Cold water storage tank tests overlook possible presence of pathogenic bacteria

Water-borne bacteria that occur in drinking water storage tanks could be missed by standard health and safety tests, scientists from Brunel University have warned in a report published on 12 April.

Coldwater storage tanks supplying public drinking water are regularly checked for harmful pathogens such as Legionella bacteria and E.coli. It is assumed that samples taken from the top of the tank are representative of the water found throughout.

However, microbiological analysis by researchers found that samples taken as standard from the top of the tank are 40% less likely to raise a red flag than samples taken from the opposite end.

One in five samples taken from the top of the tank didn’t trigger the urgent safety action that samples from the bottom of the same tank showed was needed.

“These results call into question the reliability of present measures used to protect the public from waterborne pathogenic diseases, including Legionella,” said Aji Peter, who did the research with Dr Edwin Routledge at Brunel’s Institute of Environment, Health and Societies.

Cold water storage tanks are often found on the roofs or in the basements of public buildings such as schools and hospitals, and can be a source of repeated bacterial contamination.

Current safety regulations require a sample of water be taken from under the ball valve at the top of the tank for regular microbiological monitoring, although scientists are now calling for the standard safety tests to be changed to look at samples taken from the far end of the tank, where water is likely to be warmer and hold more bacteria-feeding sediment. 

“Given the disparity between measurements taken at different ends of the tanks, monitoring at the far end would provide a much more accurate indication of microbiological contamination. This would allow appropriate precautions to be taken to protect the public from waterborne pathogenic diseases, including Legionnaires’ disease,” said Mr Peter.

As the researchers state in the report: “We propose that water storage tanks should be redesigned to allow access to the far end of tanks for routine monitoring purposes, and that water samples used to ascertain the regulatory compliance of stored water in tanks should be taken at the point at which water is abstracted for use in the building.”