Thermal treatment is one of the most commonly used techniques worldwide to control and prevent Legionella in drinking water. In practice, thermal treatment requires gallons of water for flushing practices. But how successful is this approach? Has all the flushed water proven its worth? Or has this technique become obsolete? To answer these questions, we have conducted a scientific literature review and analysis on thermal treatment.
What is thermal treatment?
Thermal treatment or thermal disinfection attempts to control Legionella bacterial growth in two ways (RIVM, 2012):
- Preventing a favorable temperature for growth in the entire drinking water installation; cold water no warmer than 25℃ (preferably < 20℃) and hot water no colder than 50℃ (preferably > 60℃);
- Preventing stagnation of water by using the entire installation, at least on a weekly basis.
Research into the effectiveness of thermal treatment
In the study of the National Institute for Public Health and the Environment in the Netherlands (in other words, RIVM (2012)), the effectiveness of several Legionella prevention techniques were investigated through a scientific literature review. Below, you will find a summary of the findings concerning thermal treatment.
Legionella can become insensitive to thermal treatment
For 20 years, Allegra et al. (2011) studied the effect of thermal disinfection on the Legionella bacteria. Their research showed that despite the application of thermal treatment the Legionella strains colonized the drinking water installations for many years. The bacterial strains differed in their sensitivity to heat and some became insensitive to thermal disinfection. These findings were also supported by the research of Farhat et al. (2010). At first, applying heat-shock therapy showed positive results. However, after the second heat-shock the Legionella count didn’t reduce (Farhat et al., 2010).
In practice, thermal treatment of cold water piping is challenging
Arvand et al. (2010) investigated the effect of temperature on Legionella contaminations in cold water pipes. It was striking to see that the number of contaminated cold water taps was considerably higher than the number of infected hot water taps (40% vs. 23%). Even the cold water taps with low temperatures were largely contaminated: < 20℃ (35% tested positive for Legionella) and < 15℃ (28% tested positive for Legionella). This indicates that a cold water tap with a temperature of < 20℃ does not guarantee a Legionella free drinking water installation (Arvand et al., 2011). Although we assume that Legionella cannot grow at a temperature below 20℃, the bacteria might grow at a different spot in the pipeline which has a higher temperature (RIVM, 2012). The problem is that it is difficult to keep the cold water pipes constantly below 25. Therefore, thermal disinfection often does not work in practice.
Thermal treatment least effective technique
Thermal treatment was mentioned as the least effective technique in the presentation of Alvin Bartels of the RIVM at the ISSO conference on Alternative Techniques in the Netherlands (see also RIVM pdf). The Legionella bacteria can become resistant against heat treatment after a while. Biofilm, the breeding ground for Legionella, often remains present in the pipelines with thermal disinfection. In addition, cold water pipes cannot be properly disinfected. Many authorities in the Netherlands that are responsible for Legionella management indicate that thermal treatment measures are difficult to comply with.
The numbers: Legionnaires’ disease is on the rise
Thermal treatment is the preferred water treatment technique worldwide. But, the numbers do not lie. Legionnaires’ disease is on the rise and locations are increasingly battling Legionella contaminations. It has become apparent that thermal disinfection is not always successful against Legionella. Why is this technique still the preferred Legionella control and prevention technique? Isn’t it time to change legislations and advice prevention techniques that do work?
To be continued: in the next campaign we’ll discuss the RIVM analysis on the effectiveness of Copper Silver Ionization.
Allegra S, Grattard F, Girardot F, Riffard S, Pozzetto B, Berthelot P. 2011. Longitudinal evaluation of the efficacy of heat treatment procedures against Legionella spp. in hospital water systems by using a flow cytometric assay. Appl Environ Microbiol. 77:1268-1275.
Arvand M, Jungkind K, Hack A. 2011. Contamination of the cold water distribution system of health care facilities by Legionella pneumophila: Do we know the true dimension? Euro Surveill. 16: pii=19844.
Bartels, A. (2019). Effectiviteit van beheerstechnieken voor legionella in drinkwaterinstallaties. ISSO-congres Alternatieve technieken
ECDC. (2017). Legionnaires’ disease. Annual Epidemiological Report for 2017. Retrieved from https://ecdc.europa.eu/sites/portal/files/documents/AER_for_2017-Legionnaires-disease_1.pdf
Farhat M, Trouilhé MC, Brand E, Moletta-Denat M, Robine E, Frère J. 2010. Development of a pilot-scale 1 for Legionella elimination in biofilm in hot water network: heat shock treatment evaluation. J. Appl. Micrbiol. 108: 1073-1082.
ISSO 55.1 Legionellapreventie in leidingwater
RIVM. (2019). Legionella. Retrieved from: https://www.rivm.nl/legionella
RIVM Rapport 703719078. (2012). Effectiviteit van beheerstechnieken voor legionella in drinkwaterinstallaties.