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 (N 2 O) 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 N 2 O 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 N 2 O production of AOB by eliminating the effects of nitrite concentration and dissolved oxygen, which are conventionally known to affect N 2 O production, by using the “conversion model,” a mathematical model developed in this study. The results showed that exogenous NO supply suppressed N 2 O production under nitrification-accelerating conditions and increased N 2 O 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 N 2 O. Conversely, when NO supply is insufficient, N 2 O production is not activated until NO production exceeds demand. These findings indicate the possibility of suppressing N 2 O production by appropriately NO concentration control in biological reactors in wastewater treatment plants.