Is flushing necessary during building closures? A study of water quality and bacterial communities during extended reductions in building occupancy

Abstract

Drinking water stagnation can lead to degradation of chlorine residual, bacterial growth (including of opportunistic pathogens and nitrifiers), and metals release from plumbing materials; however, few studies have characterized building water quality and bacterial communities during the extended stagnation periods that occurred during COVID-19 pandemic-related building closures. Additionally, despite a lack of evidence-based guidance, flushing fixtures has been recommended to restore building water quality. We aimed to evaluate the impacts of reduced building occupancy (>2 months) and weekly restorative flushing on drinking water quality, bacterial communities, and the occurrence of undesirable microorganisms in three university buildings. Reduced occupancy led to diminished chloramine and elevated intact cell counts, but values remained stable after additional weeks of limited water use. Flushing temporarily improved water quality, with chlorine and cell counts remaining stable for at least 1 day but returning to levels measured prior to flushing within 1 week. Alpha diversity was lower under more stagnant conditions, and fixture identity, not flushing, was the most influential factor on bacterial community composition, suggesting a strong influence from local biofilm. Although Mycobacterium, Legionella, Pseudomonas, Nitrosomonas, and Nitrospira were detected in samples via amplicon sequencing, concentrations measured via qPCR of M. avium complex, L. pneumophila, P. aeruginosa, and ammonia-oxidizing bacteria were very low or were undetected, supporting that stagnation alone did not lead to high occurrence of undesirable microorganisms. Findings from this study contribute to our understanding of the effects of stagnation on building water microbiomes and the efficacy of flushing to improve water quality. Under the conditions of this case study, repeated flushing on a weekly timescale during low occupancy periods was not sufficient to maintain chlorine residual and prevent bacterial growth in fixtures. Building managers need to weigh the temporary water quality benefits of flushing against the labor and water resources required considering local context.

Gradual Recovery of Building Plumbing-Associated Microbial Communities after Extended Periods of Altered Water Demand during the COVID-19 Pandemic

“…Concerns regarding opportunistic pathogen proliferation, specifically L. pneumophila , in building plumbing systems and potential exposure following reopening were heightened with the enforcement of COVID-19-related building closures . However, as reported here and elsewhere, multiple opportunistic pathogens of concern were either undetected or detected at very low levels. , Systems with chloramine as a residual disinfectant typically do not experience L. pneumophila proliferation, , even under stagnant conditions where levels are diminished or low; , this likely explains their absence at commercial building sites post-reopening. Thus, the risk of exposure to these opportunistic waterborne pathogens post-reopening was minimal.…”

“…Total chlorine concentrations were significantly lower at the commercial building sites (0.51 ± 0.68 mg L –1 ) than at the residential household sites [1.57 ± 0.6 mg L –1 ; via a Mann–Whitney U test, p = 0.005 (Table S4A)], while temperature and selected metal concentrations, including manganese, were significantly increased [via a Mann–Whitney U test, p = 1.58 × 10 –5 to 0.04 (Table S4A)], though still below the regulatory/advisory limits. Loss of disinfectant residual accompanied by increased temperatures during intermittent periods of stagnation is known to impact the microbial community structure, ,,, while increased metal concentrations at the commercial building sites likely are related to metal leaching from the piping materials or scale formation during stagnation and subsequent release …”

“…Indeed, the microbial community in the first draw as well as the final flush samples at the commercial locations was markedly enriched in bacterial taxa typically persistent in biofilm or low-flow environments or slow growers. For instance, Pseudorhodofera, Sphingomonas, Hydrogenophaga , and Phreatobacter -like species are often detected in drinking water biofilms − and water storage. , Similarly, slow-growing Mycobacterium species ( M. gordonae , M. lentiflavum , etc.) as well as autotrophic nitrifiers were far more prevalent in commercial locations than in residential locations, indicating a substantial portion of the microbial communities in the commercial locations was dominated by slow-growing or typically biofilm-associated taxa.…”

“…16 Similarly, Greenwald et al reported that the changes in microbial concentrations postflushing were sustained for only a short period of time with cell counts returning to preflush levels within 7 days. 47 In fact, they conclude that short-term flushing was insufficient in its ability to remediate the effects of long-term stagnation. The findings of our study are largely consistent with these reports in that while flushing over the short term does reduce cell counts, a long-term sustained increase in water demand over a period of months has a more substantive restorative impact.…”

“…16,47 Our study design not only provides insights into temporal changes at these two site types post-reopening but also enables estimation of the differences in temporal changes between the two site types over time. The latter is critical to determine if the impact of extended stagnation on commercial building microbial communities diminishes with increasing building occupancy and water use; previous drinking water microbiology studies related to the COVID-19 shutdown largely focused on the impact of the shutdown itself and the ability to mitigate this impact by flushing 16,47 and prevalence of L. pneumophila post-reopening across multiple drinking water systems. 59 It is important to note here that pre-shutdown baseline measurements of microbial community composition at both the commercial and residential locations would have been ideal, and the lack of these data is an important limitation of this study.…”