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 1 g 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 ± 0.01 mg N2O-N/h/g VSS, but increased with pH to a maximum of 0.53 ± 0.04 mg N2O-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 5–78 mg NH3-N/L and 0.15–4.6 mg HNO2-N/L, respectively, indicating that the observed pH effect was not due to changes in FA or FNA concentrations.

► The N2O production rate varies with pH within the range of 6.0–8.5.
► The pH effect is not due to changes in the FA and FNA concentrations.
► The N2O production rate is linearly correlated to the ammonium oxidation rate.
► Maintaining the pH between 6.4 and 7.0 mitigated the N2O production.