Lecture on purification of water by

Lecture on Purification of
Water by

Dr. Fazal Mahmood
M.B.B.S(Pb), M.C.P.S(Pak)

Purification on Large scale
 When sources of water are
Rivers,Streams,Lakes etc. then water is
Purified by
 Storage
 Sedimentation
 Filtration(rapid sand & slow sand)
 Disinfection(chlorination)

Purification on Medium Scale
 When sources of water are
wells,Springs,Tanks etc.Then water is
purified by the addition of
 Bleaching powder/Chlorinated Lime as it is
 Cheep
 Easy to use
 Reliable and safe

Purification on Small
Scale(Domestic Methods)
 BOILING: for 5-10 minutes kills almost
all organisms & removes temporary
Hardness.
 DISTILLATION: not commonly used due
to higher cost, used in Labs. etc.
 ADDITION OF CHEMICALS
 Bleaching Powder: 5% solution is used
Dose: 3-6 drops/L contact time of ½ hour.

Contnd.-----------
 Chlorine Tablets/Halazone Tablets. one
tablet/litre.
 Iodine Solution: 02 drops of 2% Soln./litre
 KMnO4: an amount that gives just pink
coloration to the Water.
 Alum: used for turbid water in a dose of
0.1-0.4 grains/5 litres of Water.

Purification of water in
Emergency/Epidemic
 Boiling: for 5-10 minutes.
 High Test Hypo chlorite: used in granular
form Dose: 01gm./10 litres of water.
 Liquid Bleach/5% Sodium Hypo chlorite
solution: Dose: 14 ml./1000 litres of water.
 Bleaching Powder: used in a dose of
2.3gm./1000 litres of water.

Purification of water on Large
Scale
 On large scale purification, water is
disinfected by addition of Chlorine after
filtration.
 On large scale, water is purified through the
following processes.

Contnd.---------
A.STORAGE:
 Not a sole measure of purification but
valuable as a preliminary to other
processes.
 There is fall in ammonia & rise in nitrates
due to oxidation of organic matter by
aerobic bacteria.

Contnd.----------
 Sedimentation, may be hastened by the
addition of Alum in a dose of 35 mg./ litre
for a detention period of 4-6 hrs.
 Sedimentation will prevent clogging of the
filters.
 Limitation to storage includes algae growth
which can be controlled by addition of
CuSo4 in a dose of 1-5 Kg. / 5000 cu.m.

Contnd.---------
B. FILTRATION:
 Filtration through sand is the oldest and
universally, most accepted method through
out the world.
 98-99% bacteria are removed along with
other impurities through filtration.

Types of Filters

 Slow sand/Biological filters.

 Rapid sand filters/Mechanical filters.

Slow Sand Filtration
 Firstused in 1804 in Scotland &
subsequently in London.
 During 19th.Century used through out the
world.
 Slow sand filter consists of concrete/brick
work rectangular basin containing carefully
selected graded sand supported on gravel
and stones.

Contnd.-----------
 ELEMENTS:
1. Supernatant (raw) water:
 Upper most layer of raw water(1-1.5M)
 It provides a constant head of water
 It provides a waiting period of 3-12 hours

Elements contnd.-----
2. Sand Bed:
 It is 1 M thick supported by a layer of fine and
then coarse gravel and finally by the stones.
3. Drainage System for filtered water:
 It consists of perforated pipes
4. Filter Control Valves:
 Valves & Devices incorporated in the outlet-
pipe system to maintain a constant rate of
filtration
 Venturi meter measures bed resistance or loss of
head.

Actions of slow sand filter
 Physical:
Mechanical straining/sedimentation
 Chemical: Oxidation of organic matter by
aerobic bacteria.
 Biological: Occurs through “Vital layer”
 Efficiency of the filter depends upon the
“Vital layer”

Vital Layer
 Slimy growth known as “Schmutzdecke”,
Vital layer, zoogleal layer or biological
layer
 The layer is slimy & gelatinous,It consists
of threadlike algae & numerous forms of
life plankton, diatoms and bacteria
 Ripening of the filter
 Vital layer is the “heart” of slow sand filter

ADVANTAGES
 Simple to construct and operate
 Cost of construction cheaper than rapid
sand filter
 Process: physical, chemical and biological
 Bacterial count reduction is 99.9% to
99.99% and E.coli reduction is 99% to
99.9%

DISADVANTAGES
 Old fashioned and outdated method of
water purification (but still in use)
 Initial cost is low but maintenance cost is
much more than rapid sand filter
 These filters need a lot of space

RAPID SAND FILTERS
 In 1885, first rapid sand filters were
installed in USA, since that time they are
gaining considerable popularity in highly
industrialized countries as well as
developing world.

