A volume reduction of approximately 30 – 80 % can be reached with sludge thickening before a further treatment. At smaller wastewater treatment plants, where the sludge is driven off regularly, thickening usually takes place directly in the sludge storage tank. The sludge is compressed at the tank bottom only by the force of gravity, while above the sludge a cloudy water layer is formed, which is taken off and led back into the inlet.

On larger plants separate thickening basins exist. These basins are equipped with slow rotating vertical rods, which create micro canals in the sludge for a better dewatering. Also pure machine thickening is gaining more significance with e.g. non-stabilised sludges, that could rot during the storage.
 

A further reduction of the sludge amount is mostly necessary after the thickening. The liquid sludge has to be dewatered and has to conform to a dry and porous form. Dewatering can be done naturally (dry beds, solar drying), however this is only possible during a long period of time. Faster and smaller, but also more cost intensive, are machine processes such as pressing (filter press) and centrifugation (centrifuge). 

For a good dewatering, size and firmness of the sludge agglomerates are important, so that these remain porous during the compression. Flocculants are often used to achieve as high as possible drying material contents at the machine dewatering and must be specifically co-ordinated with the accruing sludge.
For the choice of the correct dewatering process it is important to consider a multiplicity of further boundary conditions: Quantity, structural situation, disposal, regulations, availability, personnel etc.