Bioremediation of marine environment
contaminated with petroleum
hydrocarbon
Richard Mannion
Martin Casserly
Martin Gallagher
Table of Contents
•
Introduction
•
Environmental factors
•
Fate of contaminant
•
Chemical composition of petroleum hydrocarbons
•
Hydrocarbon degrading microbes
•
Biodegradation of hydrocarbon components
•
Factors that determine cost of oil spill
•
Two approaches to oil spill bioremediation
Bioremediation is defined as the act of adding or improving the availability of
materials (e.g., nutrients, micro organisms, or oxygen) to contaminated environments
to cause an acceleration of natural biodegradative processes. The results of field
experiments and trials following actual spill incidents have been reviewed to evaluate
the feasibility of this approach as a treatment for oil contamination in the marine
environment. The ubiquity of oil-degrading micro organisms in the marine
environment is well established, and research has demonstrated the capability of the
indigenous micro flora to degrade many components of petroleum shortly after
exposure. Studies have identified numerous factors which affect the natural
biodegradation rates of oil, such as the origin and concentration of oil, the availability
of oil-degrading micro organisms, nutrient concentrations, oxygen, climatic
conditions, and sediment characteristics.
Total petroleum hydrocarbons (TPH) are a term used to describe a large family of
several hundred chemical compounds that originally come from crude oil. Crude oil is
used to make petroleum products, which can contaminate the environment. TPH is a
mixture of chemicals, but they are all made mainly from hydrogen and carbon, called
hydrocarbons. TPH may enter the environment through accidents, from industrial
releases, or as by-products from commercial or private uses. TPH may be released
directly into water through spills or leaks.
Petroleum means "rock oil". The term petroleum is nowadays used as a common
denotation for crude oil and natural gas. Petroleum, then, is a collective term for
hydrocarbons, whether solid, liquid or gaseous. Reserves of natural gas and crude oil
have formed over millions of years as plants and animals have been broken down and
undergone chemical change at high temperature and pressure. That is why oil and
natural gas (and coal) are referred to as "fossil fuels". Petroleum is found in porous
rock-formed in large sedimentary basins, where the oil and gas has been trapped by
some kind of barrier thereby forming a reservoir. Some TPH fractions will float on
the water and form surface films. Other TPH fractions will sink to the bottom
sediments. Some TPH fractions will move into the soil where they may stay for a long
time. It is therefore the work of these microbes to degrade the hydrocarbons.
Altogether, more than 70 microbial genera are known to contain organisms that can
degrade petroleum components. The micro organisms transform contaminants to less
harmful compounds through aerobic and anaerobic respiration, fermentation, co
metabolism and reductive dehalogenation. There are certain environmental factors
that affect the bioremediation processes such as temperature, oxygen, pH, turbulence,
background concentration of inorganic nutrients and the type of contaminants. The
resulting products can be carbon dioxide, water, and partially oxidized biologically
inert by-products. Certain enzymes produced by microbes attack hydrocarbons
molecules, causing degradation. The degradation of oil relies on having sufficient
microbes to degrade the oil through the microbe’s metabolic pathways. If microbes
are not present in a system they can be added to help promote bioremediation. The
added microbes can be cultures grown from other contaminated areas or they can be
microbes genetically engineered to degrade oil. However, even when these microbes
are present, degradation of hydrocarbons can take place only if all other basic
requirements of the microbes are met.
Environmental factors
Carbon
Carbon is the most basic structural element of all living forms and is needed in greater
quantities than other elements. The nutritional requirements of carbon to nitrogen are
10:1 and carbon to phosphorus 30:1 (Atlas and Bartha 1981, p.70). Reduced organic
carbon is a source of energy for microbes because it has high energy yielding bonds in
many compounds. In the decomposition of oil, there is plenty of carbon for the micro
organism due to the structure of the oil molecule.
Nitrogen
Nitrogen is found in the proteins, enzymes, cell wall components. (Atlas and Bartha
1981, p.70).The microbes must be supplied with nitrogen in some form. Without
sufficient nitrogen the metabolism of the microbes will be altered. The main source of
nitrogen is from the atmosphere. Only a few microbes can use this molecular nitrogen
so most micro organisms require fixed nitrogen such as organic nitrogen, ammonium
ions or nitrate ions.
These other forms of nitrogen can be scarce in certain
environments, causing nitrogen to become a limiting factor in the growth of microbial
populations.
Oxygen
Biodegradation is predominantly an oxidation process. Not only do the aerobic
microbes need the oxygen to survive, bacteria enzymes catalyze the insertion of
oxygen into the hydrocarbon so that the molecule can subsequently be consumed by
cellular metabolism. Because of this, oxygen is one of the most important
requirements for the biodegradation of oil. There is usually enough oxygen to prevent
a lack of it from limiting biodegradation. The primary source of oxygen for
biodegradation is atmospheric oxygen. When oxygen is limiting the water can be
aerated.
Water
Water is an important factor as it is needed by the microbes. It makes up a large
portion of the cells cytoplasm and because most enzymatic reactions take place in
solution. Water is also needed for transport of most materials into and out of the cell.
Water is not a limiting factor in marine oil spills, but in bioremediation of oil spilled
on land it may be an important factor which needs to be controlled.
Fate of contaminant
When petrol contaminants enter an aquatic system they are subject to some physical,
chemical and biological changes. These changes contribute to the loss or alteration of
some of the components.
