Chemical disinfection of water involves using chlorine or other disinfectants like ozone gas or UV light to eliminate bacterial impurities. Chlorine is the most widely used disinfectant, either as gas, bleaching powder, or chloramines formed by mixing chlorine with ammonia. The document discusses the properties and methods of chlorine disinfection, including pre-chlorination before treatment, post-chlorination after filtration, and super-chlorination to destroy resistant organisms during epidemics. Effective disinfection requires maintaining chlorine residuals of 0.2-0.8 ppm for at least 30 minutes of contact time.

1. Chemical disinfection of
water

2. • Even after filtration, water is still found to be having certain
impurities.
• These impurities may be grouped into:
a. Colour, taste and odour.
b. Dissolved inorganic salts.
c. Iron and manganese and
d. Bacteria.
• Elimination of bacterial impurities can be carried out by using
chemical disinfectants.

3. Methods of disinfection:
1. Chlorination.
2. Excess lime treatment.
3. Potassium permanganate (KMno4)
4. Ozone gas treatment.
5. Iodine and bromine treatment.
6. Ultra-violet ray treatment.
7. Silver treatment.

4. 1. Chlorination:
• In water supply, chlorine is the disinfectant almost
universally employed.
• Too little chlorine is ineffective and too much may cause
taste and odour.
• The equipment used for chlorine application is known as
chlorinator.

5. Chemical/Physical properties of chlorine include:
• Chlorine is a yellow-green gas at room
temperature.
• Chlorine has a pungent; irritating odor similar to
bleach that is detectable at low concentrations.
• The density of chlorine gas is approximately 2.5
times greater than air, which will cause it to initially
remain near the ground in areas with little air
movement.

6. Chemical/Physical properties of chlorine include:
• Chlorine is not flammable, but may react explosively or form
explosive compounds with many common substances
(including acetylene, ether, turpentine, ammonia, natural
gas, hydrogen, and finely divided metals).
• Chlorine is slightly water soluble, and reacts with moisture to
form hypochlorous acid (HClO) and hydrochloric acid (HCl).
• Chlorine is commonly pressurized and cooled for storage
and shipment as an amber-colored liquid.

7. • The point of mixing chlorine should give a contact
period between chlorine and water (20-30 minutes
before the consumption).
• In addition to its germicidal effect, chlorine oxidizes
iron, manganese and hydrogen sulphide.
• It destroys taste and colour producing constituents.
• It controls algae and slime and acids coagulation.
• Chlorine act against Schistosomiasis cercaria in a
concentration of 2 mg/ L for 30 minutes.

8. Theory of disinfection by chlorine:
"Enzymatic theory".
• According to this theory, the disinfecting agent reacts
with certain enzymes which are necessary for the
metabolic processes of living cells and render them
inactive.
• Inactivation of enzymes causes death of living cells.

9. Because the enzymes are developed
within the cell protoplasm, disinfection
proceeds in two stages:
1. Penetration of the cell wall by the
disinfection agent.
2. Its reaction with enzymes.

10. Factors affecting chemical efficiency of
chlorine:
1. Temperature:
With rise in temperature, the rate of
reaction with enzymes increases and
so death rate of harmful bacteria also
increases.

11. 2. Concentration of organisms:
the higher the concentration of
the organisms, the greater the
percentage destruction of the
organisms.

12. 3. Amount of disinfectant:
• The higher the concentration of
chlorine, more efficient will be.

13. 4. Time of contact:
A minimum 30 to 60 minutes
contact period must be provided
before delivering water for
consumption.

14. 5. pH value:
• The change in pH value of the water
affects the disinfecting process of chlorine.
• At lower PH value, a smaller contact
period is required to kill the same number
of the organisms.

15. 6. The presence of various chemicals:
• Various impurities present in water react
with chlorine.
• This reduces the chlorine available for
disinfection purpose.

16. Chlorine dose:
• The usual dose of chlorine varies from 0.5 to 1
ppm.
• If chlorine is insufficient, the disinfection of water
will be incomplete.
• If excess of chlorine is used water will emit bad
smell and taste.

17. • Studies conducted by Public Health Service
U.S.A. has confirmed that for effective disinfection
of water, 0.2 ppm (0.2 mg/L)
free residual chlorine is the minimum requirement
under most favourable conditions.
• If conditions are adverse 0.4 to 0.8 mg/L
should be the residual chlorine.

18. Action of chlorine with water having various
impurities:
1. If water is free from organic impurities and chlorine is
added to it, hypochlorous acid (Hocl) and hypochlorite
ions (ocl) are formed. The two species (Hocl) and (Ocl)
are called free available chlorine in practice and are
responsible for the disinfection of water.
2. If water contains ammonia and organic nitrogen
compounds, monochloramine, dichloramine and
trichloramines are formed.

19. Application of chlorine:
Chlorine can be applied in the following forms:
1. Free chlorine.
2. Bleaching powder.
3. Chloramines.
4. Electrolytically.

20. 1. Free chlorine:
• Chlorine may be available in gaseous or in liquid forms.
• In gaseous form, it may be dissolved in a little amount of
water then added to water for disinfection.
• The liquid chlorine is available in the form of liquid
cylinders that are commercially supplied for use.
• The chlorine is applied to water with the help of special
instrument, called "Chlorinators or Chloronomes".

21. Chlorine cylinder

22. Advantages of free chlorine:
1. It can be stored for the long time without any
deterioration in its quality and stability.
2. The pure chlorine gas is very powerful and efficient
in killing bacteria.
3. It is economically available.
4. The chlorine dosage is precise and there is no risk
of overdose or under dose.

