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Combined Sewer Overflow (CSO)

What are Combined Sewers?

Combined sewer systems (CSS) are sewer systems that are designed to collect storm water runoff, domestic sewage, and industrial wastewater in the same pipe and bring it to the publicly owned treatment works (POTW) facilities.

During rain events, when storm water enters the sewers, the capacity of the sewer system may be exceeded and the excess water will be discharged directly to a waterbody (rivers, streams, estuaries, and coastal waters).

The untreated water may contain untreated sewage that may impact human health. For information about the general CSO wet weather advisory and links to the CSO outfall map visit the CSO Wet Weather Advisory web page.

Diagram showing sewer overflow pollution
Avoid contact recreation within waterbodies with a CSO
during or following a rainfall or snowmelt event.
Image: Moundsville WWTP, WV

What is a CSO?

A combined sewer overflow (CSO) is the discharge from a combined sewer system that is caused by snowmelt or storm water runoff. Visit the City of Brementon website, link is located under the "Links Leaving DEC Website" on the right hand side, to see animated videos of how a combined sewer system functions during dry and wet weather events.

How many CSS Communities are there in NY?

Combined sewers are found across New York State (NYS), except on Long Island. However, most CSOs are found in large cities. Most large municipal sewer systems in NYS consist of combined sewers in older downtown urban areas with separate sanitary and storm sewers serving outlying tributary suburban areas.

About ten percent of the CSOs in the United States are found in NYS. There are approximately 937 CSO outfalls in NYS. Each combined sewer system in NYS is required to have a State Pollutant Discharge Elimination System (SPDES) permit, which is issued by the NYS Department of Environmental Conservation (DEC).

The number of CSO outfalls listed in the permits of POTWs has decreased from 1300 to 937 since 1993, due to CSO abatements competed by the permittees.

What is being done to identify problems?

DEC enforces the requirements under the Wet Weather Water Quality Act and monitors permit compliance through the permit process that includes requirements of Best Management Practices (BMPs) and a Long-Term Control Plan (LTCP).

What's being done to abate the problems?

There are two types of abatement categories: water quality and technology based. Water quality based abatement options may be more expensive. The following are options for abatement of CSO discharges as part of the permittee's long-term control plan. However, these options can be expensive and may be cost prohibitive for some communities:

  • Separation of stormwater and sewer lines
  • Storage tanks to hold overflow during storm event
  • Expansion of waste treatment capacity
  • Retention basins to hold overflow during storm events
  • Screening and disinfection facilities for the overflow
  • Green infrastructure to reduce stormwater flows into combined sewer system

How is progress monitored?

Permittees submit an annual report, Combined Sewer Overflow Annual Report Form (PDF) (1.2 MB). Reports are due no later than January 31st each year to report on CSO abatement status during the previous calendar year. DEC uses information from the report to monitor permittees' progress on the implementation of their CSO abatements. Permittees report progress on:

  1. Compliance with the 15 CSO Best Management Practices;
  2. The condition and operation of the combine sewer system (CSS) components. Most importantly, the end-of-pipe measures that show trends in the discharge of CSS flows to the receiving water body, such as reduction of pollutant loadings, the frequency of CSOs, and the duration of CSOs;
  3. How water quality standards in the receiving water bodies are being met as a result of the implementations of the BMPs and/or the approved CSO control measures;
  4. Overall status of the CSO LTCP, if applicable;
  5. Key CSO control accomplishments and design and construction progress in the previous year

The new reporting format is the minimum information a permittee must provide. It is the obligation of every permittee to show full compliance with the EPA CSO Long-Term Control Policy and their SPDES permit requirements.

Wet Weather Water Quality Act

In 1994, the United States Environmental Protection Agency (EPA) issued a National CSO Control Policy. The Wet Weather Water Quality Act of 2000 requires combined sewer systems to conform to the requirements in the National CSO Control Policy. The requirements include implementing Nine Minimum Controls (NMC) and a Long-Term Control Plan (LTCP). The NMCs are technology-based controls that can be used to abate CSOs. The LTCP consists of more extensive characterization and monitoring of the combined sewer system and the receiving water, as well as selection and implementation of CSO control alternatives, with the intent of minimizing the impacts of CSOs on water quality.


More about Combined Sewer Overflow (CSO) :

  • CSO Wet Weather Advisory - Adivory information for NYS combined sewer outfalls (CSOs) during and after rainfall snowmelt events.
  • CSO Best Management Practices - Best Management Practices For Combined Sewer Overflows
  • CSO Long Term Control Plan (LTCP) - Combined Sewer Overflow Long-Term Control Plan
  • Albany Pool CSO LTCP - This page provides information on the Long Term Control Plan to significantly reduce the direct discharge of stormwater diluted with sewage in the Hudson River.
  • 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.