Combined Sewer Overflows

About 60% of New York City has a combined sewer system. In a combined sewer system, there is a single pipe that carries both stormwater runoff and sewage from buildings. This mix of stormwater and sewage is usually sent to a wastewater treatment plant. For information about the separate sewer system, which is used by about 40% of New York City, visit Municipal Separate Storm Sewer System.

Graphic of Dry Weather Conditions in the Combined Sewer System
Dry Weather Conditions in the Combined Sewer System*
Graphic of Wet Weather Conditions in the Combined Sewer System
Wet Weather Conditions in the Combined Sewer System

During heavy rainstorms, combined sewers receive higher than normal flows. Treatment plants are unable to handle flows that are more than twice the design capacity. When this occurs, a mix of stormwater and untreated sewage discharges directly into the City’s waterways. These events are called combined sewer overflows (CSOs). We are concerned about CSOs because of its effect on water quality and the recreational use of local water bodies.

*If you see discharge coming from any of New York City’s 700 combined sewer outfalls (an outfall is an outlet along the waterfront connecting the City’s sewers to the open waters) during dry weather conditions, report it to 311 or file a complaint online. Each outfall has a sign with an identification number. Be sure to give the 311 operator the outfall number.

How You Can Help Prevent CSOs

There are quite a few things New Yorkers can do to help prevent CSOs and improve the health of our local waterways.

Green Infrastructure to Prevent CSOs

“Green” infrastructure practices are designed and constructed to manage stormwater runoff when it rains. Green infrastructure slows down, absorbs, and filters stormwater runoff before it can enter the sewer system or local waterbodies. This helps reduce the occurrence of combined sewer overflows and the amount of pollution carried by stormwater runoff. Learn more about Green Infrastructure.

Grey Infrastructure to Prevent CSOs

“Grey” infrastructure typically refers to traditional infrastructure such as sewers, tunnels, and wastewater treatment plants. Expanding grey infrastructure and making better use of existing grey infrastructure enables us to store and treat more stormwater runoff. This reduces the occurrence of CSOs, helping keep untreated sewage and other pollutants out of our waterways. Learn more about our Wastewater Treatement System and our CSO Long Term Control Program.

CSO Best Management Practices Annual Reports

In accordance with the Section VIII of the State Pollutant Discharge Elimination System (SPDES) permits for the 14 New York City wastewater treatment plants, these Annual Report describe our ongoing program to reduce water quality impacts from CSOs by:

  • optimizing operation and maintenance procedures
  • using existing treatment facilities and the wastewater collection system to their maximum extent practicable
  • implementing related sewer retrofits and replacements

Major capital investments and larger-scale drainage planning efforts to reduce CSOs are covered by the CSO Long Term Control Plans.