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.