The Combined Sewer System of NYC and its Overflow

On an average rainy day in New York City (NYC), you can expect to receive a flash flood warning on your phone, or experience extreme travel delays, but you will never hear about sewage overflow.

NYC operates with a combined sewer system. This means wastewater from toilets, sinks, and drains combines with stormwater runoff after rain and snow fall, before going into a wastewater treatment facility. Since NYC is home to approximately 8.6 million people and a revolving door of tourists, the result is up to 1.3 billion gallons of wastewater being treated daily. [i] [ii]

Combined sewer systems on a dry weather day (left) and a wet weather day (right). Source: Jersey Waterworks

Currently, wastewater treatment facilities have a total capacity of 1.8 billion gallons of wastewater per day. [iii] In an ideal situation, the product of wet weather (rain and snow) should not exceed 500 million gallons as it would exceed the total capacity that can be held prior to treatment. However, we cannot control the abundance of water in our weather system, and stormwater runoff often does exceed the capacity of wastewater that can be held.

In the event of a heavy rainstorm or snowfall that results in snow melting and excess stormwater, the sewer system only has one form of relief: combined sewer outfalls. Outfalls are large open pipes that release untreated excess wastewater directly into the bodies of water surrounding NYC. Below you can see a map of the combined sewer overflows in NYC:

Locations of all the combined sewer outfall open drains in NYC. Source: NYC Department of Environmental Protection

Why is this important to residents of NYC? There are a lot more of these outfalls than residents are aware of. A total of fifty-one combined sewer outfalls exists throughout the city. Each is categorized in tiers that more or less dictate the volume of wastewater that may be discharged if the capacity of combined wastewater exceeds what treatment facilities can hold.

It is impossible to know what you may find in wastewater. The obvious is raw sewage but there are also traces of pharmaceuticals, household chemicals, industrial chemicals, and many more random objects or liquids that cannot be accounted for.

So, what are the impacts of this untreated wastewater? It has the ability to disturb the ecosystems immediately surrounding where it is released. It can also create health hazards for humans, animals, and marine life. Recreational uses become limited for those who might swim or go fishing in these areas. It also encourages algae growth that can cause oxygen depletion in the water. [iv]

Currently, the city is taking initiative to alleviate the stormwater runoff with green infrastructure. The installation of green infrastructure allows stormwater to be captured by grass, trees, shrubs, and soil so that it may be filtered naturally downward into groundwater like it is naturally supposed to. But the concrete sidewalks and streets are impenetrable, meaning water cannot go through them. This forces stormwater directly into the sewage system.

Example of green infrastructure, this one called right-of-way bioswales for curbsides. Source: NYC Department of Environmental Protection

Another way that this may be alleviated is by spreading awareness to the residents of NYC. If water use is limited during wet weather conditions, then there will be a decrease in the total amount of wastewater. This gives the stormwater an opportunity to be treated and released until the wet weather has passed.

Other ways that the city has thought to fix this problem are by separating the sewer and stormwater system, or expanding the system to include storage tanks. But these are expensive options and residents will be responsible to pay for it through taxes. It seems way more feasible to simply tell residents the truth about overflows—and perhaps they will begin to feel a bit more connected to their environment.


[i] NYC Planning. 2017. Current and Projected Populations. NYC Department of City Planning. Retrieved from

[ii] NYC Environmental Protection. 2018. New York City’s Wastewater. NYC Department of Environmental Protection. Retrieved from

[iii] AECOM. Fiscal Year 2018 Consulting Engineer’s Report. 2018. The NYC Municipal Water Finance Authority. Retrieved from

[iv] NYS Department of Environmental Conservation. Combined Sewer Overflow (CSO). Retrieved from

One thought on “The Combined Sewer System of NYC and its Overflow

  1. Hi Alyssa, I’m a data scientist who wants to take a data-driven approach to urban river swimming safety. Do you have any insight into the amount of rainfall that will activate the outflow system?

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