Smaller Aerobic Granules Significantly Reduce N₂O Production by Ammonia-Oxidizing Bacteria: Evidences from...

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-O₂/L and NO₂^- = 0 mg-N/L).