EPA 

United States 

Environmental Protection 

Agency 

Office Of Water EPA 832-B-95-003 

(4204) May 1995 

Combined Sewer Overflows 

Guidance For Nine Minimum 

Controls 
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

MAY 3 1995 OFFICE OF 

WATER 

MEMORANDUM 

SUBJECT: Guidance 

FROM: Michael B. Cook, Director (4201) 

Office of Wastewater Management 

TO: Interested Parties 

I am pleased to provide you the Environmental Protection 

Agency's (EPA's) guidance document on the implementation of the 

nine minimum controls for correction of combined sewer overflows 

(CSOs). This document is one of eight being prepared to foster 

implementation of EPA's CSO Control Policy. The CSO Control 

Policy, issued on April 11, 1994, establishes a national approach 

under the National Pollutant Discharge Elimination System (NPDES) 

permit program for controlling discharges into the nation's 

waters from combined sewer systems. 

To facilitate implementation of the CSO Control Policy, EPA 

is preparing guidance documents that can be used by NPDES 

permitting authorities, affected municipalities, and their 

consulting engineers in planning and implementing CSO controls 

that will ultimately comply with the requirements of the Clean 

Water Act. 

The nine minimum controls are identified in the CSO Control 

Policy as minimum technology-based controls that can be used to 

address CSO problems without extensive engineering studies or 

significant construction costs, prior to the implementation of 

long-term control measures. This document has been prepared to 

provide guidance to municipalities on how to implement the nine 

minimum controls and how to document their implementation. 

Documentation should be completed as soon as practicable but no 

later than January 1, 1997. 

This guidance has been reviewed extensively within the 

Agency as well as by municipal groups, environmental groups, and 

other CSO stakeholders. I am grateful to all who participated in 

its preparation and review, and believe that it will further the 

implementation of the CSO Control Policy. 

If you have any questions regarding the manual or its 

distribution, please call Norbert Huang in the Office of 

Wastewater Management, at (202) 260-5667. 
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 

in 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 he 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. 
TABLE OF CONTENTS 

1 

2 

3 

4 

5 

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

1.1 Background ...................................... 1-1 

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 Goal of this Guidance ................................ 1-6 

1.6 The Nine Minimum Controls ........................... 1-6 

1.7 Concurrent Efforts .................................. 1-8 

1.8 Related Activities ................................... 1-8 

1.9 Documentation .................................... 1-9 

Proper Operation and Regular Maintenance Programs ................. 2-1 

2.1 Elements of a Proper Operation and Maintenance Program .......... 2-1 

2.2 Considerations .................................... 2-5 

2.3 Examples of Implementation ............................ 2-5 

2.4 Documentation .................................... 2-6 

Maximization of Storage in the Collection System ................... 3-1 

3.1 Control Measures .................................. 3-1 

3.2 Considerations .................................... 3-3 

3.3 Example of Implementation ............................ 3-3 

3.4 Documentation .................................... 3-4 

Review and Modification of Pretreatment Requirements ................ 4-1 

4.1 Control Measures .................................. 4-1 

4.2 Performance and Cost ................................ 4-3 

4.3 Considerations .................................... 4-3 

4.4 Documentation of Actions Taken ......................... 4-4 

Maximization of Flow to the POTW for Treatment ................... 5-1 

5.1 Examples of Control Measures .......................... 5-1 

5.2 Considerations .................................... 5-2 

5.3 Documentation .................................... 5-3 

i May 1995 
6 Elimination of CSOs During Dry Weather ........................ 6-1 

6.1 Control Measures .................................. 6-1 

6.2 Example of Implementation ............................ 6-4 

6.3 Documentation .................................... 6-5 

7 Control of Solid and Floatable Materials in CSOs ................... 7-1 

7.1 Methods for Removing Solids and Floatables from Combined Sewage ... 7-1 

7.2 Considerations in Removing Solids and Floatables from Combined 

Sewage.. ....................................... 7-8 

7.3 Methods for Removing Floatables from the Surface of the Receiving 

Water Body ...................................... 7-8 

7.4 Considerations in Removing Floatables from the Surface of the 

Receiving Water Body ............................... 7-10 

7.5 Methods to Prevent Extraneous Solids and Floatables from Entering the 

CSS .......................................... 7-12 

7.6 Considerations in Preventing Extraneous Solids and Floatables from 

Entering the CSS .................................. 7-12 

7.7 Documentation ................................... 7-14 

8 

9 

10 

Pollution Prevention Programs to Reduce Contaminants in CSOs .......... 8-1 

8.1 Control Measures .................................. 8-1 

8.2 Performance and Cost ................................ 8-4 

8.3 Considerations .................................... 8-4 

8.4 Example of Implementation ............................ 8-5 

8.5 Documentation .................................... 8-5 

Public Notification ...................................... 9-1 

9.1 Examples of Control Measures .......................... 9-1 

9.2 Performance and Cost ................................ 9-2 

9.3 Considerations .................................... 9-2 

9.4 Documentation .................................... 9-3 

Monitoring to Characterize CSO Impacts and the Efficacy of CSO Controls ... 10-1 

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R-1 

10.1 Examples of Characterization Measures .................... 10-1 

10.2 Performance and Cost ............................... 10-4 

10.3 Considerations ................................... 10-5 

10.4 Documentation ................................... 10-5 

ii May 1995 
LIST OF EXHIBITS 

Exhibit l-l Roles and Responsibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-5 

LIST OF FIGURES 

Figure 7-1. Baffles .......................................... 7-2 

Figure 7-2. Trash Racks ....................................... 7-4 

Figure 7-3. Static Screens ...................................... 7-5 

Figure 7-4. Examples of Catch Basin Modifications ...................... 7-6 

Figure 7-5. Nets ........................................... 7-7 

Figure 7-6. Outfall Boom ...................................... 7-9 

Figure 7-7. Skimmer Boats .................................... 7-11 

Figure 7-8. Floatable Material in New York City CSOs ................... 7-13 

iii May 1995 
ACRONYM LIST 

Acronym Term 

BAT 

BCT 

BMP 

BPJ 

CSO 

CSS 

CWA 

DWO 

EPA 

I/I 

LTCP 

NMC 

NPDES 

O&M 

POTW 

WQS 

Best Available Technology Economically Achievable 

Best Conventional Pollutant Control Technology 

Best Management Practice 

Best Professional Judgment 

Combined Sewer Overflow 

Combined Sewer System 

Clean Water Act 

Dry Weather Overflow 

Environmental Protection Agency 

Infiltration/Inflow 

Long-term Control Plans 

Nine Minimum Controls 

National Pollutant Discharge Elimination System 

Operation & Maintenance 

Publicly Owned Treatment Works 

Water Quality Standards 

iv May 1995 
CHAPTER 1 

INTRODUCTION 

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 aesthetic 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 May 1995 
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: 

l Ensure that if CSOs occur, they are only as a result of wet weather 

l Bring all wet weather CSO discharge points into compliance with the technologybased and water quality-based requirements of the CWA 

l Minimize the impacts of CSOs on water quality, aquatic biota, and human health 

from CSOs. 

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: 

l Provide guidance to permittees with CSOs, NPDES permitting and enforcement 

authorities, and State water quality standards (WQS) authorities 

1-2 May 1995 
Chupter I Introduction 

l Ensure coordination among the appropriate parties in planning, selecting, designing, 

and implementing CSO management practices and controls to meet the requirements 

of the CWA 

l 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: 

l Provide clear levels of control that would be presumed to meet appropriate health and 

environmental objectives 

l 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 

l Allow a phased approach for implementation of CSO controls considering a 

community’s financial capability 

l 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: 

l-3 May 1995 
Chapter I Introduction 

l 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. 

l Permittees should give priority to environmentally sensitive areas. 

l Perrnittees 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”). 

l WQS authorities should review and revise, as appropriate, State WQS during the 

CSO long-term planning process. 

l NPDES permitting authorities should consider the financial capability of permittees 

when reviewing CSO control plans. 

Exhibit l-l 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 has developed the following guidance documents: 

l Combined Sewer Oveflows - Guidance for Long-Term Control Plan (EPA 832-B-95- 

ow 

l Combined Sewer Oveq7ows - Guidance for Nine Minimum Controls (EPA 832-B-95- 

003) 

l Combined Sewer Oveflows - Guidance for Screening and Ranking Combined Sewer 

System Discharges (EPA 832-B-95-004) 

l-4 May 1995 
Chapter 1 Introduction 

Exhibit l-l. Roles and Responsibilities 

Permittee NPDES Permitting Authority NPDES Enforcement Authority State WQS Authorities 

l Evaluate and implement NMC l Reassess/revise CSO permitting . Ensure that CSO requirements and l Review WQS in CSO-impacted 

strategy schedules for compliance are receiving water bodies 

l Submit documentation of NMC incorporated into appropriate 

implementation by January I, 1997 l Incorporate into Phase I permits enforceable mechanisms l Coordinate review with LTCP 

CSO-related conditions (e.g., development 

l Develop LTCP and submit for NMC implementation and l Monitor compliance with 

review to NPDES permitting documentation and LTCP January I, 1997, deadline for l Revise WQS as appropriate: 

authority development) NMC implementation and 

documentation Development of site-specific 

l Support the review of WQS in l Review documentation of NMC criteria 

(SO-impacted receiving water implementation l Take appropriate enforcement 

bodies action against dry weather Modification of designated use to 

l Coordinate review of LTCP overflows 

l Comply with permit conditions components throughout the LTCP - Create partial use reflecting 

based on narrative WQS development process and l Monitor compliance with Phase I, specific situations 

accept/approve permittee’s LTCP Phase II, and post-Phase II permits - Define use more explicitly 

l Implement selected CSO controls and take enforcement action as 

from LTCP l Coordinate the review and revision appropriate Temporary variance from WQS 

of WQS as appropriate 

l Perform post-construction 

compliance monitoring l Incorporate into Phase II permits 

CSO-related conditions (e.g., 

l Reassess overflows to sensitive continued NMC implementation 

areas and LTCP implementation) 

l Coordinate all activities with 

NPDES permitting authority, State 

WQS authority, and State 

watershed personnel 

l Incorporate implementation 

schedule into an appropriate 

enforceable mechanism 

l Review implementation activity 

reports (e.g., compliance schedule 

progress reports) 

l-5 May 1995 
Chapter I Introduction 

l Combined Sewer Overflows - Guidance for Monitoring and Modeling (EPA 832-B-95- 

05) 

l Combined Sewer Ove@lows - Guidance for Financial Capability Assessment (EPA 

832-B-95-006) 

l Combined Sewer Overjlows - Guidance for Funding Options (EPA 832-B-95-007) 

l Combined Sewer Overjlows - Guidance for Permit Writers (EPA 832-B-95-008) 

l Combined Sewer Overjlows - Questions and Answers on Water Quality Standards and 

the CSO Program (EPA 832-B-95-009) 

1.5 Goal of this Guidance 

The goal of this document is to help the CSO community, particularly municipal public 

works officials or planning and engineering consultants, evaluate, understand, and implement, 

as well as document, the NMC. The examples presented in this document illustrate different 

measures available to address a particular control. Appropriate control measures will be sitespecific and a municipality may select from several available measures to effectively implement 

each minimum control. EPA encourages municipalities to be creative and to explore innovative 

and cost-effective measures in implementing the NMC to address their specific CSO problems. 

