The variable removal from solution of sulfate, orthophosphate, fluoride, five simple organic acids, and natural organic matterfromtwo sources by adsorption on aluminum hydroxide was examined to assess their potential influence on coagulation during drinking water treatment. Measurements of electrophoretic mobility were conducted with adsorption studies to provide means of evaluating the impact of the adsorption of these anions on the removal of particulate material during coagulation at water treatment facilities. The three inorganic ions exhibited widely different trends in terms of removal from solution and effect on the surface charge of the aluminum hydroxide. Phosphate was nearly completely removed from solution across a wide pH range and was observed to lower surface charge and shift the isoelectric point. Sulfate was removed to a lesser extent than phosphate, lowered the surface charge on the precipitate, but did not shiftthe isoelectric point. Fluoride was well-removed through adsorption but exhibited no influence on the charge of the hydrous aluminum oxide. The organic acids similarly displayed varying abilities to be removed through adsorption and different influences on surface charge. The results indicate the importance of the number and location of functional groups and their acid/ base properties. The ability to strongly influence surface charge illustrates the impact that adsorption of these anions can have on particle stability.

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For the past decade, US water utilities have been working to optimize their treatment processes to meet increasingly stringent regulations. Of the many unit processes employed in conventional treatment, water fluoridation appears to have been overlooked as a potential obstacle to effective compliance with the new requirements. A review of treatment plant schematics collected for the Information Collection Rule found that many utilities add fluoride to the process train at points where it can interfere with particle and natural organic matter (NOM) removal. In this research, bench‐ and pilot‐scale studies provided evidence that addition of fluoride to water prior to treatment with aluminum sulfate (alum) can severely affect removal of turbidity and NOM under enhanced coagulation conditions. Utilities disregarding these effects may face increased chemical costs, lower removal of organic matter and disinfection by‐product precursors, elevated concentrations of aluminum in the finished water, shorter filter run times, and the potential for turbidity breakthrough in filters.

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