A sewage aeration tank is constructed as an elongated cylinder with a horizontal axis having a length to diameter ratio of greater than three to one. The large aeration tank is completely closed and has openings tightly surrounding sewage inlet and effluent pipes, sludge return pipes, oxygenation pipes and vent pipes. The large aeration tank and its sewage inlet, grease and trash trap, clarifier, sludge return lines from the clarifier to the aeration tank, tertiary treatment tank and outflow apparatus are buried underground. A recreational space is provided on top of the system. An air blower is housed in an equipment room of an adjacent apartment unit which the system serves, and a vent extends upward from a roof of the apartment house.