Evaluation of nitrogen removal and N2O emission in a novel anammox coupled with sulfite-driven autotrophic denitrification system: Influence of pH

Abstract A combination of anaerobic ammonium oxidation (anammox) with sulfur-autotrophic denitrification is a promising alternative for complete nitrogen removal. Herein, a novel anammox coupled with sulfite-driven autotrophic denitrification (ASDAD) is firstly proposed. The influence of pH (8.5–6.5) on nitrogen removal and N2O emission is investigated in an anaerobic sequencing batch biofilm reactor fed with low-nitrogen wastewater containing sulfite. Long-term operation demonstrates that a maximum nitrogen removal of 94.5% occurred at a pH of 7.5 and that the contribution of autotrophic denitrification to nitrogen removal increased as the pH declined. Cyclic and batch experiments verify that anammox and sulfite-autotrophic denitrification simultaneously participates in nitrogen removal, and the stoichiometry for sulfite-driven autotrophic denitrification is firstly proposed. Furthermore, N2O accumulation is also highly correlated with the pH. The N2O emission peaks at a pH of 6.5 due to the inactivated anammox and incomplete autotrophic denitrification of nitrite at such a low pH. High-throughput sequencing reveals the coexistence of anammox (Candidatus Brocadia and Candidatus Kuenenia) and sulfur-oxidizing species (Thiobacillus). Finally, the schema for nitrogen conversion and N2O formation of ASDAD is elucidated based on metagenomic metabolism. This study offers a cost-effective and environment-friendly completely autotrophic nitrogen removal process for municipal wastewater with a low carbon-to-nitrogen ratio in practice.