Recovery of spilled oil, either from the sea surface or from the shoreline usually results in the collected oil becoming mixed with a large quantity of water, debris and beach material. This greatly increases the volume of oily waste for treatment and disposal.
Ideally as much of the collected oil as possible should be reprocessed through an oil refinery or recycling plant. Unfortunately this is not often possible as the oil may have weathered or been contaminated with debris and seawater. Various methods of disposal are available including direct disposal to controlled landfill sites; use in land reclamation, roadbuilding or similar activities; and destruction by incineration or biological processes. The disposal route chosen will depend on a number of factors including the amount and type of oil and debris collected, the location of the spill, the likely costs involved and environmental, legal or practical limitations.
Type and nature of the oil and oily debris
If the oil is recovered soon after it is spilled it may still be in a fluid state and relatively free of solid debris and other contamination. In most cases, however, the collected oil will be highly viscous due to the effects of weathering and may contain large amounts of water as water-in-oil emulsion ("chocolate mousse"). Oil stranded on the shore will usually be mixed with considerable amounts of solid material, such as sand, wood, plastic or seaweed, or it may have weathered to such an extent as to be found as solid tarballs. Three different types of oily waste can generally be identified; fluid oil, heavily contaminated beach material (sand, shingle and cobbles) and oily debris (flotsam & jetsam). Each type of oily waste requires a different method of treatment and disposal.
Storage and preparation for disposal
As far as possible the different types of oily waste should be collected and stored separately as each type of waste may demand the use of different disposal options. Once the oil and oily debris has been collected, it will usually be necessary to store it temporarily to allow time for logistics to be put into place to support the optimum transport and disposal route. In a large spill, the amount of material collected may exceed the capacity of local treatment or disposal sites. In this event, a larger temporary storage site will then be required. This storage acts as a buffer between the rate of collection and the, usually slower, rate of final disposal of the waste. A range of containers such as barges, skips and drums may be used, but if they are not available, simple lined storage pits can also be effective. The containers used should allow for easy transfer of the oil. This may be a particular problem if heavy or emulsified oils are collected as the oil may need to be heated to make it fluid enough to be pumped out. If barges are to be used for the storage of such oils, it is essential they are equipped with heating coils.
Every effort should be made to reduce the amount of material for final disposal. For example, fluid oil collected in vacuum trucks is usually mixed with large amounts of water. After a period of time, the oil and water will separate into two layers and the bottom water layer can then be decanted off. Similarly, where contamination by tarballs occurs, clean sand may be separated by sieving the material collected. The clean sand can then be returned to the beach.
Recovery of oils
The first option to consider is the feasibility of using the recovered oil as a raw material or low grade fuel. In some instances, fluid oil may be blended into feedstock for use in oil refineries or with fuel oils for burning in power stations, cement plants or brick kilns. Specialist waste oil recycling contractors may also be able to take this oil. However, strict criteria will usually have to be met before waste oil can be accepted for any of these uses. Generally , the oil should pumpable (i.e. it should not be too viscous), contain few solids or debris and have a relatively low salt content to minimise corrosion.
Oil collected from the sea will be the most suitable for processing since it will usually only be necessary to separate any water collected with the oil. For free water this can be achieved by gravity separation in tanks or in vacuum trucks. The removal of water from water-in-oil emulsions is more difficult and often requires heat treatment or the use of chemicals known as 'emulsion breakers' or 'demulsifiers' mixed into the oil.
Heavily contaminated oil
Oil contaminated by water and solids can be discharged into lagoons, pits or large open topped tanks. Oil can be skimmed from the surface and the water decanted from below. The residual oily solids are disposed of by one of the methods outlined below.
On occasions it may be possible to recover oil from contaminated beach material. This usually involves washing the oiled material with water, sometimes in conjunction with a suitable chemical solvent to release the oil. The process can be carried out in heated open tanks with the released oil being skimmed from the surface, or in cement mixer trucks where released oil/water can be discharged to a tank and the oil allowed to separate.
The disposal of solid oily waste is usually strictly controlled by local or national regulations and legislation. Selection of an appropriate method from those that are technically feasible, as summarised below, will therefore need to take into account any applicable restrictions, as well as the relative costs.
Disposal of oily solid waste, mixed with domestic rubbish, to designated landfill sites is a commonly used disposal method, however in some countries (eg EU States) waste pre-treatment may be required. Modern sites are usually enclosed by an impermeable membrane to prevent substances from leaching from the site but, provided the waste is properly mixed with the domestic refuse, there is little risk of oil leaching from the site. Nevertheless, in parts of the world where such linings are not regularly used, care is needed to make sure that contamination of nearby ground and surface water does not occur.
When shorelines are only lightly contaminated with oily debris or tarballs, small quantities of the collected material may be buried at the back of the beach. However, this should only be done when there is no risk of damage to vegetation or of the oil being uncovered by winds or other activities.
