Growth of Opportunistic Pathogens in Domestic Plumbing: Building Standards, System Operation, and Design
| dc.contributor.author | Rhoads, William J. | en |
| dc.contributor.committeechair | Edwards, Marc A. | en |
| dc.contributor.committeemember | Pruden, Amy | en |
| dc.contributor.committeemember | Pearce, Annie R. | en |
| dc.contributor.committeemember | Falkinham, Joseph O. III | en |
| dc.contributor.department | Civil and Environmental Engineering | en |
| dc.date.accessioned | 2017-03-16T05:07:44Z | en |
| dc.date.available | 2017-03-16T05:07:44Z | en |
| dc.date.issued | 2017-03-15 | en |
| dc.description.abstract | Understanding and limiting public health threats resulting from exposure to opportunistic pathogens (OPs) in domestic water (i.e., hot/cold water for human use) will be one of the grand challenges for water safety in the 21st century. This dissertation anticipates some of the complexities in balancing stakeholder goals and developing building standards to limit OP growth, and advances scientific understanding of OP survival and proliferation in domestic plumbing systems. In a cross-sectional survey of water- and energy-efficient buildings, domestic water age ranged from 8 days to 6 months and resulted in pH and temperature fluctuations, rapid disinfectant residual decay up to 144 times faster than municipal water delivered to the buildings, and elevated levels of OP gene markers. This motivates future work to determine how to maintain high quality and safe water while preserving the sustainability goals of these cutting-edge buildings. Head-to-head pilot-scale experiments examining OP growth in recirculating hot water systems revealed that elevated temperature had an overarching inhibitory effect on L. pneumophila growth where temperatures were maintained. However, control was undermined in distal branches, especially when density-driven convective mixing gradients maintained ideal growth temperatures and delivered nutrients to the otherwise stagnant branches. These results resolve discrepancies reported in the literature regarding the effects of flow, and identify important system design and operational conditions that facilitate OP growth. Advancements were also made in understanding how corrosion can trigger OP growth. In Flint, MI, corrosive Flint River water damaged iron pipes, releasing iron nutrients, consuming chlorine residual, and supporting high levels of L. pneumophila in large building systems. This likely triggered two unprecedented clusters of Legionnaire's disease. In pilot-scale systems, copper released from copper pipes, but not dosed as soluble cupric, triggered release of >1,100 times more H2 into the water due to deposition corrosion. The organic carbon fixed by autotrophic hydrogen oxidation has the potential to facilitate OP growth, but more work is needed to understand the limits of this mechanism. Finally, well-controlled laboratory experiments confirmed past reports from field surveys that the use of chloramines trigger a trade-off between controlling Legionella and allowing non-tuberculous Mycobacteria to persist. | en |
| dc.description.abstractgeneral | Understanding and limiting public health threats resulting from exposure to opportunistic pathogens (OPs) in domestic water (i.e., hot and cold water intended for human use) will be one of the grand challenges for water safety in the 21st century. Unlike traditional fecal-based waterborne pathogens that have all but been eliminated through advanced treatment applied at water treatment facilities, OPs are native microbial members of drinking water and tend to proliferate in domestic plumbing. In addition to the complexity and technical nature of engineering controls applied in buildings to limit OP growth, there are many stakeholder groups with varied responsibilities and expertise in preventing, diagnosing, and/or remediating problems. Stakeholders sometimes present additional challenges when their goals have direct or indirect trade-offs with limiting OP growth in buildings. This dissertation anticipates some of the challenges to come, and advances scientific understanding of how OPs survive and proliferate in domestic plumbing systems. Water- and energy-efficient buildings, while nobly seeking to preserve precious natural resources, potentially create unintended consequences with respect to water quality. In a cross-sectional survey of green building designs, water remained within domestic plumbing for over a week to months before being used by consumers, and resulted in water quality changes that facilitated the growth of OPs. While short-term solutions exist, such as flushing water to decrease water stagnation and introduce “fresh” water into the system, this work motivates future research for how to maintain high quality and safe water while preserving the sustainability goals of these cutting-edge buildings. Systematic experiments were conducted on water heaters with a recirculating pump, which are marketed as a “green” technology for water and energy savings, to determine the effect of system design and operation on the growth of OPs. Elevated temperature was found to have an overarching inhibitory effect on growth of <i>L. pneumophila</i>, the most commonly reported OP. However, when the water heater temperature was not sufficient to completely eliminate <i>L. pneumophila</i> (51 °C), higher water temperatures actually supported high levels of <i>L. pneumophila</i> growth in infrequently used pipes by periodically disinfecting other microorganisms that are more susceptible to thermal disinfection and decreasing competition for nutrients. System design also impacted <i>Legionella</i> growth. In pipes that slowly mixed with the recirculating line (simulating a pipe running upward to a shower head from a recirculating line in the floor, for instance), <i>L. pneumophila</i> were consistently elevated relative to pipes that did not convectively mix (simulating a pipe running downward to a kitchen tap from a recirculating line in the ceiling, for instance). The slow mixing maintained ideal <i>Legionella</i> growth temperature in the pipes with mixing, even when water heaters were maintained well above thermal disinfection levels for <i>Legionella</i> (i.e., at 60 °C). This is due to continuous delivery of nutrients to the upward pipes with mixing, but not the downward pipes without it. This result is significant because it outlines scenarios encountered in real buildings where even the most effective thermal disinfection strategy can be undermined in distal branches within the building. This work also outlines the importance of corrosion in potentially triggering OP proliferation. In Flint, MI, when the water utility began distributing very corrosive Flint River water, the new water source damaged iron water pipes, releasing iron (which is a nutrient for <i>Legionella</i>) and eliminating disinfectant residual in the distribution system (which is needed to prevent <i>Legionella</i> regrowth). As a result, the corrosion supported increased <i>Legionella</i> levels and likely triggered two unprecedented clusters of Legionnaires’ disease. In the pilot-scale systems, corrosion of the water heater anode rod caused trace nutrients to evolve into the water. However, this only occurred when low levels of copper released to the water from the natural corrosion of copper pipes were present. Ionic copper, which is sometimes used to disinfect <i>Legionella</i>, did not have the same effect when it was dosed to the experiment at similar concentrations. These trace nutrients generated from copper-enhanced corrosion of the water heater anode rod are a potential source of carbon for OP growth, and may help explain the variable effects of copper that have been reported in the literature. More work is needed to fully understand this potential growth mechanism. Finally, this work confirmed past field observations that there is a trade-off between controlling <i>Legionella</i> and allowing <i>Mycobacteria</i>, another OP, to persist when using chloramines disinfectant residual. Reproducing this phenomenon in controlled laboratory settings is an important step in understanding, and ultimately preventing it. | en |
| dc.description.degree | Ph. D. | en |
| dc.format.medium | ETD | en |
| dc.identifier.other | vt_gsexam:10027 | en |
| dc.identifier.uri | http://hdl.handle.net/10919/76653 | 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 | en |
| dc.subject | water chemistry | en |
| dc.subject | corrosion | en |
| dc.subject | Temperature | en |
| dc.subject | Design | en |
| dc.subject | flow | en |
| dc.subject | Mycobacteria | en |
| dc.title | Growth of Opportunistic Pathogens in Domestic Plumbing: Building Standards, System Operation, and Design | en |
| dc.type | Dissertation | en |
| thesis.degree.discipline | Civil Engineering | en |
| thesis.degree.grantor | Virginia Polytechnic Institute and State University | en |
| thesis.degree.level | doctoral | en |
| thesis.degree.name | Ph. D. | en |