Sanitary sewer overflows (Ssos) are discharges of untreated sewage from municipal sanitary sewer systems that were not designed to carry storm-water runoff. Almost all sewer systems experience at least occasional Ssos and they occur frequently in some systems. Ssos are a major source of water pollution in lakes, rivers, and streams. Although they are illegal under the Clean Water Act, the U.S. Environmental Protection Agency (EPA) estimates that at least 40,000 Ssos occur annually nationwide.

Of the approximately 50 trillion gal (189 trillion l) of raw sewage that flow daily through about 19,500 sewer systems in the United States, it is estimated that about 1.2 billion gal (4.5 billion l) were released in Ssos in 2000. This raw sewage is discharged from manholes, bypassing pump stations and treatment plants. It flows into basements, lawns, streets, parks, streams, swimming areas, and drinking water. It is estimated that sewers back up into...