The NMC are not necessarily distinct and separate from one another. Many control measures 

can address and facilitate more than one of the controls at the same time (e.g., street sweeping 

can address both the “Control of Solids/Floatables” and the “Pollution Prevention” controls). 

With the assistance of this guidance document, municipalities with CSOs should plan and pursue 

control measures that can achieve the ultimate goal of reducing overall CSO impacts in a holistic 

manner. 

1.6 The Nine Minimum Controls 

As described in the CSO Control Policy, municipalities should immediately implement 

best available technology economically achievable (BAT) or best conventional pollutant control 

technology (BCT). At a minimum, BAT/BCT should include the nine minimum controls 

(NMC), which are determined on a best professional judgment (BPJ) basis by the NPDES 

permitting authority. The NMC are controls that can reduce CSOs and their effects on receiving 

l-6 May 1995 
Chapter 1 Introduction 

(NMC), which are determined on a best professional judgment (BPJ) basis by the NPDES 

permitting authority. The NMC are controls that can reduce CSOs and their effects on receiving 

water quality, do not require significant engineering studies or major construction, and can be 

implemented in a relatively short period (e.g., less than approximately two years). 

Implementation of the NMC is among the first steps a municipality is expected to take in 

response to EPA’s CSO Control Policy. EPA recognizes that many municipalities have made 

significant progress in implementing the NMC as a result of the 1989 CSO Strategy. 

The NMC are as follows: 

1. Proper operation and regular maintenance programs for the sewer system and CSO 

outfalls 

2. Maximum use of the collection system for storage 

3. Review and modification of pretreatment requirements to ensure that CSO impacts 

are minimized 

4. Maximization of flow to the POTW for treatment 

5. Elimination of CSOs during dry weather 

6. Control of solid and floatable materials in CSOs 

7. Pollution prevention programs to reduce containments in CSOs 

8. Public notification to ensure that the public receives adequate notification of CSO 

occurrences and CSO impacts 

9. Monitoring to effectively characterize CSO impacts and the efficacy of CSO controls. 

Each of the following chapters in this manual describes one of the NMC, its intended 

objectives, examples of control measures, considerations for implementation, and suggested 

documentation. In addition, where available, the chapters present case studies and performance 

and cost data. 

l-7 May 1995 
Chapter 1 Introduction 

1.7 Concurrent Efforts 

When evaluating and implementing the NMC, the municipality should also be undertaking 

the following activities: 

l Initiating the process to develop a long-term control plan (LTCP), including 

characterizing the CSS, CSOs, and receiving waters 

l Meeting with the NPDES permitting authority and State WQS authority to discuss: 

- The materials expected to document implementation of the NMC 

- Monitoring, regulatory, and planning requirements that will affect the preparation 

of the LTCP. 

1.8 Related Activities 

The NPDES permitting authority should undertake, among other efforts, the following 

activities: 

l Develop and issue Phase I NPDES permits requiring CSO communities to implement 

the NMC, within two years of notice from the NPDES permitting authority, but no 

later than January 1, 1997 

l Develop and issue Phase II NPDES permits requiring continued implementation of 

the NMC and implementation of an LTCP. 

If implementation of the NMC in Phase I and Phase II permits is determined to meet the 

technology-based requirements, the permit writer should not need to develop other technologybased effluent limitations. 

Therefore, implementing the NMC is among the first steps a municipality should take to 

reduce CSO impacts. Minimum controls are not temporary measures; they should be a part of 

long-term efforts to control CSOs. A community that has already implemented a CSO control 

program will likely have made substantial progress in implementing the NMC. Such a 

community is still expected to provide documentation to the NPDES permitting authority to 

l-8 May 1995 
Chupter 1 Introduction 

demonstrate how its program addresses each minimum control. The NPDES permitting 

authority should then evaluate the extent to which each minimum control is satisfied. 

The LTCP should describe the approaches for implementing and integrating the NMC 

into the long-term CSO control program. On a preliminary basis, the LTCP should describe the 

effectiveness of the NMC in reducing the frequency and magnitude of CSOs and in reducing 

impacts on receiving waters. Monitoring conducted under the NMC will likely provide such 

information as the number of overflow events or receiving water impacts, including fish kills 

or beach closures. Other impacts, such as pollutant load reductions and receiving water 

concentrations, will be ascertained through monitoring associated with LTCP development. 

1.9 Documentation 

The CSO Control Policy states that the municipality should submit to the NPDES 

permitting authority documentation on the implementation of the NMC. Documentation should 

include information that demonstrates: 

l The alternatives considered for each minimum control 

l The actions selected and the reasons for their selection 

l The selected actions already implemented 

l A schedule showing additional steps to be taken 

l The effectiveness of the minimum controls in reducing/eliminating water quality 

impacts. 

Each chapter of this manual presents examples of the information that should be 

documented for the minimum control presented in that chapter. The discussion is presented in 

the form of suggestions and objectives because each NPDES permitting authority (EPA Regional 

office or State agency) will likely have different implementation and documentation 

requirements. Meeting as early as possible with the NPDES permitting authority to determine 

its particular expectations will facilitate the NMC implementation process. 

l-9 May 1995 
Chapter I Introduction 

Generally, however, the documentation burden imposed by the NPDES permitting 

authority should be the minimum necessary to demonstrate that proper NMC measures are in 

place. The burden may vary according to the NPDES permitting authority’s customary practices 

and the municipality’s compliance record, among other factors. The NPDES permitting 

authority may choose to require the municipality to keep some records of NMC implementation 

on-site rather than requiring all documentation to be submitted. In these cases, NPDES 

inspectors can review NMC documentation that is on file during inspections. 

l-10 May 1995 
CHAPTER 2 

PROPER OPERATION AND REGULAR MAINTENANCE PROGRAMS 

The first minimum control, proper operation and regular maintenance of the CSS and 

CSO outfalls, should consist of a program that clearly establishes operation, maintenance, and 

inspection procedures to ensure that a CSS and treatment facility will function in a way to 

maximize treatment of combined sewage and still comply with NPDES permit limitations. 

Implementation of this minimum control will reduce the magnitude, frequency, and duration of 

CSOs by enabling existing facilities to perform as effectively as possible. Essential elements of 

a proper operation and maintenance (O&M) program include maintenance of suitable records 

and identification of O&M as a high management priority. 

The municipality should already have an established O&M program for its publicly 

owned treatment works (POTW). It may be very formal, with written manuals and operating 

forms and logs, or it may be informal, with few or no written manuals or established 

recordkeeping procedures. In either case, the steps involved in implementing this minimum 

control are the same: 1) assess how well the existing O&M program is being implemented, 

2) determine whether or not the O&M program needs to be improved to satisfy the intent of the 

CSO Control Policy, 3) develop and implement the improvements to address CSOs, and 

4) document any actions and report them to the NPDES permitting authority. 

2.1 Elements of a Proper Operation and Maintenance Program 

For the purposes of the CSO Control Policy, a proper O&M program generally should 

include the following: 

l The organizations and people responsible for various aspects of the O&M program 

l The resources (i.e., people and dollars) allocated to O&M activities 

l Planning and budgeting procedures for O&M of the CSS and treatment facilities 

2-1 May 1995 
Chapter 2 Proper Operation and Regular Maintenance Programs 

l A list of the facilities (e.g., tide gates, overflow weirs) critical to the performance of 

the CSS 

l Written procedures and schedules for routine, periodic maintenance of major items 

of equipment and CSO diversion facilities, as well as written procedures to ensure 

that regular maintenance is provided 

l A process for periodic inspections of the facilities listed previously 

l Written procedures, including procurement procedures, if applicable, for responding 

to emergency situations 

l Policies and procedures for training O&M personnel 

l A process for periodic review and revision of the O&M program. 

2.1.1 Organizational Structure 

The organizational structure can be shown with an organizational chart or other 

documents. The chart (or supplemental documents) should provide the names and telephone 

numbers of key personnel, the chain of command, and the relationships among various program 

components. In addition, the organizational structure should establish clear lines of 

communication, authority, and responsibility. 

2.1.2 Budget 

The O&M program records should show the resources currently available for O&M and 

the procedures for preparing and approving the annual O&M budgets. The budget should 

provide sufficient funds and personnel for routine O&M and a reasonable contingency amount 

for emergencies. Individuals responsible for day-to-day O&M should have the opportunity to 

participate in the budget preparation process so that the officials responsible for final budget 

preparation and approval are aware of O&M needs. 

2.1.3 Critical Facilities 

The O&M program should include an agreed-upon list of the most critical elements of 

the CSS and demonstrate that they receive an appropriate amount of attention. “Critical 

2-2 May 1995 
chapter 2 Proper Operah’on and Regular Maintenance Programs 

elements” are those facilities that affect the performance of the CSS, CSO volumes, or CSO 

pollutant levels. The list should include regulator structures, tide gates, pumping stations, 

diversion structures, retention basins, sections of the sewer system prone to sedimentation, all 

CSO outfalls included (or to be included) in the NPDES permit, and wastewater treatment plants 

if they are used to treat a significant portion of the wet weather flows. The list and 

supplemental documents should include a physical description of each facility and its location. 

2.1.4 Procedures for Routine Maintenance 

The existing O&M program for a particular POTW should include documentation of 

procedures for routine maintenance of the major elements of the CSS. Only the critical elements 

identified above need to be included to document implementation of this minimum control. The 

program should focus on preventative maintenance to avoid failures during critical times, such 

as a period of heavy rainfall. 

2.1.5 Non-Routine Maintenance and Emergency Situations 

The O&M program should describe response procedures for emergency situations, 

particularly those requiring funds outside the approved annual budget. The NPDES permitting 

authority will expect to see that response can be quick, without unnecessary processes and 

procedures. It would be a good practice to establish a protocol for responding to emergencies 

at night, on holidays, or on weekends. The protocol should include the names and telephone 

numbers of employees or others designated to respond to the emergency. 

Depending on the sensitivity of the receiving waters, the permittee might need to notify 

the NPDES permitting authority and the State public health agency during overflow events. It 

would be a good practice to maintain a list of people, organizations, and telephone numbers for 

appropriate regulatory agencies. 

2.1.6 Inspections 

The O&M program should describe the procedures for inspecting critical elements of the 

CSS. The NPDES permitting authority will expect the municipality to have an established 

2-3 May 1995 
Chapter 2 Proper Operation and Regular Maintenance Programs 

program for periodic inspections. The appropriate frequency of inspections will depend on the 

type of facilities, historical records of performance and failure, sensitivity of nearby surface 

waters to CSOs, adequacy of the maintenance program, and other factors. 

