Lecture 3
The final stage in the purification of water.
Disinfection is the elimination of pathogens. The
disinfectant has to satisfy the following criteria:
Capable of destroying the pathogenic
organism
•Should not leave color and toxic products after the
reaction
•Should be readily available and cheap
•Should have the property of residual concentration
•Detection is easy rapid and simple
CHOLORINATION
Chlorine is the method of choice for sterilization of water
Disinfections is synonymous with Chlorination
It’s a supplement, not a substitute to sand filtration
It kills pathogenic bacteria
Action of chlorine depends on
• Organic matter and bacterial count.
• Temperature and PH of water
• Amount of free residual chlorine
• Presence of suspended matter
CHLORINATION
• Chlorination is a process in which chlorine is
added to water for disinfection
• pH of ordinary water is 6.5 to 8
• Chlorination is more effective when water pH
is 7
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Water should be clear and free from turbidity
Chlorine demand of the water should be
estimated
The contact period
Minimum recommended concentration (0.5 mg/l)
Correct dose should be applied
Effects of chlorine
Germicidal effect
Oxidizes iron ,manganese and hydrogen sulphide
Controls algae and slime organism
Aids in coagulation
Has no effects on spores and certain virus
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ACTIONS OF CHLORINE
WATER +CHLORINE –HYDORCHLORIC ACID +HYPOCHLOROUS ACID
HCL IS NEUTRALIZED BY ALKANITY OF WATER
HYPOCHLOROUS ACID AND HYPOCHLORITE ION ARE DISINFECTANTS
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ACTION OF CHLORINE DEPENDS
ORGANIC MATTER AND BACTERIAL
CONTENT
• TEMP AND Ph OF WATER
AMOUNT OF RESIDUAL CHLORINE
• PRESENCE OF SUSPENDED MATTER
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Super chlorination: Addition of large doses of
chlorine to the water and removal of excess of
chlorine after disinfection. This method is
applied for heavily polluted water.
Chlorine is removed by an oxidizing agent like
H2S or sodium thiosulphate in the ratio of
1.8:1
Methods of Chlorination
Chlorine gas
Chloramine (chlorine + ammonia)
Perchloran (High Test Hypochlorite)
(Calcium comps. With 60-70 % available Chlorine).
• Used on large scale
• Used in generally pre-filtered water
• .1-.2 ppm (parts per million) 30 minutes contact time.
• Ammonia is added first to the water followed by
addition of chlorine. This method is more stable
remain active for longer period of time.
• Less liable to give objectionable taste and odour.
• Ratio of ammonia to chlorine is 1:4 or 1:5
Method of chlorination
MEANS OF CHLORINE GAS
• Cheaper method easy to apply
• Requires a chlorinating equipment
MEANS OF CHLORAMINES
• CHLORINE AND AMMONIA compounds in 4-1 ratio
• Slower in action but more persistent, ammonia is first added then
chlorine
MEANS OF PERCHLORAN
• Calcium compound carries 60-70 %chlorine
• Solution are prepared for disinfection of water
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It is the amount of chlorine that is needed to
destroy bacteria and to oxidize all the organic
matter and ammoniacal substances present in
the water.
The point at which the chlorine demand of water
is met is called break point chlorination.
Orthotolidine (OT)Test
• It enables both free and combined chlorine in
water to be determined with speed and
accuracy.
Orthotolidine – Arsenite (OTA)Test
• It determines the free and combined chlorine
residuals separately.
It is unstable gas, powerful oxidizing agent ,
removes undesirable color, odour and taste
and removes all chlorine from the water. Has
strong virucidal effect. Inactivates viruses in
seconds. There is no residual germicidal effect
Ozone dose required for potable water
treatment is .2-1.5 mg per litre.
UV Irradiation is effective against most
microorganisms and viruses.
This method involves the exposure of a film of
water up to 120mm thick.
The water should be free from turbidity and
suspended or colloidal constituents for efficient
disinfection.
It contains 33% of available chlorine.
2.5 Gms is required to disinfect 1000 liters of water.
Boiling for 5-10 minutes kills off most micro-organisms.
This method would get rid of all types of impurities. It is very
costly method.
A single tablet of .5 gm is sufficient to disinfect 20 liters of
water.
In Pakistan 1 tablet of 25 mg for one liter is also available
two drops of 2% iodine are sufficient for one liter
of water.
amount just sufficient to
give pink colouration.
should be used in all turbid water from .1 to .4 gm
per 5 liters of water
W.H.O has placed contaminants of drinking water
under three categories
Toxic substances
Substances that may affect health
Substances that may affect water acceptability
Substance Highest desirable level
Substances causing 5 units
Discoloration
Odour Unobjectionable
Total solids 500mg/L Unobjectionable
PH (7.0-8.5)
Hardness 2 m Eq/L
Calcium (75 mg/l)
Magnesium (30-150 mg/l) 30mg/L
Chloride (200mg/L)
Copper (.05-1.5mg/l) .05mg/L
Iron .1 mg/L
Manganese, etc. .05mg/L
Sulphate 200mg/L
Zinc 5mg/L
WHO
•Microbiological Standards
•No sample should have E.Coli in 100ml.
•No sample should have more than three Coliform
in 100ml.
•Not more than 5 % of samples throughout the year
should have coliforms in 100 ml.
•No two consecutive samples should have coliforms
in 100 ml.
•Viruses should be completely absent.
Toxic substances (upper limit of concentration)
Arsenic (0.05 mg/l)
Cadmium (0.01 mg/l)
Cyanide (0.05 mg/l)
Lead (0.01 mg/l)
Mercury (0.001 mg/l)
Selenium (0.01 mg/l)
Books Recommended
1- Public Health & Community Medicine 7th
Edition
2- Parks Text book of Preventive and Social
Medicine
19th & 20th Edition
Dr M Ayaz Bhatti 25
