In situ reactivation of aerobic granular sludge after extended idle conditions: Effect of different N-acyl-homoserine lactones (AHLs)

Prolonged idle conditions pose a major challenge to aerobic granular sludge (AGS) systems by compromising granule integrity and pollutant removal performance. This study investigates the in situ reactivation of AGS after three months of static storage using two quorum sensing molecules, N-hexanoyl-L-homoserine lactone (C6-HSL) and N-octanoyl-homoserine lactone (C8-HSL). All reactors rapidly restored COD and NH 4 + -N removal efficiencies to >92% and >98%, respectively. C6-HSL significantly accelerated phosphorus recovery, reaching removal efficiencies above 90% by day 26, compared to day 34 in the control and C8-HSL groups. C8-HSL enhanced EPS secretion and granule growth, yielding the largest granule size (1210 μm), which was 1.26-fold and 1.71-fold larger than those in the control and C6-HSL groups, respectively. Metagenomic analysis revealed comparable microbial structures at the phylum level, but distinct functional responses. C6-HSL increased the abundances of phosphorus metabolism genes ( ppk, ppx, ppa ), while C8-HSL notably upregulated genes related to the biosynthesis of tyrosine, tryptophan, and structural polysaccharides (e.g., alginate and Psl), supporting enhanced EPS production and granule stability. These results demonstrate molecule-specific regulatory roles of individual N-acyl-homoserine lactones during AGS reactivation, linking functional recovery and structural regeneration to distinct quorum sensing pathways. This study provides mechanistic and engineering insights into an energy-efficient strategy for restoring AGS performance after prolonged ambient idle conditions, with direct relevance to the stable operation and management of full-scale wastewater treatment systems.