N₂O generation via nitritation at different volumetric oxygen transfer levels in partial nitritation-anammox process

Volumetric oxygen transfer coefficient (K_La) is an important parameter affecting oxygen transfer and nitrogen conversion during biological wastewater management. However, how the generation of nitrous oxide (N₂O) responds to the variation of K_La levels is unclear. In this study, N₂O generation via the main pathway, which was confirmed to be nitritation, at different K_La levels was investigated in a partial nitritation-anammox (PN-A) reactor run as a sequencing batch reactor. The results showed that N₂O generation in the PN-A process, which was described using the maximum N₂O generation rate (MGR), the duration of N₂O generation (DG), and the N₂O generation factor, was affected by K_La levels and the limiting factors of the nitritation process, i.e., oxygen transfer and nitritation activity. DG always decreased with the increase of K_La, indicating that the process of N₂O generation lasted for a shorter time at higher K_La. With the increase of K_La, MGRs remarkably increased when oxygen transfer was the limiting factor, while slightly reduced when nitritation activity became the limiting factor. As a result, with the increase of K_La levels, the N₂O generation factor remarkably increased at lower K_La levels when oxygen transfer was the limiting factor, but significantly reduced at higher K_La levels when nitritation activity became the limiting factor. These results revealed how N₂O generation via nitritation responded to different K_La levels and the limiting factors of the nitritation process. Based on these results, practical suggestions to mitigate the N₂O generation in PN-A process operated in a batch mode were recommended.