ALL CONTENTS ARE FROM CURRENT EDITION OF PARK'S PREVENTIVE AND SOCIAL MEDICINE....

1. Purification of Water
SURAJ DHARA
MIDNAPORE MEDICAL COLLEGE

2. Purification Of Water
• Purification of is done by TWO ways:
1. Natural Methods
2. Artificial Methods

3. NATURAL METHODS
• Natural Method includes:-
 Aeration
 Sedimentation
 Sunlight
 Dilution
 Oxidation
 Plants & Animals (Aquatic)

4. Artificial Methods
• They may be carried out at:
A. Purification of Water on Large Scale
B. Purification of Water on Small Scale

5. A. Purification of Water on Large Scale
• At Large scale, it is done by Filtration,
which may be
i. Slow sand filtration
ii. Rapid sand filtration
iii. Desalination

6. B. Purification of Water on Small
Scale / Domestic Level
• At Small scale or Domestic level, is
carried out by:
i. Boiling
ii. Chemical disinfection
iii. Filtration
iv. Solar radiation

7. A. Purification of Water on Large Scale
• The purpose of water treatment is to produce water
that is safe & wholesome.
• The method of treatment to use depends upon the
nature of raw water & the desired standards of
water quality.
Ex: Ground water (wells & springs) may need no
treatment other than disinfection.
• Surface water (e.g River water) which tends to be
turbid & polluted, require extensive treatment.
• The components of a typical water purification
system comprise one or more of the following
measure.

8. A. Purification of Water on Large Scale
• A typical water supply carries the following
steps in Purifying the water:
I. Storage
II. Filtration
III. Disinfection

9. I. STORAGE
• Water is drawn out from the source and
impounded in natural or artificial reservoirs.
• Storage provides of a reserve of water
from which further pollution is excluded.
• As a result of storage, a very considerable
amount of purification takes place.
• This natural purification & we may look at
it from three points of view:

10. I. STORAGE
(a) Physical:
• By mere storage the quality of water improves.
• About 90% of the suspended impurities settle
down in 24 hours by gravity.
• The water becomes clearer.
• This also improves the turbidity of water.
• This allow penetration of light & reduce the work
of filters.

11. I. STORAGE
(b) Chemical:
• Certain chemical changes also take place
during storage.
• The aerobic bacteria oxidize the organic
matter present in the water with the aid of
dissolved oxygen.
• As a result the content of free ammonia is
reduced and a rise in nitrates occurs.

12. I. STORAGE
(c) Biological:
• A tremendous drop takes place in bacterial
count during storage due to antibiosis and
oxidation.
• The pathogenic organism gradually die out.
• It is found that when river water is stored the
total bacterial count drops by as much as 90
percent in the first 5-7 days.
• This is one of the greatest benefits of storage.
• The optimum period of storage of river water is
considered to be about 10-14 days.

13. I. STORAGE
(c) Biological:
• If the water is stored for long periods, there is
likelihood of development of vegetable growths
such as algae which impart a bad smell and
colour to water.

14. III. FILTRATION
• It is the oldest and Universal method of
purification.
• Filtration is the second stage in the purification of
water.
• This filtration reduces the bacterial content by 98-
99%, turbidity by 50 PPM to 5 PPM and colour to
colurless.

15. III. FILTRATION
• There are two types of Filters:
 SLOW SAND FILTERATION (BIOLOGICAL
FILTER).
 RAPID SAND FILTERS ( MECHANICAL
FILTERS).

16. SLOW SAND FILTRATION (BIOLOGICAL FILTER)
• They are cheap and easy to design and
occupy less space.
• Slow sand filters were first used for water
treatment in 1804 in Scotland and
subsequently in London.
• During the19th century their use spread
throughout the world.
• Even today they are generally accepted as the
standard method of water purification.

