Phosphate removal in a fluidized bed—II. Process optimization

The aggregation of fine primarily formed calcium phosphate particles with sand grains in a fluidized bed for phosphate removal was studied experimentally by means of a set-up which isolated aggregation from other processes during calcium phosphate precipitation, as well as through experiments under normal operation of the fluidized bed. The net aggregation process was described by means of a mathematical model which takes into account two competing mechanisms: orthokinetic aggregation and breakage. The net aggregation process was found to account for 60% of the phosphate removed by the fluidized bed. It was found that the orthokinetic aggregation can be improved by spreading the supersaturation more evenly throughout the reactor, and breakage can be diminished by a low energy dissipation rate in the bed. Optimization of the phosphate removal efficiency was therefore achieved by selecting sand grains of small sizes (0.1–0.3 mm) and a low superficial velocity (7•10−3 m/s), and by spreading the addition of the NaOH solution (reactant) over two dosage points. Under these conditions the phosphate removal efficiency was 80%.