An efficient DPB utilization process: The modified A2N process

The modified A2N process was proposed on basis of the A2N process. There are two waste sludge lines in the modified A2N process: an AN (anaerobic-anoxic) waste sludge line and an AO (anaerobic–aerobic) waste sludge line. Denitrifying phosphorus removing bacteria (DPB) and common phosphorus accumulation organisms (PAOs) coexisted in the modified A2N process but was relatively separated. Two streams of mixed liquor suspended solid (MLSS) were out of the anaerobic tank with one entering the anoxic tank, where DPB was the dominant microorganism, and the other the nitrification tank, where the aerobic PAOs dominated. The effects of pH and DO on process performance were investigated. Results showed that DPB might be more pH tolerable than aerobic PAOs while DPB and aerobic PAOs shared similar responses to DO. Parameters optimization was conducted and the following optimized parameters obtained: DO in nitrification tank was 2–3 mg L−1; nominal HRT for each unit (anaerobic, anoxic and aerobic units) was 3 h; SRT was 15 d. When the influent nutrient mass ratio (C/N/P) was 26.6/7.67/1 to 26.6/3/1, the average effluent COD, TN, and TP concentrations of the optimized process were 45, 10.5 and 0.7 mg L−1, respectively, with average removal efficiency 89%, 95.3% and 76.7% for COD, TN and TP.