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 ₂ ^- and free nitrous acid (FNA) was separately evaluated in continuous feeding tests using different biomass carriers. The influent NO ₂ ^- 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 ₂ ^- -N concentration in the reactor was 350 mg L ^-1 (FNA ≤0.06 mg L ^-1 ). The relationship between NO ₂ ^- concentration in the reactor and inhibition ratio increased linearly. Evaluation of the inhibitory effect of FNA by increasing the influent NO ₂ ^- 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 ₅₀ ) 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 IC ₅₀ 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 NO ₂ ^- . The evaluation of these two factors helped identify important operational indicators of the stable application of anammox processes.
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