An apparatus that treats salt-containing water by reverse osmosis, in which the salt-containing water is separated into a permeate stream and a concentrate stream, includes an inlet for the salt-containing water; an outlet for the permeate; an outlet for the concentrate, and a reverse osmosis device, wherein the inlet and the outlets are integrated into a base unit adapted for installation in a positionally fixed manner, and the reverse osmosis device is an exchangeable unit detachably connected to the base unit.