Water: treating it, and what each method misses
This page is about water that does not come from a treated mains supply: a stream, a standpipe of unknown provenance, a tap marked not drinking water, a tank that has been sitting in the sun.
Cooking from long-life ingredients uses a surprising amount of water — dried pulses and grains are bought with water and paid for in water — so on a long trip the question of what you are rehydrating things with stops being academic.
The useful thing to understand is not that treatment works. It is that each method has a specific gap, the gaps are different, and they do not overlap.
Boiling: the one with no gap
NaTHNaC:
Water can be disinfected by bringing it to a rolling boil.
The CDC gives the times:
Bring clear water to a rolling boil for 1 minute. At elevations above 6,500 feet, boil for 3 minutes.
That is it. Boiling deals with bacteria, viruses and parasites alike — no carve-outs, nothing it is “not always” effective against. Note rolling boil: steaming and the first few bubbles are not the same thing, and this is one of the few places where a visual cue maps cleanly onto the requirement.
The cost is fuel, and on a trip fuel is the constraint. A litre from cold is a meaningful fraction of a small gas canister, and if you are treating all your drinking water this way you will notice. That is the honest trade: boiling is the method with no blind spot and the highest running cost.
Chemicals: the parasite gap
Chlorine and iodine tablets are light, cheap and need no fuel. NaTHNaC:
chlorine products are usually effective, but parasites like Cryptosporidium and Giardia are not always destroyed by these products
The CDC is blunter:
kill bacteria and viruses but do not work well to kill parasites
This is the gap that catches people, because Cryptosporidium and Giardia are precisely what you would expect from surface water in a livestock landscape — which is most of upland Britain. The tablets deal with the organisms you were less worried about and not the ones you were.
Chlorine dioxide is the better chemical, and is a genuinely different product from ordinary chlorine tablets despite the similar name. Per the CDC it kills Giardia and is
somewhat effective at killing Cryptosporidium
but with the caveat that
it is still safest to filter the water first
“Somewhat effective” is doing real work in that sentence. Read it as narrowing the gap rather than closing it.
Filters: the virus gap
Filters are mechanical, need no fuel and no contact time. What they do depends entirely on pore size, and the number that matters is the absolute pore size, not nominal:
Absolute pore (hole) size of 1 micron or smaller
removes parasites but, per the CDC, not viruses or bacteria. For bacteria you need
absolute pore (hole) size of 0.3 micron or smaller
NaTHNaC recommends a filter of ≤0.2 µm to 1.0 µm used before chemical treatment.
Viruses are smaller than any of that. A filter alone is not complete treatment.
Which is why you combine them
The gaps are complementary, and that is the design. The CDC’s instruction:
Filter water before disinfecting it
Filter water before using UV light to disinfect it
The filter takes out the parasites and the sediment that chemicals struggle with; the chemical or UV stage takes out the viruses and bacteria the filter lets through. Order matters — filtering afterwards would strain out the thing you just treated and leave the treatment behind.
So, practically, three workable approaches:
| Approach | Covers | Costs |
|---|---|---|
| Rolling boil, 1 min (3 min above ~2,000 m) | Everything | Fuel, and time to cool |
| Filter (≤0.2–1 µm absolute), then chemical or UV | Everything, in two steps | Kit, weight, contact time |
| Chemical tablets alone | Bacteria and viruses; parasites not reliably | Least of anything |
The third is not a recommendation. It is what you have left if the filter clogs, and it is worth knowing exactly which risk you are accepting rather than discovering it later.
Cloudy water first. Sediment shields organisms from chemical treatment and UV both, so if the water is not clear, filter or let it settle before treating. The CDC’s boiling instruction is specific to clear water for the same reason.
Two things this page does not tell you
Chemical contamination. Everything above concerns germs, which is what the cited guidance covers. Boiling, filtering and chlorinating are aimed at living things; agricultural run-off, road drainage and old mine workings are a different category of problem, and we have not found guidance we are willing to summarise on it. So we do not offer any. Treat “this water has been treated for germs” and “this water is fine” as different statements, and be wary of a source downstream of industry or intensive farming regardless of what you do to it.
How much to carry and how to carry it. That is a planning question rather than a safety one, and it belongs in the water guide rather than here.
The habit worth having
Decide your water treatment before the trip, not at the stream. The failure mode is not usually ignorance — it is being thirsty, some distance from anywhere, with only tablets, looking at water that runs off a hillside full of sheep and deciding it is probably fine.
It might be. Cryptosporidium is the one that will not be, and no amount of looking at the water tells you.