Assessing the Potential of Granular Activated Carbon Filters to Limit Pathogen Growth in Drinking Water Plumbing Through Probiotic Versus Prebiotic Mechanisms
| dc.contributor.author | Deck, Madeline Emma | en |
| dc.contributor.committeechair | Pruden-Bagchi, Amy Jill | en |
| dc.contributor.committeemember | Falkinham, Joseph O. | en |
| dc.contributor.committeemember | Edwards, Marc A. | en |
| dc.contributor.department | Civil and Environmental Engineering | en |
| dc.date.accessioned | 2025-02-07T09:00:28Z | en |
| dc.date.available | 2025-02-07T09:00:28Z | en |
| dc.date.issued | 2025-02-06 | en |
| dc.description.abstract | Legionella pneumophila (Lp) and nontuberculous mycobacteria (NTM) are opportunistic pathogens that can be transmitted via drinking water, when tiny droplets containing the bacteria are aerosolized and inhaled during activities such as showering. The resulting respiratory illnesses, Legionnaires' Disease and NTM lung disease, are among the leading sources of drinking water associated disease in the United States and other parts of the world. Lp and NTM are both difficult to control, because they establish as part of natural biofilms that form within the interiors of pipes and fixtures that deliver drinking water to the point of use. These pathogens are especially problematic within premise (i.e., building) plumbing, where intermittent use throughout the day leads to long periods of stagnation, increased water age, warmer temperatures, and depleted disinfectant residuals that exacerbate bacterial growth. The recent advent of high throughput DNA sequencing has led to the discovery that drinking water microbiomes are diverse, complex, and largely comprised of non-pathogenic microbes. This has further led researchers to hypothesize that the microbial ecology of this diverse microbiome could be harnessed as a natural means to control Lp and NTM, i.e., a "probiotic" approach, but such an approach has not yet been demonstrated. The objective of this study was to assess this hypothesis by utilizing biologically active granular activated carbon (GAC) filters, which are already a widely used drinking water treatment both at the municipal and household scale, as a means to naturally shape the microbial ecology of downstream premise plumbing and inhibit Lp and NTM proliferation. GAC has an extremely high surface area that aids removal of organic carbon via adsorption but also provides an ideal habitat for establishment of biofilms, which removes organic carbon from the water via biodegradation. Convectively-mixed pipe reactors (CMPRs) were used for replicable simulation of premise plumbing distal taps. The CMPRs consisted of four-foot-long closed polyvinyl chloride (PVC) pipe segments with the sealed bottom portion resting in a ~48 °C water bath and with the top portion plugged and exposed to the cooler, ambient atmosphere (25 °C in this study), inducing convective mixing and resulting in an internal water temperature of 37 °C. PVC was chosen because it is common in premise plumbing and generally leaches the least organic carbon among the different types of plastic pipe. Four different influent water conditions were implemented in the experimental design: 1) Untreated, dechlorinated municipal tap water with high organic carbon and low biomass; 2) GAC-treated tap water with low organic carbon and elevated, viable biomass; 3) GAC-treated + 0.22-m pore size membrane-filtered tap water to remove both nutrients and biomass; 4) GAC-treated tap water pasteurized at 70 °C with low nutrients and elevated, killed biomass. The 0.22-m pore size membrane filter simulated the use of a building scale particle filter, while pasteurization simulated water passing through a hot water heater at an elevated temperature recommended for pathogen thermal disinfection. To understand the influence of these experimental conditions on older pipes containing mature biofilms versus new pipes that leach more organics and are being freshly colonized, a set of older pipes colonized with mature ~4-year-old biofilms were compared to newly purchased pipes. Each set of pipes was tested in triplicate for the four different experimental conditions with the full volume replaced three times a week for eight months, simulating infrequently used taps containing warm, continuously mixing water thought to create conditions at a very high risk for opportunistic pathogen growth. In the aged CMPR bulk water effluents, droplet-digital-polymerase-chain-reaction measurements showed a one-log reduction of Lp and NTM when receiving GAC-treated or GAC-treated + particle-filtered influent water versus receiving dechlorinated municipal tap water or GAC-treated + pasteurized water. These findings suggest that decreased biodegradable dissolved organic carbon achieved by GAC filtration acted to suppress Lp and NTM growth, while the additional step of biomass removal by particle filtration provided a more modest benefit. In the CMPRs consisting of new pipes, concentrations of Lp and NTMs in the effluent bulk water were similar among the experimental conditions, except that the CMPRs receiving the GAC-treated + particle-filtered influent water experienced a two-log reduction in NTMs. These results demonstrate that the colonization and proliferation of NTM within premise plumbing can be significantly controlled by limiting nutrients and biomass in the influent water. This work demonstrates the potential of harnessing GAC-treatment as a means to Control Lp and NTM in premise plumbing via nutrient removal. In scenarios where chemical disinfectants have been depleted, off-the-shelf GAC-treatment used as point-of-entry treatment to large buildings with recirculating plumbing could provide benefits that have previously been unrecognized. Alternatively, pasteurization in very hot water heaters could provide a short-term disinfection benefit, but eventually the nutrients embodied in the dead biomass undermine the positive influence of the nutrient removal provided by the GAC-treatment. Improved mechanistic understanding of probiotic strategies to opportunistic pathogen control would be needed to overcome inherent limitations to the approaches examined herein, if more effective control is desired in the absence of thermal or chemical disinfection. | en |
