COVID-19 lockdowns depleted water quality of commercial building plumbing systems

In a recent study published on preprint server medRxiv*, a team of researchers evaluated how changing water demands in the plumbing systems of commercial and residential buildings worsened water quality when exposed to water during the Coronavirus Disease 2019 (COVID-19)-induced pandemic.

The closure of commercial buildings during the pandemic led to water stagnation due to reduced flow rates in their plumbing systems, raising concerns about the microbiological safety of potable water when buildings reopen. To date, the extent of the impact of these closures on water quality is unknown.

To learn: Systematic recovery of microbial communities associated with building plumbing after prolonged periods of changing water demand during the COVID-19 pandemic. Photo credit: Quality Stock Arts /

About the study

In the present study, researchers assessed the changes in water quality parameters associated with the installation using direct and derived flow cytometric measures along with characterization of the water chemistry. Some of the parameters evaluated in this study were temperature, pH, conductivity, and dissolved oxygen in commercial buildings that were closed for long periods of time.

Due to changing water needs in these buildings during the pandemic, their plumbing systems had reduced flow rates, resulting in water stagnation for approximately 14 weeks between March 2020 and June 2020. As a result, water quality deteriorated, leading to renewed growth of microbes and opportunistic plumbing, pathogen growth (OPP) and leaching of metals such as lead.

Researchers analyzed water samples from 420 cold water faucets at locations in three commercial buildings operating at reduced capacity due to COVID-19 social distancing guidelines and four residential homes. These sites, which varied in size, age, and functionality, were within five miles of each other and received chlorinated water from the same distribution system.

Sampling began in June 2020 when these buildings reopened and lasted six months. Seven samples from each tap, including the first withdrawal sample designated Time 1 (TP1) at 0 min, and six flushed samples were collected at five minute intervals for 30 minutes (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 between all samples collected from the commercial and residential sites.

study results

Commercial construction sites experienced a 39% to 46% decrease in water demand between March 2020 and May 2020 compared to 2019. In contrast, residential areas experienced an average 6% increased water demand during this period compared to 2019 due to local lockdown orders.

TP1 samples from both sites had higher total cell concentrations (TCCs) than the final rinsed samples. However, the TCCs of the TP1s of the commercial and residential sites were approximately 8 times and 2 times higher than their TP7, respectively.

Overall, these results suggest that flushing practices counteracted the effects of stagnation at both site types, albeit more rapidly at residential sites compared to commercial sites.

In terms of microbial load, samples from commercial buildings exhibited high temperatures, low total chlorine concentrations, elevated metal levels, particularly copper (Cu) and manganese (Mg), and larger intact cell (ICC) concentrations with less diversity and uniformity of microbial communities compared to the locations of residential households. Consistent with the results of previous studies conducted over shorter time periods, these results confirm that reduced water demand led to water stagnation and adversely affected water quality.

Total chlorine, ammonium and conductivity were the primary water chemistry parameters driving the change in microbial community composition over the six-month study period. The loss of disinfectant residues during intermittent periods of stagnation was also responsible for this change in the composition of the microbial community associated with plumbing.

Further investigation of microbial community composition using Bray-Curtis distance-based redundancy analysis (dbRDA) revealed an accumulation of samples based on site type. This analysis explained about 15% of the variation in microbial community composition.

In addition, distribution of variance analysis indicated that total chlorine, ammonium, and dissolved oxygen were the primary water chemistry parameters associated with change in microbial community composition between sites, with total chlorine accounting for approximately 12% of this variation.

Finally, commercial construction site piping systems exhibited metal concentrations below prescribed concentrations and were associated with higher manganese, magnesium, zinc, copper and lead concentrations due to metal leaching from the pipes or scaling during water stagnation.


The study results showed that building closures related to COVID-19 affected the water needs and water composition of the microbial community in their plumbing systems.

While these impacts on the drinking water microbial community of commercial buildings could not be mitigated by 30-minute flushing programs after those buildings reopened, they gradually recovered over several months to eventually converge on the microbial communities at residential sites.

Taken together, the study results suggest that after prolonged periods of changing water demand compared to short-term flushing, a sustained and gradual increase in water demands was required for the recovery of the microbial communities associated with the building plumbing.

*Important NOTE

medRxiv publishes preliminary scientific reports that have not been peer-reviewed and therefore should not be relied upon as conclusive, guide clinical practice/health behavior or be treated as established information.

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