If you are the Duty Holder or appointed responsible person, e.g. head teacher, business manager, maintenance manager or property owner, you have a legal responsibility to ensure that occupants of your premises are suitably protected from water safety risk. When dealing with water hygiene issues in schools frequent reviews and updates to your Legionella risk assessment are key to keeping water systems safe.
When assessing the risk from Legionella and devising control measures, it’s of vital importance to have a clear understanding of all parts of the water system. For instance, areas within a school may not be used during holidays, but the planned preventative maintenance and monitoring schedule needs to include all foreseeable circumstances – particularly as little-used water outlets are the ones at most risk from Legionella contamination.
Good water management can reasonably centre on – but not be limited by – two main considerations:
- Ensuring that there is sufficient water turnover to prevent stagnation;
- Ensuring that cold water remains cold and hot water remains hot. For example, cold water should be less than 20°C and hot water should not be stored below 60°C. The return legs of recirculating hot water systems should not fall below 55°C in healthcare buildings and not below 50°C in non-healthcare premises.
Whilst there are many other considerations for mitigating the growth of aquatic microorganisms, such as Legionella, the presence of slow-moving or stagnant water at temperatures between 20-45°C can provide ideal conditions for such bacteria and therefore should be avoided. Slow moving or stagnant water can allow nutrients within water systems to settle on the inner surface of pipework encouraging waterborne bacteria, such as Pseudomonas species, to attach and start producing biofilm for other ‘secondary infecting pathogens’ (like Legionella) to take-up residence.
At this point, it’s noteworthy that waterborne Gram-negative bacteria such as Legionella and Pseudomonas are ‘ubiquitous’ within water systems in ‘planktonic’ or free-floating form and are often flushed to drain in well managed/maintained estates. However, when water systems harbor the ideal conditions for proliferation, then this often presents an issue that requires an expensive, multi-factorial and time-intensive response – which also sometimes has limited success (when you consider the likelihood of recurring issues from previously affected/colonised water systems). It’s well documented that biofilm bacteria can be phenotypically different from their planktonic form, in part attributable to gene expression/transfer, as well as being better-protected by the dense and complicated architecture of biofilm matrices. This often makes it very difficult if not impossible to eradicate biofilm-based contaminants from affected systems with lasting success!
As previously mentioned, systems affected by such contaminants often succumb to recurring issues and therefore prevention in this regard is very much better than cure! As such, when deciding upon a suitable risk mitigation strategy for buildings subject to ‘scheduled’ intermittent or low use, such as schools, universities etc, it may be prudent to review HSG274 Part 2, Paragraph 2.50; which offers practical guidance on how to manage buildings that have been temporarily taken out of use (known as mothballing). Whilst school and university estates increasingly have ‘dual use’ facilities that help to maintain the turnover of water by keeping buildings open to service users during holiday periods, some schools and universities do not have such facilities and therefore are tasked with safely managing water during this time.
A suitable risk assessment that considers the design of the water systems, their operating characteristics, mode of use, population vulnerability and other factors will assist in determining the required control measures. It’s advised that any mothballing strategy or procedure should be a compromise between prevention of stagnation by regular flushing, the avoidance of water wastage and protection of the water system from degradation caused by any disinfectant added.
During shorter periods of infrequent or intermittent use, microbial growth may be adequately controlled by identifying areas of infrequent use, i.e. those used less than once per week, and flushing them at least weekly (twice weekly in healthcare and other high-risk premises). Each outlet should be flushed for long enough to ensure that the entire contents of the outlet and its associated supply pipework has been replenished with fresh water. Water temperature can be a helpful indicator in this respect – by flushing for as long as it takes for the water from the outlet to stabilise at a temperature comparable to the incoming supply. In larger, more complex water systems the continual dosing of biocide may be combined with regular flushing to provide an additional level of control, however, it should be ensured that the system remains fit for its intended purpose when dosing stops.
For longer periods of disuse, it can be acceptable to leave a system filled with water without weekly flushing – provided that measures are in place to prevent unauthorised use. Draining down a system unnecessarily may leave residual moisture or pockets of water within the system, potentially created or exacerbated by high humidity, and this may subsequently provide a ‘reservoir for contamination’ or an area for biofilm to develop. Leaving the system filled with water and suitably maintained also helps to avoid issues associated with systems drying out, such as joint failure in sectional tanks and/or metal pipework corrosion. However, these systems should be thoroughly flushed, cleaned and disinfected before their return to use. Therefore, in such situations - it’s indicated to recommission these systems as though they were new – in accordance with British Standards; BS EN 806, BS 8558.
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Editors Note: The information provided in this blog is correct at date of original publication - August 2019.
© Water Hygiene Centre 2019