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 NO and free nitrous acid (FNA) was separately evaluated in continuous feeding tests using different biomass carriers. The influent NO 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 NO-N concentration in the reactor was 350 mg L (FNA ≤0.06 mg L). The relationship between NO concentration in the reactor and inhibition ratio increased linearly. Evaluation of the inhibitory effect of FNA by increasing the influent NO 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 (IC) of FNA was 0.88 mg L. 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 IC for FNA at 0.70 mg L. These results indicate that the inhibitory effect of FNA on anammox activity was greater than that of NO. The evaluation of these two factors helped identify important operational indicators of the stable application of anammox processes.