Reducing Aluminum Residuals in Finished Water

Abstract

Pilot‐ and plant‐scale tests were conducted in Minneapolis, Minn., in an attempt to reduce aluminum residuals in the finished water. At high pH, when the predominant mode of alum coagulation is sweep coagulation, increasing the intensity of chemical mixing reduced aluminum residuals only slightly. Reducing the pH with acid was effective in reducing the residuals, but posttreatment pH adjustment with lime to stabilize the water increased its hardness. It appears that the most promising means of reducing aluminum residuals is to replace part of the alum dosage with ferric chloride. At both Minneapolis and St. Paul water treatment plants, ferric chloride, at less than half the dosage of alum, reduced the residuals and resulted in longer filter runs.