Legionella and the Unholy Trinity

The three factors that contribute to legionella growth

The built water environment (BWE) is ideally suited to opportunist microbial colonisation. If bacteria and protozoa could make choices then potable water systems would be near the top of their bucket list.

This presents an on-going challenge for health care facilities. Despite a global consensus that Legionella infections in health care are preventable; they persist. Perhaps we could blame some of that on our changing demographics and increasing susceptible population. Perhaps we could also accept that we could do things better.

The simplest strategy in controlling Legionella in a BWE is to keep water moving. The movement of water reduces stagnation improves thermal control and introduces whatever disinfection is available. Unfortunately, other imperatives make water movement and temperature control a major challenge. Flow restriction (obstruction), overly complex systems (stagnation) and poor temperature control (fermentation) are often design features of buildings. I know I’ve stretched it a bit using ‘fermentation’ – but it went well with the other ‘tions’. (More on that later).

These issues are magnified by the current ‘lock-downs’ in Health and Aged Care thanks to COVID. Re-commissioning of locked down facilities needs serious consideration of the impacts of Obstruction, Stagnation and Fermentation. So let’s have a look at the data relating to these three critical factors and present some possible remedies.

Some Assumptions

Before we go any further let’s lay some foundations about your system:

Statistically it is safe to assume your system is colonised by Legionella to some degree.
The system will contain hot, warm and cold water.
Hot, warm and cold supplies are all documented sources of disease.
Eradication of Legionella is not possible, it must be controlled

Not a great starting point, but the control in that last bullet point regarding is achievable if we understand the Unholy Trinity within our facility.

Obstruction

Blocking the flow of water through your building may be intentional or unintentional. Either way it is going to slow down water causing stagnation and leading to loss of temperature control (fermentation).

Intentional blockages include aerators on taps and flow restrictors on showers. Not only do they hold back water they also act as sieves that trap debris (see below - your guess is as good as ours how a fingernail got in there!). Soon, if left to their own devices (pun intended) they are reservoirs for biofilm and Legionella and others.

Legionella will colonise contaminated aerators

Unintentional blockages are the blind ends and dead legs that have become clichés in water treatment circles. Blind ends are usually the result of changed water usage and pipes being ‘capped off’. Dead legs are, in many instances, necessary evils in recirculating water systems. Branches from the loop are all dead legs if they don’t get flushed.

A register of dead legs (locations where water doesn't get used much) is a good idea. Also consider whether the outlet in question is really necessary?

Stagnation

Apart from the dead legs that are always with us there are other causes of stagnation specifically design and configuration. Often water systems are ‘organic’ – that is they grow and branch. In response to changes in building use or building extensions the water distribution easily become stagnant in some places. Often this is caused by sub-ordinate loops. A subordinate loop is a loop off a loop. The pressure at each end of the loop is the same so the water has nowhere to go and stagnates.

Have look at the drawing below. The little boxes are reasonably self explanatory. Please note this not just a subordinate loop. The larger loop also acts as a subordinate because the water will short circuit through the smaller loop. This is a hydrualic drawing of a water system in a hospital that we recently reviewed.

Subordinate loops create dead legs for Legionella colonisation

Water testing at points in the system showed low warm water temperatures, warm cold water temperatuires, and undetectable chlorine residuals from a continuous chlorination system. Before you jump at chlorine as the problem here: the result would be the same for any disinfectant you chose. Remember disinfection is at the bottom of the Hierarchy of Control we're somewhere higher up the ladder in this post.

The problem with sub-ordinate loops is that unless someone turns on a tap on there is no physical reason for the water to move. Once movement stops stagnation starts and temperature control is lost.

Don't forget that once stagnation occurs it will begin to cause obstruction. Excessive biofilm formation will reduce pipe diameters, clog strainers, and fill aerators and shower roses. Of course the reverse is also true obstruction will create stagnation - they are intimately linked and both will cause fermentation.

Fermentation

Loss of temperature control is arguably the most critical member of the Trinity. In itself loss of temperature control will not contribute to the other two problems. However, it is the inevitable consequence of the presence of either of the other two. But it can occur when the other two are absent.

In situations where the cold supply is not very cold, or the water heating plant is not up to the job then temperature control can be an isolated problem. Temperature checks of sentinel hot, warm and cold outlets will soon let you know if there’s a problem. The sub-ordinate loop above is a case in point. The temperature checks identified that there was a problem with stagnation.

There is evidence that Legionella will persist outside the 20-50°C window. More compelling evidence is that Legionella, other bacteria, and protozoa are at their happiest inside this window. Keeping to the lower and upper limits of the window is one step in the heirarchy of control - making the system as inhospitable as possible.

Shun the ‘tions’...

.....a handy little catch phrase to deal with the Unholy trinity! And a few handy tips to do this are:

have accurate hydraulic drawings and make sure you understand them.
use temperature monitoring to verify the system is behaving itself.
avoid intentional obstructions - remove aerators or replace them with less obstructive laminar flow devices.
find and remove unintentional obstructions - cut blind ends back as far as possible.
lag pipes - hot and cold (but not together - as we have sometimes seen!)

Finally, get a Water Safety Risk Management Plan and implement it. The plan should tell you what's going on in your system and how to monitor. It should also give you a strategy to maintain control.

About Us

All of the areas mentioned above are what we specialize in. Our business delivers industry leading processes, systems, training and support as well as market leading disinfection products that will enable you to manage and reduce risk of waterborne infection. We are an ISO accredited business and hold Systems, Safety and Environmental certification, please feel free to contact us if you would like to talk more.