‘The O&M personnel should have check sheets, operating logs, and other easy-tocomplete forms readily available. The forms should prompt field personnel to check critical 

items, record their observations, and recommend corrective actions, if necessary. For example, 

an inspection should identify whether there has been an overflow, whether debris has 

accumulated and needs to be removed, whether the device would operate correctly during the 

next storm, and whether any items need repair. In addition, inspections could be conducted of 

regulator devices and interceptors, trunks, and combined sewers during dry weather for 

blockages, excessive deposition of solids, excessive infiltration/inflow, and structural 

deterioration that needs to be corrected. 

The municipality should also have an established process for the review of the completed 

inspection forms by supervisory and management personnel, submittal to the NPDES permitting 

authority. if required, and retention of the forms. In addition, the municipality should have a 

process for ensuring that necessary follow-up maintenance and repair actions, indicated by the 

inspection reports and operating logs, are scheduled and carried out. 

2.1.7 Training 

New employees should be trained in operation and safety procedures as soon as they 

begin duty, and opportunities for training and re-training of long-time employees should be 

available. Training includes an appropriate blend of classroom training and on-the-job training. 

The objective is to have well-trained employees who know their duties and how to report 

problems that require attention from CSS managers. EPA encourages the development of and 

adherence to a written policy on training. 

2-4 May 1995 
Chapter 2 Proper Operation and Regular Maintenance Programs 

2.1.8 Periodic Review of O&M Plans 

O&M practices should be reviewed periodically and modified as necessary. It is good 

practice to involve field O&M personnel in this process. The O&M plan will likely be revised 

after completion of the LTCP to include agreed-upon long-term CSO controls. (See Combined 

Sewer Oveflows - Guidance for Long-Term Control Plan. EPA, 1995c.) 

2.2 Considerations 

Frequent inspection, regular maintenance, and the timely repair of facilities, including 

tide gates and regulators, are cost-effective ways to improve the control of CSOs. The 

elimination of obstructions will increase the effective storage capacity of the system and the 

quantity of wet weather flows that can be delivered to the treatment plant. 

The effective organization of overall O&M operations, a specific commitment of 

resources for maintenance of the collection system, the assignment of sufficient personnel and 

equipment for inspection and maintenance at the appropriate frequency, and timely repairs might 

require increases in O&M budgets. In some cases, reorganization of the operational structure 

might be necessary. Ultimately, the effectiveness of an O&M program depends on the resources 

allocated and the extent to which the CSOs are caused by conditions that can be mitigated by 

O&M practices. 

2.3 Examples of Implementation 

The following list provides examples of O&M program approaches used by several 

different municipalities: 

l Lansing, Michigan, has 40 regulators that are inspected twice per week and 

immediately following any wet weather event. 

l Jersey City, New Jersey, has tide gates and 31 regulators that are inspected in 

sequence by two assigned crews, enabling each regulator to be inspected at least 

twice per month. Crews perform cleanings and minor repairs when possible. Each 

inspection is documented. 

2-5 May 1995 
Chapter 2 Proper Operation and Regular Maintenance Programs 

l Elizabeth, New Jersey, has a CSS with 41 regulators of varying design. All syphons, 

regulators, and tide gates in the system are inspected daily. All syphons are jetcleaned monthly. 

l New York City has more than 450 regulators that are inspected on a regular 

schedule. Certain regulators identified as critical are inspected more often. Pump 

stations, most of which have overflow points, are inspected daily. Of the 183 people 

who maintain these elements of the sewer system, about 50 are assigned specifically 

to regulator and tide gate maintenance and inspection, and the remainder are involved 

with pump station operation. The city also has a shoreline inspection program and 

has mapped all discharge points, including CSO outfalls, storm water outfalls, 

industrial outfalls, and highway drains. Several vessels patrol the shorelines on a 

regular basis. If a dry weather ovefflow is suspected or observed, the maintenance 

crews will attempt to correct the problem immediately. 

2.4 Documentation 

The following elements are examples of documentation that could be submitted to the 

NPDES permitting authority to demonstrate that appropriate O&M activities to reduce the 

impacts of CSOs have been considered and have been or will be implemented: 

l An identification of CSS components requiring routine operation and maintenance 

l An evaluation of operation and maintenance procedures to include regular 

inspections; sewer, catch basin, and regulator cleaning; equipment and sewer 

collection system repair, or replacement where necessary 

l An operation and maintenance manual and/or procedures for the CSS and CSO 

structures 

l Resources allocated (manpower, equipment, training) for maintenance of the CSS and 

CSO structures 

l A summary of inspections conducted and maintenance performed. 

2-6 May 1995 
CHAPTER 3 

MAXIMIZATION OF STORAGE IN THE COLLECTION SYSTEM 

As the second minimum control, maximum use of the collection system for storage means 

making relatively simple modifications to the CSS to enable the system itself to store wet 

weather flows until downstream sewers and treatment facilities can handle them. The 

municipality should evaluate more complex modifications (e.g., those requiring extensive 

construction) as part of the LTCP. 

The first step is to identify possible locations where minor modifications can be made to 

the CSS to increase in-system storage. O&M personnel should be able to identify these sites; 

the concurrent effort to characterize the system as part of the LTCP should also help. Possible 

modifications should then be analyzed to ensure that they will not cause other problems such as 

street or basement flooding. Modifications should be implemented and efforts documented for 

the NPDES permitting authority. 

3.1 Control Measures 

This section briefly discusses simple measures that can be implemented to increase the 

storage capacity of a CSS, thus decreasing the magnitude, frequency, and duration of CSOs. 

A number of these measures can also be applied to implementation of other minimum controls. 

For example, inspection and maintenance activities that increase the use of the collection system 

will also reduce dry weather overflows and increase flows to the POTW. 

l Collection System Inspection-This will enable identification of serious deficiencies 

that restrict the use of the system’s available storage capacity. Deficiencies that can 

be corrected by proper maintenance or structural repairs, or by modifications that do 

not require comprehensive engineering design and facility construction, should be 

remedied as soon as possible. For example, O&M staff can remove accumulations 

of debris or sediment and replace sections of pipe that are obviously undersized in 

relation to upstream and downstream line sizes. In addition, inspection programs can 

identify malfunctioning regulators or broken regulator weirs for repair. 

3-1 May 1995 
Chapter 3 Maximization of Storage in the Collection System 

l Tide Gate Maintenance and Repair-Leaking tide gates can admit significant 

volumes of water into the conveyance system, thereby occupying system storage and 

conveyance capacity that would otherwise be available during wet weather periods. 

A tide gate inspection and maintenance program can use sensors placed inboard of 

the gate to detect tidal intrusions during dry weather periods and alert maintenance 

crews. The sensors can also be used to detect dry weather overflows, which are 

addressed under a different minimum control. 

l Adjustment of Regulator Settings-Many regulating devices, with simple 

modifications, can be used to increase in-system storage of wet weather flows. In 

some cases, stop planks or brick/concrete weirs can be raised to increase in-system 

storage. In addition, interactive controls can be used to temporarily induce in-line 

storage of wet weather flows (e.g., a regulator setting can be manipulated 

automatically in response to depth or flow in an interceptor). 

l Retard Inflows-By using special gratings or Hydrobrakes (or comparable 

commercial devices), O&M staff can modify catch basin inlets to restrict the rate at 

which surface runoff is permitted to enter the system. Slowing inflow will enable the 

CSS to transport more flow overall by spreading out the flow over time. Eliminating 

the direct connection of roof drains and sump pumps to the collection system is also 

possible where sufficient land area is available for drainage. 

l Localized Upstream Detention-Using localized detention in appropriate upstream 

areas could provide effective short-term storage (e.g.. upstream parking areas could 

be used for temporary storage of some storm water during storm events). 

l Upgrade/Adjustment of Pump Operations at Interceptor Lift Stations-Increased 

pumping rates might be possible through repair, modification, or augmentation of lift 

stations. This would increase the available capacity in upstream portions of the 

system but would depend on the available hydraulic capacity of downstream portions 

of the collection system, as well as the processing capability of the POTW, to accept 

the increased flow rates. 

l Removal of Obstructions to Flow-This can include maintenance activities to 

remove and prevent accumulations of debris and sediment that restrict flow. Where 

flow obstruction is caused by sediment accumulations in sections with low gradients, 

sewer flushing might be an effective control measure. When a section of the 

conveyance system routinely accumulates sediment deposits at a substantial rate, 

design and installation of a permanent flushing station or an in-line grit chamber 

might be the most cost-effective approach and should be considered as part of the 

LTCP. 

3-2 May 1995 
Chapter 3 Maximization of Storage in the Collection System 

3.2 Considerations 

Maximizing the use of existing facilities is a cost-effective way to improve the level of 

CSO control without the difficulties associated with land acquisition, construction, and 

community impacts of some other control methods. Appropriate techniques, costs, and the 

degree of improvement will vary substantially with system characteristics. In cases where 

collection system maintenance has been neglected, where there are blockages or other hydraulic 

bottlenecks, or where excess capacity is available, corrective action may provide significant 

improvements in CSO control. 

Risk of upstream (street, basement) flooding goes up with increased use of the collection 

system for storage. The application of measures to expand storage capacity in the collection 

system will increase O&M requirements, and for some techniques (e.g., check dams with 

telemetering and real-time control) the increase may be significant. Storing wet weather flows 

within the collection system is likely to increase deposition through settling of suspended matter. 

Additional O&M may be necessary if subsequent flows do not resuspend and remove 

sedimentation. 

Topography and other site conditions will also limit the volume of combined sewage that 

can be stored in the system regardless of whether simple or more elaborate modifications are 

undertaken. For example, where the entire system is relatively flat, wet weather flows might 

back up relatively far into the head of the system. In addition, such a system would normally 

be designed with relatively large combined sewers to convey runoff away from city streets. As 

a result, an area with relatively flat topography can expect greater storage capacity in the 

collection system. An area with relatively steep slopes, on the other hand, would flood 

downstream areas before much of the upstream storage capacity could be used and, thus, would 

have limited storage capacity. 

3.3 Example of Implementation 

The city of Detroit installed inflatable dams in two long, large-diameter lines that extend 

from the collection system to the shoreline discharge point. The system layout prevented any 

3-3 May 1995 
Chupter 3 Maximization of Storage in the Collection System 

risk of upstream adverse effects, and installation was relatively straightforward and inexpensive. 

Detailed monitoring data are not available to quantify the benefits, but these devices are often 

effective in completely containing overflows from smaller storms and can reduce the number of 

overflows. Maintenance is minimal because contained flows drain back into the collection 

system following the storm, and no real-time operation of the devices is necessary. The dams 

simply provide more effective use of existing excess capacity within the system. 

