A legacy of 19th century municipal engineering, CSOs occur mostly in older cities like New York where the sewage system was designed to collect both wastewater and storm runoff in the same pipes. Less than four percent of U.S. municipalities, 772 out of more than 20,000, have such combined sewer systems. (See EPA’s CSO Demographics under “related info.”) Newer cities are typically designed with separate sewage and stormwater systems.

Wastewater from virtually every sink, tub and toilet in New York flows from smaller pipes to larger sewer mains, typically three to five feet in diameter. Here it combines with runoff from rainstorms as well as and all the debris and chemicals that wash off the street or are poured in storm drains. In dry weather all of New York City’s wastewater is transported to a treatment plant, where it is treated before being discharged to a water body. Combined sewage flows to the plants mostly by gravity, but is assisted in spots by pumps. (See NYC DEP’s water and sewer diagram under “related info.”)

New York has fourteen sewage treatment plants – one in Manhattan, two each in Staten Island and The Bronx, four in Queens, and five in Brooklyn. Each of these plants receives combined sewage, except the Oakwood Beach plant on Staten Island where a separate storm sewer system serves part of the borough. Each of plants receives the combined sewage flowing from a particular drainage area – for example, the West Side of Manhattan above Canal Street drains to the North River plant on West 135th Street; downtown and the East Side below 59th Street flows to the Newtown Creek plant in Greenpoint, Brooklyn; and the East Side north of 59th Street goes to the Ward’s Island plant. (See NYC DEP’s map of sewer drainage areas under “related info.”)

The total dry weather capacity of these fourteen treatment plants is 1.8 billon gallons per day, and each plant is designed to provide “primary” treatment of double that volume and “secondary” treatment of one-and-a-half times that volume. In other words, at an absolute maximum, the City’s fourteen sewage plants can remove solids from 3.6 billion gallons per day and provide biological treatment of 2.7 billion gallons per day.

During periods of heavy rainfall, the combined sewage volume quickly exceeds the wet weather capacity and overwhelms the treatment plants. In order to keep sewage from backing up in the system – where it could spurt through manhole covers or backflood into homes and businesses, as it did several times in 1999 – the City’s combined sewer system is designed to overflow during rains and discharge excess wastewater directly to the Harbor and other waterbodies. About 460 CSO discharge pipes, called outfalls, line the shores of the five boroughs.

An inch of rain falling on the roughly 120,000 acres on the City served by combined sewers produces more water than the City’s treatment plants can handle (an acre-inch of water equals approximately 27,000 gallons, yielding 3.14 billion gallons over the 120,000 acres). In addition, because the pipes lack the necessary girth to handle these increased volumes, combined sewage causes bottlenecks in the collection system, which trigger overflows from the 460 overflow pipes known as outfalls. It doesn’t take a torrential storm to overwhelm the plants or cause bottlenecks in the system – as little as one-tenth of an inch of rain can trigger an overflow.

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