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.