Cooling Tower Legionella Biofilm Monitoring
A careful reading of a 2026 field study: surface and bulk-water results can provide different evidence, but biofilm was not a universal early warning and qPCR did not establish viability.
Kerry and colleagues followed two operational evaporative cooling towers at one UK nuclear-waste site from March to October. Their results support adding representative biofilm sampling to a wider monitoring programme, not replacing bulk-water sampling or the controls required by HSG274 Part 1. The study observed broader Legionella signals in biofilm before or at the same time as some bulk-water rises, but it did not establish a dependable lead time, prove transfer from biofilm into water, compare biocides, or test ChloroKlean.
What the field study did
The researchers sampled two operational cooling towers, identified as A1 and A2, on eight occasions between March and October at one UK nuclear-waste site. At each time point they collected five biofilm and three bulk-water samples per tower. The work combined qPCR assays for Legionella targets with flow-cytometry measures of total and intact cells.
This longitudinal design is useful because it compares compartments in working towers rather than relying on a single water sample. It is still an observational study of two towers at one site. Operating conditions, interventions and local ecology limit how far its timing patterns can be generalised.
What was found, and what was not
Legionella pneumophila serogroup 1 DNA was detected exclusively in A2 biofilms and not in the paired bulk-water samples. Broader Legionella qPCR signals in biofilm preceded or coincided with rises in bulk water on some occasions. The pattern was not universal, so the paper does not provide a fixed warning interval or show that a biofilm positive will predict a later water positive.
The observations also do not prove that L. pneumophila moved from the attached community into the water. An association in timing is not a traced transmission pathway. Total and intact cell counts did not reliably predict Legionella, so general cell-count trends cannot be treated as organism-specific assurance.
qPCR and culture answer different questions
qPCR detects target nucleic acid. It does not by itself show that detected organisms were viable, culturable or capable of causing infection. Culture confirmation was not available in this study because of quarantine and sample-timing constraints. That limitation matters when interpreting the exclusive biofilm detections.
A competent monitoring plan should define what each method is intended to show, use an appropriate accredited laboratory and sampling protocol, and interpret results alongside operating history, treatment records and the written scheme. A negative bulk-water result is evidence about that sample at that time, not proof that wetted surfaces are free from Legionella.
How this fits HSG274 Part 1
HSG274 Part 1 places monitoring within a complete control programme based on risk assessment, a written scheme, competent oversight, treatment, cleaning, inspection, maintenance, corrective action and records. A surface-sampling plan may add evidence where representative and safe access is possible, but it must not displace the water sampling, microbiological monitoring or operational checks specified by the scheme.
Sampling points, methods, frequency and action thresholds must be site-specific and documented. Results should trigger the response defined in the written scheme rather than an improvised dose change. Duty holders should use the current HSG274 Part 1 tables and advice from their competent water-treatment and occupational-health-and-safety specialists.
No treatment comparison or product claim
This was field observation, not a comparative biocide trial. It does not rank chlorine dioxide, chlorine, bromine or non-oxidising biocides, and it establishes no treatment concentration or efficacy claim. ChloroKlean was not tested.
A product must be supplied under the applicable GB BPR position, whether authorised or lawfully available under relevant transitional arrangements, and used only as its permitted label allows. PT11 concerns preservation of liquid-cooling and processing systems. Whether an activity is equipment disinfection, public-health disinfection or system preservation depends on its purpose and applicable product status; PT11 is not blanket permission for every water-disinfection use.
Adding Biofilm Evidence to a Cooling-Tower Monitoring Plan
Questions for the competent team when deciding whether and how surface sampling adds useful evidence.
Start with the Written Scheme
Confirm the risk assessment, responsible roles, control limits, routine water sampling and corrective actions required under the current written scheme and HSG274 Part 1.
Define the Monitoring Question
State whether surface sampling is intended to investigate a recurring result, follow an accessible location over time or supplement routine evidence. Do not use it as a substitute for required bulk-water monitoring.
Choose Representative Points and Methods
Have competent persons select accessible, repeatable points and document collection, transport, laboratory method and quality controls. Avoid unsafe access or disrupting contaminated deposits.
Interpret Each Method Within Its Limits
Treat qPCR as detection of target DNA unless viability has been established separately. Do not infer Legionella status from total or intact cell count alone.
Trend and Act Through the Scheme
Review surface, water, treatment and operational data together. Follow the written corrective-action process for adverse results and record decisions, rather than assuming a universal biofilm lead time.
Expert Insights
About the Reviewer
Gavin Owen
Managing Director, ChloroKlean
Gavin Owen leads ChloroKlean's technical and commercial operations, bringing over 20 years of experience in industrial chemical distribution and water treatment. He oversees product development, regulatory compliance strategy, and the company's BPR compliance programme across PT2, PT4, PT5, and PT11 product types. Gavin works directly with water treatment professionals, facilities managers, and public health engineers across healthcare, leisure, food processing, and industrial sectors.
Frequently Asked Questions
Common questions about this topic, answered by our technical team.
Monitoring and Safety Limits
- This guide is educational and does not replace ACOP L8, HSG274 Part 1, a site-specific risk assessment or the written scheme.
- Do not disturb deposits, open plant or access tower internals for sampling unless a competent risk assessment and safe system of work cover chemical, aerosol, mechanical, electrical, work-at-height and confined-space hazards as applicable.
- Never treat a negative qPCR, culture or bulk-water result as proof that the whole system is free from Legionella.
- qPCR detection does not by itself establish viability, culturability or infectivity; use the laboratory method and interpretation specified by the competent monitoring plan.
- Do not infer a treatment dose, efficacy or corrective action from this study. Use only a product supplied under the applicable GB BPR position, whether authorised or lawfully available under relevant transitional arrangements, and follow its permitted label, Safety Data Sheet and the written scheme.
HSG274 Part 1 monitoring supports, but does not replace, effective control. Sampling locations, methods, frequencies, limits and corrective actions must be defined by the site-specific risk assessment and written scheme under competent oversight.
Related Resources
Continue exploring our knowledge base and product information.
Cooling Tower Water Treatment
The cornerstone guide to risk assessment, the written scheme and treatment within HSG274 Part 1.
Cooling-Water Biofilm and Legionella Risk
Background on biofilm as one part of cooling-system risk management.
PT11 Cooling Water
The scope of PT11 liquid-cooling and processing-system preservation.
Biofilm Monitoring and Verification
How surface and water evidence can be combined without overinterpreting either.
Sources & References
This article references guidance from the following authoritative sources:
- HSG274 Part 1: The control of legionella bacteria in evaporative cooling systems
HSE - Health and Safety Executive
- Biofilms as an early indicator of Legionella colonisation within evaporative cooling towers
Industry Standard - Kerry et al., Environmental Science: Water Research & Technology 12(8), 2505-2519 (August 2026)
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