How to Make Water Safe to Drink in an Emergency
First decide whether the water is a germ problem or a chemical problem. If it smells like fuel or officials say it is chemically contaminated, do not drink it and do not assume boiling or bleach will fix it. If it is a microbial risk, settle and strain cloudy water, then bring it to a rolling boil for 1 minute, or 3 minutes above 6,500 feet (CDC), or use the EPA emergency-disinfection bleach table.
Clean water is how you drink, brush teeth, wash hands, clean wounds, and prepare food. Ready.gov says to treat water of uncertain quality before using it for any of those jobs. This page summarizes the same official methods used in Chapter 4 of 90 Days Without Power by Robert Smythe.
A decision path you can follow without guessing
Skip chemical, fuel, and flood water. Prefer stored sealed water. Prefilter cloudy water. Boil if you can. Use bleach or chlorine dioxide if you cannot. Know what your filter misses.
Hidden water already in many homes includes the water-heater tank (often 30 to 50 gallons), water left in pipes after you shut the incoming supply, ice you made on purpose, and the toilet tank, never the bowl, and only if you do not use tank cleaning chemicals. Turn off power or gas to a water heater before you drain it, following the manufacturer instructions.
Pool and hot-tub water is for flushing and cleaning, not a drinking plan. Rooftop rain can pick up bird droppings and debris; collect it as a raw source and then treat it. Flowing outdoor water is usually a better raw source than a stagnant ditch, and it still needs treatment (EPA).
Boiling
CDC: a rolling boil for 1 full minute, or 3 minutes above 6,500 feet. EPA's page has also used 3 minutes above 5,000 feet. This book uses 3 minutes at or above 5,000 feet as a conservative household rule.
A rolling boil means large bubbles breaking continuously across the surface. Let the water cool and store it in clean, covered containers. Pouring it between two clean containers can improve the flat taste.
Boiling kills pathogenic bacteria, viruses, and protozoa. It does not remove heavy metals, salts, or most chemicals.
Household bleach (EPA and CDC doses)
Use only regular unscented chlorine bleach that lists sodium hypochlorite, typically 5 to 9 percent. Never use splash-less, scented, color-safe, or cleaner-added bleach.
CDC: for 5 to 9 percent bleach, add 8 drops (about 1/8 teaspoon) per gallon of clear water. Double that if the water is cloudy, colored, or very cold. Stir and wait at least 30 minutes.
EPA's emergency-disinfection table distinguishes bottle strengths: 8 drops of 6% bleach per gallon, or 6 drops of 8.25% bleach per gallon. For a quart or liter, EPA lists 2 drops of either strength. After 30 minutes the water should have a slight chlorine odor; if it does not, repeat the dose and wait 15 more minutes.
CDC notes that chlorine bleach does not work as well as boiling against some protozoa, including Cryptosporidium. When those parasites are a concern and you cannot boil, chlorine dioxide tablets used exactly as labeled or a filter rated for protozoa are the better backup.
Filters, SODIS, DIY stacks, and distillation
Most hollow-fiber straw, squeeze, and gravity filters are built for bacteria and protozoa. They often do not remove viruses unless the label says so, and they do not remove salt, fuel, or many chemicals.
A coffee filter or clean cloth is a prefilter. A DIY layered sand, gravel, and charcoal bucket is also a prefilter only; you still have to boil or disinfect. SODIS (WHO / Eawag) is a last resort: clear PET bottles, about 6 hours of full sun or 2 consecutive cloudy days, with relatively clear water.
Ready.gov describes distillation as a way to address microbes plus heavy metals, salts, and most other chemicals by collecting only condensed vapor. Their household pot-and-lid method is slow. Some volatile chemicals can travel with steam, so distillation is not permission to drink from a chemical spill or floodwater.
Browse water filters, tablets, bleach droppers, bathtub bladders, and storage jugs on the gear page, and see 90 Days Without Power for the full chapter and diagrams.
A practical order of operations when the tap is no longer trusted
Use the cleanest water you already have first, settle and prefilter cloudy water, then apply a method that matches the risk: boil or disinfect for microbes, use the right filter for what it is rated to remove, and do not pretend any kitchen method fixes chemical contamination.
