A chemical model of ferric phosphate precipitation was developed describing ferric hydroxy-phosphate precipitation either alone or together with ferric hydroxide. Conditions for formation of one or two precipitates are examined. The model also incorporates an additional mechanism for phosphate removal through adsorption of PO43− ions on the precipitate. Experimental verification of the proposed model was carried out in lab-scale batch and continuous activated sludge units fed with municipal primary effluent and at five pH values in the range of 6.5 to 8.0. Solubility of ferric phosphate in the activated sludge system was significantly different from that reported in the literature for distilled water systems and was pH-dependent with a minimum at pH of approx. 7.0. It is proposed that the composition of precipitating ferric hydroxy-phosphate can be represented by the empirical formula Fe2.5PO4(OH)4.5. Corresponding solubility product was estimated at pKsp=96.7. The adsorption mechanism has an important effect on total phosphate removal, especially at low residual phosphate concentrations.

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