How to purify drinking water?


Proper selection and protection of water sources are of prime importance for safe drinking water supply. It is always wise and economical to protect water sources from contamination than to treat the water after it gets contaminated.

Prior to the selection of water supply sources, it is very important to ensure the satisfactory quality and quantity of water. The water sources must be protected from human activities, which may contaminate the water.

Activities like mining, quarrying, hazardous waste dumping, agricultural use of fertilizers and pesticides and recreational activities must not be allowed around the water sources. In practice, it is very difficult to protest sources like rivers but all efforts should be made to keep away the various sources and activities which may contaminate the river waters.


It is very important to purify/treat the water before distribution to community. The extent of treatment required to be given to the water, however, depends upon the qualities of available raw water and the quality requirements prescribed for the intended use.

The basic purpose of water treatment is to protect the consumer/user from pathogens and impurities in water that may be offensive or injuries to human health. The water treatment systems for urban water supply schemes mainly involve followings stages:

1) Pre-Treatment

2) Sedimentation aided with coagulation


3) Filtration and

4) Disinfection

1) Pre Treatment of water

The pre-treatment of water consists of storage or disinfection of river waters. Most of the suspended impurities have to settle down under gravity during the storage process. The microbiological quality of water also improves during the impoundment of water in lakes or reservoirs, especially when combined with the exposure to ultraviolet radiation of sunlight.


Pre-disinfection is usually done when water is abstracted and treated without storage. This process destroys or reduces harmful faecal bacteria and pathogens. Another important objective of pre-treatment or pre-disinfection is to reduce the amount of ammonia in the water.

2) Sedimentation aided with coagulation

The very fine suspended mud particles and colloidal matter presents in water cannot settle down during the pre-treatment impoundment of water in the sedimentation tanks. These particles can easily be removed by increasing their size by changing them into flocculated particles.

For this purpose, certain chemical compounds like aluminum sulphate, ferric or ferrous sulphate and ferric chloride are used. These compounds commonly known as coagulants on thorough mixing form a gelatinous precipitate called floc, which absorbs and entraps the very fine mud particles, colloidal matter, and mineral particles present in the water.


This can bring about a major reduction in turbidity and in the number of pathogens. This process of adding and mixing of chemicals is called coagulation. The coagulated water is then made to pass through sedimentation tanks to remove the flocculated particles after proper settlement.

3) Filtration

Even after the pre-treatment and sedimentation aided with coagulation processes, water may contain some very fine suspended particles and pathogens. To remove or to reduce the remaining impurities still further and to enhance quality, the water is filtered through beds of fine granular materials such as sand. This process of passing the water through beds of such material (called filters) is known as filtration. The filtration process helps in removal of color, odor, turbidity and pathogens from the water.

4) Disinfection


Disinfection is the process in which microbiological organisms are destroyed by physical or chemical action, and is commonly used as the final process in the water treatment. A small amount of the active disinfectant should normally remain in the distributed water, to destroy subsequent contamination during distribution.

Chlorine is the most common and most effective disinfectant used world over, as it is cheap, reliable and easy to handle. The term chlorination is used very commonly to indicate that the water has been treated with a sterilizing agent. Other acceptable methods of disinfection are:

  • Boiling of water
  • Treatment with lime
  • Treatment with ozone gas
  • Treatment with iodine and bromine
  • Treatment with ultraviolet rays
  • Treatment with potassium permanganate

Disinfection by Chlorination

The application of chlorine during disinfection process requires certain precautions due to harmful effects of chlorine compounds and thus the careful dosing of chlorine is essential. The quantity of chlorine required for disinfecting the water depends upon the water quality, quantity and contact period, i.e. the period for which chlorine remain in contact with water before serving to consumer. However, the excess amount of chlorine gives bitter and bad taste to water.

The dose of chlorine is generally determined by experimenting various combinations in a water sample and to check the residual left after 20-minute contact period. The dose, which gives a residue of 0.1-0.2-mg/1 milligram per liter, is considered ideal.

Chlorine used for disinfection can be applied in any of the following forms:

  • In the form of liquid chlorine or chlorine gas.
  • In the form of bleaching powder.
  • In the form of chloramines and
  • In the form of chlorine dioxide

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