Chlorine has been used in the treatment of drinking water in the United States since 1908. Since the Safe Drinking Water Act of 1974 and its amendments, utilities around the country have made changes to their disinfection strategies to meet more stringent regulations for either microbial contaminants or disinfection by-products (DBPs). Specifically, utilities have investigated and implemented the use of alternatives to chlorine. Consequently, ozone and chlorine dioxide have been tested and applied at treatment plants for primary disinfection, and the use of chloramines has increased for secondary disinfection.
While it is well accepted by many in the drinking water community that many of these changes will have beneficial impacts, it is not well known how these changes in disinfection will affect distribution system water quality in the long term. Based on a comprehensive literature review and the experience of 26 participating water utilities, potential positive and adverse water quality impacts were identified and summarized when changing from chlorine to either chloramines, ozone, chlorine dioxide, or ultraviolet (UV) light. The data indicated better microbial quality, such as lower coliform levels and heterotrophic plate counts (HPC) and reduced levels of disinfection by-products like trihalomethanes and haloacetic acids. Disinfection changes also led to reduced numbers of customer complaints regarding red or discolored water and tastes and odors. However, two of the more common adverse impacts included some higher HPC levels when using chloramines or ozone and nitrification when using chloramines.