Imagine a rainstorm rolling through Philadelphia on a July afternoon. Nothing unusual — just a couple of inches over a few hours. Below the streets, however, something far less ordinary is happening. The city's sewer system, designed more than a century ago to carry both household sewage and stormwater in the same pipes, is filling beyond capacity. Within minutes, relief valves open. A torrent of raw sewage — mixed with rainwater, household waste, industrial discharge, pharmaceuticals, and anything else that found its way down a drain — begins pouring untreated into the Delaware River and the Schuylkill River.
Philadelphia is not unique. It is not even unusual. It is one of roughly 770 communities across the United States that still rely on combined sewer systems, a 19th-century design choice that becomes a 21st-century public health crisis every time it rains. Between July 2023 and June 2024, nearly 14 billion gallons of raw sewage and polluted stormwater flowed through Philadelphia's waterways. Nationally, the U.S. Environmental Protection Agency estimates that combined sewer overflows release approximately 850 billion gallons of untreated wastewater and stormwater into American rivers, lakes, and coastal waters every year.
That number deserves to sit with you for a moment. Eight hundred and fifty billion gallons. Annually.
How Combined Sewers Work — and How They Fail
Most modern cities operate separate sewer systems: one network of pipes carries sanitary sewage to a treatment plant; another network handles stormwater runoff and routes it to local waterways. But in many of America's oldest cities — concentrated in the Northeast, the Great Lakes region, and the Pacific Northwest — the two flows share a single set of pipes. This is the combined sewer system.
In dry weather, the system works as intended. Everything flows to the wastewater treatment plant, gets processed, and is discharged in compliance with Clean Water Act standards. But during heavy rain or rapid snowmelt, the volume of water entering the system can exceed the capacity of both the pipes and the treatment plant. The system faces a binary choice: back sewage up into basements and streets, or open overflow points and release the mixture — raw, untreated — directly into the nearest body of water.
Cities choose the overflow. Every time.
These discharge points are called combined sewer overflow outfalls, and they are permitted under the National Pollutant Discharge Elimination System. That's right — these discharges of raw sewage are, in a regulatory sense, legal. They are expected. They are engineered into the system.
What's in the Water
The contents of a combined sewer overflow are not merely unpleasant. They are hazardous. A typical CSO discharge contains untreated human waste, industrial effluent, bacteria including E. coli, Salmonella, Shigella, and Campylobacter, viruses such as norovirus and Hepatitis A, parasitic protozoa including Giardia and Cryptosporidium, heavy metals, oil and grease, pharmaceuticals, and household chemicals. CSO events can increase the concentration of waterborne pathogens in receiving waters by one to two orders of magnitude compared to dry weather conditions.
The health consequences are measurable. A study of Massachusetts communities along a CSO-impacted river found a 62 percent increase in the cumulative risk of acute gastrointestinal illness in the four days following a major overflow event compared to days with no CSO discharges. A separate 2015 analysis showed a 13 percent rise in emergency room visits for gastrointestinal illness in areas with combined sewers that discharged into drinking water sources following extreme precipitation.
These are not theoretical risks. They are documented, quantified, and ongoing.
The Scale of the Problem, City by City
The national statistics are staggering, but the city-level numbers make the crisis tangible.
New York City discharges approximately 18 billion gallons of combined sewer overflow per year. Newtown Creek, a single waterway in Brooklyn and Queens, receives over 1.2 billion gallons of CSO annually.
Chicago experienced a particularly severe event in July 2023, when several days of heavy rainfall forced approximately 3.2 billion gallons of untreated wastewater and stormwater into the Chicago River.
The Merrimack River in Massachusetts received over 2 billion gallons of CSO discharges from five cities in 2023 — nearly four times the average annual volume of about 550 million gallons.
In the Great Lakes region alone, more than 24 billion gallons of untreated sewage and stormwater are discharged into the lakes each year — the same lakes that provide drinking water to tens of millions of people.
These are not fringe waterways. These are the rivers people fish in, the lakes children swim in, the sources communities draw their drinking water from.
The Cost of Fixing It
The reason combined sewer systems persist is simple: replacing them is extraordinarily expensive. Separating a combined sewer system requires tearing up streets, laying new pipe networks, building new treatment capacity, and often restructuring the entire stormwater management approach for a city. The price tags are measured in billions.
Kansas City's CSO control program carries an estimated cost of $3.3 billion. Boston's updated CSO control plan is projected at up to $4.7 billion. Louisville's consent decree with the EPA originally priced its compliance plan at $1.15 billion, later extended with additional projects through 2035. Akron, Ohio has already spent $1 billion and faces another $209 million project — with residential utility rates expected to rise 20 percent to cover the cost.
