Authors: Shubham Varshney, Prerna Dobriyal*, and Shubra Singh
ICAR-Central Institute of Fisheries Education, Mumbai 400061
Corresponding author: *[Emailemail@example.com]
Crude oil is a complex mixture of hydrocarbons with 4-26 or more carbon atoms in the molecule. Crude oil is extracted usually from the upper strata of the earth. Non-hydrocarbons such as sulphurs, nitrogen, oxygen and vanadium may represent upto 25% of the oil. Oil pollution is a contamination of sea water due to an oil pour, as a result of an accident or human error is termed as oil spill.
Based on the Chemical Structure it can be classified into two broad categories:
o Aliphatic compounds
o Aromatic compounds
Sources of Oil pollution
Petroleum hydrocarbons reach the sea by many routes. It is estimated that on an average 2.5 million tonnes of oil reaches the sea per year. Of the total oil entering the sea 17% is from natural sources, while the remaining 84% is due to direct or indirect anthropogenic activities. Tanker Accidents: A large number of accidents involving oil tankers happen every year. Most accidents bring about severe ecological damage, as a large quantity of oil is discharged in a relatively small area (0.121 million tonnes). The causes of accidents are due to collision, grounding, hull failure, fire/explosion.
Bilge and Fuel Oils:
All shipping needs to pump out bilge water which invariably contains oil from the ships engines. Individually, the quantity of oil released may be small, but since all shipping contributes, the total quantity of oil entering the sea is considerable (0.252 million tons). Dry Docking: All ships, including oil tankers, require periodic docking for servicing, repairs, cleaning the hull and so on. It is essential that all oil is removed from the cargo compartments of tankers and from the empty fuel tanks of all ships to avoid the risk of explosion from petroleum gases.
Situation has now improved and the quantity of oil reaching the sea in association with dry docking has been reduced from 30,000 t (1981) to 4000 t (1989). Non Tanker Accidents: When a ship is in accident, its fuel oil may be lost to the sea. So this source of oil contamination is not negligible (0.02 milion tonnes). Marine Terminals:
Accidents through human error and pipeline failure are an inevitable accompaniment to loading oil on to tankers and discharging it at oil terminals (0.03 million tonnes).
Offshore Oil Production
The oil extracted from the seabed invariably contain some water (production water), which must be extracted before the oil is transported to the refinery. This is done by oil separators on the platform and the oil concentration in the water that is discharged is usually 40 ppm, but in total this amounts to a substantial quantity.
When an oil well is being drilled, ‘drilling muds’ are pumped down the well. These maintain a head of pressure and prevent a blow out when oil is struck, cool and lubricate the drill bit, and carry the cuttings back to the surface. The drill mud contains water and oil and the oil contaminates the water.
Blow-Outs: Uncontrolled release of oil from the well (0.05 million tonnes). Coastal Oil Refineries: The waste water discharged from oil refineries should contain only 5 ppm or less of oil (0.10 million tons). Municipal and Industrial Wastes: Domestic wastes and sewage contain a quantity of oils and greases and depending on the nature of the industry, industrial wastes may also contain a considerable quantity of petroleum hydrocarbons (lubricating oils).
Municipal Waste: Oil from cleanings, automobile washings, sewage treatment plants and the degraded asphalt from roads. Industries: Metal industries, core plants, woolen textiles, paint industries, meat and fish processing plants. Urban and River Runoff: Every time it rains, the oil and petrol, as the roads is washed down drains and into water causes and eventually reaches the sea.
Dumping at Sea
Shipping channels in estuaries and ports commonly need regular dredging. The dredging oil, which is usually dumped at sea, is contaminated with oil. Various industrial wastes and solid municipal wastes that are dumped at sea may also contain petroleum hydrocarbons.
The incomplete combustion of petrol or diesel in motor vehicles results in petroleum hydrocarbons being released into the atmosphere. These hydrocarbons are washed out in rain into the sea. Fate of Spilled Oil: When liquid oil is spilled on the sea it spreads over the surface of the water to form a thin film or an “oil slick”. The rate of spreading and the thickness of the film depend on the sea temperature, nature of oil, wind and wave action, water currents and coastal configuration.
The crude oil in the sea undergoes compositional and chemical changes from the time it spilled. Light fraction evaporates, water soluble components dissolve in the water column and immiscible components become emulsified and dispersed in the water column as small droplets. In some conditions due to photo-oxidation a water in oil emulsion is produced, this may contain 70-80 % water and forms a viscid mass known as “chocolate mouse”. It forms thick pancakes on the water or sticky masses if it comes ashore.
Toxicity of Petroleum Hydrocarbons
Oil affects the organisms in a number of ways depending upon the characteristics of the oil fractions and their concentration in water. Aromatic compounds are more toxic than aliphatics and middle molecular weight constituents are more toxic than high molecular weight tars.
Some important effects of oil on organisms are given below
• Oils have smothering effects (suffocation) on most of the aquatic animals.
• In sea birds, the microstructure and waxy nature of feathers attract oil. Buoyancy and thermal insulation of birds are affected. Oils penetrate the plumage, which facilitates the entry of water in it. Heavy oils stick to the feathers, and increase its weight to make the bird drown.
• The body temperature of the birds is kept higher by burning of stored food in the form of muscles. The oiled birds lose their capacity to catch the food efficiently, and can die because of low temperature.
