Clean Up Operations Of Oil Spills Environmental Sciences Essay

The quenching thirst and quest for a cheap source of fuel and energy to meet the demands of growing global commercialisation and industrialisation has expanded the frontiers of petroleum and it subsidiary products exploration. This irrepressible exploration often results in the pollution of the environment thus creating an imbalance between ([4])the abiotic and biotic strata of the ecosystem. Several methodologies have been developed and is put to use to undo the damage created by oil pollution. This paper gives an overview of those methods which falls into three categories —- physical, chemical and biological.

1.1. Scope and limitations

This paper explores the clean up operations using physical, chemical and biological methods in water bodies like oceans, lakes and groundwater systems.

The limitations of this report is that several technical papers related to the topic were not accessisible

1.2 Related Theory

Deliberate and accidental oil spills have and are still a major source of environmental contamination. For instance approx 6×107 barrels of oil were spread over 2×107 m 3 soil and 320 oil lakes were created across([4]) the vast desert in Kuwait during First World War .This was removed in due course and the use of three methods — physical, chemical and biological methods have been documented. Among this bioremediation is thought to be the only process that reverts the environment with minimal damage.([1])

1.2.1. Effect of oil spills on life forms

As far as it is known oil spills have a negative impact on diverse life forms. It alters the aquatic as well as the shoreline chemistry of life. The oil spills form a coating over water which inhibits the oxygen transport to the sub surface environment where various aquatic life forms reside. When it comes to the coastline it pollutes the soil and creates an adverse effect on the flora by changing the optimum soil composition.([2])

The effect of dispersants and solidifiers which are used to remove oil spills have far more adverse effects than the oil spills itself. The chemical composition of the dispersants changes the protein structure of many organisms .This pushes various vulnerable fauna to the verge of extinction. One of the recent instance is the case of bluefin tuna.

1.3. Major Oil Spill Accidents

Oil is described as a “thick liquid which does not dissolve in water” in the oxford dictionary. But in the real world ‘oil’ is always considered as a synonym to petroleum, “liquid gold”.

Since the time oil is known, oil spills are also known. An oil spill is a release of liquid petroleum hydrocarbon into the environment due to human activity and is considered as a form of pollution. It mainly refers to marine spills where oil is released into the ocean or coastal waters. Oil spills include release of oil from tankers, oil rigs, offshore platforms etc. Apart from this oil can also enter from the natural oil seeps.

The histories of oil spills are as old as the discovery of petroleum. A list of the very recent oil spill accidents are noted below:

Table 1.1: List of Oil spill accidents (source :Wikipedia)

Spill / vessel

Location

Dates

Estimated flow rate (tonnes/day)

Spilled (min tonnes)

Spilled (max tonnes)

Notes

Mumbai oil spill

 India, Mumbai, Arabian Sea

August 7, 2010 to August 9, 2010 (2 days)

not applicable

400

400

MSC Chitraand MV HYPERLINK “http://en.wikipedia.org/w/index.php?title=MV_Khalijia_3&action=edit&redlink=1″Khalijia 3collided, causing the former to tilt and spill oil and other cargo.

Xingang Port oil spill

 China, Yellow Sea

July 16, 2010 to no later than July 21, 2010 (up to 6 days)

not applicable

1,500

90,000

Two pipelines exploded at an oil storage depot near Xingang Harbourin Liaoningprovince, spilling crude into the Yellow Sea.

Jebel al-Zayt oil spill

 Egypt, Red Sea

June 16, 2010 to June 23, 2010 (8 days)

not applicable

not known

not known

Oil spilled from a platform in the Red Sea fouled beaches along the coast near Hurghada. Leak is sealed but cleanup continues and quantity remains unknown.

Deepwater Horizon

 United States, Gulf of Mexico

April 20,

2010 to July 15, 2010 (86 days)

not

applicable

492,000

627,000

After a blowout and rig fire, oil spilled into the Gulf until the well was shut-in. Relief wells are being drilled to seal the well permanently.

Leaks resulting

Taylor Energy wells

 United States, Gulf of Mexico

September 16, 2004 to present (2199 days)

0.03 – 0.05

62

96

from Hurricane Ivanhave been largely contained, but not entirely. Ocean Saratogais drilling a relief well to permanently seal the leaks.

1.4. Transport Processes

Three major modes of transport are there for oil spills and petroleum products. The first includes the([5]) surface transport of slicks which is really important as the thickness, shape and location affect the ability of dispersants to act on them. The next step is the vertical transport in which the initial dilution([7]) of dispersed oil takes place. The last step is the horizontal subsurface transport which results in the complete dilution of oil spill.([3])

FIGURE 1.1 Major open-ocean oil fate and transport processes.

SOURCE: NRC, 1985.

1.5. Summary

The first chapter of this report gives an overall idea about the oil spills, its consequences and a short literature review giving the facts and figures of recent oil spills. There are several methodologies to deal with this problem and those methods will be described in detailed in the following sections.

