Content deleted Content added
Thewellman (talk | contribs) clarify difference from CSOs |
Thewellman (talk | contribs) add reference citations |
||
Line 4:
'''Sanitary sewer overflow''' ('''SSO''') is a condition in which untreated [[sewage]] is discharged from a [[sanitary sewer]] into the environment prior to reaching [[sewage treatment]] facilities. When caused by rainfall it is also known as '''wet weather overflow'''. It is primarily meaningful in [[developed countries]], which have extensive treatment facilities. Frequent causes of SSO spills include:
* Blockage of sewer lines
* [[Infiltration/Inflow]] of excessive [[stormwater]] into * Malfunction of [[pumping station]] lifts or electrical [[power failure]]
* Broken sewer lines
In many countries there are obligations to measure and report SSO occurrence using real-time [[telemetry]] to warn the public, bathers and shellfishery operators.
Line 21 ⟶ 22:
Sewers were built before sewage treatment was practiced. Early sewers were 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>
==Blockage==▼
==Infiltration/inflow==▼
Decentralized failures in dry weather mainly occur from collection sewer line blockages, which can arise from a [[debris]] clog, line rupture 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}} Approximately half of United States SSOs attributed to blockage were caused by cooking fats dissolved in hot water for discharge to sanitary sewers which congealed as grease deposits in the cooler sewer. Solid debris including rags and clothing was responsible for approximately one-quarter of United States SSOs attributed to blockages. Roots were a contributing factor in approximately one-quarter of United States SSOs attributed to blockage.<ref name="EPARTC"/>{{rp|p. 4–28}}
By far the most common cause of Sanitary Sewer Overflow are heavy [[rainfall]] events, which can cause massive 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. 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 above.▼
Historically, one of the biggest causes of SSO operation is the overloading of the sewer during storms and there has been a call to reduce the amount of water entering sewers by introducing SuDS.▼
Dry weather blockage is less likely within combined sewers
==Malfunction==▼
▲==Infiltration/inflow==
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. This type of discharge is uncommon from combined sewers, because the combined volume of sewage and storm water discourages use of lift stations.▼
▲
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.▼
▲Historically, one of the biggest causes of SSO operation is the overloading of the sewer during storms and there has been a call to reduce the amount of water entering sewers by introducing SuDS.
▲==Blockage==
▲==Malfunction==
▲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}}
▲Decentralized failures in dry weather mainly occur from collection sewer line blockages, which can arise from a [[debris]] clog, line rupture or tree root intrusion into the line itself. 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 pipage, 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%.
▲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}}
▲Dry weather blockage is less likely within combined sewers, because combined sewers are so much larger than sanitary sewers. Combined sewers may be vulnerable to blockage at the storm water regulator, but discharges from such blockage typically enter the diversion outfall to avoid flooding private or public property.
==Human health and ecological consequences==
| |||