peak flows policy, suggests in part, assessing other ways to
reduce peak wet-weather flow volumes, such as limiting slug loadings from indirect dischargers. When
SSOs or diversions occur, wastewater is discharged that has not received treatment (in the case of SSOs)
or limited treatment (when a portion of the flow does not receive secondary treatment). During wet-
weather events, it is helpful to minimize nondomestic discharges to the extent possible, to decrease the
likelihood of toxic pollutants being discharged to the environment. Continuous commercial and
industrial discharges are difficult to minimize or eliminate during wet-weather events, but those
businesses that discharge slug loadings or batches might be able to hold their discharges until after the
wet-weather event has concluded. A review of the companies included in the industrial pretreatment
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program could identify the batch dischargers. A review of the slug discharge plans submitted to the
treatment plant could identify additional businesses to approach for minimizing or eliminating
discharges during wet-weather events.
The treatment plant should discuss the feasibility of minimizing or eliminating the batch and slug
discharges during wet-weather events with each individual industry. Agreements reached could be
incorporated into the industrial user discharge permits and or slug control plans. These agreements
might not be able to eliminate batch and slug discharges during wet-weather events that occur for an
extended period of time. Removing the nondomestic discharge from the initial wet-weather flows that
can include higher BOD and TSS concentrations and delaying them to the more dilute flows later in a
wet-weather event can decrease the impact of toxic pollutants on the environment.
Any agreements that have been reached or are being explored concerning storage of batch and slug
discharges should be included in the Utility Analysis.
d. Additional Alternatives
Routing Flows to a Different Wastewater Treatment Plant
The possibility of transferring wet-weather flows to an alternate secondary treatment plant that might
have available capacity should be evaluated. If this is a feasible alternative, a discussion of how the
transfer of wastewater would occur should be included. Items to be considered include at what flows
would the transfer begin and end, would a gate be operated manually or automatically. A brief
discussion of the peak flow capacities of the alternate treatment plant to be used should also be
included. If the treatment plants are not operated by the same entity, a description of the agreement
that should be developed should also be included.
Sewer Moratorium
Section l.e of the December 22, 2005, draft peak flows policy, suggests in part, assessing other ways to
reduce peak wet-weather flow volumes, such as limiting collection system extensions. The feasibility of
instituting a moratorium on accepting additional flows in the sanitary collection system should be
included in the Utility Analysis. Information that could be included in the discussion is any developments
that have been promised sewer capacity and the legal and economic consequences of instituting a
building moratorium. The economic analysis could include the amount of undeveloped land or land to
be redeveloped that exists in the service area.
e. Treatment of Diverted Flows
A comprehensive Utility Analysis should assess technologies, such as supplemental biological treatment,
physical/chemical treatment (e.g., ballasted flocculation, deep bed filtration, or membrane technology)
that are or could be used to provide additional treatment to peak wet-weather flows or peak wet-
weather diversions at the POTW treatment plant.
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Under the bypass regulation, a bypass occurs when there is intentional diversion of peak flows from the
secondary treatment units, regardless of whether the diverted flows are treated. If the diverted flow is
routed to a treatment unit that is itself a secondary treatment unit, it is not a bypass. The term
secondary treatment unit refers to a treatment process that meets the effluent imitations in the
secondary treatment regulations. See 40 CFR Part 133. If the diverted flows meet the effluent limits in
the secondary treatment regulations before mixing or recombination with other flows, the routing
scenario does not represent a bypass. The treatment unit(s) in that scenario would represent a parallel
treatment facility. In contrast, in situations where flows are diverted around secondary treatment units
and receive treatment that is not designed and demonstrated to meet limits based on the secondary
treatment regulations, the diversion is a bypass.
Providing Secondary Treatment for Diverted Flows
Section l.f of the December 22, 2005, draft peak flows policy says in part, that utilities should evaluate
technologies (such as supplemental biological treatment, physical chemical treatment, ballasted
flocculation, deep bed filtration, or membrane technology) that are or could be used to provide
additional treatment to peak wet-weather flows or peak wet-weather diversions at the POTW treatment
plant and the costs of implementing those technologies. An evaluation of options that would provide
treatment that meets the regulatory definition of secondary treatment at 40 CFR Part 133 for all or a
larger portion of the diverted flow than under current operations should be conducted.
