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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 were 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 OD process would possess a better DNPR potential if the IRF was controlled within the proper scope.

Mixed liquor circulates ceaselessly in the closed-loop corridor in an oxidation ditch (OD), which is significantly different from other wastewater treatment processes. The internal recirculation ratio (IRR), i.e., the ratio between circulation flow rate (QCC) and influent flow rate (QIn), and the circulatory period (T), i.e. the time consumed for the mixed liquor to complete one lap in the circular corridor, was used to quantify the internal recirculation characteristics of the OD system. In order to elucidate the characteristics and applicability of IRR and T, this study obtained the numerical relationship between IRR and T by formula derivation. It also discusses the factors influencing IRR and analyses the applications of IRR and T. The results showed that IRR = QCC/QIn = HRT/T = HRT ž IRF (HRT = hydraulic retention time of the mixed liquor in the circular corridor; IRF = internal recirculation frequency). Moreover, three kinds of parameters had an effect on IRR: QIn; reactor dimensions, i.e., length (Lmid), width (B), and height (H) of the circular corridor; and horizontal velocity of the mixed liquor in the circular corridor (v). QIn changed IRR by altering HRT. However, B, H, Lmid, and v changed IRR by altering IRF and T. Furthermore, the same IRR corresponded to many different HRT and IRF. Therefore, when QIn and QCC varied in the OD system, using HRT and IRF to evaluate the variation of QIn and QCC, respectively, was better than using IRR to evaluate their synthetical variation. IRF and T were useful for directly and precisely characterizing the circulation speed and circulation flow rate in the circular corridor, while IRR was more useful for evaluating the dilution effect of reflux on influent.

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The flashboard was installed in the circulation compartment of the modified oxidation ditch in order to regulate the mixed reflux from aerobic zone to anoxic zone. The differences of the nutrient removal efficiencies and the phosphate-removal bacteria content were researched before and after flashboard installation. The results showed that the average removal efficiencies of COD, NH4+, TN, TP were 93.3%, 87.1%, 78.1% and 96.0% respectively, and the proportion of denitrifying phosphate-removal bacteria (DPB) to total phosphorus accumulating organisms were 46.1% after the flashboard installation. However, the average removal efficiencies of COD, NH4+, TN, TP were 91.2%, 82.7%, 67.2% and 86.4% respectively, and the proportion of denitrifying phosphate-removal bacteria to total phosphorus accumulating organisms were 17.54% before the flashboard installation. So, the modified oxidation ditch with flashboard could enrich denitrifying phosphate-removal bacteria and and improve the nutrient removal efficiencies.