Abstract: A plant for the treatment of surface water comprises a safety barrier mounted at the exit from the treatment plant and constituted by a physical filter providing, at the exit from the filter, a predetermined minimum quality irrespectively of the quality of the water on entry into the filter, and a pre-treatment installation located upstream from the safety barrier, the treatment intensity of the pre-treatment installation being adjusted as a function of the characteristics of the surface water to be treated through measuring devices located at the entrance to the physical barrier. The safety barrier is advantageously a nanofiltration membrane, coupled if necessary to one or more reverse osmosis membranes mounted downstream in "series-rejection" mode. The disclosed installation can be applied to the treatment of water liable to show great variability in quality, notably to the treatment of surface water or of water with equivalent characteristics.