WATER SYSTEM SAFETY
Deposits in aerators in two hospitals with continuous disinfection.
managing risk get us around this problem. Risk-based strategies assess systems to identify problems. These assessments are made using established evidence to set control targets and determine a best practice. From there a risk management plan can be developed that not only meets compliance requirements, but equally goes beyond them to meet the specific health targets of a given facility or application.
A critical step in developing a risk- based control strategy is identifying and assessing the control measures that must be addressed. In the case of Legionella, the Unholy Trinity mentioned above embodies the three overarching problems that must be considered. The following sections give an overview of these three factors and how they interact to cause us problems.
Obstruction
Obstruction creates reservoirs for Legionella where sediments and nutrients collect and nurture biofilms. There has been a historical intentional incorporation of flow restriction and obstruction into building design. Flow restrictors, balancing valves, diffusers, and aerators, have been used to conserve water. The new emphasis on green buildings and water economy has not improved things. Deliberate obstruction of water flows has meant that residence times of water in ‘green’ buildings can be triple those of other buildings. This is good news environmentally in terms of water conservation, but bad news for Legionella control.
Of course, there are also obstructions that occur in the design of any large building water systems. A re-circulating loop feeding multiple outlets will necessarily have deadlegs installed. Good design can reduce the length (and water volume) of these deadlegs. Note that they are only deadlegs if they are not flushed; otherwise they are essential pieces of infrastructure. No Legionella in building services presentation is complete without mentioning deadlegs and blind ends. This presentation is no exception. A very
24 Health Estate Journal November 2021
common source of deadlegs and blind ends (capped off pipes) is refurbishing and repurposing buildings. Patient rooms become storage or office space, and outlets are capped at the wall rather than removed (see below). The contributions of obstruction to the Trinity are to increase stagnation, provide growth environments, and reduce temperature control by preventing or reducing water flow. Reducing or removing obstruction weakens the effects of the other two members of the Trinity.
Stagnation
As discussed, the impact of deadlegs and blind ends on elevating Legionella risks is almost a cliché these days. Much of the stagnation in a system is caused by either deliberate or neglectful obstruction. Design and maintenance also make significant contributions. Water follows the route of least resistance as it flows through a building. Ideally a system design that ‘balances’ the water system would mean flows throughout are constant and stagnation
is minimised. In reality older buildings and facilities that have had multiple renovations and extensions almost always become ‘unbalanced’. For example, connecting sub-loops to main loops immediately creates stagnation and reduced flow. Trying to balance a water system post-installation can also create stagnation by restricting flow. A variety of outlets will also foster stagnation. We’ve already discussed flow restrictors and aerators as obstructions, but they also reduce flows and cause stagnation. They also provide ideal homes for biofilm communities. Some outlet types – for instance sensor taps – can cause stagnation by limiting the flow of water from the outlet. They also introduce several construction materials that might be conducive to Legionella growth.
Very variable data
Water usage from outlets is very variable. The chart below shows some data we collected some years ago (published in HEJ in 2016). The interesting point is that Legionella-positive test results were most
Bath Sluice Pool Spa Feature Pond 1.6% 1.8% 0.1% 0.1% 0.3% 0.1%
Samples taken x outlet Nov 2013 – Oct 2015 799 samples taken
Sink 18.7%
Shower 21.6%
Basin 55.8%
Shower Basin Sink Bath Shiuce Pool Spa
Feature Pond
Figure 1: Prevalence and nature of Legionella contamination in aged care facilities in Australia. The Australian Hospital Engineer, 2016. (Jones R, Pope L, Bentham R).
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