This paper presents regulatory, technologic, and economic information for considering low -pressure membranes for the rehabilitation of an existing conventional water treatment plant (WTP). The paper presents desktop analysis, pilot testing and design performed for the rehabilitation of the WTP of the City of Lake Forest (IL). The City of Lake Forest WTP is a 12 mgd conventional treatment process using Lake Michigan as raw water. The City of Lake Forest had concerns about the ability of the WTP to meet existing and future water quality standards at its design capacity of 18 mgd. In addition, physical limitations prevented the operation of the plant at its design capacity. Furthermore, aesthetic constraints limited the expansion of the existing building. Thus, the City of Lake Forest decided to proceed with the evaluation of a low-pressure membrane alternative. The evaluation process was divided into three distinct phases. Phase 1 (desktop analysis) allowed the City of Lake Forest to select three low-pressure membrane suppliers among the seven manufacturers that had been invited to bid. The main criteria were matching the treatment and operation goals of the WTP and verify that plant layout and construction costs were acceptable to the City. Phase 2 (short term side by side pilot testing) allowed the City of Lake Forest to assess design flux, chemical cleaning interval, and powdered activated carbon (PAC) efficiency for the three pre-selected manufacturers. Phase 3 concluded the evaluation process with the selection of the membrane system based on a present worth analysis of the complete facility. As a result of the evaluation process, the City of Lake Forest hired Carollo Engineers to follow up a twelve-month pilot testing required by the Illinois Environmental Protection Agency (IEPA) and to design the new facility. The design takes into account a final plant capacity of 18 mgd. A cellulose acetate membrane with a nine-month demonstrated dead-end flux of 74 gfd @ 20C is used. The minimum chemical cleaning interval will be every three months. The low-pressure membrane process occupies one third of the initial conventional treatment footprint. The City of Lake Forest awarded the construction contract to the General Contractor by July 19th, 2002. Construction completion is scheduled by August 2004. Includes 3 references, tables, figures.