EPA 832-B-95-004
COMBINED SEWER OVERFLOWS
GUIDANCE FOR SCREENING AND RANKING
Office of Wastewater Management
U.S. Environmental Protection Agency
401 M Street, SW
Washington, DC 20460
August 1995
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NOTICE
The statements in this document are
intended solely as guidance. This
document is not intended, nor can it be
relied on, to create any rights
enforceable by any party hi litigation
with the United States. EPA and State
officials may decide to follow the
guidance provided in this document, or
to act at variance with the guidance,
based on an analysis of specific site
circumstances. This guidance may be
revised without public notice to reflect
changes in EPA's strategy for
implementation of the Clean Water Act
and its implementing regulations, or to
clarify and update the text.
Mention of trade names or commercial
products in this document does not
constitute an endorsement or
recommendation for use.
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TABLE OF CONTENTS
Page
1 Introduction 1-1
1.1 Background I'l
1.2 History of the CSO Control Policy 1-1
1.3 Key Elements of the CSO Control Policy 1-3
1.4 Guidance to Support Implementation of the CSO Control Policy 1-4
1.5 Purpose of Manual and Target Audience 1-6
1.6 Watershed Approach to Permitting 1-8
2 The Screening Process 2-1
2.1 Criterion 1 2-1
2.2 Criterion 2 2'2
3 The Ranking Process 3-1
3.1 Criterion 1 3'2
3.2 Criterion 2 3-3
3.3 Criterion 3 3~4
3.4 Criterion 4 3-6
3.5 Criterion 5 3-7
3.6 Criterion 6 3-8
3.7 Criterion 7 3-9
APPENDIX A TESTING OF THE GUIDANCE FOR SCREENING
AND RANKING COMBINED SEWER SYSTEMS A-l
REFERENCES R-*
LIST OF EXHIBIT AND TABLES
Exhibit 1-1 Roles and Responsibilities I'5
Table A-l. Sources From Which Needed Information Was Acquired for Screening
and Ranking Process Criteria A-2
Table A-2. Summary of Results Obtained in Applying the Screening and Ranking
Process to 13 CSSs A~4
August 1995
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1.1 Background
Combined sewer systems (CSSs) are wastewater collection systems designed to carry
sanitary sewage (consisting of domestic, commercial, and industrial wastewater) and storm water
(surface drainage from rainfall or snowmelt) in a single pipe to a treatment facility. CSSs serve
about 43 million people in approximately 1,100 communities nationwide. Most of these
communities are located in the Northeast and Great Lakes regions. During dry weather, CSSs
convey domestic, commercial, and industrial wastewater. In periods of rainfall or snowmelt,
total wastewater flows can exceed the capacity of the CSS and/or treatment facilities. When this
occurs, the CSS is designed to overflow directly to surface water bodies, such as lakes, rivers,
estuaries, or coastal waters. These overflows-called combined sewer overflows (CSOs)-can
be a major source of water pollution in communities served by CSSs.
Because CSOs contain untreated domestic, commercial, and industrial wastes, as well as
surface runoff, many different types of contaminants can be present. Contaminants may include
pathogens, oxygen-demanding pollutants, suspended solids, nutrients, toxics, and floatable
matter. Because of these contaminants and the volume of the flows, CSOs can cause a variety
of adverse impacts on the physical characteristics of surface water, impair the viability of aquatic
habitats, and pose a potential threat to drinking water supplies. CSOs have been shown to be
a major contributor to use impairment and aesthetics degradation of many receiving waters and
have contributed to shellfish harvesting restrictions, beach closures, and even occasional fish
kills.
1.2 History of the CSO Control Policy
Historically, the control of CSOs has proven to be extremely complex. This complexity
stems partly from the difficulty in quantifying CSO impacts on receiving water quality and the
site-specific variability in the volume, frequency, and characteristics of CSOs. In addition, the
financial considerations for communities with CSOs can be significant. The U.S. Environmental
1-1 August 1995
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Chapter 1 Introduction
Protection Agency (EPA) estimates the CSO abatement costs for the 1,100 communities served
by CSSs to be approximately $41.2 billion.
To address these challenges, .EPA's Office of Water issued a National Combined Sewer
Overflow Control Strategy on August 10, 1989 (54 Federal Register 37370). This Strategy
reaffirmed that CSOs are point source discharges subject to National Pollutant Discharge
Elimination System (NPDES) permit requirements and to Clean Water Act (CWA) requirements.
The CSO Strategy recommended that all CSOs be identified and categorized according to their
status of compliance with these requirements. It also set forth three objectives:
• Ensure that if CSOs occur, they are only as a result of wet weather
• Bring all wet weather CSO discharge points into compliance with the technology-
based and water quality-based requirements of the CWA
• Minimize the impacts of CSOs on water quality, aquatic biota, and human health.
In addition, the CSO Strategy charged all States with developing state-wide permitting strategies
designed to reduce, eliminate, or control CSOs.
Although the CSO Strategy was successful in focusing increased attention on CSOs, it
fell short in resolving many fundamental issues. In mid-1991, EPA initiated a process to
accelerate implementation of the Strategy. The process included negotiations with
representatives of the regulated community, State regulatory agencies, and environmental groups.