23. Advantages of free chlorine:
5. There is no sludge formation in this case.
6. Working of chlorinators or chlorinomes is simple
and skilled supervision is not necessary.
7. Results obtained are uniform and reliable.

24. 2. Bleaching powder : Caocl2
• The available chlorine usually varies from 35 - 37 % in
freshly prepared bleaching powder.
• Adequate disinfection may be obtained when 1 gm of
high grade bleaching powder is added to 160 - 180 liters
which when expressed in terms of free chlorine amounts
to about 0.5 part of available chlorine to million parts of
water.

25. 2. Bleaching powder : Caocl2
• The amount should be increased when more organic
matter present.
• After chlorination the water should allowed to stand in an
open tank for four hours before being used.
• Some sodium thiosulfate may be used to remove the taste
of chlorine.

26. 3. Chloramines:
• Chlorine alone is not stable in water.
• When chlorine mixed with ammonia (at the ratio 4/1), it forms
chloramines.
• chloramines are stable in water and have disinfecting properties.
• The formation of particular type of chloramine depends upon the pH
value of water.
• Ammonia dissolves quickly in water but it dose not diffuse easily and,
hence it is necessary to mix with the help of mechanical mixers.
• Ammonia may be used in the form of gas or as solution or as
ammonium sulphate of as ammonium chloride.

27. Advantages of Chloramines treatment:
1. It is more effective than chlorine alone.
2. It is less affected by the presence of organic matter.
3. The disinfectant effect lasts for longer periods.
4. The quantity of chlorine required is less.
5. It is less irritant to eyes and nose (more useful for
treating water for swimming pools).
6. There is no danger of overdose.

28. Disadvantages of Chloramines treatment:
• The only draw back of the use of
chloramine is that its bactericidal effect is
very much slower than chlorine and a long
contact time is needed.

29. 4. Electrolytically:
• Chlorine may be produced electrolytically, by
passing an electric current through a solution of
sodium chloride.
• The sodium collecting at the negative pole and
the chlorine collecting at the positive pole.
• The chlorine liberated at the positive pole reacts
with water
• Because of the cost of electric current, this
method has not found very wide application.

30. Different forms of chlorination:
1. Plain-chlorination.
2. Pre-chlorination.
3. Post-chlorination.
4. Double chlorination.
5. Super-chlorination.

31. 1. Plain-chlorination:
• When chlorination is the only applied treatment to raw
water before supplying for the consumption.
• This is usually adopted for treating relatively clear water
(turbidity less than 30 ppm) obtained from natural lakes,
small reservoirs or deep wells.
• The quantity of added chlorine is about 0.5 ppm or a
little more.

32. 2. Pre-chlorination:
• If chlorination of raw water is done before any other
treatment.
• Chlorine dose is added just prior to settling process.
• This gives sufficient contact time for it to act on water.
• It helps in reducing the amount of coagulants required.

33. The advantage of pre-chlorination:
1. It reduces taste and odour of water.
2. It reduces bacterial load on filters so filter runs
longer
3. It improves coagulation so less quantity of
coagulant is required.
4. It prevents the growth of algae in sedimentation
tanks and filters.
5. It prevents putrifaction of sludge in settling basins.

34. 3. Post-chlorination:
• When chlorine is applied to water after all the treatment of
purification.
• It is the usual practice to add chlorine after the filtration of
water.
• After chlorination water is sent for distribution for
consumption.
• Residual chlorine of about 0.1 to 0.2 ppm is maintained till
water enters the distribution system.

35. 3. Post-chlorination:
• This residual chlorine will safe guard the water
while flowing through the distribution system.
• Dosage will depend upon the character of the
water and may be 0.25 to 0.5 mg/L in order to
obtain available residue 0.1 to 0.2 mg/L as the
water leaves the plant.

36. Advantages of post chlorination:
a. It completely destroys all the pathogenic bacteria.
b. It removes taste, colour and odour from water.
c. It checks weed growth in water.
d. It completely oxidizes ammonia.

37. 4. Double chlorination:
• When chlorine is added once before filtration and once
after filtration of water.
• Double chlorination treatment has the same advantages
as those of pre-chlorination treatment.
• By this arrangement the load on filters is greatly
reduced.

38. N.B.
The combination of pre-chlorination with post-
chlorination may be advisable or even
necessary if the raw water contains large
amount of bacterial, colloidal and organic
impurities in order that a satisfactory coliform
index or MPN will be obtained in the final
effluent.

39. 5. Super-chlorination:
• The process of chlorination beyond the stage of break point
is called, the super-chlorination.
• It is considerably in excess than that necessary for
adequate bacterial purification of water.
• The residual chlorine content which under normal
conditions is about 0.1 ppm, is raised to 0.5 ppm or even up
to 2 ppm by super-chlorination.
• When there is epidemic and water is likely to contain higher
amounts of organic impurities, super-chlorination of waters
the only answer.

40. • Super-chlorinated water gives a strong odour and
taste of chlorine, which can be removed by de-
chlorination process.
• Super-chlorination can effectively destroy the
relatively resistant organisms such as viruses and
amoebic cysts.
• The dose of chlorine may be as high as 10 to 15
mg/L with contact periods of 10 to 30 minutes.
• Excess chlorine will have to be de-chlorinated.

41. De-chlorination:
• The partial or complete reduction of residual
chlorine after treatment is called the de-
chlorination.
• But it will be preferred, if de-chlorination is done
to such an extent that some residual chlorine
remains in water.

42. The usual chemicals used are:
1. Sodium thiosulfate
2. Sodium bisulphate
3. Sodium sulphate
4. Activated carbon
5. KMno4
6. Sulphur dioxide