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What are Combined Sewer Overflows (CSOs)?
A combined sewer system is a wastewater collection system owned by a municipality which transports wastewater from homes, businesses and industry, and stormwater from storm drains on our city streets and property roof leaders through a single-pipe system to a Water Pollution Control Plant. In the City of Philadelphia, during dry weather conditions (when it is not raining) and during very small storm events, combined sewers can adequately transport this mixture of sanitary, wastewater and stormwater to one of the City’s three Water Pollution Control Plants for treatment. Under heavier rainfall conditions, however, the flow in combined sewers may exceed the capacity of the pipe or treatment facility. As a result, a portion of the wastewater and stormwater may be diverted directly to a nearby stream or river to prevent the flooding of homes and streets. This is what is known as a Combined Sewer Overflow (CSO). During heavy rainfalls or sudden snowmelts, Philadelphia may experience these overflows in various locations throughout the city from any of its 164 permitted combined sewer outfalls.
CSO outfalls were designed to protect us when pipes became overloaded. CSOs benefit us by preventing backups into our homes and businesses, and by preventing flooding in the streets, and pipes from bursting underground.
Today we understand the adverse impacts CSOs have on our water quality, aquatic organisms, riparian buffers and other vital components of our streams and drinking water sources. As a response to this problem, the Philadelphia Water Department (PWD) is taking multiple innovative and successful measures to reduce these impacts.
Where are CSOs located?
There are 164 permitted CSO outfalls with in the City of Philadelphia. There are CSOs on the Delaware and Schuylkill Rivers, as well as the Pennypack, Tacony-Frankford and Cobbs Creeks.
CSO outfalls in Philadelphia
