Assessing Legionella Risks - Swan-Necks & Pigtails
The use of swan-necks, a "U" shaped pipework bend, and pigtails a "spiral" shaped bend in hot and cold water systems is often subject to much debate.
Legionella Control International's position on the issue is that they should not be used in hot and cold water systems as they have the potential to increase the risks associated with Legionella.
However, recently one of our large healthcare clients asked us to clarify our position so we turned to leading Legionella specialist Dr. Tom Makin for expert comment.
"Dead-legs of any length represent a risk if the water temperature is conducive to microbial growth ..."
Dr. Makin is co-author of the Department of Health’s “Health Technical Memorandum” HTM 04-01 dealing with Legionella control in healthcare premises; and co-author of the Health & Safety Executives “Approved Code of Practice and Guidance” for the control of Legionella bacteria in water systems L8 (revision 4).
Swan-necks and pigtails are not considered to be a significant source of legionellae if the system water temperature is always maintained at 60oC.
Problems arise when this temperature is not achieved and it falls below 50oC, the lower thermal death point for Legionella.
Many argue that swan-necks and pigtails should not be regarded as a significant source of legionellae because they are usually considerably shorter than the 2m length allowed in HTM04-01 for pipe connections to hot and cold water outlets.
Dead-legs of any length represent a risk if the water temperature is conducive to microbial growth.
The 2m length quoted in HTM04 refers to the maximum permissible length of a deadleg in blended hot and cold water outlets.
Clearly, there is no option but to have a small section of pipe from blended hot and cold water supplies to the outlet.
The length of this section of pipe can vary and may be very short e.g. where the thermostatic mixing valve is incorporated as an integral part of the blended outlet; however it should not exceed 2m in length.
The risk in such situations is mitigated because at some point these sections of pipe should have water flowing through them; and at least twice weekly in healthcare premises, if compliant with HTM04-01.
The regular use of outlets helps prevent the proliferation and accumulation of legionellae both in the outlet and associated sections of pipe.
Swan-Necks & Pigtails
In most cases there is no flow of water through swan-necks and pigtails, therefore there is no opportunity to discharge and dilute any accumulating biofilm.
One option to reduce Legionella related risks in these devices is to fit valves that would allow them to be flushed to waste at a suitable frequency.
However, this additional responsibility is often unpopular with users, particularly if there are alternative systems available that do not require flushing.
I understand there are suitable alternatives to pigtails and swan-necks in which the volume of retained water is considerably reduced.
It has been suggested that the material from which swan-necks and pigtails are constructed (copper or brass) is sufficiently antibacterial to prevent the accumulation of biofilm within the device.
It is recognised that heavy metals, particularly copper and probably also brass (although I have seen no peer reviewed evidence confirming the efficacy of the latter in controlling biofilm), have antibacterial properties and so can delay the accretion of biofilm.
However, passivity eventually occurs due to a fine coating of inorganic deposits and scale build-up on the internal surface, which can afford protection to microorganisms that alight on the surface, so enabling them to form into biofilm.
Over time, even copper and probably brass surfaces will support biofilm, particularly if exposed to warm and stagnant water as indeed many pigtails and swan necks are.
A recent study carried out by Dr Mathys and colleagues from Munster University, Germany, showed that plumbing systems with copper pipes were more frequently contaminated than those made of synthetic materials or galvanized steel.
Other studies have shown copper to be better than plastics and other materials at resisting the development of microbial colonisation in aquatic systems.
Perhaps certain types of copper are more susceptible to accretion of biofilm than others, but all at some point will succumb to microbial colonisation.
In each case, where swan-necks and pigtails have been installed it will be necessary to assess the risk of Legionella proliferation in those devices.
If a Legionella risk is shown to exist, then it should be controlled, where it is reasonably practicable to do so.
Both manufacturers and installers of these devices have responsibilities, as highlighted in the HSE’s Approved Code of Practice and Guidance (L8), with regard to helping to control the risk of Legionella infection linked to such devices.
Werner Mathys, et al. 2008. International Journal of Hygiene and Environmental Health. Volume 211, issue 1-2, pages 179-185. Occurrence of Legionella in hot water systems of single-family residences in suburbs of two German cities with special reference to solar and district heating.
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