Where available land is an issue, considering secondary treatment processes that require less space than
the existing technology can be examined. Because these technologies can require less space than the
existing secondary treatment system, an evaluation of replacing the existing secondary treatment
system with one of these technologies could also be performed. However, the conversion of the
secondary treatment system would have to be carefully planned to ensure that sufficient capacity exists
to provide treatment during construction.
One possible secondary treatment technology to be considered where land is limited is membrane
bioreactors (MBRs), which use microfiltration membranes to separate the biomass from the secondary
effluent, which are much more compact than secondary clarifiers (WEF 2005). In addition, because the
system relies on filtration, rather than settling rates, the MLSS concentration can be much higher in an
MBR than a conventional aeration basin. MBRs are often limited to a hydraulic capacity of twice the
average flow and are expensive. An advantage to the MBR process is that they are easier to operate
temporarily for the peak flow periods as an extension of the existing biological process compared to
other secondary processes (USEPA 2007). The integrated fixed-film activated sludge (IFAS) process and
moving bed biofilm Reactors (MBBRs) could also be considered for treating the diversion flows. The IFAS
process uses fixed-film treatment in combination with an activated sludge basin. The biomass in an IFAS
tank will be larger than in an aeration tank of the same size because of the added growth on the fixed-
film media. As with a conventional secondary process, an RAS line returns solids settled in the secondary
clarifier. An MBBR is designed to rely solely on the fixed-film growth that occurs on the floating media.
In addition, all sludge is wasted from the secondary clarifiers without requiring recycle to the treatment
tank. Both the IFAS and MBBR treatment systems are less susceptible to a loss of solids during peak
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flows because of the biomass located on the fixed-film media and, thus, can be an economical
alternative for temporary treatment of peak flows as an extension of the existing processes.
Providing Treatment that Does Not Meet Secondary Treatment Standards: Physical and
Chemical Treatment
Chemical addition can be used to remove TSS and the insoluble fraction of BOD. Chemical addition
should be followed by a clarifier or other quiescent zone to allow for settling. One example of chemical
addition during wet-weather treatment is chemically enhanced primary treatment (CEPT), which
involves feeding relatively large quantities of a metal salt such as ferric chloride or alum to the primary
clarifiers. Under dry-weather conditions, a metal salt can be added to the primary clarifiers to decrease
the solids and BOD entering the aeration basin. Feeding too much chemical might cause operational
problems if there is insufficient phosphorus available in the aeration basin to allow BOD removal.
However, during wet-weather events, the treatment plant personnel might want to precipitate and
settle as much of the BOD and TSS in the primary effluent as possible, which can be accomplished by
increasing the feed rate of the metal salt. Implementing CEPT would be especially important at plants
that operate primary and secondary treatment processes in parallel during wet-weather events.
Filtration, including deep bed filters, could be used to remove solids and particulate BOD from the
diverted flow. The quality of the effluent from a filter system can be enhanced by chemical addition.
Sufficient mixing should be provided between chemical addition and filtration to allow larger particles to
form that can be trapped in the filters.
Ballasted flocculation can also be used to treat diverted flows through chemical addition and a media on
which the floe can form. These floes will settle faster than a conventional process. The media is
separated from the settled floe and returned to the process. Two examples of processes using ballasted
flocculation are Actiflo and DensaDeg. (see www.epa.gov/owm/xxxx) An emerging technology is the
CoMag process, which uses magnetic particles as the ballast. A coagulant is added, followed by the
magnetic particles and a polymer. Solids are settled in a solids contact clarifier. A magnetic separator
captures any magnetic particles that did not settle in the clarifier. The magnetic particles are separated
from the solids and returned to the process, and the solids are wasted from the system.
/. Emergency Back-Up Equipment
Replacement Parts and Redundant Capabilities
Common replacement parts should be kept on hand at the POTW treatment plant in accordance with
the recommendations contained in the operation and maintenance manual. In particular, pumps that do
not a standby or back-up pump installed are required to have a spare pump on-site that can be quickly
installed in an emergency. Inadequate pumping capacity can lead to decreased hydraulic and treatment
capacity downstream, which could lead to overflows and diversions if a pump should fail before or
during a wet-weather event.
