Double pot is a method of:
It is desirable to ensure a constant dosage of chlorine to well water.

The double pot method is an improvement devised by the National Environmental Engineering Research Institute, Nagpur, India. 
This method uses two cylindrical pots, one placed inside the other. The inside height and diameter are 30 cm and 25 cm respectively, for the outer pot. A hole 1 cm in diameter is made in each pot; in the inner pot the hole is in the upper portion, near the rim and in the outer pot it is 4 cm above the bottom.
A mixture of 1 kg of bleaching powder and 2 kg of coarse sand (approx. 2 mm in diameter) is prepared and slightly moistened with water. 
The inner pot is filled with this mixture up to 3 cm below the level of the hole. 
The inner pot is introduced into the outer one, and the mouth of the latter closed with polyethylene foil. The use of two pots makes it possible to have larger holes without the risk of over chlorination.
Fluoridation
Defluoridation
Ozonisation
Chlorination