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* [[Infiltration/Inflow]] of excessive [[stormwater]] into sewer lines during heavy [[rainfall]]
* Malfunction of [[pumping station]] lifts or electrical [[power failure]]
* Broken sewer lines.
==Magnitude of the problem==▼
In many countries there are obligations to measure and report SSO occurrence using real-time [[telemetry]] to warn the public, bathers and shellfishery operators.▼
Developed countries such as the United States, [[Canada]], most [[Western Europe]]an states, [[Australia]] and [[Japan]] are struggling with a public health problem of SSO prevention. However, the
===United States===
The concept of SSO containment valves has been pioneered in the UK and they are installed to mitigate dry spills, by correlating rainfall data with SSO spill activity.▼
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 |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 |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% 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===▼
▲==Magnitude of the problem==
Advanced European countries and Japan have similar or somewhat larger percentages of SSO events compared to the U.S.{{citation needed|date=June 2015}}
In [[developing countries]], most wastewater is still not treated properly but discharged into the environment. Even a country such as the [[People's Republic of China]] discharged in 2001 about 55 percent of all sewage without treatment of any type.<ref>"[[World Bank]] Supports China's Wastewater Treatment", ''The People’s Daily'', November 30, 2001, Beijing</ref> In a relatively developed [[Middle East]]ern country such as [[Iran]], the majority of [[Tehran]]'s population has totally untreated sewage injected to the city’s groundwater.<ref>Massoud Tajrishy and Ahmad Abrishamchi, "Integrated Approach to Water and Wastewater Management for [[Tehran]], [[Iran]]", [[Water Conservation]], Reuse, and Recycling: Proceedings of the Iranian-American Workshop, National Academies Press (2005)</ref> 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.<ref>Appropriate Technology for Sewage Pollution Control in the Wider [[Caribbean]] Region, Caribbean Environment Programme Technical Report #40 1998</ref>▼
▲In many countries there are obligations to measure and report SSO occurrence using real-time [[telemetry]] to warn the public, bathers and shellfishery operators.{{citation needed|date=June 2016}}
== Causes ==
===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. 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 offensive 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. Sanitary sewers were built 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. |authorlink = |title =Sewage Treatment Plant Design |publisher =[[American Society of Civil Engineers]] and Water Pollution Control Federation |volume = |edition = |date =1959 |___location = |page =6 |isbn =}}</ref>
===Blockages===
Decentralized failures in dry weather mainly occur from collection sewer line blockages, which can arise from a [[debris]] clog or tree root intrusion into the line itself. Approximately half of SSOs in the United States are caused by blockage.<ref name="
▲Decentralized failures in dry weather mainly occur from collection sewer line blockages, which can arise from a [[debris]] clog or tree root intrusion into the line itself. Approximately half of SSOs in the United States are caused by blockage.<ref name="EPARTC"/>{{rp|p. 4–26}} [[Brown grease|Grease]] is the blocking agent in approximately half of United States SSOs attributed to blockage, and solid debris is the blocking agent for another 25 percent. [[Root]]s are a contributing factor in approximately one-quarter of United States SSOs attributed to blockage. Grease deposits are caused by cooking fats liquified with hot water for discharge to sanitary sewers. These fats congeal as solid deposits in the cooler sewer. Solid debris includes soiled clothing, [[diaper]]s, and [[sanitary napkin]]s flushed into toilets.<ref name="EPARTC"/>{{rp|p. 4–28}}
One of the main problems of a decentralized line failure is the difficulty of defining the ___location of overflow, since a typical urban system contains thousands of miles of collection pipes, and the central treatment plant has no way of communicating with all the lines, unless expensive monitoring equipment has been installed. Companies in the UK have widely deployed bulk dielectric transducers suspended in the sewers to detect high levels and to report the events back over fixed wireless data networks. In certain locations it has been said that this practice has permitted the reduction of pollution events by up to 60%.
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===Infiltration/inflow===
Approximately one-quarter of United States SSOs occur during heavy [[rainfall]] events, which can cause inflow of [[stormwater]] into sanitary sewers through damage, improper connections, or flooding buildings and lift stations in low-lying areas of the collection system. The combined flow of sewage and stormwater exceeds the capacity of the sanitary sewer system and sewage is released into homes, businesses and streets.<ref name="
▲Approximately one-quarter of United States SSOs occur during heavy [[rainfall]] events, which can cause inflow of [[stormwater]] into sanitary sewers through damage, improper connections, or flooding buildings and lift stations in low-lying areas of the collection system. The combined flow of sewage and stormwater exceeds the capacity of the sanitary sewer system and sewage is released into homes, businesses and streets.<ref name="EPARTC"/>{{rp|p. 4–26}} This circumstance is most prevalent in older cities whose subsurface infrastructure is quite old; [[Paris]], [[London]], [[Stockholm]],<ref>Monica Spendilow, ''Impeller Magazine'', ITT Flygt AB, SE-174 87 Sundbyberg, Sweden (2004)</ref> [[New York]], [[Washington, DC]], and [[Oakland, California]]<ref>''Environmental Impact Statement for the [[East Bay Municipal Utility District]] Wet Weather Overflow Project'', Earth Metrics Incorporated, prepared for the U.S. EPA and East Bay Municipal Utility District, Oakland, Ca. (1985)</ref> are typical examples of such locations. Inflow into the sanitary lines can be caused by tree root rupture of subsurface lines or by mechanical fracture due to age and overpressure from trucks and buildings.
