The use of low-pressure membrane systems has experienced exponential growth over the past five years due to their abilities to deliver drinking water and wastewater that exceed more stringent standards. With their modular design and sophisticated automation, membrane plants are now being built to deliver safety and flexibility with minimal operator intervention. Another driving force for the popularity of membranes is that their costs have never been lower. Lower costs are spurred by more efficient process operation, expanded manufacturing capacity and increased competition. The most important factor that dictates the membrane plant size or the number of membranes is the membrane's ability to handle fouling. A well-designed membrane, with high porosity and permeability, will inherently provide better flux and economics. However, to make use of those physical properties, membrane fouling must be controlled in a practical and economical manner. Pretreatment of the feed water usually has a quantifiable benefit on the sizing of membrane systems. However, pretreatment carries an associated cost and is typically applied when additional contaminant removal is required. For example, coagulation prior to membrane filtration is applied to reduce TOC in raw water. An alternative option to control fouling is through the judicious use of an "enhanced flux maintenance" (EFM) strategy that effectively increases the membrane flux, thereby reducing the cost of membranes quite significantly. Keeping the membranes clean as often and as long as possible allows the most efficient use of the membranes area and porosity. This paper presents the causes of fouling and the details of the application of the EFM process for full-scale facilities. Case histories of successful implementation of EFM's are provided. Present worth costs for normal flux and enhanced flux are given for a 36-mgd membrane plant design. Operating membranes at enhanced flux using EFM resulted in 30% savings in lifetime costs and 20% smaller footprint. The benefits of lower membrane capital and replacement costs greatly exceed costs associated with increased use of chemicals to mitigate fouling and energy to maintain a higher flux. Includes 4 references, tables, figures.