Optimization of phosphorus precipitation from swine manure slurries to enhance recovery

Laboratory experiments were conducted using magnesium chloride (MgCl₂·6H₂O, 64% solution) to force the precipitation of phosphorus and reduce the concentration of soluble phosphorus (PO₄^3-) in two swine wastes. One of the swine wastes tested contained a high concentration of PO43- (initially ≈1,000 mg/L), and the other swine waste tested contained a low concentration of PO₄^3- (initially ≈230 mg/L). The precipitation reactions were performed to determine the required reaction time, pH, magnesium addition rate and seed material for future precipitate recovery work. For the high and low concentration waste, a 10-minute reaction time at a pH of 8.6 was sufficient to remove 98 and 96% of the PO₄^3- from solution. A molar ratio of Mg²⁺:PO₄^3- of 1.6:1 was determined to be effective for PO₄^3- removal from both the low and high strength wastes. At a molar ratio of 1.6:1, the PO₃^- in the high concentration waste was reduced from 590 to 12 mg/L. In the low concentration waste, the PO₄^3- concentration was reduced from 157 to 15 mg/L. Seeding the reaction did not significantly enhance the recovery process.