Autotrophic\nSulfur-Driven Partial Denitrification\nas a Sustainable Nitrite Supply Pathway for Anammox: Insights on Enhanced\nNitrogen Removal and Microbial Synergies

Anaerobic ammonia oxidation (Anammox) has garnered growing\nattention\nas an energy-efficient bioprocess. However, the sustainable provision\nof nitrite remains a crucial challenge. This study aimed to assess\na promising alternative to existing partial nitrification- and heterotrophic\npartial denitrification (PDN)-based Anammox processes by substituting\nthe nitrite supply with autotrophic sulfur-driven PDN. After 200 days,\nthe desirable nitrogen removal efficiency of 96.1% was obtained in\nthe S-PDN coupling Anammox process (S-PDN/A) with a high-quality effluent\ntotal nitrogen of 3.1 mg N/L. Additional experiments identified S-PDN/A\nas a stepwise reaction with generated S<sup>0</sup> as an intermediate.\nInitially, S<sup>2–</sup> was oxidized to S<sup>0</sup> [21.2\nmg of S/(g of SS·h)], coupled with ultrafast denitrification\n[NO<sub>3</sub><sup>–</sup> → N<sub>2</sub>, 3.9 mg\nof N/(g of SS·h)]. Subsequently, S<sup>0</sup> served as an electron\ndonor for S-PDN (NO<sub>3</sub><sup>–</sup> → NO<sub>2</sub><sup>–</sup>), efficiently facilitating Anammox as\nthe dominant nitrogen removal pathway contributing up to 71.0% with\na reaction rate of 1.7 mg N/(g SS·h). Distinct from reported\nprevalence of Candidatus brocadia in\nthe Anammox technology for low-strength wastewater treatment, Candidatus kuenenia (0.12% → 3.4%) unexpectedly\ntriumphed due to unique ecological niche provided by S-PDN. S-PDN/A\noffers fresh insights into Anammox application, enabling a potential\nreduction of up to 100% in organics demand, 43.0% savings in aeration\nenergy consumption, and 69.9% decrease in biomass generation when\ncompared to conventional bioprocesses.