A sewer is an artificial conduit (or pipe) or system of conduits used to remove sewage (human liquid waste) and to provide drainage. In the 20th 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) (both are American English terms that are usually replaced in the United Kingdom by the term "sewer") 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.

Storm sewer function

Catchbasins 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. 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 on any new catch basins installed have inscriptions on them not to dump any waste usually with a fish imprint as well. Washington, D.C. and other cities with older combined systems have this problem 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.^[1]

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, although some of the contaminants in the runoff, such as heavy metals, oils and many chemicals, are not removed through waste water treatment systems anyway. 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 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 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.[1]

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 fumes. 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, traps 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 not there.

Notes

See also

• Brighton sewers
• 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

Images

Sewers in Paris	Sewers in Paris	Sewers in Paris	PVC sanitary sewer installation
Concrete storm sewer installation	Brick sewer in Auckland

External links

• Site on the history of sewers
• Site on dealing with Sewer and Manhole Odors.