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Emergency Back-up Power Systems
The loss of power at a treatment plant can lead to flow diversions and bypasses. Back-up power system
should be evaluated for typical power loss scenarios, as well as severe wet-weather situations such as
tornados and hurricanes. It is recommended that the treatment plant have a fixed, on-site, back-up
power generators with an emergency fuel supply. The generator should be properly sized with sufficient
output to maintain proper operations during power losses. Aeration equipment is not required to be
powered by the generator, unless the POTW has a history of experiencing long power outages that
longer than 4 hours. If power outages longer than 4 hours are typical, the generator should provide
power for minimum aeration of the activated sludge basins. Disinfection and dechlorination should be
provided during all power outages if it is needed to meet NPDES permit limits. If the plant does not
have an emergency back-up power system, the cost of such a system should be evaluated.
VI. Ability to Pay / Financial Capability Assessment
A financial assessment of the water and wastewater system should be included in conjunction with the
Utility Analysis. This is reflected in Section l.i of the December 22, 2005, draft peak flows policy, which
in part provides that the Utility Analysis should assess the community's ability to fund the peak wet-
weather flow improvements discussed in the Utility Analysis, taking into consideration current sewer
rates; planned rate increases; and the costs, schedules, anticipated financial impacts to the community
of other planned water and wastewater expenditures, and other relevant factors impacting the utility's
rate base using as a guide EPA's CSO Guidance for Financial Capability Assessment and Schedule
Development, EPA 832-B-97-004.
a. Review of Current Potable and Wastewater Costs
The costs associated with the water treatment system and the current water rates should be provided.
A comparison of the treatment costs and amount collected from the water rates should be included. In
addition, the costs associated with the wastewater system and the current wastewater rates should be
stated, as well as the total amount collected from the wastewater rates. The costs associated with
storm water management measures used to improve water quality and any associated storm water
rates can be included as wastewater costs and rates. Information on any funds that are targeted for
future wastewater improvements would also be useful if included in the Utility Analysis.
Any large projects that are planned for water or wastewater treatment plant improvements outside of
this Utility Analysis should be briefly described, as well as the money budgeted for the improvement. If
any rate increases are scheduled, the information should also be included in this section.
b. Financial Capability Assessment
Utilities should use the Combined Sewer Overflows—Guidance for Financial Capability Assessment and
Schedule Development (USEPA 1997). This document provides a planning tool for evaluating the
financial resources a community has available to implement projected wastewater controls and to assist
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in the development of implementation schedules. It describes a procedure that results in a community
cost burden ranking (high, medium, or low) that supports the development of a rational schedule for
implementing wastewater projects.
The first goal of the guidance is to introduce and explain the planning tool, which is a two-phased
approach for assessing a community's financial capability to implement controls using indicators
identified in the guidance. In the first phase, a residential indicator is calculated that characterizes the
financial impact of wastewater controls on individual households using the annual wastewater collection
and treatment costs as a percentage of median household income. In the second phase, a composite
community financial capability score is calculated using a variety of indicators that measure a
community's debt, socioeconomic, and financial conditions (e.g., bond rating, unemployment rate,
property tax collection rate). Data from the two phases are then compared to a financial capability
matrix to arrive at an overall assessment ranking of a community's ability to finance and construct
wastewater controls.
The final section of the document focuses on the schedule development process and presents details of
how environmental and financial considerations appropriately influence schedule development. The
principles of considering wastewater treatment costs as a percentage of median household income for
assessing and ranking a community's ability to afford and finance future wastewater controls can be
applied to controls necessary to reduce peak wet-weather diversions under this policy.
VII. Summary of Public Participation
The public should be provided the opportunity to review and discuss the alternatives included in the
Utility Analysis. Typical administrative procedures for the municipality should be followed. This could
include having the engineering firm that is helping prepare the Utility Analysis make presentations and
provide updates during municipal board or council meetings. Depending on the complexity of the
alternatives involved and the project impact on wastewater and sewer rates, a special meeting or series
of meetings in the community might be warranted. The information shared with the public should
include the alternatives considered, brief summaries of the design associated with feasible alternatives,
and the impact that alternatives under consideration would have on wastewater and sewer rates.
VIII. Selection of Recommended Measures for Implementation
The alternatives that the applicant recommends to implement should be summarized in this section
along with a description of the criteria used to select recommended measures and process for applying
the criteria. The alternatives may include a combination of construction projects and operational
changes in the collection system and treatment plant. A description of the proposed operation of the
treatment plant during wet-weather events should be included. In addition, a proposed financial plan
and proposed schedule should be provided.
