Optimization of Solids Separation in Dissolved Air Flotation

Sizes of flocs were analyzed to identify characteristics of the particle size distribution optimal for separation by dissolved air flotation (DAF). Optical microscopes and two particle counters were used for floc sizing. A Brightwell Technologies particle counter was found to provide floc size measurements in agreement with improved microscopic methods. The particle counter provided distribution of flocs with sizes down to 1 micron (µm). This allowed for inclusion of flocs with size ranging from 5 to 1 µm, which were excluded from the analyses in the earlier study. Four alum dosages were applied: 15, 25, 40, and 60 mg/L. The turbidity and colour of the DAF effluent at alum dosages of 25, 40, and 60 mg/L were very similar. However, the analysis of the flocs in the treated effluent revealed that, at the alum dose of 60 mg/L, particle removal was the best. Therefore, this dosage was selected as optimal for the solid/liquid separation process. The average size of coagulation flocs at 60 mg/L was approximately 30 µm, and was equal to the estimated size of air bubbles produced by the saturator. Therefore, this study confirms the finding of the earlier work claiming that the optimum DAF performance is attained when the mean floc size and the bubble size are equal. Similar size of floc and bubble indicates that flocs act predominantly as nuclei for bubble formation. This finding contributes to the knowledge of mechanisms of floc air bubble attachment in DAF.