Types of chlorination-types-of-chlorination

TYPES  OF CHLORINATION

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a) Plain chlorination

 Only chlorination and no other
treatment is given to water.
It removes bacteria, organic matter
and colour.
 It is used for clean water i.e. turbity
between 20 – 30 mg/I.
Dose is 0.5 mg/I.

b) Pre Chlorination

 In this case chlorine is added before
filtration or rather before
sedimentation and coagulation.
 Doses required are such that 0.1 to
0.5 mg/I come to filter.
Normal dose is 5 to 10 mg/litre and
prechlorination is always by post
chlorination to ensure safety

Also read Air pollution

c) Post Chlorination

 Applying chlorine at the end when
all treatment is complete is called
post chlorination.
 The dose should be such that 0.1 to
0.2 mg/lit should be left after a
contact period of 20 min.

d) Double Chlorination

 Prechlorinationpsp & post-chlorination
combinedly is called double
chlorination.

e) Breakpoint Chlorination

During disinfection process amount of
residual chlorine is less in the beginning
(stage І) during which iron, nitrite etc.
are oxidized.
In stage П chloramines and combined
residual chlorine forms. Combined
residual chlorine will gradually increase
as demand for disinfection is satisfied.
 Actually chlorine residue is tested by
DPD test which measures both
combined and free chlorine.
 Amount of residual would be slightly
less than applied chlorine because some
bacteria killing takes place.
 At point C bad smell start coming out. It
is due to the fact that oxidation of
organic matter starts at point C. Hence
the residual decreases.
In the stage III free chlorine breaks
down chloramines into nitrogen
compounds.

2NH3+3Cl2   ----->> N2+6HCl

 Chloro-organic compounds are also
destructed. At point D bad smell
suddenly disappears. This implies that
organic matter oxidation is complete.
 Any further chlorine addition simply
appears as free chlorine i.e. chlorine
breaks away from water. Thus point D
is called break point.
 In general practice chlorine is added
beyond break point to ensure a residual
of 0.2 – 0.3 mg/litre as free chorine.
This residual takes care of future
recontamination.
Note:The difference of applied chlorine
and residual chlorine is called demand
of water.

f) Super Chlorination

When excess chlorine (5 to 15 mg/l)
is added in water during epidemic
such that it gives a residual of 1 to 2
mg/lit beyond break point is called
super chlorination.
When chlorine residue is high
excess chlorine will be removed by
dechlorinating agent. The various
dechlorinating agents are :
Sodium thiosulphate (Na2S2O3) –
cheapest of all.
 Activated carbon.
 Sulphur dioxide (SO2).