Individual evaluation of nitrite and free nitrous acid inhibition on anammox activity

The anammox reaction simultaneously utilizes ammonia and nitrite as substrates; however, high nitrite concentrations act as strong inhibitors of the reaction. In this study, inhibition by NO₂^- and free nitrous acid (FNA) was separately evaluated in continuous feeding tests using different biomass carriers. The influent NO₂^- concentration was increased under pH 7.6, where FNA is less likely to affect anammox activity. A continuous test using polyethylene glycol (PEG) gel carriers containing immobilized anammox bacteria showed that the inhibition ratio was 13% when the NO₂^--N concentration in the reactor was 350 mg L^-1 (FNA ≤0.06 mg L^-1). The relationship between NO₂^- concentration in the reactor and inhibition ratio increased linearly. Evaluation of the inhibitory effect of FNA by increasing the influent NO₂^- concentration at pH 6.4, where FNA is easily formed, demonstrated that the relationship between FNA and inhibition ratio could be fitted to a sigmoid curve, and the 50% inhibitory concentration (IC₅₀) of FNA was 0.88 mg L^-1. A similar test performed using polyvinyl alcohol carriers containing anammox bacteria on their surface showed the same trend as the PEG gel carriers, with the IC₅₀ for FNA at 0.70 mg L^-1. These results indicate that the inhibitory effect of FNA on anammox activity was greater than that of NO₂^-. The evaluation of these two factors helped identify important operational indicators of the stable application of anammox processes.