Factors regulating residual aluminium concentrations in treated waters

Elevated concentrations of residual aluminium (Al) is an important issue for water treatment facilities. Concerns associated with high concentrations of residual aluminium include increased turbidity in drinking water, reduced effectiveness of disinfection, decreased carrying capacity of the water distribution system as well as potential effects on human health. The objectives of this study were to review the literature on mechanisms for controlling residual aluminium concentrations and to conduct a field study to evaluate the forms of residual aluminium in three water treatment plants. Both the literature review and results of the field study suggest that elevated concentrations of residual aluminium appear to occur largely in either of two forms; particulate aluminium or dissolved (monomeric) aluminium. To decrease concentrations of residual aluminium which largely occur as monomeric aluminium, it is necessary to maintain the pH of treated water near the pH of minimum Al(OH)₃ solubility (6.5–7.0), remembering that this value varies with temperature. Elevated concentrations of residual aluminium associated with particulate aluminium may be due to influent suspensions with a high aluminium content or relatively large amounts of aluminium coagulant per amount of particulate matter in raw water. The results of this study suggest that filtered water turbidity should be less than 0.1 NTU to minimize particulate aluminium concentrations that contribute to residual aluminium.