Longitudinal evaluation of the impact of thermal disinfection on the hot water system microbiome

Pan Ji, PhD candidate of the Department of Civil & Environmental Engineering and Water INTERface IGEP, has attended Association of Environmental Engineering & Science Professors (AEESP) Research and Education Conference 2017 in Ann Arbor, Mi on June 20-22. Pan has presented her interdisciplinary water research (Ji, P.; Rhoads, W.J.; Edwards, M.A.; Pruden, A.) in the conference: Longitudinal evaluation of the impact of thermal disinfection on the hot water system microbiome.

       Abstract: Within the built environment, hot water plumbing provides a unique niche for the proliferation of opportunistic pathogens (e.g., Legionella pneumophila). Consequently, the operating temperature of water heaters has been recommended to be above 60°C with optimal at-the-tap delivery temperature above 50°C. Although heat treatment is sometimes recommended as a short-term disinfection measure for domestic hot water systems that have been maintained at lower temperatures, little is known about the long-term impact to the resident microbial community and susceptibility to opportunistic pathogen proliferation. Conversely, the response of hot water system microbial consortia to thermal-failure event remains largely unexplored.
Two identical hot water rigs were operated in parallel to examine the impact of thermal disinfection and interrupted continuous thermal disinfection on the microbial community composition after 15-month acclimation with the local municipal tap water. Each rig consisted of an electric water heater and recirculating pipe with 18 distal taps comparing two pipe orientations (stratified downward and convectively mixing upward) and three water use frequencies (high, medium, low; 21, 3,1 flushes/week respectively) in triplicate. The water heater temperature settings had been respectively maintained at 40 and 60°C for 4 months prior to the heat treatment (i.e., both rigs being set to 60°C and distal taps maintained at elevated temperature for 30 minutes). Both rigs were returned to 40°C afterwards, representing thermal disinfection for the rig previously at 40°C and interrupted continuous thermal disinfection for the rig previously at 60°C. First-flush bulk water samples were collected 2-month and immediately prior to heat treatment, and immediately, 8-hour and 1-month afterwards. Biofilm was swabbed 2-month and immediately prior to heat treatment, and 1 month afterwards.
Analysis of 16S rRNA gene amplicon sequencing data revealed that in bulk water phase, average relative abundance of Legionella spp. decreased immediately after the heat treatment in both rigs yet increased to and beyond pre-heat-treatment levels 1-month later. While in the biofilm phase, post-heat-treatment samples had higher average Legionella relative abundance than the pre-heat-treatment ones. Heat treatment also altered bulk water microbial community composition in the two hot water rigs. Comparatively, water heater temperature setting prior to heat treatment exhibited a more dominant impact, as planktonic microbial community of the two rigs remained distinct immediately and 8-hr after the heat treatment. One-month acclimation at 40°C following the heat treatment selected for similar bulk water microbial compositions in the two rigs. In the thermal disinfection rig, the temporary phase of heat treatment imposed limited change in biofilm microbial community composition, which could be attributed to ecosystem resistance or resilience. The result further reinforced our recent report of synergistic effects of pipe orientation (convective mixing level) and water use frequency on hot water plumbing microbial community, which in this study had a stronger impact than the water heater temperature setting (40 vs 60°C) on microbial community composition. This study evaluates the longitudinal impact of heat treatment compared to other engineering measures (e.g., water heater temperature) and takes a step towards intentional control of the plumbing microbiome.

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