Contnd.---------
 TYPES:

1. Open / Gravity type (Peterson’s Filters)

2. Closed / Pressure type (Candy’s Filters)

Contnd.----------
 STEPS / PROCESSES IN RAPID SAND
FILTERS
1. COAGULATION:
 Raw water is first treated with Alum in a
dose of 5-40 mgs./L
 pH has to be adjusted by addition of lime or
soda ash as Alum needs CaCo3 to form the
flock.

Contnd.---------
2. RAPID MIXING:
 Water is mixed rapidly with Alum for a
few minutes in mixing chamber.
3. FLOCCULATION:
 From mixing chamber water is moved into
the Flocculation chamber where it rests for
half an hour.

Contd.--------
4. SEDIMENTATION:
 The coagulated water is now led into the
sedimentation tank where it is detained for
2-6 hours.
 Flocculant precipitate together with
impurities & bacteria settle down in the
tank

Contnd.--------
5. FILTRATION
 Partially clarified water is now subjected to
the rapid sand filtration.
 Depth of the sand bed is 1 M & each rapid
sand filter unit has a surface of about 80-90
Sq.M.
 Oxidation of NH3 also takes place in these
filters.

ADVANTAGES
 Rapid sand filters deal with raw water
directly
 Occupies less space
 Filtration is rapid about 40-50 times as
compared to slow sand filters
 Washing of filter is easy
 More flexibility in operation

DISADVANTAGES
 Preliminary treatment with Alum is
required
 Operation is highly skilled
 Removal of color material is less than slow
sand filter
 Reduction of bacterial count is less than
slow sand filter

Comparison of Rapid and Slow
sand filters
Rapid.S.F.
Slow.S.F.
1. Space: Occupies little space-----Occupies large area
2. Rate of filtration: 5-15 cu.m./sq.m./hr.----- 0.1-0.4
3. Effective sand size: 0.4-0.7mm-----------------0.2-0.3mm
4. Preliminary treatment: Coagulation& sedimentation--plain
Sediment
5. Washing: By back washing--------By scraping the sand
6. Loss of head allowed: 6-8 feet--------------------4feet
7. Removal of turbidity: Good-----------------------Good
8. Removal of color: Good------------------------Fair
9. Removal of bacteria: 98-99 %--------------------99.9-99.99

DISINFECTION OF WATER
 CHEMICAL METHODS / GERMICIDES
1. CHLORINE: is used for purification of water on large
scale
2. KMnO4: used for disinfection of water in wells especially
during epidemics
3. CuSO4/CaCl2: used for the removal of algae
4. Lime: used for muddy water
5. Ozonization & U.V. irradiation: both methods are used for
disinfection of water in swimming pools
6. Katadyn Silver: sand coated with silver used in slow sand
filters instead of plain sand
7. Charcoal: used as adsorbent to remove color,odor & taste

Disinfection of Wells
 The most effective & cheapest method of
disinfecting wells is by bleaching powder
 STEPS IN WELL DISINFECTION :
1. Find the volume of water in a well:
a. Measure the depth of water column..(h) M
b. Measure the diameter of well………(d) M
c. Substitute h and d in:

Cotnd…….
3.14 x (d)2 x h
Volume(litres)=------------------------------ x 100
4
d. One cubic meter = 1,000 litres of water

Find the amount of bleaching powder
required for disinfection:

Estimate the Chlorine demand of well
water by HORROCK’S APPARATUS

Contnd……..
3. Dissolve Bleaching Powder in water

4. Delivery of Chlorine solution into the well

5. Contact period

6. Orthotolidine arsenite test

Chlorination of water
 It is the most widely used method for water
purification, both on large as well as on
small scale.
 It is the most reliable and cheapest method.
 It is supplement but not the substitute to
sand filtration.

Properties of Chlorine
 Killing of pathogenic bacteria by inhibiting their
protein synthesis.
 Oxidizes Iron, Manganese & Hydrogen Sulphide.
 Destroys taste & odor producing constituents.
 It controls algae growth.
 It helps in coagulation and suspension of the
organic matter.

Action Of Chlorine
 When chlorine is added to water it forms
HCl & HOCl.
 The disinfecting action of chlorine is mainly
due to Hypochlrous acid & to a small extent
due to Hypochlorite Ions.
 Hypochorous acid is 70-80 times more
effective than Hypochlorite Ions.

