a filtration system for removing fine solids from a liquid suspension comprising a vessel for containing the liquid suspension , one or more permeable , membranes within the vessel , means for providing a pressure differential across walls of the membranes such that some of the liquid suspension passes through the walls of the membranes to be drawn off as permeate and means for withdrawing the permeate from the membranes . a screen is positioned in a flow path of the liquid suspension through to the membrane walls .