3.1. Identification of Critical Factors for Water Quality Management in Buildings
The 18 design issues identified during the SME interviews are shown in
Table 2, along with a count of the number of favorable and unfavorable mentions in both the SME interviews and the guidance documents. Six of these factors related broadly to increased water age (residence time), and were viewed negatively by both SMEs and guidance documents: distant taps, low flow fixtures, oversized pipe diameters, building size, building layout, hydraulics of system (hydraulics in this context generally referring to the absence of dead zones and speed and directness of flow through the system). Green buildings may be included as a 7th factor in this category as they typically contain low use fixtures (which were perceived as contributing to high water age) and potentially other design features, which may lead to long water ages, such as rainwater collection and greywater reuse. There is good agreement among the SMEs and guidance documents on these factors in that they are mentioned frequently and generally in a negative light. The only favorable mentions among these seven factors are the favorable mentions of hydraulics by 4 guidance documents, which relate to the benefits achieved by good hydraulics, whereas the other mentions by SMEs and guidance documents referred to negative aspects of poor hydraulics. Thus, this does not indicate an actual divergence of opinion.
Three other design factors were viewed unfavorably by both SMEs and guidance documents. Flexible shower hoses (which may leach organic carbon), pipes exposed to temperature extremes (passing through boiler rooms or uninsulated areas), and electronic (automatic) faucets were viewed as providing conditions conducive to the growth of opportunistic pathogens.
Other factors showed a mix of favorable and unfavorable mentions. In some cases, this mixture was due to actual disagreements. Thermostatic mixing valves were sometimes viewed favorably as enabling high water temperatures (which protect against opportunistic pathogen growth) to be used in the majority of the hot water system while protecting the user against scalding. Others viewed them negatively and even advocated for their removal when possible due to their creation of a favorable temperature for opportunistic pathogen growth somewhere across the thermal gradient present at the valve, which might include most or all locations downstream of the valve, and their potential to fail and allow hot and cold water to mix. Additionally, biofilms present on the valve itself could contribute to pathogen growth.
In other cases, the mix of favorable and unfavorable mentions does not reflect conflicting points of view but rather commonly held nuanced views that acknowledge favorable and unfavorable aspects of different features. For example, in-building treatment and point of use treatment were generally acknowledged as being capable of improving water quality but were also viewed as a challenge to implement correctly, involving a range of unintended effects (nitrification due to chloramination, precipitate from copper-silver ionization, corrosion and disinfection byproduct formation from disinfectants, maintenance, increased costs, and administrative burdens). Hot water recirculation was generally viewed as needed in some applications, but when implemented with inadequately high temperatures (see
Table 4 below and accompanying discussion below regarding what constitutes a sufficiently high temperature), it has the potential to create conditions favorable for pathogen growth by providing high residence times at pathogen-favorable temperatures (return lines are particularly challenging to maintain at an adequate temperature).
Pipe material considerations elicited a range of views. Several mentioned the anti-microbial properties of copper and the potential for PVC and PEX pipe to leach organic carbon, which could enhance the growth of opportunistic pathogens. Nevertheless, none of the SMEs strongly advocated for one type of pipe material over another. The issue of organic carbon leaching from plastic pipes was indicated by SMEs as a quality control issue, requiring proper testing and certification, rather than an unavoidable drawback of all plastic pipes. The antimicrobial properties of copper were seen as influencing the microbial ecology of the plumbing system but not necessarily in favorable ways, given that more problematic organisms, such as
Legionella spp., and other opportunistic pathogens capable of living inside amoebae, may be able to develop resistance under certain selective pressures, such as the presence of copper, and outcompete other microorganisms. As only one of the IGDs, the National Academies report [
30], mentioned PEX pipe, the 39 non-IGD documents were searched for information and an additional source was found to provide guidance on PEX pipe. The UK Department Health [
25] identified British Standard 6920 as being applicable, while the National Academies report [
30] indicated a need for US standards that address the potential for microbial growth. Both guidance documents identified standardized testing methods as being vital to addressing concerns with PEX pipe, and hence closely matched the SME views on this topic.
Of the 18 topics related to design factors, three factors—electric water heaters, PEX pipe materials, and green buildings—were addressed by only one of the 15 IGDs. In all three cases, the IGD which addressed the topic was the National Academies report [
30]. The 39 other (non-IGD) documents were searched for these topics, and additional information was found for one topic in one report; specifically The UK Department Health report [
25] included information on PEX pipe, as described above. None of the 39 non-IGD reports provided guidance on the remaining two topics, electric water heaters and green buildings. Electric water heaters were mentioned mostly unfavorably by SMEs and by the National Academies report [
30] as creating a thermally stratified reservoir with lower temperature water (favorable for opportunistic pathogen growth) able to pool at the bottom. Green buildings were viewed unfavorably by six of the SMEs with no favorable mentions by SMEs. The National Academy report [
30] emphasized the negative aspects while also mentioning favorable features such as energy and water conservation, and suggested incorporating water quality into certification processes for green buildings, such as the LEED standards.
