Nitrate Reverses Severe Nitrite Inhibition of Anaerobic Ammonium Oxidation (Anammox) Activity in Continuously-Fed Bioreactors

Nitrite (NO2–) substrate under certain conditions can cause failure of N-removal processes relying on anaerobic ammonium oxidizing (anammox) bacteria. Detoxification of NO2– can potentially be achieved by using exogenous nitrate (NO3–). In this work, continuous experiments in bioreactors with anammox bacteria closely related to “Candidatus Brocadia caroliniensis” were conducted to evaluate the effectiveness of short NO3– additions to reverse NO2– toxicity. The results show that a timely NO3– addition immediately after a NO2– stress event completely reversed the NO2– inhibition. This reversal occurs without NO3– being metabolized as evidence by lack of any 30N2 formation from 15N-NO3–. The maximum recovery rate was observed with 5 mM NO3– added for 3 days; however, slower but significant recovery was also observed with 5 mM NO3– for 1 day or 2 mM NO3– for 3 days. Without NO3– addition, long-term NO2– inhibition of anammox biomass resulted in irreversible damage of the cells. These results suggest that a short duration dose of NO3– to an anammox bioreactor can rapidly restore the activity of NO2–-stressed anammox cells. On the basis of the results, a hypothesis about the detoxification mechanism related to narK genes in anammox bacteria is proposed and discussed.