Characterization and mitigation of nitrous oxide (N2 O) emissions from partial and full-nitrification BNR processes...

It has been reported that a directional change from anoxic to aerobic conditions is a common trigger for nitrous oxide (N2 O) production by ammonia oxidizing bacteria (AOB). By extension, during anoxic-aerobic cycling, post-anoxic dissolved oxygen (DO) concentrations might likely play a role in the magnitude of N2 O emissions observed. The overall goal of this study was to determine the impact of three select post-anoxic DO concentrations (0.8, 2.0, and 3.0 mg O2 /L) on N2 O emissions from partial-nitrification (PN) and full-nitrification (FN) reactors subjected to anoxic-aerobic cycling and, ultimately, to explore the development of strategies to minimize N2 O emissions from PN and FN based biological nitrogen removal (BNR) processes. Statistically similar N2 O emissions were observed during anoxia for both PN (0.62 ± 0.21% N load) and FN (0.61 ± 0.070% N load) processes. In contrast, N2 O emissions were statistically lower for PN (0.86 ± 0.25% N load) than for FN (4.6 ± 2.8% N load), during the post-anoxic aerobic phase, when compared together for all three post-anoxic DO concentrations. Further, for PN, the highest N2 O emissions were observed at the highest post-anoxic DO concentration of 3.0 mg O2 /L (1.2% N load), likely due to the highest corresponding AOB specific growth rate. In contrast, for FN, the highest N2 O emissions were at the lowest post-anoxic DO concentration of 0.8 mg O2 /L (8.5% N load). The higher emissions from FN process at low DO concentrations were associated with a lag in nitrite oxidizing bacteria activity upon recovery to aerobic conditions. This lag phase contributed to transient nitrite accumulation, and in turn correlated positively to the observed N2 O emissions. Based on our findings, a gradual ramp up in post-anoxic DO concentrations can minimize N2 O emissions during PN-based BNR, whereas a completely different strategy, entailing a rapid increase in post-anoxic DO concentrations can minimize emissions during FN-based BNR operations.