It has been reported that a proportion of polyphosphate-accumulating organisms (PAOs) can denitrify or utilize nitrate as an electron acceptor. The usage of denitrifying-PAO (DN-PAO) can relieve the competition for COD between denitrification and phosphorus removal because they can treat nitrate and phosphate by using the same carbon source. To effectively use DN-PAO for biological nutrient removal (BNR), a new system was proposed in which an anaerobic phase is placed at the influent end, followed by the anoxic and external nitrification phase. In this study, the lab-scale proposed system (A2N system) was operated with a municipal wastewater 1) to confirm stable settlement of DN-PAO in the proposed system, 2) to quantitatively estimate the fraction of different groups of organisms like denitrifiers without polyphosphate accumulating capability, aerobic-PAO and DN-PAO and 3) to estimate the advantages of DN-PAO's presence in the system. Moreover, batch experiments in which anoxic and aerobic phosphate uptake rates (PUR) were measured were also carried out. The activity of DN-PAO was observed throughout the experimental period by the batch experiment. From the results of the calculation of COD utilized by each group of organisms, it was concluded that the proposed system could accumulate much more PAO (as DN-PAO) than conventional BNR systems. Moreover, they were responsible for both EBPR and denitrification.

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