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Effect of aqueous matrix on disinfection efficiency in terms of bacterial inactivation

Kyriazi Afroditi

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Summary

Water is a basic unit for sustaining life on our planet, covering, in its various forms, about 2/3 of the Earth’s surface. Nevertheless, it is a renewable, yet not inexhaustible natural resource since climate change is bringing about droughts and water scarcity, at a time when global demand for fresh water is growing rapidly and proportionately with the population. At the same time, a great part of the world’s population has insufficient access to drinking water, establishing issues deriving from water as some of the most important environmental problems of our time.At the same time, of course, proportionately with the reduction of the quantity the quality of the water resources is degraded, along with water-borne diseases that are documented worldwide and their unaccountable speed of transmission through the consumption of polluted water, all of which demonstrate the importance of the effective inactivation of pathogenic microorganisms. Thus, today more than ever both the proper use of water resources and the reuse of water are considered essential, laying the foundations for new ways of management, the expansion of the methods of processing it as well as the improvement of the existing ones.Considering the environmental conditions and needs that arise regarding the various types of the Earth’s waters, the need to study their microbiological quality emerges. Therefore, in the context of this dissertation, the control of the effectiveness of disinfection was examined both with the established method of UVC radiation and with the ascending activation of the sodium persulfate with ferric ions. The purpose is to inactivate pathogenic bacteria and faecal infection indexes in the selected aqueous matrices, but also to study the effect of the matrix on the efficiency of disinfection.Deionized water, bottled water, tap water, lake water and partially treated wastewater were selected as aqueous matrices, whereas Escherichia coli, Enterococcus faecalis and Vibrio parahaemolyticus were used as bacterial indexes. The experiments were conducted in matrix-bacterial pairs under predetermined controlled conditions. Specifically, for the method of UVC radiation the intensity of the lamp was 30W, while for the activation of sodium persulfate with ferric ions the concentrations of the solutions were 150mg / L with respect to the sulfate radical and 30mg / L with respect to divalent iron respectively. Finally, the initial bacterial concentration in each experiment was 106 CFU / mL.According to the experimental results, the method that proved to be most effective for all bacteria and in all aqueous matrices is that of UVC radiation, as the rate of reduction of bacterial populations amounts to 6 Logs in about 30 s. The effect of the matrix lies mainly in the inactivation time of the bacteria, with tap water, bottled and deionized water giving consistently less times, while lake water and wastewater taking up to twice as long to completely kill the bacterial populations. E. faecalis shows the lowest resistance to lake water, while in other aqueous matrices the results are similar to all three bacteria. As far as the method of disinfection with sodium persulfate activated with ferric ions is concerned, the extermination rates and the reaction rate are significantly lower, with the exception of experiments performed in deionized water, where the complete inactivation of all three bacteria occurs in 60 min. In general, E. coli shows the highest mortality rates in all aqueous matrices, with the waste having the lowest inactivation rate, but also a factor of k. E. faecalis, apart from deionized water, does not show fluctuations with tap water giving the lowest results. V. parahaemolyticus seems to show the greatest resistance, since its population is significantly reduced only in deionized water, while in bottled water and in wastewater the rates are extremely low. It is worth mentioning that in this method the rates are not fully in line with the mortality rates in order of effect of the aqueous matrices, as the bacterial populations initially decrease at different rates but subsequently stabilize for long periods until the end of the experiments.In summary, the effect of the aqueous matrix on the inactivation of the bacteria is obvious; the wastewater and the lake water contribute negatively to it, whereas the deionized water provides the appropriate conditions for the complete extermination of the populations.

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