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Chlorination- Disinfection of waters with chlorine is generally referred to as chlorination. Chlorine was first used as a disinfectant around 1800 A.D. by de Morveau in France and by Cruikshank in England. The primary use of chlorine in sewage disposal system was for odor control, hydrogen sulphide destruction and prevention of septicity.
Although chlorine is a strong oxidising agent its toxicity against micro-organisms is not due to its oxidation effect. Actually chlorine interferes with the vital enzymes of microorganisms and this brings out their death.
At the same time the oxidising power of chlorine is inconvenient in disinfection procedures as it is consumed in reactions with reduced compounds like ammonia. Thus reducing its effectiveness as a disinfectant.
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Chlorine compounds:Chlorine when added to water undergoes various reactions with water and other constituents in water. The first important reaction is the formation of hypochlorous acid (HOCL)
Cl2 + H2O → HOCL + HCL
Hypochlorous acid is a weak acid and often dissociates to H+ and OCl- (chlorite) ions. The extent of dissociation actually depends on the pH. For example, at a pH of 6.7 about 90% of the chlorine added to water is in the form of HOCL with the remaining 10% in the dissociated form.
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At pH 9 only less than 5% remain in the undissociated form i.e. HOCL. The disinfection potential is greater with HOCL than with OCl-. For instance, HOCL is about 100 times toxic to E.coli than OCI-. Thus 6.7 is the preferred pH for disinfection with chlorine.
However in the presence of ammonia in water, hypochlorous acid reacts with ammonium ion and successively gives monochloramine or chloramide (NH2CI), dichloramine or chlorimide (NHCI2) and trichloramine or nitrogen trichloride (NCI3)
The reactions are as follows:
NH4 + + HCIO → NH2CI + H2O + H+
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NH2CI + HCIO → NHCI2 +H20
NHCI2 + HCIO → NCI3 + H2O
These chloramines, along with other chlorine compounds, are referred to as combined chlorine residuals or combined available chlorine. In contrast, chlorine, hypochlorous acid and chlorite ion are collectively referred to as free chlorine residuals or free available chlorine.
Although the combined residuals are more stable than free residuals they are less effective as disinfectants. Thus, in an indirect way, the concentration of ammonia in water influences the amount of chlorine required for disinfection.
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