CPD PROGRAMME
immediately following periods of shutdown – maybe after lack of use in winter and spring – where poorly treated systems can accumulate significant Legionella concentrations. Ultraviolet (UV) disinfection and, of
course, pasteurisation – where the water is raised to a high temperature for a set period of time – are successfully used for bacterial control in water services. However, UV and pasteurisation provide localized treatment that will not affect downstream biofilms and, if not continuously applied throughout the system, allow the regrowth of Legionella. Through appropriate design of systems such as reducing dead legs, supplying mains water direct to water outlets, keeping connecting pipework lengths to a minimum, and keeping water storage tanks accessible for cleaning but otherwise sealed, the opportunities for Legionella multiplication may be substantially reduced. Since it is the inhalation of the bacteria within water aerosols that carries the principal risk, that risk can be reduced by not purposely creating aerosols, and by allowing drain down of components when not in use to remove the opportunity for water dead legs. Legionella is not seen as a particular risk
in direct cold water supplies that are under 20°C in operation, or for hot water supplies that are stored at 60°C and distributed to the outlet at 50°C.
Safety standards The various codes and standards require that there should be an appropriate water safety plan. The World Health Organization
100 80 60 HWS tap outlets 40
Cold water services Fire sprinklers
20 Spray humidifier 0
Mains cold water and air cooling coil condensate
Increasing risk of multiplication of Legionella Figure 5: Water temperatures and that viability of Legionella (Source: CIBSE TM13, 2002)
www.cibsejournal.com
Legionella will remain dormant
Steam humidification LTHW heating system
Hot water service storage
Spas, showers Cooling towers
No viable Legionella
Legionella will not multiply and will die in time
Legionella will multiply
(WHO)[5] splits this into three key elements:
System assessment – This includes undertaking and recording system survey to determine such things as whether Legionella bacteria is likely to be present, level of risk, whether conditions prevail to promote growth, and so on; Monitoring – This includes setting up
and monitoring control regimes to maintain appropriately safe conditions; and Management and communication –
This includes defining responsibility; documenting system assessment and monitoring; ensuring appropriately trained people; developing action plans; and planning communication requirements, both internally and to external bodies. In the case of an outbreak, the potential
source should be identified and isolated. This would normally be as a result of epidemiological information gathered by the local health authority. Isolation could be as simple as switching off a cooling tower or a fountain, or as showers being put out of use prior to sampling and subsequent disinfection. Where cooling towers are the suspected source, it is not unusual to undertake emergency treatment of water in all towers in the vicinity. Microbiological testing can determine the subtype of Legionella from infected people, and this information is used to aid identification of the specific source of the outbreak. With the increase in ‘novel’ low-energy solutions for buildings, care must be taken to avoid potential conditions where Legionella can readily multiply. For example, evaporative coolers used on the inlet to air conditioning systems have become
very popular for applications such as data centres. Although there have been no recorded cases of Legionella resulting from such installations, and with proper design and maintenance there should not be, it is important to maintain systems to control the accumulation of sediment that can provide a breeding ground for bacteria. Although the number of Legionnaires’
disease outbreaks are relatively infrequent, they are potentially life threatening and normally avoidable. By following the readily available guidance given in the documents listed below, and conforming with local codes and standards, the risk may be practically reduced so that outbreaks become not just infrequent but a rarity. © Tim Dwyer 2012
• Thanks for guidance from Geoff Brundrett and to Lew Harriman of
Mason-Grant Consulting.
Further reading: There are several documents and websites (most freely available) that provide guidance on the management processes and technical solutions:
1. CIBSE TM 13:2002 Minimizing the risk of Legionnaires’ disease. Although ten years old, this guide still maintains its position as one of the leading guidance documents for building services engineers that contains practical measures (freely downloadable to CIBSE members through the CIBSE Knowledge Portal).
2. The HSE website
www.hse.gov.uk/legionnaires has extensive information, including a link to approved COP L8 (below).
3. Health and safety executive (HSE), Legionnaires’ disease the control of Legionella bacteria in water systems approved COP L8 aims to ensure that responsible people have the information needed to comply with legal duties. It provides extensive coverage on management requirements and the control of Legionella.
4. European Guidelines for Control and Prevention of Travel Associated Legionnaires' Disease, released in November 2011, is one of the most recently published guidelines and draws on a wealth of international information. See
http://ecdc.europa. eu/en/activities/surveillance/ELDSNet/Documents/ EWGLI-Technical-Guidelines.pdf
5. Legionella prevention of legionellosis, published by World Health Organisation (WHO) in 2007, is freely downloadable from the web and provides an excellent and comprehensive reference.
6. The US Centre for Disease Control and Prevention has a great web resource covering Legionellosis freely available at
www.cdc.gov/legionella/index.htm
7. ASHRAE Standard 188P, Prevention of Legionellosis Associated with Building Water Systems. This ASHRAE standard is currently available as a downloadable review copy, and plans are to release it formally later this year. It has changed markedly from previous editions in order to establish a clear management framework and process to reduce risks.
References
A. Vidic, R., et al, Biological Control in Cooling Water Systems Using Non-Chemical Treatment Devices, ASHRAE Project Number 1361-RP, April 2010.
July 2012 CIBSE Journal
51
Temperature (o
C)
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