The sewer pipes are made of ceramic, or brick and concrete if they are older. A downhill gradient moves the sewage through the system to a treatment plant. In a storm the runoff may cause the sewage to join with storm runoff and run directly into the river. 1,700,000,000 gallons of sewage are produced each day. Grease, scum, and oil rise to the top and are collected.
Air is pumped through the water to stimulate bacteria to feed on suspended and dissolved matter.
In a final settling tank, the bacteria is removed. The water is chlorinated and pumped into the East River. Raw sewage used to go straight into the river. NYC sludge contains heavy metals, which cannot be removed by treatment. In the future, sewage sludge may be burned or landfilled, like the citys trash.