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Legionella Water System Disinfections Explained


When it comes to stored water in the work place, it is vitally important that all systems are monitored frequently.

If there is a risk of / or a suspected bacterial contamination, a full system disinfection must take place as soon as possible - to eliminate the risk of employees and visitors contracting Legionnaires disease or any other bugs.

How is a system disinfection carried out?

A system clean is carried out across the entire water system, as opposed to just at a tank or one particular outlet. It can be done in two different ways, either a thermal disinfection, which only works on the hot water system, or chemical disinfection.

In thermal disinfections the water heaters are set to operate at a very high temperature, sufficient to kill legionella bacteria, and then all the hot water taps are opened slightly to draw hot water through the entire system.

In chemical disinfections a disinfectant, often chlorine dioxide, is introduced to the water system, the taps are then opened until the disinfectant has been run through to every outlet. The taps are then closed, and the water with the disinfectant in is left to sit in the pipework for a while, to let the chemical get to work killing any bacteria.

After enough time has passed, a neutralising agent is added, and the taps are opened again to purge the disinfectant.

How often should water systems be disinfected?

None of the official guidance documents recommend carrying out regular disinfections of water systems, though you may decide to do this as part of your individual monitoring on a biennial basis – to ensure maximum and continued water safety.

There are several situations however, where a one-off full system disinfection is recommended, these include:

  • Out of specification microbiological sample results
  • If there is a case of legionellosis associated with the water system
  • If the water system has been unused for an extended period and has not been flushed. (Two to three weeks is a good rule of thumb).
  • The installation of a new water system or new components, or significant refurbishment or maintenance of current systems.
  • If tank inspections show significant contamination or stagnation
  • When recommended by your legionella risk assessment

What impact will a system disinfection have on the business?

The impact of a disinfection will vary depending what type of disinfection is carried out. Thermal disinfections are a relatively easy process, it only requires that taps be left to run for five minutes. Chemical disinfections can be much more disruptive, as the outlets cannot be used while the disinfection is carried out, this will be for at least one hour, but may be longer.

TIP - If possible, it is best to carry out chemical disinfections at times when your water system is unlikely to be used, such as early mornings, in the evening or at the weekend in order to minimise disruption to the workforce and business productivity.

Who should carry out a system disinfection?

As with all water treatment tasks, the person carrying out the disinfection should be competent and suitably trained for the task.

One easy way to help work out if the contractor you are using is competent is to see if their company is a member of the LCA (Legionella Control Association) as they conduct audits of procedures and training that their members provide their staff.

You can also directly ask the contractor to provide evidence of competence for their staff members.

Once a disinfection has been carried out, a certificate of disinfection should be provided, this will include information such as when the disinfection was carried out, and the concentration of disinfectant used.

How Euro Environmental can help

We are members of the LCA and can carry out Legionella testing or system disinfections for your organisation. We offer these service on a one-off basis or if you add regular disinfections to your health and safety policy – we can work on a contractual basis offering updates of when systems require a clean.

For more information contact the team on: 0870 7019170 or contact us via the contact us page on the website. (Link -


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