by Linda Bertelsen
New measures for Legionella in hot water systems -baggrund

Legionnaires’ disease, a severe pneumonia caused by Legionella bacteria, has become a growing challenge in Denmark and other European countries. A comprehensive Danish project on Legionella in domestic hot water systems has developed and demonstrated three innovative solutions to minimize the problem.

EUDP (Danish Energy Technology Development and Demonstration Programme) project “Legionella protection and energy efficiency for installations and supply” carried out from September 2020 to February 2023:

EUDP (Danish Energy Technology Development and Demonstration Programme) project “Legionella protection and energy efficiency for installations and supply” carried out from September 2020 to February 2023:

Published in Hot Cool, edition no. 8/2023 | ISSN 0904 9681 |

Legionnaires’ disease is a severe form of pneumonia caused by the Legionella bacteria. According to ECDC (The European Center for Disease Prevention and Control), there has been an increase in Europe in the incidence and number of deaths from Legionnaires’ disease since the early 2000s. In 2022, the incidence rate per 100,000 inhabitants reached 2.6, compared to approximately 1.3 in 2005, as the graph below indicates. The significant growth in the incidence occurred around 2016.

Coinciding with the rise in incidence, there has been a corresponding increase in the number of deaths from Legionnaires’ disease in the EU/EEA. Deaths rose from around 350 in 2005 to nearly 800 in 2022.
Denmark has had a relatively higher disease prevalence than other European countries. As a result, there has been a specific focus on the problem in Denmark.
It is generally recognized that relatively low domestic hot water temperatures may have contributed to this. The low temperature allows the growth of Legionella, and the bacteria may spread to humans in small water particles (aerosols) from showers, e.g.
Approximately two-thirds of Danish homes – and the associated domestic hot water – are heated with district heating. As for the rest of the households, heat pumps are gaining ground to a considerable extent. Regarding energy efficiency and the possibilities of using alternative energy, it is important for both energy systems to have water temperatures as low as possible. There is thus a significant temperature-related dilemma about ensuring Legionella-safe domestic water installations while simultaneously meeting the need to save energy, use renewable energy sources, and reduce the climate footprint.
An interdisciplinary project co-financed by the EUDP (Danish Energy Technology Development and Demonstration Programme) has investigated this dilemma and pointed to some solutions.
The diverse group of partners has included the Danish Technological Institute and Statens Serum Institut (SSI, under the auspices of the Danish Ministry of Health).

The project included partly a comprehensive study on previous investigations and requirements for Legionella in domestic hot water and the derived energy consumption, partly developing and demonstrating three solutions for control of Legionella in domestic hot water installations. These were, respectively, a tool for risk assessment of Legionella (Danish Technological Institute), an electric booster for monitoring and ensuring the necessary water temperature (METRO-THERM), and an innovative solution for biocide dosing (Danish Clean Water)—responsible partner in brackets.

Results from the literature study

The study was based on a major literature search followed by contacts to selected knowledge and research centers. The study included an examination of the influence parameters affecting the spread, growth, and reduction of Legionella, as well as of authority requirements, standards, and guidelines. The study showed that due to especially biofilm, the temperature requirements for controlling Legionella are often underestimated, and the Danish temperature requirements and practices are often challenging about Legionella.

Figure 1: Risk assessment for Legionella via review of the domestic water installation.
Figure 1: Risk assessment for Legionella via review of the domestic water installation.

The study also leads to the following conclusions:

  • It is generally agreed that Legionella pneumophila develops at temperatures higher than 20 – 25°C and lower than 45 – 50°C. The Legionella will die at 50°C and higher temperatures in the water. Due to, e.g., biofilm formation, it is at varying temperatures; however, it is often uncertain if the necessary temperatures have been obtained in the biofilm, although fulfilled for the water.
  • An overall water temperature of 50°C or above will generally limit the Legionella content to the recommended max. 1,000 CFU/L (Colony Forming Units per Liter). However, these temperatures are particularly challenging for energy efficiency, climate footprint, and running costs as the necessary comfort requirement for the water temperature is only 45°C.
  • Besides the water temperature and the biofilm, several other parameters impact the growth/reduction of Legionella, such as water flow conditions, water quality, pressure, and affected materials. However, the study showed that the knowledge of the conditions influencing the growth and reduction of Legionella is often relatively poorly founded or unclearly documented; e.g., it has not been clarified how temperature and flow conditions play a role together.
  • Nationally and internationally, there is relatively limited regulatory encouragement to search for alternatives to temperature protection against Legionella.
  • The new EU drinking water directive will increase the focus on Legionella as well as direct or indirect on the risk assessment of domestic water installations.

Due to the dilemma between protection against Legionella in domestic hot water and achieving high energy efficiency and low climate footprints, it is essential to focus on all potential improvement opportunities, just as new knowledge and insight into the problems must be ensured.

Figure 2: The risk assessment tool illustrating initial risk factor screening

Tool for Legionella risk assessment

The developed tool is based on the widely recognized and used method FMEA (Failure Modes and Effects Analysis) for assessing risks and opportunities for improvement of technical facilities. It has been further adapted to include impact parameters related to Legionella in domestic water installations. The tool aims to provide a simplified approach to reviewing and improving both existing and new domestic water installations.

