MIL-HDBK-1138
downward or upward between the plates; in the configuration
shown, wastewater flows downward through the plates. As this
happens, the oil droplets float upward and collect on the
underside of adjacent plates where they coalesce. The coalesced
oil droplets move up the plates and are retained in the separator
to form a floating layer that is skimmed from the surface of the
tank. Settled solids from the wastewater collect on the top side
of adjacent plates, migrating down the plates and dropping into
the bottom of the CPI vessel. In the diagram shown, treated water
flows down through the plates, and over a weir into an effluent
flume. Some manufacturers use different configurations than the
one shown.
CPI separators are smaller and easier to cover for
controlling atmospheric emissions, and they may be less expensive
than API-type separators. In practice, however, the smaller size
has sometimes been a disadvantage since it may not provide
sufficient volume to accommodate slugs of oil and it may not
provide sufficient detention time for breaking emulsions.
In some cases, the plate packs have become severely fouled. CPIs
are usually drained and hosed down routinely to clean the plates.
Operating experience over time will dictate how often this
occurs, but a minimum interval of every 6 months is appropriate.
Dissolved Air Flotation (DAF). DAF is commonly used to
5.2.2
remove oil, grease, and suspended solids from industrial
wastewaters. Typically, gravity oil/water separators are used in
front of flotation units to remove the major fraction of free or
floating oils, so flotation units are usually considered
polishing units.
In the flotation process, small gas bubbles rising
through the wastewater adhere to solids particles and oil
globules, causing them to float to the surface where they are
skimmed off. A diagram of a typical flotation system is presented
in Figure 5. The air bubbles can be added to the wastewater by a
variety of means. Diffused air flotation and induced air
flotation are the two most common types of DAF units. Both types
incorporate a flotation vessel with a baffle to retain floated
oil, an oil-skimming mechanism, and sometimes a bottom-scraping
mechanism to remove very heavy particles that do not float.
Significant mechanical equipment is associated with
these systems and should be maintained according to
manufacturers' directions. Flotation vessel surface skimming
generally is continuous, but settled sludge must be drawn off
manually. The drawoff frequency will have to be determined by
experience but could range from once daily to twice monthly, with
weekly being a reasonable starting point.
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