Different kinds of surface waters (natural and synthetic) were tested for disinfection by using membranes on a laboratory-scale apparatus equipped with an ultrafiltration membrane. Water with different hardness (5 degrees F to 50 degrees F) and organic loads were studied, and the results show that ultrafiltration can be considered as a reliable process to disinfect surface water, especially in those cases where a quick and easy transportable apparatus is needed. The results were interpreted by using the resistance model through the fit of the experimental results, and the estimated parameters are given.