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Separate Sewer System

The separate sewer system covers more than one-third of the sewer service area in Philadelphia with approximately 455 stormwater outfalls throughout the city.


A separate sewer system consists of two different sewer pipes running one on top of the other, or “piggyback.” In most instances, the sanitary pipe is below the storm pipe. The sanitary sewer pipe transports sanitary sewage collected from the laterals (plumbing connections) of homes, businesses, and industry to treatment plants. The stormwater sewer pipe carries water collected from street inlets, building downspouts, and other storm sewer lines to a nearby receiving stream and is discharged through a Stormwater Outfall.

Stormwater Outfalls

Although stormwater outfalls are located in every watershed in Philadelphia — most occur in the larger separate sewer regions of the City, particularly in the Wissahickon, Schuylkill, Pennypack and Poquessing watersheds — approximately 455 stormwater outfalls exist throughout the City.

Potentially Polluted Stormwater

Although the separate sewer system does not pose immediately apparent environmental hazards and safety concerns (as in the case of the combined sewer system), there is a potential to discharge polluted waters from runoff of buildings and streets (e.g. metals, debris, animal waste, etc.). This concern arises because the water from the stormwater sewer inlets and pipes is not treated before discharging into waterways.

Defective Laterals

Due to problems generally attributed to improper installation or lack of oversight during construction, sanitary wastewater from some properties can be transported into the storm sewers and, from there, to the streams and rivers. This intrusion of sanitary wastewater causes pollution of the streams and rivers that are the source of city’s water supply. The polluted streams and rivers also endanger the physical health and safety of residents and users of the streams.