Toxicity
Some oils have toxic hydrocarbons. These may prevent or delay microbial attack and
can slow down the process of degradation. Under favourable conditions microbes will
degrade 30-50% of a crude oil residue. However with favourable conditions and the
proper microbes all type of crude oils can be broken down, straight chained, branched
chained, crylic, aromatic, polynuclear aromatic. In most natural ecosystems the
numbers of hydrocarbon utilising microbes will initially limit the rate of hydrocarbon
degradation. After a short period of exposure to the petroleum pollutant the number of
utilising microbes increases and is no longer a problem.
Sorption
Solid surfaces can act by adsorption which is the retention of solutes in solution by
the surface of the solid material, or by absorption which is the retention of the solute
within the mass of the solid rather then on its surface. Sorption reflects the rate of
volatilisation. Adsorption may involve van der walls forces or hydrogen bonding.
Cationic molecules may be absorbed to the ion exchange sites of clay minerals or
organic matter present.
Volatilisation
Some hydrocarbons are very volatile and are lost to the atmosphere by evaporation.
Evaporation is the most important natural cleansing process during the early stages of
an oil spill, and it results in the removal of lighter-weight components in oil.
Depending on the composition of the oil spilled, up to 50 percent of the more toxic,
lighter weight components of oil may evaporate within the first 12 hours following a
spill (U.S. EPA, 1999). In terms of environmental impacts, evaporation is the most
important weathering process during the early stages of an oil spill in that it can be
responsible for the removal of a large fraction of the oil including the more toxic,
lower molecular weight components.
For oil on water, evaporation removes virtually all the normal alkanes smaller than
C15 within 1 to 10 days. Volatile aromatic compounds, such as benzene and toluene,
can also be rapidly removed from an oil slick through evaporation. However, these
volatile oil components may be more persistent when oil is stranded in sediments. The
volatile components make up 20-50% of most crude oils, about 75% of No. 2 fuel oil,
and about 100% of gasoline and kerosene. As a result, the physical properties of the
remaining slick change significantly (e.g., increased density and viscosity). Major
factors influencing the rate of evaporation include composition and physical
properties of the oil, wave action, wind velocity, and water temperature.
Tempeture
Hydrocarbon degradation has been found to occur at a wide range of temperatures.
Degradation has occurred at tempetures as low as 0oc and as high as 700c.It is an
important factor on the rate of biodegradation (Atlas 1981, p.190).It is so important
because at low temperatures, molecules move relatively slowly and colliding
molecules do not always bring about a reaction" (Atlas 1984, p.339). Raising the
temperature will increase the possibility of reactions taking place. In general the rate
of enzymatic reactions can be doubled for every 10°C. The more enzymatic reactions
the faster the biodegradation will occur. Even though temperature plays an important
part in the rate of biodegradation, it does not act alone. Concentration of the
hydrocarbon is also an important factor.
Spreading
The spreading of oil on water is one of the most important processes during the first
hours of a spill, provided that the oil pour point is lower than the ambient temperature.
The principal forces influencing the spreading of oil include gravity, inertia, friction,
viscosity and surface tension. This process increases the overall surface area of the
spill, thus enhancing mass transfer via evaporation, dissolution, and later
biodegradation.
Dispersion
Dispersion, or formation of oil-in-water emulsions, involves incorporating small
droplets of oil into the water column, resulting in an increase in surface area of the oil.
In general, oil-in-water emulsions are not stable. However, they can be maintained by
continuous agitation, interaction with suspended particulates, and the addition of
chemical dispersants. Dispersion may influence oil biodegradation rates by increasing
the contact between oil and the microbes.
Emulsification
The process of emulsification of oils involves a change of state from an oil-on-water
slick or an oil-in-water dispersion to a water-in-oil emulsion, with the eventual
possible formation of a thick, sticky mixture that may contain up to 80% water,
commonly called “chocolate mousse”. The formation and stability of emulsions are
primarily related to the chemical composition of the oils and are enhanced by wax and
asphaltic materials. Surface-active materials generated through photochemical and
biological processes are also involved in formation of the emulsions. The formation of
emulsions makes oil clean-up operations more difficult by decreasing the
effectiveness of physical oil spill recovery procedures and suppressing the natural
rates of oil biodegradation.
Photo oxidation
Photo oxidation occurs when oxygen under sunlight reacts with oil components.
Photo oxidation leads to the breakdown of more complex compounds into simpler
compounds, which tend to be lighter in weight and more soluble in water, allowing
them to be removed further through other processes.
Concentration
The concentration of pollutants is an important factor. If the concentration of
petroleum hydrocarbons is too high then it will reduce the amount of oxygen, water
and nutrients that are available to the microbes. This will create an environment where
the microbes are stressed reducing their ability to break down the oil. The
concentration of the hydrocarbon will affect the biodegradability and toxicity to the
degrading organisms. The concentration at which inhibition occurs depends on the
compound. Concentrations in the range of 1 to 100µg/ml of water are not said to be
very toxic. The concentration of the hydrocarbons will have two effects. At low
concentrations all fractions of the hydrocarbon will be attacked. At high
concentrations only fractions susceptible to degradation will be attacked. The
concentration of the contaminant will affect the number of microbes present. It has
been found that high concentrations of gasoline in contaminated water were related to
higher counts of microbes.
Viscosity
Viscosity is the property of the fluid that describes how it resists a change in shape or
movement. The lower the viscosity a fluid has, the more easily it flows. The viscosity
of petroleum is related to oil compositions and the ambient temperature. It is an
important index of the spreading rate of spilled oil.
