Testing and monitoring your water system for legionella
This page explains when and how to test your water system to ensure legionella is not present. There is also advice on what you must do when asking someone else to test or monitor your system for you.
How often you should test water for legionella
The regularity of your checks depends on your system and the outcome of your risk assessment.
For open systems, such as cooling towers, evaporative condensers and spa pools etc, routine testing should be carried out at least quarterly. However, there may be circumstances were more frequent sampling may be required.
For hot and cold water systems, which are generally enclosed (not open to the elements), significant contamination is less likely than in cooling towers etc. Microbiological monitoring is therefore not usually required. However, testing for legionella may be necessary, for example where:
- there is doubt about the control regime’s effectiveness
- recommended temperatures or disinfection concentrations are not being consistently achieved
Further guidance is available in Part 2 of Legionnaires’ disease - Technical guidance.
Testing or monitoring legionella in your water system
Where monitoring for legionella is considered appropriate, the sampling method should be carried out in accordance with British Standard BS7592 and the biocide, if used, neutralised where possible. Water samples should be tested by a UKAS-accredited laboratory that takes part in a water microbiology proficiency testing scheme such as that run by the UK Health Security Agency.
The laboratory should also apply a minimum theoretical mathematical detection limit of no more than 100 legionella bacteria per litre of sample for culture-based methods.
Interpreting legionella test results
Consider what the results mean in the context of your water system. Your subsequent specific actions will depend on your risk assessment.
Further information about what action to take when certain levels of legionella are identified can be found in Legionnaires' disease - Technical guidance:
- Part 1 for evaporative cooling systems
- Part 2 for hot and cold water systems
Appointing someone to test or monitor water for legionella
Testing of water quality may be carried out by a service provider, such as a water treatment company or consultant, or by the operator, if they are trained to do so and are properly supervised.
The type of test required will depend on the nature of the water of the system. Legionnaires' disease: Technical guidance provides further details for both evaporative cooling systems and hot and cold water systems.