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[[Image:Paris-Egouts-p1010721.jpg|thumb|Sewers in [[Paris]].]]
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'''Sewers''' [[transport]] [[wastewater]] from buildings to [[sewage treatment|treatment facilities]].  Sewers are [[pipeline]]s that connect buildings to horizontal 'mains'. The sewer mains often connect to larger mains, and then to the wastewater treatment site.  Vertical pipes, called [[manhole]]s, connect the mains to the surface.  Sewers are generally gravity powered, though [[pump]]s may be used if necessary.


'''Storm sewers''' (also '''[[storm drain]]s''') are large pipes that transport storm water runoff from [[street]]s to natural bodies of water, to avoid street flooding. When the two systems are operated separately, the sewer system that is not the set of storm drains is called a '''[[sanitary sewer]]'''.

[[Catchbasin]]s are immediately below the vertical pipes connecting the surface to the storm sewers.  While [[sewer grates]] covering the vertical pipes prevent large objects from falling into the sewer system, the grates are spaced far enough apart that many small objects can fall through.  The area immediately below the catchbasin "catches" such [[detritus]].  Water from the top of the catchbasin drains into the sewer proper.  The catchbasin serves much the same function as the "trap" in household wastewater [[plumbing]] in trapping objects. Unlike the trap, the catchbasin does not necessarily prevent sewer gases such as [[hydrogen sulfide]] and [[methane]] from escaping.  Catchbasins contain stagnant water and can be used by mosquitoes for breeding. Catchbasins require regular cleaning to remove the trapped debris.  Municipalities typically have large vacuum trucks that clean out catchbasins.

Storm sewer water may be treated or not, depending on jurisdiction.  Treatment helps purify the storm water before being restored to a natural body of water. Storm water may become contaminated while running down the road or other impervious surface, or from lawn chemical runoff, before entering the sewer.  It is a good idea to separate storm sewers from waste sewers because the huge influx of water during a rainstorm can overwhelm the treatment plant, resulting in untreated sewage being discharged into the environment.  [[Washington DC]] and other cities with older '''combined systems''' have this problem after every heavy rain.  

However, completely separating storm sewers from [[sanitary sewer]]s often means no treatment of [[stormwater]] which is not desirable either, as the ''first flush'' from storm runoff can be extremely dirty.  Runoff into storm sewers can be minimized by including ''Sustainable Urban Drainage Systems'' in to municipal plans.  Eaves troughs should not discharge directly into the storm sewer system but rather onto the ground where it has a chance to soak into the soil.  Where possible, storm water runoff should be directed to unlined ditches before flowing into the storm sewers, again to allow the runoff to soak into the ground.

Separation of undesired runoff can be done within the storm sewer system, but such devices are new to the market and can only be installed with new development or during major upgrades. They are referred to as oil-grit separators (OGS) or oil-sediment separators (OSS). They consist of a specialized [[manhole]] chamber, and use the water flow and/or gravity to separate oil and grit.

See also [[sewage treatment]], [[infiltration gallery]]

==History==

The earliest covered sewers uncovered by archeologists are in the regularly planned cities of the [[Indus Valley Civilization]]. In ancient Rome, the [[Cloaca Maxima]], considered a marvel of engineering, disgorged into the [[Tiber]]. In medieval European cities, small natural waterways used for carrying off wastewater were eventually covered over and functioned as sewers. London's Fleet is such a system. Open drains along the center of some streets were known as 'kennels' ('canals'). The 19th-century brick-vaulted sewer system of [[Paris]] offers tours for tourists.

==Sewers in fiction==

The theme of traveling through, hiding, or even residing in sewers is a common cliché in fantasy [[role-playing game]]s, [[video games]] and, to a lesser extent, [[movies]].  Some titles that include this cliché are the [[Mario]] series, [[Deus Ex]], [[Dark Cloud]] 2, [[The Matrix]], [[Resident Evil]] 2, [[The Shawshank Redemption]], [[Teenage Mutant Ninja Turtles]] and many materials related to the influential [[Dungeons and Dragons]] game. The darkness, maze-like layout, separation/connection with the city overhead (not to mention the slime) create a perfect environment for fantasy adventure. The odors and health concerns that would surround sewer travel in real life are often not addressed or are downplayed in these titles. A well-known [[urban legend]] is that of giant alligators or crocodiles residing in sewers, especially of major metropolitan areas. The sewers under [[Vienna]] serve as the setting for the [[Climax (narrative)|climactic]] scene in the film [[The Third Man]]. Sewers are also used in the novel [[Les Misérables]] as a setting for escaping and pursuit following a lost battle.

== Accidents ==

A trap is a U-shaped bend in a water conduit, as found on toilets, and wash basin outlets. Most of the time, traps allow water to pass freely but block a backflow of fumes or water.

An poorly designed sewer main in [[Guadalajara, Jalisco|Guadalajara]], [[Mexico]] had to be diverted down through a [[siphon]] to allow space for a metro railway to be built.  The siphon allowed water and waste to pass, but not fumes.  Petrol which spilled or leaked into the sewer on one side of the siphon was not able to easily escape to the safe exit on the other side, leading eventually to a build-up of fumes and an explosion killing hundreds. These explosions occurred in [[1983]] and most seriously on [[April 22]], [[1992]].

=== Lessons learned ===

The sewer siphon should have had a second siphon '''over''' the metro tunnel to allow fumes to get from one side to the other, as if the [[Metro]] tunnel were not there.

==See also==
{{Commonscat|Sewers}}
* [[Brighton sewers]]
* [[London sewerage system]]

See also:
* [[Latrine]]
* [[Outhouse]]  
* [[Pit toilet]]
* [[Chemical toilet]]
* [[Septic tank]]   
* [[Waste disposal]]
* [[Ecological Sanitation]]

==External links==
[http://www.sewerhistory.org/ Site on the history of sewers]

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[[Category:Sewerage]]
[[Category:Subterranea]]
[[Category:Water streams]]
[[Category:Construction]]

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====References====
* Adapted from the [[Wikipedia]] article, "Sewer" http://en.wikipedia.org/wiki/Sewer, used under the [[GNU Free Documentation License]]