Water Purification by Chlorination
Water chlorination is the process of adding chlorine (Cl2) or hypochlorite (mostly as sodium hypochlorite – NaOCl) to water. Chlorine is a highly efficient disinfectant, and is added to public water supplies to kill disease-causing pathogens, such as bacteria, viruses, and protozoans, that commonly grow in water supply reservoirs, on the walls of water mains and in storage tanks. In particular, chlorination is used to prevent the spread of water-borne diseases such as cholera, dysentery, typhoid, etc.
After early attempts to implement water chlorination at water treatment plants in Germany, England and the United States in the 19th century, permanent water chlorination began in 1905. It is a worldwide accepted technology of water disinfection.
Chlorination with chlorine (Cl2)
Chlorination through injection of gaseous chlorine (Cl2) is reserved for industrial-scale water treatment plants and public water works that disinfect large quantities of water. This technology cannot be used at the point of use in rural private households where there are no public water utility companies operating.
Even in many African cities where such chlorination systems are installed, the water treatment is not carried out in a reliable way. Concentration of chlorine in the water can vary to a large extent. Systems can become dysfunctional at any time, and in most cases the users are not informed. There can be a shortage of supply of chlorine in gas cylinders, as it has to be imported from foreign countries.
Gaseous chlorine is highly toxic, classified as hazardous good.
Chlorination with Hypochlorite (HOCl)
Chlorination with sodium hypochlorite is also known as the Electro-Chemical Activation (ECA) process. An electrolysis cell produces an aqueous solution of hypochlorite which purifies water. Only table salt is needed which is cheaply available. Although sodium hypochlorite is a strong oxidizer, aqueous solutions that are produced with the ECA process are not classified as hazardous materials. Therefore, hypochlorite solutions can be easily and safely handled, transported and stored.