===Malfunctions===
Another mode of system failure can include [[power outage]]s, which may disable lift station pumps and cause sewage overflow from the lift station wet well. Lift station mechanical or power failure causes approximately ten percent of United States SSOs. This type of discharge is uncommon from combined sewers, because the combined volume of sewage and storm water discourages use of lift stations. Broken sewer lines are responsible for approximately ten percent of United States SSOs.<ref name="
Power failure, [[human error]], or mechanical failure may cause similar discharge of untreated or partially treated sewage from a sewage treatment plant; but this is typically regarded as a sewage treatment plant malfunction rather than a sanitary sewer overflow. Sewage treatment plants may be designed to capture overflow from malfunctioning units and discharge it to alternative treatment facilities. Flooding of private or public property is typically avoided by discharging the overflow to an outfall designed for discharge of treated sewage.<ref name="
▲Another mode of system failure can include [[power outage]]s, which may disable lift station pumps and cause sewage overflow from the lift station wet well. Lift station mechanical or power failure causes approximately ten percent of United States SSOs. This type of discharge is uncommon from combined sewers, because the combined volume of sewage and storm water discourages use of lift stations. Broken sewer lines are responsible for approximately ten percent of United States SSOs.<ref name="EPARTC"/>{{rp|p. 4–27}}
▲Power failure, [[human error]], or mechanical failure may cause similar discharge of untreated or partially treated sewage from a sewage treatment plant; but this is typically regarded as a sewage treatment plant malfunction rather than a sanitary sewer overflow. Sewage treatment plants may be designed to capture overflow from malfunctioning units and discharge it to alternative treatment facilities. Flooding of private or public property is typically avoided by discharging the overflow to an outfall designed for discharge of treated sewage.<ref name="EPARTC">U.S. Environmental Protection Agency (EPA), Washington, D.C. (2004).[http://cfpub.epa.gov/npdes/cso/cpolicy_report2004.cfm "Report to Congress: Impacts and Control of CSOs and SSOs."] August 2004. Document No. EPA-833-R-04-001.</ref>{{rp|p. ES–3}}
==Human health and ecological consequences==
[[Image:Overflowepa.gif|thumb|230px
Human health impacts include significant numbers of [[gastrointestinal illness]] each year, although death from one overflow event is uncommon. Additional human impacts include [[beach]] closures, [[human swimming|swimming]] restrictions and prohibition of the consumption of certain aquatic animals (particularly certain [[mollusc]]s) after overflow events. Ecological consequences include [[fish kill]]s, harm to [[plankton]] and other aquatic microflora and microfauna. Turbidity increase and dissolved oxygen decrease in receiving waters can lead to accentuated effects beyond the obvious [[pathogen]]ic induced damage to aquatic [[ecosystem]]s. It is possible that higher life forms such as [[marine mammal]]s can be affected since certain seals and [[sea lion]]s are known to experience peaks in pathogenic harm.<ref>Johnson, S. P., Jang, S., Gulland, F.M.D., Miller, M., Casper, D., Lawrence, J., Herrera, J., [http://www.jwildlifedis.org/cgi/reprint/39/1/136.pdf "Characterization and clinical manifestations of Arcanobacterium phocae infections in marine mammals stranded along the central California coast"], ''Journal of Wildlife Diseases'', 39:136-144 (2003).</ref>
==Mitigation techniques==
▲===Worldwide perspective===
▲The concept of SSO containment valves has been pioneered in the UK and they are installed to mitigate dry spills, by correlating rainfall data with SSO spill activity.{{citation needed|date=June 2016}}
▲In [[developing countries]], most wastewater is still not treated properly but discharged into the environment. Even a country such as the [[People's Republic of China]] discharged in 2001 about 55 percent of all sewage without treatment of any type.<ref>"[[World Bank]] Supports China's Wastewater Treatment", ''The People’s Daily'', November 30, 2001, Beijing</ref> In a relatively developed [[Middle East]]ern country such as [[Iran]], the majority of [[Tehran]]'s population has totally untreated sewage injected to the city’s groundwater.<ref>Massoud Tajrishy and Ahmad Abrishamchi, "Integrated Approach to Water and Wastewater Management for [[Tehran]], [[Iran]]", [[Water Conservation]], Reuse, and Recycling: Proceedings of the Iranian-American Workshop, National Academies Press (2005)</ref> 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.<ref>Appropriate Technology for Sewage Pollution Control in the Wider [[Caribbean]] Region, Caribbean Environment Programme Technical Report #40 1998</ref>
==History==▼
▲Developed countries such as the United States, [[Canada]], most [[Western Europe]]an states, [[Australia]] and [[Japan]] are struggling with a public health problem of SSO prevention. However, the magnitue of the problem is much greater in most developing countries.
[[Image:Wonga wetlands sewage plant.jpg|thumb|250px
Since medieval times rulers have been aware of the impact of raw sewage improperly discharged to the environment. Before treatment systems existed in 16th century [[England]], [[Henry VIII of England|King Henry VIII]] decreed that sewage troughs should be kept flowing so that they would not stagnate in [[London]] prior to reaching the [[River Thames]] ([[London sewer system]]).
▲==History==
▲[[Image:Wonga wetlands sewage plant.jpg|thumb|250px|left|[[Sewage treatment plant]] used to create the Wonga Wetlands, [[Australia]]]]
==See also==
* [[Combined sewer]]
* [[Water pollution]]
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