What are Combined Sewer Overflows?

What are Combined Sewer Overflows?

Combined sewer overflows (CSOs) occur when combined sewer systems—networks that carry both sanitary sewage and stormwater in the same pipe—exceed their capacity during wet weather events. When rainfall generates runoff volumes beyond what the system and treatment plant can handle, the excess mixture of raw sewage and stormwater discharges directly into rivers, lakes, harbors, and coastal waters through engineered overflow points. About 700 communities in the United States have combined sewer systems, according to EPA, concentrated in older cities of the Northeast, Great Lakes region, and Pacific Northwest. Globally, combined sewers serve hundreds of millions of people in Europe, Asia, and Latin America.

Why It Matters

CSOs represent one of the most significant remaining sources of water pollution in the developed world. In its 2004 Report to Congress, the U.S. EPA estimated that combined sewer systems discharge approximately 850 billion gallons of untreated wastewater and stormwater annually—a volume equivalent to nearly 1.3 million Olympic swimming pools. These discharges contain pathogens (fecal coliform bacteria, viruses, parasites), nutrients (nitrogen and phosphorus that fuel algal blooms), suspended solids, heavy metals, pharmaceuticals, and other contaminants that impair water quality and threaten public health.

The public health implications are serious. CSO-impacted waterways frequently exceed recreational water quality standards, forcing beach closures and swimming advisories. In Milwaukee, one study found a 50% increase in children's emergency department visits for diarrheal illness three to seven days after high-volume CSO events among residents served by Lake Michigan drinking water. In England, public outrage over sewage discharges has become a major political issue: Environment Agency monitoring recorded 3.61 million hours of storm overflow spills in 2024, and 291,492 spills lasting 1.87 million hours in 2025.

Climate change is worsening the problem. Heavier rainfall events increase the frequency and volume of overflows, while urbanization continues to add impervious surface that generates more runoff.

The regulatory and financial burden is enormous. Most of the roughly 700 U.S. CSO communities now have Long-Term Control Plans (LTCPs) with enforceable schedules, some running to 2040 (GAO, 2023), and EPA's 2022 Clean Watersheds Needs Survey still counted $36.5 billion in CSO correction needs. London's Thames Tideway Tunnel—a 25-kilometer deep sewer designed to capture CSOs from 34 overflow points along the Thames—cost about £4.5 billion and was fully connected in February 2025. These investments highlight both the severity of the problem and the extraordinary cost of addressing it through conventional gray infrastructure alone.

How It Works / Key Components

Combined sewer systems were the standard design approach when modern sewerage was first constructed in the mid-to-late 1800s. The logic was straightforward: collect everything in one pipe and send it to the treatment plant. Engineers sized the systems for typical wet weather conditions and included overflow points—essentially relief valves—to prevent sewage from backing up into streets and basements during major storms. At the time, dilution by stormwater was considered adequate treatment for the overflow.

CSO control strategies fall into three categories: storage, treatment, and source reduction. Storage approaches—deep tunnels, in-system storage, and offline detention facilities—capture overflows for later treatment at the wastewater plant. Chicago's Tunnel and Reservoir Plan (TARP), known as "Deep Tunnel," includes 109 miles of tunnels with 2.3 billion gallons of storage capacity. These massive infrastructure projects are effective but extraordinarily expensive and take decades to complete.

Treatment approaches apply some level of pollutant removal to overflows before discharge. CSO treatment facilities range from simple screening and disinfection to full secondary treatment. High-rate treatment technologies—including ballasted flocculation (e.g., Actiflo) and compressed media filtration (e.g., Fuzzy Filter)—can treat wet weather flows at rates several times higher than conventional activated sludge, providing meaningful pollutant reduction at lower capital cost than full storage.

Source reduction—keeping stormwater out of the combined system in the first place—is increasingly central to CSO control strategies. Green infrastructure practices (permeable pavements, green roofs, bioretention) reduce runoff volumes entering the combined system, directly reducing overflow frequency. Philadelphia's Green City, Clean Waters program relies primarily on this approach, targeting 9,564 greened acres by 2036 to capture and treat 85% of combined sewer flow, about 8 billion gallons a year. The approach costs less than tunnel construction, delivers co-benefits, and can be implemented incrementally—but requires sustained commitment over decades.

Council Fire's Approach

Council Fire helps municipalities and utilities develop CSO control strategies that balance regulatory compliance with fiscal responsibility and community benefit. We advocate for hybrid approaches that combine targeted gray infrastructure investments with green infrastructure source reduction and real-time control optimization—maximizing overflow reduction per dollar invested while delivering stormwater management, urban greening, and community amenity co-benefits. Our climate resilience perspective ensures that control plans account for projected changes in precipitation intensity, not just historical patterns.

Frequently Asked Questions

How do cities decide between gray and green CSO solutions?

Most cities adopt integrated or hybrid approaches rather than choosing exclusively gray or green. The optimal mix depends on site-specific factors: available land for green infrastructure, soil infiltration rates, system hydraulics, regulatory timelines, and capital budgets. Deep tunnels and storage facilities provide large-volume, high-certainty CSO capture but cost billions and take a decade or more to construct. Green infrastructure is less expensive per gallon of runoff reduced and delivers co-benefits, but requires distributed implementation across thousands of sites and decades to reach full effectiveness. Consent decree negotiations increasingly allow adaptive management approaches—starting with green infrastructure implementation, monitoring results, and adjusting the gray infrastructure program based on demonstrated performance.

What role does real-time control play in CSO management?

Real-time control (RTC) uses sensors, telemetry, and automated gate/pump operations to optimize the use of existing system capacity during wet weather. By dynamically routing flows to underutilized portions of the collection system and maximizing in-system storage, RTC can reduce CSO volumes without any new construction. In South Bend, Indiana, a city-pioneered smart sewer program using real-time control, together with the first phase of the city's consent-decree projects, cut overflows to the river by 75%, from over 2 billion gallons a year to less than 500 million, by 2017. RTC is increasingly recognized as a cost-effective first step that can reduce the scale of capital investments needed to meet regulatory targets.

Are CSOs a problem that can actually be solved?

Several cities have effectively eliminated or dramatically reduced CSOs. Portland, Oregon reduced CSO volume by 94% through a combination of tunnel construction and source control. Atlanta invested $4 billion in sewer separation and storage. Complete elimination of CSOs through sewer separation—building separate storm and sanitary sewer systems—is technically feasible but prohibitively expensive for most cities. The practical goal for most communities is reducing CSOs to a frequency and volume that meets water quality standards in receiving waters—typically 4–6 overflow events per year. Full elimination may eventually be achievable through continued green infrastructure deployment, real-time control optimization, and climate-adaptive infrastructure investment over multiple decades.