Previous Year Question
Which one of the following gases is used for the purification of drinking water?
Sulphur dioxide
Chlorine
Fluorine
Carbon dioxide
Correct Answer: B
Detailed Solution
Chlorine (Cl₂) is an effective Chemical Disinfectant gas used to eliminate disease-causing microorganisms from drinking water. This water purification process is known as Chlorination. Chlorine helps prevent the transmission of Waterborne Diseases by inactivating many harmful Bacteria and Viruses present in water. It is widely used in municipal Water Treatment Plants and public water supply systems.
When chlorine gas is added to water, it reacts with water to form Hypochlorous Acid (HOCl) and Hydrochloric Acid (HCl). Hypochlorous Acid is an effective disinfectant that inactivates microorganisms by damaging their essential cellular structures and biological molecules. A portion of Hypochlorous Acid is converted into the Hypochlorite Ion (OCl⁻). The relative proportions of these two species depend on the pH of the water, and HOCl generally possesses greater disinfecting efficiency than OCl⁻.
Drinking water treatment generally involves processes such as Coagulation, Flocculation, Sedimentation, Filtration, and Disinfection. Chlorination is primarily used for disinfection. According to the World Health Organization (WHO), effective chlorination requires a minimum Free Residual Chlorine concentration of 0.5 mg/L after at least 30 minutes of contact time at a pH below 8. A minimum residual chlorine concentration of 0.2 mg/L is recommended at the point of delivery. This residual provides protection against microbial recontamination within the water distribution system.
Among the other options, Sulfur Dioxide (SO₂) is used in certain water treatment processes to remove excess chlorine through a process called Dechlorination. Fluorine (F₂) is an extremely reactive and toxic gas that is not used for conventional drinking water disinfection; it should be distinguished from Fluoride, which is used in controlled concentrations for dental health. Carbon Dioxide (CO₂) is used in certain water treatment processes to adjust pH and Alkalinity rather than as a primary disinfectant. Chlorine is effective against many pathogens, but its effectiveness is limited against certain resistant parasites, such as Cryptosporidium.