How Do You Desalinate Water? A Survival Expert Weighs In

Desalinating water is the process of removing salt from seawater to turn it into freshwater. This makes it safe for humans and animals to drink. On a large scale, desalination is expensive and uses a lot of energy, so it's not widely used. But you can desalinate seawater on a small scale for survival or as a science experiment by making your own solar still! Keep reading to learn how to desalinate seawater using a solar still or by boiling it, with expert tips from wilderness and survival expert Doug Flinders. Later, we'll explore how to desalinate salt water at home, the challenges of desalination, and the health risks of drinking seawater.

How do you desalinate seawater?

Wilderness and survival expert Doug Flinders says to dig a hole in the ground. Place a container in the hole and pour seawater around it. Stretch a tarp over the hole, weigh the edges down, and set a small rock in the center of it. Wait 3-4 hours for the water to evaporate, condense, and fall into the container.

Section 1 of 5:

How to Desalinate Seawater with a Solar Still

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Dig a hole 3–4 ft (0.91–1.22 m) across and 2 ft (0.61 m) deep. Using a shovel, a rock, or your hands, dig a hole in the ground about 3–4 ft (0.91–1.22 m) across to provide plenty of surface area for water vapors to condense. Make the hole approximately 2 ft (0.61 m) deep to leave plenty of room for the water you plan to desalinate.^{[1]
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- Survival tip: This process works best if you're in the wilderness near an ocean on a sunny day. If you're not near an ocean, you can still use the steps below to distill moisture out of the ground—just skip step 3!
Meet the wikiHow Expert

Doug Flinders is a wilderness and survivalist expert. He has a decade of experience teaching wilderness survival and two decades of backpacking and backcountry canoe expeditions.
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Put a container in the center of the hole. Set a watertight container in the hole in the middle and pile dirt or sand up about 1 inch high around it to stabilize it. An old plastic water bottle will do, or use whatever materials are available to make one. For example, you can:
- Assemble a cup or bowl with overlapping strips of duct tape.
- Set a plastic bag upright under the tarp (make sure there are no holes in it!).
- Lay a waterproof jacket in the hole and roll up the edges to make a shallow depression in the middle.
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Pour seawater into the hole around the container. Add about twice the amount of seawater it would take to fill up your container. It's okay if the dirt or sand absorbs it! ^{[2]
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Cover the hole with a tarp. If you don't have a tarp, Flinders suggests using plastic wrap, if any is available.^{[3]
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Expert Source

Doug Flinders
Wilderness and Survival Expert

Expert Interview} Ensure the tarp covers the entire hole, and weigh down the edges with rocks or piles of dirt or sand. Alternatively, stake them into the ground with plastic stakes or branches from a nearby tree.
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Set a rock or another small weight in the center of the tarp. Flinders says to lay a rock in the center of the tarp to make a slope.^{[4]
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Expert Source

Doug Flinders
Wilderness and Survival Expert

Expert Interview} If you can't find a rock, use a seashell or scoop a small mound of sand onto the tarp instead.
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Check the container for water after 3-4 hours. Depending on how hot and humid it is outside, it may take several hours for the saltwater to heat up enough to condense on the bottom of the tarp. Try to wait at least 3-4 hours, so you don't let out any air unnecessarily.^{[5]
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- How does it work? The heat from the sun will warm up the seawater in the hole and cause it to turn into vapor. "As water evaporates… it condenses on the tarp and drips into the container," says Flinders.^{[6]
  X
  Expert Source
  
  Doug Flinders
  Wilderness and Survival Expert
  
  Expert Interview} Meanwhile, the salt falls to the bottom of the hole, so you don't have to drink it.

Section 2 of 5:

How to Desalinate Seawater By Boiling It

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Build a fire and set a pot filled with seawater over it. Cover it with a lid to help it boil faster. The goal is to get it hot and steaming, so you can collect the water vapor and condense it into freshwater.
- Not sure how to start a fire in the wilderness? Read our article on how to build a campfire in 7 easy steps!
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Fold an aluminum foil funnel and set it over the pot of boiling water. Fold a long, rectangular piece of foil into a tube at one end, and leave the other end flat. Remove the lid from the pot and place the flat end of the foil "funnel" on top of the pot. Stuff the tube end into the mouth of a water bottle or container.^{[7]
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Check the container after 1 hour for freshwater. It typically takes about 1 hour to boil off 1 to 1.5 gallons of water. Your container may be full earlier if you boiled less than 1 gallon of seawater.^{[8]
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- How does it work? The vapor from the boiling water funnels through the foil, where it cools off and condenses into water droplets. The droplets drip into the container. Meanwhile, the salt and other minerals in the water fall to the bottom of the pot.