Chapter 2 – Methods of Clean Up

Cleanup and([4])recovery from an oil spill is difficult and depends upon many factors, including the type of oil spilled, the temperature of the water (affecting evaporation and biodegradation), and the types of shorelines and beaches involved.

Methods for cleaning up include:

Watch and Wait: In some cases the most appropriate way is to wait for the natural dispersion of oil. This is([5]) particularly applicable in ecologically sensitive areas like wetlands.

Dredging: used for oils dispersed with detergents which are thicker than water.

Skimming: Calm waters are required for this to be successful.

Solidifiers: They are made up of hydrophobic polymers which can both absorb and adsorb. They clean up the spills by doing a transitional change of the oil molecules from liquid to a semisolid rubber like state which can float. Solidifiers are insoluble in water, so the([5]) solidified oil can be removed quickly. The advantage is that oil will never leach out and it is proved relatively non toxic to aquatic organisms and corals. It suppresses the harmful vapours([6]) associated with Benzene, Xylene, naphtha etc which are the common constituents of aromatic hydrocarbons. The reaction time is determined by the area and size of the polymer as well as the viscosity of the oil to be contained. A solidifier called C.I. Agent (manufactured by C.I. Agent Solutions of Louisville, Kentucky) is being used by BP in granular form as well as in Marine and Sheen Booms on Dauphin Island, AL and Fort Morgan, MS to aid in the Deepwater Horizon oil spill cleanup.([7])

Vacuum and Centrifuge: A centrifuge can be used to separate the oil from water by sucking them together. After([3]) this method a tanker can be filled with pure oil which is separated from the water source. ([4])The water sucked up is drained back to the sea but a small amount of oil goes back into the sea. Because of this reason, this method is not used([4]) in the U.S. and all widely.

Controlled Burning: It can significantly bring([4]) down the amount of oil, if carried out properly. It can be carried out only in low wind speed areas or less windy days else it can cause air pollution.

Dispersants: They function as detergents, clustering around oil globules and form a micelle and allowing them to be taken away by water. Aesthetically it improves the surface waters and help([6]) in the mobilisation of oil droplets. When the bigger oil droplets are broken down into small drops it can be easily degraded and are less harmful. The disadvantage is that it can seep into the deep water and contaminate the coral. According to the recent research work, it is been found toxic to the corals.([4])

Bioremediation: involves the use of microorganisms([6]) or biological agents to break down the oil molecules.

Bioremediation Accelerator: Oleophilic, hydrophobic chemical, devoid of any microbe and can physically and chemically ([6])bond to both soluble and insoluble hydrocarbons. It acts as a herding agent in water and superficially floats molecules to the surface of the water including phenols and BTEX. Undetectable levels of hydrocarbon can be found out in the water. By overspraying([6]) it with sheen, sheen is discarded within minutes. Whether applied on land or on water, the nutrient-rich emulsion creates a bloom([7]) of local, indigenous, pre-existing, hydrocarbon-consuming bacteria. These specific bacteria will break down the hydrocarbons into water and carbon dioxide in 28 days.[3] [6] [7]

2.1. C.I. Agent

This is a material which is a blend ([7])of USDA food grade polymers that has the capability to solidify hydrocarbons. C.I. Agent is the most widely used solidifier agent and is the number one in this field. It encapsulates([4]) and solidifies the pollutants into an inert rubber like mass by means of a permanent physical bond. It is ecofriendly, non toxic, non hazardous, non corrosive and non carcinogenic. This was developed ([3])after a series of studies including consultation with the Environment Protection Agency, review of environmental regulations and a numerous protocol testing.([4])([3])([7])

This has been extensively([4]) used in marine dewatering and filtration, water monitoring etc. For water monitoring, the company has([6]) made a device called C.L.A.M to extract water samples using solid phase extraction to sequester pesticides, herbicides, and other trace organics in water.([6])

2.1.1. Benefits

Rapidly convert hydrocarbons into a rubber like mass([3])

Non hazardous, eco friendly and non carcinogenic([3])

They capture the sheen([3])

No need of special and extra tools for clean up([3])

They can be disposed easily as it comes under the classification of general waste([3])

It neither leaches nor reverts backs the hydrocarbon into its native state.([3])

Solidified hydrocarbons are mostly recycled and are used in the production of asphalt, plastic and rubber.([3])

Suppression of volatile vapours([3])

They are reusable until they are 100% absorbed([3])

No need of large labour force to clean up the spills([3])

Can extract hydrocarbons from mangroves, rocks, shoreline etc.([3])

Can function efficiently in salt water as well as fresh water([3])

Can solidify liquid hydrocarbons on ice([3])

2.2. Vacuum Centrifuge

Oil water separation devices exists other than these centrifuges. But the problem with them is that many of them use holding ponds to drain the liquid.

This machine works like a washing machine in spin cycle. If we open it up, we’ll see wet clothes stuck against the([5]) sides of the machine. The centrifuge employs the same force. It spins the oily water separating the denser liquid away from the lighter one.

