Inhibition factors and kinetic model for anaerobic ammonia oxidation in a granular sludge bioreactor with Candidatus Brocadia

Varying pH and high levels of nitrite have major inhibitory effects on anaerobic ammonium oxidizing bacteria (AnAOB) and are known to interfere with the anaerobic ammonia oxidation (Anammox) process. In this study, we evaluated the effect of pH and nitrite on Anammox process performance through a predictive kinetic model. A series of batch experiments were performed in a stably operating Anammox sequencing batch reactor system with “Candidatus Brocadia” as the dominant AnAOB taxon (>95%), by varying the nitrite concentration from 40 to 378 mg/L and the pH from 6.7 and 8.5, while maintaining the NH₄ ⁺-N concentration at 54.6 ± 5.6 mg/L. Dual substrate kinetics including a pH inhibition function was able to predict Anammox activity at higher NO₂ ⁻-N concentrations, with a nitrite inhibition constant of 210 mg/L. A synergistic negative effect on Anammox activity was observed when combining pH and nitrite stressors. Anammox activity was completely lost at low (6.7) and high (8.5) pH conditions, but resumed after adjusting to an optimal pH (7.5 ± 0.2). We found that AnAOB bacteria are susceptible to both high nitrite concentrations and fluctuating pH levels. Based on experimental evidence and modeling, pH mediates nitrite inhibition in the Anammox process due to the accumulation of free nitrous acid. Our model can predict the outcome of different operating scenarios and help select optimal operational strategies for Anammox implementation.