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Combined Sewer Overflow (CSO)

Diagram showing sewer overflow pollution
Fig. 1 Image: Moundsville WWTP, WV

A combined sewer system (CSS) collects rainwater runoff, domestic sewage, and industrial wastewater into one pipe. Under normal conditions, it transports all the wastewater it collects to a wastewater treatment plant (WWTP), where it receives treatment, before it is then discharged as treated effluent to a nearby waterbody. During wet weather (e.g., rainfall events or snowmelt), the volume of combined wastewater can sometimes exceed the capacity of the CSS or WWTP. When this occurs, untreated or partially treated combined wastewater discharges from an outfall directly to nearby streams, rivers, and other water bodies. This is called a combined sewer overflow (CSO).

CSO Impacts and Advisory

Discharges from CSO outfalls may contain bacteria or other pollutants, that may cause illness or other environmental hazards.

CSO discharges can cause:

  • Human and animal health hazards
  • Water quality impacts
  • Bathing beach closures due to bacterial contamination
  • Aesthetic impacts due to floating debris or oil slicks
  • Shellfishing bed closures
  • Algae growth
  • Reduced oxygen levels in the water

Facilities with permitted CSO discharges are required by their State Pollutant Discharge Elimination System (SPDES) permit to post signs at all CSO outfalls to alert the public that the water may be contaminated with untreated sewage after a rainfall event. During or following a significant rainfall or snowmelt event, the public should avoid contact or recreation (swimming, boating, and fishing) within the waterbody that has CSO outfalls.

CSO's in New York State

CSS are typically found in older cities, along large waterbodies that are navigable to support commerce. Most of the CSO permittees in the New York are clustered in older industrial areas. Each CSS is required to have coverage under a State Pollutant Discharge Elimination System (SPDES) permit, which is issued by DEC.

There are currently 45 communities in New York State that are permitted to own and operate a CSS. A small fraction of these have eliminated or closed their CSO outfalls, yet still are a CSS. In addition, 5 CSO permits have been issued to regional wastewater treatment plants (WWTPs), which do not own any traditional CSO outfalls within the tributary collection system. The metropolitan areas of New York City, Buffalo, Syracuse, and Albany own and operate 74% of the approximately 800 CSO outfalls in New York State. The New York City metropolitan area currently makes up about 50% of these outfalls. The "Albany Pool," which consists of the Cities of Albany, Cohoes, Rensselaer, Troy, Watervliet, and the Village of Green Island, owns and operates 11% of the total CSO outfalls.

The DEC Info Locator contains a layer that shows the locations and other information about each of the permitted CSO outfalls in New York State.

Reporting CSO Events

DEC and CSO permittees use multiple methods of reporting CSO events, including:

  • Sewage Pollution Right to Know (SPRTK) - CSO discharges must be reported to the public using NY-Alert, the NYS electronic notification system. Anyone can sign up for email or text notifications from NY-Alert and are able to receive these notifications as they are reported by each community.
  • Annual Reports - As required by their SPDES permit, each CSO community must submit a CSO Annual Report to DEC, for which DEC uses the information from the report to monitor compliance and the permittees' progress on the implementation of their CSO abatement projects. As of August 2022, DEC has transitioned to the use of the agency's nForm Portal for electronic filing of this annual report. For submission of the nForm, a Certification Statement is required to be signed and uploaded. Download the statement (PDF)
    • On 10/26/2022, NYSDEC recorded a virtual webinar to provide an overview of CSO program updates and demonstrate the new electronic submission of the Annual Report via nForm. Watch the recorded webinar.
  • Statewide CSO Annual Report - Each year, NYSDEC collects and summarizes data received in each municipality's CSO BMP annual report to produce the statewide report. This report includes overall CSO program implementation status, quantification of LTCPs, CSO Outfalls, and CSO permits. Community-specific data & information for CSO program progress is also included. The 2021 Statewide CSO Annual Report (PDF).

What is DEC Doing to Control CSO Discharges?

