Prefilt directing impermeable membrane of varying force exertion

In a membrane pressure filter which comprises a pressure chamber divided into non-intercommunicating first and second compartments by a substantially upright impermeable membrane, said first compartment being provided with an inlet for a feed mixture to be pressure filtered and containing a filtering surface, and said second compartment being provided with an inlet for a hydraulic fluid;

• said hydraulic fluid having a lower density than said feed mixture, whereby when said feed mixture and said hydraulic fluid are provided to said compartments the lower portion of said membrane tends by virtue of the consequent pressure difference acting at said lower portion of said membrane to expand into said second compartment;
• the improvement comprising:
• means selectively acting upon said lower portion of said membrane during use thereof for generating counteracting forces of magnitude increasing from the top to bottom of said portion, for opposing said expansion of said membrane into said second compartment, and said means acting over at least half the effective length of said membrane, whereby said membrane is constrained to adopt a shape such that the surface of the membrane opposite the filtering surface is substantially parallel to the filtering surface.