Chemical coagulation has been widely used as a method to mitigate membrane fouling in MF/UF membranes used for drinking water treatment. Optimization of coagulation as pre-treatment of membrane processes has not been achieved yet: the optimum condition of coagulation for conventional treatment systems is not necessarily applicable to membrane-based treatment systems. This study investigated (physically) irreversible membrane fouling in an MF membrane used with pre-coagulation by aluminum salt. In a series of bench-scale filtration tests, feed water containing commercially available humic acid or organic matter isolated from surface water was coagulated with polyaluminum chloride (PACl) under various conditions and subsequently filtered with an MF membrane with the nominal pore size of 0.1 mu m. It was found that coagulation conditions had great impacts on the degree of physically irreversible fouling. Acidic conditions improved the quality of treated water but generally caused greater physically irreversible fouling than did neutral or alkaline conditions. Also, dosage of coagulant was found to be influential on the degree of membrane fouling: high dosage of coagulant frequently caused more severe irreversible fouling. Sizes of flocs seemed to become small under acidic conditions in this study, which was indicated by high concentrations of aluminum in the permeate under acidic conditions. It is thought that small flocs produced under acidic conditions could migrate into micropores of the membrane and caused physically irreversible fouling by plugging or adsorption. These findings obtained in the bench-scale tests were verified in a long-term pilot-scale test. (C) 2008 Elsevier B.V. All rights reserved.