Sulphur-based autotrophic denitrification of wastewater obtained following graphite production: Long-term performance, microbial communities involved, and functional gene analysis

• It almost completely eliminated NO 3 – -N from graphite production-derived wastewater. • N metabolism pathways of sulphur-based autotrophic denitrification were revealed. • Thiobacillus carried functional genes for each pathway of denitrification. • Thiobacillus was tolerant to high salinity. Sulphur-based autotrophic denitrification is an energy-efficient NO 3 – -N removal process; it does not require carbon and may potentially replace traditional denitrification processes. This process was used to treat graphite production-derived wastewater and achieved almost complete removal of NO 3 – -N (concentration in effluent: 5.2 mg/L; concentration in influent: 606 mg/L) at a salinity of 15 g/L with a 30 h hydraulic retention time. A unique microbial community was established, in which the abundance of Thiobacillus increased with the increase of the NO 3 – -N concentration and salinity. Metagenomic analysis revealed that the denitrification metabolic pathway in the bioreactor was active. It also revealed the increased activation of nhaH, a gene encoding Na + /H + antiporters; proA , proB , and proC , genes encoding proline; and Trk and Kdp systems during the treatment of graphite production-derived wastewater to maintain cell function, providing valuable information about utilizing the sulphur-based autotrophic denitrification process to treat graphite production-derived wastewater.