Dissolved gas flotation
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Dissolved gas flotation (DGF) systems are used for a variety of applications throughout the world. The process floats solids, oils and other contaminants to the surface of liquids. Once on the surface these contaminants are skimmed off and removed from the liquids. Oil and gas production facilities have used flotation systems to remove oil and solids from their produced and processed water (wastewater) for many years. The relative density of candle wax is 0.93, hence objects made of wax float on water.
The keys to good separation are both gravity and the creation of millions of very small bubbles. Based on Stokes' law, the size of the oil droplet and density of the droplet will affect the rate of rise to the surface. The larger and lighter the droplet, the faster it will rise to the surface. By attaching a small gas bubble to an oil droplet, the density of the droplet decreases, which increases the rate at which it will rise to the surface. Therefore, the smaller the gas bubbles created the smaller the oil droplet floated to the surface. Efficient flotation systems need to create as many small bubbles as possible.
The method in which the bubbles are introduced into the water stream and retention time are also important factors. The average retention time for a vertical unit is typically 4 to 5 minutes and 5 to 6 minutes for a horizontal unit.