Nitrogen and phosphorus removal by GAOs and PAOs using nitrate and limited oxygen as electron acceptors simultaneously and the impact of external carbon source in the anoxic phase

In this study, the effects of both limited oxygen gradient supplies (0, 0.1–0.3, and 0.3–0.5 mg/L) and nitrate as electron acceptors and an external carbon source in the anoxic phase on nitrogen and phosphorus removal and microbial community structures were investigated. The results demonstrated that more than 90% of the NO3–-N and P removal efficiencies were achieved under limited oxygen conditions (0.1–0.5 mg/L). Improved NH4+-N removal and anaerobic COD uptake were achieved under conditions of high oxygen concentrations. The genera of Dechloromonas (DPAOs) and norank _ f _ Bacteroidetes BD2–2 (PAOs) performed P removal without and with oxygen in the reactor, respectively. The genera of Defluviicoccus and Candidatus Competibacter (GAOs) were mainly responsible for NO3–-N removal in all phases. It indicated that the GAOs and PAOs were responsible for NO3–-N and P removal with a relationship of cooperation besides the competition to carbon source. When there was an external COD in the anoxic phase, the COD and NO3–-N could be removed by the genera of Azospira and Thauera thanks to the heterotrophic denitrification. However, the P removal efficiency decreased as the vicious circle that external COD deteriorated the anoxic P removal and polyhydroxyalkanoates (PHAs) consumption, leading to the decrease of the following anaerobic P release and PHAs synthesis, and more remaining COD. Thus, the anaerobic-anoxic-nitrifying (A2N) process was suggested to be a promising approach for nutrition removal, considering the negative effects of anoxic external COD. Moreover, the formation of granular sludge was promoted under limited oxygen conditions.