Solubility
The solubility of oil in water is extremely low and depends on the chemical
composition of the petroleum hydrocarbon in question and temperature. For a typical
crude oil, solubility is around 30 mg/L (NAS, 1985). The most soluble oil components
are the low molecular weight aromatics such as benzene, toluene and xylene. This
property is important with respect to oil fate, oil toxicity and bioremediation processes
Soil sediment
Depending on where the contamination has occurred some of the hydrocarbon may
get washed up along the shore line and become trapped in the sand sediments and in
and between rocks. The type of soil present will determine whether not insitu
treatment is possible. Effective treatment of sub soils requires continual access to
nutrients by microbes to promote growth. Tight soils, those with high clay content,
are more likely to plug up and restrict the free flow of nutrients to the microbes.
However soils with high sand content which are common in a marine environment
allow nutrients and oxygen to flow and are therefore more tractable to bioremediation.
The Chemical composition of petroleum hydrocarbons
Petroleum Hydrocarbon based products are quite common today with many uses in
transport, heating and machinery function. There are three different types of
petroleum hydrocarbons products, they are; Crude oil, petroleum components and
refined oil products.
Crude oil
Crude oil is the most well known for marine destruction due to spills, it is so
destructive as it is extremely thick and heavy and just suffocates the environment
whether it be on the seashore or out to sea.
Crude oil is made up of both hydrocarbon compounds (accounting for 50-98% of its
total formation) and non hydrocarbon compounds (which may contain sulphur
nitrogen oxygen and some trace metals) in a wide range of combinations
Petroleum components
Petroleum components may be broken up into four major groups due to their
differential solubility in organic solvents. They are Saturated Hydrocarbons, Aromatic
Hydrocarbons, Resins and Asphaltines.
Saturated Hydrocarbons
Saturated Hydrocarbons include normal and branched alkanes with aliphatic
structures and cyclic alkanes with alicyclic structures.
Saturated Hydrocarbons can range in chain length from one carbon to over 40
Carbons. Saturates usually are the most abundant material in crude oil.
Aromatic Hydrocarbons
Aromatic Hydrocarbons include monocyclic aromatics examples of such aromatics
include Benzine, Toluene and Xylene and polycyclic aromatic Hydrocarbons (PAH’s)
examples of such PAH’s include Napthalene, Anthracene and Phenanthrene and
which have two or more fused aromatic rings.
PAH’s are of particular environmental concern because of their potential
carcinogenicity or which maybe converted into carcinogens through microbial
metabolic reactions
Resins
Resins include polar compounds containing nitrogen, sulphur and oxygen examples of
these include pyridines and thiophenes and these are often referred to as NSO
compounds
Asphaltines
Asphaltines consist of poorly characterised hydrocarbons with high molecular weight
compounds and possible NSO group connections also metals such as nickel,
vanadium and iron are also associated with asphaltines.
Refined oil products
Refined oil products such as petrol, kerosene, fuel oils, lubricating oils are all derived
from crude oil through certain processes such as catalytic cracking and fractional
distillation. These products have physical and chemical characteristics according to
the type of crude oil used and the process of which the product was formed. In the
catalytic cracking process unsaturated compounds or olefins (Alkenes and CycloAlkenes) which are not generally present in crude oil that can easily are formed. For
example the concentrations of Olefins are as high as 30% in petrol while about only
1% in jet fuel.
Table 1 Chemical compositions of refined petroleum products (adapted from Clark and Brown,
1977)
Distillation
Fraction
Gasoline &
naphtha
Hydrocarbon types
Saturates
olefins
Range of carbon
atoms
Typical Refined
products
04--12
Gasoline
Middle Distillate
Wide cut gas oil
Residium
Aromatics
Saturates
olefins
Aromatics
Saturates
Aromatics
Resins
Asphaltines
10--20
18--45
>40
Kerosene
Jet Fuel
Heating oil
Diesel oils
Wax
Lubricating oil
Residual oils
Asphalt
Hydrocarbon Degrading Microbes
These Micro organisms which are capable of degrading petroleum hydrocarbons and
related compounds are ubiquitous in marine habitats. There are up to 200 species of
bacteria, yeasts and fungi that have been shown to degrade hydrocarbons ranging
from methane to compounds of over a 40 carbon chain length. In the marine
environment bacterial are considered to be the most dominant of the hydrocarbon
degraders with a range of distribution that even covers extreme cold environments
such as the Antarctic and arctic regions.
The Distribution of hydrocarbon degraders or utilizing micro organisms is
predominantly related to historical exposure of the environment to petroleum
hydrocarbons. These environments with a recent or continuous oil contamination will
have a higher concentration of hydrocarbon degraders compared to a low percentage
of hydrocarbon degraders in unpolluted areas. In clean or free from the presence of
petroleum hydrocarbons the percentage of hydrocarbon degraders will make up less
than 0.1% of the total microbial community, while as in areas of oil pollution they can
constitute up to 100% of the viable micro organisms present.
It should be known that there is no single strain of bacteria capable or that has the
metabolic capacity to degrade all the hydrocarbon components. In nature
biodegradation of crude oil or its by-products typically involve a succession of species
within the microbes present.
Micro organisms classifies as they utilise non hydrocarbon that may also play a very
important role in the full removal of petroleum eventually from the environment.
Degradation of petroleum involves a progressive of sequential (in sequence)
reactions, in which certain organisms may carry out an initial attack on the petroleum
compound and turn this produces an intermediate compounds that are subsequently
utilized by a different group of organisms this leads to further degradation and
possible removal after a number of these processes have occurred.