These negotiations were conducted through the Office of Water Management Advisory Group.
The initiative resulted in the development of a CSO Control Policy, which was published in the
Federal Register on April 19, 1994 (59 Federal Register 18688). The intent of the CSO Control
Policy is to:
Provide guidance to permittees with CSOs, NPDES permitting and enforcement
authorities, and State water quality standards (WQS) authorities
1-2 August 1995
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Chapter 1 Introduction
• Ensure coordination among the appropriate parties in planning, selecting, designing.
and implementing CSO management practices and controls to meet the requirements
of the CWA
• Ensure public involvement during the decision-making process.
The CSO Control Policy contains provisions for developing appropriate, site-specific
NPDES permit requirements for all CSSs that overflow due to wet weather events. It also
announces an enforcement initiative that requires the immediate elimination of overflows that
occur during dry weather and ensures that the remaining CWA requirements are complied with
as soon as possible.
1.3 Key Elements of the CSO Control Policy
The CSO Control Policy contains four key principles to ensure that CSO controls are
cost-effective and meet the requirements of the CWA:
Provide clear levels of control that would meet appropriate health and environmental
objectives
Provide sufficient flexibility to municipalities, especially those that are financially
disadvantaged, to consider the site-specific nature of CSOs and to determine the most
cost-effective means of reducing pollutants and meeting CWA objectives and
requirements
Allow a phased approach for implementation of CSO controls considering a
community's financial capability
Review and revise, as appropriate, WQS and their implementation procedures when
developing long-term CSO control plans to reflect the site-specific wet weather
impacts of CSOs.
In addition, the CSO Control Policy clearly defines expectations for permittees, State
WQS authorities, and NPDES permitting and enforcement authorities. These expectations
include the following:
1-3 August 1995
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Chapter 1 Introduction
• Permittees should immediately implement the nine minimum controls (NMC), which
are technology-based actions or measures designed to reduce CSOs and their effects
on receiving water quality, as soon as practicable but no later than January 1, 1997.
• Permittees should give priority to environmentally sensitive areas.
• Permittees should develop long-term control plans (LTCPs) for controlling CSOs.
A permittee may use one of two approaches: 1) demonstrate that its plan is adequate
to meet the water quality-based requirements of the CWA ("demonstration
approach"), or 2) implement a minimum level of treatment (e.g., primary
clarification of at least 85 percent of the collected combined sewage flows) that is
presumed to meet the water quality-based requirements of the CWA, unless data
indicate otherwise ("presumption approach").
• WQS authorities should review and revise, as appropriate, State WQS during the
CSO long-term planning process.
• NPDES permitting authorities should consider the financial capability of permittees
when reviewing CSO control plans.
Exhibit 1-1 illustrates the roles and responsibilities of permittees, NPDES permitting and
enforcement authorities, and State WQS authorities.
In addition to these key elements and expectations, the CSO Control Policy also addresses
important issues such as ongoing or completed CSO control projects, public participation, small
communities, and watershed planning.
1.4 Guidance to Support Implementation of the CSO Control Policy
To help permittees and NPDES permitting and WQS authorities implement the provisions
of the CSO Control Policy, EPA is developing the following guidance documents:
• Combined Sewer Overflows—Guidance for Long-Term Control Plan (Publication
number 832-B-95-002)
• Combined Sewer Overflows—Guidance for Nine Minimum Control Measures
(Publication number 832-B-95-003)
• Combined Sewer Overflows—Guidance for Screening and Ranking (Publication
number 832-B-95-004)
1-4 August 1995
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Chapter 1
Introduction
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Chapter 1 Introduction
• Combined Sewer Overflows—Guidance for Monitoring and Modeling (Publication
number 832-B-95-005)
• Combined Sewer Overflows—Guidance for Financial Capability Assessment
(Publication number 832-B-95-006)
• Combined Sewer Overflows—Guidance for Funding Options (Publication number 832-
B-95-007)
• Combined Sewer Overflows—Guidance for Permit Writers (Publication number 832-B-
95-008)
• Combined Sewer Overflows—Questions and Answers on Water Quality Standards and
the CSO Program (Publication number 832-B-95-009)
1.5 Purpose of Manual and Target Audience
This guidance presents a process for screening and ranking CSSs with CSOs that have
adverse impacts on water quality, aquatic life, or human health. Its primary purpose is to give
NPDES permitting authorities (i.e., EPA Regions and States with approved NPDES programs)
a method of prioritizing the issuance of NPDES permits to communities with CSSs. A
secondary purpose is to give communities with multiple CSOs to multiple receiving water bodies
a tool for ranking CSOs. Ranking CSOs will give the communities a basis for allocating
resources to eliminate or control, in accordance with the CSO Control Policy, CSOs with the
most significant impacts and to maximize the environmental benefits achieved for the resources
expended. It can also help target monitoring needs. The screening and ranking process relies
primarily on information readily available for most CSSs, such as a general knowledge of known
or expected impacts from CSOs, estimates of CSO flows and their characteristics, and receiving
water characteristics.
This guidance is not designed or intended to be used as a tool to prioritize Federal
enforcement actions. Decisions to initiate an enforcement action are generally based on site-
specific data and information and in accordance with the NPDES permitting authority's
enforcement management system.
