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Classification and Control of Water Pollution

Class of Pollutants


A) Inorganic pollutants
1) Salts, trace elements like copper, zinc, arsenic etc., metals coming out from chromium plating industry.

Affects the human health and aquatic animals

2) Metals and complex compounds.

Metals disturb the water system. Algae cannot grow properly. In such surroundings, photosynthesis is affected and increases air pollution indirectly.

3) Cyanides, hydrogen sulphides, carbondioxide, nitrogen dioxide and sulphites

pH of the water varies and becomes toxic to aquatic animals.

4) Algal nutrients: Nutrients like carbon dioxide, hydrogen, oxygen, nitrogen, nitrates, phosphates, sulphates and micro nutrients like born, chlorine, copper, iron, manganese, vanadium, and zinc etc.

Eutrophication of the pond causes excess growth of the algae and subsequently the ponds get dried up.

5) Heavy metals like lead and mercury.

Water becomes toxic.

6) Fluorides present in water.

Water cannot be used for drinking purposes; Bones and teeth of human beings also get affected.

B) Organic Pollutants
Waste coming from industries and agricultural fields.

Water becomes toxic.

C) Sewage from domestic, commercial, food processing and industrial effluents.

Consumes dissolved oxygen.


Fluorides concentration up to 3 ppm. in drinking water is harmless. When it exceeds 3 ppm. it cannot be used for drinking. In the districts of Nalgonda, Guntur, and Prakasam in Andhra Pradesh, the water contains excess of fluorides. It reacts with calcium present in the body (especially teeth and bones) to form calcium fluorides.
By this reaction, the colour of the teeth turns yellow. For the same reason, the bones become weak and the disease is called "Fluorosis".

As the water containing fluorides in harmful, it is very essential to detect and estimate the fluoride content in the water. Fluorides can be detected very easily from the reaction of fluorides with zirconium - Alizarin S.dye. (The mixture of zirconium - Alizarin - S is coloured) Fluoride reacts with Zirconium to form zirconium fluoride, which is colourless. The colour of the dye becomes weak with the increase in the amount of fluorides. Oil pollution of seas, due to spilling of the oil during transport and other means has increased in recent years, because of increase in use of oil by all the countries of the world. Every year 25 billions of crude oil is transported across the seas and sea water is getting polluted. Aromatic substances get volatilized, but the nonvolatile substances are absorbed by the plants and shrubs and get propagated through the food chain process. These are carcinogenic too.

Some times the water becomes over nutritious when organic substances from agriculture and industry are thrown into the water resources like ponds and lakes. This type of nutritious lake is called as "Eutrophic lake".
Eutrophic Lake
It can support the luxuriant growth of algae; the lakes get filled with sediment and ultimately become dry. Preventing these waste substances from entering it can protect lakes.

Control of Water Pollution

Treatment of Domestic Sewage Water
Generally, the substances coming out from the houses contain less toxic substances than those coming from industries. These can be decomposed in the presence of arobic bacteria. To oxidize 1 mg. Of carbon, 2.67 mgs. of oxygen is required.
The total amount of oxygen consumed by micro organisms in decomposing the pollutants, is called B.O.D. BOD value of the water is an index for the quality of water. Generally, in polluted water it is high. BOD value of domestic sewage is around 165 ppm. The following process is commonly used for improving the quality of water.

The water is first passed through sieves and decanted. After the removal of sediment, the decanted water is subjected to oxidation (using aerobic bacteria) that removes all soluble organic matter. The water is again decanted, and lime is added to precipitate out the phosphates in the form of calcium phosphate. The precipitate is removed, and now, the BOD value of the water comes down to 1 ppm. It is then chlorinated before releasing into the pond. The sediment is dried and made to react with anaerobic bacteria (to destroy organic matter) and thrown away.
Treatment of Domestic Waste


Treatment of Industrial Waste Water
The water that gets polluted by the industrial wastes cannot be used for drinking purposes. Aquatic life also gets disturbed. These waste substances vary from industry to industry. Some are less toxic and biodegradable while some are highly toxic and non-biodegradable.

Defluoridation Techniques for Drinking Water
It is not good for health if the fluoride ion (F-) concentration exceeds 3 ppm in drinking water. Therefore it is essential that fluoride ion be removed from the drinking water. The methods and principles underlying these methods of removing fluoride ion are collectively called defluoridation techniques. It is not easy to remove fluoride ions from drinking water. The main reason for this is that fluoride ion is very small. It is therefore not possible to use the ion exchange methods generally used for removal of bigger ions such as Cl-, SO4 2- , Ca++ .For the removal of fluoride ion, special methods are therefore necessary.

Ion Exchange Method
Sometime back defluoron - 1 and defluoron-2 synthetic resins were used to remove fluoride ions. The filters were packed with resins and water was sent through them. Fluoride ions were removed by ion-exchange process. These methods were expensive and required lot of skill to handle them. Hence the use of these methods has been discontinued.

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