The role of autoinducer-2 in aerobic granulation using alternating feed loadings strategy

•Alternating organic loading rate (OLR) could promote cell adhesiveness.•Alternating OLR was favorable to large molecular weight EPS.•A positive relationship between AI-2 and granulation process is revealed.•A mechanism of AI-2 controlled aerobic granulation was proposed. Quorum sensing (QS) plays an important role in aerobic granulation while how QS system regulates the formation of aerobic granules needs further discussion. This study cultivated activated sludge in two identical sequencing batch reactors (R1 and R2) at different influent organic loading rate (OLR) strategies: R1 was operated using constant OLR (around 8.0kg/m3d), while R2 was operated at alternating OLR (4.0–17.0kg/m3d). Microbial aggregates appeared in R2 on day 19, while the morphology of sludge in R1 changed little compared with the initial sludge. The concentration of autoinducer-2 (AI-2) in R2 showed an ascending trend, along with the increase of cell adhesiveness. The total extracellular polymeric substances (EPS) amount and large molecular weight EPS of R2 rose steadily, which was different from R1. Some bacteria able to self-aggregate and promote EPS secretion were exclusive in R2. A mechanism about aerobic granulation at alternating OLR was proposed.