Today you are going to learn all about water pollution caused mainly by sewage and use of excess fertilizers. This is a part of the ecosystem chapter in any biology book of Cambridge O-level. I will provide pictures for maximum understanding and write in the easiest of ways. If you any errors in this article feel free to comment. Like and share to other students for helping them in their studies.


Chemical pollutants, waste products, and things like paper and thrown cans and plastic bags mixed up in water is called sewage. The substances mixed in it are feces, urine, and industrial waste. This also has microorganisms in it which sewage-dump-montrealdecompose the organic matter in the sewage and respire aerobically using up all of the oxygen. When the sewage enters into a river or lake, the oxygen in the air is used to oxidize the water which turns it into fresh water when it enters the lake or river. However the excess nutrients and dumped products in it are harmful for the water and the aquatic life. Whether the sewage is dumped or not in rivers and lakes depends on whether the sewage is treated or not. If it is not treated then it will reach the rivers and lakes and can have the following disadvantages:

  • Sewage has various bacteria and viruses in it. If they enter our source of drinking water they will cause diseases like cholera and typhoid.
  • Sewage can provide excess nutrients which causes the problems of eutrophication.
  • The bacteria and the viruses in the sewage respire aerobically and give off noxious gases like hydrogen sulfide and ammonia. This helps to cure polluted the water but in untreated sewage, the microorganisms use up all the oxygen and kills the aquatic life.

These problems are controlled in advanced countries but are tolerated in non developed and developing countries making this a problem.


Eutrophication is a Greek word meaning “well fed”. This means when a river or a lake has excess nutrients then we say that eutrophication has occurred. It may be by:

  • Waste products of animals and artificial fertilizers are run off by rain water.algal bloom
  • The fertilizers have nitrates in it which is absorbed by the ground water.
  • The sewage which enters the river.

The nitrates in the artificial fertilizers are needed by the plants and crops to grow. When it reaches the rivers passing around the farm area, the aquatic plants start to grow. But the microscopic algae grows faster and causes a population explosion. This state of algae is called algal bloom. In this state, the the algae covers the surface of the water and prevents the light from reaching the aquatic plants.

The plants are then unable to photosynthesize meaning less oxygen is produced. Other than that when the algae dies the microorganisms decompose it and respire aerobically, using up all of the oxygen of the water. It is difficult for the water to be oxidized again because the surface of the water is covered by algae. Soon the fish and other aquatic animals die.

Inorganic Pollutants in Sewage

There are many inorganic pollutants produced by various factories. Some of these are poisonous like mercury, lead, and cadmium. In developed countries, laws are strict regarding proper treatment of inorganic substances but in countries which are yet to develop and are developing this is still tolerated.

mercuryThe reason this should be focused on is that inorganic materials do not break down and remain in the environment for a long time. In shorter words they are non-biodegradable. When ingested either by drinking polluted water or by eating fish from such region, it does not break down by the enzymes and certainly does not lessens along the food chain unlike energy. It rather accumulates in the body, this is called bio-accumulation. As the inorganic material moves up the food chain it increases rather than decreasing and this is called bio-magnification.

An example of this is the Minamata disease. A factory in Minamata, Japan, released mercury on to the sewage. The sewage was untreated and soon theMinamata disease inorganic waste ended up in the bay. The fish collected from the bay was ingested by hundreds of people and soon these people developed the Minamata disease. It is basically mercury poisoning. It affects the nervous system and damages it. The symptoms are; numbness, weakness in muscles, blindness and damage to hearing and speech. In worst cases it causes paralysis due to nervous system disorder, coma and death. This disease is also transferred to a fetus from the placenta. This is why even today the carriers of the disease exceed thousands of people.

This article is helpful for spreading awareness among people of this threat that can destroy our lives and the lives of the animals. With this I conclude my article.

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