Disinfection with chlorine dioxide

Chlorine dioxide is a strong oxidant. It attacks cell components of microorganisms and also penetrates the resistant shells of protozoa such as Giardia and Cryptosporidium. That distinguishes it from chlorine and iodine at normal concentrations, which can hardly touch Cryptosporidium.

Its effect, however, is not a yes-or-no matter but a question of time. Bacteria and viruses are inactivated quickly. Giardia takes longer, and Cryptosporidium oocysts – the resistant resting forms of this parasite – only after a long contact time. Manufacturers often state several hours for this.

As with chlorine: cold slows the reaction, and turbidity shelters pathogens and uses up the active ingredient. Clear, not too cold water and the full contact time are therefore the basis for a safe result.

Many packs show several times: a shorter one for bacteria and viruses and a much longer one for protozoa. The most common mistake is drinking after the short time even though parasites are a concern. For surface water with livestock, wild animals or people nearby, the longest stated time always applies.

If the water is cold or slightly cloudy, use the manufacturer's longer figure, if one is given. It helps to treat water in the evening and drink it in the morning, or to work with two bottles in turn: one is drunk while the other is still working.

If you have no time, chlorine dioxide alone is not the right choice when Cryptosporidium is suspected. Then boiling, or a filter that retains protozoa combined with a short contact time against viruses, is the better solution.

Tablets are usually individually packed and designed for a fixed volume of water, such as a certain number of litres. They are light, easy to dose and easy to carry. Do not split them to treat smaller amounts, and do not estimate the water volume – otherwise the concentration is no longer right.

With two-part drops, two solutions are mixed. They react with each other for a set time before the mixture goes into the water. This is more flexible, because the amount can be adjusted, but also more error-prone: a wrong mixing ratio, too short an activation time or mixed-up bottles change the result.

Moisture, heat and light harm both forms. Damp or expired tablets lose strength. Store them dry, cool, protected from light and out of children's reach.

Use only products approved for drinking-water disinfection and follow their instructions exactly. Chlorine dioxide preparations promoted as a „remedy“ to be ingested are hazardous to health. They are not water treatment, and nothing in this text recommends drinking such products.

Drink only after the full contact time has passed. After dosing, wet the threads and the rim of the lid with the treated water so that no untreated drops remain there. If larger amounts of the concentrate are swallowed by accident, or if symptoms occur, call the local emergency number (112 in the EU) or a poison control centre.

Chlorine dioxide does not remove chemicals, heavy metals, salt or turbidity. Water from contaminated sources or seawater does not become drinkable with it.

On trips without fuel, chlorine dioxide is a light and compact solution, especially when there is enough time, for example overnight or during a long break. Many people find the aftertaste milder than with chlorine.

A very common combination is with a hollow-fibre filter. The filter retains bacteria and protozoa immediately but lets viruses through. The chlorine dioxide then covers the viruses, for which a comparatively short contact time per the manufacturer is enough. That way you quickly get water treated against all three groups of pathogens.

In winter, chlorine dioxide is an alternative to boiling meltwater when fuel is short. Because of the cold, however, waiting times become considerably longer – plan for them deliberately and keep the water as close to your body as possible.