Two mixed liquid phases,([5]) such as water and oil, are drawn into the annulus between the contactor body and the rotor. ([5])Liquids gravitate downward in the annulus where rotational liquid motion is slowed by radial vanes in the bottom plate. It enters a hole at the base of the rotor, the liquid phases are([5]) then centrifugally separated into a duel vortex([7]) because of the density difference between the two fluids.([7]) In the case of water and oil, because of the density difference, heavier water exits the rotor from a hole at the top of the unit, while the lighter oil is recovered near the central shaft.

A major challenge for these centrifuges to work in the BP oil spill is that the chemicals used as surfactants and dispersants([7]) to disperse the oil complicated the cleaning process. Phase separation is the technique used. In this method one phase is required to move in([3]) one direction and the other phase in the other direction. If small droplets are stuck in the water it affects the efficiency of the process.([7])

2.3. Dispersants and surfactants

A number of significant steps have been put into action to control the frequent occurrence of oil spills. But the consumption ([5]) of petroleum and its products and the vast production and distribution system to meet the demands makes them inevitable. Oil dispersants include surfactants,([3]) solvents and other compounds which minimises the effect of oil spills by changing the physical and chemical nature of oil. By increasing the amount of oil that gets mixed into the water, they can reduce the potential shoreline contamination by surface slicks. Although dispersants help in reducing the impact of oil spills the use of them is been a controversial issue for a long time. The activity of the dispersants is by its ability to form an interface between oil and water and thus facilitating the breakage of fuel hydrocarbons.([5])

2.3.1 Composition of Dispersants

A dispersant composition which is good enough to break down the crude petroleum and its components([6]) that float on the water generally consists of a combination of a selected non ionic surfactant and an anionic surfactant in a suitable solution. The non ionic surfactant part is an ester ([4])of a polyalkoxylated sorbitol or sorbitan and a fatty acid. Anionic surfactant can be picked([7]) from a group of calcium sulphonate, magnesium sulphonate, sodium sulphonate, triethanolamine sulphonate and isopropylamine sulphonate. Solvents used are glycols, glycol ethers and C 5 to C 10 alcohols. This dispersant composition is particularly suited for cold and saline water.

The dispersant that was used and is been still used on a large scale in the BP oil spill is corexit. The major ingredient of this is propylene glycol. ([6])Propylene glycol is widely used as an anti freeze and in detergents. The active ingredients of this composition is mixed with water. The active ingredients([7]) include sodium lauryl sulphate, cocamidopropyl betaine, ethoxylated nonylphenol, lauric acid diethanolamide, diethanolamine and propylene glycol. Preferred compositions are made up of the ingredients in an aqueous base : from 0.02 % to 2.25% by total weight([6]) of sodium lauryl sulphate, from 0.02% to 1.95% by total weight of cocamidopropyl betaine,0.002% to 0.25% by total weight of ethoxylated nonylphenol, from 0.04% to 4.25% of lauric acid diethanolamide, from([7]) 0.02% to 1.85%by total weight of diethanolamine and from([6]) 0.02% to 1.85% by total weight of propylene glycol.

There are various other dispersants under different trade names and compositions but they are patented and are kept as trade secrets.

2.3.2. Toxological use of Dispersants

The most difficult([4]) decision that natural resource managers and oil spill responders face during a spill is the environmental tradeoffs and ecological security associated with the use of dispersants. The main objective([6])of the dispersants is to transfer oil from water surface into water column. When dispersants are applied before they reach the shoreline, surface dwelling organisms like sea birds and intertidal species like mangroves are not affected. There are more likely chances for fish and other benthic species to get affected.

Once the use of dispersants([5]) is expanded to the near shore, estuarine and perhaps even fresh water systems, the tradeoffs become more complicated. For example, the protection of sensitive habitats, such as tropical coral reefs and mangroves, is a priority in oil spill response decisions. The recent([7]) findings have shown that the oil floating above sub tidal reefs is not a threat to corals. It can have a long lasting effect on the adjacent mangrove system if it is allowed to reach the shoreline and the persistence([7]) of oil there for a long time can act as a source of oil pollution in the adjacent coral reefs. The trade off would be to rethink about the use of dispersants.

2.3.4. Surface transport

If the slick it is impractical to apply dispersants at the surface.The surface slicks can be contained by using Langmuir([6]) circulation cells or other convergence mechanisms.. If surface convergence is very negligible, the most effective method is to apply dispersant within the cell before oil gets a chance to mix with the sea water. But this is a little difficult method, so attention is paid to means that dispense the dispersants([7]) directly into the plume. This should be carried out as close to the sea floor to minimize the dilution and then get the desired level of dispersant -to -oil ratio without bearing the cost and environmental consequences of using the excessive amounts of dispersant([6]) ([7])

2.3.5. Making Decisions about the use of Dispersants

A variety of perspectives should be considered should be considered about the use of dispersants in removing surface slicks. It is important to recognize, however, that avoiding a decision to apply dispersants([3]) due to lack of sufficient information or understanding may place some resources at risk that otherwise would be protected if dispersants were used effectively. So the real key to making effective and wise decisions about the use of dispersants is to have([4]) complete knowledge about the advantages and disadvantages of it and having a full understanding about their impacts and the alternative sources available.([3])([4])

.

