Chlorine Dioxide vs Ozone for Cooling Towers and Drinking Water

ClO₂

Chlorine Dioxide

Sustained residual across distribution system
Penetrates biofilm in fill packs and pipework
No bromate formation in bromide-containing water
No on-site generator required - lower capital cost
Effective across pH 4-10

O₃

Ozone (O₃)

Highest oxidation potential of common biocides
Fast point-of-dose efficacy
No residual - secondary disinfection still required
Forms bromate when bromide is present in source water
High capital cost (on-site generator required)

Detailed Comparison

Detailed comparison of chlorine dioxide versus ozone
Feature	Chlorine Dioxide	Ozone
Residual Disinfection	Yes Measurable residual across distribution	No Half-life ~20 minutes in water
Biofilm Penetration	Excellent Penetrates EPS matrix	Surface only Reacts too fast to diffuse into biofilm
Bromate Formation	None Does not oxidise bromide to bromate	Yes Regulated DBP in bromide-containing water
THM / HAA Formation	Generally very little Control chlorite, chlorate and other by-products	Generally little Control bromate and other oxidation products
Capital Cost	Low Liquid biocide, simple dosing pump	High On-site generator and contactor required
Energy Use	Low No on-site generation	High Continuous electrical generation
Legionella Efficacy	Proven >4-log reduction documented	Proven at point of dose But no downstream residual protection
pH Range Effectiveness	pH 4-10 Performance largely independent of pH	Reduced at high pH Faster decomposition above pH 8
GB BPR Status	Transitional Supply ChloroKlean: applicable in-situ precursor route and stated PT5/PT11 uses	Check Each Product Verify authorisation or applicable transitional status by route and product type

When to Choose Each

Choose Chlorine Dioxide When:

You need a measurable residual across long pipe runs
Biofilm control in cooling tower fill packs is essential
Source water contains bromide and you need to avoid bromate
Capital budget rules out an on-site ozone generator
ACoP L8 / HSG274 compliance with continuous biocide is required
Multiple distributed dosing points need simple, reliable injection

Consider Ozone When:

Single point-of-dose application with no distribution network
Removal of taste, odour, and colour at a treatment works
High organic load requiring strong oxidation before filtration
Capital budget for a generator and contactor is available
A secondary residual disinfectant is already in place downstream

Why Choose ChloroKlean Plus L20

If you're considering switching to chlorine dioxide, ChloroKlean Plus L20 is purpose-built for industrial and commercial applications.

Supplied for GB PT5 / PT11 Uses

ChloroKlean Plus L20 is supplied in Great Britain under transitional arrangements for the applicable in-situ precursor route and its stated PT5 and PT11 uses. This is not a claim of product authorisation; no EU or Northern Ireland authorisation is claimed.

Regenerative ClO₂ Chemistry

Stabilised, regenerative formulation delivers sustained antimicrobial action at low doses (0.1-0.5 ppm). No on-site generator, no contactor, no ozone destruction unit.

Independently Tested

BS EN 13623:2020 testing confirms >4-log Legionella reduction in cooling water. Documented biofilm penetration in field installations where ozone left residual fouling.

Regulatory and Scientific References

This comparison is informed by the following authoritative sources. Always refer to the latest published guidance.

WHO Guidelines

World Health Organization (WHO)

Guidelines for Drinking-water Quality: Chlorine Dioxide

WHO describes chlorine dioxide use and the need to control its chlorite and chlorate by-products.

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HSE HSG274 Part 1

Health and Safety Executive (HSE)

Legionnaires' disease: Technical guidance for cooling towers

HSG274 Part 1 sets out the technical requirements for biocide selection and residual maintenance in cooling tower systems.

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US EPA D/DBPR

US Environmental Protection Agency

Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules

EPA D/DBPR sets limits on bromate (10 µg/L) and other regulated by-products formed during ozonation of bromide-containing source water.

View source

DWI Guidance

Drinking Water Inspectorate (DWI)

List of approved products and chemicals for use in public water supply

For public water supply in England and Wales, DWI Regulation 31 approval and its conditions apply to the specific product and use. That approval is separate from GB BPR product authorisation or transitional status and does not imply approval of ChloroKlean.

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EU BPR 528/2012

European Parliament and Council

Biocidal Products Regulation

EU BPR applies in the EU and Northern Ireland. Great Britain has a separate GB BPR regime, where status must be checked by product, product type and active-substance precursor route; applicable transitional supply is distinct from product authorisation.

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HSE GB Review Programme

Health and Safety Executive (GB BPR)

GB Review Programme for existing biocidal active substances

HSE explains the transitional arrangements that may permit continued supply while an existing active-substance precursor route and product-type combination is under review.

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HSE GB Article 95

Health and Safety Executive (GB BPR)

GB Article 95: the basics

HSE explains the GB Article 95 active-substance supply-chain requirement. Article 95 evidence does not establish finished-product authorisation.

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Frequently Asked Questions

Ozone has a higher oxidation potential than chlorine dioxide (2.07 V vs 0.95 V), so it reacts faster at the point of dosing. However, ozone has no residual and a half-life of around 20 minutes in water, so it cannot maintain protection across a distribution system. Chlorine dioxide provides sustained biocidal action and a measurable residual, which is usually more important for cooling towers and drinking water networks under HSG274.

Yes. Ozone oxidises naturally occurring bromide in source water to bromate, a regulated disinfection by-product. The US EPA limit is 10 µg/L. Chlorine dioxide does not oxidise bromide to bromate, so it is preferred where source water bromide is elevated or where bromate compliance is marginal.

Cooling towers under HSG274 and ACoP L8 require a measurable biocide residual throughout the recirculating system, including the fill packs and basin where biofilm forms. Ozone is consumed too quickly to provide that residual, and is rarely cost-effective at typical cooling tower volumes. Chlorine dioxide penetrates biofilm in the fill, maintains residual, and dosing equipment is far simpler than ozone generation. See ChloroKlean for cooling towers.

Yes, in most cooling tower and distributed water applications. Chlorine dioxide dosing uses a simple metering pump and storage tank instead of an ozone generator, contactor, and destruction unit. The transition usually involves draining and cleaning the system, then commissioning a ClO₂ dosing rig sized to maintain 0.1 to 0.5 ppm residual. ChloroKlean's technical team can specify equipment and dosing for the changeover.

There is no blanket UK approval for chlorine dioxide or every chlorine dioxide product. For public water supply in England and Wales, the specific product and use must meet applicable Drinking Water Inspectorate Regulation 31 requirements and conditions; Scotland and Northern Ireland have equivalent but separate regimes. For GB BPR, verify one of two routes. For an authorised product, obtain its GB authorisation number and check the authorised uses and conditions. For transitional supply, obtain evidence that the specific active-substance precursor route and PT5 combination remains covered by the GB Review Programme, check applicable deadlines and the product label, and verify the GB Article 95 supply chain. Article 95 evidence is not product authorisation. ChloroKlean is supplied in Great Britain under transitional arrangements for its stated PT5 uses; no EU or Northern Ireland authorisation is claimed.

No. Ozone decomposes rapidly in water with a half-life of around 20 minutes, depending on pH and temperature. It leaves no measurable disinfectant residual, which is why systems using ozone almost always require secondary disinfection with chlorine, chloramine, or chlorine dioxide downstream of the ozone contactor.

Replace Ozone with a Simpler GB-Supplied Alternative

ChloroKlean Plus L20 is supplied in Great Britain under GB BPR transitional arrangements for its stated PT5 and PT11 uses, providing residual chlorine dioxide without the capital cost or complexity of an ozone generator.