Action of chlorine

DISINFECTING ACTION OF CHLORINE

• When chlorine is added to water, it forms hypochlorous acid or hypochlorite ions.
• This causes the destruction of all microorganisms
• The hypochlorous acid is unstable and breaks into hydrogen ions and hypochlorite ions
• The above reaction is reversible and depends on the pH value of water
• Hypochlorous acid in itself is extremely effective in destroying microorganisms when compared to its constituent ions (H+ & OCl-)
• HOCl exists without dissociating into its constituent ions at pH range below 5.
• Hence the pH value of water during chlorination during chlorination is maintained slightly less than 7
• The germicidal action of free chlorine or combined chlorine is explained by the theory of enzymatic hypothesis which states that chlorine enters the cell walls of bacteria, reacts with, and kills the enzymes that are essential for metabolic processes of living organisms thereby bringing about disinfection.

DOSES OF CHLORINE

• The amount of chlorine required for disinfection depends upon the impurities present. When chlorine is added to water, it first reacts with inorganic impurities (S2-, Fe2+, Mn2+, NO2-, etc) that convert chlorine into chloride.
• Excess chlorine after this point is consumed by ammonia to form chloramines
• Simultaneously, chlorine reacts with organic impurities. Some organic impurities completely oxidise chlorine, while some chloro-organics formed have some oxidising power.
• Chlorine consumed in ALL the above reactions represents chlorine demand of water.
• Once chlorine demand of water is satisfied, chlorine appears as free chlorine
• Free chlorine and chloramines are responsible for germicidal action on bacteria and pathogens.
• It is considered safe if residual chlorine is 0.2ppm, 10 minutes after chlorination
• Generally, chlorine dose is increased during rainy season and during epidemics

FORMS OF CHLORINE

• Chlorine is generally applied as free chlorine
• in the form of liquid chlorine or gas
• Chlorine is generally applied as combined chlorine
• in the form of hypochlorites or bleaching powder
• in the form of chlorine tablets
• in the form of chloramines (mixture of ammonia and chlorine)
• in the form of chlorine dioxide

Molecular chlorine can be carried and applied to water to be treated either in a liquid form or gaseous form

Types of chlorination

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