In this paper, pilot data are presented for different pressure driven ultrafiltration (UF) systems operating on four different source waters, including groundwater, two surface waters, and a wastewater. The data present fouling rate as a function of flux both above and below the sustainable flux. The data show that fouling rate increases exponentially with flux, the rate of rise being determined by feed source and process design. The analysis develops algorithms for UF/MF systems from the pilot data, thereby providing guidelines for design and operation. By monitoring permeability, on-going operations can ensure that long term plant performance remains steady, fouling remains under control, and that recovery of fouled membranes is performed in a timely manner to avoid any permanent loss of capacity, or irreversible damage to the membranes. The recommended methodology could be applied to any of the commercial UF/MF systems currently offered in the water/wastewater industry. Includes 6 references, tables, figures.