Abstract
Controlling organic carbon levels in municipal water has been hypothesized to limit downstream growth of bacteria and opportunistic pathogens in premise plumbing (OPPPs). Here, the relationships between influent organic carbon (0–15,000 µg ozonated fulvic acid /L) and the number of total bacteria [16S rRNA genes and heterotrophic plate counts (HPCs)] and a wide range of OPPPs (gene copy numbers of Acanthamoeba polyphaga, Vermamoeba vermiformis, Legionella pneumophila, and Mycobacterium avium) were examined in the bulk water of 120-mL simulated glass water heaters (SGWHs). The SGWHs were operated at 32–37 °C, which is representative of conditions encountered at the bottom of electric water heaters, with water changes of 80% three times per week to simulate low use. This design presented advantages of controlled and replicated (triplicate) conditions and avoided other potential limitations to OPPP growth in order to isolate the variable of organic carbon. Over seventeen months, strong correlations were observed between total organic carbon (TOC) and both 16S rRNA gene copy numbers and HPC counts (avg. R2 > 0.89). Although M. avium gene copies were occasionally correlated with TOC (avg. R2 = 0.82 to 0.97, for 2 out of 4 time points) and over a limited TOC range (0–1000 µg/L), no other correlations were identified between other OPPPs and added TOC. These results suggest that reducing organic carbon in distributed water is not adequate as a sole strategy for controlling OPPPs, although it may have promise in conjunction with other approaches.
Highlights
Legionella pneumophila is the most frequently reported agent of waterborne disease outbreaks in the United States [1] other opportunistic pathogens such as Mycobacterium avium, Acanthamoeba spp., Naegleria fowleri and Pseudomonas aeruginosa are increasingly recognized as public health concerns in potable water systems [2,3,4]
Similar controls seem unlikely to be effective in premise plumbing hot water systems given that the very high levels of heterotrophic plate counts (HPCs) colony counts (>106 CFU/mL) and 16S rRNA gene copies were maintained even when no organic matter was added to the tap water synthesized from ultrapure water in which carbon was purposefully destroyed by UV
Transient correlations sometimes existed between organic carbon and HPC colony counts or 16S rRNA gene copy numbers in SGWH effluents, correlations for M. avium were limited to lower levels of added organic carbon (< 1000 μg/L total organic carbon (TOC)) and no correlations were observed for A. polyphaga, L. pneumophila, or V. vermiformis
Summary
Legionella pneumophila is the most frequently reported agent of waterborne disease outbreaks in the United States [1] other opportunistic pathogens such as Mycobacterium avium, Acanthamoeba spp., Naegleria fowleri and Pseudomonas aeruginosa are increasingly recognized as public health concerns in potable water systems [2,3,4]. In contrast to traditional fecal pathogens, opportunistic pathogens in premise plumbing (OPPPs) tend to be resistant to disinfectant and subject to regrowth in the distribution system and premise plumbing. Research is needed to determine how OPPPs might be controlled by manipulating various factors at the treatment plant, in the distribution system, and in premise plumbing. These include altering the source water chemistry, chemical/biological treatments, distribution system design, temperature control, dissolved oxygen levels, plumbing materials, and in-building disinfection strategies [8,9,10]
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