| Abstract: | The mechanism involved in the decrease of phosphate in a high rate pond (HRP) is investigated. The calcium concentration in the water (1.25–3.75 mM) and the high pH obtained in the reactors (8–10), result in the precipitation of calcium phosphate minerals. Although calcium hydroxyapatite Ca5(PO4)3OH, pK5 = 57] is the thermodynamical stable state, the phosphate concentration is determined by the solubility of the amorphous tricalcium phosphate Ca3(PO4)2, pK5 = 25.2]. This is confirmed by the calculation of the theoretical predicted solubility as well as various experiments. The relationship between pH and phosphate concentration may be used to optimize the efficiency of this type of reactor to produce a minimum concentration of phosphate in the effluent. |
