Enhancement of phosphorus removal in a low temperature a²/O process by anaerobic phosphorus release of activated sludge

An anaerobic phosphorus release tank was introduced to an anaerobic-anoxic-aerobic (A²/O) process treating domestic sewage to enhance the phosphorus removal at low temperature. Phosphorus release of the activated sludge from the second sedimentation tank was evaluated at 14°C by batch cultures, and the nutrient removal in the modified low temperature A ²/O process was further investigated at the same temperature. The results showed that the feasible sludge retention time was 14 h for sequencing batch reaction and 12 h for continuous flow operation. The ratio of raw sewage to activated sludge from the second sedimentation tank was 1:1 in volume to meet the demand of carbon resource for the growth of phosphorus release microbes. The feasible chemical oxygen demand (COD) loading rate of the activated sludge in the phosphorus release tank was 0.015-0.02 g COD/g MLSS (mixed liquor suspended solids) and the nitrate concentration should be less than 5 mg/L. The phosphorus release was doubled when the sludge was blended intermittently and gently. The anaerobic phosphorus release of the activated sludge improved the phosphate removal remarkably, as well as the removal of NH₄ ⁺-N and total nitrogen (TN) in the modified low temperature A ²/O process. The effluent COD, NH₄⁺-N, TN and total phosphorus could meet a stricter discharge standard.