Anaerobic ammonium oxidation (anammox) is widely considered as a promising N removal technology to treat ammonium rich wastewater. In the anammox process, nitrite (NO2-) is used as an electron acceptor to oxidize ammonium (NH4+) yielding environmentally inert dinitrogen (N2) gas as the main product. However, studies showed that NO2- also causes substrate toxicity, potentially causing the failure of the anammox reactors. In this project, we investigated the mechanism of NO2- toxicity to anammox process and proposed to utilize nitrate (NO3-) addition to recover anammox reactors failing due to NO2- inhibition. The effectiveness of NO3- addition in preventing anammox failure was studied in batch- and continuous-fed-experiments in which anammox was purposely subjected to NO2- stress. The results show that higher recoveries of performance were obtained with increasing concentrations of added NO3-, with the highest recovery of 80% at 0.8 mM NO3- at pH 7.0. In order to study the relationship between proton gradient and the recovery performance by NO3-, carbonyl cyanide m-chlorophenyl hydrazine (CCCP) was used to dissipate the proton gradient of anammox bacteria. The results suggested that reactivation of NO2--inhibited anammox process by NO3- is irrespective of the proton gradient, supporting a proposed hypothesis that the detoxification mechanism is the NO2-/NO3- antiporter activity encoded by narK present in anammox cells. To further evaluate the feasibility of using NO3- to recover failing anammox reactors, three UASB were used to mimic the system with normal performance and failing reactors with/without NO3- addition, respectively. A complete performance recovery was obtained within 4 days when a 3-day exposure of 5.0 mM NO3- was applied to a failing system. These results collectively show that NO3- addition can salvage a failing anammox reactor, thereby minimizing economic loss and assure the safety of WWTP to the greatest extent.
