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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 seweragesewerage,
system for the removal and disposal of chiefly liquid wastes and of rainwater, which are collectively called sewage. The average person in the industrialized world produces between 60 and 140 gallons of sewage per day.
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a tank for the treatment of small amounts (up to 25 cu m daily, less frequently, up to 50 cu m) of household sewage. A septic tank is an underground horizontal settling reservoir with one to three compartments, through which the sewage flows in sequential order. After preliminary treatment (clarification) in the septic tank, the sewage is subjected to biological purification in beds of underground filtration or in sand-gravel filters. Up to 90 percent of the suspended material is retained in a septic tank.
septic tank[′sep·tik ‚taŋk]
A single-story, watertight, on-site treatment system for domestic sewage, consisting of one or more compartments, in which the sanitary flow is detained to permit concurrent sedimentation and sludge digestion. The septic tank is constructed of materials not subject to decay, corrosion, or decomposition, such as precast concrete, reinforced concrete, concrete block, or reinforced resin and fiberglass. The tank must be structurally capable of supporting imposed soil and liquid loads. Septic tanks are used primarily for individual residences, isolated institutions, and commercial complexes such as schools, prisons, malls, fairgrounds, summer theaters, parks, or recreational facilities. Septic tanks have limited use in urban areas where sewers and municipal treatment plants exist. See Concrete, Reinforced concrete, Structural materials
Septic tanks do not treat sewage; they merely remove some solids and condition the sanitary flow so that it can be safely disposed of to a subsurface facility such as a tile field, leaching pools, or buried sand filter. The organic solids retained in the tank undergo a process of liquefaction and anaerobic decomposition by bacterial organisms. The clarified septic tank effluent is highly odorous, contains finely divided solids, and may contain enteric pathogenic organisms. The small amounts of gases produced by the anaerobic bacterial action are usually vented and dispersed to the atmosphere without noticeable odor or ill effects. See Sewage, Sewage treatment