A free chlorine conversion occurs when a water system that typically uses chloramine removes ammonia (needed to form chloramine) from the treatment process, and disinfects the water with only chlorine. Chlorine is more effective than chloramine at inactivating certain types of bacteria. Excess ammonia, which can accumulate in a chloramine-treated distribution system over time, is a source of food for specific types of bacteria that are harmless to people. These bacteria can make it difficult for public water systems to maintain a disinfectant residual, which means that microorganisms that are harmful to people can grow. The chlorine conversion is a common practice by many public water systems throughout the country to reduce the number of the bacteria so that a satisfactory disinfectant residual can be maintained throughout the distribution system. Chlorine conversions can be used as a preventative strategy or to stop nitrification (the microbial process that converts ammonia and similar nitrogen compounds into nitrite and nitrate), which can diminish water quality. According to a 2016 EPA survey, 25 to 40 percent of the utilities that use chloramine reported using free chlorine burns to control nitrification.
Public water systems should notify their customers prior to a chlorine conversion as changes in taste and odor may briefly occur. Although customers may be concerned about taste and odor changes, no associated health risks are expected during the temporary conversion.