Combined Sewer System

Across approximately 75 percent of Pittsburgh, sewage and stormwater are routed into the same underground sewer pipes. During dry weather, the sewage from buildings flows into these combined sewer pipes and is carried to the wastewater treatment plant to be cleaned then discharged into the river.

When it rains, storm drains on our streets direct the stormwater into the sewer pipes that are already carrying sewage. Excess stormwater can overwhelm the capacity of those pipes, causing some of the combined stormwater and sewage to overflow into the rivers at the sewer outfall. 

Side-by-side diagrams of the combined sewer system during dry and wet weather, showing where sewage and stormwater go

Combined sewer overflows, also known as CSOs, make it dangerous to come in contact with river and stream water during overflow events. Combined sewage and stormwater can also back up into basements and surcharge up through manholes when the pipes are overwhelmed, causing public health issues.

PWSA is continuing to enhance our stormwater program to address stormwater management issues such as CSOs, basement sewage backups, stormwater pollution, and flooding. We are using solutions such as sewer improvements and green stormwater infrastructure projects that mimic nature to capture and filter rainwater before it enters sewer pipes. Learn more on our Stormwater webpage. 

Combined Sewer Overflow Advisory Alerts

The Allegheny County Sanitary Authority (ALCOSAN) treats our region's wastewater, and once treated, discharges it into the Ohio River where that treated water becomes drinking water for downstream communities. ALCOSAN also manages numerous combined sewer outfalls located along our rivers and streams. During heavy rains, these outfalls will sometimes overflow causing combined sewage to spill into local waterways. 

To alert the public of possible river contamination from combined sewer overflows, ALCOSAN has developed the Sewer Overflow Advisory Key (SOAK) and CSO Flag Program. These programs caution recreational users to limit contact with river water when boating, fishing, water skiing, or engaging in other recreational activities during overflow events.

If you would like to receive these alerts or learn more about SOAK, please visit By subscribing, you will receive text messages or emails when an overflow occurs; be alerted when the overflow condition stops, and when the system returns to dry weather conditions - typically 48 hours later.

During overflow events, ALCOSAN will raise orange CSO flags at designated points along the waterway to notify the public. Within Pittsburgh those locations include:

  • Peggy Harbor
  • Three Rivers Rowing Association
  • Washington's Landing
  • Riverquest
  • Riverfront Park
Find Your Sewershed

Learn more about the combined or separated sewer area that you live in using our Find Your Sewershed webmap.