Response surface methodology for optimization of phosphate removal and recovery by dolomitic lime from sludge digestion supernatant

Dolomitic lime was utilized to recover phosphate from sludge digestion supernatant. The linear, quadratic and interaction effects of dolomitic lime dosage (X₁), initial reacting pH values (X₂) and temperature (X₃) on the removal efficiency were investigated using Box-Behnken Design experiments and response surface methodology. An empirical mathematical model describing the relationship between response values (phosphate removal rates, denoted as Y) and the influence factors was presented. The results demonstrated that regression equation fit well with experimental data (R² equals to 0.9680). X₁, X₂, X₃, X₁X₃, X₁² and X₂² had significant effects on phosphate recovery efficiency. Considering simplicity and efficiency of operation, the optimum conditions of dolomitic lime 300 mg·L^-1, initial pH 9.0 and 25.0°C were selected. Under the optimum conditions the removal rate of phosphate was 89.08% with 0.39% of relative deviation between the experimental and predicted values. Scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction were performed to characterize the composition of the recovered products. It was shown that the recovered products consisted of ammonium magnesium phosphate and amorphous calcium phosphate, as well as impurity, i. e. calcium carbonate.