Towards a Sustainable Residential Hot Water Infrastructure: Optimizing Public Health, Water Savings, and Energy Goals

Virginia Polytechnic Institute And State University, Blacksburg VA

Investigators

Abstract

1336650 (Pruden). Residential hot water infrastructure is a critical nexus of water, energy, and public health. In the U.S., its total energy demand exceeds that of the water and wastewater utility sectors combined, while building plumbing systems (especially hot water) are now the primary source of waterborne disease outbreak. In particular, Legionella pneumophila, non-tuberculosis mycobacteria, Pseudomonas aeruginosa, and Acanthamoeba are especially challenging because they establish and grow within the hot water distribution system itself. Unfortunately, there is currently a void of practical research to guide rational selection of optimal hot water systems. Thus, the overall goal of the proposed research is to advance sustainability of our residential hot water infrastructure by conducting the first integrated assessment of performance in terms of public health, water savings, energy goals, and overall system vulnerabilities. Three specific research Objectives (O1-O3) are: O1. Conduct a controlled, head-to-head study of standard, recirculating, and on-demand water heater configurations over a range of temperatures and water demands and compare their performance in terms of energy consumption and microbial quality of the water; O2. Examine the interplay of varying water chemistries, pipe materials, and water heater configuration on scaling, corrosion, energy loss, and microbial quality of the water; and O3. Develop a multi-criteria decision analysis tool to identify the most sustainable configuration(s) for a specific context, with consideration of consumer barriers to implementation. Although 3.3-5.5% of total U.S. energy demand is used in residential water heating systems, and such systems are now the primary source of waterborne disease outbreak, there exists a critical gap in fundamental knowledge needed to select an optimal water heater. Extensive head-to-head comparisons will be made of water and energy demands incurred by three representative water heater configurations. Application of next-generation DNA sequencing tools will provide a truly pioneering understanding of water heater impacts on pathogens and the broader microbial ecology. Preliminary results suggest that hydrogenotrophic bacteria may utilize hydrogen produced by magnesium anodes (common in tank heaters to reduce corrosion) and, in turn, fix and release detrimental levels of organic carbon into the water. Other preliminary results indicate that copper may possess beneficial properties for inhibiting Legionella. While this research focuses on electric energy sources as an important first step, the fundamental understanding gained, such as effects of temperature stratification, scaling, and roles of plumbing components, can be readily extrapolated to other energy sources. The optimal water heater is likely to vary based on a variety of local constraints. Although on-demand is expected to have the lowest initial water and energy demands, and the least potential for pathogen amplification, it is not likely to be a feasible option in waters of high scaling potential. Effects of all operating conditions on disinfectant residual levels, a critical protective barrier against pathogens, will also be a vital factor for overall performance evaluation. Evidence is mounting that existing ?green? advice may be creating misguided policy with long-term negative repercussions on water-energy consumption and public health. This research seeks fundamental knowledge and a holistic perspective to support rational decision-making by consumers, public health officials, and regulators on selection of optimal hot water systems. Scientifically-defensible recommendations considering multiple dimensions of performance are necessary to achieve truly sustainable water systems. The proposed effort incorporates a significant social science component and sustainability ranking approach in support of the development of a multi-criteria decision making tool. Results will be available on public websites, presented at green building and water engineering conferences, and published in the peer-reviewed literature. The project will also support three graduate students, who will participate in the Water INTERface Interdisciplinary Graduate Education Program and will be trained across fields of green engineering, water chemistry, and environmental microbiology. Undergraduate researchers will also participate through the Interdisciplinary Water Science and Engineering NSF REU site.