2.3.6. Oil Dispersants, an Environmental Crapshoot

Bluefin tuna is a majestic deep water giant which is threatened by over fishing, had just lost a bid for protection as ([4])an endangered species when the oil gushed into the Gulf of Mexico which is the spawning ground for the fish.([5])

Now as a quick and effective response to the oil, the use of dispersants has further put a question mark to the fate of this species. The chemicals which are used in very large volumes and in previously untested ways([3]) can come with a big trade off. The reason behind this is that nobody can actually foresee or predict the impact of these on aquatic flora and fauna. ([4])People have no clue how it will affect the ecosystem. Once this chemical enters the lowest link in the ecological pyramid, it can come to the humans in much larger quantities through the process of biomagnifications. Dispersants are toxic. The higher concern now is what happens in dispersing the oil, which is far more dangerous to the living beings. ([7])

Fig 2.1 – Dr. Steve Wilson of Stanford University tags a 700-pound bluefin tuna off Canada with a satellite monitoring tag. The fish was tracked in 2009 as it travelled to the Gulf of Mexico, where the fish spawn, now the site of the Deepwater Horizon oil spill.

Actually what the dispersants do([6]) is the trade off of one ecosystem for the other. When the dispersant results in fewer oily ergets in the marsh, it affects the bluefin tuna. The area covered by the oil spill coincides with the only known spawning ground of one of two bluefin populations existing now. It spends 10 months of a year in the cold waters of the North Atlantic and swim thousands of miles to reach Gulf of Mexico and lay their eggs in the warm waters([4]) between April and June.During the initial stages of development, the larvae float 10 to 15 feet below the water surface. No one is certain whether the oil hampers the growth or will destroy the eggs but this is a common fear among the marine biologists.([3])

The biodiversity of the bluefin is([4]) at the edge of extinction .If it loses one year of new bluefins it will have a catastrophic impact on the population of them.([3])

Fig 2.2 – A Basler BT-67 aircraft releases dispersant over an oil slick from the Deepwater Horizon disaster off Louisiana on May 5.

Gulf of Mexico is also the spawning ground for several other species including marlin, sword fish and yellow tuna.

The effect of some of the chemical components which can stay in the water for years and the impact they have is still not sure. It can affect many species of marine flora and fauna. ([7])

The biggest mystery is how dispersants affect the ecosystem way ([3]) below the ocean floor. BP and EPA suggests that enough monitoring systems are installed to understand ([5]) what is happening and make sure nothing is affected or pushed beyond the edge.

The scientists([6]) and marine biologists and anthropologists say that the monitoring plan is not made available outside for the review and it raises questions about the lack of transparency of the plan.A very little knowledge is there about the deep water ecosystem and the action of dispersants under extreme conditions.([7])

Fig 2.3 – Greenpeace staff member Lindsey Allen tests water in a heavily oiled marsh near South Pass, La., on May 19. Despite use of dispersants and thousands of feet of containment booms, some of the slick is beginning to wash up in the delicate coastal ecosystem.

As is known today, the large oil droplets which are broken into smaller particles are degraded the microbes in the sea floor. If the([4]) microbes won’t degrade them it poses a major threat to the coral reefs deep down the ocean floor which provides shelter and acts as nurseries to different kinds of fishes.([3])

The massive use of dispersants may also further deplete the oxygen content contributing to the appearance of a dead zone.([5])

In short if this([3]) matter is not dealt with serious and immediate concentration, it can bring an ecosystem somewhere deep down the ocean to a halt and its impact on the global ecosystem would be catastrophic.([4])

2.4. Summary

This section gives an overview of the different methods of clean up , mainly stressing on physical and chemical methods. The above discussion proves that the use of dispersants should not be wide spread due to their adverse effects on various life forms.

Chapter 3 – Microbial Bioremediation

3.1. Introduction

Chemical degradation involves direct application of chemical oxidants into polluted soil and water sources thereby completely altering the native molecular chemistry and biology. In ([2])contrast biological treatment involves the break down of the pollutant into non toxic sub particles by microbial activity. Thus bioremediation can be defined as the application of living organisms to get rid of the environmental pollutants in soil, water and air. ([1])

The microbes called oleophilic bacteria or oil eating microbes are those that use oil as a source of food. ([8])Oil is made up of only hydrocarbons that oleophilic microbes can act on. The fundamental process of cleaning the oil spill starts by the application of a special solution to the oil spill which contains oleophilic or oil eating microbes. First of all the surface area is increased by breaking down the oil particles into the size of a molecule. This initiates the([2]) oxygenation process. Once it starts, the dormant microbes will become active again and start their ‘oil meal’. Nutrients present in the OEM solutions helps the microbes to survive.

