The Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) approach was used to predict the mechanisms regarding the influence of humic acid together with calcium ions on membrane fouling in an ultrafiltration (UP) system. The polymeric UF membranes composed of polyvinylchloride (PVC) were selected to investigate membrane fouling. The results indicated that membrane fouling was aggravated when the total interaction energy caused by humic acid combined with calcium ions decreased. Calcium ions aggravated membrane fouling, compared with water samples without calcium ions. Increasing concentrations of calcium ions resulted in greater membrane fouling due to their chelation with humic acid. Increased pH influenced the interaction between calcium ions and humic add, resulting in an increase in the total interaction energy, which reduced membrane fouling. Prechlorination reduced the interaction between calcium ions and humic acid, and resulted in an increase in the total interaction energy of the water sample, indicating that prechlorination may reduce membrane fouling, due to characteristic changes in organic matter. (C) 2014 Elsevier BY. All rights reserved.