Microbial and isotopomer analysis of N2O generation pathways in ammonia removal biofilters

Ammonia removal biofilters can be a potential source of nitrous oxide (N2O) production as a result of microbial nitrification and denitrification. In this study, these two N2O generation pathways was quantified using isotopic site preference values (SP, 33‰ for nitrification and 0‰ for denitrification) in a 204-d operation. Tests with two moisture conditions (45% and 55%) and three inlet NH3 concentrations (35, 18 and 0 ppmv) were performed. A 55+% NH3 removal efficiency was achieved in biofilters with 35 and 18 ppmv ammonia supply, but no significant difference (p > 0.05) was found between the moisture treatments. Results showed that biofilters were clearly net sources of N2O, and biofilters with higher moisture content generated significantly (p < 0.05) higher N2O concentration. The N2O generation did not stop even after the biofilters were terminated. The percentage of inlet NH3–N converted into N2O–N were 5.2%, 8.5% for biofilters with 45% moisture content, and 14.8%, 10.8% for those with 55% moisture content. Gene abundance of amoA and nosZ in packing materials (taken on days 64, 107, 140, 180 and 204) increased due to NH3 input reaching the highest on day 140 and then decreased in response to reduced NH3 supply on day 180 and 204. The changes of SP values suggested a shift between nitrification and denitrification with regard to N2O generation. Overall, the nitrification was the dominant pathway for N2O generation, but uncertainty exits as well. This study confirmed that NH3-loaded biofilters were net sources of N2O, and use of SP-N2O may be helpful in better understanding the processes responsible for such emissions.