AWWA ACE61814

In February 2004, the San Francisco Public Utilities Commission [PUC] changed its residual drinking water disinfectant from chlorine to monochloramine in order to achieve new disinfectant byproduct standards in water delivered to its customers. Prior to the conversion, the Centers for Disease Control [CDC] in Atlanta, Georgia contacted the PUC with a proposal to study the effect of the new disinfectant on the occurrence of Legionella in the drinking water distribution system. Other local health agencies were also interested in this issue, which resulted in a final study team that included representatives from the State of California Department of Health Services [DHS], the California Emerging Infections Program [CEIP], and the San Francisco Department of Public Health [DPH]. Sites in 53 buildings were sampled six times each: three times before the conversion to monochloramine and three times after the conversion. Since Legionella bacteria are known to survive and multiply within biofilms in large buildings with extensive hot water systems, only buildings that were at least three stories were eligible for sampling. At each time point, we collected a water sample from the primary water heater and four paired water and biofilm samples from sink faucets or showerheads. Temperature, pH, and free and total chlorine residual were measured at each site. Samples were sent to CDC for Legionella culture. Laboratory testing was blinded to the identities of the buildings. Prior to the conversion to monochloramine, 37 (70%) of sampled buildings were colonized with Legionella on one or more occasion and 347 (24%) of 1,405 samples grew Legionella. After the conversion, 5 (9%) of buildings were ever colonized and only 12 (<1%) of 1,420 samples were positive. The reduction of Legionella colonization within buildings suggests that monochloramine provides better residual disinfection than free chlorine for limiting the growth of these bacteria in hot water systems. Includes abstract only.