Survival of bioaerosols in HVAC system photocatalytic filters

• Stable bioaerosols were generated within operating environment of HVAC systems.
• Reproducible coatings of aerosolised E. coli bacteria were applied on filters.
• Damages to E. coli cells were studied through SEM and epifluorescent microscopy.
• Total inactivation of cells was observed in irradiated HEPA photocatalytic filters.
• Contact between TiO2 and bacteria was a critical point for inactivation efficiency.

The survival of an experimental bioaerosol in photocatalytic and non-photocatalytic filters was investigated under UV-A or UV-C radiation exposure. To ensure the generation of reproducible bioaerosols in real operating conditions of Heating, Ventilation and Air Conditioning (HVAC) systems and their homogeneous coating on filters, a reactor with an optimal geometry was developed. The bacterial aerosol was generated from standardised bacterial suspensions and the Gram-negative bacterial model Escherichia coli was used as the experimental organism. The bioaerosol was characterised in terms of particle size distribution, reproducibility and stability over time.The coating of aerosolised cells on the filters was then studied. The total number of aerosolised E. coli cells was different from the amount of cultivable bacterial cells extracted from filters, probably resulting from the stress of the aerosolisation process. Damages to the outer membrane of E. coli cells were clearly observed through SEM and epifluorescent microscopy analysis. The influence of important physical parameters in the reactor-aerosolisation duration, frontal velocity and relative humidity- on the amount of aerosolised cells coated on filters were also investigated.Finally, the effects of UV-A or UV-C radiation were assessed on aerosolised microorganisms coated on HEPA photocatalytic filters and on photocatalytic or non-photocatalytic filters containing activated charcoal. The HEPA photocatalytic filters demonstrated a better disinfection efficiency with full damages of the aerosolised bacterial cells, probably resulting from an optimal contact between TiO2 coating and the microorganisms. In contrast, the use of filters with activated charcoal resulted in the apparition of an inactivation threshold that could be attributed to the penetration of aerosolised 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.

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