We investigate the characteristics of free-surface vortex (FSV) formation and aeration processes in a self-ingested reactor. A disk with a protruded ridge rotating in a water tank is used as a simplified model. First, flow velocity fields on the FSV surface and the disk are obtained analytically using a semi-empirical method. From the bottom view of the impeller, we observe bubbles separated from two air pockets formed behind the ridge as the interface of the air pockets becomes fluctuated. Then, void fraction distribution along the vertical direction by rising bubbles is established theoretically and confirmed by experimental measurements. These results and the measured total void fraction data of the tank are used to show that the turbulence cascade energy near the disk is converted into the surface tension energy of bubbles and is responsible for aeration.