|a tank, usually below ground, for containing sewage to be decomposed by anaerobic bacteria|
sewage-disposal unit used principally for single residences not connected to sewer lines. It consists ordinarily of either a single- or double-compartment concrete or steel tank buried in the ground, so arranged that settled sludge is in contact with the waste water as it flows through the tank and thence to a tile drain field. Solids are decomposed by the anaerobic bacterial action of the sludge. After several years the accumulation of sludge interferes with the action, and a scavenger unit must pump the sludge out of the tank for disposal elsewhere.
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