17. SLOW SAND FILTRATION (BIOLOGICAL FILTER)
• They contain the concrete made basin, which
contains the selected graded sand, supported
on stones. The contents are
1) Supernatant Water Head 1 – 1.5 meter for
Raw Water
2) Sand Bed, which consists of sand particles,
supported by fine and coarse gravel.
3) Drainage system
4) Filter Control Valve

18. 1) SUPERNATANT WATER HEAD
• The supernatant water above the sand bed,
whose depth varies from 1 to 1.5 meter, serves
two important purpose.
1. It provides a constant head of water so as to
overcome the resistance of the filter bed &
thereby promote the flow of water through the
sand bed.
2. It provides waiting period of some hours (3
to 12 hours) for the raw water to undergo
partial purification by sedimentation, oxidation
& particle agglomeration. The level of
supernatant water is always kept constant.

19. 2. SAND BED
• It is very important part of the sand bed.
• The thickness of the sand bed is a about 1
meter.
• The sand grains are carefully chosen so that
they are preferably rounded and have an
effective diameter b/w 0.2 to 0.3 mm.
• The sand should be clean and free from clay &
organic matter.
• The sand bed is supported by a layer of
graded gravel 30 to 40 cm deep which
prevents the fine grain being drained into the
drainage pipes.

20. 2. SAND BED
• The sand bed presents a vast surface area,
one cubic meter of filter sand presents some
15,000 sq.meter of surface area.
• Water percolates the through the sand bed
very slowly, process taking 2 hours or more.
• It is subjected to a number of purification
processes- mechanical straining,
sedimentation, adsorption, oxidation &
bacterial action, all playing their part.
• The designed rate of filtration of water usually
lies b/w 0.1 to 0.4 m3/ hour/ per square meter
of sand bed surface.

21. 2. SAND BED
Vital Layer/ (Biological Filter)
• When the filter is newly laid, it acts merely as a
mechanical strainer & can not truly be
considered as Biological.
• But very soon the surface of the sand bed gets
covered with a slimy growth known as “
Schmuztadecke” “Vital Layer” “Zoogleal layer”
or “Biological Layer”.
• This layer is slimy & gelatinous and consist of
threadlike algae and numerous forms of life
including plankton, diatoms and bacteria.

22. 2. SAND BED
• The formation of vital layer is known as “ripening
of the filter.
• It may take several days for the vital layer to form
fully and when fully formed it extends for 2-3 cm
into the top portion of the sand bed.
• The vital layer is the “heart” of the slow sand filter.
• It removes organic matter, holds back bacteria and
oxidizes ammoniacal nitrogen into nitrates and
helps in yielding bacteria-free water.
• Un till the vital layer is fully formed, the first few
days filtrate is usually run to waste.
• It is Scrapped, when it becomes thick.

23. 3. UNDER-DRAINAGE SYSTEM
• At the bottom of the filter bed is the under-drainge
system.
• It consist of porous or perforated pipes which
serve the dual process of providing an outlet for
filtrated water and supporting the filter medium
above.
• Once filter bed been laid, the under-drainage
system can not be seen.

24. FILTER BOX
• The filter box is an open box, usually
rectangular in shape, from 2.5 to 4 meters
deep & is bulit wholly or partly below ground.
• The walls may be made of stone, brick or
cement.
• The filter box consist from top to bottom:
1.Supernatant water 1 to 1.5 meters.
2. Sand Bed 1.2 meters
3. Gravel Support 0.30 meter
4. Filter bottom 0.16 meter

25. 4. FITER-CONTROL
• Filter control is equipped with certain valves and
devices which are incorporated in the outlet-pipe
system.
• The purpose of these devices is to maintain a
constant rate of filtration.

26. FITER-CLEANING
• Naturally the filter may run for weeks or even months
without cleaning.
• When the bed resistance increases to such extent that
the regulating valve has to be kept open fully.
• It is time to clean filter bed, since any further increase
in resistance is bound to reduce the filtration rate.
• At this stage supernatant water is drained off & the
sand bed is cleaned by “scrapping” off the top portion
of the sand layer to the depth of 1 to 2 cm.
• After several years of operation & say 20 to 30
scrapings, the thickness of the sand bed will have
reduced to about 0.5 to 0.8 meter.
• Then the plant is closed down & a new bed is
constructed.

