Combined Sewer System

While New York City currently uses a combined sewer system (CSS), many municipalities around the country are beginning to use Municipal Separate Storm Sewer Systems (MS4s). MS4s are systems that provide a separate pipe for storm water, which is typically discharged, untreated, into local bodies of water. Unlike CSS, which use a singe pipe for both waste water and storm water, MS4s have a separate pipe for sewage — the sanitary sewer. However, New York City currently uses a CSS, whose treatment plans for sewage is outlined below.

NYC’s Current Sewage Treatment Plans

New York City currently has 14 wastewater treatment plants: 5 in Brooklyn, 4 in Queens, 2 in Staten Island, 1 in the Bronx, 1 in Manhattan, and 1 on Ward’s Island in the East River off of Manhattan. The city uses a five-step treatment process for the treatment of waste water: preliminary treatment, primary treatment, secondary treatment, disinfection, sludge treatment, and sludge de-watering.

Preliminary Treatment

The waste water, which has been collected in pipes all around the city, flows into one of the 14 wastewater treatment plants. The effluent gets filtered through screens, which strain out any large trash (natural and man-made), including soda cans, newspapers, and tree limbs and sticks. The garbage is kept out of the system to prevent it from damaging the sewage apparatus, and is sent to landfills.

Primary Treatment

After the trash is filtered out, the effluent is pumped into sedimentation tanks, and settles there for one to two hours. Because the water’s flow is slow, heavy solids begin to sink, while lighter solids float to the top of the water. The floating solids (including grease and small bits of plastic) are skimmed off. The heavy solids are called primary sludge, and cyclone degritters — which use centrifugal force to separate the grit within the heavy solids from the sludge — clean it up. The grit, which includes sand, coffee grinds, and gravel, is washed and sent to a landfill. The remaining sludge is siphoned off to a separate part of the facility, while the now-partially treated waste water leaves the sedimentation tanks.

Secondary Treatment

In the activated sludge process, which lasts three to six hours, air and “seed sewage” are pumped into the waste water, promoting the growth of microorganism that eat much of the residual organic pollutants in the water. The feeding of the microorganisms also produce heavy particles, which then settle, along with other heavy solids, as the water flows into the final settling tanks. Called secondary sludge, some of the material is then circulated back to act as “seed sewage” for the activated sludge process. The secondary sludge that is not sent back is combined with the primary sludge from the primary treatment.


The wastewater that was aerated in the secondary treatment then gets disinfected with sodium hypochlorite (the main ingredient in bleach solutions). Immediately after treatment, the waste water is discharged into local waterways.

Sludge Treatment

The sludge that is collected in the primary and secondary treatments is 90% water; before it can be treated, it is sent to thickening tanks to collect and separate from the water. The water is then sent back to secondary treatment to aerate, while the sludge moves to tanks where it is heated to at least 95ºF for fifteen to twenty days. Anaerobic bacteria, which live in oxygen-free environments, grow naturally and consume much of the organic matter in the sludge. Their digestion process modifies the chemical structure of the sludge, converting much of it into methane gas, carbon dioxide, and water. The methane is often used to produce electricity and provide heat for the treatment plants. Recently added fuel cells in four of the plants help create more electricity out of the methane gas and carbon dioxide.

Sludge Dewatering

The sludge that remains after the heating process, called digested sludge, flows into dewatering tanks. Using centrifugal force, the water and solids are separated. The water is reprocessed, and the sludge, now called biosolids, is mixed with organic polymers to create biosolid “cakes.” The cakes are used in fertilizers.

Biosolids as Fertilizers The biosolids that are used for fertilizers can be toxic; despite the cleaning process, they contain heavy metals, hormones, pathogens, and pharmaceuticals. However, biosolid cakes can be tested for dangerous heavy metals like lead and mercury, and a variety of pathogens can be killed by a simple heating process.


All references last accessed April 10, 2010

“CSO Best Management Practices,” New York State Department of Environmental Conservation. Web.

“Stormwater Discharges From Municipal Separate Storm Sewer Systems (MS4s),” U.S. Environmental Protection Agency. Web.

“New York City’s Wastewater Treatment System,” New York City Department of Environmental Protection, 2007. Web-PDF.

“New York City’s Wastewater Treatment System–Wastewater Treatment Plants,” Web.

“New York City’s Wastewater Treatment System–Wastewater Treatment Process,” Web.

“New York Underground,” National Geographic Online. Web.

Table of Contents:


o CSO in NYC
Combined Sewer System
Eliminating CSO

o Drinking Water in NYC
Buried Streams

o Holland

o Water Around the World

o PLANYC Initiatives

Leave a Reply