This paper presents fundamental data on the settling of the iron phosphate precipitate in the presence and the absence of polyelectrolytes. Supernatant “equilibrium” phosphorus concentrations are also examined. Tests were conducted with model phosphate solutions in a modified jar test apparatus. The results show that adequate phosphorus removal (greater than 90%) required a minimum molar iron to phosphorus ratio of 1.5. The settling rate of iron phosphate flocs is such that complete settling in clarifiers operated at overflow rates of 600-800 igpd/ft2 cannot be ensured. The addition of a polyelectrolyte increases settling rate at least 5 fold. Phosphorus removal was less effective in the presence of condensed phosphates. Both ultimate phosphorus removal and settling rates were adversely affected by the presence of SO4= ions. An equation is developed to predict the maximum permissible iron dosage for a given initial solution alkalinity.

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