The fouling behavior of different NOM fractions in combination with two micro-particles (0.5-10 mu m and -45 mu m) and one nano-particle (5-15 nm) on UF membrane was investigated. Three NOM models, Aldrich humic acid (HA), bovine serum albumin (BSA) and dextran (DEX) were employed as representatives of humic substances, proteins and polysaccharides in natural waters. For all the NOM models, a significant synergistic fouling effect was observed between the organic fraction and the two micro-particles, as evidenced by the proportional increase of fouling resistance with the increase of particle concentration from 10 to 50 mg/L. Whereas at higher particle concentration of 100 mg/L, the synergistic fouling resistance became balanced, or even decreased in comparison with that at 50 mg/L, indicating that there might be a critical concentration range for the micro-particles, in which the particles caused the most pronounced synergistic fouling effect with the NOM fractions. As for the nano-particle of 5-15 nm, much lower synergistic fouling resistance was observed as compared with that of the micro-particles for all the NOM models. This work highlighted the importance of particle size and particle concentration (i.e. mass ratio of particle/NOM) on the combined membrane fouling behavior for UF of natural waters. Besides, the interplay of different NOM fractions in the particle layer also played a crucial role in UF membrane fouling, as demonstrated by the much higher synergistic fouling resistance caused by the HA-BSA-DFX mixture in the presence of different particles, as compared with that by HA alone. (C) 2013 Elsevier B.V. All rights reserved