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a. Financial Plan
A discussion of the capital funding options considered should be included in the Utility Analysis.
Alternative funding options, such as privatization of the wastewater system can be explored.
The municipality should have a revenue system in place to pay for any increased operation and
maintenance costs associated with the alternatives to be implemented. In addition, funding to pay any
interest on the bonds or loans used to fund the projects may be necessary. A proposed schedule for rate
increases should be developed and included in the Utility Analysis. The current wastewater and sewer
rates should be restated.
A summary of the selected financial option(s) should be provided. A schedule for the application of any
funding that has not already been obtained should be included.
b. Proposed Implementation Schedule
The proposed schedule for implementing each selected alternative should be provided. For projects that
are planned to be completed in the near future, a more detailed breakdown of milestones should be
provided.
c. Estimate of Projected Frequency, Duration, and Volume of
Diversions
Section l.k of the December 22, 2005, draft peak flows policy provides, in part, the utility analysis should
provide projections of the POTW treatment plant effluent improvements and other improvements in
collection system and treatment plant performance that could be expected if the technologies, practices
or other measures discussed in the Utility Analysis are implemented. Information on the impact on the
diversion flows for each measure to be implemented should be provided.
The basis for the future wet-weather flows should include the projections for population and service
area expansion from Section 11 Lb of this guidance, projected treatment plant capacity after completion
of the improvements, and projected l/l upon completion of reduction efforts. The anticipated frequency,
duration, and volume of future diversions after the improvements are completed should be provided. In
addition, the projections of the final effluent quality during wet-weather flows should be included.
d. Demonstration of No Additional Feasible Alternatives Beyond the
Recommended Measures
POTW treatment plant operators seeking approval of peak wet-weather diversions at a treatment plant
as an anticipated bypass should demonstrate to the NPDES permitting authority there are no additional
feasible alternatives beyond the measures recommended herein to peak wet-weather diversions at the
time of their NPDES permit application or NPDES permit renewal. This could typically be done by
May 2009
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Draft Guidance on Preparing a Utility Analysis
showing that implementing additional controls would create a high financial impact or are otherwise
infeasible.
IX. Proposed Monitoring Protocol
Section l.j of the December 22, 2005, draft peak flows policy provides, in part, that the utility should
propose a protocol for monitoring the recombined flow at least once daily during diversions for all
parameters for which the POTW treatment plant has daily effluent limitations or other requirements
(e.g., monitoring only requirements) and ensure that appropriate representative monitoring for other
monitoring requirements of the permit, the total volume diverted, and the duration of the peak wet-
weather diversion event are tracked.
The utility should project the POTW treatment plant effluent improvements and other improvements in
collection system and treatment plant performance that could be expected if the technologies, practices
or other measures discussed in the Utility Analysis are implemented. Future data needs should also be
identified.
Utilities should ensure that functional flow metering equipment is in place for determining the volumes
and rates of flow through the full secondary treatment process and during the potential diversion flow
routes. In addition, access for sampling to assess the process performance of individual units during
peak wet-weather conditions should be provided.
X. Proposed Plan for Public Notice of Diversion
POTW treatment plants should provide a plan that describes the framework for how and when the
public and other entities would be notified of bypasses that exceed any effluent limitation in the permit
or that may endanger health due to a likelihood of human exposure. The plan should be developed in
consultation with appropriate authorities at the local, county, and/or state level, and describe how,
appropriate local, county and/or State agencies, such as downstream drinking water facilities and
public health agencies, as well as the public would be notified of bypasses that exceed any effluent
limitation in the permit or that may endanger health due to a likelihood of human exposure the public.
The plan should who will be notified and the specific type of information that would be reported.
The plan should include a description of lines of communication and the identities of responsible
officials.
The proposed framework would not need to dictate the specific procedures or the specific information
that would be provided through public notification; rather, indicate how the POTW treatment plant
operators will establish and update, in consultation with appropriate local, county and/or State
agencies, case-by-case notification procedures that would depend on the nature and duration of the
diversion and the responsibilities of different local entities.
The framework should describe the criteria to be used to evaluate if a given bypass event may endanger
health due to a likelihood of human exposure. The criteria would reflect the uses of potentially affected
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Draft Guidance on Preparing a Utility Analysis
waters as well as other relevant factors. The development of these criteria should be coordinated with
the NPDES authority, local health officials, drinking water suppliers, and other key potentially affected
entities.