In this recommended screening and ranking process, the NPDES permitting authority uses
the available information to assess an individual CSS. The screening process involves two
1-6 August 1995
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Chapter 1 . Introduction
criteria. If the NPDES permitting authority determines through the screening process that the
CSS has a high likelihood of causing significant adverse impacts, the CSS may be assessed (i.e.,
scored) using the ranking process, which has seven criteria. Chapters 2 and 3 of this guidance
discuss the screening and ranking processes, respectively. They present each criterion, the
associated scoring, and the rationale for its use in the screening or ranking process. The scores
for all ranking criteria may be totaled to determine priorities.
NPDES permitting authorities should develop and issue NPDES permits for those
communities with the highest point totals and proceed, in order, to the communities with the
lowest point totals.
This guidance can also be used to rank individual CSO outfalls within a CSS, to identify
CSOs requiring prompt attention, to better allocate limited resources, and to prioritize any
necessary modifications under individual CSO permits. Ranking individual CSO outfalls is
particularly useful whenever resources or other constraints limit an NPDES permitting
authority's or a community's ability to address all of its CSS and CSO problems simultaneously.
In applying this recommended screening and ranking process, it is important to recognize
that, as stated in the CSO Control Policy,
EPA expects a permittee's long-term CSO control plan to give the highest
priority to controlling overflows to sensitive areas. Sensitive areas, as
determined by the NPDES authority in coordination with State and Federal
agencies, as appropriate, include designated Outstanding National
Resource Waters, National Marine Sanctuaries, waters with threatened and
endangered species and their habitat, waters with primary contact
recreation, public drinking water intakes or their designated protection
areas, and shellfish beds.
EPA also recognizes, however, that technical and financial constraints may limit a
permittee's ability to implement controls for all CSOs to sensitive areas at the same time. This
document can help establish priorities to phase in permitting efforts across multiple CSSs and
CSOs to many sensitive areas, as well as CSOs to less sensitive areas.
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Chapter 1 Introduction
1.6 Watershed Approach to Permitting
In response to the 1989 EPA National Combined Sewer Overflow Control Strategy, 30
States have received approval or conditional approval for CSO permitting strategies. EPA
expects States to evaluate the need to revise their CSO strategies for consistency with the 1994
CSO Control Policy. This represents an opportunity for NPDES permitting authorities to
reconsider their CSO permitting priorities in light of current or suspected environmental impacts,
watershed permitting initiatives, and other factors. States and EPA Regions should review these
strategies and establish appropriate permitting priorities for implementation of the CSO Control
Policy. In establishing CSO permitting priorities, the NPDES permitting authority should
consider factors such as the environmental impacts of CSOs (e.g., beach closings, human health
hazards, and potential risk to endangered species). The NPDES permitting authority should also
consider requiring immediate action for CSOs to areas that meet the CSO Control Policy's
definition of "sensitive areas." This document provides guidance on establishing permitting
priorities for CSSs and provides permittees with a tool for prioritizing individual CSOs within
their CSSs to allow for effective allocation of resources.
EPA encourages States to use a watershed approach to set permitting priorities. Under
a watershed approach, all surface water, ground water, and habitat stressors within a
geographically defined area are understood and addressed in a coordinated fashion, as an
alternative to addressing individual pollutant sources in isolation. To support States that want
to implement a comprehensive statewide watershed approach, the Office of Water has developed
guidance and training designed to assist communities and natural resource agencies that are
pursuing a watershed approach. One part of the effort is the release of the NPDES Watershed
Strategy. This Strategy encourages NPDES permitting authorities to evaluate water pollution
control needs on a watershed basis. The CSO Control Policy supports the goals of the NPDES
Watershed Strategy and urges communities to work with NPDES permitting authorities to
coordinate CSO control program efforts with other point and nonpoint source activities within
the watershed.
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Chapter 1 Introduction
Applying a watershed approach to the CSO control program is particularly timely and
appropriate since the ultimate goal of the CSO Control Policy is development of long-term CSO
controls that will provide for the attainment of WQS. Since pollution sources other than CSOs
are likely to be contributing to the receiving water and affecting whether WQS are achieved, the
NPDES permitting authority needs to consider and understand these other sources.
NPDES permitting authorities can use this document to prioritize other wet weather
sources, as well as CSOs. Assessing wet weather sources on a watershed basis will allow the
NPDES permitting authority to effectively allocate resources for the greatest improvement in the
quality of the receiving water bodies within the watersheds under its jurisdiction. For
watersheds with interstate consideration, the respective NPDES permitting authorities should
establish an ongoing dialogue to address mutual concerns for improving the watersheds' quality.
The CSO Control Policy promotes ongoing interaction between the NPDES permitting
authority and the permittees during CSO control program planning and implementation. Such
interaction is critical to the success of a CSO program and is important in the screening and
ranking process. As the NPDES permitting authority compiles available information for the
screening and ranking process, the permittee can also contribute valuable information.
