An anaerobic membrane bioreactor coupled with online ultrasonic equipment (US-AnMBR) was operated for sludge digestion under a high volatile solids (VS) loading rate of 3.7 g VS L-1 d(-1) for 54 d. The ultrasound intentionally used for membrane fouling control enhanced membrane filtration. Mechanisms of membrane fouling controlled by the ultrasound were investigated in this study. The results of digested broth properties, membrane filtration resistance distributions and the scanning electron microscope observation comprehensively indicated that the ultrasound primarily controlled the cake layer fouling, but did not modify the digested broth properties to enhance membrane filtration. The contents of extracellular polymeric substances (EPS) in the residual cake layer on the membrane surface signified that the loose cake layer was removed by the ultrasound, whereas the compacted cake layer strongly adhering to the membrane surface with higher EPS concentrations remained. Furthermore, the ultrasound slightly affected the gel layer, as demonstrated by a reduction in gel layer fouling resistance. Based on the analysis of gel layer compositions, the ultrasound was proven to be able to control the adsorption of the soluble organic matters rather than the deposition of the inorganic particles into the gel layer. (c) 2013 Elsevier B.V. All rights reserved.