Combined sewer
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[[Image:CSO_diagram_US_EPA.jpg|right|thumb|400px|Combined Sewer System]]
A '''combined sewer''' is a type of [[sewer]] system which provides partially separated channels for sanitary [[sewage]] and [[stormwater]] [[surface runoff|runoff]]. This allows the [[sanitary sewer]] system to provide backup capacity for the runoff sewer when runoff volumes are unusually high, but it is an antiquated system that is vulnerable to [[Combined sewer#Combined Sewer Overflows|combined sewer overflows]] during peak rainfall events.
In many older cities, the collection of sanitary sewage--the sewage that emanates from homes or businesses--was collected by a single sewage system. This system also collected the stormwater runoff from the streets and roofs. Since this type of combined collection system included both rainwater and sewage from homes and businesses it is referred to as a '''combined sewer system''' or a CSS. The cities' rationale during that time (late 19th to early 20th century) was that it would be cheaper to build just a single system. Since it was expensive to build collection lines, they were only designed to handle certain size storms. These sewers were designed with relief structures that bypassed the treatment plant when there was excessive flow during a large storm. The excess water would exit the sewer system and into a nearby body of water through a relief sewer to prevent back-up into the street or homes.
== Combined Sewer Overflows ==
A '''combined sewer overflow,''' or CSO, is an apparatus built into a combined sewer system.<ref name="CSO Policy">U.S. Environmental Protection Agency, Washington, D.C. [http://www.epa.gov/npdes/pubs/owm0111.pdf "Combined Sewer Overflow (CSO) Control Policy."] ''Federal Register,'' 59 FR 18688. April 19, 1994.</ref> The arrangement is designed to allow a certain amount of flow to discharge into a water course untreated to keep the system from becoming surcharged in storm conditions. They often contain a screen, which may be either a mechanical or static arrangement, depending on the frequency of spills per year. During heavy [[rainfall]] when the stormwater exceeds the sanitary flow, the sewage from homes would be diluted.
==Extent and Effects of CSOs in United States==
There are about 772 communities in the [[United States]] with combined sewer systems.<ref>U.S. EPA. [http://cfpub.epa.gov/npdes/cso/demo.cfm "Combined Sewer Overflows: Demographics."] Accessed 2008-01-30.</ref> CSO discharges during heavy storms can cause serious [[water pollution]] problems in these communities. Pollutants from CSO discharges can include [[bacteria]] and other [[pathogen|pathogens]], [[toxicity|toxic]] chemicals, and debris. The U.S. [[United States Environmental Protection Agency|Environmental Protection Agency]] (EPA) issued a policy in 1994 requiring municipalities to make improvements to reduce or eliminate CSO-related pollution problems.<ref name="CSO Policy"/> In 2000 [[United States Congress|Congress]] amended the [[Clean Water Act]] to require the municipalities to comply with the EPA policy.<ref>Wet Weather Quality Act of 2000, Section 112 of Division B, Pub.L. 106-554, December 21, 2000. Added section 402(q) to Clean Water Act, {{USC|33|1342(q)}}.</ref>
==Mitigation of CSO Impacts==
Recently, municipalities have begun to look for ways to mitigate the environmental effects of such overflow locations. One solution is to build a CSO facility, which consists of some low level treatment, storage, and return of the sewage to the normal system. As described above, in the initial setup, water would flow through the relief structure out into a body of water. In this new arrangement, the water would be diverted through a channel into a treatment building. Typically, only [[sewage treatment|mechanical treatment]] (screening of solids) would be completed. The sewage which previously flowed into the water, would flow into a large [[storage tank]] - typically underground. That tank would have the capacity to hold runnoff from all but the largest storms which occur once every 100 years or less. Once the storm passes, the facility's pumps would send the retained water back into the system to be treated under the normal dry-weather process. The result of this effort is the near elimination of raw sewage flowing into the body of water. An example of this type of system is currently being constructed for the [[New York City]] Department of Environmental Protection (NYCDEP) in [[Brooklyn]] at [[Paerdegat Basin]].
The 2004 EPA '''Report to Congress''' on CSO's provides a review of available technologies to mitigate CSO impacts.<ref>U.S. EPA. "[http://cfpub.epa.gov/npdes/cso/cpolicy_report2004.cfm Report to Congress: Impacts and Control of CSOs and SSOs."] Chapter 8. August 2004. Document No. EPA-833-R-04-001.</ref>
==Combined sewers vs dual piping==
Combined sewerage should not be confused with [[dual piping]]. Combined sewers separate [[Rainwater|roof water]] (water from rain, ice, hail, ...) and the actual toilet water coming from dwellings. This to reduce strain on the system. The non-potable water in these pipes is simply led away from dwellings to a place where they can be filtered (and used again or simply discarded in streams, rives, ...) Dual piping however, is a potable [[water supply]] pipe, which is used by private people to deliver water both for drinking and other puposes (washing textile, personal hygiene/sanitation, ...)
==See also==
*[[Sanitary sewer overflow]]
*[[Dual piping]]
*[[Sewage treatment]]
*[[Storm drain]]
*[[Stormwater]]
*[[Water pollution]]
==References==
{{reflist}}
==External links==
*[http://poopthebook.com/blog/2007/10/19/the-great-stink/ Brief history of combined sewer overflows, 1858-present]
*[http://cfpub.epa.gov/npdes/home.cfm?program_id=5 U.S. EPA - Combined Sewer Overflows]
*[http://www.water.ky.gov/permitting/wastewaterpermitting/KPDES/seweroverflows/cso_questions-1.htm What are combined sewers and combined sewer overflows?] - Kentucky Division of Water
[[Category:Environmental engineering]]
[[Category:Hydraulic engineering]]
[[Category:Physical infrastructure]]
[[Category:Sewerage]]
[[Category:Water pollution]]