Sanitary sewer overflow: Difference between revisions

Revision as of 12:09, 15 July 2015

Revision as of 06:13, 6 September 2015

Line 1:

: ''Not to be confused with [[Combined sewer#Combined sewer overflows (CSOs)|combined sewer overflow (CSO)]]''

[[File:Sewer overflow RI EPA.jpg|thumb|right|A [[manhole]] cover unable to contain a sanitary sewer overflow]]

'''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. In Europe the term [[Combined sewer|Combined Sewer Overflow]] (CSO) is often used. When caused by rainfall it is also known as '''wet weather overflow'''. It is primarily meaningful in [[developed countries]], which have extensive treatment facilities. The mainFrequent causes of SSO and CSO dry spills areinclude:

* [[Infiltration/Inflow]] of excessive [[stormwater]] into [[sanitary sewer|sewer]] lines during heavy [[rainfall]]

* Rupture or blockage of [[sanitary sewer|sewerage]] lines

* Malfunction of [[pumping station]] lifts or electrical [[power failure]]

* Human operator error at treatment plant facilities

Whatever the name SSO and CSO the net effect on the environment is the same and inIn many countries there are obligations to measure and report theSSO occurrence using real-time [[telemetry]] to warn the public, bathers and shellfishery operators. In Europe the revised Bathing Water Directive call upon water companies to actively monitor CSOs and to publish league tables on bathing water quality, often referred to as the Blue Flag system. The process of monitoring is called event duration monitoring (EDM), which on critical CSO is set to two-minute sampling and live reporting. On less critical CSOs the sampling rate is 15 minute.

The concept of CSO / SSO containment valves is something that has been pioneered in the UK and they are installed to mitigate what called dry spills, by correlating rainfall data with CSO / SSO spill activity.

In all cases both EDM monitoring and in particular CSO spill containment requires real-time data and this usually is delivered either using the cell phone network or radio telemetry, where the occurrence of a spill is detected using a form of flow detection transducer (FDT), rather than a level transducer that is used for blockages.

==Magnitude of the problem==

Line 21 ⟶ 19:

[[Image:Aparissewer.jpg|thumb|230px|Sanitary sewer line in [[Paris]] more than two meters in diameter.]]

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>

By far the most common cause of Sanitary Sewer Overflow are heavy [[rainfall]] events, which can cause massive influx of [[stormwater]] into sewerage lines. The combined flow of wastewater and stormwater exceeds the capacity of the sewer system and sewage is released into local waterways to prevent flooding in 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.

==Infiltration/inflow==

By far the most common cause of Sanitary Sewer Overflow are heavy [[rainfall]] events, which can cause massive influxinflow of [[stormwater]] into seweragesanitary linessewers through damage, improper connections, or flooding buildings and lift stations in low-lying areas of the collection system. The combined flow of wastewatersewage and stormwater exceeds the capacity of the sanitary sewer system and sewage is released into local waterways to prevent flooding in 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 CSO / 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.

==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. 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.

Other modes of system failure can include [[power outage]]s, which may disable lift station pumps or parts of the treatment plant operations themselves (in fact, any mechanical system failure within a treatment plant can create a circumstance leading to overflow); breakdown of rotating arms of [[trickling filter]]s; jamming of line gates; clogging of filters or grates, etc. Furthermore, some forms of [[human error]] can infrequently lead to diversion of sewage and result in an overflow event.

==Blockage==

Historically, one of the biggest causes of CSO / 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.

ItDecentralized hasfailures beenin mentioneddry thatweather mainly occur from collection sewer line blockages, which can arise from a [[debris]] clog, line rupture or tree root intrusion into the otherline primaryitself. cause One of overflowthe main problems of a decentralized line failure is duethe todifficulty blockagesof 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%.

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.

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.

==Human health and ecological consequences==