Growth of Opportunistic Pathogens in Domestic Plumbing: Building Standards, System Operation, and Design

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Virginia Tech

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.

Legionella, water chemistry, corrosion, Temperature, Design, flow, Mycobacteria