Legionella Prevention in Water Systems
How chlorine dioxide delivers sustained Legionella control in hot and cold water systems — meeting ACOP L8 and HSG274 compliance requirements while eliminating the biofilm that harbours Legionella bacteria.
Legionella pneumophila remains one of the most significant waterborne pathogens in the UK, causing Legionnaires' disease — a potentially fatal form of pneumonia. The Health and Safety Executive (HSE) estimates that there are between 300 and 600 reported cases in England and Wales each year, though the true incidence is believed to be significantly higher due to under-diagnosis. Every building with a water system that stores or distributes water at temperatures between 20°C and 45°C is potentially at risk, making Legionella prevention a critical responsibility for duty holders across healthcare, hospitality, commercial property, and industrial sectors. Chlorine dioxide (ClO₂) has emerged as the gold standard for Legionella control because of its unique ability to penetrate and destroy the biofilm matrix where Legionella resides, multiplies, and is shielded from conventional disinfectants.
Understanding the Legionella Risk in Building Water Systems
Legionella bacteria are naturally present in freshwater environments at low concentrations. They become a health risk when they multiply to dangerous levels within engineered water systems — particularly hot and cold water systems, cooling towers, spa pools, and any system that creates water aerosols. The conditions that promote Legionella growth are well understood: water temperatures between 20°C and 45°C, stagnation in dead legs and low-use outlets, sediment and scale that provide nutrients, and critically — biofilm that provides a protected habitat for bacterial colonisation.
Biofilm is the single most important factor in Legionella persistence within water systems. Research published in Applied and Environmental Microbiology has demonstrated that over 95% of Legionella bacteria in water systems reside within biofilm, not in the bulk water. This means that any Legionella control strategy that only addresses bulk water disinfection — without tackling biofilm — will fail to eliminate the root cause of contamination. Conventional chlorine-based disinfection is particularly vulnerable to this limitation, as free chlorine is rapidly consumed by the organic matrix of biofilm before reaching the bacteria within.
The legal framework for Legionella control in the UK is built around the Health and Safety at Work Act 1974, the Management of Health and Safety at Work Regulations 1999, and the HSE's Approved Code of Practice L8 (ACOP L8). These require duty holders to identify and assess Legionella risk, prepare a written scheme of control, implement appropriate prevention and control measures, keep records, and appoint a responsible person to oversee the programme.
Why Chlorine Dioxide Is the Most Effective Legionella Control Method
Chlorine dioxide (ClO₂) is recognised by water treatment professionals, the HSE, and international health bodies as the most effective chemical disinfectant for Legionella control. Unlike free chlorine, chlorine dioxide is a selective oxidiser that penetrates biofilm without being consumed by the organic components of the biofilm matrix. This allows it to reach and kill Legionella bacteria living within biofilm — the primary reservoir of contamination in building water systems.
ChloroKlean Plus L20 delivers this biofilm-penetrating capability through regenerative chlorine dioxide chemistry. Rather than providing a single high-dose treatment that rapidly depletes, Plus L20 maintains a sustained, controlled release of ClO₂ that provides continuous protection between dosing events. This sustained residual prevents recolonisation and progressively cleans the system over time — reducing biofilm thickness, lowering total microbial counts, and improving the consistency of compliance sampling results.
Independent studies have shown chlorine dioxide to be effective against Legionella pneumophila at concentrations as low as 0.1–0.5 mg/L — significantly lower than the chlorine concentrations required for equivalent efficacy. At these concentrations, chlorine dioxide does not form trihalomethanes (THMs) or chloramines, avoids the corrosive effects of high-dose chlorine treatment on copper and other pipework materials, and maintains efficacy across a pH range of 4–10.
For duty holders managing Legionella risk under ACOP L8, chlorine dioxide addresses the three key challenges simultaneously: it disinfects the bulk water, penetrates and removes biofilm, and provides a measurable residual that can be monitored throughout the distribution system. This comprehensive approach is why an increasing number of NHS trusts, care homes, hotels, and commercial buildings are switching from chlorine to chlorine dioxide for Legionella prevention.
ACOP L8 and HSG274: What Duty Holders Must Do
ACOP L8 (Legionnaires' disease: The control of legionella bacteria in water systems) sets out the legal duties of employers, self-employed persons, and those in control of premises to identify and manage the risk from Legionella. HSG274 provides the detailed technical guidance for implementing these duties in practice — with Part 2 covering hot and cold water systems and Part 3 covering other risk systems including cooling towers.