27. ADVANTAGE OF SLOW SAND FILTRATION
• Simple to construct & operate.
• The cost of construction is cheaper than that of
rapid sand filters.
• The physical, chemical & bacteriological
quality of filter water is very high.
• It will reduce total bacterial count by 99.99
percent & E. coli by 99.9 %.

28. RAPID SAND FILTERS ( MECHANICAL FILTERS)
• In 1885 the first rapid sand filters were
installed in the USA.
• Since that time, they have gained considerable
popularity even in developing countries.
• Rapid Sand Filters are of two types:
• A. Gravity type (e.g Paterson’s Filter)
• B. Pressure type ( Candy’s Filter)
• Both types are in use.
• Following steps are involved in the purification
of water by Rapid Sand Filters.

29. RAPID SAND FILTERS ( MECHANICAL FILTERS)
• Following steps are involved in the purification of water
by Rapid Sand Filters.
• 1. COAGULATION:
• The raw water is first treated with a chemical coagulant
such as alum.
• The dose of alum varies from 5-40 mg or more per
liter, depending upon the turbidity & colour,
temperature and the pH value of water
• 2. RAPID MIXING:
• The treated water is then subjected to violent agitation
in a “mixing chamber” for few minutes.
• This allows a quick and thorough dissemination of
alum throughout the bulk of water, which is very
necessary.

30. RAPID SAND FILTERS ( MECHANICAL FILTERS)
• 3. FLOCCULATION:
• The next phase involves a slow & gentle stirring of
the treated water in a “Flocculation Chamber” for
about 30 minutes.
• The mechanical type of flocculation is the most
widely used.
• It consist of a number of paddles which rotates at
2 to 4 rpm with the help of motors.
• It results in the formation of thick, copious, white
flocculent precipitate of aluminum hydroxide.
• The thicker the precipitate or flock diameter, the
greater the setting velocity.

31. RAPID SAND FILTERS ( MECHANICAL FILTERS)
• 4.SEDIMENTATION:
• The coagulated water is now led into
sedimentation tanks where it is detained for
periods varying from 2-6 hours, when the
flocculent precipitate together with impurities &
bacteria settle down in the tank.
• At least 95 % of the flocculent precipitate needs to
be removed before the water is admitted into the
rapid sand filters.
• The precipitate or sludge is which settles at the
bottom is removed from time to time without
disturbing the operation of the tank.

32. RAPID SAND FILTERS ( MECHANICAL FILTERS)
• 4.SEDIMENTATION:
• For proper maintenance the tank should be
cleaned regularly from time to time, otherwise they
may become a breeding ground for molluscs and
sponges.
• 5.FILTERATION:
• The partly clarified water is now subjected to rapid
sand filters.

33. RAPID SAND FILTERS ( MECHANICAL FILTERS)
Mixing
Chamber
Flocculation
Chamber
Sedimentation
Tank
Fitters
Chlorine
Clear
water
storage
RiverWater
Consumption
Alum

34. FILTER BEDS
• Each unit of filter bed has a surface of about 80 to
90 m2 (900 sq. feet).
• Sand is the filtering medium.
• The effective size of the sand particles is b/w 0.4
to 0.7 mm.
• The depth of the sand bed is usually about 1
meter.
• Below the sand bed is a layer graded gravel 30 to
40 cm deep.
• The gravel supports the sand bed and permits the
filtered water to move freely towards under the
drain.

35. FILTERATION
• As filtration proceeds, the “alum-floc” not removed
by sedimentation is held on the sand bed.
• It forms a slimy layer layer comparable to the
Zoogleal layer in the slow sand filters.
• It absorbs bacteria from the water & effects the
purification.
• Oxidation of ammonia takes place during the
passage of water through the filters.
• As filtration proceed the suspended impurities and
bacteria clog the filters.
• The filters soon become dirty & begin to loose
their efficiency.