Basic types([2]) of hydrocarbons are: straight chain HC’s, branched chain HC’s and six member ring HC’s. ([9])These are broken down into carboxylic acid or fatty acids by the OEM, which are further disintegrated into energy and carbon atoms which inturn are used in TCA cycle to generate energy. At the end the oil is broken down into its basic constituents—- carbon, Carbon dioxide and water which are non toxic. For survival oleophilic microbes need water, air and a source of nutrient such as oil. In order to carry out bioremediation they need an ambient environment with a temperature of -2 to 60 degree Celsius and a pH of 5.5 to 10. Other inhibitory factors are lack of moisture, oxygen content or mineral concentration or accumulating waste concentration. Once these factors are corrected, OEM’s can be set into motion.([1])

Fig 3.1: Biodegradation of oil

3.2. Necessity of Bioremediation

Most of the ([10])hydrocarbons are slowly and sparingly soluble in water whereas the aromatic compounds such as the benzene dissolves fast and is hazardous to living beings. They can([8]) invade the membranes surrounding cells and affect the normal functions. Fortunately oleophilic microbes can feed on them and disintegrate them into their basic constituents. The water in the ocean itself helps the process by carrying minerals and oxygen to the microbes. There are many other organisms other than the bacterial strains which can break down the oil spills. This include some higher organisms([1]) too like some particular fungal species, yeasts etc. They obtain both energy and tissue building materials from the oil spills. Pseudomonas, the fuel eating bacteria has a major inclination towards hydrocarbons, the main component of fossil fuels.([2])

Through the genetic engineering techniques, researchers and scientists can improve and enhance the ability of these oil eating microbes. ([8])Some attempts have been there to develop an oil eating ‘super bug’. Anyways even without these superbugs, there is a high capacity([2]) for the ocean water to biodegrade petroleum. Very recently under islands in the Arctic Ocean has shown proofs of the biodegradation of hydrocarbons([1]) even in the very cold winter season. Thus it shows that the principle cure for oil spills is found all over the oceans and has been with us for quite some time.

If left all alone the oil spills will be naturally([10]) degraded by the biological and non biological mechanisms. This raises the question of the need of the application of OEM solution. Potential short term as([14]) well as long term damages can be caused by the oil spill. Crude oil coats can adversely affect the sea life and can alter the coastal ecosystem. So if the bioremediation is left to the nature, the oil will plague the environment indefinitely. By ([9]) the application of oleophilic bacteria we are speeding up the process of degradation and thus making the environment more safe and sound.([12])

Even so many ([12]) years after bioremediation the bacterial strains still persists in the site of the oil spill in a variety of morphological forms such as coccus, bacillus, spirillum, filamentous etc. These([8]) cells always tend to remain together in the form of a cluster. Microbial degradation of the oil can be enhanced by the addition of inorganic nutrients such as nitrogen and phosphorous to oil spill area. This will remarkably increase the rate of oil disintegration.([14])

Fig 3.2: Chromatogram of 1% Tapis crude oil (1) Before microbial degradation (2) degradation by microbial consortium.

3.3. Bioremediation in extreme environments

Many of the hydrocarbon contaminated areas are characterised by extreme ranges of temperature, pH, salt concentrations and pressure. As the([11]) scope of this project is limited to water bodies, the discussion will only be about low temperature and acidic pH.

The nature and extent of microbial bioremediation is affected mainly by the temperature of the environment.([10]) Bioavailability and solubility of aliphatic and poly aromatic hydrocarbons are temperature dependant.

3.3.1. Cold Habitats: Psychotrophic microbes

The biodegradation of petroleum compounds has been reported in several cold marine ecosystems like Arctic seawater and Antarctic sea water and sediments. There are several indigenous psychotropic microbes in the cold habitats which are efficient oil degraders. It is proved by many studies that indigenous microbes are much more efficient in bioremediation than the introduced microbial strains. Several oil spill accidents in the cold regions have shown that the bioremediation has been limited by the availability of nutrients like nitrogen and phosphorous which is confirmed by field studies. ([11])

The catabolic pathways of microbes which are responsible for hydrocarbon degradation including the alk, xyl and nah pathways are very common in the cold regions. From the arctic samples available, so far only alkB+ psychrophiles are isolated and not alkB- physchophriles. Very rarely microbes carry out both alk and nah pathways. The most predominant species in the cold regions are Rhodococcus species and psychotropic Pseudomonas putida([11]).

3.3.2. Saline Environment : Halophilic Organisms

As majority of the oil spills occur in oceans which is saline in nature, a little bit of understanding about the bioremediation in saline environment is needed. The more promising degraders in halophiles are([10]) eubacteria when compared to archae. Eubacteria has a low intracellular concentration and their enzymes needed for disintegration are much more conventional as in they don’t need salt. The use of these microbes which can degrade in the presence of salt will ([12]) prevent the cost of dilution to lower the salinity or the removal of salt by reverse osmosis.

An inverse relationship is prevalent between salinity and solubility of hydrocarbons. The increase in the absorption of hydrocarbons with the increase in salinity is a result of the salting out effects occurring in both the solution as well as the solid([12]).

