Chlorination | Type of chlorination | Residual Chlorine

What is chlorination | Type of chlorination | What is residual chlorine |

The process involves water treatment plant is screening, sedimentation, coagulation, filtration, disinfection and distribution. First process of the water treatment plant is screening; Screening is the process  of the water treatment plant removing the floating particles such as glasses, wood, dead animals, bottles etc are removed by the screening process. After the screening process the raw water flows from the sedimentation tank. The working principle of a sedimentation tank is to remove the suspended particles such as sand, silt, clay etc. Sedimentation is a natural process (Gravity). Again sedimentation processes are done then raw water flows from the coagulation tank; The working principle of the coagulation tank is to remove the colloidal particles. Sometime sedimentation and coagulation are done simultaneously is called plain sedimentation, after the coagulation process raw water flow from filtration tank; Filtration is the process to removing the dissolved particles from water than water flow from disinfection tank; Densification is the process of removing the harmful bacteria it means remove the pathogens. Generally chlorine is called Universal disinfectant.

What is Chlorination?

ChlorinationWhen  disinfection is done by chlorine that process is called by chlorination. It means harmful bacteria are removed by chemical method with the help of chlorine because chlorine is called Universal disinfectant. [ What is brick, type of brick, composition of brick]

Chlorine demand is calculated by following formulas

Cl2 demand = Cl2 dosage – Residual chlorine


    • If ph 5, chlorine does not react with raw water and remains free chlorine.
    • Freely available chlorine (HOCl + OCl and Cl2)
    • Most destructive chemical is HOCl and it is 80% effective compared to OCl  negative ions. Hence PH value of water slightly below 7.
    • Most predominant dischloramine whose PH value is less than 7.

Types of chlorination

Generally seven types of chlorination are done in water treatment plants.

    1. Plain chlorination
    2. Pre chlorination
    3. Post chlorination
    4. Double chlorination
    5. Super chlorination
    6. De chlorination
    7. Break point chlorination

Plain chlorination

Plain chlorinationPlain chlorination indicates that only the chlorine treatment is done, no other chemical treatment has been given to the raw water. Only chlorine is used for the water treatment process. Chlorine is mixed with raw water to prevent bacteria and control the growth of algae, organic matter and colour. Plain chlorination is used for treating relatively clear water having turbidity less than 20 mg / lit. [Properties of brick]

Generally chlorine is added in the plain chlorination treatment process is 0.5 mg / lit.

Pre chlorination

Pre chlorination Pre chlorination is the process of chlorine added to the raw water before any treatment of raw water; it means chlorine is added to the raw water before filtration, before coagulation or before sedimentation.

Pre chlorination helps in improving coagulation and sedimentation because it reduces the load on the sedimentation tank and filtration tank. Pre chlorination processes reduce the taste and odour of water.

Generally the dosage of chlorine in pre chlorination lies between 5 to 10 mg / lit.

Post Chlorination

Post ChlorinationPost chlorination or chlorination is the normal standard process of applying chlorine at the end of the water treatment process. When all the treatment has been completed. This treatment is done during normal public supply water.

Post chlorination is adopted after the filtration process and before the water enters into the distribution pipe.

The dosage of chlorine in post chlorination lies between 0.1 to 0.2 mg / lit. and has some residual chlorine.

Double chlorination

Double chlorination Double chlorination indicates that the water has been chlorinated twice. It means chlorine is added more than one point is called double chlorination.

The pre chlorination and post chlorination are generally used in double chlorination. This type of chlorination is done in those waters having highly contaminant and Contains large amounts of bacteria and turbidity is greater than 50 mg / lit.

Super chlorination

Super chlorinationSuper chlorination is a term which indicates  the addition of excess amounts of chlorine to the water.

This type of treatment may be adopted in special cases like highly polluted water or during treatment of water which is discharged from various types of industry.

Generally the dosage of chlorine  in super chlorination lies between 5 to 15 mg / lit   and having some  chlorine  is between 1  to 2 mg  /   lit.


Dechlorination The process of removing excess chlorine from raw water is called dechlorination. Dechlorination generally requires when super chlorination is done. it gives a better order and test

The dechlorination may be carried out by adding certain chemicals to the water and aeration process. These chemicals are called the dechlorinating agent. The common dechlorinating agent are Sulphur dioxide (so2), Sodium thiosulphate (Na2S2O3),  Sodium sulphate (Na2SO3), Sodium bi-sulphate (NaHSO4) etc.

Break Point Chlorination

Break Point ChlorinationThe break point chlorination method which gives an idea about demand of chlorine to be added in the raw water.

The amount of chlorine at which all microbes are destroyed, that amount of chlorine is called break point chlorination. When the chlorine is added beyond the break point chlorination all chlorines are converted into the residual chlorine.

type of chlorination
Break Point Chlorination

Residual Chlorine

Residual Chlorine – When chlorine is added into the water, It first reacts with Fe and Mn and oxidises it. After that it reacts with Ammonia and forms chloramines and finally reacts with the microorganism to destroy them completely. If any amount of chlorine left over that is going to store in the tank, that chlorine is called Residual chlorine. Residual chlorine prevents the contaminant of water in future during water supply. [ Chain Surveying]

In water treatment plant testing of residual chlorine is one of the most common tests. Water that has finished testing and water is ready to distribute the distribution pipe.


 Q1. What is the importance of chlorination?

Answer – The importance of chlorination is to remove the harmful bacteria in raw water.

Q2.  What is the role of chlorine in a water treatment plant?

 Answer – The role of chlorine in water treatment plants is disinfection.

Q3. How to calculate chlorine demand for water.

Answer – Chlorine demand is calculated by following formula

Cl2 demand = Cl2 dosage – Residual chlorine

 Q4. How to calculate chlorine in water.

Answer – The breakpoint chlorination is a method  to calculate the amount of chlorine in water treatment plant.

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