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Assessment of disifection effectiveness of various techniques for the inactivation of bacterial pathogens in water

Tsirou Myrto

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Summary

Inadequate access of a large part of the world's population to safe drinking water is one of the major environmental problems of our time. Addressing this problem requires the optimization of conventional water treatment processes, in order to enhance their effectiveness and energy efficiency. Water disinfection is defined as its release from pathogenic micro-organisms and is carried out by killing the micro-organisms or by restricting their growth and reproduction.There are various ways of disinfection such as chlorination, ozonation, UV disinfection, common and successful but costly and uneconomic ways. This has led to the search for new, environmentally friendly ways of disinfection such as utilizing solar radiation through the mechanism of photocatalysis.In this thesis, the efficacy of solar radiation in the presence of catalyst, chlorination and UVC as disinfection methods was studied in the laboratory. The micro-organisms tested were, each, in water solution. Experiments were performed by predetermining the intensity of UVC radiation, the chlorine dose as well as the concentration of microorganisms, the amount of catalyst and the volume of the final solution. The microorganisms studied were Escherichia coli, Klebsiella pneumoniae and Staphylococcus aureus, while the catalysts used were magnesium titanium (MgTiO3) and zinc titanium (ZnTiO3) (perovskites).Finally, the lipid peroxidation of the bacterial cell membrane tested how effective each disinfection method was. Determination of lipid oxidation by the value of the formed malondialdehyde (MDA) was done by the thiobarbituric acid method.According to the experimental results, the method that proved to be most effective for all bacteria is UVC radiation, since the reduction order of bacterial population was 6 Logs in about 1 min. Regarding the chlorination method, both concentrations yielded relatively good results, with the reduction order of bacterial population being 4 Logs at 60 min. The solar radiation method with ZnTiO3 catalyst was more effective for Staphylococcus aureus bacteria as the reduction of bacterial population was 6 Logs for 50mg/L and 5 Logs for 25mg/L in 4 h, while for the other two bacterias it was 4 Logs for both concentrations at 4 h. For the catalyst MgTiO3, it was used only to inactivate the bacterium Escherichia coli. The most effective concentration was 50mg/L at 3 h, with a reduction order of 6 Logs.Finally, by controlling the amount of malondialdehyde produced through lipid oxidation, we observe that the most efficient disinfection method is UVC radiation. The amount of MDA produced in this method was more than any other method, especially for the bacterium S.aureus, as it was increased by 57% from the next method.

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