Carry two ways to make water safe, not one good filter

Quick answer: Carry a hollow-fiber filter as your main method and a backup that can't break: chlorine dioxide tablets, a stove, or both. Test the filter at home. Sleep with it in your bag when nights drop below freezing.

The failure you can't see

It's 5:40 a.m. at 9,000 feet. Frost on the tent. You pull the filter out of the side pocket of your pack, where it spent the night, and it works fine. Water comes out the clean end. You drink it all day.

It may have been frozen solid at 3 a.m.

Hollow-fiber filters, the Sawyer Squeeze and Katadyn BeFree kind, work by pushing water through bundles of tiny straws with pores around 0.1 micron. Water that freezes inside those straws expands and can crack them. The cracks are invisible. The filter still pumps and still looks clean, but it's now letting through the very things it was built to stop. Manufacturers say the same thing: if it froze, it's done.

That's the worst kind of failure, because nothing tells you it happened.

So in cold weather, the filter sleeps in your sleeping bag. In the footbox, or in a stuff sack near your chest. It's a ten-second habit that protects a forty-dollar piece of gear and the stomach you're attached to.

What you're actually filtering out

Three kinds of trouble live in backcountry water.

• Protozoa like giardia and cryptosporidium. These are the common ones, and any 0.1 micron filter stops them.
• Bacteria like E. coli, salmonella and campylobacter. Hollow-fiber filters stop these too.
• Viruses, which are smaller than the pores. Hepatitis A and norovirus are the ones to know. Filters alone don't stop them.

In most U.S. backcountry, protozoa and bacteria are the realistic threat, which is why a filter is the standard tool. Viruses matter more when you're near heavy human traffic, downstream of a campground, or traveling outside the country. That's where a backup that handles viruses earns its place.

The three ways a filter quits on you

It clogs. This is the most common one. Silty water, snowmelt, a slow stream in late summer. Flow drops to a trickle and you're squeezing a bag for ten minutes to fill one bottle. Backflush it with the syringe that comes with the Sawyer, or swish and shake the BeFree in clean water. A pump-style filter with a ceramic element can be scrubbed and cleaned in the field.

Better yet, don't feed it mud. Let the water settle in a pot or bottle for twenty minutes, or pour it through a bandana or a coffee filter first. Fill from the clearest water you can reach. A small pool below a riffle beats a stagnant edge every time.

It freezes. Covered above. Treat it as dead if you can't be sure.

It gets damaged. A drop onto rock, a pack that got crushed under something heavy, a filter left in a hot truck and then a cold night. If something happened to it and you have doubts, you don't have a filter. You have a bag with a straw.

The backup that weighs nothing

Chlorine dioxide tablets or drops, Katadyn Micropur or Aquamira, weigh almost nothing and take up less room than a lighter. They handle bacteria, viruses and protozoa. Read the label before you leave. The wait time is about thirty minutes for most things and as long as four hours for cryptosporidium. Cold water and cloudy water slow chemicals down, so run cloudy water through the bandana first and give cold water extra time.

Tablets have a weakness too. They're slow, and they taste a bit like a swimming pool. But they can't crack, clog or freeze in a way that fools you. That's the whole point of a backup.

The second backup is the stove you're already carrying. The CDC says a rolling boil for one minute makes water safe, and three minutes above 6,500 feet. It costs you fuel and time, and it leaves you with hot water that needs cooling. It's also the only method here that kills everything and has no expiration date.

You don't need all three. A filter plus tablets is the lightest pairing. A filter plus a stove works fine if you were bringing the stove anyway. What you don't want is one method and nothing else.

Test it before you trust it

Nobody wants to learn their filter is broken at mile eleven. At home, the night before:

1. Fill the dirty bag or bottle from the kitchen sink.
2. Run a full liter through the filter and time it.
3. Backflush it, then run another liter.
4. Check for leaks at every connection.
5. Let it dry fully before it goes in the pack, so it doesn't grow something in storage.

If flow is much slower than you remember, backflush again. If it's still slow, replace it. A filter that's tired at the sink will be useless at the trailhead.

How much to carry, and where to get it

A rough rule is about half a liter per hour of moderate hiking, more in heat or at altitude. Water weighs about 2.2 pounds per liter, so carrying three liters is over six pounds. That's why you filter along the way and carry only enough to reach the next source.

Check your route before you leave. Look at the map, ask the ranger station, and read recent trip reports. Springs dry up. Streams in late summer may be a puddle or a trickle. If your plan depends on a creek that turns out to be dry, you need a dry-camp carry that you didn't pack for. If you're planning your first overnight, the checklist in Your First Overnight Backpacking Trip and What to Leave Home covers how much to carry and what to cut.

Pick sources the way you'd pick a campsite. Moving water over standing. Upstream of camp, trails, stock and obvious animal use. Away from beaver ponds and cattle, which are common culprits for giardia. For a campsite that gives you a clean way out if something goes wrong, see Pick a campsite you can bail from in forty minutes.

Leave the water better than you found it

Do your washing, brushing and dishes at least 200 feet from any lake or stream. Carry water to your spot in a bucket or collapsible container instead of washing at the edge. Soap, even the biodegradable kind, doesn't belong in the water you or the next person will drink. Dig catholes at the same 200-foot distance, and pack out toilet paper if the area requires it.

And if you find a bottle or a wrapper at the water's edge, carry it out. You'll be thanked by someone you never meet.

Where this sits in the bigger kit

Water is one of the items on the old list for a reason. The Ten Essentials and Why Veterans Still Carry Them is built on the same idea as this piece. Redundancy isn't paranoia. It's the difference between an inconvenience and a rescue.

Pack the filter. Pack the tablets. Put the filter in your bag when the temperature drops. Then forget about it and enjoy the walk.