Abstract: Improved methods apparatus are provided for introducing and entraining air into a waste liquid, thereby increasing the quantity of dissolved oxygen in the liquid to enhance the aerobic treatment process. A power driven propeller is preferably positioned at the discharge end of conical-shaped transition housing, and draws in air while waste water is controllably introduced to the transition housing from below the surface of the water. The housing upstream from the propeller is primarily an air chamber with incoming streams of water, while the housing downstream from the propeller is filled with waste water having minute entrained air bubbles. The water and entrained air are thus discharged downwardly, and the axis of the equipment preferably inclined to increase circulation in the waste water pond or tank. The techniques of the present invention have relatively low power requirements while substantially increasing gas concentration in the liquid.