top of page
Separate Sewers

Municipal Separate Storm Sewer System

Separate Sewers and Stormwater Runoff

In addition to the our Combined Sewer System, NYC has a municipal separate storm sewer system (MS4) which collects polluted stormwater runoff from nearly 40% of New York City's sewered land. The Separate Storm Sewer System is not connected to our wastewater treatment plants - meaning polluted stormwater runoff from our streets and buildings, which sweeps up litter, oil, pet waste, and sediment, among other things, is not treated before it is discharged into our waterways. There are some instances where a building is (usually inadvertently) connected to the MS4 pipes causing its untreated wastewater (from toilets, sinks, showers, etc) to discharge directly into our waterways, this is called an illicit discharge. The NYC DEP is implementing a plan to address the MS4 system's stormwater runoff and illicit discharge pollution in our waterways. Scroll down for more information about the plan and how citizens can help. 

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

The "dry weather" image on the left shows how wastewater and stormwater systems are entirely separate.
Sewage from residential and commercial buildings flows to a treatment plant.  
Anything that enters the stormwater system, on the other hand, is discharged untreated into waterways.

Separate Sewers in New York City


SWIM Coalition is closely monitoring New York City’s Municipal Separate Storm Sewer Systems (MS4) Permit process to ensure measures are being taken to address the pollution caused by stormwater runoff, a problem inadequately addressed by the City.


Polluted stormwater runoff is the major reason that almost 40% of surveyed US water bodies do not meet water quality standards! If unmanaged, polluted stormwater can result in the destruction of fish, wildlife, and aquatic habitats; a loss in aesthetic value; and threats to public health through contaminated food, drinking water, beaches, and recreational waterways. Read more in water quality.


Stormwater Management Permit and Plan


Under the US Clean Water Act (CWA), sources of stormwater discharge that harm water quality must be addressed through a permit system known as the MS4 permit. The State Department of Environmental Conservation (DEC) issued an MS4 Permit to New York City in August, 2015. Now, NYC is undergoing a multi-year process to research and develop a Stormwater Management Plan (SWMP) to reduce the pollutants discharging into the water through the stormwater system.

There are 14 NYC agencies with significant obligations under the MS4 permit, with efforts being coordinated by the Department of Environmental Protection (DEP). Image credit DEP.

Everyone has a role to play 

The initial runoff during rain, the first flush, is the most polluted because it sweeps up the pollutants on the ground. Any litter, debris, or oil on the street will end up in the waterways. Help keep streets clean. Share your knowledge with friends and neighbors. 


If you work in construction or plumbing, or own property, the MS4 Permit will affect you. Check out our MS4 Factsheet for more information.

If you see illegal dumping or discharge coming from an outfall pipe
on a dry-weather day, report it!

Call 914.232.8642 or 

MS4 Key facts


The CWA's MS4 permit requires the City to:

Map stormwater outfalls
to identify the source of pollutants in stormwater


Find and eliminate illegal stormwater outfalls

Reduce water pollution stemming from new construction and development

Regulate industrial sources of water pollution

Educate the public

The City’s detailed Stormwater Management plan can be found here.

bottom of page