The flotation process is useful for removing solids, oils, and grease from contaminated water. Dissolved air is released into the influent contaminated water that enters the flotation tank. The dissolved air bubbles attach to solid particles, oil droplets, or grease, thereby decreasing their specific gravity. Through buoyancy, these materials float to the surface of the tank where they can be skimmed. Figure 13  illustrates a typical dissolved air flotation unit.
Removal of suspended solids using the dissolved air flotation process ranges from 40 to 65% without the use of chemicals and 80 to 93% with chemicals such as alum, ferric chloride, or polymers. Oil and grease removal ranges from 60 to 80% without chemical addition and 85 to
95% with chemical addition. Common surface loading rates range from 500 to 8000 gal/day/ ft2. Imperial Oil Company, Inc. presented a case history for its treatment of storm runoff by dissolved air flotation [44,45].
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