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	None Does not chlorinate organics	None Forms aldehydes/ketones instead
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
UK BPR Status	PT5/PT11 Compliant Full GB BPR compliance	PT5/PT11 Available Also requires PT authorisation

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

"Ozone is a phenomenal oxidiser at the point of dosing - but the moment treated water leaves the contactor, you have no protection against biofilm regrowth. In a UK cooling tower running under ACoP L8, that's the wrong trade-off. Chlorine dioxide gives you the same biocidal kick, a measurable residual at every sample point, and none of the bromate risk that comes with ozonating bromide-containing make-up water."

Gavin Owen, Managing Director, ChloroKlean

BPR-compliant disinfection specialist

Why Choose ChloroKlean Plus L20

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

BPR PT5 / PT11 Compliant

ChloroKlean Plus L20 holds full UK GB BPR compliance for drinking water (PT5) and cooling system preservation (PT11). Direct replacement for ozone in distributed systems.

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 recognises chlorine dioxide as an effective drinking water disinfectant that does not form trihalomethanes, haloacetic acids, or bromate.

View source

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.

View source

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

The DWI maintains the list of approved chemicals for treatment of water destined for public supply in England and Wales, including chlorine dioxide formulations.

View source

EU BPR 528/2012

European Parliament and Council

Biocidal Products Regulation

Both chlorine dioxide and ozone require PT5 (drinking water) or PT11 (cooling system) authorisation under the UK GB and EU BPR.

View source

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.

Yes. Chlorine dioxide is approved for use in UK public drinking water supply, subject to product listing under the Drinking Water Inspectorate approval scheme and GB BPR (PT5) authorisation. WHO Guidelines for Drinking-water Quality also list chlorine dioxide as an effective disinfectant.

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, Compliant Alternative

ChloroKlean Plus L20 delivers BPR-compliant, residual chlorine dioxide for cooling towers and drinking water - without the capital cost or complexity of an ozone generator.