Thickening of sludge

Sludge thickening is the process used to increase the solids content of sludge by the separation and removal of a portion of the liquid phase from sludge. Thickening results in a saving in total unit costs compared to sludge digestion and dewatering processes without sludge thickening. Three types of thickening are commonly practised, viz. gravity thickening, flotation thickening, and centrifugal thickening. Gravity thickening is based on the settling/sedimentation of suspended or floccu-lated particles due to the downward force created by the gravitational field of the earth. Thickening by centrifugation involves the settling or removal of sludge particles under the influence of centrifugal forces. The solid–liquid separation in a rotary drum thickener is achieved by coagulation flocculation of sludge and drainage of free water through a rotating porous media. The working principle of gravity belt thickening is solid–liquid separation by coagulation-flocculation of sludge and drainage of free water from the slurry through a moving fabric-mesh belt.