Process Requirement for Phosphorus Removal at Activated Sludge Sewage Treatment Plants

Abstract

The results of bench-scale and full-scale studies for nutrient removal at four activated sludge plants in the City of London were reported. The objective of 1 mg/l of phosphorus in final effluent was met by the addition of ferric chloride or lime. The choice of chemical, the required chemical dosages and the efficiency of phosphorus removal were found to be dependent on the location of the chemical addition point, the hydraulic loading of clarifier and the conditions of the sludge. Pacing of chemical pump by the plant flow signal was found to be satisfactory. Good sedimentation of the phosphate floc was obtained with overflow rate of 600 IGPD/ft2 using lime and over 1000 IGPD/ft2 using ferric chloride. Lime increased the solid content but decreased the volume of the sludge. Ferric chloride did not significantly alter the characteristic of the sludge other than the colour.