The flow structure in the separation zone of a DAF pilot plant and the relation with bubble concentration

Fluid dynamics is believed to be of crucial importance for the removal mechanism in Dissolved Air Flotation. The paper is a presentation of experimental studies on velocity and air-content profiles in a DAF pilot plant. The correlation between air content and flow patterns is discussed. Two flow structures are detected. A stratified flow structure is defined by a horizontal transport of the water in a upper, less-dense layer, and a lower, downwards-vertical transport in the lower, dense layer. Between the layers a return flow is found. The short-circuiting flow is defined as an immediate downward deviation towards the outlet arrangement at the bottom of the tank, probably causing decreasing effluent quality. Density gradients, caused by differences in air content in the tank, are believed to generate a stratification of the water body, thus causing the water to be transported in layers. Measurements of air-content profiles correlate to the measured flow structures, also indicating a separation ofthe water body in two layers with a distinct boundary.