Actions of Chlorine depends
upon the……..
 Organic matter
 Metals
 Bacterial content
 Temperature & pH of water.
 Suspended matter and,
 Amount of free residual chlorine in water

Contnd………
 Efficiency
of chlorine increases, when it
combines with Ammonia and……

 Itdecreases, when it combines with
Sulphides, Ferrous salts & inert matter in
suspension form

Principles of Chlorination
 Water to be chlorinated must be free from
turbidity
 Chlorine demand of water should be
estimated
 Contact period of one hour is essential to
kill the bacteria and viruses
 Minimum recommended concentration for
free chlorine is 0.5mg. / liter

Chlorine demand of water
 Is the difference between the amount of
chlorine added to water and the amount of
residual chlorine remaining at the end of a
specific period of contact (usually 60 min.)
at a given temperature and pH of water.
 ACUAL DOSE OF CHLORINE:
It is the sum of chlorine demand of water
and free residual chlorine

Methods of Chlorination
1. Chlorine gas
2. Chloramine
3. Perchloron (H.T.H.)
TYPES OF CHLORINATION:
a. Superchlorination
b. Break Point Chlorination

Tests for the presence of
Chlorine in Water
 Orthotolidine (O.T.) Test

 Orthotolidine-Arsenite (O.T.A.) Test

 Starch Iodide Test

Surveillance of drinking Water
1. Sanitary survey
2. Sampling
3. Bacteriological surveillance
4. Biological examination
5. Chemical surveillance

Sampling of Water
 Collection, Storage & Transportation of water
samples from……
 Taps
 Rivers
 Streams
 Tanks
 Wells
A. For Physical & Chemical Examination
B. For Bacteriological Examination
 PERTICULARS OF A SAMPLE

Standards of Water
 PHYSICAL STANDARDS
 Turbidity
 Color
 Odor
 Taste

Contnd………
 CHEMICAL STANDARDS
 Hardness
 Chlorides
 Ammonia
 Nitrites
 Nitrates
 Absorbed Oxygen
 Dissolved Oxygen

Contnd……..
 Toxic substances and a specific group of
chemicals causing Non-Specific diseases of
water
 Poly-nuclear Aromatic Hydrocarbons and
Radioactive substances ( a group of
chemical substances of water affecting the
acceptability of water.

Bacteriological Standards
1.BACTERIAL STANDARDS :
W.H.O. recommended these standards in
1984 for Public water supplies
 Ideally all samples taken from the
distribution system should be free from
Coliform organisms
 In Practice this standard is not always
attainable

Recommended Standards for
Water samples collected
 Throughout any year 95 % samples 100 ml.
each should not contain any Coliform
organisms
 No samples should contain E.coli
 No sample should contain more than 03
Coliform
 Any two consecutive samples should not
contain Coliform

Contnd………
 Incase of Individual / Small water
supplies such as from household Tap, Well,
Spring etc.
 Coliform count should not exceed 10 /100
ml. and E.coli should be nil

Contnd………..
2. VIRAL STANDARDS :
 W.H.O. Standards fix the limit for viruses
such as Eneroviruses, Retroviruses &
Adenoviruses in water supplies at 01 Plaque
forming unit
 Faecal Bacteroiphage & Enteropathogenic
viruses should be completely absent

Bacteriological Indicators
These are based on organisms indicative of
faecal pollution, These organisms include!

1. E.coli and Coliform group as a whole
2. Faecal Streptococci
3. Clostridium Perfringens
4. Examination for Protozoa, Parasitic
worms & Larvae is also recommended

Contnd……..
 Reasons why Coliform organisms are chosen
as indicators of Faecal Pollution
1. Constantly present in human intestine an
average person excretes 200-400 billions / day
of these organisms in faeces
2. They are foreign to potable water supplies
3. Easily detectable by ordinary cultural methods
4. Survival time is much better than other
organisms
5. Have greater resistance to natural forces of
purification

Contnd……..
 FAECAL STREPTOCOCCI :

 Faecal Strepococci occur regularly in much
smaller number than E.coli
 Their presence is regarded as confirmatory
evidence of recent faecal contamination of
water in doubtful cases

Contnd………
 CLOSTRIDIUM PERFRINGENS :

 They are spore bearing organisms
 Their presence along with Coliform indicate
recent water contamination
 While the presence of their spores but the
absence of Coliform confirms remote
contamination of water

Lecture on purification of water by

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