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Guidance on the principles of Legionella risk assessment in industrial process water systems


This paper is part of a series of guidance documents produced by the Water Management Society for the control of Legionella in ‘Other Risk’ systems.


In order to help its members, The Water Management Society (WMSoc) has undertaken to draw together people with expertise in the subject and those with knowledge in the control of Legionellae, as a working group. The group has been charged with the task of outlining the principals of the control of Legionella in these systems. The controls will be risk assessment led and, due to the large variation in types and complexity of systems in industry, this document will attempt to clarify the principles of Legionella risk assessment of them. The Society recognises that not all of the many types of process water systems will have been mentioned in this paper.


However, the general principles discussed will be applicable, even though a particular process or system has not been named.


Micro-organisms including potential human pathogens (a pathogen is a micro-organism which can cause infections) such as Legionellae will grow in any water system where:

  • The temperature of the water is likely to be between 20oC and 45oC in any part of that system.

  • There are sediments, biofilms (or slimes) sludge or other nutrients, which will support microbial survival and growth, as will the presence of scale deposit and/or corrosion in the system.

  • There are stagnant areas, for example where water is held in the system for long periods, or there are areas of low flow.

  • If the system can produce a spray or aerosol at any time during normal operation or when being maintained there is an increased risk of susceptible persons breathing that aerosol and contracting Legionnaires’ disease.

There are many types of process water system in use. These include:

  • Machine washers.

  • Cutting fluid systems.

  • Parts washing.

  • Paint preparation.

  • Paint spray booths.

  • Leak testing.

  • Body washing.

  • Water jet cutters.

The industries in which process water systems are found include:

  • Manufacturing and Engineering.

  • Chemicals.

  • Oil.

  • Pharmaceuticals.

  • Glass.

  • Textiles.

  • Plastics.

  • Steel Manufacture.

  • Food, dairy and brewing.

  • Mineral extraction.

Before commencing work on a Legionella risk assessment it is critical that the scope of the assessment is properly defined, alongside a number of other considerations; these are explored in the paper.


Water Management Society February 2023


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Guidance on the Principles of Legionella Risk Assessment in Industrial Process Water Syste
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