Effect of initial microbial density on inactivation of Giardia muris by ozone

Inactivation of microorganisms by disinfectants frequently shows non-linear behavior on a semilogarithmic plot of log survival ratio versus time. A number of models have been developed to depict these deviations from Chickʹs Law. Some of the models predict that the log survival ratio (at a particular disinfectant dose and contact time, even in absence of demand) would be a function of the initial concentration of microorganisms (N0), while other models do not predict such an effect. The effect of N0 on the survival ratio has not been deliberately tested. This work examined the inactivation of Giardia muris by ozone in batch systems, deliberately varying the disinfectant dose and N0. It was found that the models predicting a dependency of survival on N0 gave a better description to the data than models that did not predict such a dependency. Hence there is an apparent decrease in disinfection efficiency of ozone against Giardia muris (at pH 8 and 15°C) as the initial microorganism concentration decreases. This phenomena should be taken into account by both disinfection researchers and by process design engineers.