How Does Pipework Size Influence Legionella Growth in Building Water Systems?
In this article, the water safety experts at Legionella Control International look at the role and importance of good building services design, and how, if it’s done correctly it can influence the control of legionella and other waterborne pathogens in building plumbing systems long after practical completion and building hand over have taken place.
The article asks if legionella can grow in water pipes, how it does it and how long it takes. It then moves on to look at water design considerations, asking how pipe sizing, flow rates and occupier demand influences the growth of legionella and what can be done to control it.
A version of this article looking at the importance of water pipe sizing and how it can influence the growth of legionella in building water systems first appeared in Legionella Control International’s newsletter. To get it in your inbox, sign up for free here.
Calculating the size and volume of building water systems
There’s plenty to read about legionella and how if it’s left un-controlled it can adversely impact hot and cold water systems and the safety of the people using them. The sizing of pipework is far less frequently mentioned though, perhaps because this forms part of the engineering design and building services installation process. When a new plumbing system is ready for installation, the water storage tanks should be sized correctly, and the distribution pipes should be of the correct length and diameter if the system is going to work properly. Making an error at this stage could potentially lead to issues with legionella and other waterborne pathogens later on.
The larger the pipes, the more water can flow through them. However, this isn’t always the best approach to take. Most domestic water systems will be installed according to what is already there in the case of a simple replacement. This article will delve into the sizing of pipework in more detail, to consider how it might affect the growth of legionella bacteria.
Can legionella grow in water pipes?
If given the opportunity, yes, it can. Hence why it is vital to take all possible measures to prevent – or at least limit – this from happening.
Dead ends or dead legs of pipework, i.e., runs that are no longer used, should always be removed where possible. Legionella likes to grow in stagnant water, which will collect in such dead legs. Additionally, pipework that is only occasionally used should be regularly flushed through to keep the water moving and remove any bacteria that might be lurking there.
How long does it take for legionella to grow in pipes?
How quickly legionella grows in pipes depends on the conditions, but in some instances, it can multiply to the extent that it becomes an issue within just two weeks. Often it can take longer, but once it takes hold, it can be difficult to get under control. Hence why prevention is the best tactic.
How does pipe size affect legionella growth?
Firstly, it’s important to consider flow rate, i.e., the rate at which water flows through the pipe. If the pipe size is too large for the application, the flow rate of water through it will drop. For example, running a tap should fill the relevant pipes with water. This ensures the entire pipe is flushed through. If the pipe is too large, the flow of water will run through the bottom of the pipe instead of filling it. This means the upper part of the pipe isn’t touched by the flow of water.
This in turn increases the risk that legionella will get a foothold in the water system. The inside of the pipe will obviously be wet, and with water not flushing the entire pipe through, it can allow for the development of biofilms (slime) along the upper section of the pipework. This gives legionella and other waterborne bacteria additional protection and something to feed on. In turn, it will allow the bacteria to multiply and begin to spread throughout the water system if given the chance.
Furthermore, even if a system is dosed with special chemicals such as chlorine dioxide or silver stabilised hydrogen peroxide, to combat legionella, the chemicals won’t reach all parts of the pipe either. It is possible that biofilms attached to the upper part of the pipe remain untouched by whatever is used to disinfect the system.
In short, then, the combination of pipe size and water flow rate must be calculated in advance to ensure both work together to prevent legionella from becoming an issue.
How does water demand affect pipework size?
Domestic properties are rarely affected in this manner because they are simple installations. Legionella is not thought to be a major risk to domestic properties. The rule of thumb if upgrading a plumbing system is generally to install a new system based on what was there before.
However, the larger the building, the more complex it is, and the more involved the water systems will be. It’s therefore more important to follow the guidance given in documents such as BS 8558:2015, a guide to the design, installation, testing and maintenance of services supplying water for domestic use. It gives UK guidance for the water supply services industry on all aspects of sampling and monitoring of hot and cold water. It also gives guidance on underground pipework within the curtilage of a building. The water systems in buildings might be sufficient and straightforward, but calculating the size of the overall water system will obviously be more challenging.
Similar considerations should also be borne in mind for commercial premises and other non-residential premises with necessarily large water systems. The pipework must be sufficient to carry the flow to meet peak demand, while still protecting the pipes as much as possible. The target is always to cater for those using the water system, without generating the conditions that would allow legionella and similar bacteria to develop.
Is legionella a bigger risk when the water flow rate is lower?
Yes, lower water flow rates can encourage the growth of legionella. We’ve already seen how stagnant water can encourage legionella to establish itself within a system (or part of it) and multiply there. Similarly, a lower flow rate makes it more likely that biofilms will be able to attach and develop to the pipe walls. A larger pipe will have the same effect, since the flow will not fill the pipe, as seen earlier.
Calculating demand on the water supply is another element to think about. It wouldn’t be sensible or economical to make sure the water system would be able to accommodate the maximum amount of demand that might be placed on it 24 hours per day. This is only rarely likely to occur. However, the system and associated pipework should be adequate enough to provide ample water supplies to all those who need them. It’s a fine balance – and one that must consider the presence of waterborne bacteria.
In conclusion: The design of any water system is a key factor in controlling legionella and other waterborne pathogens
Most domestic water systems will look much the same. What works for one property will likely work for another. However, we can see that there are risk factors connected to water storage tanks and pipe sizing that can appear with every system. That’s why the building services design stage of a building is so important. The building services engineers must consider who is using the water system, the demands that will be placed on it, and how those demands can influence the flow of water (or stagnation) through the available tanks and pipes. Getting it spot on at this stage will play a huge role in reducing the legionella risk from the start – even though care must still be taken to address it.
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