Environmental Engineering Reference
In-Depth Information
oil-saturated sorbents. Moreover, lighter oils
tend to be released more easily than heavier oils.
Blowers and fans are often used for application
of loose sorbents. However, in windy conditions,
application of clay and vermiculite is particu-
larly difficult. Also, clay and vermiculite are in-
jurious to health if inhaled as they spread in the
surrounding area as dust (EPA
1999c
; Al-Majed
et al.
2012
).
surfactant and, therefore, the dispersant is less
available to interact with oil (Chapman et al.
2007
). Efforts are currently active to increase the
salt content of surfactant in order to reduce its
solubility in sea water (George-Ares et al.
2001
).
Close monitoring of economic implication
of addition of dispersants has to be done before
undertaking dispersant application. Spraying of
dispersants may be decided against in areas close
to aquaculture activities, where dispersant-taint-
ed oil may interfere with spawning of fish. Dis-
persant spraying is often undertaken to prevent
potential damage to coastal amenities, intertidal
marine life, and aquatic birds, even at the cost of
potentially tainting fish stocks. Therefore, the de-
cision on whether dispersants should be applied
or not has to be taken only after weighing care-
fully the cost effectiveness of the operation and
conflicting priorities for protection of different
resources from potential damage (ITOPF
2013
).
Dispersants have been used with varying suc-
cess in real incidents. In case of M/V Red Seagull
(1998) and Sea Empress (1996), successful use
of dispersants was reported, whereas in case of
Natuna Sea incident in Singapore Strait (2000),
it was proved ineffective. In case of M/V Red
Seagull, oil spilled was light to medium Arabian
crude oil that was dispersed readily on applica-
tion of dispersants, whereas Nile Blend crude
oil spilled in Natuna incident had high viscosity.
Moreover, the weather conditions (calm weather
and little wave energy) at Natuna oil spill further
exacerbated the situation. Thus, it can be inferred
that successful chemical dispersion results from
accurate understanding of components of spilled
oil, weather, and sea conditions (Chapman et al.
2007
).
Dispersants suffer from limitations and, hence,
their use has to be carefully planned and judi-
ciously controlled. The effectiveness of disper-
sant application needs to be carefully examined
and should be immediately stopped once it is no
longer effective. Submerged flow-through sys-
tem using ultraviolet fluorescence spectrometry
(UVF) and
in situ
florometry can be used to mon-
itor oil concentration and confirm visual observa-
tion (successful chemically dispersed oil yields
a brown-colored plume) about amenability of oil
to chemical dispersion. Currently dispersants are
9.4.2
Chemical Approaches
Dispersants are mixture of surfactants and sol-
vents, which when sprayed on oil slick accelerate
the rate of natural dispersion (EPA
1999a
). They
are capable of rapidly removing medium- and
light-weight oil from the sea surface to the water
column, wherein by the action of wave energy,
oil slick breaks into minute droplets and gets di-
luted rapidly. Microbial action is at the oil-water
interface—dispersion of oil into minute droplets
increases the area of microbial colonization. Mi-
croorganisms degrade the oil droplets and prevent
formation of water-in-oil emulsions. Dispersants
are most effective when applied immediately
after oil spill, before the evaporation of lightest
components of oil. Wax and asphaltene content
of the oil affect the manner of weathering, emul-
sification, and dispersion of oil in sea (Strm-
Kristiansen et al.
1997
). Therefore, knowledge
of properties of spilled oil and how they change
with weathering is an important factor for deter-
mining the use of dispersants (Chapman et al.
2007
). The state of sea and probable weather
conditions in the approaching weeks after the oil
spill also affect the success of oil spill cleanup by
dispersants. Rough and windy seas tend to inhibit
interaction of oil and dispersant, as they are often
overwashed by waves. Thus, dispersants unable
to interact with oil are washed or blown off into
sea. Application of dispersants in calm conditions
is effective only if the wave energy is predicted
to increase with reasonable time period (Nedwed
et al.
2006
). Use of dispersants is generally dis-
couraged in unique ecological areas, such as Bal-
tic Sea, where sea is shallow and is characterized
by low-water exchange and low salinity (35‰).
Low salinity encourages increased solubility of
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