Boiling times, bleach and iodine doses, filtration types and what they remove, UV purification, combined treatment, what purification cannot remove, and pre-filtering sediment.
Priority: Remove large particles first (sediment filtering), then kill biological pathogens (boil, chemical, UV), then address chemical contamination (activated carbon filter, distillation). Most emergency situations require only the middle step.
⚠️ No single method removes all contaminants. Chemical contamination (heavy metals, pesticides, industrial chemicals) requires activated carbon filtration or distillation — boiling, bleach, iodine, and UV do NOT remove these.
Boiling is the most reliable method to kill biological pathogens — bacteria, viruses, and protozoa.
| Condition | Boiling Time |
|---|---|
| Sea level to 2,000 m (6,600 ft) | 1 minute rolling boil |
| Above 2,000 m (6,600 ft) altitude | 3 minutes rolling boil |
| Visibly contaminated / cloudy water | Pre-filter through cloth; then boil 3 minutes |
Rolling boil = large bubbles breaking the surface continuously. A simmer is not sufficient.
Let water cool before drinking; store in covered clean containers.
Use unscented household bleach. Check the concentration on the label.
| Bleach Concentration | Dose per Litre | Dose per Gallon (US) |
|---|---|---|
| 1% sodium hypochlorite | 10 drops | 40 drops |
| 6–8.25% sodium hypochlorite | 2 drops | 8 drops |
| Greater than 8.25% | Do not use | — |
Process:
Shelf life of bleach: Bleach loses potency over time. Stock no more than 1-year supply; use fresh stock for emergency treatment.
Iodine tablets (tetraglycine hydroperiodide) are compact and suitable for go-bags.
| Water Condition | Dose | Wait Time |
|---|---|---|
| Clear water | 1 tablet per litre (5 mg) | 30 minutes |
| Cold water (< 15°C / 59°F) | 1 tablet per litre | 60 minutes |
| Cloudy or coloured water | 2 tablets per litre | 60 minutes |
Limitations:
| Filter Type | Bacteria | Viruses | Protozoa | Sediment | Chemical | Notes |
|---|---|---|---|---|---|---|
| Hollow fibre (LifeStraw, Sawyer) | Yes | No | Yes | Partial | No | Lightweight; does not treat viruses |
| Ceramic filter | Yes | No | Yes | Yes | Partial | Long life; fragile |
| Activated carbon filter | Limited | No | No | Yes | Yes (some) | Best for taste/chemicals; not biological |
| Reverse osmosis | Yes | Yes | Yes | Yes | Yes (most) | Requires pressure; slow; removes most contaminants |
| Pump filter + chemical treatment | Yes | Yes (chemical) | Yes | Partial | No | Combination is thorough |
| Gravity filter (Platypus, Katadyn) | Yes | No (most) | Yes | Yes | No | No effort; slow; check for virus coverage |
Key gap: Most portable filters do NOT remove viruses. In high-risk environments (disaster areas with contaminated water supplies, developing-world use), combine filtration with chemical treatment or use a filter specifically rated for viruses.
UV purification uses ultraviolet light to deactivate the DNA of microorganisms, rendering them unable to reproduce.
| Water Clarity | Effectiveness |
|---|---|
| Clear water | Excellent — kills bacteria, viruses, and protozoa including Cryptosporidium |
| Cloudy/turbid water | Reduced — particles shield organisms from UV; pre-filter required |
Process:
Limitations: Requires battery power; does not remove chemical contamination; reduced efficacy in turbid water.
Chlorine dioxide is more effective than bleach or iodine, including against Cryptosporidium. Recommended for serious backcountry and emergency use.
| Water Condition | Tablets | Wait Time |
|---|---|---|
| Clear water | 1 tablet per 1 litre | 30 minutes |
| Cloudy water | Pre-filter; 1 tablet per litre | 4 hours |
| Cryptosporidium concern | 1 tablet per litre | 4 hours |
For highest confidence in contaminated water sources:
This combination addresses bacteria, viruses, protozoa, and sediment.
| Contaminant | Boiling | Bleach | Iodine | Carbon Filter | Reverse Osmosis |
|---|---|---|---|---|---|
| Heavy metals (lead, arsenic) | No | No | No | Limited | Yes |
| Pesticides / herbicides | No | No | No | Partial | Most |
| Industrial chemicals | No | No | No | Partial | Most |
| Radiological contamination | No | No | No | No | Partial |
| Saltwater (desalination) | No | No | No | No | Yes |
| Nitrates | No | No | No | No | Yes |
For chemical contamination: Activated carbon filtration removes some organic chemicals; reverse osmosis or distillation provides the most comprehensive chemical removal. In a disaster involving chemical spill or industrial contamination, do not use local surface water unless you have RO capability.
Distillation (boiling and collecting steam condensate) removes almost all biological and chemical contaminants but is very slow and fuel-intensive.
Before any treatment method, removing sediment improves effectiveness and protects filter membranes:
| Situation | Action |
|---|---|
| Clear water, have fuel | Boil: 1 minute at sea level, 3 minutes at altitude |
| Clear water, no fuel | Bleach: 2 drops per litre (6–8% bleach); wait 30 min |
| Clear water, have tablets | Iodine or chlorine dioxide per dosage above |
| Cloudy water | Pre-filter through cloth; then double treatment time |
| Need virus protection (not just bacteria) | Use bleach/iodine/CIO₂ OR UV — hollow fibre filters alone do not kill viruses |
| Chemical contamination suspected | Do not use; seek alternative source; activated carbon or RO required |
| In high altitude wilderness | Boil 3 minutes; UV wait doubled for cold water |
| Combining methods for maximum safety | Cloth pre-filter → hollow fibre filter → chemical treatment |
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