Sewers transport wastewater from building 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 stormwater 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 necesssarily prevent sewer gases such as hydrogen sulfide and methane from escaping. Catchbasins contain stagnant water and can be used by mosquitos 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 stormwater before being restored to a natural body of water. Stormwater 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 older U.S. cities with combined sewer overflows have this problem after every heavy rain.
However, separating storm sewers from sanitary sewers often means no treatment of stormwater which is not good either. Runoff into storm sewers can be minimized by municipal planning. Eavestroughs 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, stormwater runoff should be directed to unlined ditches before flowing into the storm sewers, again to allow the runoff to soak into the ground.
See also sewage treatment
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.