Smaller Aerobic Granules Significantly Reduce N2O Production by Ammonia-Oxidizing Bacteria: Evidences from Biochemical and Isotopic Analyses

The mitigation of nitrous oxide (N2O) is of primary significance to offset carbon footprints in aerobic granular sludge (AGS) systems. However, a significant knowledge gap still exists regarding the N2O production mechanism and its pathway contribution. To address this issue, the impact of varying granule sizes, dissolved oxygen (DO), and nitrite (NO2-) levels on N2O production by ammonia-oxidizing bacteria (AOB) during nitrification in AGS systems was comprehensively investigated. Biochemical and isotopic experiments revealed that increasing DO or decreasing NO2- levels reduced N2O emission factors (by 13.8 or 19.5%) and production rates (by 0.08 or 0.35 mg/g VSS/h) via weakening the role of the AOB denitrification pathway since increasing DO competed for more electrons required for AOB denitrification. Smaller granules (0.5 mm) preferred to diminish N2O production via enhancing the role of NH2OH pathway (i.e., 59.4-100% in the absence of NO2-), while larger granules (2.0 mm) induced conspicuously higher N2O production via the AOB denitrification pathway (approximately 100% at higher NO2- levels). Nitrifying AGS systems with a unified size of 0.5 mm achieved 42% N2O footprint reduction compared with the system with mixed sizes (0.5-2.0 mm) under optimal conditions (DO = 3.0 mg-O-2/L and NO2- = 0 mg-N/L).