MIL-HDBK-1138
5.2.1.1
Conventional Gravity-Type Separators.
Conventional gravity separators are typically rectangular
in-ground or above-ground tanks with maximum widths of 20 feet
(6 m). A diagram of a typical conventional gravity separator is
presented in Figure 3. Influent and effluent channels are
normally located on opposite ends of the separator. The influent
typically passes an inlet section that contains a slotted baffle
to distribute influent evenly throughout the depth of the
separator. For units without sludge collectors, there may also
be a bottom baffle in the separator; this retains settled solids
in the front part of the separator to reduce cleaning
requirements. Other separators may have automatic sludge removal
equipment that will rake accumulated sludge to a sludge hopper
where it can be pumped from the tank periodically.
In either case, the sludge level should be monitored
routinely, and sludge should be removed when it occupies 10
percent or more of the separator volume. All conventional
gravity separators have a surface baffle at the outlet end to
retain floating oil and grease. The grease is removed by pumping
or by activating a rotary drum or slotted pipe that allows the
surface material to drain to a drum or oil holding tank. The
depth of the surface oil layer should be checked regularly, and
surface skimming should be conducted routinely. Experience
gained from operating a conventional gravity separator in a
specific application will indicate the required intervals for
checking and skimming the oil layer. For example, although the
oil layer might need to be checked and skimmed daily, this
interval could range from several times per day to several times
per month, depending on the rate of oil accumulation on the
separator surface.
One criterion to use is that the oil layer should be
monitored and skimmed as often as is necessary to prevent an
excess amount of oil from being flushed through the separator by
an unexpected hydraulic surge (e.g., rainfall). Thus, the
frequency may also depend on the sensitivity of downstream
processes to increased oil loading. The frequency will have to
be determined by experience but it likely will be such that the
floating oil layer does not exceed about 2 inches (5 cm) (some
operators prefer that there be no floating oil layer on the
separator).
Corrugated Plate Interceptors. CPIs are typically
5.2.1.2
supplied by vendors and are based on proprietary designs. A CPI
consists of a tank containing a number of parallel corrugated
plates mounted from 0.8 to 1.6 inches (2 to 4 cm) apart and
inclined at an angle to the horizontal. A diagram of a typical
CPI is presented in Figure 4. Wastewater may flow either
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