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Combined Sewers

Combined Sewers Anchor

Combined Sewer Overflows

SWIM Coalition is working with New York City, New York State, and the U.S. Environmental Protection Agency toward the goal of fishable, swimmable water quality. This requires reducing and eliminating the volume of polluted water discharged from combined sewer overflows (CSOs) through citywide green infrastructure installations, grey infrastructure improvements, and conservation and efficiency.

Combined Sewer Systems


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. This system is known as a combined sewer system. 


When it rains (as little as a fraction of an inch in some places), the sewer system’s capacity is overwhelmed and the mix of polluted stormwater and raw sewage is discharged from an outfall. This is called a combined sewer overflow, or CSO.

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. 

How can this be solved?

When untreated household sewage is released along with stormwater, our rivers, beaches, and bays become contaminated with pathogens. Stormwater runoff alone contains litter, pet waste from streets and parks, and other pollution regularly found on streets and buildings, like cigarette butts, motor oil, and brake dust.


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

long term control plans

The City of New York is required to create green and grey infrastructure plans called Long Term Control Plans (LTCPs). The goal of these plans is to reduce CSO discharges citywide over a specific period of time, including 10 designated high-priority watersheds.

SWIM members and partners regularly follow up on the content and implementation of these plans - reviewing them, providing comments and testimonies in courtrooms, conference rooms, and public hearings. SWIM monitors 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

Visit the SWIM library for more resources on local LTCPs.

CSO Key facts


New York City has 460 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

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