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 (approximate to 1.5 mol m(-3)) and phosphorus (approximate to 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 acid water, the following reactions: (1) Fully reversible precipitation of hydroxydicalcium phosphate (Ca2HPO4(OH)(2), HDP) as an intermediate. (2) Formation of hydroxyapatite (Ca-5(PO4)(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 degrees C) and a heat of dissolution (Delta H-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). (C) 1998 Elsevier Science Ltd. All rights reserved.