Surveys on the performance of nitrogen, phosphorus and BOD removal were carried out in a full-scale conventional, i.e. continuously aerated, sequencing batch reactor (SBR) activated sludge process which was constructed ten years ago, treating wastewater from a small factory producing soybean fermentation products. The old SBR plant, originally designed only for BOD removal, is operated with one cycle per day and aerated continuously for 19 hours per day from the start of working and wastewater inflow followed by sedimentation and decanting the next morning before work. In spite of continuous and a constant rate of aeration, DO increased rapidly immediately after the beginning of aeration, whereas it decreased down to less than 0.5 mg l−1 with the inflow of wastewater. DO increased again after work with little inflow of wastewater. The diurnal fluctuation of DO enhanced both nitrification and denitrification simultaneously in a single tank reactor with continuous aeration and resulted in high performance of nitrogen removal beyond expectation as well as BOD. The anaerobic condition and subsequent increase in DO also enhanced release and luxury uptake of phosphorus, i.e. biological removal of phosphorus. Increase in the strength of wastewater enhanced denitrification without deterioration of BOD removal. Phosphorus removal, however, was deteriorated by insufficient sludge production.

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