References
Ten State Standards Great Lakes - Upper Mississippi River Board of State and Provincial Public Health
and Environmental Managers. 2004. Recommended Standards for Wastewater Facilities. Health
Education Services Division. Albany, NY.
Jenkins D., G. Daigger, M. Richard. 2003. Manual on the Causes and Control of Activated Sludge Bulking,
Foaming, and Other Solids Separation Problems. CRC Press. 3rd Edition.
Tozer, H. 2007. Study of Five Phosphorus Removal Processes Selects Comag to Meet Concord, MA's
Stringent New Limits, In Proceedings of the Specialty Conference on Nutrients 2007, Water
Environment Federation, Alexandria, VA, 2007, pp. 1492-1509.
USEPA(U.S. Environmental Protection Agency). 1997. Combined Sewer Overflows—Guidance for
Financial Capability Assessment and Schedule Development. EPA 832-B-97-004.
. U.S. Environmental Protection Agency,
Washington, DC.
USEPA, 1991. Handbook: Sewer System Infrastructure Analysis and Rehabilitation, EPA/625/6-91/030,
191; Existing Sewer
USEPA, 1989 EPA Technology Transfer Handbook: Retrofitting POTWs, Hegg.B.A, DeMers, L.D.
WRC Engineering. A Guide to Short Term Flow Surveys of Sewer Systems, WRC Engineering (Undated),
National Association of Sewer Service Companies (NASSCO) "Manual of Practice"
USEPA (U.S. Environmental Protection Agency). 2007. Emerging Technologies for Wastewater Treatment
and In-Plant Wet Weather Flow Management. EPA 832-R-006. U.S. Environmental Protection
Agency, Washington, DC.
WEF (Water Environment Federation). 2005. Biological Nutrient Removal Operation in Wastewater
Treatment Plants. WEF Manual of Practice No. 30. Water Environment Federation. Alexandria,
VA.
WEF (Water Environment Federation). 2006. Guide to Managing Peak Wet Weather Flows in Municipal
Wastewater Collection and Treatment Systems. Water Environment Federation. Alexandria, VA.
WEF 1994. Existing Sewer Evaluation and Rehabilitation, WEF MOP FD-6, Water Environment
Federation. Alexandria, VA.
May 2009
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Draft Guidance on Preparing a Utility Analysis
Handbook: Sewer System Infrastructure Analysis and Rehabilitation, EPA/625/6-91/030,191; Existing
Sewer
Existing Sewer Evaluation and Rehabilitation, WEF MOP FD-6, 1994; A Guide to Short Term Flow Surveys
of Sewer Systems, WRC Engineering (Undated), the National Association of Sewer Service Companies
(NASSCO) "Manual of Practice"
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Draft Guidance on Preparing a Utility Analysis
Appendix A: Peak Wet-Weather Flows Utility Analysis
Completeness Checklist
The December 22, 2005, draft peak flows policy provides that permittees that are seeking approval of
peak wet weather diversions as an anticipated bypass at a POTW treatment plant serving a sanitary
sewer collection system should submit, along with their permit application, a comprehensive utility
analysis (UA) to the NPDES authority. The UA, permit application and other information in the permit
record is intended to support an authority's determination as to whether or not there is a feasible
alternative to peak wet weather diversions at a POTW treatment plant serving a separate sanitary sewer
collection system. This Checklist is to assist NPDES authorities in a completeness review of a UA to
evaluate if the permittee has provided information in a format that is understandable and consistent
with the December 22, 2005, draft peak flows policy. The goal of the checklists is to assess and
document whether the permit and administrative record provide a complete, comprehensive, and
transparent record of permit development. As such, the checklist responses are not intended to judge
the "correctness" or "incorrectness" of permit limits and conditions. Rather, the checklists are intended
to guide a comprehensive evaluation of the NPDES permit development process by a knowledgeable
EPA or state NPDES permit reviewer.
Assessment criteria presented in this document are not binding; the permitting authority may consider
other approaches consistent with the bypass regulation at 40 CFR 122.41(m). When an NPDES authority
assesses the merits of a UA, it should make each assessment on a case-by-case basis and will be guided
by the applicable requirements of the bypass regulations, taking into account information related to the
particular situation. EPA may change the elements of this assessment in the future.