Start with sealed commercial bottles. Next, water you stored in clean food-grade containers. After that, water already in the home that you can isolate, such as the water heater if the tank is intact and you follow a safe drain procedure from the manufacturer or a plumber’s guidance. Do not drink from a heater that may have been chemically treated in a way you do not understand, and never open a tank that could scald you.
If you must use outside water, pick the clearest flowing source you can reach safely. Avoid water with a chemical sheen, a strong fuel smell, or obvious sewage. Floodwater is a mix of microbes, chemicals, and debris; CDC and health departments treat it as unsafe to drink. Settling in a bucket and pouring off the clearer top layer, then pouring through a cloth or coffee filter, makes later disinfection more reliable. A homemade sand, gravel, and charcoal column is a prefilter only. It is not a certified drinking-water filter.
Boil when you can: a rolling boil for 1 minute, or 3 minutes above 6,500 feet (CDC). This book also uses a conservative household rule of 3 minutes at or above 5,000 feet because EPA’s emergency disinfection page has described a 3-minute boil above 5,000 feet. Let the water cool in a clean, covered container. Improve flat taste by pouring between two clean vessels.
When you cannot boil, EPA’s emergency disinfection page gives household unscented bleach doses for microbial risk: 8 drops of 6% bleach per gallon, or 6 drops of 8.25% bleach per gallon; double those amounts if the water is cloudy, colored, or very cold; wait 30 minutes. CDC also describes about 8 drops (or a little less than 1/8 teaspoon) of 5% to 9% unscented bleach per gallon. Use a dropper, not a kitchen guess. If the water still has no faint chlorine smell after 30 minutes, repeat the dose and wait another 15 minutes, following the same official pages.
Chlorine dioxide tablets such as household products labeled for water treatment have their own contact times. Read the packet. They are often used when people want a method that has a claim against cryptosporidium, which ordinary household bleach does not reliably handle. That is a label-and-CDC-filter issue, not a reason to skip boiling when boiling is available.
SODIS, described by WHO and the Swiss research institute Eawag, uses clear PET bottles, relatively clear water, and sunlight: about 6 hours in full sun, or about 2 days when the sky is cloudy. It is a last resort, not a desert-summer lifestyle tip, and it does not remove chemicals or heavy metals.
Distillation - a solar still or a pot with an inverted lid and a collection cup - can separate water from salt and from many (not all) chemicals. Some volatile organic compounds can travel with steam, which is why a fuel-smelling source is the wrong input. Distillation is slow. Treat it as a specialty method after you understand the risk, not as your only plan for a family of four.
What not to drink, and how this ties to a boil-water notice
Do not drink untreated surface water, floodwater, radiator water, swimming-pool water as your main supply, or any water under a chemical 'do not use' notice. A boil-water advisory is a microbial notice; boiling is the matching tool.
Swimming pools are treated with chemicals you do not want to drink in volume, and the water is not a planned household supply. Radiator water and water from toilet bowls are not drinking sources. The tank behind a toilet is sometimes discussed as unused clean water if it held only tank water and no chemical cleaners; if you have dropped in a toilet-tank cleaner, do not drink it.
When your utility issues a boil-water advisory, follow that notice even if your stored water plan is mature. The matching how-to is what to do during a boil water advisory. The EPA Safe Drinking Water Hotline is 1-800-426-4791.
Chapter 4 of 90 Days Without Power by Robert Smythe is built as a complete “making any water safe” field guide, with decision diagrams and Watch and Learn videos. The Top 10 gear list leads with filters and storage because water is the first bulk problem in a long outage.
Questions people ask
How long should I boil water?
CDC says a rolling boil for 1 minute, or 3 minutes above 6,500 feet. This site also uses a conservative 3-minute boil at or above 5,000 feet because EPA has described that elevation split.
Can I use scented bleach?
No. Official emergency disinfection directions call for regular unscented household bleach. Scented, splash-less, and color-safe products are the wrong chemical for this job.