Nitrous oxide (N2O) emission from biological nitrogen removal (BNR) processes has recently received more research attention. In this study, two lab-scale BNR systems were used to investigate the effects of various operating parameters including the carbon to nitrogen (C/N) ratio, ammonia loading, and the hydraulic retention time on N2O production. The first system was operated in a conventional BNR mode known as the Ludzack–Ettinger (LE) process, consisting of complete denitrification and nitrification reactors, while the second one was operated in a shortcut BNR (SBNR) mode employing partial nitrification and shortcut denitrification, which requires less oxygen and carbon sources. As the C/N ratio was decreased, a significant increase in N2O production was observed only in the anoxic reactor of the LE process, indicating that N2O was released as an intermediate of the denitrification reaction under the carbon-limited condition. However, the SBNR process did not produce significant N2O even at the lowest C/N ratio of 0.5. When the SBNR process was subjected to increasing concentrations of ammonia, N2O production from the aerobic reactor was rapidly increased. Furthermore, the increasing production of N2O was observed mostly in the aerobic reactor of the SBNR process with a decline in hydraulic retention time. These experimental findings indicated that the increase in N2O production was closely related to the accumulation of free ammonia, which was caused by an abrupt increase of the ammonium loading. Consequently, the partial nitrification was more susceptible to shock loading conditions, resulting in a high production of N2O, although the SBNR process was more efficient with respect to nitrogen removals as well as carbon and oxygen requirements.
Keywords: denitrification - nitrification - nitrite accumulation - nitrous oxide - shortcut biological nitrogen removal (SBNR)