The separation unit of screw press is inter-stacked by the fixed ring and the moving ring with a small gap between which the screw shaft runs through. When the screw shaft rotates, the moving ring is driven to be in circular motion. Water from the mud pie after flocculation will be filtered out from the gap between and with the moving ring in circulation, stoppage is prevented. Gap between the fixed ring and the moving becomes small little by little along the outward direction of the filter cake from the concentration area to the dewatering area. And forced dewatering is made under the backpressure formed on the backpressure plate at the mud outlet.

The Sludge delivery pump conveys sludge to the mud inlet. The flocculating agent is measured and sent to the flocculation mixing tank. There, sludge and flocculating agent are stirred, mixed and flocculated fully into larger flocs in the blenders and then enter the volute separation unit from the volute sludge inlet. In the separation unit, flocs on the one hand concentrate under the gravity and on the other hand move towards the dewatering area with the spiraling effect. The gap gets narrow and the forcing watering is furthered under the influence of the backpressure plate and the filter cake is discharged from the sludge outlet.