Contamination by sewage or animal faeces is the greatest danger associated with water for drinking. This is because sewage from human or animal sources may contain the microorganism that cause disease. The use of disinfection to kill or inactivate microorganisms is necessary if the untreated water contains such organisms. Several disinfection methods are used in water treatment. Disinfection with ultraviolet (UV) irradiation and chlorine are the most widely used methods.