Water pollution is one of the most serious environmental challenges.
It occurs when water is contaminated by such substances as human and
animal wastes, toxic chemicals, metals and oils. Pollution can affect
rain, rivers, lakes, oceans and the water beneath the earth, called
Polluted water may look clean or dirty, but it all contains bacteria,
viruses, chemicals or other chemicals than can cause illness or even
death. Impurities must be removed before such water can be used safely
for drinking, cooking or washing. Some industries must clean the water
before it can be used in their manufacturing processes.
Water pollution has become a serious problem in most countries. As a
result, governments have passed laws limiting the amounts and kinds of
wastes that can be dumped into water. In many parts of the world,
cities and towns release untreated sewage into rivers, lakes and
coastal waters. Also, pollution that does not come from a direct
point, such as a sewerage outlet or factory drain, is largely
Sources of water pollution
There are three main sources of water pollution. These sources are (1)
industrial wastes, (2) sewage and (3) agricultural chemicals and
· Industrial wastes. In many countries industries discharge pollutants
that include many toxic chemicals. Industries discharge much chemical
waste directly into natural bodies of water. In addition, the burning
of coal, oil and other fuels by power plants, factories and motor
vehicles releases sulphur and nitrogen oxides into the air. These
pollutants cause acid rain, which enters streams and lakes.
High levels of mercury have been found in fish far from industrial
areas. the main sources of the mercury appear to be emissions to the
atmosphere from coal-fired boilers or municipal incinerators.
Some industries pollute water in yet another way. they use large
quantities of water to cool certain equipment. Heat from the equipment
makes the water hot. The industries then discharge the hot water into
rivers and lakes, heating those bodies of water. Such heating that
harms plants or animals is known as thermal pollution.
· Sewage consists of human wastes, garbage and water that has been
used for washing or bathing. Most of the sewage goes through treatment
plants that remove solids and such dissolved substances as the
nutrients nitrogen and phosphorous.
· Agricultural chemicals and wastes. Water from rain or melted snow
flows from farmland into streams, carrying chemical fertilisers and
pesticides that farmers have used on the land. Animal wastes also can
cause water pollution, particularly from feed lots with many animals.
Cattle, sheep and poultry raised on feed lots do not distribute their
wastes over widespread pastureland. Instead, much of their wastes runs