Electron buffer formation through coupling thiosulfate-dependent denitratation with anammox in a single-stage sequencing batch reactor

The combination of thiosulfate-dependent denitratation and anammox in a single-stage reactor provides a feasible way to improve total nitrogen removal. The molar ratios of NH₄⁺/NO₃^- and S₂O₃^2-/NO₃^- were confirmed to be two key factors affecting the reactor performance. The optimal total nitrogen removal efficiency of 99.4% was achieved at NH₄⁺/NO₃^- of 0.75 and S₂O₃^2-/NO₃^- of 0.85. The multiple thiosulfate oxidation pathways contribute to electron buffers generated in the system. A novel isotope labeling method using ¹⁵N was applied to reveal N transformation pathways and a 3-step model was proposed. The nitrate was first converted to nitrite or nitric oxide (NO) by sulfur-oxidizing bacteria. In the second step, both nitrite and NO were utilized by anammox bacteria. Finally, the nitrate generated from anammox could be removed using sulfur deposits as electron donors. The findings provide a potential solution for mainstream nitrogen removal.