Abstract: A reactor for introducing gas into a fluid comprises a mixing tank (12, 43) for the fluid and a partition means (13,45) for dividing the tank into at least two chambers (21, 89) and (23, 91). The reactor further comprises a first pump means (14, 47) located in one of the chambers for circulating the fluid downwards in one chamber and then upward in the other chamber and an aerator assembly (29, 49) for aerating a sidestream of the fluid and introducing the aerated fluid into the tank for mixing the aerated fluid with the circulating fluid in the tank. The aerator assembly comprises a bank of venturi aerators (38, 57) each having an aerator inlet; an aerator outlet; and a constriction intermediate the aerator inlet and the aerator outlet for creating a region of reduced pressure in the fluid, the constriction being elongate in a section transverse to the direction of flow of the fluid through the constriction; and a means for introducing the gas into the constriction to aerate the fluid.