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Sanitary sewer overflow


Sanitary sewer overflow (SSO) is a condition in which untreated sewage is discharged from a sanitary sewer into the environment prior to reaching sewage treatment facilities. When caused by rainfall it is also known as wet weather overflow. It is primarily meaningful in developed countries, which have extensive treatment facilities. Frequent causes of SSO spills include:

Developed countries such as the United States, Canada, most Western European states, Australia and Japan are struggling with a public health problem of SSO prevention. However, the magnitude of the problem is much greater in most developing countries.

The U.S. Environmental Protection Agency (EPA) estimates that at least 23,000 to 75,000 SSO events occur in the United States each year. EPA estimated that upgrading every municipal treatment and collection system to reduce the frequency of overflow events to no more than once every five years would cost about $88 billion as of 2004. This cost would be in addition to approximately $10 billion already invested. Although the volume of untreated sewage discharged to the environment is less than 0.01 percent of all treated sewage in the United States, the total volume amounts to several billion gallons per annum and accounts for thousands of cases of gastrointestinal illness each year.

Advanced European countries and Japan have similar or somewhat larger percentages of SSO events compared to the U.S.

In developing countries, most wastewater is still not treated properly but discharged into the environment. The People's Republic of China discharged about 55 percent of all sewage without treatment of any type, as of 2001. In a relatively developed Middle Eastern country such as Iran, the majority of Tehran's population has totally untreated sewage injected to the city’s groundwater. In Venezuela, a below-average country in South America with respect to wastewater treatment, 97 percent of the country’s sewage is discharged untreated into the environment.


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