36. FILTERATION
• When the “loss of head” approaches 7-8 feet,
filtration is stopped & the filters are subjected to a
washing process known as “Back Washing”.

37. BACK WASHING
• Rapid sand filters need frequent washing daily or
weekly depending upon the loss of head.
• Washing is accompanied by reversing the flow of
water through the sand bed which is called Back
Washing.
• It removes impurities & cleans the sand bed.
• The whole process of washing takes about 15
minutes.
• In some rapid sand filters compressed air is used
as part of back washing.

38. ADVANTAGE OF RAPID SAND FILTRATION
• Rapid sand filter can deal with raw water directly.
No preliminary storage is required.
• The filter bed occupy less space.
• The initial cost is high but becomes cost
effective in future.
• Filtration is rapid, 40 to 50 times that of slow sand
filters.
• The washing of the filters is easy.
• There is more flexibility in operation.

39. III. METHOD OF CHLORINATION
• For disinfection on large scale water chlorine is
applied either as:
 Chlorine Gas.
 Chloramines.
 Perchloron.
• Chlorine gas is the first choice , bcz it is cheap, quick
in action, efficient and easy to apply.
• But chlorine gas is an irritant to the eyes &
poisonous.
• A special “Chlorination equipment” known as
Paterson’s chloronome is required for measuring,
regulating and administration of chlorine gas to
water.

40. DISINFECTION/ CHLORINATION
• Chlorination is one of the greatest advances in water
purification.
• It is supplement, not a substitute to sand filtration.
• Chlorine kills pathogenic bacteria, but it has no effect
on spores & certain viruses (e.g Polio, viral
hepatitis).
• Apart from its germicidal effect, chlorine has several
important secondary properties in water treatment.
• It oxides' iron, manganese & hydrogen suphide.
• It also helps in destroying some odour producing
constituents, so improves the taste and odour.
• It controls algae and slim organisms & acid
coagulation.

41. USE OF CHLORINE IN PURIFICATION OF WATER
• Chlorine can be used in different ways to
purify the water.
1. Simple Chlorination
2. Chloramination
3. Super chlorination followed by
Dechlorination

42. SIMPLE CHLORINATION
• When Chlorine is added to water, there is a
formation of hypochloric & hypochlorous acids.
• The hypochloric acid is neutralized by the alkalinity
of the water.
• The hypochlorous acid ionizes to form hydrogen ions
& hypochlorite ions.
• H2O+ Cl2 HCl + HOCL
• HOCl H + OCL
• The disinfection action of chlorine is mainly due to
the hypochlorous acid & due to small extent due to
the hypochlorite ion.
• Hypochorous acid is the most effective form of
chlorine for water disinfection.

43. SIMPLE CHLORINATION
• Chlorine acts best as a disinfection when the
pH of water is around 7 bcz of predominance
of hypochorous acid.
• When the pH value exceeds 8.5 it is un reliable
as a disinfectant because about 90 % of the
hypochlorous acid gets ionized to hypochlorite
ion.

44. CHLORAMINATION
• In this process NH3 is added to water First
and is followed by addition of Chlorine.
• It exerts its effect on bacteria by forming
chloramines.
• The contact time must be 2 hours.
• These are more effective and stable for
longer period of time.
• The Ratio of NH3 to Cl2 is 1:4 or 1:5

45. BREAK POINT CHLORINATION
• The addition of chlorine to ammonia in water
produces chloramines which do not have same
efficiency as free chlorine.
• Further addition of chlorine in water will cause
reduction in residual chlorine due destruction of total
chloramines by added chlorine.
• The end product do not represent any residual
chlorine.
• This fall in the residual chlorine will continue with
further increase in chlorine dose & after a stage the
residual chlorine begins to increase in proportion to
the added dose of chlorine.

46. BREAK POINT CHLORINATION
• This point at which the residual chlorine appears
& when all combined chlorine have been
destroyed is Break point chlorination.
• Now here if further Cl2 is added this will cause
rise in Residual Cl2 (May be free or combined).
• This is direct proportion to chlorine added.