Key requirements include conducting a Legionella risk assessment by a competent person, preparing and maintaining a written scheme of control that details the prevention strategy, implementing temperature control (storing hot water at 60°C or above, distributing at 50°C or above), flushing low-use outlets, cleaning and disinfecting the system as needed, and monitoring water quality through regular sampling and testing.
Where temperature control alone is insufficient to manage Legionella risk — for example in complex healthcare systems, buildings with extensive dead legs, or systems where compliance temperatures cannot be achieved throughout — chemical disinfection with chlorine dioxide provides an additional layer of protection. HSG274 Part 2 specifically references chlorine dioxide as an effective supplementary disinfection method, noting its ability to control biofilm and maintain a measurable residual throughout the distribution network.
Record-keeping is a critical compliance requirement. Duty holders must maintain records of risk assessments, the written scheme of control, monitoring results (temperature checks, water sampling), maintenance activities, training records, and any remedial actions taken in response to positive Legionella detections. These records may be inspected by the HSE or local authority environmental health officers.
Implementing a Chlorine Dioxide Legionella Control Programme
Transitioning to chlorine dioxide for Legionella control involves a structured approach: system assessment, product selection, dosing system design, commissioning, monitoring, and ongoing management. ChloroKlean's technical team supports clients through each stage, from initial assessment to long-term operational support.
The first step is a comprehensive system survey to understand the water system configuration — hot water generation and storage, distribution pipework, outlet inventory (including low-use and dead-leg outlets), existing treatment regime, and historical sampling data. This survey informs the dosing strategy, including the number of dosing points, target residual concentrations, and monitoring locations.
For most building hot and cold water systems, continuous low-level dosing of ChloroKlean Plus L20 at 0.3–0.5 mg/L provides effective ongoing Legionella control. Where systems have existing biofilm contamination (evidenced by persistent positive Legionella samples or elevated TVC results), an initial elevated-dose treatment at up to 5 mg/L may be recommended to remediate the system before transitioning to maintenance dosing.
Monitoring involves regular measurement of chlorine dioxide residuals at sentinel and indicator outlets throughout the system, combined with routine Legionella sampling in accordance with the written scheme of control. ChloroKlean provides guidance on monitoring frequency, sampling protocols, and interpretation of results. The progressive improvement in microbiological results following chlorine dioxide implementation provides objective evidence of treatment effectiveness.
Key Data & Statistics
99.9%
Legionella pneumophila kill rate at <0.5 ppm ClO₂
EN 13623 testing
20–45°C
Danger zone temperature range for Legionella growth
ACOP L8
HSE Approved Code of Practice for Legionella control
4-log
Reduction in Legionella achievable with chlorine dioxide in 30 minutes
Published efficacy data
Legionella Control Methods Compared
Comparison of water treatment approaches for Legionella prevention in building water systems under ACOP L8.
| Feature | Chlorine Dioxide | Thermal Disinfection | Copper-Silver Ionisation | Chlorination |
|---|---|---|---|---|
| Biofilm Penetration | Excellent — removes Legionella harbourage | None — bacteria protected in biofilm | Limited — ions cannot penetrate biofilm | Poor — surface reaction only |
| Continuous Protection | Yes — measurable residual throughout system | No — point-in-time treatment only | Yes — ionic residual | Yes — but rapidly consumed |
| Dead Leg Treatment | Effective — residual reaches dead legs | Ineffective — water does not circulate to dead legs | Partial — depends on flow | Limited — depleted before reaching dead legs |
| Temperature Independence | Effective at all water temperatures | Requires >60°C throughout system | Temperature independent | Less effective at higher temperatures |
| Scalding Risk | None — chemical treatment | Significant — requires TMVs at outlets | None | None |
| DBP Formation | Chlorite only | None | None | THMs, chloramines |
| ACOP L8 Compliance | Recognised treatment method | Recognised but limited effectiveness | Recognised | Recognised |
| Ongoing Monitoring | Simple residual testing | Temperature monitoring at all outlets | Ion concentration monitoring | Residual chlorine testing |
Based on ACOP L8, HSG274 Part 2, and published Legionella control research.
How to Implement Legionella Prevention with Chlorine Dioxide
A step-by-step guide for duty holders implementing chlorine dioxide Legionella control in building water systems.
