Legionella and water hygiene: Transitioning to greater building occupancy

18 Jun 2021

There is much conjecture surrounding the return to the office and what this will do to occupancy levels. A hybrid mix of home and office based working seems to be the dominant theme at the moment, certainly in the short to medium term. If this scenario does transpire, what impact will this have on water quality and, in particular, the growth of legionella bacteria? Mike Rose, Divisional Director – Health and Safety, discusses how water systems need to be managed and how to address the associated challenges.


Maintaining safe water systems

Water systems - whether hot and cold water systems, cooling towers, water features or other type of risk system - always operate best when there is a good throughput of water on a consistent basis. In this situation, opportunities for stagnation occurring are significantly reduced which minimises the opportunity for bacterial growth. The ability to control temperatures, especially cold water, is significantly improved with high levels of water usage. This becomes even more important as we enter the summer season with higher ambient temperatures which can have a significant impact on mains water temperatures as well as outlet temperatures within buildings.

In many ways the initial lockdown, and for many months afterwards, provided a much easier situation to control as everyone was working from home, with very low levels of building occupancy. It was very clear that full flushing regimes and isolating tanks to reduce capacity were the order of the day.

The conundrum now is finding the right balance between maintaining safe water systems and reducing activities that were implemented in response to the initial lockdown. It should also not be forgotten that these activities are both labour intensive, a result of having to manually flush outlets, as well as impacting sustainability goals through water wastage. The transition to a more typical management regime will also reduce costs and improve the environmental profile of buildings.


Finding the optimum water management regime

Changing your water management process is dependent on building occupancy, water system complexity, the mix of landlord and tenant services, and whether it is tank or mains fed.

When it comes to assessing if there is an opportunity to reduce flushing regimes, it is important to have a clear understanding of occupancy levels throughout the building, especially so in multi – let buildings. Maintaining an attendance log by floor will identify which tenants are returning or not and in what numbers and frequency. Where tenants aren’t returning, this will indicate that on these floors the flushing regime may need to be maintained. On floors where there are improved occupancy numbers, consideration can be given to reducing flushing.                                       

Ultimately, in hot and cold water systems, where temperature is the primary control mechanism, it is the temperature of the water that will determine the extent to which flushing can be reduced. If temperatures are being measured at the required levels and within the necessary time period, this will indicate that the water system is being regularly used and flushing can be reduced.  During times of normal occupancy levels most buildings will always have some little used outlets, flushed on a weekly basis. It is likely that, irrespective of building occupancy levels, these will still remain little used and as such need to be left on the flushing schedule.

Every building and water system is unique and will have its own operating characteristics. By recognising this we are able to support clients through the provision of UKAS accredited legionella risk assessments and sampling, providing assurances that legislative requirements are being addressed and a safe water system is being maintained.


If you would like further support in legionella risk management, please contact Mike Rose

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Mike Rose

Divisional Director - Health & Safety T: +44 20 3691 0500 Email

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