The combined influence of natural organic matter (NOM) and colloidal particles on the fouling of thin-film composite nanofiltration (NF) membranes is systematically investigated. Combined fouling is compared to the individual fouling behaviors (i.e., colloid or NOM alone) with respect to fouling mechanisms and the effect of concentration factor (or recovery). Results demonstrate that (1) "cake-enhanced osmotic pressure" (CEOP) is a key fouling mechanism for individual colloidal fouling, (2) NOM-calcium complexation is the dominant factor governing individual NOM fouling, and (3) combined fouling is affected by both CEOP and NOM-calcium complexation. The extent of flux decline for combined fouling, however, is less than what inferred from additivity of the individual contributions of colloidal and NOM fouling to flux decline. This observation implies that the contributions of the fouling mechanisms appear to be relatively less significant for combined fouling compared to their separate influences on individual colloidal and NOM fouling. An increase in colloidal stability in presence of NOM and the competition between colloids and NOM for calcium are likely explanations for this behavior. It is further shown that NF membrane salt rejection increases noticeably in case of combined fouling compared to individual colloidal fouling due to the formation of an active rejecting layer by the accumulated NOM on the membrane surface. Results from combined fouling runs involving EDTA treatment confirm that both CEOP and NOM-calcium complexation take place simultaneously. (c) 2005 Elsevier B.V. All rights reserved.