The invention relates to a device for purifying raw water. The device comprises an inlet for the raw water, a conduit system connected to the inlet, a pump system incorporated in the conduit system for carrying the feed water under pressure through the conduit system, and separating units incorporated in the conduit system for separating the feed water into a pure fraction (permeate) and an impure fraction (concentrate), in addition to outlets for the pure and the impure fraction connected to the conduit system. The device comprises panels which are held at a mutual distance and between which the separating units are situated, wherein the panels are constructed from mutually coupled panel modules which form a bearing frame for the at least one separating unit, which panel modules comprise through-feed conduits which in connected situation form at least a part of the conduit system. The device can be constructed in flexible manner with a number of panel modules as required. The invention also relates to a panel module evidently intended for such a device.