A chlorine residual is the low level of chlorine remaining in the water after the initial application. The presence of chlorine residual in drinking water indicates that a sufficient amount of chlorine was added initially to disinfect the water and to inactivate the bacteria and some viruses that cause disease. Also, the water is protected from re-contamination during storage. It is an important tool to safeguard against potentially harmful disease causing bacteria. also known as pathogens. By having a consistent residual in the distribution system, the district can monitor the residual throughout the system and detect if there are any operational or water quality issues. Perhaps, the greatest health development of the 20th century was the usage of chlorine to disinfect the drinking water to eliminate water borne diseases from spreading easily and provide safe drinking water to the public.