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NYC's Combined Sewer System

Combined Sewers Anchor

Combined Sewer Systems and Overflows


In many older cities, sewer systems and stormwater systems were often built as a single system of conveyances. Rain and melting snow drain into the same set of pipes that carry sewage water from the toilets and sinks of our homes and businesses to waste water treatment plants. This system is known as a combined sewer system. 


New York City’s 150-year old combined sewer system carries both sewage from buildings and all the polluted stormwater runoff from our streets to 14 City-operated wastewater treatment plants (WWTPs). When it rains (as little as one tenth of an inch per hour), New York City's combined sewer system’s capacity is overwhelmed and the mix of polluted stormwater from our streets and untreated, raw sewage from our toilets, sinks, and showers is discharged directly into our waterways making them unsafe to touch. This is called a combined sewer overflow, or CSO.   Click here to see a map of the combined sewer area of NYC.

Combined sewer overflow events in NYC cause our sewer treatment plants to release over 20 billion gallons of raw sewage and polluted runoff into our waterways every year through a series of outfall points along the city’s coastline where people fish, wade, kayak and come into contact with the water year round. CSO discharges are the largest ongoing source of pollution in NYC's waterways. 


Click here to learn more about combined sewer overflow events (from a video produced by WNYC, public radio) and what you can do in your own home to help!

CSO Key facts


New York City has 450+ CSO outfalls 
from the Harlem River to the Gowanus Canal


Combined sewer systems make up ~60% of New York City's sewershed

CSO outfalls discharge
over 20 billion gallons
of polluted water
into the New York Harbor every year

Click the image to enlarge. Credit: Open Sewer Atlas.

The "dry weather" image on the left shows how wastewater and stormwater systems are interconnected and eventually flow to a treatment plan. However, heavy rains, depicted in the image on the right, overwhelm the system. To handle the excess, untreated water is discharged into our waterways through CSO outfalls. 

Image: Kingsland Wildflower Green Roof with the Newtown Creek Wastewater Treatment Plant's Digester Eggs in the background. 

How can this be solved?


To lessen the chance of an combined sewer overflow (CSO) event, we must reduce the amount of water flowing into the sewer system in the first place! This can be accomplished by several means: 

  • Green infrastructure solutions - capturing rainwater before it runs off a property using rain barrels, green roofs, and rain gardens

  • Grey infrastructure solutions - preventing overflows by storing rainwater and sewage through increased pipeline or wastewater treatment plant (WWTP) capacity

  • Water-efficient devices and strategies - limiting household waste going to WWTPs by encouraging the use of water-saving appliances (dishwashers, toilets) and more conscious water use. When it's raining outside, don't take a long shower, do laundry and run the dishwasher! You have the power to shorten your shower! 

Image: SWIM's guide for water users, organizations, and citizens to respond to the City’s LTCPs.

CSO long term control plans for New York City

The New York City Department of Environmental Protection (DEP) is required to develop Long Term Control Plans (LTCPs) to reduce the overflows that discharge into ten high-priority waterbodies. Use our Clean Water Steward Handbook, left, to learn more about the plans and how they will impact our neighborhoods and waterways. 

These plans include implementing green and grey infrastructure with the goal of reducing CSO discharges citywide over a specific period of time. Before the plans get implemented, they have to be approved by the New York State Department of Environmental Conservation (DEC). As required by the 1972 federal Clean Water Act (CWA), the U.S. Environmental Protection Agency oversees the State’s management of these plans to ensure that they meet “fishable, swimmable” water quality standards.

Image: SWIM's LTCP Action Alert fact sheet.

SWIM Coalition's LTCP Campaign

SWIM members and partners are closely monitoring the content and implementation of the City's proposed plans - reviewing them, providing comments and testimonies in courtrooms, conference rooms, and public hearings. SWIM monitors the plans for:

  • actual reduction of CSO discharges

  • sufficient attention given to green infrastructure

  • incorporation of long-term solutions

  • effective attainment of the community’s water quality goals

  • attainment of fishable and swimmable water quality standards year-round


Check the SWIM library for more resources.

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