The fouling phenomenon and associated fouling mechanisms of the cross-flow type nanofiltration membrane were investigated using the modified Hermia empirical model and the resistance-in-series model. The effects of pH on membrane permeate flux and the rejection of natural organic matter (NOM) were also studied. Natural water and the effluent of a rapid sand filter (SF) were used as the source waters. The results showed that when the raw water was treated, the highest flux decline occurred at pH 5 due to weak electrostatic repulsion between the charged membrane and NOM. Moreover, a two-step fouling process in flux decline before 7 h and after 7 h was observed during the treatment of the raw water. Ultimately, physically reversible rather than irreversible fouling was the main fouling mechanism. Standard blocking was not a fouling mechanism for either of the water samples treated at any pH level, indicating that the foulants were not adsorbed onto the inner walls of membrane pores. For raw water, intermediate blocking was the major fouling mechanism at all pH levels, whereas for the SF effluent, gel layer formation played the major role in membrane fouling at all pH levels. (C) 2011 Elsevier B.V. All rights reserved.