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DISSOLVED AIR FLOTATION (DAF)
Flotation is a process used to separate solid or liquid particles from a liquid phase and is achieved by introducing fine air bubbles into the liquid phase. The bubbles attach to the particulate matter and the buoyant force of the bubbles is sufficient to cause the particle to rise to the surface of the reaction vessel. A mechanical surface scraper/skimmer is used to remove the floating material from the surface of the reaction vessel. Flotation is most commonly used as a means of removing small or light particulate matter that is difficult to remove by conventional settlement or a free oil suspension in water. With the DAF process there are two methods utilised to create the necessary fine bubbles:
- Smaller systems pressurise the entire flow (by means of a pump with compressed air introduced at the pump suction). The flow is retained in a tank or pressure vessel for a short period to allow the entrained air to dissolve. The pressurised flow is then admitted (via a pressure reducing valve) to the flotation tank where the dissolved air comes out of solution in the form of minute bubbles throughout the entire volume of liquid.
- With larger systems, a portion of the effluent from the DAF plant (usually 15 to 100%) is recycled, pressurised and semi-saturated with air. This recycled flow is mixed with the unpressurised stream immediately prior to entry to the flotation tank where again the air comes out of solution in the form of minute bubbles causing the attached particulate matter to rise.
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