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Combined Sewer Overflow (CSO)

Discharges from Combined Sewer Overflows (CSO)

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

A combined sewer overflow (CSO) is the discharge from a combined sewer system that is caused by snowmelt or stormwater runoff. Combined Sewers are sewer systems that collect stormwater runoff, domestic sewage, and industrial wastewater in the same pipe and bring it to the wastewater treatment facility. They are designed to overflow during wet weather.

During rain events, when stormwater 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) See Fig. 1.

CSO Impacts

Discharges from CSO outfalls may contain mixtures of domestic sewage, stormwater runoff, and sometimes, industrial wastewater, including high levels of suspended solids, toxic chemicals, floatable material and other pollutants. Avoid contact while recreating within waterbodies with a CSO during or following rain or snowmelt.

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

CSO's in New York State

Combined sewers can be found in all parts of the state, with the exception of Long Island, mainly 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 suburban areas. Each combined sewer is required to have coverage under a municipal wastewater treatment plant's State Pollutant Discharge Elimination System (SPDES) permit, which is issued by DEC.

There are currently about 800 CSO outfalls in New York State. This is a decrease from about 1300 in 1993, due to CSO abatements completed by the permittees.

The Combined Sewer Overflow (CSO) Outfalls Google Earth Map (requires Google Earth to be installed on your computer) shows the locations and other information about the permitted CSO outfalls in New York State.

Reporting and Fixing CSO's

DEC and permittees use a combination of reporting and CSO abatement work to reduce the impact and occurrence of CSOs:

  • Sewage Pollution Right to Know (SPRTK) - CSO discharges must be reported to the public using NY-Alert.
  • Permits - SPDES permit holders with CSOs must comply with fifteen CSO Best Management Practices. They are also required to develop a Long Term Control Plan to reduce CSO impacts.
  • Funding - Grants may be available to offset the costs of CSO improvements:
    • Water Quality Improvement Project (WQIP) program
    • NYS DEC/EFC Wastewater Infrastructure Engineering Planning Grant (EPG)
  • Annual Reports - DEC uses information from the report to monitor permittees' progress on the implementation of their CSO abatements.

Reducing CSOs

As part of the permittees long-term control plan, municipalities present options to reduce the frequency and volume of CSO discharges. However these options can be expensive and may be cost prohibitive for some communities. Some options 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

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.