Effects of Flow Structure on Particle Separation in Dissolved Air Flotation

University dissertation from Lund University, Water and Environmental Engineering, Box 118, 221 00 Lund, Sweden

Abstract: Dissolved Air Flotation (DAF) is a separation process in water and wastewater treatment. The removal function is based on reducing the density of the particle by producing aggregates of the particles and added micro-bubbles of air, thus lifting the aggregates to the water surface, where they can be removed as sludge. One parameter that is believed to be of crucial importance is fluid dynamics. The hypothesis is that complex hydraulic effects occur inside a DAF process for clarification, and that these effects have consequences for the separation of particles. Hydraulic loading, air content and internal geometry are believed to control the hydraulic behaviour inside a specific DAF tank. By mapping the flow structure and defining the relationship to these factors, it should be possible to partially predict the separation performance of the unit. The measurements were performed in a rectangular pilot tank. The hydraulic loading over the separation zone was varied in nine cases, from 11.3 m/h to 24.7 m/h, and the recycle rate varied from 5% to 15% of the head flow, generating air contents of 3-12 ml air/l water. Changes in internal geometry were achieved by modifications of the contact zone shaft wall. The flow structure was defined through measurements with an Acoustical Doppler Velocimeter and the separation efficiency was determined through measurements of suspended solids. Numerical and analytical analyses of the flow structure were performed with tracer measurements. A conceptual model was defined based on the cascade box model and the dispersion plug-flow model. The results show that a stratification of the water body exists inside the separation zone. The structure is generated by density gradients caused by spatial differences in air content. The stratification is achieved for the low to average hydraulic surface loading and for the average to high air-content. For the low air-content, a short-circuit flow structure was developed. There appears to be a relationship between the flow structure and the removal efficiency of particles. A high concentration of suspended solids seem to influence the short-circuit flow structure by a transformation towards a stratified flow structure. This, in turn, was observed to improve the separation of particles as well, further strengthening the hypothesis that the flow structure is important for the separation function and that a stratified structure should be maintained. Numerically, there seem to be a relationship between the estimated number of boxes in a cascade box model, calculated from the measured time-concentration curve (retention time distribution) from a tracer test. Visually, the time-concentration curve displays a high, narrow peak for the stratified flow structure and a low, more spread-out distribution for the short-circuit flow structure. Analytical analysis of the stratified flow structure seems feasible with the cascade box model or the dispersion plug-flow model in the upper layer of the stratification. However, the model can only be used for defining the stratified flow structure.

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