Volumetric oxygen transfer coefficient (K L a) is an important parameter affecting oxygen transfer and nitrogen conversion during biological wastewater management. However, how the generation of nitrous oxide (N 2 O) responds to the variation of K L a levels is unclear. In this study, N 2 O generation via the main pathway, which was confirmed to be nitritation, at different K L a levels was investigated in a partial nitritation-anammox (PN-A) reactor run as a sequencing batch reactor. The results showed that N 2 O generation in the PN-A process, which was described using the maximum N 2 O generation rate (MGR), the duration of N 2 O generation (DG), and the N 2 O generation factor, was affected by K L a levels and the limiting factors of the nitritation process, i.e., oxygen transfer and nitritation activity. DG always decreased with the increase of K L a , indicating that the process of N 2 O generation lasted for a shorter time at higher K L a. With the increase of K L a , 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 L a levels, the N 2 O generation factor remarkably increased at lower K L a levels when oxygen transfer was the limiting factor, but significantly reduced at higher K L a levels when nitritation activity became the limiting factor. These results revealed how N 2 O generation via nitritation responded to different K L a levels and the limiting factors of the nitritation process. Based on these results, practical suggestions to mitigate the N 2 O generation in PN-A process operated in a batch mode were recommended. [Display omitted] • N 2 O generation via nitritation at different K L a levels was revealed. • The limiting factors of the nitritation process changed with the increase of K L a. • N 2 O generation via nitritation was affected by both K L a levels and the limiting factors. • Practical suggestions to mitigate the N 2 O generation were recommended. [ABSTRACT FROM AUTHOR]