A novel method for the clarification of activated sludge, using sludge blanket filtration, was proposed. The process was tested continuously with activated sludge over 250 days. The proposed process is based on DAF separation, where the mixed liquor is distributed to the surface of the tank, and the clarified effluent is partially recycled with compressed air to the bottom. As a result, a floating blanket of sludge is formed which provides efficient separation of the mixed liquor suspended solids. Various factors, including the extent of sludge bulking, surface loading rate, and the depth of sludge blanket, were evaluated for their effects on the effluent water quality. A pilot-scale study demonstrated that separation by downflow Sludge Blanket Clarification (dSBC) showed better performance than other conventional separation technologies using microbubbles, such as DAF. The average effluent solids concentration from dSBC was about 3.0 mg/l, even with a relatively large amount of bulking sludge with a total surface loading of 60 m(3)/m(2)/d and air/solids ratio (A/S) of 0.011. This result has not been achievable with either gravitational sedimentation or DAF, even when the total surface loading rate was reduced to 20 m(3)/m(2)/d, one third of the value applied for dSBC. Based on these results, the dSBC process may provide a more efficient option for achieving both higher and more stable water quality in an activated sludge system.