In this study, an anaerobic/aerobic/anoxic process (referred to as an AOA process) using a sequencing batch reactor (SBR) was proposed for simultaneous phosphorus and nitrogen removal from wastewater. The AOA process was stably operated over more than one year when a certain amount of carbon substrate (40 mg-C/L in a reactor) was supplemented to inhibit aerobic phosphate uptake. The average nitrogen and phosphorus removal efficiencies were 83% and 92%, respectively. It was demonstrated that phosphate-accumulating organisms (PAOs) capable of utilizing nitrite as an electron acceptor, the so-called denitrifying phosphate-accumulating organisms (DNPAOs), could exist in the AOA process. Moreover, the ratio of anoxic phosphate uptake rate (PUR) to aerobic PUR (anoxic/aerobic PUR ratio), which indicates the fraction of DNPAOs in total PAOs, was experimentally evaluated. The results indicate that the AOA process has a much larger anoxic/aerobic PUR ratio than the conventional A2O (anaerobic/anoxic/aerobic) and AO (anaerobic/aerobic) processes. In conclusion, the AOA process allows DNPAOs to take an active part in simultaneous nitrogen and phosphorus removal in an SBR when a suitable amount of carbon substrate is supplied at the start of aerobic conditions.
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