Solubility equilibrium models and/or adsorption models enable the prediction of phosphorus residuals in an effluent of a wastewater treatment plant over a wide range of operating conditions. A generally valid model cannot be given because the model parameters depend on wastewater composition, e.g. on dissolved and particulate water constituents, above all on phosphorus speciation.

Chemical precipitation is able to bind ortho-phosphates proportionally to added chemicals and polyphosphates to a great extent. The P-residuals consist mainly of organically bound phosphorus and some residual polyphosphate. In wastewater treatment plants with enhanced biological phosphorus removal (EBPR), an extensive use of precipitation chemicals could suppress the EBPR-ability of the sludge.

In EBPR-plants a part of the phosphorus can also be eliminated by chemical precipitation without added chemicals. In experiments with inactivated sludge derived from a wastewater treatment pilot plant, 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 the following reactions:

  • – Fully reversible precipitation of hydroxydicalcium phosphate (Ca2HPO4(OH)2, HDP) as an intermediate, with a solubility product of 10−22.6 (kmole m−3)5 (20°C).

  • – Formation of hydroxyapatite (Ca5(PO4)3OH, HAP) from HDP. HAP is regarded as a kinetically stable endproduct.

If the solubility of HDP is exceeded, then 15.5 gpm−3d−1 of inorganic calcium phosphate (HAP) is fixated irreversibly in the sludge.

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