Boiling water for one minute at a rolling boil is the most reliable way to make it safe to drink. But boiling isn’t always practical, and it’s not the only option. Depending on your situation, you can also use chemical disinfectants, portable filters, distillation, or even sunlight. Each method handles different threats, so understanding what’s in your water helps you pick the right approach.
Start by Clearing Sediment
Before treating water by any method, remove visible particles. Sediment, debris, and cloudiness reduce the effectiveness of every disinfection technique. If the water looks murky, let it settle in a clean container, then pour the clearer water off the top through a clean cloth, coffee filter, or paper towel. This pre-filtering step doesn’t make water safe on its own, but it makes the next step work far better.
Boiling: The Most Reliable Method
Bringing water to a full rolling boil for one minute kills bacteria, viruses, and parasites. At elevations above 6,500 feet (about 2,000 meters), water boils at a lower temperature, so extend the boil to three minutes to compensate. Once boiled, let the water cool naturally and store it in a clean, covered container.
Boiling is effective against virtually every type of biological contaminant, including hardy parasites like Cryptosporidium that resist chemical treatment. The downside is that it requires fuel, a heat source, and time. It also won’t remove chemical pollutants, heavy metals, or dissolved solids.
Household Bleach in Emergencies
Unscented liquid household bleach can disinfect water when boiling isn’t possible. The EPA recommends using bleach that contains either 6% or 8.25% sodium hypochlorite, with no added fragrances, dyes, or cleaners. Check the label before using it.
For one gallon of water, add 8 drops of 6% bleach or 6 drops of 8.25% bleach. For a single quart or liter, 2 drops of either concentration is enough. If the water is cloudy, colored, or very cold, double those amounts. Stir the water and let it stand for at least 30 minutes before drinking. You should detect a faint chlorine smell after treatment. If you don’t, repeat the dose and wait another 15 minutes.
Bleach kills bacteria and viruses effectively but does not reliably kill parasites like Cryptosporidium and Giardia. If parasitic contamination is a concern (common in untreated surface water from lakes and streams), combine bleach treatment with filtration or choose a different method.
Chlorine Dioxide and Iodine Tablets
Chemical tablets sold for camping and emergency kits fall into two main categories. Iodine tablets kill bacteria and viruses but, like bleach, don’t work well against parasites. Chlorine dioxide tablets are more versatile. They can kill Giardia and are somewhat effective against Cryptosporidium, though filtering the water first is still the safest approach when parasites may be present.
Follow the contact time on the tablet packaging carefully. Chlorine dioxide tablets typically need 30 minutes to four hours depending on the target organism and water temperature. Cold water requires longer wait times. These tablets are lightweight and easy to carry, making them a practical option for hiking and travel.
Portable Water Filters
Water filters physically strain out contaminants based on pore size. For removing bacteria and parasites, look for a filter rated at 0.2 microns or smaller. At that pore size, a filter blocks bacteria, protozoan cysts like Giardia and Cryptosporidium, and sediment. Filters with an absolute pore size of 1 micron will catch most parasites but may let smaller bacteria through.
Most portable filters do not remove viruses, which are far smaller than bacteria. If viral contamination is a concern (more common in developing countries or areas with sewage contamination), you’ll need to pair filtration with a chemical disinfectant or UV treatment. Some higher-end portable purifiers combine a filter element with a chemical or UV stage specifically to address viruses.
Reverse osmosis systems use membranes with pore sizes as small as 0.02 microns, small enough to remove bacteria and most other dissolved contaminants. These are typically home-installed systems rather than portable devices.
Distillation for the Broadest Protection
Distillation removes the widest range of contaminants of any single method. It works by boiling water into steam, leaving impurities behind in the boiling chamber, then cooling the steam back into liquid in a separate container. The heat kills bacteria, viruses, and parasites, while the evaporation-and-condensation process removes dissolved solids, heavy metals like lead, fluoride, nitrate, iron, manganese, chlorine, and even radioactive particles.
The limitation is that some chemicals with boiling points close to water’s, such as certain pesticides and volatile organic compounds like benzene and toluene, can vaporize along with the steam and end up in the collected water. Distillation is also slow and energy-intensive, making it better suited for home use than field situations.
Solar Disinfection (SODIS)
If you have no fuel, no chemicals, and no filter, sunlight can disinfect small quantities of water. Fill a clear plastic (PET) bottle with water, leaving a small air gap at the top. Lay the bottle on its side in direct sunlight. On a clear or partly cloudy day, six hours of sun exposure is enough. If the sky is more than half covered with clouds, leave the bottle out for two consecutive days. When water temperature reaches at least 50°C (122°F), one hour is sufficient.
SODIS works through a combination of UV radiation and heat. It’s effective against bacteria, viruses, and many parasites, but it only works with relatively clear water in small volumes (typically 1 to 2 liters per bottle). It’s a last-resort method, widely used in parts of the world where other treatment options aren’t available.
Storing Treated Water Safely
Treating water is only half the job. Poor storage can recontaminate clean water within hours. The CDC recommends storing safe water in durable containers made of plastic, ceramic, or metal. Ideal containers have a single small opening (about 5 to 8 centimeters) that can be sealed tightly, and a narrow neck so you can pour without dipping hands or utensils into the water.
Never use containers that previously held bleach, pesticides, gasoline, or other toxic chemicals, even if they’ve been rinsed. Residual chemicals can leach into the water. Store containers in a cool location out of direct sunlight, away from gasoline, solvents, or pesticides. Label each container as “drinking water” and note the date you treated it.
Choosing the Right Method
No single method handles every possible contaminant, so matching your approach to your situation matters. Here’s a practical breakdown:
- At home during a boil advisory: Boiling is simplest. One minute at a rolling boil handles everything biological.
- Hiking or camping near streams and lakes: A 0.2-micron filter removes parasites and bacteria. Pair it with chlorine dioxide tablets if viruses are a concern.
- Emergency with no heat source: Household bleach disinfects against bacteria and viruses. Double the dose for cloudy water.
- Concern about heavy metals or chemical pollution: Distillation or reverse osmosis are the only methods that address dissolved metals and many chemical contaminants. Boiling, bleach, and filters won’t help with lead or nitrate.
- No supplies at all: SODIS with a clear plastic bottle and six hours of sun provides meaningful protection.
When in doubt, combining methods gives you the broadest protection. Filtering first to remove sediment and parasites, then treating with a chemical disinfectant or boiling to kill viruses and bacteria, covers nearly every biological threat you’re likely to encounter.