Mixotrophic denitrification for enhancing nitrogen removal of municipal tailwater: Contribution of heterotrophic/sulfur autotrophic denitrification and bacterial community

The heterotrophic and autotrophic denitrification system can be used to remove wastewater nitrogen effectively. However, the relationship between nitrogen removal performance and microbial community composition variation needs to be explored further. Therefore, a combined heterotrophic‑sulfur autotrophic biofilter (HSAD) was established to remove nitrogen from municipal tailwater. As methanol dosage increased from 12 mg/L to 36 mg/L, NO3--N removal efficiency increased from 86.1% to 98.9%, and the generation of SO42- in the effluent was controlled within 167.6-113.2 mg/L under the condition of 30 mg/L NO3--N in influent and 3 h hydraulic retention time. Increasing methanol dosage promoted the synergism of heterotrophic denitrification (HD) and sulfur autotrophic denitrification (SAD). Different denitrification performance was associated with the microbial community composition. Proteobacteria, Bacteroidetes, and Chloroflexi were major phyla with cumulative abundance of over 70% and Proteobacteria was predominate in all samples. Denitrifying bacteria, such as Ferritrophicum, Thiobacillus, Thauera and Comamonas dominated in different operation stages of mixotrophic reactor. The decrease in dominant HD bacteria accompanied with the increase in SAD bacteria, and the SAD bacterial richness declined with the rise of HD contribution in the total denitrification process. Correlation networks analysis indicated that the dominant bacteria had positive or negative correlation with each other, but a stable coexistence state of microbial community structure was formed under the mixotrophic conditions. This work deepens our understanding of HSAD and reveals the interconnection between nitrogen removal mechanism and microbial community composition variation.