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===United States===
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.<ref>{{cite web |url=https://www.epa.gov/npdes/sanitary-sewer-overflows-ssos |title=Sanitary Sewer Overflows |author=<!--Staff writer(s); no by-line.--> |date=2015-11-16 |access-date=2023-02-17 |website=National Pollutant Discharge Elimination System |publisher=U.S. Environmental Protection Agency (EPA) |___location=Washington, D.C.}}</ref> 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.<ref name="EPA-RTC">{{cite report |date=August 2004 |title=Report to Congress: Impacts and Control of CSOs and SSOs |url=https://www.epa.gov/npdes/2004-npdes-cso-report-congress |access-date=2023-02-17 |publisher=EPA |id=EPA-833-R-04-001}}</ref> 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.<ref name="EPA-RTC" />{{rp|Ch. 6}}
===Worldwide perspective===
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===Engineering aspects===
[[Image:Aparissewer.jpg|thumb|230px|Sanitary sewer line in [[Paris]] more than two meters in diameter.]]
Sewers that were built in the early stages of urbanization were usually built before [[sewage treatment]] was implemented.<ref>''See'' [[History of water supply and sanitation]].</ref> Early sewers were simple drainage systems to remove [[surface runoff]] with any waste material it might contain. These drainage systems became [[combined sewers]] when sewage from kitchens, baths, and toilets was added; and the discharge became offensive. Early sewage treatment plants were built to treat the sewage during dry weather; but it was infeasible to treat the larger volume of mixed sewage and precipitation runoff from combined sewers during wet weather. Some cities built sanitary sewers to keep sewage from being mixed with surface runoff so the sewage could be efficiently treated during both wet and dry weather.<ref>{{cite book |last=Okun |first=Daniel A. |title =Sewage Treatment Plant Design |publisher =[[American Society of Civil Engineers]] and Water Pollution Control Federation |date =1959 |page =6 }}</ref> (About 860 communities in the U.S. continue to use combined sewers.<ref>{{cite web |url=https://www.epa.gov/npdes/combined-sewer-overflow-frequent-questions |title=Combined Sewer Overflow Frequent Questions |author=<!--Staff writer(s); no by-line.--> |date=2017-12-20 |access-date= |website=National Pollutant Discharge Elimination System |publisher=EPA}}</ref>)
===Blockages===
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In the 19th century, sewage treatment plants were first developed and installed in the U.S. and parts of Europe, and the concept of SSO was identified. SSOs were not recognized as a widespread environmental problem until the rise of environmental awareness in the 1960s. Around that time government agencies in the U.S. began identifying locations and frequencies of SSOs in a systematic way. Local governments heard complaints of citizens, and beach closure protocols were systematised to reduce risks to public health.
After passage of the [[Clean Water Act]] in 1972, the U.S. spent billions of dollars on upgrades to sewage treatment plants, with some associated repairs and improvements to the associated collection systems, where the overflows occur. EPA continues to provide funding for low-interest loans to communities for addressing SSO problems, through the [[Clean Water State Revolving Fund]].<ref>{{cite web |url=https://www.epa.gov/npdes/sanitary-sewer-overflow-sso-frequent-questions |title=Sanitary Sewer Overflow (SSO) Frequent Questions |author=<!--Staff writer(s); no by-line.--> |date=2015-11-16 |access-date= |publisher=EPA}}</ref>
In the 1990s [[Japan]], the [[UK]] and a number of other European countries began earnest investigation of some of their countries’ overflow issues.{{citation needed|date=June 2016}}
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