A membrane module (3) comprising a plurality of porous hollow membranes (8). The membranes (8) extend between and are fixed at each end in a header (5, 6). One header (6) has one or more of openings (12) formed therein. The openings (12) are in fluid communication with a source of gas and/or liquid (13, 14, 15). The other of the headers (5) is sealingly connected to and in fluid communication with a head-piece (9). The head-piece (9) is adapted to couple to an associated head-piece of a further module to form a rack of modules (17). A potting head (6) for use in mounting porous hollow membranes (8) is also disclosed comprising a preformed potting element (29). The potting element (29) includes one or more cavities (30) for receiving curate potting material which, in use, supports said membranes (8).