The effect of pH on N₂O production under aerobic conditions in a partial nitritation system

Ammonia-oxidising bacteria (AOB) are a major contributor to nitrous oxide (N₂O) emissions during nitrogen transformation. N₂O production was observed under both anoxic and aerobic conditions in a lab-scale partial nitritation system operated as a sequencing batch reactor (SBR). The system achieved 55±5% conversion of the 1g NH₄⁺-N/L contained in a synthetic anaerobic digester liquor to nitrite. The N₂O emission factor was 1.0±0.1% of the ammonium converted. pH was shown to have a major impact on the N₂O production rate of the AOB enriched culture. In the investigated pH range of 6.0-8.5, the specific N₂O production was the lowest between pH 6.0 and 7.0 at a rate of 0.15±0.01mgN₂O-N/h/g VSS, but increased with pH to a maximum of 0.53±0.04mgN₂O-N/h/g VSS at pH 8.0. The same trend was also observed for the specific ammonium oxidation rate (AOR) with the maximum AOR reached at pH 8.0. A linear relationship between the N₂O production rate and AOR was observed suggesting that increased ammonium oxidation activity may have promoted N₂O production. The N₂O production rate was constant across free ammonia (FA) and free nitrous acid (FNA) concentrations of 5-78mg NH₃-N/L and 0.15-4.6mg HNO₂-N/L, respectively, indicating that the observed pH effect was not due to changes in FA or FNA concentrations. © 2011 Elsevier Ltd.