A vital role of dissolved oxygen

The oxygen that dissolves in water has a vital role to play. Oxygen is not very soluble: water is saturated with oxygen when concentration of dissolved oxygen is only 10 p. p. m. (parts per million). Water-living creatures and plants and all fish depend on this dissolved oxygen. When the level of dissolved oxygen falls below 5 p. p. m., aquatic plants and animals start to suffer.

Water has a natural ability to purify itself when it becomes contaminated. This ability depends on the dissolved oxygen and the bacteria present in water. When organic matter (plant and animal debris) gets into the water, aerobic bacteria (bacteria that need oxygen) use this matter as food. They use dissolved oxygen to oxidize debris to harmless products, such as carbon dioxide and water, with the release of energy.

Oxidation reactions inside bacteria:

Organic pollutants (plant and animal debris) + oxygen (dissolved) = carbon dioxide (harmless products) + water + energy (needed by bacteria)

If water is to remain life-supporting, the oxygen which the bacteria use must be replaced by the dissolving of more oxygen. Sometimes rivers are overburdened with organic debris, for example, when untreated sewage is discharged into them. When this happens the dissolved oxygen is used up more rapidly than it is replaced, and the aerobic bacteria die. Then anaerobic bacteria (bacteria which do not need oxygen) take over. They attack the organic matter to produce unpleasant decay products, such as ammonia, hydrogen sulphide and methane.

For some synthetic materials, e.g. plastics, there are no bacteria able to oxidize them. Such materials are non-biogradable, and they persist for a very long time in water.

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Spills: how we polute the sea

In the early morning of Saturday 18 March 1967, the Torrey Canyon sailed a dangerous course off the coast of southern Britain. At full speed and in broad daylight, it rammed Pollard Rock. A massive crack appeared in the bottom of the tanker and 95,000 tons of oil leaked into the sea. The action of tide and wind carried oil to the beaches of Britain and France. Twenty-five thousand sea birds died. Large-scale tanker disasters of this type make world news. It is important to remember that many more, smaller accidents take place very often. A United States Coastguard survey showed that, in one year alone, there was at least one leakage or spill by an oil tanker every five days. Most of these accidents take place close to the shore and around ports. Beaches and wildlife suffer.

How do we control pollution?

1. Burning up the oil.

On 25 March 1967, oil slicks reached Cornish beaches. When the government found out that nothing more could be done to salvage the Torrey Canyon and to control the oil slick, they had to take emergency measures. Royal Navy planes bombed the tanker in order to set fire to the oil slick.

2. Detergent sprays.

Detergents can help to break up the oil spill and make it less dangerous to beaches and wildlife. Bouts with detergent sprays can be send out from ports. Royal Navy fire ships with spray apparatus can be used. Helicopters are used to check the size and spread of the oil slick.

3. Protecting wildlife.

Members of environmental groups such as Greenpeace help to collect oil-covered birds from the beaches. Detergents and water are used to clean off the oil. If oil remains on a bird, it prevents flight and the chemicals can kill. The bird dies of poison and starvation.

4. Barrages.

A barrage is a floating barrier which can be placed around an oil slick. This can only be done if seas are calm and the oil slick is not too big. The oil inside the barrage can then be burnt off. Barrages could be built along a bay or stretch of coastline to protect beaches and wildlife. The Army might be called in to help with this exercise.

2. Make sure if you remember the meaning of the following verbs. Consult a dictionary if necessary:

Sail, ram, appear, leak, suffer, reach, salvage, break up, clean off, prevent.

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