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Stoves and water

Purifier vs filter

Water purifier or filter: what is the difference?

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.

Purifier vs filter at a glance

Filter removes
Bacteria, protozoa
Purifier adds
Viruses
Backcountry US
Filter is standard
Sewage risk areas
Purifier

The difference is viruses

The distinction is about viruses and nothing else. Filters use pore size, and viruses at roughly 0.02 microns are an order of magnitude smaller than the 0.1 to 0.2 micron pores in a backpacking filter. Straining them out isn't practical at usable flow rates, so purifiers reach for something else: chlorine dioxide, iodine, UV light, or an electro-adsorptive medium that catches particles smaller than its physical pores.

Where you need a purifier

Where you are decides which you need. Waterborne viruses come primarily from human sewage, so in remote North American backcountry, where the realistic hazards are giardia and cryptosporidium from wildlife, a filter is the appropriate tool and the one most guidance recommends. Travel where water may be contaminated with human waste and viruses become a genuine risk.

The middle path most people take

There's a practical middle path a lot of people use: carry the filter and add chlorine dioxide tablets as a lightweight backup. That covers the virus gap for a few dollars and a few grams, gives you a fallback if the filter clogs or freezes, and weighs less than carrying a purifier you rarely need. Chemical treatment does need contact time, and against cryptosporidium chlorine dioxide needs several hours.

What this means when you're choosing

  • Which one your destination actually needs, which is a question about sewage
  • Chlorine dioxide tablets as a light backup, which many people carry
  • Whether a purifier's flow rate is one you will tolerate

The common mistake

Buying a purifier for North American backcountry

Waterborne viruses come from human sewage, and in remote wilderness the risk is low enough that essentially nobody carries one. You pay in money, weight and flow rate for a capability you will not use. Save it for international travel.

Common questions

Do I need a purifier for hiking in the US?

For remote backcountry water, a filter covers the realistic risks, which are protozoa and bacteria from wildlife. Water downstream of towns, farms or heavy human use is a different question and worth treating more thoroughly.

Are chemical tablets as good as a filter?

They handle viruses, which filters don't, and they're slow against cryptosporidium, needing up to four hours with chlorine dioxide. They also don't remove sediment or improve taste. Carried together, tablets and a filter cover each other's gaps well.

Where this is defined

Where this comes up

Guides on Tom's Hiking where purifier vs filter 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.

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.

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