The use of booms and skimmers to recover floating oil and dispersants to enhance natural dispersion are well established methods of responding to oil spills. However, a number of techniques have been promoted over the years as alternative or complementary measures. These include in-situ burning and bioremediation.
In-situ burning is the term given to the process of burning oil slicks at sea, at or close to the site of a spill. Burning may be seen as a simple method which has the potential to remove large amounts of oil from the sea surface. In reality, there are a number of problems which limit the viability of this response technique. These include: the ignition of the oil; maintaining combustion of the slick; the generation of large quantities of smoke; the formation and possible sinking of extremely viscous and dense residues; and safety concerns.
The decision whether or not to burn a slick at sea is often contentious. Issues such as the distance of the oil from the damaged vessel or from a populated area; the potential toxicity of the resultant smoke; the nature of the oil; the likelihood of the burn being successful; and the fate of any unburned residues all require careful attention before attempts are made to ignite the oil.
Fire proof containment boom and an ignitor will most probably be required for a burn to be undertaken. In the time needed for these to be put in place, the spilled oil will have undergone considerable weathering. Up to 25% by weight, of many light and medium crude oils may well have evaporated. This loss of the lighter fractions of the oil through evaporation makes ignition difficult. Although fresh crude oils will burn easily, such oils are generally more difficult to ignite once they have lost in excess of 20% by weight. Ignition and any consequent burning are further hindered by other weathering processes, in particular by the formation of oil-in-water emulsions and by the fragmentation and scattering of the slick over a wide area by winds and currents.
Ignition can be achieved using a variety of devices ranging from a diesel soaked rag to more sophisticated equipment, such as the Helitorch. The latter device is basically a flame-thrower which is suspended beneath a helicopter and is generally accepted as being one of the safest methods of ignition in trained hands. However, this is not readily available worldwide.
For a successful in-situ burn the layer of oil on the sea surface needs to be at least 2-3mm thick to counter the cooling effect of the wind and sea. However, spreading of the slick means that the layer of oil is often much thinner than this. The thickness of the oil layer can nevertheless be built up by containing it against a barrier. This can occur naturally against ice sheets or the shoreline, or by corralling it using fire resistant booms. However, the effective containment of oil using booms can be difficult and time consuming, even in ideal weather and sea conditions.
The state of the sea can limit the success of any burn. Short, steep waves will reduce the efficiency of the burn and choppy seas may extinguish the fire altogether. Once alight, the slick itself needs to reach sufficiently high temperatures to keep the fire burning. However, as the slick becomes thinner due to the volatilisation and removal of the lighter fractions, the cooling effect of the wind and sea eventually extinguishes the fire. As a result of some of these difficulties, a significant quantity of oil may remain unburned at sea.
Vast quantities of black smoke can be produced from in-situ burning. Following an accidental fire on board the CASTILLO DE BELLVER (South Africa, 1983), clouds of black smoke resulted in an oily rain falling on farms up to 80km inland contaminating sheep and wheat. Fortunately, most of this residue was subsequently washed away. The accidental ignition of the cargo on board the AEGEAN SEA (Spain, 1992) caused dense clouds of black smoke to threaten the town of La Coruña leading to temporary mass evacuation. In addition, black soot coated several buildings which required cleaning.
Although both incidents are not examples of an intentional in-situ burn, they illustrate the possible consequences of burning oil when the smoke will be carried across inhabited areas. Health and airborne pollution concerns could prove to be a major obstacle in gaining permission to employ the technique in an actual spill and alternative methods of clean-up may be more appropriate.
The viscous residue that can be left following in-situ burning resembles the consistency of toffee, and is difficult to recover both at sea and from the shoreline. Of even greater concern is the potential for some residues to sink. Sunken residue has the potential to smother or poison bottom dwelling (benthic) species. It can also contaminate fishing gear or nearby shorelines, following storms or changes in the current or tide, as occurred following the explosion and fire on board the BETELGEUSE (Ireland, 1979) and the HAVEN (Italy, 1991). In the response to the spill from the HONAM JADE (South Korea, 1983) crude oil was deliberately ignited. As a result, a dense residue formed which sank and seriously contaminated shell fish beds. When oil does sink to the sea bed and cause problems, the scope for recovering it is limited.
