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Combined Sewer System

The combined sewer system covers almost two-thirds of the sewer service area in Philadelphia.


Serving more than three-quarters of the city's residents, the combined sewer system is in the oldest and densest parts of the city, including Center City, South Philadelphia, West Philadelphia, North Philadelphia, Bridesburg/Kensington/Richmond, East Mt. Airy and East Germantown, as well as parts of near Northeast Philadelphia.

Combined Sewer Overflows

During dry weather, the combined sewer system and wastewater treatment plants have the capacity to transport and treat all the sanitary sewage entering the system. However, when flow in the sewer increases as a result of rainfall or snowmelt, the sewer pipes or treatment plants may reach their capacity. When this happens, the EPA permits Philadelphia, as it does with other cities with combined sewer systems, to discharge excess wastewater into nearby waterbodies to prevent health and human safety issues that may result from localized flooding in neighborhoods and in treatment plants.

There are 164 combined sewer outfalls (CSOs) along the Delaware and Schuylkill rivers and the Cobbs, Tookany/Tacony-Frankford, and lower Pennypack creeks.

The unfortunate side effect of preventing flooding by allowing CSOs to discharge water is the contamination and erosion of our waterways. As stormwater travels over impervious surfaces, it picks up pollutants, and this polluted water mixes with raw sewage in the combined sewer. When there are overflows, these contaminants end up in our rivers and creeks, causing the waterbodies to be unsafe to recreate in for about 24 hours. The large rush of excess wastewater also scours river and creek beds. The pollution and scouring make it difficult for native plant and animal species to survive, giving way to invasive species and a degraded landscape.

Reducing the amount of stormwater that ends up in the combined sewer system can reduce the number of overflows, allowing our waterways to become healthier.

Related Issues


CSOs affect water quality and are often linked to flooding events, as the volume of precipitation is too great for the sewer system to accommodate.

What we're doing to address these issues


The purpose of this notification system is to alert the public of the possible Combined Sewer Overflows (CSOs) from Philadelphia's combined sewer system outfalls. This system is based on the Philadelphia Water Department's extensive flow monitoring network that has been maintained since 1995 via level sensors that record data throughout the combined sewer system. PWD currently operates and maintains monitoring equipment at or near the 164 CSOs throughout the city.