Parameters to characterize the internal recirculation of an oxidation ditch

  • Shao Po Wang 1. Tianjin Key Laboratory of Aquatic Science and Technology, Tianjin Chengjian University, Tianjin 300384, China; 2. School of Environmental & Municipal Engineering, Tianjin Chengjian University, Tianjin 300384, China; 3. Municipal Experimental Teaching Demonstration Center of Environmental and Municipal Engineering, Tianjin Chengjian University, Tianjin 300384, China
  • Jing Jie Yu 1. Tianjin Key Laboratory of Aquatic Science and Technology, Tianjin Chengjian University, Tianjin 300384, China; 2. School of Environmental & Municipal Engineering, Tianjin Chengjian University, Tianjin 300384, China; 3. Municipal Experimental Teaching Demonstration Center of Environmental and Municipal Engineering, Tianjin Chengjian University, Tianjin 300384, China
  • Hua Ji Ma 1. Tianjin Key Laboratory of Aquatic Science and Technology, Tianjin Chengjian University, Tianjin 300384, China; 2. School of Environmental & Municipal Engineering, Tianjin Chengjian University, Tianjin 300384, China; 3. Municipal Experimental Teaching Demonstration Center of Environmental and Municipal Engineering, Tianjin Chengjian University, Tianjin 300384, China
Keywords: oxidation ditch, hydraulic retention time, internal recirculation ratio, circulatory period, internal recirculation frequency

Abstract

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 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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Published
2021-04-29
Section
Technical note