*2.1.5. Chemical*

Chlorine can be used as liquid or gas form. It is a very strong, oxidizing agent. Both the forms (liquid and gas) can be stored and used from gas cylinders under pressure. The chlorine cylinders can be 150 pounds. Small drinking water systems commonly use cylinders of 150 pounds.

Hypochlorous and hypochlorite ions are formed when chlorine is mixed with water. The hypochlorous ion is a better disinfectant which is formed in greater concentration at low pH concentrations. The hypochlorite and hypochlorous ions will be present in equal concentrations at pH 7.3. At pH above 8.3, the hypochlorite ion predominates which is not a better disinfectant. So, better disinfection is achieved at low pH. To avoid the formation of trihalomethanes and haloacetic acids, chlorination should be applied after treatment [5] (**Figure 1**).
