| Summary: | In waste-water treatment plants with enhanced biological phosphorus removal (EBPR), a part of the phosphorus can be eliminated by chemical precipitation. In experiments with inactivated sludge containing relatively high concentrations of dissolved calcium (≈1.5 mol m -3 ) and phosphorus (≈1 mol m -3 ), a pH-sensitive and partly reversible precipitation of calcium phosphates was observed at pH values below 8.0. A dynamic model was formulated on the basis of these observations. It contains, in addition to equilibrium equations for the deprotonation(s) of phosphoric acid, carbonic acid and water, the following reactions: (1) Fully reversible precipitation of hydroxydicalcium phosphate (Ca 2 HPO 4 (OH) 2 , HDP) as an intermediate. (2) Formation of hydroxyapatite (Ca 5 (PO 4 ) 3 OH, HAP) from HDP. HAP is regarded as a kinetically stable product. A parameter estimation performed for the surface complex HDP predicts a solubility product of 10 -22.6 M 5 (20 °C) and a heat of dissolution (ΔH 298 ) of -88.4 kJ mol P -1 . If the solubility of HDP is exceeded, then inorganic calcium phosphate (HAP) is fixed irreversibly in the sludge with a rate of 0.5 mol P -3 d -1 .
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