component of a modern sewage-treatment system. In a large metropolitan system, waste water is passed through a series of tanks following coarse screening to remove large objects. Grit chambers settle out sand and cinders; primary settling tanks remove heavy organic solids; and aeration tanks mix the incoming sewage with activated, bacteria-rich sludge and fine air bubbles, causing the rapid oxidation of organic matter. Final settling tanks remove the remaining suspended solids from the mixture.

The German engineer Karl Imhoff developed a two-layer tank that collects sediments in its upper layer and permits the accumulated solids to slide down to the lower compartment, where bacterial action takes place. Newer tanks have pumping mechanisms that permit more rapid treatment, though Imhoff tanks continue to be widely used for settling prior to filtration through sand beds. See also activated-sludge method.