Commonly, two broad classes of sources of water pollution are recognized. One is ‘point pollution sources, where the source is a well-defined location, e.g., a sewage outlet or a factory drain, the pipe through which factory discharges its waste water into streams. Such a pollution source can be checked effectively with appropriate technical skills. The other category consists of non-point pollution sources, which are spread over larger areas. For example, the water that runs off farms, grazing land, construction sites’, abandoned mines and pits, etc., carries silt (soil particles) via diffuse and undefined routes into streams and lakes. Water pollution control in this case is not easy and requires constant, widespread efforts on a large scale.
Water pollution also upsets various processes that occur naturally in water. These processes which use oxygen dissolved in water, help to make wastes harmless. Let us study some of these processes.
If you allow a water body, containing limited amount of degradable organic wastes, to recover, it will! purify itself in course of time. Apart from clearing the water body of pathogenic bacteria, this activity is important for regenerating the health of a water body by releasing nutrients for utilization by green plants that were locked up in the organic matter. The length of time taken in this process depends on the organic load and temperature of the surroundings.
Any natural water course contains dissolved gases normally found in air in equilibrium with the atmosphere. Thus, fish and other aquatic life obtain oxygen for respiration. The amount of oxygen which water holds at saturation depends on temperature and follows the law of decreased solubility of gases with a temperature increase. If you increase temperature of water the dissolved gases will escape in the form of bubbles.
ADVERTISEMENTS:
Degradable or oxidisable substances in waste water deplete oxygen through action of bacteria and related aerobic organisms which feed on organic waste materials. The aerobic microorganisms use available dissolved oxygen for their respiration and organic wastes for their food. If the amount of such wastes is large and if this activity proceeds at a rate fast enough to depress seriously the oxygen level, the fish and other fauna of a water body are adversely affected. If the oxygen is entirely used up, a condition of exhaustion occurs which suffocates the aerobic organisms in a natural water body. Under such conditions the water body is said to be eutrophied and is likely to smell offensively and give an ugly look, because its self-purification capacity is hampered.