This paper assesses the results obtained from a microfiltration pilot plant operating with effluent water from an activated sludge reclamation plant as compared with those predicted by two models: the "resistances in series" and the "blocking laws" models. The microfiltration unit consisted of hollow-fibre membranes and it worked with direct flow. the characteristics of the feed water varied with time depending on the treatment plant operation and on the characteristics of raw feed water. Therefore, it was impossible to work with a desired composition and the unit worked with a real and variable feed water. Two sorts of experiments were conducted: a series in which flow rate was kept constant (ranging from 6-7 m(3)/h during 60 min), and a second block, also in 60-min cycles, where neither the flow nor the transmembrane pressure were controlled. Finally, the experimental data were in good agreement with the model.