Legionella Control under ACOP L8
A duty holder's guide to ACOP L8 and HSG274 obligations and the product-specific assessment of chlorine dioxide within a written scheme.
ACOP L8 sets out the framework for controlling Legionella risk in water systems. HSG274 provides guidance for evaporative cooling, hot and cold water, and other risk systems. This page summarises duty-holder responsibilities and the questions to address where a chlorine dioxide product is considered. It does not claim that ChloroKlean Plus L20 guarantees compliance, control or a particular dose.
The ACOP L8 Framework in Brief
Approved Code of Practice L8 (ACOP L8), Legionnaires' disease: The control of legionella bacteria in water systems, sets out what employers and people in control of premises must do under the Health and Safety at Work etc. Act 1974 and the Control of Substances Hazardous to Health (COSHH) Regulations to prevent Legionnaires' disease.
The duty holder must: identify and assess sources of risk, prepare a written scheme for preventing or controlling the risk, implement and manage the scheme, keep records, and appoint a competent responsible person.
HSG274 is the practical guidance underpinning ACOP L8. Part 1 covers evaporative cooling systems, Part 2 covers hot and cold water systems, and Part 3 covers other risk systems including humidifiers, spa pools, and other water uses.
Where Chlorine Dioxide Fits in the Scheme
HSG274 discusses chlorine dioxide within relevant water-treatment programmes. That does not establish a universal use pattern, residual target or sampling frequency. The selected product conditions and competent written scheme govern application and verification.
Biofilm can contribute to Legionella persistence, but chlorine dioxide biofilm evidence is specific to the organism, matrix, product, concentration and contact time. It does not prove Plus L20 removal of embedded populations or an end to repeat-positive cycles.
For persistent Legionella positives, investigate system design, temperatures, hydraulics, cleaning, biofilm, residual delivery and monitoring. Any Plus L20 regime must follow current product conditions, and outcomes must be reported from the specific case-study sampling rather than promised generally.
The Written Scheme of Treatment
The written scheme is the operational record of how the system is controlled. It must include: an up-to-date schematic of the water system, the assessed risks, the control measures (including the biocide chemistry and dose), the monitoring regime (residual, microbiology, temperatures), the responsible person, and review and audit arrangements.
Switching to a regenerative ClO2 chemistry requires the written scheme to be updated to reflect the new product, the dose strategy, the residual target at distal points, and the revised sampling regime. The transition itself should be documented including baseline sampling, transition window, and confirmatory sampling.
Records must be kept for a minimum of five years and made available for inspection. Audits should confirm the scheme is being followed and that the responsible person remains competent.
Common Failure Modes and How ClO2 Helps
Residual drop at distal points. Measure representative points and investigate demand, hydraulics and dosing. A regenerative description does not establish that fresh chlorine dioxide continues to be released throughout the loop.
Biofilm-protected Legionella in dead legs and fittings requires design review, cleaning and a verified treatment plan. Chlorine dioxide has condition-specific biofilm evidence, but is not a substitute for removing avoidable dead legs and should not be described as the strongest available control.
Recurrent positives require review of temperature, system design, operation, cleaning, treatment and monitoring. A competent assessment may consider supplementary chemical treatment, but no chemistry guarantees a second defence.
Operator complexity depends on the product, equipment, training, controls and monitoring. Do not assume a single chlorine dioxide programme is simpler than shock or dual-biocide arrangements.
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
ACOP L8
UK statutory framework for Legionella control in water systems
HSG274
HSE practical guidance underpinning ACOP L8 (Parts 1-3)
5yr
Minimum retention period for ACOP L8 records
No generic residual
ACOP L8 compliance depends on the written scheme, operation and verification
Implementing ClO2 in an ACOP L8-Scope System
Sequence for transitioning a water system onto regenerative chlorine dioxide while remaining within the ACOP L8 framework.
Confirm Scope and Duty Holder Roles
Identify the duty holder, the responsible person, and any external competent water treatment specialist. Confirm the system is in scope of ACOP L8 and which HSG274 part applies.
Update the Risk Assessment
Update the Legionella risk assessment to capture the proposed change in biocide chemistry. Capture any change in dose hardware, monitoring points, or sampling regime.
Revise the Written Scheme of Treatment
Amend the written scheme to specify ChloroKlean Plus L20, the target free ClO2 residual at distal points, the dose strategy, and the revised microbiology and residual sampling regime. Have the responsible person sign off.
Baseline Microbiology and Residual
Take a full baseline before transition: Legionella, TVC, ClO2 residual at all sampling points, temperatures. This is the dataset improvement will be measured against.
Transition and Confirmatory Sample
Begin dosing only under the revised scheme. Use the commissioning and confirmatory sampling schedule set by the competent risk assessment and document the transition in the operating record.
Settle to Steady-State Monitoring
Once the scheme's acceptance criteria are met, use its ongoing monitoring and audit frequencies. Review after system changes, adverse trends or results, and at the interval set by the competent risk assessment.
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.
Frequently Asked Questions
Common questions about this topic, answered by our technical team.
Important Safety and Compliance Information
- ACOP L8 compliance is a statutory obligation under UK health and safety law. This page is technical guidance only and does not replace ACOP L8, HSG274, or specialist advice.
- Duty holders must appoint a competent responsible person and engage competent water treatment specialists for ACOP L8-scope systems.
- Any change in biocide chemistry on an ACOP L8-scope system must be reflected in the written scheme of treatment and supported by appropriate risk assessment.
- ChloroKlean Plus L20 concentrate is a hazardous oxidiser. PPE per the Safety Data Sheet is required for handling.
- Residual monitoring at distal points is essential. Dose-point residual is not evidence of system coverage.
This page is guidance only and does not replace ACOP L8, HSG274, the product Safety Data Sheet, or the written scheme of treatment for the specific system. Always engage a competent person and a competent water treatment specialist for ACOP L8-scope systems. 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.
Related Resources
Continue exploring our knowledge base and product information.
ChloroKlean Plus L20 Dosing Guide
How product-specific dose and monitoring decisions should be obtained and verified.
Regenerative Chlorine Dioxide Explained
How claimed release behaviour and biofilm outcomes should be assessed.
Chlorine Dioxide vs Chlorine for Cooling Towers
Comparison evidence base for biocide selection in HSG274 Part 1 systems.
Hospital Legionella Case Study
Marketing summaries depict later non-detection in hospital water; original sampling records are unavailable, and elimination or treatment causation is not established.
Legionella Control Overview
Wider context for Legionella control in UK water systems.
Compliance and BPR
GB BPR transitional-supply information for stated ChloroKlean PT5 and PT11 uses.
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 1: Evaporative cooling systems
HSE - Health and Safety Executive
- HSG274 Part 2: Hot and cold water systems
HSE - Health and Safety Executive
- HSG274 Part 3: Other risk systems
HSE - Health and Safety Executive
- Biocidal Products Regulation (BPR, Regulation (EU) 528/2012)
ECHA - European Chemicals Agency
- HTM 04-01: Safe water in healthcare premises
UK Government - NHS England
- 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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