Inexact mechanism of aerobic granulation still impedes optimization and application of aerobic granules.In this study,the extended Derjaguin,Landau,Verwey,and Overbeek (XDLVO) theory and physicochemical properties were combined to assess the aggregation ability of sludge during aerobic granulation process qualitatively and quantitatively.Results show that relative hydrophobicity of sludge and polysaccharide content of extracellular polymeric substances (EPS) increased,while electronegativity of sludge decreased during acclimation phase.After 20 days’ acclimation,small granules began to form due to high aggregation ability of sludge.Since then,coexisted flocs and granules possessed distinct physicochemical properties during granulation and maturation phase.The relative hydrophobicity decreased while electronegativity increased for flocs,whereas that for granules presented reverse trend.Through analyzing the interaction energy using the XDLVO theory,small granules tended to self-grow rather than self-aggregate or attach of flocs due to poor aggregation ability between flocs and granules during the granulation phase.Besides,remaining flocs were unlikely to self-aggregate owing to poor aggregation ability,low hydrophobicity and high electronegativity.