Even smaller cities face crushing obligations. Anderson, Indiana — population roughly 54,000 — has invested $82.8 million to eliminate 15 of its 28 CSO outfalls, with total costs projected at $200 million. Lowell, Massachusetts faces a $195 million sewer upgrade.
For many of these communities, the infrastructure costs are being imposed through EPA consent decrees — legally binding agreements that require municipalities to reduce overflows to comply with the Clean Water Act. These consent decrees set specific compliance timelines, mandate particular infrastructure improvements, and carry penalties for failure. They are, in effect, the federal government telling cities: fix this, or else.
The cities, in turn, pass the costs to ratepayers. In communities already struggling with aging infrastructure and declining tax bases, this creates a genuine equity crisis. The people least able to absorb utility rate increases are often the ones living closest to the overflow outfalls.
Green Infrastructure: A Partial Answer
There is a growing recognition that the problem cannot be solved with pipes alone. Green infrastructure — rain gardens, bioswales, green roofs, permeable pavement, rainwater harvesting systems — offers a complementary approach by managing stormwater at its source, before it ever enters the combined sewer system.
The logic is straightforward: if you can keep rainwater out of the sewer, you reduce the volume that triggers overflows. The EPA estimates that green infrastructure can offer cost savings ranging from 15 to 80 percent compared to traditional gray infrastructure approaches. DC Water is implementing a $90 million green infrastructure plan that reduces the necessary size of a $2.6 billion storage tunnel project. Seattle Public Utilities is combining green infrastructure with storage capacity and system optimization to capture 99 percent of its CSOs.
Green infrastructure also delivers co-benefits that concrete tunnels cannot: reduced urban heat island effects, improved air quality, enhanced property values, increased biodiversity, and better quality of life in neighborhoods that often receive the least public investment. Richmond, Virginia has committed $10 million to green infrastructure projects specifically to reduce runoff entering its combined sewer system.
But green infrastructure is not a silver bullet. It can reduce the frequency and volume of overflows, but in cities with severely undersized systems, physical separation or deep tunnel storage remains necessary. The most effective approaches combine both strategies.
Climate Change Is Making It Worse
Combined sewer systems were designed for the rainfall patterns of a different era. Climate change is delivering heavier, more intense precipitation events — exactly the conditions that overwhelm these systems. The Merrimack River's fourfold increase in CSO discharges in 2023 was driven in significant part by precipitation that far exceeded historical norms.
This means the infrastructure gap is not static. It is widening. Cities are chasing a moving target, trying to build capacity for storms that are growing more severe faster than engineers can design for them. The 850 billion gallons discharged annually is not a ceiling. It is a number that will grow unless investment accelerates dramatically.
What Transparency Looks Like
One of the most frustrating aspects of the CSO crisis is how little most people know about it. Combined sewer overflow outfalls are physical structures — pipes that discharge into waterways at specific, known locations. They are permitted. They are monitored, at least in theory. Yet in most communities, there is no real-time public notification when an overflow occurs. You can be kayaking on a river downstream of a CSO outfall and have no idea that raw sewage entered the water an hour earlier.
The EPA's full implementation of electronic CSO reporting — Phase 2 of the 2015 NPDES Electronic Reporting Rule — was not expected until December 2025. Until that data flows into publicly accessible databases, the true scope of the problem in many communities remains opaque. Citizens deserve to know when their waterways are being used as emergency sewage conduits. That information should be immediate, accessible, and impossible to miss.
Where We Stand
At the EPR Foundation, we believe that environmental protection begins with honest accounting. Combined sewer overflows represent one of the most significant — and most underreported — water quality challenges in the United States. The infrastructure is old. The costs are enormous. The health risks are documented. And for decades, the problem has been largely invisible to the public because it happens underground, out of sight, and only surfaces when it rains.
We are not calling for blame. These systems were built in an era before modern environmental science, before the Clean Water Act, before anyone understood what untreated sewage does to an aquatic ecosystem at scale. The engineers who designed them were solving a different problem — keeping sewage off city streets — and they succeeded.
But we are calling for urgency. Eight hundred and fifty billion gallons of raw sewage entering American waterways every year is not a legacy problem. It is a present-tense crisis. It demands federal investment at a scale commensurate with the infrastructure deficit, transparent real-time reporting so communities know what is entering their water, and creative approaches that combine gray and green solutions to stretch every dollar further.
The rain is not going to stop. The sewage is not going to treat itself. The only question is whether we choose to act with the seriousness this problem demands — or continue to let it wash downstream, unseen and unaccounted for.