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Water Pollution


The major sources of water pollution are:

Domestic sewage - this contains mineral and organic substances, e.g., refuse, faces, urine and decomposed dead plants and animals.

Industrial sewage - this depends on the nature of industry, e.g., foam - from soap and detergent industries, effluents from beer/ alcohol industries, dyes from textiles mills. Notice that industrial sewage is also called effluent. Agricultural activities - organic compounds from pesticides, e.g., gammaline-20, Aldrin, Dieldin and DDT; nitrates, phosphates, sulphates, and ammonium from fertilizers and from farm animal droppings.

Oil and mineral prospecting - large organic molecules on water surface due to oil spillage. Accidents - collision of oil tankers at sea; linking of nuclear waste from storage containers. Effects of Water Pollution On Aquatic life - water is the source of food and air to support the survival of aquatic organisms such as fish and plants. When water becomes polluted, its properties changes and this could affect the survival of the organisms in it.

Some of the changes in the properties of water due to pollution that can be observed are:

- Change in color.

- Change in the amount of free oxygen.

- Change in the amount of free solid.

- Change in the chemical oxygen depend (COD).

- Change in the biochemical oxygen demand (BOD).

On Human Beings – polluted water is unfit for human consumption. This may result in the scarcity of water for consumption and for other uses.

- Pollution of water may cause fish and other edible aquatic life forms to die, thereby depriving man of important protein source.

- Man may be poisoned from feeding on poisoned fish or other aquatic life forms.

Types of Water Pollutants

Water pollutants can be categorized into two on the basis of whether they can decompose naturally or not. Hence, we have the biodegradable pollutants and the non-biodegradable pollutants. The biodegradable pollutants are naturally occurring organic matter, which can be decomposed by bacteria.

Example, faeces, refuse and dead plants and animals. Aerobic bacteria use dissolved oxygen in the water to oxidize or decompose biodegradable organic matter in water into products such as CO2, H2O, NO3-, SO42- and PO43-.

Notice that due to the activities of these bacteria, if there is much organic matter in the water, the concentration of dissolved oxygen in the water may fall to a level at which even the aerobic bacteria may not survive (fish and other aquatic organisms may not survive also - this is why faeces and refuse are disallowed from being emptied into the rivers or lagoons or any mass of water).

The decomposition process may then be taken over by anaerobic bacteria, which produce gases such as methane (CH4), hydrogen sulphide (H2S) and ammonia (NH3) (Notice that non of these contains oxygen) - these are the rotten – smelling gases often indicative of polluted waters.

The non-biodegradable pollutants are synthetic materials, such as plastics, which cannot be decomposed by bacteria. These types of pollutants are most difficult to treat.

Biochemical Oxygen Demand

The amount of oxygen needed to decompose all the biodegradable organic matter in water is known as the biochemical oxygen demand (BOD). The BOD is measured by taking a water sample and assessing the concentration of dissolved oxygen. A second sample taken at the same time is held in a sealed container at a constant temperature for 5 days and then assessed for dissolved oxygen. BOD is simply the difference between initial and final dissolved oxygen concentrations.

This difference is due to the activities of the aerobic bacteria, which use up dissolved oxygen in the water to decompose biodegradable organic pollutants in the water. BOD is a useful measure of pollution, since a high reading would suggest high level of pollution, and that the concentration of oxygen might fall to a level at which many aquatic organisms will not survive.   

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