Legionella and monochloramine in San Francisco

In February 2004, San Francisco changed its residual drinking water disinfectant from chlorine to monochloramine to ensure continued compliance with disinfectant by-product regulations. In anticipation of the switch, the U.S. Centers for Disease Control and Prevention [CDC] initiated a study of the effect of the change in disinfectant on the occurrence of Legionella in the drinking water distribution system. The study was a collaboration among CDC, San Francisco Public Utilities Commission, California Department of Public Health and San Francisco Department of Public Health. The study included 53 buildings of at least three stories. Samples were collected from each building three times before the conversion to monochloramine and three times after. In each round, nine samples were collected from each building: bulk water from the primary water heater for the building plus bulk water and biofilm swab samples from four distal sites. Free and total chlorine residuals, temperature and pH were measured at each sampling location. Samples were immediately shipped to the CDC where laboratory testing was blinded to building identity. When chlorine was the residual disinfectant in municipal drinking water, Legionella spp. were cultured from building water systems on 96 (60%) of 159 occasions, and 37 (70%) of 53 buildings were colonized with Legionella in at least one of three sampling rounds. After conversion to monochloramine, Legionella were found on 7 (4%) of 159 occasions in 5 (9%) of 53 buildings. The Legionella reduction at these sites demonstrates the efficacy of monochloramine in controlling these bacteria in hot water systems.