Efficiency of (photo)catalytic filters to remove bioaerosols : application to HVAC system
Résumé
Indoor air quality has become a major public health concern and it potentially poses a threat to the health of all types of populations. Actually, populations in developed countries spend more than 80 % of their time in confined indoor environments. Indoor air pollutants emanate from various sources, chemical or biological. Most of the studies deal with the removal of chemical sources but biological treatment of indoor air remains an emerging research theme. Indeed, the removal of biological agents is a complex issue. None of the traditional technologies for indoor air decontamination, such as activated charcoal filters, HEPA filters with or without UV radiation, ozonation, air ionization, are completely effective. Moreover, these technologies still need further investigation. The aim is not only to trap the microorganisms present in the indoor air, but also to alter them irreversibly. In this context, the combination, within air treatment plants, of both the photocatalytic and the filtering process is a promising technology that would combine the benefits of both processes to efficiently trap and alter chemical and microbiological pollutants through total mineralisation. Our researches deals with the effects of UV-A and UV-C radiation on the coating of aerosolised cells of fungal spores before and after their germination but also on an experimental bioaerosol of bacteria in presence of photocatalytic and non-photocatalytic air filters commonly used in Heating, Ventilating, and Air Conditioning (HVAC) systems. E.coli and Aspergillus Niger are used as model of bacteria and fungi because E.coli is commonly used to evaluate indoor air cleaning technologies and A.Niger is known for its resistance to any environmental stress conditions, and especially the spores which contain a black fungal spore pigment, protecting them from UV radiation exposure. In presence of E.coli or A.Niger the photocatalytic filters without charcoal demonstrated better disinfection efficiency with full damages of the cells, probably resulting from an optimal contact between TiO2 coating and the microorganisms. In contrast, the use of filters with activated charcoal, even with UV-C, resulted in the apparition of an inactivation threshold that could be attributed to the penetration of cells within the activated charcoal layer and the absence of contact with the photocatalyst. However, UV-C photocatalysis was able to inactivate faster and, at the same time, mineralise biological pollutants than UV-A. In the case of A.Niger, the effects of UV radiation were also assessed on spore germination for both types of filters. The inactivation of spores in illuminated photocatalytic filters resulted in an irreversible inhibition of the fungal germination under UV-A or UV-C radiation. In contrast, fungal spores were able to germinate in non-photocatalytic filters despite previous exposure to both types of UV radiation. After spore germination disinfection efficiency is lower than experiments whereby spores had just been applied onto the filters, due to the absence of contact between the biological pollutants and the photocatalyst coating. Our results demonstrated the interest to use photocatalytic filters ensuring optimal contact between pollutants and TiO2 coating to lead to a total inactivation of fungal spores in filters of HVAC systems.