Engineering consultant, Clive Broadbent, provides risk management guidelines relating to the prevention of Legionnaires' disease.

Legionnaires’ disease is an affliction that may always be with us while we have a microorganism that is widespread in the natural aquatic environment and, unsurprisingly, also in the built environment.

In Australia we are fortunate to have a highly informed medical community to handle human treatments and we have in place excellent standards and guidelines for building water systems to assist in precluding Legionella.

Facility management concerns about the hazards presented by Legionella have led to the use of risk management methodologies in our bailiwick of Legionella control measures associated with the building stock. Initially, the standard in use has been, and still is, AS/NZS 3666.

This standard was initially introduced to handle the range of microbial hazards at buildings. It was first introduced in 1987 and has been incorporated in many regulations since about 1989.

In 1998, a performance-based alternative was introduced to the suite of parts to this standard and is concerned primarily with cooling water systems. Known as AS/NZS 3666.3 – Air-handling and water systems of buildings – Microbial control – Part 3: Performance-based maintenance of cooling water systems, this standard has provided a practical means of managing the hazards presented by these systems.

It has been modified several times since its inception, although arguably only at the edges, and remains essentially the same document as originally conceived with its requirements well proven. Any person who has responsibility for the design, installation, operation and maintenance of open water systems should study and observe the statements specified in this document.

One aspect of the standard that is of special interest is the list of system risk features requiring evaluation (Table 2.1 in the standard). These include aspects such as presence of dead legs, water cleanliness, physical condition of the plant and need for safe access at the plant.

There are five overall risk categories that act as headings for the risk factors involved. These are:

Stagnant water
Nutrient availability
Poor water quality
Deficiencies in the cooling tower system
Location and access to cooling towers

A number of the risk factors are described as Key Performance Indicators. These mostly comprise readily measurable parameters such as conductivity of the circulating water, concentration of bacteria, water pH and water temperature.

To differentiate the KPI indicators from others provides one method of addressing relative risk (the adage: what can be measured can be managed).

It also enables priorities to be set in programs for remedial measures that may require funding. The standard also sets out control actions for the presence of Legionella and/or excessively high general bacterial counts, thereby providing a risk evaluation response sequence albeit one that is necessary after the event. Prevention is always preferable to cure, however.

Other risk hierarchy methodologies are available according to the jurisdiction in which the plant is located.

One approach to the grading of risk levels is as follows:

High risk – may directly encourage growths or allow transmission of pathogenic organisms, e.g., evidence of significant dead legs;

Medium risk – indirectly compromises the ability of a system to inherently discourage bacterial growths, e.g., poor corrosion control;

Low risk – what is achievable in practice by close attention to the risk factors described in Table 2.1 in AS/NZS 3666.3.

These risk levels could well be useful for water-based systems other than cooling towers and including decorative fountains, spa pools, aeration ponds, warm water systems in aged care homes, car washing plant and the like, all of which have been associated with outbreaks of Legionnaires’ disease.

The risk from the microbial hazards associated with any water system can never be zero but can readily be controlled. An understanding of the factors causing growth of Legionella, and implementation of measures shown to control such microbes, is needed and is facilitated by use of a sound risk assessment approach.

About the author

Clive Broadbent AM, FIEAust, FAIRAH, FASHRAE is a Canberra-based engineering consultant who has written extensively on research findings and control measures for Legionella.


comments powered by Disqus