Section 3 of 5:

How to Desalinate Salt Water at Home

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Pour 1 in (2.5 cm) of salt water into a large, clear glass bowl. Fill the bowl up with salt water until the surface comes up about 1 in (2.5 cm) from the bottom.^{[9]
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Research source} The bowl should be wide and tall enough to completely contain a mug or a small cup.
- If you live near an ocean, go out and fill a thermos with seawater to use for this experiment! If not, make your own saltwater by stirring salt into plain drinking or tap water until it tastes salty.
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Set a mug in the bowl and cover it with plastic cling wrap. Ensure the rim of the mug doesn't rise past the rim of the bowl. The bowl should be wide and tall enough to completely contain the mug. Stretch the cling wrap tightly over the mug and the sides of the bowl, leaving no open areas around the bowl's rim.^{[10]
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- Place a small weight on top of the cling wrap above the cup. Use a small rock, a marble, or a heavy magnet. The plastic will sag slightly in the center of the cup due to the weight.
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Set the bowl in direct sunlight for 3-4 hours. Look for a windowsill or a spot on your deck outside that gets lots of direct sunlight. Place the bowl on top of it and leave it for 3-4 hours to build up condensation.^{[11]
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- After several hours in the sun, the water in the bowl will evaporate and rise, forming water droplets on the underside of the cling wrap. The water droplets should then drip into the cup.
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Check the cup for fresh water. Remove the plastic wrap and drink the water in the cup. If you don't taste any salt, that means you've successfully desalinated your water!^{[12]
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- How does it work? As the sun warms up the water in the bowl, the water begins to evaporate. Salt is a mineral and can't evaporate, so it simply falls to the bottom of the bowl.
- When the water vapors rise, the cling wrap prevents them from escaping. When the vapors touch the underside of the cling wrap, they condense into water droplets.
- As the water droplets get bigger, they also get heavier. This causes them to travel down toward the center of the cling wrap, where it sags, and fall into the cup!

Section 4 of 5:

Challenges of Desalinating Seawater

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Desalination doesn't produce much freshwater. Desalinating seawater produces about half as much freshwater. If, for example, you started with 1 gallon of seawater, you'd only get ½ gallon of freshwater from it.^{[13]
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The desalination process takes several hours. If you're stranded somewhere and need to desalinate seawater to drink it, you're going to be thirsty for a long time. Desalinating takes several hours, and it's important to seal up the solar still as long as possible to prevent the water vapor and heat from escaping. Otherwise, you'll have waited all that time for nothing!
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Desalination is extremely expensive on a large scale. Turning seawater into freshwater costs more money than any other water source. Not only does it cost upwards of $1 billion to construct a desalination plant, but running the plant is expensive, as well!^{[14]
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Section 5 of 5:

Risks of Drinking Seawater Without Desalinating It

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The high salt content in seawater can damage your kidneys. As humans, we need to consume a small amount of salt through our food to help balance the chemicals and electrolytes in our bodies. If there's any salt left over, it passes out through our urine. But seawater has far more salt than our bodies need. If you drink seawater, your kidneys won't be able to process all the excess, and you would swiftly get dehydrated. Even if you kept drinking more salty seawater, you would still be thirsty. You could eventually pass away from drinking lots of seawater if you don't get freshwater or medical attention.^{[15]
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Instead of letting the sun do all the work, can I boil the water?

Community Answer

That's essentially what large desalination plants do. Just keep in mind the heat so you don't melt any plastic.

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What happens to the salt?

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The salt remains in solution in the larger bowl, just in greater concentrations. If the concentration becomes high enough, it will precipitate (i.e., form solid salt) in the bottom of the bowl.

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What happens if you leave it out for more than three or four hours?

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Nothing bad will happen to the water if you wait longer than three or four hours.

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