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

At a full-scale single-stage nitritation-anammox reactor, off-gas measurement for nitric oxide (NO) and nitrous oxide (N(2)O) was performed. NO and N(2)O 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 N(2)O 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 N(2)O 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 N(2)O emission from bioreactors for reliable emission estimates. Nitrite accumulation clearly resulted in increased NO and N(2)O concentrations in the off-gas, yielding higher emission levels. Oxygen limitation resulted in a decrease in NO and N(2)O 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 N(2)O emission, which stresses the importance of efficient aeration control to limit NO and N(2)O emissions.