Although anaerobic ammonia oxidation (anammox) is a cost-effective nitrogen removal process, nitrous oxide (N2O) production will greatly reduce the advantages of this process. It is important to identify the N2O emission pathways and then reduce the N2O production in anammox system. To date, very limited research has been done to investigate the N2O production and N2O emission pathways in anammox biofilter. In this study, N2O production were investigated under different filtration rates in anammox biofilter for treating wastewater with low nitrogen concentrations, and N2O emission pathways were analyzed with batch tests using N2O microsensor and stable isotope mass spectrometry. The results showed N2O production increased with the increase of filtration rates in anammox biofilter, where the N2O 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 NH4+-N and NO2--N removal efficiencies reached 99 % and N2O 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 N2O production. The batch tests and stable isotope mass spectrometry analysis indicated that nitrifier denitrification, hydroxylamine oxidation and endogenous heterotrophic denitrification were N2O 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 N2O production. Therefore, the production of N2O in anammox system is a problem that cannot be ignored and should be paid more attention to.
Keywords: Anammox biofilter; Filtration rates; Microbial communities; N(2)O emission pathways; N(2)O production.
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