Metagenomic Insight Reveals the Microbial Structure and Function of the Full-Scale Coking Wastewater Treatment System: Gene-Based Nitrogen Removal

Microbial communities play crucial roles in pollutant removal and system stability in biological systems for coking wastewater (CWW) treatment, but a comprehensive understanding of their structure and functions is still lacking. A five month survey of four sequential bioreactors, anoxic 1/oxic 1/anoxic 2/oxic 2 (A1/O1/A2/O2), was carried out in a full-scale CWW treatment system in China to elucidate operational performance and microbial ecology. The results showed that A1/O1/A2/O2 had excellent and stable performance for nitrogen removal. Both total nitrogen (TN; (17.38 ± 6.89) mg·L⁻¹) and ammonium–nitrogen (${{\mathrm{NH}}_4}^{+}$-N; (2.10 ± 1.34) mg·L⁻¹) in the final biological effluent satisfied the Chinese national standards for CWW. Integrated analysis of 16S ribosome RNA (rRNA) sequencing and metagenomic sequencing showed that the bacterial communities and metagenomic function profiles of A1 and O1 shared similar functional structures, while those of A2 significantly varied from those of other bioreactors (p < 0.05). The results indicated that microbial activity was strongly connected with activated sludge function. Nitrosospira, Nitrosomonas, and SM1A02 were responsible for nitrification during the primary anoxic–oxic (AO) stage and Azoarcus and Thauera acted as important denitrifiers in A2. Nitrogen cycling-related enzymes and genes work in the A1/O1/A2/O2 system. Moreover, the hao genes catalyzing hydroxylamine dehydrogenase (EC 1.7.2.6) and the napA and napB genes catalyzing nitrate reductase (EC 1.9.6.1) played important roles in the nitrification and denitrification processes in the primary and secondary AO stages, respectively. The mixed liquor suspended solids (MLSS)/total solids (TS), TN removal rate (RR), total organic carbon (TOC) (RR), and ${{\mathrm{NH}}_4}^{+}$-N (RR) were the most important environmental factors for regulating the structure of core bacterial genera and nitrogen-cycling genes. Proteobacteria were the potential main participants in nitrogen metabolism in the A1/O1/A2/O2 system for CWW treatment. This study provides an original and comprehensive understanding of the microbial community and functions at the gene level, which is crucial for the efficient and stable operation of the full-scale biological process for CWW treatment.