3.4 Documentation 

The following elements are examples of documentation that could be submitted to the 

NPDES permitting authority to demonstrate the municipality’s efforts to implement this control, 

as well as the control’s effectiveness in reducing CSO impacts: 

l An analysis/study of alternatives to maximize collection system storage 

l A description of procedures in place for maximizing collection system storage 

l A schedule for implementation of minor construction associated with maximization 

of collection system storage 

l Documentation of actions taken to maximize storage 

l Identification of any additional potential measures to increase storage in the existing 

collection system, but which require further analysis, and which will be evaluated in 

hydraulic studies conducted as part of the LTCP. 

3-4 May 1995 
CHAPTER 4 

REVIEW AND MODIFICATION OF PRETREATMENT REQUIREMENTS 

Under the third minimum control, the municipality should determine whether 

nondomestic sources are contributing to CSO impacts and, if so, investigate ways to control 

them. The objective of this control is to minimize the impacts of discharges into CSSs from 

nondomestic sources (i. e., industrial and commercial sources, such as restaurants and gas 

stations) during wet weather events, and to minimize CSO occurrences by modifying inspection, 

reporting, and oversight procedures within the approved pretreatment program. Once 

implemented, this minimum control should not require additional effort unless CSS 

characterization and modeling indicate that a pollutant from a nondomestic source is causing a 

specific health, water quality, or environmental problem. 

This review can be conducted as part of a municipality’s pretreatment program. If a 

community does not have an approved local pretreatment program, it should still determine 

whether nondomestic sources are contributing to CSO impacts. A municipality with no known 

nondomestic sources should implement this minimum control by periodically reevaluating 

whether it has nondomestic discharges. All municipalities should provide documentation to the 

NPDES permitting authority on the assessment of nondomestic source impacts and on efforts to 

mitigate any impacts from such sources, as appropriate. 

4.1 Control Measures 

The following steps are appropriate for municipalities with local pretreatment programs 

as well as municipalities that receive nondomestic discharges but are not required to develop a 

formal pretreatment program. 

4.1.1 Inventory Nondomestic Discharges to the Combined Sewer System 

The municipality should first prepare an inventory of all nondomestic discharges to the 

collection system. The inventory should include information on the volume of flow and the 

pollutant types and concentrations in the discharge. By identifying the locations where 

4-1 May 1995 
Chapter 4 Review and Modification of Pretreatment Requirements 

nondomestic discharges enter the CSS on a map of the system, the potential impact of the 

nondomestic discharge on the CSO will be more clear. Municipalities with existing pretreatment 

programs should have all of this information readily available because as part of approved 

pretreatment programs, they are required to identify and locate all possible industrial users (in 

accordance with 40 CFR 403.8(f)(2)(i)). 

If the number of nondomestic users is large enough to preclude review of all facilities, 

the municipality should focus on the facilities with the greatest potential impact with regard to 

CSOs. This determination can be based on the size of the discharge, the concentration of 

pollutants that might be contributing to water quality criteria exceedances, or the proximity of 

the nondomestic user’s discharge point to the CSO outfall. 

4.1.2 Assess the Impact of Nondomestic Discharges on CSOs 

The second measure is to assess the impact of nondomestic discharges on CSOs by 

comparing the total quantity of nondomestic flow to the total flow from all sources. When 

nondomestic facilities are concentrated in certain areas, the comparison should be based on flows 

from areas contributing to specific overflow points. 

When appropriate, this assessment can also include the identification of nondomestic 

sources that are significant contributors of specific pollutants implicated in water quality 

problems. A more detailed assessment may be appropriate for cases in which nondomestic 

discharges contribute significantly to discharge volume and pollutant loading. 

4.1.3 Evaluate Feasible Modifications 

The third measure is to evaluate feasible modifications to the approved pretreatment 

program if the assessment indicates that nondomestic sources might contribute significantly to 

CSOs. Both the feasibility and the effectiveness of modifications are site-specific. The 

prohibition of batch discharges or a requirement for some form of detention to prevent 

discharges during wet weather events should be considered. Once such controls are in place, 

a procedure for scheduling releases might be necessary to avoid post-event overflows. If such 

4-2 May 1995 
Chapter 4 Review and Modification of Pretreatment Requirements 

procedures are necessary, scheduled releases can be included in the semi-annual monitoring 

reports of significant industrial users and the need for industrial slug discharge control plans 

required in 40 CFR 403.8(f)(2)(v). 

All POTWs with approved pretreatment programs are required to notify and obtain 

approval from the approval authority for all substantial pretreatment program modifications. 

Substantial modifications include changes to legal authorities, local limits (if made less 

stringent), and control mechanisms. In addition, POTWs with approved pretreatment programs 

must notify the approval authority of any nonsubstantial pretreatment program modification. 

Section 403.18 of the General Pretreatment Regulations contains more information on the 

requirements for pretreatment program modifications. 

4.2 Performance and Cost 

The degree to which pretreatment program modifications can reduce CSOs will be highly 

variable and site-specific. The costs for conducting an inventory of nondomestic sources and 

reviewing existing pretreatment program requirements are expected to be nominal because most 

of the required information is readily available. The affected nondomestic dischargers will incur 

most of the costs for implementing modified requirements. Where delayed-release volume 

control is employed, however, regulating and inspecting release schedules will add to the 

municipality’s O&M responsibilities. 

4.3 Considerations 

Industrial and commercial sites in CSO areas might have limited space available for 

temporary on-site storage of process wastewaters. Such situations might warrant development 

of appropriate release schedules and operational controls. 

Where the relative contribution of nondomestic flow to the total dry weather flow is 

small, or where the fraction of the CSS service area dedicated to nondomestic use is small, the 

effect of increasing pollutant control might be insignificant. When nondomestic users contribute 

4-3 May 1995 
Chapter 4 Review and Modification of Pretreatment Requirements 

a problem pollutant in a substantial quantity and effective pretreatment modifications are 

feasible, modification of the pretreatment program might improve CSO control significantly. 

4.4 Documentation of Actions Taken 

The NPDES permitting authority will need documentation demonstrating diligent effort 

to evaluate this control. The NPDES permitting authority will also need a clear understanding 

of the planned modifications and expected pollution control benefits. The following list provides 

suggested documentation: 

l If the municipality does not have any significant nondomestic dischargers or is not 

authorized to administer its own pretreatment program, it should provide information 

sufficient to substantiate this fact. 

l If the municipality does not have any significant nondomestic dischargers and is 

authorized to administer its own pretreatment program, it should provide: 

- An inventory of nondomestic dischargers 

- An assessment of the impact of nondomestic discharges on CSOs and receiving 

waters 

- An assessment of the value and feasibility of modifications to existing 

pretreatment programs. 

l If modification of the pretreatment program is appropriate, the municipality should 

provide the following information: 

- A description of the modification 

- A schedule for implementing the modifications, including amending sewer use 

ordinances, if needed 

- An estimate of the loading reduction expected from the modification in pounds 

of biochemical oxygen demand and suspended solids, or other pollutants of 

concern. 

l If modifications to the pretreatment program are not proposed, the permittee should 

provide justification. 

4-4 May 1995 
CHAPTER 5 

MAXIMIZATION OF FLOW TO THE POTW FOR TREATMENT 

The fourth minimum control, maximizing flow to the POTW, entails simple modifications 

to the CSS and treatment plant to enable as much wet weather flow as possible to reach the 

treatment plant. The objective of this minimum control is to reduce the magnitude, frequency, 

and duration of CSOs that flow untreated into receiving waters. Municipalities should identify 

and evaluate more complex CSS and POTW modifications as part of their LTCPs. 

5.1 Examples of Control Measures 

EPA suggests that the following minimum measures be considered in implementing this 

control: 

Determine the capacity of the major interceptor(s) and pumping station(s) that deliver 

flows to the treatment plant. Ensure that the full capacity is available by using the 

O&M suggestions presented in Chapter 2. 

Analyze existing records to compare flows processed by the plant during wet weather 

events and dry periods and determine the relationships between performance and 

flow. 

Compare the current flows with the design capacity of the overall facility, as well as 

the capacity of individual unit processes. Identify the location of available excess 

capacity. 

Determine the ability of the facility to operate acceptably at incremental increases in 

wet weather flows and estimate the effect on the POTW’s compliance with the 

effluent limits in its permit. Increased flows may upset biological processes, for 

example, and decrease performance for an extended period after the wet weather 

flows have subsided. 

Determine whether any inoperative or unused treatment facilities on the POTW site 

can be used to store or treat wet weather flows. 

Develop cost estimates for any planned physical modifications and any additional 

O&M costs at the treatment plant due to the increased wet weather flow. 

5-1 May 1995 
Chapter 5 Maximization of Flow to the POTW for Treatment 

5.2 Considerations 

Implementation of this control requires particular attention to regulatory considerations 

as well as treatment and capacity considerations. Although many POTWs have the physical 

capacity to accept increased flows during wet weather events, the following regulatory and 

technical issues must be addressed, however, in order to ensure that flow maximization provides 

a net environmental benefit. 

5.2.1 Regulatory Considerations 

POTWs are generally subject to EPA’s secondary treatment regulations (40 CFR 

Part 133). which specify numeric effluent limits for biochemical oxygen demand and total 

suspended solids, as well as a minimum removal percentage (85 percent) for secondary 

treatment. Secondary treatment requirements are enforceable conditions in POTW permits. 

Section 133.103(a) and (e), however, provide relief for POTWs with CSSs that process 

elevated flows (and more dilute influents) by allowing for the possibility of a waiver of the 

percentage removal requirement. (Waivers are not available from effluent concentration limits, 

however.) The decision to apply a waiver and the recalculation of the removal percentage are 

made on a case-by-case basis. 

The CSO Control Policy states that a bypass of secondary treatment may be justified 

when the LTCP identifies the cut-off point at which the flow will be diverted from secondary 

treatment and demonstrates that conveyance of wet weather flow to the POTW for primary 

treatment is more beneficial than other CSO abatement alternatives. Section 122.41(m) outlines 

the criteria under which a bypass may be allowed. 

5.2.2 Technical Considerations 

Maximizing the use of existing facilities to treat wet weather flows that would otherwise 

overflow without treatment is a desirable element of a control program, especially when CSOs 

to sensitive areas are eliminated or reduced. The more effectively existing facilities are utilized, 

the less total CSO control costs are likely to be under the LTCP. Some increases in the cost of 

5-2 May 1995 
Chapter 5 Maximization of Flow to the POTWfor Treatment 

operating the POTW can be expected, but the result is likely to be more cost-effective than 

control efforts at upstream overflow locations. 

Plant performance will degrade somewhat at a certain point because of the increased 

influent flow. The optimal volume of wet weather flow might be constrained by provisions of 

existing discharge permits and the ability to obtain modified provisions for increased flows 

during wet weather events. An engineering study will usually be necessary to determine the 

ultimate effects of increased flow on the plant’s treatment capacity and effluent quality. 

