Aerating apparatus for aerating liquids such as sewage sludges, is improved by providing a rotor which moves in a path on the surface of the liquid and which moves the liquid in a direction at an angle of between 90* and 180* with respect to the path of motion of the frame. Alternatively, a device of this type is improved by providing a rotor which moves in a path on the surface of the liquid the rotor axis of which is inclined at an angle of up to 90* with respect to the horizontal.