Nitrous Oxide Formation and Consumption by Denitrifying Bacteria in Wastewater Treatment

This research examined the formation and consumption of nitrous oxide (N2O) by denitrifying bacteria during wastewater treatment processes. The reduction kinetics of N2O were determined for both a mixed culture of denitrifying bacteria and a pure culture of Paracoccus pantotrophus. The formation of N2O was studied in a denitrifying biofilm. Additionally, a mathematical model was developed to describe the formation and consumption of N2O in suspended-growth and biofilm systems. The batch kinetic studies showed that some denitrifying bacteria grow well on N2O as a sole electron acceptor. For the mixed culture, the maximum rates of substrate utilization were significantly higher than the rates for nitrate (NO3-) or nitrite (NO2-) reduction. Also, N2O was found to select for a less diverse community than NO3-or NO2-. Enzyme expression studies on P. pantotrophus revealed that the presence of N2O induced the N2O reductase (NosZ), but it did not induce the NO3- reductase (NarG) or NO2- reductase (NirS)The denitrifying biofilm study revealed that N2O is produced for range of bulk liquid dissolved oxygen concentrations. However, the highest N2O accumulation occurred at the base of the biofilm, and the N2O flux from the biofilm was low. The modeling work predicted N2O formation due autotrophic nitrifying bacteria and heterotrophic denitrification bacteria under varying oxygen concentrations. Processes were examined separately and then combined together. When all the processes were combined in the biofilm model, a complex trend emerged revealing the interplay between oxygen concentration and microbial community composition.