Pittsburgh Sewage Overflow: River Discharges vs. Basement Backups

Ask three people in this region what a Pittsburgh sewage overflow is and you may get three answers. One means a pipe outlet spilling into a river, another means a manhole popping, and the third means a basement that smells like a drain.

They share a cause, which is rain arriving faster than the pipes can carry it away. They do not share an owner or a safety rule, so it helps to pull them apart, starting with how the Pittsburgh combined sewer works.

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The river version, as ALCOSAN defines it

ALCOSAN describes a combined sewer overflow, or CSO, as stormwater and untreated, diluted sewage in a single pipe overloading the sewer system and flowing into rivers and streams. To warn river users, it runs advisory programs from April 1 until October 31 and raises orange flags at designated points along the waterways when overflows are under way.

Nine flag sites are listed, among them James Sharp Landing in Sharpsburg and the Braddock Boat Ramp. ALCOSAN also says river water quality usually does not return to normal until up to 48 hours after an overflow ends, and that the advisories have nothing to do with drinking water.

Rainwater running along a wet street past a manhole cover toward a curbside storm drain.

When it is your own basement

Across approximately 75 percent of the city, sewage and stormwater travel in the same underground pipes. In dry weather, that Pittsburgh combined sewer carries household sewage to the treatment plant. When it rains, street drains feed stormwater into the same pipes, and Pittsburgh Water says the excess can overwhelm them.

The authority's list of consequences runs past the rivers: combined sewage and stormwater can back up into basements and surcharge up through manholes when the pipes are overwhelmed. An orange river flag says nothing about your own drain. Nothing on ALCOSAN's advisory page links the flags to basements.

Who owns which pipe

Pittsburgh Water maintains the sewer main and a small portion at the joint known as a wye, while the entire lateral belongs to the property owner. Beyond the city line, the arrangement is regional: according to ALCOSAN's Clean Water Plan, 83 customer municipalities own and operate their own collection sewers, and ALCOSAN owns and operates the major interceptor and treatment system.

3 Rivers Wet Weather calls Allegheny County unique for that reason, with 83 municipalities running their own collection systems that all feed one treatment facility. A borough resident therefore starts with the borough, not with ALCOSAN.

Reports of overflows and backups blur together; what separates a backup from an overflow explains the terms as EPA and utilities use them.

Why storms keep making headlines

Pittsburgh Water states that as storms become more intense, it sees more frequent sewer overflows and floods, and it describes the number of flooding and backup events as increasing. It also admits that some portions of the public sewer lack the capacity to handle heavy rainfall or snowmelt.

Projects aimed at the overflows

Pittsburgh Water points to sewer improvements and green stormwater projects meant to capture and filter rainwater before it enters the pipes, and a rehabilitation round for infrastructure it calls often more than a century old, beginning in Hazelwood. ALCOSAN calls wet weather treatment capacity a cornerstone of its overflow strategy, with its plant expansion page giving a target of 600 mgd by the end of 2029.

If it reaches your house

When an overflow turns into sewage inside the home, the practical steps are on the sewage backup guide, and the storm pattern is covered on the page about basement sewage after heavy rain. For how this website fits in, the referral service overview explains it in a few lines.

If the smell is the problem rather than the spill, where to take a sewage odor report sorts out the utility, the health office and the septic provider.

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Frequently asked questions

No. ALCOSAN says its alerts began in 1995 to give public notice when overflows and runoffs increase the likelihood of river contamination. Its advisory page does not tie them to basements.
ALCOSAN answers that question with a plain no. It says drinking water treatment processes are adjusted to address changes in river water quality.
Per ALCOSAN's FAQ, a sewer back-up goes to the municipality, either its public works department or its sewer authority, because the local provider handles basement backups.