Importance of Bubble Bed Characteristics in Dissolved-air-flotation

Abstract

Dissolved Air Flotation (DAF) is a water treatment process widely employed in various facilities such as water treatment plants, sewage treatment plants, and seawater desalination plants. In this process, tiny bubbles are released, which adhere to suspended particulates causing them to float to the surface where they may then be removed by a skimming device. Although the removal efficiency of DAF can be improved by increasing bubble bed depth, not much is known about the effect of the characteristics of the bubble bed on the removal efficiency. Therefore, the effect of bubble bed depth and compactness on the removal efficiency was investigated in this study. The results showed a trend of increasing bubble bed depth and compactness with increases in pressure and recycle ratio. In particular, the change in depth was mainly attributable to recycle ratio, and that in compactness was mostly due to pressure. As the depth and the compactness of bubble bed increased, the removal efficiency also increased up to 99% at maximum and there was a large difference in the removal efficiency of tiny particulates < 30 μm. At the same depth, removal efficiency increased with increases in compactness. At higher loading rates (over 10 m3/m2·h), greater removal efficiency was obtained with deep beds. The depth of the bubble bed however did not play a significant role at lower loading rates (5 m3/m2·h). Under this experimental condition, when the loading rate doubled, the bubble bed also needed to be doubled to achieve the same removal efficiency. In cases where it was difficult to increase the bubble bed, the compactness was increased instead by increasing pressure.