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API Separator

API Separator

  • API Separator

    The API oil-water separator is a gravity separation device designed by using Stokes Law to define the rise velocity of oil droplets based on their density and size. The design of the API oil-water separator is based on the specific gravity difference between the oil and the wastewater because that difference is much smaller than the specific gravity difference between the suspended solids and water. Based on that design criterion, most of the suspended solids will settle to the bottom of the separator as a sediment layer, the oil will rise to top of the separator, and the wastewater will be the middle layer between the oil on top and the solids on the bottom. Typically, the oil layer is skimmed off and subsequently re-processed or disposed of, and the bottom sediment layer is removed by a chain and flight scraper (or similar device) and a sludge pump. The water layer is sent to further treatment consisting usually of a dissolved air flotation (DAF) unit for further removal of any residual oil and then to some type of biological treatment unit for removal of undesirable dissolved chemical compounds.


\Parallel plate separators are similar to API separators but they include tilted parallel plate assemblies (also known as parallel packs). The underside of each parallel plate provides more surfaces for suspended oil droplets to coalesce into larger globules. Any sediment slides down the topside of each parallel plate. Such separators still depend upon the specific gravity between the suspended oil and the water. However, the parallel plates enhance the degree of oil-water separation. The result is that a parallel plate separator requires significantly less space than a conventional API separator to achieve the same degree of separation.

API oil/water separators are designed to remove oil droplets down to 150 micron, which is not nearly enough removal necessary to stay compliant today.  Most API oil/water separators are not going to do any better than 50 parts per million as opposed to most state & federal regulations that today require below 20 ppm.

Most industries have abandoned building these large gravity settling vessels because they are simply too costly to justify. Most are built of concrete and with the high cost of labor and the need to reinforce these tanks with steel, new construction costs rise quickly. Plus API units have an excessively large footprint and are far too much of a real estate burden to consider this as a viable option.

Those who have API oil/water separators acknowledge they are easy to operate and maintain on a day-to-day basis, but when it is time to clean out the tank, the cost to pump out the sludge and clean this large space is just a nightmare. Some units have a chain and scrapper mechanism. This helps move the solids to one side to be pumped out, but the automation piece for these are usually as much as the construction of the pit itself—and require significant maintenance as well.