1-9 August 1995
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CHAPTER 2
THE SCREENING PROCESS
To rank CSSs using this guidance, the NPDES permitting authority should first identify
through the screening process CSSs with the greatest likelihood of causing significant adverse
impacts. The screening can be based primarily on information available in documents recently
prepared by States under Sections 303(d) and 305(b) of the CWA. Supplemental information
can be obtained from sources such as State health departments, the National Oceanic and
Atmospheric Administration (NOAA), the United States Geological Survey (USGS), news
organizations, permittees, and consultants. (Table A-l in Appendix A lists the sources of
information obtained for 13 CSSs across the United States during a test of this screening and
ranking process.) If information necessary for the screening is not available, the screening
system should not be used.
2.1 Criterion 1
Does any CSO in the CSS discharge into a receiving water body recently listed in the
State's 303(d), 305(b), or other similar reports as not attaining use goals or as
having impacts that could be caused by CSOs?
• Yes - Assume CSOs are a contributing problem and proceed to the ranking
criteria, given in Chapter 3.
• No - Proceed to Criterion 2 of the screening process.
Rationale: Under Section 305(b) of the Clean Water Act, each State is required to
submit to EPA, on a biennial basis, a report that, among other things, describes the quality of
all surface waters within the State and provides recommendations regarding point and nonpoint
source control programs and actions to achieve the water quality goals of the Act. Under
Section 303(d) and EPA's implementing regulations, 40 CFR §130.7(b), each State is also
required to submit to EPA, again on a biennial basis, a list of water quality-limited segments that
still require total maximum daily loads (i.e., those waters that do not or are not expected to
attain water quality standards after implementation of technology-based or other controls). The
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Chapter 2 _^____ TTrg Screening Process
Section 303(d) lists also identify the pollutants of concern and, sometimes, the contributing
sources.
For many States, these reports and lists provide information adequate to identify water
bodies that do not attain applicable water quality standards, the nature of the impacts, and
possibly whether CSOs are a primary or probable source of these impacts. When a water body
receiving CSOs is listed as not attaining water quality standards or the goals of the Act because
of pollutants or effects typically associated with CSOs (e.g., high bacteria counts), States should
assume, absent information to the contrary, that CSOs contribute to the problem. In such cases,
the NPDES permitting authority should continue to evaluate the CSS using the ranking process.
Another set of lists developed by the States may also be of some limited use. These lists,
which were developed in 1989 or 1990 under CWA Section 304(1), identify waters not attaining
water quality standards or the goals of the Act. In addition, for waters impaired by point source
discharges of toxics, the lists identified the point sources of those pollutants. The Clean Water
Act does not require States to update these lists; nevertheless, they might be useful screening
devices in appropriate cases.
2.2 Criterion 2
Does other available information indicate that CSO-related adverse impacts might
be occurring and that permitting and a CSO control program might be a high
priority?
• Yes - The NPDES permitting authority should begin discretionary review of
other available information to indicate whether the CSS should be
included for evaluation using the ranking process. Proceed to the ranking
process given in Chapter 3.
• No - Infer that significant adverse CSO impacts do not occur and remove the
CSS from further consideration for prioritized action.
Rationale: This screening criterion provides the States and EPA Regions with the
flexibility to include in the ranking process those CSSs with CSOs to a receiving water body that
is not included in Section 303(d) or 305(b) reports. Under Screening Criterion 2, for example,
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Chapter 2 The Screening Process
the NPDES permitting authority may decide to include in the ranking process those CSSs in
which solid and floatable materials are discharged in close proximity to recreational waters or
raw sewage is discharged to commercial and recreational fishing areas, even if the water body
is not listed in the previously mentioned reports.
Note that removal of a CSS from the screening and ranking process at this stage does not
mean that it should be removed permanently from consideration in permitting and enforcement
actions. Removal simply means that control of the CSS should not be the primary focus of the
NPDES permitting authority. EPA expects that the NPDES permit for such a CSS, when
issued, will contain appropriate CSO requirements.
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CHAPTER 3
THE RANKING PROCESS
CSSs that are identified in the screening process as most likely to cause significant
adverse impacts should be ranked through a seven-criterion process using site-specific
information. Information needed for ranking may be available from many sources, including
NPDES permits, NPDES permit applications, 305(b) reports, and compliance and enforcement
reports. When adequate information cannot be obtained from these sources, new information
can be obtained from site visits or from other outside sources (e.g., consultant reports and data
from other agencies, such as USGS), as noted in more detail below. Information from outside
sources on the CSSs and CSOs under evaluation can be invaluable during the ranking process.
The NPDES permitting authority should make every reasonable effort to obtain the information
necessary to give each CSS a score under each ranking criterion. If a particular criterion does
not apply to a community (e.g., if a community has no dry weather overflows under Criterion
2), it should receive a score of zero.
In ranking individual CSOs, each individual score should be used. In ranking each CSS,
the CSSs that receive the highest point totals from the ranking process should be judged as likely
to cause the greatest impacts and should, in most cases, be the highest priority for NPDES
permitting. Clearly, this represents a simplistic approach to the ranking of CSSs for NPDES
permitting. EPA expects that additional analysis may be necessary and that in some cases it may
be desirable to compare systems using "second tier" scores to reflect additional impacts.
