The effects and mechanism of alkalinity on the phosphate recovery from anaerobic digester effluent using dolomite lime

The alkalinity plays an important role in phosphate removal. Precipitate in the Mg²⁺–Ca²⁺–NH₄ ⁺–PO₄ ³⁻ system has been researched to understand the phosphate recovery from anaerobic digester effluent. This project investigated the usefulness of a chemical equilibrium model, Visual Minteq3.0, for prescribing the ionic form of CO₂. In the present study, a series of jar test was conducted using dolomite lime (DL) to evaluate the function of alkalinity during the precipitate reaction. Over a range of PO₄ ³⁻–P (20, 60, and 120 mg/L), the addition of alkalinity (0–8 g/L) was found to decrease the removal rate of phosphate. However, the result showed that the impact was definite when the maximum of inhibition reached. The removal rate of phosphate reduced 4.80, 11.49 and 9.19 % under the effect of alkalinity. The recovered products were detected and analyzed by X-ray diffraction, scanning electron microscope and energy dispersive spectrometer, which were proved to be struvite and calicite. Finally, our experiment showed that the struvite precipitation was kinetically favored over hydroxyapatite. The precipitation mechanisms in the DL-treated wastewater were researched and explained by the kinetic experiment.