Separation characteristics of inorganic particles from rainfalls in dissolved air flotation: A Korean perspective

Many reservoirs have been constructed in Korea to store water resources utilizing the terrain of the land. In general, dam source waters contain algae species that have densities close to that of water. Consequently algae are difficult to remove by conventional gravity sedimentation (CGS), while dissolved air flotation (DAF) is known to be an effective process for the purpose. The same source waters usually also have high turbidities due to mineral soil particles in the wet summer season. Systematic studies on the effect of high turbidity on the DAF process are very limited. In this work, DAF and CGS experiments were carried Out to investigate water treatment characteristics and removal efficiencies under various COD/SS and chlorophyll-a/SS ratios. A kinetic DAF process model was employed to describe bubble-floc collision and agglomeration, as well as the rising velocity of bubble-floc agglomerate. Our results showed that the initial collision-attachment efficiency for the clay floc size range of 100 - 400 mu m was a relatively low value of 0.3. The removal efficiency by DAF was greater than by CGS when chlorophyll-a/SS ratio was high. It was also found that sedimentation prior to flotation is required for the effective separation of large clay floes caused by runoff. Our experimental and theoretical results also suggest that the DAF process requires carefully operation in Korea, especially, in the rainy summer season.