The most famous halophilic microbes are Streptomyces albaxialis and an n alkane degrading member of the Halobacterium group.([12])

An approach to improve the efficiency of degraders in saline environment is to cause an impairment in the phenotype of osmotolerance to a crude oil degrading consortia consisting of four Pseudomonas strains. ([10]) The E. Coli pro U operon was sub cloned and then introduced into each of the strains. The transformed microbes proved that the expression of salt tolerance phenotype did not destroy or slow down the hydrocarbon degrading capability. But the survival of these transformed organisms in situ conditions has yet to be proven.

3.3.3. Deep Sea Environments: Barophilic (piezophilic) microbes

The deep oceans as well as other habitats like deep sediments or oil fields or deep groundwater are affected by high pressure. Barophiles are those type or organisms that require high([11]) pressure normally more than the atmospheric pressure to thrive. There is very little knowledge about the degradation of hydrocarbons by the barophiles. Contaminants with greater density than that of the ocean waters will sink to the benthic zone, where the hydrostatic pressure is remarkably high. Because of high pressure and very low temperatures in the deep waters, the microbial activity over there is very low.

Some of the barophilic microbes are Pseudomonas sp., Flavobacterium sp., Aeromonas and Vibrio.([11])

3.4. Modes of Biodegradation

Microbes derive the energy for cellular production and maintenance by transferring electrons from electron donors to electron acceptors. As a result oxidation of electron donors takes place and reduction happens for electron acceptors. Most common electron donors are at oil spill sites are natural organic carbon and fuel related organic compounds including BTEX(Benzene, toluene, ethylbenzene, xylene). Oxygen, nitrogen and sulphate are the most common electron acceptors found in the ground water. In the aquifer matrix iron (iii) is the most common electron acceptor. Bioremediation can happen due to aerobic respiration, iron (iii) reduction, sulphate reduction or Methanogenesis, denitrification depending on several conditions like electron acceptors and nutrients present, alkalinity and pH conditions. Electron acceptors are utilised by the microbes in a particular order during the metabolisation of fuel hydrocarbons. The primary electron acceptor used first is the dissolved oxygen. After the dissolved oxygen is completely used up , anaerobic organisms typically use the electron acceptors in a preference order —-nitrate, iron (iii), sulphate and finally carbon dioxide. The prevalent environmental conditions and the competition between the organisms will finally decide which processes to dominate in a given site. ([1]) ([9]) ([8])

Table 3.1: Benzene biodegradations

3.4.1. Aerobic Biodegradation

The stoichiometry of benzene biodegradation is given in the above table. To metabolise 1 mg of benzene completely ([13])3.08 mg of oxygen is needed, in the absence of microbial cell production. This concludes that 1 mg of oxygen is enough to completely degrade 0.32 mg of benzene.

Expressed Assimilative capacity (EAC) of a particular TEAP(Terminal Electron Accepting Process) is the total amount of fuel hydrocarbons that ([13])has been degraded by the groundwater source using the stoichiometry given in the above table. The following equation is used to find the EAC of ground water for aerobic respiration.

EAC DO= 0.32 (OB-OM)

([13])Where

EAC DO = expressed assimilative capacity, aerobic respiration

0.32 = mg/L BTEX degraded per mg/L of dissolved oxygen consumed

O B = background dissolved oxygen concentration (mg/L)

O M = dissolved oxygen concentration in plume (mg/L)

A reduction in the dissolved oxygen concentration is the proof that the indigenous microbe community has started it course of action. In general we can say that the dissolved oxygen concentration will be lower when compared to the background levels in the site of contamination. ([13])

3.4.2. Anaerobic Degradation

Once the contaminant enters the water body, there will be a rapid decrease in the dissolved oxygen content due to the fast blooming microbial population. This ends up with the establishment of([10]) anaerobic conditions near and in the contamination site. In order to carry out anaerobic respiration certain conditions must be met which includes availability of carbon sources and nutrients, absence of dissolved oxygen and optimum ranges of pH, temperature etc. ([9])

Anaerobic degradation can occur through different mechanisms like denitrification, sulphate reduction, iron (iii) reduction or methanogenesis depending on the electron acceptors used. Ultimately which TEAP dominates is being decided by the environmental conditions[8] and the competition among the organisms. In a typical water body which has very least dissolved oxygen reserves denitrification happens first followed by iron (iii) reduction which in turn is followed by sulphate reduction and finally by methanogenesis. ([10])

3.4.2.1. Denitrification

After all the oxygen molecules have been used up facultative anaerobic microbes will use nitrate as the terminal electron acceptor.([10])

The stoichiometry of benzene bioremediation through denitrification is presented in the table given above. To metabolise 1 mg of benzene completely in the absence of microbial production 4.8 mg of nitrate is required. This implies that 1 mg of nitrate can annihilate 0.21 mg of benzene. The relation that can be used for the determination of EAC is([10])

EAC N=0.21 (N B- N M)

