Chlorine Dioxide CT Values and Contact Time: Why One Number Cannot Cover Every Organism

What CT is and where it comes from

CT expresses exposure as the disinfectant concentration (mg/L) multiplied by contact time (minutes), giving units of mg·min/L. It descends from the Chick-Watson model, in which the log reduction of a population is proportional to concentration raised to an empirically fitted exponent, multiplied by time. Regulators use CT tables to specify how much exposure a treatment step must deliver for a stated log reduction of a stated organism at a stated temperature and pH.

Two assumptions sit inside every CT figure: that the concentration was actually maintained through the contact period, and that the organism, matrix and temperature match those for which the value was derived. Chlorine dioxide decays through reaction with organic matter, biofilm and some materials, so the concentration applied and the concentration experienced can differ; and the second assumption is the subject of this guide.

The 2026 Water Research kinetics study

Seo and colleagues (Water Research, Volume 300, 1 August 2026, article 125919; doi 10.1016/j.watres.2026.125919) measured CT-based inactivation kinetics of chlorine dioxide against Escherichia coli O157:H7, methicillin-resistant Staphylococcus aureus (MRSA) and Bacillus subtilis in both vegetative and spore form, in buffered water and in river water. Inactivation followed second-order CT kinetics. Rate constants for vegetative cells ranged from about 0.13 to 0.54 L per mg·min, whereas B. subtilis spores gave about 0.0031 L per mg·min, roughly forty to a hundred and seventy times lower.

Electron microscopy showed concentration-dependent membrane destabilisation, vesiculation, deformation and leakage, and biochemical assays showed protein release and lipid peroxidation, linking loss of culturability to progressive envelope damage and oxidative stress rather than instant lysis. The authors attribute the species differences to cell-envelope structure and oxidative-stress responses. Chlorine dioxide also degraded an extracellular plasmid-borne antibiotic resistance gene target in a size-dependent way, predominantly through base damage rather than strand breakage. In river water, E. coli inactivation followed a CT relationship comparable to buffered water, with little effect of filtration.

Why one CT value cannot represent every microorganism

If the rate constant for a spore is two orders of magnitude lower than for a vegetative cell, the CT needed for the same log reduction is two orders of magnitude higher. A CT that gives four logs of E. coli may give a fraction of one log of spores; protozoan cysts and oocysts, certain viruses and biofilm-embedded cells each sit somewhere else on that scale. This is why regulatory CT tables are organism-specific and why 'the CT for chlorine dioxide' is not a meaningful phrase without naming the target, temperature, pH and matrix.

The organism-specific spread also interacts with the first assumption above. A CT calculated from an initial dose over-estimates exposure if the residual decays, and the over-estimate matters most for the organisms that need the highest CT. Where spores, cysts or established biofilm are the concern, measured residual through the contact period, rather than applied dose, is the relevant concentration.

What these values are and are not

They are laboratory kinetic constants for named strains under the study's conditions, useful for understanding relative susceptibility and for designing further studies. They are not ChloroKlean dosing instructions, not a product efficacy claim, and not a substitute for the concentration, contact time and conditions stated on an authorised product's label or in a site's written scheme. The study did not test any ChloroKlean product, did not examine biofilm, and ran in buffered and river water rather than in a distribution system, CIP circuit or cooling tower.

For a specific application, the questions to answer are: which organism or indicator is the target, what log reduction is required and by whom, what residual can be maintained through the contact time at the site's temperature and demand, and how that will be verified. Those answers come from the relevant guidance, the product authorisation and site data, not from a single CT figure.

Evidence and uncertainty

Published biofilm studies are valuable for understanding mechanisms, but their conditions may not match a particular installation. Species, surfaces, deposits and operating conditions should be recorded when interpreting evidence.

For safety-critical systems, decisions should be documented through the relevant risk assessment and management plan.

Choosing the next question

A useful next step is to identify what is known, what is inferred and what needs verification. This avoids treating a general reference as a site diagnosis.

Where a product is considered, confirm the intended use, authorisation and label directions independently of this educational guide.

Use this sequence to frame investigation and control; it is not a dosing protocol.

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