Water supply engineering topic:- Disinfection of water.

1. DISINFECTION OF WATER

2. What is Disinfection?
• Disinfection is the process of the removal, deactivation or killing of pathogenic
microorganisms.
• It is also known as sterilization.
• Microorganisms are deactivated, resulting in the termination of growth and
reproduction such that they represent no significant risk of infection.

3. Why disinfection is done?
• When microorganisms are not removed from drinking water, drinking water usage will cause
people to fall ill.
• When water leaves the filter plant, it is still found to contain some of the impurities. These
impurities can be grouped as: -
1. Bacteria, Viruses, Protozoa
2. dissolved inorganic salts,
3. Color, odor and taste,
4. iron and manganese.
Therefore, for removing such impurities disinfection is done and the chemical or material used for
killing the bacteria are known as disinfectants.

5. Requirement of Disinfectants
• It should be able to destroy all types of pathogens and in whatever number present in the water.
• Function properly regardless of any fluctuations in the composition or condition of the water.
• Function within the temperature range of the water.
• Not cause the water to become toxic or unpalatable.
• Be safe and easy to handle.
• Be easy to determine its concentration in the water.
• Provide residual protection against recontamination.

6. TYPES OF DISINFECTION
Disinfection can be done by two methods:
1. Physical Method:-
• Heating to Boiling
• Incineration
• Irradiation with X-rays or UV rays
2. Chemical Method
• Chlorination
• Ozonation

7. DISINFECTANTS USED
• For Chemical Disinfection of water the following disinfectants can be used:
-Chlorine (Cl2)
- Ozone (O3)
- Halogens: bromine (Br2), iodine (I)
- Bromine chloride (BrCl)
- Metals: copper (Cu2+), silver (Ag+)
- Alcohols
- Soaps and detergents

8. • For Physical Disinfection of water the following disinfectants can be used:
-Ultraviolet light (UV)
-Electronic radiation
-Gamma rays

9. Minor methods of disinfection
• Boiling of water
• Treatment with excess lime
• Treatment with ozone
• Treatment with iodine and bromine
• Treatment with u-v rays
• Treatment with KMno4
• Treatment with silver

10. Following are the minor method of disinfection
1.Boiling :
• The bacteria present in water can be killed by boiling it for long time. This is most
effective method of disinfection of water .
• However, this method cannot be used to disinfect water on a large scale for public
supplies.
2.Excess lime treatment:
 It has been found that when pH value of water is greater than 9.5, E-coli and other
bacteria present in water cannot survive.
 Thus enough lime is added to water to raise its pH value to 9.5 or more most of
the bacteria will be killed.

11. 3.IodineTreatment:
• In this method iodine is used for disinfection of water. Iodine when added to water forms
hypoiodous acid (HIO) and the dissociation of the hypoiodous acid resulting in the
formation of hypoiodite. The quantity of this disinfect is limited to 8ppm.
4. Potassium permanganate treatment:
• It works as a powerful oxidising agent and it is found to be effective in killing cholera
bacteria. However, it is less effective in killing bacteria of other water – borne diseases.

12. 5. Silver treatment :
• It has been found that silver has the property of disinfecting water as it is
effective in destroying the bacterial spores and algae present in
comparatively clear water.
• It can be introduced in water either in form of silver salt or by immersing
silver coated electrodes in water and applying an electrical potential of
about 100 volts.
• It is also known as Electro-Katadyn process.

13. CHLORINATION
• Chlorination is the process of adding the element chlorine to water as a method of water
purification to make it fit for human consumption as drinking water.
• Chlorine is the most widely used disinfectant. Chlorine compounds can destroy disease-causing
organisms quickly — usually after 30 minutes.
• Chlorine is applied to water to be treated, through an instrument called as chlorinator or
chloronome.

14. WHY DO WE CHLORINATE WATER?
• A leading advantage of chlorination is that it has proven effective against bacteria and viruses.
• The chlorination process is fairly easy to implement when compared to other water treatment
methods.
• It is an effective method in water emergency situations as it can eliminate an overload of pathogens
relatively quickly.

15. Various forms in which Chlorine can be applied
1.As free chlorine:
-In the form of liquid chlorine or as chlorine gas.
2. As combined chlorine:
- In the form of hypochlorite or bleaching powder.
- In the form of chlorine dioxide.

17. TYPES OF CHLORINATION
1. Plain Chlorination: Only chlorine treatment and no other treatment. It helps to remove bacteria
and color water and it also controls the growth of algae. It is used in case of emergencies when
full-fledged treatment cannot be given. Also adopted for clean water. The used quantity of
chlorine required for plain chlorination is about 0.5 mg/l or more.
2. Pre-chlorination: Chlorine is added before raw water enters sedimentation tanks. It helps in
reducing the number of coagulants and improves coagulation. It reduces the taste and odor of
water. The chlorine dose should be such that about 0.1-0.5 mg/l of residual chlorine comes to
the filter plant. The normal doses required are as high as 5-10 mg/l.

18. 3. Post chlorination: This is the standard form of chlorination in which chlorine is added to water
as it leaves filters and before it enters the distribution system. The residual chlorine present in water
is useful for its protection against contamination in the distribution system. The dosage of chlorine
should be such as to leave residual chlorine of about 0.1 to 0.2 mg/l
4. Double chlorination: The term double chlorination is used to indicate that water has been
chlorinated twice. It consists of pre-chlorination and post-chlorination. Adopted when raw water is
highly contaminated and contains a large amount of bacterial life.

19. 5. Break Point Chlorination: Break point chlorination gives an idea of chlorine to be added to the
water so as to ensure the desired amount of residual chlorine. It represents, that much dose of
chlorination, beyond which any further addition of chlorine will appear as free residual chlorine.
Figure indicating break–point chlorination.

20. 6. Super chlorination: The application of chlorine to water beyond the stage of breakpoint is known
as super chlorination. It is most commonly added at the end of filtration. It is adopted whenever there
is a breakout of waterborne diseases. The huge quantity of chlorine which is added in super
chlorination is such as to give about 1-2 mg/l of residue beyond the break point.
7. Dechlorination: Dechlorination is the process of removing excess chlorine by dechlorinating
agents. This is to avoid chlorinous taste from water. Chemicals used for dechlorination are sulphur
dioxide, sodium thiosulphate, sodium bio sulphate, activated carbon, potassium permanganate,
ammonia etc.