DEC and CSO permittees use several methods to perform CSO abatement work to reduce or eliminate the impact and occurrence of CSOs:

  • Permits - SPDES permit holders with CSOs must comply with fifteen CSO Best Management Practices (BMPs). They are also required to develop and implement a Long-Term Control Plan (LTCP) that identifies solutions to reduce CSO impacts.
  • Funding - Grants may be available to offset the costs of CSO improvements, including:
    • Water Quality Improvement Project (WQIP) program
    • NYS DEC/EFC Wastewater Infrastructure Engineering Planning Grant (EPG)
    • NYS Water Infrastructure Improvement Act (WIIA) grant program

In 1994, the United States Environmental Protection Agency (USEPA) adopted a CSO Control Policy (leaves DEC website), which established minimum requirements for CSO communities to achieve and set forth goals that would allow for the communities to achieve the goals of the Clean Water Act. Since adoption, DEC has taken several steps to reduce and/or eliminate CSOs. DEC has further categorized implementation of the CSO Control Policy into three phases:

  1. implementation of CSO Nine Minimum Control Measures (NMCs);
  2. development and implementation of LTCPs; and
  3. Post Construction Compliance Monitoring to gauge the effectiveness of the LTCPs.

Phase 1: Implementation of CSO BMPs

Each community that has a SPDES permit to operate a CSS has had their permit modified to include 15 CSO BMPs. The BMPs are technology-based controls, designed to implement operation and maintenance procedures, utilize the existing treatment facility and collection system to the maximum extent practicable, and implement sewer design, replacement, and drainage planning, to maximize pollutant capture and minimize water quality impacts from combined sewer overflows. BMPs do not require major engineering studies or construction.

The 15 CSO BMPs developed by DEC include the NMCs required by the USEPA CSO Control Policy, but also require additional measures specific to NYS. The 15 BMPs are:

1. CSO Maintenance/Inspection - Requires a written maintenance and inspection program for the CSS, CSO outfalls, and regulators, which includes routine dry weather inspection of CSOs and regulators. Also requires CSO and regulator inspections following wet-weather events for unmetered and unmodeled systems, to determine whether a CSO event occurred.

2. Maximum Use of Collection System for Storage - Requires the permittee to optimize collection system operation and maintenance to minimize the discharge of pollutants, while maximizing in-system storage capacity to minimize CSO events.

3. Industrial Pretreatment - Requires the permittee to consider CSOs in the calculation of local limits for indirect dischargers and to maximize capture of non-domestic wastewater during wet weather events.

4. Maximize Flow to POTW - Requires the permittee to treat the maximum flow possible during wet weather, through the POTW. Each SPDES permit for a treatment plant requires specific minimum treatment flows for each process unit.

5. Wet Weather Operating Plan - Requires development and update of procedures for operation of unit processes to treat maximum flows while not appreciably diminishing effluent quality or destabilizing treatment.

6. Prohibition of Dry Weather Overflow - Prohibits the discharge of wastewater from a CSO outfall during dry weather.

7. Control of Floatable and Settleable Solids - Requires the implementation of measures required under BMPs 1, 2, 4, and 5 to eliminate or minimize the discharge of floatable material and settleable solids. Also requires the consideration of additional control measures, including maintenance, infrastructure, and public education, should aesthetic problems occur and persist.

8. Combined Sewer System Replacement - Requires that the replacement of combined sewers shall be done with separate sanitary and storm sewers to the greatest extent possible.

9. Combined Sewer/Extension - Requires that when combined sewers are to be extended, it shall be designed using separate sewers.

10. Sewage Backups - Requires the permittee to prohibit further connections to the sewer system if there are documented, recurring instances of sewage backing up into houses or from surcharging manholes.

11. Septage and Hauled Waste - Prohibits the discharge of septage or hauled wastes upstream of a CSO. Septage or hauled wastes shall only be accepted at the treatment plant, or at approved locations outside the treatment plant, during dry weather to minimize the discharge of these wastes from a CSO outfall.

