Simultaneous anammox and denitrification process shifted from the anammox process in response to C/N ratios: Performance, sludge granulation, and microbial community.

An up-flow anaerobic sludge blanket (UASB) reactor was operated to investigate the performance, sludge granulation, and microbial community dynamics of the simultaneous anammox and denitrification (SAD) process at different C/N ratios by inducing a shift from the anammox process. The nitrogen removal efficiency (NRE) increased with an increase in the C/N ratio at lower C/N ratios (≤ 0.5). The NRE and nitrogen removal rate (NRR) were 90.81% and 1.75 kgN m-3d-1, respectively, at a C/N ratio of 0.5. The contribution of the anammox process to nitrogen removal (Eanammox) decreased sharply to 51% from 81% as a result of the decreased activity of the anaerobic ammonia-oxidizing bacteria (AnAOB) at a C/N ratio of 0.9. The EPS production and the ratio of the protein fraction of EPS (EPSp) to the carbohydrate fraction of EPS (EPSc) increased with the increase in the C/N ratio (0.0-0.9), which induced sludge aggregation and resulted in larger particle size. The enrichment of denitrifying bacteria (DNB) on the surface of the AnAOB due to EPS bridging further enhanced sludge granulation. The Illumina sequencing results showed that the dominant genus of AnAOB (Candidatus Jettenia) initially increased with an increase in the C/N ratio at lower levels and then decreased sharply at a C/N ratio of 0.9. The relative abundance of Candidatus Jettenia was promoted by decreasing the C/N ratio, which supported the recovery of the AnAOB activity. Denitratisoma, Bacillus, Acinetobacter, and Pseudomonas were responsible for denitrification throughout the entire operational period.