A two-stage anaerobic-aerobic sequencing batch reactor (SBR) system (PAF-SBR) was developed to enhance biological phosphorus removal in the sequencing batch reactors. The system performance was evaluated against a conventional SBR system in parallel lab-scale reactors at room temperature, using the degritted raw wastewater as the feed. The SRT for the anaerobic SBR which is named PAF (Primary Acid Fermenter) was 12 days, and for both the BNR reactors was 10 days. All reactors were run at 3 cycles per day.

A significant difference (P=0.01) was observed between the performances of the two systems. The Ortho-P concentration in the effluent from the PAF-SBR was mostly below 0.5 mg/L while in the conventional SBR was generally above 1.5 mg/L. Lack of availability of carbon (mean VFA/PSol.=1.1) and long anoxic/anaerobic period were the major causes of inefficient removal of phosphorus in the conventional SBR system. The use of anaerobic stage however increased the mean VFA/PSol. to 11.3 which enhanced Bio-P removal in the PAF-SBR system. Prefermentation also improved the sludge consistency and settleability in the following SBR unit.

The results indicated that by using the perfermentation step, the anoxic/anaerobic period in the BNR-SBR could be controlled and reduced to less than 50 minutes, which would reduce the total cycle time from 8 hr to 6 hr.

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