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 mech anism in Dissolved Air Flotation. The paper is a presentation of experiment al studies on velocity and air-content profiles in a DAF pilot plant. The c orrelation between air content and flow patterns is discussed. Two flow str uctures are detected. A stratified flow structure is defined by a horizonta l transport of the water in a upper, less-dense layer, and a lower, downwar ds-vertical transport in the lower, dense layer. Between the layers a retur n flow is found. The short-circuiting flow is defined as an immediate downw ard deviation towards the outlet arrangement at the bottom of the tank, pro bably causing decreasing affluent quality. Density gradients, caused by dif ferences in air content in the tank, are believed to generate a stratificat ion of the water body, thus causing the water to be transported in layers. Measurements of air-content profiles correlate to the measured flow structu res, also indicating a separation of the water body in two layers with a di stinct boundary.