The Guadalupe Blanco River Authority (GBRA) began operation of the Western Canyon Regional Water Treatment Plant (WCRWTP), located in central Texas, in 2006. The facility utilizes a coagulation / microfiltration membrane process to treat reservoir water. The overarching goal of the project was to deliver finished water with free chlorine residual to help ensure compatibility with the existing disinfection practices of wholesale customers. Coagulation was required to remove organics in order to reduce disinfection byproducts. Ferric sulfate was initially used as a coagulant. The membrane process experienced shorter than anticipated runs between chemical cleanings during the initial startup period and subsequent operations. The membranes also did not return to the expected filtration performance after cleaning. The clean in place (CIP) chemical waste had to be hauled by truck to a nearby facility, causing unplanned significant costs. The membrane manufacturer (Pall Corporation), GBRA and Malcolm Pirnie began a collaborative effort to identify measures to correct the operational challenges. Polyaluminum chloride (PACl) was considered as an alternative to ferric sulfate. PACl coagulants from several manufacturers were evaluated. Bench scale testing was performed, and then a rigorous pilot testing protocol was developed. A close dialogue with the Texas Commission on Environmental Quality (TCEQ) was required to obtain prompt approval of the pilot testing program. Once approved, pilot testing was performed utilizing an on-site membrane pilot system. The results revealed a preferred PACl candidate compared to ferric sulfate. Numerous operational and design aspects were considered and used to prepare the WCRWTP for the changeover. The changeover was conducted in July 2007. To date, GBRA has experienced successful operation using PACl as a coagulant, as indicated by rate of trans-membrane pressure development and flux recovery after cleaning. This paper presents the methodologies used to plan for and implement the coagulant changeover, and the consequences including residuals management, overall cost impacts, and lessons learned. Includes table, figures.