How To Distill Water
Boil it, catch the steam, and let it drip into a cup. Distillation removes germs, salt and heavy metals in one step. It also burns about a pound of propane per gallon, and it misses one thing.
Fill a pot halfway, hang a cup from the inside of the upside-down lid, and boil for 20 minutes. The water that drips off the lid into the cup is distilled. That’s the whole method, and it’s the only home method that removes germs, salt and heavy metals in one step.
The catch is output. Plan on small batches for badly contaminated water, not a daily supply.
Every other method on the purifying water guide leaves something behind. Boiling kills germs and concentrates the chemicals. Bleach does nothing for lead. Most filters do nothing for salt. Distillation takes the water away from the problem instead: you boil it, catch the steam, and let it turn back into water somewhere else. Most of what was dissolved stays in the pot.
The pot method, step by step
Ready.gov gives the home version. You need a pot with a lid, a cup, and some string.
- Fill the pot about halfway.
- Tie the cup to the handle on the inside of the lid, so it hangs right side up when the lid is upside down.
- Check that the cup isn’t touching the water.
- Boil for 20 minutes.
In Ready.gov’s words, “The water that drips from the lid into the cup is distilled.” The upside-down lid works as a funnel. Steam hits the cooler metal, condenses, runs down toward the handle and drips into the cup.
Three things decide how much you collect:
- Lid temperature. A cool lid condenses more. Some people set ice or a bag of cold water on top.
- The seal. Steam escaping around the rim is water you don’t get back.
- The cup. A wide cup catches more drips than a narrow one.
Expect a cup, not a pot. Most of the steam condenses on the lid and runs back down the sides into the pot.
| What is in the water | Distillation | Boiling | Verdict |
|---|---|---|---|
| Bacteria, viruses, protozoa | Removed | Killed | Both work |
| Lead, arsenic, other heavy metals | Removed | Concentrated | Distill, never boil |
| Salt and nitrate | Removed | Concentrated | Distill, never boil |
| Gasoline, solvents, VOCs | Not reliably removed | Not removed | Use stored water |
What it removes, and the one thing it misses
The CDC’s list for distillation is long: parasites, bacteria, viruses, arsenic, barium, cadmium, chromium, lead, nitrate, sodium and sulfate. Ready.gov adds that it removes “heavy metals, salts and most other chemicals,” along with microbes that resist boiling and chlorine.
The miss is anything that boils easily. The CDC says distillation does not remove volatile organic compounds, solvents or certain pesticides. Those turn to vapor near or below the temperature water does, and they ride the steam right into your cup.
That gap matters most in the situation where people reach for distillation: floodwater near a gas station, a spill, a fuel leak. If you smell fuel, don’t distill it. Use stored water and wait for testing.
The fuel problem
Distilling costs so much because you’re not just heating water. You’re turning it into steam, and that takes about seven times the energy it took to bring it to a boil.
The math for one gallon, starting at room temperature:
- Heating 8.3 pounds (3.8 kg) of water from 68°F to 212°F takes about 1,270 kJ.
- Turning that water into steam takes about 8,540 kJ more.
- Total: roughly 9,800 kJ, or about 2.7 kWh.
The inputs are the specific heat of water, 4.186 kJ per kg per degree Celsius, and its heat of vaporization, 2,257 kJ per kg.
That’s the floor, with no heat lost. A camp stove loses a lot of heat around the sides of the pot. At an assumed 40 percent efficiency, which varies a lot with the stove and the wind, one gallon costs roughly a pound of propane (about 21,600 BTU). So one small camp cylinder buys you about one gallon of distilled water.
Now compare that with need. Ready.gov’s baseline is one gallon per person per day, and the daily water needs calculator shows what that means for your household. For most families it rules distillation out as a main supply.
Solar stills
A solar still skips the fuel and runs on sunlight. It also produces very little.
The figures collected on Wikipedia’s solar still entry give the scale. A dug pit still about 16 inches across yields around 100 to 150 mL a day, which is half a cup. A 1952 military inflatable still made 1.4 to 2.4 liters a day, still less than one person’s gallon. The same entry notes that a pit still can cost more water in sweat to dig than it gives back.
A solar still is a way to get some drinkable water out of seawater or brackish water when you have nothing else. It won’t keep a household going. Solar stills get their own page.
Countertop distillers
Electric countertop distillers do the pot method with a fan-cooled coil and a timer. They make a gallon at a time, unattended. The physics doesn’t change: each gallon still needs about 2.7 kWh of heat, so a unit that runs for hours per gallon is normal. They also need power, which is often the thing you don’t have when you need one.
When to distill
Distill when the water has something nothing else can remove, meaning salt, nitrate or heavy metals, and when you only need a little: medicine, baby formula, a few days of drinking water for one person.
For germs alone, boiling or bleach is faster and far cheaper. For volume, a filter system plus disinfection beats any still. Distillation vs. filtration vs. boiling puts the three side by side.