Micro organisms capable of degrading petroleum hydrocarbons
Bacteria
Yeast and Fungi
Achromobacter
Acinetobacter
Alcaligenes
Anthrobacter
Bacillus
Brevibacterium
Cornybacterium
Flavobacterium
Nocardia
Pseudimonas
Vibro
Aspergillus
Candida
Cladosporium
Penicillium
Rhodotorula
Sporobolomyces
Candida
Trichoderma
Biodegradation of Hydrocarbon components
Petroleum components have been classified into four major groups as mentioned
earlier they are Saturates, Aromatics, Resins and Asphaltines.
Saturates
In general the n group alkanes are the easiest degradable component of saturated
hydrocarbons.
Biodegradation of the n group alkanes with molecular weights up to C44 has been
demonstrated while as Alkanes in the C10 to C 26 are considered the most frequently
and readily utilized hydrocarbons.
The predominant mechanism of the n group Alkanes degradation involves the
terminal oxidation to the corresponding alcohol, aldehydes or fatty acid functional
group.
Highly branched iso-prenoid alkanes such as pristine and phytane which were earlier
thought to be resistant to degradation through study have been found to be readily
degradable.
Complex alicyclic compounds such as hopanes and steranes are among the most
persistant petroleum compounds in the environment.
Aromatics
Aromatics are generally more resistant to biodegradation although but some low
molecular weight aromatics such as naphthalene may actually be oxidised before
many saturates.
Mono aromatic hydrocarbons are toxic to some micro organisms due to their solvent
action on the cell membranes, but mono aromatic hydrocarbons in low concentrations
are extremely easily biodegraded through aerobic respiration.
Polycyclic aromatic hydrocarbons with between 2-4 rings are less toxic and
biodegradable with the level of formation complexity.
Polycyclic aromatic hydrocarbons with five or more rings can be degraded only by
co-metabolism, which the microbes fortuitously (accidentally) transform non growth
substrates while metabolising hydrocarbons of a simpler combination or other primary
substrates found within the oil.
The bacterial degradation of aromatics normally involves the formation of a diol
followed by a ringed cleavage and finally the formation of a di-carboxylic acid.
Fungi and other types of eukaryotes normally oxidise aromatics by mono-oxygenases
an enzyme produced that acts somewhat similar to the bacterial metabolism although
a trans-diol is formed instead.
Asphaltines and Resins
Up until very recently very little was known about the biodegradation of both Resins
and Asphaltines, as this was due to their complex structures which were extremely
difficult to analyse although with new technology ever improving this has made it
easier to analyse such complex compounds to gain a better understanding into the
structure and biodegradation of these compounds.
They have been considered to be refractory (stubborn) to biodegradation, however
recently evidence has shown that Asphaltine components can be biodegraded through
co-metabolism.
Some Resins particularly with a low molecular weight Resin fraction can also be
biodegraded at low concentrations.
Summary
To summarise the susceptibility of petroleum hydrocarbons to microbial degradation
generally occurs in the following order:
n-alkanes<branched alkanes<low molecular weight aromatics<cyclic alkanes
although this pattern is not universal. The composition of oils may greatly differ in
heterogeneity (combinations) which in turn will affect the rate of biodegradation by
the microbial population present.
The degradation rate for the constituents may vary significantly for different oils.
Many of the complex branching, cyclic, and aromatic hydrocarbons, which otherwise
would not be biodegraded individually, although can be oxidised through cometabolism in an oil mixture due to the abundance of other substrates which can be
easily metabolised within the mixture of components present within the oil.
The biological fate of the oil components in an oil mixture still requires further
research. Particularly efforts should be made to establish a data base regarding the
biodegradability of different types of oils and their components such as petroleum
products.
Factors That Determine The Cost Of Oil Spills
There is a general agreement that the main factors influencing the cost of spills are :

Type of oil

Amount spilled

Pattern of spillage

Location

Clean-up response

Termination of Clean-up
The interactions between these factors are complex.
Type of Oil Spilled
One of the most important factors that determine the seriousness and therefore the
ultimate cost of an oil spill, one of the most important is the type of oil.
Generally light refined products (e.g. petrol, diesel) and light crude oils do not persist
on the surface of the sea for a long time due to rapid evaporation of the volatile
components and because they disperse and dissipate naturally, especially in rough
seas. This occurred during the Braer incident in the Shetland Isles, UK in January
1993. A combination of light crude oil and severe weather conditions resulted in the
entire cargo of 85,000 tonnes was lost. The spill had little contact with the shoreline,
even though the tanker was stranded on the coast. Clean-up costs in this case were
extremely low (about US $ 0.5m), especially due to the large quantity of oil involved.
At the other end of the spectrum of oil types are heavy crudes and heavy fuel oils.
These oils are highly persistent when spilled due to the high proportion of nonvolatile
components and high visocity. These oils have the potential, to travel great distances
from the original spill location. The downside to this, is that the clean-up of heavy oil
spills can be very difficult, and be very costly. This is illustrated by two of the most
expensive tanker spills of all time, the Erika off France and the Nakhodka off Japan.
Both involved relatively small amounts of oil (17,500 tonnes in the case of the
Nakhodka and about 20,000 tonnes in the Erika) spilled a distance from the coast.
Severe weather affected offshore recovery operations, allowing the highly persistent
oil to spread over a large area of sea, which lead to extensive coastal contamination.
The cost of cleaning up of heavy fuel oil relative to the quantity spilled is also
demonstrated by the Tanio, which crashed off the north coast of Brittany, France in
1980. In this case the clean-up of the 14,500 tonnes of heavy fuel oil cargo
contaminated over 200 km of the Brittany coastline was just as difficult and as costly
to clean up as the 223,000 tonnes of crude oil from the Amoco Cadiz which had
contaminated the same area almost exactly two years earlier.
