In order to elucidate optimum operational parameters, five series of experiments with bench scale “A/O and MBR (membrane biological reactor)” devices were carried out. The HRTs of each anoxic and oxic bioreactor were maintained for 2˜6 h and for 3˜6 h, respectively. The internal recycle ratios were also changed from 100% to 300% of the influent feeding rate by peristaltic pump. There were no recognizable differences in the values of BOD removal efficiency, being around 99%, in the series of experiments. The same was true for T–N removal efficiency, which ranged about 60%. The similarity of the serial experiments for each parameter indicates that the extraordinarily high concentration of microorganisms in the bioreactor, i.e. 10,000–15,000 mg/l, strongly controls the entire process regardless of any moderate changes of the operational parameters.

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