How Water Disinfects in Water Treatment Plant | Disinfection

In this article, we explain what is disinfection of water, how water disinfects in water treatment plants, various methods of disinfection, advantages, and disadvantages of each method.

What is Disinfection?

At the initial stage, various water treatment activities like water screening, sedimentation and filtration were done to purify the water. The process usually involves physical purification of water, but disinfection procedures are performed on the water to remove bacterial as well as other germs contaminants.

Requirements for Disinfection

Due to the initial water treatment processes performed on the water during the various stages, about 99% of the germs are killed or eliminated, but only a very small amount of the germs, bacteria, and other impurities remain. Such biological (bacterial) contaminants should be very harmful to health.

To remove such types of germs, viruses, and bacteria, disinfectant chemicals are added into the water to kill such biological (bacterial) contaminants. This chemical process of water treatment is known as Disinfection of water.

The simplest method of disinfection is the boiling process by which the germs of typhoid, cholera, and other diseases are killed. But it is difficult and expensive on a large scale. 

Other methods include purification of water by chlorination, zonation, and ultraviolet treatment. 

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Methods of Disinfection of water

Methods of disinfection are classified into the following three methods:

  1. Chlorination method
  2. Ozonization
  3. Ultraviolet ray treatment

The essential properties of disinfectant chemicals should be as under to use for disinfection Process:

  • The chemicals used should be harmless to the consumers.
  • Its effectiveness, as well as effects, should last longer. 
  • It should be easily available. 
  • It should be easy to use (Simply applicable). 
  • It should not be more expensive. 

1. Chlorination

Chlorination is a generally accepted and widely used method of disinfection. This method uses chlorine or its various compounds of chlorine to kill germs, bacteria, and other impurities.

The following forms of chlorine are used to perform water treatment chlorination processes:

  • Gaseous chlorine
  • A liquid form of chlorine
  • Chlorine in the form of bleaching powder 
  • Di-oxide form of chloride
  • Chloramine form of chlorine

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Advantages of chlorination

  • Chlorine is readily available in gaseous, liquid, or powder form. 
  • Chlorine in water can dissolve at an instant rate of 7000 mg/lit. 
  • Residual of chlorine in water is harmless. 
  • It has an immediate and heavy toxic effect on micro-organisms in water so that the metabolism of bacteria can be stopped immediately. 
  • Long-lasting effects
  • It is a reliable and economical procedure. 

Chlorination Dosages required for chlorination

The dosage of chlorine required to disinfect water depends on the amount and type of bacterial contaminants in it. Different samples of the same size of water are taken experimentally to determine the amount of chlorination. Different amounts of chlorine are added to it. After 10 to 20 minutes of contact period, its residual quantity is obtained. The minimum amount of chlorine from all these observations which gives a residue of 0.05 to 0.02 mg/lit is called the amount of chlorine required for chlorination. 

The following formula is useful for calculating the amount of chlorine:

Required quantity of chlorine in kilogram

= (Vol. of sample in liter x Qty of chlorine in mg/lit)/10,000,00 :

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Various forms of the chlorination process

The chlorination process is classified into the following types depending on the stage of use of chlorine for processing disinfection of water and the expected results of chlorination.

  1. Plain chlorination
  2. Pre-chlorination
  3. Post-Chlorination
  4. Free Residual or Break-Point Chlorination
  5. Super Chlorination
  6. De – Chlorination
  7. Double Chlorination
1. Plain Chlorination

Sometimes in case of emergency use of water and also such water is of good quality only treatment other than chlorination should not be done. This type of chlorination is called “plain chlorination”.

For plain chlorination, usually, 0.5 ppm of chlorine is added. In addition to disinfection, simple chlorination removes organic impurities from the water as well as color. 

2. Pre-chlorination

Chlorine is often added around 0.1 to 0.5 ppm before the water filtration process to reduce the germination on the filter. This type of chlorination is called “pre-chlorination”. 

Dosage of chlorine is added to the water before it enters the basin for pre-chlorination or before it enters the suction pipe. 

Due to pre-chlorination, oxidation of excess substances in water removes taste and odor. In addition, the cleaning period of the filter can be extended.

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3. Post-chlorination

The practice of giving extra doses in such a way that about 0.10 to 0.20 ppm of free chlorine is available after completion of all water treatment procedures is called “post-chlorination”. 

Post chlorination is considered standard treatment. Generally, every waterworks scheme adopts this action. 

