How to Control Legionella in Firefighting Sprinkler Systems?
In this article the legionella specialists at Water Treatment Services consider the risks posed by Legionella bacteria in wet firefighting sprinkler systems, and how it can be controlled to keep people safe.
The article considers whether wet sprinkler systems pose a significant risk from legionella, common factors that can increase risk, how to identify them, and what measures can be taken to control the risks from legionella to within acceptable levels.
Is legionella a risk in wet firefighting sprinkler systems?
Any water system, whether hot or cold, can potentially pose a risk of Legionella bacteria being present. Indeed, it would be unusual to examine any such system and find no evidence of legionella there at all. However, in the UK and elsewhere there are safe levels that need to be maintained, to ensure the bacteria does not reach levels where it could colonise a water system and pose potential harm to those using it.
Some water sources are obvious – cooling towers, shower heads, taps, water features, hot tubs, ornamental fountains, and so on. Others, however, are less so. Would you think of firefighting sprinkler equipment, for example? Few would, but since this also involves water by its very nature, it must also be considered as a legionella risk factor.
Do fire sprinklers pose a significant risk of legionella?
Reassuringly, the answer is no. Certain water sources and systems pose a far greater risk than others. You’re far more likely to encounter Legionella bacteria by using a little used shower head or tap than you ever would be if you encountered a sprinkler system designed to put out fires.
Of course, while the risk is minimal – or certainly far less notable – in this situation, it is not altogether removed. However, it is crucial to retain perspective of the situation. This was stated in the RISCAuthority document, Recommendations for minimising the impact of legionella in firefighting systems.
Removing or coping with potential legionella hazards in sprinkler systems
The RISCAuthority document also makes it clear that you should do all you can to remove any potential legionella hazards from sprinkler systems if you can. For example, if there are redundant runs of pipework in the firefighting system, they’re best removed rather than trying to maintain and clean them. Any area where water could sit and become stagnant poses a far higher risk of allowing Legionella bacteria to spread, compared to an area where water regularly flows – or does not contain any water at all.
Having removed any redundant areas of pipework or equipment from the system (which may include tanks that are no longer in use), you’ll already have reduced the overall risk of Legionella bacteria spreading through the system. At this point, you should consider how best to maintain the firefighting system to reduce the risk still further.
Do wet fire suppression systems pose a significant legionella risk?
Fortunately, wet fire suppression systems don’t pose a significant risk. Research has shown that Legionella bacteria can be inhaled if present in water droplets of a certain size. However, water-based firefighting systems typically generate droplets that are far larger than those present in other sources. Currently, there have been no formally recorded cases of Legionnaires’ disease being caused by wet firefighting sprinkler equipment.
| Firefighting system/equipment | Average droplet size /micrometre |
|---|---|
| Sprinkler systems | >1000 |
| Deluge systems | >1000 |
| Water spray systems | 300-1000 |
| Watermist systems | 100-300 |
| Fixed foam systems | 300-1000 |
| Kitchen suppression systems | 150-300 |
| Fire hose reels | 300-1000 |
| Water-based portable fire extinguishers | 100-1000 |
| Water-based mobile fire extinguishers | 300-1000 |
Of course, there is still a need to maintain such equipment to make sure there is no legionella risk involved. The equipment itself should be maintained to avoid the chance of bacterial build-up when not in use. Moreover, the water source that supplies such equipment should be maintained and checked for safety. In some cases, the water could be taken from a public water supply. In contrast, some water sources could be private, which may involve different risk factors.
This, therefore, highlights that there are always two areas to consider when looking to control legionella risk from any firefighting equipment. Firstly, there is the installation itself, along with any associated pipework. Secondly, there is the water supply that feeds the installation.
In most cases, stagnant water represents the biggest risk factor for legionella growth. If a fire hose was situated outdoors and had some residual water left inside it, this could warm to a degree that could encourage bacterial growth. This would be more likely during the summer months, of course, if the hose was out in the sun and not properly emptied. Making sure hoses are fully emptied of water before being stored for future use could help reduce the risk posed by stagnant water.
| Component | Bacteria growth | Pipework & System | Water supply | ||||
|---|---|---|---|---|---|---|---|
| Property | Public mains | Elevated private reservoir | Gravity tank | Automatic tank | Pressure tank | ||
| Temperature range | 25-45 °C | Ambient | Ambient | Ambient | Ambient | Ambient | Ambient |
| Stagnant water | + | + | # | + | + | + | + |
| Contamination | |||||||
| Organic | + | # | # | ? | # | # | # |
| Iron | + | + | + | + | + | + | + |
| Oxygen | + | #++ | + | + | + | + | + |
| Light | + | # | # | ? | # | # | # |
| Bacterial growth | – | # | # | ? | # | # | # |
+ condition present
# condition not present
? condition possible
++ Deoxygenation from rust formation
What are the conditions legionella needs to grow and multiply in water systems?
Legionella bacteria requires certain conditions to stand a chance of multiplying to take over a water system or piece of equipment. It requires the temperature to be between 20-45 degrees Celsius. It’s possible that firefighting equipment and the water supply that feeds it could generate such temperatures. Since all scenarios stay at ambient temperature, it may depend on the time of year as to whether the equipment or water source would come within the range required for legionella to multiply. Furthermore, suitable nutrients and oxygen are also required – something else that could be present in the water supply, regardless of where it comes from.
How to minimise the presence of legionella in fire sprinkler systems?
When a wet firefighting system of any kind is installed, it is imperative to make sure proper consideration is paid to the presence of Legionella bacteria and its potential to grow. It is essential to make it as difficult as possible for the bacteria to colonise the system. We have seen that it is less likely to occur in this type of situation, but this does not mean you shouldn’t take precautions to protect those who may be exposed to the firefighting equipment, whether it is in use at any point or not.
Avoid redundant areas of pipework. Keep all pipework that is required to an absolute minimum. Furthermore, make sure there are no areas where water could be left to sit and stagnate. Make sure you do not use any materials that may degrade and allow for the formation of biofilms or the release of iron particles. Both these are sources of food for Legionella bacteria.
Legionella risk assessments should help you identify and remove potential risks
A full legionella risk assessment is designed to identify the possible risks that any firefighting sprinkler system may pose. Furthermore, it should help you understand the steps you should take to get the risk factors down to a minimum level. Be sure you are competent enough to undertake the legionella assessment; if not, hire an expert, such as Water Treatment Services to conduct it on your behalf.
In conclusion, we can see that the risks from legionella are indeed lower concerning wet firefighting equipment when compared to other water sources and outlets. However, we should not be complacent. By identifying the best actions to take and creating a plan to take those actions, the risks can drop still further. Such actions may include regular checks, annual testing or as required by the legionella risk assessment, and removal of unnecessary items of equipment or pipework.
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