NO production and emission pathways in anammox biofilter for treating wastewater with low nitrogen concentrations.

Although anaerobic ammonia oxidation (anammox) is a cost-effective nitrogen removal process, nitrous oxide (NO) production will greatly reduce the advantages of this process. It is important to identify the NO emission pathways and then reduce the NO production in anammox system. To date, very limited research has been done to investigate the NO production and NO emission pathways in anammox biofilter. In this study, NO production were investigated under different filtration rates in anammox biofilter for treating wastewater with low nitrogen concentrations, and NO emission pathways were analyzed with batch tests using NO microsensor and stable isotope mass spectrometry. The results showed NO production increased with the increase of filtration rates in anammox biofilter, where the NO emission factor increased from 0.012 % at 1.0 m/h to 0.496 % at 3.0 m/h. And the optimal operation condition was at filtration rate of 1.5 m/h, where NH-N and NO-N removal efficiencies reached 99 % and NO concentration was the lowest. qPCR showed that anammox bacteria, nitrifying bacteria and denitrifying bacteria were all present in anammox biofilter, with anammox bacteria in the highest abundance. And nitrifying bacteria and denitrifying bacteria provided the possibility of NO production. The batch tests and stable isotope mass spectrometry analysis indicated that nitrifier denitrification, hydroxylamine oxidation and endogenous heterotrophic denitrification were NO production pathways in aerobic zone and anoxic zone of anammox biofilter, respectively. In addition, batch tests under different conditions showed no oxygen environment could reduce NO production. Therefore, the production of NO in anammox system is a problem that cannot be ignored and should be paid more attention to.