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


Combined Sewer Overflows (CSOs)

Sometimes, during heavy rain and snow storms, combined sewers receive higher than normal flows. Treatment plants are unable to handle flows that are more than twice design capacity and when this occurs, a mix of excess stormwater and untreated wastewater discharges directly into the City’s waterways at certain outfalls. This is called a combined sewer overflow (CSO). We are concerned about CSOs because of their effect on water quality and recreational uses. 

Recent DEP construction projects have included upgrades in key wastewater treatment facilities, storm sewer expansions and the construction of several large CSO retention tanks to further mitigate this chronic source of pollution. Existing infrastructure developments have increased DEP’s standardized CSO capture rate from about 30% in 1980 to over 80% today. Some of the most recent increases can be attributed to the implementation of additional CSO control measures such as the Spring Creek and Flushing Bay CSO Retention Facilities that came online in 2007, and the Paerdegat Basin and Alley Creek CSO Retention Facilities, which came online in 2010.

DEP has a broad citywide effort to better manage stormwater using a variety of innovative, sustainable green infrastructure. Improved stormwater management is an important component of Mayor Bloomberg's PlaNYC initiative and Sustainable Stormwater Management Plan.  Green infrastructure, or source controls, are a set of techniques that detain or retain stormwater runoff through capture and controlled release, infiltration into the ground, vegetative uptake and evapotranspiration thereby reducing the need for end-of-pipe stormwater storage and treatment systems.

Related Links

CSO Consent Order Documents


Green Infrastructure