Disinfection kinetics of Legionella pneumophila by ultraviolet irradiation was investigated. The change in viable cell concentration with exposure time could be divided into three steps: lag step in which little change in viable cell concentration was observed, fast disinfection step and slow disinfection step. The slow disinfection step was not observed at the initial cell concentrations below about 106 cfu/mL. The disinfection kinetics were well described with two parameters; lag time and disinfection rate constant of the fast disinfection step. The effects of UV intensity, temperature and initial cell concentration in the kinetic parameters were investigated. With increasing initial cell concentration, the lag time decreased and the disinfection rate constant increased. The effects of initial cell concentration on the kinetic parameters were considered to be attributed to the decrease in the effective UV irradiation intensity due to the partial shield of UV light by the disinfected cells. The empirical correlations were presented for predicting the lag time and disinfection rate constant. Furthermore, UV disinfection of L. pneuophila in a model hot-tub connected with external irradiation chamber was also discussed.

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