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 f luid dynamics of the process for optimisation. This study describes a metho d using an Acoustic Doppler Velocimeter for measuring local water velocitie s 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% redu ction 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 an d two major flow patterns were observed. Stratified how was characterised b y a horizontal flow in a bubbly upper layer and a plug-like flow in the bub ble-free lower layer. The break-through dow was seen to short-circuit the w ater to the outlet, which most probably could be the reason for bad effluen t quality at higher surface loading. Finally, three-dimensional flows were discovered, which also could have a deteriorating impact on the effluent qu ality. (C) 1999 Elsevier Science Ltd. All rights reserved.