Abstract: The clarifier in any activated sludge sewage treatment plant must provide (a) a quiescent environment to allow gravity settling of the bio-mass (sludge) grown within the aeration tank, (b) the decanting of clear supernatant thus produced, and (c) the timely removal of all accumulated clarifier sludge. This invention improves prior clarifier performance by (a) providing vacuum assistance to gravity settling, (b) trapping all sedimentation within an aerated sludge containment zone, (c) providing clarifier sludge aeration to prevent denitrification, (d) utilization of spent aeration air for energy-free return sludge pumping, (e) the elimination of density currents and induced currents created by return sludge pumping, and (f) providing uniform hydraulic sludge collection eliminating the need for mechanical collection. The lower portion of the clarifier walls must be inward sloped to provide a hopper bottom.