Water treatment outdoors: what actually needs filtering

The theory of water treatment is well covered elsewhere. This is about the version you meet in practice.

The three things you are treating for

Protozoa such as giardia and cryptosporidium are large, common in surface water and effectively removed by any decent filter. Bacteria are smaller and are also removed by filters rated to the usual 0.1 to 0.2 micron range.

Viruses are smaller again, pass through most filters, and are the reason chemical or UV treatment exists. Which of these actually matters depends far more on where you are than on which brand you bought.

Where you collect water matters more than how you treat it

Fast-flowing water above any human or livestock activity carries a fraction of the load of a slow pool below a grazing field. Spring sources emerging from rock are generally the best available option, and are worth a detour.

Silty water clogs filters and consumes chemical treatment, so letting sediment settle before treating is worth the fifteen minutes. The specific things worth walking upstream to get above are a path or track crossing, a shelter, a campsite and any place stock gather, because each of those puts contamination into a stream at one identifiable point rather than diffusely.

Filters are convenient and have a failure mode

Hollow-fibre filters are fast, need no waiting time and taste of nothing, which is why most people use them. They clog gradually, they are destroyed by freezing, and a frozen filter shows no visible damage while no longer filtering.

Over a season, backflushing regularly and keeping a filter in a sleeping bag on freezing nights covers both problems. A filter that has slowed noticeably is telling you it is close to finished, and the other routine error is cross-contamination — dirty hands, a dirty bottle thread, the two hoses swapped in the dark — which puts back exactly what the filter took out.

Chemical treatment is slow, light and thorough

Chlorine dioxide handles viruses, bacteria and — given enough time — cryptosporidium, which most other chemicals do not. Contact time is the catch: thirty minutes for most pathogens and up to four hours for crypto in cold water.

It weighs almost nothing, which makes it an excellent backup even when a filter is the primary method. Cold and cloudiness both extend the time required, and the doses printed on the packet assume clear water at moderate temperature, so treating cold silty water in a hurry is the usual way this method quietly fails.

Boiling is the one that always works

Bringing water to a rolling boil inactivates everything of concern, at any altitude you are likely to be walking at. The cost is fuel and time, which is why it is a fallback rather than a routine method. It also does nothing about chemical contamination or silt, neither of which any consumer method addresses.

Reaching a rolling boil is itself sufficient, because the organisms of concern are inactivated well below boiling point on the way up, so the widely repeated instruction to hold it there for ten minutes burns fuel to no purpose at all.

The exception is usually weather, which overrides most planning.

What none of these do

No practical field method removes agricultural chemicals, heavy metals or industrial contamination. Water downstream of intensive arable land or old mine workings is a source-selection problem, not a treatment problem.

Local knowledge is the only real tool here, which is a good reason to ask somebody who lives there. Activated carbon elements improve taste and reduce some organic compounds, which is worth having and is not the same thing as making contaminated water safe, and no consumer device claims otherwise if you read its specification carefully.

The takeaway

Choose the source carefully and the treatment method stops mattering much.

The route rewards patience more than equipment.