How to Purify Water When You Can't Trust the Source
When the tap stops running, or the water coming out of it is suspect, purifying water comes down to three moves. Start with the cleanest water you can find. Clear it if it's cloudy. Then kill the germs: bring it to a rolling boil for at least one minute (three minutes above 5,000 feet), or, if you can't boil, use the bleach dose in the U.S. Environmental Protection Agency's emergency disinfection guidance and wait 30 minutes.
Here's the catch most quick answers leave out. Every one of those methods targets germs. EPA says boiling or disinfection "will not destroy other contaminants, such as heavy metals, salts, and most other chemicals." So the first decision isn't which method to use. It's whether the water you have can be made safe at all.
This guide is for water of unknown quality: a storm knocked out service, you're drawing from a creek or lake, or floodwater reached your well. If your utility has issued a boil notice and your tap still runs, our boil water advisory guide covers that situation step by step.
Step 1: Start With the Cleanest Water You Can Get
Treatment works best on water that was decent to begin with, so the source matters more than people expect. EPA's guidance on where to find water looks like this:
- Bottled or stored water first. EPA calls bottled water your best choice, followed by water you prepared and stored ahead of time.
- Water already inside your house. Melted ice cubes and water drained from your hot water tank or pipes are options EPA names.
- Flowing surface water. River or lake water can work, and EPA says flowing water is generally better than still, stagnant water.
Skip any water with floating material in it, a dark color or a questionable odor. EPA says not to use it.
These are germ treatments, full stop. The Washington State Department of Health says emergency treatments like these work only against bacteria or viruses, and that if you suspect chemicals, oils or other poisonous substances in the water, you shouldn't drink it. If fuel or chemicals may have gotten into your water, use bottled or stored water and check with your local health department. And if you can't treat suspicious water at all, Ready.gov advises putting off drinking it as long as possible, without letting yourself get dehydrated.
Step 2: Clear Cloudy Water Before You Treat It
Cloudy water gets a quick cleanup first. Let it sit so the particles settle. Pour off the clearer water, then run it through a clean cloth, paper towel or coffee filter. That's EPA's method, and it uses whatever's in your kitchen drawer.
This step isn't about looks. Particles get in the way of treatment. The CDC's traveler water guidance says small particles can block germs from UV light and that solar disinfection doesn't work on cloudy water, and EPA doubles its bleach dose when water is cloudy.
A coffee filter doesn't make water safe to drink, though. It just gets the water ready for the step that does.
Step 3: Kill the Germs
You've got four realistic options, depending on whether you have heat, household bleach, tablets or sunlight.
Boil It
Boiling is the most reliable choice when you have fuel. EPA, citing the World Health Organization, says boiling "is sufficient to kill pathogenic bacteria, viruses and protozoa." Bring the water to a rolling boil for at least one minute, let it cool on its own, and store it in clean containers with covers.
Altitude is where the agencies differ. EPA says to boil for three minutes above 5,000 feet. CDC's traveler guidance puts that line at 6,500 feet. If you're anywhere above 5,000 feet, boil for three minutes and you've met both.
Boiled water tastes flat. EPA suggests pouring it from one clean container to another several times, or adding a pinch of salt to each quart or liter. For what boiling leaves behind, and why it can concentrate some contaminants, see our breakdown of whether boiling water actually purifies it.
Use Unscented Household Bleach
If you can't boil, EPA's next option is regular, unscented chlorine bleach that's labeled for disinfection and sanitization. Skip scented, color-safe or bleaches with added cleaners. Use a fresh bottle, or one stored at room temperature for less than a year, and find a clean dropper.
Check the label before you count drops. EPA's dose depends on whether your bleach is 6% or 8.25% sodium hypochlorite:
| Water to treat | 6% bleach | 8.25% bleach |
|---|---|---|
| 1 quart or liter | 2 drops | 2 drops |
| 1 gallon | 8 drops | 6 drops |
| 2 gallons | 16 drops (1/4 teaspoon) | 12 drops (1/8 teaspoon) |
| 4 gallons | 1/3 teaspoon | 1/4 teaspoon |
| 8 gallons | 2/3 teaspoon | 1/2 teaspoon |
Double the dose if the water is cloudy, colored or very cold. Stir, then let it stand for 30 minutes. The water should have a slight chlorine odor. If it doesn't, EPA says to repeat the dose and wait another 15 minutes. If the chlorine taste is too strong, pour it between clean containers and let it stand for a few hours.
You'll find other bleach charts online, including some from state health departments, and their numbers don't always match EPA's. Pick one agency's instructions and follow them all the way through instead of mixing charts.
Bleach has a blind spot, though. CDC says disinfectants kill most harmful viruses and bacteria but aren't as effective against tougher parasites such as Cryptosporidium and Giardia. Its traveler guidance describes using a disinfectant to make a small amount of filtered water safe to drink. A coffee filter only clears particles, though. If parasites are a worry, boil the water, or run it through a filter certified for cyst reduction before you add bleach.
Tablets and Iodine
Disinfection tablets containing chlorine, iodine or chlorine dioxide are sold online and at pharmacies, according to EPA. Strengths vary, so follow the package directions. If parasites worry you, look for chlorine dioxide. CDC says those tablets can be effective against Cryptosporidium when you follow the manufacturer's instructions correctly.
Plain tincture of iodine from a first aid kit also works in a pinch. EPA's dose is five drops of 2% tincture per quart or liter, or 10 drops if the water is cloudy or colored, then at least a 30-minute wait. CDC says not to drink iodine-treated water if you're pregnant, have thyroid problems or a known sensitivity to iodine, and not to use it for more than a few weeks at a time.
