English

Noun

sewers

1. Plural of sewer

A sewer is an artificial conduit (or pipe) or system of conduits used to carry and remove sewage (human liquid waste) and to provide drainage. In the 21st century developed world, sewers are usually pipelines that begin with connecting pipes from buildings to one or more levels of larger underground horizontal mains, which terminate at sewage treatment facilities. Vertical pipes, called manholes, connect the mains to the surface. Sewers are generally gravity powered, though pumps may be used if necessary.

Storm sewers (also storm drains) are large pipes that transport storm water runoff from streets to natural bodies of water or absorptive areas (also considered as part of the sewer), 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. In the United Kingdom legal system storm sewers are not differentiated from foul sewers (sanitary sewers) both being referred to as sewers. However in practice storm sewers are most commonly called Highway drains or gulleys. Catch-basins referred to below are therefore called gully-pits in the UK.

Storm sewer function

Most storm sewers are provided with gratings or grids prevent large objects from falling into the sewer system. Catchbasins are immediately below the vertical pipes connecting the surface to the storm sewers. While sewer grates covering the vertical pipes with fairly widely spaced grating bars so that the flow of water is not impeded. Consequently many small objects can fall through. Most of these objects are caught by the catchbasin immediately below the grating. 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. In the United States of America, unlike the trap, the catchbasin does not necessarily prevent sewer gases such as hydrogen sulfide and methane from escaping. However in the United Kingdom, where they are called gulley-pots, they are designed as true water-filled traps and do block the egress of gases and rodents. Most catchbasins will contain stagnant water during the drier parts of the year and can be used by mosquitoes for breeding. Catchbasins require regular cleaning to remove the trapped debris. Municipalities typically have large vacuum trucks that perform this task.

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 run-off, before entering the sewer. The separation of storm sewers from sanitary sewers helps to prevent sewage treatment plants becoming over-whelmed by the huge influx of water during a rainstorm. This can result resulting in untreated sewage being discharged into the environment.

Usually storm sewers are designed to drain the storm water to rivers or streams as previously described. In the city of Cleveland, Ohio, for example, all new catch basins installed have inscriptions on them not to dump any waste, and usually include a fish imprint as well. Washington, D.C. and other cities with older combined systems have this problem due to a large influx of storm water after every heavy rain. Some cities have dealt with this by adding large storage tanks or ponds to hold the water until it can be treated. Chicago has a system of tunnels underneath the city for storing its stormwater.

However, completely separating storm sewers from sanitary sewers 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 (UK term) or low impact development practices (U.S. term) into 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.

History

The earliest covered sewers uncovered by archaeologists 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 River Fleet is such a system. Open drains along the center of some streets were known as 'kennels' (= canals, channels). The 19th century brick-vaulted sewer system of Paris offers tours for tourists.

Sewers in non-fiction

The image of the sewer recurs in European culture as they were often used as hiding places or routes of escape by the scorned or the hunted, including partisans and resistance fighters in WWII. The only survivors from the Warsaw Uprising and Warsaw Ghetto made their final escape through city sewers. Some have commented that the engravings of imaginary prisons by Piranesi were inspired by the Cloaca Maxima, one of the world's earliest sewers.

Sewers in fiction

The theme of traveling through, hiding, or even residing in sewers is a common cliché in media, where unsanitary conditions or the strong smell of sewage are seldom mentioned. A famous example of sewer dwelling is Teenage Mutant Ninja Turtles.

Sewer alligators

A well-known urban legend, the sewer alligator, is that of giant alligators or crocodiles residing in sewers, especially of major metropolitan areas. The Thomas Pynchon novel, 'V.', features extended passages in which one of the protagonists, Benny Profane, works with a fictional New York City task force to track alligators in the city sewers. His goal is to bag the great albino alligator, reputed to inhabit the system. This literary conceit grows from the persistent urban legend that baby pet alligators, flushed down toilets by tourists returning from Florida, continue to live and flourish in the pipes below.

Two public sculptures in New York depict an alligator dragging a hapless victim into a manhole.http://www.npr.org/templates/story/story.php?storyId=4111768

Alligators have been known to get into combined storm sewers in the Southeastern United States. Closed-circuit television by a sewer repair company captured an alligator in a combined storm sewer on tape. http://youtube.com/watch?v=Mzh1lhV_RXE

Accidents

A sewer main in Guadalajara, Mexico had to be diverted down through an inverted siphon (culvert) to allow space for a metro railway to be built. The inverted siphon allowed water and waste to pass, but not gases. Petrol which spilled or leaked into the sewer on one side of the inverted siphon could not easily escape to the safe exit on the other side, and petrol vapor accumulated and finally exploded killing hundreds. These explosions occurred in 1983 and most seriously on April 22, 1992.

A sewer trap is a U-shaped bend in a water conduit, as found on toilets, and wash basin outlets. Most of the time, snares are used to block the fumes, but not the waste and water.

Lessons learned

The sewer inverted 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 absent.

Notes

See also

• 1929 Ottawa sewer explosion
• 1992 explosion in Guadalajara
• Brighton sewers
• Buchan trap
• Chemical toilet
• Ecological sanitation
• Humanure
• Latrine
• London sewerage system
• Orangi Pilot Project
• Outhouse
• Pit toilet
• Septic tank
• Sewage treatment
• Sewer gas destructor lamp
• Waste disposal
• Water pollution
• Grease interceptor
• Infiltration gallery

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