Chlorination

Chlorination

In this article we discuss about Chlorination

After reading you will be able to answer

1) what  is Chlorination

2) why Chlorination

3) forms of chlorine address

4) favourable conditions for chlorination,etc 

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Chlorination

At pH < 5, chlorine does not react with water and remains as free chlorine.

(HOCl OCl andCl ) 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.

Moreover chlorine will immediately 

react with ammonia present in water to 

form chloramines.

Chloramines

 Chloramines are combined form of 

chlorine. It is less effective than free 

chlorine (25 times lesser). But they are 

stable and remain in water for greater 

duration.

 In the usual chlorine treatment, in 

which PH is kept slightly less than 7, 

dichloramine is most predominate.

 These disinfectant kill those enzymes 

which are essential for the metabolic 

process of living organism.

Doses of chlorine should be sufficient so 

as to leave a residue of 0.2 mg per litre 

after 10 minutes of contact period. This 

does is called chlorine demand of water.

 The residual chlorine is tested by DPD 

(Diethyl-Paraphenylene diamine) test

FORMS IN WHICH CHLORINE IS ADDED

a) free chlorine (liquid or gaseous form)

b) Hypochlorite’s (Bleaching powder)

c) Chloramines (ammonia + chlorine)

d) Chlorine dioxide (ClO2)

FREE CHLORINE

Liquid form is mostly used. 

If temperature is below 10°C, liquid 

chlorine will get frozen into ice crystals 

which will stick and choke the lines of 

feeding. Hence liquid cylinder is kept 

at 32 - 48°C.

Steel cylinder burns in dry chlorine at 

temperature greater than 92°C. Hence 

high temperature is avoided. 

 Chlorine forms explosive mixture with 

carbon monoxide.

 Chlorine is applied through an 

equipment called chlorinator.

 Free Chlorine can be stored for long 

time without being deteriorated.

Chlorine dose can be easily measured in 

liquid forms. Hence under loading and 

overloading is less frequent. Chlorine is 

a powerful disinfectant and remains in 

water for a long time when ammonia is 

present.

No Sludge is formed in its application as 

may be produced in Hypochlorites and 

chloramines.

HYPOCHLORITE OR BLEACHING

OCl-and -HOCl are the disinfectant in 

this case. This process is called hypochlorination.

Bleaching powder

100% pure hypochlorite should contain 

free available chlorine equal to OCl 

value of the compound.

 E.g. 142 gm of Ca(Clo)2 will contain 102 gm of OCl 

i.e. 

free available chlorine in 100% pure 

calcium hypochlorite is 70%. But the 

actual chlorinating ability is lesser 

because bleaching powder is unstable 

and goes on losing its chlorine content 

when exposed to atmosphere.

 Hypochlorites are generally not used in 

modern days because they increase pH 

because to they are having lime content. 

Hypochlorite contains very low amount 

of chlorine.

It is used for swimming pools only.

USE OF CHLORAMINES

Chloromines are weaker disinfectants 

(25 times lesser than chlorine). Hence 

either higher dose or longer contact 

period is used.

Chloramines are stable and can remain 

in water for a long time contrary to 

unstable chlorine which evaporates 

after some time. Hence they provide

greater safeguard against future 

pollution.

They are weaker as compared to free 

chlorine but do not cause bad taste 

when left as residue.

 When phenol is present in water 

chloramines are mostly used because 

chlorine with phenols gives bad taste. 

But chloramines with phenol does not 

give any taste.

For producing chloramines ammonia is 

added to filtered water before adding 

chlorine.

Amount of ammonia should be 

1/3 to 1/4 of the amount of chlorine. They are added in water and mixed for 20 

minutes to 2 hours before adding 

chlorine. This contact period of

ammonia should be higher when phenol 

is present.

 Ammonia adding instrument is called 

ammoniator.

CHLORINE DIOXIDE (ClO2)

It is highly effective (2.5 times stronger than free chlorine).

ClO2 is highly unstable hence should 

be used immediately after production

 It may also be used when phenol is 

present and can also remove organic 

impurities.

 pH range is 8 – 10.

Normal dose is 0.5 – 1.5 mg/litre