A simplistic quantitative microbial risk assessment (QMRA) based on the maximum risk curve (r = 1) was developed for Legionella within a water distribution system. Both biofilms and a thermophilic isolate of acanthamoebae were shown to increase the resistance of Legionella to conventional thermal disinfection by between one and two logs respectively. The level of risk presented to consumers was shown to exceed the USEPA 10-4 benchmark in many cases tested. This was caused, in part, by the sensitivity of the risk model but also through a lack of reliable dose-response data for Legionella. Notwithstanding this, the current study provided comparative information on the efficacy of conventional disinfection against Legionella. Combined chlorine was shown to reduce the risk of infection by as much as 1-log when compared to free chlorine, although thermal disinfection provided the most effective means of risk reduction. Biofilm detachment and the interaction of Legionella with acanthamoebae were two important ecological factors that significantly increased the risk of legionellosis, and thus should be further considered in the refinement of QMRA models.

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