Clean-up techniques

Three possible methods of removing the oil:

1. Dispersant Chemicals:

These are chemicals which enhance the natural dispersion of oil. This is the most common method used for oil slicks, especially when mechanical recovery is impossible. They are used to reduce the extent of damage caused by the floating oil. However, there are some limitations on chemical dispersants as they can cause damage themselves if they are not controlled.

• How they work: these chemicals increase the rate of natural dispersion. When the dispersant is sprayed onto the oil slick the oil forms droplets of variable size (this process is called dispersion), the larger droplets float and the smaller droplets remain in suspension. Dispersants have two main components, a solvent and a surfactant.
• Methods of application: workboats can be used for small spills in confined areas of water, but for the larger, off-shore oil slicks large aeroplanes are used and for small oil slicks near the shore small aeroplanes and helicopters are used. A spray of "rain drop" sized droplets is used in order not to lose too much in wind drift.

• Limitations: chemical dispersants have little effect on oil with a high viscosity as the chemicals run off the oil before the solvent can work. Oils that the dispersants have an initial effect on become resistant after a while because the viscosity increases as the more volatile oils evaporate.

2. Bioremediation

This is the name for the 2 processes used to promote the natural biodegradation process. The two processes are bioaugmentation (application of microbes to degrade oil) and biostimulation (addition of nutrients).

• Bioaugmentation: this is used in many waters (both sea and fresh) to degrade waste such as oil as well as raw sewage and industrial discharges. Microbes are added to the water to feed on these wastes. The species of microbes added will not compete with the naturally occuring species so that degradation is as efficient as possible.
• Biostimulation: for the microbes to work efficiently there has to be enough phosphorus, nitrogen and carbon available. In the case of an oil spill, the carbon level escalates and the nitrogen and phosphorus levels are too low to allow the microbes to work at optimal efficiency. Fertilisers containing these 2 elements are added so the microbes are able to degrade the oil.
• Limitations: even though these processes seem attractive, they cannot be taken at face value. If bioremediation is used on oil floating on the surface, the materials added will quickly dilute and be lost. Oxygen is also required, which is not present in the oil itself, only at the water/oil interface. Bioremediation is therefore not suitable for the removal of large volumes of oil. The processes of bioremediation and bioaugmentation currently available are too slow to prevent the majority of the oil from reaching the shoreline. They can be physically and biologically harmful in some habitats such as salt marshes.

3. In-situ burning

This is the name given to the burning of the oil while it is still at sea. Technically this seems like a good method which will remove most of the oil, however in reality there are a number of problems with this technique including: production of a lot of smoke, formation of residues, ignition of oil, and safety concerns.

• Smoke: when oil burns large clouds of black smoke occur which react with the normal clouds and causes oily rain which contaminates farm crops and animals. Two oil accidents were related to on board fires on the Castillo de Bellver (in 1983 on the coast of South Africa) and the Aegean Sea (in 1992 on the coast of Spain), both accidents caused problems with local areas: the Castillo de Bellver cause contamination of sheep and wheat and the Aegean Sea cause temporary evacuation of the city of La Coruna. These were not intentional in-situ burning however these can be effects of the smoke produced by burning the oil.

• Residue: the residues formed by in-situ burning are heavily viscous and are extremely hard to remove from the sea and the shorelines. These residues affect fishing gear, boats and the shoreline. Some of the residues sink and can poison the sealife and is almost impossible to recover.
• Ignition: the time it takes for the ignition and fire safety devices to be set up many of the lighter fractions of the oil will have already evaporated making ignition more difficult. Ignition can be done by different methods, from simple petrol or diesel soaked cloths to sophisticated devices such as the helitorch (a flamethrower attached to a helicopter).

Pictures are from the ITOPF website

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