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).