The Inadequacy of Flushing in Maintaining Water Quality in Gravity-Fed Supply Systems with Storage Tanks during Prolonged Lockdowns

The efficacy of fixture flushing in intermittently supplied, gravity-fed water networks during extended lockdowns is unclear. A laboratory-scale premise water supply system with a secondary storage tank was operated over 52 weeks, comparing daily flushing (DF) versus flushing every 3 days (O3DF) to preserve water quality. Tap water from buildings and overhead tanks (OHTs) collected before the pandemic served as benchmarks for typical water quality under normal usage. The water quality index followed the trend DF = O3DF < building < OHTs, indicating both flushing regimes resulted in “poor” water quality. Moreover, O3DF samples had higher levels of 16S rRNA gene copies, sul1, and intI1 than DF. Biofilm analyses showed sul1, intI1, and Legionella pneumophila DNA markers were most abundant in elbows, followed by corroded pipe sections, then taps. Importantly, heavy metal content exceeded drinking water standards in some flush samples: 32% of DF and 31% of O3DF samples surpassed lead limits, while 48% of DF and 35% of O3DF exceeded iron limits. These results indicate that intermittent flushing alone fails to ensure safe water quality in storage tank systems during prolonged stagnation, emphasizing the need for updated maintenance and risk mitigation strategies during extended low-occupancy periods like lockdowns.