Gravity Separation

are in common use: a) conventional, rectangular-channel units, commonly called API separators because they are usually based upon design standards developed by the American Petroleum Institute; and b) parallel plate separators. In either case, removal is a function of residence time, specific gravity of the oil, oil droplet size, fluid salinity, and fluid temperature.

Well designed and operated API gravity separators are capable of removing oil globules with a diameter greater than 150 microns and achieving effluent levels of free oil as low as 100 mg/L. Parallel plate separators are generally designed to remove oil globules greater than 60 microns in diameter, and can meet effluent limits as low as 50 mg/L of free oil. The total oil content of the effluent will be greater, depending on the amount of emulsified and dissolved oil present. Other factors will also affect the efficiency of oil removal, including oil-specific

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gravity, droplet size, OWS hydraulic retention time, and temperature.

While gravity separators are designed to remove oil, they also function as a sedimentation unit. As a result, solid particles more dense than water will tend to settle out and provisions must be included to remove accumulated solids.

Knowledge of the solids content of the influent wastewater stream is particularly important in the selection of parallel plate separators because they are prone to increased maintenance and clogging problems.

5.5.1.1 Conventional Gravity Separators. A typical

conventional separator system is shown in Figure 11. The separator itself has three chambers separated by baffles: an influent chamber, the main separator chamber, and an effluent chamber. The operation of these chambers is described below:

a) Influent Chamber. The influent chamber is used to

remove free oil that has already separated from the oil/water mixture during conveyance to the unit. Two baffles separate the influent chamber from the larger, main settling chamber. The upper baffle is placed at the top of the water level and extends three quarters of the way to the bottom. It prevents the

floating oil and scum from entering the main chamber, and allows it to be skimmed off through an overflow pipe. The lower baffle extends from the bottom and is used to direct the wastewater to the top of the main chamber and to prevent short-circuiting.

b) Main Separator Chamber. In the main separator

chamber, the oily wastewater flows from one end to the other under quiescent conditions. The wastewater velocity is kept very low, typically less than 3 feet per minute (0.9 m/min) to prevent turbulent mixing. For flat-bottom chambers, removal of settled solids is typically accomplished by taking the chamber out of service; the chamber is drained and accumulated solids are removed either manually or by a vacuum truck. If the floor is sloped, the solids can be removed from the hopper or V-bottom trough by

pumping or gravity discharge while the unit is still in service. Where large amounts of solids are anticipated,

mechanical equipment may be provided to move the solids to the collection point. A chain-drive mechanism is most common. Attached between a pair of chains are crosspieces, or “flights," extending the full width of the tank or bay and spaced at

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now also being constructed of alternative materials. Settled solids are dragged to the solids hopper at one end of the tank and removed.

An oil-skimming device should be provided at the end of the separation chamber. The rotatable, slotted-pipe skimmer is the most common type. Other oil-skimming devices include belt skimmers and floating skimmers. The waste oil collected by the skimmer is discharged to a waste oil holding tank. The tank should be designed so that confined space entry is not required for operation or maintenance.

Figure 11

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c) Effluent Chamber. The effluent chamber is also

separated from the main chamber by upper and lower baffles. Wastewater flows under and over the baffles into the effluent chamber. From the effluent chamber, the treated water can be discharged to the sewerage system or to additional treatment if necessary.

5.5.1.2 Parallel Plate Separators. A typical parallel plate

separator system is shown in Figure 12. Parallel plate separators function on the same principles as conventional gravity separators, but they require less space and are theoretically capable of achieving lower concentrations of effluent oil. The size of the unit is reduced by incorporating an array of closely spaced parallel plates within the separator chamber, thereby increasing the surface settling area. Flow through a parallel-plate unit can be two to three times that of an equivalently sized conventional separator. The oil is removed by passing the wastewater at laminar velocity through the pack of closely spaced plates, which are constructed at various inclines ranging from 45 to 60 degrees. These oil droplets rise and are trapped along the bottom of the plates. The oil droplets coalesce and gradually move upward along the bottom of the plates, eventually collecting at the surface of the tank. The plates aid in separation in the following ways:

a) Preventing short-circuiting of the oily waste

b) Increasing effective settling area

c) Enhancing contact/agglomeration of o il particles

Suspended solids settle to the bottom and are collected in a sludge well. From the well, sludge is pumped or withdrawn by gravity to waste. If sludge transfer is by gravity

displacement, an automatic valve is usually provided. The plates in parallel-plate separators may be made of an oleophilic (oil attracting) material, such as polypropylene, fiberglass, or nylon, to promote coalescence of oil droplets. For this reason, the units are sometimes referred to as coalescing plate

separators. Coalescing separators are usually recommended only for light oil loadings when a higher level of oil removal is required, the wastewater stream contains minimal solids

concentrations, and the facility is committed to the additional maintenance procedures required to keep the coalescing pack free of debris. The plates may also be constructed in a corrugated

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configuration with alternate troughs and ridges, such as in the so-called Corrugated Plate Interceptor (CPI).

Figure 12

Parallel Plate Separator