Photocatalytic inactivation of Legionella Pneumophila and aerobic bacteria consortium in water over TiO2/SiO2 fibres in a continuous reactor

A continuous photoreactor, working in a total recycle mode and irradiated by a low-pressure Hg lamp, has been used to study the bactericidal effect of a photocatalyst, formed by TiO2embedded in SiO2fibres, on Legionella pneumophila and a consortium of common gram-negative aerobic bacteria (Escherichia coli, Klebsiella sp., Pseudomona sp. And Proteus sp.) in water. The kinetic modeling of the inactivation process, carried out with the measured values of viable bacteria concentration at the outlet of photoreactor, evidenced that for each pass inside the photoreactor the ratio between the outlet and inlet cell concentrations was of order of 0.01 for the inactivation of L. pneumophila. For the other aerobic bacteria, which are usually taken as reference in photocatalytic bacteria inactivation studies, this ratio was of order of 0.3 for the first hour of illumination, while upon prolonged irradiation (up to 9 h) this ratio increased to 0.7. Several factors inducing this latter decrease of efficiency are possible, as e.g. competition for photocatalytic attack between microorganisms and organic compounds released by damaged bacteria or photoinduced alteration of a small fraction of still viable bacteria making them less interactive with the photocatalyst. The inactivation mechanism normally proposed for common bacteria involves an initial attack of the photogenerated radicals to the outer membrane; the consequent membrane dispersion allows the radicals to damage the cytoplasmic membrane. The higher lethality of the photocatalytic method observed towards Legionella (in comparison to the other aerobic bacteria) is explained considering that the radicals attack the Legionella secretion system, which is adapted for high virulence and would become activated for and through adhesion to the TiO2surface. This attack would then be able to inactivate L. pneumophila without dispersing the outer membrane. Apart from this, the water flow through the catalyst fibres can facilitate the bacteria transport towards the anatase surface, and additionally the generated shear stress may help adhesion, at least for some bacteria as E. coli, contributing further to improve the photokilling efficiency; both factors may contribute to the efficiency of this photoreactor configuration.