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Stoves and water

Chlorine dioxide

Also called ClO2, chlorine dioxide tablets

What is chlorine dioxide water treatment?

Chlorine dioxide is the chemical released by backcountry purification tablets and two-part drops. It kills bacteria, viruses and Giardia, and is the only tablet chemistry the CDC calls somewhat effective against Cryptosporidium, which takes up to four hours.

Chlorine dioxide at a glance

Kills
Bacteria, viruses, Giardia
Cryptosporidium
Somewhat effective, about 4 hours
Clear-water wait
About 15 minutes
Regulated by
US EPA pesticide registration

How is chlorine dioxide different from chlorine and iodine?

Chlorine dioxide is a different molecule from the chlorine in bleach or in sodium dichloroisocyanurate tablets, and it behaves differently. It works across a wider pH range and tends to leave less taste behind. It also gets through the tough outer wall of protozoan cysts better than chlorine or iodine does.

That last part is what matters on the trail. The CDC says chlorine and iodine don't work well against parasites, while chlorine dioxide kills Giardia and is somewhat effective against Cryptosporidium.

How long does chlorine dioxide take to work?

Against bacteria and viruses in clear water, about 15 minutes. Against Cryptosporidium, or in cold or cloudy water, up to four hours. Katadyn's Micropur MP1 box prints that four-hour treatment time, and Andrew Skurka cites the same figure for Aquamira's drops. In practice that means a two-bottle rotation: treat one while you drink from the other.

Tablets or two-part drops?

Tablets combine the ingredients in one foil-sealed dose, usually one tablet per liter. Two-part drops keep a chlorine dioxide solution and an acid activator in separate bottles, which you mix for five minutes before adding them to the water.

Drops treat far more water per ounce carried, while tablets are simpler and harder to get wrong. Both are registered with the US EPA under the federal pesticide law, and the registration number on the pack is the one to look up.

What this means when you're choosing

  • An EPA registration number printed on the pack
  • Cryptosporidium named on the label with a treatment time
  • Individually foil-sealed tablets if it will ride in a pack for months

The common mistake

Drinking at fifteen minutes when Crypto is the risk

Fifteen minutes covers bacteria and viruses in clear water. Cryptosporidium needs hours, and cold or silty water slows everything down. The CDC's advice is to filter first and then disinfect, so the filter handles the parasites and the disinfectant covers viruses.

Common questions

Is chlorine dioxide the same as bleach?

No. Household bleach is sodium hypochlorite, which releases chlorine. Chlorine dioxide is a different compound, tastes milder, and is more effective against protozoan cysts.

Can I use chlorine dioxide instead of a filter?

Some ultralight hikers do, and they live with the longer wait. It won't remove sediment, though, so cloudy water still benefits from straining through a cloth or a filter first.

Where this is defined

Where this comes up

Guides on Tom's Hiking where chlorine dioxide changes which product is right for you.

More in stoves and water

Canister stove

A canister stove screws directly onto a sealed cartridge of pressurised isobutane-propane. It lights instantly, simmers well and needs no priming, which is why it's the default backpacking stove almost everywhere fuel canisters are sold.

Integrated stove system

An integrated stove system pairs a burner with a matched pot that locks on, usually with a heat exchanger ring on the base. It boils water faster and uses less fuel than a separate stove and pot, at the cost of versatility.

Regulated valve

A regulated valve holds a stove's fuel pressure steady as the canister empties or cools. Without one, output drops off through the canister's life and falls sharply in the cold; with one, the flame stays consistent much further down.

Isobutane-propane blend

Isobutane-propane is the fuel blend inside standard threaded backpacking canisters, usually around 80 percent isobutane and 20 percent propane. Propane provides cold-weather pressure, isobutane provides most of the burn time.

Hollow fiber filter

A hollow fiber filter passes water through a bundle of tiny straw-like tubes whose walls are perforated with pores around 0.1 to 0.2 microns. It's the technology in almost every modern backpacking filter, and a single freeze can destroy it invisibly.

Micron rating

A micron rating states the largest particle a filter lets through, in microns. Backpacking filters are typically rated 0.1 or 0.2 microns, which stops bacteria and protozoa. Viruses are around 0.02 microns and pass straight through.

Flow rate

Flow rate is how fast a filter delivers clean water, usually quoted in liters per minute. The advertised figure is measured on a brand-new filter with clean water, and real flow drops as the element clogs with sediment.

Backflushing

Backflushing pushes clean water backwards through a filter to dislodge the sediment clogging its pores. It's routine maintenance on a hollow fiber filter, and it's the difference between one that lasts a season and one that lasts years.

Purifier vs filter

A filter physically strains out bacteria and protozoa. A purifier also deals with viruses, which are too small to strain, using chemistry, UV light or a much finer element. Most backcountry hiking needs a filter; international travel often needs a purifier.

Soft flask

A soft flask is a collapsible bottle, usually 250 to 750 ml, carried in the front pockets of a hydration vest. It shrinks as you drink so it doesn't slosh, and you can see exactly how much water you have left.

Hydration bladder

A hydration bladder is a flat water reservoir carried inside a pack with a drinking hose routed over the shoulder. It holds 1.5 to 3 liters, packs flat against your back and lets you drink without stopping or taking the pack off.

Bite valve

A bite valve is the silicone mouthpiece on the end of a hydration reservoir's drink tube. It stays shut until your teeth compress a slit in it, then self-seals when you let go, so water only flows while you are drinking.

Quick disconnect

A quick disconnect is the two-part coupling in a hydration hose that lets you unclip the tube from the reservoir without threading it back through your pack. Both halves usually self-seal, so you can pull the bag out to refill and neither end drips.

Hard-anodized aluminum

Hard-anodized aluminum is aluminum whose surface has been grown into a thick, very hard oxide layer by an electrochemical bath. In cookware it gives a pot that spreads heat far more evenly than titanium and resists scratching far better than bare aluminum.

Vacuum insulation

Vacuum insulation is two stainless steel walls with the air pumped out of the gap between them. With no air to carry heat, cold water stays cold and hot water stays hot for most of a day, and the outside of the bottle never sweats.

Tritan

Tritan is a clear, tough copolyester plastic made by Eastman without BPA. It is what most modern hard plastic water bottles, including the Nalgene and the CamelBak Chute, are made from, because it resists cracking and staining and survives the dishwasher.

By Tom · Last reviewed . Back to the glossary