Abstract: The apparatus comprises a rotor, plurality of spaced transverse ribs disposed along the axis of the rotor and a plurality of longitudinal ribs having the shape of an impeller of a pump, the plurality of longitudinal ribs being mounted on each of said transverse ribs. The plurality of longitudinal ribs and transverse ribs constitute a biological contactor, thereby causing wastewater to be treated by passing through the biological contactor. The longitudinal ribs are obliquely installed in an appropriate angle with the perpendiculars of the exterior tangent arc of the contactor like the impeller of a pump, thereby making wastewater pass efficiently through the space between the ribs with centrifugality by rotation of the rotor.