A method and filtration module (5) for providing gas bubbles within an
array of vertically disposed porous hollow membranes (6) to clean the
outer surfaces of said membranes (6) when the array is immersed in a
liquid by feeding (10) the gas bubbles into the array transversely of the
vertical axis of the array. In one preferred form, the gas bubbles are
retained within the array using a sleeve (11) surrounding the array (6)
at least along part of its length.