Abstract: A membrane filtration system (5) including: a feed containing vessel (7); a membrane module (6) including one or more hollow permeable membranes (8) for filtering feed liquid, located in the feed containing vessel (7); means for applying a pressure differential across walls of the membranes (8) to induce filtration therethrough; a feed liquid reservoir (9) in fluid communication with the membrane module (6) to supply feed liquid thereto; a source of feed liquid (12); a source of gas (10) and means (11) for selectively producing a flow of gas from the gas source (10) and feed liquid from the feed liquid source (12) through the membrane module (6) and into the feed liquid reservoir (9). The flow of gas producing gas bubbles within the feed liquid to clean the membranes (8).