How Storms Impact Sewer Systems
In many communities—including much of Allegheny County—stormwater and wastewater can enter the same sewer pipes. On dry days, wastewater flows to the ALCOSAN treatment plant, where it is cleaned before being returned safely to the river.
During heavy rain, stormwater can rush into the system and increase flows quickly. Common sources include:
- Roof drains and downspouts
- Yard drains and driveway runoff
- Inflow from aging pipes and manholes
When stormwater volume exceeds capacity, overflow conditions may occur to help prevent backups into homes, streets, and businesses.
What Is a Combined Sewer Overflow (CSO)?
A combined sewer overflow (CSO) is a discharge that can occur during wet weather when stormwater overwhelms a combined sewer system. CSOs may release a mix of stormwater and diluted wastewater into waterways.
Why River Protection Matters After Rain
CSO conditions can temporarily impact water quality. Even when rain stops, impacts can linger—especially in areas where overflow conditions occurred. That’s why ALCOSAN provides public notification tools so river users can make informed decisions about when and where to recreate.
What ALCOSAN Is Doing to Protect Rivers
Protecting rivers during storms requires long-term improvements across the region. ALCOSAN’s work focuses on reducing wet-weather overflows and improving water quality through strategies such as:
Store and treat wet-weather flows
Capturing and treating more water during storms reduces the need for overflows and supports cleaner rivers.
Targeted system improvements
Reducing stormwater entering sewer lines helps manage peak flows during heavy rain.
Clean Water Plan investments
Modernizing regional infrastructure helps reduce overflow impacts over time.
Partnerships across the region
Collaboration helps keep stormwater out of sanitary sewers and improves wet-weather performance.
Stay Informed: Sewer Overflow Advisories and SOAK Alerts
Wet weather conditions can change rapidly. ALCOSAN provides tools to help the public stay informed during river recreation season, including the Sewer Overflow Advisory Key (SOAK) and the CSO Flag Alert Program.
Sign up for SOAK Alerts (Text/Email)
SOAK Alerts notify subscribers when overflow conditions are possible, when overflows have ceased, and when the system returns to dry weather operation 48 hours later (water quality impacts can persist).
Learn more and subscribe on ALCOSAN’s Sewer Overflow Advisories page.
You can also visit the advisories page anytime for current status updates and recent advisories.
How You Can Help Protect Rivers During Storms
Small actions can reduce stormwater impacts and support cleaner waterways.
Reduce stormwater entering sewers
Redirect downspouts to lawns or landscaped areas where appropriate and keep stormwater out of sewer connections.
Reduce runoff at home
Use rain gardens, native plants, or permeable surfaces to help soak up water and slow runoff.
Keep clogs out of the system
Keep fats, oils, and wipes (even “flushable” wipes) out of sinks and toilets—especially during wet weather.