Date of Award
Doctor of Philosophy (PhD)
Environmental Engineering and Earth Science
Biofilm growth on engineered surfaces poses a risk to public health and presents a challenge in several industries, such as the food industry, as biofilms harbor a wide range of microorganisms, including opportunistic respiratory pathogens. In the context of drinking water, conventional secondary disinfectants such as free chlorine and chloramines are not known to fully prevent biofilm formation at relevant concentrations necessitating the development of scalable, effective interventions. The issue of biofilms and their role as a host to a wide range of pathogenic microorganisms can be more effectively addressed at the point-of-dispensing. Unlike building-point-of-entry or point-of-use devices interventions, there are no wetted surfaces downstream from point-of-dispensing devices where biofilms can grow.Available point-of-use technologies target planktonic bacteria in bulk water, neglecting possible biofilm formation downstream from the disinfection process. The approach of current point-of-use technologies may not eradicate downstream biofilms given that the biofilm establishment is not a result of substandard treatment at the treatment plants but of conditions that promote their growth within DWDS and premise plumbing. Hence, targeting the biofilm at the point-of-dispensing may better address the challenge by introducing ultraviolet-C (UVC) LED-equipped fluid handling devices. Despite the widespread applications of UVC radiation for water and food disinfection, its use for inhibiting surface colonization is at the outset of the research. The emerging compact UVC LEDs with rapidly increasing efficiencies have the potential to alter the technology horizon of UVC disinfection. Such advancements enable their incorporation in confined spaces to inhibit surface colonization on inaccessible surfaces such as those in premise plumbing and the food industry. In this approach, UVC LED-equipped devices will target biofilm formation by continuously or intermittently irradiating internal surfaces. Such applications necessitate knowledge of the response of the biofilm-forming bacteria on a surface continuously exposed to UVC radiation, which enables the prediction of biofilm formation under different intensities. This research was carried out with three main objectives: firstly, to develop experimental setups and protocols that enable biofilm growth under continuous irradiation; secondly, to determine minimum lethal UVC intensity and develop a predictive intensity response model; thirdly, examine the effect of growth rate, which is governed by parameters such as temperature and time, on the biofilm-UVC intensity response. Herein, the apparatus and methodology that allowed for biofilm growth under controlled UVC irradiation intensities and quantification of their growth rates were developed. Furthermore, a biofilm-UVC intensity response model was created for the first time, enabling quantitative prediction of Escherichia coli (E. coli) biofilm formation rates as a function of surface irradiation intensity (λ = 254 nm). Although biofilm formation was suppressed by more than 95% under the intensities employed, a minimum nonzero threshold of surface biovolume was observed even when comparatively high UVC intensities were used. This minimum threshold was attributed to the deposition of colloidal material and bacterial secretions, which provided shielding against UVC photons. Such shielding likely enabled biofilm growth and has implications for the long-term efficacy of continuous UVC irradiation. Furthermore, the dependences of the parameters of the developed biofilm-UVC intensity response model on baseline growth rate (growth rate without irradiation) were determined. The temperature was used as the criterion for manipulating the baseline growth rate. Biofilm formation under continuous irradiation at a constant intensity was observed to intensify when the flow cell temperature was increased. Increasing the temperature by 10°C resulted in an increase in biovolume by 193% under intensities of 59.5-60 µW/cm2. Evaluation of the model parameters confirmed the hypothesized shielding effect arising from the deposition of extracellular colloidal materials diminishing the UVC intensity. It was observed that as the growth rate increases, the intensity response diminishes attributed to the higher rate of colloidal depositions shrinking received intensity by underlying bacterial cells. The shielding effect was further investigated by conducting 12- days long biofilm growth experiments. After 48 h, a breakthrough in biofilm growth under continuous UV irradiation was observed, followed by a steady increase in the next 10 days attributed to the synthesis and deposition of shielding materials.
Torkzadeh, Hamed, "Biofilm Deterrence on Surfaces Exposed to Continuous Ultraviolet-C Radiation As a Tool for Mitigating Bacterial Pathogens: Developing a Biofilm-UVC Intensity Response Model" (2021). All Dissertations. 2871.