GENERAL CONSIDERATIONS The concept of the downflow bubble contactor and the U-tube application involves mixing air and water in ratios of from 10 to 20 per cent air by volume. Subsequent to mixing, the two phase fluid is retained in the system for sufficient time to allow mass transfer of gas to occur. Both nitrogen and oxygen are transferred if the gas is air. Figure 1 shows two U-tube configurations. The device used for entrainment of air can be a plug type aspirator, a Venturi aspirator, a simple cascade, or compressed air. The mixed phase fluid is then passed vertically downward in a cocurrent flow. The bouyant force of the gas causes a hold up or slippage of the the gas with respect to the water and back mixing occurs. Thus, the residence time of the air in the downflow segment is greater than that of the surrounding liquid. Liquid velocities of from two to eight fps are generally used to maintain cocurrent bubble flow. Bubble density also varies with cross section. The AIR INDUCED