Apparatus incorporating potted hollow fibre membranes has a solid mass of potting material having a first face, a plurality of hollow fibre membranes sealed in the potting material. Portions of the membranes adjacent open ends of the membranes protrude from the first face. The potting material extends to and adhesively secures to the inner periphery of a permeate collection means to form a permeate collection zone. The permeate collection means may have walls enclosing a plenum adapted to receive a gas. The apparatus may have an air tube passing through the potting material and having an end in communication with a plenum or manifold adapted to receive a gas.