An alternative technique to that of sedimentation is flotation. This uses gas bubbles to increase the buoyancy of suspended solids. The gas bubbles attach to the particles and make their effective density lower than that of the water. This causes the particles to rise through the water to float to the top. Flotation may be achieved by several methods but the most effective form is dissolved air flotation. In this process (Figure 27) air is dissolved in water at elevated pressures and then released as tiny bubbles (30–120 μm) by reducing the pressure to atmospheric level.
The principal advantages of flotation over sedimentation are that very small or light particles that settle slowly can be removed more completely and in a shorter time. Once the particles have reached the surface, they can be collected by a skimmer. Flotation does, however, require careful control to achieve high quality output.