A submerged membrane assembly including a membrane having a first surface, a second surface, and a vertical axis, and which is permeable between the surfaces by molecules of less than a predetermined size. A first fluid compartment in fluid communication with the first membrane surface and that contains at a first column height a first fluid having a first specific gravity, a second fluid compartment in fluid communication with the second membrane surface and that contains at a second column height a second fluid having a second specific gravity, and means for changing the second specific gravity. The second column height being selected relative to the first column height to produce a selected pressure differential across the membrane along the vertical axis at the first specific gravity and the changed second specific gravity.