Microbial water quality in clean water tanks following inspection and cleaning

Increased bacterial counts are often registered in drinking water leaving clean water tanks after the tanks have been emptied, inspected and cleaned by flushing. To investigate the reason for the increased bacterial concentrations and consequently limit it, samples from two clean water tanks before, during and after cleaning of the tanks were analysed. Bacteria were quantified, the dominating bacterial groups were identified and re-growth potential in the water was estimated. Bacterial counts did not exceed drinking water guideline values but ATP concentrations in the water were high right after start-up of the tanks, which may indicate that a substantial part of the bacteria in the drinking water leaving the tanks originated from the sand filter. This was supported by 16S DNA analyses.