A filter press is a pressure filter used for dewatering sludge. Often a low dose of polymer is required if the solids need assistance to speed dewatering. One advantage of filter presses over belt presses is that they require a lower polymer input due to higher pressures and finer cloths used.

A filter press comprises a number of plates, each fitted with a durable permeable cloth. In normal operation the recessed plates are pushed together, by hydraulic ram, forming sealed voids (or chambers) between the plates when closed.
 

The pressure used for dewatering in a chamber press is provided by the feed pump. During the filtration cycle, the sludge is pumped into the chambers, initially at a high rate, reducing as the chambers are filled and the pressure in the chambers builds up. The cloths retain the solids allowing the water to pass through as filtrate.

Once the pressure in the press reaches a set point and the filtrate is reduced to a trickle, the dewatering cycle is complete.

The next stage of the process is to empty the press by opening the plate chambers and allowing the dewatered cake "biscuits" to drop out. This entire operation can be a manual, semi automated or fully automated process.

Once the press is empty, the plates are returned to the closed position ready for further batches.