• Several fur-bearing animals in sea, like seals, sea lions, sea otters and musk-rats may be affected by oil like birds, when oil penetrates the fur.
• Small animals can be caught in oil envelops and die.
• Sedimentary animals near the shore also die due to oil, as they are unable to move to safe distances away from oil.
• Oil layers increase the temperature, which may be critical for several organisms, particularly in the tropics. Tropical coral are greatly affected as the prevailing temperature is already very near to their tolerance limits.
• The poisonous effects of oil may be of much greater severity than the mechanical effects. A few aromatics like benzene and its derivatives have a very high penetration power into the body of organisms. They can penetrate the lipid layers of all membrane, thus, affecting the permeability by modifying the spacing of protein molecules on each side of the lipid layer. Brain and nerve cells of animals which depend upon fatty substances are also affected in this manner. A few straight chain and cycloparaffins also cause damage to the nervous system.
• The effects of the oil on lipids are also manifested in the maldevelopment of eggs and larvae following oil spills.
• A few toxic constituents of oils like non-hydrocarbons such as naphthanic acids and those containing nitrogen, sulphur and oxygen together with carbon and hydrogen are more soluble and can affect the enzymatic system and other vital biomolecules in the body.
• Transiting in fish, crustacean, molluscs and others.
• Effect on feeding and mating behaviour in several species. At 10 ppm reduces it the food finding capacity in lobster. 1% of oil stops feeding in mussels. Sexual behaviour of crabs is also modified at 0.0001 ppm.
• It decreases the filter feeding capacity, survival and fecundity in animals.
• It also leads to embryo toxicity and disruption of ionic regulation in sea birds.
• Some polyaromatic hydrocarbons (PAHs) like benzo (a) pyrene, dibenzo (a) pyrene and benzo(a) anthracene are potential carcinogens to a number of animals.
• Several PAHs are mutagenic and teratogenic. Abnormal offsprings have been reported in several aquatic organisms.
• At 0.01 ppm of oil, eggs hatch late and irregularly. Larvae are deformed. Young stages of lobsters are greatly affected at 1 ppm.
• Oils affect the migration behavior in salmon.
Control of Oil Pollution: Some common methods to control the oil pollution from marine areas and other bodies of water are briefly described below:
Mechanical Containment for Oil Pollution
The oil spills spread rapidly in the form of slicks which can be contained by means of some mechanical barriers. One such method is the use of floating containment barriers or booms. These are made up of relatively compatible plastic materials, such as polyethylene, polyurethane, polyvinyl chloride, nylon, neoprene, epoxies and polypropylene and contain inflated sacs at regular intervals for floatation. Any floating or spreading oil can be contained in them from where it can be removed by some other means.
Another technique is the use of bubble and current barriers which involves the generation of a surface current in opposite direction of spread of the oil by means of spray nozzles or by generating bubbles at a depth. A simple bubble discharger can be constructed with a submerged pipe with numerous air discharge pores along its length. Its major use is at oil loading terminals where it encircles them to protect the spread of spilled oil.
Oil Pollution through Dispersion Agents
The natural process of emulsification of oil in the water can be speeded up by spraying chemical dispersants on the oil slick from ships or aircrafts. However, dispersants are not effective against heavy oils and weathered oil and some of these chemicals may be toxic to life and can consume oxygen during their biodegradation. Dispersants are useful for treating small quantities of fresh oil.
Slick Lickers for Curbing Oil Pollution
A variety of slick lickers have been developed in which continuous belt of absorbent material dips through the oil slick and is passed through rollers to extract the oil. Slick lickers can deal with only a small quantity of oil and are useful in harbours and shelter waters polyurethane foams, polyethylene fibres, shredded polystyrene foam, wood cellulose fibres etc can also absorb oil.
Sinking Agents for Oil Pollution
The use of these agents is made only in deep waters where no fish catch is usually made. These agents are oleophilic and hydrophilic in nature with a high density. [The examples are: chalk treated with stealic acid, barite treated with latex, asbestos, carbonized treated sand, treated-fly ash etc.]
Oil Pollution Control Through Burning Agents
Burning can provide a rapid disposal of a large quantity of oil in short period. However the volatile substances from the oil are rapidly evaporated leaving behind the oil which requires some support for combustion. A few agents supporting the combustion are straw, cellular glass beads and silage treated fume silica. All these substances work as wicking agents and can support combustion.
Biodegradation/ Bioremediation for Controlling Oil Pollution
Several components of the oil can be degraded by the micro-organisms. A number of experiments have demonstrated the possible use of bacteria in producing single cell protein from certain oil industry wastes. However the use of bacteria in natural degradation of oil in sea is still under experimental stage.
Shubham Varshney: Pursuing post-graduation in Fish Biotechnology from CIFE, Mumbai. He has written 3 popular articles, 1 review article and 1 textbook in the field of fisheries. Email: firstname.lastname@example.org
Shubra Singh: Pursuing post-graduation in Dept. of Fish Genetics and Breeding CIFE Mumbai. Keen in research oriented activities such as innovation, discoveries and other outputs.
Prerna Dobriyal: Currently pursuing post-graduation in Dept. of Fish Genetics and Reproduction in CAU, Imphal. Likes to research in this field.