This papers presents the results of two studies performed to measure the size distribution of bubbles at different locations in dissolved air flotation tanks. Measurements were performed at the Albert Water Treatment Works and in two separate tanks at the Graincliffe Water Treatment Works (Yorkshire Water). At Albert, and in one of the tanks at Graincliffe, bubbles were generated using conventional pressure reduction nozzles. Bubbles in the second tank at Graincliffe were generated using needle valves. Bubble size distributions are presented and variations in the size distributions at different locations in the flotation tanks are explained. It is observed that bubble clustering plays an important role in dissolved air flotation and that smaller bubbles are preferentially incorporated into these clusters. Macrobubble formation by the needle valves reduces the efficiency of the flotation process. It is concluded that flotation is enhanced by the use of smaller bubbles.

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