What Happens If A Test Shows Legionella Bacteria Is Present?

What Happens If A Test Shows Legionella Bacteria Is Present?
If you are wondering what happens if routine testing shows that Legionella bacteria is present, we have translated into layman’s terms the process involved according to HSG274: Legionnaires’ disease Part 2: The control of Legionella bacteria in hot and cold water systems.

This information applies to hot and cold water systems only.


The minority of samples test positive for Legionella
In this instance, the system will be resampled to confirm findings. Should similar results be found again, a risk assessment will be carried out and the control measures will be reviewed to identify the best course of action.


The majority of samples test positive for Legionella
This indicates that the system is most likely colonised. Once again a risk assessment will be carried out and control measures will be reviewed, but it is recommended that the system be disinfected.


Disinfecting hot and cold water systems
Disinfection can only take place when the water system is offline. There are two methods available: Thermal Disinfection and Chemical Disinfection. It must be noted that both of these processes create a hazard to users through either chemical exposure or scalding – therefore it is essential that a risk assessment is carried out and specific procedures followed to ensure total safety while either of these processes are carried out.


“Signage and outlet warning labels should be fitted to all areas to alert occupants of the building for whom the risk is greater (such as the very young, elderly or those with sensory loss) not to use these outlets.” HSG274 Part 2.


Thermal Disinfection – This process involves raising the hot water system to a temperature at which the Legionella bacteria will not survive, drawing the very hot water through every outlet and then flushing the system slowly to keep contact time with the hot water for as long as possible.


“Thermal disinfection may prove to be ineffective where parts of the calorifier or water system fail to reach the required temperature for a long enough period.” HSG274 Part 2.


Chemical Disinfection – This process involves adding chlorine or chlorine dioxide to a cold water system, and drawing it through to every outlet, which is closed to maximize contact time. After disinfection has taken place, the solution should be completely flushed ready for the system to be brought back online.


Confirming Disinfection
Resampling should be carried out between two and seven days after the system is refilled. Resampling cannot be done immediately after disinfection as there is a chance that a false negative result will be obtained. Additional sampling should be carried out at regular intervals to ensure that the bacteria is under control.


“Once the water system is colonised with Legionella, it may prove extremely difficult to reduce numbers to undetectable levels and periodic positive Legionella results may recur. Under such circumstances steps should be taken to make sure the risk assessment reflects this and control measures should be devised to ensure that, although likely to be present at low levels, Legionella cannot multiply to dangerous levels.” HSG274 Part 2.


To book a risk assessment or get advice on maintenance and treatment call us on (0)1206 266828.

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