Sanitary Sewer Overflows

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Sanitary sewer overflows

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 basements 400,000 times a year in the United States.

Ssos pose a serious threat to human health and to the environment . Raw sewage can introduce disease-causing organisms into drinking water and swimming and fishing areas. People may swallow contaminated water or eat contaminated fish or shellfish. Diseases also can be contracted by breathing in organisms from sewage or by absorption through the skin. Although most such illnesses are mild stomach upsets, there is a risk of life-threatening diseases such as cholera , dysentery, and hepatitis. Ssos are a major cause of beach closures. In 2000, an Sso discharged an estimated 72 million gal (273 million l) of raw sewage into the Indian River, causing drinking-water advisories and beach closures throughout much of Florida. Sewage contamination can rob water of oxygen and result in floating debris and harmful blooms of algae. Sewage from Ssos can destroy habitat and harm plants and animals.

Ssos often occur when excess rainfall or snowmelt in the ground enters leaky sanitary sewers that were not built to hold rainfall or which do not drain properly. Excess water also can enter sewers from roof drains that are connected to sewers, through broken pipes, or through poorly connected service lines. In some cities, as many as 60% of Ssos originate at service connections to buildings. Sometimes new subdivisions or commercial buildings are connected to systems that are too small. Ssos also can occur when pipes crack, break, or become blocked by tree roots or debris. Sediment , debris, oil, and grease can build up in pipes, causing them to break or collapse. Sometimes pipes settle or shift so that the joints no longer match. Pump or power failures also can cause Ssos. Many sewer systems in the United States are 30100 years old and the EPA estimates that about 75% of all systems have deteriorated to where they carry only 50% or less of their original capacity. Ssos are also caused by improper installation or maintenance of the system and by operational errors. Some municipalities have had to spend as much as a billion dollars correcting sewer problems or have halted new development until the system is fixed or its capacity increased.

Most Ssos are avoidable. Cleaning, maintenance, and repair of systems, enlarging or upgrading sewers, pump stations, or treatment plants, building wet-weather storage and treatment facilities, or improving operational procedures may be necessary to prevent Ssos. In some cases backup facilities may be required. Many municipalities are developing flow-equalization basins to retain excessive wet-weather flow until the system can handle it. The EPA estimates that an additional 12 billion dollars will be required annually until 2020, if the nation's sewer systems are to be repaired.

In 1995 representatives from states, municipalities, health agencies, and environmental groups formed the Sso Federal Advisory Subcommittee to advise the EPA on Sso regulations. In May 1999 President Bill Clinton ordered stronger measures to prevent Ssos that close beaches and adversely affect water quality and public health. Each EPA region is now required to inventory Sso violations and to address Ssos in priority sewer systems. In January 2001 the EPA's Office of Wastewater Management proposed a new SSO Rule that will expand the Clean Water Act permit requirements for 19,000 municipal sanitary sewer collection systems to reduce Ssos. Furthermore, some 4,800 additional municipal collection systems will be required to obtain permits from the EPA. Both the public and health and community officials will have to be notified immediately of raw sewage overflows that endanger public health.

[Margaret Alic Ph.D. ]



Office of Water. Benefits of Protecting Your Community from Sanitary Sewer Overflows. Washington, DC: U.S. Environmental Protection Agency, 2000.


Bell Jr., Robert E., and Maggie L. Powell. "Pending SSO Regulations." Water Engineering & Management 148, no. 4 (April 2001): 3037.

Hobbs, David V., Eva V. Tor, and Robert D. Shelton. "Equalizing Wet Weather Flows." Civil Engineering 69, no. 1 (January 1999): 569.

Whitman, David. "The Sickening Sewer Crisis: Aging Systems Across the Country are Causing Nastyand Costly Problems." U.S. News & World Report (June 12, 2000): 1618.


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American Public Works Association, 2345 Grand Blvd., Suite 500, Kansas City, MO USA 64108-2641, (816) 472-6100, <>

U.S. Environmental Protection Agency Office of Wastewater Management, Mail Code 4201, 401 M Street, SW, Washington, DC, USA 20460 (202) 260-7786, Fax: (202) 260-6257, <>