Screw Press. A recent development in sludge dewatering equipment, used primarily in the pulp and paper industry, is the screw press. Screw presses are most effective for primary sludges, producing cake solids of 50-55%, but are also appropriate for primary and secondary blended sludges.
Sludge is conditioned and thickened prior to dewatering. A slowly rotating screw, analogous to a solid bowl centrifuge, conveys and compresses the solids.
The screw has the same outer diameter and pitch for the entire length of the press. In some models, the diameter of the screw shaft increases toward the discharge end of the screw press to enhance dewatering. The compression ratio (the ratio of free space at the inlet to the space at the discharge end of the screw) is selected according to the nature of the material to be dewatered and the dewatering requirement. Dewatered cake is discharged as it is pressed against the spring or hydraulically loaded cone mounted at the end of the screw press.
The drum of the screw press consists of a fine strainer screen, a thicker punched holding plate, and a reinforcement rib.
Filtrate is collected in the collecting pan located under the screw press, and the cake is transported to the next stage.