Water pollution

Water pollution

Water pollution is defined as: “It is the contamination of water with undesirable and harmful substances.

Polluted water has certain characteristics –

  • Bad taste
  • Offensive odor
  • Unpleasant color
  • Unchecked growth of weeds
  • Oil or grease floating on surface


  • For drinking purposes, water should be tested for fluoride ion concentration. 
  • Its deficiency in drinking water is harmful to man and causes diseases such as tooth decay etc.
  • Soluble fluoride is often added to drinking water to bring its concentration up to 1 ppm.
  • F ion concentration above 2 ppm causes brown mottling of teeth. At the same time, excess fluoride (over 10 ppm) causes harmful effect to bones and teeth.


  • Drinking water gets contaminated with lead when lead pipes are used for transportation of water. 
  • The prescribed upper limit concentration of lead in drinking water is about 50 ppb. 
  • Lead can damage kidney, liver, reproductive system etc.


  • Excessive sulphate (>500 ppm) in drinking water causes laxative effect, otherwise at moderate levels it is harmless.

Nitrate :

  • The maximum limit of nitrate in drinking water is 50 ppm. Excess nitrate in drinking water can cause disease such as methemoglobinemia (‘blue baby’ syndrome).


Maximum concentration (ppm or mg/dm3)














Causes of Water Pollution:
(i) Pathogens: Pathogens are the bacteria and the other organisms that enter water from domestic sewage and animal excreta.
Human excreta contain bacteria such as Escherichia coli and Streptococcus faecalis. It causes gastrointestinal diseases.

(ii) Organic Wastes: Organic matter such as leaves, grass, trash etc. can pollute water.
– Excessive growth of phytoplankton within water also pollute water.
– Large numbers of bacteria in water can consume oxygen dissolved in water by decomposing organic matter present in water.
– If the concentration of dissolved oxygen in water is below 6 ppm, the growth of fish gets inhibited.
– If too much of organic matter is added to water, all the available oxygen is used up. This can cause the death of the aquatic life.



  • It is Biological Oxygen Demand.
  • BOD is the amount of oxygen that will be consumed by bacteria or other aerobic microorganisms while decomposing organic matter under aerobic conditions.
  • Biological Oxygen Demand (BOD) is a standard criterion for pollution assay in aquatic systems.
  • It is generally used as an indicator of the amount of organic pollution in a water sample. 
  • So, the BOD in polluted waters will generally be higher than in clean water.
  • Clean water would have BOD value of less than 5 ppm whereas highly polluted water could have a BOD value of 17 ppm or more


  • It is dissolved oxygen.
  • This measurement indicates the actual amount of free oxygen dissolved in water and is an important requirement to support aquatic life in our streams and rivers.
  • A good quality water will have high DO.


  • It is Chemical Oxygen Demand.
  • COD measures the amount of oxygen that will be consumed by the chemical breakdown, or oxidation (degeneration) of organic pollutants in water.
  •  It is the important index for the amount of organic pollution in a water sample. 
  • The COD is less specific than BOD as it measures the oxygen consumption for any pollutant that can be chemically oxidized, versus BOD which is only that portion which is biodegradable.
  • COD is determined by oxidizing the organic matter with acidified (50% H2SO4) potassium dichromate solution.

Eutrophication :

  • Eutrophication is enrichment of the inorganic nutrient in natural waters. 
  • This leads to an increased production of algae and macrophytes which kills animal life by depriving it of oxygen and results in subsequent loss of biodiversity is known as Eutrophication.

Reason for Eutrophication-

Fertilizers contain phosphates as additives. 

  • The addition of phosphates in water enhances algae growth. Such profuse growth of algae, covers the water surface and reduces the oxygen concentration in water.
  • This leads to anaerobic conditions, commonly with accumulation of abnoxious decay and animal death.
  • Thus, bloom-infested water inhibits the growth of other living organisms in the water body.

Most of the detergents available are biodegradable. 

  • The bacteria responsible for degrading biodegradable detergent feed on it and grow rapidly. While growing, they may use up all the oxygen dissolved in water. 
  • The lack of oxygen kills all other forms of aquatic life such as fish and plants.

Control measures of eutrophication:

  • Waste water like sewage should be discharge into river or other water system only after proper treatment. For example nitrate and phosphate should be removed from waste water before disposal into river.
  • Algal bloom should be removed from water. Algae should not be disposed into water after killing them because it further generate plant nutrient during decomposition.
  • Use phosphate free detergents to reduce eutrophication
  • Growth of algae in water can also be controlled by applying algicides such as CuSO4.
  • Prevents flooding of water from agricultural soil containing fertilizers to water system.

Chemical Pollutants

(i) Industrial Wastes: Chemical reactions carried in the industrial units also pollute water to a great extent. For example, lead, mercury, nickel, cobalt etc. These chemicals give very bad effect to the groundwater and waterbodies are polluted due’ to the chemical reactions known as leaching.
Organic chemicals like petroleum products also pollute many sources of water e.g., major oil spills in oceans.

(ii) Pesticides: These are mostly chlorinated hydrocarbons, organophosphates and metallic salts etc. They dissolve in water to small extent and pollute it. Since all the pesticides are toxic in nature, they are injurious to both plants and animals.

(iii) Polychlorinated biphenyls (PCBS): These are the chemical compounds used as fluids in transformers and capacitors. These are released in atmosphere as vapours. They mix with rain water and thus contaminate the water.

(iv) Eutrophication: The process in which algae like organisms reduce dissolved oxygen in water is called as eutrophication. It is harmful for aquatic life.