What is the main reason for pH control during disinfection?

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SSC JE CE Previous Year Paper 9 (Held on 30 Oct 2020 Morning)
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  1. To inhibit the reaction of chlorine with water
  2. To ensure that powerful residual hypochlorous acid (HOCl) is formed
  3. To ensure a good water pH in the distribution system
  4. To ensure only HCl is formed

Answer (Detailed Solution Below)

Option 2 : To ensure that powerful residual hypochlorous acid (HOCl) is formed
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Detailed Solution

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Explanation:

Disinfection  may be defined as the process of destruction or inactivation of harmful micro-organism in water either by physical process or chemical process.

Note:

pH plays an important role during disinfection

  • At pH < 5, chlorine does not react with water and remains as free chlorine.
  • (HOCL +OCL-) are combined called freely available chlorine. Out of these forms of freely available chlorine, HOCL is most destructive. It is 80% more effective than OCL- ion. Hence pH of water should be maintained slightly below 7

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For chemical disinfection of water the following disinfectants can be used:

  • Chlorine (Cl2)
  •  Chlorine dioxide (ClO2)
  • Hypo chlorite (OCl-)
  • Ozone (O3)
  • Halogens: bromine (Br2), iodene (I)
  • Metals: copper (Cu2+), silver (Ag+)
  • (KMnO4)
  • Alcohols
  • Soaps and detergents
  • Hydrogen peroxide
  • Several acids and bases


For physical disinfection of water the following disinfectants can be used:

  • Ultraviolet light (UV)
  • Electronic radiation
  • Gamma rays
  • Sounds
  • Heat 
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