Legionella Control under ACOP L8
A duty holder's guide to meeting ACOP L8 and HSG274 obligations for Legionella control in hot and cold water systems and evaporative cooling systems, using regenerative chlorine dioxide.
ACOP L8 sets out the statutory framework for controlling Legionella bacteria in water systems in the United Kingdom. It is supported by the HSG274 guidance documents covering evaporative cooling systems (Part 1), hot and cold water systems (Part 2), and other risk systems (Part 3). This page summarises the duty holder responsibilities under that framework and explains how regenerative chlorine dioxide - specifically ChloroKlean Plus L20 - fits a compliant written scheme of treatment. It is not legal advice; it is technical guidance for facilities engineers, water treatment specialists, and responsible persons.
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
Chlorine dioxide is recognised in HSG274 as a suitable biocide for both evaporative cooling systems (Part 1) and hot and cold water systems (Part 2). It is dosed continuously to maintain a free ClO2 residual at distal points across the system, with periodic Legionella sampling to confirm control.
The key reason for selecting ClO2 over alternative chemistries in ACOP L8-scope systems is biofilm penetration. Legionella lives in biofilm; chemistries that act only on the bulk water leave the colonising population intact. Regenerative ClO2 reaches into the biofilm matrix and removes the embedded population, which is what ends repeat-positive cycles.
For systems with persistent Legionella positives, transitioning to a sustained sub-0.5 ppm ClO2 regime under ChloroKlean Plus L20 commonly clears the system within the first treatment cycle. This is documented in our hospital, cooling tower, and leisure case studies.
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. Many systems hit target residual at the dose point and zero at the far end of the loop. Sustained-residual regenerative ClO2 mitigates this because the active continues to liberate fresh ClO2 as it travels the loop. Distal sampling must still confirm coverage.
Biofilm-protected Legionella in dead legs and fittings. Shock-dose chemistries do not reach these. ClO2 diffuses into biofilm and oxidises the embedded cells. Where dead legs are unavoidable, sustained ClO2 with documented monitoring is the strongest available control.
Recurrent positives after temperature-only control. Hot and cold water systems controlled by temperature alone (target 60 C flow, 50 C return, less than 20 C cold) sometimes still throw positives where temperatures slip in service. Adding ClO2 dosing provides a second defence and is recognised in HSG274 Part 2.
Operator complexity. Shock chemistries and dual-biocide programmes are harder to run consistently. A single sustained ClO2 chemistry is simpler to dose, monitor, and audit.
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
<0.5ppm
Typical sustained ClO2 residual achieving ACOP L8-compliant control
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 per the revised scheme. Run intensified residual monitoring during the first 30 days. Take confirmatory Legionella samples at 14 and 28 days. Document the transition in the scheme and the operating record.
Settle to Steady-State Monitoring
Once the system is in spec, settle into the steady-state monitoring frequency in the scheme. Review trend data monthly. Audit the scheme annually or after any system change.
Expert Insights
"The single most important thing under ACOP L8 is that the residual you measure at the far end of the loop matches what your written scheme says it should be. Everything else - chemistry choice, dose calculation, sampling frequency - flows from getting that right."
Gavin Owen
Managing Director, ChloroKlean
"If a system has any history of recurrent Legionella positives, the question is almost never about peak dose. It is about whether the biocide reaches the biofilm and whether the residual is continuous. Regenerative chlorine dioxide answers both."
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 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.
Related Resources
Continue exploring our knowledge base and product information.
ChloroKlean Plus L20 Dosing Guide
Dose rates and monitoring targets for the PT5 and PT11 applications referenced in ACOP L8 and HSG274.
Regenerative Chlorine Dioxide Explained
Why sustained residual matters for biofilm-resident Legionella populations.
Chlorine Dioxide vs Chlorine for Cooling Towers
Comparison evidence base for biocide selection in HSG274 Part 1 systems.
Hospital Legionella Case Study
Real-world clearance of persistent Legionella in an NHS estate using ChloroKlean Plus L20.
Legionella Control Overview
Wider context for Legionella control in UK water systems.
Compliance and BPR
GB BPR compliances for ChloroKlean products across PT5 and PT11.
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
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