Thiosulfate-driven autotrophic and mixotrophic denitrification processes for secondary effluent treatment: Reducing sulfate production and nitrous oxide emission

Three ecological floating beds (EFBs) with different additional electron donors including sodium thiosulfate, mixed electron donors of sodium thiosulfate and sodium acetate and without additional electron donors were established to compare the differences of nitrogen removal efficiency, nitrous oxide emission, microbial community and functional gene between autotrophic and mixotrophic denitrification. Results showed denitrification efficiency was nearly 100% in both autotrophic and mixotrophic process when electron donors were sufficient while that ranged from 4 to 43% without additional electron donors. Sodium acetate addition could effectively decrease sulfate concentration in effluent and nitrogen oxide flux. In addition, high-throughput sequencing analysis revealed autotrophic denitrifying bacteria were dominant in autotrophic denitrification while autotrophic, facultative and heterotrophic denitrifying bacteria coexisted in mixotrophic denitrification, and there was no dominant genus. For EFB with mixed external autotrophic and heterotrophic electron donors, it can not only achieve better denitrification efficiency, but also reduce the emission of nitrous oxide.