Kinetics of biologically induced phosphorus precipitation in waste-water treatment

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 (Ca2HPO4(OH)2, HDP) as an intermediate. (2) Formation of hydroxyapatite (Ca5(PO4)3OH, 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 M5 (20°C) and a heat of dissolution (ΔH298) of −88.4 kJ molP−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 molP −3 d−1.