The impact of stagnation on the microbial quality of constructed water systems after COVID-19 shutdowns.

In response to COVID-19 pandemic, governments all over the world limited the movement of people and mandated temporary closure of different institutions. While, these measures helped to reduce the spread of COVID-19, stagnant water can cause water quality deterioration. Stagnation is considered in context with the proliferation of pathogenic and facultatively pathogenic bacteria which pose potential health risks to humans. The objective of this study was to document the hygienic microbiological status of different water systems after the first shutdowns (between 18th March 2020 and 18th May 2020) in Hungary in comparison with a reference period (between 3rd January 2020 to 17th March). During the reference period drinking waters were compliant > 95% of total samples to the parametric values. After the short period shutdowns, the ratio of tnon-compliant drinking water samples was 6.6%: mainly Pseudomonas aeruginosa (7%) and coliform bacteria (5%) resulted poor water quality. The microscopic analysis of drinking waters showed that after low water demand the values of non-compliant samples also increased due to the proliferation of Amoebozoa and other Protozoa species. The compliant pool waters' ratio was also high in the reference period (97-99%), while after the shutdowns more samples were positive in both pool operation type (fill-and-drain pools and pools with recirculation) due to the proliferation of P. aeruginosa (14%) and micrococci (12%). Legionella non-compliant samples in hot tap water did not show significant difference during both studied periods (15%) although after stagnation the Legionella CFU (colony forming unit) values of the samples increased markedly.