Modelling of floc-bubble aggregate rise rates in dissolved air flotation

The use of relatively simple, but conceptually sound mathematical models, i s a powerful tool to identify and understand parameters that are critical t o a process. In this paper, a model is presented which addresses the rise r ate of floc-bubble aggregates in the DAF separation zone. The model uses St oke's Law as a point of departure, which is then progressively extended to incorporate the non-sphericity of the aggregates, the non-laminar nature of their movement, the fractal nature of the flocs, the physical constraints of attaching bubbles to a floc, and the limit on bubble numbers imposed by a typical air dosing system. The main findings are: There are two distinctly different DAF domains, namely a small floc domain and a large floc domain. In the small floc domain, the bubble size, the air volume, chemical dosing and the degree of flocculation have to be optimize d and accurately controlled, while the large floc domain is less sensitive to these parameters. The calculated rise rates are significantly lower than some of the latest p ilot testing results reported from the USA; a surprising finding at first. There are, however, numerous modelling simplifications that may explain thi s discrepancy. The most probable reasons are the further agglomeration of t he aggregates in the separation zone due to differential rise rates or the recirculating flow within the white-water blanket, and the complex and poor ly understood flow patterns within the separation zone.