The nature of the damage caused by a spill will also vary due to the type of oil. Light
refined products may constitute a fire and explosive hazard if spilled in confined
situations, which may lead to a wide variety of third party claims due, for example, to
temporary closure of port areas or nearby industry. These oils also tend to be more
toxic than heavier oils. This can lead to mortalities of marine plants and animals if
high
concentrations of light oil enter the water through wave action and then are rapidly
diluted by natural sea movements. Also these oils may cause the tainting of edible
fish, shellfish and other marine products, as occurred in the Braer where the main
affected product was of high value farmed fish. This was the major part of the US $
50m compensation claims. All these effects will usually be localized and in the case
of light oils since the toxic components are also the ones that evaporate most rapidly.
Fish and shellfish also lose the oil components that cause taint once clean water
conditions return.
Heavy crude, emulsified crude and heavy fuel oils, have generally lower toxicity, and
can become a threat to seabirds and other wildlife that become physically coated or
smothered. More problems can arise if the overall density of the oil increases further
(e.g. due to mixing with sediment in coastal waters) it gets to the stage that residues
sink. This can lead to a the prolonged contamination of the sea beds. All these
problems can result in extended clean-up costs and large third party damage claims
for economic loss, as illustrated by the spills of heavy fuel oil cargo from the
Nakhodka and Erika.
Amount Spilled
The amount of oil spilled is a very important factor in determining costs. If there is no
variation in other factors, a 100,000 tonne spill will result in far wider contamination,
and will require a far more extensive clean-up response, because greater damage will
result in much higher costs than, say, a 10,000 tonne spill. However, the relationship
is not linear. Etkin (1999), believes that the clean-up costs on a per a tonne basis
decreased significantly with increasing amounts of oil spilled. Which means the cost
of cleaning up small spills is much grater than for large spills.
This trend makes it tempting to conclude that it is alright to calculate average costs of
spills of different sizes. This is a simplistic approach ignores the complexity and interrelations between the factors that cause the considerable variation in the cost of
similar sized incidents. This show that simple comparisons between the costs of
individual spills based on the single parameter of the cost per unit of spill can be
highly misleading.
Pattern of Spillage
As well as total spill volume, the pattern of oil loss can be important as well. For
example, the clean-up operation for a single large release of oil may be high but may
be completed in a matter of weeks. The resulting damage to marine resources and
amenities may also be short-term. But the same quantity of oil lost over several
months from a damaged tanker close to the coast may require a major clean-up effort,
resulting in repeated cleaning of amenity areas and long-term effects on fishery
resources and tourism. The best example of this is the Betelgeuse, a tanker that
exploded and sank at a terminal at Whiddy Island in Bantry, County Cork on the 8th
January 1979 with a 42 crew members killed along with 7 oil terminal workers also
killed. Because of the on going release from the various parts of the wreck it was
necessary to maintain the clean-up response consisting of oil collection and chemical
dispersal at sea, defensive booming of sensitive shorelines and regular beach clean-up
for some 21 months. It must be remembered that the total amount of oil spilled during
this period probably amounted to no more than 1,500tonnes, it is clear that the cost
(US $120m) of the response was far high than it would have been had the same
quantity of oil been spilled in a single release.
Location
The location of a spill can have a huge bearing on the costs of an incident as it will
determine the requirements for the clean-up response, as well as the degree of damage
to the environment and economic resources. All oils, if they remain at sea long
enough, will dissipate through natural processes. When a tanker spills oil far from the
coast the response will more often than not often be confined to aerial surveillance of
the slick to monitor its movement. The cost of responding to oil spills can be low.
This is illustrated by the fact that the three largest tanker spills of all time – Atlantic
Empress off Tobago, West Indies in 1979 (287,000 tonnes), Castillo De Bellver off
South Africa in 1983 (252,000 tonnes) and Abt Summer off Angola in 1991 (260,000
tonnes) – these resulted in very low clean-up and damage costs because no major
quantities of oil reached coastlines. Had a similar volume and type of oil been spilled
near a sensitive coastlines (as, for example, occurred in the Amoco Cadiz in France in
1978), the clean-up requirements would have been totally different, as would have
been the impact on fisheries, tourism and other sensitive economic and environment
resources. The costs would have alsoe been much greater.
The physical characteristics of the spill site (e.g. prevailing winds, tidal range,
currents, water depth) as well as its distance from the coast are important as they have
a major bearing on the clean-up response at sea and a successful salvage operation.
Shoreline contamination will also be determined. The high cost of the shoreline cleanup in both the Erika and Nakhodka incidents was due to the extensive coastal
contamination (some 400 km in the Erika and over 1,000 km in the Nakhodka), which
was a result of the highly persistent nature of the oil and its spread from the incident
location that was some distance offshore.
Other site-specific factors influence the cost of oil spill clean-up
1. the vulnerability of different shoreline types,
2.
the extent to which they are self-cleansing,
3. the feasibility of undertaking manual clean-up (e.g. accessibility, likelihood of
clean-up causing more damage than the oil itself),
4. the availability and cost of local labour and many.
Clean-up Response
As a general rule, considerable effort and money is devoted to trying to deal with oil
spills at sea, in an attempt to prevent the damage and public outcry often associated
with extensive pollution of inshore waters and shorelines.
