MIL-HDBK-1005/9A
effluent are summarized in Table 12. Gravity separators alone
will not remove emulsified oils. The methods described in
paragraph 3.9.5 are required to break emulsions.
3.10.1.2 Equipment. Consider wastewater flow rate, oil droplet
size and distribution, the concentration of oil and grease and
suspended solids, differences in specific gravities of wastewater
components, and operating temperatures in selection and
specification of equipment. Gravity separators are usually used
as pretreatment unit processes since effluent quality fluctuates
significantly due to large variances in the quality of the
influent wastewater. Construct separators in parallel to provide
continuity of operation during individual unit repair, cleaning,
or inspection. If waste volumes are small and adequate off-line
holding tank capacity is provided, a single separator may be
used.
3.10.1.3 Equalization (Surge Tank). Provide side holding tank
or basin to equalize and store oily wastewater flows prior to
oil-water separation. Holding facilities can be concrete or
steel tanks. They should be covered or under a roof in a rainy
climate or where wildlife is present.
3.10.1.4 Grit Removal. Use grit removal equipment to remove
solids particles larger than 200 microns such as sand, metals,
and rags prior to entering load equalization tanks.
3.10.2
Load Equalization Tank (LET). The LET is a batch
operated, gravity oil-water separator. Oily wastes are
discharged to the LET for a predetermined collection period.
Wastes are settled, the oil skimmed off to storage, and sludge
withdrawn for further processing and disposal. Clarified water
is passed on for additional treatment or discharge.
a) Provide at least two LETs for sequential fill and
draw operation. Each tank should have a capacity equal to the
average flow for 7 days. Typical settling time is 24 hours.
Sludge should be withdrawn daily. Longer LET operating periods
or large volume upstream receiving tanks should not be used since
they promote anaerobic conditions and H2S gas production.
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