Quorum sensing unveils the sludge floccule-assisted stabilization of aerobic granules in granule-dominated sequencing batch reactors.

Floccules are another major form of microbial aggregates in aerobic granular sludge systems. Previous studies mainly attributed the persistence of floccules to their relatively faster nutrient uptake and higher growth rate over aerobic granules; however, they failed to unravel the underlying mechanism of the long-term coexistence of these two aggregates. In this work, the existence and function of the floccules in an aerobic granule-dominated sequencing batch reactor were investigated from the view of quorum sensing (QS) and quorum quenching (QQ). The results showed that though the floccules were closely associated with the granules in terms of similar community structures (including the QS- and QQ-related ones), they exhibited a relatively higher QQ-related activity but a lower QS-related activity. A compatible proportion of floccules might be helpful to maintain the QS-related activity and keep the granules stable. In addition, the structure difference was demonstrated to diversify the QS- and QQ-related activities of the floccules and the aerobic granules. These findings could broaden our understanding of the interactions between the coexistent floccules and granules in aerobic granule-dominated systems and would be instructive for the development of the aerobic granular sludge process.