Commission a Legionella Risk Assessment
Engage a competent person to conduct a Legionella risk assessment of your water system in accordance with ACOP L8. The assessment should identify all water systems, evaluate risk factors (temperature profiles, stagnation points, dead legs, vulnerable occupants), and recommend control measures including chemical disinfection where appropriate.
Review Your Written Scheme of Control
Based on the risk assessment, review or create your written scheme of control. This document details the control measures in place, monitoring procedures, responsibilities, and corrective actions. Include chlorine dioxide dosing as a specified control measure with target residual concentrations and monitoring locations.
Contact ChloroKlean for a System Assessment
Contact ChloroKlean on +44 333 772 7379 or email hello@chloroklean.com. Our technical team will assess your water system, review historical sampling data, and recommend the appropriate ChloroKlean Plus L20 dosing strategy — including dosing equipment specification, target residuals, and monitoring schedule.
Install Dosing Equipment and Commission the System
Install the chlorine dioxide dosing system at the recommended injection points. Commission with ChloroKlean Plus L20, setting target residual concentrations in line with the technical recommendation. Verify residuals at sentinel outlets to confirm distribution throughout the system.
Implement Ongoing Monitoring and Record-Keeping
Establish routine monitoring of ClO₂ residuals at sentinel and indicator outlets. Continue Legionella sampling in accordance with your written scheme of control. Maintain records of all monitoring results, dosing quantities, maintenance activities, and any corrective actions. Review results with ChloroKlean's technical team to optimise dosing and confirm treatment effectiveness.
Expert Insights
"Legionella prevention is not just about treating water — it is about treating the system. Biofilm is where Legionella lives, and chlorine dioxide is the only practical disinfectant that reaches bacteria inside biofilm. That is why we developed Plus L20 with sustained release chemistry — to provide continuous protection, not just momentary treatment."
Gavin Owen
Managing Director, ChloroKlean
"We work with NHS trusts, care home groups, and hotel chains across the UK who have switched from chlorine to ChloroKlean Plus L20 for Legionella control. The results are consistent — better microbiological compliance, reduced biofilm, and lower chemical usage compared to their previous treatment regime."
Gavin Owen
Managing Director, ChloroKlean
"ACOP L8 compliance is a legal duty, not a recommendation. Every building with a hot and cold water system needs a Legionella risk assessment, a written scheme of control, and an effective prevention strategy. Chlorine dioxide gives duty holders a proven tool to manage that risk effectively."
Gavin Owen
Managing Director, ChloroKlean
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 authorisation 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.
Related Products
BPR-authorised chlorine dioxide products available from ChloroKlean.
ChloroKlean Plus L20
Regenerative liquid chlorine dioxide with sustained antimicrobial activity. The primary product for Legionella control programmes — effective against Legionella, Pseudomonas, E. coli, and biofilm at less than 0.5 ppm.
ChloroKlean PT5-200
Chlorine dioxide solution for municipal and large-scale drinking water system treatment. BPR PT5 authorised for potable water Legionella control.
Frequently Asked Questions
Common questions about this topic, answered by our technical team.
Important Safety Information
- Chlorine dioxide products must be stored, handled, and used in accordance with the Safety Data Sheet (SDS) and product label directions.
- Appropriate personal protective equipment (PPE) must be worn when handling concentrated products, including chemical-resistant gloves, safety goggles, and suitable protective clothing.
- Legionella risk assessments and written schemes of control must be carried out by competent persons in accordance with ACOP L8.
- Chemical disinfection with chlorine dioxide is a supplementary control measure and does not replace the requirement for temperature control, flushing, and other measures specified in the written scheme of control.
- In drinking water systems, monitor chlorite and chlorate by-product levels to ensure compliance with Drinking Water Inspectorate standards.
- Seek professional advice from a competent water treatment specialist before implementing chlorine dioxide Legionella control.
This information is provided for guidance only and does not replace professional Legionella risk assessment, your written scheme of control, or applicable regulatory requirements. Always refer to ACOP L8, HSG274, and the current product SDS.
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Contact ChloroKlean for a free Legionella risk consultation and product recommendation.
Sources & References
This article references guidance from the following authoritative sources:
- ACOP L8: Legionnaires' disease - Control of legionella bacteria in water systems
HSE — Health and Safety Executive
- HSG274 Part 2: The control of legionella bacteria in hot and cold water systems
HSE — Health and Safety Executive
- Legionnaires' disease: Technical guidance Part 3 (HSG274)
HSE — Health and Safety Executive
- Guidelines for Drinking-water Quality - Chlorine Dioxide
WHO — World Health Organization
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