Engineering operation and control logic for stable Candidatus Nitrosoglobus-mediated acidic partial nitritation

Acidic partial nitritation (Acidic PN) mediated by Candidatus Nitrosoglobus (Ca. Nitrosoglobus) offers a promising route for stable nitrite accumulation. However, the engineering parameters and control logic required for practical reactor operation remain insufficiently established. In this study, two airlift reactors (E-Reactor and D-Reactor) were operated to investigate the enrichment of Ca. Nitrosoglobus and the operational boundaries of acidic PN. In the E-Reactor, stable nitrite accumulation (>90%) was achieved during a stepwise increase in nitrogen loading from 0.25 to 1.05 kg N m⁻³ d⁻¹, accompanied by the enrichment of Ca. Nitrosoglobus to 74.86% relative abundance. Batch activity tests identified broad pH adaptability, an optimum temperature of approximately 30 ± 1 °C, relatively low free ammonia tolerance, and high free nitrous acid tolerance. Additionally, in situ analyses under optimized conditions were used to determine substrate and oxygen affinity coefficients (K_s and K_o). Kinetic characterization in the D-Reactor operated under the optimized conditions revealed a biomass yield (Y) of 0.0976 g VSS g⁻¹ N, a decay coefficient (K_d) of 0.0155 d⁻¹, ammonium and oxygen affinity coefficients of 22.8 mg N L⁻¹ and 0.77 mg O₂ L⁻¹, respectively, and an oxygen-limited minimum sludge retention time (SRT) of 10-14 d at DO concentrations of 1-2 mg O₂ L⁻¹. These quantified parameters provide a practical basis for defining optimal operation window and biomass retention requirements for stable acidic PN and sustained Ca. Nitrosoglobus enrichment, thereby supporting reliable upstream nitrite production and future integration with downstream processes such as anammox.