This paper presents the results of a bench-scale investigation to determine whether phosphatecan be added during rapid mix to reduce the filtered water aluminum concentration in order tominimize aluminum precipitation in distribution systems. Tests showed that applyingorthophosphate during rapid mix following alum addition was an effective method to reduceresidual aluminum concentration and decrease the potential for aluminum precipitation in thedistribution system. Orthophosphate addition reduced residual aluminum concentrations by asmuch as 350 μg/L. Orthophosphate reduced residual aluminum concentration under all testconditions (pH 7.0 - 8.0, 4°C- 31°C) except at the lowest temperature, where aluminumconcentrations fell below the detection limit, obscuring the effect of the orthophosphate.Orthophosphate loss to floc particles ranged from 22% to 85% of the applied dose. Addingorthophosphate before alum addition, rather than after alum addition, resulted in greaterphosphate loss to floc particles, without improving aluminum reduction.
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