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Aluminum residual control using orthophosphate

Applying orthophosphate during rapid mix following alum addition is an effective method for reducing residual aluminum concentrations and decreasing the potential for aluminum precipitation in distribution systems. Orthophosphate addition during bench‐scale tests reduced residual dissolved aluminum concentrations by as much as 350 μg/L. The tests involved adding alum and phosphate to Lake Michigan water followed by continuous mixing for up to 8 h. Analysis of filtered samples showed that orthophosphate addition reduced residual aluminum concentrations under all test conditions (pH 7.0–8.0, 4–31oC) except at the lowest temperature, at which aluminum concentrations fell below the detection limit. Orthophosphate loss to floc particles ranged from 22 to 85% of the amount dosed, with greater losses at the lowest pH value tested. Applying orthophosphate before rather than after adding alum resulted in greater phosphate loss to floc particles without improving aluminum removal.