To provide a filter press capable of performing a long time press dewatering treatment by applying a technology of injecting compressed air to diaphragm filter plates and supplying pressurized water.

In the diaphragm pressing process, the diaphragm filter plates 6, 6 of a large capacity are disposed at both ends of a filter plate 4 and the diaphragm filter plate 5 alternately disposed in parallel, pressure in diaphragm chambers 12, 13 of the diaphragm filter plate 5 and diaphragm filter plates 6, 6 of the large capacity is increased to 0.85 MPa by compressed air, the pressurized water is pressed into the diaphragm chambers 12, 13 to raise the compressed air pressure to 1.6 MPa, press-in of the pressurized water is stopped, and when the pressing pressure of the diaphragm chambers 12, 13 drops to 1.4 MPa, the pressurized water is pressed in again to raise the pressure of the compressed air to 1.6 MPa. The pressure of the compressed air for the long time press dewatering is adjusted by the pressurized water to elongate the pressing time by the air of the diaphragm chambers of the large capacity, to halve the operation time of a pressurized water pump, to thereby save the energy cost.

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