Newsletter Sign-up Printer Friendly Format Translate This Page Text Size Small Medium Large


Combined Sewer Overflows (CSOs)

New York City, like other older urban communities, is largely serviced by a combined sewer system where stormwater that falls on roofs, streets, and sidewalks, and wastewater from homes and businesses, are carried through a single sewer line to treatment plants. The City’s 14 treatment plants can manage and treat to federal Clean Water Act standards all the wastewater created in New York City on a dry weather day, or about 1.3 billion gallons on average. On a rainy day they have the capacity to clean more than twice the dry weather flows. However, during intense precipitation events, the stormwater that falls on the City’s impervious surfaces exceeds that capacity and overflows can be discharged into local waterways, otherwise known as a Combined Sewer Overflow (CSO). If the overflows were not discharged, the City’s treatment plants would be flooded and severely damaged and wastewater could backup into homes and businesses.

To reduce CSO’s, DEP has upgraded key wastewater treatment facilities, expanded and separated storm sewers, constructed large CSO retention tanks, and has incorporated the nation’s largest green infrastructure program to further mitigate this source of pollution. The City’s standardized CSO capture rate has risen from about 30% in 1980, to over 80% today. DEP has committed $4.1 billion to these projects and has completed a series of Long Term Control Plans which commit an additional $4.4 billion to further mitigate the water quality impact of CSO events.

DEP has also launched the NYC Green Infrastructure Plan which combines traditional infrastructure upgrades and the integration of green infrastructure to capture and retain stormwater runoff before it can ever enter the sewer system and contribute to CSOs. To date, more than 4,000 rain gardens have been constructed across the city, with thousands more planned through 2030. In addition to installing green infrastructure along streets and sidewalks, DEP is also partnering with other City agencies and non-profits to add stormwater-capturing green elements such as retention basins, permeable pavement and trees, to playgrounds in schoolyards and parks citywide.

Related Links

CSO Consent Order Documents


Green Infrastructure