Landfill sites are often licensed under specific conditions, therefore acceptance of waste into landfills may be limited to permissible types, volumes, or contaminant levels of waste.
When oil is first spilt it is a flammable material. However, within a few hours at sea it loses any volatile components and picks up a high proportion of water. Consequently burning the oil itself without first removing the water is very difficult . Burning oily debris directly in the open is not a recommended method of disposal, except in very remote areas, due to the resultant smoke levels. When oil is burnt by this method it also tends to spread and to penetrate into the ground. Additionally, a tar-like residue may remain since it is rarely possible to achieve complete combustion.
These problems can be overcome by using an incinerator which contains the waste to be burnt and which, by burning an auxiliary fuel, generates the high temperatures necessary for total combustion. Although many types of incinerator are available, rotary kiln and open hearth furnaces are the most appropriate for oils containing large amounts of solid debris. Industrial and domestic waste incinerators may not have sufficient capacity to deal quickly with the large quantities of oily waste and so substantial storage facilities may be required. Large pieces of debris must be removed prior to burning.
A number of helicopter transportable incinerators have been developed for use on site in remote locations. Similarly, a kiln can be assembled from low cost materials such as 45 gallon drums which can handle small quantities of oil contaminated beach material.
The applicability of both direct burning and incineration depends largely on local legislation and environmental conditions. In addition, the costs of incineration are often considerably higher than other techniques and this should be taken into account if this method of disposal is selected.
Oiled Beach Materials
Although oiled beach materials may be disposed of in landfill or buried at the back of the beach other methods are available for their disposal.
Stabilisation is a method that can sometimes be used with oiled sand. This entails mixing the oiled sand with an inorganic substance such as quicklime (calcium oxide) powder to form an inert product. The oil binds with fine particles which prevent leaching. However, this method is not appropriate for cobbles or if the sand is mixed with large amounts of wood, seaweed or other debris. Stabilised material can usually be disposed of with fewer restrictions than untreated oily sand and can be used in land reclamation or similar applications.
Although quicklime appears to be the best binding agent, other materials might also be used such as cement and pulverised fuel ash from coal fired power stations. The cost effective use of this technique does, however, depend upon a plentiful supply of stabilising material close to the spill location and suitable arrangements to deal with the quantities of inert stabilised material produced.
Oil and oily wastes are broken down by biological processes. The term "bioremediation" is used for methods which attempt to accelerate this natural process. One such technique is landfarming whereby the oil and debris is spread over an area of land. Biodegradation of oil by micro-organisms only takes place at the oil-water interface so that the oil must be first mixed with a moist substrate. It may take as long as three years before the bulk of the oil is broken down although this can often be shortened by aeration and the application of fertilisers.
Landfarming is only likely to be applicable to relatively small spills because of the large amount of land required. Ideally the land selected should be of low value, located well away from drinking water supplies and should not be permeable. Once the oil has degraded, the soil may be capable of supporting a wide variety of plants including trees and grasses.
There are a number of bioremediation products on the market which contain oil-degrading micro-organisms. Attempts to use these products in actual spill situations have met with very little success, mainly due to the oil concentrations being too high and the difficulty in maintaining required nutrient levels.
The large amounts of waste that can be collected during an oil spill clean-up operation may become a major problem if the question of disposal is not dealt with in the appropriate contingency plan.
The plan should include details of those disposal options applicable to the area covered by the plan. Local disposal should be given priority so as to minimise the cost and logistical problems of transporting the waste over long distances. Nevertheless, the disposal methods used will largely depend on the presence of suitable disposal facilities and, where appropriate, the local availability of raw materials. In the event of a major spill, it is likely that more than one technique will be required to deal with the quantities of oil to be handled.
Large amounts of waste are likely to be collected relatively quickly during a clean-up and beach-head storage sites are likely to be rapidly overwhelmed. Contingency plans should therefore also include the location of a variety of temporary storage sites. If there are sufficient temporary storage sites the problem of disposal can be approached in stages so that each disposal route is not overloaded.
Points to remember
- Disposal of oil is a major problem, particularly following shoreline clean-up, when large amounts of oily debris can be collected.
- Contingency plans for major spills need to include details of all oil disposal techniques which can be utilised for the area covered by the plan, including details of legislative and regulatory requirements.
- In determining potential waste recovery and disposal routes, local waste legislation should be adhered to, and relevant authorities consulted.
- Sites and methods for the temporary storage of both liquid and solid waste should be identified to act as a buffer between collection of waste from the sea or shore and final disposal.
- Disposal of recovered oil and oily waste should only be considered after all possibilities of processing it for use as a fuel or raw material have been exhausted.
- The costs of disposal, including handling, separation and transport, are likely to be high and make up a significant proportion of the overall costs of the clean-up operations.