A Legionella control case study – when routine fails

Speaking during an update on the activities of the various IHEEM Technical Platforms at July’s 2025 IHEEM Authorising Engineers conference, Harry Evans, an IHEEM-Registered Authorising Engineer (Water) discussed a past experience in his AE capacity advising a school on addressing a challenging Legionella issue. In a cautionary tale, he described how he, the school’s own engineer, and its management, soon realised the school’s caretaker had failed to complete many of the essential preventative, and subsequently, remedial tasks he had claimed to have undertaken, with disastrous results.

After years in complex, high-pressure hospital environments, a newly appointed hospital engineer and long-serving Responsible Person (RP) is drawn to the promise of a quieter role at Flushoaks Primary School — a fictional but representative single-storey facility located somewhere in the south of Scotland. The school serves just under 300 pupils, and on paper at least, presents a textbook example of a ‘low-risk’ building. There are no showers, with a direct-fired water heater handling hot water generation, cold water stored in a 1200-litre tank, and with the site incorporating just 60 outlets and a single external bib tap.

Upon arrival, the engineer anticipates an environment of procedural ease and reduced stress. His first routine question to colleagues feels innocuous: “How many Legionella samples do you take each year?” The response was jarring: “What are Legionella samples?”

Note: This is not a hospital environment, and routine Legionella sampling is not typically required. Primary schools lack augmented care areas, control failures, or high-risk features, that would necessitate regular testing.

Initial inspection and immediate escalation

Alarmed by the lack of awareness among staff, the engineer undertakes a visual inspection of the cold-water storage tank. What he finds is far removed from the serene simplicity he had imagined: significant biofilm formation, layers of debris, and substantial sediment. It’s a scene that triggers deep professional concern and an instinctive escalation.

The engineer contacts a trusted water treatment specialist — one with whom he had collaborated previously in hospital settings. Twelve water samples are swiftly collected from key outlets across the building. When the results return, they indicate systemic contamination: Legionella is detected in eight of the 12 outlets sampled. The nature and spread of contamination demand decisive action. The water treatment specialist recommends:

• A full clean and disinfection of the cold-water tank.
• A complete system-wide disinfection of the building’s water services.

The engineer agrees immediately. There is no room for compromise.

Post-treatment sampling is undertaken, and results appear encouraging; all outlets test clear. A sense of control, albeit fragile, begins to re-emerge.

Monitoring, recurrence, and mounting concerns

The engineer recommends that monthly water quality monitoring is introduced, as recommended by the water treatment specialist, encompassing testing for:

• Legionella species.
• TVCs at both 22 °C and 37 °C.
• E. coli and coliforms.
• Pseudomonas aeruginosa.

However, this sense of control proves short-lived. Legionella reappears in multiple locations, raising 
serious doubts about the effectiveness of disinfection 
and the integrity of the system. A second full system disinfection is recommended by the water treatment specialist.

Recognising the complexity and persistence of the problem, the school’s engineer — now far removed from his hoped-for tranquillity — calls upon an Authorising Engineer (Water). The stakes are rising, and specialist insight is urgently needed.

AE intervention and technical assessment

The AE’s involvement marks a shift from reactive treatment to strategic investigation. He begins with a structured review, asking:

• Is there a Legionella risk assessment?
• Is there a current Legionella written control scheme?
• Were pre- and post-flush samples correctly taken?
• What Legionella species were present, and at what levels?
• What is the turnover rate of the cold-water storage tank?
• Are temperatures across the system compliant?
• Is any residual disinfectant detectable at outlets?

The findings paint a sobering picture:

• A Legionella risk assessment and written control scheme were in place.
• The cold-water tank had not been adequately cleaned during previous interventions.
• Post-flush sampling was incorrectly conducted, invalidating much of the earlier assumptions of systemic contamination.
• Sentinel monitoring revealed cold water temperatures above 20 °C.
• Water meter readings indicated monthly, not daily, turnover of the storage tank.
• Residual disinfectant was undetectable at all test locations.

Systemic colonisation

Although the Legionella species identified were non-pneumophila, the water treatment company acknowledged the situation as one of serious systemic colonisation. The AE arranged a meeting with the regional Water Undertaker, and confirmed a critical design limitation: the school is served by a long supply main, which dissipates chlorine disinfectant before it reaches the building.

