You can make water drinkable by removing or killing the pathogens and contaminants in it. The most reliable method is boiling: bring water to a rolling boil for one minute, and it’s safe to drink. But boiling isn’t always practical, and it doesn’t remove chemicals or sediment. Depending on your situation, you may need a combination of methods to get truly clean water.
Start by Clearing Cloudy Water
Before you disinfect water, you need to deal with cloudiness. Murky water contains suspended dirt, organic matter, and other particles that shield bacteria and parasites from heat, UV light, and chemical treatment. If you skip this step, none of the disinfection methods below will work as well as they should.
The simplest approach is to let water sit undisturbed in a container for several hours. Heavier particles settle to the bottom, and you can carefully pour or siphon the clearer water off the top. If you’re in a hurry, you can strain water through a clean cloth, coffee filter, or cotton shirt to catch the larger debris. Neither method makes water safe to drink on its own, but both improve the effectiveness of whatever purification step comes next.
Boiling: The Simplest Purification Method
Boiling kills bacteria, viruses, and parasites effectively. The CDC recommends bringing clear water to a full rolling boil for one minute at most elevations. If you’re above 6,500 feet, boil for three minutes instead, because water boils at a lower temperature at higher altitudes and needs more time to kill pathogens.
Once boiled, let the water cool naturally in a clean container with a lid. Boiled water can taste flat because dissolved air escapes during heating. Pouring it back and forth between two clean containers a few times adds air back in and improves the taste. Keep in mind that boiling does not remove chemical contaminants, heavy metals, or salt. If your water source might contain those, you’ll need a different approach.
Household Bleach for Emergency Disinfection
When you can’t boil water, unscented household bleach is a proven backup. The EPA recommends using regular liquid bleach with no added fragrances, colors, or cleaners. For 8.25% sodium hypochlorite bleach (the standard concentration sold in most stores), the amounts are:
- 1 quart or liter: 2 drops for clear water, 4 drops for cloudy water
- 1 gallon: 6 drops for clear water, 12 drops for cloudy water
- 2 gallons: 1/8 teaspoon for clear water, 1/4 teaspoon for cloudy water
- 8 gallons: 1/2 teaspoon for clear water, 1 teaspoon for cloudy water
After adding the bleach, stir the water and let it sit for at least 30 minutes. You should notice a faint chlorine smell. If you don’t, repeat the dose and wait another 30 minutes. The bleach must be fresh, stored at room temperature, and less than a year old to work reliably.
Portable Filters and What They Actually Remove
Water filters work by forcing water through material with pores small enough to physically trap contaminants. The key number is the pore size, measured in microns. Understanding this helps you pick the right filter for the right threat.
Parasites like Giardia range from about 1 to 15 microns, and bacteria range from roughly 0.2 to 10 microns. A filter rated at 0.2 microns or smaller will catch both. Most backpacking-style pump filters and gravity filters hit this threshold and are effective against bacteria and parasites. However, viruses are far smaller, ranging from 0.004 to 0.1 microns. Standard portable filters cannot reliably remove viruses. If viruses are a concern (common in developing countries or areas with sewage contamination), you’ll need to pair your filter with a chemical or UV disinfection step.
Filters also don’t remove dissolved chemicals or heavy metals unless they contain activated carbon or a specialized membrane. If you’re filtering water from a source that might have pesticide or industrial runoff, look for a filter that specifically lists chemical reduction in its specs.
UV Light Treatment
Ultraviolet light in the UV-C range (wavelengths between 200 and 300 nanometers, with peak effectiveness around 260 nm) damages the DNA of bacteria, viruses, and parasites so they can’t reproduce or infect you. Portable UV purifiers, typically pen-shaped devices you swirl in a bottle for 60 to 90 seconds, use this principle.
UV treatment works well on clear water, but it’s much less effective on cloudy or murky water because particles block the light from reaching pathogens. Always pre-filter or settle turbid water before using a UV device. These devices also require batteries or a charge, so they’re not ideal as your only option in extended emergencies.
Solar Disinfection (SODIS)
If you have no equipment at all, sunlight can disinfect water. The SODIS method involves filling clear plastic bottles (PET, the type most disposable water and soda bottles are made from) with water and placing them in direct sunlight. On a sunny day, six hours of exposure is enough. On overcast days, you need up to 48 hours. During continuous rainfall, SODIS doesn’t work and shouldn’t be relied on.
The water needs to be reasonably clear for this to work. Bottles should be no larger than two liters, because UV light can’t penetrate deeply enough in bigger containers. Lay the bottles on their sides on a reflective surface like a metal roof for best results. SODIS won’t remove chemicals, but it’s effective against most bacteria, viruses, and parasites when done properly. It’s widely used in parts of Africa, Asia, and Latin America where other treatment options aren’t available.
Distillation for the Cleanest Water
Distillation is the most thorough purification method available. It involves boiling water into steam, then collecting the steam as it condenses back into liquid. Because contaminants don’t evaporate with the water, this process removes up to 99.5% of impurities, including dissolved salts, heavy metals like lead, nitrates, fluoride, iron, and many organic compounds.
You can build a basic still at home with a large pot, a heat-resistant bowl, and a lid turned upside down. Fill the pot with water, float or elevate the collection bowl inside it, and invert the lid so steam condenses on the underside and drips into the bowl. Place ice on the inverted lid to speed condensation. It’s slow and fuel-intensive, but it’s the only home method that effectively handles saltwater or water with chemical contamination.
Building a DIY Sand and Charcoal Filter
In a survival or long-term off-grid situation, you can build a gravity filter using natural materials. The basic structure is a cone or cylinder (a cut plastic bottle works well) with layers of filtering material stacked inside.
Start with a thick plug of grass or clean cloth at the very bottom to hold everything in place. Above that, fill most of the container with crushed charcoal, ideally from a hardwood fire (not briquettes, which contain additives). Top the charcoal with a layer of sand. Water poured in at the top passes through the sand first, which catches larger particles, then through the charcoal, which absorbs some chemicals and smaller impurities.
This type of filter improves taste and clarity significantly, but it does not reliably kill bacteria or viruses. You should still boil or chemically treat the filtered water before drinking it. Think of this as a pre-treatment step, not a complete solution.
Iodine Tablets: Useful but Limited
Iodine tablets are lightweight and popular with hikers, but they come with restrictions. They’re effective against most bacteria and viruses, though they take longer to work in cold water and are less reliable against certain parasites like Cryptosporidium.
The bigger concern is safety with extended use. Excess iodine intake can cause thyroid problems, particularly hypothyroidism. People with existing thyroid conditions, pregnant women, and those on lithium are especially sensitive. While a commonly cited guideline suggests limiting iodine-treated water to three weeks of continuous use, the threshold for problems varies widely between individuals. For short trips, iodine tablets are a reasonable option. For anything longer, switch to a different method.
Combining Methods for the Safest Results
No single method handles every possible contaminant. The most reliable approach layers multiple steps together. A practical sequence for most situations: first, settle or strain the water to remove visible sediment. Then filter it through the finest filter you have. Finally, disinfect with boiling, bleach, or UV light. If your water source might contain chemicals or dissolved metals, distillation or an activated carbon filter is the only way to address those.
For everyday emergency preparedness, keeping a gallon of unscented bleach and knowing the correct drop count gets you surprisingly far. For backcountry travel, a 0.2-micron filter paired with chemical or UV treatment covers nearly every pathogen you’ll encounter. Whatever method you choose, starting with the clearest water you can find makes every subsequent step more effective.