Membrane fouling is a major obstacle to the wide application of microfiltration membrane bioreactor (MF MBR). Protein-like substances have been known to contribute significantly to membrane fouling in MF MBRs. In this study, the effects of ionic strength and divalent cations on MF membrane fouling by protein were investigated. Fouling experiments showed that MF membrane fouling by protein was enhanced with increasing ionic strength and calcium ion addition. Measured interaction energies among protein molecules and between protein molecules and clean MF membrane confirmed the fouling trends of MF membrane. It was consistently shown that solution chemistries that induced higher fouling rates of MF membrane were associated with greater attractive and lower repulsive interaction energies among protein molecules and between protein molecules and clean MF membrane. Analysis of fouled MF membrane surface roughness further indicated that MF membrane surface easier. (C) 2013 Elsevier B.V. All rights reserved.