AWWA ACE68863

This powerpoint presentation begins by providing a brief historical overview of the study, including the following: "classical" equilibrium model for carbonate + orthophosphate system suggests best results between about pH 7.2 and 7.8 based on solubility predictions; reports of difficulties achieving LCR Action Level with DS pH > 8; and, wide variation in tabulated and published equilibrium constants for key lead solids and aqueous complexes, which can affect predictions and treatment choices. The experimental setup of test systems is presented, along with pipe preparation, chemical analyses performed, data analysis approach, and integration with scale mineralogy. Presentation conclusions indicate the following: confirmed prior prediction of best lead solubility reduction by orthophosphate between pH 7 and 8; confirmed problem with orthophosphate performance at pH in low- to mid-8's; mechanism is not clear but near several phase boundaries; not simple charge, colloid stability or particle size issue; and, Saturation Index approach promising, but more complicated than expected because of disequilibrium, even after 1 year, important role of metastable phases (like Cu(II)), multiple phases simultaneously present, and some unidentified solid phases present.