The effect of pH on N_2O production under aerobic conditions in a partial nitritation system

摘要

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 1g 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 NH3-N/L arid 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.