One of the critical issues for the successful application of ultrafiltration in water treatment is membrane fouling due to dissolved organic matter, which negatively affects productivity, product quality, and process costs. The aim of the present study is to contribute to the understanding of fouling phenomena by organic matter and the efficiency of the backwashing technique, which is applied in practice to restore membrane flux. Fouling experiments are carried out, in a single fiber apparatus, using humic acid solutions as model substances representative of naturally occurring organic matter; they are aimed at identifying the significance of distinct fouling mechanisms and their degree of reversibility. A new model is presented which takes into account the simultaneous action of all fouling mechanisms and describes the experimental results successfully. An important parameter considered in the study is the concentration of calcium ions, which promote humic acid aggregation and influence the rate of flux decline, the reversibility of fouling and rejection. A relatively rapid irreversible fouling takes place due to internal pore adsorption, which persists for a long time. In parallel, with time, pore blocking becomes important and a fouling cake develops on the membranes. The effects of the latter are partly reversed by backwashing, but their combined influence persists over a long time giving rise to a prolonged flux decline. (c) 2005 Elsevier B.V. All rights reserved.