Maintaining and storing a water filter

Portable filters hold back particles and germs mechanically: the water has to pass through tiny pores, and anything larger than those pores gets stuck. In hollow-fibre filters these are many thin tubes with porous walls; in ceramic filters it is a block of fine-pored fired material. Single-celled parasites and bacteria are comparatively large and are held back by many filters.

Viruses, by contrast, are much smaller than bacteria. Many outdoor filters do not reliably hold them back. Dissolved substances such as salt or chemicals also pass through ordinary filters – no hiking filter can make seawater or chemically contaminated water drinkable. So: disinfect filtered water as well, for example by boiling, and follow official advice from the authorities.

What your filter actually does is stated in the manufacturer's information. The CDC Yellow Book describes which filter types hold back which groups of germs, and the WHO stresses that any water treatment only works when it is used and maintained correctly. Maintenance is therefore not an extra but part of the protection.

It is normal for a filter to get slower over time. Particles lodge in the pores and the flow drops. Backflushing or cleaning per the manufacturer's instructions partly reverses this. Letting cloudy water settle first or pre-filtering it through a clean cloth spares the filter and extends its life.

The dangerous damage is the kind that does not reduce flow but opens paths for germs unnoticed. Frost is the best-known case: water expands when it freezes and can damage hollow fibres or membranes without anything being visible. A fall onto hard ground, cracks in the housing or hairline cracks in ceramic can also cancel the protection. A filter that suddenly runs noticeably fast is a reason for suspicion.

Many manufacturers therefore advise no longer treating a filter that may have frozen or been dropped as reliable. Some describe an integrity test you can use to check whether the filter is still intact. If there is no such test or it fails, the filter is replaced. Until then, disinfect the water as well or boil it.

Every filter has a dirty side (raw water) and a clean side. The best filtering is useless if a few drops of raw water get onto the mouthpiece, the outlet or into the clean-water container. A single contamination can make the filtered water unsafe again.

So label containers clearly, for example with coloured tape for “dirty”, and never mix them up. Hold hoses and lids so that raw water cannot drip onto the clean side. Wash or sanitise your hands if possible before touching the clean side. Backflush only with the water the manufacturer specifies – often that is already filtered or clean water.

This separation is a habit that needs practice. In groups it helps if one person is in charge of filtering and everyone knows which container is which.

Read the instructions before first use: how is the filter prepared, backflushed, cleaned and, if needed, sanitised? How long does it last? How do you protect it from frost? Take the instructions or a copy with you, because every model is different.

After the trip, clean the filter, sanitise it if the manufacturer provides for this, and let it dry completely before packing it away. A filter stored damp can grow germs or mould inside. Store it dry, frost-free and protected from dust. A filter kept for emergencies should be taken out and checked regularly – otherwise you will not know in an emergency whether it still works.

Replace the filter when its stated lifespan is reached, an integrity test fails, the housing is damaged or barely any water gets through despite cleaning. Ceramic filters are usually scrubbed carefully; after a fall, check them especially thoroughly for cracks.

Contaminated water can cause diarrhoea and other illnesses. So: disinfect filtered water as well, for example by boiling, especially when sewage, settlements or grazing animals are nearby. When the authorities issue boil-water notices or warnings, always follow them.

When in doubt: better to boil once too often than once too little. A filter is a helpful tool for removing cloudiness and many germs, but it is not a free pass. If someone becomes seriously ill after drinking, loses a lot of fluid or becomes very weak, get medical help or call the local emergency number (112 in the EU).