Most oils are insoluble in water, aiding in their separation. Furthermore, most insoluble oils are lighter than water, and therefore they float on its surface. A few oils are heavier than water, and these settle to the bottom of water.
Wastewater treatment facilities must also make separate provisions to separate and collect these heavy oils.
Oils that create the greatest problem are those whose density is close to that of water. These oils separate from water slowly—in some cases, too slowly for normal gravitational separation to be effective.
These soluble oils can be naturally soluble but are rendered soluble or miscible due to man's activities. Oils can be rendered miscible by adding detergents and emulsifiers, or using mechanical processes that result in homogeniza-tion. In all cases, soluble and miscible oils are in the same category for treatment.
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