The effect of pH on N 2O production under aerobic conditions in a partial nitritation system

Abstract

Ammonia-oxidising bacteria (AOB) are a major contributor to nitrous oxide (N 2O) emissions during nitrogen transformation. N 2O 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 1 g NH 4 +-N/L contained in a synthetic anaerobic digester liquor to nitrite. The N 2O emission factor was 1.0 ± 0.1% of the ammonium converted. pH was shown to have a major impact on the N 2O production rate of the AOB enriched culture. In the investigated pH range of 6.0–8.5, the specific N 2O production was the lowest between pH 6.0 and 7.0 at a rate of 0.15 ± 0.01 mg N 2O-N/h/g VSS, but increased with pH to a maximum of 0.53 ± 0.04 mg N 2O-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 N 2O production rate and AOR was observed suggesting that increased ammonium oxidation activity may have promoted N 2O production. The N 2O production rate was constant across free ammonia (FA) and free nitrous acid (FNA) concentrations of 5–78 mg NH 3-N/L and 0.15–4.6 mg HNO 2-N/L, respectively, indicating that the observed pH effect was not due to changes in FA or FNA concentrations.