An aeration/backwash device (16) for use with a porous membrane filtration module (5) including one or more membranes (6) extending longitudinally between vertically spaced upper (7) and lower headers (8) into which the ends of the membranes are potted. The membranes (6) having a permeable wall which, in use, is subjected to a filtration operation wherein feed containing contaminant matter is applied to one side of the membrane wall and filtrate is withdrawn from the other side of the membrane wall. The aeration/backwash device (16) adapted to at least partially surround a portion of said membrane module (5) and including a communication chamber (17) having spaced through-openings (18, 19) in fluid communication with the chamber (17) and the membrane module (5). In use, gas is supplied to the chamber (17) and communicated to the membrane module (5) through the through-openings (18, 19) to aerate the membranes within the membrane module and liquid backwash is withdrawn from and/or fed into the membrane module (5) through the throughopenings into the chamber (17).