Scaling Up of Plugflow-Dissolved Air Flotation

Abstract

In a conventional rectangular flotation basin, acceptable separation efficiencies in excess of 90% can rarely be achieved with an operational surface loading in excess of 5 m/h, because of the great degree of dispersion, turbulence, and dead space present in the basin. A pilot-scale coaxial cylinder-type flotation column was built to fulfill the required plugflow performance, to reduce dead space, and to optimize the size. This coaxial plugflow flotation column was used in a pilot-scale flotation unit capable of handling 10 m³/h of wastewater, operating at a maximum surface loading of 12 m/h. Near-plugflow was established using baffles in the different sections. Flotation efficiencies in excess of 90% were obtained for industrial wastewater at high surface loadings (up to 12 m/h) and a low recycle ratio (9%). Efficiencies obtained in a traditional unit are significantly lower. Experience with the coaxial unit has enabled us to formulate scale-up guidelines for the plugflow dissolved air flotation, and the use of a modular design is required. The process can be described by a model using two parameters, i.e., the bubble-floc attachment efficiency, α_pb, and the air-to-solid volume ratio in the agglomerate, β. Fitting experimental data and model predictions defines both model parameters, which are in line with the literature data and with values obtained in a laboratory-scale unit, thus justifying the possible use of small-scale experiments for scale-up purposes.