Correct Answer - Option 4 : chlorine
Concept:
A disinfectant is a chemical that kills the disease-causing micro-organisms.
The most commonly used chemicals as primary disinfectants are chlorine, chlorine dioxide and ozone. Among them Chlorine is the most widely used primary disinfectant throughout the world.
Properties of a good disinfectant:
1. It should be persistent enough to prevent the regrowth of organisms in the distribution system so that the water will be safe from future recontamination.
2. It must be toxic to micro-organisms at concentrations well below the toxic thresholds of humans and higher animals so that it cannot negatively affect the animals or humans.
3. It should have a fast rate of kill of micro-organisms.
4. It should be active and stable and environmentally safe.
5. It should not leave damage and odour.
Alum is added in water in order to remove the very fine suspended particles in water by the process of coagulation and sedimentation. It also removes phosphorous from water.
Lime is added in water to lower the hardness of water. It is generally removed the temporary hardness i.e. hardness due to multivalent cations of carbonate and bicarbonate ions in water.
