Legionella Prevention in Water Systems
How chlorine dioxide may fit a site-specific Legionella control programme under ACOP L8 and HSG274, with product-specific evidence and verification.
Legionella pneumophila can cause Legionnaires' disease, a potentially fatal form of pneumonia. Duty holders must assess and control foreseeable risk in relevant water systems. Chlorine dioxide is one chemical option within the wider ACOP L8 and HSG274 framework, not a gold standard or a substitute for system design, temperature, cleaning, hydraulics and monitoring. Its performance and biofilm effects depend on the product and the conditions used.
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 can contribute to Legionella persistence, but its importance and the distribution of organisms vary by system and study method. A bulk-water result alone does not describe attached growth. Control therefore combines system design, temperatures, cleaning, hydraulics, treatment and verification rather than promising elimination of a single root cause.
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.
Assessing Chlorine Dioxide for Legionella Control
Chlorine dioxide is one chemical option within a Legionella control programme; HSE guidance does not name one universally most effective disinfectant. Its performance depends on the product, organism and matrix, demand, concentration, contact time, hydraulics and monitoring.
Plus L20 is described as a regenerative chlorine dioxide formulation, but sustained release must be demonstrated by residual measurements in the actual system. A residual alone does not establish prevention of recolonisation, progressive cleaning, lower microbial counts or improved compliance results.
Published studies report outcomes for Legionella pneumophila under their stated matrix, concentration and contact-time conditions. They do not establish a Plus L20 dose or direct superiority at equivalent concentrations. Chlorine dioxide forms no chloramines and generally very little THM/HAA; chlorite and chlorate must be controlled by dosing and verified by measurement.
For duty holders managing Legionella risk under ACOP L8, residual measurement can be one operational check within a wider programme. It does not prove biofilm removal or microbiological control throughout the distribution system.
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, a competent assessment may consider supplementary chemical treatment alongside design and operational corrections. HSG274 Part 2 discusses chlorine dioxide, but that reference does not establish universal biofilm control, residual coverage or product efficacy.
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.
The permitted product conditions and a competent site assessment must set any continuous or remedial Plus L20 dose. General study concentrations must not be converted into product instructions. Persistent positives require investigation of hydraulics, temperature, cleaning, biofilm, monitoring and the whole written scheme.
Monitoring can include chlorine dioxide residuals at selected outlets and Legionella sampling where the written scheme requires it. Trend results against the baseline and investigate exceptions; do not presume progressive improvement following implementation.
GB BPR status and verification
ChloroKlean products are supplied for their stated Great Britain uses under applicable GB BPR transitional arrangements while the relevant chlorine dioxide precursor-route and product-type combinations remain in the GB Review Programme. This is not a claim of full product authorisation. No EU or Northern Ireland authorisation is claimed.
Verify the exact precursor route and product type, current Review Programme status and deadlines, the applicable GB Article 95 supply chain, product label and other national requirements. Article 95 listing is a supply-chain requirement, not product authorisation, an efficacy assessment or an HSE endorsement.
Key Data & Statistics
Verify
Legionella outcomes must match product evidence and site conditions
20–45°C
Danger zone temperature range for Legionella growth
ACOP L8
HSE Approved Code of Practice for Legionella control
Condition-specific
Legionella reductions require the named organism, matrix, concentration, contact time and method
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 evidence | Depends on product and test conditions | Depends on heat coverage and cleaning | Depends on product and system conditions | Depends on product and test conditions |
| Coverage and persistence | Verify at representative points | Verify temperature coverage | Verify at representative points | Verify at representative points |
| Dead legs | Design correction remains necessary | Design correction remains necessary | Design correction remains necessary | Design correction remains necessary |
| Temperature response | Verify under site conditions | Temperature programme is condition-specific | Verify under site conditions | Verify under site conditions |
| Scalding Risk | None - chemical treatment | Significant - requires TMVs at outlets | None | None |
| DBP Formation | No chloramines; generally very little THM/HAA; measure chlorite/chlorate | Process-specific by-products | Process-specific by-products | THMs and chloramines can form |
| ACOP L8 fit | Must form part of a site-specific written scheme | Must form part of a site-specific written scheme | Must form part of a site-specific written scheme | Must form part of a site-specific written scheme |
| Ongoing monitoring | Validate method and locations | Validate method and locations | Validate method and locations | Validate method and locations |
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 representative residual monitoring and continue Legionella sampling in accordance with the written scheme. Record monitoring, dosing, maintenance and corrective actions. Competent review of the complete results must determine whether the programme remains within product conditions and meets its acceptance criteria.
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.
Related Products
BPR-compliant chlorine dioxide products available from ChloroKlean.
ChloroKlean Plus L20
Regenerative liquid chlorine dioxide; current product evidence and site verification govern organisms, concentration, contact time and intended use.
ChloroKlean PT5-200
Chlorine dioxide solution supplied for stated PT5 uses under applicable GB BPR transitional arrangements; separate drinking-water requirements also apply.
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. ChloroKlean products rely on applicable GB BPR transitional arrangements for stated Great Britain uses; Article 95 is not product authorisation, and no EU or Northern Ireland authorisation is claimed.
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Contact Us
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
- GB Review Programme for existing biocidal active substances
HSE - Health and Safety Executive
- GB Article 95: the basics
HSE - Health and Safety Executive
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