Varied mixed carbon source ratios affect nitrous oxide emissions in aerobic granular sludge systems

The aerobic granular sludge (AGS) system is highly-sensitive to the mixed carbon sources ratios. Despite the combination of sodium acetate/glucose is widely applied in AGS systems, a knowledge gap in the nitrous oxide (N₂O) emission mechanisms and production pathways was yet filled. In this study, the impacts of sodium acetate/glucose (S/G) ratios of 3, 1 and 1/3 on N₂O emissions, production pathways and sink capability were systematically unveiled in AGS systems by biochemical batch tests and microbial community analysis. The results revealed that during the sole denitrification, higher N₂O reduction rates (i.e., 16.7 and 12.3 mg/g VSS/h in sole and multiple nitrogen oxide schemes, respectively) and greater N₂O-sink capability (i.e., 3.77) were invariably obtained with higher sodium acetate ratios (i.e., S/G = 3), probably due to the enrichment of denitrifiers harboring N₂O reduction genes and key enzymes. Nevertheless, higher sodium acetate ratios relatively induced a higher N₂O emissions (i.e., 4.02 %) during the sole nitrification, driven by the higher contribution of nitrifier denitrification (34.1 %) and increasing Nitrosomonas (i.e., ammonia-oxidizing bacteria) abundance. Conversely, the elevated glucose ratio (i.e., S/G = 1/3) enhanced nitrite-oxidizing bacteria (NOB) abundance, thereby suppressing nitrifier denitrification pathway-derived N₂O emissions. The counterbalancing effects of elevated sodium acetate ratios on mitigating N₂O production during nitrification and higher glucose proportions promoting N₂O reduction in denitrification collectively resulted in slight difference in N₂O emissions factors (0.43-0.5 %, p > 0.05) throughout cyclic operations. The gained insights can be as a reference by wastewater treatment plants (WWTPs) to reduce the N₂O footprint in AGS systems.