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.
First reaction is usually "wait, what?" Followed by "why does that matter?"
What Is a Combined Sewer System?
Most newer cities and suburbs have separate infrastructure: one set of pipes for stormwater runoff, another for sewage. In Philadelphia, roughly 60 percent of the city runs on a combined system where both share the same pipes. This includes most of the older housing stock in Northwest Philadelphia, covering neighborhoods like Mt. Airy, Germantown, Chestnut Hill, and Roxborough.
During dry weather, this works fine. Everything flows to treatment plants as intended. The problems show up during heavy storms, when a surge of water pushes through the system all at once. Treatment plants have limits. 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 the Wissahickon.
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 measurable 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 that history has 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 with combined sewer systems, concentrated mostly 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 these neighborhoods were built. Where else was the water supposed to go?
This isn't a Philadelphia failure. It's a Philadelphia reality. 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 buying older housing stock. Combined sewers are part of the package, and the city is actively working to improve the system.
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 things worth knowing:
- Your sewer lateral: This is the pipe connecting your home to the municipal sewer line. In neighborhoods like Germantown, Roxborough, and East Falls, many of these laterals are original clay or cast iron. Keeping your lateral clear and in good shape matters. See my post on trees and root intrusion for more on why.
- Backwater valves: In rare cases, a heavy storm can cause a backup regardless of how clean your lateral is. A backwater valve prevents that sewage from flowing back into your home. A licensed plumber can confirm how your specific property is tied in and whether one makes sense.
- Green stormwater infrastructure: Some blocks in Northwest Philly have rain gardens, permeable pavers, or tree trenches installed by the city as part of Green City, Clean Waters. These are a good sign.
This Is Worth Understanding, Not Worrying About
Philadelphia's combined sewer system surprises almost every buyer coming from newer construction. It's not something your home inspector will flag as a defect, because it isn't one. It's urban infrastructure with a long history and a funded plan for improvement.
Every city has tradeoffs. Dense, walkable neighborhoods with 100-year-old rowhomes are one of the best things about living here. Understanding what comes with that is part of making a genuinely informed decision.
If you're buying in Mt. Airy, Chestnut Hill, Germantown, or anywhere in Northwest Philly and want to know 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.
Henry is a Philadelphia-based REALTOR® serving buyers and sellers in Northwest Philadelphia and Montgomery County, PA. Questions? Get in touch.
