Fouling of nanofiltration (NF) membranes by pre-treated Aldrich humic acid (PAHA), alum coagulant, calcium, and their combinations was systematically investigated in a dead-end filtration system in conjunction with analysis of the specific cake resistance. The severity of flux decline was greatly decreased when alum coagulant was added regardless the presence of calcium. Cake compressibility and fouling mechanism were found to be different in each system: non-compressible for system with only PAHA, compressible for system with PAHA and calcium, and in the presence of alum more than one fouling mechanisms co-existed. Specific cake resistances of these systems were determined using the membrane fouling index (MFI) curve, and the values were compared with the empirical ones. A correlation between foulant attachment and specific cake resistance was observed. Finally, a new attachment coefficient in the fouling index was proposed. With this new coefficient, the foulant attachment is accounted for in the determination of specific cake resistance using the MFI curve. (C) 2009 Elsevier B.V. All rights reserved.