Chlorine dioxide in drinking water: limit values, chlorite and chlorate

After treatment, drinking water may contain at most 0.2 mg/l of chlorine dioxide; at most 0.4 mg/l may be added. Its use produces the reaction products chlorite and chlorate. The Drinking Water Ordinance sets limit values for both: 0.20 mg/l for chlorite and 0.070 mg/l for chlorate for continuous dosing.

Laboratory glassware for determining residual chlorine dioxide concentrations in water

Which limit values apply to chlorine dioxide and its by-products?

The figures come from two sources: the list of permitted treatment substances under Section 20 of the Drinking Water Ordinance, kept by the German Environment Agency, and Annex 2 of the Drinking Water Ordinance, with the chemical limit values.

Parameter	Value	Source	Note
Chlorine dioxide, addition	at most 0.4 mg/l	UBA list, Part I c	Purpose: disinfection
Chlorine dioxide after treatment	0.05 to 0.2 mg/l	UBA list, Part I c	Minimum and maximum content
Chlorite	0.20 mg/l	TrinkwV, Annex 2	deemed to be met if no more than 0.20 mg/l of chlorine dioxide is added
Chlorate	0.070 mg/l	TrinkwV, Annex 2	0.20 mg/l for temporary dosing; met with chlorine dioxide if no more than 0.20 mg/l is added
Trihalomethanes, total	0.050 mg/l	TrinkwV, Annex 2	Reaction product of chlorine, hypochlorites and ozone
Bromate	0.010 mg/l	TrinkwV, Annex 2	Reaction product of ozone and hypochlorites

Sources: UBA list under Section 20 TrinkwV, as of January 2023; Drinking Water Ordinance, Annex 2.

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What are chlorite and chlorate?

Chlorite and chlorate are reaction products that form when chlorine dioxide reacts or breaks down in water. The list under Section 20 TrinkwV names both explicitly as "reaction products to be observed" for chlorine dioxide. For chlorite it adds that a maximum value of 0.2 mg/l must be met after treatment is complete, and that this is deemed to be met if no more than 0.2 mg/l of chlorine dioxide is added.

For chlorate, the list names two maximum values: 70 µg/l for continuous addition and 200 µg/l for temporary addition, where disinfection cannot otherwise be assured. The Drinking Water Ordinance gives the same values in Annex 2, in milligrams: 0.070 and 0.20 mg/l.

Does chlorine dioxide form trihalomethanes?

The list under Section 20 TrinkwV names only chlorite and chlorate as reaction products for chlorine dioxide. It lists trihalomethanes for chlorine, sodium and calcium hypochlorite, and for ozone. Unlike chlorine, chlorine dioxide reacts with organic substances in water predominantly by oxidation rather than chlorination. That is why chlorite and chlorate, not trihalomethanes, are the by-products to watch for with chlorine dioxide.

Who has to monitor the values?

Anyone who treats drinking water continuously with a treatment substance must have the remaining concentration after treatment tested daily by trained personnel. This is set out in Table 1a of the UBA list. Munich's Department of Health requires that, for disinfectants in the drinking water installation, the concentration be determined and documented daily at the tap closest to the dosing point and, where applicable, at hydraulically unfavourable points.

If an operator uses a treatment substance, they must inform connected parties and consumers in writing about the start of its use and about the concentration in the drinking water, under Section 26 TrinkwV. How the residual concentration is measured is explained on the page Sampling and measurement.

What does this mean for using biostream® ZERO?

The dosing instructions distinguish two cases. At the low dosage, which according to the instructions corresponds to the Drinking Water Ordinance, the product remains in the water, and according to the dosing instructions the water can be used as drinking water. Levels 1 to 3, by contrast, are installation disinfections: after the contact time, containers and pipes are completely emptied and refilled with drinking water. According to the dosing instructions, any remaining product is then diluted enough for the water to be used as drinking water again.

The dosing calculator calculates the quantity of concentrate for your volume of water.

Frequently asked questions about limit values

How much chlorine dioxide may be added to drinking water?

At most 0.4 mg/l. After treatment, the content must be between 0.05 and 0.2 mg/l. Both values are set out in the list of permitted treatment substances under Section 20 of the Drinking Water Ordinance.

Does chlorite in drinking water have to be tested?

Under Annex 2 of the Drinking Water Ordinance, the chlorite limit value of 0.20 mg/l is deemed to be met if no more than 0.20 mg/l of chlorine dioxide is added. Testing can generally be waived if disinfection with chlorine dioxide does not take place.

What applies to chlorate for temporary dosing?

For temporary dosing, a chlorate limit value of 0.20 mg/l applies instead of 0.070 mg/l. For disinfection with chlorine dioxide, it is deemed to be met if no more than 0.20 mg/l of chlorine dioxide is added.

Why is the water drinkable again after installation disinfection?

Because the disinfecting water is discarded after the contact time. After installation disinfection, thorough flushing is required to rinse out any remaining disinfectant; this is also stated in the DVGW guidance on drinking water supply at events. With biostream® ZERO, containers and pipes are completely emptied and refilled with drinking water; for dispensing systems, the dosing instructions state that flushing with twice the system's water volume is sufficient.

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