To reduce the spread of COVID-19, students and workers in many sectors were placed under stay-at-home orders, leading to significant decreases in water demand in many non-residential areas. Previous research has associated increased water age with increased microbial abundance, metals, and other contaminants in distribution systems and building plumbing. In the United States, most hospitalizations and deaths due to waterborne pathogens are caused by biofilm-associated opportunistic pathogens (e.g., NTM, Pseudomonas, Legionella), costing $2.39 billion annually. However, uncertainty remains regarding conditions that lead to water quality issues. Additionally, there is still a need for clear consensus guidelines on how to maintain or restore water quality after building closures. This study consisted of 2 arms: (i) survey of water quality and trial of tap flushing in full-scale campus buildings after 2-4 months of reduced occupancy; (ii) bench-scale experiments involving annular reactors that transitioned to increased water age and decreased pipe wall shear forces sequentially. In campus buildings, stagnant water had high flow-cytometric cell counts, but no samples had culturable L. pneumophila. Iron, copper, and nitrate were detectable but never exceeded EPA limits. Measured lead surpassed the EPA action limit in 4% of samples. Building tap samples collected immediately after flushing had higher total chlorine (181-368% increase) and lower intact cell counts (0.6-1.3 log reduction) than samples collected immediately prior, although flush times were not sufficient to return water quality to building influent levels. In the day and week after flushing, the bacterial cell counts rebounded as the chlorine residual decreased again. In bench-scale reactors, both bulk water and biofilm cell counts and metabolic activity increased dramatically with increased water age; however, reducing pipe wall shear forces did not have much impact on the concentration of microorganisms in the bulk water.
