Nitrous oxide emission from nitrifying activated sludge dependent on denitrification by ammonia-oxidizing bacteria

Abstract

Nitrous oxide (N 2O) is emitted during the aerated nitrification process of wastewater treatment, but its mechanism is not understood. In this study, we employed a model system to clarify the mechanism of N 2O emission, utilizing the activated sludge derived from a piggery effluent. Aerated incubation of the sludge with ammonium ( NH 4 + ) or hydroxylamine (NH 2OH) resulted in the emission of a significant amount of N 2O. The emission stopped when the nitrification substrate ( NH 4 + or NH 2OH) was exhausted. When NH 4 + was replaced with nitrate ( NO 3 - ) and nitrite ( NO 2 - ), no N 2O was emitted. This result suggests that the N 2O emission under nitrifying conditions did not depend on the oxidation of NO 2 - by nitrite-oxidizing bacteria (NOB) or denitrification by heterotrophic denitrifiers but depended on the oxidation of NH 4 + by ammonia-oxidizing bacteria (AOB). When NO 2 - , the product of nitrification by AOB, was added to the NH 4 + -oxidizing system, N 2O emission was enormously enhanced, suggesting that N 2O was formed via denitrification. Diethyldithiocarbamate (DCD), an inhibitor of copper-containing nitrite reductase (NirK), strongly blocked N 2O emission from NH 2OH. Furthermore, the expression of the gene ( nirK) encoding NirK of AOB was detected in the sludge exposed to the nitrifying conditions. The results showed that N 2O emission during the nitrification process depends on denitrification by AOB that reside in the activated sludge. This study provides direct evidence for the cause of N 2O emission from activated sludge (non-pure culture).