Abstract
Ultraviolet germicidal irradiation (UVGI) has gained recognition as an effective method for antimicrobial purposes. This technology has been widely employed in various indoor air disinfection layouts, including upper-room and in-duct configurations. Traditionally, mercury lamps were used to emit 254 nm germicidal ultraviolet-C (UVC) light. However, concerns have been raised about the sustainability of the materials used and the potential hazards associated with such disinfection methods. To address these issues, alternative light sources with different wavelengths have been developed and investigated. Recently, there has been growing interest in 222 nm far-UVC due to its demonstrated effective antimicrobial capabilities, while being less harmful to humans compared to 254 nm UVC. However, research on its air disinfection performance is limited. Therefore, the objective of this thesis is to explore the feasibility of using 222 nm far-UVC for indoor air disinfection. To achieve this aim, a series of experiments and numerical simulations were conducted. The disinfection performance was evaluated by measuring the inactivation efficacy of far-UVC irradiation against various species of microorganisms in different application scenarios. Additionally, approaches for enhancing disinfection effectiveness were identified.Initially, the 222 nm far-UVC was applied in conjunction with an upper-room UVGI system. Two gram-negative bacteria, namely Escherichia coli (E. coli) and Salmonella enterica (S. enterica), one gram-positive bacteria, namely Staphylococcus epidermidis (S. epidermidis), and two bacteriophages, namely MS2 and P22, were selected to assess the inactivation efficacy of the upper-room far-UVC in a real-size chamber in different scenarios. These scenarios included varying air mixing conditions and chamber scales to simulate realistic application scenarios. The disinfection performance of the current system was assessed by analyzing the changes in microbial concentration over time during UV treatment. The results were compared with that of 254 nm UVC to find advantages of 222 nm far-UVC light. The comparison showed that the upper-room 222 nm UVGI system exhibited higher efficacy in inactivating gram-negative bacteria, but lower efficacy in inactivating gram-positive bacteria compared to 254 nm UVC irradiation. This disparity can be attributed to the differences in microbial structures and their intracellular contents.
Further, the air disinfection performance of upper-room far-UVC was investigated both experimentally and numerically, with a focus on examining the effect of different source arrangements. The study involved manipulating irradiation angles, irradiation modes, and lamp positions. The results indicated that the performance was influenced by the irradiation angles and positions of lamps, while lamps with a fixed 90° irradiation angle presented similar disinfection performance to that of the rotating lamps. A model based on the radiative view factor revealed the relationship between the lamps’ arrangements and provided chamber-average far-UVC irradiance: the long space in front of a lamp is more conducive to the irradiation performance of the lamp than the wide space beside a lamp. Although a rotating lamp can irradiate a chamber with various irradiation angles, the equivalent chamber-average irradiance was similar to that provided by the lamps with a fixed 90° irradiation angle. Consequently, the rotating lamp did not enhance air disinfection beyond the performance achieved by lamps with a fixed 90° irradiation angle.
Then, the air disinfection performance of upper-room far-UVC was further investigated under poorly mixed air conditions, considering the different locations of UV sources and microbial release. The results indicated that the linear function was not suitable for fitting the UV inactivation process over time in such conditions. Instead, a bi-exponential function was found to accurately describe the changes in microbial concentration during UV irradiation. The disinfection performance under poorly mixed air conditions depends not only on the arrangements of the UV sources but also on the pattern of microbial release. The fitted disinfection processes demonstrated that upper-room UVGI could perform superior air disinfection under poorly mixed air conditions when specific microbial distributions were present. However, adequate air mixing is necessary to achieve significant microbial reduction, such as two- or three-log reductions.
Finally, the feasibility of applying a single-pass air far-UVC disinfection system for indoor air disinfection was explored using different geometries and installations of the UV sources. Two types of systems were examined: externally installed rectangular module lamps (referred to as EME devices) and internally installed UV tubes (referred to as ITE devices). Considering the challenge in achieving a high far-UVC dose at high flow rates, the baseline systems were further modified by extending the UV exposure pathway and incorporating UV reflective materials. The baseline ITE system presents higher inactivation efficacies against S. enterica, S. epidermidis, and MS2, with the values of 94.4%, 25.8, and 37.8%, respectively, compared to the baseline EME system, which recorded efficacies of 81.4%, 16.9%, and 25.1%. After modifications, both EME and ITE systems achieved over 50% inactivation efficacies against the three microorganisms. Based on the disinfection efficacy of the modified systems, it was anticipated that the systems would provide reliable air disinfection performance, as supported by a comparison with upper-room far-UVC light in a hypothetical environment.
| Date of Award | 5 Aug 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Chi Keung Alvin LAI (Supervisor) |