[Phosphorus Removal Performance in EBPR System under Extra-low Dissolved Oxygen Condition]

Two sequencing batch reactors (SBRs) were operated with alternating anaerobic/oxic (An/O) for 127 days to study the phosphorus removal efficiency and process performance of the EBPR under different dissolved oxygen gradient. For the reactor (R₁) in which DO was not controlled and measured as high as 6 mg·L^-1 during the aerobic stage, a stable phosphorus removal performance was achieved in the former 65 days during the operational period. The phosphorus removal efficiency was greater than 95.9% and the total phosphorus in effluent was less than 0.5 mg·L^-1. However, the phosphorus removal performance subsequently deteriorated after 65 days and completely broke up until 97 day due to long-term excessive aeration. Throughout the operational period,only 39.4% of effluent TP met the national wastewater discharge standard A. For the test reactor (R₂) in which DO was controlled at the levels of 2, 1, 0.5, 0.2, 0.1 mg·L^-1 during the aerobic stage, respectively, the phosphorus removal performance showed a slight fluctuation at the beginning of each stage but rapidly increased to a stable state. According to the statistics, 94.6% of effluent TP in reactor R₂ could meet the national wastewater discharge standard A throughout the operational period. In only 6 days out of the 127 days, effluent TP failed to meet the national standard and the process performance was far better than that of R₁. The specific phosphate uptake rate under DO=2 mg·L^-1 was found to be nearly the maximum, but it was greatly influenced by the low DO. In addition, limited filamentous bulking resulted from low organic matter degradation rate under extra-low DO level (0.1 mg·L^-1) was observed. Nevertheless, 100% of effluent TP during this stage could meet the national standard. As a result, the phosphorus removal in micro-aerobic EBPR system for saving energy was feasible.