4. Break-point chlorination

In this method, the chemical action takes place in four stages as shown in the figure by gradually adding chlorine to the water.  This process is called “free-residual chlorination” or “breakpoint chlorination”. 

The chlorination in this method can be described as follows:

In the first stage, disinfection takes place and the amount of residues of chlorine is less. 

In the second stage, the amount of combined residual chlorine starts to increase. Maximum disinfection is done during this stage. 

The third stage involves the splitting of chloramines and chloro-organic compounds. Oxidation of organic matter in the water is continued by adding chlorine after which the water becomes odorless and tastes bad. 

In the fourth and fourth stages, bad odors and tastes in water are removed and oxidation of organic matter is eliminated. Free chlorine is obtained at point D as shown in the figure which is known as ‘break point’.

Experimentally, chlorination can be done to prevent water contamination until free chlorine of about 0.1 to 0.2 ppm is found after the breakpoint. 

5. Super Chlorination

Chlorine levels in water need to be increased to control the situation when an infectious disease outbreak occurs and the amount of organic impurities in the water increases. In such cases, excess chlorine is added after breakpoint chlorination. This action is known as the super-chlorination process. 

Adding extra chlorine to water in this way can be done during any one or more phases. Extreme chlorination of 0.5 to 2 ppm is usually done in the post-break point phase or this can be added even after filtration action. 

Super chlorination causes the water to have an excessive odor of chlorine and also retains the taste, so the excess chlorine in the water is de-chlorinated and removed before it reaches the public. 

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6. De-chlorination

Due to Super chlorination or any other reason, the taste of water changes when the amount of chlorine in the water is too high. It also has bad odor. Such water is de-chlorinated. 

During this de-chlorination, a small amount of chlorine residue is retained which is carried forward in the water supply system and also helps in disinfection in the further system. 

Aeration method for de-chlorination of excess chlorine in water, exposure to sun or use of chemical compounds is adopted. 

The following compounds are commonly used for de-chlorination. 

  • Sodium bisulfate
  • Sodium sulfate
  • Sodium thiosulfate
  • Activated carbon
  • Potassium permanganate
  • Sulfur dioxide, etc. 
7. Double Chlorination

Double chlorination is done to disinfect contaminated raw water with excessive germs. For double chlorination of water, chlorine is added at more than one point during disinfection.

Pre-post chlorination and post-chlorination are usually done. Double chlorination can result in more disinfection from contaminated water, thus reducing the load on filters and other treatments as well as reducing the amount of greenery in the water. 


Ozone is an excellent disinfectant. The effectiveness of ozone in disinfecting water is due to its oxidizing power. Ozone is a temporary isotope of oxygen. In which there are three atoms of oxygen. It is denoted by the ‘O3‘. When ozone is converted into stable particles i.e. its oxygen becomes O2 then it produces undeveloped oxygen (Nascent Oxygen, O).

Undeveloped oxygen reduces the organic matter present in the water, without producing an offensive taste or odor. A dose of 2 to 3 mg/lit of ozone gives 0.1 mg/lit residue after 10 minutes. Ozonization is a natural way of disinfection. 

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  • Ozone removes bacteria as well as color, odor, and taste. 
  • Because ozone is unstable, ozone residues remain when water reaches the distribution system.
  • Ozonized water taste is good, while chlorinated water tastes bitter. 
  • It is more expensive than chlorination. 
  • Electricity is required to form ozone. 
  • It is not suitable for shallow water. 
  • Ozone requires an intricate tool called Ozoniser. 

Ultra-violet ray treatment

Ultraviolet rays are invisible rays of light with a wavelength of 1000 to 3000m. The sun’s rays also contain ultraviolet rays which can be used for disinfection. 

U-V rays are produced by passing an electric current through a mercury vapor placed in a quartz bulb in a laboratory. The water that is to be disinfected is passed over this bulb. 

This method is effective for clean water. U-V rays can effectively penetrate only up to 30 cm depth of water. That is why it is used in small installations e.g.  Used only for swimming pools. 

It is also used for disinfection of water used for surgical purposes in hospitals. 

  • Its use does not produce any taste or odor in the water.
  • There is no danger of overdose. 
  • The cost of producing such rays is high. 
  • Residual action does not occur with the use of such rays. 

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Frequently Asked Question on Disinfection of Water

Why chlorine is used for purification of water?

Chlorine is mostly used chemical for water purification because it is easily available, speedily dissolve, kill more than 90% bacteria-viruses and the residual of chlorine is also harmless.

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