5.3 Documentation 

The municipality should submit documentation demonstrating a diligent effort to evaluate 

alternatives for increasing flow to the POTW. The municipality should also describe any 

measures being implemented. The following list provides some examples of documentation that 

could be submitted: 

l A description of any planned physical changes that are part of this control 

l A cost estimate and implementation schedule for each of the changes listed above 

l An estimate of the expected decrease in frequency and magnitude of CSOs and, when 

possible, an estimate of the loading reduction in pounds of biochemical oxygen 

demand and suspended solids, as well as other pollutants of concern 

l A description of the additional studies and analyses, if any, that will probably be 

performed during LTCP development. 

5-3 May 1995 
CHAPTER 6 

ELIMINATION OF CSOs DURING DRY WEATHER 

The fifth minimum control, elimination of CSOs during dry weather, includes any 

measures taken to ensure that the CSS does not overflow during dry weather flow conditions. 

Since the NPDES program prohibits dry weather overflows (DWOs), the requirement for DWO 

elimination is enforceable independent of any programs for the control of CSOs. DWO control 

measures include improved O&M (see Chapter 2), as well as physical changes to regulator and 

overflow devices as described in this Chapter. 

6.1 Control Measures 

Control measures that can be implemented to eliminate CSOs during dry weather flow 

conditions include inspection of the system to identify DWOs, correction of the DWOs, 

notification to the NPDES permitting authority when a DWO has occurred, and submittal of a 

description of the corrective actions taken. 

6.1.1 Identification 

In order to record and enumerate DWOs, a visual inspection program of sufficient scope 

and frequency is needed to provide reasonable assurance that any occurrence will be detected. 

Details of program methods and frequency of inspections will vary, depending on the size of the 

service area, characteristics of the CSS, number of overflow points, and past history of DWOs 

at particular locations. DWOs can be identified by O&M crews or the public. 

Regulators should be a principal focus of inspection activity since they are probably the 

most common originating point for DWOs. Inspection at accessible locations in the outfall line 

or at the outfall itself could be sufficient, however, if the tide or river stage does not obscure 

results. Because regulator mechanisms are subject to blockage or damage that might cause them 

to malfunction, they should be inspected repeatedly for the presence of DWOs. Observations 

should be scheduled to coincide with higher flow periods in the diurnal dry weather flow cycle. 

6-1 May 1995 
Chapter 6 Elimination of CSOs During Dry Weather 

O&M plans should include explicit procedures for inspecting DWOs. Although the 

frequency of inspections depends on site-specific factors, EPA recommends biweekly 

inspections, as well as inspections after wet weather events. Monthly observations might be 

adequate for potential DWO locations where the regulator mechanism is in good repair and the 

system has adequate hydraulic capacity to reduce the likelihood of DWOs in the absence of 

regulator malfunction. In contrast, at locations where the system’s hydraulic capacity is limited 

or where the regulator’s design, age, and state of repair are questionable, inspection several 

times a week may be warranted. 

A number of techniques can support visual inspection activities. Some techniques, such 

as chalking, block testing, or use of mechanical devices, can also be used to document CSO 

occurrences (refer to Chapter 10). These relatively simple techniques provide some flexibility 

when visual observation cannot be made. The physical presence of an observer on an 

appropriate schedule is still necessary to maintain and reset mechanical devices, however. 

For large systems that have many overflow points, automatic devices can indicate DWO 

events by activating each time the water level reaches a predetermined point. Such devices, 

when connected to a central location, can also be used as remote alarms for the maintenance 

crew. Installation of automatic detection devices, particularly for large CSSs, can significantly 

reduce the cost of visual observation and O&M. 

If an outfall is located far from a regulator, sanitary sewer connections might be 

downstream of the regulator chamber. In such a case, the presence of a DWO cannot be 

determined from observations at a regulator. This situation should be checked by reference to 

as-built drawings, by observations at appropriate manholes, or where possible, by direct 

observation of the outfall. 

Shoreline inspections can be made from a boat or on foot in cases where outfall pipes are 

exposed above the surface of the receiving water. Inspections should be conducted, therefore, 

at low tide in estuarine water bodies or at the low river stage in rivers subject to large variations 

in river stage. Personnel familiar with the locations of CSO outfalls and other permitted outfalls 

6-2 May 1995 
Chapter 6 Elimination of CSOs During Dy Weather 

should conduct inspections during periods of dry weather flow when CSO outfalls are expected 

to be dry. 

6.1.2 Correction of DWOs 

Dry weather overflows caused by operational problems can generally be alleviated by one 

or several of the following methods: 

l Adjustment of Regulator Settings-Population growth in an area tributary to a 

regulator can result in flows greater than the design flow of the regulator. Some 

regulators can simply have a gate adjusted/raised or a weir elevated to pass the peak 

dry-weather flow to the interceptor. In other circumstances, a regulator might have 

to be replaced. 

l Repair/Rehabilitation of Regulators-Frequently, regulators with hydraulically or 

mechanically actuated gates can become stuck in the bypassing position because of 

damage, deterioration, or inadequate maintenance. This may allow dry weather flows 

to enter the outfall. Simple repairs can correct some of these problems. 

l Maintenance of Regulators-The orifice through which dry weather flows pass from 

the regulator to the interceptor can become blocked with trash and refuse and result 

in a DWO. Routine inspection and maintenance will eliminate such blockages. 

Debris and relatively large items can be removed manually. Jet washing with a hose 

can remove grease, sediment, and fiber buildup from relatively small orifices. 

l Maintenance of Tide Gates-Tide gates can fail to close properly because of 

obstruction by trash or timber, corroded or warped gates, or deteriorated gaskets. 

As a result, receiving water can enter the CSS and increase the dry weather flow 

sufficiently to produce DWOs at downstream locations. Routine inspections, removal 

of obstructing debris, and prompt repair of defective tide gates can correct this cause 

of DWOs. 

l Interceptor Cleaning-Sediments, tree roots, and other items can restrict flow and 

result in DWOs at upstream locations in interceptors. Restrictions can be removed 

through sewer flushing, power rodding, balling, jetting, power bucket machines, or 

other common maintenance methods. 

l Sewer Repair-Ground water can enter the sewer system by infiltration and, when 

combined with peak sanitary sewage flow, can exceed the capacity of the regulator. 

Where specific DWO problem locations can be linked to defects in localized sewer 

segments, repair may be appropriate as a minimum control measure. For widespread 

infiltration problems, a comprehensive infiltration/inflow (I/I) control program would 

likely be a necessary component of the LTCP. 

6-3 May 1995 
Chapter 6 Elimination of CSOs During Dry Weather 

Unlike DWOs caused by operational problems, DWOs caused by structural problems (e.g., 

insufficient interceptor capacity) may require long-term construction that is addressed through 

the LTCP. 

6.1.3 Notification 

The municipality should establish a procedure to promptly notify the NPDES permitting 

authority that a DWO has occurred. The timing for such notification might vary given the 

characteristics of the CSS and the frequency of DWOs. NPDES regulations (40 CFR 

122.41(l)(6)) require NPDES permits to contain provisions that require permittees to report 

within 24 hours any noncompliance that can endanger health or the environment. 

The municipality should prepare and submit DWO summary reports at regularly 

scheduled intervals. EPA suggests a quarterly reporting schedule during the initial stages of 

CSO control to assist in documenting initial conditions and identifying trends. A less frequent 

basis might be appropriate after the first or second year, once the main features of the DWO 

situation have been established. These reports should document the DWOs that occurred during 

the reporting period, causes and problems noted by the inspections, corrective actions taken, 

results of such actions, and the status of ongoing inspection and remediation activities. 

6.2 Example of Implementation 

A study by the city of New York determined that DWOs from its CSS were caused 

primarily by clogging or blockage of regulators employing weirs, orifices, and drop pipes, as 

well as by mechanical failure of automatic regulators. The city implemented a regulator 

improvement program in 1988, based on a first phase effort that developed an inventory of the 

system and problems and recommended actions to reduce DWOs. Implementation of the 

program reduced DWOs by about 94 percent, from approximately 2 to 0.12 percent of the total 

dry weather flows (DEP 1991). Principal elements of the program were the reorganization of 

maintenance operations to clarify the responsibilities between treatment plant and collection 

system operations, an increase in collection system maintenance staff from 33 to 50, and the 

acquisition of additional vehicles and equipment (jet flushers, vactors). These actions, coupled 

6-4 May 1995 
Chapter 6 Eliminah’on of CSOs During Dry Weather 

with timely inspection and maintenance of regulators, have improved the city’s DWO problem 

dramatically. 

6.3 Documentation 

The following suggested documentation should demonstrate to the NPDES permitting 

authority a municipality’s efforts to correct DWOs: 

l A summary of alternatives considered and actions taken to identify and correct DWOs 

l A description of the procedures for notifying NPDES permitting authorities of DWOs 

and a summary of reports submitted 

l A summary of periodic reports on progress toward eliminating DWOs 

l A plan for complete elimination of all DWOs as part of the LTCP. 

6-5 May 1995 
CHAPTER 7 

CONTROL OF SOLID AND FLOATABLE MATERIALS IN CSOs 

The sixth minimum control is intended to reduce, if not eliminate, visible floatables and 

solids using relatively simple measures. Simple devices including baffles, screens, and racks 

can be used to remove coarse solids and floatables from combined sewage, and devices such as 

booms and skimmer vessels can help remove floatables from the surface of the receiving water 

body. In addition, as discussed in the next chapter, pollution prevention measures such as street 

sweeping can prevent extraneous solids and floatables from entering the CSS. 

Several other minimum controls (e.g., increased use of the collection system for storage 

and maximization of flow to the POTW) are also likely to reduce solids and floatables on an 

incidental basis. The NPDES permitting authority might require evaluation and implementation 

of some measures specifically aimed at reducing coarse solids and floatables in any CSOs. The 

LTCP will need to address the effectiveness of the minimum control measures and evaluate other 

methods (e.g., swirl concentrators and mechanically cleaned screens) for removing solids and 

floatables. 

7.1 Methods for Removing Solids and Floatables from Combined Sewage 

Several simple measures can be used to remove solids and floatables from combined 

sewage before they reach the receiving stream. These include baffles, screens, catch basin 

modifications, and nets. 

7.1.1 Baffles 

Floatables can be captured relatively easily within the collection system with baffles 

placed at overflow locations (Figure 7-1). The effectiveness of baffles will depend on the 

specific design of the diversion points for the overflows. Baffles are generally simpler than 

screens and other methods, and have lower capital and O&M costs. Their removal effectiveness 

is likely to be lower, however, because turbulence in the flow stream tends to entrain floatables, 

especially those that are relatively close to neutral buoyancy. 

7-1 May 1995 
Chapter 7 Control of Solid and Floatable Materials in CSOs 

Figure 7-1. Baffles 

May 1995 
Chapter 7 Control of Solid and Floatable Materials in CSOs 

7.1.2 Trash Racks 

A trash rack is a set of vertical bars designed to remove coarse and floating debris from 

CSOs (Figure 7-2). Trash racks are usually used to prevent floatables from exiting storm water 

detention ponds and from entering and clogging the pond outlet pipes. Trash racks can be used 

in a similar manner for CSO floatables, as long as enough outfall pipe or land space is available 

for a small structure and the outfall is high enough above the receiving water to facilitate regular 

maintenance. 