Permittee:
Permit Number:
Reviewer(s):
Documents Reviewed:
Date Review Completed:
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Evaluation Criteria
v>
a>
>
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Page#
Comments
I. Existing Flow Information
Flows at the Plant
1A
Base Flows - Does section A.6
of Form 2A provide data on the
annual average daily flow rate
at the plant for the previous
three years?
IB
Does the utility analysis
provide additional data to
adequately characterize base
flows at the plant?
2A
Peak Flows - Does section A.6
of Form 2A provide the
maximum daily flow rate
received at the plant for the
previous three years?
2B
Peak Flows - Does the Utility
Analysis provide additional
data on peak flows received at
the plant?
2C
Does the peak flow data
provided in Form 2A
adequately describe peak flow
conditions during sustained
periods of wet weather?
3
Does the Utility Analysis
provide a summary of long-
term rainfall records
4
Does the permittee present
sufficient data for an adequate
range of storms and other
conditions (such as the
occurrence of wet weather
SSOs in the collection system,
antecedent soil conditions, etc.)
to demonstrate an adequate
understanding of the
relationship between rainfall
conditions and influent flow
rates at the plant?
5
Does the permitte identify all
locations where diversions
have occurred (e.g., prior to
primary, around secondary,
around tertiary) during the
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previous three years?
6
Does the applicant provide
information on the frequency
and duration of diversions
occurring in the previous three
years?
7
Does the applicant provide
estimates of the volume of
diversions at the plant
occurring in the previous three
years?
Flows in the Collection System
8
Does the Utility Analysis
identify dates when wet
weather induced SSOs occur in
the collection system?
9
Does the permittee present
sufficient data for an adequate
range of storms and other
conditions to demonstrate an
adequate understanding of the
relationship between rainfall
conditions and SSO volumes in
the collection system?
10
Does the permit applicant
provide summaries of studies
involving flow monitoring that
have been conducted to
evaluate rates and sources of
I/I in the collection system?
11
Has the applicant identified all
municipal satellite collection
systems in the collection
system? (see section A.4 of
Form 2A).
12
Does the permit applicant
provide flow monitoring data
or data from pump stations to
estimate the I/I levels
contributed by each municipal
satellite collection system?
13
Does the applicant provide a
summary of the results of past
and on-going efforts to reduce
I/I?
14
Has the applicant adequately
described its existing legal
authority for lack of authority]
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to reduce I/I from municipal
satellite collection systems?
15
Has the applicant adequately
described its existing legal
authority (or lack of authority)
to reduce I/I from private
building laterals?
II. Information Regarding the Existing Treatment Plant
16
Does the permittee provide a
process flow diagram or other
information that adequately
describes the treatment process
for average daily flows (those
associated with during dry-
weather conditions)?
17
Are design capacities provided
for each component of the
treatment plant?
i. Preliminary treatment
ii. Storage
iii. Primary clarification
iv. Secondary treatment units
v. Secondary clarification
vi. Disinfection
vii. Outfall
18
A
Did the applicant indicate that
stress testing has been
conducted to identify the
maximum wet weather
capacity of capacity limited
treatment processes?
18
B
Is a summary of the results of
any stress testing for capacity-
limited treatment process
components provided?
19
Does the permittee provide a
process flow diagram or other
information that adequately
describes the treatment
process used when wet
weather diversions are
occurring?
18
Is a clear description provided
of where in the process peak
flow diversions occur, where
they are routed and which
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treatment units are routed
around?
19
Is a clear description provided
of the flow rate that triggers the
onset of peak flow diversions?
20
Is the treatment process for
peak wet weather flows clearly
defined?
21
Does section A.8.d of Form 2A
(or the Utility Analysis)
indicate whether the treatment
works discharge or transports
treated or untreated
wastewaters to another
treatment works?
III. Estimates of Future Diversions
22
Does the UA provide estimates
of future peak wet weather
diversions?
23
What information used to
estimate future peak weather
diversions is provided in the
UA?
i. Population growth
estimates
ii. Estimates of current wet
weather SSOs (that may
require additional flows to
the treatment plant)
iii. Projected collection
system changes
(improvements or
deterioration)
iv. Projected treatment plant
improvements
v. Any projected reductions
in treatment plant capacity
vi. Projected changes in
weather patterns
24
Is the model used to predict
future diversions described in
the UA?
