Experimental Verification of Nitric Oxide as the Novel Indicator for Nitrous Oxide Emission Control in Biological Wastewater Treatment

Nitrous oxide (N2O), known to be the highly greenhouse effect gas, is emitted from the biological wastewater treatment process. However, microbial control methods to reduce N2O emission have not been established. Therefore, in this study, we focused on nitric oxide (NO) which is a precursor of N2O produced by ammonia-oxidizing bacteria (AOB), to clarify the mechanisms of N2O production by AOB and to experimentally verify the effectiveness of NO as a novel engineering indicator for reducing N2O production. We verified the emission characteristics and interrelationships of NO and N2O during the ammonia oxidation process using nitrifying bacteria cultivated in the laboratory-scale batch reactor. The results indicated that N2O production by AOB can be predicted by observing NO emissions. It was also confirmed that AOB regulates the supply and demand of NO by themselves depending on the amount of ammonia oxidation, and produce N2O as the way to decompose excess NO. Furthermore, we tested the applicability of our new findings to the full-scale wastewater treatment plant and experimentally demonstrated the possibility of reducing N2O production by controlling NO concentration or chemical NO removal, even in a highly diverse microbial community that includes denitrifying bacteria.