Polyaluminum Coagulants for Drinking Water Treatment: Chemistry and Selection

Abstract

Selection of polyaluminum (PAXs) coagulants is examined for different raw water types and solid-liquid separation processes. PAXs were tested with various basicities and sulfated versus non-sulfated. Four widely different water supplies were studied in terms of alkalinity, turbidity, and concentration and nature of natural organic matter (NOM). The performance of the PAXs was evaluated for turbidity and NOM removal. Solubility and A1 speciation of the PAXs differ from alum. Hight basicity PAXs, for example, are more soluble than alum below pH 7 and contain a highly charged Al₁₃ polymer for coagulation. High basicity PAXs produce less filter bed headloss than alum and other PAXs in direct filtration applications. An important finding is direct filtration of low turbidity supplies is not feasible, even when the TOC is low (2.5–3.5mg/L), if the NOM is composed of some aquatic humic matter. DAF is very effective in treating low turbidity supplies of low and high alkalinity, and supplies high in natural color. All of the coagulants gave good DAF performance, but generally the medium and high basicity PAXs were particularly effective. Generally, best sedimentation performance is achieved using medium and high basicity PAXs with sulfate. A silica containing PAX can also improve settling. NOM controls coagulant dosages for high TOC supplies containing aquatic humic matter. For the latter type supply, stoichiometric relationships were found between the optimum coagulant dosage and raw water TOC or UV (254 nm). Good removals of NOM were found, and generally a broader dosing range of good treatment was found when using high basicity PAXs.