12. Control of Run-off - Requires compliance with both NYSDEC Erosion and Sediment Control practices and the NYSDEC Stormwater Management Design Manual in areas served by combined sewers to reduce the impacts of runoff from development and redevelopment activities.

13. Public Notification - Requires the establishment and maintenance of standardized identification signs for informing the public of the presence of a CSO outfall.

14. Characterization and Monitoring - Requires the permittee to characterize the CSS, determining the frequency of overflows and identifying CSO impacts, to aid in development and implementation of the LTCP.

15. Annual Report - Requires the permittee to submit a standardized report form which summarizes details of the municipality's CSS, implementation of the CSO BMPs and the LTCP in the prior year, while also discussing planned or anticipated activities to be conducted over the upcoming year.

Phase 2: Development and Implementation of Long-Term Control Plans

The USEPA CSO Control Policy required each CSO community to develop and implement a Long-Term Control Plan (LTCP) that evaluates alternatives and recommends a series of improvement projects that, when implemented, will reduce, or eliminate CSO discharges and ensure that any remaining CSO discharges do not cause or contribute to violations of water quality standards. The LTCP is an extensive evaluation of the collection system and identifies the best, yet reasonable and feasible, improvements to reduce the frequency, duration, and intensity of CSO events. Every community in NYS that was required to submit an LTCP to DEC has done so. Most of the communities are still in the process of implementation of the LTCP.

An LTCP includes a system wide evaluation of the combined sewer system (CSS), and the hydraulic relationship between the CSS, precipitation, wastewater treatment plant capacity and CSOs. As part of the LTCP, the permittee must evaluate alternatives that will reduce or eliminate the frequency and volume of CSOs, and then develop a plan and implementation schedule to do so. However, these alternatives can be expensive and may be cost prohibitive for some communities. Common alternatives considered include:

  • Separating stormwater and sewer lines
  • Installing storage tanks to hold overflow during storm event
  • Expanding wastewater treatment capacity
  • Creating retention basins to hold overflow during storm events
  • Screening and disinfecting the overflow
  • Using green infrastructure to reduce stormwater flows

Every LTCP should include each of the following:

  • Characterization, monitoring, and modeling activities which guide the selection and design of effective CSO controls,
  • A public participation process that actively involves the affected public in the decision-making to select long-term CSO controls,
  • Consideration of sensitive areas as the highest priority for controlling overflows,
  • Evaluation of alternatives that will enable the permittee to select CSO controls that will meet the CWA requirements,
  • Cost/performance considerations to demonstrate the relationships among a comprehensive set of reasonable control alternatives,
  • Operational plan revisions to include agreed-upon long-term CSO controls,
  • Maximization of treatment at the existing WWTP for wet weather flows,
  • An implementation schedule for CSO controls, and
  • A post-construction compliance monitoring program to adequately verify compliance with water quality standards and determine the effectiveness of CSO controls.

Under the USEPA CSO Control Policy requirements for LTCPs, there are two approaches that a permittee can use to assess alternative controls to ensure compliance with Clean Water Act requirements: Presumption and Demonstration.

Presumption Approach

The presumption approach requires, following implementation of the approved LTCP, the permittee must meet one of the following criteria:

  1. no more than 4-6 overflow events per year that do not receive minimum treatment, or
  2. the elimination or capture for minimum treatment of no less than 85% by volume of the combined sewage collected during precipitation events on a system-wide annual average basis, or
  3. the elimination or removal of no less than the mass of the pollutants, identified as causing water quality impairment during the characterization, monitoring, and modeling effort.

Minimum treatment is defined in the CSO Control Policy as: primary clarification, solids and floatables disposal, and disinfection of effluent.

Demonstration Approach

The demonstration approach requires that the approved LTCP must meet EPA's requirements by addressing each of the following:

  1. the planned control program is adequate to meet water quality standards and protect receiving waters designated uses, and
  2. the CSO discharges remaining after implementation of planned control programs will not preclude the attainment of water quality standards or the receiving waters designated uses or contribute to impairment, and
  3. the planned control program will provide the maximum pollution reduction benefits reasonably attainable, and
  4. the planned control program is designed to allow cost effective expansion or retrofitting if additional controls are subsequently determined to be necessary to meet water quality standards or receiving waters designated uses.

