Sierra Pacific Power Company (SPPCo), located in Reno, Nevada, recently began using polyaluminum chloride instead of alum as the primary coagulant at one of its water treatment facilities. Shortly thereafter, higher aluminum residuals were detected in the finished water. Bench-scale, pilot-scale, and full-scale studies using carbon dioxide to reduce pH and minimize residual aluminum concentrations were conducted. These studies demonstrated that carbon dioxide was cost effective for lowering pH, decreasing aluminum residual, and improving treatment while reducing chemical dosages. Includes tables, figures.