While all of the design topics were covered in at least one guidance document, no one guidance document covers all the factors identified by the SMEs. When a topic is addressed by a guidance document in some cases both favorable and unfavorable aspects are covered but in other cases, multiple documents would need to be consulted to appreciate the range of views on a topic (e.g., the WHO 2011 [
22] mention only negative aspects of copper pipe while the WHO 2007 [
21] and the National Academies report [
30] provide both favorable and unfavorable aspects of copper pipe). This indicates a need for a more comprehensive synthesis of available guidance so that designers and facility managers can access information on all these topics from a single source or a compendium enabling the user to access information from multiple sources.
While no single guidance document covers all the topics, the National Academies report [
30] covers more of the topics, 16 out of 18, than any other document. The second and third highest number of topics were covered by the WHO (2007) [
14] and the CDC (2003), with eight and seven topics addressed, respectively. It is also notable that the National Academies report [
30] covers favorable and unfavorable aspects of eight topics, while for the remaining eight topics (flexible shower hoses, distant taps, low flow fixtures, electronic faucets, electric water heaters, oversized pipes, pipes exposed to temperature extremes, and hydraulics/poorly balanced flow), the National Academies of Science, Engineering, and Medicine (NASEM) report [
30] and all SME views are negative. Thus, when the National Academy report [
30] includes only negative aspects of these topics, one might reasonably conclude that there are few or no highly salient favorable aspects of these topics, rather than concluding that the report missed important favorable aspects of the topics.
The 11 important operational factors identified during the SME interviews are summarized in
Table 3. Similar to the design factors, issues leading to higher water age were cited frequently and consistently viewed as unfavorable (low demand, infrequently used fixtures, high water age) by both SMEs and guidance documents. High carbon and nutrient water were also viewed unfavorably due to their potential to support microbial growth.
Another set of issues pertains to the maintenance of a disinfectant residual in the plumbing system (these are related to water age as residual decays with time). These include maintaining a disinfectant residual (consistently viewed as favorable by SMEs and guidance documents) and low residual being provided by the water supply (negatively viewed by three SMEs but not explicitly addressed by any guidance documents). The surface water treatment rule requires surface water suppliers to maintain detectable residual levels for surface water systems [
34]. While neither the surface water treatment rule nor the guidance documents considered here provide specific guidance for building water quality managers, monitoring for residual at the influent could serve as a check on this issue. Three SMEs and four guidance documents mentioned other aspects of water quality (i.e., other than residual levels) from the utility, and generally included distribution system issues such as main breaks, loss of pressure, cutoff of supply, and sloughing of materials from distribution system pipes (see National Research Council 2006 report [
34] and Propato and Uber 2004 [
35], for a consideration of water quality in the utility distribution system piping).
The use of chloramine rather than chlorine was noted as favorable by 10 SMEs and four guidance documents. None of the responses reviewed identified chloramine as an unfavorable factor, but there was a great deal of nuance and uncertainty expressed on the issue. Chloramine is viewed as more persistent and better able to penetrate biofilms but has the drawback that it allows for the possibility of nitrification if the residual is not maintained. There was some degree of consensus that chloramine is more effective for limiting Legionella spp. growth, but much less clarity was apparent regarding its impacts on mycobacteria. Mycobacteria were consistently viewed as more resistant to disinfection. Some view the use of chloramines as having the ability to open up ecological niches for mycobacterial growth by preferentially removing competitive bacteria. Others saw chloramine as useful for limiting the growth of mycobacteria even though it could not eliminate them.
Temperature control was frequently mentioned as an important operational factor. The qualitative adage of “keeping the hot water hot and the cold water cold” was endorsed by 6 SMEs and 12 guidance documents. Control of the temperature setpoint on the water heater was mentioned favorably by 14 SMEs and seven guidance documents with 0 unfavorable mentions. Flushing received 12 favorable and three unfavorable mentions by SMEs and 10 favorable and 0 unfavorable mentions by guidance documents. Favorable views saw flushing as bringing in fresh residual disinfectant and controlling microbial growth. A long duration between uses was seen as a possible factor in stressing the biofilm and leading to detachment and mobilization of opportunistic pathogens into bulk water. There was a contrary view that noted that flushing brings more nutrients and carbon to the pipe biofilm, which can enhance growth. Overall, the impacts of flushing likely depend on a complex interplay of residual levels, carbon and nutrient levels, and frequency of flushing.
The operational factors tend to fall at least generally into somewhat clearer favorable vs. unfavorable categories, which is in contrast to the design factors, many of which were noted as having both favorable and unfavorable aspects. Flushing is the only factor that has both favorable and unfavorable mentions. However, views on chloramines were nuanced and included some mentions of potential tradeoffs as noted above, although these were not identified as clearly unfavorable aspects of chloramines.
The only operational issue identified as important by the SMEs but not addressed in any of the guidance documents was low residual from the utility. Some of the topics were addressed by most of the guidance documents (10 addressed water age, 12 indicated that hot water should be kept hot and cold water kept cold, 12 noted the importance of residual, and 11 addressed flushing). This topic coverage is markedly better than the situation for the design factors, where none of the factors were addressed by more than five of the guidance documents. Of the 10 topics addressed by the guidance documents, the NASEM report [
30] is the only guidance document that covered all 10. The second highest percentage of topic coverage is eight by the European Guidelines Working Group, 2017 [
23], and the third highest is seven by both the WHO, 2007 [
14] and the CDC [
15]. As noted above, the NASEM report [
30] also had the best topic coverage of the design topics.