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Chapter 3 The Ranking Process
3.1 Criterion 1
If any CSOs within the CSS pose a direct risk to public health or contribute to the
non-attainment of designated uses on an ongoing basis, or if the potential impacts
from CSOs are significant to areas designated under Federal or State law as sensitive
or protected resources, assign points as listed below:
• Discharges to waters experiencing beach closings or where there is a significant
risk to public health from direct contact with pollutants in CSOs:
Score 250 points.
• Discharges to Outstanding National Resource Waters, National Marine
Sanctuaries, or waters with threatened and endangered species and their habitat;
public drinking water intakes or their designated protection areas; or shellfish
beds: Score 200 points.
Rationale: The primary purpose of this criterion is to identify CSSs with CSOs that
endanger public health and affect water quality. This criterion is assigned a high point total
because it addresses observed impacts often associated with CSOs. The high point score for the
first category in this criterion is consistent with the risks that the pollutants in CSOs pose to
public health. Potential impacts to the sensitive areas listed under the second category are
included because, as identified in the CSO Control Policy, they generally need the highest levels
of protection.
Information required to determine the score for this criterion is often available from State
and local public health officials, the NPDES permit, the NPDES permit application, and the
305(b) report. NPDES permit applications and permits contain the specific locations of CSO
outfalls. Commonly, 305(b) reports identify whether the use of a water body is impaired and
whether municipal sources are responsible; these reports may not give a specific location or
specifically identify CSOs as a contributing or primary cause of the impairment. However, if
the 305(b) report does not provide adequate information, an appropriate State agency often can
help in completing evaluations under this criterion. Local offices of State and Federal natural
resource management agencies (e.g., fish and game agencies or the U.S. Fish and Wildlife
Service) can provide information on sensitive resources.
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The Ranking Process
3.2 Criterion 2
If dry weather overflows (DWOs) occur within the CSS, score the following points
depending on the frequency of the DWOs:
• Chronic DWOs (i.e., they occur on a regular basis and are not caused by an
occasional blockage of a regulator by debris):
Score 150 points.
• Infrequent DWOs caused by infrequent maintenance:
Score 75 points.
Rationale: Dry weather flows include sanitary flows, industrial flows, and infiltration
from ground water. DWOs result when dry weather flow is discharged from a CSO outfall.
Many CSSs continue to have DWOs for a variety of reasons, including illegal connections to the
CSS causing flows that exceed the system's design capacities, plugging of underflow (dry
weather) screens, tidal or high stream flow intrusions, damaged or poorly designed flow-
regulating equipment, undersized interceptor sewers, and insufficient plant capacities. Ground
water may infiltrate into old, poorly designed, or poorly maintained CSSs, causing their design
capacities to be exceeded. Because DWOs are not diluted by storm water, they can cause
significant impacts in receiving waters.
NPDES regulations prohibit DWOs, and both the 1989 National CSO Control Strategy
and the 1994 CSO Control Policy target the expeditious elimination of all DWOs. Both
documents recommend that NPDES authorities take appropriate enforcement actions to eliminate
all such discharges and to ensure that all CSOs comply with technology-based and water quality-
based requirements of the CWA. This criterion has a relatively high maximum score (150
points) because DWOs are undiluted by storm water and, thus, are likely to cause impacts and
because DWOs are prohibited.
A CSS would automatically receive a score of 150 points if the DWOs are occurring
because of structural problems such as an undersized pipe. The score of 75 points addresses
infrequent DWOs that result from inadequate operation and maintenance programs and
3.3 August 1995
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Chapter 3
The Ranking Process
procedures. The owner/operator of the CSS should be able to mitigate or eliminate these DWOs
by implementing a more aggressive operation and maintenance program.
In many cases, the municipal and State personnel will know the dry weather status of a
system. In some cases, however, the CSS may not have been studied and may not be well
characterized. In these cases, the permittee will generally need to evaluate dry weather flows,
which can often be accomplished by relatively simple observations.
3.3 Criterion 3
Depending on the type of water body receiving the CSO, as well as the body's
turbulence and mixing characteristics (energy), score points according to the
following table:
Water Body Type
Estuarine and Wetland
Near-Shore Oceanic
Offshore Oceanic
Lakes and Ponds
River
Streams
Low
Energy
100 points
60 points
30 points
100 points
40 points
60 points
Medium
Energy
N/A
40 points
15 points
N/A
20 points
40 points
High
Energy
N/A
20 points
10 points
N/A
10 points
20 points
N/A = Not applicable
Rationale: Investigations done in North America and Europe provide information on the
relative susceptibility of various water body types to CSO and storm water impacts. Using this
information, water bodies most likely to suffer impacts from CSOs can be identified and
categorized based on two factors: type of water body (e.g., estuary, river) and its relative
energy (i.e., low, medium, or high). Water body energy describes the degree of turbulence and
mixing in the receiving water body. Water bodies that flow rapidly and have noticeable
turbulence will mix and flush more quickly than standing water systems and, therefore, are more
likely to disperse any pollutant loadings from CSOs before they cause substantial impacts. Thus,
3-4
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Chapter 3 The RankinS Process
flowing water systems with high energy receive proportionally lower scores than low energy
flowing systems and standing water systems. This criterion assumes that lakes and ponds are
always considered low energy due to minimal mixing.
