Evaluation of Direct Effect of Nitric Oxide on Nitrous Oxide Production by Ammonia-oxidizing Organisms

Abstract

We have tried to investigate the applicability of nitric oxide (NO) , which has been drawing attention as an information transmitter substance for most organisms, as a new operational factor for nitrous oxide (N₂O) production by ammonia-oxidizing bacteria (AOB) . Nitrifying sludge was cultivated in a lab-scale sequencing batch reactor fed with an inorganic substrate. NO was blown into the reactor in the aeration or mixing phase during cycle operations. The N₂O production and nitrifying performance in the aeration phase were monitored. The obtained results showed that NO enhanced both nitrification performance and N₂O production in most cases. However, the direct effect of NO on N₂O production was not clear, because both dissolved oxygen (DO) and nitrite concentrations, which are well-known factors strongly affecting N₂O production, also varied. Accordingly, the mathematical model, named the Conversion Model, was developed to eliminate the effects of DO and nitrite. The model analysis revealed that the NO supply in the aeration phase has the direct decreasing effect on N₂O production by AOB, whereas the NO supply in the mixing phase has the direct increasing effect on that. These findings suggest that proper control of the NO concentration could lead to the reduction of N₂O production by AOB.