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.