What effect does water hardness have on Legionella?

Hard water is that which contains a lot of calcium and magnesioum salts that may precipitate when heated to form scale on pipe and heat exchanger surfaces.

A study in Italy found that:

” Total hardness protected Legionela pneumophila serogroup 1 and other species. As sites frequently use water softening systems, the high levels of Legionella pneumophila serogroup 1 colonization of water may also depend on this technical device. Hardness is generally considered a risk factor for Legionella colonization, and in our investigation this was confirmed for Legionella pneumophila serogroups 2 to 14. A negative correlation with hardness and chemical oxygen demand has been described in other studies, but Legionella species were not examined. As a lower number of bacteria were found in dental unit water systems using soft water than in dental unit water systems using hard water, we suggest that less microbial competition or modification of biofilm formation and/or composition in soft water might represent a risk factor for Legionella pneumophila serogroup 1 colonization more than hardness per se.”

Borella et al., ‘Legionella Contamination in Hot Water of Italian Hotels’.
Calcium carbonate salts form regular “building blocks” which can form layers of scale.

These precipitated hardness salts then form an environment that can allow bacteria to grow on and within the scales produced. This scale can then provide protection to bacteria “hiding out” within then avoiding temperature and biocide addition.

A water softener acts by replacing the salts of calcium and magnesium with sodium salts which are soluble and hence do not precipitate to form scale within a system. So the use of a water softener can act to keep systems clean of scale and hence reduce the risk of Legionella bacteria having an environment in which to grow and survive.

Extract from HSE HSG274 Part 1 describing effects of scale formation in cooling systems

Clearly then scale has a profound effect of Legionella bacteria within a system and hence all actions necessary should be taken to ensure a scale free environment.

If you want to discuss the effect of hardness salts in your system and how to manage Legionella then contact us via email on general@collatonconsultancy.com

Collaton Consultancy Limited are a specialist consultancy offering technical advice on Legionella, Pseudomonas and water treatment. We also act as Authorising Engineer (Water) in healthcare sites and as Expert Witnesses. We also offer both online and face to face training in a host of subjects.

More information can be found on the following websites:



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