7.1.3 Static Screens 

Static screens (usually vertical bar racks) are manually cleaned screens similar to trash 

racks (Figure 7-3). Static screens are typically used in sewage treatment plants for preliminary 

treatment and at pump stations for the removal of debris to protect facility pumps and other 

internal working areas. They can be used to control coarse solids and floatables in areas where 

adequate construction space exists and where the outfalls are above the water level of the 

receiving water body to facilitate maintenance. 

7.1.4 Catch Basin Modifications 

Catch basin modifications include the installation of horizontal grating restrictions, catch 

basin outlet restrictors (e.g., hanging traps, hoods), and vertical throat restrictions (Figure 7-4). 

Restricting the amount of flow that enters the catch basins will also reduce the amount of street 

litter that enters the catch basin and the CSS. Before modifying catch basins, it is necessary to 

evaluate whether restricting the catch basin inflow rate will cause unacceptable street flooding. 

In addition, regular maintenance is necessary to remove trapped floatables and other debris from 

the catch basin. 

7.1.5 End-of-Pipe Nets 

Nets can be used to separate floatables from CSOs (Figure 7-5). In general, simple 

placement of a net across the face of an outfall is not practical because factors such as the 

discharge velocity and receiving water currents can threaten the integrity and influence the 

7-3 May 1995 
Chapter 7 Control of Solid and Floatable Materials in CSOs 

Access Screens --, 

a Combined 

Sewer Outfall 

ro 

Trash Rack ! 

SECTlON A-A 

AccessSaeens - 

PLAN 

Figure 7-2. Trash Racks 

May 1995 
Chapter 7 Control of Solid and Floatable Materials in CSOs 

Combined Sewer 

ToPOlW - 

ToPOlVV 

PLAN VIEW 

Combined Sewer 

Combined Sewel 

Overflow 

To 

POTW 

Dktwge 

Figure 7-3. Static Screens 

7-5 May 1995 
Chapter 7 Control of Solid and Floatable Materials in CSOs 

CUfb 

Street //xl\ 

/,x\\ 

Figure 7-4. Examples of Catch Basin Modifications 

May 1995 
Chupter 7 Control of Solid and Floatable Materials in CSOs 

Fbatable net 

Comtined Sewer Outfall 

Fbatabk baffle 

Side Curtain Pontoons 

----------T 

, \ 

, 

I I 

Nybn Net I Side Curtain I 

SIDE ELEVATION 

Figure 7-5. Nets 

7-7 May 1995 
Chapter 7 Control of Solid and Floatable Materials in CSOs 

efficiency of a netting system. Usually, a netting installation takes the form of an in-water 

containment area deflecting CSO flow through a set of netted bags. Floatables are retained in 

the bags and removed for disposal. The containment system should be sized to handle the 

volume and force of the CSO. Nets have the potential to work well in lake, tributary, or 

quiescent estuarine waters at least a few feet deep with an outfall at or close to the level of the 

water surface. Because these devices are constructions within the natural boundaries of the 

waterway, however, some NPDES authorities might not approve them. 

7.2 Considerations in Removing Solids and Floatables from Combined Sewage 

The principal advantage of the removal devices described in Section 7.1 is that they 

remove larger visible materials from CSOs. One or more of the illustrated screening methods 

could be considered as a control measure where physical site conditions permit. 

The principal disadvantage of these devices is the demand on existing O&M program 

personnel and budget resources for regular and timely maintenance to clean the screens and 

dispose of retained materials. Clogged screens will either result in unplanned discharges at other 

overflow points or produce backups, which cause street or basement flooding. Clogged screens 

will also cause head loss in the sewer system or act as a barrier in the system and cause 

surcharges. 

7.3 Methods for Removing Floatables from the Surface of the Receiving Water Body 

Solids and floatables can also be removed from the receiving water body after discharge. 

This section briefly describes two commonly-used devices. 

7.3.1 Outfall Booms 

Simple vinyl oil collection booms, or more elaborate containment systems with specially 

fabricated flotation structures and suspended curtains, can be placed in the water around outfalls 

to contain materials with positive buoyancy (which remain on the surface even in turbulent 

pipeline flows) and materials with neutral buoyancy (which will surface only under the relatively 

quiescent conditions of the containment zone) (Figure 7-6). Once contained behind booms, 

7-8 May 1995 
Chapter 7 Control of Solid and Floatable Materials in CSOs 

r Comtined Sewer 

Outfall 

IBoom 

I- Pilings 

PLAN VIEW 

Flotation Collar 7 

SECTION A-A 

PLAN MEW 

SECllON AA 

Figure 7-6. Outfall Boom 

7-9 May 1995 
Chapter 7 Control of Solid and Floatable Materials in CSOs 

floatables can be removed by hand, skimmer vessels, or vacuum trucks. Booming systems can 

also be deployed downstream of one or several outfalls in a river. 

Site-specific conditions should be considered in the evaluation, design, and placement of 

any boom system. Ambient water velocity, CSO exit velocity, provision for a stilling area, 

allowance for submerged material to rise to the surface, selection of a cleanup method, and the 

anchoring of the system are all important factors. Because booms are constructions within the 

natural boundaries of the waterway, however, some NPDES permitting authorities might not 

approve them. 

7.3.2 Skimmer Boats 

Skimmer boats remove floating materials within a few inches of the water surface and 

are being used in cities including New York, Baltimore, and Chicago (Figure 7-7). These 

vessels range from less than 30 feet to more than 100 feet in length. They can be equipped with 

moving screens on a conveyor belt system to separate floatables from the water or can lower a 

large net into the water to collect the materials. Skimmer vessels can be used in water bodies, 

including back embayments, lakes, reservoirs, and sections of harbors, where currents do not 

carry floatables away from the CSO outfall area. They might not be effective in areas where 

fast-moving river or estuary currents rapidly carry floatables downstream or where other 

conditions impede retrieval. Vessels can also be employed in open water areas where slicks 

from floatables form due to tidal and meteorological conditions. 

‘1.4 Considerations in Removing Floatables from the Surface of the Receiving Water 

Body 

Simple outfall booms are relatively inexpensive. If the shoreline geometry is favorable, 

they can be effective in preventing floatables from reaching areas of the water body of higher 

visibility and sensitivity. More elaborate containment systems, although much more expensive, 

might be appropriate if CSO outfalls are large but few in number. 

7-10 May 1995 
Chapter 7 Control of Solid and Floatable Materials in CSOs 

I- Off-Loading 

Conveyor 

r Control Cabin 

/- 

Pick-Up Conveyor 

4 Engine 

n-mL Propulsion Unit 

SIDE VIEW 

- D~llle 

r Off-Loading 

\ Conveyor 

Control Cabin 7 

r Pontoon ,- Pick-Up Conveyor 

l- Prop&&Unit L Engine L Pontoon wN7s 

TOP WEVV 

Figure 7-7. Skimmer Boats 

7-11 May 1995 
Chapter 7 Control of Solid and Floatable Materials in CSOs 

Skimmer boats are relatively expensive to purchase and operate. They might satisfy 

minimum technology criteria if they provide an alternative to the individual control of a large 

number of widely-dispersed CSO outfalls. In addition, skimmers might be a feasible alternative 

if geometry and currents make it possible to intercept the floatables before they reach sensitive 

waterfront areas and beaches. A single skimmer could be used in a cost-effective manner, for 

example, to clean several containment systems and to recover slicks in open waters. 

The principal disadvantage of booms and skimmer boats is that floatables enter the 

receiving water before removal. The more effective the containment, the more unsightly the 

appearance of the containment area. Containment can temporarily downgrade the conditions of 

the receiving waters between cleanings. Therefore, the systems must be cleaned frequently and 

as soon as possible following overflow events. As mentioned previously, capital and O&M costs 

for skimmer boats might exceed minimum technology criteria but provide a cost-effective interim 

program. 

7.5 Methods to Prevent Extraneous Solids and Floatables from Entering the CSS 

An extensive monitoring program conducted by the city of New York suggests that most 

floatables in CSOs (about 95 percent) originate as street litter. The remainder includes personal 

hygiene items flushed down toilets, which are some of the more objectionable material causing 

beach closings (Figure 7-8). 

Accordingly, source control programs that address the prevention or removal of street 

litter and the proper disposal of personal hygiene materials can contribute greatly to the control 

of floatables. The next chapter identifies practices to reduce the introduction of such materials 

into the CSS. 

7.6 Considerations in Preventing Extraneous Solids and Floatables from Entering the 

css 

Source control techniques for reducing floatables can offer a relatively cost-effective 

method for preventing floatable materials from appearing in overflows. Citizen action or 

7-12 May 1995 
Chapter 7 Control of Solid and Floatable Materials in CSOs 

r Vials (2.7%) 

r Utensils (3.4%) 

l-! Styrofoam Cups (4.9%) 

Juice Bottles (6.4%) Candy mappers (29.2 

Caps/Lids (7.2%) 

Plastic Bags/Fragments (18.2%) W 

Straws (15.9%) 

Plastic Items 

’ Cigarette Butts (12. 1%) 

Styrofoam (18.2%) 

All Floatables 

Note: Percentages based upon number of items 

Figure 7-8. Floatable Material in New York City CSOs 

7-13 May 1995 
Chapter 7 Control of Solid and Floatable Materials in CSOs 

education programs can also raise public awareness of the problems associated with CSOs and 

of the need for the broader control programs. 

7.7 Documentation 

The following list provides examples of documentation that could be submitted to 

demonstrate diligent effort in evaluating this minimum control and a clear understanding of the 

measures being implemented: 

l An engineering evaluation of procedures or technologies considered for controlling 

solid and floatable materials 

l A description of CSO controls in place for solid and floatable materials 

l A cost estimate (including resource allocation) and implementation schedule for each 

of the control measures being implemented 

l An estimate of the decrease in solids and floatables expected from the minimum 

control efforts 

l Documentation of any additional controls to be installed or implemented. 

7-14 May 1995 
CHAPTER 8 

POLLUTION PREVENTION PROGRAMS TO REDUCE CONTAMINANTS IN CSOS 

The seventh minimum control, pollution prevention, is intended to keep contaminants 

from entering the CSS and thus receiving waters via CSOs. Congress enacted the Pollution 

Prevention Act of 1990 to establish a national strategy for pollution prevention. Section 6602(b) 

of the Act establishes the following hierarchy for pollution management efforts: 

l Pollution should be prevented or reduced at the source whenever feasible. 

l Pollution that cannot be prevented should be recycled in an environmentally safe 

manner’ whenever feasible. 

l Pollution that cannot be prevented or recycled should be treated in an environmentally 

safe manner whenever feasible. 

l Disposal or release of pollution into the environment should be employed only as a 

last resort and should be conducted in an environmentally safe manner. 

