The effect of the oxidation reduction potential (i.e., ORP) control system on the simultaneous removal of nitrogen and phosphorus, utilizing the oxic-anaerobic-oxic process, was investigated. A full-scale wastewater treatment plant which was designed as an extended aeration process was modified for this process to study. The optimum ORP at the first basin for the simultaneous removal was 125 mV in this experiment. In the first oxic basin the nitrification reaction and the denitrification reaction occurred simultaneously to maintain the optimum level of the ORP value. The averaged values of the effluent of this process were 1 mg/l of total nitrogen (T-N), 0.5 mg/l of total phosphorus (T-P) and 5 mg/l of BOD respectively. It was also clearly shown by the statistic analysis that the ORP control was more effective in comparison with the control of dissolved oxygen (DO) concentration for the operating method of this process. Additionally, it was supposed that the simultaneous nitrification and denitrification using the ORP control system was a reliable nitrogen removal system for renovating the conventional activated sludge process because the hydraulic retention time of the first basin used in this study was equal to that of the conventional process.

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