A method of operating a membrane separation device in a continuous flow process includes in the membrane separation device, separating a supply liquid into a first partial stream and a second partial stream, wherein flow rate of the second partial stream and/or recovery rate of the second partial stream is influenced by at least one influencing variable that varies during operation, and varying the flow rate of the first partial stream to compensate for a change of the at least one influencing variable to control flow rate and/or recovery rate of the second partial stream to a preset value.