I once added up what I'd spent on bottled water across a three-week trip and it was genuinely depressing — not just the money, but the small mountain of empty plastic I'd personally created and left behind in a country that had nowhere good to put it. A €30 filter would have replaced all of it. I bought one as soon as I got home, which is the wrong order to do these things in, but here we are.
A portable water filter or purifier lets you turn questionable water into drinkable water, on the spot, without waiting or much faff. For a traveller that means two things: you can refill from streams on a walk instead of carrying a day's supply, and you can drink from taps you're not sure about instead of funding the bottled-water industry one room-temperature half-litre at a time.
But — and this is the part most posts skip — "filter" and "purifier" are not the same thing, and buying the wrong one for where you're going is a real mistake. Let me make the difference clear before anything else.
Filter vs. purifier: the one thing you must understand
The nasties in water come in three sizes. Protozoa (like giardia) are big. Bacteria are smaller. Viruses are tiny.
- A filter — the common hollow-fibre kind — physically strains water through pores around 0.1–0.2 microns. That reliably removes protozoa and bacteria. It does not remove viruses, which are small enough to slip through.
- A purifier goes further and deals with viruses too, using either a finer method, a chemical (chlorine dioxide, iodine), or UV light that scrambles the bugs' DNA.
Why it matters: in clean wilderness and most of Europe, waterborne viruses are a low risk, so a simple filter is plenty for streams and dodgy-tasting taps. In parts of the world where human sewage gets into the water supply, you want a purifier or a chemical backup. Match the tool to the destination. Don't take a basic filter somewhere with a real virus risk and assume you're covered.
A filter handles the bugs you'd meet on a mountain. A purifier handles the bugs you'd meet downstream of a city. Know which trip you're on.
The main types, and who they suit
- Squeeze / bottle filters. A filter built into a soft flask or bottle, or that screws onto one. Fill, squeeze or sip, drink. Light, cheap, fast, brilliant for day walks and travel. My default.
- Straw filters. Drink directly from the source through the straw. Cheapest and simplest, but you can't carry the clean water onward, which limits them.
- Gravity filters. Fill a dirty bag, hang it, let gravity push water through into a clean bag. No effort, great for groups or a base camp. Bulkier.
- Pump filters. Pump water through by hand. Reliable from shallow or awkward sources, but slow and fiddly.
- UV purifiers. A pen you stir in clear water to kill everything, viruses included. Excellent and fast, but needs batteries and clear water (it won't help muddy water — pre-filter that).
- Chemical tablets/drops. Tiny, weightless, kill viruses too, no moving parts. The catch: you wait 30 minutes and the water can taste faintly of pool. The perfect lightweight backup, less perfect as your only method.
What to look for
- The right protection for your destination (see above — this is the decision that matters).
- Flow rate. A slow filter is one you'll resent. Newer hollow-fibre filters are quick; pumps and heavily-clogged filters are slow.
- Lifespan and replaceable cartridges. Filters are rated for hundreds or thousands of litres; check whether you can replace the cartridge or have to bin the whole thing.
- Weight and pack size, since the point is to carry it without noticing.
- The freezing catch. A wet hollow-fibre filter that freezes is ruined — the ice cracks the fibres invisibly and it no longer protects you. If you're somewhere cold, keep it in your jacket or sleeping bag.
Roughly what it costs
- Tablets/drops: a few euros for a course. Buy some regardless as a backup.
- Squeeze and straw filters: €25–45. Outstanding value.
- Gravity and pump systems: €60–120.
- UV purifiers: €70–120, plus batteries.
The cost argument writes itself: a €30 filter pays for itself against bottled water in about a week of travel, and keeps going for years.
Where to buy
Outdoor and camping shops, travel-health sections, and online. Buy the replacement cartridges at the same time if your model uses them — they're easy to find at home and impossible in the field. Test the thing in your kitchen sink before you leave so you're not learning how it works while thirsty on a hillside.
My honest take
A squeeze filter lives in my bag for any trip with walking involved, and a strip of chemical tablets rides along as a featherweight backup for when I'm somewhere I don't trust. On the Theth-to-Valbona walk in Albania it meant I carried half the water I otherwise would have and topped up from the streams. Around the coast and beaches of Lofoten it was more about refilling from taps without buying plastic. It tucks into the side pocket of a packable daypack and I forget it's there until I need it.
Be honest with yourself about where you're going and buy accordingly — a simple filter for clean-water trips, a purifier or chemical backup where viruses are a real risk. Then enjoy never queuing for warm bottled water at a trailhead kiosk again. More of what earns its place in my bag on the gear page.