Experimental Evaluation and Modeling of an In-duct Ultraviolet Germicidal Irradiation (UVGI) System for Bioaerosol Disinfection
Creators: Luo, Hao
Publisher: name: University of Alberta Library
Year: 2024

<p>Airborne pathogen transmissions have played critical roles in the previous COVID-19 pandemic and various respiratory epidemics, such as influenza, severe acute respiratory syndrome (SARS), etc. Inhaling airborne pathogens can cause adverse health effects, resulting in respiratory infections, severe illnesses, and, in some cases, fatalities. One practical approach to combat airborne transmission is to use ultraviolet-C (UVC) lamps. When integrated with ventilation and filtration measures in heating, ventilation, and air conditioning (HVAC) ducts or room settings, these systems effectively inactivate airborne microorganisms and mitigate airborne pathogen transmission. However, its broader implementation in commercial, residential, and industrial buildings has been limited by the complexity of designing systems for optimal UV inactivation efficiency. Specifically, for the in-duct UV germicidal irradiation (UVGI) systems (this thesis’s focus), the effects of system designs, operating conditions, and bioaerosol characteristics on the inactivation efficiency are insufficiently investigated or interpreted. Therefore, the main objectives of this study are twofold: first, to investigate the effects and underlying mechanisms of influencing factors, including the lamp arrangements, duct wall materials, relative humidity (RH), bioaerosol particle size, and microorganism species, on UV inactivation efficiency through comprehensive experimental and modeling works; and second, to integrate the insights into a practical flowchart, supplemented with a case study, to guide the design and implementation of UVGI technology in HVAC ducts.<br>
For this purpose, a pilot-scale duct system with twin-tube low-pressure mercury UV lamps emitting 254 nm radiation was designed and constructed. This study experimentally examined the impacts of the abovementioned influencing factors on UV inactivation of airborne microorganisms. Airborne MS2 and E. coli, commonly used surrogates for pathogenic viruses and bacterium, were employed in the UV disinfection tests. Additionally, a comprehensive mathematical model was developed to characterize the in-duct UVGI system. This model integrated a new view factor-based model to predict irradiance distribution and an improved genomic model to predict UV rate constants of airborne single-strand RNA (ssRNA) viruses. The UV irradiance model considered multiple factors, such as direct emissive irradiance, specular reflection irradiance, diffusive reflection irradiance, and shadowing effects caused by the arrangement of multiple lamps. The UV rate constant model considered genomic damage, protein capsid damage, and the ratio of aerosol and liquid to represent the UV-induced inhibition of genome replication inside host cells, the prevention of the virus attachment, entry, and genome penetration into the host cell, and the bridge of the UV rate constant between the liquid-based matrix and the airborne state. Finally, computational fluid dynamics (CFD) simulation was utilized to calculate the average received UV dose and predict the disinfection efficiency of the in-duct UVGI system. The mathematical model and CFD simulations were validated using experimental data. <br>
The results demonstrated effective inactivation of airborne microorganisms by UV irradiation, as measured by reduction in live virus/bacteria titers (using conventional culturing methods) and damage to viral genomes (using quantitative polymerase chain reaction (qPCR)). The design of the duct system played a crucial role in regulating irradiance distribution inside the duct. The combination of increasing the number of UV lamps, placing them perpendicular to airflow in the same row (2 lamps scenario), and using more diffusively reflective duct materials resulted in a higher and more uniform irradiance distribution inside the duct, thus providing better disinfection performance. In addition, operating conditions significantly impacted UV inactivation performance, where increasing RH (25% to 60%) initially increased and then decreased inactivation efficiency due to the combined effects of the bioaerosol water sorption and viral structural damage. Moreover, bioaerosol characteristics critically determined the performance of the UVGI system. Larger bioaerosols posed a more significant challenge for inactivation than smaller bioaerosols due to potential virion aggregations, particle aggregations, and larger salt crystals. In addition, different microorganisms exhibit distinct UV rate constants, thus critically defining the UV inactivation efficiency. <br>
In the end, this thesis proposed a comprehensive UVGI system design flowchart, integrating with the abovementioned insights. A case study was included to demonstrate the practical application of the flowchart. It summarized existing in-duct UVGI system designs from the literature and predicted their germicidal performance, specifically focusing on mitigating possible airborne transmissions of ssRNA viruses. Ten designs were identified as suitable for achieving 90% inactivation of all potential airborne ssRNA viruses.</p>