|
|
|
Oil slick: Impact and control
Typically, the ecological damage of oil discharge is more severe if the discharge occurs in a coastal area than if it occurs in open sea. Oil slick affects the sea birds. Extensive mortality of marine mammals occurs as the result of oil discharge. Fish life, larvae and planktonic eggs would be subjected to mortalities, when subjected to oil (both crude and refined oils). Benthic organisms (both subtidal and intertidal), zooplankton species including pelagic fish larvae will face heavy losses. Due to the disruption of the food chain and the destruction of marine grazing animals the ecological imbalance would be profound. Aromatic hydrocarbons, including PAH (polynuclear aromatic hydrocarbons), the oil’s constituents may bio-concentrate in the tissue of primary marine life. These are then transferred to higher members in the food chain and may reach a critical level through bio-magnification and bio-accumulation. Since PAH are carcinogenic, consumption of seafood with relatively high levels of PAH, may cause carcinogenesis. Oil slicks gives out an oily smell and in the case of the Clifton episode create a nuisance for residents. One can also see black oil waves rolling in which gives an unaesthetic view of the environment. In older people, oily odours create constant headache and reduce the appetite. Flow of oil spills to the intakes of water treatment plants or power plants would disrupt the functions of these facilities. Presence of oil, when the temperatures are high, always give rise to fears of fire breakout. CONTROL A boom has four basic components. These are flotation element, skirt, ballast and tension member. Flotation element is a float, which keeps the boom floating. Skirt is the oil retention component which is made of plastic or synthetic rubbercoated fabrics. Skirt acts as a barrier and its depth determines the efficiency of the boom. After the oil spillage is contained with the boom the next step is oil removal. Physical, chemical and biological methods are employed to remove oil. Physical methods are relatively more popular. Chemical methods are less popular due to the introduction of additional substances. Biological methods’ (use of bacteria) use is somewhat restricted when the spill is on a large scale. Physical methods include oil skimmers and oil barriers. Oil skimmers are a mechanical device designed to remove oil or oil/water mixture. Skimmers can be mechanical or oleophilic, depending upon their method of oil uptake. The design of mechanical skimmers is based on the principle of difference in densities of oil and water. There are four categories of mechanical skimmers: weir skimmers (the top of weir is positioned little below the water surface which makes the oil flow to a sump, from where it is pumped); vortex skimmers (oil concentrates at the centre of vortex, created by an impeller for subsequent pumping); direction-suction skimmers (floating intake sucks oil surface layer) and conveyor belt skimmers (inclined conveyor belt conveys oil to a sump). Concept of oleophilic skimmers is based on the “adsorption” properties that oil has for certain materials. Materials, like stainless steel, polyurethane, polypropylene and aluminium can adhere oil onto their surface. The four categories of oleophilic skimmers are: disc skimmers (oil sticks to a number of discs, made of oleophilic material, which is scrapped by scrappers into a sump); drum-type skimmers (a horizontally-mounted rotating drum is used); belt skimmers (similar to conveyor belt system) and rope skimmers (floating rope is run between two pulleys). Pumps are important adjuncts of skimmers. It is essential that correct-sized pumps are chosen so that the pumping rates match the handling facilities’ volumetric capacities. The horsepower of prime mover, pumping rates, delivery head, diameters of suction and delivery pipes, type of pumps (vacuum, vertical and centrifugal) and maximum viscosity limit should be carefully worked out and specified. Sorbents are also used for oil spill control. Sorbents are materials which can fix oil by absorption (soaking), or by adsorption (adhering onto its surface). Examples are natural fibre in synthetic bags, non-woven polypropylene, heat-treated crushed peats, pads, blankets, pine bark and, thermally expanded volcanic rock. Sorbents must have density lower than that of water and must be hydrophobic, to be able to fix oil only and remain floating when saturated with oil. Sorbents are not advisable for large spills as the volume of sorbents required must be equal to the volume of oil to be removed. Large volumes of sorbents would result in transport and handling problems. Dispersion of oil is a natural process for the elimination of oil. It is, however, a slow process. The drifting oil may affect the shorelines or may be emulsified (water-in-oil emulsions), which would make the cleaning operation relatively more arduous. To hasten the oil dispersion process chemical dispersants are used. Chemical dispersants (both conventional and concentrated) are mixtures of surface active agents, wetting agents and solvents, reduce tension at the oil/water interface, breaking the oil slick into droplets. The chemical dispersants are not applied on oils having relatively high viscosity (higher than 1,000 centistokes at ambient temperature). Use of chemical dispersants has some disadvantages. Kerosene-based dispersants with high aromatic content are toxic and are therefore banned in many countries. Some dispersants tends to concentrate and increase the uptake of PAH in marine organisms. When selecting the option of use of dispersants two aspects must be carefully considered: one, the effects of the dispersant itself; and second, the effects of the dispersant/oil mixture. In most cases the toxicity of dispersed oil is of great concern. Certain chemical dispersants inhibit microbial activity and damage cell membranes and cell permeability. Since the aim of the use of dispersants is to convert oil into oil droplets deep into the water column, this would make the oil available to other forms of marine life which may have escaped the oil film when it was on the surface of water. Nekton and other filter feeders may come in contact with the dispersed oil droplets causing mortalities. Finer droplets have elevated immediate acute toxicity to small marine organisms (10 micrometer range). Finely dispersed oil droplets become a more widespread contaminant and may cause long-term environmental and ecological impacts. EMERGENCY RESPONSE CENTRE Foreseeing these problems, the writer developed a mega, project of establishment of emergency response centre in Karachi. In addition to oil spills (both inland and onshore) the centre was to handle three other distinct types of emergencies. These were: industrial emergencies (industrial accidents involving toxic chemicals like the Bhopal incident) emergencies occurring during transportation of hazardous goods and radiological emergencies. In Sindh, the persons, who supported the project were Dr. Mutawakkil Kazi, the then Additional Chief Secretary (Dev.), P&D department, now Chief Secretary, Sindh, and Mr. A.G. Pirzada, Chief Economist, P&D department, government of Sindh. Rs. 50 million were allocated for the project and Rs. 10 million were released during the outgoing financial year. Japanese grant was also lined-up for the project. At the national level, under NEAP (National Environmental Action Plan) program, the project received tremendous support. The project now stands transferred to the City District Government Karachi (CDGK). The Chief Secretary, Sindh, may direct the CDGK to reactivate the project. | ||
In the absence of any rational handling strategy and prompt emergency response, the oil spill is bound to take its toll on the habitat, ecosystem and human life |
|
|
|
