Drinking water coagulation with polyaluminum coagulants: Mechanisms and selection guidelines
In recent years, polyaluminum coagulants have received considerable interest as drinking water coagulants. Many types of polyaluminum coagulants are commercially available, including polyaluminum chloride (PACl), aluminum chlorohydrate, and polyaluminum sulfate (PAS). Unlike alum, these products differ in their basicity and strength, and can contain varying amounts of other substances, such as sulfate, silica, and calcium. The chemistry of polyaluminum coagulants is not well known, coagulation mechanisms have not been studied as extensively, and comprehensive guidelines on the selection and use of these different polyaluminum coagulants currently do not exist.
In this research, several polyaluminum coagulants were examined for five different raw water types and three solid-liquid separation processes. Coagulants of different chemistry were evaluated: alum, PACl, PAS, and aluminum chlorohydrate. PACls of low, medium, and high basicities, with or without sulfate were examined. The solubility and Al speciation of the coagulants were investigated. As well, treatment experiments were performed with five widely different water supplies with sedimentation, dissolved air flotation (DAF) and direct filtration. The performance of the coagulants was evaluated for turbidity and NOM removal. Guidelines were developed for choosing polyaluminum coagulants and selecting doses based on water quality parameters, coagulant characteristics, and treatment process.
The results of this research indicate that significant differences exist between the solubility of the PACls, PAS, aluminum chlorohydrate, and alum. Aluminum chlorohydrate had the highest solubility and the highest pH of minimum solubility, followed by the PACls, PAS, and alum. For the PACls, the pH of minimum solubility tended to increase with increasing basicity. The Al speciation data indicated that high basicity PACls contain the largest fraction of dissolved polymeric species in solution over the broadest pH range. The polymeric fraction present, or the pH range over which polymer was present, decreased as basicity was decreased.
The selection of the appropriate coagulant for a given water treatment application was found to depend on the chemical characteristics of the coagulant, the characteristics of the raw water, and the treatment process used. High basicity PACls were found to be effective for all model waters under a wide variety of treatment conditions. PACls with added sulfate or silica were found to be especially effective for sedimentation applications. The presence of sulfate was detrimental for treatment by direct filtration. All of the PACls were effective for DAF treatment applications, although raw water alkalinity did have some influence on coagulant selection.
0543: Civil engineering