7.3 Pollution by Petroleum in Oceans and Seas: Role of Microorganisms in Oil Degradation and Remediation 185
(c) Dispersants
The adverse economic and environmental
effects of offshore oil spills are greatest when
the oil slick reaches the shoreline. On account
of this, much effort is put into preventing off-
shore oil spills from reaching the shoreline. In
calm seas, use of skimmers and booms to col-
lect the oil at sea is the conventional method of
cleanup and recovery. In situ burning (see
below) is also used in such a situation. In rough
seas, skimming or burning the oil is not effective,
and the use of chemical dispersants appears to be
the most suitable for cleaning up the oil spill.
Dispersants are chemicals that promote the
formation of tiny oil droplets, and delay the ref-
ormation of slicks. They contain surfactants
and/or solvent compounds that cause the oil
sleek to break into small droplets in a process
known in oil parlance as dispersion. After oil
spills, oil droplets break down because of waves
and currents during the dispersion process.
Water and oil droplets then combined to form
water-in-oil emulsions, which have high viscos-
ity. Dispersants inhibit emulsion formation and
promote oil dispersion. Thus, they can remove
the spilled oil from the water surface and reduce
the impact to environment, especially to the
shoreline and sensitive habitats. The generated
small oil droplets get transported or transferred
into the water column due to wave action and
sea turbulence. They subsequently move away
from the contaminated area in accordance with
the prevailing currents. They can then more eas-
ily adsorb onto suspended particulate matter
and/or more easily biodegrade. After an oil spill,
oil droplets break down because of waves and
currents during the dispersion process.
Very light oils are, however, not easily dis-
persible because the formed droplets have to be
very small to overcome buoyancy. For this rea-
son, a high dosage of dispersant is required to
cause the formation of such small droplets in
very light oils. Very heavy oils on the other
hand are much more resistant to dispersion
because their high viscosity prevents the disper-
sant from penetrating them. Such penetration is
necessary to produce dispersed oil droplets.
In very calm seas, the applied dispersant is
not very effective as it tends to run off the oil
and gathers in small pools within the slick. In
very rough seas the use of large amounts of dis-
persants might not be needed because a high
degree of dispersion occurs naturally due to the
mixing effect of the waves.
The evaluation of dispersant effectiveness
used for oil spills is commonly done using tests
conducted in laboratory flasks, which attempt
to mimic the conditions at sea using tests such
as the swirling flask devised by the US EPA.
The advantages of dispersants over other
types of remediation methods are that they can
be used over a wide area through the use of air-
craft and also in remote areas. Furthermore, in
instances where other methods do not work
well such as the use of booms in high seas, dis-
persants work excellently. By dispersing the oil
it is brought in closer contact with microorgan-
isms, which biodegrade it.
On the other hand, many dispersants are
toxic and may have lethal or long-term effects
on plankton, animals, or fish. They are ineffec-
tive on oils which are viscous because of their
content of the heavier fractions of oil. Finally,
they are not very effective in calm waters.
(d) In situ burning
In in situ burning of oil, the controlled burning
of an oil spill takes place on the water’s surface.
It requires minimal equipment although some
specialized equipment and training is required.
In burning oil, no need exists for collecting and
transporting recovered oil. The burning leads to
the production of green house gases and may
leave toxic residues in the water. Because the
oil is gasified during combustion, the need for
physical collection, storage, and transport of
recovered product is reduced but can cause air
pollution. At times, burning also leaves a toxic
residue on the surface of water, thereby causing
more pollution, rather than removing it from
the natural environment.- Biological methods of remediation (bioremediation)
Bioremediation is the process of using biological
means in remediating or ameliorating the effects of
oil pollution. In general terms, it is the remediation
of any pollution by the use of microorganisms in
transforming harmful organic compounds, such as
oil, into nontoxic and less dangerous compounds.
Seawater contains a range of microorganisms that