Chlorine Gas is the most widely used chemical for disinfecting all varieties of water. It possess very residual oxidizing capacity and its therefore useful for destruction of organic matter/waterborne pathogens. It is least expensive form of chlorine to use. The typical amount required for water treatment is 1-16 mg/L of water and it always depends on the quality of water that needs to be treated.
Water which is intended for human consumption must be free from bacteria/organic matter/pathogens. This can only be achieved by disinfecting the water by injecting chlorine gas in required concentration.
There are number of reasons that affect the process of disinfection, out of all these major reasons, would be the concentration and dosage of chlorine and the contact time. The longer the chlorine is in contact with microorganisms, more effective is the process.
Chlorine which is intended for human consumption needs time to disinfect the pathogens/micro-organism present in the water body.