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Combined sewer
A combined sewer is a sewage collection system of pipes and tunnels designed to also collect surface runoff. Combined sewers can cause serious water pollution problems during combined sewer overflow (CSO) events when wet weather flows exceed the sewage treatment plant capacity.〔U.S. Environmental Protection Agency (EPA), Washington, D.C. (2004).("Report to Congress: Impacts and Control of CSOs and SSOs." ) August 2004. Document No. EPA-833-R-04-001.〕 This type of sewer design is no longer used in building new communities (because current design separates sanitary sewers from runoff), but many older cities continue to operate combined sewers.〔Metcalf & Eddy, Inc. (1972). ''Wastewater Engineering.'' (New York: McGraw–Hill.) p.119.〕
== Background ==
Most sewage collection systems of the 19th and early to mid 20th century used single-pipe systems that collect both sewage and urban runoff from streets and roofs. This type of collection system is referred to as a combined sewer system. The rationale for combining the two was that it would be cheaper to build just a single system.〔Burrian, Steven J., et al. (1999). ("The Historical Development of Wet-Weather Flow Management." ) EPA, National Risk Management Research Laboratory, Cincinnati, OH. Document No. EPA/600/JA-99/275.〕 Most cities at that time did not have sewage treatment plants, so there was no perceived public health advantage in constructing a separate "surface water sewerage" (the term used in the UK) or "storm sewer" (the term used in the U.S.) system.〔
When constructed, combined sewer systems were typically sized to carry three〔 to 160 times the average dry weather sewage flows. It is generally infeasible to treat the volume of mixed sewage and surface runoff flowing in a combined sewer during peak runoff events caused by snowmelt or convective precipitation. As cities built sewage treatment plants, those plants were typically built to treat only the volume of sewage flowing during dry weather. Relief structures were installed in the collection system to bypass untreated sewage mixed with surface runoff during wet weather, protecting sewage treatment plants from damage caused if peak flows reached the headworks.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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