As already discussed, oil spills will on occasions dissipate naturally and not pose a
threat to sensitive coastal resources. On other occasions there may be little that can be
done due to bad weather or other particular circumstances. The decision not to
respond, is a difficult one, because it will be viewed by politicians, public and the
media as unacceptable. An active response is sometimes used even when technical
opinion is agreed that it is unlikely to have a major benefit. This is usually due to the
fact that oil spilled on the surface of the sea spreads, rapidly, and will extend over an
area that is too great to be cleaned-up. Also there could be holding and collection
systems imposed by winds and waves, and there could be reduced effectiveness of
chemical dispersants on high viscosity oils. Response in such circumstances can lead
to high clean-up costs for little or no benefit in stopping the oil’s impact on coastlines.
However, there are exceptions: the spill of 2,450 tonnes of heavy fuel oil cargo from
the Baltic Carrier off Denmark showed that considerable success can be achieved
offshore when conditions are favourable and the recovery operation is well coordinated. In this case, approximately 900 tonnes, around one third of it’s volume,
was collected by a fleet of twelve recovery vessels from three countries. This greatly
reduced the extent of shoreline contamination. Also, in the Sea Empress incident in
Wales, UK in 1996, a combination of natural and chemical dispersion, 450 tonnes in
total of chemical dispersant was applied from a plane, this was claimed to have a
major impact in removing at least 18,000 tonnes of crude oil from the sea surface.
It is often stated that shore clean-up is much more costly than offshore clean-up. But
offshore clean-up is almost always incomplete leaving the bulk of the oil to be dealt
with on the shore. One reason why shore clean-up is often mostly cheaper than an
offshore clean-up is that it usually relies on manual recovery methods and locallyavailable equipment. In contrast, offshore clean-up requires considerable amounts of
expensive equipment, vessels, aircraft and trained operators, which might have to
begotten from a distant location. However, a more important factor determining the
cost of shoreline clean-up is the amount of cleaning that is required before the
contaminated area will be considered acceptable. The removal of bulk oil from a
heavily contaminated shoreline is relatively straight forward and can often be done
quickly, depending on the type of shoreline (e.g. rock, sand, mud), ease of access and
other incident and site-specific factors.
As the amount of shoreline contamination is gradually reduces more and more effort
is required to effect an improvement. The operation has little returns with rapidly
escalating costs as the operation moves into the secondary and final clean-up stages
phases.
Termination of Clean-up
All shore clean-up activities should be constantly evaluated to ensure that they remain
appropriate as circumstances change. Any operation should be stopped once if it has
been shown to be ineffective, or likely to cause unacceptable damage to
environmental or economic resources.
The standards set for clean-up vary from country to country and depend on national
attitudes. For example, amenity beaches oiled just before or during the holiday season
will usually need to be cleaned rapidly to a high level to permit their use in order to
minimise lost income by hoteliers and others involved in the tourism industry. This
may require the use of ‘aggressive’ clean-up techniques such as bulldozers on sandy
beaches and high pressure washing of nearby rocks, even at the risk of causing
additional environmental damage. On the other hand, areas like salt marshes and
mangrove swamps that are of great ecological importance may be better left to clean
themselves naturally in view of their sensitivity to physical disturbance. Similarly, it
will usually be appropriate and least damaging to the flora and fauna to leave natural
processes such as wave action and scouring to deal with any residual oil on rocky
shores in remote areas.
Bioremediation Technologies
There are two main approaches to oil spill bioremediation. 1) bioaugmentation
(seeding), in which oil-degrading microbes are added to supplement the existing
microbial population, and 2) biostimulation, in which the growth of indigenous oil
degraders is stimulated by the addition of nutrients or other growth-limiting cosubstrates and/or habitat alteration.
Bioaugmentation
What is a microbe ? A microbe is a microscopic organism or bacteria. The microbes
used in bioaugmentation are found naturally throughout the world within the
environment. The microbes are chosen and custom blended based on their ability to
degrade and remediate various hydrocarbons This option for oil-bioremediation has
been used since the 1970s. The principal for adding oil-degrading microbes is that
indigenous microbial populations may not be able to degrading the wide range of
potential substrates which are present in the complex mixtures such as petroleum
(Leahy and Colwell, 1990). Bioaugmentation may also be used when the indigenous
hydrocarbon-degrading population is low, when speed of decontamination is the
primary factor, and when seeding may reduce the lag period to start the
bioremediation process (Forsyth et al., 1995).
Laboratory studies on bioaugmentation have had mixed results. Aldrett et al. (1997)
tested 12 commercial microbial cultures for bioremediation of Alaska North Slope
crude oil in the lab. After 28 days, four products showed an enhancement of oil
biodegradation with significantly higher degradation rates of alkanes and aromatics
when compared to a nutrient control. In another shaker-flask experiment, Hozumi et
al. (2000) investigated the effectiveness of a microbial product in treating a heavy oil
spilled from Nakhodka using the thin layer chromatography-flame ionization
detection (TLC-FID) analysis. They found that around 35% of the oil was degraded
with addition of the microbial product. A major surprise,was the asphaltene fraction
had the highest loss among the four major oil components, this may show that oil loss
was actually due to biodegradation rather than some quality control problem with the
chemical analysis. Some laboratory studies found that microbial seeding may enhance
oil degradation in seawater but not in freshwater environments (Leahy and Colwell,
1990). Bioaugmentation may be effective in laboratory studies where environmental
conditions are well controlled, but its effectiveness is not guaranteed in the field.
The creation of a “superbug” that combines the genetic information from many
organisms and that has the ability to degrade a variety of different types of
hydrocarbons has also been considered. Friello et al. (1976) successfully produced a
multiplasmid-containing Pseudomonas strain capable of oxidizing aliphatic, aromatic,
terpenic, and polyaromatic hydrocarbons. Thibault and Elliot (1980) also developed a
multiplasmid P. putida strain that can simultaneously degrade some lighter alkanes
and aromatics. However, the survival of this strain in the environment has a question
mark over it. The issues of safety, containment, and the potential for ecological
damage must be fully thought out before field testing of these organisms can be
conducted (Leahy and Colwell, 1990). There is also the problem of public perception
over the release of “foreign” and especially “genetically engineered” microorganisms
into the environment.
