Typical flow diagrams incorporating biological processing are presented below. Depending on the source of the sludge, thickeners may be used contingent upon the method of sludge stabilization, dewatering, and disposal. Following biological digestion, any of the several methods shown may be used to dewater the sludge, the choice influenced by economic evaluations and local conditions. Gravity thickening is accomplished in a tank similar in design to a conventional sedimentation tank. Normally, a circular tank is used.
Dilute sludge is fed to a center - feed well.
The feed sludge is allowed to settle and compact, and the thickened sludge is withdrawn from the bottom of the tank. Conventional sludge - collecting mechanisms with deep trusses or vertical pickets are used to stir the sludge gently, thereby opening up channels for water to escape and promoting densification. The supernatant flow that results is returned to the primary settling tank or to the headworks of the treatment plant. Gravity thickening is most effective on primary sludge. A photo of gravity thickener is shown below.