Summary and conclusions

The Danish EPA decided in 2001 to examine Danish drinking water for Legionella, as no such investigation had previously been made. This report describes the screening of Danish drinking water for Legionella. The purpose of the screening was to investigate whether Legionella occurs in drinking water, and, if so, in which concentration – first, to confirm the theory that Legionella does not occur in Danish drinking water in concentrations which present any health risk and, secondly, to investigate whether drinking water is a possible source of Legionella found in the hot water systems.

The investigation was initiated by a pilot study at three water supplies. The purpose was to decide how to do the sampling, and, at the same time, to evaluate any effects of the modifications including increased sample volumes (10, 50 and 100 litres) and filtration of the water in the field in connection with the sampling itself, introduced to the method of analyses (DS 3029:2001 Enumeration of Legionella – Concentration and colony count on solid medium – Spread plate method). Equipment for sampling has been developed by Eurofins Danmark A/S. Based on the pilot study it was decided that 10 litres of water ab the water supply should be filtered at the final screening, whereas it was impossible to sample or filter sufficient amounts of raw water from the water supplies. It was furthermore important to use the correct filter type (0.45 µm mixed esters of cellulose: nitrocellulose and cellulose acetate) in order to filter the water, as it appeared to be impossible for the water to penetrate a 0.45 µm filtre of cellulose nitrate.

Legionella was not detected in the samples from the three water supplies. Despite the large sample volume the background flora was not so concentrated that it presented a problem in terms of overgrowth of Legionella at the plates. Leaving the samples at room temperature for 48 hours did not have a positive effect on the recovering of stressed Legionella, as growth of background flora made it difficult to distinguish any Legionella colonies from the background flora. MWY-agar (Modified Wadowsky Yee-agar) was tested in parallel with GVPC agar (Buffered Charcoal Yeast Extract agar with selective supplement) to ascertain whether one medium appeared better for selection and detection of Legionella than the other. It was not possible to reach an unambiguous conclusion of the pilot study, and the two agar types were therefore used in parallel in the investigation.

The final screenings of drinking water included 27 water supplies – at which water ab the water supply and the distribution network were examined. In addition water from the distribution net of eight other water supplies was analysed. The screening at the water supplies took place at the end of a long hot summer (2002), whereas the screening of the distribution net took place in a cold period (2003). Water ab the water supply and tap water were analysed, 10 litres per sample, whereas samples of biofilm from the edge of filter and filter material were analysed whenever possible. No Legionella were detected in the samples taken ab the water supplies. Legionella was detected in two of 37 water samples from the distribution net at a low level of approx. 4 – 40 cfu/litre considered to be harmless for the ingestion of drinking water.

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Version 1.0 Marts 2005, © Miljøstyrelsen.