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Disinfection of Wells during an Emergency – The Double Pot Method

Disinfection of Wells during an Emergency – The Double Pot Method
E.g. an outbreak of cholera
• During an emergency, it is desirable to ensure a constant dosage of chlorine to well water
• One method of doing this is the ‘Double pot method’
• The method is an improvement devised by the National Environmental Engineering Research Institute (NEERI), Nagpur, India
• There are two cylindrical pots
• One pot is placed inside the other
• Outer pot:
 Inside height is 30 cm
 Inside diameter is 25 cm

Horrock’s Apparatus

Horrock’s apparatus is used for estimating the dose of bleaching powder needed to disinfect 455 Liters of water
It is used mostly while disinfecting the water in a well

Contents of ‘Horrock’s Apparatus’
• 6 white cups, each of 200 ml capacity
• One black cup with a circular mark on the inside
• 2 metal spoons (each holds 2g of bleaching powder when filled up to the brim)
• 7 glass stirring rods
• One special pipette
• Two droppers
• Starch - iodide indicator solution
• Instruction folder

5 Principles of Chlorination

For adequate chlorination, following rules should be followed:
1. The water to be chlorinated should be clear and free from turbidity.
– Turbidity impedes efficient chlorination
2. The chlorine demand of the water should be estimated first
– The chlorine demand of water is the difference between
• the amount of chlorine added to the water and the amount of residual chlorine remaining in a sample of water
• at the end of a specific period of contact (usually 60 min),

Water Supplies Disinfection with Chlorination

Chlorine is a supplement and not a substitute to sand filtration
-It kills pathogenic bacteria
-But has no effect on spores and certain viruses (e.g. polio, viral hepatitis) except in high doses
In addition to being germicidal, it has other properties
-Oxidizes iron, manganese and hydrogen sulphide
-It destroys some taste and odour producing constituents
-It controls algae and slime organisms and aids coagulation
How does chlorine act as disinfectant?
When chlorine is added to water, it forms:
-Hydrochloric acid (HCl) and

Criteria for a Chemical to be Suitable as Water Disinfectant in Water Supplies

Criteria for a Chemical to be Suitable as Water Disinfectant in Water Supplies
A chemical can be suitable for use as a disinfectant in water supplies if it satisfies the following criteria:
1. Should be able to destroy the pathogenic organisms in the water,
• within the contact time available and
• This capability should not be unduly affected by the range of the physical and chemical properties of the water e.g. temperature, pH and mineral constituents
2. Should not leave residues which
• render the water toxic or
• impart colour or

Slow Sand Filter (Biological Filter)

Slow Sand Filter
(Biological Filter)
• Elements of a slow sand filter
1. Supernatant or raw water
2. Bed of graded sand (on a support of gravel)
3. Under – drainage system
4. A system of filter control valves
1. Supernatant water:
• Depth varies from 1 - 1.5meters
• Serves two important purposes:
a) Provides a constant head of water
• This overcomes the resistance of the filter bed and promote downward flow of water through the sand bed

Water Purification at a LARGE SCALE (for water supply)

Safe & Wholesome Water
Water intended for human consumption should be both safe and wholesome.
This has been defined as water that is –
a. free from pathogenic agents
b. free from harmful chemical substances
c. pleasant to the taste, i.e., free from colour and odour; and
d. usable for domestic purposes
Water is said to be polluted or contaminated when it does not fulfil the above criteria
Without ample and safe drinking water, we cannot provide health care to the community

Orthotolidine (OT) test and Orthotolidine – Arsenite (OTA) test

Free chlorine combines with ammonia and nitrogen of organic matter to form ‘Chloramines’
Chloramines are not efficient residual disinfectants
Yet these get included in the Total chlorine content of the water to be tested
We intend to measure only the ‘free residual chlorine ’ as this only will provide protection from subsequent contamination
We can measure ‘free chlorine ’ using OT test 10 seconds reading
We can measure ‘total chlorine ’ using OT test 5 minutes reading
‘Combined chlorine ’ = total chlorine - free chlorine


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