Elucidating the Impact of Low DO on Enhanced Biological Phosphorus Removal Under Aerobic and Anoxic Conditions at Full-Scale

Abstract

Research on low dissolved oxygen (DO) enhanced biological phosphorus removal (EBPR) at full-scale remains limited, a knowledge gap this study aims to fill by investigating EBPR performance and microbial community shifts at a Water Resource Recovery Facility (WRRF) transitioning to low DO conditions. Average DO concentrations decreased from 2.62 mg O₂/L in 2019 to 0.80 mg O₂/L in 2023. Simultaneously, average effluent orthophosphate concentrations decreased from 0.57 mg P/L to 0.29 mg P/L, despite the elimination of metal salt addition for chemical precipitation in 2023. Average effluent total phosphorus concentrations remained between 0.47 and 0.67 mg P/L across varying DO concentrations, which reached below 0.50 mg O₂/L. Batch tests conducted over a four-year period indicated higher phosphorus release and aerobic uptake rates when full-scale DO concentrations were below 1 mg O₂/L. Phosphorus release rates increased from 8.9 ± 1.0 to 12.1 ± 0.6 mg P/g MLVSS/hr, while aerobic phosphorus uptake rates increased from 3.6 ± 0.6 to 5.3 ± 0.4 mg P/g MLVSS/hr. Microbial analysis revealed a community shift toward taxa containing polyphosphate-accumulating organisms (PAOs) with estimated relative abundances between 0.12% and 3.62%. High rates of denitrification fueled by internally stored carbon during the anoxic phase were correlated with elevated aerobic phosphorus uptake rates. Batch tests in the latter two years indicated that anoxic phosphorus uptake rates accounted for 3% to 40% of the aerobic uptake rates, suggesting that the reduction in DO concentrations from 2019 to 2023 may have facilitated anoxic phosphorus uptake capacity.