Simultaneous Use of Polyphosphate for Sequestration and Antiscaling

Abstract

Bench-scale testing methods examined polyphosphate dose− response relationships for sequestration and antiscaling. Tests in water from nine utilities with two different phosphate chemicals indicated that the minimum dose for effective sequestration was a linear function of iron, manganese, magnesium, and calcium concentrations. Chemical A had an optimal polyphosphate dose (in mg/L as P) equal to 58.5[Fe] + 59.7[Mn] + 0.041[Ca + Mg] + 0.4669 (unit mM). Chemical B was similar but had about 33% more dependency on calcium and magnesium hardness and tended to sequester manganese more effectively on a mole Mn/mol P basis (p = 0.001). Chemical B also reverted to orthophosphate at about three times the rate of Chemical A. Color in samples was strongly correlated with particulate manganese (R 2 = 0.79), while turbidity was mostly correlated with particulate iron (R 2 = 0.601), and neither color nor turbidity measurements perfectly agreed with visual observations of discoloration. Bench-top scaling tests indicate that a significantly higher dose was needed for sequestration than for scale inhibition in all systems tested. At three utilities tested, at least 3.6 times more phosphate was needed for sequestration than scale inhibition, but lab tests indicated that antiscaling could sometimes control the minimum dose at extremely high levels of calcium.