Autotrophic Sulfur-Driven Partial Denitrification as a Sustainable Nitrite Supply Pathway for Anammox: Insights on Enhanced Nitrogen Removal and Microbial Synergies

Abstract

Anaerobic ammonia oxidation (Anammox) has garnered growing attention as an energy-efficient bioprocess. However, the sustainable provision of nitrite remains a crucial challenge. This study aimed to assess a promising alternative to existing partial nitrification- and heterotrophic partial denitrification (PDN)-based Anammox processes by substituting the nitrite supply with autotrophic sulfur-driven PDN. After 200 days, the desirable nitrogen removal efficiency of 96.1% was obtained in the S-PDN coupling Anammox process (S-PDN/A) with a high-quality effluent total nitrogen of 3.1 mg N/L. Additional experiments identified S-PDN/A as a stepwise reaction with generated S⁰ as an intermediate. Initially, S^2– was oxidized to S⁰ [21.2 mg of S/(g of SS·h)], coupled with ultrafast denitrification [NO₃^– → N₂, 3.9 mg of N/(g of SS·h)]. Subsequently, S⁰ served as an electron donor for S-PDN (NO₃^– → NO₂^–), efficiently facilitating Anammox as the dominant nitrogen removal pathway contributing up to 71.0% with a reaction rate of 1.7 mg N/(g SS·h). Distinct from reported prevalence of Candidatus brocadia in the Anammox technology for low-strength wastewater treatment, Candidatus kuenenia (0.12% → 3.4%) unexpectedly triumphed due to unique ecological niche provided by S-PDN. S-PDN/A offers fresh insights into Anammox application, enabling a potential reduction of up to 100% in organics demand, 43.0% savings in aeration energy consumption, and 69.9% decrease in biomass generation when compared to conventional bioprocesses.