Thermodynamic Simulation Of Phosphate Crystallization For Phosphorus Recovery In Environment

It is very important to remove and recover phosphorus (P) from wastewater so as to prevent eutrophication of waters and to recover and reuse P resource. To understand the effect of solution condition on P recovery from wastewater it is necessary to carry out systematic study on phosphate crystallization, and to study phosphate crystallization chemistry by using geochemical water quality model program is of significance for research and engineering practice of P recovery.PHREEQC Program from the U. S. Geology Survey was chosen, and the probable solution condition systems for P recovery were designed. A great deal of calculations on the thermodynamic parameters of phosphate calcium and phosphate magnesium salts have been carried out and modeled; the effects on phosphate crystallization have been understood.It shows that the saturation index (SI) of calcium phosphate is the logarithmic functions of Ca and phosphate concentrations, and increases with the increasing of their concentrations. SI is the polynomial function of pH value; the SI values of hydroxyapatite (HAP) and tricalcium phosphate (TCP) increase with the increase of pH value; while the SI of octacalcium phosphate (OCP) and dicalcium phosphate dehydrate (DCPD) have highest values at pH 9.0～9.5 and 7.0～7.5, respectively. SI is the linear function of alkalinity and decreases with its increase. SI value is the logarithmic function of ionic strength and deceases with its increase. .The SI of HAP is positively correlated to temperature, but other calcium phosphates show their complex temperature effect. The SI of carbonate is the logarithmic function of Ca and carbonate concentrations, respectively; and it is also the polynomial function of pH value, decreasing with the increase of ionic strength and being positively correlated to temperature. Carbonate and phosphate may compete in crystallization.The SI of magnesium ammonium phosphate (MAP) is the logarithmic function of the concentrations of its constitutional Mg ion, ammonium ion and phosphate ion, and the SI of magnesium phosphate (MP) is the logarithmic function of the concentrations of its constitutional Mg ion and phosphate ion. The SI values of MAP and MP are the polynomial functions of pH value; and the optimum pH for MAP crystallization is around 9.0, while it is at 9.5～10.5 for MP. The SI values of MAP and MP decrease with the increases of alkalinity and ionic strength, and being the polynomial functions of temperature correlating to reaction conditions.The thermodynamic simulation on the process conditions of synthetic swine wastewater treatment shows that the SI of calcium phosphate and magnesium phosphate can be increased by increasing pH, but the SI value of carbonate also increases. The crystallization of calcium phosphate can be promoted by dosing Ca but this may inhibit the crystallization of magnesium phosphate, and vice versa. However, both dosing Ca and Mg may increase the SI of calcium and magnesium carbonate. The SI values of phosphate salts can be greatly improved by increasing pH and dosing Ca and Mg, but meanwhile the SI of calcium carbonate also increases. With a comprehensive thinking, to adjust solution pH is one of the most effective measures for phosphate crystallization adjustment.