Chemicals commonly considered for use as disinfectants in municipal drinking water treatment are chlorine, chloramines, chlorine dioxide, and ozone. Considerations such as disinfection power, ease of application, and low cost have led in the past to the use of free chlorine as the primary disinfectant. Discovery of trihalomethanes, formed by the action of free chlorine upon natural organic materials has led to a reexamination of this practice. A change to an alternative disinfection practice has now either occurred or is being contemplated by utilities that otherwise have difficulty meeting the maximum contaminant level requirement for trihalomethanes.