The document discusses the importance of water disinfection to eliminate harmful microorganisms and ensure microbial safety in drinking water. Various disinfection methods are highlighted, including physical (filtration, boiling), radiation (UV), and chemical methods (chlorination, ozonation). The text outlines the objectives, ideal properties of disinfectants, and the advantages and disadvantages of different treatment options.

1. WATER SUPPLY ENGINEERING
(ENVIRONMENT ENGINEERING I)
DISINFECTION OF WATER
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ASST. PROF. PRACHI DESSAI

2. Disinfection of water
The chemical used for killing these bacteria are known as disinfectants and the process is known is
known as disinfection
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ASST. PROF. PRACHI DESSAI

3. Microorganisms – Found in almost all environments
Microorganisms
SOIL AIR
WATER
DOMESTIC/INDUSTR
IAL WASTE
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ASST. PROF. PRACHI DESSAI

4. Introduction
The presence of turbidity’s, color, minerals etc.
may not be dangerous but the presence of even single harmful organism will be definitely dangerous
thereby making disinfection as the most important process.
The disinfection not only removes the existing bacteria from the water at the plant but also ensures
their immediate killing even afterwards in the distribution system.
The contamination of water during its transit from the treatment plant to the place of its consumption
is also thus, prevented by disinfectants.
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ASST. PROF. PRACHI DESSAI

5. Disinfection Objectives and Approaches
Objective of disinfection is to produce microbially safe water by removing, inactivating or killing
pathogens. It primarily aims for safety from the risk of waterborne diseases.
Approaches: Disinfection of water can be achieved by:
Removal of pathogens: Filtering out all the pathogenic microorganisms
Inactivation or killing: Destroying the cellular structure of the micro-organisms or disrupting its
metabolism, biosynthesis or ability to grow/reproduce.
◦ In the case of bacteria, inactivation describes the subsequent inability of the microorganism to
divide and form colonies.
◦ For viruses, inactivation measures the inability of the microorganism to form plaques in host cells.
◦ For protozoan Cryptosporidium oocysts, it measures the inability of the microorganism to
multiply, thereby preventing consequent infection of a host by Cryptosporidium.
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ASST. PROF. PRACHI DESSAI

6. Properties of an Ideal Disinfectant
Broad spectrum: active against all microbes
Fast kinetics: produces rapid inactivation
Non-toxic; Non-flammable; Non-explosive
Effective in the presence of interfering constituents
Compatible with various materials/surfaces
Stable for the intended exposure period
Provides a residual, if desirable
Easy to generate and apply
Economical
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ASST. PROF. PRACHI DESSAI

7. Typical Methods for Disinfection
Physical methods: Filtration or Boiling
Radiation methods: Ultrasonic, Electronic, or UV irradiation
Chemical methods: Chlorination, Ozonation, or Other chemical agents
The most common disinfectant used is chlorine, which is used in different forms including Chlorine
gas, Hypochlorite, Chloramines and Chlorine dioxide.
Other chemical agents which have been used for disinfection include iodine, bromine, hydrogen
peroxide, ozone, copper/silver ions.
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ASST. PROF. PRACHI DESSAI

8. Boiling of water
i. Simple method
ii. Boiling to 100°C, kills most of the
pathogenic organisms, particularly viruses
and bacteria causing waterborne diseases.
iii. At least 20 minutes.
iv. Boiling requires a source of heat
v. Effective but not practically possible for
public water supplies
vi. Does not take care of future contamination
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ASST. PROF. PRACHI DESSAI

9. Treatment with ultraviolet rays
It can be produced by passing electric current
through mercury enclosed in quartz bulbs.
The water to be treated with ultraviolet rays
should, however, be less turbid and low in
colour.
Normally it should be colorless and turbidity
should not exceed 15 NTU.
The depth of water over the bulbs should not
generally exceed 10cm or so because these rays
can effectively penetrate through this much
distance only.
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ASST. PROF. PRACHI DESSAI

10. Treatment with ultraviolet rays
Advantages
◦ Does not impart any addition taste or odour to water , as no chemicals are added.
◦ Scope for treating small quantities of water. E.g. Hospitals, Dispensaries & Domestic
Disadvantages
◦ Method is very costly
◦ Needs technical knowhow
◦ Possesses possibilities of interruption due to failure of electricity.
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ASST. PROF. PRACHI DESSAI

11. Treatment with Ozone
It is produced by passing high tension electric current through the stream of air in a closed chamber
The nascent oxygen so produced is powerful oxidizing agent and removes the organic matter and
bacteria from water.
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ASST. PROF. PRACHI DESSAI

12. Treatment with Ozone
Advantages:
◦ Unstable nothing remains in water by the time it reaches in water to distribution system.
◦ Removes the colour, taste and odour from water in addition to removing bacteria from it.
◦ Water becomes tasty and pleasant
Disadvantages:
◦ Costlier than chlorination
◦ Needs electricity for its manufacture and hence can be used when electricity is available easily
and cheaply
◦ No residuals can be maintained hence it does not ensure safety against possible contamination.
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ASST. PROF. PRACHI DESSAI

13. Treatment with Lime
Addition of excess lime kills bacteria and removes the
bacterial load by about 99.3-100% from highly polluted
waters
Bacteria gets killed due to increased pH value resulted
by the addition of various lime
Disadvantages:
◦ Treatment like recarbonation for the removal of
excess lime for the further purification of water need
to be done hence not useful
◦ Cannot protect water from recontamination, hence
not used.
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ASST. PROF. PRACHI DESSAI

14. Treatment with iodine and bromine
They are not used for treating any large scale public
supplies but may be used for treating small water
supplies for army troops, private plants, swimming
pools etc.
The quantity of this disinfectants may be limited to
about 8 ppm and contact period of five minutes is
generally enough.
These disinfectants are now a days are also available in
form of pills and are thus very handy.
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ASST. PROF. PRACHI DESSAI

15. Treatment with potassium permanganate
This is used as popular disinfectant for disinfecting well
water in villages which are generally contaminated with
lesser amount of bacteria.
Normal doses of this disinfectant varies between 1-2mg/l
with a contact period of 4-6hrs.
Advantages
◦ Cheap
◦ Killing bacteria
◦ Oxidizes the taste producing organic matter hence it is
added in small doses to chlorinated water also.
◦ Used as an algaecide
◦ Removing colour and iron from water.
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ASST. PROF. PRACHI DESSAI

16. Treatment with potassium permanganate
Disadvantages
◦ Can’t guarantee 100% removal of bacteria.
◦ It can possibly remove organisms causing cholera , but
is of little use against other disease organisms.
◦ Water treated with potassium permanganate with the
passage of time , produces a dark brown precipitate ,
which is noticeable as a coating on porcelain vessels
and is difficult to remove without scouring.
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ASST. PROF. PRACHI DESSAI

17. Recommended Readings
i. Mark J. Hammer; Water and Waste Water Technology; Prentice Hall of India.
ii. S. K. Garg; Water Supply Engineering; Khanna Publ.
iii. B. C. Punmia, A. K. Jain; Water Supply Engineering; Laxmi Publication.
iv. G. S. Birdie; Water Supply Engineering and Sanitary Engineering; Dhanpat Rai.
v. R. C. Rangwala; Water Supply Engineering, Charotar Publ. House.
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