Chlorine Dioxide, Chlorite and Chlorate: Which UK Drinking-Water Limits Actually Apply

What the World Health Organization actually says

The WHO Guidelines for Drinking-water Quality (fourth edition, incorporating the first and second addenda) set provisional guideline values of 0.7 mg/L (700 µg/L) for chlorite and 0.7 mg/L for chlorate. They are designated provisional because using chlorine dioxide, or aged hypochlorite, as a disinfectant may cause them to be exceeded, and WHO states that difficulty meeting them must never be a reason to compromise adequate disinfection. WHO notes that when chlorine dioxide is used as final disinfectant at typical doses, chlorite is normally below 0.2 mg/L.

WHO has not established a guideline value for chlorine dioxide itself, because any chlorine dioxide remaining at the tap is reduced to chlorite and chloride on ingestion and the chlorite and chlorate values are considered protective. WHO gives a taste and odour threshold for chlorine dioxide of 0.2 to 0.4 mg/L. Any statement that WHO sets chlorite at 0.5 mg/L, or sets a 0.5 mg/L limit for chlorine dioxide, is incorrect.

The UK condition for water entering public supply

In England and Wales, water undertakers may only use treatment chemicals approved under Regulation 31 of the Water Supply (Water Quality) Regulations 2016 (and the equivalent Welsh regulations), administered by the Drinking Water Inspectorate. The DWI's conditions for on-site generation of chlorine dioxide state that the combined concentration of chlorine dioxide, chlorite and chlorate shall not exceed 0.5 mg/L, expressed as chlorine dioxide, in the water entering supply. This is a single combined figure, expressed as chlorine dioxide, applying at the point where treated water enters the public distribution system. It is a condition of product approval for public supply, not a WHO value and not a concentration of chlorite alone.

Scotland and Northern Ireland operate their own regulations and regulators, with equivalent approval regimes. Private water supplies in England and Wales are regulated separately under the Private Water Supplies Regulations, enforced by local authorities with DWI oversight. Whether and how a combined 0.5 mg/L condition applies to a given private supply or on-site treatment installation depends on the applicable regulations and the product's approval; it should not be assumed to be universal.

HSE guidance for chlorine dioxide inside buildings

HSE's HSG274 Part 2, the technical guidance on controlling Legionella in hot and cold water systems, addresses chlorine dioxide used within buildings. It reminds duty-holders that national conditions of use require the combined concentration of chlorine dioxide, chlorite and chlorate in the drinking water not to exceed 0.5 mg/L as chlorine dioxide, and states that maintaining a chlorine dioxide residual of 0.1 to 0.5 mg/L (as total oxidant) at an outlet is usually sufficient to control Legionella in the preceding pipework, with higher residuals sometimes necessary in heavily colonised systems. Where control cannot be achieved without total oxidant exceeding 0.5 mg/L at outlets used for drinking, HSG274 says those outlets should be labelled unsuitable for drinking or fitted with suitable point-of-use activated-carbon filtration.

HSG274 also sets an expectation of monitoring: chlorine dioxide and total oxidant/chlorite checked monthly near the injection point to verify conversion yield, at least 0.1 mg/L chlorine dioxide at sentinel taps monthly, and an annual survey of representative outlets. The 0.1 to 0.5 mg/L figure is therefore an operating range within a legal ceiling, not a WHO limit and not a product claim.

Putting the numbers in their places

In summary: 0.7 mg/L chlorite and 0.7 mg/L chlorate are WHO provisional health-based guideline values for each ion individually. 0.5 mg/L as chlorine dioxide is the UK condition for the combined total of chlorine dioxide, chlorite and chlorate in water entering public supply, echoed by HSE for building systems. 0.1 to 0.5 mg/L is HSE's typical operating range for chlorine dioxide residual at outlets for Legionella control. None of these is a ChloroKlean specification, and this guide does not set dosing for any product or site.

For any specific installation, confirm which regulations apply (public supply, private supply, building water system, or a non-potable process), what the product's authorisation and DWI approval status permit, and what the site's written scheme requires, with a competent adviser.

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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