3
$\begingroup$

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?

$\endgroup$
  • 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 at 11:42
4
$\begingroup$

Usually cell death would follow first order kinetics.

$$X=X_0e^{-kt}$$

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.

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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.