A multi-barrier system for Legionella control is recommended by the World Health Organisation and many regulatory and advisory bodies. So what does one look like? An easy way to explain the principles is to consider how to keep your trousers up! Most people are keen to keep their pants on. So what strategies do you need to achieve this goal? Furthermore what has this to do with multi-barrier systems?

Multi-barrier System

In building water systems there are multiple points at which a plumbing or engineering intervention can be used to control microbial contaminants, for example aerators. These points are the ‘barriers’. If we have control at a number of barriers then it is a multi-barrier system. If there are controls at the barriers then we control the consequences. Frequently the consequences get misunderstood as barriers in themselves. This is not actually true. Things like stagnant water, loss of temperature control and nutrient build up are not directly controllable. So they are not barriers. Rather, they are the consequences of loss of control in the system. So let’s use the ‘trouser model’ to explain this in more detail.


Keeping your trousers up!

There are a number of things to consider if you’re planning to avoid a trouser failure consequence. Not looking down and hoping it won’t happen is not one of them. Pulling them up quickly and hoping no one noticed is probably not advisable! Something more pro-active is a better way to go. Most of the steps you can take are pretty obvious. For the sake of this example we will go into them in detail. The first step in trouser management is good design.

Good design

A well designed pair of trousers is a very good start. Trousers that fit well, and perhaps have room for a little expansion are going to make things much easier. Having to let trousers out or adjust them after purchase adds to the cost and never looks quite as good. Everyone likes a bargain – but you only get what you pay for. The same is true of a building water system. Good building system design that is fit for purpose and has capacity for expansion is an ideal starting point.

  Identifying the barriers

Once you’ve bought your trousers there are few options for change. So what barriers are there, or can be built into them? There are four or maybe five barriers you can use. Installing a trouser system with a zip and button, elastic waist band, belt and braces (suspenders in the US!). These barriers are all individually capable of doing the job. You may choose to only use one or two options. Or depending on how averse you are to a trouser falling incident may wish to use them all. The more barriers you put in place the less chance of the adverse incident. Keep in mind that more barriers means more maintenance. A multi-barrier system that is simple enough to let you get out of the front door in the mornings is pretty important.

Let’s assume you have installed all the barriers what advantage does it give you? Simply put, if one barrier fails, for instance a button pops off, the other barriers will keep the trousers up until you can fix the button. There’s no need for a costly panic. There’s no need to stop what you’re doing. So a multi-barrier system allows you to continue normal operations if a barrier fails and still avoid negative consequences. Of course you have to notice that the button has popped off to take remedial action. This means you must commit to maintenance.

Maintaining the barriers

Maintaining barriers is an essential part of the system. Checking the buttons are not loose and zip closes, testing the elastic waist-band, buckling your belt and adjusting your braces are an essential part of your routine. Routinely checking for wear and tear on these barriers becomes quite easy. What is more important is it gives you peace of mind. You won’t be constantly looking down – or avoiding looking down! This is exactly the principle behind routine maintenance and monitoring as part of a Water Safety and Risk Management Plan. A periodic review that all is operating to plan is then all you need to hold your head high!


The word ‘verification’ (some use ‘validation’) seems to confuse people as it relates to Legionella. Using the trouser scenario it simply means looking down occasionally to check things are in place, or taking a look in the mirror. Sometimes you may want to get an outside opinion and ask the dreaded question ‘how do I look?’. Applying this to building water systems using NATA accredited or nationally accredited testing services is more likely to get you an honest answer. Asking a friend or neighbour is likely to get a more ‘diplomatic’ response! As always these tests are ‘snapshots’ just like looking in the mirror. They do not solve any problems but reassure you that what you have in place is working.

“Verification monitoring
Verification monitoring involves taking samples that are tested for specific parameters, usually at an analytical laboratory accredited for the analytical method being used. As a result, analytical results of samples taken as part of verification monitoring are often obtained more than 24 hours after the sample is taken (up to 10 days in the case of Legionella culture testing). Verification monitoring is used to confirm the quality of the water supply and determine whether the existing control measures are effective. Corrective actions can be undertaken as a result of adverse results; however, the corrective actions need to take into consideration the time lag between sampling and reporting of results.” – enHealth Guidelines for Legionella Control 2015

Building Water Systems

How do these principles apply to building water systems? Well, the goal is to minimise microbial growth by avoiding stagnation, controlling temperature, limiting nutrients and effective disinfection. Understanding system design is the first step in achieving these goals. Next identify barriers where you can apply practical and cost effective controls. Work out a routine maintenance plan. Finally establish a verification program to ensure what you’re doing is working.

Obviously this is not a simple process. An effective multi-barrier system will require qualified individuals in multiple disciplines to implement an effective water safety and risk management plan. It will also require a reality check. A top notch plan that either you can’t afford or can’t implement is not much use at all. However, understanding what a multi-barrier system is a first step in successful management of building water systems. The ultimate goal is protecting public health and a consequence of that is peace of mind.

Further Reading

Canadian Government (2002) From Source to Tap: The multi-barrier approach to safe drinking water

EnHealth (2015) Guidelines Legionella Control.

WHO (2005), Water Safety Plans