Selection of alum and polyaluminum coagulants: principles and applications

This paper presents guidelines for the selection and use of polyaluminum chloride (PACl) coagulants and alum in terms of raw water quality and treatment method. The concentration of natural organic matter (NOM) was found to be the most important parameter affecting coagulant dose. The nature of the NOM, as measured by the specific ultraviolet absorbance (SUVA), was useful for predicting the degree of NOM removal expected. Raw water turbidity and NOM did not influence the type of coagulant that was most effective. Raw water alkalinity, as it relates to the pH of coagulation, was found to be very important for choosing one coagulant type over another. PACl basicity should be matched to raw water alkalinity, so that coagulation pH is as close as possible to the pH of minimum solubility of the coagulant. The solids separation process used for treatment was also found to be important for coagulant selection. Raw waters coagulated with PACls containing sulfate were found to have the best settling characteristics, but showed the highest headloss rates in direct filtration applications. Dissolved air flotation (DAF) performance was relatively insensitive to coagulant type.

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