Question

What are the types of chlorination?

Answer 1

1. Chlorination is a method of disinfecting water to make it fit for human consumption through the addition of Chlorine or chlorine-containing substances.
2. Chlorine is available as a dry powder/pellet of Calcium hypochlorite $\mathrm{Ca}{\left(\mathrm{ClO}\right)}_2$ (also known as bleaching powder) or as a liquid of Sodium hypochlorite ($\mathrm{NaOCl}$). The different types of chlorination processes are mentioned below.
3. Simple Chlorination: It is the process of adding a small volume of Chlorine solution to the top of the water well and circulating the Chlorine throughout the whole system. This process can be effective for existing water supply systems but not for new ones. Even for existing systems, it should be used only for the first treatment
4. Super Chlorination: Super chlorination is the process in which abnormally large amounts of Chlorine are added to water in very short periods which ensures the disinfection of water. This is generally used when the water has a very high bacteria presence or comes from a polluted source. It's also an important part of swimming pool maintenance.
5. Dechlorination: This process is carried out after super chlorinating the water because super chlorination can corrode components of the water supply system and can lead to foul-smelling water. Dechlorination removes the Chlorine from water. It is achieved by adding either Sulphur dioxide (${\mathrm{SO}}_2$) or Sodium sulphite (${\mathrm{Na}}_2{\mathrm{SO}}_3$).
6. Prechlorination: This is an industrial water treatment process which involves adding Chlorine to the raw water before sending it for further treatment. Prechlorination helps in the water treatment process by aiding coagulation, controlling algae growth, and eliminating micro-organisms such as protozoa, bacteria and viruses.
7. Postchlorination: This is the final step of industrial water treatment. It is the addition of Chlorine to water to prevent microbial growth. The Chlorine concentration should be 0.1 to 0.2 ppm, ideally. Since adding Chlorine to water makes it acidic, lime ($\mathrm{CaO}$) is added along with Chlorine to counter the acidity.
8. Breakpoint Chlorination: When Chlorine is added to water it reacts with and breaks down the contaminants. Thus, any Chlorine added gets “used up” to sanitize the water. Breakpoint chlorination involves the addition of Chlorine till all of it has been used up to break down the contaminants and no freely available chlorine is left.
9. Multiple Chlorination: Multiple chlorination involves the addition of Chlorine at multiple points during the water treatment. It becomes necessary when the water being treated has high bacterial content. Pre- and Post-chlorination processes are known together as Double chlorination.
10. Thus, chlorination is the process of disinfecting water through the addition of Chlorine. Chlorination processes can be carried out in various ways depending on the type of water being treated.