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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A coagulation-anaerobic hydrolysis-aeration process was used to treat the waste cutting fluid which was discharged from a metalworking plant, and the operating conditions of each unit were optimized in this paper. The results showed that 9 g/L polyaluminum chloride and 0.3 g/L cationic polyacrylamide were added in the coagulation stage, the TOC was removed by 78.94% and the BOD5/COD of the waste cutting fluid increased from 0.046 to 0.312 before and after coagulation; The coagulation effluent was further treated by anaerobic hydrolysis-aeration, and the TOC removal efficiencies of the biological process and the whole chemical-biological process were 92.77% and 98.48% respectively; Adding glucose as a cosubstrate into the anaerobic hydrolysis can improve the TOC removal efficiency, when the TOC content ratio of coagulation effluent to glucose solution was 7:3, the TOC removal efficiencies of the biological process and the chemical-biological process reached 97.16% and 99.40%, and the total oil removal efficiency of the whole process reached 99.99%; The effluent quality parameters of the coagulation-anaerobic hydrolysis (with cosubstrate glucose)-aeration process met the Class C limits specified in the Wastewater Quality Standards for Discharge to Municipal Sewers (GB/T 31962-2015); that is, the effluent COD, TN, TP and total oil were below 300 mg/L, 25 mg/L, 5 mg/L and 110 mg/L respectively, and the effluent pH was between 6.5–9.5.