Similarly, potential impacts to flowing waters are stratified because smaller flowing
systems (i.e., streams) may not as readily or rapidly flush themselves of accumulated sediments
and associated pollutants as would larger systems (i.e., rivers). Because systems with greater
sediment accumulation rates are more prone to environmental or human health impacts, they are
given more points than waters relatively less prone to sediment accumulation. This criterion can
contribute a maximum of 100 points to a system's total score, substantially lower than that
possible in each of the first two criteria. This is because the emphasis of this guidance is first
on actual or highly probable impacts to receiving water bodies, which are emphasized under the
first two ranking criteria, and then on potential impacts having a lesser degree of certainty,
which are evaluated under this and the next three criteria. If a CSS has CSOs occurring to more
than one type of water body with various energy levels, then scores for each receiving water
body are not combined. Rather, the CSS is assigned the score based on the receiving water
body and energy level with the highest point value.
Because of Regional differences relevant to the meanings of streams and rivers, etc., this
document does not define these terms. Instead, the NPDES permitting authority should provide
clear and appropriate definitions of all terms when using this guidance.
Information necessary for this criterion is generally contained in the NPDES permit. If
NPDES permits are not available or if additional information on the characterization of a
receiving water body is needed, information can generally be obtained from in-state offices of
the USGS or State water resources offices.
3-5 August 1995
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Chapter 3 ^ The Ranking Process
3.4 Criterion 4
If the measured or estimated proportion of the flow rate(s) of all CSO outfalls to the
receiving water flow rate (including CSO flow) in streams or rivers is:
• More than 50 percent:
Score 50 points.
• Twenty-five to 50 percent:
Score 30 points.
• Less than 25 percent:
Score 10 points.
Note that since the proportion of CSO flow rate(s) to receiving water flow rate
cannot be calculated for lakes and estuaries, they should automatically receive
30 points.
Rationale: This criterion continues the projection of probable impacts from CSOs to
water bodies begun in Criterion 3. It is based on the assumption that impacts increase as the
proportion of CSO flow increases relative to receiving water flow. It might be difficult to
evaluate the CSS under this criterion if flow information is lacking.
Authorized States and/or EPA Regional offices maintain enforcement or compliance
records for many CSOs. These records can provide information on CSO occurrences, volumes,
durations, and frequencies. When data are not available, Section 308 information requests or
new or revised permit requirements can, as appropriate, require monitoring programs to gather
needed information. Alternatively, the CSO flow can be estimated using one of several available
modeling approaches. A model can predict peak runoff flow rates resulting from recurring
precipitation rates for the watershed drained by the CSO. The approximate flow volume
discharged from the CSO outfall is then computed by subtracting the treatment capacity (i.e.,
flow conveyed to the POTW treatment plant) of the CSS from the sum of the projected peak
runoff and dry weather flow volumes predicted by the model.
Useful stream and river flow information may be available from the USGS network of
stream and river gage stations.
3-6 August 1995
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Chapter 3 ^ne Ranking Process
3.5 Criterion 5
If a drinking water intake is within 10 miles (downstream in flowing water systems)
of any CSO outfall in the CSS, score the following points:
• Within 5 miles:
Score 100 points.
• Between 5 and 10 miles:
Score 50 points.
Rationale: CSOs might contaminate drinking water supply systems and cause widespread
human health problems associated with pathogens or toxic materials. Most drinking water
treatment facilities with intakes located near CSO outfalls have developed various operational
and treatment strategies to avoid such problems. But unforeseen problems, including illegal new
connections or discharges of toxic wastes to the CSS, might occur, or new drinking water
intakes might be constructed. While routine treatment of drinking water supplies is likely to
protect public drinking water supplies from CSOs in most cases, impacts may still occur. Thus,
while the association between CSOs and impacts to drinking water sources may be rare, the
consequences may be rather severe. Therefore, this criterion yields a score of 100 points if the
intake is within 5 miles and 50 points if it is between 5 and 10 miles of a CSO outfall.
The information necessary for this criterion should be available at the State or local
public health agency offices or other State offices responsible for monitoring or regulating
drinking water intakes and drinking water supplies.
(Note: During the test of this guidance, this criterion was the only one to score zero for
every permittee tested. Where CSOs occur to salt or brackish water, the reason for this score
is obvious. Most of the other permittees included in this test have a long history of water
quality problems in the water bodies affected by CSOs. It is likely that drinking water supply
intakes are not located near CSO outfalls in such cases.)
3-7 August 1995
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ChaPter 3 The Ranking Process
3.6 Criterion 6
If the composition of wastewater flows prior to any CSO outfall (based on dry
weather flows) in the CSS includes:
• More than 50 percent industrial and commercial discharges or significant
individual sources of potentially toxic materials:
Score 50 points.
• Thirty to 50 percent industrial and commercial discharges or significant
individual sources of potentially toxic materials:
Score 25 points.
• Less than 30 percent industrial and commercial discharges or significant
individual sources of potentially toxic materials:
Score 0 points.
Rationale: This criterion uses the surrogate measure of CSO industrial/commercial
contributions to address the potential impact of CSOs on the quality of the receiving water body.
