An official website of the United States government.

This is not the current EPA website. To navigate to the current EPA website, please go to This website is historical material reflecting the EPA website as it existed on January 19, 2021. This website is no longer updated and links to external websites and some internal pages may not work. More information »

National Pollutant Discharge Elimination System (NPDES)

Combined Sewer Overflows (CSOs)

A combined sewer system (CSS) collects rainwater runoff, domestic sewage, and industrial wastewater into one pipe. Under normal conditions, it transports all of the wastewater it collects to a sewage treatment plant for treatment, then discharges to a water body. The volume of wastewater can sometimes exceed the capacity of the CSS or treatment plant (e.g., during heavy rainfall events or snowmelt). When this occurs, untreated stormwater and wastewater, discharges directly to nearby streams, rivers, and other water bodies.

Combined sewer overflows (CSOs) contain untreated or partially treated human and industrial waste, toxic materials, and debris as well as stormwater. They are a priority water pollution concern for the nearly 860 municipalities across the U.S. that have CSSs.

CSO Control Policy

EPA’s CSO control policy is a national framework for controlling CSOs through the NPDES permitting program. It provides guidance on how communities with CSOs can achieve Clean Water Act (CWA)(274 pp, 571 K, About PDF) goals in a flexible, cost-effective manner. The CSO control policy also defines expectations for regulated communities, state water quality standards (WQS) authorities, and NPDES authorities.

In support of the CSO control policy, EPA developed the following guidance documents:

Top of Page