A clear trickle and a muddy ditch after a storm are not the same job. People get sick from water that looked fine. The work is settle or cloth-filter the grit, then boil or chemically treat for enough time. A filter is not the whole treatment.
This is for water you have to take from a creek, a pond, or a roadside ditch — not for storing tap water you already trust. For that, see how to store water for 30 days.
Read the water first
Stand upstream of where people swim, dogs enter, and cows stand. Walk a few minutes if you can. Look for a dead animal, a pipe, an algae mat, a fuel sheen, or a field that just got sprayed. If the water smells like sewage, chemicals, or dead fish, leave it. Treatment methods in this article are for biological risk in ordinary surface water. They are not a cleanup for fuel, pesticides, or heavy runoff.
Prefer moving water over still. Prefer a gravel or sand bottom over silt. Prefer upstream of a town, not below it. None of that makes the water safe. It makes the job smaller.
Avoid water with a rainbow sheen that does not break when you poke it — that is often petroleum. Avoid bright green soup. Avoid anything downstream of a mine, a dump, or a large parking lot after rain if you have another choice.
Collect from the middle of the flow, not the bank scum. Dip a clean pot. Do not stir the bottom. If you scoop from the shallows, let the jar sit and pour off the clearer top.
Clear the grit before you treat
Grit, silt, and organic junk clog filters and shield microbes from heat and chemicals. Clear it first.
Let the water sit in a covered bucket until the heavy stuff drops. Thirty minutes helps. A few hours is better if the water is brown. Pour the clearer water off the top. Leave the sludge.
If you need it sooner, pour through a clean cloth — a bandanna, a pillowcase, a coffee filter if you have one. You are not purifying. You are taking out the chunks so the next step can work.
Do this even if you plan to boil. Muddy water tastes worse after boiling and leaves a film in the pot. Do this especially if you plan to use a filter. A filter full of silt is a paperweight.
Keep a dedicated "dirty" bucket and a dedicated "clear" bucket if you are processing more than a pot. Do not dip the clean cup back into the creek.
Boil
Boiling is the method you can trust when you have heat and time. Bring the cleared water to a rolling boil — big bubbles that do not stop when you stir — and keep it there.
At typical U.S. elevations, a full rolling boil is the standard home practice. At higher elevation, water boils at a lower temperature, so keep it rolling longer. A common field rule is to add time as you go up; if you are in the mountains, do not treat a brief simmer as done. Let it roll.
Cover the pot if you can. You lose less water and you keep debris out. Let it cool, covered, before you pour it into a clean bottle.
Boiling does not remove chemical pollution. It does not make fuel-tainted water safe. It does kill the bacteria, viruses, and protozoa that are the usual creek problem.
If fuel is short, boil only what you will drink and cook with. Washing water is a different standard — see how to wash up without running water.
Chemical treatment
Chemical treatment is for when you cannot boil: no stove, no fire, not enough fuel, or you need water in a closed bottle.
It needs full contact time. That means the chemical is mixed through the whole volume and then left alone for the full wait. Cold water takes longer. Cloudy water takes longer and may not be a good candidate until you settle it.
Follow the product label for the tablets or drops you actually have. If you are using unscented household bleach, follow current CDC and EPA emergency guidance and the label. We are not dosing from here. Do not invent a parts-per-million recipe from memory, a forum post, or a number you think you remember from a class.
After you add the chemical, stir or cap and invert so it reaches the lid threads. Wait the full time before you drink. If the water is very cold, wait longer than the warm-water time on the label.
Chemical treatment has a taste. That is not a sign it failed. A strong chemical smell that does not fade after the wait can mean you overdid it or the water was dirtier than the method allows. When in doubt, start over with clearer water.
Chemicals are weaker against some protozoa than boiling is, especially if the water was not cleared first. That is one reason settle-then-treat is not optional.
Filter vs purify, in plain language
A filter strains particles. How small depends on the filter. Many camping filters are built to take out protozoa and most bacteria. They are not a complete treatment for viruses, and they do nothing for dissolved chemicals.
Purify, in plain language, means you have used heat or a chemical (or a purifier specifically built for viruses) to deal with the things a filter may miss. Filter then boil is a solid pair. Filter then chemical is a solid pair if you honor the contact time.
A filter is not complete treatment. Clear-looking water out of a filter can still carry viruses. Treat it as improved, not finished, unless the device and the situation say otherwise — and even then, boiling is the backup you should know.
Gravity filters are slow and hands-off. Pump filters are faster and take work. Either one clogs if you skip the settle-and-cloth step. Backflush or clean per the instructions you have. A clogged filter that you force can crack a seal or push dirty water around the element.
Do not put untreated creek water into the same bottle you drink from unless that bottle is the output side of a finished process. Dirty cap, dirty hands, dirty scoop — most failures are re-contamination, not a failed boil.
When each method fails
Boiling fails when the water is chemically contaminated, when you never reached a rolling boil, or when you pour boiled water back into a dirty jug. It also fails if you run out of fuel and stop at a simmer.
Chemical treatment fails when the water is still cloudy, when the wait is short, when the water is near freezing and you used a warm-water time, or when the product is old and you guessed the dose. It fails if you use scented bleach, splashless bleach, or anything with additives. Those are not drinking-water products.
A filter fails when it is clogged, cracked, frozen, or used on water it was not built for — silt, glacial flour, or a chemical spill. It fails when you drink from the intake hose. It fails when you treat it as purification.
All methods fail if you start with water you should have walked away from.
If you have a choice, boil. If you cannot boil, clear the water, then use a labeled chemical method with full contact time. If you have a filter, use it as the middle step, not the last word.
Once you can make a pot of creek water drinkable, store a real reserve so you are not doing this every day. Next: how to store water for 30 days.