Legionella bacteria in water systems – an invisible risk worth preventing

Water coming out of the tap can look perfectly clean and clear, but you cannot judge its real composition by eye. Under certain conditions, microorganisms can multiply in a building’s water system – one of them is Legionella, a bacterium that can cause severe pneumonia, also known as Legionnaires’ disease.

Buildings with large and complex water systems need particular attention, for example hotels, spas and water parks, hospitals, nursing homes, sports and health centers, apartment buildings and other buildings with many water outlets, extensive pipework or where water can stand still for long periods (schools, kindergartens).

At the same time, the Legionella risk is not only a problem for large buildings. Favorable conditions for bacterial growth can also arise in home water systems, especially when water is not used for a longer time – for example in the summer, when people spend a long time at a summer cottage or on vacation and household water use is low or absent altogether. In that case the water stays standing in the pipes and can warm up to a temperature that promotes the growth of microorganisms.

That is why, after a longer absence, it is worth paying attention to the home water system as well and letting the water that has been standing in the pipes run out before resuming normal water use.

How does Legionella reach people?

Legionella bacteria cause a disease called legionellosis. The illness is most often associated with the bacterial species Legionella pneumophila.

Infection occurs mainly when a person inhales tiny droplets of water, or aerosol, contaminated with the bacteria. Aerosol can be produced by, for example, showers, hot tubs, taps, humidifiers, cooling systems and other devices that spray water.

That is why, in the case of Legionella the main question is not what the water looks like or how it tastes. The problem can be present even when nothing unusual can be noticed in the usual properties of the water.

The risk of becoming seriously ill is higher for the elderly, smokers, and people with chronic diseases or weakened immunity. That is precisely why special attention must be paid to Legionella prevention in hospitals, nursing homes and other buildings where people belonging to risk groups stay.

Legionella has also been found in the water systems of Estonian buildings

The problem is not unfamiliar in Estonian conditions. In a study carried out by the Health Board in 2023*, water samples were taken from the water systems of hospitals, nursing homes, prisons and selected accommodation facilities.

In the study, the Legionella content exceeded 1000 CFU/1000 ml in 29.7 percent of the samples taken from hospitals and 15.6 percent of those taken from nursing homes. In prisons, four samples out of six exceeded the same level.

The results do not mean that all such buildings have a Legionella problem, but they illustrate why conscious monitoring of the condition of the water system is necessary.

Standing warm water creates a suitable environment for the bacterium

Legionella growth is favored above all in water whose temperature is roughly between 20 and 50 °C. A particularly important risk factor, however, is several circumstances coinciding.

Bacterial growth can be promoted by standing water and low water flow, rarely used pipe sections and water outlets, biofilm that has formed on the inner surfaces of the pipes, and limescale, rust and sediment. Attention should also be paid to boilers, hot water tanks, shower heads, tap aerators and other parts of the system that are not maintained regularly.

For example, a problem can arise in a hotel room that has not been used for a long time, in a shower room that is rarely used, or in a dead-end section of pipework where fresh water moves rarely. A similar situation can arise after periods when the whole building or part of it has been in low use.

Raising the boiler temperature alone may not be enough

One of the most important factors limiting Legionella growth is water temperature. As a guideline, the temperature of hot water leaving the boiler should be at least 60 °C, and cold water should stay below 25 °C, if possible below 20 °C.

At higher temperatures the risk of scalding has to be taken into account, and the water system must be designed so that the water reaching the user is safe.

However, temperature alone does not solve every problem. If biofilm has formed on the inner surfaces of the pipes or sediment and limescale have accumulated in the system, microorganisms may be better protected from external influences.

That is why Legionella prevention means a holistic approach: regular maintenance of the water system, correct temperatures, sufficient water circulation, flushing of little-used water outlets and, where necessary, professional cleaning and disinfection of the system.

UV disinfection – a chemical-free option

One way to control microorganisms is disinfection based on ultraviolet radiation, i.e. UV disinfection.

In a UV unit, water is passed through a chamber in which UV radiation deactivates microorganisms by damaging their genetic material and preventing reproduction. An important advantage of the method is that no chemicals need to be added to disinfect the water, and a correctly sized and maintained system does not change the taste or smell of the water.

A UV unit can be well suited, for example, for disinfecting incoming water or for reducing the microbiological risk in a specific part of the water system. This solution has an important limitation, however: the disinfecting effect takes place at the moment the water passes through the unit.

UV radiation does not keep acting in the water, and therefore it does not protect the pipework located further from the UV unit against new microbiological growth. Nor does a UV unit remove biofilm that has already formed on the inner surface of the pipes.

That is why the suitability of UV disinfection depends on the specific water system, its construction, the water quality and the problem you want to solve. For the unit to work efficiently, regular maintenance and sufficient UV transmittance in the water are important.

When the problem affects the whole pipework

In more complex water systems, a solution whose disinfecting effect extends into the pipework may be needed.

One possible method is chlorine dioxide disinfection. Chlorine dioxide is used to control microorganisms, and in a properly designed system it allows the disinfecting effect to be maintained in the water network as well. When attention is needed not only for incoming water but for the entire water system inside the building, this can be a suitable solution.

Chlorine dioxide is also used to control problems related to biofilm, which is why its logic of use differs from UV disinfection, for example.

The Grundfos Oxiperm Pro offered by Miridon produces chlorine dioxide on site and doses it into the water system in a controlled way. The suitability of the unit, dosing and operating parameters must always be determined based on the specific building’s water system, the water quality and the applicable requirements.

There is no single solution for every building

Legionella risk reduction has no single device or method that would automatically suit every water system. In some systems it may be enough to adjust water temperatures and circulation, avoid standing water and carry out regular maintenance. In another, UV disinfection may be justified. 

In a larger or more complex pipe system, controlled dosing of a disinfectant and a solution covering the whole system may turn out to be necessary instead. The most important thing is to find out why conditions suitable for bacteria have arisen in the water system and, where possible, to eliminate the cause of the problem.

Miridon helps you find the right solution

Miridon has long experience and knowledge in developing water treatment and water system solutions. When needed, our specialists can help assess your existing water system and choose a disinfection solution that takes into account its construction, purpose of use and water quality.

Clean water is not only what we see with our eyes. A well-designed, correctly used and regularly maintained water system helps prevent problems that might otherwise go unnoticed.

Does your building’s water system need checking or more effective protection? Contact us – we will help assess the situation and find a suitable solution.

miridon@miridon.ee
+372 506 6775

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*Health Board, “Occurrence of Legionella in Estonian drinking water”, 2024 (study carried out in 2023).
https://terviseamet.ee/uudised/terviseameti-uuring-milline-olukord-legionellaga-eesti-hoonesisestes-veevarkides