The confounding effect of nitrite on N2O production by an enriched ammonia-oxidizing culture

The effect of nitrite (NO2(-)) on the nitrous oxide (N2O) production rate of an enriched ammonia-oxidizing bacteria (AOB) culture was characterized over a concentration range of 0-1000 mg N/L. The AOB culture was enriched in a nitritation system fed with synthetic anaerobic digester liquor. The N2O production rate was highest at NO2(-) concentrations of less than 50 mg N/L. At dissolved oxygen (DO) concentration of 0.55 mg O2/L, further increases in NO2(-) concentration from 50 to 500 mg N/L resulted in a gradual decrease in N2O production rate, which maintained at its lowest level of 0.20 mg N2O-N/h/g VSS in the NO2(-) concentration range of 500-1000 mg N/L. The observed NO2(-)-induced decrease in N2O production was even more apparent at increased DO concentration. At DO concentrations of 1.30 and 2.30 mg O2/L, the lowest N2O production rate (0.25 mg N2O-N/h/g VSS) was attained at a lower NO2(-) concentration of 200-250 mg N/L. These observations suggest that N2O production by the culture is diminished by both high NO2(-) and high DO concentrations. Collectively, the findings show that exceedingly high NO2(-) concentrations in nitritation systems could lead to decreased N2O production. Further studies are required to determine the extent to which the same response to NO2(-) is observed across different AOB cultures.