Short Note on Chlorination of Water

CHLORINATION OF WATER :-

The process of disinfection of water with the use of chlorine in its various forms is called the chlorination of water. It is cheap, reliable, easy to handle, easily measurable and above all, it is capable of providing residual disinfecting effects for long period, thus affording complete protection against future recontamination of water in the distribution system.

Generally following three methods are adopted for chlorination of water using different chlorine producing agents.

(a) Bleaching powder solution is added to the water, the chlorine available in bleaching power is used to disinfect the water. This method is suitable only for smaller requirement of water.

(b) Chlorine is produced by electrolysing a solution of brine or common salt and the chlorine so produced is used for disinfection.

(c) Chlorine is obtained directly from the cylinders filled with pure chlorine in gaseous or liquid form. This method is generally adopted for larger size of water supply plants The residual free chlorine available one-hour after chlorination should be 0.5 mg/L.Chloroscopes to know residual chlorine and Horrocks apparatus to assess chlorine demand of water should be available with all Engineering staff in-charge of chlorination and all Health Inspectors. 

DISINFECTING ACTION OF CHLORINE:-

Cl2 +H2 O ƥH>5 HOCL + HCL

Hypochlorous Acid

In the ƥH range below 5, chlorine does not react and remains as elemental chlorine The hypochlorous acid is unstable and may break into hydrogen ions and hypochlorite ions

HOCL ƥH>8 H+ + OCL-

ƥH<7 Hypochlorite ions

The above reaction is reversible and depends upon the ƥH value of water. Out of these forms of free available chlorine, the hypochlorous acid is most destructive, being about 80 times more effective than hypochlorite ions. For this reason, the ƥH value of water during chlorination is generally maintained slightly less than 7, thereby keeping more HOCL in solution compared to OCL ions. Moreover, the chlorine will immediately react with ammonia present in water to form various chloramines as:-

NH3 + HOCL NH2 CL + H2 O

(Mono chloramines)

NH2 CL + HOCL NHCL2 + H2 O

(Di- chloramines)

NHCL2 + HOCL NCL3 + H2 O

(Nitrogen Tri-chloramines)

The chloramines so formed are stable and found to possess disinfecting properties. When the added chlorine has consumed all the ammonia available in water, then it would persist as free chlorine. The combined chlorine with ammonia in the form of chloramines is much less effective in causing disinfection compared to the free chlorine, being about 25 times less effective.

DOSES OF CHLORINE:- The amount of chlorine required for water depends upon the inorganic and organic impurities present in it. When chlorine is added to water, it first of all reacts with the inorganic impurities like S- - , Fe++, Mn++, NO2- etc, that convert the chlorine into chloride, which has no residual oxidizing powder. Excess chlorine, after this point is consumed by ammonia to form chloramines. Simultaneously, chlorine will also react with organic impurities present in water. The chlorine consumed in all the above reactions represents the CHLORINE DEMAND OF WATER. When once it gets satisfied, the chlorine will appear as FREE CHLORINE. In general, most of the waters are satisfactorily disinfected if the free residual chlorine is about 0.5 mg/liter, 30 minutes after the chlorine is applied.

VARIOUS FORMS IN WHICH CHLORINE CAN BE APPLIED:-

1. In the form of liquid chlorine or as chlorine gas (As free chlorine).

2. In the form of hypochlorite or Bleaching Powder.

3. In the form of chloramines i.e. a mixture of ammonia & Chlorine.

4. Use of Chlorine Dioxide gas.

5. Use of Chlorine tablets.