Sewers transport wastewater from buildings to treatment facilities. Sewers are pipelines that connect buildings to horizontal 'mains'. The sewer mains often connect to larger mains, and then to the wastewater treatment site. 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, 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.

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. Washington DC and other cities with older combined systems have this problem after every heavy rain.

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 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



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 video games and, to a lesser extent, movies. Some titles that include this cliché are the Mario series, Deus Ex, Dark Cloud 2, Resident Evil 2, The Shawshank Redemption, and Teenage Mutant Ninja Turtles. The odors and health concerns that would surround such practices 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 climactic scene in the film The Third Man.


A sewer main in 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.

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

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.

Related articles

External links

Site on the history of sewers (

da:Kloak (byplan) de:Kanalisation fr:Égout nl:Riool ja:下水道


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