Conventional Gravity Separators

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MIL-HDBK-1005/16

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:

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.

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

specific intervals. Flights have been wooden in the past but are