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In this study, the start of partial nitrification (activated sludge)-anaerobic ammonia oxidation (immobilized fillers)-denitrification (immobilized fillers) and stable nitrogen removal in the treatment of rare earth element tailings wastewater were explored. The results showed that, at 25–30 °C, partial nitrification could be rapidly established by controlling the combination of average free ammonia concentration (25.4–60.2 mg/L) and dissolved oxygen concentration (0.5 mg/L). Adopting the stepwise substrate (three stages) and shortening hydraulic retention time (HRT) methods, the total nitrogen removal rate of anaerobic ammonia oxidation (anammox) immobilized fillers reached up to 0.99 kg N•(m3•d)−1. Adopting the C/N ratio of 4.5:1 and shorter HRT methods, the total nitrogen removal rate of denitrification immobilized fillers reached up to 1.07 kg N•(m3•d)−1. The activities of the anammox and denitrification immobilized fillers were increased by 8.39 and 4.46 times, respectively. At steady state operation, the average effluent ammonia nitrogen concentration was 3.01 mg/L, with an average effluent total nitrogen concentration of 5.11 mg/L, which met the requirements of the "Emission Standard for Pollutants from Rare Earth Industry" GB 26451–2011. High-throughput sequencing technology showed that the relative abundance ratios of Nitrosomonas and Nitrospira changed from 0.36:1–108.05:1. The dominant bacterial genera of the anammox immobilized fillers was Candidatus Kuenenia , and that of denitrification immobilized fillers were Pseudomonas and Thauera. This process provides a new technical approach for rare earth element tailings wastewater treatment, with high significance in engineering. [Display omitted] • This system achieved excellent nitrogen removal performance. • Controlling the high FA and low DO could rapidly establish partial nitrification. • Controlling the FA (above 5 mg/L) and low DO achieved a stable partial nitrification. • AnAOB and denitrifying bacteria could grow larger biomass in immobilized fillers. • Oxygen consumption, COD demand and sludge production would be largely reduced. [ABSTRACT FROM AUTHOR]

The present study sought to treat the ion-adsorption type rare earth mine wastewater in a pilot plant system using the nitritation-denitrification process with immobilized biological fillers. Meanwhile, the microbial community structure in the immobilized fillers was analyzed. The ammonia nitrogen removal rate of the nitritation fillers reached 0.43 kg N m−3 d−1 on day 61 (28.1 °C) and the total nitrogen removal rate of the denitrification fillers reached 0.51 kg N m−3 d−1 on day 58 (27.8 °C) under the hydraulic retention time of 3.4 hr and a fillers volume ratio of 8% (v/v). The average NH 4 +-N and TN removal efficiencies of the nitritation-denitrification process were 96.9% and 96.8%, respectively, during the steady state operation period (day 26–72). The results also indicate that high DO (5.5–6.5 mg L−1) could induce rapid growth of nitrite-oxidizing bacteria (NOB) in the nitritation fillers, even though the initial NOB proportion is extremely low. The optimal operational performance of the denitrification fillers reactor was the COD/(NO 2 −-N + NO 3 −-N) ratio of 3.7–4.2. The high-throughput sequencing analysis showed that the functional bacteria in the immobilized fillers could maintain a competitive advantage. The abundance of Nitrosomonas in the nitritation fillers increased from 6.58% to 21.24%, while Thauera and Ottowia were the dominant denitrifying bacteria genera in the denitrification fillers. [Display omitted] • Immobilized fillers were used for nitrogen removal from rare earth mine wastewater. • High DO (5.5–6.5 mg L−1) could induce rapid growth of NOB in the nitritation fillers. • A COD/(NO2 −-N + NO3 −-N) ratio of 3.7–4.2 was optimum for the denitrification fillers. • Functional bacteria in the immobilized fillers could maintain a competitive advantage. [ABSTRACT FROM AUTHOR]