| name | purify-water |
| description | Purify water from wild sources using boiling, filtration, and chemical methods. Covers source assessment and priority ranking, sediment pre-filtering, method selection (boiling, chemical, UV, filter), altitude-adjusted boiling procedure, chemical treatment dosages, and safe storage practices. Use when needing drinking water in a wilderness setting, when available water sources are of unknown quality, in an emergency survival situation where dehydration is a risk, or when making water safe for cooking or wound cleaning.
|
| license | MIT |
| allowed-tools | Read |
| metadata | {"author":"Philipp Thoss","version":"1.0","domain":"bushcraft","complexity":"intermediate","language":"natural","tags":"bushcraft, water, purification, survival, wilderness, filtration"} |
Purify Water
Purify water from wild sources to make it safe for drinking using field-available methods.
When to Use
- You need drinking water in a wilderness setting without access to treated water
- Available water sources are of unknown quality (streams, rivers, lakes, ponds)
- Emergency survival situation where dehydration is a risk
- You need to make water safe for cooking or wound cleaning
Inputs
- Required: A water source (flowing or still)
- Required: A container (metal pot, bottle, or improvised vessel)
- Optional: Purification supplies (chemical tablets, filter, UV pen)
- Optional: Fire-making capability for boiling (see
make-fire)
- Optional: Cloth or natural filter materials for pre-filtering
Procedure
Step 1: Assess and Select the Water Source
Not all water sources carry equal risk. Choose the best available source.
Water Source Priority Ranking (best to worst):
┌──────┬─────────────────────────┬────────────────────────────────────┐
│ Rank │ Source │ Notes │
├──────┼─────────────────────────┼────────────────────────────────────┤
│ 1 │ Spring (at the source) │ Lowest contamination; still treat │
│ 2 │ Fast-flowing stream │ Moving water has fewer pathogens │
│ │ (above human activity) │ than still water │
│ 3 │ Large river │ Dilution helps but agriculture/ │
│ │ │ industry upstream is a concern │
│ 4 │ Large lake │ Collect from open water, not shore │
│ 5 │ Small pond or puddle │ High pathogen and parasite risk │
│ 6 │ Stagnant pool │ Last resort; heavy treatment needed│
└──────┴─────────────────────────┴────────────────────────────────────┘
Warning Signs (avoid if possible):
- Dead animals nearby
- Algae bloom (blue-green scum)
- Chemical odor or oily sheen
- Downstream of mining, agriculture, or settlements
- No surrounding vegetation (may indicate toxic soil)
Collect water from below the surface (avoid surface film) and away from the bank edge.
Expected: Clear or slightly turbid water from the best available source, collected in a clean container.
On failure: If only poor sources are available (stagnant, turbid), proceed but plan for aggressive pre-filtering (Step 2) and use multiple purification methods (belt-and-suspenders approach). If no water source is found, look for indicators: green vegetation in valleys, animal trails leading downhill, insect swarms at dawn/dusk, and listen for running water.
Step 2: Pre-Filter Sediment
Remove particulate matter before purification. Sediment reduces the effectiveness of chemical treatment and clogs filters.
Improvised Gravity Filter (layered in a container with a hole at the bottom):
┌─────────────────────┐ ← Open top: pour water in
│ Grass / cloth │ ← Coarse pre-filter
│ Fine sand │ ← Removes fine particles
│ Charcoal (crushed) │ ← Adsorbs some chemicals and odors
│ Gravel │ ← Structural support
│ Grass / cloth │ ← Prevents gravel from falling through
└────────┬────────────┘
│
Filtered water drips out
Materials:
- Container: birch bark cone, hollow log, cut plastic bottle, sock
- Sand: fine, clean sand (rinse first if possible)
- Charcoal: from a previous fire (NOTite ash — charcoal only)
- Gravel: small stones, rinsed
For simple sediment removal, strain water through a bandana, t-shirt, or multiple layers of cloth.
Expected: Visibly clearer water with reduced turbidity. Charcoal layer removes some odor and taste.
On failure: If water is still very turbid after filtering, let it settle in a container for 30-60 minutes. Carefully decant the clearer top layer. Repeat the settling or filtering process. Note: pre-filtering does NOT make water safe to drink — it prepares it for purification.
