The correct answer is Chlorine.Chlorine elements can be used to treat/purify water.Chlorination is one of many methods that can be used to disinfect water.It is a chemical disinfection method that uses various types of chlorine or chlorine-containing substances for the oxidation and disinfection of what will be the potable water source.Chlorine inactivates a microorganism by damaging its cell membrane. Once the cell membrane is weakened, chlorine can enter the cell and disrupt cell respiration and DNA activity.The main objective of this chlorine addition is to disinfect the water and maintain chlorine residuals that will remain in the water as it travels through the distribution system.Any type of chlorine that is added to water during the treatment process will result in the formation of hypochlorous acid (HOCI) and hypochlorite ions (OCI-), which are the main disinfecting compounds in chlorinated water.The amount of chlorine that is required to disinfect water is dependent on the impurities in the water that needs to be treated.Chlorination is able to provide a residual to reduce the chance of pathogen regrowth in water storage tanks or within the water distribution system.Chlorination has been proven to be very effective against bacteria and viruses.However, it cannot disinfect all waterborne pathogens. Certain pathogens, namely protozoan cysts, are resistant to the effects of chlorine.Additional InformationChlorine was first discovered in Sweden in 1744.At that time, people believed that odors from the water were responsible for transmitting diseases.Chlorination can be done at any time/point throughout the water treatment process. There is not one specific time when chlorine must be added.