Production of NO and NO during shortcut nitrification: characteristics and mechanisms.

Nitric oxide (NO) and nitrous oxide (NO) are significant greenhouse gases, yet their production mechanisms remain inadequately understood. To elucidate the production mechanisms of NO and NO during biological nitrogen removal, the variations in dissolved NO and NO concentrations during autotrophic shortcut nitrification were analysed in a sequencing batch reactor. The findings indicated that increasing the influent ammonium concentration from 120 mg-N/L to 240 mg-N/L led to a transition of complete nitrification to shortcut nitrification, achieving >80% nitrite accumulation. This shift was primarily attributed to free ammonia-mediated inhibition of nitrite-oxidising bacteria. The changes in NO and NO concentrations indicated that the peak levels of NO in the liquid during the aerobic phase of autotrophic shortcut nitrification were consistently lower than those of NO. Transient NO accumulation occurred primarily during the initial aeration phase, due to incomplete hydroxylamine oxidation. In contrast, NO production was predominantly influenced by nitrite concentration, and its principal source was observed to be the nitrifier denitrification pathway. These findings advance our understanding of microbial nitrogen cycling mechanisms and offer practical guidance for minimising greenhouse gas emissions in engineered systems.