WATER SYSTEM SAFETY
common in basins. Why is this? – most probably because they are used for the least amount of time. Compare the time you spend with the taps running in the shower to the time you spend washing your hands, and it’s obvious which is flushed the best. The data demonstrates that the Legionella will gravitate to and colonise stagnant areas. It also shows the value of flushing. As an aside, encouraging handwashing at basins rather than disinfectant gels is worth consideration. It is cheap, effective, and flushes the outlet. Following on from this early work we have been able to demonstrate that sensor taps and basins don’t get flushed enough. Data from a thermostatic mixing valve installed on 220 outlets in an anonymous hospital showed that flushing time was usually less than 20 seconds. It also showed that the length of time was not sufficient to reduce the bacterial content of the water.
The combination of obstruction and stagnation leads to loss of temperature control. A large building will maintain its internal temperature in a window conducive to Legionella growth (20-30˚C). Water that is not moving will move towards that temperature. Warm water cools and cold water warms. This leads to the third member of our Trinity – fermentation.
Fermentation
It is clear that temperature is a critical factor in Legionella control. As pointed out already, obstruction and stagnation can combine forces to set the right temperature and environment for Legionella growth – by allowing time for the water temperature to move up or down the gradient towards that of the surrounding environment.
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R3 0 103 104 55˚C/0 min R1 R2 R1 105 0 103 104 55˚C/1 min
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R3 0 103 104 55˚C/5 min 105 0 103 104 55˚C/7 min Thiazole orange
Figure 2: Flow ctyometry and vital staining of L. pneumophila SG 1 at 55˚C (Atif Nisar et al 2021, In Press).
The temperature requirements for Legionella growth are widely published. Most regulations will quote a chart that shows Legionella only grows between 20˚C and 45˚C, and will die in time at a temperature above 45˚C. These charts are ‘de-facto’ facts. Real-world evidence has shown Legionella multiplication above 50˚C, and survival above 60˚C for significant periods.
What is a significant period? It’s long enough to survive pasteurisation of outlets as a decontamination strategy. Worse still, repeat pasteurisations tend to improve Legionella survival by selecting tolerant strains. This is published data that hasn’t reached as far as regulation.
Uncharted territory
Before we throw the baby out with the bath water, temperature control is very important. Reducing cold water temperatures and maintaining hot water temperatures does reduce the growth rate of Legionella. From 20˚C the growth rate of Legionella is an upwards sliding scale up to around 35˚C, and a downwards sliding scale from there to beyond 45˚C. This is published data that is well covered in the regulations. What happens at the extremes of the temperature scale is the uncharted territory.
Some recent work we have completed has focused on the effects of higher
R3 105 –102 0 102 103 104 55˚C/10 min
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November 2021 Health Estate Journal 25
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