Portable UV and Sunlight
Portable units that deliver a measured dose of UV light can help disinfect small amounts of clear water, according to CDC. Clear is the key word. CDC doesn't recommend them for cloudy water.
Sunlight is slow, but it costs nothing. For emergencies, CDC describes filling clear plastic soda bottles, from 0.3 to 2 liters, with clear water, shaking them, and setting them on a roof or rack in the sun for 6 hours, or 2 days if it's cloudy. Here's a quick test from the same guidance: if you can't read newspaper headlines through the bottle, filter the water before you try it.
What a Filter Can and Can't Do
A filter is only as good as its pore size, and most portable filters aren't built to stop everything. CDC's traveler guidance breaks it down like this:
- Most portable filters can remove parasites if the pore size is small enough, but they don't remove bacteria or viruses.
- A filter certified under NSF/ANSI Standard 53 or 58 for cyst reduction removes parasites when it's used properly. Check that the listing names cyst reduction, not just lead or chlorine. It won't remove viruses or all bacteria.
- Reverse osmosis removes bacteria and viruses, and it can remove salt too.
That's the logic behind pairing a filter with a disinfectant. The filter takes out particles and parasites, and the chemical step handles the germs a filter's pores let through. Check the label and follow the manufacturer's instructions for whichever filter you use.
If your utility issues a boil notice, keep boiling even if you have a filter under the sink or on the counter. Our boil water advisory guide explains why a household filter isn't a substitute, and what to do with it once the notice lifts.
For Cabins, Boats and Long Outages
Treating a pot at a time gets old fast when an outage drags on or you live off the grid. That's the job Crystal Quest® built its Off-Grid Portable Reverse Osmosis Water System to do. It draws water from a lake, stream or river and runs it through an ultrafiltration membrane, a reverse osmosis membrane and a coconut-based carbon block, powered by its own solar panel and battery.
It's rated for about 200 gallons per day and weighs about 50 to 55 pounds, so it belongs at a cabin, on a boat or in a disaster kit, not in a backpack. It was listed at $2,889 in September 2026.
Emergency Water Treatment Methods Compared
Here's how the options line up, based on the EPA and CDC guidance above. The table describes each type of treatment the way those agencies do. It isn't a performance rating for any specific product.
| Method | Bacteria and viruses | Parasites | Chemicals and metals | What you need |
|---|---|---|---|---|
| Boiling | Yes | Yes | No | Heat source, pot, 1 to 3 minutes at a rolling boil |
| Unscented bleach | Most | Less effective | No | Fresh bleach, dropper, 30 minutes |
| Chlorine dioxide tablets | Most | Can work on Cryptosporidium if used as directed | No | Tablets, package wait time |
| Iodine | Most | Less effective | No | 2% tincture, 30 minutes; not in pregnancy, with thyroid problems or iodine sensitivity, or for more than a few weeks |
| Portable UV | Some germs, clear water only | Not listed separately by CDC | No | Batteries, small batches of clear water |
| Most portable filters | No | Yes, with a small enough pore size | Depends on the filter | Follow with disinfection |
| Reverse osmosis | Yes | Not listed separately by CDC | Depends on the system | Power and pressure |
Look down the chemicals column. Every heat, chemical and light method on this list is a germ treatment, which is why picking the right source in Step 1 matters so much.
If Floodwater Reached Your Private Well
A flooded well needs more than a pot of boiling water. EPA says a well that's been flooded should be disinfected and tested after the flood, and your state or local health department can advise you on next steps. Until the well has been disinfected and tested, rely on bottled or stored water.
Our guide to shock chlorinating a well walks through the disinfection itself, including how to protect a water softener or RO system while you do it.
Then test the water before anyone drinks from the well again. Our guide to testing well water walks through that side of it.
Build a Small Water Kit Now
Ready.gov, FEMA's preparedness site, recommends storing at least one gallon of water per person per day for several days, for drinking and sanitation. Children, nursing mothers and sick people may need more, and in very hot temperatures water needs can double.
Then keep the treatment basics together so you aren't hunting for them during an outage:
- Regular, unscented bleach, replaced before it's a year old, plus a clean dropper
- Coffee filters or clean cloths for clearing cloudy water
- Clean containers with covers
- A pot and a heat source that doesn't depend on household power
- Chlorine dioxide tablets as a lightweight backup
Planning water for long outages or off-grid living?
Tell us your water source and setup, and we'll help you figure out what fits. For cabins, boats and long outages, the Off-Grid Portable RO treats lake, stream and river water on solar power.
Frequently Asked Questions About Purifying Water
Can I use granular calcium hypochlorite instead of liquid bleach?
Yes, but it takes two steps and a lot more care. EPA's method starts by dissolving one heaping teaspoon, about a quarter ounce, of high-test granular calcium hypochlorite in two gallons of water to make a chlorine solution, working in a ventilated area with eye protection. Stir until the particles have dissolved. You then add one part of that solution to 100 parts of the water you're treating, about one pint per 12.5 gallons. That solution is only for dosing, so never drink it straight. It's a very powerful oxidant, so follow the label for handling and storage.
Is the water in my water heater safe to drink?
It's a usable emergency source, not a guaranteed safe one. EPA lists draining your hot water tank or pipes as a way to find water at home, and it says to treat water from any source with the boiling or disinfection steps before you drink it.
