Explanation: Chlorination: Chlorination is used for: Disinfection, control of microorganisms, removal of ammonia, control of taste and odour, destruction of organic matter, hydrogen sulphide oxidation. Types of chlorination: [ alt="ft9 (51-86) images Q63" src="//storage.googleapis.com/tb-img/production/17/03/ft9%20%2851-86%29_images_Q63.PNG" style="height: 219px; width: 464px;"> 1. Plain chlorination: This term is used to indicate that only the chlorine treatment and no other treatment has been given to the raw water It is used for clean water, i.e turbidity between 20 - 30 mg/l Dose is 0.5 mg/l 2. Pre-chlorination: It is the process of applying chlorine to the water before filtration or rather before sedimentation and coagulation. Normal dose is 5 to 10 mg/l and pre-chlorination is always followed by post chlorination 3. Post chlorination: It is the normal standard process of applying chlorine in the end, when all other treatments have been completed. 4. Double chlorination: It simply means that the water has been chlorinated twice 5. Break-point chlorination It means an extent of chlorine is added to water. It represents that much dose of chlorination, beyond which any further additional chlorine will appear as free residual chlorine. 6. Super chlorination When excess chlorine is added to water during epidemic such that it gives a residual of 1 to 2 mg/l beyond break point is called as super chlorination. 7. Dechlorination When chlorine residue is high, excess chlorine will be removed by dechlorinating agent. The various dechlorinating agents are: i) Sodium Thiosulphate (Na2S2O3H2O) ii) Activated carbon iii) Sulphur dioxide (SO2) iv) Sodium bisulphate (NaHSO3) v) Ammonia as NH4OH vi) Sodium sulphate (Na2S2O3)