New York City's Wastewater Treatment System
Wastewater Treatment Process
Every day, wastewater goes down toilets and drains in homes, schools, businesses and factories and then flows into New York City's sewer system. Runoff from rain and melting snow, street and sidewalk washing, and other outdoor activities flows into catchbasins in the streets and from there into the sewers. In some New York City neighborhoods, runoff from the streets is carried by separate storm sewers directly to local streams, rivers and bays. In most areas of the City, sanitary and industrial wastewater, rainwater and street runoff are collected in the same sewers and then conveyed together to the City's treatment plants. This is known as a combined sewer system. Sometimes, during heavy rains or snow, combined sewers fill to capacity and are unable to carry the combined sanitary and storm sewage to the plants.When this occurs the mix of excess storm water and untreated sewage flows directly into the City's waterways. This is called combined sewer overflow (CSO). Approximately 70 percent of the City sewers are combined.
Wastewater treatment plants, also called sewage treatment plants or water pollution control plants, remove most pollutants from wastewater before it is released to local waterways. At the plants, physical and biological processes closely duplicate how wetlands, rivers, streams and lakes naturally purify water. Treatment at these plants is quick, taking only about seven hours to remove most of the pollutants from the wastewater. In the natural environment this process could take many weeks and nature alone cannot handle the volume of wastewater that New York City produces.
At the City's wastewater treatment plants, wastewater undergoes five major processes: preliminary treatment, primary treatment, secondary treatment, disinfection and finally, sludge treatment. Primary and secondary treatments remove about 85% to 95% of pollutants from the wastewater before the treated wastewater is disinfected and discharged into local waterways. Sludge, the byproduct of the treatment process, is digested for stabilization and is then dewatered for easier handling. The resulting material, known as biosolids, is then applied to land to improve vegetation or processed further as compost or fertilizer. (See illustration — “Diagram of Treatment Process” on page 16-17.)
Several stories underground, wastewater flows into the plants from sewers connected to New York City's homes and businesses. The incoming wastewater, called influent, passes through screens consisting of upright bars, spaced one to three inches apart. These bars remove large pieces of trash including rags, sticks, newspaper, soft drink cans, bottles, plastic cups and other similar items. This protects the main sewage pumps and other equipment. The garbage is transported to landfills. The main sewage pumps then lift the wastewater from the screening chamber to the surface level of the plant.back to top
Next, the wastewater enters primary settling tanks, also called sedimentation tanks, for one to two hours. The flow of the water is slowed, allowing heavier solids to settle to the bottom of the tank and the lighter materials to float. At the end of the process, the floatable trash, such as grease and small plastic material, rises and is skimmed from the top of the tanks surface.
The settled solids, called primary sludge, are then pumped through cyclone degritters — devices that use centrifugal force to separate out sand, grit (such as coffee grinds) and gravel. This grit is removed, washed and taken to landfills.
The degritted primary sludge is pumped to the plant's sludge handling facilities for further processing. The partially treated wastewater from the primary setting tanks then flows to the secondary treatment system.back to top
Secondary treatment is called the activated sludge process. This is because air and “seed” sludge from the plant treatment process are added to the wastewater to break it down further. Air pumped into large aeration tanks mixes the wastewater and sludge that stimulates the growth of oxygen-using bacteria and other tiny organisms that are naturally present in the sewage. These beneficial microorganisms consume most of the remaining organic materials that are polluting the water and this produces heavier particles that will settle later in the treatment process.Wastewater passes through these bubbling tanks in three to six hours.
The aerated wastewater then flows to the final settling tanks which are similar to the primary settling tanks. Here the heavy particles and other solids settle to the bottom as secondary sludge. Some of this sludge is re-circulated back to the aeration tanks as “seed” to stimulate the activated sludge process. The returned sludge contains millions of microorganisms that help maintain the right mix of bacteria and air in the tank and contribute to the removal of as many pollutants as possible.
The remaining secondary sludge is removed from the settling tanks and added to the primary sludge for further processing in the sludge handling facilities.Wastewater passes through the settling tanks in two to three hours and then flows to a disinfection tank.back to top
Even after primary and secondary treatment, diseasecausing organisms may remain in the treated wastewater. To disinfect and kill harmful organisms, the wastewater spends a minimum of 15-20 minutes in chlorine-contact tanks mixing with sodium hypochlorite, the same chemical found in common household bleach. The treated wastewater, or effluent, is then released into local waterways. Disinfection is an essential step because it protects the health of people who use local beaches and enjoy other recreational activities on or near the water.back to top
The following are typical stages of the sludge treatment process.
The sludge produced by primary and secondary treatment is approximately 99% water and must be concentrated to enable its further processing. Thickening tanks allow the sludge to collect, settle and separate from the water for up to 24 hours. The water is then sent back to the head of the plant or to the aeration tanks for additional treatment.back to top
After thickening, the sludge is further treated to make it safer for the environment. The sludge is placed in oxygenfree tanks, called digesters, and heated to at least 95 degrees Fahrenheit for between 15 to 20 days. This stimulates the growth of anaerobic bacteria, which consume organic material in the sludge. Unlike the bacteria in the aeration tanks, these bacteria thrive in an oxygen-free or “anaerobic” environment. The digestion process stabilizes the thickened sludge by converting much of the material into water, carbon dioxide and methane gas. The black sludge that remains after digestion has the consistency of pea soup and has little odor. This is called digested sludge.
Methane gas is often used as an energy source at the City's wastewater treatment plants. The gas may be used in engines to produce electricity or directly drive plant equipment. Gas is also used in boilers to provide heat for digestion and plant-wide buildings. Currently, DEP and the New York Power Authority (NYPA) have jointly installed fuel cells at four of the City's water pollution control plants; 26th Ward, Red Hook, Oakwood Beach and Hunts Point. Fuel cells convert the methane gas and carbon dioxide into heat and electricity that is then used to operate the plants. This technology contributes to New York City's efforts to enhance clean air operations at its facilities. There is a significant reduction in air emissions as a result of using fuel cells.
Digester sludge is pumped from sludge storage tanks to a dewatering facility. At some treatment plants, where there are no dewatering facilities on site, the sludge is transported for processing through a pipeline or by a sludge boat to a plant that has a dewatering facility. (See pictures of sludge vessels.)back to top
Dewatering reduces the liquid volume of sludge by about 90%. New York City operates dewatering facilities at eight of its 14 treatment plants. At these facilities, digested sludge is sent through large centrifuges that operate like the spin cycle of a washing machine. The force from the very fast spinning of the centrifuges separates most of the water from the solids in the sludge, creating a substance knows as biosolids. The water drawn from the spinning process is then returned to the head of the plant for reprocessing. Adding a substance called organic polymer improves the consistency of the “cake”, resulting in a firmer, more manageable product. The biosolids cake is approximately 25 to 27percent solid material.back to top