Disinfection of Indoor and In-duct Bioaerosols Using Ultraviolet C and Air Ionization


Student thesis: Doctoral Thesis

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Awarding Institution
Award date19 Feb 2021


The purpose of this thesis was to develop and evaluate the effectiveness of novel and sustainable technologies for air decontamination in indoor environments and ventilation ducts. To achieve this aim, a series of air disinfection investigations into factors influencing the performance of various air decontamination technologies and the response of different species of microorganisms to these antimicrobial agents were conducted.

A novel compact upper-room ultraviolet germicidal irradiation system was developed using light-emitting diodes as the UV source (UR-UVGI-LED). Its effectiveness was compared with the traditional mercury-lamp-based upper-room ultraviolet germicidal irradiation system (UR-UVGI-MV). Test microorganisms were atomized and exposed to UR-UVGI-LED (or UR-UVGI-MV) device. The disinfection performance of both UR-UVGI-LED and UR-UVGI-MV systems was comparable for E. coli and S. marcescens. However, there was a significant difference between the efficacy of the UR-UVGI-LED and UR-UVGI-MV to inactivate S. epidermidis. The estimated effective susceptibility values for bacteria were substantially higher (ratio varies between 11 and 31) using the UR-UVGI-LED than with the UR-UVGI-MV.

To enhance the efficacy of the UR-UVGI-LED, moving irradiation was explored. The results showed no significant differences between decay rates for straight-ahead and moving irradiations under well-mixed conditions. Compared to well-mixed conditions, the performance of the straight-ahead irradiation under poorly-mixed conditions decreased by 52.90 to 79.38 percent. The performance of the moving irradiation under poorly-mixed conditions was higher than the straight-ahead irradiation by 28.81 to 99.43 percent. Thus, rotating irradiation offers great potential for application in environments where bioaerosols are unevenly distributed in a built environment. 

The in-duct air decontamination investigated the independent applications of air ions and ultraviolet waveband C (UVC), both in conjunction with the effects of environmental factors on their performance. Also, the impact of duct reflectance on the performance of in-duct UVC lamps was evaluated. Different polarities of ionizers were explored and the irradiance of the UVC lamp was varied. The results showed an inverse correlation between air velocity or relative humidity (RH) and disinfection efficiencies of air ions and UVC lamps. The disinfection efficacy of the UVC lamp was higher at an air temperature of 20-21°C than 15-16 °C and 25-26 °C. Whereas, the efficacy of air ions decreased as temperature increased. The susceptibility values of bacteria with the UV lamp showed a similar pattern described above for air temperature and RH. The results also showed that the efficacy of the UV lamp decreased in a black surface duct than in the clean duct. On the whole, the disinfection efficacy depends on the irradiance of the UVC lamp and the polarity of air ions. The efficacy of the UVC lamp increases with irradiance and positive air ions demonstrated higher efficacy than negative air ions. The ozone emission rates computed for the two air ion polarities were very small. 

Moreover, the synergistic effect of combined disinfection methods was studied. The combination of air ions and UV disinfection methods demonstrated improvements in disinfection efficacy and major synergistic effects. The coupled positive air ions and UV exhibited increased disinfection efficiency and significant synergistic effects for S. epidermidis (p = 0.008) while the coupled negative ions and UV showed a significant synergistic effect on E. coli (p < 0.001) and S. typhimurium (p = 0.04). The susceptibility constants of bacteria under positive air ions were 1.70, 3.10, and 2.95 times higher than and negative air ions for E. coli, S. typhimurium, and S. epidermidis, respectively. Finally, this thesis emphasized that S. epidermidis exhibited the greatest resistance to all methods of air decontamination, suggesting that the structural composition of microorganisms had a very strong influence on their survival.