Biostimulation
Biostimulation involves the addition of rate-limiting nutrients to accelerate the
biodegradation process. In most shoreline ecosystems that have been heavily
contaminated with hydrocarbons, nutrients are likely the limiting factors in oil
biodegradation.
What are the nutrients? In the bioremediation area nutrients are sometimes called
biocatalysts or biostimulants. These include products or ingredients which support the
stimulation and growth or reproduction of microbes, namely nitrogen and phosphorus.
The nutrient types and concentrations change depending on the oil properties and the
environmental conditions. Wrenn et al. (1994) studied the effects of different forms of
nitrogen on biodegradation of light Arabian crude oil in respirometers. They found
that in poorly buffered seawater, nitrate is a better nitrogen source than ammonia
because acid production associated with ammonia metabolism may inhibit oil
biodegradation. It should also be remembered that ammonia is less likely to be lost
from the system by washout due to its higher adsorptive capacity to organic matter.
But the disadvantage of ammonia is that it is toxic to many marine species. Using
nitrate as a biostimulation agent, Venosa et al. (1994) determined that approximately
1.5 to 2.0 mg N/L supported near maximal biodegradation of heptadecane
immobilized onto sand particles in a microcosm study. Du et al. (1999) investigated
the optimal nitrogen concentration for the biodegradation of Alaska North Slope
crude oil in continuous flow beach microcosms at a loading of 5g-oil/kg sand. The
results showed that nitrate concentrations below approximately 10 mgN/L limited the
rate of oil biodegradation. The higher nutrient requirement was attributed to the more
complex substrate (crude oil).
(Rosenberg and Ron, 1996). Stated that approximately 150 mg of nitrogen and 30 mg
phosphorus are consumed in the conversion of 1 g of hydrocarbon to cell material.
Therefore, a commonly used strategy has been to add nutrients at concentrations that
approaches a stoichiometric ratio of C:N:P of 100:5:1. This theory suggests that
manipulating the N:P ratio may result in the enrichment of different microbial
populations, and the optimal N:P ratio can be different for degradation of different
compounds (such as hydrocarbons mixed in with other biogenic compounds in soil).
However, the practical use of these ratio-based theories remains a challenge.
Particularly, in marine shorelines, maintaining a certain nutrient ratio is impossible
because of the dynamic washout of nutrients resulting from the action of tides and
waves. Oil biodegradation largely takes place at the interface between oil and water.
Therefore, the effectiveness of biostimulation depends on the nutrient concentration in
the interstitial pore water of oily sediments (Atlas and Bartha, 1992; Bragg et al.,
1994). The nutrient concentration should be maintained at a level high enough to
facilitate bacterial growth.
Commonly used nutrients include water-soluble nutrients, solid slow-release
nutrients, and oleophilic fertilizers. Each type of nutrient has its advantages and
disadvantages. General characteristics of these nutrients and important factors
affecting their persistence in the field, such as waves and tides, and physical intrusion
effects, can be seen in table below.
Water-soluble nutrients
Commonly used water-soluble nutrient products include mineral nutrient salts (e.g.
KNO3, NaNO3, NH3NO3, K2HPO4, MgNH4PO4), and many commercial inorganic
fertilizers (e.g. the 23:2 N:P garden fertilizer used in Exxon Valdez case). They are
usually applied in the field through the spraying of nutrient solutions or spreading of
dry granules. This approach has been effective in enhancing oil biodegradation in
many field trials (Swannell et al., 1996; Venosa et al., 1996). Compared to other types
of nutrients, water-soluble nutrients are more readily available and easier to
manipulate to maintain target nutrient concentrations in interstitial pore water.
Another advantage of this type of nutrient over organic fertilizers is that the use of
inorganic nutrients eliminates the possible competition of carbon sources. The field
study by Lee et al. (1995a) indicated that although organic fertilizers had a greater
effect on total heterotrophic microbial growth and activity, the inorganic nutrients
were much more effective in stimulating crude oil degradation. However, watersoluble nutrients also have several potential disadvantages. First, they are more likely
to be washed away by the actions of tides and waves because of their water-solubility.
The field study in Maine demonstrated that water-soluble nutrients can be washed out
within a single tidal cycle in high-energy beaches (Wrenn, 2000, see section 2.6.2).
Second, inorganic nutrients, ammonia in particular, should be added carefully to avoid
reaching toxic levels. Existing field trials, however, have not observed acute toxicity
to sensitive species resulting from the addition of excess water-soluble nutrients
(Mearns et al., 1997; Prince et al., 1994). Third, water-soluble nutrients may have to
be added more frequently than slow release nutrients or organic nutrients, resulting in
more labor-intensive, costly, and physical intrusive applications.
