MIL-HDBK-1138
5.3
Problems with Oil/Water Separator Applications.
Oil/water separators are used to remove small amounts of oil and
other petroleum products from wastewaters prior to further
treatment. The military has historically purchased and installed
gravity oil/water separators under the assumption that it has
only an oil/water separation problem to solve. However, the most
common military applications seldom involve simple oil-and-water
mixtures.
Waste streams generated from military applications
frequently contain significant quantities of dirt, cleaning aids
(detergents, solvents), fuels, flotable debris, and various other
items common to military equipment and activities. Oil/water
separators are not designed to separate these other products.
An oil/water separator must not be used as a catch-all for wastes
generated from any maintenance activity, and maintenance
personnel need to be made aware of this fact. Improper use can
result in illegal discharges of hazardous substances to
stormwater systems or WWTPs. Even when discharges are not
illegal, misuse of these systems can upset treatment plants,
cause discharge permit violations, increase sludge disposal
costs, and eliminate beneficial reuse of wastewater or sludge.
The following factors directly affect the efficiency,
use, and management of oil/water separators: frequency and
intensity of influent flow, design capacity, emulsifying agents,
periodic maintenance practices, type of separator system, and
other contaminants contained in the waste stream. Installation
personnel should be familiar with these factors so they properly
design, select, install, operate, and maintain these systems.
A separator that is being used improperly should be reported to
the environmental office.
Frequency and Intensity of Flow. The longer the
5.3.1
residence time of the waste stream in the oil/water separator,
the more efficient it will be at separating oil. Contaminated
water enters a receiving chamber of the separator where the flow
velocity of the wastewater is reduced, thereby allowing heavy
solids to settle while larger oil droplets float to the top of
the compartment. Further separation continues in a separation
chamber where smaller droplets of oil separate from the water and
join the larger droplets previously separated. The oil layer
that has accumulated on the top of the water spills over an oil
skimmer into a holding area; the wastewater then flows, or is
pumped, to the stormwater or sanitary sewer system.
A longer separation time increases the efficiency of
the oil/water separator by allowing a greater amount of oil to
rise to the top of the wastewater. Therefore, restricting the
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