Two methods of disinfection of municipal water are as follows  :

**1. By adding bleaching powder

      Bleaching powder reacts with water as,

      CaOCl2 + H2O → Ca(OH)2 + Cl2

        Cl2 + H2O → HCl + HOCl

                                              Hypochlorus acid

              HOCl → HCl + [O]

                                           Nascent oxygen**

In these reactions above, the HOCl deactivates the enzymes present in the cells of micro-organism.Thus 

the metabolic activity of micro-organism gets affected, thereby making the micro-organism inactive.Finally

it dies.

Disadvantages of bleaching powder

1. ​If used in excess,imparts bad smell and taste to treated water.
2. It produces excess calcium thereby making water more hard.

**2.  Chlorination

    The reaction of chlorine is as,**

Cl2 + H2O → HCl +HOCl

With very small concentration of chlorine. HOCl,thus formed destroys the bacteria ,

as it is a powerful germicide.

                                          The apparatus used for chlorination is called as 'chlorinator'.Application of Cl2

can be as gas or concentrated solution in water.

                                                                      The raw water and concentrated chlorine solution(0.3-0.5 ppm Cl2)

is passed through the chlorinator. The water and Cl2 solution get mixed thoroughly in the chlorinator due to baffle plates,

and sterilised water is collected using outlet.

The dis-infection is governed by nature of substances used, and sterilisation by chlorination is governed by temperature

of (1.) Water (2.) time of contact (3.) pH of water.

Factors affecting efficiency of chlorine

(I) Temperature of water : The efficiency increases with rise in temperature.

(II) Time of contact : With time, efficiency goes on decreasing.

(III) pH of water : At lower pH values (between 5-6.5) the efficiency is higher.

Advantages of using chlorine as disinfectant

1. The use of chlorine is more effective and economical.

2. It can be used at high as well as low temperatures.

3. It does not introduce any impurity in treated water.

4.It is stable,requires small space for storage and does not deteriorate on keeping.

   Chlorine is the most ideal disinfectant.

Disadvantages of using chlorine as disinfectant

1. If excess of chlorine is added,it produces bad taste and disagreeable color.

2. Excess chlorine produces irritation on mucus membrane.

3. It is not affective at higher pH values.

The treated water should not contain more than 0.1 - 0.2 ppm of free chlorine.