Residential water heater cleaning and occurrence of Legionella in Flint, MI
Graphical abstract
Introduction
After switching from Lake Huron to the Flint River as the drinking water source in April 2014, widespread distribution system water quality problems occurred in Flint, MI, ultimately leading to a Federal Emergency declaration (Edwards et al., 2015; Rose, 2015; Masten et al., 2016; FEMA, 2016). Citywide lead-in-water contamination and elevated lead in children’s blood (Pieper et al., 2017, 2018; Hanna-Attisha et al., 2016) prompted a switch back to Lake Huron source water purchased from Detroit in October 2015. In January 2016, authorities announced that outbreaks of Legionnaires’ disease had occurred in summers of 2014 and 2015. Although the switch back to the Lake Huron water source in October 2015 reinstated corrosion control and improved disinfectant residuals, there was lingering concern about persistent public health threats.
All available data collected in early 2016 demonstrated relatively low levels of L. pneumophila in small single-story buildings (Schwake et al., 2016a, Schwake et al., 2016b; Rhoads et al., 2017; FACHEP, 2016), suggesting that L. pneumophila occurrence and Legionnaires’ disease incidence were primarily associated with large buildings. However, the State of Michigan and the Federal Emergency Management Agency were concerned about possible high levels of Legionella bacteria in home water systems, especially due to the high rates of water main breaks and red water from 2014 to 2016. Nationally, it is believed that 96% of reported Legionnaires’ disease is not associated with outbreaks, with the origin of 64% of reported cases never reliably being identified (Hicks et al., 2011; Shah et al., 2018). Residential plumbing systems are a potential source of unidentified Legionnaires’ disease (Stout et al., 1987, 1992; Pedro-Botet et al., 2002; Straus et al., 1996). Byrne et al. (2018) detected L. pneumophila serogroup 6 strains in 12% of Flint homes (n = 130) in late 2016, which differed from serogroup 1 clinical strains detected by the urine antigen test. All available clinical isolates were also found to belong to serogroup 1, while isolates recovered from tap water in early 2016 were found to belong to serogroup 1 or serogroup 6 (Garner et al., 2019), but of a different sequence type of serogroup 6 than that reported by Byrne et al. (2018).
Following the Federal Emergency declaration in Flint, there was concern that Legionella could have colonized water heater sediments during the water crisis, creating a long-term reservoir for Legionella. Sediments are known to react with disinfectants and reduce their efficacy, while also providing nutrients, shelter, and surface area for attached microbial growth, including for Legionella (Lu et al., 2015; Qin et al., 2017). Sediments also decrease heat transfer and energy efficiency (Weingarten, 1992). After the Flint Water Crisis, there were also general concerns that accumulation of iron and lead from water main breaks and corrosion, might increase disinfectant loss, microbial growth, and lead exposure, although hot water is not considered by EPA to be potable water used for consumption (drinking or cooking).
Current recommendations for water heater cleanout, including flushing, vacuuming, and/or dissolving sediments, are designed to remove hardness scale (Weingarten, 1992; Widder and Baechler, 2013). Water heater flushing protocols were developed following the Elk River 4-Methylcyclohexanemethanol (MCHM) spill. However, the protocols were not rigorously designed, validated, or verified in the field and there are concerns that flushing might have even increased consumer exposure to MCHM (Omur-Ozbek et al., 2016). Eventually, science-based recommendations were developed (Casteloes et al., 2015; Hawes et al., 2017; Ragain et al., 2019) for the water soluble chemicals of concern, but they were not designed for removal of contaminants that settled or deposited in plumbing.
In surveying the occurrence of Legionella, it is also important to evaluate all relevant factors that could contribute to its proliferation. In particular, water heater set point has been identified as a critical parameter for Legionella colonization of residential plumbing systems, with settings less than 49 °C (120 °F) facilitating Legionella growth in the storage tanks (Lee et al., 1988). While Michigan code allows residential water heater temperature settings up to 60 °C (140 °F), default or resident-preferred temperatures are typically lower due to a desire to decrease energy demand, reduce scaling, and prevent scalding. Gas heaters tend to have lower prevalence of Legionella than electric heaters, likely due to heating from the tank bottom as opposed to side mounted electrical elements (resistors) that create stratification (Alary and Joly, 1991; Dufresne et al., 2011; Brazeau and Edwards, 2013). In addition, while free chlorine levels in the distribution system had improved markedly by summer 2016 relative to during the period the city was using the Flint River water (Rhoads et al., 2017; Zahran et al., 2018), residents were using little water due to high water bills, readily available bottled water, and concerns about the safety of bath water (Roy and Edwards, 2019). There was also concern that the improved disinfectant residuals were not being consistently delivered to the residential plumbing due to low water use.
Here we evaluated the hypothesis that a thorough water heater sediment cleaning event would improve water quality at hot water outlets by reducing inorganic contamination, disinfectant demand, and Legionella proliferation and release. We conducted a comprehensive field sampling campaign during the Flint, MI Federal Emergency response, deploying a rigorous water heater sediment cleaning protocol at 30 single-family Flint residences located near the epicenter of the reported Legionnaires’ disease outbreaks at McLaren Hospital (MDHHS, 2018). The goal was to provide insight into the potential for water heater flushing to reduce risk of exposure to Legionella and other water contaminants following major corrosion or other massive contamination events.
Section snippets
Sample collection
Two sampling campaigns were conducted in July 2016, including 30 homes near the two hospitals in Flint, MI where the majority of the Legionnaires’ disease cases and highest water ages were reported. Six samples were collected from each home before and 5–6 days after performing the water heater cleaning protocol. Three of the samples were stagnant first draws from the (1) cold kitchen outlet, (2) hot kitchen outlet, and (3) shower head or bathtub spout sample of mixed hot and cold water.
Qualitative notes on cleaning protocol
For new heaters (<1 year old) or a few heaters that had been flushed regularly by the homeowner (n = 3), only 30–45 min was required to perform the entire cleaning protocol and achieve clear flushed water. However, in the typical case (n = 21), the procedure required between 90 and 120 min before the water from the tank ran clear of sediment. For worst-case heaters (n = 6), clear water was never achieved and flushing was discontinued at 2 h.
During the second week of sampling, after the heaters
Conclusions
The one-time cleanout of sediment accumulated in Flint, Michigan water heater tanks:
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Reduced the amount of particulate inorganics released from the drain valve;
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Had no major impact on the level of inorganics sampled at other outlets (kitchen, shower) within homes, indicating that water heater sediments do not serve as a regular source of inorganics at hot water point of use outlets; and
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At least temporarily reduced the occurrence and levels of Legionella spp. and L. pneumophila gene copy numbers,
Funding
This work was funded by a grant from the Michigan Department of Environmental Quality (MDEQ). The author’s views expressed in this publication do not necessarily reflect the views of the MDEQ.
Declaration of competing interest
The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to thank the Flint residents that participated in this study. We would also like to thank members of the Flint Water Study team for dedicating their time to conduct the sampling, in particular: Pan Ji, Rebekah Martin, Laurel Strom,Owen Strom, Ni Zhu, Kimberley Hughes, Mariana Martinez, David Otto Schwake, Jeffrey Parks, Dongjuan Dai, Haniyyah Chapman, Philip Smith, Ethan Edwards, Ailene Edwards, Connor Brown, and Kandance Donalson.
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Cited by (0)
- 1
Current affiliation: The American Cleaning Institute, Washington, DC.
- 2
Current affiliation: Virginia Tech, Department of Population Health Sciences, Blacksburg, VA.