Mechanical dewatering process utilizing a nonuniform screw conveyor

A process and an apparatus for mechanically dewatering municipal sewage sludge or peat. The undewatered feed material is passed into the first end of a cylindrical porous wall and pressurized within the cylindrical wall by a rotating screw conveyor, which also transports the solids toward the second end of the cylindrical wall. The screw conveyor comprises a central shaft which has at least two built-up sections of gradually increasing diameter providing a compression ratio of 2.5:1.0. The flight depth of the screw conveyor increases by a factor greater than 2.0 after each built-up section. The distance between the edge of the screw conveyor blade and the inner surface of the cylindrical wall and the structure of the openings in the wall have specific dimensional limitations. Fibrous additives may be used to aid in dewatering peat and secondary sludges.