Water Purification Cheat Sheet

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

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.

Household Bleach (Sodium Hypochlorite)

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:

1. Pre-filter cloudy water through clean cloth or coffee filter.
2. Add correct dose of unscented bleach.
3. Stir and let stand 30 minutes (60 minutes if water is cloudy or cold).
4. Water should have a slight chlorine smell. If not, repeat the dose and wait 15 more minutes.
5. If chlorine smell is very strong after treatment, let stand uncovered for a few minutes before drinking.

Shelf life of bleach: Bleach loses potency over time. Stock no more than 1-year supply; use fresh stock for emergency treatment.

Iodine 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:

• Iodine does NOT reliably kill Cryptosporidium (protozoan parasite)
• Not for use by pregnant women, people with thyroid conditions, or those allergic to iodine
• Not recommended for continuous use (> 3 weeks)
• Taste is unpleasant — vitamin C (ascorbic acid) added after treatment neutralises taste and colour

Water Filtration — Types and What They Remove

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 (SteriPen and Similar)

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:

1. Pre-filter cloudy water to improve clarity.
2. Insert UV device; activate per manufacturer instructions.
3. Stir device for the required treatment time (typically 60–90 seconds for 1 litre).
4. Drink immediately — UV purification has no residual effect.

Limitations: Requires battery power; does not remove chemical contamination; reduced efficacy in turbid water.

Chlorine Dioxide Tablets (Aquatabs, Potable Aqua CIO₂)

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

Combined Treatment — Maximum Protection

For highest confidence in contaminated water sources:

1. Pre-filter: Run through cloth, coffee filter, or clean sand to remove sediment.
2. Filter: Use hollow fibre or ceramic filter to remove bacteria and protozoa.
3. Chemical treatment or UV: Add bleach, iodine, or chlorine dioxide tablets to address viruses; or use UV device on clear pre-filtered water.

This combination addresses bacteria, viruses, protozoa, and sediment.

What Purification Does NOT Remove

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.

Pre-Filtering Sediment

Before any treatment method, removing sediment improves effectiveness and protects filter membranes:

1. Settling: Allow water to sit undisturbed for 30+ minutes; pour off upper layer.
2. Cloth filter: Pour through T-shirt, bandana, or multiple layers of clean fabric.
3. Sand and gravel filter: Improvised layered filter — fine sand over coarse sand over gravel; catches large particles.
4. Coffee filter: Effective for small sediment particles.

Quick Reference

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