Abstract: The invention relates to a screw compressor for compressing matter that passes through the screw compressor. The screw compressor comprises a screw having an outer thread with a pitch that decreases in the longitudinal direction of the screw, and a casing that encases the screw. The casing has an inner side that faces the thread of the screw so that matter may be passed ahead through the screw compressor between the screw and the inner side of the casing in a direction from an inlet end of the screw compressor, where the pitch of the screw thread is greater, to an outlet end of the screw compressor, where the pitch of the screw thread is smaller. Appliances are arranged on the inner side of the casing to prevent matter that passes through the screw compressor from rotating along with the screw. Inside the screw, a conduit is arranged for introduction of liquid. The invention also relates to a method for compressing and dewatering.