| dc.description.abstractgeneral | Legionella pneumophila (Lp) and nontuberculous mycobacteria (NTM) are bacterial pathogens that are the leading source of drinking water-associated disease in the US. Unfortunately, they are not effectively controlled by protections put in place by the US Safe Drinking Water Act (SDWA). Firstly, they cause respiratory infections, which are spread when tiny droplets of water are inhaled during activities such as showering, whereas the SDWA is specifically designed to protect against ingested pathogens. Secondly, unlike fecal-derived organisms (e.g. E. coli) that are the focus of the SDWA, Lp and NTM grow naturally in drinking water distribution systems, especially in premise (i.e., building) plumbing, where water is warmer and more stagnant. Therefore, even if water leaving the treatment plant is devoid of Lp or NTM, this does not guarantee that the consumer's tap water will be Lp- or NTM-free. Also, even though chlorine or other chemical disinfectant is required by the SDWA to be added to the water leaving the treatment plant to control downstream microbial growth, the disinfectant can be depleted or absent within the premise plumbing itself. Additionally, both Lp and NTM tend to more naturally resist chemical disinfectants than fecal-derived organisms. This research is aimed at overcoming these challenges, opening the door to new approaches to controlling Lp and NTM in premise plumbing. Historically, any microbial growth occurring in drinking water has been viewed as problematic, as it usually indicates the chemical disinfectant is inadequately protecting consumers. However, this work explores whether having an abundant community of beneficial bacteria could improve microbial water quality by competing against pathogens for limited space for attachment and nutrients. Such an approach would be analogous to the use of probiotics in humans, to establish a beneficial gut flora that is less susceptible to pathogen invasion. Granular activated carbon (GAC) filters are often used at drinking water treatment plants and by consumers as a point-of-use (e.g., installed on the kitchen tap or in a refrigerator) or whole-house treatment to remove any contaminants of concern and improve the taste and odor of tap water. The granules within GAC filters have a high surface area that helps remove contaminants, but also provides an environment where microbes can live and thrive. As water enters the filter, beneficial microbes can break down any remaining nutrients in the water (e.g., organic carbon and nitrogen). Additionally, the water leaving the filter carries high levels of microbes that grow on the GAC filter that are shed as water passes through. The resulting water with reduced nutrients and higher concentrations of potentially beneficial microbes could create a competitive environment that alters growth of harmful bacteria, like Lp and NTM, in downstream portions of plumbing. The incoming cold water is also warmed by the building envelope, which increases bacterial growth rates. Thus, the underlying hypothesis of this research is that GAC treatment could provide a combination of reduced nutrients and competitive microbes as water enters downstream premise plumbing and reduce the growth of Lp and NTM. However, GAC-treated water within a building can be further altered by other treatments, like a very hot water heater, which would heat and kill the microbes flowing through it, or a particle filter, which could remove the microbes in the water. This work also seeks to understand how these additional treatments might improve or interfere the nutrient reduction and addition of competitive microbes provided by GAC treatment. This research explores how all these different scenarios affect the growth of Lp and NTM using a lab-scale simulated premise plumbing system constructed out of polyvinyl chloride (PVC) pipe that is a common plumbing material used in homes. Water that was added to the pipes was prepared in four different ways to test the probiotic control hypothesis across distinct experimental conditions that replicate the different influent water scenarios. The four conditions were implemented over the course of eight months with regular chemical and biological analyses conducted to understand the effects of the different influent waters on Lp and NTM. It was discovered that premise plumbing with mature biofilms receiving GAC-treated water or GAC-treated + particle-filtered water contained ~90% less Lp and NTM than premise plumbing receiving non-filtered municipal tap water. However, if the GAC-treated water passes through a water heater, the capacity to limit Lp or NTM growth was lost. While GAC filters are currently thought of as an instantaneous treatment that removes contaminants from water, this work demonstrates how GAC treatment might provide prolonged benefits to water, after it has passed through the filter on its journey to a shower head or faucet. Increased understanding of the exact mechanisms of limited pathogen growth gained by this research can lead to new and effective approaches to protect people from contracting diseases caused by Lp and NTM. | en |
| dc.description.degree | Master of Science | en |
| dc.format.medium | ETD | en |
| dc.identifier.other | vt_gsexam:42419 | en |
| dc.identifier.uri | https://hdl.handle.net/10919/124527 | en |
| dc.language.iso | en | en |
| dc.publisher | Virginia Tech | en |
| dc.rights | In Copyright | en |
| dc.rights.uri | http://rightsstatements.org/vocab/InC/1.0/ | en |
| dc.subject | Legionella pneumophila | en |
| dc.subject | Nontuberculous mycobacteria | en |
| dc.subject | Probiotic | en |
| dc.subject | Premise Plumbing | en |
| dc.title | Assessing the Potential of Granular Activated Carbon Filters to Limit Pathogen Growth in Drinking Water Plumbing Through Probiotic Versus Prebiotic Mechanisms | en |
| dc.type | Thesis | en |
| thesis.degree.discipline | Civil Engineering | en |
| thesis.degree.grantor | Virginia Polytechnic Institute and State University | en |
| thesis.degree.level | masters | en |
| thesis.degree.name | Master of Science | en |
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