Step 3: Select Purification Method
Choose based on available tools and conditions.
Purification Method Comparison:
┌───────────────┬────────────┬───────────┬────────────┬──────────────────────┐
│ Method │ Kills │ Time │ Requires │ Limitations │
│ │ bacteria/ │ │ │ │
│ │ viruses/ │ │ │ │
│ │ parasites │ │ │ │
├───────────────┼────────────┼───────────┼────────────┼──────────────────────┤
│ Boiling │ Yes/Yes/Yes│ 1-3 min │ Fire, metal│ Fuel, time, does not │
│ │ │ (rolling) │ container │ remove chemicals │
├───────────────┼────────────┼───────────┼────────────┼──────────────────────┤
│ Chlorine │ Yes/Yes/ │ 30 min │ Tablets or │ Less effective in │
│ dioxide tabs │ Yes │ │ drops │ cold/turbid water │
├───────────────┼────────────┼───────────┼────────────┼──────────────────────┤
│ Iodine │ Yes/Yes/ │ 30 min │ Tablets or │ Taste; not for │
│ │ Partial │ │ tincture │ pregnant/thyroid │
│ │ │ │ │ conditions; weak │
│ │ │ │ │ against Crypto │
├───────────────┼────────────┼───────────┼────────────┼──────────────────────┤
│ UV pen │ Yes/Yes/Yes│ 60-90 sec │ UV device, │ Requires clear water;│
│ │ │ per liter │ batteries │ battery dependent │
├───────────────┼────────────┼───────────┼────────────┼──────────────────────┤
│ Pump/squeeze │ Yes/No*/ │ Immediate │ Filter │ Most don't remove │
│ filter │ Yes │ │ device │ viruses (*unless │
│ │ │ │ │ 0.02 micron) │
├───────────────┼────────────┼───────────┼────────────┼──────────────────────┤
│ SODIS (solar) │ Yes/Yes/ │ 6-48 hrs │ Clear PET │ Slow; needs sun; │
│ │ Partial │ │ bottle, │ only 1-2 L at a time │
│ │ │ │ sunlight │ │
└───────────────┴────────────┴───────────┴────────────┴──────────────────────┘
Decision logic:
- Have fire + metal pot? → Boil (most reliable)
- Have chemical tablets? → Chemical treatment
- Have filter + tablet combo? → Filter then treat (belt-and-suspenders)
- Sunny day + clear PET bottles? → SODIS as a backup method
- Multiple methods available? → Use two for maximum safety
Expected: A clear decision on which purification method(s) to use based on available tools.
On failure: If no standard purification tools are available, boiling is the default — it requires only fire and a heat-safe container. Even a single-wall metal water bottle can be used for boiling. In a dire emergency, a container can be improvised from a rock depression or green bamboo section placed near flames.
Step 4: Boil the Water
The most reliable field purification method. Kills all pathogen classes.
Boiling Procedure:
1. Bring water to a ROLLING boil (large bubbles breaking the surface)
2. Maintain rolling boil for:
- Sea level to 2000 m / 6500 ft: 1 minute
- 2000-4000 m / 6500-13000 ft: 3 minutes
- Above 4000 m / 13000 ft: 5 minutes
3. Remove from heat
4. Allow to cool in the covered container
5. If taste is flat, pour between two containers several times to aerate
Altitude Adjustment:
Water boils at lower temperatures at altitude.
At 3000 m / 10000 ft, water boils at ~90°C / 194°F.
Longer boiling compensates for the lower temperature.
Fuel Estimate:
Boiling 1 L requires roughly 15-20 min of sustained fire
depending on container, wind, and starting temperature.
Expected: Water reaches a vigorous rolling boil and is maintained for the appropriate duration. After cooling, the water is safe from biological pathogens.
On failure: If you cannot maintain a rolling boil (wind, weak fire), extend the boiling time. If the container leaks or cracks, transfer to another vessel. If no metal container is available, you can boil water in a wooden, bark, or hide container using hot rocks: heat stones in the fire for 20+ minutes, then transfer them to the water container with tongs or sticks. Avoid river rocks (may crack or explode from trapped moisture).
Step 5: Apply Chemical Treatment
Use when boiling is impractical or as a secondary treatment.
