septic tank, underground sedimentation tank in which sewage is retained for a short period while it is decomposed and purified by bacterial action. The organic matter in the sewage settles to the bottom of the tank, a film forms excluding atmospheric oxygen, and anaerobic bacteria attack the solid matter, causing it to disintegrate, liquefy, and give off gases. The gases are discharged from a vent and the liquids overflow through an outlet into a disposal field where they can leach into the soil. Here aerobic bacteria purify the liquid. The Imhoff septic tank, an improvement over the ordinary septic tank, is still used in the United States; it is a two-story structure with the upper compartment used for settling the sewage, the lower one for the anaerobic disintegration of sludge. A sloping floor enables solid material to slide to the lower compartment, where, since the sludge is separated from the material in the sedimentation compartment, the action is more rapid. A cesspool is a simpler underground structure that allows the liquids to leach directly into the soil while retaining the solids. The solids are not as efficiently decomposed as in a septic tank and more frequent cleaning is necessary. Also, as the effluent is likely to contain more coliform bacteria than that of a septic tank, cesspools pose a greater threat to water supplies. Septic tanks and cesspools are usually used in rural areas. For urban sewage-disposal systems, see sewerage.