The relationship between the deformation of bubbles and effective stress through the solid phase in a froth layer of dissolved air flotation, and the transmission efficiency of the effective stress are experimentally and theoretically investigated by use of waste activated sludge. As conclusions: (1) The bubbles are deformed from sphere to ellipsoid of revolution by the difference between the horizontal and vertical stresses. Greater deformation takes place for larger bubbles under higher stress conditions. (2) The froth shows a certain extent of solid-like characteristics. The effective stress in froth, therefore, has an anisotropic characteristic. The ratio of the vertical and horizontal components of the effective stress decreases with the increase in liquid fraction. (3) The transmission efficiency of effective stress by the solid phase is 1 for the bubbles with diameter less than 200µm, and decreases with increasing diameter for bubbles with diameter more than 200µm; and (4) Dissolved air flotation is carried out with high efficiency by use of bubbles with diameter less than 200µm.

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