Chlorine dioxide (ClO₂) is a neutral, yellow-green gas that dissolves in water to form a powerful oxidising biocide. It disinfects water and surfaces by oxidation rather than chlorination, so it controls bacteria, viruses and biofilm without forming trihalomethanes, and it stays effective across pH 4 to 10.
Reviewed by Gavin Owen, Managing Director, ChloroKlean. Figures reflect UK and WHO guidance; always confirm dosing against the product label and a site-specific risk assessment.
Chlorine dioxide disinfects by selective oxidation: it strips electrons from the cell walls and key proteins of microorganisms, causing rapid cell death. Because it oxidises rather than chlorinates, it behaves very differently from traditional chlorine.
Chlorine dioxide is a strong oxidising biocide that damages microbial cell walls and disrupts protein synthesis. It works by oxidation, not chlorination, so it does not form chlorinated by-products.
ClO₂ penetrates and breaks down biofilm, reaching the bacteria - including Legionella - that shelter within it where traditional chlorine struggles.
It stays effective across a wide pH range of roughly 4 to 10, unlike chlorine which loses effectiveness above pH 7.5.
Because it does not chlorinate organic matter, chlorine dioxide does not produce trihalomethanes (THMs) or haloacetic acids (HAAs), and it does not form chloramines with ammonia.
Chlorine dioxide is not the same as chlorine. It is a more selective oxidiser that stays effective across pH 4-10, penetrates biofilm, and does not form trihalomethanes (THMs) or chloramines. Chlorine, by contrast, loses effectiveness above pH 7.5 and can form regulated disinfection by-products.
For a side-by-side breakdown of efficacy, by-products, pH range and cost, see the full comparison.
Chlorine dioxide is authorised across several Biocidal Products Regulation (BPR) product types and used throughout UK water treatment, healthcare, leisure and food sectors.
Potable water treatment in municipal and private supplies, controlling Legionella, coliforms and other waterborne pathogens.
Biocidal treatment of cooling towers and closed circuits, controlling Legionella and biofilm in line with HSG274 guidance.
Slime and biofilm control in paper mills, biogas plants and other industrial process water systems.
Surface and water disinfection in swimming pools, spas and public leisure environments.
Used as directed at the low concentrations typical of water treatment, chlorine dioxide has a long safety record. It has been used by water utilities, hospitals and food processors worldwide for decades, and all ChloroKlean products are BPR-compliant following mandatory human health and environmental assessments.
Approved under PT5 and used globally for potable water at residuals of 0.1-0.5 mg/L.
Reliable disinfection across a wide pH range where chlorine underperforms.
Oxidises rather than chlorinates, so it forms no trihalomethanes or chloramines.
Typical chlorine dioxide dose and residual figures by application. These are general reference ranges - always confirm against the product label, local regulations and a site-specific risk assessment.
| Application | Typical ClO₂ dose / residual | Contact / notes | Key standard |
|---|---|---|---|
| Potable (drinking) water residual | 0.1-0.5 mg/L residual ClO₂ | Maintained through the distribution system; residual monitored routinely. | DWI / WHO drinking-water guidance |
| Legionella control - hot & cold water systems | 0.3-0.5 mg/L continuous residual (up to ~1 mg/L for remediation) | Continuous proportional dosing; sample at sentinel and distal outlets. | HSE HSG274 Part 2 / ACOP L8 |
| Cooling towers - continuous dose | 0.1-0.5 mg/L residual | Maintained during operation to control biofilm and Legionella. | HSE HSG274 Part 1 |
| Cooling towers - shock dose | Up to ~1-5 mg/L short-term | Periodic shock/remedial dose for heavily fouled systems. | HSE HSG274 Part 1 |
| Food processing / CIP rinse | Typically up to ~3 mg/L on food contact surfaces | Contact time per label; final rinse to potable standard where required. | HACCP / PT4 label directions |
| Spa & pool water | ~0.1-0.5 mg/L residual (product-specific) | Low-dose continuous control of biofilm, Legionella and Pseudomonas. | HSE HSG282 / PWTAG guidance |
This article references guidance from the following authoritative sources:
WHO - World Health Organization
HSE - Health and Safety Executive
HSE - Health and Safety Executive
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ChloroKlean is a UK principle stockist of BPR-compliant chlorine dioxide products. Contact our team for product selection and dosing guidance.