This revelation reframes the issue. Flushoaks Primary wasn’t simply failing on flushing or cleaning — it was operating under flawed assumptions about system input and treatment resilience.

In the light of these shortcomings, the AE advised a series of targeted interventions to restore control and establish defensible compliance:

• Legionella training for school staff.
• A review of the Legionella risk assessment and written control scheme.
• Competency checks on contracted-out services (water treatment company).
• Correct post-flush sampling, using validated. procedures (BS7592), resulting in a marked reduction in positive samples.
• Installation of a simple chlorine dosing system at point-of-entry to restore and maintain residual disinfectant.
• Thorough cleaning and disinfection of the cold-water storage tank, carried out with verification.
• Daily flushing of outlets, with flow volumes sufficient to promote turnover and distribute disinfectant.

These corrective actions were implemented promptly. The school regained a level of water safety assurance. The local authority, which had raised concerns about the risk to pupils, received documented evidence of remediation. School closure was averted. The story was far from over, however.

• The Human Factor:
when people undermine process

Several months later, monitoring revealed rising Legionella counts, and the engineer once again found himself facing a technical and reputational storm. Daily flushing logs were signed diligently. On paper, compliance was perfect, but suspicion crept in. The AE suggested a simple method of verification: check the water meter before the caretaker arrives, and again after he completes his flushing routine. This would reveal actual flow volumes without needing direct confrontation. The results were telling: meter readings showed virtually no water use during the designated flushing period.

Confronted with this data, the caretaker admitted: “I’ve never flushed the outlets. It’s a waste of water and my time.” This admission reframed the entire case. No matter how well-designed the procedures, or how technically sound the interventions, they were ultimately dependent on human behaviour — and in this instance, one unverified routine had nullified all progress.

This scenario exposes systemic vulnerabilities that extend beyond Flushoaks Primary. In many buildings:

• Despite its critical role in water safety, flushing is frequently delegated to non-specialist staff — such as cleaners, housekeepers, or caretakers — who often receive minimal training. This indicates that it is viewed as an ancillary task, rather than a critical control measure within a coherent safety strategy.
• Documentation is treated as proof of action, rather than a prompt for verification.
• The consequences of non-compliance are poorly understood by those tasked with frontline actions.
• Oversight is assumed, not actively exercised.

These vulnerabilities do not reflect individual failure alone; rather they represent a cultural blindspot in management. One signed sheet, when unverified, can mask widespread microbial risk. One person’s inaction can negate thousands of pounds’ worth of intervention, and hundreds of hours of strategic effort.

• Lessons learned:
beyond compliance to assurance

Flushoaks Primary’s case serves as a critical reminder for Responsible Persons (RPs), AE(W)s, Estates managers, and Water Safety Groups. Key ‘takeaways’ include:

• Flushing must be verified — whether through spot checks, meter audits, or automated monitoring.
• Signed records are not self-validating; performance audits should underpin routine activities.
• Staff training is essential, not optional; those responsible for water hygiene must understand the ‘why’, not just the ‘how’.
• Systemic failure can masquerade as routine compliance, especially in ‘low-risk’ buildings.
• Design simplicity doesn’t guarantee operational safety; long pipe runs, poor turnover, and temperature elevation, present real threats.

In water safety management, complacency is dangerous. The ‘low-risk’ label can lull teams into false security, making them blind to quiet but consequential failures.

• Final reflections:
engineering and accountability

For the hospital engineer, the transition to Flushoaks Primary was meant to represent a gentle chapter in a long career. Instead, it became a crash course in cultural vulnerability, procedural drift, and the real-world impact of overlooked routines.

Authorising Engineers must remain vigilant not just for design flaws, but for signs of cultural breakdown. Water hygiene is both a technical and behavioural discipline — robust control relies equally on accurate data, thorough training, and verified action. The lesson from Flushoaks is simple but profound: When routine fails, safety fails — and when flushing lacks impact, all other work may be for nothing.

•  Note from the author: Although Flushoaks Primary School is a fictitious establishment, the issues explored in this case study are derived from real-life scenarios, and reflect recurring challenges within the world of healthcare and public sector water hygiene. All the images were created by me.