An apparatus for mixing a liquid and a gas in a tank includes a means for circulating the liquid in the tank and a sparge ring for dispersing gas into the tank. The circulating means includes a plurality of blades extending downwardly from a bottom surface of a disc so that rotation of the blades creates a suction area. The sparge ring introduces gas into the tank in a direction that bypasses the suction area to prevent the gas from flowing into the suction area. Preferably, the sparge ring has a discharge means with a radius that is larger than the distance from the center of the disc to the outer tips of the blades.