Experimental studies of the fluid dynamics in the separation zone in dissolved air flotation

Research in the field of Dissolved Air Flotation calls for studies on the fluid dynamics of the process for optimisation. This study describes a method using an Acoustic Doppler Velocimeter for measuring local water velocities in a DAF pilot plant, and presents the results from measurements done in the separation zone. A test of the ADV in bubbly water indicates a 50% reduction of the measured velocity in water containing higher concentrations of micro-bubbles. The bubble content and the hydraulic loading were shown to significantly influence the flow pattern at surface loading in the range 10–20 m/h. Micro-bubbles generated density gradients in the flotation tank and two major flow patterns were observed. Stratified flow was characterised by a horizontal flow in a bubbly upper layer and a plug-like flow in the bubble-free lower layer. The break-through flow was seen to short-circuit the water to the outlet, which most probably could be the reason for bad effluent quality at higher surface loading. Finally, three-dimensional flows were discovered, which also could have a deteriorating impact on the effluent quality.