Backwash and cleaning method

A method of concentrating the solids of a liquid suspension by providing a pressure differential across the walls of permeable, hollow membranes ( 6 ) immersed in the liquid suspension ( 9 ). The liquid suspension ( 9 ) is applied to the outer surface of the porous hollow membranes ( 6 ) to induce and sustain filtration through the membrane walls ( 12 ) wherein some of the liquid suspension passes through the walls ( 12 ) of the membranes ( 6 ) to be drawn off as clarified liquid or permeate from the hollow membrane lumens ( 11 ), and at least some of the solids are retained on or in the hollow membranes ( 6 ) or otherwise as suspended solids within the liquid ( 9 ) surrounding the membranes ( 6 ). The method includes periodically backwashing the membrane pores using the permeate by applying a gas at a pressure below the bubble point to the membrane lumens ( 11 ) to progressively displace at least some of the liquid permeate within the lumens ( 11 ) through the membrane pores resulting in removal the solids retained on or in the hollow membranes into the bulk liquid ( 9 ) surrounding the membranes ( 6 ).