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The type of O/WS most frequently used by the Air Force is the gravity separation system. The performance of gravity separation systems is a function of the relatively low water solubility of petroleum products in water and their different specific gravities. (NOTE: The specific gravity of a petroleum product is defined as its density divided by the density of water. Since the density of petroleum products is less than that of water, they will float.) Solids, if present in the waste stream, will generally collect at the bottom of the O/WS holding tank and can be periodically removed when the tank is drained for maintenance.
A drain connected to an oil/water separator may be perceived as a convenient place to dispose of any type of liquid waste or sludge. This erroneous assumption can result in illegal discharges of hazardous substances to installation sewer systems (which eventually discharge to surface waters) or wastewater treatment plants. The illustration in Figure 1 shows, in simplified form, the operation of a typical gravity O/WS system.
Occasionally, simple gravity type O/WSs do not remove enough oil for the resulting wastewater to meet regulatory discharge requirements. In these cases, coalescing oil/water separators, which are essentially enhanced gravity-type O/WSs, are needed to achieve greater separation efficiency.
According to Stoke's Law, a 100-micron diameter oil droplet will rise approximately 6 inches in water every ten minutes. A 20-micron diameter oil droplet will take over two hours to rise the same distance. Because an oil droplet must rise approximately 48 inches to reach the water surface in a typical gravity- type oil/water separator, smaller droplets may pass through uncollected. Coalescing (binding together) the smaller oil droplets makes them larger and more buoyant, causing them to rise faster. Coalescing oil/water separators may use inclined plates placed within the separation chamber, which provide only a short vertical distance (1/4") for the small droplets to travel before they encounter a fixed surface. Here they can coalesce with other droplets and continue to rise along the plates to the water's surface. Another coalescing method uses a filter made of fine oleophillic (oil "loving") fibers such as polypropylene. The fine oil droplets attach to the fibers as the wastewater flows through. As the droplets get larger, they become buoyant enough to detach from the fibers and rise to the surface, where they can be collected.
What is an oily water Separator ،