Deciphering the evolution of the functional genes and microbial community of the combined partial nitritation-anammox process with nitrate build-up and its in situ restoration

Abstract

The operation of a combined partial nitritation-anammox (CPNA) process was hampered by the overgrowth of nitrite-oxidizing bacteria (NOB), which was then successfully in situ restored by the combination of hydroxylamine (NH₂OH) dosing and sludge retention time (SRT) control. To elucidate the microbial evolution on gene level and the microbial mechanism responding to the NO₃⁻ build-up and in situ restoration in the CPNA process, the qPCR and high-throughput sequencing were used to investigate the evolution of the N-transformation genes and microbial community in this paper. Results showed that the concentrations of amoA and hao were both decreased as well as the hzo along with the NO₃⁻ build-up, however, the nitrite-oxidation encoding gene nxrA was increased. Pearson correlation analysis demonstrated that the gene nxrA was significantly correlated with the NO₃⁻. After restoration, the abundances of amoA, hao and hzo were increased to 26.58%, 43.56% and 26.17%, respectively, while the growth of nxrA was effectively limited. The obtained high abundance of narG gene (37.85%) as well as the significant correlation of hzo–narG and hzo–nirS illustrated the in situ restoring mechanism, i.e., the intensive denitrification of the accumulated NO₃⁻ to NO followed by the stimulation of the anammox by NO. Additionally, MiSeq sequencing showed a new microbial community was rebuilt after restoration and the Haliscomenobacter and Nitrosomonas were the dominant genera. The abundance of anammox genus Candidatus Kuenenia was also increased to 8.0%. The significant correlation between nirK and Ignavibacterium indicated that the filaments played an important role in the denitrification of the accumulated NO₃⁻.