([10])Where

EAC N = Expressed Assimilated Capacity, denitrification

0.21 = mg/L of BTEX degraded per mg/L of nitrate consumed

N B = Background Nitrate Concentration

N M = nitrate concentration measured in the site

3.4.2.2. Iron (iii) Reduction

Once the dissolved oxygen and nitrate reserves are over iron (iii) can be used as an electron acceptor. The reduction of ferric to ferrous form of iron through microbial mediation in soil is a common occurrence.([10])

The stoichiometry of benzene degradation via iron (iii) reduction is given in the table. 41.1 mg of iron (iii) is required to completely degrade([10]) 1 mg of benzene under the condition of no microbial cell production. 0.047 mg of benzene can be degraded by the production of 1 mg of iron (iii). The equation for EAC can be expressed as:

EAC Fe= 0.046 (Fe M- Fe B)

[10]Where

EAC Fe = expressed assimilative capacity

0.046 = mg/L BTEX degraded per mg/L of iron (ii) produced

Fe M = iron (ii) measured at the site of contamination

Fe B = background concentration of iron (ii)

3.4.2.3. Sulphate Reduction

Sulphate reducing bacteria comes into play when the first two are exhausted. Very little is known about the mechanism of sulphate reducing bacteria.([10])

The stoichiometry is given in the above table. 1 mg of benzene is completely metabolized by 4.6 mg of sulphate. So it can be understood that 1 mg of sulphate is capable of destroying 0.22 mg benzene([10])

EAC S= 0.21 (S B- S M)

([10])Where

EAC S = expressed assimilative capacity , sulphate reduction

0.21 = mg/L BTEX degraded per mg/L sulphate consumed

S B = background sulphate concentration (mg/L)

S M = sulphate concentration in the contamination site

3.4.2.4. Methanogenesis

The presence of elevated concentration levels of methane indicates methane fermentation. When this ([10])happens to be at a site of BTEX contamination , BTEX will be the substrate for methanogenesis.

The stiochiometry for this reaction too is noted in the table above. 0.78 mg of methane is produced by the([10]) metabolism of 1 mg of BTEX. Conversely 1 mg of methane produced will destroy 1.28 mg of BTEX.

EAC M= 1.28 (Methane M- Methane B)

([10])Where

EAC M = Expressed Assimilative Capacity , methanogenesis

1.28 = mg/L BTEX degraded per mg/l methane produced

Methane M = Methane concentration measured in the plume(mg/L)

Methane B = Background concentration of methane (mg/l)

For decades ([10]) it was thought that the aerobic respiration was the dominating mechanism aiding in the degradation of hydrocarbons. Astonishingly it has turned out that anaerobic processes account for 90% of the biodegradation .Some research on this matter have shown that the role of aerobic respiration is very limited.

3.4.3. EHC-O: Compound for in situ Bioremediation

EHC-O is a buffered source for slow release oxygen and inorganic nutrients for enhancing the bioremediation of([14]) soil and water. This compound accelerates the catabolic activity of the indigenous microbes, thereby enhancing the rate of destruction of the contaminant. The method of injection depends on the application. For e.g. it can be made into powder and mixed with soil and placed at the bottom of an excavated site where the soil had been removed before. Techniques such as hydraulic fracturing or direct injection through Geoprobe rods can be used to inject the slurry.

Aerobic bioremediation is facilitated by this combination of materials. This compound accelerates the contaminant([14]) disposal by enhancing the catabolic activity of the native microbes. In nature, bioremediation is often limited by the nitrogen availability.

This mechanism acts as a delivery system providing any of the following to enhance the rate of biodegradation :

An electron acceptor ( preferably oxygen or nitrate)

Nutrients (nitrogen, phosphorous)

An energy source (carbon)

Of these the rate limiting and the two most important ingredients of any delivery system are electron acceptors and nutrients.([14]) Oxygen is provided by a calcium peroxide based source. Hydration gives molecular oxygen at a rate of 15% of initial mass of the material, according to the following reaction

2CaO2 + 2H2O = O2 + 2Ca (OH)2

Diffusion or advection([14]) is the mode of transport of molecular oxygen to the sub surface environments. This oxygen is then utilised as the terminal electron acceptor in aerobic biodegradation.

The source of nitrogen is in the form of ammonia to make sure that the microbes use this to utilise the source of energy.

The other important compound in EHC-O is Zeolite which has a very high cation exchange capacity (CEC). It has two important roles in the delivery system. First of all it releases ammonia very ([14])slowly over time than giving it off very rapidly following the injection. The second function is to trap calcium and thereby preventing or reducing self encapsulation. Self encapsulation normally occurs in the groundwater systems containing calcium or magnesium. These cations will react with phosphates and form precipitates over CaO2 thereby hampering the release of oxygen. Zeolite has cation exchange sites which will bind to Ca / Mg and prevent or reduce encapsulation.

EHC-O is very effective for full range of fuel hydrocarbons with a very few notable exceptions . Short chain,([14]) more water soluble and less molecular weight compounds are degraded more rapidly.

