Settling of Activated Sludge in Horizontal Tanks

The settling properties of activated sludge in the aerator effluent from 20 activated-sludge plants were examined in settling tubes. A study of flow conditions and efficiencies of the secondary sedimentation tanks at the plants, all of the horizontal type, was conducted. It was found that density currents caused low volumetric efficiency, but at actual loading conditions all settleable material was removed. The apparent depth of the bottom density current was found in all cases to be about 2 ft. Unsettleable floc in the settled water occurred noticeably when the concentration of sludge in the aeration tanks was low. The investigations indicate that considerable improvements in the design of secondary settling tanks are possible. The design must be made with respect to the highest concentration of activated sludge liquor that will be used in the process.