The antimicrobial activity of ozone (OZ) and chlorine dioxide (CD) is difficult to determine, not only due to the short inactivation times, but also because the concentrations of the disinfectants decrease during the reaction time. These difficulties were avoided by designing a continuous flow reactor, where steady state concentrations of the antimicrobial agents as well as of the microorganisms can be obtained, depending on the inactivation velocity. From the inactivation velocity, the time necessary for a 99.99% reduction and corresponding Ct values can be derived. This was done at different concentrations for E. coli, L. Qineumophila, B. subtiüs at pH 6 and pH 8 and at 5°C and 15°C, and for the Coliphages MS 2 and PhiX 174 at pH 8 and 5°C. The inactivation rates obtained this way are rather short as compared to results of other authors. Ct values decrease with increasing concentrations of OZ and of CD (n> 1), except with B. subtilis. The relative susceptibilities to CD are: B. subtilis < E. coll < L. pneumophila < PhiX 174 < MS 2. The greater susceptibility of Legionella as compared to E. coli was confirmed in a batch test. CD proved to be more efficient at the higher temperature and at the higher pH. The relative susceptibilities to OZ were: B. subtilis < L. pneumophila < MS 2 < PhiX 174 < E. coli. OZ seemed to be somewhat more effective at the lower temperature, while higher pH resulted in faster destruction.

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