A concentrator for concentrating the fine solids of a liquid feed suspension has a shell within which there is a bundle of microporous fibers. Pressurized feed suspension is introduced through an inlet and passes over the external walls of the fibers with the clarified liquid being drawn from the lumens of the fibers and the concentrated feed being discharged through an outlet. The solids retained within the shell are removed by first applying a pressurized liquid and then applying a gas at a pressure sufficient to ensure that the gas will pass through the larger pores of the fibrs to remove the solids from the shell through the outlet to an external collection point. The fibers are enclosed within a variable volume diaphragm located within a diaphragm holder by spaced plugs. The volume of the diaphragm is altered by varying the pressure of a fluid introduced through the inlet to the space between the diaphragm and the diaphragm holder.