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==Magnitude of the problem==
EPA estimates that about 40,000 SSO events occur in the [[United States]] each year.<ref>U.S. Environmental Protection Agency (EPA). Washington, DC. [http://cfpub.epa.gov/npdes/home.cfm?program_id=4 "Sanitary Sewer Overflows and Peak Flows"], Updated February 2012.</ref> The Agency 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 dollars as of 2004.<ref name="EPA-RTC">
EPA. [http://cfpub.epa.gov/npdes/cso/cpolicy_report2004.cfm "Report to Congress: Impacts and Control of CSOs and SSOs"], Executive Summary. August 2004. Document No. EPA-833-R-04-001.</ref> This cost would be in addition to approximately ten billion dollars already invested. Although the volume of untreated sewage discharged to the environment is less than 0.01% of all treated sewage in the United States, the total volume amounts to several billion U.S. gallons per annum and accounts for thousands of cases of gastrointestinal illness each year.<ref name="EPA-RTC" />{{rp|Ch. 6}} Advanced European countries and Japan have similar or somewhat larger percentages of SSO events.<ref>{{
==Engineering aspects==
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