Pittsburgh’s Problem with Combined Sewage Overflow

Combined sewage overflow is a system that collects storm water and sewage all in one pipe. The combined water makes its way to a wastewater treatment plant, where it is then treated and eventually released back into a nearby waterbody. In times of heavy precipitation, combined sewage overflow systems often exceed the capacity of the treatment plant and must be released as raw effluent into a waterbody to release pressure in the system. CSOs are a major problem in approximately 772 cities in the United States, including Pittsburgh. Pittsburgh’s sewers can only accommodate about a tenth of an inch of rainfall before the treatment plants cannot treat all of the influent and begin releasing raw effluent into our area's rivers.

  Graphic showing the process of combined sewage overflows. Photo by NY Department of Environmental Conservation..

Graphic showing the process of combined sewage overflows. Photo by NY Department of Environmental Conservation..

Aside from fixing the city’s complicated infrastructure, developing green spaces around the city is a useful mitigation technique. Green infrastructure diverts precipitation from falling onto impermeable surfaces and heading directly towards the storm drains. Instead, the water permeates the green spaces where it is able to percolate through the soil. This absorption gives the sewer system more time to treat the excess rainwater and wastewater. Green spaces also increase urban livability and general happiness of a city, adding value to the landscape.

The issue of combined sewage overflow is an obstacle that many aging cities with old infrastructure are faced with. However, through infrastructure changes and the addition of green spaces, cities can mitigate the problem and move onto a greener and more sanitary future.

The Allegheny County Sanitary Authority recently developed a Wet Weather Plan to be fully implemented by 2026 that will reduce the Combined Sewage Overflow occurrences and make infrastructure improvements.

Marissa Rollman