A water purification device includes a fluid pump, a diaphragm unit having a diaphragm, which separates a primary side from a secondary side, and a consumer having a pressure requirement. The fluid pump is connected with the primary side and the consumer is connected with the secondary side. In order to supply the consumer with water under a high pressure, the fluid pump supplies the pressure (P2) for the consumer through the diaphragm. A pressure control valve is arranged upstream of the diaphragm unit to control the pressure on the primary side in dependence of the pressure on the secondary side.