Chlorine is also one of the elements of the diatomic molecule (Cl2) and has been part of our daily lives for years. As for the early history of chlorine, chlorine was first discovered in the thirteenth century and was first used by the Swedish chemist Carl Wilem Scheele mentioned in the introduction in 1774. It is said that the most serious threats to health are not caused by chemical substances, but by very infectious organisms or bacteria present in the water we drink and use every day. Chlorine is an important economical disinfectant and kills most pathogenic bacteria in water. However, chlorine disinfection produces a wide variety of byproducts. A class of chlorination byproducts, known as trihalomethanes (THMs), are suspected of being carcinogenic. Because of concern about these byproducts in the water supply, chlorine is now kept at minimal levels and other disinfection methods are used more frequently. Chloramines form more stable disinfectants and pose less risk of harmful byproducts, but they cost more to use. Other methods focus on removing organisms through coagulation, sedimentation and improved filtration. The functions of chlorination are to disinfect water or wastewater, decolorize water or fabrics, sanitize and clean surfaces, remove iron and manganese, and reduce odors. However, the odor of some compounds, such as some phenols, is aggravated by reaction with chlorine. Some soluble metals can be made insoluble through oxidation with chlorine (soluble Fe2+ is oxidized to insoluble Fe3+), making the metal easier to remove through sedimentation or