When It Rains Hard, New York Sewers Dump Into the Rivers
The city's oldest pipes carry household waste and rainwater together, and treatment plants cannot clean both at once.
Practical Engineering · What's Under Your Feet in New York City?On a dry day in New York, everything flushed down toilets, sinks, and showers flows to a treatment plant to be cleaned. When a heavy rain hits, some of it goes straight into the rivers instead, untreated.
The reason is the plumbing. In much of the city, one set of pipes carries both household sewage and rainwater that runs off streets and roofs. Treatment plants can handle the first, but not the first plus a storm. When the volume gets too large, the system releases the excess through roughly 400 overflow pipes along the waterways.
It was built that way on purpose. Much of the sewer network went in before modern pollution laws, when the goal was simply to move waste out of town as quickly as possible. Sending rainwater into the same pipes even helped at the time, because it diluted the sewage being dumped directly into the harbor and rivers.
Now that dilution is the problem. Sewers sit deeper than drinking water lines, partly because they flow downhill by gravity and partly so a leak cannot seep through the soil toward drinking water. Some of the oldest lines are hand-laid brick, and the largest are big enough to walk through standing upright. As those lines join together on the way to a plant, the combined flow of sewage and stormwater can become enormous.
The city cannot simply rebuild a century of underground pipes at once. It operates under a consent decree, a court-backed deal with the state that pauses fines as long as New York works toward ending the overflows. The fixes include curbside rain gardens that soak up stormwater, and huge underground tanks such as the one at Newtown Creek that store the surge until plants can catch up. In the newest neighborhoods, the two flows run in entirely separate pipes, so rain no longer flushes sewage into the rivers.
Questions people ask
What is the difference between a combined sewer and a separate sewer?
A combined sewer carries household sewage and rain runoff in the same pipe. A separate system uses one pipe for sewage, which goes to a treatment plant, and another pipe for rainwater, which usually drains to a river. Much of the older system in New York is combined, while newer parts use separate pipes.
What is a consent decree?
It is a formal agreement approved by a court. In this case, the state allows the overflows to continue without fines while the city builds fixes on an approved schedule.
How does a rain garden help with sewers?
They are small planted basins built into sidewalks that capture street runoff and let it soak into soil. They do not stop an overflow by themselves, but thousands of them reduce the surge that reaches treatment plants during a storm.