Chemical Treatment Dosages:
┌─────────────────────┬──────────────────┬────────────┬─────────────────────┐
│ Chemical │ Dose per liter │ Wait time │ Notes │
├─────────────────────┼──────────────────┼────────────┼─────────────────────┤
│ Chlorine dioxide │ Per manufacturer │ 30 min │ Most effective │
│ tablets │ (usually 1 tab │ (4 hrs for │ chemical method; │
│ (e.g., Aquamira, │ per 1 L) │ Crypto) │ kills all pathogens │
│ Katadyn Micropur) │ │ │ │
├─────────────────────┼──────────────────┼────────────┼─────────────────────┤
│ Iodine tablets │ 1-2 tablets per │ 30 min │ Weak against │
│ │ liter │ │ Cryptosporidium │
├─────────────────────┼──────────────────┼────────────┼─────────────────────┤
│ Tincture of iodine │ 5 drops per │ 30 min │ Double dose for │
│ (2%) │ liter (clear) │ (60 min if │ cloudy water │
│ │ 10 drops per │ cold/turbid│ │
│ │ liter (cloudy) │ ) │ │
├─────────────────────┼──────────────────┼────────────┼─────────────────────┤
│ Household bleach │ 2 drops per │ 30 min │ Must be unscented, │
│ (5-8% sodium │ liter (clear) │ │ plain bleach; │
│ hypochlorite) │ 4 drops per │ │ check expiry date │
│ │ liter (cloudy) │ │ │
└─────────────────────┴──────────────────┴────────────┴─────────────────────┘
After treatment, water should have a slight chlorine/iodine smell.
If no smell is detected, add half the original dose and wait another 15 min.
Cold/turbid water adjustment:
- Temperature below 5°C / 40°F: double the wait time
- Turbid water: double the dose OR pre-filter first (recommended)
Expected: Treated water has a faint chemical smell after the wait period, indicating adequate disinfection. Water is safe from bacteria and viruses; chlorine dioxide is also effective against parasites.
On failure: If tablets are expired (no smell after treatment), use a double dose or combine with another method. If the taste is objectionable, let the water stand uncovered for 30 minutes to off-gas, or pour through an improvised charcoal filter to improve taste. If chemical treatment is your only method and you suspect Cryptosporidium (common near livestock), wait the full 4 hours for chlorine dioxide or combine with filtration.
Step 6: Store Safely
Purified water can be recontaminated through dirty containers or hands.
Safe Storage Practices:
1. Store in clean, dedicated containers (do not reuse unpurified containers)
2. If reusing a container, rinse it with a small amount of purified water first
3. Keep containers sealed or covered
4. Mark or separate "raw" and "purified" containers
(e.g., tie a knot in the purified bottle's paracord handle)
5. Avoid reaching into containers with hands — pour, don't dip
6. In warm weather, consume within 24 hours
7. Re-treat water that has been stored more than 24 hours
Hydration Planning:
- Minimum: 2 L / 0.5 gal per day (sedentary, cool weather)
- Active: 4-6 L / 1-1.5 gal per day (hiking, hot weather)
- Plan purification capacity to meet daily needs
Expected: Purified water remains safe in clean, sealed containers. A system is in place to avoid cross-contamination between raw and treated water.
On failure: If containers are limited, designate one as "raw" (collection only) and another as "clean" (purified only). Scratch or mark them distinctly. If you suspect recontamination, re-treat the water before drinking.
Validation
Common Pitfalls
- Skipping pre-filtering: Sediment reduces chemical effectiveness and clogs filters. Always pre-filter turbid water
- Incomplete boiling: A few bubbles on the bottom is not a rolling boil. Wait for vigorous, surface-breaking bubbles
- Ignoring altitude: Water boils at lower temperatures at altitude. Increase boiling time accordingly
- Chemical under-dosing: Cold or turbid water requires more chemical or longer contact time
- Cross-contamination: Using the same container for raw and purified water, or handling the drinking rim with dirty hands
- Relying on a single method for worst-case sources: For stagnant or livestock-adjacent water, use two methods (e.g., filter + chemical, or boil + chemical)
Related Skills
make-fire — required for the boiling method; fire also provides warmth while waiting for chemical treatmentforage-plants — some plants indicate nearby water sources (willows, cattails, cottonwood); foraged food may require clean water for preparation
