You can purify water through boiling, chemical disinfection, filtration, UV light, distillation, or solar exposure. The simplest and most reliable method is boiling: bring water to a rolling boil for one minute, and it’s safe to drink. Each method has tradeoffs in speed, effectiveness, and what it actually removes, so the best choice depends on your situation and what contaminants you’re dealing with.
Boiling: The Most Reliable Method
Boiling kills bacteria, viruses, parasites, and virtually every other living pathogen in water. Bring clear water to a full, rolling boil and keep it there for one minute. If you’re at an elevation above 6,500 feet, boil for three minutes instead, since water boils at a lower temperature at altitude and needs the extra time to fully disinfect.
Let the water cool naturally before drinking, and store it in clean containers with tight lids. Boiling won’t remove chemical contaminants, heavy metals, or sediment, but for biological threats it’s the gold standard. The main downside is that you need fuel and a heat source, which aren’t always available.
Household Bleach Disinfection
Unscented liquid household bleach is a surprisingly effective disinfectant and one of the primary methods recommended by both the CDC and EPA for emergency water treatment. You need regular bleach with sodium hypochlorite as the active ingredient, not splashless or scented varieties.
For bleach with a 5% to 9% concentration (the most common type on store shelves), add 8 drops per gallon of water. That’s about a quarter teaspoon. For smaller quantities, use 2 drops per quart. Stir it in and let the water sit for at least 30 minutes. After that wait, the water should have a faint chlorine smell. If it doesn’t, add the same amount again and wait another 15 minutes.
If the water is cloudy, colored, or very cold, double the bleach amount. Cold water slows the chemical reaction, and particles in cloudy water can shield pathogens from the chlorine. Whenever possible, filter or strain cloudy water through a clean cloth before adding bleach.
Portable Water Filters
Filters work by physically blocking pathogens based on size, which means the pore size of the filter determines what it can and can’t remove. To remove parasites like Giardia and Cryptosporidium, you need a filter with an absolute pore size of 1 micron or smaller. To remove bacteria, you need 0.3 microns or smaller. Most quality backpacking and emergency filters meet both of these thresholds.
Viruses, however, are far smaller than bacteria and pass through standard portable filters. Removing viruses requires nanofiltration or reverse osmosis systems, which aren’t practical in most field or emergency situations. The CDC recommends adding a chemical disinfectant (bleach, iodine, or chlorine dioxide) to filtered water to kill any viruses and remaining bacteria that may have slipped through. Filtering first, then disinfecting, gives you the most complete protection.
Chemical Tablets and Drops
Chlorine dioxide, iodine, and chlorine tablets are lightweight and easy to pack, making them popular for hiking and travel. Each product has specific dosage instructions on the packaging, and following them exactly matters.
Iodine tablets and drops kill most pathogens but will not kill Cryptosporidium, a common waterborne parasite. Chlorine dioxide tablets will kill Cryptosporidium if used according to their directions. That distinction matters if you’re treating water from streams or lakes where animal contamination is likely.
Temperature plays a big role with iodine. Cold water significantly slows disinfection. As a rough rule, for every 10°C (18°F) drop in water temperature, you need to double the contact time. Typical instructions call for 20 to 35 minutes of wait time at moderate temperatures, but near-freezing water can require 40 minutes or more for full parasite inactivation. If you’re treating ice-cold mountain stream water, patience is essential.
Iodine Safety Concerns
Iodine is not safe for everyone. Pregnant women should avoid iodine-treated water because excess iodine can affect fetal thyroid development, and the safe upper limit during pregnancy is uncertain. People with thyroid conditions or iodine allergies should also use a different method. For anyone else, iodine is fine for short-term use but shouldn’t be your only water treatment method for weeks on end.
UV Light Treatment
Portable UV devices (like the SteriPEN) kill bacteria, viruses, and parasites by damaging their DNA so they can’t reproduce. You submerge the device in a bottle of water, activate it, and stir for the time specified by the manufacturer, typically 60 to 90 seconds per liter.
The catch is that UV light only works in clear water. Suspended particles create shadows where pathogens can hide from the light. If your water is murky, you need to filter or settle it first. UV devices also require batteries or charging, which adds a point of failure in extended backcountry or emergency situations.
Solar Disinfection (SODIS)
If you have no fuel, no chemicals, and no filter, sunlight itself can disinfect water. Fill clean, clear plastic bottles (standard PET soda or water bottles work well) with clear water and lay them on their sides in direct sunlight. On sunny days, six hours of exposure is enough. On cloudy days, you need a full two days. During continuous rain, this method won’t work.
The water needs to be reasonably clear for SODIS to be effective. The UV radiation and heat from sunlight work together to kill pathogens, but particles in the water block the UV rays just as they do with portable UV devices. SODIS is a last-resort method, slower than any other option on this list, but it costs nothing and requires no equipment beyond a plastic bottle.
Distillation
Distillation is the only purification method that removes both biological contaminants and dissolved chemicals, salts, and heavy metals. It works by evaporating water and collecting the steam, which leaves impurities behind. This is how you can make seawater or brackish water drinkable.
A basic solar still uses the sun’s heat to evaporate water under a clear cover, with condensation dripping into a collection container. The output is modest: a conventional solar still produces roughly 2.5 to 4 liters per square meter per day, depending on temperature and sunlight. Modified designs with reflectors can push that to around 6.5 liters. That’s enough for survival but not enough to be convenient.
You can also distill water over a fire by boiling it in a pot and capturing the steam with an angled lid or tube that drains into a separate clean container. It’s slow and fuel-intensive, but it handles contamination scenarios that no other field method can touch, including water contaminated with chemicals or dissolved minerals.
Handling Cloudy or Murky Water
Every purification method works better with clear water, so pre-treating murky water is worth the effort. The simplest approach is to let water sit undisturbed in a container for several hours, then carefully pour off the clearer water on top. Straining through layers of clean cloth, a bandana, or a coffee filter removes larger sediment and debris.
For heavily turbid water, flocculation can help. This involves adding a substance that causes fine suspended particles to clump together and settle. Alum (aluminum sulfate), available at most pharmacies and grocery stores, is a traditional coagulant. A small pinch, roughly 20 milligrams per liter, stirred into the water and left to settle for 30 minutes, can dramatically improve clarity. After the particles settle to the bottom, pour off the clear water and then disinfect it with your chosen method.
Combining Methods for Best Results
No single portable method covers every type of contaminant. Standard filters miss viruses. Chemical treatments miss Cryptosporidium (except chlorine dioxide). Boiling misses chemical contamination. UV light fails in murky water. The most reliable approach is to combine two methods that cover each other’s gaps.
The most practical combination for most people: filter first, then disinfect chemically or with UV. Filtering removes sediment, parasites, and bacteria while improving water clarity. Chemical disinfection or UV then kills viruses and anything the filter missed. If you’re only going to carry one thing, a filter with a 0.3-micron or smaller pore size paired with a few chlorine dioxide tablets covers nearly every realistic scenario you’d encounter.