A stirring device for activated sludges contained in a basin includes a stirring body having a hyperboloidal wall with a lower circumferential surface, and a funnel-shaped recess underside the hyperboloidal wall; a shaft attached to the stirring body; an air supply line adapted to extend along a bottom of the basin and having a vertical section to supply air into the recess of the stirring body; a disk arranged underside the stirring body and spaced away from the lower circumferential surface to form an annular gap therewith; and shear ribs formed under the lower circumferential surface to extend along the lower circumferential surface and surround the annular gap. The vertical section of the air supply line extends coaxially through the disk in relation to the shaft so that when the stirring body is rotating, the shear ribs distribute air bubbles through the annular gap.