Bottle with Filter Water

Filter bottles, drinking from the stream you are standing in

A bottle with filter water built into it removes the need to carry every litre of the day from home. Water goes in from a stream, a tap of uncertain quality or a standpipe, and comes out through a cartridge in the lid or the straw as you drink. The range runs from rugged hard-sided bottles to folding designs that flatten when empty, a half-litre size for short outings, and camping, outdoor, tactical and survival versions sold with a carry cover.

What does a bottle filter deal with, and what does it not?

Filters work mechanically: the cartridge is a physical barrier, so it holds back what is large enough to be caught, along with sediment and much of the taste that comes with it. What passes through a barrier depends entirely on the pore size a maker quotes, which is why the specification on the product page is the thing to read rather than a general claim. Chemical pollution, salt water and industrial run-off are outside what any bottle filter is designed to handle, and moving upstream of livestock and roads still matters. Where the day's water is carried from home instead, a plain flask from hiking water bottles saves the cartridge for when it is needed.

How do you use one without spoiling the water?

Keep the dirty side and the clean side apart. The outside of the bottle, the cap thread and your hands have all been in the stream; the mouthpiece has not, and it should stay that way. Fill from flowing water rather than a still pool, scoop from below the surface rather than off the top, and let sediment settle before filling if the source is cloudy. Wipe the threads before closing, and never drink straight from an unfiltered bottle mouth out of habit.

Why does a bottle with filter water slow down?

Because the filter is doing its job. Everything held back accumulates on the surface of the cartridge and gradually closes it, so the effort needed to draw water rises. Most designs can be backflushed, which pushes clean water the wrong way through the element and clears part of that build-up. Two things end a cartridge early: silt drawn straight off a riverbed, and freezing, which damages the internal structure invisibly and cannot be reversed.

What should you check before choosing one?

• Capacity: a half-litre bottle refills constantly but weighs nothing when empty, which suits routes with water all along them.
• Cartridge life: quoted per bottle, and worth knowing before a long trip so a spare travels with you.
• Shape: a folding bottle packs flat once drunk, while a rigid one survives being dropped on rock.
• Mouth size: a wide opening fills quickly from a shallow stream where a narrow one refuses to.

When is a filter bottle the wrong tool?

When the water is for more than one person. Filling and drinking one bottle at a time is slow work for a group or a camp cooking meal, and a gravity water filter hung from a branch does the same job unattended while you set up. Bladders, flasks and filters all sit together in water & hydration for anyone weighing up the whole system.