A process for mechanically dewatering fibrous organic waste such as sewage sludge. The undewatered waste is passed into the first end of a cylindrical dewatering zone having a porous outer wall. A helical blade rotated within the dewatering zone pressurizes the waste and moves it to the outlet at the second end of the dewatering zone. A filter media comprising a cylindrical substantially unagitated layer of fibrous material derived from the waste is retained within an annular space located between the outer edge of the helical blade and the inner surface of the porous wall. The longitudinal support rods of the porous wall are located on the inner surface of the porous wall and protrude into the fibrous material.