Ultrafiltration using a hollow fiber membrane is increasingly used in water treatment for the removal of particles, turbidity and microorganisms to meet stringent water quality requirements. Fouling by natural organic matter (e.g. humic acid and fulvic acid) is a critical problem in membrane processes. In this work, experiments were performed to evaluate the effects of metal ions on humic acid fouling of a hollow fiber ultrafiltration membrane. Fouling was severe in the presence of metal ions, especially at high concentration of Ca(2+) or Fe(3+). The accelerated fouling is considered to be due to the formation of intermolecular crosslinking between humic acid molecules, because the humic acid aggregate size increased in the presence of Ca(2+). Addition of ethylendiaminetetraacetic acid (EDTA) decomposed the aggregates and prevented metal ion-induced acceleration of fouling. Scanning electron micrograph images clearly revealed deposition of humic acid on the membrane after filtration, and that the presence of metal ions increased humic acid deposition, which led to accelerated fouling. (C) 2011 Elsevier B.V. All rights reserved.