Hollow-fibre and membrane filters

Inside the filter are many fine, hollow plastic fibres. Their walls are a membrane with pores that, in typical outdoor filters, measure around 0.1 to 0.2 micrometres. A micrometre is a thousandth of a millimetre. The water is pushed or sucked through these walls from outside to inside or the other way round.

Everything larger than the pores is held back. Single-celled parasites such as Giardia and Cryptosporidium are large enough for this, and so are bacteria. Most viruses, however, are much smaller and can pass through the membrane. Dissolved substances – salts, many chemicals, heavy metals – are smaller still and pass straight through.

What matters is the absolute pore size, the largest opening allowed to occur. A single crack in a fibre or a leaking spot in the housing acts like a big hole through which raw water flows past the membrane. That is why an intact filter is so important.

Hollow-fibre filters fill exactly the gap that chlorine leaves with parasites: they retain protozoa including Cryptosporidium immediately, with no waiting, no taste and no fuel. Bacteria also stay caught in the membrane.

Pores of this size are not enough for viruses. Where people live, sewage is discharged or there has been flooding, viruses must be expected. Then additionally disinfect the filtered water with chlorine or chlorine dioxide, or boil it. In remote areas with little human impact, viruses are considered a lower risk – but that is an assessment of the surroundings, not a property of the filter. In an emergency, and whenever you are unsure, additional disinfection is the safe way.

Some manufacturers offer models with much smaller pores or combined systems that are explicitly rated for viruses as well. Rely only on claims documented for your exact model. No hollow-fibre filter helps against chemicals and salt – if you suspect them, choose another source.

Before use, check the housing, seals and connections for cracks and make sure the filter has never frozen. Collect the clearest water possible; let cloudy water settle first or pour it through a cloth. Mud clogs the membrane quickly, and the flow becomes laborious.

Keep the raw side and the clean side strictly separate. The outlet must not come into contact with raw water or dirty hands. Even one drop from the dirty-water bag running down to the outlet makes the filtrate unsafe again. Fill into a clean container that is meant only for treated water.

Backflush the filter regularly, using clean, already treated water, never raw water. This loosens deposits from the membrane and restores the flow. Storage and drying follow the manufacturer's instructions – some models should be stored moist, others dry.

Frost is the invisible enemy. If water freezes in the fibres, it expands and can make them crack. From the outside you see nothing, but pathogens can slip through the cracks. In frost, carry the filter close to your body and take it into your sleeping bag at night. After it has frozen through, do not trust it any more unless the manufacturer provides a test.

Falls and knocks can also damage the membrane or housing without it being noticeable. A sudden, unusually fast flow can be a warning sign. If in doubt, additionally disinfect the filtrate or switch to another method.

Remember: the filter makes water clearer and holds back many pathogens, but it does not make it safe to drink in every case. Viruses, chemicals and salt remain possible. Whenever the authorities issue advice, such as boil-water notices, always follow it.

Squeeze and straw filters are small and light: water is pressed from a soft bag through the filter or sucked directly. They suit individuals on trips where water is plentiful and weight matters.

Gravity filters consist of a hanging raw-water bag from which the water runs on its own through the filter into a clean container. That is practical for groups and in camp, because you can do other things meanwhile. Pump filters are another design, in which you push the water through the membrane by muscle power.

Hollow-fibre filters are strongest in combination: first settle or pre-filter, then filter, then – where viruses are possible – disinfect chemically. That way each step covers the weaknesses of the others, in keeping with the multi-barrier principle.