We don’t often hear information about Legionella in the news. That’s a good thing. Because when we do, it typically means there is a local outbreak and people are hospitalized. Last weeks this was the case. The outbreak in Evergem was even fatal for a number of people. Sadly, Legionella infections are more common than typically assumed. Fire and carbon monoxide intoxication are well knows as building related casualties. On a yearly basis however, more people die because of a Legionella infection then CO intoxication. Looking at the data in the Netherlands, there are 50-70 casualties because of fire, 20 to 30 due to Legionella, and on average 12 related to CO intoxication. The numbers for Legionella should be considered as a severe underestimation. Very often the infection is not classified as Legionella, as the symptoms are similar to the ones for flue or pneumonia. Hence, the cause is not identified, and the number of registered infections remains low. Recently a urine test was developed that allows to render the right diagnosis.

By default, Legionella is present in lakes, rivers, and our tap water. The bacteria enter the building, but at a very low and harmless concentration. However, in domestic hot water systems the conditions may allow the bacteria to grow significantly. The dominant parameters for growth are temperature and flow rate. Only at temperatures between 20°C and 45°C the bacteria grows to harmful concentrations. At temperatures above 50°C the bacteria dies: 90% of the bacteria die when heated up to 60°C for 5 minutes. Hence, domestic hot water installations typically provide water at 60°C, even though we only use it at 38°C. In small installations this approach is safe, but in large installations with long pipes and high heat losses, the tap water temperature may be rather low, inducing increased risks. When the water remains stagnant for a long time, e.g. at hotel rooms, the risk increases even more.

For high-risk installations, the legal limit when vigilance is required is 1 000 CFU – colony forming units. In public installations tests are done on a regular basis to check the concentrations. Certainly in hospitals this is important, because the result of a measurement only becomes available after 10 days.


In the context of our Solar Decathlon project and the PhD of Elisa Van Kenhove, measurements were done on an apartment building with 1 400 inhabitants. Concentrations up to 71 000 CFU were found. This building is not covered by the scope of current building codes, as it is not a public building. However, due to the aging population and daycare center, people may be sensitive to infections and there is a considerable risk. Due to our research, the daycare center was closed off immediately, and disinfection was initiated. Based on inquiries with inhabitants, it was shown that a disproportionate number of people have chronic lung problems in the building. This highlights the importance of more focus on these buildings.

As the process of measuring Legionella concentrations requires time and energy, an advanced simulation model was developed at Ghent University which allows to predict the Legionella concentrations at different locations in a building over time. The model comprises the biological parameters, as well as the thermal and hydraulic equations to model the growth. This allows to understand the installations and risks better, predict problems, and develop disinfection protocols.

Additionally, by levelling the controls to the risk profile, energy can be saved. Producing the tap water at these high temperatures requires a lot of energy, even though we use the water at lower temperatures. The advanced modelling control algorithm allows to reduce the set point from 60°C to lower temperatures, while reducing the infection risk by applying specific heat shocks to the water. For the apartment building with 1 400 inhabitants, up to 43% of energy can be saved. And as well: simultaneously the infection risk is reduced. For new constructions a reduction in energy use between 30 and 40% is possible. This advanced technology is demonstrated in The Mobble.

Ghent University aspires to contribute to a more safe environment with systems that use less energy. The technology can also be applied in hospitals, retirement homes, hotels, schools, sport infrastructure, offices, and residential complexes. The fact that we can produce safer systems with less energy is something we are really proud off!

Figure 1: In low energy buildings heating domestic hot water really becomes dominant in the energy use. The Ghent University patented controller allows the reduce energy use up to 40%

​Figure 2: Due to energy losses in the pipes the tap water temperature decreases, with a clear correlation with legionella concentrations.

Figure 3: Legionella grows between 20°C and 45°C. Hence, standard domestic hot water systems provide water at 60°C to reduce concentrations.