Water Purification Guide
Select your water source and available materials for specific treatment steps.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Clean drinking water is the most critical survival need. Contaminated water can cause severe illness from bacteria, viruses, and parasites. This interactive guide walks you through the best purification method based on what you actually have available.
- Select your water source. Different sources carry different risks. A clear stream has lower turbidity than standing water, which affects which methods work best and whether pre-filtering is needed.
- Check your available materials. Select everything you have on hand. The guide picks the most reliable method from your available options, following the same priority order recommended by the CDC and EPA.
- Follow the step-by-step instructions. Each step includes specific dosages, wait times, and warnings. Pay close attention to warnings about limitations of each method.
- Read the additional notes. These provide important context about your specific situation, including when a second treatment step is recommended.
All dosages and wait times are sourced from CDC, EPA, and WHO guidelines for emergency water treatment.
About the Water Purification Guide
This guide follows the water treatment priority order recommended by the CDC and EPA for emergency situations. Boiling is considered the most reliable single-step method because it kills all categories of pathogens: bacteria, viruses, and protozoa (including Cryptosporidium). Chemical treatments like bleach and iodine are effective against most pathogens but have limitations. The CDC recommends a rolling boil for 1 minute (3 minutes above 6,500 feet). For bleach treatment, the EPA specifies 8 drops of 6% bleach per gallon or 6 drops of 8.25% bleach per gallon, doubled for cloudy water. SODIS (solar water disinfection) is recognized by the WHO as a viable method in resource-limited settings.
Frequently Asked Questions
Does boiling water remove all contaminants?
Boiling kills bacteria, viruses, and parasites, including Cryptosporidium. However, it does not remove chemical contaminants, heavy metals, or dissolved solids. For water suspected of chemical contamination (such as near industrial sites), boiling alone is not sufficient. A rolling boil for 1 minute is effective at sea level. Above 6,500 feet, boil for 3 minutes because water boils at a lower temperature at higher elevations.
How much bleach do I need to purify water?
For 6% sodium hypochlorite bleach, add 8 drops per gallon. For 8.25% bleach (the most common concentration sold today), add 6 drops per gallon. If the water is cloudy, double the amount. Stir and wait 30 minutes. The water should have a slight chlorine smell after treatment. If not, repeat the dose and wait 15 more minutes. Use only regular, unscented bleach with no added cleaners or surfactants.
Why does iodine not kill Cryptosporidium?
Cryptosporidium has a tough outer shell (oocyst) that is highly resistant to chemical disinfectants like iodine and chlorine. The concentrations and contact times used in field water treatment are not sufficient to penetrate this shell. Boiling, UV treatment, and filtration with a filter rated to 1 micron or smaller are effective against Cryptosporidium. Chlorine dioxide requires up to 4 hours of contact time to inactivate Crypto.
Is the SODIS method really effective?
Yes. SODIS (Solar Water Disinfection) is recognized by the World Health Organization and has been studied extensively since the 1980s. UV-A radiation and heat from sunlight inactivate pathogens in water. It requires clear PET plastic bottles (recycling number 1), water with low turbidity (below 30 NTU), and 6 hours of direct sunlight (or 48 hours under cloudy skies). It works best in tropical and subtropical regions between latitudes 35N and 35S. It is not effective through glass or tinted plastic.