The Basics of Oil Spill Cleanup - Chapter 9

CHAPTER 9 Spill-treating Agents Treating the oil with specially prepared chemicals is another option for dealing with oil spills. An assortment of chemical spill-treating agents is available to assist in cleaning up or removing oil. It should be noted, however, that approval must be obtained from the appropriate authorities before these chemical agents can be used. In addition, these agents are not always effective and the treated oil may be toxic to aquatic and other wildlife. DISPERSANTS Dispersant is a common term used to label chemical spill-treating agents that promote the formation of small droplets of oil that “disperse” throughout the top layer of the water column. Dispersants contain surfactants, chemicals like those in soaps and detergents, that have molecules with both a water-soluble and oil-soluble component. Depending on the nature of these components, surfactants cause oil to behave in different ways in water. Surfactants or surfactant mixtures used in dis-persants have approximately the same solubility in oil and water, which stabilizes oil droplets in water so that the oil will disperse into the water column. This can be desirable when an oil slick is threatening a bird colony or a particularly sensitive shoreline. Two major issues associated with the use of dispersants — their effectiveness and the toxicity of the resulting oil dispersion in the water column — have generated controversy in the last 30 years. Some opposition was based on unsubstantiated and outdated information from trials or actual use of dispersants many years ago. Some products used in the late 1960s and early 1970s were highly toxic and severely damaged the marine environment. Others were not effective and resulted in wasted effort. Both these issues will be discussed in this section. ©2000 by CRC Press LLC Photo 82 Dispersant does not mix with or disperse heavy oils.The dispersant in this photo, which appears white, mixes into the water column without significantly dispersing the Bunker C oil. (Environment Canada) Effectiveness of Dispersants The effectiveness of a dispersant is determined by measuring the amount of oil that it puts into the water column and comparing it to the amount of oil that remains on the water surface. When a dispersant is working, a white to coffee-coloured plume of dispersed oil appears in the water column and can be seen from ships and aircraft. This plume can take up to half an hour to form. If there is no such plume, it indicates little or no effectiveness. Effectiveness is influenced by many factors, including the composition and degree of weathering of the oil, the amount and type of dispersant applied, sea energy, salinity of the water, and water temperature. The composition of the oil is the most important of these factors, followed closely by sea energy and the amount of dispersant applied. Dispersion is not likely to occur when oil has spread to thin sheens. Below a certain thickness, the applied dispersant will interact with the water and not the oil. As discussed in Chapter 4, some oils are prone to natural dispersion, particularly those that contain large amounts of saturates. For example, diesel fuel, which con-tains mostly saturates, disperses both naturally and when dispersant is added. The amount of diesel that disperses when dispersants are used compared with the amount that would disperse naturally depends primarily on the amount of dispersant entering the oil. On the other hand, oils that consist primarily of resins, asphaltenes, and larger aromatics or waxes will disperse poorly even when dispersants are applied ©2000 by CRC Press LLC Photo 83 Effective dispersion of oil is accompanied by the formation of white to cream-coloured clouds of dispersed oil in the water column. (Imperial Oil) and will in fact separate to some degree and remain on the surface. For this reason, certain products such as Bunker C are very difficult or impossible to disperse with chemical treating agents available today. Laboratory studies have found that there is a trade-off between the amount (or dose) of dispersant applied and the sea energy at the time of application. In general, it was found that more dispersant is needed when the sea energy is low to yield the same amount of dispersion as when the sea energy is high. The effect of sea energy when the same amount of dispersant is used on several different types of oil is shown in Table 9. In the tests summarized in the table, the dispersant was applied at a dispersant-to-oil ratio of 1:10 or 10% of the volume of the oil as testing has shown that this ratio is optimal for test conditions. It can be seen that dispersants are more effective when sea energy is high than when it is low. Table 9 Typical Dispersant Effectiveness Dispersant Effectiveness At Low Sea Energy At High Sea Energy Oil (Percent of Oil in the Water Column) Diesel 60 95 Light crude 40 90 Medium crude 10 70 IFO 180 5 10 Bunker C 1 1 The relationship between the amount of dispersant applied and the sea energy for a light crude oil and a typical dispersant is shown in Figure 27. As can be seen, ©2000 by CRC Press LLC Figure 27 Typical relationship between dispersant amount and sea energy. a very large amount of dispersant is required when sea energy is low. In fact, this amount of dispersant would be very difficult to get into oil under most normal circumstances. At low sea energies and with oils that disperse poorly, more dispersant is required at the interface between the oil and the water, to the point that a typical application of surfactant would not be adequate. Effectiveness of dispersants is difficult to determine as it is hard to accurately measure both the amount of oil in the water column and the oil remaining on the surface. While these are easier to measure in the laboratory, testing procedures vary greatly and may not always be representative of actual conditions. When testing in the lab, important factors influencing effectiveness, such as sea energy and salinity, must be taken into consideration. Results obtained from laboratory testing do not necessarily reflect what would take place in actual conditions, but should be viewed as a yardstick only. It is even more difficult to measure effectiveness in the field than it is in the lab. Measurements taken in the field are best viewed as estimates as it is difficult to take sufficient measurements at frequent enough time periods to accurately measure the concentration of oil in the water column. Accurately determining how much oil is left on the surface is also a difficult task as there are no common methods for measuring the thickness of an oil slick and the oil at the subsurface often moves differently than the oil on the surface. Application of Dispersants Dispersants are applied either “neat” (undiluted) or diluted in sea water. Aerial spraying, which is done from small and large fixed-wing aircraft as well as from helicopters, is the most popular application method. Spray systems on small aircraft used to spray pesticides on crops can be modified to spray dispersant. Such aircraft can perform many flights in one day and in many different conditions. Their capac- ©2000 by CRC Press LLC Photo 84 This large spray application system is being loaded into a Hercules aircraft. (Gord Lindblom) ities vary from about 250 to 1,000 L of dispersant. Transport aircraft with internal tanks can carry from 4,000 to 12,000 L of dispersant. Large transport aircraft such as Hercules fitted with portable spray systems can carry about 20,000 L that could treat 400,000 L of oil at a dispersant-to-oil ratio of 1:20. At a thickness of 0.5 mm, this oil would cover about 400,000 m2 or 0.4 km2. This treatment could be applied in as little as an hour after loading the dispersant and as many as eight flights could be flown in a day, depending on the distance from the airport to the spill. When using large aircraft, however, it can be difficult to obtain the amount of dispersant required. A co-op typically stores 100 drums or about 20,000 L of dispersant, that would be sprayed in one flyover. Further flights would have to await the arrival of more dispersant from other co-ops or production sources. An entire country’s supply of dispersant can easily be consumed in one day if large aircraft are used. When using helicopters, spray buckets are available in many sizes from about 500 to 2,000 L. If applied at a dispersant-to-oil ratio of 1:20, 10,000 to 40,000 L of oil could be treated. If the slick were 0.5 mm thick, this would cover about 10,000 to 40,000 m2 (or about 0.01 to 0.04 km2). Each bucket would take about 1 to 2 hours to fill and spray over the oil. As a spill countermeasure, this rapid coverage of such a large area is appealing. Spray systems are available for boats, varying in size from 10- to 30-m wide spray booms to tanks from 1,000 to 10,000 L. As dispersant is almost always diluted ©2000 by CRC Press LLC ... - tailieumienphi.vn