Bench, pilot and full-scale testing showed that the addition of phosphate during alum coagulation reduces soluble (dissolved) aluminum (Al) concentrations in filtered water. During pilot testing, a dose of approximately 0.5 milligrams per liter (mg/L) as PO4, orthophosphate decreased aluminum residuals by 25 percent, a blended phosphate (60 percent poly, 40 percent ortho) decreased Al by 31 percent, and polyphosphate decreased Al by 17 percent compared to no phosphate addition. While all three forms of phosphate decreased aluminum residuals, visual observations made during pilot testing showed that floc particles were smaller and didn't settle as well when the blended or polyphosphate was added. Turbidity readings on pilot plant filtered water confirmed that polyphosphate and blended phosphate addition slightly increased filtered water turbidity levels. Due to the smaller floc particles observed with blended and polyphosphate, only orthophosphate addition was investigated full-scale. Follow-up full-scale testing confirmed that orthophosphate addition decreased aluminum residuals without affecting finished water turbidity levels. Includes reference, tables, figures.