Oil, like many natural substances, will biodegrade over a period of time into simple compounds such as carbon dioxide, water and biomass. Bioremediation is the term used to describe a range of processes which can be used to accelerate natural biodegradation. More specifically biostimulation is the application of nutrients, and bioaugmentation or seeding is the addition of microbes specially selected to degrade oil.
Biodegradation occurs as a result of the oxidation of certain components of spilled oil by microbes such as bacteria, fungi, unicellular algae and protozoa. The rate at which this natural process occurs is limited by several factors including the temperature, and the levels of microbes, nutrients and oxygen present in the immediate environment. Other factors such as the chemical composition and the amount of weathering of the spilled oil are also important.
To work effectively, microbes require sufficient levels of carbon, nitrogen and phosphorus. The relative levels of these elements present is defined as the C:N:P ratio. When a spill occurs a huge amount of carbon from the oil becomes available leaving a shortfall in the corresponding nitrogen and phosphorous levels. Biostimulation by the application of fertilisers can be used to adjust the balance in the C:N:P ratio and enhance the degradation rate by the indigenous (i.e. naturally occurring) microbial community.
Oil degrading microbes are distributed widely throughout the world's coastal areas and are more abundant in coastlines adjacent to chronically polluted waters such as those which receive industrial discharges and untreated sewage. In these areas the addition of microbes will probably not be necessary. Even in regions where the oil-degrading community is likely to be less abundant it is unlikely that bioaugmentation will significantly enhance biodegradation.
Some commercially available products do combine oil-degrading microbes collected from assorted areas of the world with nutrient supplements. Their application at a spill site can result in the introduction of alien species resulting in concerns about their potential impact. However, in most cases it is likely that introduced species will not compete effectively with those species naturally occurring.
Limitations of Bioremediation
Although the idea of bioremediation is attractive, its practical use is restricted. In particular, bioremediation should not be used on oil on the sea surface since any materials added are likely to be rapidly diluted and lost from the slick. Although bioremediation may improve the rate of degradation of floating slicks the process is still too slow to prevent the vast majority of the oil reaching the shoreline.
Natural biodegradation can be most usefully accelerated when bioremediation is used on land, in landfarming. Here the physical, chemical and biological factors that affect bioremediation can be controlled to provide optimum conditions for biodegradation. Use of this process on the shoreline is more controversial as the same level of control is impossible to obtain in the marine environment.
The oxygen required for biodegradation to occur is only available at the oil-water interface and not within the oil itself. As a result, where this process is used the amount of oil degraded will be greater if the oil is dispersed in water or distributed thinly over and through sediments. Bioremediation is therefore not suitable for removing large amounts of oil and should only be considered where the concentration of oil is low as a final polishing technique. Heavily contaminated beach material will have to be the removed before bioremediation can be started.
In any case, the capability of bioremediation is limited as some of the more complex components of the oil may remain partially or totally undegraded. For example, resins and asphaltenes biodegrade very slowly, if at all.
Biostimulation or bioaugmentation in sensitive environments, such as salt marshes and mud flats, may cause unacceptable physical and biological damage. Excessive biostimulation may alter the nature of these areas irreparably by altering the natural balance of species and encouraging the growth of alien plant species. Bioremediation products should be applied with care and the methods used must be specifically tailored to the environment and pollutant at each contaminated site.
Uses for Bioremediation
The success of bioremediation has been assessed on land through landfarming but the evidence has not conclusively shown that bioremediation works for oiled shorelines. Nevertheless, it is possible that bioremediation may have a role to play in situations where nutrient levels are severely depleted. At this point in time too many unanswered questions remain about cost-effectiveness and environmental concerns to allow concrete conclusions to be made about the viability of this process.
Although bioremediation may be a useful tool, it is certainly not a 'miracle cure'. Frequently, the concentration of oil residue remaining after significant biodegradation has taken place, i.e. the 'end-point', will be the same whether nutrients or microbes are added or not; the difference being that the 'end-point' may be reached quicker. Even so the time scale for bioremediation is of the order of months.