The impact of inorganic colloidal water constituents on fouling in low-pressure membrane systems was evaluated by carrying out ultrafiltration (UF) experiments using synthetic waters containing model foulants. Humic acid (HA), sodium alginate (SA), and bovine serum albumin (BSA) were chosen as model natural organic matter (NOM) foulants, and silicon dioxide and alpha-aluminum oxide were chosen as model inorganic colloidal foulants. The synthetic solutions were filtered using one lab-scale flat sheet UF system (i.e., unstirred filtration cell) and one pilot-scale hollow fiber UF system (i.e., custom made hollow fiber modules), both of which employed polyethersulfone (PES) membranes. Filtering the inorganic colloids by themselves did not result in any significant fouling, as evidenced by the negligible permeability decline and high hydraulic reversibility. However, when combined with NOM, severe permeability decline and hydraulic reversibility reduction were observed. Moreover, the permeability decline and reversibility loss of the combined inorganic colloids and NOM were more severe than the sum of the individual effects from organics and inorganic colloids, suggesting that combining the two acts to enhance fouling (i.e., synergistic fouling effects). Aluminum oxide tended to result in more severe synergistic fouling compared to silicon dioxide, suggesting that the inorganic colloid material character, especially its surface charge, affects the severity of combined fouling. Rejection of TOC and analysis of the fouled membranes by scanning electron microscopy indicated higher NOM adsorption by aluminum oxide colloids, which lead to a larger amount of foulants in the cake layer and a worse reversibility. Both the lab- and pilot scale systems exhibited similar fouling behavior, thus indicating that the lab-scale system provides a meaningful representation of the pilot-scale system, even though the operating conditions are quite different. This study highlights the importance of interactions between foulants with respect to fouling in low-pressure membrane systems and showed conclusively that inorganic colloids play an important role in fouling of NOM. Moreover, present results underline the impact of colloidal surface charge on the resulting hydraulic and chemical fouling reversibility during ultrafiltration of natural waters containing NOM and inorganic colloids. (C) 2016 Elsevier B.V. All rights reserved.