Revealing the hydrodynamic effects on phosphorus recovery as vivianite in stirring and aeration systems through PIV experiments and theoretical calculations

Hydrodynamic conditions have an essential influence on the precipitation and agglomeration processes of crystals. However, the influence of hydrodynamics under different operating conditions on vivianite crystallization is commonly ignored in the phosphorus recovery process. To this purpose, the effects of hydrodynamics on vivianite properties in stirring and aeration systems were investigated with the aid of particle image velocimetry (PIV). Hydrodynamic parameters of shear rate (G), flow velocity, Kolmogorov microscale (eta), turbulent dissipation rate (epsilon), and turbulent kinetic energy (k) were calculated, and the generated vivianite properties were recorded. The analysis of flow field distribution showed that the mixing state under the stirring system was more homogeneous and formed a smaller hydraulic dead zone, which may be more favorable for the formation and growth of crystals. Compared with the aeration system at the same G, the stirring system has higher hydrodynamic parameters, producing stronger turbulent intensity and higher shear rate. The higher recovery efficiency, as well as larger crystal size in the stirring system, demonstrated that higher shear rates (95-309 s-1) increased the crystallization rate, ultimately improving the crystal size and settling performance. Significant relationships between hydrodynamic parameters and the harvested crystal size showed that the flow velocity, eta, epsilon, and k were suitable hydrodynamic parameters for evaluating the growth of vivianite. This work is expected to provide the theoretical basis and technical support for the design and operation of vivianite recovery reactors.