Density separation is a process whereby the water and contaminant are separated based on their individual densities. This treatment is often used for the pretreatment of suspended solids or floating immiscible products that could be present in pumped groundwater. For suspended solids, the most commonly used equipment is clarifiers, settling chambers, and sedimentation basins as discussed in Chapter 8. For immiscible products, such as oil and grease, the most commonly used equipment is oil-water separators. Both suspended solids and oil and grease must generally be removed from contaminated groundwater prior to further treatment because these materials can foul instruments and interfere with other processes. Furthermore, oil and grease and suspended solids can damage the environment and cause a significant pollution problem to the receiving body of water.
The most common oil-water separators are the American Petroleum Institute (API) gravity separators and the parallel-plate separators. The design of an oil-water separator is based on the amount of oil present in the water, the oil droplet size distribution, the presence of surfactants, the specific gravity of the oil, and the water temperature. A step-by-step procedure for the design of an oil-water separator is in Corbitt (1990). Once the oil or floating product is at the surface, it can be removed from the water by slotted pipes, dip tubes, or belt or rope skimmers.
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