If you've ever wondered how when there are heavy storms New York City's sewer system gets overwhelmed (check out this video of the Gowanus Canal for the stinking graphic details), there's a new EPA report detailing it all for you. The short reason:
Many communities have separate sewer systems for wastewater collection - an independent sewer system that carries sewage from buildings and another for rainwater, also referred to as stormwater. The stormwater is sent directly to lakes, rivers and streams, while domestic sewage is transported to wastewater treatment plants, where it is treated to remove pathogens and other contaminants.
Combined sewer systems, on the other hand, are designed to transport sewage, industrial wastewater and rainwater runoff in the same pipes to wastewater treatment plants.
Most of the time, combined sewer systems are able to transport all of the wastewater to a treatment plant, where it is treated and then discharged into a water body. During periods of heavy rainfall or snowmelt, however, the volume of wastewater traveling through a combined sewer system can exceed the capacity of the sewer system or treatment plant.
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