It is based on the following assumptions: (1) possible discharges to the CSS of potentially
hazardous materials, including oils, greases, and spilled materials, are greatest for industrial
users and intermediate for commercial users, (2) runoff volumes would be greatest from
industrial and commercial areas because of their high proportions of impervious surfaces and the
likelihood of runoff contamination is higher in these areas, and (3) most residential areas have
relatively higher rates of wet weather infiltration, lower traffic volumes, and thus lower
potentials for the release of toxic chemicals in significant quantities.
State agencies generally do not have the information needed for this criterion. Often, the
permittee's staff or consultant reports prepared for the permittee are the best sources of this
information. When this information is not otherwise available, USGS topographic maps can be
used to delineate the drainage basin. Then, land-use or zoning maps available for most cities
can be laid over the USGS maps, and the percent composition of the area can be delineated
using planimetry or a related method.
August 1995
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Chapter 3 The Ranking Process
3.7 Criterion 7
For any site-specific concern not addressed through the other criteria that is a major
concern to the NPDES permitting authority:
Score 0 to 200 points.
Rationale: This criterion recommends that the NPDES permitting authority increase the
score and rank of any CSS where special concerns not addressed in other criteria are attributable
to actual or potential impacts from the system. Permit writers can assign a score based on best
professional judgment and the relative impacts of the system. Concerns considered under this
criterion might include CSOs that threaten aesthetics or human health. For example, if
floatables from CSOs compromise the aesthetics in an area used for recreational boating, this
criterion might receive a score of 100. If the concern is a threat to human health (e.g., CSOs
entering streets or basements), a permit writer should assign a score of 200 for this criterion.
The value of this criterion was illustrated during the test of this guidance (see Appendix
A). If it were not for this criterion, the CSS for Sacramento, California, would have scored
only 50 points, primarily because Criteria 1 to 6 focus on impacts to receiving waters. For
Sacramento, however, CSO impacts on receiving waters appear to be relatively minor, but there
is a major problem with CSOs onto city streets and into homes and commercial basements in
the older sections of the city. Because of this impact to human environments, an additional
score of 200 points was assigned under this criterion.
3-9 August 1995
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APPENDIX A
TESTING OF THE GUIDANCE FOR SCREENING
AND RANKING COMBINED SEWER SYSTEMS
EPA tested the usability and effectiveness of the screening and ranking process for CSSs using
information available for 13 CSSs in 11 cities and 7 EPA Regions. All of the CSSs evaluated
were identified previously as causing serious water quality impacts. For most of these systems,
remediation is already underway or being planned. In brief, the evaluation determined that the
screening and ranking process described in this guidance provides useful information that is
relevant for ranking CSO problems of the 13 CSSs examined and is relatively easy to apply.
A.I Methods
Table A-l presents the locations of the CSSs examined in this evaluation and the source of each
major category of information used. EPA Headquarters and Regional offices provided
applicable NPDES permits, NPDES permit applications, enforcement and compliance reports,
305(b) reports, and other relevant information. State agencies also were contacted to obtain
additional needed information that was not available from EPA. Generally, enough information
was compiled by this point to allow complete evaluation of most CSSs through the first six
ranking criteria. In some cases, however, more detailed information had to be obtained from
the permittees and, sometimes, their consultants.
A.2 Results and Conclusions
Information in NPDES permits and in 305(b) reports, which are often available from EPA
Regional offices, was sufficient to complete the screening process for some CSSs. In all cases
but one, NPDES permits were useful in identifying specific CSO outfall locations for each CSS.
The 305(b) reports adequately identified specific use attainability problems in Connecticut,
Maine, Massachusetts, Michigan, New York, Oregon, and Pennsylvania, but CSOs were not
always shown as likely causes. Additional information about CSSs in Maine, Pennsylvania, and
California was necessary to confirm the occurrence of surface water impacts from CSOs or other
CSO-related problems. Using all ranking criteria generally required information from EPA,
State, and municipal sources (Table A-l).
A-l
-------
Appendix A
Testing of the Guidance for Screening and Ranking CSSs
Table A-l. Sources From Which Needed Information Was Acquired for Screening and
Ranking Process Criteria3
City
Region 1
Hartford, CT
Bridgeport, CT
South Portland, ME
Gloucester, MA
Holyoke, MA
Sources
for
Screening
Criterion
1
Sources for Ranking0
Criterion
2
Criterion
3
Criterion
4
Criterion
5
E
E
S
E
E
E
E
S
E
E
S
S
S
S
E
Sb
E
S
E
E
Sb
E
S
E
S
Sb
S
S
S
E
Region 2
Brooklyn, NY
E
P
P
P
P
P
Region 3
Philadelphia, PA
NPDES Permit
#0026662
Philadelphia, PA
NPDES Permit
#0026689
Philadelphia, PA
NPDES Permit
#0026671
Region 4
Chattanooga, TN
Region 5
Inkster, Ml
Region 9
Sacramento, CA
Region 10
Portland, OR
E
E
E
E
E
S
E
E
S
E
E
E
E
E
E
E
E
E
S
S
S
S
P
S
E
E
E
S
C
S
Criterion
6
Sb
S
P
S
E
C
P
P
P
S
C
E
E
E
E
E
S
E
E
P
P
P
S
S
P
Criterion
7
E
Key: E = EPA Regional Offices
S = State Agencies
P = Permittees
C = Consultants
If information for a criterion was obtained from more than one source, only the most local
source is given. Consultant reports obtained from the EPA Regional office are identified by
E and those obtained from a State agency are identified by S.