47. Break Point Chlorination

48. SUPERCHLORINATION FOLLOWED BY
DECHLORINATION
• Super chlorination followed by Dechlorination
comprises the addition of large doses of chlorine to
the water and removal of excess chlorine after
disinfection.
• Here we add excessive Cl2. After a suitable contact
time, this Cl2 is removed by addition of sulphur
Dioxide in the Ratio of 1:8.
• This procedure satisfies the Cl2 demand of water and
kills cysts, ova and bacteria which are not killed in
ordinary chlorination.
• This method is applicable to heavy polluted waters,
whose quality fluctuates greatly.

49. PRINCIPLES OF CHLORINATION
• First of the all, the water to be chlorinated should be
clear & should be free from turbidity. Turbidity
impedes efficient chlorination.
• Chlorine demand of water should be estimated.
• Contact period. The presence of free chlorine for a
contact period of at least one hour is essential to kill
bacteria & viruses.
• Chlorine has no effect on spores, protozoal cyst,
helminthic ova, except in higher doses.
• The minimum recommended concentration of free
chlorine is 0.5 mg/Liter for one hour.
• The free residual chlorine provides a margin of
safety against microbial contamination which may
occur during storage & distribution.

50. ORTHOTOLIDINE (OT) TEST
• Both free & combined chlorine in water to be
determined with speed and accuracy.
• Reagent : analytical grade of orthotolidin dissolved
in 10% solution hydrochloric acid.
• 0.1 ml of reagent is added to 1 ml of water
containing chlorine, turns yellow (in both cases).
• Color is matched with standards / color discs.
• OT reacts with free Cl instantaneously (within 10s)
but more slowly with combined Cl (after a lapse,
say 15 – 20 mins).
• Nitrites, Fe & Mn all produce yellow color with OT.

51. ORTHOTOLIDINE-ARSENIC (OTA)
TEST
• Modification of OT test.
• Determine free & residual Cl separately.
• Errors caused by presence of interfering
substances i.e. nitrite, Fe & Mn are overcome.

52. B. Purification of Water on Small Scale
• At Small scale or Domestic level, is
carried out by:
i. Boiling
ii. Chemical disinfection
iii. Filtration
iv. Solar radiation

53. i. BOILING
• It is satisfactory method of purifying water for
house hold purposes.
• To be effective the water must be brought to a
“rolling boil’ for 10 to 20 minutes.
• It kills all bacteria, spores, cysts, ova & yields
sterilized water.
• Boiling also removes temporary hardness by
driving off carbon dioxide & precipitating the
calcium carbonate.
• The taste of water is altered, but this is
harmless.

54. • Excellent method of purifying water but offers no
residual protection against subsequent microbial
contamination.
• It should stored in same container in which it is
boiled to avoid contamination during storage.

55. ii. CHEMICAL DISINFECTION
1. Bleaching Powder:
• Bleaching powder or chlorinated lime (CaOCl₂) is a
white amorphous powder.
• When freshly made it contains about 33% of
“available chlorine”.
• It is however unstable compound.
• On exposure to air, light & moisture, it rapidly losses
its chlorine content.
• But when mixed with lime it retains its strength known
as “stabilized bleach”.
• Bleaching should be stored in a dark, cool, dry place
in a closed container.

56. ii. CHEMICAL DISINFECTION
1. Bleaching Powder:
• The principle in chlorination is to ensure a free
residual chlorine of 0.5 mg/L at the end of one hour
contact.
• Highly polluted & turbid waters are not suited for
direct chlorination.

57. ii. CHEMICAL DISINFECTION
2. Chlorine Solution:
• Chlorine solution may be prepared from Bleaching
powder.
• If 4 kg of bleaching powder with 25% available chlorine
is mixed with 20 liters of water, it will give a 5 %
solution of chlorine.
• Ready-made chlorine solution in different strengths are
available in the market.
• Like bleaching powder, the chlorine solution is subject
to losses on exposure to light or on prolong storage.
• Bleaching should be stored in a dark, cool, dry place in
a closed container.

