Flushing water regularly inadequate to protect water quality in gravity-fed water supply systems with storage tanks during long-term lockdown

Flushing of water is recommended to prevent the growth of opportunistic pathogens, corrosion, and deterioration of the water supply network during short-term lockdowns. However, the efficacy of flushing fixtures in water supply networks with intermittent gravity-fed supply during long-term lockdowns is unknown. A laboratory-scale premise water supply network with a secondary storage tank connected to a gravity-fed drinking water distribution network was operated for 52 weeks to compare the efficacy of flushing water once a day (1D) vs. once in three days (3D) to preserve water quality. The tap water sampled from buildings (building) and overhead tanks (OHTs) prior to the pandemic served as a benchmark for comparison. The water quality trended as 1D = 3D < building < OHTs, with both the 1D and 3D yielding poor quality water. The water in 3D had higher levels of 16S rRNA gene copies, sul1, and intI1 relative to 1D; this difference was more pronounced (2-3 orders of magnitude) during summer. The levels of sul1, intI1, and DNA marker of Legionella pneumophila in biofilms sampled at the end of the operation of the laboratory-scale premise water supply network trended as elbows > visibly corroded pipes-sections > taps. The levels of heavy metals in some tap water samples exceeded the WHO recommendations and BIS standards for drinking water: 32% of 1D samples (30/93), and 31% of 3D samples (29/93) exceeded the standards for Pb, (10 ppb); while 48% (45/93) of 1D samples and 35% (33/93) of 3D samples exceeded the standards for Fe in drinking water (300 ppb).