AWWA WQTC57139

This paper presents the results of a bench-scale investigation to determine whether phosphate can be added during rapid mix to reduce the filtered water aluminum concentration in order to minimize aluminum precipitation in distribution systems. Tests showed that applying orthophosphate during rapid mix following alum addition was an effective method to reduce residual aluminum concentration and decrease the potential for aluminum precipitation in the distribution system. Orthophosphate addition reduced residual aluminum concentrations by as much as 350 ug/L. Orthophosphate reduced residual aluminum concentration under all test conditions (pH 7.0 . 8.0, 4 C . 31 C) except at the lowest temperature, where aluminum concentrations 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. Adding orthophosphate before alum addition, rather than after alum addition, resulted in greater phosphate loss to floc particles, without improving aluminum reduction. Includes 18 references, table, figures.