Water Purification
In an emergency, your tap water may become contaminated or unavailable. Knowing how to purify water from natural sources can keep your family safe. Here are five proven methods, from the simplest to the most resourceful.
Contaminated water kills
Waterborne diseases such as cholera, typhoid and giardia can cause severe illness within hours. After floods, earthquakes or infrastructure failures, municipal water may be unsafe even if it looks clear. When in doubt, purify. If authorities issue a boil-water advisory, follow it strictly.
Before you start: pre-filtering
Always pre-filter cloudy or murky water
If the water contains visible particles, sediment or colour, filter it through a clean cloth, coffee filter or layered sand before applying any purification method. This improves effectiveness and taste. Let very murky water settle in a container for 30 minutes, then carefully pour off the clearer water from the top.
Method 1: Boiling
The most reliable method
Boiling is the single most effective way to kill all types of pathogens: bacteria, viruses and parasites. It requires no special equipment, only a heat source and a container.
- Bring water to a vigorous rolling boil (large bubbles, not just simmering)
- Maintain the boil for at least 1 minute
- At altitudes above 2,000 metres: boil for 3 minutes (lower atmospheric pressure means water boils at a lower temperature)
- Allow water to cool naturally. Do not add ice (it may be contaminated)
- To improve the flat taste of boiled water, pour it back and forth between two clean containers several times to add oxygen
Advantages
Kills 100% of biological contaminants. No chemicals needed. Works with any water source. Simple and universally understood.
Limitations
Requires fuel or a heat source. Uses time and energy. Does not remove chemical contaminants, heavy metals or dissolved solids. Water must cool before drinking.
Method 2: Bleach treatment
Chemical disinfection with household bleach
Ordinary unscented household bleach containing 5-6% sodium hypochlorite can disinfect water effectively. This is the same method used by water treatment plants, just on a smaller scale.
- Clear water: add 2 drops of bleach per litre (or 8 drops per gallon)
- Cloudy water: add 4 drops of bleach per litre (double the dose)
- Stir well and wait at least 30 minutes before drinking
- After 30 minutes, the water should have a slight chlorine smell. If not, repeat the dose and wait another 15 minutes
- Use only plain, unscented bleach. Never use bleach with added fragrances, surfactants or thickeners
- If bleach concentration is 1% (some European brands), use 10 drops per litre of clear water
Bleach shelf life
Liquid bleach loses potency over time. After 6 months of storage, its effectiveness diminishes. Rotate your emergency bleach supply annually. Store in a cool, dark place, away from children.
Method 3: Purification tablets
Compact and portable
Water purification tablets are lightweight, easy to store, and have a long shelf life (typically 3-5 years unopened). They are ideal for emergency bags and car kits.
- Chlorine tablets (e.g. Aquatabs): 1 tablet per litre, wait 30 minutes. Effective against bacteria and most viruses
- Chlorine dioxide tablets (e.g. Micropur Forte): 1 tablet per litre, wait 30-120 minutes depending on temperature. Effective against bacteria, viruses, and Cryptosporidium
- Iodine tablets: 1 tablet per litre, wait 30 minutes (longer for cold water). Note: not suitable for pregnant women, people with thyroid conditions, or long-term use beyond 3 weeks
- Always follow the manufacturer's instructions for dosage and wait time
- Available at pharmacies, outdoor shops and online
Method 4: Filtration
Mechanical removal of contaminants
Portable water filters physically remove pathogens by forcing water through a fine membrane or ceramic element. The key specification is pore size.
- 0.2 micron (or smaller): removes bacteria (e.g. E. coli, Salmonella) and protozoa (e.g. Giardia, Cryptosporidium)
- 0.02 micron (hollow-fibre): also removes most viruses
- Activated carbon filters: improve taste and reduce some chemical contaminants, but do not reliably kill pathogens alone
- Popular brands: LifeStraw, Sawyer, Katadyn, MSR
- Gravity-fed filters are ideal for groups (no pumping required)
- Pump filters work faster for individual use
- Clean and maintain filters according to the manufacturer's instructions. Backflush when flow rate decreases
Important
Most portable filters do NOT remove viruses (except 0.02 micron models). In areas where viral contamination is a risk, combine filtration with boiling or chemical treatment for complete protection.
Method 5: SODIS (Solar Disinfection)
Using sunlight to kill pathogens
SODIS is a simple, no-cost method developed by the Swiss Federal Institute of Aquatic Science and Technology (Eawag). It uses UV-A radiation and heat from the sun to inactivate pathogens.
- Fill a clean, clear PET plastic bottle (1-2 litre) with pre-filtered water. Do not use glass (it blocks UV). Do not use coloured or scratched bottles
- Place horizontally in direct sunlight on a reflective surface (corrugated metal roof is ideal)
- Sunny conditions: leave for at least 6 hours
- Cloudy conditions: leave for 2 consecutive days
- Overcast or rainy: SODIS is not effective. Use another method
- Water temperature above 50°C combined with UV greatly accelerates disinfection
- Effective against bacteria, viruses and most protozoa
When to use SODIS
SODIS is best suited as a backup method when you have no fuel, no chemicals and strong sunlight. It is widely used in developing countries and recommended by the WHO. In Portugal, sunny summer days provide excellent conditions for SODIS.
Comparison table
| Method | Kills bacteria | Kills viruses | Kills protozoa | Time needed | Equipment | Cost |
|---|---|---|---|---|---|---|
| Boiling | Yes | Yes | Yes | 1-3 min + cooling | Heat source, container | Free (fuel needed) |
| Bleach | Yes | Yes | Partial | 30 min | Bleach, dropper | Under 1 EUR |
| Tablets | Yes | Yes | Varies | 30-120 min | Tablets | 5-15 EUR |
| Filtration | Yes | Some models | Yes | Immediate | Filter device | 20-80 EUR |
| SODIS | Yes | Yes | Most | 6 h - 2 days | PET bottle, sunlight | Free |
Water you should NEVER attempt to purify
These water sources remain dangerous even after treatment
- Water contaminated with chemicals, fuel, pesticides or industrial waste: boiling and filtration do not remove dissolved chemicals
- Water with a strong chemical smell (petrol, solvents, ammonia)
- Water from radiator systems (contains antifreeze, which is toxic)
- Flood water in urban or industrial areas (sewage, chemicals, debris). Filter and treat only as an absolute last resort
- Swimming pool water is safe for hygiene use but excessive chlorine makes it unsuitable for drinking without dilution
Building your water purification kit
Recommended supplies
- Portable water filter (0.2 micron minimum)
- Purification tablets (at least 50 tablets per person)
- Unscented household bleach (small bottle) and a medicine dropper
- Clear PET bottles (for SODIS backup)
- Camping pot or metal container for boiling
- Coffee filters or clean cloth for pre-filtering
- See our water storage guide and drought preparedness guide for related information
The best method is the one you have
In a real emergency, you may not have access to your preferred purification method. Learn all five methods so you can adapt to whatever resources are available. Store at least two different purification options in your emergency bag. When in doubt, boil. It works every time.