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.