Boiling water at a rolling boil for one minute is the simplest and most reliable way to make it safe for drinking. But it’s not the only option. Depending on your situation, whether you’re dealing with a boil-water advisory, camping in the backcountry, or preparing for an emergency, different treatment methods work better for different threats. Here’s how each one works and when to use it.
Start by Clearing Cloudy Water
Before you disinfect water by any method, you need it to be reasonably clear. Particles suspended in cloudy water shield bacteria and parasites from heat, chemicals, and UV light, making every treatment method less effective.
If your water is visibly murky, let it sit undisturbed in a clean container for several hours. Gravity does the work: suspended particles gradually sink to the bottom. Once they’ve settled, carefully pour or siphon the clearer water off the top into a separate container. If you have a cloth like a clean cotton shirt or bandana, pouring the water through it removes larger debris. This step alone won’t make water safe, but it dramatically improves the performance of whatever disinfection method you use next.
Boiling
Boiling is the gold standard for killing living pathogens. One minute at a rolling boil inactivates bacteria, viruses, and encysted parasites like Giardia and Cryptosporidium. You don’t need a thermometer or any supplies beyond a heat source and a pot.
If you’re above about 6,500 feet (2,000 meters), water boils at a lower temperature, so extend the boil to three minutes. At very high elevations, around 10,000 feet, water boils near 90°C instead of 100°C, and the extra time compensates for that difference. Keeping the lid on your pot during and after boiling also helps: research published in Emerging Infectious Diseases found that boiling in a covered vessel for three to five minutes destroyed even heat-resistant bacterial spores by more than 99.99%.
The main downside of boiling is practicality. It requires fuel, takes time to cool, and does nothing to remove chemical contaminants like heavy metals or pesticides. It’s best suited for situations where biological contamination is the concern.
Disinfecting With Household Bleach
Unscented liquid household bleach is cheap, widely available, and effective against bacteria and viruses. The EPA recommends using regular bleach that contains either 6% or 8.25% sodium hypochlorite, with no added fragrances or cleaners.
For clear water, the dosing is straightforward:
- 6% bleach: 8 drops per gallon (2 drops per quart)
- 8.25% bleach: 6 drops per gallon (2 drops per quart)
Double those amounts if the water is cloudy, colored, or very cold. After adding the bleach, stir or shake the container and let it sit for at least 30 minutes before drinking. You should detect a faint chlorine smell. If you don’t, repeat the dose and wait another 15 minutes.
One important limitation: bleach and iodine do not reliably kill parasites like Cryptosporidium. If parasites are a concern, such as with untreated surface water, you’ll want to combine chemical disinfection with filtration or boiling.
Chemical Tablets for Hiking and Travel
Portable water purification tablets are the go-to for backpackers and travelers. Two main types are widely sold: iodine tablets and chlorine dioxide tablets. They work differently and have different strengths.
Iodine tablets kill bacteria and viruses effectively but, like bleach, don’t work well against parasites. They also leave a noticeable taste and aren’t recommended for long-term use, especially during pregnancy or for people with thyroid conditions.
Chlorine dioxide tablets are the better all-around option. They kill bacteria and viruses and are also effective against Giardia. They have some activity against Cryptosporidium as well, though not complete. The trade-off is time: chlorine dioxide tablets typically require 30 minutes to four hours of contact time depending on the target pathogen, so read the instructions on your specific product carefully. Many hikers treat their water at each stop and let it sit while they walk.
Filtration
Water filters physically remove pathogens by forcing water through tiny pores. The key number is the filter’s absolute pore size, measured in microns. Different pore sizes stop different threats:
- 1 micron or smaller: Removes parasites and amoebas like Giardia and Cryptosporidium
- 0.3 microns or smaller: Also removes bacteria like Salmonella and E. coli
- Nanofiltration or reverse osmosis: Required to remove viruses like norovirus and hepatitis A
Most portable pump filters and gravity filters sold for camping use pore sizes around 0.2 microns, which handles parasites and bacteria but not viruses. For most North American backcountry water, that’s sufficient because waterborne viruses are uncommon. For international travel or floodwater, where viruses are a greater concern, you’d want to pair filtration with chemical disinfection or use a purifier rated for viruses.
When shopping for a home filter, look for certification to NSF Standard 53 or 58 for cyst removal. Reverse osmosis systems, which are common under-sink units, handle the full range of biological and many chemical contaminants.
UV Light and Solar Disinfection
Ultraviolet light damages the DNA of microorganisms, preventing them from reproducing. Portable UV devices like the SteriPEN use battery-powered UV-C light to treat a liter of water in about 60 to 90 seconds. They’re lightweight and effective against bacteria, viruses, and parasites, but they require batteries, only work in clear water, and treat small volumes at a time.
Solar disinfection (SODIS) uses sunlight instead of a device. Fill a clean 2-liter PET plastic bottle (the kind most soda and water bottles are made from) with water that isn’t too cloudy, ideally below 30 NTU, which roughly means you can see through it. Lay the bottle on its side in direct sunlight for six hours on a sunny day. On overcast days, extend the exposure to 48 hours. On days of continuous rain, SODIS doesn’t work reliably. PET plastic transmits the UVA and visible light wavelengths that do the disinfection work, and these bottles are available almost everywhere in the world, making SODIS a practical last-resort method in resource-limited settings.
Distillation
Distillation is the most thorough treatment method available. It works by boiling water into steam and then condensing that steam back into liquid in a separate container. Everything that doesn’t evaporate, including bacteria, viruses, parasites, dissolved minerals, heavy metals like lead, fluoride, nitrate, radioactive particles, and most organic compounds, gets left behind in the boiling chamber.
This makes distillation uniquely useful when you’re worried about chemical contamination, not just biological threats. Boiling alone kills pathogens but leaves dissolved chemicals in the water. Distillation removes both. The downside is speed and energy: countertop distillers typically produce about one gallon every four to six hours. It’s better suited for home use or long-term emergency preparedness than for treating water on the go.
Storing Treated Water Safely
Treating water is only half the job. Poor storage recontaminates it. Use containers specifically made for food or beverage contact. Polyethylene plastic, the material used in most heavy-duty water jugs and 55-gallon drums, is FDA-approved for this purpose. Glass jars work too, though they’re heavier and breakable.
Avoid milk jugs and thin plastic bleach bottles. These are manufactured from biodegradable plastic that breaks down relatively quickly and can leach off-flavors or harbor bacteria in scratches. Two-liter soda bottles with screw-on lids are a better lightweight option.
Keep stored water in a cool, dark place. If you’ve treated it with bleach, the residual chlorine helps prevent microbial regrowth, but it fades over time. Rotate your stored water supply every one to two years. Label containers with the date you filled them so you don’t lose track.
Choosing the Right Method
No single method is perfect for every situation. Here’s a quick way to decide:
- Emergency at home with power: Boil for one minute, then store in food-grade containers.
- Emergency without power: Use bleach at the doses above, or distill over a camp stove if chemical contamination is a concern.
- Backpacking or camping: A portable filter (0.2 microns) handles most North American water. Add chlorine dioxide tablets if viruses are a risk.
- International travel: Combine filtration with chemical or UV treatment for the broadest protection.
- Resource-limited setting: SODIS with PET bottles works when no other options are available.
For the most complete protection, combining two methods, such as filtering first and then disinfecting chemically or with UV, covers the widest range of contaminants. Filtration handles parasites that resist chemicals, and chemical or UV treatment handles viruses that slip through most filters.