Abstract: The invention pertains to a process for the production of pure water, in which raw water, or feed water similarly, is fed to a treatment apparatus (1) that has at least one semi-permeable membrane (4), in conjunction with which a partial quantity of the raw water passes through the membrane (4) as pure water under the driving force of a pressure differential, and the remaining quantity of raw water that flows past the membrane leaves the treatment apparatus (1) as concentrate. In order to counteract any excessive soiling of the membrane (4), and to attain the longest possible operational and membrane service lifetimes, it is provided that the direction of flow of the raw water past the membrane (4) is reversed at intervals of time. The invention also pertains to a treatment apparatus (1) for the production of pure water, with at least one semi-permeable membrane (4).