Oil traps are used in oily wastewater treatment to remove free-floating oils
[24]. Oleophilic (oil-attractive) plates attract and retain small oil droplets under
laminar flow conditions. Stokes’ law, the physical law governing the rise and fall
rate of a droplet or particle in a fluid stream, can predict the settling time of solid
particles and the rise time of oil droplets, which have specific gravities higher or
lower than that of the main fluid. The plates may be made of an oleophilic material,
such as polyethylene, fiberglass, or nylon, to promote coalescence of oil droplets.
For this reason, the units are sometimes referred to as coalescing plate separators.
Parallel or coalescing plate separators are usually recommended only for light oil
loadings when a higher level of oil removal is required and when the wastewater
stream contains low solids concentrations.
Gravity grease traps, centrifugal separators, and rarely electroflotation methods
are also used as the other removal methods for free fats present in the oily wastewater. The majority of oil-water separators used is conventional gravity separators
that rely on the different densities of oil, water, and solids for successful operation.
The wastewater is fed to a vessel sized to provide a quiescent zone of sufficient
retention time to allow the oil to float to the top and the solids to settle to the bottom.
For the construction of gravity grease traps, durable construction materials must be
used because the wastewater of vegetable oil refining industry has high sulfate
content due to the use of acids at some steps of the production. In a recent study,
it was demonstrated that gravity separation procedure, in which the wastewater was
kept in quiescence in a separation column and the aqueous phase was collected from
the bottom, yielded oil and grease removals over 90% and COD removals between
58 and 90% after 24 h [26]. This is a proper procedure as a primary treatment for
vegetable oil refinery wastewaters, and it would improve overall efficiency of
wastewater treatment plant.
3.3.2 Flotation
Flotation processes are designed to remove all suspended particles, colloids, emulsions, and even some ions or soluble organics that can be precipitated or adsorbed on
suspended solids (SS) [27]. It is widely used in the treatment of oily wastewaters. In
practice, the natural flotation process differs from the flotation with air. In the first
case, the material particles lighter than the water (oils, greases) eventually associated
with gas bubbles have the tendency to rise to the stationary liquid surface. In the
flotation process with air, the material particles heavier than the water are transported
to the liquid surface by attaching them with air bubbles [28]. In the second case, the
air could be introduced in water in very fine bubbles by the mechanical agitation
flotation with dispersed air [29]. The separation of materials lighter than water, e.g.,
edible oil/fat, can be enhanced by using flotation. This technique does not facilitate
the separation of emulsified fat-oil-grease (FOG) from wastewater and, hence, is
used in the vegetable oil refining sector for the removal of free FOG [11].
Flotation requires the generation of small bubbles which can be produced by
dispersing air into the wastewater, applying a vacuum to the wastewater, dissolving
340
D. Kaya and Y.-T. Hung
[24]. Oleophilic (oil-attractive) plates attract and retain small oil droplets under
laminar flow conditions. Stokes’ law, the physical law governing the rise and fall
rate of a droplet or particle in a fluid stream, can predict the settling time of solid
particles and the rise time of oil droplets, which have specific gravities higher or
lower than that of the main fluid. The plates may be made of an oleophilic material,
such as polyethylene, fiberglass, or nylon, to promote coalescence of oil droplets.
For this reason, the units are sometimes referred to as coalescing plate separators.
Parallel or coalescing plate separators are usually recommended only for light oil
loadings when a higher level of oil removal is required and when the wastewater
stream contains low solids concentrations.
Gravity grease traps, centrifugal separators, and rarely electroflotation methods
are also used as the other removal methods for free fats present in the oily wastewater. The majority of oil-water separators used is conventional gravity separators
that rely on the different densities of oil, water, and solids for successful operation.
The wastewater is fed to a vessel sized to provide a quiescent zone of sufficient
retention time to allow the oil to float to the top and the solids to settle to the bottom.
For the construction of gravity grease traps, durable construction materials must be
used because the wastewater of vegetable oil refining industry has high sulfate
content due to the use of acids at some steps of the production. In a recent study,
it was demonstrated that gravity separation procedure, in which the wastewater was
kept in quiescence in a separation column and the aqueous phase was collected from
the bottom, yielded oil and grease removals over 90% and COD removals between
58 and 90% after 24 h [26]. This is a proper procedure as a primary treatment for
vegetable oil refinery wastewaters, and it would improve overall efficiency of
wastewater treatment plant.
3.3.2 Flotation
Flotation processes are designed to remove all suspended particles, colloids, emulsions, and even some ions or soluble organics that can be precipitated or adsorbed on
suspended solids (SS) [27]. It is widely used in the treatment of oily wastewaters. In
practice, the natural flotation process differs from the flotation with air. In the first
case, the material particles lighter than the water (oils, greases) eventually associated
with gas bubbles have the tendency to rise to the stationary liquid surface. In the
flotation process with air, the material particles heavier than the water are transported
to the liquid surface by attaching them with air bubbles [28]. In the second case, the
air could be introduced in water in very fine bubbles by the mechanical agitation
flotation with dispersed air [29]. The separation of materials lighter than water, e.g.,
edible oil/fat, can be enhanced by using flotation. This technique does not facilitate
the separation of emulsified fat-oil-grease (FOG) from wastewater and, hence, is
used in the vegetable oil refining sector for the removal of free FOG [11].
Flotation requires the generation of small bubbles which can be produced by
dispersing air into the wastewater, applying a vacuum to the wastewater, dissolving
340
D. Kaya and Y.-T. Hung