Emission of nitrous oxide and nitric oxide from a full-scale single-stage nitritation-anammox reactor

Abstract

At a full-scale single-stage nitritation-anammox reactor, off-gas measurement for nitric oxide (NO) and nitrous oxide (N2O) was performed. NO and N2O are environmental hazards, imposing the risk of improving water quality at the cost of deteriorating air quality. The emission of NO during normal operation of a single-stage nitritation-anammox process was 0.005% of the nitrogen load while the N2O emission was 1.2% of the nitrogen load to the reactor, which is in the same range as reported emission from other full-scale wastewater treatment plants. The emission of both compounds was strongly coupled. The concentration of NO and N2O in the off-gas of the single-stage nitritation-anammox reactor was rather dynamic and clearly responded to operational variations. This exemplifies the need for time-dependent measurement of NO and N2O emission from bioreactors for reliable emission estimates. Nitrite accumulation clearly resulted in increased NO and N2O concentrations in the off-gas, yielding higher emission levels. Oxygen limitation resulted in a decrease in NO and N2O emission, which was unexpected as oxygen limitation is generally assumed to cause increased emissions in nitrogen converting systems. Higher aeration flow dramatically increased the NO emission load and also seemed to increase the N2O emission, which stresses the importance of efficient aeration control to limit NO and N2O emissions.