58. ii. CHEMICAL DISINFECTION
3. Chlorine tablets:
• Under various trade names ( E.g Halozine Tab
are available in the market.
• They are good for disinfecting small quantities
of water.
• A single tablet of 0.5 g is sufficient to disinfect
20 liters of water.

59. 4. High test hypochlorite:
• It is a calcium compound contains 60 to 70 %
available Cl.
• More stable than bleaching powder &
deteriorates much less on storage.

60. ii. CHEMICAL DISINFECTION
5. Iodine:
• Iodine may be used for emergency disinfection of
water.
• 2 drops of 2% ethanol solution of iodine will be
sufficient for one liter of water.
• A contact time of 20 to 30 minutes is needed for
effective disinfection.
• Iodine is unlikely to become a municipal water supply
disinfectant in a broad sense.
• High cost & the fact that the element is physiologically
active (Thyroid activity) are its major disadvantages.

61. ii. CHEMICAL DISINFECTION
6. Potassium Permanganate:
• Once it was widely used, but now it is no longer
recommended for water disinfection.
• Although a powerful oxidizing agent, it is not a
satisfactory agent for disinfecting water.
• It may kill Vibrios Cholera but have little effect on other
organism.
• It has other drawbacks such as altering the colour,
smell & taste of water.

62. iii. FILTERATION
• Water can be purified on a small scale by filtering
through ceramic filters such as Pastuer Chamberland
filter, Berkefeld Filter & Katadyn Filter.
• Filter candles usually remove bacteria found in drinking
water but not the filter-passing viruses.
• Filter candles are liable to be logged with impurities &
bacteria.
• They should be cleaned by scrubbing with a hard brush
under running water.
• Only clean water should be used with ceramic filters.

63. Assurance of water quality
• This is done by FOUR ways
1) Physical examination
2) Chemical examination
3) Bacteriological examination
4) Radiological examination

64. Physical examination
• In this we see for
a) Turbidity < 5 nephelometric turbidity unit
b) Colour 15 True colour unit (TCU)
c) Taste and odour (should be palatable)
d) Temperature cool
e) No smell

65. Chemical Examination
a) PH 6.5 – 8.5
b) Chloride 250 mg/Litre
c) Iron .3 mg/litre
d) Na 200 mg/Litre
e) Hardness 200 mg/litre
f) Copper 1 mg/litre
g) Zinc 3 mg/litre
h) Arsenic .01 mg/litre
i) Lead .01 mg/litre

66. Bacteriological Examination
• It should be free from bacteria, viruses
and helminths and protozoa
• To see for bacteria’s we perform
a) Presumptive Coliform test It is done on
McConkey’s media for 48 hours
We take

67. a) Presumptive Coliform test
Bacteriological Examination
i) 50 ml of H2O to 50 ml of D.S medium 1
tube
ii) 10 ml of H2O to 10 ml of D.S medium 5
tubes
iii) 1 ml of H20 to 5 ml of D.S medium 5
tubes
iv) 1 ml of H2O to 5 ml of D.S medium 5
tubes

68. Presumptive Coliform test
Bacteriological Examination
• Presence of acid and gas confirms the
presence of E.coli. Those who grow at
37°C → N. Faecal E.coli, Those who
grow at 44 °C → Faecal E.coli.

69. b) Colony Count
• It is done on Agar plate by taking 1ml of
water
1) At 22°C for 72 hour 20-100 colonies
2) At 37°C for 48 hour 0-10 colonies, will
prove that water is potable.

70. Radiological Examination
i) Alpha photon emitter 15 pci/litre
ii) Beta Photon emitter 4 m ram / y
iii) Combined radium 5 Pci/litre

71. WHO Criteria for Water
1) No sample should have E. coli in 100 ml
of water
2) Not > 3 Coliform should be present in
100 ml of water
3) Not > 5% of sample through out the year
should have coliform in 100 ml of H2O
4) No TWO consecutive sample should
have coliform in 100 ml of water