If you're moving to Philadelphia from the suburbs or another city, there's a good chance nobody told you this yet... the house you're buying very likely has a combined sewer system. That means the rainwater draining off your roof flows through the same underground pipes as the wastewater from your toilets and sinks.
I know. First reaction is usually, "wait, what?" Followed by, "why does that matter?"
What Is a Combined Sewer System?
Most newer cities and suburbs have built their water and sewer infrastructure as separate systems. One set of pipes carries stormwater runoff, another handles sewage. In Philadelphia, roughly 60 percent of the city has a combined system where both runoff and wastewater from homes and buildings share the same pipes.
During dry weather (and the vast majority of the time), this works fine. No issues. Everything flows to treatment plants as intended. Problems tend to arise when a heavy storm sends a surge of water through your homes pipes and into the larger system at once. Treatment plants have limits, and it can happen that when those limits are hit, the overflow goes directly into local rivers and creeks, untreated. According to Environment America, about 1.5 billion gallons of that overflow reaches Philadelphia's waterways in a given year.
Philadelphia's Water Department tracks this through a tool called CSOcast. When an overflow is active, the affected waterway is unsafe for swimming or recreation and may stay that way for up to 72 hours after the storm passes. Worth bookmarking if you spend time near the Schuylkill or Delaware.
Philadelphia launched its Green City, Clean Waters program back in 2011 specifically to reduce these overflow events over time. It's a long-term investment in green infrastructure, and it's been making a difference. The initiative is on track to meet its pollution reduction goals by 2036.
Why Philadelphia Is Built This Way
Philadelphia is one of the oldest major cities in the country, and density like ours doesn't come without consequences. When most of this city's housing stock was constructed, combined sewers were the standard approach. According to the EPA, "There are approximately 700 communities in the United States that have combined sewer systems and experience combined sewer overflow (CSO) discharges. These communities are mostly located in the northeast and around the Great Lakes." There was no practical alternative for managing stormwater in a tightly packed urban grid at the time many of these communities were built. Where else was the water supposed to go?
This isn't a Philadelphia failure. It's a Philadelphia reality, and the city has been investing in green infrastructure, separated sewer projects, and stormwater management programs for years to address the long-term picture. If you're buying in Mt. Airy, Germantown, Roxborough, or most of Northwest Philadelphia, you're in older housing stock. Combined sewers are part of the package.
What This Means for You as a Buyer
For most buyers, day-to-day life is completely unaffected by this. You're not going to notice it. The combined sewer system isn't a defect in your home. It's the public infrastructure underneath your street.
A few practical things to be aware of:
- Your sewer lateral: Your sewer lateral connects your home to the larger municipal sewer service. In areas with combined sewer systems, it's important to keep your sewer lateral in good shape and free of obstructions. See my post about tree roots.
- Backwater valves: There are cases where no matter how clear your sewer lateral is, a heavy rainstorm can cause a backup. That's where a backwater valve can help. A licensed plumber can confirm how your specific home is tied in and whether a backwater valve makes sense for your property.
- Green stormwater infrastructure: Some blocks have rain gardens, permeable pavers, or tree trenches installed by the city. These are part of the long-term solution and are a great sign.
The Bottom Line
Philadelphia's combined sewer system surprises almost every buyer coming from newer construction. It's not something your home inspector is going to necessarily flag as a defect, because it isn't one. It's urban infrastructure with a long history and a real plan for improvement.
Like any city, Philadelphia has tradeoffs. Dense, walkable neighborhoods with 100-year-old rowhomes with nowhere good to send their rain water are one of them. Understanding what comes with that is part of making a genuinely informed decision.
If you're buying in Northwest Philly and want to talk through what to look for during inspections, reach out. That's what I'm here for.
Sources: Philadelphia Water Department CSOcast, Green City, Clean Waters, EPA — Combined Sewer Overflow Outfalls.
