A device for purifying liquid in a reservoir and including a vacuum chamber (1) having an inflow opening (2) and an outflow opening (3) adapted to communicate with the liquid in the reservoir through a feed conduit (4) and a return conduit (5), respectively. The return conduit (5) is provided with a pump (6) and the vacuum chamber is connected with a vacuum pump (7). The device further includes a filter cartridge (8) provided with a plurality of filter elements, each having a flow side communicating with a central passage (26) and a flow side opening into the periphery of the filter cartridge (8). The filter cartridge (8) is arranged inside the vacuum chamber, the central passage (26) of the filter cartridge communicating with the inflow opening (2) of the vacuum chamber.