Electroreduction to Mitigate Nitrous Oxide Production by Ammonia-Oxidizing Bacteria: Mathematical Modeling and Experimental Verification

This study verified the potential use of nitric oxide (NO) as a useful engineering indicator for controlling and predicting nitrous oxide (N₂O) production in wastewater treatment processes. Nitrifying bacteria were cultivated on inorganic substrates in a laboratory-scale batch reactor, and we examined the effects of artificially NO supply control on N₂O production by ammonia-oxidizing bacteria (AOB) and bacterial communities during the nitrification process. Furthermore, we attempted to extract only the direct effects of NO on N₂O production of AOB by eliminating the effects of nitrite concentration and dissolved oxygen, which are conventionally known to affect N₂O production, by using the “conversion model,” a mathematical model developed in this study. The results showed that exogenous NO supply suppressed N₂O production under nitrification-accelerating conditions and increased N₂O production under nitrification-suppressing conditions. Furthermore, experiments using syringes as microbial reactors indicated that AOB may regulate NO production according to their NO demand, which is required as electron acceptors in ammonia oxidation. Hence, when NO supply is excessive, NO production is suppressed, and the excess NO is converted to N₂O. Conversely, when NO supply is insufficient, N₂O production is not activated until NO production exceeds demand. These findings indicate the possibility of suppressing N₂O production by appropriately NO concentration control in biological reactors in wastewater treatment plants.