This study investigated the influence of the unique internal recirculation characteristics of an oxidation ditch (OD) system, namely, the internal recirculation frequency (IRF) on denitrifying phosphorus removal (DNPR). The ratios of denitrifying polyphosphate-accumulating organisms (DPAOs) to polyphosphate-accumulating organisms (PAOs) under different IRF conditions were measured using a batch experiment. On this basis, the variation of nutrient transformations was studied using the IRF changes by the mass balance method. The results showed that, for the OD system that had an anaerobic zone upstream from the circular corridor and set anoxic and aerobic zones along the circular corridor, when the IRF was between 3.4 h−1 and 7.5 h−1, the DPAOs/PAOs ratio reached about 50%. Approximately 20% of the total phosphorus (TP) was removed and over 11% of the total nitrogen (TN) was transformed into nitrogen gas by the DNPR process, and meanwhile the total removal efficiencies of the TP and TN were over 93% and 80%. When the IRF was greater than 11.5 h−1, the TN removal efficiency decreased significantly, and this was not conducive to simultaneous nitrogen and phosphorus removal. The results indicated that the OD process would possess a better DNPR potential if the IRF were controlled within the proper scope.

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