Smaller Aerobic Granules Significantly Reduce NO Production by Ammonia-Oxidizing Bacteria: Evidences from Biochemical and Isotopic Analyses.

The mitigation of nitrous oxide (NO) is of primary significance to offset carbon footprints in aerobic granular sludge (AGS) systems. However, a significant knowledge gap still exists regarding the NO production mechanism and its pathway contribution. To address this issue, the impact of varying granule sizes, dissolved oxygen (DO), and nitrite (NO) levels on NO production by ammonia-oxidizing bacteria (AOB) during nitrification in AGS systems was comprehensively investigated. Biochemical and isotopic experiments revealed that increasing DO or decreasing NO levels reduced NO 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 NO production via enhancing the role of NHOH pathway (i.e., 59.4-100% in the absence of NO), while larger granules (2.0 mm) induced conspicuously higher NO production via the AOB denitrification pathway (approximately 100% at higher NO levels). Nitrifying AGS systems with a unified size of 0.5 mm achieved 42% NO footprint reduction compared with the system with mixed sizes (0.5-2.0 mm) under optimal conditions (DO = 3.0 mg-O/L and NO = 0 mg-N/L).