Volume 100, Issue 10     October 2008    ISSN 1551-8833

One third of respondents in a survey of US water utilities reported using or planning to use chloramines for secondary disinfection, primarily to maintain a disinfectant residual and minimize formation of disinfection by-products (DBPs). However, nitrification is a problem in chloraminated systems. Nitrification has been controlled via breakpoint chlorination—periodic switching from chloramines to free chlorine—but the consequences of this strategy are not well understood. The authors evaluated the effects of a one-month switch from chloramines to free chlorine. They concluded that disinfectant switching alone will terminate nitrification but is unlikely to provide long-term nitrification control. Systems that have used chloramines for a long time are more likely to have nitrification problems. Cast-iron pipe also might increase nitrification potential. Potential negative effects of disinfectant switching include increased DBP concentrations and periods of low disinfectant residual. These results can help utilities decide whether or how to use disinfectant switching as a nitrification control strategy.—JZ