PURPOSE: To permit high-efficiency aeration by forming a cylindrical casting in such a manner that at least a part of the central part of the inside casing provided on the discharge side of an impeller for delivering a liquid to be treated can be rotated and discharging gas from the rotating part.
CONSTITUTION: The impeller 3 which is driven by a driving device 2 to feed the liquid to be treated is disposed in the cylindrical casing 1 and the inside casing 5 is coaxially provided on the discharge side thereof to form an annular discharge flow passage 6 between said casing and the cylindrical casing 1. The casing 5 is so formed that at least a part of the central part 11 thereof can be rotated. A gas outflow port 7' is opened to such rotating part and the inside of the rotation part is constructed as an oxygen-contg. gas chamber 12. As a result, the liquid flow and the rotating body have a large velocity difference to form the finer bubbles and to disperse the fine bubbles in the liquid flow by the powerful shearing effect. The amt. of the gas to be dissolved into the liquid is thereby increased and the extremely high-efficiency aeration is executable.