How to control Legionella - Part 2: the risks and options…

by Water Hygiene Centre, on 08-08-2018
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How to control Legionella - Part 2: the risks and options…

As discussed in Part 1, temperature has limited effectiveness when treating biofilm bacteria (especially mature biofilm) but colonisation of a system with biofilm bacteria may (in the main) be avoided should water temperature be kept within specification and the water within the system kept in regular movement/turned over. 

Why are my water samples testing positive for Legionella and other waterborne bacteria?

When outlets test positive for bacteria of ‘human importance’, it’s first of all important to question ‘why’ and to ascertain to what extent the water system has been compromised (ie, how big is the loss of control…?).  


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With this in mind, it’s often prudent to establish ‘if’ contaminated outlets have been in regular use or flushed – as necessary, as it’s commonplace for infrequently used and or poorly maintained outlets to test positive for waterborne bacteria.


Sampling 2Therefore, not all water sample results testing positive for the ‘target organism’ substantiate the use of disinfection methods as a strategy of control before the following have been appraised:

  • The number of colony forming units recovered (cfu/volume sample) in a particular sample;

  • Pre and post flush test results;

  • The number of outlets affected;

  • Adherence to established legionella flushing regimes – in accordance to the organisational Water Safety Plan / Written Scheme;

  • Adherence to established cleaning practices – in accordance to the organisational written scheme [water safety plan];

  • Adherence to monitoring of water systems through pre-planned maintenance (PPM’s);

  • Outstanding recommendations and priority actions detailed in the site-specific risk minimisation scheme;

  • Remedial activity both within or outside the affected premises – whereby remediation works outside of the affected premises may have impacted upon water quality at the point of entry to a building (water undertaker works, fire service etc)


Whilst the above list is by no means exhaustive, it encourages the identification of the root-cause of the problem and therefore favours a more targeted rather than indiscriminate approach to applying a routine disinfection strategy. If we consider the properties of an ‘ideal disinfectant’ such as:


  • Long lasting residual;

  • Not impacted by pH, temperature or organic contaminants and corrosion scale;

  • No health and safety considerations for use;

  • Broad spectrum effectiveness (effective against a wide-range of bacterial contaminants);

  • Cost effective;

  • Non-corrosive/damaging to water systems – including aesthetics as well as integrity of water systems;

  • Environment friendly – including waste production.


What are my options for controlling Legionella and other waterborne bacteria?

We can quite quickly arrive at the conclusion that sadly there is no such thing as an ideal disinfectant, but by identifying and understanding causal issues with our water systems and taking a system-specific approach to water quality then we will be improving our chances of an ‘ideal outcome’ (ie removing waterborne bacteria from our water systems or at least controlling them within safe limits).


Various chemical methods of disinfection can offer a solution/support with the periodic cleaning of water storage vessels through to the commissioning of new water systems whereby disinfection forms part of the commissioning process.


Chlorine Dioxide can be indicated specifically for the removal of biofilm from water systems where the use of Chlorine would be comparatively less effective and both oxidising biocides may be indicated for treatment of water systems on a permanent basis whereby concerns exist surrounding the effectiveness of the control scheme.


In such situations it is however indicated to be mindful of the dosing residual to ensure that water remains within drinking water specification (fluid category 1). If for example system-contamination with mature biofilm is suspected then the use of chemistries such as chlorine dioxide may be indicated at a higher dose – resulting in the system being taken offline until all chemical has been safely flushed through the system.


Biofilm-1Whilst the effectiveness of such chemistries as ‘biofilm removers’ has been well-documented, the oxidising nature of these chemicals may inevitably impact upon the integrity and subsequent lifespan of water systems – if relied upon as a mainstay control strategy. Therefore, caution should be exercised in this regard and focus placed on using the traditional method of control – where reasonable and practicable to do so.


Another recognised control strategy being copper-silver (Cu-Ag) ionisation has demonstrated good effectiveness against Legionella bacteria but are not without limitation as the broad-spectrum effectiveness of this strategy is not well known.


Whilst it offers advantages over oxidising biocides by not impacting on the integrity of the water system, silver (Ag) ions have demonstrated to stain sanitary ware and the adequate management of these systems requires maintenance resource – to; maintain the efficacy of electrodes (especially in hard water areas – whereby water softeners are required and associated maintenance) and completion of required chemistry tests to ensure the desired concentration of Cu-Ag throughout the treated system.


POUF [point of use water filter]Finally, point of use (POU) water filters may provide a ‘physical disinfection’ option whereby the primary concern is protecting patients from waterborne pathogens at the point of care. Whilst these filters may remove the requirement to; isolate outlets from clinical use and or move patients to unaffected hospital areas (whilst required remedial works are undertaken), such filters may slow the water flow to the outlet which may in turn increase the risk of bacterial contamination behind the filter and local to the outlet.


The Responsible Person [Water] / Authorised Person [Water] needs to be aware of the condition of the water systems which they are responsible for.


Where a temperature strategy has been shown to not be effective in maintaining control then alternative controls strategies should be considered [long term / short term application].


Whilst no single disinfection regimen is without flaw, a combination of disinfection regimens may offer the most robust results following an adequate appraisal of the water system to be treated.


If you have questions regarding the issues raised above or you would like to speak with one of our consultants please click here to get in touch.

Editors Note: The information provided in this blog is correct at date of original publication - August 2018. 

© Water Hygiene Centre 2018


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About the author

Water Hygiene Centre

The Water Hygiene Centre was established in 2009 to address the lack of independent water hygiene consultancy within the industry. From our humble beginnings, we have established ourselves as a market leader, helping clients identify and minimise the risk of waterborne contamination and disease, whilst improving compliance performance.

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