The UK’s HSG 274 Part 2 describes thermal disinfection as:
“thermal disinfection, ie by raising the HWS temperature to a level at which legionella will not survive, drawing it through to every outlet, and then flushing at a slow flow rate to maintain the high temperature for a suitable period (the contact time). This method is only applicable to HWS and is commonly used as a rapid response. It may be less effective than chemical disinfection and may not be practicable where the hot water supply is insufficient to maintain a high temperature throughout.”
The Healthcare version, HTM0401 Part B, defers to HSG 274 Part 2. As a result many people recommend thermal disinfection as an alternative to chemical disinfection, so does it work?
If a hot water system has biofilm or deposits (sludge, scale, corrosion products) it is known that the high temperature water is not guaranteed to penetrate them and kill any bacteria within them. Further amoeba, in which Legionella spend part of their life cycle, also have protection strategies to minimise the effects of temperature.
Two scientific papers are worth understanding. The first one (“Effect of Thermal Shock During Legionella Bacteria Removal on the Corrosion Properties of Zinc-Coated Steel Pipes”, Juliusz Orlikowski, Jacek Ryl, Agata Jazdzewska, and Stefan Krakowiak, published in 2016) concluded that:
“The direct cause of perforations found in the hot water pipeline in the investigated buildings has been identified as the polarity reversal of zinc coating. The aforementioned process was driven by a high temperature of water in the system due to thermal treatment for Legionella removal…”
This paper showed that temperatures associated with thermal disinfection led to corrosion of pipework, which in itself would then lead to an increase in iron, a known material important to the growth of Legionella bacteria.
The second paper of interest (“Legionella Persistence in Manufactured Water Systems: Pasteurization Potentially Selecting for Thermal Tolerance”, Harriet Whiley, Richard Bentham and Melissa H. Brown, published in 2017) concluded that:
“Up to 25% of L. pneumophila cells survived heat treatment ◦ of 70 C, but all of these were in a viable but non-culturable state. This demonstrates the limitations of the culture method of Legionella detection currently used to evaluate disinfection protocols. In addition, it has been demonstrated that pasteurization and nutrient starvation can select for thermal tolerant strains, where L. pneumophila was consistently identified as having greater thermal tolerance compared to other Legionella species.”
This paper therefore suggests that raising the temperature to that used in thermal disinfection rather than killing Legionella led to thermal tolerance being developed which is the opposite of what was required from the process.
Consequently, it may be fair to say that thermal disinfection is not guaranteed to work in trying to remove Legionella bacteria from hot water systems and consideration of using chemical means of removing Legionella should receive more attention.
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