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Environmental Chemistry - Causes of water pollution | 11th Chemistry : UNIT 15 : Environmental Chemistry

Chapter: 11th Chemistry : UNIT 15 : Environmental Chemistry

Causes of water pollution

(i) Microbiological (Pathogens) (ii) Organic wastes (iii) Chemical wastes

Causes of water pollution


(i) Microbiological (Pathogens)

Disease causing microorganisms like bacteria, viruses and protozoa are most serious water pollutants.

They come from domestic sewage and animal excreta. Fish and shellfish can become contaminated and people who eat them can become ill. Some serious diseases like polio and cholera are water borne diseases.Human excreta contain bacteria such as Escherichia coli and Streptococcus faecalis which cause gastrointestinal diseases


(ii) Organic wastes:

Organic matter such as leaves, grass, trash etc can also pollute water. Water pollution is caused by excessive phytoplankton growth within water.

Microorganisms present in water decompose these organic matter and consume dissolved oxygen in water.



Eutrophication is a process by which water bodies receive excess nutrients that stimulates excessive plant growth ( algae, other plant weeds).This enhanced plant growth in water bodies is called as algae bloom.

The growth of algae in extreme abundance covers the water surface and reduces the oxygen concentration in water. Thus, bloom-infested water inhibits thegrowth of other living organisms in the water body. This process in which the nutrient rich water bodies support a dense plant population, kills animal life by depriving it of oxygen and results in loss of biodiversity is known as eutrophication.


Biochemical oxygen demand(BOD)

The total amount of oxygen in milligrams consumed by microorganisms in decomposing the waste in one litre of water at 20oC for a period of 5 days is called biochemical oxygen demand (BOD) and its value is expressed in ppm.

BOD is used as a measure of degree of water pollution. Clean water would have BOD value less than 5 ppm whereas highly polluted water has BOD value of 17 ppm or more.

Chemical Oxygen Demand (COD)

BOD measurement takes 5 days so another parameter called the Chemical Oxygen Demand (COD) is measured.

Chemical oxygen demand (COD) is defined asthe amount of oxygen required by the organic matter in a sample of water for its oxidation by a strong oxidising agent like K2Cr2O7 in acid medium for a period of 2 hrs.


(iii) Chemical wastes:

A whole variety of chemicals from industries, such as metals and solvents are poisonous to fish and other aquatic life.

Some toxic pesticides can accumulate in fish and shell fish and poison the people who eat them. Detergents and oils float and spoil the water bodies. Acids from mine drainage and salts from various sources can also contaminate water sources.


Harmful effects of chemical water pollut-ants: 

i. Cadmium and mercury can cause kidney damage.

ii. Lead poisoning can leads to the severe damage of kidneys, liver, brain etc. it also affects central nervous system

iii. Polychlorinated biphenyls (PCBs) causes skin diseases and are carcinogenic in nature.


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