Nitrogen removal performance and bacterial flora analysis of a partial nitrification-anaerobic ammonium oxidation-denitrification system treating rare earth element tailings wastewater

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 Nitrosomonasand Nitrospirachanged 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 Pseudomonasand Thauera. This process provides a new technical approach for rare earth element tailings wastewater treatment, with high significance in engineering.