This is commonly accomplished by two different ways. In an Imhoff tank, fresh sludge is passed through a slot to the lower story or digestion chamber where decomposition by anaerobic bacteria takes place resulting in liquefaction and a reduction in the volume of the sludge. After digesting for 6 to 9 months, the result is called "digested" sludge and may be disposed of by drying and then landfilling. It has value as fertilizer, being similar to humus. Alternately, the fresh sludge may be continuously extracted from the tank by mechanical means and passed on to separate sludge digestion tanks which operate at higher temperatures than the lower story of the Imhoff tank and as a result digest much more rapidly and efficiently.
See also by-product