Water Purification Decision Matrix — Choosing the Right Method

Boiling is the most reliable single method and your best default if fuel is available. Heat water to a rolling boil for at least 1 minute (3 minutes above 2000m elevation). This kills all bacteria, viruses, protozoa, and parasites. Boiling is effective regardless of water turbidity. However, boiling does not remove chemical contaminants or heavy metals — it may concentrate them if water evaporates. Use boiling when you have fuel access and suspect microbial contamination but not chemical pollution.

Boiling requires significant fuel and time. Do not rely on boiling alone if the water smells of fuel or chemicals.

Chlorine is fast, portable, and effective against bacteria and viruses. Household bleach (5% sodium hypochlorite) can disinfect water: use 2 drops per liter of clear water, or 4 drops per liter if water is turbid or colored. Let the water sit for at least 30 minutes before drinking. Chlorine is ineffective against cryptosporidium and does not remove chemical contaminants. Bleach tablets are lightweight for emergency kits. Check the smell after 30 minutes — you should detect a faint chlorine odor, indicating adequate disinfection.

Chlorine does not protect against cryptosporidium or heavy metals. Do not exceed recommended doses.

Iodine tablets or solution (2% iodine) provide portable disinfection: use 5 drops per liter of clear water, 10 drops if turbid. Wait 30 minutes. Iodine is effective against bacteria and viruses but less reliable against cryptosporidium. Iodine is not suitable for pregnant women or people with thyroid conditions. Iodine imparts a bitter taste and can be masked with powdered drinks. Long-term daily use (over weeks) is not recommended due to thyroid effects.

Do not use iodine if pregnant or thyroid-compromised. Not recommended for extended use. Less effective than chlorine against some pathogens.

Filtration physically removes particles, protozoa, and bacteria larger than the filter pores. Filters with 0.2–0.5 micron pores remove most bacteria and giardia/cryptosporidium. However, filtration alone does not reliably remove viruses, which are 0.02–0.1 microns. Cloth filters (three layers of tightly woven cotton or clean cloth) remove visible particles but not pathogens. Use filtration as a first step to improve water clarity and reduce contamination load before chemical treatment or boiling.

Do not rely on filtration alone for viral protection. Combine with chemical treatment or boiling.

SODIS (Solar Disinfection) uses UV radiation and heat to kill pathogens at no cost. Fill a clear PET plastic bottle (1.5–2L) with water, cap it loosely, place it in direct sunlight for at least 6 hours in clear weather, or 2 days if cloudy. UV-A and temperature (above 50°C / 122°F) kill bacteria and viruses. SODIS works best for bacteria and viruses but is less effective against cryptosporidium and does not remove chemical contaminants. This method requires time, clear bottles, and access to sunlight but is ideal when fuel is unavailable.

SODIS requires 6+ hours of direct sun. Do not use for heavily turbid water. Ineffective against chemical contaminants.

The most comprehensive approach combines filtration with chemical treatment. Filter the water first to remove particles, bacteria, and protozoa, then apply chlorine or boil. This protects against the full range of microbial threats: filtration removes protozoa (especially cryptosporidium) that resist chemical disinfectants, while chemical treatment targets viruses that filtration may miss. In high-contamination scenarios (urban flood water, heavily polluted sources), filter + chlorine offers the broadest protection without relying on fuel.

Urban flood water (high chemical and pathogen risk): Boil + filter (chemical removal requires distillation, which is impractical; boiling kills pathogens and filtration may reduce some chemicals). Avoid if petroleum smell is present. River water (moderate bacteria/virus/protozoa): Filter + chlorine is effective and portable. Well water (low contamination if sealed): Chlorine alone is usually sufficient, but boil if contamination is suspected. Seawater (salt and pathogens): Standard methods do NOT work — seawater requires distillation (boiling to capture condensation). No standard purification removes heavy metals, petroleum, or pesticides. If water smells of fuel or chemicals, do not drink it; boiling and filtration will not help.