Phase 3: Post-Construction Compliance Monitoring (PCCM)

All CSO communities, even those which may not have been required to develop a LTCP, are required to develop and implement a PCCM Program. The community must conduct a PCCM Program to verify compliance with water quality standards and protection of designated uses, as well as to ascertain the effectiveness of the implemented CSO controls. The PCCM Program involves various activities such as data collection, data validation, and monitoring. Since part of the intent of the PCCM Program is to gage the effectiveness of the CSO controls, the Program is not typically started until installation of the selected CSO controls are completed.

Communities are required to develop an approvable workplan, the PCCM Plan, that details the monitoring protocols to be followed, including the necessary effluent and ambient monitoring and, where appropriate, other monitoring protocols such as biological assessments, whole effluent toxicity testing, and sediment sampling. Following approval of the PCCM Plan, the community must implement the Program, often initially for a 2-year period, the produce a PCCM Report that details the findings of both the effectiveness on CSO controls and attainment of water quality standards.

Once a community enters the PCCM Program phase, the SPDES permit is often modified to include a long-term, perpetual requirement to implement the PCCM Plan on a 5-year basis. This perpetual requirement ensures continued compliance with the CSO Control Policy and permitted requirements. Should a PCCM Program identify shortcomings in either CSO control effectiveness or water quality standard attainment, DEC may reopen the SPDES permit to include additional requirements.

Green Infrastructure for Wet Weather

What is Green Infrastructure?

The term green infrastructure (GI) describes a variety of site design techniques and structural practices used by communities, businesses, homeowners, and others for managing stormwater. On a larger scale, green infrastructure includes preserving and restoring natural landscape features (such as forests, floodplains, and wetlands), and reducing the amount of land covered by impervious surfaces. On a smaller scale, GI practices include green roofs, pervious pavement, bioretention, rain gardens, vegetated swales, planters, and stream buffers.

Why is Green Infrastructure Important?

As it flows over the ground and impervious surfaces, stormwater (rain and melting snow) collects debris, chemicals, sediment, and other pollutants. Those pollutants may then end up in nearby lakes, rivers, and streams where people swim, fish, play and draw drinking water, or in local sewer systems where more problems can arise.

While some sewer systems are capable of handling large volumes of stormwater, many are not. Combined sewer systems (CSS) that carry stormwater, domestic sewage and industrial wastewater can be overwhelmed by rainwater and melting snow. These CSSs can send untreated waste into nearby waterbodies through combined sewer overflows. GI practices can help control stormwater at its source, remove pollutants, and reduce the amount of stormwater runoff and waste that ends up in sewer systems and local waterbodies. While the CSO Control Policy includes PCCM as the final piece of the LTCP implementation, it is often referred to as Phase 3.

Benefits of Green Infrastructure

When managing stormwater, green infrastructure practices can be less expensive than expanding, or building new, sewer and water treatment systems. GI practices also have several secondary benefits including aesthetic improvements, cleaner air, energy savings, urban cooling, climate change mitigation and improved human health.

The USEPA maintains a green infrastructure webpage with details on the benefits described above (see the Links Leaving DEC's Website section in the right-hand column of this page for a direct link).

Green Infrastructure to Control CSOs

Several communities have adopted green infrastructure as a tool to help mitigate CSOs under their LTCP. The Albany Pool, Buffalo Sewer Authority, The New York City Department of Environmental Protection (NYCDEP), Onondaga County Department of Environmental Protection, and several other communities have all implemented green infrastructure under their LTCP to reduce the amount of stormwater entering the combined sewer system and help to reduce the amount of CSOs.

Additional Information

More about Combined Sewer Overflow (CSO) :

  • New York City CSO - This page provides information on the 2011 modification to the CSO Consent Order. The public will be linked to the CSO Order.
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