IV. Potential Measures to Reduce Diversions
25
Attachment 1 provides a table
that can be used to track
potential measures discussed
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in the Utility Analysis and the
type of information provided
for each alternative.
27
Does the Utility Analysis
describe the process by which
by which alternatives were
developed?
V. Ability to Pay / Financial Capability
The Costs of Providing Potable Water
28
Has the applicant provided
information on current costs of
providing potable water to the
community, including annual
operations and maintenance
expenses and annual debt
service?
29
Has the applicant provided
estimates of planned water
expenditures that will
significantly increase the cost
of providing potable water to
the community?
Wastewater Costs
30
Has the applicant provided
information on current costs of
providing wastewater
collection and treatment to the
community, including annual
operations and maintenance
expenses and annual debt
service?
31
Has the applicant provided
estimates of planned
wastewater expenditures other
than those associated with
reducing wet weather
diversions that will
significantly increase the cost
of providing wastewater
collection and treatment to the
community?
32
Has the applicant provided
adequate estimates of the costs
of implementing recommended
measures to reduce wet
weather diversions?
33
Has the applicant provide
adequate information on the
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following financial indicators:
i Current annual sewer
rates
ii Estimated costs per
household of implemented
recommended measures
iii Unemployment rate
iv Median household income
v Property tax revenue
collection rate
34
Has the applicant identified
plans to increase wastewater
rates and use other sources of
revenue to fund the
improvements recommended
in the utility analysis?
35
Has the applicant provided a
description, including a
schedule, of plans to increase
wastewater rates or otherwise
raise revenue to fund the
improvements recommended
in the UA?
VI. Selection of Feasible Alternatives
36
Does the Utility Analysis
described the approach used to
screen and narrow the list of
alternatives and list the
screening criteria?
37
Does the Utility Analysis
explain the reasons for
selecting recommended
alternatives?
38
Does the Utility Analysis
project the performance of the
treatment plant and collection
system after the recommended
alternatives are implemented
(e.g., improvements to the
treatment plant effluent,
projected average annual
number of diversions,
improvements in the collection
system)?
39
Does the proposed schedule for
implementing feasible
measures provide adequate
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detail and clarity?
40
Does the applicant indicate
how EPA's "CSO Guidance for
Financial Capability
Assessment and Schedule
Development" was considered
in the development of the
proposed schedule?
VII. Projected Effluent Quality
41
Does the application provide
data or other information that
indicates that the blended
effluent will meet permit
limitations, including WQBELs,
under existing conditions?
42
Does the application provide
data or other information that
indicates that the blended
effluent will meet permit
limitations, including WQBELs,
under various alternatives?
VIII. Proposed Monitoring Protocol
43
Has the applicant proposed a
protocol for monitoring the
recombined flow at least once
daily during diversions for all
parameters for which the
POTW treatment plant has
daily effluent limitations?
44
Does the proposed monitoring
protocol provide for
appropriate representative
monitoring for the total volume
diverted and the duration of
the diversion event?
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Attachment 1: Utility Alternatives Table
A = Potential Alternative
B
C
D
E
F
G
Changes in wet weather operations at the plant: reduced residence times at key
unit operations
Changes in wet weather operations at the plant: Changes in flow configuration
Changes in wet weather operations at the plant: Other
Increased structural capacity of individual treatment units
Increasing capacity of existing storage units
Providing additional storage units
Flow reductions: I/I measures in portions of the collection system operated by
the permittee
Flow reductions: I/I measures in portions of the collection system operated by
the municipal satellite collection systems
Flow reductions: I/I measures in portions of the collection system operated by
the permittee
Flow reductions: I/I measures for building laterals
Flow reductions: Control of slug loadings during wet weather
Flow reductions: Routing flows to a different treatment plant.
Sewer moratorium
Treatment of diverted flows (e.g., supplemental biological treatment, physical
chemical treatment, ballasted flocculation, deep bed filtration, membrane
technology)
A = Potential Alternative
B = Was this alternative identified in the UA?
C = Does the UA provide cost estimates for this alternative?
D = Does the UA propose a schedule for implementation?
E = Does the UA provide estimates of the reduction in the number and volume of diversions expected after the
alternative is implemented?
F. Does the UA project the improvement in treatment plant effluent expected after the alternative is
implemented?
G. Does the UA recommend this alternative as a 'feasible alternative'?
May 2009
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