The objective of this minimum control is to reduce to the greatest extent possible the 

amount of contaminants that enter the CSS. Most of the suggested measures involve behavioral 

change rather than construction of storage or treatment devices. 

8.1 Control Measures 

Pollution prevention measures such as street cleaning, public education programs, solid 

waste collection, and recycling can keep contaminants from entering the CSS. 

8.1.1 Street Cleaning 

Street litter can be removed by mechanical or manual street cleaning or by street flushing 

during dry weather periods. Daily street cleaning in critical areas might be necessary to 

significantly reduce CSO floatables. Street cleaning will not control litter from off-street areas. 

Parked cars prevent the removal of litter and other materials from curbsides. Enforced parking 

8-1 May 1995 
Chapter 8 Pollution Prevention Programs to Reduce Contaminants in CSOs 

regulations (e . g ., alternate side of street parking at different days of the week) and public 

awareness about the necessity of street cleaning are necessary for effective litter removal. 

8.1.2 Public Education Programs 

Anti-litter campaigns can reduce the amount of street litter and household items that enter 

CSOs and storm water outfalls. Public education programs can encourage the proper disposal 

of sanitary and personal hygiene items, which cause the greatest public concerns and can close 

beaches. Education programs can also advise the public about proper application of fertilizers, 

pesticides, and herbicides. 

Education methods can include public service announcements, advertising, stenciling of 

street drain inlets, and distribution of information with water or sewer bills. In addition, these 

programs can also include elements that focus on commercial and industrial establishments. 

8.1.3 Solid Waste Collection and Recycling 

Trash receptacles along city streets should reduce the amount of litter on streets, if the 

receptacles are properly placed, maintained, and cleaned. Street litter in some key densely 

populated areas can be reduced by collecting domestic curbside garbage more frequently. 

Recycling programs can reduce the amount of street litter. 

8.1.4 Product Ban/Substitution 

Many materials that foul beaches, including polystyrene, do not degrade in the 

environment. Some oceanfront communities have banned the sale of certain food products 

packaged with these materials. In various areas nationwide, cities and environmental groups 

have worked with businesses to eliminate the production and sale of fast food items packaged 

with these materials. 

8-2 May 1995 
Chapter 8 Pollution Prevention Programs to Reduce Contaminants in CSOs 

8.1.5 Control of Product Use 

Public facilities or public agencies can control the use of problem materials (e.g., 

fertilizer and pesticides in parks, application of de-icing salt in areas where discharges occur to 

fresh water bodies). 

8.1.6 Illegal Dumping 

Public education, notices in appropriate places, and enforcement programs can be used 

to control illegal dumping of tires, used motor oil, and other materials into waterways, storm 

drain inlets, catch basins, or onto the ground. 

8.1.7 Bulk Refuse Disposal 

Designated municipal disposal facilities accept materials such as home renovation debris 

that are not accepted by normal curbside garbage collection. Commercial establishments can 

be encouraged to accept used or waste materials including used crankcase oil, worn tires, and 

dead batteries. 

8.1.8 Hazardous Waste Collection 

Designated areas should be established, either on a permanent or periodic (annual or 

semi-annual) basis, where any type of household hazardous waste can be brought for collection 

and environmentally safe disposal. Permanent disposal sites can be established for collection of 

. hazardous wastes. 

8.1.9 Water Conservation 

Water conservation will reduce dry weather sanitary flow and increase the volume of 

combined sewage that can be retained in the CSS and treated at the POTW treatment plant. 

Water conservation at larger industrial facilities might reduce dry weather flow significantly. 

Unless dry weather flows represent a large portion of the combined sewer flow causing 

overflows, however, the effect of this activity might be limited. 

8-3 May 1995 
Chapter 8 Pollution Prevention Programs to Reduce Contaminants in CSOs 

8.1.10 Commercial/Industrial Pollution Prevention 

Municipalities should actively promote pollution prevention for commercial and industrial 

establishments located in their combined sewer areas. Such establishments, particularly those 

with waste oil or hazardous waste storage, can be required through the local sewer use ordinance 

or sewer use rules and regulations to develop and implement an appropriate pollution prevention 

plan and apply best management practices (BMPs) to minimize pollutant discharges into storm 

drains in the combined sewer areas. 

The EPA guidance, Storm Water Management for Industrial Activities: Developing 

Pollution Prevention Plans and Best Management Practices (EPA, 1992) can be used as a 

reference. Another EPA document, Municipal Wastewater Management Fact Sheets - Storm 

Water Best Management Practices (EPA, 1993), provides useful guidance on pollution 

prevention practices. 

8.2 Performance and Cost 

The degree to which pollution prevention can reduce contamination of receiving water 

bodies through CSOs is not known. In theory, the costs for each unit of pollution reduced 

through prevention should be less than it would be to collect and physically treat that same unit 

at the CSO. In some circumstances, however, source control measures sufficient to provide 

effective pollution control over a diffuse area could be more costly than control measures at CSO 

outfalls. For example, the effectiveness and overall costs for street cleaning depends on the 

frequency of cleaning, the number of cars on the street, the degree of enforcement of altemateside-of-the-street parking regulations, and the volume of litter. In some cases, it would be more 

cost-effective to screen CSOs at a centralized location than to clean the streets often enough to 

effectively control pollutants. 

8.3 Considerations 

Frequently, the actions that prevent or reduce the introduction of specific pollutants into 

a CSS will be cost-effective in reducing the amount of pollution discharged in CSOs. 

8-4 May 1995 
Chapter 8 Pollution Prevention Prowarns to Reduce Contaminants in CSOs 

Even in cases where pollution prevention measures provide limited tangible benefits, they 

can have two important ancillary benefits. Reductions in the quantity of pollutants entering the 

conveyance system will reduce the O&M effort on any overflow control that may be 

implemented as part of a CSO control program. In addition, public participation in pollution 

prevention programs will serve to heighten awareness of CSO issues and might increase public 

support for the overall program. 

The measures discussed above generally involve the cooperation of the general public. 

Many measures involve changes in such habits as to how materials are generated and disposed. 

The municipality can educate and encourage the public but will have limited control over the 

degree of implementation and, hence, limited control over the actual pollutant reductions. 

8.4 Example of Implementation 

Eugene, Oregon, has a comprehensive public outreach effort to raise community 

awareness of storm water management issues. This effort involves telephone surveys to 

determine community awareness, quarterly newsletters mailed to all city residents (more than 

69,000 copies of each issue), educational events, civic and club presentations, and handouts. 

Although Eugene’s effort focuses on storm water, similar efforts can be implemented to inform 

the public about CSO problems to improve the effectiveness of pollution prevention programs. 

8.5 Documentation 

The following list presents examples of documentation that could be submitted to 

demonstrate diligent effort in evaluating this minimum control and a clear understanding of the 

measures being implemented: 

l A summary of the alternatives considered 

l A list and description of the measures planned for implementation and the name of 

the individual or department responsible 

l A cost estimate and the implementation schedule 

8-5 May 1995 
Chapter 8 Pollution Prevention Programs to Reduce Contaminants in CSOs 

l An estimate of the benefits expected from the minimum control actions 

l Samples of the public educational materials planned for use 

l A list of pollution prevention plans that have been developed, if appropriate. 

8-6 May 1995 
CHAPTER 9 

PUBLIC NOTIFICATION 

The intent of the eighth minimum control, public notification, is to inform the public of 

the location of CSO outfalls, the actual occurrences of CSOs, the possible health and 

environmental effects of CSOs, and the recreational or commercial activities (e.g., swimming 

and shellfish harvesting) curtailed as a result of CSOs. Public notification is of particular 

concern at beach and recreation areas directly or indirectly affected by CSOs. Potential risk is 

generally indicated by the exceedance of relevant water quality criteria. 

The most appropriate mechanism for public notification will probably vary with local 

circumstances, such as the character and size of the use area and means of public access. The 

measure selected should be the most cost-effective measure that provides reasonable assurance 

that the affected public is informed in a timely manner. 

9.1 Examples of Control Measures 

The following list highlights potential measures for notifying the public about CSO 

events: 

l Posting at Affected Use Areas-Posting at the affected use areas (e.g., along a 

beach front) might be most appropriate when use restrictions are temporary. 

l Posting at Selected Public Places-Posting at selected public places (e.g., a public 

information center at a park or beach) might be appropriate in the case of longer-term 

restrictions or where a relatively narrow segment of the public is likely to be affected 

and can be reached via the public places selected for display. 

l Posting at CSO Outfalls-Posting at CSO outfalls is advisable where outfalls are 

visible and the affected shoreline areas are accessible to the public. 

• Notices in Newspapers or on Radio and TV News Programs-Newspaper or 

radio/TV notices might be appropriate for situations that are not routine or are 

unusually severe in terms of impact or public sensitivity, such as beach closings. 

9-1 May 1995 
Chapter 9 Public Notification 

l Letter Notification to Affected Residents-Letters to affected residents would be 

appropriate primarily for situations that reflect longer-term restrictions and that do 

not require prompt notification. This approach is most likely to reach all potentially 

affected parties and provides an opportunity to give more detailed information. 

l Telephone Hot Line for Interested Citizen Calls-A telephone hotline might be 

appropriate in cases where restrictions on a use (e.g., beach closures) occur relatively 

frequently, affect a large number of people, and might change daily. 

9.2 Performance and Cost 

As a minimum control, public notification actions have no direct effect on reducing 

overflows and pollutant loads from CSO systems, or on minimizing water quality impacts. 

Notification, however, will diminish the potential risk of adverse public health effects. Such 

actions will also increase public awareness and might increase public support for CSO control 

programs. 

The cost of an adequate public notification procedure will vary with the method(s) 

employed and with the size of the potentially affected population. In general, costs should be 

nominal. For example, many areas already have programs for beach postings. The media might 

provide newspaper or TV announcements as a public service. Letter notifications are usually 

appropriate only in a few situations. Although a telephone hot line might be more costly, this 

might be an effective public service in certain situations. 

9.3 Considerations 

The principal advantage of a notification program is the reduced exposure of the general 

public to potential public health risks. 

Limitations associated with this minimum control are related to the degree of assurance 

that the notification method(s) selected will provide the necessary information to the appropriate 

audience. Many municipal agencies probably have the staff and mechanisms for implementing 

this control; others will have to develop the necessary organizational arrangements and allocate 

other resources to comply effectively. 

9-2 May 1995 
Chapter 9 Public Not$ication 

Posting at CSO outfalls might be more difficult in cases where outfalls are on private 

property. 

9.4 Documentation 

Following are examples of documentation that could be used to demonstrate diligent 

effort in evaluating this minimum control and a clear understanding of the measures considered: 

l A list and description of the measures planned for implementation and the name of 

the individual or department responsible 

l The procedures or protocol for issuing notices 

l Samples of the public educational materials (e.g., circulars or notices) used or 

planned for use and a photograph of a typical sign, if applicable 

l A list of the locations where signs are posted (or will be posted) 

l A log of CSO occurrences and associated public notification. 