This information was acquired from a state-chartered utility group, which serves a number
of municipalities.
USGS offices in individual States provided stream flow information for municipalities that
discharge to flowing waters.
A-2
-------
Appendix A Testing of the Guidance for Screening and Ranking CSSs
Table A-2 summarizes the results of each screening and each ranking process for the 13 CSSs.
The test of this process suggested that the information most frequently needed to assess CSSs
seems to be readily available from the EPA Regional or State offices.
The screening and ranking process as described in this guidance was reasonably easy to follow
and provided useful information for ranking the severity of problem associated with CSSs. The
process proved general enough to allow assessment of all CSO problems encountered. In
addition it helped bring together valuable information and provided a useful method to evaluate
and rank environmental impacts typically associated with CSOs. All CSSs evaluated during this
test were identified previously as having CSO problems. By applying the techniques described
in this guidance, all CSSs were ranked for priority permitting, receiving scores ranging from a
high of 555 to a low of 250 points.
A-3
-------
Appendix A
Testing of the Guidance for Screening and Ranking CSSs
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REFERENCES
American Public Works Association. 1981. Urban Stormwater Management. Special Report
No. 49.
American Society of Civil Engineers. 1960. Design and Construction of Sanitary and Storm
Sewers. ASCE Manual of Engineering Practice, No. 37. New York, NY.
American Society of Civil Engineers. 1969. Design and Construction of Sanitary and Storm
Sewers. ASCE Manual of Engineering Practice, No. 37. New York, NY.
Chow, V.T., D.R. Maidment, and L.W. Mays. 1988. Applied Hydrology. McGraw-Hill Book
Company, New York, NY.
Elder, J.R. 1990. The regulation of CSOs and storm water in the United States. Pages 85-93
in H.C. Torno (editor). Urban Stormwater Quality Enhancement - Source Control,
Retrofitting, and Combined Sewer Technology. American Society of Civil Engineers,
New York, NY.
Ellis, J. B. (editor). 1989. Urban Discharges and Receiving Water Quality Impacts. Pergamon
Press, New York, NY.
Field, R. and R.E. Pitt. 1990. Urban storm-induced discharge impacts. Water Environment
and Technology 2(8):64-67.
Freedman, P.L. and J.K. Marr. 1990. Chapter 3 - Receiving water impacts. Pages 79-117
in P.E. Moffa (editor). Control and Treatment of Combined Sewer Overflows. Van
Nostrand Reinhold, New York, NY.
Nix, S. 1990. Mathematical Modeling of the Combined Sewer System. In Control and
Treatment of Combined Sewer Overflows. Peter E. Moffa (editor).
NOAA. 1963. Probable Maximum Precipitation in the Hawaiian Islands. Hydrometeorological
Report No. 39. Washington, DC.
NOAA. 1966. Probable Maximum Precipitation, Northwest States. Hydrometeorological
Report No. 43. Washington, DC.
NOAA. 1969. Interim Report, Probable Maximum Precipitation in California. Hydrometeoro-
logical Report No. 36, revised Oct. 1969. Washington, DC.
NOAA. 1977. Probable Maximum Precipitation Estimates, Colorado River and the Great Basin
Drainage. Hydrometeorological Report No. 49. Silver Spring, MD.
R-l
-------
_ References
NOAA. 1978. Probable Maximum Precipitation Estimates, United States East of the 105th
Meridian. Hydrometeorological Report No. 51. Washington, DC.
NOAA. 1982. Application of the Maximum Precipitation Estimates, United States East of the
105th Meridian. Hydrometeorological Report No. 52. Washington, DC.
NOAA. 1984. Probable Maximum Precipitation Estimates—United States between the
Continental Divide and 103th Meridian. Hydrometeorological Report No. 51. Silver
Spring, MD.
U.S. EPA. 1989. National Combined Sewer Overflow Control Strategy. 54 FR 37370. Office
of Water, U.S. Environmental Protection Agency, Washington, DC.
U.S. EPA. 1991. Guidelines for the Preparation of the 1992 State Water Quality Assessments
(305(b) Reports). Office of Water, Washington, DC.
U.S. EPA. 1991. Guidance for Water Quality-based Decisions: the TMDL Process. Office of
Water, U.S. Environmental Protection Agency, Washington, DC. EPA 440/4-91-001.
U.S. EPA. 1994. NPDES Watershed Strategy.
U.S. EPA. 1994. National Combined Sewer Overflow (CSO) Control Policy. 59 FR 18688.
Office of Water, U.S. Environmental Protection Agency, Washington, DC.
U.S. Soil Conservation Service. 1975. Urban Hydrology for Small Watersheds. Technical
Release No. 55, January 1975.
Viessman, Jr., W., G.L. Lewis, and J.W. Knapp. 1977. Introduction to Hydrology, Second
Edition. Harper and Rowe, New York, NY.
Water Pollution Control Federation. 1989. Combined Sewer Overflow Pollution Abatement,
Manual of Practice FD-17. Water Pollution Control Federation (Water Environment
Federation), Alexandria, VA.
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