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Application of a Factorial Design to Study Chemically Mediated Phosphorus Removal

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A 24 factorial design was used to study four factors thought to be significant in chemically mediated phosphorus removal in waste water. These factors include dose, pH, mixing and water hardness. By comparing two extremes of four factors simultaneously, it was found that solution pH, iron dose and mixing intensity are statistically significant, at a 95% confidence interval, in removing phosphate from solution by precipitation with iron. Water hardness showed an effect on P removal, but not as significant as the other factors. A high dose of iron (10 mg Fe/L for 1 g P/L) is orders of magnitude better at removing phosphate compared to a low dose (5 mg Fe/L). As expected from previous lab studies, low pH (pH 6) reached a lower residual P concentration than high pH (pH 8), since pH 8 is approaching the pHpzc

and the attractions between the negatively charged P species and positively charged Fe species would be reduced. High mixing intensity (376 s−1) achieves lower residual P concentrations compared to low mixing intensity (23.5 s−1) since at high mixing intensity the iron flocs are kept small, providing more surface area for P to adsorb. Significant two-way interaction between dose and pH as well as dose and mixing, indicate these variables need to be considered simultaneously when describing or modeling chemically mediated phosphorus removal with iron. Rate constants for the adsorption reactions to remove P from solution (k1) and the reverse reaction (Psolid→Psolution, k−1) were determined for the 16 experiments performed. It was found that a first order reaction fits well with the adsorption process and initial removal by co-precipitation is assumed to be instantaneous. The wide range of rate constants indicates that modeling the kinetic processes of chemically mediated P removal will potentially involve changing rate constants but, more work needs to be performed.
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Keywords: Phosphate; chemically mediated phosphorus removal; factorial design; nutrient removal

Document Type: Research Article

Publication date: 2011-01-01

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