9-3 May 1995 
CHAPTER 10 

MONITORING TO CHARACTERIZE CSO IMPACTS AND THE EFFICACY OF 

CSO CONTROLS 

The ninth minimum control involves visual inspections and other simple methods to 

determine the occurrence and apparent impacts of CSOs. This minimum control is an initial 

characterization of the CSS to collect and document information on overflow occurrences and 

known water quality problems and incidents, such as beach or shellfish bed closures, that reflect 

use impairments caused by CSOs. Changes in the occurrences of such incidents can provide a 

preliminary indication of the effectiveness of the NMC. 

This minimum control is the precursor to the more extensive characterization and 

monitoring efforts to be conducted as part of the LTCP to assess changes in pollutant loadings 

or receiving water conditions. EPA’s manual Combined Sewer Overflows - Guidance for 

Monitoring and Modeling (EPA, 1995d) addresses monitoring and modeling program 

requirements associated with the LTCP. The manual provides detailed guidance on how to plan, 

design, and implement a monitoring program that will enable determination of pollutant loadings, 

receiving water quality impacts, and design of structural CSO controls to implement the LTCP. 

10.1 Examples of Characterization Measures 

This section describes how to characterize the CSS, determine the frequency of 

overflows, and identify CSO impacts. 

10.1.1 General Characteristics of the Combined Sewer System 

The municipality should first obtain maps, tables, and other general information on the 

characteristics of the system, including the layout of the CSS, the population served (including 

percent associated with the combined portion of the system), locations of CSO outfalls, and 

locations and designated uses (e.g., swimming, shellfishing) of receiving waters. This will 

10-1 May 1995 
Chapter 10 

Monitoring to Characterize CSO Impacts 

and the Efficacy of CSO Controls 

provide a spatial reference for records of overflows and use-related incidents developed under 

this minimum control. 

10.1.2 Overflow Occurrences 

The municipality should record the number of CSO overflows at as many outfalls as 

feasible. Small municipalities with few outfalls should be able to document overflows at each 

outfall. Large systems should work with the NDPES permitting authority to select a percentage 

of outfalls that represents the entire drainage area and sensitive locations. EPA’s monitoring and 

modeling guidance (EPA, 1995d) provides more detailed information on selecting an appropriate 

number of outfalls for monitoring. 

The municipality should record the date and time of each overflow event through visual 

observation or by an appropriately placed flow or level sensor. In addition, the municipality 

should measure and record the total daily rainfall, using a suitably placed rain gage. 

At a minimum, monitoring under this minimum control should develop information on 

the frequency of overflows at individual points in the system. EPA also recommends the 

development of information on the duration and magnitude of overflow events, where feasible. 

Such information can enhance the implementation of CSO controls and can enable measurement 

of the effectiveness of particular control measures. 

Monitoring of flow and quality at the level necessary to calibrate models and/or estimate 

pollutant loadings is addressed in EPA’s monitoring and modeling guidance (EPA, 1995d), as 

well as the monitoring/modeling section of the Combined Sewer Overflows - Guidance for LongTerm Control Plan (EPA, 1995c), and may be beyond the intended scope of minimum control 

monitoring. 

In cases where a calibrated model of the CSS exists (or when one becomes available), 

model projections may be used to determine the frequency and location of overflow events. 

10-2 May 1995 
Chapter 10 

Monitoring to Characterize CSO Impacts 

and the Efficacy of CSO Controls 

The following measures can be applied to detect overflows: 

l Visual Inspection-This requires the physical presence of an observer at each CSO 

point during each storm event. An extended presence during long rainfall events 

would be necessary, unless a history of the types of storm events producing 

overflows provides a reliable basis for selective visual inspection. This technique 

may be appropriate for very small CSSs with only a few outfalls. In general, 

however, some type of visual inspection aid will be necessary. 

l Visual Inspection with Inspection Aids-Techniques such as chalking and block 

testing can lower the personnel requirements for detecting overflows. Observations 

at each overflow location are necessary before and after a forecasted event. 

- A chalk mark can be drawn on a wall of a regulator chamber, leading, for 

example, from the top of an overflow weir to the top of the chamber. The mark 

will wash off when the water level rises high enough to overflow the weir. This 

provides a crude indication of maximum water level. 

- Wood blocks are placed in positions where an overflow will displace them. The 

blocks can be anchored with retrieval lines for repeated use. This technique is 

particularly suitable for use with weirs. 

- Simple mechanical or electrical counting devices, designed to be activated by 

water level or movement of a valve or gate, are installed at appropriate locations. 

The device is triggered each time some physical condition associated with an 

overflow takes place. Inspection before and after every event is not necessary to 

develop an accurate count of the number of overflow events. 

l Automatic Measurement-Automatic monitoring equipment records the output on 

strip charts and provides the output in digital form. This reduces personnel 

requirements but can add significantly to monitoring costs. The equipment is 

relatively sophisticated and requires a knowledge of the system’s hydraulic 

characteristics. In cases where automatic flow or level sensing devices are used with 

automatic samplers, monitoring efforts may be coordinated with a sampling program. 

Automatic devices can be either connected to an electrical power supply or battery 

operated, and backup power supplies should be provided. Automatic devices can be 

installed at remote or difficult-to-reach locations to reduce personnel requirements. 

- Velocity meters can be placed in outfall lines and will record results on strip 

charts or data cards. They are useful in less-than-ideal conditions, including 

outfall lines with leaking tide gates or uncontrolled discharges. They can be 

calibrated to distinguish tidal velocities from velocities during overflow events. 

The flow direction can also be determined with appropriate units. Multiple 

10-3 May 1995 
Chapter IO 

Monitoring to Characterize CSO Impacts 

and the Efficacy of C’S0 Controls 

meters, set at different depths and installed along with a level recorder, have been 

used to monitor discharge quantities in very large outfall lines. 

- Level recorders may be placed in manholes or pipes or behind weirs to provide 

information on overflow depth. When properly calibrated for the site and with 

the level recorded in real time on strip charts, the magnitude and duration of an 

overflow can be determined. Some devices store the data in memory for later 

downloading to a personal computer for analysis of data and preparation of 

reports. 

10.1.3 Incidents Relating to CSO Impacts 

The municipality should develop a routine report to record and summarize information 

available from other sources (e.g., the Coast Guard, local volunteer groups) on the water quality 

or use of waters affected by the CSOs. The report may include information on the following 

activities: 

l Beach closings or postings 

l Shoreline washup of floatables 

l Fish kills 

9 Hazards to small boat navigation 

l Street/basement flooding. 

10.2 Performance and Cost 

The inspection and reporting activities involved in implementation of this control will 

generally be applicable to other minimum controls. Therefore, limited incremental costs or 

additional personnel requirements are expected. 

This minimum control should provide useful information on the general performance of 

the CSS and the effect of control measures. It also will assist in characterizing the nature and 

relative severity of receiving water impacts. 

As part of the LTCP, the municipality will probably need to develop a comprehensive 

monitoring program. The monitoring approaches described here should be incorporated into this 

104 May 1995 
Chapter 10 

Monitoring to Characterize CSO Impacts 

and the Efficacy of CSO Controls 

comprehensive monitoring program. The cost of such a program will vary widely with location, 

depending on the size and characteristics of both the combined sewer system and the affected 

water bodies. 

10.3 Considerations 

The information collected under this control should provide a perspective on existing 

conditions and a basis for identifying progress that has been achieved by the application of other 

minimum control measures. Reports of receiving water impacts will assist in providing some 

indication of the actual, potential, or suspected adverse impacts due to CSOs. Furthermore, if 

a comprehensive inspection or monitoring program is already part of an LTCP, it might be 

considered adequate to meet the objectives of this minimum control. 

Potential limitations include the cost of monitoring programs and the possibility that 

overly ambitious monitoring requirements might compete for resources otherwise available for 

implementation of CSO controls. An appropriate balance should be the objective in a11 cases. 

It is essential, however, that a monitoring program measure existing conditions and assess the 

performance of the minimum control measures. 

10.4 Documentation 

The municipality should consider the following items for inclusion in documentation for 

the NPDES permitting authority to demonstrate diligent effort in implementation of this 

minimum control: 

l Identification of CSO locations in the CSS 

l A summary of observed incidents (i.e., the number and location of overflow events. 

as well as duration, volume, and pollutant loadings, if available) 

l A summary of existing water quality data for receiving water bodies 

l A summary of receiving water impacts that are directly related to CSOs (e.g., beach 

closing, floatables wash-up episodes, fish kills) 

10-5 May 1995 
Chapter 10 

Monitoring to Characterize CSO Impacts 

and the Efficacy of CSO Controls 

l An assessment of the effectiveness of any CSO control measures already implemented 

(e.g., reduction of floatables, fish kill incidents) 

l Development of a long-term monitoring plan for the LTCP, as appropriate. 

May 1995 
ADDITIONAL REFERENCES 

New York City Department of Environmental Protection. 1991. Regulator Improvement 

Program: Annual Analysis of Bypassing. 

U.S. Environmental Protection Agency. 1992. Storm Water Management for Construction 

Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 

832-R-92-005. 

U.S. Environmental Protection Agency. 1992. Storm Water Management for Industrial 

Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 

832-R-92-006. 

U.S. Environmental Protection Agency. 1993. Investigation of Inappropriate Pollutant Entries 

Into Storm Drainage Systems: A User’s Guide. EPA 600/R-92/238. 

U.S. Environmental Protection Agency. 1993. Municipal Wastewater Management Fact 

Sheets - Storm Water Best Management Practices. EPA 832-F-93-013. 

U. S. Environmental Protection Agency. 1993. Manual - Combined Sewer Overflow Control. 

EPA 625/R-93/007. 

U.S. Environmental Protection Agency. 1995a. Combined Sewer Overflows - Guidance for 

Financial Capability Assessment (EPA 832-B-95-006) 

U.S. Environmental Protection Agency. 1995b. Combined Sewer Overflows - Guidance for 

Funding Options (EPA 832-B-95-007) 

U.S. Environmental Protection Agency. 1995c. Combined Sewer Overflows - Guidance for 

Long-Term Control Plan (EPA 832-B-95-002) 

U. S. Environmental Protection Agency. 1995d. Combined Sewer Overflows - Guidance for 

Monitoring and Modeling (EPA 832-B-95-005) 

U.S. Environmental Protection Agency. 1995e. Combined Sewer Overflows - Guidance for 

Nine Minimum Controls (EPA 832-B-95-003) 

U. S. Environmental Protection Agency. 1995f. Combined Sewer Overflows - Guidance for 

Permit Writers (EPA 832-B-95-008) 

U.S. Environmental Protection Agency. 1995g. Combined Sewer Overflows - Guidance for 

Screening and Ranking Combined Sewer System Discharges (EPA 832-B-95-004) 

U. S. Environmental Protection Agency. 1995h. Combined Sewer Overflows - Questions and 

Answers on Water Quality Standards and the CSO Program (EPA 832-B-95-009) 

R-1 May 1995