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OT Test enables Total, free and combined chlorine in water
The reagent is ‘analytical grade Orthotolidine’, dissolved in 10% solution of HCl
When this reagent is added to the water containing chlorine , it turns yellow
The intensity of yellow colour is proportional to the concentration of chlorine
This yellow colour is produced by both free and combined chlorine residuals
However, OT reacts with free chlorine instantaneously but reacts more slowly with combined chlorine
Add 0.1 ml of reagent to I ml of water

Steps in well disinfection:
1. Find out the volume of water present in the well using the given formula
liters
2.Find the amount of bleaching powder required for disinfection, Using ‘Horrock’s Apparatus’
3. Dissolve the required amount of bleaching powder in water
4. Delivery of the chlorine solution into the well
5. Allow a “Contact period” – one hour

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 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

After calculating the chlorine demand from a sample of water, the calculated amount of chlorine is applied as
• Chlorine gas or
• Chloramine or
• Perchloron

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),

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
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

Are of two types:
1. Gravity type (Paterson’s filter)
2. Pressure type (e.g. Candy’s filter)
Steps in purification of water using rapid sand filter
1. Coagulation
2. Rapid mixing
3. Flocculation
4. Sedimentation and THEN
5. Filtration: using the rapid sand 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