API (American Petroleum Institute) Separators are frequently used in the treatment of refinery wastewater that has been contaminated by oil and oil-bearing sludge. Separators use the difference in specific gravity to allow heavier liquids to settle below lighter liquids. The lighter liquid is skimmed off, while the heavier liquid remains behind.
In a typical API separator , wastewater is first collected in a pretreatment section that allows sludge removal. A diffusion barrier slowly allows the wastewater to flow down the separator towards the outlet while the lighter oil fractions can be skimmed off.
Downstream, other processes are used to more completely remove entrained oil in all forms and to condition the water to meet the specifications for release into a stream or body of water.