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Action required to prevent legionella issues as water quality deteriorates

Published: 8 March 2021 - Neil Mead

As England gradually comes out of national lockdown, Tom O'Sullivan, Head of Operations at Guardian Water Treatment urges facilities managers and building maintenance teams to prioritise legionella prevention and water system maintenance to stop the ongoing trend of sample failure rates caused by underused buildings.


With people told to work from home, many commercial and office buildings closed their doors for a second time earlier this year. During periods of inactivity such as this, regularly flushing water systems by running off taps and outlets must be treated as one part of a water treatment strategy, not as a stand-alone solution. 

Occupancy rates have dropped by up to 15%

"Manual flushing cannot replicate normal water usage in buildings designed to host thousands of people when occupancy rates have dropped as low as 10-15%*, as is still the case in some parts of central London," says Tom.

"As well as being inadequate, this approach could be contributing to a general increase in microbiological contamination, posing risks to both returning occupants and valuable assets."

Data gathered from across tens of thousands of laboratory samples shows a 50% increase in the failure rate of TVC (Total Viable Count - an indicator of the general level of microbiological contamination) samples over the past 6 months, when compared to the same period last year. 

Legionella samples have risen by 20%

More worryingly, the rate of detection for positive legionella samples have also risen by around 20% year on year since April, in conjunction with the period of low occupancy.

Tom continues: "Legionella is a ticking-time bomb in underused buildings if water systems are allowed to stagnate.  Where maintenance has been neglected over lockdown, build-up of biofilm in the system provides essential nutrients to support the growth of harmful pathogens, exacerbating the problem - this could explain why we are 20% more likely to find positive legionella samples this year compared with 2019."

Flushing does play a vital role in preventing stagnation, but it should form part of a holistic approach to legionella prevention and water treatment in general, with strategies tailored to occupancy rates and building usage.

Guardian Water Treatment is hosting a series of free webinars offering bespoke advice to responsible parties looking for expert support in this area. 

For more information and to sign up, email

Tom concludes: "Many commercial buildings have been pushed to their limits since the pandemic hit and are about to be tested again. Facilities managers must put bespoke and robust water treatment strategies in place now to ensure a smooth and safe return to operations when lockdown ends."

* Figures from The Crown Estate, accessed at:


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