What Are Sanitary Sewer Overflows?

A sanitary sewer overflow (SSO) occurs when raw or partially treated sewage escapes from the sanitary sewer collection system before it can reach a wastewater treatment facility. The sewer collection system is the network of underground pipes, manholes, lift stations, and force mains that carries household toilet waste, sink water, and industrial wastewater from homes and businesses to treatment plants. When that system breaks down — for any reason — sewage has to go somewhere, and it usually goes where it should not.

SSOs can manifest in several ways: sewage bubbling up from manholes onto streets and sidewalks, raw waste backing up into home basements through floor drains, discharge flowing into storm drains that empty to rivers and harbors, or direct discharge through illegal or emergency connections into surface waters. The consequences range from a homeowner dealing with a flooded basement to beachgoers swimming in contaminated water miles from the actual spill point.

SSOs are federally regulated as unpermitted discharges under the Clean Water Act. When they happen, utilities are generally required to notify their state environmental agency, contain the spill if possible, clean up affected areas, and file a report documenting what occurred. That sewer overflow data is what powers this map.

Many people confuse sanitary sewer overflows with combined sewer overflows (CSOs). The distinction matters because they involve different infrastructure, different regulatory frameworks, and different solutions.

This site tracks SSOs only. CSO data is available separately through EPA's NPDES database but requires different analysis.

The 23,000–75,000 annual SSO estimate comes from EPA surveys conducted in the early 2000s. Given continued infrastructure aging and the expansion of mandatory reporting, some researchers believe the true number is higher. Many SSOs — particularly small ones in rural areas or those that do not reach surface waters — go undetected entirely.

At the infrastructure level, the United States operates approximately 1.87 million miles of sewer pipe, much of it installed between 1940 and 1970. With a typical useful life of 50–75 years depending on material and conditions, enormous portions of this network are past their service life or approaching it. The American Society of Civil Engineers gives American wastewater infrastructure a D+ rating in its 2025 Infrastructure Report Card and estimates a $271 billion funding gap over the next two decades.

Reporting completeness varies significantly by your state. Some states capture most reportable events in the federal ECHO database; others have significant underreporting gaps that mean the true scale of overflows in those areas is much larger than what appears in the data.

Raw sewage is a complex biological hazard. Human waste carries a range of pathogens that can cause serious illness, and contact with sewage-contaminated water — even incidental contact, like wading through a flooded street — poses real risk.

Recreational exposure is particularly well-documented. Studies consistently find elevated illness rates among people who swim in water bodies within 72 hours of nearby sewer overflow events. Surfers, swimmers, kayakers, and children playing in beach sand are at elevated risk. Workers who come into direct contact with spilled sewage during cleanup operations face the highest acute exposure risk.

Communities near frequent overflow locations — often lower-income communities and communities of color — bear a disproportionate burden. Multiple studies have found that sewer infrastructure in lower-income zip codes tends to be older, more poorly maintained, and less likely to receive timely repairs or capital upgrades.

Raw sewage entering surface waters carries enormous nutrient loads — primarily nitrogen and phosphorus — along with pathogens, pharmaceuticals, microplastics, and organic matter. The immediate and downstream effects on aquatic ecosystems can be severe.

Dissolved oxygen depletion is among the most acute effects. Bacteria decomposing the organic matter in sewage consume oxygen rapidly, creating hypoxic or anoxic conditions that suffocate fish and other aquatic life. A large SSO into a small stream can cause a fish kill within hours.

Harmful algal blooms (HABs) are a longer-term consequence of nutrient pollution from repeated overflow events. Nitrogen and phosphorus fuel explosive algae growth; when those blooms die and decompose, they again deplete oxygen and sometimes produce toxins dangerous to wildlife, pets, and humans. Lake Erie, the Chesapeake Bay, and numerous coastal waters have experienced worsening HABs partly attributable to sewer system discharges.

Shellfish bed closures are another visible consequence. Oysters, clams, and mussels filter enormous volumes of water and concentrate pathogens from any contamination events. State shellfish programs routinely close harvest areas after SSOs in nearby waters — sometimes for weeks.

Sewer utilities in the United States operate under National Pollutant Discharge Elimination System (NPDES) permits issued by state environmental agencies (or directly by EPA in states without delegated authority). These permits are the primary regulatory tool for controlling what enters the nation's waterways.

The EPA's SSO Reporting Rule (effective 2024, Phase 2 eRule) requires permittees to electronically report all SSOs to EPA through the NetDMR system or state-authorized equivalents. This reporting requirement is relatively recent — many states only began submitting comprehensive SSO data to the federal ECHO database after 2022–2024. This means historical data coverage is uneven, and some states are still ramping up compliance.

Utilities with significant SSO histories may also be subject to Capacity, Management, Operations, and Maintenance (CMOM) program requirements — essentially a mandated audit and improvement plan for their entire collection system. CMOM requirements are often imposed through enforcement consent decrees when a utility has a history of chronic overflows.