Storm sewers (also storm drains) are large pipes that transport stormwater runoff from streets to natural bodies of water, to avoid street flooding.
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 immediately below the catchbasin "catches" such detrius. 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 gas[?] 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, as the stormwater may have been contaminated while running down the road or other impervious surface 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 overwelm the treatment plant resulting in untreated sewage being discharged into the environment. Washington DC has this problem whenever there is a heavy rain.
However, separating storm sewers from sanitary sewers often means no treatment of storm sewage 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 ground. 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