Kinetics of phosphorus removal and struvite formation by the utilization of by-product of magnesium oxide production

A byproduct of the magnesite calcination process to produce magnesium oxide (BMP) was applied for struvite production from an anaerobic digester liquor. The experiments were carried out at batch conditions. The effect of (Mg:P) molar ratio of pure MgO reagent and BMP was firstly evaluated for synthetic liquor. A first-order kinetic model was applied to the experimental results obtained and the feasibility of this model was demonstrated. Equilibrium concentrations (Ce) and the reaction rate constants (K) were determined for all the cases evaluated. Values of Ce were lower for pure MgO than for BMP while the values of K were higher for pure MgO (0.039-0.048 min−1) than for BMP (0.09-0.017 min−1). The experiment was also carried out with real liquor at molar ratios (Mg:P) in the range of 1.5-3.6 for BMP and 1.5 for MgO, respectively. The values of Ce decreased with the increase of molar ratio showing values for real liquor lower than those obtained with synthetic liquor. The values of K increased from 0.018 to 0.029 min−1 for real liquor and molar ratios in the range of 1.5-3.6, respectively and these values were higher than those obtained with the synthetic liquor (0.018-0.024 min−1) at similar molar ratios. The value of K was significantly higher for MgO (0.045 min−1) than that obtained for BMP (0.039 min−1) for real liquor and for synthetic liquor at the same molar ratio (Mg:P) = 1.5. The higher pH and the presence of suspended particles in the real liquor probably contributed to enhance the struvite formation. It was found that particle size influenced the rate of reaction. Sieved material with particle size lower than 0.04 mm gave values of K higher than those obtained with particle sizes higher than 0.04 mm and ground material with particle size lower than 0.04 mm and raw material. The results obtained demonstrated that BMP is a good reagent for the removal of phosphorus and to produce struvite.