Granular nutrients (slow-release)
Many attempts have been made to design nutrient delivery systems that overcome the
washout problems characteristic of intertidal environments (Prince, 1993). Use of
slow release fertilizers is one of the approaches used to provide continuous sources of
nutrients to oil contaminated areas. Slow release fertilizers are normally in solid forms
that consist of inorganic nutrients coated with hydrophobic materials like paraffin or
vegetable oils. This approach may also cost less than adding water-soluble nutrients
due to less frequent applications. Slow release fertilizers have shown some promises
from oil bioremediation studies and applications. For example, Olivieri et al.(1976)
found that the biodegradation of a crude oil was considerably enhanced by addition of
a paraffin coated MgNH4PO4. Another slow-release fertilizer, Customblen (vegetable
oil coated calcium phosphate, ammonium phosphate, and ammonium nitrate),
performed well on some of the shorelines of Prince William Sound, particularly in
combination with an oleophilic fertilizer (Atlas, 1995a; Pritchard et al., 1992;
Swannell et al., 1996). Lee et al. (1993) also showed that oil biodegradation rates
increased with the use of a slow release fertilizer (sulfur-coated urea) compared to
water-soluble fertilizers.
However, the major challenge for this technology is control of the release rates so that
optimal nutrient concentrations can be maintained in the pore water over long time
periods. For example, if the nutrients are released too quickly, they will be subject to
rapid washout and will not act as a long-term source. On the other hand, if they are
released too slowly, the concentration will never build up to a level that is sufficient to
support rapid biodegradation rates, and the resulting stimulation will be less effective
than it could be. The field trials on of the shorelines of Prince William Sound showed
that on certain beaches, Customblen granules were apparently washed away before
any significant enhancement of bioremediation was recorded (Swannell et al., 1996).
Several recent studies have shown that a slow release nutrient (Max Bac, a product
similar to Customblen) failed to demonstrate enhancement of oil degradation because
the nutrient release rate was too slow to affect oil biodegradation (Croft et al., 1995;
Sveum and Ramstad, 1995).
Oleophilic nutrients
Another approach to overcome the problem of water-soluble nutrients being rapidly
washed out was to utilize oleophilic organic nutrients (Atlas and Bartha, 1973;
Ladousse and Tramier, 1991). The rationale for this strategy is that oil biodegradation
mainly occurred at the oil-water interface; since oleophilic fertilizers are able to
adhere to oil and provide nutrients at the oil-water interface, enhanced biodegradation
should result without the need to increase nutrient concentrations in the bulk pore
water. A well-known oleophilic fertilizer is Inipol EAP 22, a microemulsion
containing urea as a nitrogen source, lauryl phosphate (the phosphorus source), 2butoxy-1-ethanol as a surfactant, and oleic acid to give the material its
hydrophobicity. This fertilizer has been subjected to extensive studies under various
shoreline conditions and was successfully used in oil bioremediation on of the
shorelines of Prince William Sound. Other oleophilic organic fertilizers include
polymerized urea and formaldehyde, and some organic fertilizers derived from natural
products such as fishmeal (Lee et al., 1995a; Rosenberg et al., 1992; Sveum and
Ramstad, 1995). The effectiveness of oleophilic fertilizers also depends on the
characteristics of the contaminated environment such as action of wave and tide, and
sediment types. Based on several earlier studies, Sveum et al. (1994) indicated that
oleophilic fertilizers proved to be more effective than water-soluble fertilizers when
the spilled oil resided in the intertidal zone. But they have no advantages in enhancing
oil biodegradation in the supralittoral zone where water transport is limited. Inipol
EAP 22 was found to be more effective in coarse sediments than in fine sediments
due to the difficulty in penetration for the oleophilic fertilizer in fine sediments
(Sveum and Ladousse,1989). Variable results have also been produced regarding the
persistence of oleophilic fertilizers. Some studies showed that Inipol EAP 22 can
persist in a sandy beach for a long time under simulated tide and wave actions (Santas
and Santas, 2000; Swannell et al. 1995). Others found that Inipol EAP22 was rapidly
washed out before becoming available to hydrocarbon-degrading bacteria (Lee and
Levy, 1987; Safferman, 1991). Another disadvantage with oleophilic fertilizers is that
they contain organic carbon which may be biodegraded by microorganisms in
preference to petroleum hydrocarbons (Lee et al., 1995a; Swannell et al., 1996), and
may also result in undesirable anoxic conditions (Lee et al., 1995b; Sveum and
Ramstad, 1995). In summary, the effectiveness of these various types of nutrients will
depend on the characteristics of the contaminated environment. Slow-release
fertilizers may be ideal nutrient sources if the nutrient release rates can be well
controlled. Water-soluble fertilizers are likely more cost-effective in low-energy and
fine-grained shorelines where water transport is limited. And oleophilic fertilizers
may be more suitable for use in high-energy and coarse-grained beaches. However,
successful application of bioremediation products will always require appropriate
testing and evaluation based on the specific conditions of each contaminated site.
Type of nutrients
Advantages
Water soluble
Readily available. Easy to
manipulate for target
nutrient concentrations. No
complicated effect of
organic matter
Rapidly washed out by
wave and tide. Laborintensive, and physical
intrusive applications.
Potential toxic effect
Slow release
Provide continuous
sources of nutrients and
may be more cost effective
than other types of
nutrients
Maintaining optimal
nutrient release rates could
be a challenge
Oleophilic
Disadvantages
Able to adhere to oil
Expensive.
and provide nutrients at the Effectiveness is
oil-water interface
Variable.
Containing organic
carbon, which may
compete with oil
degradation and result
in undesirable anoxic
conditions
Bioaugmentation appears less effective than biostimulation because:
1) hydrocarbon degraders are ubiquitous in nature and, when an oil spill occurs, the
influx of oil will cause an immediate increased response in the hydrocarbondegrading populations; but,
2) if nutrients are in limited supply, the rate of oil biodegradation will be less than
optimal; thus,
3) supplying nutrients will enhance the process initiated by the spill, but adding
microorganisms will not, because they still lack the necessary nitrogen and
phosphorus to support growth.