Abstract: A filter cartridge (5) with a housing enclosing a filter volume filled to about 80% with a filter granulate (15) is screwed into a joint (7) of a connecting piece (1) by means of a port (12) ending in an outflow opening (13) and being connected to an outlet line formed by a second fitting (3), and surrounded with a distance by a casing (4) that is screwed to the connecting piece (1). An inlet line formed by a first fitting (2) opens into an intermediate volume (6) lying between the casing (4) and the outer side of the filter cartridge (5) in such a way that untreated water enters into the outlet line via the intermediate volume (6) and through the bottom (9) which is perforated by slots (10), the filter granulate (15), the intermediate floor (11) that is likewise slotted and the port (12). In order to reduce the flow resistance, the port (12) is provided with bypass openings (14) to allow untreated water to pass also directly out of the intermediate volume (6) into the port (12).