Nitrogen removal performance and microbial community analysis of immobilized biological fillers in rare earth mine wastewater

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 NH4+-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/(NO2−-N + NO3−-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.