In a recent study posted to the medRxiv* preprint server, a team of researchers evaluate how altered water demand in the plumbing systems of commercial and residential buildings deteriorated the water quality when they were closed for a prolonged time during the coronavirus disease 2019 (COVID-19)-induced pandemic.
The closure of commercial buildings during the pandemic resulted in water stagnation due to reduced flow volumes in their plumbing systems, which led to concerns related to the microbiological safety of drinking water when buildings reopened. To date, the extent of the impact of these closures on water quality remains unknown.
Study: Systematic recovery of building plumbing-associated microbial communities after extended periods of altered water demand during the COVID-19 pandemic. Image Credit: Quality Stock Arts / Shutterstock.com
About the study
In the present study, using direct and derived flow cytometric measures, along with water chemistry characterization, the researchers evaluated plumbing-associated changes in water quality parameters. Some of the parameters that were assessed in this study included temperature, pH, conductivity, and dissolved oxygen in commercial buildings that were closed for a prolonged time.
Due to altered water demand in these buildings during the pandemic, their plumbing systems had reduced flow volumes that led to water stagnation for about 14 weeks between March 2020 and June 2020. As a result, water quality deteriorated, which caused microbial regrowth, opportunistic plumbing pathogen (OPP) growth, and the leaching of metals like lead.
The researchers analyzed water samples from 420 cold water taps of sites at three commercial buildings that worked at reduced capacity due to COVID-19 social distancing policies and four residential households. These sites, which differed in size, age, and functionality, were situated within five miles of each other and received chlorinated water from the same distribution system.
The sample collection began in June 2020 when these buildings reopened and continued for six months. Seven samples from each tap, including the first draw sample, denoted as time point 1 (TP1) at 0 min, as well as six flushed samples, were collected at five minutes intervals for 30 min (TP2, 5 min; TP3, 10 min; TP4, 15 min, TP5, 20 min, TP6, 25 min, and TP7, 30 min).
The Mann-Whitney U test was used to assess the differences among all the samples collected from the commercial buildings and residential household sites.
Commercial building sites had 39% to 46% lower water demand between March 2020 and May 2020 as compared to 2019. Contrastingly, the residential sites had witnessed an average of 6% increased water demand during this time as compared to 2019 due to local lockdown orders.
TP1 samples from both sites had higher total cell concentrations (TCCs) than the final flushed samples. However, TCCs of the TP1 of the commercial and residential building sites were approximately eight-fold and two-fold higher than their TP7, respectively.
Overall, these findings suggest that flushing practices countered stagnation-induced effects at both types of sites, though more quickly at the residential household sites as compared to the commercial building sites.
Concerning the microbial loads, samples from the commercial buildings had high temperatures, low total chlorine concentrations, elevated levels of metals, particularly copper (Cu) and manganese (Mg), as well as greater intact cell concentrations (ICC) with less diverse and even microbial communities as compared to the residential household sites. Consistent with the results of previous studies conducted over shorter time scales, these findings confirm that reduced water demand led to water stagnation and impacted water quality.
Total chlorine, ammonium, and conductivity were the primary water chemistry parameters driving change in microbial community composition across the study period of six months. Loss of disinfectant residual during intermittent periods of stagnation was also responsible for this change in plumbing-associated microbial community compositions.
Further exploration of microbial community composition using Bray-Curtis distance-based redundancy analysis (dbRDA) showed clustering of samples based on the site type. This analysis explained approximately 15% of the variation in the microbial community composition.
Moreover, variance partitioning analysis showed that total chlorine, ammonium, and dissolved oxygen were the primary water chemistry parameters associated with a change in microbial community composition between sites, wherein total chlorine led to about 12% of this variation.
Lastly, the plumbing systems of commercial building sites showed metal concentrations below the regulatory concentrations and were associated with higher manganese, magnesium, zinc, copper, and lead concentrations due to metal leaching from the pipes or scale formation during water stagnation.
The study results demonstrated that COVID-19-related building closures impacted water demand and the microbial community composition of water in their plumbing systems.
While this impact on the drinking water microbial community of commercial building sites could not be mitigated through 30-minute flushing regimes after reopening of these buildings, it gradually recovered over several months to finally converge towards the microbial communities at the residential household sites.
Taken together, the study findings suggest that after extended periods of altered water demand, a sustained and gradual increase in water demand was needed for the recovery of building plumbing-associated microbial communities as compared to short-term flushing.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.