Apparatus for mixing air with liquid, an aerator or oxygenator, comprises a motor or other power drive generally mounted in vertical position on a floating or a fixed structure, the motor having a shaft extending downwards through an opening in the center area of the float or fixed structure and bearing on its lower part a screw pump portion located in a cylindrical volute section just below the water level. When the apparatus is operating, the waste-water is sucked by means of the screw pump from the bottom of the body of liquid at which the apparatus is positioned and is pushed upwards in order to be diffused by means of a centrifugal impeller having outwardly curved blades, at least one of which extends downward to merge with a spiral shaped portion of a blade on the screw pump. Increased efficiency of the apparatus as an aerator is achieved, allowing the exit velocity from the centrifugal impeller to be approximately equal to, or even greater than, the upward velocity through the screw pump portion, by increasing the kinetic energy of the fluid during passage through the centrifugal impeller or at least minimizing losses of kinetic energy. The resulting high exit velocity from the centrifugal impeller produces higher aerator efficiencies than have been achievable with prior art devices.