Nitrite build-up effect on nitrous oxide emissions in a laboratory-scale anaerobic/aerobic/anoxic/aerobic sequencing batch reactor

Abstract

Biological wastewater treatment processes with biological nitrogen removal are potential sources of nitrous oxide (N₂O) emissions. It is important to expand knowledge on the controlling factors associated with N₂O production, in order to propose emission mitigation strategies. This study therefore sought to identify the parameters that favor nitrite (NO₂^-) accumulation and its influence on N₂O production and emission in an anaerobic/aerobic/anoxic/aerobic sequencing batch reactor with biological nitrogen removal. Even with controlled dissolved oxygen concentrations and oxidation reduction potential, the first aerobic phase promoted only partial nitrification, resulting in NO₂^- build-up (ranging from 29 to 57%) and consequent N₂O generation. The NO₂^- was not fully consumed in the subsequent anoxic phase, leading to even greater N₂O production through partial denitrification. A direct relationship was observed between NO₂^- accumulation in these phases and N₂O production. In the first aerobic phase, the N₂O/NO₂^- ratio varied between 0.5 to 8.5%, while in the anoxic one values ranged between 8.3 and 22.7%. Higher N₂O production was therefore noted during the anoxic phase compared to the first aerobic phase. As a result, the highest N₂O fluxes occurred in the second aerobic phase, ranging from 706 to 2416 mg N m^-2 h^-1, as soon as aeration was triggered. Complete nitrification and denitrification promotion in this system was proven to be the key factor to avoid NO₂^- build-up and, consequently, N₂O emissions.