I'm coding some stuff for water heaters and one of the functions is anti-Legionella.

Anyway, I've read on wikipedia that 90% Legionella dies in first two minutes if the water temperature is 60°C, but I'm interested in how long should the temperature be 60°C for 99.99999% of Legionella to die? (I guess I can never get to 100%)

Follow-up question: On what intervals should this be done so that the water isn't contained? Should it be one week, two weeks, several days?

Final question: If the water in a water heater has 60°C for 30 minutes every 2 weeks, would the water be safe?

  • 1
    $\begingroup$ If the water can be repeatedly contaminated, you would probably need to continuously keep its temperature at the disinfection level to limit the risk of contamination to 0.00001%. $\endgroup$
    – Jan
    Jul 10, 2019 at 11:42

1 Answer 1


Usually cell death would follow first order kinetics.


where $X$ is the number of cells at time $t$, $X_0$ is the initial number of cells and $k$ is the death rate constant. You can get the constant by solving for 90% reduction in 2 minutes. Then just substitute and get the time required for 99.99999% reduction — 10 minutes.

For your follow up questions, see the same wikipedia article.

Moist heat sterilization (superheating to 140 °F (60 °C) and flushing) is a nonchemical treatment that typically must be repeated every 3–5 weeks.

It also depends on how much is the bacterial load in the input.

Other sources (Farhat et al. 2012; Whiley et al., 2017) suggest 70°C treatment for 30 minutes.

However, Legionella has been shown to survive heat treatment of 70°C/30min (reviewed by Whiley et al., 2017). Legionella can also be "protected" by other micro-organisms such as certain types of Amoeba, in the water.

So, probably multiple levels of disinfection is needed.

  • $\begingroup$ Ok, great, this is just what I needed $\endgroup$ Jul 10, 2019 at 12:11

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