An aerator assembly for wastewater treatment includes a draft tube and an air supply assembly. The draft tube includes a sidewall and presents open top and bottom tube ends. The air supply assembly includes an air supply conduit and a diffuser body. The diffuser body has an inlet aperture and a bubble generator connected to the inlet. The inlet aperture is connected to the air supply conduit such that the bubble generator receives air from a source of air, to which the air supply conduit is connected. The bubble generator has a plurality of air openings for generating fine air bubbles. The diffuser body is sealingly engaged to the sidewall adjacent the bottom tube end so as restrict upward flow of wastewater through the draft tube past the diffuser body.