Aeration-driven microbial aggregation in aerobic granular sludge systems for low-strength wastewater treatment

This study explores the effect of low aeration rates on aerobic granulation in sequential batch reactors (SBRs) treating real low-strength wastewater (1.00 ± 0.24 kgCOD soluble .m -3 .day -1 ). Three pilot-scale reactors, R1, R2 and R3, each with a 200 L capacity, were operated at aeration rates of 0.15, 0.30, and 0.60 cm.s -1 , respectively. The results show that the two higher aeration rates promoted the formation of stable granular sludge with enhanced settling properties, with granule sizes reaching up to 1.4 ± 0.3 mm and 1.9 ± 0.7 mm and settling velocities of up to 17 ± 1 and 18 ± 0 m.h -1 in R2 and R3, respectively. The lowest aeration rate resulted in biofloc formation, whereas the intermediate rate supported the highest abundance of DPAO achieving removal efficiencies of 77 ± 7% for soluble COD, 85 ± 21% for ammonium and 55 ± 21% for phosphate. The highest rate achieved removal efficiencies of 76 ± 8% for soluble COD, 75 ± 22% for ammonium and 29 ± 13% for phosphate. The relative abundance of Dechloromonas increased significantly in R2 at 239 days, comprising 12.6% of the microbial community. Additionally, Thiothrix persisted in bioflocs at 0.15 cm.s -1 throughout the operation. The study also validated a correlation between the F/M ratio and EPS levels only when AGS was formed, highlighting the complex interaction between operational parameters and biomass stability. Initial microbial aggregation varied according to aeration rates, influencing the establishment of the AGS core structure, which prevented biomass washout during startup. • Dechloromonas genus thrived in AGS at 0.30 cm.s -1 aeration rate. • Thiothrix persisted in bioflocs at 0.15 cm.s -1 throughout the operation. • The F/M ratio and EPS correlation was validated only when AGS was formed. • Biofloc formation prevented biomass washout at startup.