This paper discusses some of the important factors related to membrane system performance and provides examples based on side-by-side testing of different membrane systems. Data from two systems treating various water supplies were compared to examine the impact of water quality on the relative performance of these systems. Among other variables, the pretreatment differed among the testing locations. As expected, water quality with the permeate from the membrane systems was consistently very high in terms of turbidity and particle count. As such, results presented focus on hydraulic performance of the membranes.