Two pilot systems were set-up: a two-stage system consisting of an up-flow anaerobic sludge blanket (UASB) reactor followed in series by a membrane bioreactor (MBRUASB), and a single aerobic MBR system (MBRaer). Apart from a comparable chemical oxygen demand (COD), the influents of both MBRs were different as the MBRUASB was fed by UASB treated molasses wastewater while the MBRaer was directly fed by a more diluted synthetic molasses wastewater. This influenced microbial growth, soluble microbial products (SMP) and bound extracellular polymeric substances (EPS) in the MBRs, different MBR performances (membrane filtration and fouling behavior), and membrane cleaning results were found between the two MBRs. Scanning electron microscopy (SEM) analysis, energy-dispersive X-ray (EDX) analysis and the different cleaning results indicated that the membrane fouling layer in the MBRUASB contained more inorganic components than that in the MBRaer. This, together with the high level of SMP and the smaller particle size in the sludge resulted in a much higher fouling rate in the MBRUASB than in the MBRaer under sub-critical flux operation. (C) 2011 Elsevier B.V. All rights reserved.