A filter device includes at least one filter housing (1) in which at least one filter element (21) is arranged. Inside the filter housing (1), the filter element separates a clean side (9) from an untreated side (13). On the clean side (9) of the filter housing (1) at least one storage unit (3) having at least one movable separating element (47) is connected. The separating element divides the storage unit (3) into two working chambers or pressure chambers (49) (43).

A filtering apparatus has a plurality of filtering elements (9) in a filter housing (1), a filter inlet (45) and a filter outlet (43). During the filtration operation, one of the filtering elements (9) can be back-flushed by a back-flushing device (13, 21). The back-flushing device has a pressure control device (13) including a hydraulic accumulator (13). One accumulator fluid chamber (67) can be filled during the filtration operation with a quantity of filtered fluid and can be connected for a back-flushing operation via a back-flushing guide (21) to the clean side (59) of the filtering clement (9) to be cleaned. Another accumulator fluid chamber (79) can be subjected to a media pressure.

A water purifier, including a housing, a piston cylinder, a base, and a dust cap. The housing includes a water inlet end and a water outlet end, and a composite filter material and an ultrafiltration membrane are disposed in the housing to constitute a filter element. The piston cylinder is sleeved on the housing. The upper end of the piston cylinder is provided with a suction nozzle. The base includes an upper part and a lower part. The upper part includes a lower bayonet and an upper bayonet. The lower part includes a multipurpose interface and a water inlet nozzle. The dust cap is disposed on the upper end and covers the suction nozzle. The central part of the dust cap protrudes downward to form a flexible sealing plug. The water inlet end of the housing is secured to the lower bayonet of the base.