Wastewater Treatment and CSOs
In the mid-19th century, it was common for residents and businesses to dump sewage and garbage in the streets, with above ground streams and drains discharging into nearby waterbodies. To address unsanitary conditions, City engineers began constructing what would eventually become the New York City sewer system.
As was common at the time, the sewer system originally discharged into nearby waterways. Eventually, in order to improve water quality, the City began to build wastewater treatment plants to accept and process wastewater.
New York City is served primarily by a combined sewer system, which carries a combination of both storm water run-off and sanitary flow in the same pipe. Currently, the City’s 14 treatment plants can treat all of the wastewater produced on a dry day. During wet weather, the plants can treat two times the dry weather flow. However, when the volume of flow exceeds the capacity of the system, the combined system is designed to discharge the excess flow from certain outfalls into the City’s waterbodies, including the Gowanus Canal. These wet weather discharges or combined sewer overflows (CSOs) are necessary to prevent upstream flooding, sewer back-ups, and damage to the treatment plants. There are a total of ten CSO outfalls that can discharge to the Canal and three storm sewer outfalls that discharge only storm water.
The City is divided up into drainage areas, with treatment plants serving a specific drainage area. The Gowanus Canal stretches across two drainage areas served by two treatment plants, the Owls Head plant, which began operating in 1952, and the Red Hook plant, which began operating in 1987.