In order to distribute a high-grade water quality using resources with high turbidity peaks as well as high bacterial counts, it is necessary for small units to use reliable and easy to operate treatments. For a few years, membrane filtration techniques have been used frequently in many countries including France and the US. Operations of ultrafiltration (UF) indicated that with this process, it is possible to produce at the effluent of the treatment plant a water with a low number of particles. However, the effect of such a shift of water quality in the distribution system has not been studied thoroughly. During this study, two distribution systems were sampled before and after the beginning of ultrafiltration plant operation. In system A, the new treatment (i.e. UF) reduced turbidity, iron and manganese concentrations as well as bacterial counts. Similarly, in system B, a fast improvement of water quality was observed. After a few weeks, a 50% reduction of the average turbidity on all sampling points of the system was noticed, concentrations of iron decreased in a 30-50% range. Chlorine dosage used for disinfection at the treatment plant was reduced significantly allowing a stable chlorine residual around 0.1 mg/l and maintaining a high bacterial water quality. These studies indicated that some parameters evolution are coming directly from pipes or deposits. These phenomena show that it is necessary to survey networks which always modifies water quality even with a good water quality at the clearwell. The treatment plant and its network are physicochemical and biological reactors which depend on one another.