Gas/liquid exchange apparatus

A gas injection or aeration device of the kind adapted to be immersed in a liquid and supplied with gas under pressure so that the gas emerges through diffusion nozzles of the device as a large quantity of small bubbles to form a gas/liquid mixture, wherein the diffusion nozzles are disposed in an upwardly extending bore of an open-ended conduit submerged in the liquid, the nozzles being located between 20 and 100 centimeters below the surface of the liquid and at least 20 centimeters below the upper open end of the conduit, and wherein the nozzles are disposed at a region where the bore of the conduit is restricted to an area which is between one sixth and one twelfth of the area of the remainder of the bore above said restricted region, the diffusion nozzles being constituted by a plurality of holes each arranged to introduce gas into the bore in a direction perpendicular to the direction of flow of liquid past said holes, said holes being arranged laterally of at least two pipes which pipes extend transversely across the bore of the conduit, the said restricted area of the bore comprising at least two elongated passages defined at least in part by said pipes, and wherein the diffusion nozzles open directly into these passages, the holes constituting the diffusion nozzles having diameters not exceeding 10 millimeters and the width of the passages not exceeding four times the diameter of the holes in order to establish laminar flow of liquid adjacent said holes to shear off bubbles of gas leaving said holes thereby producing a large number of small bubbles having a resultant combined large surface area, the flow of liquid past said holes inducing turbulence in the region of the riser tube above the diffusion nozzles to thus promote gas/liquid interchange.