Rather than relying on statistically based risk analysis, the tool considers risk factors associated with each component of the installation (figure 1). These risk factors are assessed through a set of questions based on the identified impact parameters that significantly influence Legionella occurrence (figure 2).

The assessment considers both the hot water installation, including possible circulation, and the drinking water supply itself; that means it recognizes the potential for Legionella growth in lukewarm water pipes above 20°C. An overall risk assessment is determined by adding up the risk factors identified during the overall review of the domestic water installation.

The purpose of the risk assessment is to identify specific areas where Legionella risks are present and opportunities for improving the installations, and it also includes considerations for supplementary temperature and biocide treatment. Through subsequent activation of these options, the overall risk can be reduced.

A pilot version, the tool has been tested on practical domestic water installations at KAB, a large Danish housing company representing 70,000 homes. Several numbers of properties were inspected and assessed, with a comparison to Legionella analysis from samples. For one of the installations, the risk analysis and the tests showed critical conditions. Implementing an improvement measure involved installing a biocide system with hypochlorous acid (see later). However, the improvements were slightly less pronounced in the risk assessment due to the presence of old valves and lines that were not yet replaced.

3a) The electric booster under test in lab
Figure 3a: The electric booster tested in the lab for domestic hot water circulation.
3b) Complete control with attached sensors and cables

Electric booster unit ensuring temperature monitoring and optimal control.

The electric booster unit ensures monitoring and optimal control of the temperature of the domestic water installation based on knowledge of Legionella growth and reduction from the literature search. Also, it can give an alarm if some of the temperature requirements are below the limit values and thus reduces the risk of using the hot water installation. Furthermore, it conducts a thermal heat treatment of 60°C of the water producer and the circulation, if present, when the control has calculated that there is a theoretical possibility that the Legionella content has doubled.

Figure 4
Figure 4: Demo results (field test) of electric booster for a non-circulated hot water system.
Figure 5
Figure 5: Generator for hypochlorous acid installed for demo (field test 2).

The thermal heat treatment works best on systems with circulation, as the entire domestic water system, apart from taps, connecting pipes, and dead ends, is heated to 60°C. For systems without circulation, it is necessary to open the taps at the end of the lines to obtain this heating of the entire system.

In continuation of a series of laboratory tests (figure 3), a booster was demonstrated to a single-family house with district heating and a Legionella content significantly over 1,000 CFU/L (figure 4). The system was without circulation, and the heating of the domestic hot water took place in a plate heat exchanger having a standby function, which means that the water in the plate heat exchanger was kept at 44°C during standby, and only during tapping was the temperature raised to 53°C.

Derived from the heat exchanger result, the booster conducted a 60°C heating of the domestic water producer every night. However, it was confirmed that this heating did not affect the infected installation. In fact, there was a sharp increase in the number of Legionella during the four-week summer holiday without water consumption, and it was then decided to conduct a disinfection of the complete installation by opening the taps. According to the immediately achievable via the booster, the disinfection was carried out at 63°C. This resulted in a Legionella content far below the recommended max. 1,000 CFU/L, despite a lower disinfection temperature than the recommended min. 65 – 70°C. Further, the single demo test indicated that the electric booster can prevent Legionella formation in a new/cleaned system.

Innovative biocide solution for protection at lower temperatures

The project also tested an innovative solution from Danish Clean Water that can protect against Legionella and, at the same time, provide energy savings. A generator produces and doses hypochlorous acid (HOCl) into the hot water systems that kill planktonic Legionella at lower temperatures and remove biofilm. HOCI is produced via salt, water, and electrolysis and has been demonstrated to be more reactive than chlorine in combating Legionella. In addition, it is a mild disinfectant that is environmentally friendly and safe for humans.

The solution was tested at two residential sites with severe Legionella problems (up to 120,000 CFU/L) in hot domestic water (Figures 5 & 6). Field test 1 was a small property in Southern Jutland, and field test 2 was a large, completed property near Copenhagen. After the HOCl generator was switched on, viable Legionella was drastically reduced in the water, even though the temperature in both cases was lower than the 50°C required in Denmark. To ensure effective dosing, the project incorporated water flow measurement and strategically positioned sensors for robust control.

The field tests underpin the effectiveness of hypochlorous acid not only for disinfection but also for maintaining low Legionella values in ongoing operations. However, a potential risk can be that the biocide does not get around in the entire system due to dead ends, like temperature treatment. This requires special attention and documentation.

The field tests showed that the biocide solution can efficiently reduce Legionella counts and keep a low, tolerable level – at lower temperatures. This allows for saving energy and reducing climate footprints.  Consequently, the project proposes that the solution be included formally in authority requirements, providing that it meets specific documentation requirements for its efficiency throughout the installation.

Figure 6-1
Figure 6: Demo results (field tests 1 and 2) for the hypochlorous acid generator.
Figure 6-2
Results for field study 2

The overall project is documented through several professional reports in Danish as well as through an English final summary report of 20 pages: https://www.teknologisk.dk/_/media/87227_EUDP%20Jnr64020-1099%20Legionellasikring%20-%20Final%20report%2020230228.pdf

For further information please contact: Kaj L. Bryder, kbr@teknologisk.dk

“New measures for reducing Legionella in hot water systems” was published in Hot Cool, edition no. 8/2023. You can download the article here: