Every now and then there’s a headline-grabbing outbreak of Legionnaires’ disease, usually involving fatalities and serious illness in a public building. Thankfully, these tragic events are rare. However, Legionnaires’ disease, and the incidence of the Legionella bacteria that causes it, are much more common than many might think.
The reality is that we simply do not know how many cases of Legionnaires’ disease occur each year in the UK because, in healthy people with robust respiratory systems, it often simply manifests with cold or flu-like symptoms and goes undiagnosed, unrecorded and unmonitored. For the very young and very old, those with pre-existing conditions, compromised health or respiratory conditions, however, Legionnaires’ disease is far from a short-lived and relatively innocuous bug; it’s a killer.
And it’s a killer with an extraordinary ability to breed exceptionally quickly. Legionella bacteria can breed in any stagnant water within the 25 to 45ºC temperature range, doubling in number every eight hours. While stomach acids will destroy any ingested bacteria, water vapour from a flushing toilet, a shower or a running tap can enter the respiratory system and cause Legionnaires’ disease.
The healthcare sector is well aware of the risks of Legionella and hospitals put strict flushing regimes in place for minimising the risk of stagnant water in the pipework in accordance with HTM04 requirements. Taps and sentinel points must be regularly checked using thermal probes and temperatures must be recorded and kept to demonstrate that water is reaching 50°C in order to control growth of Legionella. However, such regimes are vulnerable to human error, and it is vital to fully understand the risk factors and how these can be engineered out of the plumbing installation through improved design and specification.
Understanding the risks
While ensuring that hot water temperatures exceed the risk zone for Legionella reduces the likelihood of bacteria growth, if the design and layout of the pipework allows stagnant water to collect; there remains a risk that hot water will cool to between 25 and 45ºC – the temperature at which Legionella bacteria is most prolific, unless it is properly and regularly flushed from the system.
What’s more, if a biofilm has formed within the pipework, this tacky, chemical treatment-resistant coating will enable the Legionella to become embedded on the surface. Similarly, in locations where there is a high calcium content in the water supply, calcium deposits that build up on metal-surfaced pipework can provide a perfect environment for harbouring Legionella.
Any areas of the system that are not in regular use are at highest risk of collecting stagnant water and creating the conditions where Legionella can breed. In a hospital environment, this could include bathrooms at the end of corridors, in outpatient units or areas of the hospital that have been closed to patients for refurbishment, for example. There is also a risk that danger zones could be built into the pipework system as part of a remodelling project that involves changes or additions to plumbing services. In this scenario, changes to the configuration of the pipework could result in ‘dead-ends’ (redundant sections of the pipework that remain in place despite the fact that they are capped off so water no longer flushes through), or ‘dead-legs’ (places on the end of the pipework network that are seldom used), both of which are at higher risk of stagnant water and Legionella growth.
Better by design
For any hospital building services refurbishment the first consideration should be specification of the pipe to reduce the risk of Legionella growth on the network. By switching from traditional copper piping to MLC (multi-layer composite) pipe or PE-X polyethylene pipe, the specifier can eliminate the risk of calcification and ensure a smoother internal surface to enable better water flow on the network. Some common joint methods on copper piping can provide small crevices that can harbour bio-films and bacterial growth, whereas MLC and PE-X pipe systems that seal on the inside bore provide a smooth marriage of the pipe and fitting and a more hygienic connection. This simple change in specification can remove some of the conditions that support Legionella growth.
The configuration of the pipework is also essential as this can ensure that dead-ends and dead-legs on the system are eliminated, even if the installation involves parts of the network that will not be in daily usage. The traditional ‘tee’ installation uses fewer lengths of pipe and more tee connections to direct water flow specifically and only to the water outlet activated by the user. This approach can help to maximise water pressure, improve speed of delivery of hot water to the point of use and reduce water consumption but it also allows potential dead-legs on the system by failing to flush water through any pipework for services that are not in use. As a result, a tee configuration can potentially allow stagnant water to accumulate, creating a risk of Legionella on the network.
Conversely, a ‘serial’ installation can avoid dead-legs on the network provided that the most frequently used water outlet – usually the toilet in a standard a bathroom – is placed as the final item on the network. Where this is the case, water regularly flushes through the pipe to reach the frequently used service, flushing away the potential for stagnant water and the Legionella it might harbour in the process. From a cost perspective, it may appear less attractive than a tee pipework design but, while it uses more pipe, it requires fewer connections, making it faster and less expensive to install as well as reducing Legionella risk.
The most effective pipework configuration of all for avoiding stagnant water on the system and mitigating Legionella risk is a loop installation. Used commonly in Scandinavia and Germany, where Legionnaires’ disease is more closely monitored, a loop installation connects a small set of services and flushes water through the whole local pipe network every time any water outlet on the system is activated, flushing all pipes regardless of where the water is used. It requires fewer connections and fittings and enables excellent water pressure and speed of hot water delivery whilst completely eliminating dead-legs.
Legionella will always remain a risk that requires vigilance and monitoring. However, with best practice management regimes and improved installation design and specification, the risk can be significantly reduced.