Engineering Chemistry S Datta

WATER TREATMENT 363
• These filters are rectangular tanks, which contain layers of fine sand at the top of one
meter thick, at middle of coarse sand of 0.5 meter thick and at bottom a gravel of 0.5
meter thick.
Sterilization
(i) By addition of bleaching powder (1 kg per 1000 kiloliters).
Here sterilization is effected by the HOCl generated by the following reaction:
Ca(OCl)Cl + H 2
O ⎯⎯→ Ca(OH) 2
+ Cl 2
Cl 2
+ H 2
O ⎯⎯→ HCl + HOCl
and this HOCl kills the germs.
Disadvantages of this process:
(i) Bleaching powder makes water unnecessary hard and sometimes imparts a bad taste
to the water.
(ii) By direct chlorination: Here also the generated HOCl kills the pathogenic bacteria.
Liquid chlorine is the most effective reagent, 0.3-0.5 ppm chlorine is sufficient.
Advantage of using Chlorine. It is effective and economical; chlorine does not make
water hard and requires a little space for storage.
Disadvantages. Excess chlorine imparts unpleasant taste and odour to the treated
water. Dissolved chlorine causes irritation to mucus membrane and lowers the pH of water
below 6.5. The treated water should contain chlorine less than 0.2 ppm.
(ii) Break-point Chlorination
It is seen that when liquid chlorine is added or chlorine gas is passed through water the
consumption of chlorine makes the available chlorine less and after some time the available
chlorine increases. This is due to the consumption of chlorine in oxidising some oxidisable
organic matter present in water at first. Then available chlorine again increases due to
decomposition of chloro-organic compounds.
This free chlorine is responsible for destruction of pathogenic bacteria in water. The
point at which the residual chlorine begins to appear is known as break point and at this point
water is devoid of bad taste and odours and is bacteria free.
Advantages of break point chlorination
• Organic matters present in water are completely oxidised leaving behind odour-free
water and sometimes colour-free water.
• Pathogenic bacteria are destroyed completely.
• It prevents the growth of weed in water in future.
(iii) Sterilization by Chloramine
When chlorine and ammonia are mixed in the ratio 2:1 of volume chloramine is formed.
Cl 2
+ NH 3
⎯⎯→ Cl.NH 2
+ HCl
Chloramine is much more effective than chlorine in its bactericidal (bacteria killing)
action. Here also HOCl is liberated.
Cl.NH 2
+ H 2
O ⎯⎯→ HOCl + NH 3
HOCl itself is bactericidal and here the liberated nascent oxygen is also bactericidal
HOCl ⎯⎯→ HCl + [O].