A new biological nutrient removal process, anaerobic-oxic-anoxic (A/O/A) system using denitrifying polyphosphate-accumulating organisms (DNPAOs), was proposed. To attain excess sludge reduction and phosphorus recovery, the A/O/A system equipped with ozonation tank and phosphorus adsorption column was operated for 92 days, and water quality of the effluent, sludge reduction efficiency, and phosphorus recovery efficiency were evaluated. As a result, TOC, T-N and T-P removal efficiency were 85%, 70% and 85%, respectively, throughout the operating period. These slightly lower removal efficiencies than conventional anaerobic-anoxic-oxic (A/A/O) processes were due to the unexpected microbial population in this system where DNPAOs were not the dominant group but normal polyphosphate-accumulating organisms (PAOs) that could not utilize nitrate and nitrite as electron acceptor became dominant. However, it was successfully demonstrated that 34-127% of sludge reduction and around 80% of phosphorus recovery were attained. In conclusion, the A/O/A system equipped with ozonation and phosphorus adsorption systems is useful as a new advanced wastewater treatment plant (WWTP) to resolve the problems of increasing excess sludge and depleted phosphorus.
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