Fig. 3.3: Effect of EHC-O injections

3.5. Mixed Culture Vs Pure Culture

Microbial cultures could be used as pure cultures as well as mixed cultures for bioremediation purpose. ([11])The advantages of using mixed culture instead of pure culture are widely demonstrated. It is due to the synergistic interactions among the organisms in the mixed culture. ([13])The mechanisms by which the petroleum degrading microbes benefit from these interactions are complex. One possible explanation is that one species will remove the toxic metabolites of the preceding species. Other explanation is that the second species degrades the metabolites completely which is degraded partially by the first species.

Further research should be conducted to understand the role played by each member of the consortium. In an experiment demonstrating a consortium of 8 strains constituting members of 6 genera which can degrade petroleum products. Interestingly, only([12]) 5 of them were able to survive in pure cultures containing hydrocarbons. It should be noted that when 3 strains were removed the effectiveness of the culture reduced remarkably. ([13])However this supports the theory that each member in the community has a very important role and may need to depend upon the other species for its own survival. The metabolites produced by these bacterial strains are non toxic, environment friendly and non hazardous.

3.6. Treatment of Marine Oil Spills

The frequent occurrence of oil spills along the coastline and in the marine waters has prompted the development and establishment of several methods to deal with it. A variety of techniques come under the biological methods. This involves the use of bio surfactants to clean([9]) up oil spills, use of plant materials like straw as an adsorbent, use of biological polymers to form a coating on the oil surfaces to prevent or reduce oil adhesion or the use of several additives to increase the microbial action on the oil slicks.([1])

Biological methods have proven to be the most effective in removing thin layer of oil spills whereas physical and chemical methods have failed. Due to the low temperature and very saline waters it is difficult to carry out bioremediation in the ocean. The effect of dilution has been counteracted by the development and application of oleophilic[9] formulations which cam maintain the concentration of nutrients and microbes at the oil – water interface, where there is a higher degree of bioremediation. Very recent development is the use of immobilisation of hydrocarbon absorbing materials like polyurethane foam, alginate wax and a microcapsule system.([1])

3.7 Groundwater Bioremediation

The most common procedure for in situ groundwater bioremediation is bio restoration. In this technique ([12]) oxygen and nutrients are introduced at the site of contamination where indigenous micro flora use that and destroy the unwanted fuel hydrocarbons. ([9])

As the first step as much as free oil or the fuel hydrocarbons are removed by one of the possible([10]) physical means. Without the use of this bioremediation will not be effective as the bulk source will continue adding new chemicals to the ground water. Then the three nutrients such as phosphorous, nitrogen and oxygen salts are introduced through the injection wells and are circulated.

The hydrogeology of the site affects the success of the bio restoration. Success will be very difficult and problematic if the hydrogeology is very complex. More over for the smooth transport of nutrients the subsurface environment should be permeable enough.([9]) This process of water movement called hydraulic conductivity is a very critical parameter for the possible outcome.

3.8. Comparison of Bioremediation with other Methods

Bioremediation has no adverse effect on the environment([8])

Very low cost of maintenance as the microbes are the “machines”([8])

Low risk of containment and transportation as most of them are carried out in situ.([8])

Even though start up and annual operation charges are high, it is compensated by reduced labour and supplies, monitoring needs, report and management.([8])

The end products of bioremediation are non toxic, some of which are even useful for ecosystem.([8])

Can be coupled with other treatments and form a treatment train.([8])

As biological systems are involved, no risk of public health or any long term liability.([8])

3.9. Practicality of Bioremediation

Inputs from experts in microbiology, geology, engineering, chemistry , soil science , engineering etc are needed for application in bioremediation techniques. For specific contamination sites the expertise should be expanded to([12]) other areas also. The three important aspects in bioremediation processes include microbial composition, geology of polluted site, containment type, chemical conditions at the polluted site.

Certain requirements must be met inorder to([12]) make the biological entities carry out bioremediation. This include organisms in appropriate density to carry out the process, the chemical substrate shouls be in an available form to the microbes, favourable environmental conditions and an electron acceptor- donor system, availability of essential([12]) nutrients for the survival of microbes.

For a successful bioremediation to happen, these requirements must be carefully balanced.([12])

3.10. Summary

The goal of bioremediation is to convert toxic hydrocarbons into undetectable non toxic compounds which does not pose a threat to the ecosystem. Bioremediation is carried out by certain genera of microbes under favourable circumstances through different modes. But this process takes much more time to show the result as compared to the use of dispersants or any other physical method.

Chapter 4- Conclusion

Oil spills have been a major debate topic in the last few years due to the increased frequency of occurrence and the threats they are presenting to us and several other life forms. Just like how we have suppressed several other threats like this, we found out various methods and equipments for cleanup operations. Some of them are cheap and low maintenance; some are costly and high maintenance; some are ecofriendly and easily disposable; some are a threat to the marine and land ecosystems. Now it is at our discretion what to use, when to use and how to use not only to protect and safeguard us but also the fellow creatures who lack the ability to think, discern and take decisions like us. Several physical, chemical and biological methods are devised to correct the damage done by these spills. Still a lot more thought should be put to get a more efficient process which is as safe as bioremediation and less time consuming like any physical or chemical method.